Recently, the production of RDF is being advocated by waste management planners and governments [31]. 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 [15] [19] [31] [63], Milena et al. [64] found an increase in LHV of 33% compared to the raw solid waste, 2) the homogeneity of the physico-chemical composition of the product [31] [32] [64] [65], 3) ease of storage, handling, transport and conveying of the biomass to the reactor [31] [32] [65] [66], 4) reduction of pollutant emissions during gasification [64].

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 [31] [32] [61] [64] [67]. The sorting and sampling phase presented in Table 2 give an overview of the SRF production line [24]. 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 [19]. Thus, these types of waste, such as glasses and metals, are not favourable to gasification [14] and have a huge impact on the gasification process [19]. Then, each subsample related to each imposed category is crushed, sieved, mixed and blended according to the desired composition [19] [24]. The mixed waste is then densified into briquettes or pellets [12] [24] [63]. Table 3 shows the different types of RDF production in the literature. In their work, Caputo et al. [31] 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 [73]. Seven different types of RDF have been classified by the American Society for Testing and Materials (ASTM) [74], 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/m³ in the form of pellets or briquettes [31].

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

mixture of waste with a high calorific value is essential [31]. This implies a composition in which plastic occupies an important place [75]. 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 [32] [65] [70]. 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.

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 [19]. 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

of the gasifier [44]. The cold gas efficiency of RDF gasification can reach a value of more than 70% [14] [37] [78] [79]. 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 [77]. 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 [19].

Then, the type of reactor used and the operating conditions of the gasifier such as the mass flow rate [4] [19], the oxidant used, the temperature inside the reactor, the equivalence ratio (ER), the residence time [4] [80] and the catalyst [26] influence the quality of the gasification products. The ER is a more important parameter than the bed height and the fluidisation speed [37]. An ER that is too high (above 0.4) leads to the dilution of the syngas, the CO₂ content of the gas increases whereas the H₂, CH₄ and CO decrease. On the other hand, an ER that is too low (below 0.2) means an incomplete gasification process [81]. Indeed, an appropriate ER (0.2 - 0.4) should be used to improve the production rate of combustible gases (CO, H₂ and CH₄) [44]. An increase in ER is accompanied by a rise in temperature in the gasification zone [24]. 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 CO₂ content [24] [37] [38] [40] [81]. Then, if the ER is gradually increased, the amount of H₂ and CH₄ produced will decrease. These phenomena are explained by the oxidation reaction of H₂ 2 and the steam reforming reaction of methane 7 [24]. An optimal ER is defined by optimal CO, H₂ and CH₄ contents resulting in an optimal LHV of the syngas [24] [25]. Moreover, temperature is the most influencing parameters in gasification process, it influences the composition of the syngas and the product distribution [81]. The increase in temperature favours the production of CO and H₂ and reduces the production of CO₂ and CH₄ [38]. High CO and H₂ contents mean high LHV values, cold gas efficiency and carbon conversion efficiency [13] [37] [44]. Therefore, a higher temperature is favourable to the production of H₂ rich gas [14] and CO [43] [44] and to the increase of energy efficiency [24]. The H₂/CO molar ratio increases with increasing temperature in air, oxygen and CO₂-gasification and decreases in steam-gasification [82]. Hence, it can be said that higher temperatures combined with moderate ER favours the production of H₂ and CO. Furthermore, a low temperature is selective in the types of raw materials [14].

Generally, the products of RDF gasification are: syngas, condensable organics (tars and water vapour) [22] [83] and solid products (char and ash) [35]. RDF gasification shows a positive effect on H₂ gas concentration, syngas yield and LHV of the gas [38]. A high quality syngas has a high content of H₂ and CO and a low content of CH₄, CO₂ and gaseous hydrocarbons such as C₂H₄ and C₂H₆ [4] [19] [43] [83]. The aspect of the RDF gasification results is a contribution from each of the RDF components [32]. The results show that gasification of plastic 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 [84] and is predominantly composed of: CH₄ and C₂H₄. Also, gasification of high oxygen biomass such as food waste, paper and wood produces more CO and CO₂ 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 [70]. Therefore, plastic-biomass co-gasification will be able to produce good quality syngas, i.e., low tar content and high LHV [32] [65] [70]. 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 [37]. Then, the gasification using CO₂ as gasifying agent yields more CO and CH₄ and produces syngas with higher LHV compared to other gasifying agent [82]. Moreover, steam gasification produces high quality syngas free of nitrogen and its oxides and with a very high concentration of H₂ [14] [26] [37]. Currently, steam is the most widely used gasification agent [25]. The injection of steam into the reactor favours the production of H₂ and CO₂ while decreasing the CO yield. Moreover, the steam decreases slightly the heating value and the cold gas efficiency (CGE) by decreasing the process temperature [85]. At a temperature equal to or higher than 900˚C, the production of H₂ and CO₂ intensifies and the LHV and the syngas yield improve [25]. Furthermore, the composition of the syngas produced by plasma gasification depends on the source of the plasma gas. Plasma with N₂ produces more H₂ gas while plasma from steam produces syngas formed mainly by CO and H₂. 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 [11]. With plasma gasification, inorganic materials such as minerals and metals in the ash are transformed into harmless, vitrified slag consisting of: Silicon dioxide (SiO₂), Aluminium oxide (Al₂O₃), Calcium oxide (CaO), Iron (III) oxide (Fe₂O₃), Sodium oxide (Na₂O) and Magnesium oxide (MgO) [23] [25] [43]. Following the US EPA (United States Environmental Protection Agency) leech tests, the vitrified slag can be used for construction purposes [86].

The gasification of RDF is more difficult due to its heterogeneous nature. Indeed, all available technologies have been developed to work on pure biomass [68]. The barriers to solid waste gasification are: the presence of tars, chars, particulates, NH₃, H₂S, HCl, Cl₂ and chlorinated organics in the syngas [13] [33] [87]. 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 [13]. Moreover, chlorine, cadmium and lead are often concentrated in the fuels produced [73]. However, polyvinyl chloride (PVC), a source

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 [38]. According to Rotter et al. [73] 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 NH₃ formation in the syngas [87]. Due to a high percentage of plastics in RDF, the high amount of volatile matter in RDF leads to the formation of tars [68]. 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 [38]. Therefore, efficient gas cleaning is recommended to reduce the amount of these impurities in the syngas [33] [38] [83] [87]. The tar causes the blockage of the reaction vessel [88] [89] and the reduction of the calorific value and the syngas yield [80]. 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 [2]. Nevertheless, high plastic content in RDF can generate a high temperature and damage the gasifier [65].

The co-gasification of RDF and biomass is a promising pathway to produce 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 [91]. Indeed, co-gasification with RDF is also beneficial for biomass because this technology has the potential to reduce ash melting at high gasification temperatures [69]. 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 [4]. At the laboratory scale, co-gasification of coal/biomass mixtures is promising compared to individual gasification of coal and biomass [93] [94]. 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 H₂, CO, CO₂ and CH₄ [40] [94] [95]. 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 [40] [94]. 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, H₂, hydrocarbon and tar content of the syngas [94]. 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 [95].

• The gasification of coal is associated with the production of tar, which constitutes 15% to 20% of the total production of energy [83], and with the formation of NH₃ and H₂S because of the presence of S and N in the coal. Then, the presence of SiO₂ in the coal participates in the reduction of the tar, the SiO₂ acts as an in-situ catalyst [94].

• 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 [40].

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 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 [96]. According to Goncalves et al. [96] 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 H₂ content of the syngas respectively. As in the case of gasification, higher temperature and ER favour the syngas yield during co-gasification [69]. Saleh et al. [24] 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 H₂ production. Additionally, two-stage gasification, i.e. the use of two different reactors, is an efficient way of producing H₂ 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 [13], fluidised bed plasma gasification of solid waste is a promising technology. By combining a fluidised bed gasifier with a plasma gasifier, Ray et al. [15] 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 H₂ and to obtain a tar-free syngas [38].

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 [38]. In fact, the catalyst enhances the production of H₂ and CO and decreases the production of tar, coal, CO₂, C₂H₄ and C₂H₆ [26]. Nickel is the catalyst generally used [70] [97]. The Nickel-based catalyst ensures the elimination of tar, accelerates the methane reforming reaction and the gas-to-water conversion reaction [80].

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 [13] [80], gasification and pyrolysis chars [88] and alkaline minerals in gasification bottom ash [18] 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 [43]. 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 [89]. Moreover, the use of olivine as a bed material is effective in removing tar [4] [80] [93]. The latter acts both as a heat carrier, catalyst, and increases gas production [4]. Figure 1 shows a schematic diagram of input and output of the gasification system. The

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.