277499 Hydrogen Production From Fermenting Mixtures of Pentose and Hexose Sugars Produced From the Steam Explosion of Switchgrass
Increasing demand for fossil fuel supplies, international conflicts, pollution, energy security and greenhouse gas emissions have caused heighten concerns for industrialized and non-industrialized nations to develop alternative energy sources. Hydrogen (H2) is emerging as a potential contender because it is CO2 neutral if it is produced from agriculture products and it has a high energy yield.
Among the different H2 production technologies, the dark fermentation route presents considerable advantages because the process is robust, it can operate under moderate temperature conditions, a wide variety of low value agriculture residues can be utilized and feedstock sterilization is not required (Gomez et al., 2011). During dark fermentation, H2 methane and carbon dioxide are the ultimately end products. Theoretically, only 33% of the electron equivalents from a hexose sugar source is transformed into H2 assuming acetate is the only reduced carbon byproduct. This is equivalent to 4 mol×mol-1 glucose or 0.467 L H2×g-1 COD. The remaining 66% is diverted into the formation of the volatile fatty acids (VFAs) and alcohols (Bartacek et al., 2007).
Low H2 yields are attributed to the presence of H2 consuming microorganisms (Weissman and Benemann, 1977). Increasing the H2 yield can be accomplished by using microbial stressing agents such as heat, pH, chemicals as well as engineering design parameters to suppress or control the growth of H2 consumers (Terentiew and Bagley, 2003). Previous studies have shown that pH, hydraulic retention time (HRT) and culture treatment are important factors which can affect the H2 yield (Lay, 2000; Ueno et al., 1996). Long chain fatty acids (LCFAs) have been shown to reduce the growth of H2 consumers. According to Cata Saady et al. (2012), linoleic acid (LA; (C18:2)), a H2 consumer inhibitor, is able to increase the H2 yield.
Hydrogen production from simple sugars such as glucose has been demonstrated in continuous flow reactors (Hafez et al., 2009). However, using feedstocks containing pure sugars is not a practical approach because of our dependence on agriculture food products to produce fuels and the financial implications related to the supply and demand of food products to consumers. In comparison, H2 production utilizing feedstocks consisting of hexoses plus pentoses from low value lignocellulosic agriculture residues will likely contribute to a sustainable H2 economy and solve issues related to the over use of agriculture food products. A fast growing crop such as switchgrass (SG) with low nutrient requirements (McLaughlin et al., 1999) is considered to be a suitable energy crop in North America and its production is likely to increase several fold (Sokhansanj et al., 2009). In this study, we examined H2 production from a hemicellulose rich fraction generated from the steam explosion of SG. Hydrogen production from SG was assessed using mixed anaerobic cultures in an up-flow anaerobic sludge blanket reactor (UASBR) under varying pH and HRT conditions with and without a methanogenic inhibitor (LA).
The raw material SG was steam exploded at 190 oC for 10 min under high pressure conditions. The steam exploded liquor was hydrolyzed with 2.5 % of H2SO4 to produced monomeric sugars. The steam explosion reaction conditions were optimized to minimize the formation of furans. The liquor was withdrawn from the reaction vessel and treated with a resin (Amberlite XAD-4, Rohm and Haas, PA) to remove microbial inhibitors such as furan derivatives (furfural and 5- hydroxymethyl furfural) (Weil et al., 2002). This treated liquor was used as a fed to the reactor at a concentration of 5 g COD.L-1.
Experiments were conducted in UASBRs using mixed anaerobic cultures obtained from a wastewater treatment facility located at a brewery in Guelph, Ontario. The UASBRs were operated in continuous mode and at 37 oC. All experiments were conducted in duplicate. Hydrogen and methane were analyzed using a gas chromatograph (GC) configured with a thermal conductivity detector (TCD). Volatile fatty acids (VFAs) and alcohols were measured using an ion chromatograph (IC) (data not shown). The GC and IC methods were performed in accordance with Cata Saady et al. (2012). The COD for the influent and effluent products were measured according to Greenberg et al. (1998). The microbial communities were characterized using nested-PCR of16S rRNA gene using terminal restriction fragment length polymorphism (T-RFLP). Details of the protocol are described by Chaganti et al. (2012).
The pretreated SG consisted of approximately 60% pentoses and 40% hexoses. The major sugars present were xylose and glucose in the liquid hydrolysate. Hydrogen production from SG hydrolysate was investigated using mixed anaerobic cultures in the UASBRs under continuous operational conditions. A maximum H2 yield of 0.31±0.01 L H2.g COD-1 corresponding to 67% of theoretical yield was obtained at pH 5.0 and an HRT of 12 h at an LA concentration at 2,000 mg.L-1. At an HRT at 12 h, decreasing the pH from 7.0 to 5.0, compared with control (without LA addition), the LA treated cultures showed an increased in H2 yield to approximately 65% (Fig. 1 a). Whereas when decreasing the HRT from 16 to 8 h at a pH of 6.0 compared with control, a 77% increase in the H2 yield was observed in the LA treated cultures (Fig. 1 b). Among the factors examined, pH and inhibition of the culture was more pronounced when compared to the HRT. The VFAs (filtered), alcohols (filtered), plus gas COD of the effluent, accounted for approximately 80-100% of the influent COD. Archaea (Methanosarcina sp and Methanobacterium sp.) were dominant at pH 7.0 without any inhibitor; however, when reducing the pH and increasing the LA concentration, methanogens were washed out and Clostridium sp were dominant. Decreasing the HRT from 16 to 8 h caused a reduction in growth of homacetogens such as Bacteroides sp., and Morella thermoaceticum. These results show that the H2 yield increases as the quantity of H2 consumers are reduced or suppressed in the bioreactors.
Switchgrass is a potential low value biomass which could be used to produce H2 using mixed anaerobic cultures. UASBR operational parameters (HRT and pH) and a methanogenic inhibitor (LA) can be manipulated to increase the H2 yield. Elevated H2 yields were observed when the cultures were inhibited with LA, the pH was decreased to 5.0 and the HRT was reduced to 8 h.
Figure 1: Hydrogen yield from steam exploded switch grass liquor
(a) at constant HRT (12 h) (b) at constant pH (6.0)
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