domain: spi phys sde

text:
Jordy Charly

Isidore Rabetanetiarimanana

Mamy Harimisa Radanielina

Hery Tiana Rakotondramiarana

Dominique Morau

Smart Grid and Renewable Energy A Survey of the Gasification of Residual Household Waste

Keywords: Cogasification, Household Waste, Refuse-Derived Fuel, Waste to Energy, Waste Characterization

Solid waste is a promising renewable fuel that can substitute conventional fuel. According to the researchers, thermoconversion of solid waste such as municipal solid waste or residual household waste (RHW) is beneficial to society. However, due to its heterogeneity, the gasification of RHW is more complex. This review article discusses the steps that RHW must undergo before its thermoconversion and the state of the art of solid waste gasification. First, characterisation methods of RHW are surveyed. Second, the properties of RHW, the production lines of refuse derived fuel (RDF) from RHW, the influence of RDF composition and operating parameters such as equivalence ratio and temperature are reviewed. Moreover, RDF gasification products, scientific barriers and proposed solutions are evaluated. In conclusion, concerning emissions, costs and technical aspects related to each thermochemical process, it can be said that gasification is a promising technique for the recovery of RHW. However, studies on cogasification of waste and biomass on a pilot-industrial scale are still scarce and synergistic effects of this cogasification need to be clarified.

Introduction

The depletion of fossil resources is a great dilemma to which we will have to find solutions. Indeed, the energy transition is imperative and a sustainable alternative is needed for the years to come. Nowadays, studies on the effective and effi-DOI: 10.4236/sgre.2022.1311017 269 Smart Grid and Renewable Energy cient use of renewable resources are being developed. The development of renewable energies is crucial in order to mitigate the emission of greenhouse gases (GHG) which is related to the abusive use of fossil fuels [START_REF] Ahmad | Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation[END_REF]. The exploitation of fossil fuels is responsible for the emission of 10.65 billion tons of CO 2 per year [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Moreover, the dependence on conventional resources generates geopolitical tensions linked to the instability of oil prices and markets [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. The international climate conference COP21 in Paris in 2015 encouraged countries to contribute to the mitigation of GHG emissions [START_REF] Gioutsos | Cost-Optimal Electricity Systems with Increasing Renewable Energy Penetration for Islands across the Globe[END_REF]. For the year 2020, the European Union (EU) is obliged to reduce by 20% the emission of GHG [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF].

According to the literature, biomass is a promising source of carbon and liquid renewable fuels [START_REF] Ojha | Copyrolysis of Lignocellulosic Biomass with Waste Plastics for Resource Recovery[END_REF] [6] [7] to replace conventional hydrocarbon. The availability of biomass resource compared to fossil fuels and other intermittent renewables is one of the reasons why biomass energy conversion is very important.

In addition, an effective way to reduce CO 2 emissions and promote sustainable development is to recover waste. However, the waste (municipal solid waste, agricultural residues, etc.) conversion pathway is under-exploited. Waste management plays a major role in the implementation of the circular economy, which remains an overarching objective of the EU [START_REF] Tristan | The Hidden Dynamics of Household Waste Separation: An Anthropological Analysis of User Commitment, Barriers, and the Gaps between a Waste System and Its Users[END_REF] [START_REF] Andersson | Direct and Indirect Effects of Waste Management Policies on Household Waste Behaviour: The Case of Sweden[END_REF]. Waste management, particularly municipal solid waste (MSW), is a major challenge on a global scale [START_REF] Kumar | An Empirical Model for Prediction of Household Solid Waste Generation Rate-A Case Study of Dhanbad, India[END_REF]. Currently, the annual global production of MSW is 2 billion tons [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF] and this will rise to 2.5 billion tons by 2025 [START_REF] Munir | Plasma Gasification of Municipal Solid Waste for Waste-to-Value Processing[END_REF] [START_REF] Lu | Slow Pyrolysis of Municipal Solid Waste (MSW): A Review[END_REF]. Only 11% of MSW produced is recovered in the form of energy [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. In the United States, 13% of MSW is recovered and 53% is landfilled [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF] [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF]. In Europe, in order to increase the share of renewable energy in energy production, a new principle on MSW management has been put in place. The principle is based on a waste management hierarchy such as reduction, re-use, recycling (mechanical or chemical), energy recovery and disposal [START_REF] Andersson | Direct and Indirect Effects of Waste Management Policies on Household Waste Behaviour: The Case of Sweden[END_REF] [15] [START_REF] Guérin | The Importance of Characterizing Residual Household Waste at the Local Level: A Case Study of Saguenay, Quebec (Canada)[END_REF]. In France, this principle is reflected in the LTECV law (Loi de Transition Energétique pour la Croissance Verte) [17]. This law prioritises the modes of waste treatment: 1) favouring reuse and recycling of waste, 2) ensuring material and energy recovery of recoverable waste, and 3) eliminating non-recoverable final waste by landfill. The main objectives of the LTECV law are the use of SRF (Solid Recovery Fuel) to produce heat and electricity, i.e., the recovery of waste sorting refusals while ensuring that this is not done to the detriment of the prevention of waste production and recovery in the form of materials, and energy recovery by pyro-gasification of waste that cannot be recycled according to the available channels.

The current means of waste treatment are not efficient from energy, economic and environmental point of view. Most of the solid waste is landfilled and this technique remains the most widespread one [START_REF] Munir | Plasma Gasification of Municipal Solid Waste for Waste-to-Value Processing[END_REF] [START_REF] Ouadi | A Review of the Valorization of Paper Industry Wastes by Thermochemical Conversion[END_REF]. Landfilling is associated with many disadvantages such as the emission of methane gas due to the anaerobic digestion of solid waste, the land occupation, the inability to manage a large quantity of waste and the pollution of groundwater [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF] [START_REF] Malkow | Novel and Innovative Pyrolysis and Gasification Technologies for Energy Efficient and Environmentally Sound MSW Disposal[END_REF]. Apart from landfilling, incineration is the most applied waste treatment worldwide [START_REF] Andersson | Direct and Indirect Effects of Waste Management Policies on Household Waste Behaviour: The Case of Sweden[END_REF] [12] [START_REF] Vlček | Environmental Effects Modelling of Thermal Processes at Waste Incineration[END_REF]. However, the harmful emissions, hazardous products and other dis-DOI: 10.4236/sgre.2022.1311017 270 Smart Grid and Renewable Energy advantages of these two techniques do not allow them to effectively manage waste disposal [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF]. Indeed, the development of thermochemical methods is imperative in order to eliminate and recover household waste in the form of energy. Pyrolysis and gasification are promising alternatives [19] [22]. These thermochemical methods are more efficient in terms of gas production and liquid fuels [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF]. Currently, pyrolysis is at the heart of scientific research and has great potentials [12] [13]. In addition, gasification is a technology well adapted to the conversion of solid waste into syngas [START_REF] Zhang | Eulerian Model for Municipal Solid Waste Gasification in a Fixed-Bed Plasma Gasification Melting Reactor[END_REF] [24] [START_REF] Zhang | Performance Analysis of Municipal Solid Waste Gasification with Steam in a Plasma Gasification Melting Reactor[END_REF] and makes it possible to valorise waste of different natures at the same time [START_REF] He | Syngas Production from Catalytic Gasification of Waste Polyethylene: Influence of Temperature on Gas Yield and Composition[END_REF].

Household waste (HW) consists of green and bulky waste, recyclable waste and RHW [START_REF] Jacobsen | Cost Comparison between Private and Public Collection of Residual Household Waste: Multiple Case Studies in the Flemish Region of Belgium[END_REF]. RHW is the mixed waste produced in households that remains after source separation of hazardous waste and recyclables, such as biological waste, paper, cardboard, metals, glass and wastes electrical and electronic equipment [28] [29]. Recently, the gasification of RHW has attracted the attention of researchers. RDF or SRF are auxiliary fuels formed from a mixture of non-hazardous wastes [START_REF] Pio | Co-Gasification of Refused Derived Fuel and Biomass in a Pilot-Scale Bubbling Fluidized Bed Reactor[END_REF]. Generally, RDF formed from RHW is composed of: plastic, paper, cardboard, food residues, textiles, wood and other types of biomass. With a LHV (Lower Heating Value) greater than 10 MJ/kg, RDF from RHW has the potential to be used as a raw material for gasification [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF] and may even substitute fossil fuels [START_REF] Malkow | Novel and Innovative Pyrolysis and Gasification Technologies for Energy Efficient and Environmentally Sound MSW Disposal[END_REF]. Because of its low production cost compared to biomass, the use of RDF from solid waste can launch the gasification sector [START_REF] Aluri | Pyrolysis and Gasification Studies of Model Refuse Derived Fuel (RDF) Using Thermogravimetric Analysis[END_REF] [START_REF] Barba | The Gibbs Free Energy Gradient Method for RDF Gasification Modelling[END_REF]. Like biomass, RDF can be simultaneously converted into syngas, biofuels and renewable solid fuel resources [START_REF] Widjaya | Gasification of Nonwoody Biomass: A Literature Review[END_REF]. However, due to its heterogeneity, the gasification of RDF is more complex. The lack of understanding of the contribution of each of the RDF components on the aspects of the obtained products prevents extending the conclusion to other RDF with different components. In order to have an overview of the gasification of RHW, this paper presents in the second section the advantages and disadvantages of gasification compared to other processes. In the third section, the different methods of characterising RHW are discussed. Moreover, the next section highlights the properties of RHW. The fifth section discusses the production of RDF from solid waste and the gasification of RDF such as the influential parameters, the products, the scientific barriers and the suitable solutions according to the literature.

Gasification

Gasification is a decomposition reaction of carbonaceous matrices at high temperature for a short residence time and with a quantity of oxygen lower than the stoichiometric value [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF] [START_REF] Rong | Co-Gasification of Sewage Sludge and Woody Biomass in a Fixed-Bed J. C. I. Rabetanetiarimanana[END_REF]. Generally, the gasification temperatures are higher than 800˚C and can reach more than 1300˚C depending on the solid used and the process employed [START_REF] Chen | Pyrolysis of Torrefied Biomass[END_REF]. If the gasification temperature reaches 1400˚C, the process no longer requires an external heat source, the combustion of part of the raw material provides heat to facilitate gasification [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. The products obtained are: syngas composed of H 2 , CO, CH 4 and CO 2 , coal, liquid fuels and ash. The stages of the gasification process are: drying, pyrolysis, oxidation and reduction DOI: 10.4236/sgre.2022.1311017 271 Smart Grid and Renewable Energy [START_REF] Chen | Pyrolysis of Torrefied Biomass[END_REF] [37] [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. The oxidation reactions are exothermic reactions while gasification is an overall endothermic reaction [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. The gases produced are used to produce heat or electricity [START_REF]Pyrolyse et gazéification de la biomasse pour la production d'electricite[END_REF].

Advantages: Gasification does not record a GHG emission. It is a sustainable solution for waste management [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. The low presence of oxygen slows down the formation of dioxins, furans and NO x and prevents the oxidation of metals [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF] [25] [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. Apart from mercury and cadmium, alkalis and heavy metals are retained in the fly ash [25] [37]. Gasification is a process that produces 65% less toxic residues than incineration [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. Consequently, the cost of eliminating the residues is reduced. This process is cost-effective [START_REF] Rong | Co-Gasification of Sewage Sludge and Woody Biomass in a Fixed-Bed J. C. I. Rabetanetiarimanana[END_REF] and has a higher conversion efficiency [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF]. Secondly, one of the advantages of gasification over pyrolysis is that all kinds of waste, whether organic waste, plastics of different composition or even plastics mixed with other feedstocks can be co-valorised in a gasifier [2] [38] [40]. This technology can be developed on a large scale and can eliminate up to 90% of the volume of MSW [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Furthermore, through the Fischer-Tropsch process, clean syngas can be transformed into liquid fuels that can be used to produce electrical or mechanical energy [13] [26].

Disadvantages: Gasification faces many technical barriers [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. The presence of tar, coal and ash in the syngas implies a long series of syngas purification processes [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF] [13] [START_REF] Ray | The Deployment of an Advanced Gasification Technology in the Treatment of Household and Other Waste Streams[END_REF] [20] [START_REF] Aznar | Plastic Waste Elimination by Co-Gasification with Coal and Biomass in Fluidized Bed with Air in Pilot Plant[END_REF]. Secondly, this technology requires a pre-treatment of the waste such as sorting, drying and grinding [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Indeed, this process consumes a significant amount of energy [2] [13]. In addition, the treatment of waste by gasification requires a high operational and management cost that varies between 40 and 100$ per ton of MSW [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. Also, the high quantities of coal, metals and organic pollutants in the ash and coals pose a problem especially to human health [START_REF] Ray | The Deployment of an Advanced Gasification Technology in the Treatment of Household and Other Waste Streams[END_REF]. All these issues are related to the experimental and testing phases, therefore, a lot of information is missing for the implementation of a large-scale gasification project.

There is not a big difference between the investment costs of plasma gasification, conventional gasification and pyrolysis in economic terms [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. The total investment cost of Lahti Energia's Kymijärvi II power plant in Finland, the world's first and largest power plant that runs solely on syngas from RDF/SRF is 160 M€.

The RDF/SRF from industrial, commercial and household waste processed in this gasification power plant in operation produces 50 MW of electricity and 90 MW of heat. Moreover, the Vaskiluoto power plant in Vaasa, Finland, based on a fluidized bed reactor has a total investment cost of 40 M€ for 140 MW [41].

The major differences between the three types of processes appear in the operating and maintenance costs. The operating and maintenance cost of pyrolysis is 7 times higher compared to the other treatment methods [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF].

In general, the main gasification reactions are:

1) Drying: This is the reduction of the moisture content at low temperature.

2) Pyrolysis: This is the production of syngas, biochar and liquid tar from the condensation of pyrolysis steam. DOI: 10.4236/sgre.2022.1311017 272 Smart Grid and Renewable Energy

3) Oxidation: This is the partial combustion of carbon (Equation ( 1)), hydrogen (Equation ( 2)) and carbon monoxide formed from the oxidation of carbon (Equation ( 3)). The products obtained are CO, H 2 O and CO 2 . An oxidation reaction is an exothermic reaction that occurs at a temperature above 500˚C [START_REF] Magdziarz | Analysis of the Combustion and Pyrolysis of Dried Sewage Sludge by TGA and MS[END_REF]. The heat generated by these reactions ensures the rapid heating of the reactor [43] [44].

1 2 C 0.5O CO; 111 kJ mol H - + ∆ = - ⋅ ↔ ( 1
)
1 2 2 2 H 0.5O H O; 242 kJ mol H - ∆ = - ⋅ ↔ + (2) 1 2 2 CO 0.5O CO ; 283 kJ mol H - ∆ = - ⋅ ↔ + (3)
4) Reduction: This is the gasification reaction of char with the products of combustion, i.e. reactions with steam, H 2 and CO 2 [START_REF] Munir | Plasma Gasification of Municipal Solid Waste for Waste-to-Value Processing[END_REF] [25] [START_REF] Rong | Co-Gasification of Sewage Sludge and Woody Biomass in a Fixed-Bed J. C. I. Rabetanetiarimanana[END_REF]. Indeed, the reduction reaction occurs in the absence of oxygen. The reduction reaction includes: the Boudouard reaction (Equation ( 4)), water-gas reaction (Equation ( 5)), water-gas shift reaction (Equation ( 6)) and methane steam reforming reaction (Equation ( 7)) [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. In addition, steam and dry reforming of hydrocarbons are described by Equation ( 8) and Equation ( 9) respectively.

1 2 C CO 2CO; 172 kJ mol H - + ∆ = ⋅ ↔ (4) 1 2 2

C H O H CO; 131 kJ mol H

- + ∆ = ⋅ ↔ + (5) 1 2 2 2

CO H O H CO ; 41 kJ mol H

- + + ∆ = - ⋅ ↔ (6) 1 4 2 2 CH H O 3H CO; 206 kJ mol H - + + ∆ = ⋅ ↔ (7)
( )

2 2 C H H O 2 H CO n m n n m n + ↔ + + (8) ( ) 2 2 C H CO 2 H 2 CO n m n m n + ↔ + (9)
Table 1 shows the succession of these different types of reactions which depends on the configuration of the reactor used.

Residual Household Waste Characterization

The characterisation of waste is a primordial action for its treatment, it allows it to be managed in an efficient and sustainable manner [16] [45]. In addition, it is essential for the modelling of environmental impacts [START_REF] Doležalová | The Changing Character of Household Waste in the Czech Republic between 1999 and 2009 as a Function of Home Heating Methods[END_REF] and the evaluation of waste qualities [10] [47]. The identification of the physico-chemical properties of a material fraction yields useful information in relation to the treatment, recycling of waste and the design of equipment necessary for treatment [START_REF] Guérin | The Importance of Characterizing Residual Household Waste at the Local Level: A Case Study of Saguenay, Quebec (Canada)[END_REF]. In addition, characterisation allows the sources of chemical components present in the waste to be recognised and thus to improve the conditions for thermal treatment or energy recovery of the latter [28] [48]. Furthermore, it is of great importance to make the immediate and ultimate analysis, to determine the calorific values of the waste in order to evaluate the feasibility of an energy recovery [49] [50].

Worldwide, there is very little data on the chemical composition of the material fractions present in household waste. This is probably due to the difficulty and cost of waste characterisation procedures which face numerous barriers [START_REF] Riber | Chemical Composition of Material Fractions in Danish Household Waste[END_REF].

In the literature, no work mentions the existence of a conventional method for waste characterisation [16] [46]. According to Brunner et al. [START_REF] Paul | Alternative Methods for the Analysis of Municipal Solid Waste[END_REF], there are 3 methods of waste characterisation and the choice of methods depends on the objective of the study: direct waste analysis, analysis of products by incineration or composting and the analysis of market products.

Direct Analysis of Waste

Direct analysis of waste is the most common method used by scientists to study waste management. Recently, the European Commission has stated its intention to increase the recycling rate to 70% by 2030 and to strengthen the circular economy [START_REF] Sahimaa | Method for Residual Household Waste Composition Studies[END_REF]. To reach its objective, Europe has implemented source separation systems at household level to easily separate recyclable and non-recyclable materials [START_REF] Doležalová | The Changing Character of Household Waste in the Czech Republic between 1999 and 2009 as a Function of Home Heating Methods[END_REF]. The reinforcement of waste recycling needs information on the composition of the waste to properly develop a solid recovery system [START_REF] Eisted | Characterization of Household Waste in Greenland[END_REF]. In general, the method of characterisation varies according to the means of a collection of the parent samples. Collection is either at the household level [START_REF] Essonanawe | Municipal Solid Waste Composition: Sampling Methodology, Statistical Analyses, and Case Study Evaluation[END_REF], or with vehicle loads [START_REF] Dahlén | Methods for Household Waste Composition Studies[END_REF] or in landfills [START_REF] Guérin | The Importance of Characterizing Residual Household Waste at the Local Level: A Case Study of Saguenay, Quebec (Canada)[END_REF]. The steps of the direct analysis are: stratification, sampling and physico-chemical analysis. Stratification consists of dividing the study area according to the following factors: the season (winter and summer), the location (urban and rural areas), the standard of living, the custom, the type of dwelling (single-family and multi-family), the size of the property and the holiday periods [START_REF] Guérin | The Importance of Characterizing Residual Household Waste at the Local Level: A Case Study of Saguenay, Quebec (Canada)[END_REF] [52] [55] [56] [57]. In practice, the decision on the number of strata to consider depends on the objective of the survey [28] [54].

According to Parfitt et al. [START_REF] Sahimaa | Method for Residual Household Waste Composition Studies[END_REF] the maximum number of strata should be 5 in order to avoid a large total sample size. Due to the stratification, fewer samples are needed, which increases the precision of the results [START_REF] Sahimaa | Method for Residual Household Waste Composition Studies[END_REF]. The quality of the data on waste composition is strongly influenced by the sampling procedure [START_REF] Essonanawe | Municipal Solid Waste Composition: Sampling Methodology, Statistical Analyses, and Case Study Evaluation[END_REF].

The absence of international standards for waste characterisation means that DOI: 10.4236/sgre.2022.1311017 274 Smart Grid and Renewable Energy there are several sampling procedures in the literature [START_REF] Dahlén | Comparison of Different Collection Systems for Sorted Household Waste in Sweden[END_REF] [53] [START_REF] Dahlén | Methods for Household Waste Composition Studies[END_REF]. Table 2 shows the different stages of waste sampling.

Properties of RHW

RHW is composed of recyclable and non-recyclable materials. The Waste to Energy (WTE) process consists of converting non-recyclable waste (excluding glass and metal) into heat, electricity or liquid fuel [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. The non-recyclable waste consists of organic materials such as plastics, food residues, textiles and wood, rubbers and inorganic materials such as minerals and metals. The composition of Wastes varies from region to region and non-organic wastes are abundant in developed countries [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. Then, the calorific value of RHW depends on its chemical composition.

Wastes containing more plastics have a high calorific value due to the high C (above 60%) and H content and the low O content (between 0% -4%). In contrast Table 2. The sampling steps.

Steps Description

Collect of mother sample

From waste storage site:

Use the amount of load from a waste transporter truck [START_REF] Johansson | Copyrolysis of Woody Biomass and Plastic Waste in Both Analytical and Pilot Scale[END_REF] It should represent at least a whole week [START_REF] Sahimaa | Method for Residual Household Waste Composition Studies[END_REF] [58] At household level:

Collect wastes from 40 to 857 households [START_REF] Kumar | An Empirical Model for Prediction of Household Solid Waste Generation Rate-A Case Study of Dhanbad, India[END_REF] IEA: the weight varies from 0.5 to 12 tonnes [START_REF] Dahlén | Methods for Household Waste Composition Studies[END_REF] EU: collect a minimum of six samples from each stratum [START_REF] Sahimaa | Method for Residual Household Waste Composition Studies[END_REF] or 45 m 3 of waste sample in total [START_REF] Dahlén | Methods for Household Waste Composition Studies[END_REF] Sub-sampling The production of plastic waste continues to increase and will reach 13% of global MSW production by 2025 [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Nearly 8% of conventional crude oil is used to produce plastics, which are generally used for packaging [5] [38]. Currently, between 26% -36% of plastic waste is recycled or incinerated, the rest is landfilled [5] [26]. Due to its low thermal conductivity, sticky nature and high volatile content leading to the remarkable formation of tar, the thermochemical conversion of plastics has scientific barriers [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. However, chlorine-free plastics have a HHV of 43 MJ/kg [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Furthermore, the pyrolysis of industrial plastic waste is attracting more attention from researchers [START_REF] Johansson | Copyrolysis of Woody Biomass and Plastic Waste in Both Analytical and Pilot Scale[END_REF]. The products obtained from the thermochemical conversion of plastics can be used for electricity generation, transportation and heating. Indeed, the components of RHW are all good candidates for syngas production [START_REF] Steubing | Heat, Electricity, or Transportation? The Optimal Use of Residual and Waste Biomass in Europe from an Environmental Perspective[END_REF]. For successful conversion of plastics, a high heat transfer rate, resolution of operational issues related to its sticky nature, accurate residence time to promote tar cracking and the use of an in-situ catalyst are required [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. Paper has a low calorific value (less than 10 MJ/kg), high moisture content (above 60 wt%) and an ash content above 45 wt%. Therefore, the process of converting paper into syngas is a challenge [START_REF] Ouadi | A Review of the Valorization of Paper Industry Wastes by Thermochemical Conversion[END_REF].

Gasification of RDF from Solid Waste

The Production of RDF from Solid Waste

Recently, the production of RDF is being advocated by waste management planners and governments [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF]. RDF is an auxiliary fuel made from the sorting or mixing of solid waste such as MSW and RHW and accepted in most waste combustion systems [31] [61]. Indeed, RDF is made up of plastic films, paper, non-recoverable soiled cardboard, residues unsuitable for reuse, wood, rubber and textiles [30] [62]. The production of SRF is more complex and costly than that of RDF and takes into account regulations imposed by the government. For example, for France, according to article R541-8-1 decree of 23 May 2016, an SRF must be prepared from non-hazardous waste, have a LHV on raw SRF greater than or equal to 12 MJ/kg, be sorted, free of ferrous and non-ferrous metals as well as inert materials and have the following properties: mercury (Hg):

3 mg/kg dry matter/chlorine (Cl): 15 g/kg dry matter/bromine (Br): 15 g/kg dry matter/total halogens (bromine, chlorine, fluorine and iodine): 20 g/kg dry matter. Then, according to the European standard EN 15.359, the chlorine and mercury content, the size and the LHV of the SRF must be determined [30] [61].

The results of this complexity are: improved fuel quality (physico-chemical properties and calorific value), higher production cost compared to RDF and lower mass yield. The advantages of RDF production are: 1) the increase in the calorific value of the fuel guaranteeing a good heat transfer during gasification [START_REF] Ray | The Deployment of an Advanced Gasification Technology in the Treatment of Household and Other Waste Streams[END_REF] , 3) ease of storage, handling, transport and conveying of the biomass to the reactor [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF] [32] [START_REF] Chalermcharoenrat | Optimization of Combustion Behavior and Producer Gas Quality from Reclaimed Landfill through Densify RDF-Gasification[END_REF] [66], 4) reduction of pollutant emissions during gasification [START_REF] Milena | Refuse Derived Fuel (RDF) Production and Gasification in a Pilot Plant Integrated with an Otto Cycle ICE through Aspen plus™ Modelling: Thermodynamic and Economic Viability[END_REF].

In general, the stages of waste treatment (MSW or RHW) can be summarised as follows: waste separation, screening, shredding, size reduction, classification, drying and densification [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF] [32] [START_REF] Sarc | Production, Quality and Quality Assurance of Refuse Derived Fuels (RDFs)[END_REF] [64] [START_REF] Barba | Theoretical and Experimental Procedure for Scaling-Up RDF Gasifiers: The Gibbs Gradient Method[END_REF]. The sorting and sampling phase presented in Table 2 give an overview of the SRF production line [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. During the sorting of the collected waste, recyclable and non-combustible waste is not included in the sample but is sent to the manufacturing industries [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF]. Thus, these types of waste, such as glasses and metals, are not favourable to gasification [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] and have a huge impact on the gasification process [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF]. Then, each subsample related to each imposed category is crushed, sieved, mixed and blended according to the desired composition [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF] [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. The mixed waste is then densified into briquettes or pellets [START_REF] Lu | Slow Pyrolysis of Municipal Solid Waste (MSW): A Review[END_REF] [24] [START_REF] Couto | Numerical and Experimental Analysis of Municipal Solid Wastes Gasification Process[END_REF]. Table 3 shows the different types of RDF production in the literature. In their work, Caputo et al. [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF] studied the concept of an RDF production installation and found that the processes allowing for a high calorific value are: trommel screening, manual sorting, magnetic separation, shredding, trommel screening, crushing, trommel screening, densification/pelletisation. Indeed, mechanical operations alone are insufficient as manual intervention is always necessary during the production of RDF [START_REF] Rotter | Material Flow Analysis of RDF-Production Processes[END_REF]. Seven different types of RDF have been classified by the American Society for Testing and Materials (ASTM) [START_REF] Manser | Practical Handbook of Processing and Recycling Municipal Waste[END_REF], the differences are in the RDF production line. The most common types used in the research are Type 3 and Type 5. Type 3 is defined by shredding of MSW, extraction of metals, glass and other inorganic materials while Type 5 is a densified fuel of more than 600 kg/m 3 in the form of pellets or briquettes [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF].

The choice of RDF composition is very important because the physico-chemical properties of the fuel and the quality of the syngas will depend on it. The Table 3. RDF production line.

Type of production

Reference classification, sorting, separation of ferous, non-ferous metals and heavyweight inert materials and confectioning of the fuel Lorber et al. [START_REF] Lorber | Waste to Energy by Preparation of Quality Controlled Solid Recovered Fuels (SRF)[END_REF] size screening, magnetic separation, coarse shredding, refining separation and pelletization Barba et al. [START_REF] Barba | Theoretical and Experimental Procedure for Scaling-Up RDF Gasifiers: The Gibbs Gradient Method[END_REF] sorting, screening, shredding, size reduction, sieving, drying and densification Milena et al. [START_REF] Milena | Refuse Derived Fuel (RDF) Production and Gasification in a Pilot Plant Integrated with an Otto Cycle ICE through Aspen plus™ Modelling: Thermodynamic and Economic Viability[END_REF] size reduction, drying, screening, sorting, metal and glass separation, pelletization Aluri et al. [START_REF] Aluri | Pyrolysis and Gasification Studies of Model Refuse Derived Fuel (RDF) Using Thermogravimetric Analysis[END_REF] trommel screen, hand sorting, magnetic separation, shredder, trommel screen, milling, trommel screen and densification Caputo et al. [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF] refuse separation, shredding, densification mixture of waste with a high calorific value is essential [START_REF] Caputo | RDF Production Plants: I Design and Costs[END_REF]. This implies a composition in which plastic occupies an important place [START_REF] Khosasaeng | Effect of Equivalence Ratio on an Efficiency of Single Throat Downdraft Gasifier Using RDF from Municipal Solid Waste[END_REF]. However, a mixture in which plastic dominates is a mixture with a high volatile matter content. Therefore, the proportion of wood, paper, cardboard and food residues should not be neglected in order to have a balanced content of fixed carbon and ash [START_REF] Aluri | Pyrolysis and Gasification Studies of Model Refuse Derived Fuel (RDF) Using Thermogravimetric Analysis[END_REF] [65] [START_REF] Hwang | Characterization of Products Obtained from Pyrolysis and Steam Gasification of Wood Waste, RDF, and RPF[END_REF]. The moisture content of the mixture varies mainly with the proportion of biomass such as food residues, wood, paper and cardboard and ranges from 1 -25 wt%. However, the composition can vary depending on the origin of the waste, the season, the production technique and the sorting and sampling technique.

Table 4 shows the results of the proximate analysis of RDF.

Influential Parameters of RDF Gasification

Apart from the gasifier parameters, the physical and chemical characteristics of the feedstock influence the quality of the syngas. The mass composition of the waste is important, therefore an ultimate analysis of the RDF is necessary prior to gasification [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF]. Next, the moisture content of the feedstock affects the gasification process, high moisture content reduces the syngas quality and cold gas efficiency [37] [77] which is an important factor in determining the performance [START_REF] Murakami | Optimized Approach of High Cold Gas Efficiency of Woody Biomass in a Fluidized Bed Gasifier with Triple-Beds[END_REF]. Then, the size of the raw material plays a major role, a size of 2 mm ensures a good heat transfer facilitating the thermal decomposition of the waste [START_REF] Littlejohns | Analysis of the Performance of an Integrated Small-Scale Biomass Gasification System in a Canadian Context[END_REF]. In addition, the LHV which is relative to the composition of the biomass is often used as a parameter indicating the potential of the raw material [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF].

Then, the type of reactor used and the operating conditions of the gasifier such as the mass flow rate [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF] [START_REF] Gabbar | Comparative Study of MSW Heat Treatment Processes and Electricity Generation[END_REF], the oxidant used, the temperature inside the reactor, the equivalence ratio (ER), the residence time [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF] [80] and the catalyst [START_REF] He | Syngas Production from Catalytic Gasification of Waste Polyethylene: Influence of Temperature on Gas Yield and Composition[END_REF] influence the quality of the gasification products. The ER is a more important parameter than the bed height and the fluidisation speed [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. An ER that is too high (above 0.4) leads to the dilution of the syngas, the CO 2 content of the gas increases whereas the H 2 , CH 4 and CO decrease. On the other hand, an ER that is too low (below 0.2) means an incomplete gasification process [START_REF] Chen | Hydrogen-Rich Syngas Production from Municipal Solid Waste Gasification through the Application of Central Composite Design: An Optimization Study[END_REF]. Indeed, an appropriate ER (0.2 -0.4) should be used to improve the production rate of combustible gases (CO, H 2 and CH 4 ) [START_REF] Ahmed | Gasification of Refinery Sludge in an Updraft Reactor for Syngas Production[END_REF]. An increase in ER is accompanied by a rise in temperature in the gasification zone [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. According to Boudouard's reaction 4 and le Chatelier's principle, this temperature rise will lead to an increase in the CO content and a reduction in the CO 2 content [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF] [37] [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF] [40] [START_REF] Chen | Hydrogen-Rich Syngas Production from Municipal Solid Waste Gasification through the Application of Central Composite Design: An Optimization Study[END_REF]. Then, if the ER is gradually increased, the amount of H 2 and CH 4 produced will decrease. These phenomena are explained by the oxidation reaction of H 2 2 and the steam reforming reaction of methane 7 [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. An optimal ER is defined by optimal CO, H 2 and CH 4 contents resulting in an optimal LHV of the syngas [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF] [START_REF] Zhang | Performance Analysis of Municipal Solid Waste Gasification with Steam in a Plasma Gasification Melting Reactor[END_REF]. Moreover, temperature is the most influencing parameters in gasification process, it influences the composition of the syngas and the product distribution [START_REF] Chen | Hydrogen-Rich Syngas Production from Municipal Solid Waste Gasification through the Application of Central Composite Design: An Optimization Study[END_REF]. The increase in temperature favours the production of CO and H 2 and reduces the production of CO 2 and CH 4 [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. High CO and H 2 contents mean high LHV values, cold gas efficiency and carbon conversion efficiency [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF] [37] [START_REF] Ahmed | Gasification of Refinery Sludge in an Updraft Reactor for Syngas Production[END_REF]. Therefore, a higher temperature is favourable to the production of H 2 rich gas [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] and CO [START_REF] Byun | Demonstration of Thermal Plasma Gasification/Vitrification for Municipal Solid Waste Treatment[END_REF] [START_REF] Ahmed | Gasification of Refinery Sludge in an Updraft Reactor for Syngas Production[END_REF] and to the increase of energy efficiency [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF]. The H 2 /CO molar ratio increases with increasing temperature in air, oxygen and CO 2 -gasification and decreases in steam-gasification [START_REF] Islam | Effect of Different Gasifying Agents (Steam, H 2 O 2 , Oxygen, CO 2 , and Air) on Gasification Parameters[END_REF]. Hence, it can be said that higher temperatures combined with moderate ER favours the production of H 2 and CO. Furthermore, a low temperature is selective in the types of raw materials [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF].

The Products of RDF Gasification

Generally, the products of RDF gasification are: syngas, condensable organics (tars and water vapour) [START_REF] Werle | A Review of Methods for the Thermal Utilization of Sewage Sludge: The Polish Perspective[END_REF] [83] and solid products (char and ash) [START_REF] Rong | Co-Gasification of Sewage Sludge and Woody Biomass in a Fixed-Bed J. C. I. Rabetanetiarimanana[END_REF]. RDF gasification shows a positive effect on H 2 gas concentration, syngas yield and LHV of the gas [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. A high quality syngas has a high content of H 2 and CO and a low content of CH 4 , CO 2 and gaseous hydrocarbons such as C 2 H 4 and C 2 H 6 [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF] [19] [START_REF] Byun | Demonstration of Thermal Plasma Gasification/Vitrification for Municipal Solid Waste Treatment[END_REF] [START_REF] Liu | Aerosol Problems in Coal Gasification Research[END_REF]. The aspect of the RDF gasification results is a contribution from each of the RDF components [START_REF] Aluri | Pyrolysis and Gasification Studies of Model Refuse Derived Fuel (RDF) Using Thermogravimetric Analysis[END_REF]. The results show that gasification of plastic DOI: 10.4236/sgre.2022.1311017 279 Smart Grid and Renewable Energy produces more C1 -C4 hydrocarbons due to high volatile content, low fixed carbon content and low or no oxygen content. Indeed, plastic gasification produces more tar than coal. The syngas produced by the gasification of plastic-rich RDF has a high LHV [START_REF] Ramos | Syngas Production Strategies from Biomass Gasification: Numerical Studies for Operational Conditions and Quality Indexes[END_REF] and is predominantly composed of: CH 4 and C 2 H 4 . Also, gasification of high oxygen biomass such as food waste, paper and wood produces more CO and CO 2 and less hydrocarbons. At a temperature of around 900˚C or with a high ER, the ash in biomass acts as a catalyst in the gasification of RDF and leads to the rapid cracking of the plastic polymers [START_REF] Hwang | Characterization of Products Obtained from Pyrolysis and Steam Gasification of Wood Waste, RDF, and RPF[END_REF]. Therefore, plastic-biomass co-gasification will be able to produce good quality syngas, i.e., low tar content and high LHV [START_REF] Aluri | Pyrolysis and Gasification Studies of Model Refuse Derived Fuel (RDF) Using Thermogravimetric Analysis[END_REF] [65] [START_REF] Hwang | Characterization of Products Obtained from Pyrolysis and Steam Gasification of Wood Waste, RDF, and RPF[END_REF]. Table 5 shows the results of RDF gasification of different composition.

For air gasification, half the volume of syngas produced is formed by nitrogen. At high temperatures (above 900˚C), the use of air as an oxidising agent shows little improvement in the calorific value of the syngas [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. Then, the gasification using CO 2 as gasifying agent yields more CO and CH 4 and produces syngas with higher LHV compared to other gasifying agent [START_REF] Islam | Effect of Different Gasifying Agents (Steam, H 2 O 2 , Oxygen, CO 2 , and Air) on Gasification Parameters[END_REF]. Moreover, steam gasification produces high quality syngas free of nitrogen and its oxides and with a very high concentration of H 2 [START_REF] Lee | High-Temperature Steam Gasification of Municipal Solid Waste, Rubber, Plastic and Wood[END_REF] [26] [START_REF] Niu | Simulation of Syngas Production from Municipal Solid Waste Gasification in a Bubbling Fluidized Bed Using Aspen Plus[END_REF]. Currently, steam is the most widely used gasification agent [START_REF] Zhang | Performance Analysis of Municipal Solid Waste Gasification with Steam in a Plasma Gasification Melting Reactor[END_REF]. The injection of steam into the reactor favours the production of H 2 and CO 2 while decreasing the CO yield. Moreover, the steam decreases slightly the heating value and the cold gas efficiency (CGE) by decreasing the process temperature [START_REF] Sebastiani | Steam-Oxygen Gasification of Refuse Derived Fuel in Fluidized Beds: Modelling and Pilot Plant Testing[END_REF]. At a temperature equal to or higher than 900˚C, the production of H 2 and CO 2 intensifies and the LHV and the syngas yield improve [START_REF] Zhang | Performance Analysis of Municipal Solid Waste Gasification with Steam in a Plasma Gasification Melting Reactor[END_REF]. Furthermore, the composition of the syngas produced by plasma gasification depends on the source of the plasma gas. Plasma with N 2 produces more H 2 gas while plasma from steam produces syngas formed mainly by CO and H 2 . The combination of plasma and steam gasification has a positive effect on the LHV of the syngas, the syngas yield and the conversion of char [START_REF] Munir | Plasma Gasification of Municipal Solid Waste for Waste-to-Value Processing[END_REF]. With plasma gasification, inorganic materials such as minerals and metals in the ash are transformed into harmless, vitrified slag consisting of: Silicon dioxide (SiO 2 ), Aluminium oxide (Al 2 O 3 ), Calcium oxide (CaO), Iron (III) oxide (Fe 2 O 3 ), Sodium oxide (Na 2 O) and Magnesium oxide (MgO) [START_REF] Zhang | Eulerian Model for Municipal Solid Waste Gasification in a Fixed-Bed Plasma Gasification Melting Reactor[END_REF] [25] [START_REF] Byun | Demonstration of Thermal Plasma Gasification/Vitrification for Municipal Solid Waste Treatment[END_REF]. Following the US EPA (United States Environmental Protection Agency) leech tests, the vitrified slag can be used for construction purposes [START_REF] Horton | Implementation of a Molecular-Level Kinetic Model for Plasma-Arc Municipal Solid Waste Gasification[END_REF].

The Scientific Barriers to RDF Gasification

The gasification of RDF is more difficult due to its heterogeneous nature. Indeed, all available technologies have been developed to work on pure biomass [START_REF] Materazzi | Performance Analysis of RDF Gasification in a Two Stage Fluidized Bed-Plasma Process[END_REF]. The barriers to solid waste gasification are: the presence of tars, chars, particulates, NH 3 , H 2 S, HCl, Cl 2 and chlorinated organics in the syngas [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF] [33] [START_REF] Pröll | Removal of NH 3 from Biomass Gasification Producer Gas by Water Condensing in an Organic Solvent Scrubber[END_REF]. The high content of chlorine (NaCl and PVC) in the waste is the cause of the formation of certain chlorinated products and a source of emission of toxic dioxins and furans [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF]. Moreover, chlorine, cadmium and lead are often concentrated in the fuels produced [START_REF] Rotter | Material Flow Analysis of RDF-Production Processes[END_REF]. However, polyvinyl chloride (PVC), a source DOI: 10.4236/sgre.2022.1311017 282 Smart Grid and Renewable Energy high-quality syngas from solid waste [69] [90]. The addition of biomass or coal leads to an increase in the fraction of fixed carbon in the mixture and promotes the maintenance of the gasification reaction (Equations ( 4) and ( 5)). The formation of char from the pyrolysis reaction is very important in order to accelerate the water-gas reaction (Equation ( 5)) which produces hydrogen and carbon monoxide [30] [70]. Therefore, the addition of biomass improves the quality of the gas produced and optimises the temperature distribution inside the reactor [30] [65]. Moreover, biomass ash is rich in alkali and alkaline earth metals (sodium, potassium, magnesium and calcium) and has effectively catalytic effects during co-gasification with other fuels [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]. Indeed, co-gasification with RDF is also beneficial for biomass because this technology has the potential to reduce ash melting at high gasification temperatures [START_REF] Cai | Synergistic Effects of Co-Gasification of Municipal Solid Waste and Biomass in Fixed-Bed Gasifier[END_REF]. However, the lack of understanding of phenomena such as the synergistic effects of biomass-RDF co-gasification and the lack of published data on biomass-RDF co-gasification remain barriers that require attention [69] [92].

On the other hand, the gasification of the coal-biomass mix is a new challenge. The mastery of a clean fossil fuel technology is very promising and advantageous because of the abundance and stability of coal prices [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF]. At the laboratory scale, co-gasification of coal/biomass mixtures is promising compared to individual gasification of coal and biomass [START_REF] Aigner | Co-Gasification of Coal and Wood in a Dual Fluidized Bed Gasifier[END_REF] [START_REF] Mallick | Co-Gasification of Coal and Biomass Blends[END_REF]. The existence of the synergistic effect obtained by jointly feeding biomass and coal into a gasifier may improve the conversion process and the quality of the products obtained. Generally, the syngas obtained from the co-gasification of the coal/biomass mixture is composed of H 2 , CO, CO 2 and CH 4 [START_REF] Aznar | Plastic Waste Elimination by Co-Gasification with Coal and Biomass in Fluidized Bed with Air in Pilot Plant[END_REF] [94] [START_REF] Mastellone | Co-Gasification of Coal, Plastic Waste and Wood in a Bubbling Fluidized Bed Reactor[END_REF]. The distribution of the products and the composition of the syngas depend on the following parameters: the temperature of the gasifier, the ER, the composition of the raw materials, the heating rate and the gasification agent used [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] Mallick | Co-Gasification of Coal and Biomass Blends[END_REF]. There are different types of coal/biomass blends in the literature, such as coal/plastic blend, plastic/wood/coal blend and wood/coal blend. According to the literature:  Increasing the ratio of biomass in the biomass/coal mixture leads to an increase in the CO, H 2 , hydrocarbon and tar content of the syngas [START_REF] Mallick | Co-Gasification of Coal and Biomass Blends[END_REF]. Then, the increase of the wood ratio in the wood/coal mixture leads to the production of cleaner syngas with a low LHV due to the production of coal, the reduction of the presence of hydrocarbons and the absence of tar [START_REF] Mastellone | Co-Gasification of Coal, Plastic Waste and Wood in a Bubbling Fluidized Bed Reactor[END_REF].  The gasification of coal is associated with the production of tar, which constitutes 15% to 20% of the total production of energy [START_REF] Liu | Aerosol Problems in Coal Gasification Research[END_REF], and with the formation of NH 3 and H 2 S because of the presence of S and N in the coal. Then, the presence of SiO 2 in the coal participates in the reduction of the tar, the SiO 2 acts as an in-situ catalyst [START_REF] Mallick | Co-Gasification of Coal and Biomass Blends[END_REF].  A mixture with a high plastic content produces syngas with a high LHV. The syngas yield is low. However, a mixture with a high coal content has a high syngas yield and produces syngas with a low LHV [START_REF] Aznar | Plastic Waste Elimination by Co-Gasification with Coal and Biomass in Fluidized Bed with Air in Pilot Plant[END_REF]. Moreover, pre-treatment of RDF by carbonisation at 300˚C -400˚C for 15 -60 mn has been shown to be effective. The advantages of carbonisation of RDF DOI: 10.4236/sgre.2022.1311017 283 Smart Grid and Renewable Energy before gasification are: reduction of the emission of hazardous pollutants due to the elimination of some inorganic components such as calcium or chlorine, an increase of density, calorific value, carbon, ash and fixed carbon content of the RDF [START_REF] Nobre | Upgrading of Refuse Derived Fuel through Torrefaction and Carbonization: Evaluation of RDF Char Fuel Properties[END_REF]. According to Goncalves et al. [START_REF] Nobre | Upgrading of Refuse Derived Fuel through Torrefaction and Carbonization: Evaluation of RDF Char Fuel Properties[END_REF] the HHV of RDF after carbonisation is between 20.1 -26.2 MJ/kg.

Then, modifying the reactor design to increase the temperature inside, or gasification in two or more stages, can reduce and increase the tar and H 2 content of the syngas respectively. As in the case of gasification, higher temperature and ER favour the syngas yield during co-gasification [START_REF] Cai | Synergistic Effects of Co-Gasification of Municipal Solid Waste and Biomass in Fixed-Bed Gasifier[END_REF]. Saleh et al. [START_REF] Saleh | Improved Municipal Solid Waste Gasification Efficiency Using a Modified Downdraft Gasifier with Variations of Air Input and Preheated Air Temperature[END_REF] showed that the creation of an air inlet in a zone between pyrolysis and oxidation leads to an increase in temperature and consequently to a reduction of 30% -50% in tar content and a 20% increase in H 2 production. Additionally, two-stage gasification, i.e. the use of two different reactors, is an efficient way of producing H 2 and tar-free syngas. The combination of a conventional gasification reactor with a plasma reactor gives a good result. In this case, the cracking of the tars and vitrification of the inorganic ash fraction takes place in the plasma reactor. According to Mukherjee et al [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF], fluidised bed plasma gasification of solid waste is a promising technology. By combining a fluidised bed gasifier with a plasma gasifier, Ray et al. [START_REF] Ray | The Deployment of an Advanced Gasification Technology in the Treatment of Household and Other Waste Streams[END_REF] found high energy conversion (74% -90%) and carbon conversion (95% ± 1.6%). Then, the use of 2 reactors, one for pyrolysis and the other for catalytic reforming of the volatiles produced by pyrolysis, allows to enhance the production of H 2 and to obtain a tar-free syngas [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF].

Furthermore, the use of a catalyst is important in order to increase the efficiency of gasification, improve the gas yield by decreasing the production of liquid and char, reduce the tar content and avoid the thermodynamic restriction of the gas to water displacement reaction (Equation ( 6)) due to a high-temperature [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. In fact, the catalyst enhances the production of H 2 and CO and decreases the production of tar, coal, CO 2 , C 2 H 4 and C 2 H 6 [START_REF] He | Syngas Production from Catalytic Gasification of Waste Polyethylene: Influence of Temperature on Gas Yield and Composition[END_REF]. Nickel is the catalyst generally used [START_REF] Hwang | Characterization of Products Obtained from Pyrolysis and Steam Gasification of Wood Waste, RDF, and RPF[END_REF] [START_REF] Kobayashi | Woody Biomass and RPF Gasification Using Reforming Catalyst and Calcium Oxide[END_REF]. The Nickel-based catalyst ensures the elimination of tar, accelerates the methane reforming reaction and the gas-to-water conversion reaction [START_REF] Heidenreich | New Concepts in Biomass Gasification[END_REF].

The reduction of the tar content of the syngas is of primary importance. Apart from the Nickel catalyst, other types of catalysts such as zeolites, dolomites [START_REF] Mukherjee | A Review on Municipal Solid Waste-to-Energy Trends in the USA[END_REF] [80], gasification and pyrolysis chars [START_REF] Wang | Pyrolysis Municipal Sludge Char Supported Fe/Ni Catalysts for Catalytic Reforming of Tar Model Compound[END_REF] and alkaline minerals in gasification bottom ash [START_REF] Ouadi | A Review of the Valorization of Paper Industry Wastes by Thermochemical Conversion[END_REF] could be used as adsorbent or catalyst for tar decomposition. The use of bottom ash and fly ash as a catalyst both improves the quality of the syngas and treats the highly toxic residues [START_REF] Byun | Demonstration of Thermal Plasma Gasification/Vitrification for Municipal Solid Waste Treatment[END_REF]. Ash from biomass gasification contains C, O, Si, alkali metals, alkaline earth metals, non-metals, metalloids and a significant amount of Ca (above 40%). A good catalyst such as calcium oxide (CaO) can be obtained by open thermal calcination of the Ca present in these residues for the production of biodiesel from vegetable oil [START_REF] Luque | Carbonaceous Residues from Biomass Gasification as Catalysts for Biodiesel Production[END_REF]. Moreover, the use of olivine as a bed material is effective in removing tar [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF] [80] [START_REF] Aigner | Co-Gasification of Coal and Wood in a Dual Fluidized Bed Gasifier[END_REF]. The latter acts both as a heat carrier, catalyst, and increases gas production [START_REF] Stendardo | High Quality Syngas Production via Steam-Oxygen Blown Bubbling Fluidised Bed Gasifier[END_REF]. diagram summarizes the state of the art in solid waste gasification and highlights the variation in gasification products as a function of waste composition, catalysts and reactor type used and operating conditions such as temperature, gasification agent and equivalence ratio.

Conclusions and Suggestions

Gasification is ideal for the production of syngas from RHW because all kinds of waste can be recovered from this process. The aspect of the result of the gasification of RDF is a contribution from each of the RDF components. The gasification of plastic produces more C1 -C4 hydrocarbons due to a high volatile content, low fixed carbon content and low or no oxygen content. Plastics readily decompose into tar rather than char. In addition, gasification of high oxygen biomass such as food waste, paper and wood produces more CO and CO 2 and less hydrocarbons. Indeed, the plastic: biomass ratio is an important parameter in the gasification of RDF. A plastic-rich RDF has a low syngas yield while a biomass-rich RDF produces a low calorific value syngas. Plastic-biomass co-gasification produces good quality syngas, i.e., low tar content and high calorific value. In short, to have a better quality of gas produced, an RDF must have a high calorific value and a sufficient amount of fixed carbon and ash. The latter has a catalytic effect on the decomposition of RDF. Plastic and biomass provide the calorific value and the fixed carbon and ash content of RDF respectively.

However, the quality of the syngas produced depends not only on the composition of the RDF but also on the operating conditions of the reactor, the type of DOI: 10.4236/sgre.2022.1311017 285 Smart Grid and Renewable Energy reactor and the type of gasification agent used.

In addition, solid waste gasification has a higher cold gas efficiency than 70% and a higher conversion efficiency. Steam and plasma gasification produce syngas with a high calorific value and low tar content. By removing tars, coals and ash from the syngas, it can be used for electricity generation in small power plants below 10 MW. However, the high amounts of char, metals and organic pollutants in the ash and char are concerns especially with human health.

Taking into account the emissions, costs and technical aspects of each process, it can be said that gasification is a promising technique for the valorisation of RHW. However, studies on co-gasification of waste and biomass and gasification on a pilot-industrial scale are still scarce in the literature. So far, the synergistic effect of co-gasification of biomass and RDF is not entirely clear.

  Figure 1 shows a schematic diagram of input and output of the gasification system. The DOI: 10.4236/sgre.2022.1311017 284 Smart Grid and Renewable Energy

Figure 1 .

 1 Figure 1. Schematic diagram: Input and output of the gasification system.

Table 1 .

 1 Chemical reactions in reactor.

	Type of reactor	Chemical reactions	Reference
		Drying
	Downdraft reactor	Pyrolysis Combustion and tar cracking	[10] [35]
		Reduction
		Drying
	Updraft reactor	Pyrolysis Char gasification or reduction	[25]
		Combustion
		Oxidation reaction with oxygen
	Plasma reactor	Reduction with steam, H 2 and CO 2	[11]

Decomposition of tar and hydrocarbons DOI: 10.4236/sgre.2022.1311017 273 Smart Grid and Renewable Energy

  waste, waste formed by food residues, garden waste and biomasses has a low calorific value due to the excess of oxygen and moisture content[12] [13]

	[59].
		Hand sorting of mother sample into multiple categories
		[16] [28] [50] [52] [54] [56]
		Weight: 100 kg [28]
		IEA: 9 to 19 categories (kitchen and garden waste, paper
		and cardboard, plastics, metals, textiles, fuels, non-fuels,
		fine fractions and glass) with 2 or 3 levels of categories
		[28] [54]
	Prepare sample for the laboratory	Mass reduction by: coning and quartering [45] [50] [54] and the flat pile [45] [52] [53] Sample weight: 2 -4 kg for each category [52]
		Drying for 1 or 2 days (biowaste: in a laboratory oven at
		105˚C for 24 hours [47] [51] [52] [56], other categories:
		twice at 80˚C for 24 hours until constant weight [52])
		Shredding of dry samples by Retsch SM300 or SM400 or
	Drying and shredding	SM2000 chipper [52] or the ARP CS 2000 shredder [45]
		Shredding of non-combustible waste and glass by Jaw
		crusher and a vibrating disc mill, respectively
		Mass reduction by the Riffle splitter: 5-10 g sample for
		each category [45] [52]
	Proximate and
	ultimate analysis

Proximate analysis: 1 g of sample Ultimate analysis: 1-5 g of sample

[START_REF] Zhou | An Overview of Characteristics of Municipal Solid Waste Fuel in China: Physical, Chemical Composition and Heating Value[END_REF]

[53] DOI: 10.4236/sgre.2022.1311017 275 Smart Grid and Renewable Energy to plastic

Table 4 .

 4 RDF proximate analysis.

	Plastic 20.5%
	Paper 16%
	Cardboard 22.5%	19.6	1.59	15.47	78.04	4.9	[67]
	Textiles 31%
	RDF	15.2	6	26.3	67.6	12	[64]
	RDF	11.9	19.1	6.4	59.6	14.9	[68]
	RDF	16.03	22.92	9.35	58.57	9.16	[69]
	Plastics paper and food wastes	24.8 (HHV)	3.44	-	-	4	[32]
	Paper and fiber 50%
	Wood 28%
	Plastics 9%	17.9	12.9	10.4	76.7	-	[70]
	Food waste 7%
	Incombustibles 6%
	Paper 70%
		24.6	7.9	5.4	86.7	-	[70]
	Plastics 30%
	MSW 50%
		21	12.1	11.6	64.8	11.5	[71]
	Commercial and industrial wastes 50%
	RDF	18.6	1.1	16.3	82.6	25	[72]

RDF Composition LHV of RDF [MJ/kg] Ash [wt%] FC [wt%] VM [wt%] Moisture [wt%] Reference

Acknowledgements

The authors greatly appreciated the financial support from Région Réunion and Fonds européen de développement régional (FEDER).

of Cl, constitutes the majority of polymers produced and distributed worldwide.

In the case of Europe, after polypropylene (PP) and polyethylene (PE), PVC constitutes 10% of polymer production in 2017 [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. According to Rotter et al. [START_REF] Rotter | Material Flow Analysis of RDF-Production Processes[END_REF] the presence of these elements in most waste streams limits the ability to reduce pollution associated with the gasification of the fuels produced. Furthermore, the nitrogen in the biomass used in RDF, for example, wood is the source of NH 3 formation in the syngas [START_REF] Pröll | Removal of NH 3 from Biomass Gasification Producer Gas by Water Condensing in an Organic Solvent Scrubber[END_REF]. Due to a high percentage of plastics in RDF, the high amount of volatile matter in RDF leads to the formation of tars [START_REF] Materazzi | Performance Analysis of RDF Gasification in a Two Stage Fluidized Bed-Plasma Process[END_REF].

The obstacles to the valorisation of plastics by gasification are: the presence of inorganic materials in the plastic, the chemical characteristics of the plastics, the high content of volatile matter [START_REF] Lopez | Recent Advances in the Gasification of Waste Plastics. A Critical Overview[END_REF]. Therefore, efficient gas cleaning is recommended to reduce the amount of these impurities in the syngas [33] [38] [83] [START_REF] Pröll | Removal of NH 3 from Biomass Gasification Producer Gas by Water Condensing in an Organic Solvent Scrubber[END_REF]. The tar causes the blockage of the reaction vessel [START_REF] Wang | Pyrolysis Municipal Sludge Char Supported Fe/Ni Catalysts for Catalytic Reforming of Tar Model Compound[END_REF] [89] and the reduction of the calorific value and the syngas yield [START_REF] Heidenreich | New Concepts in Biomass Gasification[END_REF]. Moreover, the ash produced by the gasification of RDF, composed of alkali and alkaline earth metals, heavy metals and non-toxic organic compounds, contaminates water and soil. At high concentrations, bottom ash from RDF gasification has toxic effects on human HepG2 and MRC-5 cells [35] [68]. Besides, the gasification of plastics remains a challenge despite its high calorific value. However, on a regional or national scale, the mixture introduced into the gasifier contains at least 10% plastics [START_REF] Sipra | Municipal Solid Waste (MSW) Pyrolysis for Bio-Fuel Production: A Review of Effects of MSW Components and Catalysts[END_REF]. Nevertheless, high plastic content in RDF can generate a high temperature and damage the gasifier [START_REF] Chalermcharoenrat | Optimization of Combustion Behavior and Producer Gas Quality from Reclaimed Landfill through Densify RDF-Gasification[END_REF].

Solutions from Literature

The co-gasification of RDF and biomass is a promising pathway to produce

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this pa-


T Widemann
email: thomas.widemann@obspm.fr

E Marcq

C Tsang

N Mueller

D Kappel

J Helbert

M. D Dyar

THE VENUS EMISSIVITY MAPPER -INVESTIGATING THE ATMOSPHERIC STRUCTURE AND DYNAMICS OF VENUS' POLAR REGION

Introduction:

The Venus Emissivity Mapper (VEM) is the first flight instrument specially designed to map the surface of Venus using the narrow atmospheric windows around 1 µm [1]. VEM is proposed for the European Space Agency's M5/EnVision proposal in combination with a high-resolution radar mapper (see Abstract #1937). Mapping of Venus with VEx/VIRTIS using the 1.02 µm thermal emission band can be viewed as a proof-of-concept for an orbital remote sensing approach to surface composition and weathering studies for Venus [2][3][4][5][6][7].

Thermal brightness on Venus' night side is mainly modulated by the lower clouds, imaged at 2.3 µm by the Akatsuki IR2 camera [8]. Thanks to the circular polar orbit geometry of M5/EnVision, VEM has the unique capability to (1) better constrain the microphysics of the lower cloud particles in three spectral bands at 1.195, 1.310 and 1.510 µm at a spatial resolution of ~10 km, and (2) investigate short-timescale cloud dynamics and thus local wind speeds by tracking cloud features in both polar regions.

Cloud parameters: Global cloud layers (~45 to 70 km) drive the energy balance of the atmosphere and hence climate at the Venus' surface [9]. While much progress has been made since the early suggestion that the Venus clouds are H 2 O-H 2 SO 4 liquid droplets [10], several cloud parameters are still poorly constrained, particularly in the lower cloud layer and optically thicker polar regions [11][12][13]. Observations at small horizontal scales are of great importance to microphysical models of cloud and haze systems [14]. VEM has the capability to better constrain the microphysics (vertical, horizontal, time dependence of particle size distribution, or/and composition) of the lower cloud particles in three spectral bands at 1.195, 1.310 and 1.510 µm at a spatial resolution of ~10 km.

Wind measurements: Venus displays the bestknown case of polar vortices evolving in a fast-rotating atmosphere. Few wind measurements exist in the polar region due to unfavorable viewing geometry of currently available observations. Cloud-tracking data indicate circumpolar circulation close to solid-body rotation. E-W winds decrease to zero velocity close to the poles. N-S circulation is marginal, with extremely variable morphology and complex vorticity patterns [15][16][17] (Fig. 1).

Circular polar orbit geometry would provide an unprecedented study of both polar regions within the same mission. VEM's pushbroom method will allow shorttimescale cloud dynamics to be assessed, as well as local wind speeds, using repeated imagery at 90 minute intervals.

Tracking lower cloud motions as proxies for wind measurements at high spatial resolutions will greatly benefit modeling of the vortices' physics. Convective modeling demonstrates that there will be cloud-level convection at high latitudes, so repeated imagery at 90 minute intervals will help constrain the time evolution of cloud-level convection as well as wave-generating dynamical instabilities [17]. This will also allow a direct comparison of the N-S wind regimes and their temporal evolution at several time scales.

Figure 1 -

 1 Figure 1 -Polar projection of South vortex morphology and cloud tracking zonal wind velocities near 1.74 µm [17].


Frederic Andre
email: frederic.andre@thalesgroup.com

F G Minenna

Khalil Aliane

Yves Elskens

Alexandre Poye

Jerome Puech

Fabrice Doveil

Progress with DIMOHA for fast time-domain simulations of traveling-wave tubes

Keywords: traveling-wave tubes TWTs, DIMOHA, simulation, AM/AM, AM/PM, nonlinear signals, time-domain, waveparticle interaction, Hamiltonian dynamics, N-body dynamics, large-signal amplifiers

come   L'archive ouverte pluridisciplinaire

Progress with DIMOHA for fast time-domain simulations of traveling-wave tubes Frédéric André * § , Member, IEEE, Damien F. G. Minenna * , Khalil Aliane † ‡ * , Yves Elskens ‡ , Alexandre Poyé ‡ , Jérôme Puech † , Fabrice Doveil ‡

I. INTRODUCTION

Nowadays, to simulate the wave-particle interaction occurring in the slow-wave structures (SWSs) of traveling-wave tubes (TWTs) [START_REF] Minenna | The Traveling-Wave Tube in the History of Telecommunication[END_REF], one can rely on two main options : specialized frequency codes or particle-in-cell (PIC) codes in time-domain. We choose a third option based on our discrete model with hamiltonian approach (DIMOHA). The two salient features of this model are that it relies on model order reduction (MOR) to represent electro-magnetic waves and that its physics is expressed by a Hamiltonian function. MOR allows huge reduction in the number of degrees of freedom describing the waves compared to PIC models. The Hamiltonian formalism results in a so called symplectic time integrator that preserves important qualitative features of the physics at play. The simulation code derived from this approach is very fast compared to PIC codes and possibly more accurate than specialized frequency models, but also has some limitation. To assess these claims, we engaged into the simulation of actual TWTs and compared the different codes and the measurements. Results from this endeavor will be presented.

II. ALGORITHM

DIMOHA is a specialized model in time-domain and largesignal regime based on an N -body (a.k.a. many body) hamiltonian description [START_REF] Minenna | Electromagnetic power and momentum in N-body Hamiltonian approach to wave-particle dynamics in a periodic structure[END_REF], [START_REF] André | Hamiltonian description of self-consistent wave-particle dynamics in a periodic structure[END_REF], [START_REF] André | Hamiltonian description of the electron and its radiated field in a periodic structure[END_REF], [START_REF] André | First comparison of new TWT discrete model with existing models[END_REF], [START_REF] Minenna | Electron-wave momentum exchange and time domain simulations applied to traveling wave tube[END_REF], [START_REF] Minenna | DIMOHA: Traveling-wave tube simulations including band edge and multiple-carriers operations[END_REF]. The symplectic algorithm [START_REF] Minenna | DIMOHA: A Time-Domain Algorithm for Traveling-Wave Tube Simulations[END_REF] we built is developed jointly between Thales AVS/MIS, Aix-Marseille Université-CNRS and the Centre National d' Études Spatiales in France. It is written in modern Fortran and parallelized with the Message Passing Interface (MPI) libraries. DIMOHA bears several advantages : 1) it allows arbitrary waveform (not just field envelope), including continuous waveform (CW), multiple carriers, or digital modulations (e.g. phase-shift keying); 2) the algorithm is much faster than PIC codes, thanks to a field discretization allowing a drastic degrees-of-freedom reduction, along with a robust symplectic integrator; 3) it supports any periodic slow wave structure (SWS) design such as a helix or folded waveguides, including structures with pitch tapering; 4) it reproduces harmonic generation, reflection, oscillation, and distortion phenomena; 5) it handles nonlinear dynamics, including intermodulations, trapping, and chaos; and 6) the number of input parameters required is low (listed in Table I), so that DIMOHA is easy to use.

III. VALIDATION

The validity of the discrete model was analytically assessed [START_REF] Minenna | Recent discrete model for small-signal analysis of traveling-wave tubes[END_REF] by comparing it with Pierce's four-waves theory in frequency-domain. Several works in time-domain were performed to validate our algorithm. We performed [START_REF] Minenna | Electromagnetic power and momentum in N-body Hamiltonian approach to wave-particle dynamics in a periodic structure[END_REF] a numerical comparison against MVTRAD (a frequency code developed by Thales) for a fictitious (lossless) TWT. We already compared [START_REF] Minenna | DIMOHA: A Time-Domain Algorithm for Traveling-Wave Tube Simulations[END_REF] DIMOHA against measurements from a 150W Ku TWT (a commercial helix TWT with tapered pitch) and against PIC codes for folded waveguide TWTs.

Since then, simulations of folded waveguide TWTs were improved. We compared [START_REF] André | Technology and Assembly of a Wband Traveling Wave Tube for New 5G High Capacity Networks[END_REF] DIMOHA against PIC codes CST-PS and MAGIC3D for a 94 GHz folded waveguide TWT. Currently, we are testing our algorithm against measurements from helix TWTs at 14W in X-band, 170W in Ka-Band and 80W in Q-band (see Fig. 1). Multiple reflections, possibly causing oscillations, are being investigated (see Fig. 2).

Finally, we are performing simulations of a 4 meters long TWT. This is the experimental device of the PIIM laboratory at Aix-Marseille University-CNRS used to investigate turbulence and chaos in plasmas.

IV. CONCLUSION

DIMOHA is a promising tool that bears several advantages for both industrial and research activities. It also provides a new approach to analyze telecommunications signals for operators. Currently, a version is under industrialisation (with a graphical user interface) to be implemented as a new numerical solution for TWT design. In this respect, the large range of tested tubes helps us to burn-in DIMOHA. New features are under consideration, like 2.5-D, 3-D and frequency-domain versions.

[ 8 ]Fig. 1 .

 81 Fig.1. Simulations of the output power at various input powers and frequencies for an 80W Q-band tapered helix TWT[START_REF] André | 80 to 100 Watts TWT in Q-band for Space Downlink Communication[END_REF] with DIMOHA. It took 22 minutes per point with 16 processors for a numerical resolution of ±0.005 W. On a small computer with 4 processors, it took 16 minutes per point for a resolution of ±0.25 W.

Fig. 2 .

 2 Fig. 2. 50 ns long simulation of the electric field amplitude along the tube axis for the 80W Q-band tapered helix TWT. For the orange curve, we added two reflecting defects, one after the local attenuator (VSWR of 1.05) and one at the helix output end (VSWR >2).

TABLE I INPUTS

 I AND OUTPUTS OF DIMOHA FROM REF.

ACKNOWLEDGMENT

This work was granted access to the HPC resources of Aix-Marseille Université funded by the project Equip@Meso (ANR-10-EQPX-29-01) of the program "Investissements d'Avenir" supervised by the Agence Nationale de la Recherche.