## 5 Thermochemical Conversion of Carbonaceous Feedstocks
### Gasification  
Biomass gasification represents a way to increase the use of biomass for energy production, permitting widespread use of biomass [109; 110; 111]. It consists of the conversion of solid/liquid organic compounds, in which partial oxidation of the carbon in the feedstock occurs, leading to the formation of a gas/vapor phase and a solid phase: The gas phase, otherwise called synthesis gas or syngas, can be used for power generation or can be upgraded for biofuel production. The solid phase (char) includes unconverted organic fraction and ash. During the gasification process, the carbonaceous biomass is converted into syngas, a mixture consisting mainly of H\({}_{2}\), carbon monoxide (CO), CO\({}_{2}\), and methane (CH\({}_{4}\)). This is generally carried out in the presence of a suitable gasifying agent or oxidants, such as air, oxygen, steam, or carbon dioxide. In this process, four main steps can be distinguished:  
* Oxidation (exothermic);
* Drying (endothermic);
* Pyrolysis (endothermic);
* Reduction (endothermic). The classification of gasification is based on several parameters, such as the types of gasifiers, gasification temperature, heating (direct or indirect), and gasifying agent. According to the biomass, gasification technology, and operating conditions [112; 113; 114; 115], the Lowest Heating Value (LHV) of syngas can vary from 4 to 13 MJ/Nm\({}^{3}\), while the LHV of char can range from 25 to 30 MJ/kg. Figure 6: Schematic diagram of the methane steam reforming process, adapted from Ref. [108]. Gasification reactors are classified into three main categories [116]: cross-flow reactors (>50 MW\({}_{\text{th}}\)), fluidized-bed reactors (5-100 MW\({}_{\text{th}}\)), and fixed-bed reactors (100-10,000 kW\({}_{\text{th}}\)). The former requires a very high temperature (2000 \({}^{\circ}\)C [116]), pure oxygen, and steam as a gasifying agent in addition to high-quality fuels, so this category does not seem to be the best solution for coupling with MS technologies. Regarding fluidized-bed reactors, which are certainly effective when high exchange coefficients are required and where the fluidization fluid coincides with the heat transfer fluid, the associated technological difficulties and high costs make it convenient for CSP to apply this solution only to large plants (>100 MW\({}_{\text{th}}\)). In addition, since fluidized-bed reactors have an operating temperature of 700-800 \({}^{\circ}\)C throughout the reactor, it would not be feasible to combine them with a commercial CSP plant, which can only supply heat up to a maximum temperature of 550 \({}^{\circ}\)C (using MS as the HTF). Fixed-bed reactors, on the other hand, work at temperatures between 800-1400 \({}^{\circ}\)C, with low reaction rates, and are the most common (78%) [116]. Briefly, reactors in this category can be classified into three types, differentiated by the modes of gas phase efflux: up-draft, cross-draft, and down-draft. The up-draft reactor (1-10 MW\({}_{\text{th}}\)) is the simplest type and has a maximum flow rate of about 4 t/h, with strict grain-size requirements (see Figure 7). Biomass enters from the top, while the gasification agent inlet is at the base of the reactor. Above the inlet, the gasification agent finds the grid where biomass that has already released volatiles is oxidized. In this zone, it is possible to exceed 1000 \({}^{\circ}\)C. This phase promotes both the formation of ash, which precipitates through the grate and is collected at the bottom, and the production of hot gases. Moving upward, heat is transferred to the zone where the reduction process takes place. Further up, the hot gases meet the pyrolysis zone, where volatile components are released, and then the drying zone, where the biomass is dehydrated.