Cellulose-hydrogen production from corn stalk biomass by anaerobic fermentation

The study is reported in Issue 54(8) (April, 2009) of Chinese Science Bulletin based on its important academic significance as well as potential commercial and environmental benefits in the future.

Hydrogen is an ideal fuel, producing only water upon combustion, and has been deemed as one of the most promising carriers of new energy for the future. From the aspect of energy security and environmental protection, bio-H2 production from renewable crop straw wastes has been an exciting area of bio-energy production because of its environmentally friendly and energy saving process[1,2] . By far the majority of study, however, are confined to using pure carbohydrates and carbohydrate-rich wastewater[3,4] .

It is reported that the annual yield of natural cellulosic biomass in China exceeds 0.7 billion tons, in which the amount of corn stalk is around 220 million tons[5]. The bio-conversion of corn stalk into cellulose-hydrogen is challenging the scientific community because of their complex chemical structures and hard biodegradation. A little information is available on the cellulose-hydrogen production using corn stalk as feedstock so far[6,7].

From the above reason, the H2-producing characteristic of lesser panda as inoculum, pretreatment methods of substrate and verification tests were significantly investigated in the present work.

The hydrogen production potential of 89.2 mL/g-TS and 109.0 mL/g-TS was observed by cattle manure and lesser panda manure at the pretreated corn stalk of 15 g/L, initial pH 7.0 and 36℃, respectively. Based on the results, the pre-incubated lesser panda manure was employed as H2-producing micro-flora in this study.

As far as we know, the pretreatment of substrate played a vital role in the effective conversion of corn stalk into cellulose hydrogen by mixed culture. Thus, the influences of three pretreatment methods on the yields of soluble saccharides (SS) and H2 were particularly discussed at the fixed substrate of 15 g/L and initial pH 7.0, respectively. 1) Dilute acid pretreatment: The yields of SS and H2 were significantly dependent on the acid types and acid concentration. The maximum SS yield of 212 mg/g-TS at the lactic acid concentration of 0.4%, 343 mg/g-TS and 350 mg/g-TS at the HCl concentration of 1.0%, and H2SO4 concentration of 2.0% were observed, respectively. The maximum H2 yield of 125 mL/g-TS at 0.4% lactic acid 0.4%, 129 mL/g-TS at 1.0%HCl concentration, 151 mL/g-TS at H2SO41.5% concentration was observed, respectively.

Thereafter, with further increase of acid concentration, the trend was reversed. The results are consistent with our previous studies, in which higher anion concentrations of Cl− and SO42− inhibited heavily the growth of the hydrogen-producing bacteria and led to the decrease of bio-hydrogen production capacity; 2) Acid-enzyme coupling pretreatment: The anaerobic atmosphere is beneficial to the enzymatic hydrolysis of corn stalk and the hydrogen production. The yields of SS and H2 increased rapidly with the increase in the enzyme dosage from 348 mg/g-TS and 134 mL/g-TS at 1.1 IU/g to maximum 468 mg/g-TS and 165 mL/g-TS at 17.6 IU/g; 3) Solid bio-pretreatment: The process of the bio-pretreatment was accompanied by the generation of lactic acid, the microbe additive loading significantly effected the yields of SS and lactic acid in bio-pretreated corn stalk. An increase of the SS yield could be observed in the range of dosage 2.5 g/kg to 7.5 g/kg, the maximum SS yield of 212 mg/g-TS occurred at dosage 7.5g/kg, and then with further increase in microbe additive dosage the trend was reversed. Furthermore, the mechanism studies of hydrogen production from pretreated corn stalk indicated that the enhanced H2 yield related to the direct bio-degradation of the hemi-cellulose and cellulose besides the contribution of the generated SS and lactic acid in the bio-pretreated corn stalk during the bio-hydrogen fermentation.

The verification tests were performed in a 5 L continuously stirred anaerobic bioreactor (CSABR) with 3 L mixture at fixed bio-pretreated corn stalk of 15 g/L, pH 5.5, 36℃ and HRT for 10 h. The maximal H2 yield, H2 content and H2 production rate was observed at 175.6 mL/g-TS, 57.2% and 14.5 mL/g-TS•h−1, respectively. During the optimal bio-hydrogen production period, the ORP value stayed in the range of −445 mV to −455 mV, which was consistent with that in previous reports. During H2 fermentation progresses, butyric acid, acetic acid and alcohol as main metabolic by-products were left at the reactor, during which butyrate and acetate accounted for about 70-80% of VFAs, and there was no significant methane observed in the CSABR. The CSABR operated steadily for 170 h with higher H2 yield and lower H2 partial pressure level, and the pH value could be easily adjusted by online control system.

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