Feedstock and gasification

Due to the flexibility of the microbes to ferment syngas with diverse composition, virtually any carbonaceous materials can be used as feedstock for gasification. Non-food biomass that can be employed as feedstock for gasification includes agricultural wastes, dedicated energy crops, forest residues, and municipal organic wastes, or even glycerol and feathers [1620]. Biomass is available on a renewable basis, either through natural processes or anthropogen­ic activities (e. g. organic wastes). It has been estimated that out of a global energy potential from modern biomass of 250 EJ per year in 2005, only 9 EJ (3.6%) was used for energy gener­ation [18]. The use of existing waste streams such as municipal organic waste also differenti­ate itself from other feedstocks such as dedicated energy crops because these wastes are available today at economically attractive prices, and they are often already aggregated and require less indirect land use. Alternatively, gasification of non-biomass sources such as coal, cokes, oil shale, tar sands, sewage sludge and heavy residues from oil refining, as well as reformed natural gas are commonly applied as feedstocks for the FTP and can also be used for syngas fermentation [15, 21]. Furthermore, some industries such as steel manufac­turing, oil refining and chemical production generate large volume of CO and/or CO2 rich gas streams as wastes. Tapping into these sources using microbial fermentation process es­sentially convert existing toxic waste gas streams into valuable commodities such as bio­fuels. The overall process of gas fermentation is outlined in Figure 1.


Figure 1. Overview of gas fermentation process

Prior to gasification, biomass generally needs to go through a pre-treatment process encom­passing drying, size reduction (e. g. chipping, grinding and chopping), pyrolysis, fractiona­tion and leaching depending on the gasifier configuration [22, 23]. This upstream pre­treatment process can incur significant capital expense and add to the overall biomass feedstock cost, ranging from US$16-70 per dry ton [22]. Gasification is a thermo-chemical process that converts carbonaceous materials to gaseous intermediates at elevated tempera­ture (600-1000oC), in the presence of an oxidizing agent such as air, steam or oxygen [16, 22]. The resulting syngas contains mainly CO, CO2 H2 and N2, with varying amounts of CH, water vapour and trace amount of impurities such as H2S, COS, NH3, HCl, HCN, NO» phe­nol, light hydrocarbons and tar [17, 22, 24]. The composition and amount of impurities of syngas depends on the feedstock properties (e. g. moisture, dust and particle size), gasifier type and operational conditions (e. g. temperature, pressure, and oxidant) [17, 22]. Table 1 summarizes typical composition of syngas and other potential gas streams derived from various sources.

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