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Energies 2020, 13, 420 38 of 96 Rotating-cone pyrolysis reactors employ centrifugal force to press feedstock particle against reactor walls [498,499]. Finally, in vacuum pyrolysis reactors the feedstock is pyrolyzed under vacuum conditions (3.5–10 kPa) and do not require a carrier gas. The heating rate and the residence time are slow; therefore, the solid yield is high [500,501]. 4.3. Gasification The gasification process thermochemically converts char and heavier hydrocarbons into gas or liquid with a lower molecular weight, such as hydrogen and carbon monoxide. The gasification reaction is an endothermic process that requires high temperatures (higher than 1200 ◦C). Typically, the energy required to sustain the process is provided by partial oxidation of solid pyrolysis products (mostly char) in a medium (oxygen, air, steam, hydrogen or combinations of these). The partial oxidation involves fuel consumption, either fossil fuel for allothermal gasification or up to 30% of biomass feedstock in autothermal gasification [502]. A viable alternative to the partial combustion is represented by sensible solar heat concentrated to a gasifier fed by only steam [503]. Avoiding the partial oxidation, the product syngas is not contaminated by combustion products such as NOx and SOx, CO2 emissions are erased, and biomass of fossil fuels are not consumed to enhance the reactions [504]. Moreover, the calorific value of the product gas is improved due to the amount of energy provided by the solar irradiation [505]. The biomass can be heated either directly in a cavity reactor configuration. Still, tar, coke or ash formation could foul the glass window or indirectly with an intermediated tubular absorber that conducts the heat to the feedstock but is affected by higher losses [506]. When solar irradiation is absent, the gasification operates with inert gas recycle loops [502]. The gasification medium affects the energy content of the product gas: air holds a high nitrogen fraction that dilutes the product lowering the heating value (4–7 MJ/Nm3), the withdrawal of pure oxygen is cost-effective, but the lower heating value is 10–18 MJ/Nm3. Steam enhances the hydrogen content (higher H/C ratio), and the lower heating value is 10–14 MJ/Nm3, but the water gas reaction is endothermic and requires high temperature [507]. The addition of oxygen to the steam medium provides the heat needed by endothermal reactions though partial oxidation that also enables drying and pyrolysis process. Hydrogenation that results in a product gas with a lower heating value of 40 MJ/Nm3 and catalytic steam gasification is also used [508]. 4.3.1. Gasification Process Main gasification reactions involve carbon, carbon monoxide, carbon dioxide, hydrogen, water and methane. The overall gasification reaction may be summarized by the biomass devolatilization equation [509]. Solar-assisted gasification is fed by steam. Therefore, only reduction reactions (Equations (55)–(62)) occur [510]. CxHyOz(pyrolysis products) + O2 + H2O → CH4 + CO + CO2 + H2 + H2O + Char + Tar (48) In the partial oxidation or combustion zone, heterogeneous reactions occur between pyrolysis products, generating carbon dioxide and water vapor [511,512]. C + 12 O2 → CO −394kJ/mol Partial oxidation reaction C + O2 → CO2 −394kJ/mol Complete oxidation reaction CO + 21 O2 → CO2 −284kJ/mol CO oxidation reaction (49) (50) (51)PDF Image | Green Synthetic Fuels
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