Biohydrogen production using starch as the carbon substrate

• Main Authors: Ji-Fang Wu, 吳季芳
• Other Authors: Jo-Shu Chen
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/40463016341556532484

碩士 === 國立成功大學 === 化學工程學系碩博士班 === 94 === Hydrogen is a promising energy carrier of the future. Starch is well suited to act as a cost-effective substrate for biohydrogen production, since it is the second abundant organic resources (next to cellulose) from plants. Dark hydrogen fermentation from original starch encountered problems with poor hydrogen producing efficiency since hydrolysis of starch is often a rate-limiting step. Moreover, the soluble metabolites (e.g., volatile fatty acids, alcohols) produced during dark H2 fermentation required further treatment. Therefore, this study applied enzymatic hydrolysis step to hydrolyze starch and utilized hydrolyzed starch as the substrate of dark fermentation. Meanwhile, the soluble metabolites (e.g., volatile fatty acids) were decomposed by photosynthetic bacteria to produce more H2 via photo-fermentation. This three-step process allowed biological production of H2 from starch. This study investigates the effect of environmental factors on starch hydrolysis of an indigenous strain Caldimonase taiwanensis On1T to identify the optimal operation conditions. The tested temperature, pH, agitation rate, substrate concentration were 45-60oC, 5.5-9.5, 100-200 rpm and 10-50 g/L, respectively. The results show that the best starch hydrolysis efficiency occurred when temperature, pH, agitation rate, substrate concentration were 55oC, 7.5, 150 rpm, and 50 g/L, respectively, resulting in a reducing sugar production rate of 1.72 g/h/L. Moreover, the reducing sugar production appeared to increase as the starch concentration increased from 10-50 g/L, suggesting that there was no substrate inhibition during the range of starch concentration examined. The reducing sugar yield was 0.46-0.53 g reducing sugar per g starch. The original and hydrolyzed starch was used as the substrate to produce H2 through dark fermentation. In these batch H2 production experiments, five pure Clostridium isolates, namely, Cl. butyricum CGS2, Cl. butyricum CGS5, Cl. pasteurianum CH1, Cl. pasteurianum CH5, Cl. pasteurianum CH7, as well as mixed cultures were used. The results show that all of the pure and mixed cultures were able to utilize the hydrolyzed starch for H2 production, especially for Cl. butyricum CGS2 and Cl. pasteurianum CH5. The Cl. butyricum CGS2 strain exhibited the highest H2 production rate of 165.33 mL/h/L and a H2 yield of 6.40 mmol H2/g COD (overall H2 yield of 5.74 mmol H2/g starch). Cl. pasteurianum CH5 had the maximum H2 production rate of 175 mL/h/L and a H2 yield of 6.66 mmol H2/g COD (overall H2 yield of 6.36 mmol H2/g starch). For using original starch as the carbon substrate, the two Cl. pasteurianum strains did not produce H2, while H2 production was observed for Cl. butyricum and the mixed cultures. The latter gave a higher H2 production rate of 58 mL/h/L and a H2 yield of 3.14 mmol H2/g starch. Apparently, using hydrolyzed starch as the substrate led to much better H2 production efficiency. Furthermore, continuous H2 production from starch hydrolysate with Cl. butyricum CGS2 shows a H2 production rate of 0.51 L/h/L at HRT=12 h with a H2 content of 51%, H2 yield of 2.03 mol H2/mol glucose (10.56 mmol H2/g COD), and specific H2 production rate of 0.14 mmol/g VSS/d. In photo H2 fermentation experiments using a photosynthetic bacterium Rhodopseudomonas palustris WP3-5, acetate and butyrate were used as the carbon substrate. When acetate concentration was 3000 mg COD/L, the H2 production rate increased with a decrease in HRT. The best H2-producing performance took place at HRT=12 h, giving a H2 production rate of 42.05 mL/h/L and a H2 yield of 1.39 mol H2/mol acetate. Moreover, when using butyrate (2500 mg COD/L) as the carbon substrate, the H2 production rate did not increase with decreasing HRT. When HRT was 48 h, the H2 production rate and yield were 20.25 mL/h/L and 2.54 mol H2/mol butyrate, respectively. In summary, this study demonstrates the feasibility of bioH2 production from starch via a three-stage approach (i.e., enzymatic starch hydrolysis, dark fermentation, and photo-fermentation). More detailed and improved operation strategies will be identified to establish key technologies for the starch-to-H2 bioprocess.