30 June 2009
Producing hydrogen from corn stalks
By using bio-pretreated corn stalk, it is now easier to produce cellulose-based hydrogen.
The beauty of hydrogen as a fuel is it produces only water upon combustion. Leaders have called hydrogen 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, said researchers at the Department of Chemistry, Zhengzhou University, China. By far the majority of study, however, has so far focused on using pure carbohydrates and carbohydrate-rich wastewater. 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. The bio-conversion of corn stalk into cellulose-hydrogen is challenging the scientific community because of their complex chemical structures and hard biodegradation, the researchers said. Little information is available on the cellulose-hydrogen production using corn stalk as feedstock so far.
Researchers looked at the pretreatment of how substrate could play a vital role in the effective conversion of corn stalk into cellulose hydrogen by mixed culture. Researchers investigated the influences of three pretreatment methods on the yields of soluble saccharides (SS) and H2 at the fixed substrate of 15 g/L and initial pH 7.0 and 36°C, respectively.
The three treatment methods:
Dilute acid pretreatment: The yields of SS and H2 were significantly dependent on the acid types and acid concentration. Researchers observed 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%. 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 occurred. Thereafter, with further increase of acid concentration, they were able to reverse the trend. The results are consistent with 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.
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.
Solid bio-pretreatment: The process of the bio-pretreatment combined with the generation of lactic acid, the microbe additive loading significantly affected the yields of SS and lactic acid in bio-pretreated corn stalk. An increase of the SS yield occurred 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 reversed. Furthermore, the mechanism studies of hydrogen production from pretreated corn stalk indicated 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 occurred 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°C, and HRT for 10 h. The maximal H2 yield, H2 content, and H2 production rate occurred 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 stayed 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 easily adjust via online control system.
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