F Fabry

L Fulcheri Mines

SYNTHESIS OF CARBON BLACKS AND FULLERENES FROM CARBONACEOUS WASTES BY 3-PHASE AC THERMAL PLASMA

Keywords: Carbon Black, Fullerene, Carbonaceous Wastes, Thermal Plasma, material recycling

In the current context of conventional fossil resource depletion, global warming and rising waste, thermal plasma appears as an interesting alternative compared to conventional thermochemical processes (combustion, gasification, pyrolysis, cracking…) in the fields of energy and nanomaterial synthesis. The particular processing conditions due to thermal plasma (i.e. high temperatures and high enthalpy densities) allow using raw materials with Low Heating Value such as organic wastes (dried sewage sludge, plastics, used tires, sawdust…) or biomass (liquid, solid or gaseous) in the fields of energy and nanomaterial synthesis. This paper reviews the last developments of a 3-phase AC plasma technology for the treatment of domiciliary and industrial wastes for nanomaterial synthesis, particularly the preliminary results reached for the production of carbon blacks and fullerenes from thermochemical processing by plasma of plastics (HDPE pellets) and carbon black soot resulting from tire pyrolysis. Carbon blacks obtained from HDPE are well organized with physical characteristics similar to acetylene black and plasma treatment of tire carbon black soot shows that it is possible to produce fullerenes with this carbon precursor and with nitrogen plasma gas at atmospheric pressure.

1-INTRODUCTION

In the current context of conventional fossil resource depletion, global warming and rising waste, thermal plasma appears as an interesting alternative compared to conventional thermochemical processes (combustion, gasification, pyrolysis, cracking…) in the fields of energy and nanomaterial synthesis. Indeed, conventional methods based on Low Heating Value of the raw material present some limitations that might be overcome through plasma, particularly in terms of: material yield, gas purity, energy efficiency, dynamic response, compactness, flexibility... The injected plasma power can be adjusted independently of the heating value of the treated material. These particular processing conditions due to the thermal plasma (i.e. high temperatures and high enthalpy densities) allow using raw materials with Low Heating Value such as organic wastes (dried sewage sludge, plastics, used tires, sawdust…) or biomass (liquid, solid or gaseous) in the fields of energy and nanomaterial synthesis [START_REF] Fabry | Waste gasification by thermal plasma: a review[END_REF][START_REF] Gomez | Thermal plasma technology for the treatment of wastes: a critical review[END_REF]. An original semi-industrial scale plasma technology using a three-phase AC source is presently working at the Centre PERSEE, MINES ParisTech, PSL. This technology has been developed initially for the synthesis of carbon nanoparticles such as fullerenes, carbon blacks, nanotubes… This technology has evolved since 1993 and has reached a high level of reliability, unique at this scale [3 -5]. Since few years, this three phase AC plasma technology has been modified and adapted in order to operate as well as in neutral or reducing medium than under oxidizing conditions for waste and biomass valorization in the field of energy and nanomaterial synthesis [START_REF] Takali2 | Development of a 100 kW plasma torch for plasma assisted combustion of low heating value fuels[END_REF]. This paper reviews the last developments of the 3-phase AC plasma technology for the treatment of domiciliary and industrial wastes for nanomaterial synthesis, particularly the preliminary results reached for the production of carbon blacks and fullerenes from thermochemical processing by plasma of plastics (HDPE pellets) and carbon black soot resulting from tire pyrolysis.

2-MATERIALS AND METHODS

Since 1993, researches on synthesis of carbon nanomaterials by plasma have been carried out at the laboratory of the Centre PERSEE -MINES-ParisTech -PSL in collaboration with academic and industrial partners (CNRS-PROMES France, Erachem Belgium and Timcal Belgium). For carbon nanoparticle synthesis, the originality of this plasma process consists in the substitution of the energy of the flame resulting of the incomplete combustion of a hydrocarbon by a thermal plasma in which a hydrocarbon precursor (gaseous, liquid or solid) is directly injected in order to crack it into carbon black and hydrogen. These researches have allowed the development of an original 3-phase AC plasma technology [3 -5] at a pilot scale and a partnership with a Californian star-up (Monolith Materials) for the analysis and design of one pilot plant (in USA) for production of carbon black and hydrogen by thermal plasma [7]. This technology has evolved since 1993 and has reached a high level of reliability, unique at this scale. The process set-up at Sophia-Antipolis at a pilot scale is illustrated in Figure 1. The process can be briefly described as follow: the plasma is powered by a 3-phase AC plasma power supply (600 Hz, 0-400 A, 263 kVA maximum power). Each of the three phases of the power supply is connected to the graphite electrodes of a 3-phase plasma torch located at the upper part of a reactor. Thermal plasma is generated by an arc discharge between the three graphite electrodes.

Carbon Black synthesis

In Figure 2 is represented the configuration used for the synthesis of carbon black (configuration 1 -solid line). For the treatment of plastic wastes (HDPE pellets), a special powder injection system is employed to mix the solid carbonaceous precursor with a suspending gas to transport the mixture inside the reactor. This aerosol flows across the plasma, reaching the highest temperature region in the reactor. Due to the high enthalpy density obtained, the plastic waste is cracking completely while passing through the plasma zone. The internal shape of the reactor has been designed to improve the conditions for cracking by a strong confinement of the gas flow. The internal part of the plasma reactor is composed of a high temperature insulator lining (solid graphite and graphite felt) in order to increase the heat density as well as to decrease the thermal losses from the hot gas through the walls. The external kiln of the reactor is a double wall water cooling cylinder. The system is equipped with a PTFE bag filter where product and gas are separated and collected. A gas network allows providing different plasma gases or mixtures (N 2 , Ar, He, CO, H 2 ) to the plasma system. The process pressure is controlled to be at atmospheric pressure.

The pilot is composed of:

 a 100 kW 3-phase AC plasma torch with graphite electrodes, located on the upper part of the reactor,  a high temperature reactive zone in which the plastic wastes (HDPE pellets) or carbon black soot resulting from tire pyrolysis (Powder) is injected with plasma gas,  a hot wall reactive zone (2 meters high),  a cooling and carrying zone,  a filtration system where solid product (CB or fullerene soot) and gas (N 2 -H 2 or N 2 ) are separated. The plastic selected as feedstock for the test is a HDPE powder from Atofina : FINATHENE® 56020 S. This is a very high molecular weight high density polyethylene produced by the slurry loop low pressure polymerization process. It has been especially developed for the manufacturing of blow moulded rigid containers offering a high impact resistance and stackability behaviour (transport of dangerous goods). This grade can also be used in sheet extrusion and compression. FINATHENE® 56020 S is available as white powder (average particle size: 800 microns in a range 100 -1600 microns and with a specific surface of 0.3 m 2 .g -1 ).

In Table 1 are summarized the main operating conditions for the synthesis of carbon black from HDPE used as carbonaceous precursor. In the following chapter, this paper will present the most significant results about the characterization of the carbon black synthesized from HDPE using BET and SEM and a comparison between an industrial carbon black grade (Acetylene Black -Y50A -SN2A) and a HDPE plasma carbon black is proposed.

Fullerene synthesis

This 3-phase AC plasma technology, initially designed and optimized for the treatment of hydrocarbons and the synthesis of novel grades of carbon black, has been modified and adapted in order to process solid carbonaceous powder for the continuous synthesis of fullerenes, fullerene soot or carbon nanotubes [8 -9]. The process combines the high temperature arc method with continuous gas-phase synthesis by injecting solid carbon precursors into a thermal arc plasma. The adaptations lead to the overall process scheme illustrated in Figure 2 (configuration 2 -solid line + dashed line). In this case, solid powder is carried away together with the plasma gas to the arc zone. The main difference with the classical arc process for fullerene and nanotube synthesis is that the input carbon rate is no more limited to the electrode erosion but can be controlled totally independently. A quenching/sampling system collects the high temperature gas at a predetermined position, cools it down rapidly to the conditions chosen for the production of fullerenes (quenching) and extracts the resulting product from the reactor. The gas is filtered and a part of it is reinjected into the reactor (dashed line in the Figure 2). The fraction that corresponds to the initial flow rate entering the reactor as plasma gas is exhausted. This set-up (with recirculation) allows the extraction of a gas volume superior to the initial flow of plasma gas and therefore, disconnects the dependency of these two parameters, which leads to an additional degree of freedom in relation to process operating conditions. Absolute airtightness of the system is monitored by continuous measurement of the oxygen concentrations at several points of the system.

The plasma process addresses two types of products:

• Fullerenes, mainly C 60 and C 70 molecules,

• Fullerenic nanostructures based on carbon nanoparticles (carbon-black-like) with particular fullerenic surface structure.

The control of the three main processing parameters: temperature profile, residence time in the reactor and reactant concentration is of great importance to generate tailor-made particles. Thus, the system has been designed to adjust the residence time and the quenching rate by changing the gas extraction point from the reactor and the gas flow rate in the quenching/recirculation loop. The reactant concentration is adjusted by the control of the plasma gas/solid precursor ratio.

Experimental procedure for Fullerene synthesis

Today, the main processes applied in fullerenes production are the laser ablation, the arc discharge, arc jet plasmas, concentrated solar flux, resistive or inductive heating and combustion-based methods [START_REF] Fulcheri | The influence of the carbon precursor, carbon feed rate and helium gas flow rate on the synthesis of fullerenes from carbon powder in an entrained flow 3-phase AC plasma reactor operating at atmospheric pressure[END_REF]. In the major part of these processes, the process is more or less the same and can be described as follow with the arc discharge as example: in the first step of the arc discharge, carbon vapor composed of atomic species and very small clusters is produced by the sublimation of carbon in an inert atmosphere through an electric arc maintained between two electrodes. In a second step, the carbon vapor is expended in an inert gas. In a third step, the carbon vapor is quenched and a part of the small clusters self-assemble to produce fullerenes. As presented before, the plasma process for fullerene production is derived from the arc process but the main difference consists in injecting a carbon precursor as an aerosol (with nitrogen gas) through the 3-phase plasma. After traveling through the hottest temperature region (> 4000 K), the carbon is partially vaporized. The rate of the vaporized part of the carbon particles is function of the initial size of the particles and the residence time in the high temperature zone (plasma zone). After the re-condensation of the gaseous carbon on the nonvaporized part of the carbon precursor and an annealing step around 2000 K, the mixture of carbon nanoparticles and plasma gas is rapidly extracted from the hot region (quenching) and cooled down to room temperature by means of a gas recirculation loop which also contains the filtering system. The point of gas extraction from the reactor and the gas flow rate in the recirculation cycle determine the residence time of the carbon precursor in gas phase and the quenching rate.

Once collected in the filter, Soxhlet Toluene extraction was performed on the soot and the extract was analyzed by UV-Vis spectrophotometry. The extract concentration in fullerenes C 60 and C 70 was evaluated from the two absorption peaks at 330 nm and 470 nm [START_REF] Hare | Preparation and UV / visible spectra of fullerenes C 60 and C 70[END_REF].

In Table 2 are summarized the main operating conditions for the synthesis of fullerenes from carbon black soot resulting from tire pyrolysis used as carbonaceous precursor.

3-RESULTS AND DISCUSSION

Carbon Black synthesis

TGA, BET and SEM analyses have been carried out on several samples taken in different locations in the reactor and the filter in the aim to analyse the particular structures of the plasma carbon blacks in function of the origin of the samples. Plasma carbon blacks have been sampled in three zones of the process: sample A in the bag filter, sample B along the graphite walls of the reactor and sample C at the lower part of the reactor. In Table 4 are summarized the main results about these three samples, the raw material (HDPE pellets) and an industrial carbon black (Y50A acetylene carbon black from SN2A). (1) Sample contains HDPE-like particles. Product was sieved prior to testing but the finest particles could not be removed.

Raw material (HDPE)

One can observed a small decrease of the specific surface area of the carbon black sampled on the surface of the inner graphite walls of the reactor (sample B). It is mainly due to the annealing of the carbon black deposit by the high temperature plasma during the process. One can observed too that the sample C contains some HDPE-like particles. These particles are the biggest HDPE pellets which have not been cracked during their processing due to the initial size of these particles and the residence times in the plasma zone which is not optimized for this kind of carbon precursor. Sample A is the more representative of the final product obtained from HDPE cracking by thermal plasma and its specific surface area is very interesting, in the range values of industrial carbon blacks like acetylene carbon black (Y50A).

Figure 3 represents a typical SEM image of some aggregates and agglomerates of HDPE carbon black synthesis by thermal plasma. Figure 4 represents a typical SEM image of some aggregates and agglomerates of Y50A acetylene carbon black from SN2A. One can observe that both morphologies are very close, like previously observed on carbon black synthesis by thermal plasma from others various precursors (methane, ethylene, Pyrolysis Fuel Oil (PFO) or Colza Oil [START_REF] Probst | Quality and performance of carbon blacks from plasma process[END_REF]). This is one of the particularities of the 3-phase AC thermal plasma carbon blacks. Independently of the carbon precursors, the carbon blacks produced are always very similar morphologies due to the thermal history of the carbon particles in the reactor which is not only function of the carbon precursor but mainly of the temperature of the thermal plasma and the nature of the plasma gas. The structure of the plasma CBs is the result of the growing of the particles at some temperatures between 1800 K and 2500 K. The 3-phase AC plasma process allows the production of new carbon grades thanks to reaction conditions unreachable by the conventional combustion processes.

Fullerene synthesis

In the plasma process, insoluble carbon soot is generated together with soluble fullerenes. The fullerenes have been extracted from the fullerene soot with organic solvents (toluene) by applying Soxhlet-extraction. The C 60 and C 70 fullerene molecules present in the separated fractions have been qualitatively and quantitatively identified and measured by UV/VIS spectroscopy.

In Table 5 are given the wt % of C 60 and C 70 measured after the plasma treatment with nitrogen plasma gas. Table 5: wt % of the C 60 and C 70 fullerenes synthesized by 3-phase AC thermal plasma with nitrogen plasma gas.

Previously [START_REF] Fulcheri | The influence of the carbon precursor, carbon feed rate and helium gas flow rate on the synthesis of fullerenes from carbon powder in an entrained flow 3-phase AC plasma reactor operating at atmospheric pressure[END_REF], during the processing of carbon power (CB or graphite powder) by 3-phase AC thermal plasma, best yields were obtained using helium as plasma gas, slightly lower with argon while nearly no fullerenes were found with nitrogen. These results were in line with results obtained with other high temperature methods (arc, laser or solar). In the case of nitrogen, this was probably due to the formation of stable binary species at high temperature, such as CN, which stops the production of carbon vapor species (C, C 2 , C 3 …) as these clusters have been identified as fullerenes precursors. In the present case, the impurities in the carbon precursor could play the role of catalyst promoting the formation of fullerenes during the plasma treatment. In the literature, it is the first time such fullerene yields are synthesized with nitrogen plasma gas at atmospheric pressure.

So far the process of continuous fullerene synthesis is not fully optimized, but current yields typically of the order of 1 % (toluene extractable) have been obtained. At carbon flow rates of several hundred grams per hour, fullerene production rates of the order of 10 g.h -1 could be achieved.

4-CONCLUSIONS

3-phase AC plasma systems open a new area for the production of carbon nanostructures. These technologies are characterized by a very high versatility and flexibility. It has been demonstrated in several papers that the very similar technology can be used for the production of a wide range of carbon nanostructures ranging from carbon black over fullerenes to carbon nanotubes with a high product selectivity. In addition, plasma systems allow the conversion of any organic carbon precursor: liquid, solid or gaseous to valuable carbon nanostructures. This paper reviews the last developments of the 3-phase AC plasma technology for the treatment of domiciliary and industrial wastes for nanomaterial synthesis, particularly the preliminary results reached for the production of carbon blacks and fullerenes from thermochemical processing by plasma of plastic wastes (HDPE pellets) and carbon black soot resulting from tire pyrolysis. Carbon blacks obtained from HDPE are well organized with physical characteristics similar to acetylene black and plasma treatment of tire carbon black soot shows that it is possible to produce fullerenes with this carbon precursor and with nitrogen plasma gas at atmospheric pressure.

Carbon Black:

The main difference with other gas-phase synthesis processes (mainly combustion processes) is that the enthalpy applied to the system can be controlled totally independently by means of an external electric power supply. As a consequence, this allows the creation of thermodynamic conditions unreachable by conventional means. In particular, very high temperatures can be obtained (higher than 3,000 K up to 10,000 K) thus allowing the formation of specific species (C 1 , C 2 , etc), which could play an important role as precursor for new carbon black grades. In the conventional carbon black market, it will allow the use of new raw materials, such as waste oils, vegetable oils, etc, a more rational use of the raw material (100 % carbon yield), the production of carbon black free from CO 2 emission on the site and the production of hydrogen as by-product.

Fullerenes and fullerene soot:

The plasma process is addressing two types of products: • Fullerenes, mainly C 60 and C 70 , • Fullerene soot based on carbon nanoparticles (carbon-black-like) with particular fullerenic surface structures. So far the process of continuous fullerene synthesis from carbon black soot resulting from tire pyrolysis is not fully optimized, but current yields typically of the order of 1 wt % (toluene extractable) are obtained. At carbon flow rates of several hundred grams per hour, fullerene production rates of the order of 10 g.h -1 could be achieved. No other technology has so far been able to reach such high fullerene production rates on a continuous basis at atmospheric pressure and with nitrogen. The 3-phase AC plasma process can be considered as an improved highly flexible process with an enormous potential for further up-scaling to an industrial size and fullerene production at commercially viable cost.

In the emerging domains, the plasma process will allow the production of carbon materials of nano-sized structure from carbonaceous wastes, which are without any doubt amongst the most promising resources for future industrial applications and this 3-phase AC plasma process can be considered as a highly flexible process with an enormous potential for further up-scaling to an industrial size at commercially viable cost.

Figure 1 :

 1 Figure 1: Overall picture of the 3-phase AC plasma reactor (2 meters high).

Figure 2 :

 2 Figure 2: Scheme of the plasma reactor for the synthesis of carbon blacks (configuration 1 -solid line) and for the synthesis of fullerenes and nanotubes (configuration 2 -solid line + dashed line).

Figure 3 :

 3 Figure 3: HDPE Plasma Carbon Black (Sample A).

Figure 4 :

 4 Figure 4: Y50A Carbon Black.

Table 1 :

 1

	Property	Unit	Value
	Average Power	kW	50
	Average Current	A	220
	Average Voltage	V	175
	Plasma gas flowrate (N 2 )	Nm 3 .h -1	2
	Carrier gas flowrate (N 2 )	Nm 3 .h -1	2
	HDPE Flowrate	Kg.h -1	0.25

Main operating conditions for the synthesis of carbon black from HDPE used as carbonaceous precursor (average error ± 10%).

Table 2 :

 2 Main operating conditions for the synthesis of fullerenes from carbon black soot resulting from tire pyrolysis used as carbonaceous precursor (average error ± 10%).In Table3are summarized the main characteristics of the carbon black soot resulting from tire pyrolysis (CCT-R 632 from Carbon Clean Tech) used as carbonaceous precursor.

	Property	Unit	Value
	Average Power	kW	36
	Average Current	A	180
	Average Voltage	V	150
	Plasma gas flowrate (N 2 )	Nm 3 .h -1	4
	Carrier gas flowrate (N 2 )	Nm 3 .h -1	4
	Carbon black soot flowrate	Kg.h -1	0.8
	Recirculation loop gas flowrate (N 2 )	Nm 3 .h -1	200

Table 3 :

 3 Composition/information on ingredients (CCT-R 632 from Carbon Clean Tech MSDS).

Table 4 :

 4 Characterization of the plasma samples.

th International Conference on Engineering for Waste and Biomass Valorisation -May 23-26, 2016 -Albi, France


Laurent Fulcheri
email: laurent.fulcheri@mines-paristech.fr

Vandad Rohani

Sabri Takali

Frédéric Fabry

François Cauneau

A new electro-burner concept for biomass and waste combustion

Keywords: Biomass/Waste, Combustion, Plasma, Electro-burner

Raw biomass resources and many wastes are composed of poor-LHV organic matter (LHV < 20 MJ/kg). Their use as renewable fuels for heat or power generation is challenging, particularly when they are in solid form. Indeed, their combustion in air is critical and it is not possible to build autonomous burners independently of external conditions without an assistance. Three main options are currently studied: (i) the co-combustion, in which the poor-LHV fuel flame is supported thanks to an additional rich fuel, (ii) the oxy-combustion (iii) the electro-combustion, consisting in the generation of thermal plasma for activating and assisting the combustion. This last option is highly interesting because it only requires electricity (having low carbon content if renewable electricity). Depending on the nature of the feedstock, the electric power of the plasma does not exceed 1% to 5% of the flame power. Most plasma electro-burners technologies today on the market use DC plasma torches. These technologies suffer from many drawbacks among which: limited electrodes lifetime, poor reliability, important water cooling needs, need of AC/DC transformers, etc. leading to high CAPEX and OPEX. With the objective to go over these limits and reduce OPEX and CAPEX while increasing reliability, the Center PERSEE MINES-ParisTech has been working in the development of an original three-phase AC plasma technology to be integrated in a plasma electro-burner. This paper presents the main achievements on the plasma technology with a special focus on the limitation of electrodes erosion thanks to an active thermo-chemical gas sheathing.

In the present context of conventional fossil fuel depletion and global warming, energy production from biomass and organic wastes is a major challenge for the future of society and environment. Most of crude renewable organic resources are characterized by a low heating value and their direct combustion in air can only be achieved in a high temperature environment. Due to the increase of renewables in the energy mix, together with fluctuation of electricity demand, modern thermal power plants using low heating value feedstock are more and more led to operate under dynamic conditions including low temperature operation conditions during start up, heat up and low load phases. To face these evolutions in operation modes, the most developed technique consists in using assistance to combustion thanks to specific additional burners working with rich fossil fossils, generally natural gas or heavy fuel. This solution is constraining, costly and environmentally damaging since it not only implies the combustion of expensive rich fossil fuels but also increased operating cost due to additional burners and side elements. For few years, new alternative solutions for the assistance of low heating value feedstock combustion have emerged. Among these news techniques, the plasma assistance combustion is particularly promising and attractive. The principle is based on the controlled thermochemical activation of the combustion thanks to the use of plasma whose power generally does not exceed few percent of the combustion thermal power. Such systems, called plasma electro-burners present number of advantages versus traditional technologies based on partial oxidation or combustion among which: their ability to deliver and tune the enthalpy and/or temperature through an external energy supply with temperatures that cannot be achieved with combustion methods, their very short dynamic response, their limited environmental impact (no direct CO2 emissions), their flexibility and ability to operate under very different atmospheres (neutral, reducing or oxidant), and their compactness due to their very high volumetric energy density which opens the way to process intensification. Most plasma electro-burners technologies today on the market use DC plasma torches. These technologies suffer from a certain number of weaknesses and drawbacks among which: limited electrodes lifetime, poor/limited reliability, important water cooling needs, need of AC/DC transformers leading to high Capital (CAPEX) and Operational (OPEX) costs. With the objective to go over these limits and reduce OPEX and CAPEX costs while increasing reliability, the Center PERSEE MINES-ParisTech has been working for many years in the development of an original 3-phase AC plasma technology to be integrated in a plasma electro-burner.

2-BIOMASS AND WASTE

In 2050, primary energy consumption would reach 22.6 Gtep i.e. three fold the 2000 consumption (7.38 Gtep). Simultaneously the share of electricity would rise from 34 % to 42.5 %. According to B1-GIEC and IMACLIM-CIRED scenarii, the share of biomass in the energy mix would reach in 2050 a part comprised between 9.7 % and 14.4 % (2.2 to 3.25 Gtep). In conventional thermal power plants where coal plays today a major role, the decrease of coal share to face up to critical environmental issues will necessarily involve an increase use of biomass and wastes. Indeed, as shown on Table 1, the economic and environmental advantages of this renewable resource are obvious: (i) a highly competitive MWh price versus other renewable energy technologies (wind, hydro or solar) and fossil fuels, (ii) an expected CAPEX (2030) among the lowest on the market and (iii) greenhouse gases emissions around 30 kg CO2 equivalent per MWh corresponding to 10 % of fossil fuel emissions and same order of magnitude than other renewable energy technologies. Biomass can be considered as a form of solar energy storage during the photosynthesis process. It is often considered as a clean energy. However, In many cases, non-sustainable biomass resource exploitation can create important environmental damages and it is important to keep in mind that biomass can be considered as a clean and renewable energy resource only on condition that it is accompanied by a sustainable policy. Biomass can have different origins (agricultural, farming or food industry residues, sewage sludge, municipal wastes, dedicated energy plantations…) and it can be classified into three main categories depending on its state: gas, liquid or solid. With 10 Mtep in 2011, France is the first European country in terms of wood exploitation [START_REF]Ministère de l'écologie, du développement durable et de l'énergie[END_REF]. We will focus in the following on solid biomass only.

3-SOLID BIOMASS FOR ENERGY PRODUCTION

Solid biomass can be used for electricity production and its use in thermal power plant is increasing since the resource is more abundant than biogas and biofuels. Most developed countries engaged towards a clean energy transition policy follow this strategy with the aim to "decarbonize" their energy but also to limit their fossil fuel dependency.

In France, solid biomass resource for electricity production per year (residues from forest industry and wood transformation industry, straw…) is estimated to 5 Mtep plus 0.5 Mtep of sewage sludge (potential of non-exploited resource). Most of this biomass is characterized by LHV (Low Heating Value) varying between 6 to 20 MJ/kg, Chlorine and Sulphur contents generally lower than 0.1 % and very high water content generally ranging from 30 to 50 % of the mass, hence the need for drying it before they use for energy purposes [START_REF] Haider | Chaudières de puissance, à déchets, à biomasse et de récupération[END_REF]. We give on Table 1 China which is the country having highest energy consumption and CO2 emissions is currently massively investing on biomass integration technologies in the energy mix. In 2010, 5.5 GW electricity production from biomass was achieved among which 62% was directly produced by wood-energy [START_REF] Xingang | Development goal of 30 GW for China's biomass power generation: Will it be achieved?[END_REF]. The objective for 2020 is to reach 30 GW; this will need a USD$ 3.1 billion investment. Today the number of thermal power plants in China using biomass is estimated between 500 and 700 [START_REF] Xingang | Development goal of 30 GW for China's biomass power generation: Will it be achieved?[END_REF]. In order to support this policy, a $USD 0.039 subsidized price for electricity production from biomass is currently applied. Thermal processes for large scale energy production from biomass can be classified into three main categories: pyrolysis, gasification and combustion. This later is the most common since it accounts around 97% in the global bioenergy production.

The high water content is generally a major drawback for biomass combustion and sustaining a continuous and stable flame need humidity content lower than 65 % [START_REF] Jenkins | Combustion properties of biomass[END_REF]. Fixed and fluidized beds are the most developed technologies for solid biomass combustion [START_REF] Nussbaumer | Combustion and Co-combustion of Biomass: Fundamentals, Technologies, and Primary Measures for Emission Reduction[END_REF]. Fixed bed being far away the most simple and spread option for small units while fluidized bed technologies allow achieving higher yields are most suitable for big installations.

The self-sustained and clean combustion of low heating value feedstock such as biomass is not obvious and is today an important scientific and technical challenge. The main drawback is that the combustion of low heating value feedstock in air can only be realized at high temperature which makes it highly dependent on the furnace thermal conditions. At low temperature, such as encountered during start up, heat up and low load regimes feedstock having a low heating value cannot be used without this add of an external energy source.

In order to exceed these drawbacks, three options for the combustion assistance of low heating value feedstock are today under development: (i) oxy-combustion, (ii) co-combustion (or co-firing) and (ii) plasma-assisted combustion.

In oxy-combustion, pure oxygen, generally produced by cryogenic processing with purity higher than 95%, is used as combustive. This allows obtaining a stable flame while limiting NOx production. The flue gases being mainly composed of CO2 and Steam this possibly opens the way towards the direct capture of CO2 in post combustion. The main drawback of this technique relies on the high cost of oxygen production which is around 0.20 -0.22 kWh per kg of oxygen (cryogenic separation) [START_REF] Zheng | Oxy-Fuel Combustion for Power Generation and Carbon Dioxide (CO2) Capture[END_REF].

As example, a technical and economical assessment of a 460 MW oxy-combustion coal power plant gives a cost comprised between 72 and 83 MW for the production of oxygen corresponding around 15-18 % of the total electric power [START_REF] Skorek-Osikowska | Thermodynamic and economic analysis of the different variants of a coal-fired, 460 MW power plant using oxy-combustion technology[END_REF]. In addition with increased operational costs (OPEX), investment costs (CAPEX) for an oxy-fuel combustion power plant is also high: 2 089 €/kW against 1 323 €/kW for a supercritical coal power plant [START_REF] Skorek-Osikowska | Thermodynamic and economic analysis of the different variants of a coal-fired, 460 MW power plant using oxy-combustion technology[END_REF].

The principle of co-combustion consists in using an extra fuel, generally fuel oil or natural gas to assist the combustion of a low heating value feedstock. It is today the most developed method used during the startup and heat up phases.

4-PLASMA ASSISTED COMBUSTION

Plasma, which represents about 99% of the universe, is often presented as the forth state of matter. It is composed of electrons, ions, molecules at fundamental or exited state. Thanks to existence of charge carriers (electrons and ions), plasmas are characterized by a high electric conductivity. Plasma systems can be classified into two main categories: non thermal plasma where temperature (energy) of heavy particles (molecules and ions) is much lower than those of electrons and thermal plasmas where all the species have approximately the same temperature (energy). In an environment having a low carbon content electricity, such as what can be expected in the future with a large scale development of renewable energy (wind, solar, …), plasmas are particularly attractive and may open the way towards breakdown technologies and solutions able to answer to major tomorrow' energy and environment challenges. Indeed plasmas allow: (i) an energy input without direct CO2 emissions which is: robust, flexible, instantaneous reactive and tunable in a very wide range of operating conditions (atmosphere, pressure…) and likely to lead to temperatures that may exceed temperatures encountered in conventional combustion processes, (ii) to increase efficiency and reduce environmental impacts of a wide range of industrial processes thanks to their ability to generate unique and very reactive chemical species. As shown on Figure 1, plasmas can interact with combustion by different means [START_REF] Ju | Plasma assisted combustion: Dynamics and chemistry[END_REF] dealing with thermal, chemical and transport phenomena. On the thermal side, plasmas allow to increase the temperature and speed up the chemical reactions, including oxidation according to Arrhenius law. On the chemical side, plasmas can produce many active radical and species such as for example O, OH and H that have been identified as playing a major role in the chain combustion / oxidation mechanisms. On the transport side, generally considered as an indirect effect, plasma can participate to the breaking of big fuel molecules thus modifying the combustion kinetics. Other effects linked with electromagnetic forces acting of a source of momentum can have a great impact on the fluid dynamics and heat and mass transfer.

Figure 1: Role of plasma in combustion enhancement [START_REF] Ju | Plasma assisted combustion: Dynamics and chemistry[END_REF].

Plasma assisted combustion presents a great potential in many industrial sectors such as internal combustion engines [START_REF] Ju | Plasma assisted combustion: Progress, challenges, and opportunities[END_REF][START_REF] Bergthorson | A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines[END_REF][START_REF] Merola | Optical characterization of combustion processes in a DISI engine equipped with plasma-assisted ignition system[END_REF], turbo machines and gas turbines [START_REF] Starikovskiy | Plasma-assisted ignition and combustion[END_REF][START_REF] Warris | Ignition and flame stabilization by plasma jets in fast gas streams[END_REF] and thermal power plants [START_REF] Askarova | Plasma-Supported Coal Combustion in Boiler Furnace[END_REF]. Spark ignition of internal combustion engines, plane reactors or gas turbines is the most common "plasma system". In a standard spark plug, an electrical breakdown is created between two electrodes allowing ionizing and cracking molecules of the mixture air-fuel. This ignites the combustion mechanisms locally and the flame can propagate towards the overall mixture. On this area, new advanced ignition technologies are currently under development by most engine, and turbo machines manufacturers.

5-PLASMA ELECTRO-BURNERS FOR COAL COMBUSTION

Plasma electro-burners can generate a stable flame with a low heating value feedstock even in a low temperature environment thanks to the thermo-chemical plasma activation. For economic and environmental reasons, this technology has today an exponential development in China in coal power plants in replacement of traditional burner (Figure 2). 80 % of new coal power plants installed in China are equipped with plasma burners: in October 2012 more than 627 boilers equipped with plasma burners were in operation in China corresponding to a total installed capacity around 265 GW (total power plants). Plasma technologies cover boiler with powers ranging from 50 to 1000 MW. The yearly savings of auxiliary fossil fuels was estimated to 5 million tons equivalent to 3.5 billion euros. Figure 3 shows a picture of the Tianjin Guodian coal power plant (China) equipped with plasma burners. In France, researches in the field have been carried out in 1984 by BERTIN and Cie. These researches led to a patent [START_REF] Galant | Réacteur haute température associé à une zone de plasmagenèse, en particulier brûleur mixte électricité-combustible[END_REF]. The reactor has two stages respectively upstream and downstream the plasma zone. The injection of fuel as plasma gas aims at reducing the electrode erosion. The plasma chamber is equipped with a convergent nozzle in the upstream area and a divergent nozzle in the downstream area.

In 1994, a patent was filled by the French utility EDF [START_REF] Leleu | Electrobrûleur à gaz à apport d'énergie électrique et amorçage assisté[END_REF]. The system had several tangential air injection ports to create a vortex in the plasma gas and was working in DC.

6-CONCEPT OF PLASMA ELECTRO-BURNER FOR BIOMASS/WASTE COMBUSTION

With the recent necessity to limit the atmospheric CO2 concentration, Europe progressively turns away from coal and wish to replace it by regenerative solid biomass, including organic wastes. A lot of industrial sectors are concerned by this transition, going from energy sector to metallurgy, cement work, etc. Nevertheless, this change is not so trivial at technological level because the difference between these two fuel types can be significant (LHV, dryness, particle size and variability of the quality). Burners are within the first devices concerned by this change. In 80/90's, several plasma electro-burners were invented and developed for pulverized coal combustion in order to offer by means of a low electricity consumption (P elec = few % of P comb ) the possibility for the plants to operate with one unique fuel while increasing the operation flexibility (see previous section). By the way, the current Chinese plasma electro-burners presented previously derived from them. This new category of burners has been somewhat put aside in Europe because of the slowing down of the European coal technology development. Today, with the drive to develop biomass sector, there is a resurgence of interest. But, a plasma electro-burner designed for coal combustion cannot be directly used for combustion of common biomass. It requires revisions: (i) minor ones, if adequate associated technologies of biomass pre-treatment (ultra-drying, roasting, pyrolysis) and injection (grinding and pulverization) are specially developed and set-up in order to upgrade the fuel and get comparable properties as coal, or (ii) major revisions, even complete redesign, if the goal would be to directly used weakly treated or raw biomass.

In any case, the technological challenges are significant, and the best solution would certainly need, at the same time, development efforts on the fuel pre-treatment processes and/or the injection technology, and on the plasma electro-burner technology itself. The existing coal plasma electro-burners require a particle size of 50-100 m [START_REF] Messerle | Plasma assisted power coal combustion in the furnace of utility boiler: Numerical modelling and full-scale test[END_REF]. The biomaterials able to be convenient and grinded at this size could be dried non-filamentous ones such as wood-char, dried sewage sludge, roasted kernel, bone meal and dried leaf. So, a special technology for grinding and pulverizing at a micron-size these materials must be designed.

On the burner side, it is important to note that the existing coal plasma electro-burners suffer from a lack of robustness due essentially to the plasma torch technology used inside them. The severe electrodes erosion coming from the combination of oxidative conditions and particles impact reduces drastically the life-time of the burners. Thus, whatever the fuel to be used, a consequent development effort must be done on the plasma torch technology before a large industrial deployment. Despite the technological challenges, a biomass plasma electro-burner technology (Figure 4) would offer evident advantages. A robust tunable device allowing generating from biomass pulverized in air and low electricity consumption a stable flame independently of external thermal conditions would be a great advance for many sectors. Obviously, the quality of the combustion offered by the burner would be determinant for targeting its applications. For example, glass work sector is more demanding than energy sector on the quality of exhaust gases. Overall, in addition to the flexibility gain brought, the use of such biomass burners would allow guaranteeing for the future thermal installations a total independence from fossil fuels and making huge economy of atmospheric CO2. Assuming in the near future, a systematic regeneration of the biomass and availability of free-carbon electricity (renewable energy deployment), a biomass burner would lead to avoid the consumption of several million tons of fossil fuels. As an example, a 100 MW th /25 MW e biomass power plant needs to use 325 tons of oil per year for ensuring its flexibility and consequently emits 1980 tons/year of CO2. The use of plasma electro-burners in this case would require only 180 MWh carbon-free electricity per year for avoiding this CO2 pollution.

7-THREE-PHASE AC PLASMA TECHNOLOGY

Arc plasma technologies can be split into two main categories: DC and AC plasma torches [START_REF] Boulos | Thermal plasmas: fundamentals and applications[END_REF][START_REF] Solonenko | Thermal plasma torches and technologies[END_REF]. DC plasma torches usually have a tip cylindrical geometry with the tip, generally cathode, located at the upstream axis of a narrow cylindrical nozzle, generally anode. The plasma plume is characterized by a very high velocity (several hundred m/s) resulting from the combined effect of the narrow nozzle diameter and the severe thermal expansion of the plasma gas due to strong gas heating. Despite continuous technological and material improvements DC plasma torches suffer from one critical weak spot: electrode erosion resulting in limited electrode lifetime due to the combination of very strong thermal, mechanical, and chemical constrains. Another drawback of DC plasma torches is the need for delicate and costly electronics for the AC-DC current rectifier which generally accounts about 30 % of the overall capital and operating costs. On the other hand, AC plasma technologies have attractive features to exceed some of the critical drawbacks of DC plasma technologies [START_REF] Fulcheri | Three Phase AC Arc Plasma Systems: A Review[END_REF]. In multiphase AC plasma systems, several arcs can coexist in a larger discharge chamber volume creating a large volume zone as the arcs are generally much less confined than in classical DC plasma torches. The plasma average temperature can consequently be significantly lower than in DC systems. With the plasma temperature decrease, radiation and convection heat losses can be strongly reduced, increasing the overall electro-thermal efficiency. Unlike DC, multiphase AC plasma systems can be supplied thanks to simple transformers, thus significantly reducing OPEX and CAPEX costs while increasing reliability and scalability. Each electrode acts successively as anode and cathode, leading to a more symmetrical operating mode than in DC systems and limited electrode erosion. The convection transfer within the plasma can be enhanced by the strong arc motion resulting from the self-induced MHD forces. Finally, AC plasma systems can use different electrode materials that can be water-cooled (copper, stainless steel, tungsten…) or consumable (graphite…).

In 1973, at CNRS-Odeillo, France, Bonet started working on the development of a threephase AC plasma technology for the high temperature treatment of refractory materials [START_REF] Bonet | Contribution to the theoretical study of a refractory spheroidal particle evaporation in ac thermal plasma[END_REF].

The research led to a very intense technological effort, and to the study and analysis of a wide range of prototypes and design options together with electrode materials and plasma gases. Between 1979 and 1980, the French Lafarge cement company in collaboration with the Limoges University and CNRS-Odeillo led a collaborative research program on raw cement powder decarbonization based on Odeillo's three-phase AC technology. The feasibility of the process was demonstrated but it was abandoned probably for economic reasons.

In 1986, the French engineering company Bertin in collaboration with the French utility company EDF worked on the development of a hybrid 500 kW electro-burner prototype [START_REF]The graphite electrodes electro-burner[END_REF][START_REF] Reybillet | Bertin electro-burner: description and mockup tests[END_REF]. The plasma technology was inspired from Odeillo's technology. Despite significant technological achievements, the concept never reached the industrial scale and was abandoned in the 1990s. The development of the three-phase AC technology started again in 1993 in collaboration between PROMES-CNRS and PERSEE MINES-ParisTech in the field of hydrocarbon cracking for the co-synthesis of carbon black and hydrogen [START_REF] Fulcheri | From methane to hydrogen, carbon black and water[END_REF][START_REF] Fulcheri | Plasma processing: a step towards, the production of new grades of carbon black[END_REF][START_REF] Fulcheri | Comparison between new carbon nanostructures produced by plasma with industrial carbon black grades[END_REF][START_REF] Fulcheri | Comparison between two carbon nanostructures furnace and acetylene blacks[END_REF]. The R&D continued between 1997 and 2003 through three successive European projects. In parallel with the production of carbon black, it was demonstrated the three-phase AC plasma technology was also particularly suitable for the large scale synthesis of fullerenes and carbon nanotubes [START_REF] Gruenberger | Production of carbon nanotubes and other nanostructures via continuous 3-phase AC plasma processing[END_REF][START_REF] Okuno | Synthesis of carbon nanotubes and nano-necklaces by thermal plasma process[END_REF]. In 2002, the 250 kW three-phase AC plasma equipment, initially located at PROMES-CNRS Odeillo, was moved to PERSEE MINES-ParisTech Sophia Antipolis. Between 2004 and 2009, the development of the technology continued in collaboration between MINES-ParisTech and TIMCAL with the objective of bringing the technology at a pilot scale. Since 1993, four international patents have been granted, more than 30 papers were published in peer review journals [START_REF] Ravary | Thermal and Hydrodynamic modelling of a 3-phase plasma reactor, contribution to the development of an industrial process for the production of carbon black[END_REF][START_REF] Ravary | Influence of the Electromagnetic forces on momentum and heat transfer in a 3-Phase AC plasma reactor Plasma[END_REF][START_REF] Ravary | Analysis of the behavior of the arcs in a three phase AC plasma reactor[END_REF][START_REF] Fulcheri | Carbon Nanostructures by plasma[END_REF][START_REF] Rehmet | Theoretical and experimental study of a 3-phase AC plasma torch associated to a gasification process[END_REF][START_REF] Rehmet | 3D unsteady state MHD modeling of a 3-phase AC hot graphite electrodes plasma torch[END_REF][START_REF] Rehmet | A comparison between MHD modeling and experimental results in a 3-Phase AC arc plasma torch. Influ electrode tip geom[END_REF][START_REF] Rehmet | High speed video camera and electrical signal analyses of arcs behavior in a 3-Phase AC arc plasma torch[END_REF][START_REF] Rehmet | Unsteady state analysis of free-burning arcs in a 3-phase AC plasma torch. Comparison between parallel and coplanar configurations[END_REF][START_REF] Takali | Etude théorique d'un électrobruleur industriel dote d'une torche à arc triphasée pour la valorization énergétique de combustibles à faible pouvoir calorifique[END_REF][START_REF] Jones | Global reaction schemes for hydrocarbon combustion[END_REF][49][50][51][52][53] and four PhD theses carried out.

In his PhD, Fabry 1999 [START_REF] Fabry | Study of a plasma process for the synthesis of carbon black by high temperature hydrocarbons pyrolysis and product characterization[END_REF] focused on the development of experimental devices, diagnostic tools for the three-phase AC reactor, experimental analyses based on macroscopic heat and mass balances, as well as experimental plasma flow diagnostics. Meanwhile, Ravary [START_REF] Ravary | Thermal and Hydrodynamic modelling of a 3-phase plasma reactor, contribution to the development of an industrial process for the production of carbon black[END_REF] demonstrated that the flow was highly influenced by the arc motion. The presence of three electrodes creates a mutual electromagnetic influence of the arcs which deeply modifies the. To analyze this complex behavior, a high-speed cinema camera was used to film the arc zone. Movies showed the importance of the electromagnetic forces applied on the arcs which consequently had very characteristics motions. On the theoretical side, the forces have been evaluated in a simplified geometry. This estimation proved to be in qualitatively good agreement with the phenomena. A simplified model derived from the electromagnetic force model was used as a source of momentum as input of the CFD model [START_REF] Ravary | Influence of the Electromagnetic forces on momentum and heat transfer in a 3-Phase AC plasma reactor Plasma[END_REF][START_REF] Ravary | Analysis of the behavior of the arcs in a three phase AC plasma reactor[END_REF]. A comprehensive summary of achievements obtained between 1993 and 2003 is given in [START_REF] Fulcheri | Carbon Nanostructures by plasma[END_REF].

In his PhD, Rehmet 2013, [START_REF] Rehmet | Theoretical and experimental study of a 3-phase AC plasma torch associated to a gasification process[END_REF] deepened the analysis of the three-phase AC plasma system on non-reactive conditions (nitrogen, syngas) through an original approach carried out in parallel on the theoretical and experimental sides. The experimental study was based on high speed video camera and electrical signal analyses. The theoretical analysis was made through the development of an unsteady 3D Magneto-Hydro-Dynamic (MHD) model using the CFD software Code_Saturne. A parametric study based on current, frequency, plasma gas flow rate and electrode configurations (coplanar and parallel), was carried out. These studies confirmed and highlighted the dominant influence of the electromagnetic phenomena. In a paper published in 2013, Rehmet [START_REF] Rehmet | 3D unsteady state MHD modeling of a 3-phase AC hot graphite electrodes plasma torch[END_REF] presented MHD numerical model. The experimental results published in a follow up paper [START_REF] Rehmet | A comparison between MHD modeling and experimental results in a 3-Phase AC arc plasma torch. Influ electrode tip geom[END_REF][START_REF] Rehmet | High speed video camera and electrical signal analyses of arcs behavior in a 3-Phase AC arc plasma torch[END_REF] showed the arc behavior was totally in line with modeling results. Results showed that the electrode jets due to Maecker effect were playing an essential role. The influence of the electrode configuration was studied in [START_REF] Rehmet | Unsteady state analysis of free-burning arcs in a 3-phase AC plasma torch. Comparison between parallel and coplanar configurations[END_REF]. In the coplanar electrode configuration, the arcs remain confined within the inter-electrode gap and arc behavior is mostly controlled by electrode jets while, in the parallel electrode configuration, the arcs length increases. In this configuration, arcs are pushed toward the axial and radial directions by the combined effect of electrodes jets and Lorentz forces along the arc column. This work showed the plasma power could efficiently be controlled by artfully adjusting the electrodes layout. At this time, the 3-phase AC plasma technology with graphite electrode was able to run under any neutral or reducing plasma gas (argon, helium, nitrogen, hydrogen, carbon monoxide, syngas,…) but hardly under oxidizing conditions (air, oxygen, carbon dioxide,…) due to strong electrodes erosion. This issue was addressed by Takali in 2015. In his PhD dissertation [START_REF] Takali | Etude théorique d'un électrobruleur industriel dote d'une torche à arc triphasée pour la valorization énergétique de combustibles à faible pouvoir calorifique[END_REF], Takali studied the development of a 100 kW plasma torch working in air to be embedded in an industrial electro-burner in making possible an operation in oxidizing environment by reducing as much as possible the air erosion of the graphite electrodes. For this purpose, different methods were imagined, investigated and tested on both theoretical (numerical modeling) and experimental sides. The by far best solution involved the injection of a carbonaceous element under finely controlled conditions acting as sheathing agent at the vicinity of the electrodes. The challenges lies in being able to control the oxidation to locally keep an atmosphere having a mixture closed to syngas (mixture CO-H2) which behaves neutral at high temperature with carbon (neither oxidant nor reducing). To illustrate this, Figure 5 represents the composition (molar fractions at thermodynamic equilibrium) of a mixture (C / O 2 / N 2 ) versus temperature for four increasing C/O2 ratios: 0.2, 1, 2 and 4 respectively. It appears clearly that cases (a) and (b) must be avoided since it would strongly erode graphite electrodes while cases (c) and (d) must be researched since it would not chemically interact with solid carbon. Among the different cheap carbonaceous element likely to be used as electrodes sheathing, natural gas is particularly attractive. Indeed, its partial combustion in air would produce a mixture of carbon monoxide and hydrogen (syngas). In order to study this, a CFD numerical model based on the Jones-Lindstedt reduced kinetic model [START_REF] Jones | Global reaction schemes for hydrocarbon combustion[END_REF] Methane is injected around the electrodes at three flow rates: 1.5 Nm 3 .h -1 , 3 Nm 3 .h -1 and 6 Nm 3 .h -1 respectively. As shown on Figure 6, methane flow rate shows a huge influence on spatial distribution of the gas composition. At high methane flow rate, methane is blown toward the discharge zone and reaction with oxygen takes place below the electrodes. For this reason, the areas with high CO and H 2 concentrations are localised undernith the electrodes. When decreasing the methane flow rate from 6 Nm 3 .h -1 to 3 Nm 3 .h -1 , CO and H 2 are produced along the transversal area of the électrode. In this case, as a result of the methane velocity decrease, the residence time at the vicinity of the electrodes is higher and in line with the kinetics of the partial oxidation. On the other hand, when reducing the methane flow rate to 1.5 Nm 3 .h -1 , the methane velocity become too low to sheath electrodes and the CO and H 2 remains very low. Experiments confirmed the great interest of this solution together with the strong sensitivity of methane flow rate on the local concentration of CO and and H 2 and consequenly on the erosion rate of the electrodes. In our specific case, an optimal methane flow rate around 3 Nm 3 .h -1 .. was found giving erosion rates as low as 0.1 to 0.01 g/kWh similar to erosion rates obtained with nitrogen. To finish, Figure 7 illustrates drawings of the three-phase AC plasma torch specially designed for being integrated at a later stage in a burner and its combustion test bench. The behavior of the torch in reactive conditions, i.e. with pulverized solid biomass injection in air, is currently under study. The first results obtained are very encouraging in terms of combustion efficiency. The different biomass fuels injected at different sizes of pulverization (wood, dried sewage sludge) are nearly 100% burned (conversion into CO2 and H2O) even though the biomass flow rate is low compared to the plasma power because of the limitation of the thermal power capacity of the test bench. The test bench is equipped with a special window in order to visualize the plasma and the generated flame with a high-speed camera. The flame obtained is extremely stable and the erosion rate of the torch electrodes is of the same order of magnitude as in neutral condition operation thanks to the active thermo-chemical gas sheathing technique set-up. A complete experimental study of biomass combustion will be reported in a next paper.

8-CONCLUSION

In the current context of global warming, the use of poor LHV biomass and waste for heat and power production will certainly play a major role in the next coming years. Unfortunately, poor LHV feedstock suffers from a major weakness, particularly when in solid form: they can hardy burn in autonomous burners without an external assistance. For few years, a new alternative solution derived from the coal industry has emerged: the plasma assistance combustion. This solution is particularly attractive in the perspective low carbon electricity production thanks to large scale deployment of renewable energy.

The principle is based on the thermochemical activation of the combustion thanks to the use of a thermal plasma whose power generally does not exceed few percent of the combustion thermal power. Such systems, called plasma electro-burners present number of advantages versus traditional technologies among which: their ability to deliver and tune the enthalpy and/or temperature through an external energy supply with temperatures that cannot be achieved with combustion methods, their capacity to be an intense source of radicals in air, their very short dynamic response, their limited environmental impact (no direct CO2 emissions), their flexibility and ability to operate under very different atmospheres, and their compactness. Most plasma electro-burners technologies today on the market use DC plasma torches. These technologies suffer from many drawbacks among which: limited electrodes lifetime, poor reliability, important water cooling needs, need of AC/DC transformers, etc. leading to high CAPEX and OPEX.

With the objective to exceed these limits and reduce OPEX and CAPEX while increasing reliability, the Center PERSEE MINES-ParisTech has been working in the development of an original three-phase AC plasma technology to be integrated in a plasma electro-burner. This technology based on graphite electrodes was initially designed for operating under inert or reducing conditions (cracking of methane, gasification, etc.). The main barrier for using this technology under oxidizing conditions was linked with the huge electrode erosion in the presence of oxygen. This challenge has succeeded thanks to an active thermo-chemical gas sheathing of the graphite electrodes. This paper presents the state of art and the main achievements on the plasma technology with a special focus on the limitation of electrodes erosion. It paves the way towards the design of a future robust and flexible biomass plasma electro-burner.

Figure 2 :

 2 Figure 2: Scheme of a pulverized coal power plant: overall (left) and boiler (right).

Figure 3 :

 3 Figure 3: Tianjin Guodian coal power plant (China).

Figure 4 :

 4 Figure 4: Scheme of a biomass plasma electro-burner.

Figure 5 :

 5 Composition (molar fractions at thermodynamic equilibrium) of a mixture (C / O 2 / N 2 ) versus temperature for four increasing C/O2 ratios: 0.2, 1, 2 and 4 respectively

  From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1 O2 mass fraction From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1 CH4 mass fraction From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1 CO mass fraction From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1 CO2 mass fraction From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1 H2O mass fraction From left to righ Q CH4 = 6, 3 and 1.5 Nm 3 .h -1

Figure 6 :

 6 Figure 6: Influence of methane flow rate on distribution of the gas component mass fractions. The air flow rate is here fixed and equal to 65 Nm 3 .h -1 .

Figure 7 :

 7 Figure 7: Drawings of the three-phase AC plasma torch and its biomass combustion test bench.

Table 1 : Characteristics of biomass used in the Polish Poloniec power plant (mass fractions) for co-firing with coal.

 1 the typical characteristics of biomass feedstock used in a Polish thermal power plan (Poloniec) for the co-combustion with coal.

		Mixure (80	Wood	Agro	Straw-	Sun	Palm
		% wood	ships		pellets	flower	nut shell
		20 % agro)
				Raw Biomass
	Humidy (%)	35,90	42,40	11,55	13,00	11,50	9,70
	Ash content (%)	1,80	0,29	2,63	-	2,90	2,00
	LHV (MJ.kg -1 )	10,50	9,40	15,85	14,80	16,00	17,90
	Density (kg.m -3 )	350	300	437,5	600	600	300
				Wet Biomass
	Volatile (%)	--	80,00	80,78	83,10	80,00	80,00
	Ash (%)	2,81	0,50	3,95	6,90	3,28	2,20
	C (%)	49,15	49,75	49,01	45,83	49,51	53,69
	H (%)	5,96	6,12	5,86		5,91	5,80
	N (%)	0,25	0,05	0,87	0,80	0,93	0,49
	O (%)	--	43,50	40,25	40,63	40,21	37,78
	S (%)	0,05	0,04	0,11	0,15	0,16	0,04
	Cl (mg.kg -1 )	--	100	550	1 000	700	300
	P (mg.kg -1 )	--	100	1 150	1 000	1 600	300
	alkalin (Na+K) acid soluble (mg.kg -1 )	--	600	6 650	10 000	11 000	1 300

Table 2 :

 2 has been developed in the fluent CFD code (« Laminar finite-rate » model). Kinetics data for the Jones and Lindstedt mechanism.

	Réactions	A	b	E	Réf
		0,44 10 12	0 2,49 10 8 [CH 4 ] 0,5 [O 2 ] 1,25	[170]
		0,30 10 9	0 2,49 10 8 [CH 4 ] [H 2 O]	[170]
		0,68 10 16	-1 3,32 10 8 [H 2 ] 0,25 [O 2 ] 1,5	[170]
		7,06 10 17 0,27 10 10 6,71 10 10	0 8,06 10 9 [H 2 ] -0,75 [O 2 ][H 2 O] 0 1,66 10 8 [CO] [H 2 O] 0 1,14 10 8 [CO 2 ] [H 2 ]	[172] [170] [173]

th International Conference on Engineering for Waste and Biomass Valorisation -May 23-26, 2016 -Albi, France


Kerry Taylor

Robert Woodcock

Susan Cuddy

Peter Thew

David Lemon

A Provenance Maturity Model

Keywords: provenance, reproducibility, lineage, pedigree, requirements

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

Introduction

As the trend towards data-intensive cyberscience picks up pace, scientists are becoming increasingly concerned about data provenance. As consumers of data, scientists need to know: Where did this data come from? Is it good enough for me to use? Can I trust it? As producers of data and expert opinion, scientists need to ensure their results are scientically credible, repeatable, and justied by the methods used and the reasoned interpretations made. Knowledge of the history of a piece of information (where it came from, what it was generated for, and the workow that generated it) is known as provenance, although the terms audit trail, lineage and pedigree are common synonyms. Understanding the provenance of a piece of information can be as important as the information itself. Using provenance, it should be possible to understand whether or not a piece of information is t for the intended purpose or whether the information should be trusted.

While these general concerns are quite widespread, implementing software systems for longterm provenance management needs much more thought about requirements for the business context, capture, representation and storage, retrieval, and usability. From extensive interactions with hydro-and geo-scientists in Australian science agencies we found widespread confusion around how to move from high level descriptions of outcomes towards statements of requirements for selection of tools and methods for implementation. This problem is magnied in multidisciplinary science projects.

In this paper a novel tool for developing requirements for provenance is presented, the Provenance Maturity Model (PMM). The PMM has been developed together with a method for provenance requirements elicitation that is not further discussed here. It has been specically developed for a context of multiple, disconnected stakeholders who are generally unaware of the drivers, challenges and tradeos of provenance management, but it can be useful in any scenario where an implementation of provenance management is required, such as hydrological modelling [START_REF] Shu | Modelling provenance in hydrologic science: A case study on streamow forecasting[END_REF], agricultural research [START_REF] Compton | Sensor data provenance: SSNO and PROV-O together at last[END_REF], emergency management [START_REF] Power | Harmonising web feeds for emergency management[END_REF] and chemistry lab notebooks [START_REF] Adams | A semantic lab notebook report on a use case modelling an experiment of a microwave-based quarantine method[END_REF]. The PMM can also be used to classify existing or aspirational tools and approaches to provenance to aid in tool selection after requirements are established.

Our development of the PMM was driven by our experience in the Bioregional Assessment program of the Australian Government; a program to understand the potential impacts of coal seam gas and large coal mining on water resources and water-related assets. It is a complex intergovernmental resource development decision-making process, with expectations of signicant long term commercial and public interest in the decisions to be made. While there was very strong awareness of the necessity for provenance amongst the stakeholders, we struggled to discuss the breadth and depth of the impact of simple requirements statements. Elements of maturity have a major impact on the cost, distance from current practice, the ability to capture, and the ability to interoperate over organisational and disciplinary boundaries.

The content of the PMM was assembled by the authors' analysis of remarks and expectations of stakeholders in that and previous resource-exploitation scenarios. A survey of the research literature was also used to insert capabilities and maturity points that we may have missed. The PMM then provided a way to hold the conversations that were necessary to understand these issues where previously we had role, process and scope confusion. We needed the additional dimension of maturity to facilitate the conversation along with a context of cost-benet and risk. [START_REF] Adams | A semantic lab notebook report on a use case modelling an experiment of a microwave-based quarantine method[END_REF] The PMM Like the Capability Maturity Model (CMM) [START_REF] Paulk | Capability maturity model, version 1.1[END_REF], the PMM contains a matrix of capabilities described at ve levels of maturity. The capabilities are grouped into Provenance Business, Data Management, Provenance Capture, Provenance Representation and Storage, Provenance Retrieval, and Usability. In the scope of a process, when do we start capturing provenance and when does it end?

Granularity of capture

The smallest unit of a process that generates a provenance record -from a complete workow down to individual exe- These capabilities discuss the static aspect of provenance information; in between creating it and using it Provenance format Do the provenance records adhere to a standard, and is that standard an international one or bespoke to the organisation?

Provenance language

How formalised is the content of the provenance records;

for example free text or a strictly controlled language (with specic meanings)

Provenance security Can the provenance records be validated or can they be corrupted or altered after collection; is there a way to de- Can the provenance records identify the component that is the source of error or dierence within a workow with respect to an alternative?

Cross-disciplinary application Is everyone talking the same language? Are they being forced inappropriately to talk the same language?

The ve levels of maturity (columns) are labelled and described as follows where the original CMM title Repeatable has been replaced with Tactical to avoid conicts with the use of that term in the study of provenance. The descriptions of each level have been modied to be more appropriate for provenance.

Initial(Chaotic) It is characteristic of provenance treatment at this level that it is (typically) undocumented and in a state of dynamic change, tending to be driven in an ad hoc, uncontrolled and reactive manner by users or events. This provides a chaotic or unstable approach to provenance management and provenance services and certainly implies that any services developed will be of very low functionality and scope.

Tactical It is characteristic of provenance treatment at this level that many aspects of data production and management are carried out with record-keeping in mind. Some attention is being made to ensure that identied processes are repeatable in some circumstances, possibly with consistent results. Discipline is unlikely to be rigorous, but where it exists it may help to ensure that following an audit trail is possible under stress.

Dened It is characteristic of provenance treatment at this level that there are sets of dened and documented standard processes established and routinely followed. These standard pro-cesses both require and enable standard tools to be developed and used. Such tools both assist in the implementation of the processes and oer provenance services to derive value from provenance across the organization or community.

Managed It is characteristic of provenance treatment at this level that it has become uncontroversial: moved into the background as well-managed practices that are embedded in the fabric of business, travelling smoothly from project to project. It is consistently and eectively controlled and widely used.

Optimising It is a characteristic of provenance management at this level that the focus is on continually improving performance of provenance management itself as a lever for continuous improvement of performance of the underlying scientic or administrative processes, through both incremental and innovative technological improvements. Provenance is a highly valued component of business delivering transparency, accountability and knowledge management.

The full PMM matrix of 33 capabilities by 5 maturity levels is included at the end of this paper.

Evaluation

The PMM was used for requirements elicitation for three projects of two government science agencies in August 2013, over a two-day workshop. Background material was provided, including the PMM evaluation of some existing tools, many of which were known to the participants, such as ISO-19115-LE [START_REF]2009 geographic information -metadata -part 2: Extensions for imagery and gridded data[END_REF] and Prov-O [START_REF] Lebo | PROV-O: The PROV ontology[END_REF]. The workshop included a half-day of presentations on the nature of provenance and some known tools and methods; brief presentations on the selected projects' goals; an introduction on the PMM and how to use it; a hands-on application of PMM to the three projects in project groups; a subsequent requirements documentation exercise; a plenary analysis of consequent requirements for the agency as a whole; and the joint development of an architecture sketch for the agency-wide provenance management. Those who were present for the full two days excluding the PMM developers and facilitators, that is 13 people, were surveyed at the beginning and again at the end of the workshop.

The survey participants were invited to respond to 21 questions of which 18 were phrased on a 5-level balanced Likert scale and all 21 included requests for free-text comments. In some cases participants indicated responses either in between or spanning consecutive levels; in all cases these have been treated as if the lowest (i.e. least positive) level was selected.

The lowest-scoring question overall was the pre-workshop question Please rate your familiarity in dealing with issues or tools relating to provenance (1=not all aware, 5=extremely aware) , for which the minimal response was 1, the maximum 5, and the average 3.2. The highestscoring question overall was the post-workshop The approach to provenance management for your project will benet by the application of the PMM as you have used it in the workshop (1=strongly disagree, 5=strongly agree) . The minimum response was 3 and the maximum was 5, with average 4.8. Other high-scoring questions referred to the contribution of the PMM towards developing user requirements. The lowest-scoring post-workshop question (average 3.7) referred to the ease of determining evaluation criteria, an element of the PMM application methodology that is out of scope for this paper. We can conclude that the participants found the PMM worthwhile.

The participants were also invited to suggest needs for clarication or other improvement to the PMM version that was used in the workshop. The PMM presented here has had some wording and sequencing adjustments since the workshop to take account of those suggestions.

Related Work

We have evaluated some existing tools and methods with respect to the PMM in order to provide some benchmarks to assist in interpretation of the PMM, and on the other hand, to assist project designers to locate tools that might help them achieve desired maturity levels. In this context, Prov-O [START_REF] Lebo | PROV-O: The PROV ontology[END_REF] features due to its potential contribution towards high-maturity provenance Representation and Storage, therefore also contributing to high maturity Retrieval and Usability.

Because of both its underlying exible graph representation, and the ontology inference coupled with domain-specic ontology extensions, it is possible to use it to traverse widely diering domain-oriented provenance records through semantic, executable mappings to Prov-O, such as described in [START_REF] Compton | Sensor data provenance: SSNO and PROV-O together at last[END_REF]. This can be done even when the primary record-keeping may be entirely ontology-unaware, such as is enabled by the mapping from ISO19115 Lineage to Prov-O [START_REF] Shu | ISO 19115 lineage ontology[END_REF]. An alternative ISO19115-driven extension to Prov-O has been developed [START_REF] Lopez | Linked map VGI provenance schema[END_REF] that would be useful when an early commitment to Prov-O is made. We can envisage the particular utility of the Prov-O graph representation to support dynamic provenance assembly for federated information systems like [START_REF] Taylor | Charging for information services in service-oriented architectures[END_REF], [START_REF] Ackland | Semantic service integration for water resource management[END_REF] and [START_REF] Woodcock | AuScope's use of standards to deliver earth resource data[END_REF] that support retrieval of data products or query-answering over compositions of resources. In unpublished work, we are also exploiting the inference capability to support arbitrary provenance comparison, building on the graph matching of [START_REF] Liu | Towards semantic comparison of multi-granularity process traces[END_REF].

Recent work on provenance for an integrated ecosystem approach to management of large marine ecosystems [START_REF] Di Stefano | Enabling the integrated assessment of large marine ecosystems: Informatics to the forefront of science-based decision support[END_REF] demonstrates a high level of maturity for Capture, whereby a Web application for the development of data products and charts, tables, and map visualisations also keeps track of steps taken then embeds the provenance in the nal PDF report. The related Global

Change Information system will demonstrate a high level of maturity for Data Management, particularly for identier management over a heterogenous contributor community [START_REF] Ma | Capturing provenance of global change information[END_REF].

The notion of Research Objects [START_REF] Page | From workows to research objects: An architecture for preserving the semantics of science[END_REF], especially computational research objects [START_REF] De | Towards computational research objects[END_REF], contributes to a very high level of maturity in Usability, with provenance records very closely tied to executable components for repeatability and reusability and also to the scientic practice.

Conclusion

We present the Provenance Maturity Model as a part of a structured approach to developing requirements for provenance management in data-intensive science. The PMM lays out many characteristics of provenance management in a matrix where preferred options may be considered and selected in the context of some evaluation criteria. We found that scientists recognise the value of a structured approach to requirements elicitation that ensures the depth and breadth of the issues are considered and that aspects are not overlooked. The value of the framework in clarifying the language in an industry standard approach is also appreciated.

We recommend using the PMM in a workshop environment once the scientic content of a project is well enough understood to commence. We recommend developing an evaluation criteria then proceeding to check cells in the PMM by consensus, adding additional rows if necessary.

Later, system and software requirements can be developed in a conventional way, with frequent reference to the instantiated PMM. A record of tools and business processes that have previously been benchmarked by the PMM can help to ll in a solution architecture. Much later, the PMM can help to review provenance goals and to consider advancing the maturity.

In future work we would like to re-evaluate the PMM in an alternative cultural, organisational and problem context, and also to track the inuence on project results through project life-cycle case studies.

  When was the provenance record made with respect to the execution of the process: during the process step-by-step, at specic stages or after the fact?Software tools used in processWhat information is captured on software used within a workow & the workow system itself; for example version number?Hardware/platform Information on the hardware environment when a workow is executed Provenance capture integration How integrated is the collection of provenance records in the process execution? For example does the carrying out of the process generate provenance transparently or are the provenance records obtained by post-processing log les or independent data entry?Provenance quality How trustworthy is the provenance record that is kept? Sophistication of automation How automated is the capture of provenance record; for example manually or embedded into workow processes

  category of capabilities relates not so much to provenance as to the management of the underlying data over which provenance operates. It is included here because certain features are necessary preconditions to provenance maturity

Table 1 :

 1

		PMM Capabilities(Rows)
	PROVENANCE BUSINESS	This category relates to the adoption and commitment to
		provenance management. It also contains a grab bag of is-
		sues that need to be addressed for any particular prove-
		nance venture
	Longevity	How long are provenance records designed to be kept?
	Software maturity	What is the maturity of the software being used; for exam-
		ple does it have a known history of usage elsewhere or is it
		unknown to the organisation?
	Organizational awareness: cul-	Is there a culture that understands and appreciates prove-
	ture and behaviour	nance?
	Value recognition	Does the organisation care about provenance?
	Governance	How formalised is the approach to provenance?
	Perspective	The social extent of provenance sharing and interpretability

PROVENANCE CAPTURE

This category refers to the processes, methods and tools for initial capture, or record-keeping, of provenance Decision making and consultation What records are kept on decisions made by people and groups and how integrated are these records to a provenance database? Temporal scope (Start and end points)

Acknowledgement The authors thank the anonymous reviewers and the many collaborators for their questions and insight, especially Neal Evans and Brian Hanisch of Geoscience Australia.

Provenance Business This category relates to the adoption and commitment to provenance management. It also contains a grab bag of issues that need to be addressed for any particular provenance venture.

Capability