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Energies 2019, 12, 809 22 of 41 The reactions (6)–(8) take place at high temperature and the final composition of the products is controlled also by the equilibrium of the water gas shift (WGS) reaction: CO+H2O↔CO2 +H2 ∆H=−41kJ/mol (9) Due to the high gasification temperatures, methane is the only byproduct that can be present in the products in any significant amount. An increased temperature reduces the methane production as well as the H2/CO molar ratio of the syngas by promoting the production of CO. High pressures reduce the amounts of CH4 and CO2. When gasification is optimized to provide a syngas mixture rich in CO and H2, temperatures above 1250 ◦C and pressures at 25–80 bar are usually used [199]. A syngas rich in H2 and CO is particularly useful for the synthesis of useful chemicals and fuels through the well-known Fischer-Tropsch (FT) process. The Fischer-Tropsch reaction was discovered in 1923 by Franz Fischer and Hans Tropsch, when they observed that CO and H2 may react to provide gasoline, diesel and other heavy distillate oils over iron (Fe) and cobalt (Co) catalysts [200]. Ever since the method has found success in various industrial applications for the production of useful chemicals and fuels from syngas produced by the gasification of coal or from the partial oxidation/steam reforming of natural gas. The technologies using gasification of coal and production of liquid fuels from syngas through Fischer-Tropsch synthesis are usually symbolized as XTL, where X refers to the raw material of the gasification process. Hence, one can discern the CTL, GTL, WTL and BTL technologies which produce liquid fuels from the Fischer-Tropsch synthesis by syngas produced by the gasification and/or steam reforming of coal (X = C), gas (X = G), wastes (X = W) or biomass (X = B). Fischer-Tropsch synthesis is a highly exothermic surface polymerization reaction which converts H2 and CO into saturated or unsaturated hydrocarbons of 1–40 carbon atoms over heterogeneous catalysts such as iron (Fe) or cobalt (Co). H2 and CO are absorbed on the surface of the catalyst and dissociate to form monomer units such as CH2 which represents a methylene group of a normal saturated hydrocarbon. The reaction proceeds with the surface polymerization of these groups into larger chains according to the scheme: 2nH2 + nCO → (CH2)n + nH2O ∆Ho250 ◦C = -185.8 kJ/mol for n = 1 (10) which eventually produces a crude oil mixture of saturated hydrocarbons, unsaturated hydrocarbons, alcohols and carbonyls according to the following reactions [201,202]: (2n + 1)H2 + nCO → CnH2n+2 + nH2O (11) 2nH2 + nCO → CnH2n + nH2O (12) 2nH2 + nCO → H(CH2)n OH + (n − 1)H2O (13) nCO + (2n − 1)H2→ (CH2)n O + (n − 1)H2O (14) At the same time, several other simultaneous reactions may occur such as the reversible WGS reaction (9) which forms CO2. The main products of the Fischer-Tropsch process are straight saturated hydrocarbons and unsaturated hydrocarbons (alkenes) through reactions (11) and (12), respectively, but the exact composition of the crude oil product depends on the H2/CO molar ratio, the thermodynamic conditions and the catalyst used on the reaction bed. According to the reaction (11) the production of one monomer CH2 requires approximately 2.1 mol of H2 and therefore a H2/CO molar ratio equal to 2.1. Fe catalysts promote the WGS reaction (9) and therefore allow the utilization of syngas with H2/CO molar ratios below 2.1 [203]. Co catalysts, on the other hand, inhibit the WGS reaction and as a result require H2/CO molar ratios between 2.06 and 2.16 [204]. The Fischer-Tropsch process usually takes place at 200–350 ◦C and 10–60 bar over Fe or Co catalysts. However, two technologies may be further recognized: (a) highPDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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