ABSTRACT
However, the syngas produced by gasification contains impurities, depending on the type of feedstock and conversion technology applied. Typical are organic tar components and BTX (benzene, toluene, and xylenes), inorganic nitrogen, sulfur, and chlorine containing impurities such as NH3, HCN, H2S, COS, and HCl, as well as volatile metals (regarding biomass in particular Na and K), dust, char, and soot. Larger hydrocarbons summarized as tars being produced at low temperature gasification processes, reduce the primary syngas yield, may cause fouling of downstream equipment, coat surfaces, and plug pores in filters and sorbents. Other contaminants are corrosive or poisons to the catalysts in the subsequently following synthesis stages. For raw syngas cleaning, conventional technology is available. Tar constituents and BTX may be removed by either thermal or catalytic cracking, or by scrubbing with an oil-based medium followed by gasifier recycle. The other above mentioned impurities are removed by standard wet gas cleaning technologies. Using absorbing liquids like refrigerated methanol or amines, for example, by the Rectisol or Selexol process, CO2 and sulfur compounds are removed in separate fractions in a multistage process, resulting in a pure CO2 product and an H2S/COS-enriched Claus gas fraction suitable for sulfur production. In advanced “dry” hot gas cleaning, the residual contaminants are removed by chemical sorbent materials at elevated temperatures up to 800°C. In the case, that gasification occurs already at those pressures required in the subsequently following synthesis, dry pressurized gas cleaning is expected to enable significant energetic benefits. For biomass gasification, a variety of technologies exist. In view of the large-scale production of high-quality syngas, as required for synfuel production, mainly the types of gasifiers shown in Fig. 10.2 have to be considered here. Direct or indirect gasification using fluidbed and entrained-flow gasifiers is possible with potential plant capacities of up to several hundred megawatt of thermal fuel input capacity. In indirect gasification the energy required is provided by an external heat source, for example, by hot flue gas, while in direct or autothermal gasification thermal energy is provided by partial internal combustion under addition of an oxidizing agent to the fuel feed. Main differences between the two approaches are compiled in Table 10.1.
