4.3 Inhibitory Compounds

In general, the syngas mixture contains constituents such as ethylene (C2H4), ethane (C2H6), acetylene (C2H2), tar, ash, char particles, and gases containing sulfur and nitrogen (Ahmed et al., 2006). These impurities impair the fermentation process through scale forma­tion in the pipes/joints, and inhibition of microbial catalysts and enzymes resulting in low cell growth and product yield. Datar et al. (2004) reported a cell dormancy, hydrogen uptake shutdowns, and a shift in metabolic pathways from acidogenesis to solventogenesis and vice versa, when the syngas was used without conditioning. Introduction of a 0.025-p. m filter to remove tar, ash, and other particulate matter from the biomass-derived producer gas was able to overcome cell dormancy (Ahmed and Lewis, 2007). Further, the authors reported that nitrous oxide (NO) was found to be a potential inhibitor of hydrogenase enzyme activity, which reduced the available carbon for product formation. In order to eliminate the inhibitory effects of NO, some studies suggested to improve the gasification efficiency or to scavenge NO by chemicals including sodium hydroxide, potassium permanganate, or sodium hypo­chlorite. In a separate study, Klasson et al. (1993) reported that the growth of C. ljungdahlii was not significantly affected by H2S concentrations as high as 5.2% (v/v).

It is evident that the biomass-generated syngas has inhibitory compounds that have adverse effects on syngas fermentation efficiency. Some of these impurities can be reduced by biomass pretreatment. Turn et al. (2003) reported that the fuel characteristics of sugarcane
bagasse could be improved by pretreatments including milling and leaching. The authors reported a reduction of N, S, and Cl content of the sugarcane bagasse by 13%, 36%, and 62% from their ultimate analysis values (% dry basis) of 0.48%, 0.22%, and 0.65%, respectively. By combining the pretreatments, milling-leaching-milling, the authors reported a further reduction in N, S, and Cl contents by 27%, 82%, and 94%, respectively, from their initial contents. These pretreatments can be implemented to other lignocellulosic biomass feedstocks in order to reduce the production of nitrogen and sulfur compounds during gasi­fication. Takara and Khanal (2011) introduced a new concept of wet or green processing of biomass for upfront juice extraction for coproduct generation, and the utilization of clean fiber for biofuel production. The authors reported the elimination of nitrogen compounds from the biomass to provide clean biomass feedstock for thermochemical conversion.