Chantal Block

Augustina Ephraim

Elsa Weiss-Hortala

Doan Pham

Ange Nzihou

Carlo Vandecasteele

D Pham Minh

Co-pyrogasification of Plastics and Biomass

Keywords: Biomass, Plastics, Co-pyrogasification, Waste, Waste management

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The annual worldwide production of plastic materials mainly from fossil fuel increased from 1.7Mt/y (1.7 10 9 kg/y) in 1950 to 322 Mt/y in 2015 [START_REF] Plasticseurope | Plastics-the facts 2016, Analysis of european latest plastics production, demand and waste data[END_REF]. China (27.8%) was the largest producer in 2015, followed by the rest of Asia (incl. Japan) (21.0%), Europe (18.5%) and NAFTA (18.5%) [START_REF] Plasticseurope | Plastics-the facts 2016, Analysis of european latest plastics production, demand and waste data[END_REF]. In Europe (EU28 + NO, CH) plastics production amounted in 2015 to 58 Mt/year [START_REF] Plasticseurope | Plastics-the facts 2015, Analysis of european latest plastics production, demand and waste data[END_REF] and 25.8 Mt (2014) of post-consumer plastics waste was generated. Treatment of postconsumer plastic waste is thus an important and growing challenge. The 7 most common plastics are, with between brackets the % of European plastics demand for each polymer in 2014 (plastics Europe, 2015): polypropylene (19.2%), PP or (C3H6)n; low-density polyethylene (17.2%), LDPE or (C2H4)n; high-density polyethylene (12.1%), HDPE or (C2H4)n; polyvinylchloride (10.3%), PVC or (C2H3Cl)n; polyurethane (7.5%), PUR or C17H16N2O4; poly(ethylene terephthalate) (7.0%), PET or (C108O4)n; and polystyrene (7.0%), PS or (C8H8)n. The EU recommends a waste treatment hierarchy with the following order of preference [START_REF]Council of the European Union[END_REF]: preparing for reuse > recycling > recovery > landfill To date, in many countries landfill is still the first option for disposal of postconsumer plastics waste; in the year 2014, 38 % of the plastic waste was landfilled in the EU [START_REF]Regulatory barriers for the circulatory economy : Lessons from ten case studies, Final report[END_REF] whereas 68.5 % was landfilled in the US [START_REF]Advancing Sustainable Materials Management: 2014 Fact Sheet[END_REF]. A landfill ban exists however in Switzerland, Austria, The Netherlands, Germany, Sweden, Luxembourg, Denmark, Belgium and Norway, so that landfill is negligible in these countries. Value (material, energy) can be recovered from postconsumer plastic waste by the following treatment routes [START_REF] Al-Salem | Recycling and recovery routes of plastic solid waste (PSW): A review[END_REF], [START_REF] Al-Salem | The valorization of plastic solid waste (PSW) by primary to quaternary routes: From re-use to energy and chemicals[END_REF]:

 Mechanical recycling, in which end-of-life plastic waste is used as feedstock to manufacture plastics via mechanical means. The treatment may include size reduction, separation of contaminants and of other plastics, milling, washing, drying. The more complex and contaminated the waste, the more difficult is mechanical recycling. It is therefore the preferred recovery route for homogeneous and relatively clean plastic waste, provided end markets exist for the recyclate.  Chemical or feedstock recycling, whereby plastics are converted into smaller molecules (plastic monomers, syngas), suitable for use as feedstock for the production of the original or other products. The most important technologies for feedstock recycling are pyrolysis and gasification (pyro-gasification). Pyrogasification has the advantage that it can be applied for many sorts of plastics, but also for biowaste, wood….. In each case syngas is obtained, which can be combusted to generate electricity. It is however preferable to use the syngas obtained from gasification to produce various chemicals (H2, methanol, ammonia, urea) and also fuels (e.g. Fischer-Tropsch diesel). Feedstock recycling (and particularly pyrogasification) has greater flexibility and is more tolerant to impurities than mechanical recycling, but is capital intensive.  Energy recovery. The waste can be combusted to produce energy in the form of steam, heat and/or electricity. Alternatively, the waste can be gasified to give synthesis gas (syngas), which can be used to generate steam via combustion boilers, or, after removal of particles and tar [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF], [START_REF] Bosmans | Waste-to-clean syngas: avoiding tar problems[END_REF], to produce electricity in a gas engine, or, for high efficiency electricity production, in a gas turbine or fuel cell [START_REF] Castaldi | What's new and what are the prospects in the field of energy and added-value materials from biomass and waste?[END_REF]. In 2014 in Europe 29.7% of postconsumer plastic waste was recycled (by far the most by mechanical recycling, in 2012 only 0.3% went to feedstock recycling [START_REF] Plasticseurope | Plastics-the facts 2013, Analysis of european latest plastics production, demand and waste data[END_REF]; 39.5% went to energy recovery; 30.8 % was landfilled) [START_REF] Plasticseurope | Plastics-the facts 2016, Analysis of european latest plastics production, demand and waste data[END_REF]. This would appear a rather positive situation, but it should not be forgotten that much of the plastics collected in the EU in view of recycling or energy recovery are in fact exported to China, where their fate is less certain [START_REF] Velis | Plastic waste -recycling via export ? Implications for resource and value recovery[END_REF]. Pyro-gasification is feedstock recycling, if the gas obtained is used for producing materials from plastic waste; it must be classified as recovery if the gas (or oil) is used for producing heat and/or electricity (or as biofuel). A lot of potential remains for pyro-gasification (both as feedstock recycling or as energy recovery) of plastic waste, as this would reduce the volume of material sent to landfill thus saving landfill space, and allow valorization as material or energy. Pyro-gasification (with air, pure oxygen, steam and carbon dioxide or their mixtures as gasifying agent) of pure plastics is treated in many publications: Xiao et al., 2007 [13], Ongen, 2016 [START_REF] Ongen | Methane-rich syngas production by gasification of thermoset waste plastics[END_REF], Toledo et al., 2011 [15], Erkiaga et al. 2013 [START_REF] Erkiaga | Syngas from steam gasification of polyethylene in a conical spouted bed reactor[END_REF], Wu and Williams, 2010 [START_REF] Wu | Pyrolysis-gasification of plastics, mixed plastics and realworld plastic waste with and without Ni-Mg-Al catalyst[END_REF], Acomb et al., 2014 [18], Wilk and Hofbauer, 2013 [START_REF] Wilk | Conversion of mixed plastic wastes in a dual fluidized bed steam gasifier[END_REF], Kannan et al., 2013 [20], Cho et al., 2013 [21], [START_REF] Cho | Production of low-tar producer gas from air gasification of mixed plastic waste in a two-stage gasifier using olivine combined with activated carbon[END_REF], Cho et al., 2014 [23], Lopez et al., 2015 [24], Arena et al., 2011 [25], Arena and Gregorio, 2014 [START_REF] Arena | Energy generation by air gasification of two industrial plastic wastes in a pilot scale fluidized bed reactor[END_REF], Kim et al., 2011 [27], Lee et al., 2013 [28], Salbidegoitia et al. 2015 [START_REF] Salbidegoitia | Steam gasification of printed circuit board from ewaste: Effect of coexisting nickel to hydrogen production[END_REF]. Pyrogasification of mixed plastics is treated by Friengfung et al., 2014 [30], Martinez-Lera et al., 2013 [START_REF] Martínez-Lera | Design and first experimental results of a bubbling fluidized bed for air gasification of plastic waste[END_REF], Saad and Williams, 2016 [START_REF] Saad | Pyrolysis-Catalytic-Dry Reforming of Waste Plastics and Mixed Waste Plastics for Syngas Production[END_REF]. It is clear that pyro-gasification as waste treatment method does not so much target pure plastics, or plastics that can easily be purified, as these are preferably treated by mechanical recycling, but rather mixed plastics and plastics mixed with or contaminated by other waste. The main issues in using plastics alone during pyro-gasification and catalytic gasification with in bed material are related to the product composition and the reactor technology. Plastic waste degradation may pollute bio-oil or syngas with corrosive and toxic components (HCl, benzoic acid, tar and so on) that will lower the industrial potential of the technologies. Regarding reactor technology, it should be carefully designed for suitable feeding, mass and thermal transfers [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF]- [START_REF] Anuar Sharuddin | A review on pyrolysis of plastic wastes[END_REF]. Moreover, operational problems may occur with plastics gasification, relative to plastics feeding, and in an air blown bubbling fluidized bed, relative to agglomeration of the bed material [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF]. Biomass or waste biomass is a suitable feedstock for pyro-gasification, and one of the favorite feedstocks in literature [START_REF] Kumar | Thermochemical Biomass Gasification: A Review of the Current Status of the Technology[END_REF]- [START_REF] Balat | Main routes for the thermo-conversion of biomass into fuels and chemicals. Part 1: Pyrolysis systems[END_REF]. An advantage of pyro-gasification (like for other thermochemical conversion technologies as combustion and pyrolysis) is that almost any type of biomass may be used as feedstock, including wood waste from industry or from MSW, agricultural residues, forestry residues, byproducts or waste from the food or feed industry and from the bio-industry, and organic municipal waste. The ideal feedstock for pyro-gasification is not so much pure biomass e.g. wood or food waste, where many other possibilities exist for use or recycling (construction, furniture, fiber panels for wood and wood waste; feed for food waste) or for energy recovery (pellets for central heating, for CHP, etc.). According to Eurostat data (Eurostat, 2012), in 2011 the production of roundwood, including wood and wood waste, accounted for 429 million m 3 for the 27 countries in Europe in 2011. Among this roundwood it could be assumed that 10-15% is wood waste. In the US, the availability of biomass is unequally distributed over the country and represents 408 million tonnes dry/year (7.5% of urban wood waste: MSW wood, tree trimming waste, construction/demolition wood) [START_REF] Milbrandt | A geographic perspective on the current biomass resource availability in the united states[END_REF]. Taking into account the wood density, this value is quite the double of the roundwood availability in Europe. Regarding wood waste, the classification proposes 4 classes: grade A (clean wood, wood from pallets, packaging, and so on); grade B (industrial wood processing, including construction, demolition, furniture materials and so on); grade C (fuel grade, this is composite of wood, rubber, paint and so on, such as MDF, chipboard) and grade D (hazardous wastes contained metals impregnated and treated woods).

Co-gasification of plastics and biomass allows to improve the quantity of the syngas, as well as its composition (e.g. H2/CO), thus increasing compliance with subsequent applications [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF]. Additional motivations for co-gasifying biomass and plastics are [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF] :

-It helps to overcome difficulties of seasonal biomass availability [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF], [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF].

-Increasing the fraction of plastics in a biomass-plastic feed, during steam gasification, would increase conversion of the fuel to gas, and reduce the production of char and tar [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Moghadam | Syngas production from palm kernel shell and polyethylene waste blend in fluidized bed catalytic steam co-gasification process[END_REF]. -It allows plastics to be fired in a downdraft gasifier [START_REF] García-Bacaicoa | Gasification of biomass/high density polyethylene mixtures in a downdraft gasifier[END_REF].

-The operational problems mentioned earlier relative to pyro-gasification of plastics alone can also sometimes be solved by co-gasification of plastics and biomass [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF]. Robinson et al., 2016 [34] showed e.g. that, when an air-blown bubbling fluidized bed was fed with composite wood/PET pellets (50/50) problems with coking were prevented. This would be due to a better thermal transfer and fluidization using the composite pellets since the density difference between wood (0.3 to 0.7 g/cm 3 ) and PET (1.35 to 1.38 g/cm 3 ) is significant. Taking into account the different densities of the raw materials, pelletization will tend to improve reactor processing, especially in fluidization technology. However, pelletization using additive, as used by Robinson et al. 2016 [34], may introduce organic and/or inorganic elements that could act as inhibitors or catalysts of the pyro-gasification process, or may introduce pollutants in the syngas. -Co-gasification may be the method of choice for packaging waste consisting of plastics together with wood, paper, or cardboard and difficult to separate from it.

-Brachi et al. also mentioned that the use of biomass and polymeric fuel blends (PET and tyre blends) has proven a useful strategy for overcoming the limitations and operational problems related to using only polymeric fuels, such as the tendency of plastics to become viscous and sticky when heated, the seasonal availability of some types of biomass, e.g. olive husk, nut shells and grape residues, and the low energy density of biomass. They also claim that co-gasification of PET and wood enables a product gas composition such that no further conditioning in a water-gas shift reactor is required in view of downstream methanol production, thus simplifying the overall process. Robinson et al., 2016 [34] suggested a nice application for small, remote communities: nonbiodegradable plastic waste, such as water bottles or plastic food containers are targeted and diverted from the community landfill, and co-gasified with biomass, in the form of composite pellets. The combustible gases can be used to generate power, in order to replace power supplied by diesel engines, which were so far the only way for power production in the considered remote locations and resulted in high electricity prices. There exists a significant amount of literature on pyro-gasification of mixes of different sorts of biomass, with one type of plastic, with plastics mixes, etc., a large part of which appeared in the last few years, but we do not know a review paper on this topic. The present paper will review this literature, in a broad sense: sometimes coal is added to plastics-biomass mixes, and several papers also consider mixtures of coal with plastics. When these papers are relevant for the present discussion, they will be included. The review paper will compare the behaviour of the mixtures with that of the pure components. Particular attention will be paid to the product yield and distribution (amounts of gas, solid residue and liquid obtained), and to the composition of the gas mixture, all this in relation to potential applications of the products obtained. The purpose of generating producer gas or syngas is of course its subsequent application. The syngas generated may contain several components, which can cause serious problems (fouling, clogging, corrosion, catalyst poisoning, etc.) during subsequent use of the syngas. Table 1 summarises for each relevant syngas contaminant, the emission concentration in waste gasification, the concentration limit for each application, and the concentration limit of the EU emission standard. It is clear that for all relevant parameters, the limits set by the quality requirements of the applications are, in general, lower than the emissions. Two-step oxidation where the syngas is combusted to produce heat, used to drive an externally fired cycle is relatively insensitive to syngas quality, but the emission of pollutants is a major environmental issue and the results should comply with the EU emission standards [START_REF] Consonni | Waste gasification vs. conventional Waste-To-Energy: A comparative evaluation of two commercial technologies[END_REF].

Table 1: Main syngas contaminants with their emissions in waste gasification and the target levels of the major applications [START_REF] Anuar Sharuddin | A review on pyrolysis of plastic wastes[END_REF], [START_REF] Balat | Main routes for the thermo-conversion of biomass into fuels and chemicals. Part 2: Gasification systems[END_REF]- [START_REF] Ruiz | Biomass gasification for electricity generation: Review of current technology barriers[END_REF]. When pyro-gasification is followed by downstream synthesis, in addition to the maximum values for contaminants given in Table 1, a specific composition of the syngas is required, e.g. for methanol production 71% of H2 and 19% of CO are required (Table 2). Also the total concentration of inert gases (N2, and Ar) must be as low as possible, typically below 2%. These inert gases are not really toxic for the synthesis, but they reduce the partial pressure of the reactants in recycle processes [START_REF] Boerrigter | Syngas production and utilisation[END_REF]. To bring the produced syngas in line with the requirements Tables 1 and2, gas treatment is required, including gas purification, where contaminants are removed that would otherwise interfere with the subsequent use of the syngas (Table 1), or conditioning where undesirable major gas components are removed or converted, and the ratio of the relevant main components is adjusted to the appropriate value. The water gas shift reaction allows e.g. to convert CO in CO2 and H2 upon addition of steam. Tar is one of the most critical components and its removal is essential for syngas application. Recently several review papers were published on tar removal or reduction from syngas from biomass or plastics pyro-gasification [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF], [START_REF] Bosmans | Waste-to-clean syngas: avoiding tar problems[END_REF], [START_REF] Arena | Tar removal during the fluidized bed gasification of plastic waste[END_REF]- [START_REF] Hervy | Multi-scale characterisation of chars mineral species for tar cracking[END_REF]. It is clear that each purification or conditioning step makes the process more complicated and more expensive. Therefore it would be ideal if one could approach as much as possible the required specifications, without further treatment steps. There exist traditional primary measures that can be used to modify the ratio of main components in the syngas and to remove tar. These consist in optimising the gasifier configuration and the operating conditions (temperature, equivalence ratio, pressure, gasifying agent, additives or catalysts in the bed). In addition we believe that selecting a suitable plastic, biomass and plastic/biomass ratio allows also to approach the desired gas composition. Therefore the paper will subsequently discuss the influence of the feedstock, the gasifying agent, the temperature, the equivalence ratio, and catalysis.

Contaminant mg/Nm

Pyro-gasification

2.1.Principle, gasifying agents and reactions

Pyro-gasification is a thermal treatment usually including the following steps: drying, pyrolysis or devolatilization, char gasification, and (partial) oxidation. There is no sharp boundary between these different steps, they often overlap. The term pyro-gasification is used as we do not only consider the gasification step, but we include also the pyrolysis step, and it is difficult to distinguish pyrolysis and gasification, as 'pyrolysis' under N2 gas will also give gasification products because of the water (moisture) and oxygen present in the feedstock and in the feedstock molecules. The feedstock, which in the case of biomass contains typically 10 to 20% of water, is first dried and then undergoes pyrolysis or devolatilization. This occurs in the absence of oxygen gas and at temperatures between 300 and 800°C. The products of pyrolysis may be classified into three principal types, the relative yields of which depend on the heating rate, the pyrolysis temperature, the residence time in the reaction zone and the composition of the waste [START_REF] Bridgwater | Renewable fuels and chemicals by thermal processing of biomass[END_REF]:

 Solid (mostly char)  Liquid (tars, heavy hydrocarbons, oil, water)  Gas (CO2, H2O, CO, C2H2, C2H4, C2H6, C6H6, etc.). Secondary reactions may then take place involving the volatile products. A low heating rate, a high temperature and a long residence time favour conversion to char; similar conditions but with a longer residence time conversion to gas; and finally fast heating, intermediate temperature and short residence time favours conversion to oil [START_REF] Antal | The Art, Science, and Technology of Charcoal Production[END_REF]- [START_REF] Abnisa | A review on co-pyrolysis of biomass: An optional technique to obtain a high-grade pyrolysis oil[END_REF]. For biomass, high lignin tends to increase the char yield, high cellulose and hemicellulose the gas and oil yields [START_REF] Varhegyi | Kinetics of the Thermal-Decomposition of Cellulose, Hemicellulose, and Sugar-Cane Bagasse[END_REF], [START_REF] Couhert | Is it possible to predict gas yields of any biomass after rapid pyrolysis at high temperature from its composition in cellulose, hemicellulose and lignin?[END_REF]. Pyrolysis is followed by gasification, which can take place in the same reactor or in a subsequent one, the overall process will be called pyro-gasification. Gasification takes place at high temperature (the heat being supplied directly or indirectly), ranging from 500 to 1500°C, and requires a gasifying agent like air, pure oxygen, steam, carbon dioxide or a combination thereof. Under these circumstances, the large molecules are further converted into lighter molecules and eventually into permanent gases (CO, H2, CO2, CH4, and light hydrocarbons), tar, char, and ash. Tar and char result from incomplete biomass conversion. The final gas product is either:

-producer gas, a mixture of gas produced by gasification at relatively low temperature, 700 to 1000°C, and composed of CO, H2, CO2, CH4 and trace amounts of higher hydrocarbons, inert gases from the gasifying agent and other contaminants e.g. char particles. It is usually combusted in a boiler for heat, or in an internal combustion gas engine to generate electricity or combined heat and power, and the heating value depends on the type of gasifying agent and gasification process; -(bio)syngas (synthesis gas), a mixture of CO and H2, which after clean-up to remove impurities can be used as feedstock for the chemical industry to produce organic molecules.

For simplicity we will speak of syngas or product gas, and will not distinguish between both. Key reactions that occur in gasification are, depending on the gasifying agent:

C + H2O  CO + H2 H 0 298 = +131 kJ/mol (1) (primary water gas reaction)

C + CO2  2 CO H 0 298 = +173 kJ/mol (2) (Boudouard reaction)
CO + H2O  CO2 + H2 H 0 298 = -42 kJ/mol (3) (water gas shift reaction, WGS)

The carbon in the reactions refers to the char generated by earlier pyrolysis. Reactions (1) and ( 2), as well as the combination of ( 1) and ( 3), the secondary water gas reaction, are highly endothermic.

To provide the heat for drying, pyrolysis and gasification, some exothermic combustion is allowed by injecting air or pure oxygen, the most important oxidation reactions being: 8) and [START_REF] Bosmans | Waste-to-clean syngas: avoiding tar problems[END_REF] show that syngas can be produced with steam (steam reforming) or with CO2 (dry reforming). If the gasifying agent contains no or little oxygen, the thermal energy necessary for drying, pyrolysis, and endothermic reactions comes from exothermic combustion, or another source of energy, outside (allothermic) the gasifier. If it contains steam or CO2 and oxygen, the heat is generated within the reactor by reaction with oxygen (autothermic).

C + 0.

Gasifiers

Various technologies are employed for the gasification of solid fuels. Most frequently used in practice are fixed bed reactors (updraft and downdraft), and fluidized-bed reactors (bubbling and circulating) (Figures 1 and2). However, rotary kilns, moving grate systems, and plasma gasifiers were all used on occasion (Figures 3 and5). Plasma reactors provide very high operating temperatures up to some 5000 °C and ensure complete destruction of toxic compounds, so they are ideal for treatment of some hazardous waste streams. The economic viability of this technology for the applications considered here remains to be proven. In an updraft (counter-current) fixed bed reactor the feedstock enters from the top and moves downwards, whereas the gasifying agent (air, oxygen, steam or carbon dioxide) is introduced below, so that it and the product gas flow upward. An updraft reactor typically gives a syngas with a high tar concentration [START_REF] Belgiorno | Energy from gasification of solid wastes[END_REF], as the tar formed in the pyrolysis zone is carried upward, away from the high temperature zone above the grate where feedstock combustion takes place, and is thus not (partially) oxidized. In a downdraft (co-current) reactor, the feedstock is added from the top whereas the gasifying agent is introduced at the sides. As the gas obtained is withdrawn to the bottom, all products (including tar) pass through the high temperature zone at the base of the reactor, just above the grate, where the tar is oxidized, so that the tar concentration is much lower. In a fluidized bed gasifier the bed material consisting of fine solids, is fluidised by the action of the gasification medium that flows in through the nozzle bottom [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF], [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF], [START_REF] Pohorely | Gasification of coal and PET in fluidized bed reactor[END_REF], [START_REF] Van Caneghem | Fluidized bed waste incinerators: Design, operational and environmental issues[END_REF]. The bed material can be inert (sand) or catalytically active (dolomite, olivine, etc.). The velocity of the gas through the bed determines the bed expansion: low gas velocities give a bubbling fluidized bed (BFB), high velocities cause the bed to expand more and result in entrainment from the reactor of bed particles, which are captured by a downstream cyclone and recirculated (circulating fluidized bed, CFB) into the bed. Shredded feedstock is fed into the fluidized bed. The constant turbulence of the fuel and the bed material ensures an intense contact between the bed material and the fuel, resulting in high rates of heat transfer between inert material, feedstock and gas. The processes of drying, pyrolysis, oxidation and reduction take place more or less homogeneously in the entire, nearly isothermal bed. This results in intermediate tar levels and low unconverted carbon. The operating temperature is usually between 700 and 900°C and the pressure ranges between 0 and 70 bar. A definite disadvantage is bed sintering when a feedstock containing ash of high content and low melting temperature is used [START_REF] Billen | Predicting Melt Formation and Agglomeration in Fluidized Bed Combustors by Equilibrium Calculations[END_REF]. A fluidized bed reactor is an updraft gasifier. Gasifiers can also be classified according to the way heat is supplied for the endothermic gasification reactions: directly or indirectly. In a directly heated gasifier (autothermic), part of the biomass is (partly) combusted in the gasifier, raising the temperature and providing the heat needed for the endothermic gasification reactions. In an indirectly heated gasifier (allothermic), biomass or un-gasified char is combusted separately and heat exchanger tubes conduct the heat to the gasification chamber. A novel fluidized bed gasification concept was described by Wilk and Hofbauer, 2013 [66], a 2-stage gasifier. It comprises 2 separate reactors: a gasification reactor with steam as fluidizing medium, where gasification takes place, and a combustion reactor with air as fluidizing medium, where combustion takes place. Bed material circulates between the 2 reactors and connects them thermally, carrying heat from the combustion zone to the gasification zone. Feedstock is inserted in the gasification reactor, where it reacts with steam and forms the product gas. The remaining un-gasified char is transported to the combustion reactor with the circulating bed material, where it is combusted with air and heats up the bed material. The bed material is separated from the flue gas and returned to the gasification reactor where it supplies the heat for the endothermic gasification reactions. Two different gas streams are obtained: product gas and conventional flue gas. Wilk and Hofbauer, 2013 [START_REF] Wilk | Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels[END_REF] showed that this 2-stage gasifier can be used for co-gasification of plastics and biomass. Several authors promoted two-stage reactors for pyro-gasification, whereby in the first stage pyrolysis takes place, and the evolved gases from pyrolysis are passed to the second reactor where (catalytic) dry reforming by carbon dioxide or reforming by steam occurs [START_REF]Production of steam cracking feedstocks by mild cracking of plastic wastes[END_REF]- [START_REF] Barbarias | Pyrolysis and in-line catalytic steam reforming of polystyrene through a two-step reaction system[END_REF]. Serrano et al. 2012 [START_REF] Serrano | Developing Advanced Catalysts for the Conversion of Polyolefinic Waste Plastics into Fuels and Chemicals[END_REF] and Williams 2016 [START_REF] Saad | Pyrolysis-Catalytic-Dry Reforming of Waste Plastics and Mixed Waste Plastics for Syngas Production[END_REF] highly recommended two-stage 'pyrolysis catalysis' as this improves contact between pyrolysis products and catalyst, enables the reacted catalysts to be recycled, makes the process more controllable (temperature at each stage; greater control of catalytic process conditions) and finally makes that waste residues and dirt associated with the plastics remain in the pyrolysis unit.

Pyro-gasification of biomass/plastics mixtures

The composition of the feedstock has an important influence on the distribution, composition and characteristics of the gas, liquid and solids produced by pyro-gasification. Operational parameters such as temperature, equivalence ratio, gasifying agent (O2, air, steam, CO2), ratio steam to fuel or carbon dioxide to fuel, and used catalyst, play also a significant role.

Overview

Table 3 gives an overview of relevant papers on the pyro-gasification of plastics along with biomass, with biomass and coal, and occasionally with coal alone.

Properties and thermogravimetric behavior of plastics and biomass

Most of the researches in thermochemical conversion of plastics have been carried out with thermoplastics polymers, such as PET, PE, PVC, PP, PS or a mixture of these compounds. The plastic structure is a long hydrocarbon chain that could contain aromatic cyclic groups or oxygenated groups. From their chemical formula, these polymers except for PET and PVC, are only composed of carbon and hydrogen. However, they may also contain low contents of oxygen due to the presence of additives, impurities or moisture. In addition, it has been shown that the volatile matter content is very high (at least 94% except for PET).

It should be noticed that PET, PE (LDPE and HDPE) and PP, first melt and then decompose; for PVC, degradation and melting temperature are very close; for PS the degradation temperature is clearly lower than the melting temperature [START_REF] Matsuzawa | Acceleration of cellulose co-pyrolysis with polymer[END_REF].

Many authors gave TG curves for biomass, wood and different types of plastics [START_REF] Jakab | Thermal decomposition of polypropylene in the presence of wood-derived materials[END_REF]- [START_REF] Sorum | Pyrolysis characteristics and kinetics of municipal solid wastes[END_REF], [START_REF] Sharypov | Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases[END_REF], [START_REF] Jimenez | Binary mixtures based on polycaprolactone and cellulose derivatives[END_REF], [START_REF] Cai | Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere[END_REF], [START_REF] Brebu | Co-pyrolysis of pine cone with synthetic polymers[END_REF], [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF], [START_REF] Çepelioğullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF], [START_REF] Kumagai | Interactions of beech woodpolyethylene mixtures during co-pyrolysis[END_REF], [START_REF] Dewangan | Co-pyrolysis of sugarcane bagasse and low-density polyethylene: Influence of plastic on pyrolysis product yield[END_REF], [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF], [START_REF] Chen | Synergistic effect on thermal behavior and char morphology analysis during co-pyrolysis of paulownia wood blended with different plastics waste[END_REF]- [START_REF] Zhou | Thermogravimetric characteristics of typical municipal solid waste fractions during co-pyrolysis[END_REF]. Figure 6 gives the TG and DSC curves as a function of temperature of HDPE, PS, PVC and for poplar wood (PW) and a waste wood sample (W3). The behaviour can be summarized as follows. Desorption of water starts below 100°C for the wood samples. HDPE melts at around 130°C, which results in a sharp endothermic peak in the DSC curve, a well-defined melting temperature being characteristic of polymers with a high degree of crystallinity. Contrary to HDPE (95% crystalline), PVC and PS are amorphous (<15% crystalline) and melt over a wide temperature range, so that no peak can be observed.

Between 200 and 500 °C, all plastic and wood samples lose mass due to devolatilization. The plastics have different thermal stabilities, because of their different polymer chain structures.

The dissociation energies of C-H, C-Cl, C-C bonds in polymers are 414, 339 and 347 kJ/mol, respectively [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF], which can explain that HDPE has the highest thermal stability and PVC the lowest, as shown by the TG curves. For PVC, the char residue yield corresponds to 3.4%, whereas HDPE and PS are completely decomposed. Other thermogravimetric analyses conducted with virgin plastics in nitrogen showed also a char yield of about 10wt% from PET and less than 3wt% from LDPE, PP and PS [START_REF] Jin | Evaluation of the co-pyrolysis of lignin with plastic polymers by TG-FTIR and Py-GC/MS[END_REF], these char yields are highly correlated to the degradation mechanism pathways.

In general, biomass has a higher oxygen, a higher moisture and higher ash content, but a lower carbon content than plastics, and contains less volatiles than plastics. Polyolefines (PE, PP) and PS consist only of carbon and hydrogen; PET contains 30 to 40 % of oxygen; PVC contains 56.5% of chlorine. Waste plastics can have different compositions as they usually consist of a mixture of different (contaminated) plastics. The two wood samples in Figure 6 have a similar thermal behaviour with devolatilization in the range of 190 to 400 °C. Wood consists mainly of cellulose, hemicellulose and lignin. On heating, hemicellulose decomposes first (190 to 290°C), followed by cellulose (290 to 360°C) and then lignin, which decomposes over a broad temperature range (360 to 500°C). After devolatilization, charring occurs whereby a char is formed. For the wood samples char residues correspond to 12 to 35%. Thus, wood yields in general more char than plastics.

Influence of feedstock type on the pyro-gasification products

During thermal conversion, plastics are transformed into gas, bio-oil and char. The industrial development of pyro-gasification processes is held back by the production of unwanted byproducts, which decreases efficiency and sometimes requires very costly treatments [START_REF] Demirbaş | Gaseous products from biomass by pyrolysis and gasification: effects of catalyst on hydrogen yield[END_REF], [START_REF] De Wild | Biomass valorisation by staged degasification[END_REF]. For char, a technology for downstream recovery of solid is required, otherwise char accumulation would clog plug-flow reactors. Gaseous phase recovery should limit tar condensation as this leads to fouling and blocking of the process equipment [START_REF] Kiel | Primary measures to reduce tar formation in fluidised-bed biomass gasifiers[END_REF]. In addition, gas composition (e.g. H2/CO and contaminants) should be determined in view of the subsequent application and the post-treatment process.

It appears from the TG curves previously discussed, and from other studies that pure plastics give very little char compared to wood [START_REF] Pohorely | Gasification of coal and PET in fluidized bed reactor[END_REF], [START_REF] Kunwar | Plastics to fuel: a review[END_REF]- [START_REF] Wu | TG/FTIR analysis on copyrolysis behavior of PE, PVC and PS[END_REF]; the same is true for real waste plastics [START_REF] Singh | Time and temperature depended fuel gas generation from pyrolysis of real world municipal plastic waste[END_REF].

Several studies have shown that the type and composition of biomass/plastics feedstock influences the distribution of the pyro-gasification products (gas, tar/oil, char) and their characteristics and composition [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF], [START_REF] Pinto | Cogasification of coal and wastes in a pilot-scale installation 1: Effect of catalysts in syngas treatment to achieve tar abatement[END_REF], [START_REF] Mastellone | Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor[END_REF], [START_REF] Mastellone | The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor[END_REF].

Gas composition

Lopez et al. (2015) [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF] studied the effect of HDPE (0-100%) co-feeding on the catalytic steam gasification of biomass (spouted bed reactor; 900°C; S/F=1); co-feeding uses two different entries followed by a mixer which is particularly suited for very different densities. The gas yield increased with the HDPE content of the feed and was for pure HDPE more than 2.5 times higher than for biomass; the tar content in the gaseous stream decreased by a factor of 10 going from 58.2 g/Nm 3 on a dry basis for biomass to 5.1 g/Nm 3 for HDPE; the char yield decreased from 4.3% for biomass to almost negligible (0.3%) for HDPE. The great differences between the product distributions for HDPE and biomass were explained by the complex composition of the lignocellulosic biomass compared to plastics, related to the short residence time in the reactor. The biomass is pyrolysed into gas (H2, CO, CO2, CH4), tar and char. HDPE, made up of long chains of hydrocarbons, is cracked at high temperature following a random chain scission mechanism and gives a high yield of gas mainly light olefins, a small amount of liquid products (tar), mainly aromatic hydrocarbons, and no char. Both the tar and char yield decreased with increasing HDPE content, and were even lower (positive synergistic effect) than predicted by linear interpolation from the yields for the separate feed. 25% HDPE in the feed reduced tar from 58.2 to 32.0 g/Nm 3 , 50% HDPE reduced it further to 9.7 g Nm 3 .

Increasing the amount of HDPE in the mixture from 25 to 50%, increased the H2 concentration from 40% to 57%, but a further increase of HDPE did not give more H2. When the amount of HDPE increased from 0 to 100%, the CH4 concentration decreased from 20 to 6%, CO decreased only slightly and CO2 first decreased and then increased slightly. 7) [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF]. Dong (2016) [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF] pyro-gasified (FB, 650°C, 15 min) 4 potential components of MSW (poplar wood, cardboard, food waste and PE) under 3 reaction atmospheres (N2, steam, and CO2). Under all 3 atmospheres and for the pure components the H2 and CH4 concentrations increased in the order: poplar wood<cardboard<food waste<PE; for CO and CO2 the order was opposite.

Co-gasification of PE with wood, cardboard or food waste, as binary mixtures, increased the concentration of H2 and CH4 and decreased the concentration of CO and CO2 even more than expected by linear calculation from the individual components, due to a synergistic effect. Wilk and Hofbauer (2013) [START_REF] Wilk | Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels[END_REF] co-gasified (DFB, steam gasification, 850°C) soft wood pellets and different types of plastics (up to 100%): virgin and recycled PE from packaging, MSW plastics and plastics from treatment of end-of-life vehicles treatment (shredder light fraction, SLF). It appeared that the H2 concentration on average remained almost constant upon addition of plastics to wood. For mixtures, non-linear effects were observed, but no unique trend in H2 formation as a function of the share of plastics could be given for the different plastic materials, or even for different sorts of PE (recycled or virgin). Linear interpolation overestimated the H2 yield from SLF and MSW plastics, but underestimated it for pure and recycled PE. The concentration of CO and CO2 decreased with an increasing share of plastics in the mixture. The decrease was steeper for PE plastics than for SLF (shredder light fraction originating from endof-life vehicles) and MSW plastics, which is according to Wilk and Hofbauer (2013) [START_REF] Wilk | Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels[END_REF] related to the amount of oxygen in the feedstock (wood>MSW>SLF>PE~recycled PE) (Figure 8). With increasing plastic fraction, CH4 and CnHm concentrations as well as tar yield increased. A significant decrease in char production was only noticed for virgin PE. Zaccariello and Mastellone (2015) [START_REF] Zaccariello | Fluidized-Bed Gasification of Plastic Waste, Wood, and Their Blends with Coal[END_REF] reported that, when 20% recycled plastics was mixed with natural wood (BFB, 850°C), the concentrations of H2, CO, CO2 decreased and CH4, CnHm, and tar increased. These results are partially at variance with the literature discussed so far. No information was however provided on the 'recycled plastics' or on the 'natural wood' used. According to Robinson et al. ( 2016) [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF], pyro-gasification (FB with bed material olivine, 725 to 875°C, air) of wood-PET pellets has a worse performance than wood pellets alone: tar concentrations were much higher and heating values were lower; moreover the H2 concentration in the gas was lower, whereas concentrations of CO2 and CO were somewhat higher than for wood pellets. Robinson et al. ( 2016) [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF] do not provide an explanation for these observations, but these results could indicate that PET should better be avoided in cogasification of plastics and biomass. This should not really be a problem as methods for PET recycling exist. The high tar concentrations in the syngas tents reported by Robinson et al.

(2016), were also observed by Brachi et al, 2014 [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF] (FB, steam + oxygen + nitrogen, 745-856°C).

Alvarez et al., (2014) [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF] added 20% PS to biomass and noticed, compared to pure biomass, an increase of the gas yield and of the oil yield, and a decrease of the char yield. They compared the behavior of polyolefin and PS when added to biomass: PS addition resulted in a lower gas yield, higher oil yield, and slightly higher char yield. H2, CO and CO2 concentrations in the gas were higher, those of CH4 and CnHm lower. It should be noticed that Wu and Williams., (2010) [START_REF] Wu | Pyrolysis-gasification of plastics, mixed plastics and realworld plastic waste with and without Ni-Mg-Al catalyst[END_REF] came to similar conclusions, when comparing pure polyolefines with PS. Ephraim a , (2016) [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF] (FB, semi continu, N2 atmosphere, 750°C) noticed with increasing PS addition to biomass, a slight increase in H2 concentration up to 30% PS and then a decrease; concentrations of CH4, CnHm, CO and CO2 decreased over the whole plastic content range (0-100%). Since PS contains an aromatic cycle, Alvarez et al. (2014) [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF] concluded that cracking requires higher temperatures (they used 600°C for pyrolysis and 800°C for steam gasification, with or without catalyst).

Ephraim a , (2016) [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF] also studied the pyrogasification of biomass with PVC. With increasing amount of PVC in the mixture, gas yield (mainly HCl) and H2 concentration increased significantly, CH4 and CnHm concentration remained nearly constant, tar yield decreased and char yield first increased up to about 30% of PVC and subsequently decreased with higher PVC concentration.

Kaewluan and Pipatmanomai (2011) [START_REF] Kaewluan | Gasification of high moisture rubber woodchip with rubber waste in a bubbling fluidized bed[END_REF], gasified (BFB, air, ER 0.3 to 0.5, 800°C) high moisture (9.5 to 27%) rubberwood chips with rubber waste in a bubbling fluidized bed reactor. Addition of 20% waste rubber to 27% moist rubberwood chips, increased the bed temperature from 700°C to 770°C, and showed several advantages compared to the gasification of air-dried biomass, such as increased H2 production due to the presence of water and a higher heating value of the syngas.

Lee et al. ( 2014) [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] investigated the production of clean gaseous fuels (syngas) produced from the gasification of MSW, rubber, plastic and wood (fixed bed, 700°C, using 1000°C steam). A high-temperature gasification process with steam at 1000°C was applied, to generate syngas whose concentration is, mainly because of the water gas shift reaction, dominated by H2 and can be used as gaseous fuel. There were only minor differences between the results obtained for the different feedstocks, proving that steam gasification can convert any material containing C, H and O, into a gaseous fuel with 50-60 % H2, about 10 % CO and CO2 and about 3% CH4, despite different characteristics of the feedstocks (C between 43 and 81%; O between 3 and 40%; moisture between 0 and 15%; LHV between 14 and 32%). Among the four feedstocks studied, plastics gave the highest H2 concentration and the lowest concentrations of CO, CO2 and CH4 concentration, wood feedstock gave the lowest H2 concentration and the highest CO concentration. The highest LHV value was obtained for rubber (10.8 MJ/m 3 , the lowest for plastics (7.8 MJ/m 3 ).

Also the type of biomass has an important influence on the decomposition of polymers and was studied by different authors [START_REF] Brebu | Co-pyrolysis of pine cone with synthetic polymers[END_REF], [START_REF] Çepelioğullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF], [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF], [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF], [START_REF] Lathouwers | Yield Optimization and Scaling of Fluidized Beds for Tar Production from Biomass[END_REF]- [START_REF] Kawamoto | Strong interactions during lignin pyrolysis in wood -A study by in situ probing of the radical chain reactions using model dimers[END_REF]. Cepeliogullar and Pütün, (2014) [START_REF] Çepelioğullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF], [START_REF] Çepelioğullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF], studied the co-pyrolysis (semi-continuous reactor) of PET and PVC, each mixed with 4 different types of biomass. They observed for the PET mixtures, that the oil yield increased with the lignin content of the biomass, or with the amount of char produced.

For the PVC-biomass mixtures gas yield increased with increasing lignin content of the biomass. Zhou et al. (2015) [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF], co-pyrolysized PVC with xylan, cellulose or lignin (1/1 mixtures). They measured for xylan and lignin an increase in gas yield and a decrease in char yield, whereas cellulose gave the opposite results. The decrease in HCl yield was much higher for lignin than for the other biomass compounds. Yang et al. (2016) [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF] investigated the occurrence of aliphatic and aromatic hydrocarbons in the oil obtained by fast co-pyrolysis of LDPE and different types of biomass. They observed a positive synergistic effect for the aliphatic hydrocarbons; the synergistic effect for the aromatic hydrocarbons was positive or negative, depending of the type or biomass in the feedstock.

Tar and char

As mentioned previously, pure plastics provide small amount of tar, due to their high amount of volatile matter. As a result the amount of char obtained from pyro-gasification of biomass and plastic mixture is often decreased by the presence of plastics. The char yield is basically calculated over the total mass of the feedstock. However, this is interesting to define a biomass char yield which should be calculated from the mass of the residue over the initial mass of biomass. Table 4 shows the values of experimental char yield found in the literature, as well as the calculated wood char yield. For these examples, the values of this parameter are not so far from the char yield obtained experimentally, meaning that the decrease of char yield is mainly related to a "dilution phenomenon". Finally, the composition of tar in pyro-gasification is also significantly modified by co-feeding plastics with biomass, compared to the individual components. The addition of plastics to biomass reduces the concentration of phenols and furans in the tar fraction; phenols and furans contain oxygen and are typically associated with tars from wood gasification [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF], [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF], [START_REF] Wilk | Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels[END_REF], [START_REF] Sharypov | Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases[END_REF], [START_REF] Paradela | Study of the co-pyrolysis of biomass and plastic wastes[END_REF], [START_REF] Önal | Bio-oil production via co-pyrolysis of almond shell as biomass and high density polyethylene[END_REF], [START_REF] Abnisa | Pyrolysis of mixtures of palm shell and polystyrene: An optional method to produce a high-grade of pyrolysis oil[END_REF], [START_REF] Xue | Fast pyrolysis of biomass and waste plastic in a fluidized bed reactor[END_REF]. Lopez et al., (2015) [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF] obtained for the gasification of a biomass-HDPE mixture (1/1) a tar composed of 80% of aromatic and polyaromatic hydrocarbons, mainly naphthalene and toluene, and of only 20% of phenols. Ruoppolo et al.

(2010) [START_REF] Ruoppolo | Fluidized Bed Cogasification of Wood and Coal Adopting Primary Catalytic Method for Tar Abatement[END_REF], measured a significant decrease in phenol and a significant increase in phenanthrene for an increasing amount of plastic in a biomass-PE mixture. Robinson et al. ( 2016) [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF] in their study of the gasification of commercial hardwood and plastic PET waste (BFB; 725-875°C) divided tar components into 5 groups: light nonoxygenated tar compounds (benzene, toluene, xylenes, etc.), phenolic compounds, naphthalene and similar compounds, PAHs (from fluorene to coronene) and gravimetric tar (compounds too heavy for gas chromatography). Gases produced from wood pellets contained mainly phenolic compounds. Gases produced from wood-PET pellets contained ca. 10 times greater concentrations of light non-oxygenated compounds than gases produced from wood pellets. Abnisa et al. (2014) [START_REF] Abnisa | Pyrolysis of mixtures of palm shell and polystyrene: An optional method to produce a high-grade of pyrolysis oil[END_REF] measured a pH of 2.5-2.7 for tar obtained by the pyrolysis of mixtures of palm shell and PS and attributed it to the presence of low molecular weight carboxylic acids. Cepeliogullar and Pütün (2014) [START_REF] Çepelioğullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF] investigated the co-pyrolysis of agricultural waste with PET and with PVC, and measured significant amounts of benzoic acid and of PAHs in the tar of respectively a 1/1 biomass-PET mixture, and a 1/1 biomass-PVC mixture.

The pyrolysis of PVC with other feedstocks such as biomass waste, coal or other plastics has been recently reviewed by [START_REF] Yu | Thermal degradation of PVC: A review[END_REF] [START_REF] Yu | Thermal degradation of PVC: A review[END_REF]. Blends containing low PVC content (e.g. 2 wt%) would cause significant difference amount of chloro-organic compounds in pyrolysis oil. For example, [START_REF] Uddin | Dechlorination of Chlorine Compounds in Poly(vinyl chloride) Mixed Plastics Derived Oil by Solid Sorbents[END_REF] [START_REF] Uddin | Dechlorination of Chlorine Compounds in Poly(vinyl chloride) Mixed Plastics Derived Oil by Solid Sorbents[END_REF] studied the pyrolysis of different blends of PVC with PE, PP and PS. Around 3-12 % of chlorine were found in liquid oil fraction. The formation of chloro-organic compounds was assumed as the result from the combination of HCl, formed from PVC degradation, and organic compounds, formed from other feedstocks [START_REF] Yu | Thermal degradation of PVC: A review[END_REF]. [START_REF] Yuan | High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor[END_REF] [START_REF] Yuan | High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor[END_REF] studied the thermal degradation of pure PVC granules or PVC pipe scraps. The dechlorination efficiency could be complete above 320°C. The formation of chloro-organic compounds could be limited by using metals oxides as adsorbents during PVC pyrolysis. Among different metal oxides investigated, [START_REF] Masuda | Pyrolysis study of poly(vinyl chloride)-metal oxide mixtures: Quantitative product analysis and the chlorine fixing ability of metal oxides[END_REF] [START_REF] Masuda | Pyrolysis study of poly(vinyl chloride)-metal oxide mixtures: Quantitative product analysis and the chlorine fixing ability of metal oxides[END_REF] showed that La2O3 and ZnO were the most efficient to inhibit the formation of chlorobenzene. On the other hand, for the dechlorination of pyrolysis oil, catalytic processes are preferred, as previously reviewed by [START_REF] Mastellone | Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor[END_REF] and Zaccariello and Mastellone (2012) [START_REF] Mastellone | The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor[END_REF] concluded from their experiments that different types of fuels (biomass, plastics, coal) may be substituted by each other, without requiring important changes to the gasifier. They highlighted that each component may change the product distribution and the concentration of the components in the gas. In general, but some papers came to other conclusions, wood promotes the formation of a solid char composed of almost pure carbon and reduces the presence of heavy hydrocarbons in the syngas, thus leading to a cleaner syngas, with less heavy hydrocarbons and tar, but with a lower calorific value. The presence of plastics on the other hand, in general increases the yield of H2, light hydrocarbon and gas, affording a higher heating value syngas [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], [START_REF] Wilk | Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels[END_REF], [START_REF] Ahmed | Characteristics of syngas from cogasification of polyethylene and woodchips[END_REF], [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF], [START_REF] Mastellone | The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor[END_REF].

Conclusions

It is clear that the information given, even for 1 plastic (PE or PP) mixed with biomass is not always completely consistent. Product distributions for biomass and plastics and their mixtures, as well as concentrations of the gases, depend in addition to the plastic and biomass composition on various other parameters (reaction time, gasifying agent, temperature, heating rate, catalysts, etc., these factors will be discussed later), so that it is not surprising that sometimes conflicting results are reported in literature [START_REF] Johannes | Synergy in co-pyrolysis of oil shale and pine sawdust in autoclaves[END_REF]. However for polyolefines (PE and PP) and PS/biomass mixtures an almost general consensus exists: with increasing PE or PP, gas yield increases, tar and char decrease, H2 increases, CH4, hydrocarbons, CO and CO2 decrease. For PET/biomass mixtures much less information is available but the data of Robinson et al. ( 2016) [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF] for air gasification indicate almost opposite trends than for PE or PP. For real plastic waste, little information is available and it is usually contradictory [START_REF] Wu | Pyrolysis-gasification of plastics, mixed plastics and realworld plastic waste with and without Ni-Mg-Al catalyst[END_REF], [START_REF] Wilk | Conversion of mixed plastic wastes in a dual fluidized bed steam gasifier[END_REF], [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF], [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF]. It would be interesting to investigate the 'real plastics fractions' obtained during separate collection of a mixture of some plastics (e.g. by Fost Plus in Belgium), and also the resulting fractions obtained during separation of these plastics.

Synergistic effect during the thermoconversion of plastics and other feedstocks (biomass, coal)

As mentioned before, synergistic effects occur during the thermoconversion of plastic/biomass, plastic/biomass/coal and plastic/coal mixtures. This means that the experimental values, for example the gas, oil or char yield, or the gas composition, differ from those calculated by linear interpolation on the basis of the results for the pure components in the feedstock. The mechanisms of these synergistic effects were studied by different authors [START_REF] Jakab | Thermal decomposition of polypropylene in the presence of wood-derived materials[END_REF]- [START_REF] Sorum | Pyrolysis characteristics and kinetics of municipal solid wastes[END_REF], [START_REF] Marin | Copyrolysis of wood biomass and synthetic polymers mixtures. Part II: characterisation of the liquid phases[END_REF], [START_REF] Sharypov | Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases[END_REF], [START_REF] Sharypov | Influence of reaction parameters on brown coal-polyolefinic plastic co-pyrolysis behavior[END_REF], [START_REF] Cai | Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere[END_REF], [START_REF] Brebu | Co-pyrolysis of pine cone with synthetic polymers[END_REF], [START_REF] Ahmed | Characteristics of syngas from cogasification of polyethylene and woodchips[END_REF], [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF], [START_REF] Costa | Study of the experimental conditions of the co-pyrolysis of rice husk and plastic wastes[END_REF], [START_REF] Çepelioğullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF], [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF]- [START_REF] Dewangan | Co-pyrolysis of sugarcane bagasse and low-density polyethylene: Influence of plastic on pyrolysis product yield[END_REF], [START_REF] Chen | Synergistic effect on thermal behavior and char morphology analysis during co-pyrolysis of paulownia wood blended with different plastics waste[END_REF], [START_REF] Matsuzawa | Acceleration of cellulose co-pyrolysis with polymer[END_REF], [START_REF] Çepelioğullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF], [START_REF] Narobe | Co-gasification of biomass and plastics: Pyrolysis kinetics studies, experiments on 100 kW dual fluidized bed pilot plant and development of thermodynamic equilibrium model and balances[END_REF] . Plastics (PS, PE, PP, PET, PVC) devolatilise in a higher temperature range (300-500°C) than biomass (200-400°C) (see 3.2.), making interactions and synergy possible between plastics and the volatiles thereof, with char from biomass. The char plays the role of radical donor in the initiation of the polymer chain scission (H-transfer from plastic to coal/biomass) and may also adsorb volatiles from polymers. Dong (2016) [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF] observed a higher synergy between food waste and PE than between poplar wood or cardboard and PE, and attributed this to food waste char being more porous and thus having more catalytic effect on gas-solid phase reactions. Catalytic properties of char from thermo-conversion of biomass were intensively studied for a large family of chemical processes [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF], [START_REF] Min | Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming[END_REF]- [START_REF] Dupont | How inorganic elements of biomass influence char steam gasification kinetics[END_REF].

Compared to other plastics, polyolefins decompose more easily in the presence of cellulosic materials. During the pyrolysis of biomass-polyolefin mixtures, the onset temperature of the polymer decomposition is lowered as was demonstrated for PP in the presence of beech wood [START_REF] Jakab | Thermal decomposition of polypropylene in the presence of wood-derived materials[END_REF], [START_REF] Marin | Copyrolysis of wood biomass and synthetic polymers mixtures. Part II: characterisation of the liquid phases[END_REF]. Also, the co-pyrolysis of almond shell with HDPE was found beneficial for oil production and thus for the decomposition of HDPE [START_REF] Önal | Bio-oil production via co-pyrolysis of almond shell as biomass and high density polyethylene[END_REF]. [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF] [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF] studied the fast co-pyrolysis of woody biomass with up to 50 wt.% of LDPE: at 600°C and under inert atmosphere, the oil yield was highest for the 1/1 LDPE/ biomass mixture, higher than for wood or LDPE alone. [START_REF] Dorado | Origin of carbon in aromatic and olefin products derived from HZSM-5 catalyzed co-pyrolysis of cellulose and plastics via isotopic labeling[END_REF] [START_REF] Dorado | Origin of carbon in aromatic and olefin products derived from HZSM-5 catalyzed co-pyrolysis of cellulose and plastics via isotopic labeling[END_REF] added different polymers (PE, PP, PS, PET) to biomass, in view of reducing coke formation to increase catalyst (HZSM-5) lifetime. It appeared that PE and PP were most efficient in the conversion of cellulosic materials: during pyrolysis these polyolefines decompose into olefins that react with the oxygenated primary pyrolysis products from biomass, avoiding coke formation. PET and PS produced lower olefin concentrations. Biomass/PS mixtures and biomass/PET mixtures, behave in a different way than biomass/polyolefines mixtures during thermal conversion [START_REF] Brebu | Co-pyrolysis of pine cone with synthetic polymers[END_REF], [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF], [START_REF] Chen | Synergistic effect on thermal behavior and char morphology analysis during co-pyrolysis of paulownia wood blended with different plastics waste[END_REF], [START_REF] Çepelioğullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF]. For PET, a variety of oxygen-containing products and aromatic hydrocarbons are released upon degradation. These may react with the primary char from wood, thus maximize the char [START_REF] Chen | Synergistic effect on thermal behavior and char morphology analysis during co-pyrolysis of paulownia wood blended with different plastics waste[END_REF].

. [START_REF] Oyedun | Co-pyrolysis of Biomass and Plastics waste: A Modelling Approach[END_REF], (2014 b ) [START_REF] Oyedun | Co-pyrolysis of Biomass and Plastics waste: A Modelling Approach[END_REF], [START_REF] Oyedun | Mixed-waste pyrolysis of biomass and plastics waste -A modelling approach to reduce energy usage[END_REF] used two modelling approaches to study the synergetic effect of the co-pyrolysis of bamboo/PS blends. A significant interaction between bamboo and PS was shown from the mass loss, volatile evolving rate and the overall energy used. The energy of the mixed-blends was lower than this calculated from the pyrolysis of the separate components of the blends, bamboo and PS. Oyedun et al. (2014 a ) [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF], also observed a much larger deviation between experimental and predicted TGA's under N2 atmosphere for PS/biomass than for HDPE/biomass blends and attributed this to the decomposition temperature of the lignin fraction of biomass (400°C) overlapping better with the PS decomposition temperature interval (400-425°C) than with the one of HDPE (455-510°C).

According to Jakab et al. ( 2001) [START_REF] Jakab | Thermal decomposition of mixtures of vinyl polymers and lignocellulosic materials[END_REF] the decomposition of PS is correlated with the amount of char produced from the added biomass. The amount of char increases in the order cellulose<beech wood<lignin<charcoal and has an increasing impact on the decomposition of PS.

The thermal degradation of PVC is more complex than for other polymers: whereas most polymers decompose in one stage, PVC decomposes in two stages. Dechlorination starts lower than 200°C at the surface, resulting in cyclization/aromatization, and becomes significant at 300°C, while aromatic hydrocarbon release starts at 350°C and become significant above 450°C [START_REF] Zhou | Understanding the pyrolysis mechanism of polyvinylchloride (PVC) by characterizing the chars produced in a wire-mesh reactor[END_REF]. The first stage of PVC degradation almost perfectly matches the temperature interval of biomass and lowers the degradation temperature of biomass, enhancing the release of volatiles and increasing the char yield [START_REF] Sorum | Pyrolysis characteristics and kinetics of municipal solid wastes[END_REF], [START_REF] Matsuzawa | Evaluation of char fuel ratio in municipal pyrolysis waste[END_REF], [START_REF] Çepelioğullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF], [START_REF] Chen | Synergistic effect on thermal behavior and char morphology analysis during co-pyrolysis of paulownia wood blended with different plastics waste[END_REF], [START_REF] Matsuzawa | Acceleration of cellulose co-pyrolysis with polymer[END_REF], [START_REF] Çepelioğullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF]. Ephraim (2016 a ) [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF] observed a significant positive synergy for a wood /PVC mixture (7/3) on char and tar yield: the char and oil yield increased by 44 and 30% respectively, relative to the yield predicted by linear interpolation from the yields for the pure components. A corresponding, but negative synergy in gas (HCl) was observed [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF]. Also [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] and Zhou et al.

(2015) [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF] observed a decrease in HCl when co-pyrolysing biomass and PVC. The chlorine of PVC was preferentially found in the 'oil fraction'. According to Ephraim (2016 a ) [START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF], it is mainly present as HCl gas dissolved in the oily phase after condensation, in contradiction with [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] and Zhou et al. (2015) [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF], who suggest that some chlorine of the PVC might be present in organic chlorinated compounds.

Operational parameters

Of course selecting the feedstock mixture for pyro-gasification is just a start, and it may also be dictated by practical considerations (availability, cost, etc.). The final product yields and product distribution, as well as the gas composition depend also on several operational parameters. These will now be discussed.

Gasifying agent

The gasifying agent is a key factor in determining the quality of the syngas and its subsequent use. Common gasifying agents are air, oxygen, steam, carbon dioxide and mixtures thereof. Steam gasification is endothermic, so heat must be supplied to the process. If some oxygen is present in the gasifying medium, it can give the heat needed for gasification, so that oxygen/steam mixtures are often used. CO2, or a mixture of steam and CO2, can also be used as gasifying medium.

In their study of the co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed reactor, Pinto et al., 2003 [81] studied the influence of the gasification medium: different combinations of air and steam were investigated. It was shown that increasing the oxygen (in air) concentration reduced hydrocarbons and tar due to partial combustion reactions, but the gas produced was diluted due to nitrogen coming in with air, thus reducing the heating value. Steam and air gasification gave similar gas yields; steam gasification favoured steam reforming reactions, whilst air gasification favoured combustion reactions. Therefore, H2 and hydrocarbon concentrations were higher with steam gasification than with air gasification, whereas CO and CO2 were lower, as replacing air by steam reduced combustion of char and volatiles. Similarly, higher fractions of oxygen (oxygen/(oxygen +steam)) decreased the content of H2, methane and other hydrocarbons, and increased CO and CO2 concentrations.

As discussed by [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF] [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF], air is the most common gasifying agent for reasons of cost. With air a product gas with low heating value (<6MJ/m 3 ), and containing 7 to 12 % of hydrogen is obtained. To produce gas with a high heating value (10-20 MJ/Nm 3 ), pure oxygen may be used, and thus avoiding dilution by nitrogen, although the cost is higher. Pure steam is also a popular gasifying agent: it gives syngas with a high hydrogen content (50-55%), but also with a high tar content, and is very endothermic. Mixtures of steam and oxygen require less external heat, and give a medium heating value (12 to 14 MJ/ m 3 ) syngas. Lee et al. (2014) [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] investigated the production of clean gaseous fuels (syngas) produced from the gasification of MSW, rubber, plastic and wood. The results of steam gasification (700°C, using 1000°C steam) were compared to air-blown (900°C) gasification. The main difference was the H2/CO ratio: in steam gasification the H2/CO ratio was 12, compared to only 1 in air-blown gasification. The LHV of the syngas from steam gasification was more than twice that obtained with air-blown gasification. The differences between both gasifying agents were due to extra water gas shift reaction in steam gasification and N2 dilution in air-blown gasification Dong (2016) [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF] studied pyro-gasification (650°C, 15 min) of 4 components of MSW (poplar wood, cardboard, food waste and PE) under 3 reaction atmospheres (N2, steam, and CO2). Both steam and CO2 (dry gasification) are effective in enhancing syngas yield and change syngas properties compared to N2. In the case of poplar wood the syngas yield rises from 0.070 m 3 /kg (N2) to 0.088 (steam) and 0.074 (CO2), respectively, for the other wastes a similar increase was noticed. The higher syngas yield with CO2 and H2O than with N2 is attributed to the occurrence of the Boudouard and water gas reactions, producing CO or CO2.

Figure 9: Syngas composition for poplar wood and PE under N2, steam and CO2 [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF] and H2 from char [START_REF] Ahmed | Kinetics of woodchips char gasification with steam and carbon dioxide[END_REF]. A simulation analysis by Ephraim, (2016 b ) [START_REF] Ephraim | Simulation of biomass char gasification in a downdraft reactor for syngas production[END_REF] provides insight into the evolution of the complex reaction mechanisms along the char bed of a downdraft gasifier under varying concentrations of mixed reaction atmospheres (O2, H2O and CO2). Their results have shown that the reaction atmosphere strongly influences the gasification reaction rates in the gasifier entrance zone, whereby the increase in O2 concentration (0 to 3 vol%) strongly increases the rates of combustion, steam gasification, Boudouard and WGS reactions, whereas increasing H2O (5 to 20 vol%) raises only steam gasification and WGS rates. Furthermore, CO2 (5 to 20 vol%) only affects Boudouard and WGS reaction rates. Based on their analysis, Ephraim, (2016 b ) [START_REF] Ephraim | Simulation of biomass char gasification in a downdraft reactor for syngas production[END_REF] concluded that O2 and H2O atmospheres exert the most influence on the syngas composition (H2/CO ratio). Figure 9 gives the syngas composition for poplar wood and PE under N2, steam, and CO2 [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF].

In the presence of steam the water gas shift reaction (reaction (3)) is promoted explaining the H2 and CO2 increase and the CO decrease, compared to N2. CO2 gasification slightly increases H2, increases CO and decreases CH4, which can be explained by the dry reforming reaction [START_REF] Bosmans | Waste-to-clean syngas: avoiding tar problems[END_REF], and the Boudouard reaction (2). Table 5 gives an overview of the H2/CO ratio from pyro-gasification under different reaction atmospheres of MSW single components [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF]. Steam gasification increases the H2/CO2 molar ratio for all components compared to pyrolysis (N2 atmosphere). For 3 of the components it is in or very close to the range 1.5 to 3.0 desirable for Fischer-Tropsch synthesis [START_REF] Butterman | CO2 as a Carbon Neutral Fuel Source via Enhanced Biomass Gasification[END_REF]. When CO2 is used as gasifying agent, a lower H2/CO molar ratio (0.61 to 1.87) is obtained than with steam, making the syngas from poplar wood and cardboard more suitable as chemical raw material [START_REF] Mitsuoka | Gasification of woody biomass char with CO2: The catalytic effects of K and Ca species on char gasification reactivity[END_REF]. Also for a mixture of the 4 components the H2/CO molar ratio is highest (1.79) for steam gasification, compared to 1.70 for pyrolysis (N2 atmosphere) and 1.29 for CO2 gasification. An experimental study of the co-gasification of biomass residue and polymeric wastes (tyre rubber and PET) in an oxygen-enriched air and steam atmosphere, was carried out by [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF] [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF] in a fluidized bed gasifier, to investigate the possibility of producing a syngas suitable for methanol synthesis. It appeared that to optimize the process also in terms of reduced tar production, it was preferable to operate the bed above 750 °C, with a steam/ fuel ratio above 0.7. No significant differences in gas composition were observed moving from the PET-totyre-based pellets, although the first ones produced less tar and particles. With polymeric waste in the fuel blend (in quantities exceeding 20 wt.%), suitable selection of the operating conditions, the gas composition was such that no further water-gas shift reactor was needed; only carbon dioxide removal suffices to meet the requirements of H2/(3CO2 + 2CO) ≈ 1.05 for the downstream methanol production.

The heating value of the product gas is lower with air as gasifying medium, as the gas is diluted by nitrogen. With pure oxygen a gas with higher heating value is obtained, but at higher cost. When pure steam is used for gasification, the product gas is free of N2, has a higher heating value. H2 and hydrocarbon concentrations are higher with steam gasification than with air gasification, whereas CO and CO2 were lower, as replacing air by steam reduced combustion of char and volatiles, selection of the gasifying agent thus affords an ideal means of adapting the H2/CO ratio to the requirements of the subsequent application.

Temperature

Pinto et al. ( 2002) [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF] (Figure 10) studied steam gasification of biomass (pine wood sawdust) and co-gasification of biomass with plastic (PE) in the 730 to 890 °C temperature range in a fluidized bed. It was shown that temperature was the parameter that influenced the gas composition most. For pure pine and for pine with 10 to 60% of PE, the effect of increasing temperature was similar: H2 concentration increases; CO2 concentration increases and then, from 830°C on, decreases; CO first decreases somewhat and then, from 830°C on, increases again or remains constant, but the effects are small; CH4, CnHm and tar decrease; char formation decreases.

Figure 10: Effect on bed temperature on gas composition for co-gasification of pine with 40% of PE and a steam ratio of 0.8 (w/w) [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF] The concentration increase of H2 with temperature can be explained by the majority of reactions leading to H2, i.e. the primary and secondary water gas reaction, as well as the reforming reactions ( 8) and ( 9) being endothermic. The results for CO2 and CO can be explained by the water gas shift reaction being dominant in the 730-830 °C range, reaction (3), which decreases CO concentrations and increases CO2. In the temperature range 830-900 °C the dominant reactions are the water gas reaction (reaction (1)), increasing CO), and the Boudouard reaction (reaction (2)), decreasing CO2 and increasing CO). CH4 and CnHm decrease with increasing temperature due to endothermic cracking and reforming reactions as do tar and char [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF]. This behaviour is, in the considered temperature range, in good qualitative agreement with the thermodynamic equilibrium calculations for pure carbon and steam as reported by Kodama, 2003 [152], except for CO where the calculations predict an increase with temperature over the entire temperature range considered. In a subsequent paper [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF] [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF] studied the co-gasification of coal, biomass and plastics wastes (FB, air/steam mixtures). Again, the temperature strongly influenced the composition of the syngas. For a mixture of 60% (w/w) of coal, 20% of pine and 20% of PE wastes under air atmosphere, a temperature increase from 750 to 890°C decreased the concentration of methane and of other hydrocarbons by 30 to 63%, while the hydrogen concentration increased by around 70%. [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF] [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF] gasified blends of plastics waste with pine wood sawdust and coal (FB, air, dolomite as catalyst, 750-880°C). The effect of increasing bed temperature on gas composition for the most important components is as follows: H2 increases, CO decreases first and then, from about 830°C increases, CO2 decreases mainly above 830°C, and CH4 seems to increase somewhat and C2Hn remains constant. This is in good agreement with the results of [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], ( 2003) [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF], except for CH4 and C2Hn. Taba et al., (2012) [START_REF] Taba | The effect of temperature on various parameters in coal, biomass and CO-gasification: A review[END_REF] reviewed pyro-gasification of coal, biomass, (sometimes) plastics and their co-gasification in different types of gasifiers, with different particle sizes, in different types of gasifier, under different operating conditions, in different media (air, steam, CO2, airsteam, N2, O2-steam, etc.). The results of co-gasification (mainly the production of H2, CO2, CO, CH4, and other hydrocarbons) as a function of temperature are reviewed and compared to these obtained by gasification of coal and biomass alone. Of course there may be a limitation to the tolerable temperature (typically 750-900°C, in a fluidized bed gasifier), as it may e.g. affect construction materials in the gasifier, and lead to ash melting and agglomeration. The results were obtained in the 740-1000°C range, but mainly in the 750-900 °C range. From all the individual curves, each obtained by one group of authors in one series of experiments under consistent experimental conditions, and reported by Taba et al., (2012) [START_REF] Taba | The effect of temperature on various parameters in coal, biomass and CO-gasification: A review[END_REF], we deduced the following more or less general trends: -H2 (expressed as concentration, %) increases with temperature. The agreement between the curves presented is however not too good, as can be expected since they were obtained under a range of different circumstances, and instead of increasing some curves are horizontal or even decrease slightly with temperature; -CO2 first increases slightly with temperature, and then decreases, but other curves seem to decrease from the beginning -CO first decreases slightly with temperature, and then a somewhat above 800°C starts to increase with temperature; some curves are, however, horizontal or even decrease over the entire temperature range considered. -CH4 in general decreases as a function of temperature, but some curves remain constant or even increase slightly. There is in general a good agreement with results mentioned earlier. From all the data presented, Taba et al. 2012 [START_REF] Taba | The effect of temperature on various parameters in coal, biomass and CO-gasification: A review[END_REF] claim that among the parameters having a direct effect on the gasification process, temperature is most significant. We do not agree completely with them on this aspect. Temperature is indeed an important parameter, but if we consider the figure where H2 production is given as a function of temperature, it appears that one of the steepest curves shown increases (only) by a factor 1.7 over the 730 to 900°C temperature range. On the other hand, the lowest and highest curves of H2 concentration as a function of temperature show a difference by a factor of at least 20. This difference is due to other factors than temperature, such as feedstock, particle size, type of gasifier, gasifying medium (air, steam, CO2, air-steam, N2, O2-steam, etc.), use of catalyst or not, catalyst type, other operating conditions… So the figures rather show that the effect of other factors may be much larger than the mere effect of temperature. The influence of other parameters will be considered further. Temperature influences the product distribution, the syngas yield and its composition, a high temperature is required for high carbon conversion and to obtain a low tar content. In general, with increasing temperature the H2 concentration increases, CH4, CnHm and tar decrease; char formation decreases.

4.3.Equivalence ratio

In their study of the co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed reactor, [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF] [START_REF] Pinto | Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixtures in a fluidized bed system[END_REF] showed that the increase of the O2/fuel ratio favoured partial combustion reactions and led therefore to an increase in CO and CO2 concentrations and to a reduction of the H2, CH4 and other hydrocarbon concentrations. Moreover a higher O2 (from air) /fuel ratio led to an increase of the quantity of gas products, but lowered the heating value of the gas products because of the diluting effect of N2. [START_REF] Mastellone | The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor[END_REF] [START_REF] Mastellone | The O2-enriched air gasification of coal, plastics and wood in a fluidized bed reactor[END_REF] investigated the gasification of mixtures of coal, plastic waste (PE and PP) and wood in a fluidized bed reactor under O2-enriched atmosphere. Increasing the inlet O2 concentration from 21 to 35% increased strongly the formation of H2, CO, CH4, C2H4, and decreased the N2 content. The equivalence ratio, ER, has a strong influence on the product gas composition and on the type and amount of tar in the product gas. Indeed, if more oxygen is available it may react with the hydrocarbons present. A too high ER would yield decreased H2 and CO concentrations and increased CO2 concentrations in the syngas and thus lower heating values of the syngas. In their study on co-gasification of plastic waste with coal and biomass (FB, air, dolomite catalyst, 750-880°C), [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF] [START_REF] Aznar | Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant[END_REF] showed that, when ER is increased from 0.30 to 0.46, the concentrations of all relevant gas components (H2, CO, CO2, CH4, C2Hn) and the LHV, decrease. Obviously dilution by N2 contributes to this decrease, but the relative decrease is not the same for all components: it is low for CO2, but high for hydrocarbons. Increasing ER reduces the char yield significantly, and the tar yield somewhat.

4.4.Steam/fuel (S/F) ratio

Pinto et al., (2002) [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF], in order to vary the S/F ratio, kept the steam flow rate constant and changed the mass flow rate of the waste mixture, thus ensuring a constant residence time. It appeared that the influence of S/F ratio was less pronounced than that of temperature for all compositions (pine and pine/PE mixtures) studied. For a 40/60 PE/biomass mixture at 835°C, with S/F increasing from 0.5 to 0.8, a slight increase of the H2 concentration occurs reaching a maximum at S/F = 0.75; CO, CH4 and CnHm slightly decrease and CO2 slightly increases. Additional information can be obtained from among others Ruoppolo et al., (2012) [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF] and [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF] [START_REF] Brachi | Fluidized bed co-gasification of biomass and polymeric wastes for a flexible end-use of the syngas: Focus on bio-methanol[END_REF].

4.5.Conclusion

It was shown that, next to the feedstock, also other factors such as temperature, gasifying medium (air, steam, CO2, air-steam, N2, O2-steam, etc.), equivalence ratio (ER), and S/F ratio play a role. H2 and hydrocarbon concentrations are higher with steam gasification than with air gasification, whereas CO and CO2 are lower. Selection of the gasifying agent thus affords an ideal means of adapting the H2/CO ratio to the requirements of the subsequent application.

Temperature is the parameter that has the largest influence: it influences the product distribution, and the syngas yield and composition. A high temperature is required for high carbon conversion and to obtain a low tar content. In general, with increasing temperature the H2 concentration increases, CH4, CnHm and tar decrease; char formation decreases. The equivalence ratio, ER, has a strong influence on the product gas composition and on the type and amount of tar in the product gas, the influence of S/F ratio is limited. Next paragraph discusses another important factor, the catalyst.

Catalysis in the thermoconversion of plastic/biomass waste

A lot of research effort has been, and is still being devoted, to developing effective and feasible methods for tar reduction. Catalysts may play a significant role in this context. Moreover, in addition to the gasifier configuration and the operating conditions discussed earlier, catalysts have also an important impact on product distribution and gas composition, as they accelerate the reforming and cracking reactions of pyrolysis products, increasing H2 production [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF], [START_REF] Bosmans | Waste-to-clean syngas: avoiding tar problems[END_REF], [START_REF] Toledo | Catalytic Air Gasification of Plastic Waste (Polypropylene) in a Fluidized Bed. Part II: Effects of Some Operating Variables on the Quality of the Raw Gas Produced Using Olivine as the In-Bed Material[END_REF], [START_REF] Devi | A review of the primary measured for tar elimination in biomass gasification processes[END_REF], [START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF]. Catalytic cracking/reforming is used for in-bed tar conversion, as well as for tar conversion downstream of the gasifier (reactor outlet or separate reactor). Catalytic pyrolysis and gasification of biomass [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF], [START_REF] Asadullah | Biomass gasification gas cleaning for downstream applications: A comparative critical review[END_REF], [START_REF] Richardson | A short overview on purification and conditioning of syngas produced by biomass gasification: Catalytic strategies, process intensification and new concepts[END_REF], [START_REF] Woolcock | A review of cleaning technologies for biomassderived syngas[END_REF] and of plastic waste [START_REF] Anuar Sharuddin | A review on pyrolysis of plastic wastes[END_REF], [START_REF] Miandad | Catalytic pyrolysis of plastic waste: A review[END_REF] has recently been reviewed. The most used solid catalysts are dolomite, olivine, zeolites, metal oxides, supported-metals catalysts, and char generated from thermoconversion processes. Nibased catalysts were found most effective for the considered purpose. Also in the thermoconversion of plastic-biomass, plastic-coal-biomass, and coal-biomass mixtures, catalysts improve the quality of the syngas [START_REF] Robinson | Comparison of the air-blown bubbling fluidized bed gasification of wood and wood-PET pellets[END_REF], [START_REF] Pinto | Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement[END_REF], [START_REF] Pinto | Cogasification of coal and wastes in a pilot-scale installation 1: Effect of catalysts in syngas treatment to achieve tar abatement[END_REF]. [START_REF] Pinto | Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement[END_REF] [START_REF] Pinto | Effect of catalysts in the quality of syngas and by-products obtained by co-gasification of coal and wastes. 1. Tars and nitrogen compounds abatement[END_REF] (BFB, 850-900°C, gasifying agent: mixture of steam and oxygen) compared different catalysts for pyrogasification of high ash coal and pine: low-cost natural minerals (non-calcinated and calcinated dolomite and olivine), Ni based catalysts (Ni-dolomite, Ni-Mg) and 2 commercial metal catalysts G-72D (ZnO) and C49 TRX (Co-Mo-oxides). They found Ni-based catalyst most effective, as these led to the highest decrease in hydrocarbons and to the highest increase in H2 release. [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF] [START_REF] Ruoppolo | H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel[END_REF] compared the performance of the gasification of biomass and biomass-plastic mixtures, with and without steam, for a quartzite and for a Ni-based catalyst (FB, 800-900°C). Regardless of the fuel, steam addition increased the production of H2 and reduced the tar concentration, but in general the Ni-based catalyst increased the H2 yield and decreased tar production more than steam. However, for the plastic-biomass mixtures the tar concentration could not be brought below 40 g/m 3 with a combination of steam and catalyst. [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF] [START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF] investigated the co-pyrogasification (steam; 2 stage fixed bed, first bed, 600°C; second bed, 800°C and Ni/Al2O3) of plastics (PP, HDPE, PS and plastic waste) and wood sawdust with and without a Ni/Al2O3 catalyst. The use of the catalyst gave a sharp increase in gas yield and H2 production and a decrease in CO and light hydrocarbons: for PP the H2 production in the syngas increased from 36.1 to 52.1% and the H2/CO ratio from 1.2 to 2. The Ni/Al2O3 catalyst promotes the water gas shift and the steam reforming reactions (reaction, 3, 8 and 11). Pinto et al. ( 2009), (2014) [START_REF] Pinto | Cogasification of coal and wastes in a pilot-scale installation 1: Effect of catalysts in syngas treatment to achieve tar abatement[END_REF], [START_REF] Pinto | Hot treatment and upgrading of syngas obtained by co-gasification of coal and wastes[END_REF] obtained similar results in the cogasification (steam gasification, 850°C) of coal/biomass, coal/plastic and coal/biomass/plastic mixtures, using two fixed bed reactors; the first one with dolomite, the second one with a Nibased catalyst. Both catalysts increased the H2 concentration and decreased CO, CH4 and hydrocarbons. With the dolomite reactor a tar reduction of ca. 80% was obtained, and after the Ni-based catalyst reactor no more tar was detected. Pinto et al. [START_REF] Pinto | Cogasification of coal and wastes in a pilot-scale installation 1: Effect of catalysts in syngas treatment to achieve tar abatement[END_REF], [START_REF] Pinto | Hot treatment and upgrading of syngas obtained by co-gasification of coal and wastes[END_REF] concluded that the arrangement of two catalytic fixed bed reactors is suitable to deal with a wide range of feedstocks. For carbonaceous materials, giving high contents of hydrocarbons and tar, the use of two sequential catalytic reactors would ensure complete destruction of tar and production of a gas suitable for a wide range of applications. This configuration is also suitable to treat gases with high contents of sulphur and halogen compounds; they are retained in the reactor with dolomite, thus ensuring a longer life for the more specific catalyst for tar abatement. Not only the catalyst, also the catalyst support plays an important role in tar conversion. According to different authors [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF], [START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF], [START_REF] Ammendola | Dual bed reactor for the study of catalytic biomass tars conversion[END_REF], [START_REF] He | Syngas production from catalytic gasification of waste polyethylene: Influence of temperature on gas yield and composition[END_REF], Ni/Al2O3 catalysts have the highest activity and efficiency. This is at variance with the findings of [START_REF] Song | Production of gaseous fuel from refuse plastic fuel via co-pyrolysis using low-quality coal and catalytic steam gasification[END_REF] [START_REF] Song | Production of gaseous fuel from refuse plastic fuel via co-pyrolysis using low-quality coal and catalytic steam gasification[END_REF] who compared different catalysts, Ni, NiO and Mg, with and without Al2O3 and Fe2O3 support (promotor) in the steam co-pyrolysis of a 1/1 mixture of low-quality coal and refuse plastic fuel. The efficiency of the catalyst for gas, H2, CO, CH4 and C2-C4 yield increased in the order Ni < NiO < Mg, better results being obtained with the Fe2O3 promotor than with the Al2O3 promotor.

Non-nickel metals and alkali metals also show catalytic characteristics [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF]. Different authors demonstrated that, for the pyro-gasification of biomass, alkali metals are effective in reforming tar and improving the quality of the obtained gas [START_REF] Yanik | Biomass gasification in supercritical water: II. Effect of catalyst[END_REF]- [START_REF] Habibi | Co-gasification of Biomass and Non-biomass Feedstocks: Synergistic and Inhibition Effects of Switchgrass Mixed with Sub-bituminous Coal and Fluid Coke During CO 2 Gasification[END_REF]. [START_REF] Habibi | Co-gasification of Biomass and Non-biomass Feedstocks: Synergistic and Inhibition Effects of Switchgrass Mixed with Sub-bituminous Coal and Fluid Coke During CO 2 Gasification[END_REF] [START_REF] Habibi | Co-gasification of Biomass and Non-biomass Feedstocks: Synergistic and Inhibition Effects of Switchgrass Mixed with Sub-bituminous Coal and Fluid Coke During CO 2 Gasification[END_REF] studied the co-gasification of potassium-rich switchgrass with bituminous coal. Addition of switchgrass to coal mixtures hindered the gasification of coal, as the mobile alkali elements were sequestered by reaction with aluminosilicate minerals in coal ash to form inactive alkali aluminosilicates, such as KAlSi3O8 and KAlSiO4. Addition of more switchgrass ash to coal char, so that there was an excess of potassium, enhanced the coal gasification. This research area was recently reviewed by [START_REF] Nzihou | A review of catalysts for the gasification of biomass char, with some reference to coal[END_REF] [START_REF] Nzihou | A review of catalysts for the gasification of biomass char, with some reference to coal[END_REF]. They concluded that the best catalysts for promoting biomass char gasification are group I metals. Calcium is, next to potassium the most common active metal found in biomass, but is far less effective. Alkali metals, heavy metals and other inorganic components are present in plastic waste, and may serve as catalysts for the degradation of biomass-plastic mixtures, as mentioned, but not yet thoroughly investigated, by different authors [START_REF] Lopez | Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor[END_REF], [START_REF] Mastellone | Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor[END_REF], [START_REF] Taba | The effect of temperature on various parameters in coal, biomass and CO-gasification: A review[END_REF], [START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF], [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF], [START_REF] Kříž | Hydrogen from the two-stage pyrolysis of bituminous coal/waste plastics mixtures[END_REF], [START_REF] Kaewpanha | Steam co-gasification of brown seaweed and land-based biomass[END_REF]. Plastics contain indeed small amounts of various inorganic elements, used as catalysts during polymerization processes. The most common polymerization catalytic concept was discovered by Ziegler and Natta in the 1950's. A Ziegler-Natta catalyst is a complex formed by reaction of a transition metal compound (halide, alkyl, aryl, or alkoxy derivative) of a group IV-VIII transition metal (e.g. Ti, V, Cr, Mo, Co, Rh, Ni) called catalyst, with a metal alkyl or aryl halide of a Group I-III metal (e.g. Al), called co-catalyst [START_REF] Huang | Ziegler-natta catalysts for olefin polymerization: mechanistic insights from metallocene systems[END_REF]. These elements remain in the polymer matrix. Furthermore, metals are also added to the plastic matrix during the manufacture of plastic products: Pb, Cd, Cr, Hg, Br, Sn, Sb or Zn, are added as pigments, fillers, UV stabilizers, and flame retardants [START_REF] Piorek | Feasibility of analysis and screening of plastics for heavy metals with portable X-ray fluorescence analyzer with miniature X-ray tube[END_REF], [START_REF] Guney | Heavy Metals in Toys and Low-Cost Jewelry: Critical Review of U.S. and Canadian Legislations and Recommendations for Testing[END_REF], in variable contents depending on the end-use. For example, plastic toys or jewellery may contain different metals such as Pb, Cr, Cd, Ba, As, and Hg. as recently reviewed by Guney and Zagury (2012) [START_REF] Guney | Heavy Metals in Toys and Low-Cost Jewelry: Critical Review of U.S. and Canadian Legislations and Recommendations for Testing[END_REF]. Al-Qutob et al. ( 2014) [START_REF] Al-Qutob | Determination of Different Trace Heavy Metals in Children's Plastic Toys Imported to the West Bank/Palestine by ICP/MS-Environmental and Health Aspects[END_REF] also highlighted the presence of various metals in plastic toys for children. High concentrations of Pb, Sb, Al, Cd, Cr, Fe, Cu, Ni, Hg, Zn, Sn, Cl, P (up to 15 g/kg, for Al) were found in the plastic fraction of waste electrical and electronic equipment [START_REF] Morf | Metals, non-metals and PCB in electrical and electronic waste -Actual levels in Switzerland[END_REF]. Contamination of soil and sediments by metal leaching from plastic waste, has been reported [START_REF] Brennan | Analysis of Plastics, Fibres and Textiles for Metals Content Using ICP-OES[END_REF]- [START_REF] Singh | Integrated Plastic Waste Management: Environmental and Improved Health Approaches[END_REF], again an indication that the considered metals are present in the plastic waste. In addition, plastic waste may be contaminated with metal-containing compounds after its use. We would recommend to further investigate the role of metals during the thermoconversion of plastic and biomass/plastic waste, taking into account the content and the speciation of metals in plastics.

General conclusion

Co-gasification of plastics and biomass in pyro-gasification has several advantages: it may help to overcome difficulties with seasonal availability of biomass, and, as a form of material recycling, improve the position of plastics treatment relative to the waste treatment hierarchy; it may also potentially solve technical problems related to difficult feeding of plastics. The major advantages are however that in general more fuel is converted to gas; less char and tar are produced than for pure biomass; adding plastics to biomass allows modifying the composition of the producer gas and adapting it to subsequent applications. The most suitable plastics for co-gasification with biomass are PE, PP and PS. For these plastics, an almost general consensus exists: with increasing PE, PP or PS, gas yield increases, tar and char decrease, H2 increases, CH4, hydrocarbons, CO and CO2 decrease. For PET/biomass feed, the behaviour appeared different, and PVC gave much HCl in the gas. Thus, selecting a suitable plastic/biomass mixture as feedstock allows to obtain a product distribution and a product gas composition that approximates more closely to the end-use requirements than using biomass alone. Of course, not only the feedstock, but also the gasifying agent, and operating parameters such as temperature, equivalence ratio (air or oxygen), steam/fuel ratio, as well as the catalyst determine the product distribution and product gas composition. These parameters can also be optimised, in view of achieving the ideal product distribution or gas composition for the subsequent process. In view of the large number of parameters this is not an easy task. We believe that this paper will help in selecting the most relevant ones. A systematic study of the optimisation methods available and the difficulties encountered is recommended, as it will help to develop the technology for the co-pyrogasification of plastics and biomass. If the resulting product gas does not comply to a sufficient extent with the requirement for the gas mixture, further syngas clean-up is necessary. The tar yield can be decreased further by applying secondary measures such as mechanical/physical methods, catalytic cracking and/or thermal treatment. Moreover, if needed, one can apply secondary reforming to remove CH4, shift conversion to convert CO into H2 and CO2 and to remove water. It is clear that these steps add additional complexity to the installation and increase costs, mainly for small systems. We believe that there is currently little information on the co-pyrogasification of real plastic fractions, e.g. from separate collection or from separating mixed plastics, light ASR, etc., with different sorts of biomass (including contaminated biowaste). Indeed, both such plastics and biowaste may contain metals and other impurities. The latter may constitute potential catalysts or pollutants, which may also influence the required post-treatment. Furthermore, the synergies observed during co-pyrogasification of biomass and plastics need to be further studied in order to explain the mechanisms involved in more depth.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1: Fixed bed reactors [71]Figure2: Fluidised bed reactors[START_REF] Knoef | Energy from biomass[END_REF]

Figure 6 :

 6 Figure6: TG and DSC curves as a function of temperature of HDPE, PS, PVC and for poplar wood (PW) and a waste wood sample (W3)[START_REF] Ephraim | Valorization of wood and plastic waste by pyro-gasification and syngas cleaning[END_REF].

Figure 7 :

 7 Figure7: Gas composition and hydrogen production for different biomass/PP ratios[START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF]

Figure 8 :

 8 Figure 8: CO2 in dry gas product (SLF= shredder light fraction originating from end-of-life vehicles; PE regrind=derived from packaging waste foil; PE=virgin PE) [66].

  [START_REF] Kumagai | Novel Ni-Mg-Al-Ca catalyst for enhanced hydrogen production for the pyrolysis-gasification of a biomass/plastic mixture[END_REF] [START_REF] Kumagai | Novel Ni-Mg-Al-Ca catalyst for enhanced hydrogen production for the pyrolysis-gasification of a biomass/plastic mixture[END_REF] proved the importance of Ca in combination with Ni-based catalysts. They synthesized Ni-Mg-Al-Ca catalysts with different Ca contents and calcinated at different temperatures, to enhance the in situ CO2 absorption and to increase the hydrogen production from the pyro-gasification of a wood sawdust/ PP mixture. The highest hydrogen yield (39.6 mol H2/g Ni) with an H2/CO ratio of 1.90 was obtained for the catalyst with the molar ratio Ni/Mg/Al/Ca = 1/1/1/4. NiO catalyses the gasification of the biomass/plastic pyrolysis products, giving H2, CO, CO2 and hydrocarbons; CO2 adsorption by CaO shifts the equilibrium of the water-shift reaction, thus increasing the H2 production (Figure11).

Figure 11 :

 11 Figure 11: Pyrolysis and hydrogen production from wood sawdust (WS) and PP in the presence of Ni-Mg-Al-Ca catalyst[START_REF] Kumagai | Novel Ni-Mg-Al-Ca catalyst for enhanced hydrogen production for the pyrolysis-gasification of a biomass/plastic mixture[END_REF]

Table 2 :

 2 Main gas composition specifications for selected applications[START_REF] Boerrigter | Syngas production and utilisation[END_REF],[START_REF] Butterman | CO2 as a Carbon Neutral Fuel Source via Enhanced Biomass Gasification[END_REF],[START_REF] Van, Drift | Duurzaam synthese gas: een brug naar een duurzaam energie-en grondstoffenherziening[END_REF]

	Synthesis	H2	for	Ammonia	Methanol	Fischer-		Oxo	-
		refinery				Tropsch		alcohols
	H2	>98%		75%	71	60		60
	CO	<10-50 ppm CO+CO2	19	30		40
				<20 ppm
	CO2	<10-50 ppm CO+CO2	4-8%
				<20 ppm
	N2			25%
	Inert	N2, Ar, CH4	Ar, CH4	N2, Ar, CH4	CO2,	N2,
		balance		as low as	as low as	Ar, CH4
				possible	possible	low
	H2/N2			 3
	H2/CO					1.5-3		1-1.5
	H2/(2CO+3CO2)				1.3-1.4
	Process			350-550°C 300-400°C 200-350°C 85-200°C
	temperature
	Process pressure >50 bar		100-250 bar 50-300 bar 25-60 bar	15-350 bar

Table 3 :

 3 Co-pyrolysis/gasification of plastics/biomass; plastic/biomass/coal and plastic/coal mixtures.

	Reference	Fuel	Reactor and parameters	Research
	Jakab et al., 2000	PP and wood flour,	TG/MS; GC/MS	Thermal decomposition
	[75]	lignin, cellulose,		of PP
		charcoal
	Jakab et al., 2001	Vinyl polymers and	TG/MS; GC/MS	Thermal decomposition
	[76]	lignocellulose		of vinyl polymers
	Sorum et al., 2001	Pyrolysis of cellulose	TG, 600°C: rotary kiln, 400-	Biomass-PVC pyrolysis
	[77]	with PE, PP, PS,	500°C	mechanisme
		PVC
	Matsuzawa et al.,	Co-pyrolysis of	TG; DSC	Thermal decomposition
	2001 [78]	cellulose and PVC;		of polymers
		MSW
	Marin et al. 2002 [79] Co-pyrolysis wood	TG, 600°C	Characterization of
		and polyolefines		liquid phases
	Sharypov et al., 2002	Beech wood, pine,	TG	Influence of
	[80]	cellulose lignin +		experimental conditions
		MDPE, isotactic and		on solids, liquids and
		atactic PP		gases
	Pinto et al., 2002 [33] Pine + PE (0-60%)	-Circular in cross-section	Influence of T, O2 flow
			gasifier	rate, S/F, PE content
			-air/steam gasification
			-T: 720-900°C
			-S/F: 0.5-0.8
	Pinto et al., 2003 [81] 60% coal, 20% pine,	-Pilot scale fluidised bed	Effect of T and
		20% PE	reactor.	gasification medium (air
			-750-900°C	+ steam)
			-gasification medium: O2,
			steam, O2+steam
			-O2/fuel: 0.03-0.33 kg/kg (dry
			and ash free)
			-O2/steam: 0.02-0.28 kg/kg
			-flow rate feedstock: 3.3-5.5
			kg/h
			-steam flow rate: 2-5 kg/h
			-particle size: 1.2-2 mm
	Sakurovs, 2003 [82]	Coking coal + PP,	PMRTA (Proton magnetic	Influence of polymers
		PS, polyacrylonitrile,	resonance thermal analysis)	on the fluidity of coking
		polyphenylene		coal-polymer blends
		sulfide

  [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF] [START_REF] Pinto | Cogasification study of biomass mixed with plastic wastes[END_REF] observed in the steam co-gasification of biomass and PE (circular in cross-section gasifier, 835 °C), that with increasing amounts of PE in the feed, H2 increased, CO and CO2 decreased, methane first decreased and then increased, and hydrocarbons and tar also decreased. With increasing PE, from a PE content of 20% the gas composition remained almost constant with a maximal hydrogen concentration of about 50%.Ahmed et al. (2011)

[START_REF] Ahmed | Characteristics of syngas from cogasification of polyethylene and woodchips[END_REF]

(steam gasification; 900°C) observed for the gasification of PE/woodchip mixtures peak values for syngas yield, H2 yield, and hydrocarbon yield for PE percentages of about 80% PE.

[START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF]

[START_REF] Alvarez | Hydrogen production from biomass and plastic mixtures by pyrolysis-gasification[END_REF]

studied the co-gasification (two-stage fixed bed; pyrolysis 600°C; steam gasification 800°C; with and without Ni-based catalyst) of wood sawdust and 20% PP, HDPE, PS or real plastics. The highest gas yield and lowest tar and char yields were obtained for the polyolefines. With increasing amounts of polyolefines in the biomass/plastic mixtures, H2 and C1-C4 concentrations increased and CO and CO2 decreased. Increasing the amount of PP in the woodchip/PP mixture from 0 to 20 %, increased the gas yield from 51.6 to 57.0% and the H2 concentration from 30.3 to 36.1%. The H2 concentration significantly increased further to 52.1%, in the presence of a Ni-based catalyst. The release of H2 is also favoured in the gasification of coal/PE and biomass/coal/PE mixtures (steam; 850°C) (Figure

Table 4 :

 4 Comparison of char yield (experimental) and wood char yield (calculated)

		Alvarez [106]		Xue 2015 [114]
	wood wt%	100	95	90	80 wood wt%	100	80
	PP wt%	0	5	10	20 HDPE wt%	0	20
	char yield %	20.9	19.9	18.7	17.3 char yield %	11.1	8.3
	wood char yield %	20.9	19.9	18.8	16.7 wood char yield %	11.1	8.9

Table 5 :

 5 H2/CO of the syngas from pyro-gasification of MSW single-components[START_REF] Dong | MSWs gasification with emphasis on energy, environment and life cycle assessment[END_REF].

	Reaction	Poplar wood	Cardboard	Food waste	PE
	atmosphere
	N2	0.64	1.53	2.49	3.07
	Steam	1.57	3.90	2.77	3.14
	CO2	0.61	0.86	1.81	1.87