Fuels via Thermal Biomass Conversion

The low melting point and high surface mobility of NiS also accelerate the sintering process of Ni crystallites. Since the formation of NiS is exothermic, activity loss can be partially recovered by raising the reaction temperature, which, however, also accelerates thermal degradation of the catalyst and increases carbon formation through cracking reactions. 

In the specific case of biomass gasification, several alkaline salts and heavy metals and metal oxides particles may act as additional poisons by enhancing the sintering of the Ni crystallites or by being adsorbed on the Ni sites [44]. While acid supports such as alumina react with alkali to form crystalline phases, basic supports (like MgO) do not react directly with them however, alkali causes coverage of the surface and plugging of the pores. 

Another cause of activity loss is carbon deposition, which can be avoided if a high steam to carbon (S/C) ratio is employed [45, 46], However, economic evaluations indicate that the optimum S/ C ratio tends to be low. The presence of tars in the reforming reactor enhances coking and it is the main cause of carbon formation in reforming a gas from biomass thermal conversion [29]. 

Depending on the reason for converting the produced gas from biomass gasification into synthesis gas, for applications requiring different H2/CO ratios, the reformed gas may be ducted to the water-gas shift (WGS, Reaction 4) and preferential oxidation (PROX, Reaction 5) unit to obtain the H2 purity required for fuel cells, or directly to applications requiring a H2/CO ratio close to 2, i.e., the production of dimethyl ether (DME), methanol, Fischer-Tropsch (F-T) Diesel (Reaction 6) (Fig. 7.6). 

Since CO acts as a poison to the proton exchange membrane fuel cell in the 50 ppm range, it has to be removed before feeding the H2 enriched gas to the fuel cell. This CO removal occurs in the PROX reactor, where Pt/Al203 catalysts are common, even though some interest in Au-based catalysts is growing due the lower cost of the active phase [48]. 

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