| Summary: | 碩士 === 逢甲大學 === 綠色能源科技碩士學位學程 === 103 === Hydrogen partial pressure is one of the very important environmental factors for fermentative hydrogen production. This study used the anaerobic granular sludge bed (AnGSB) to investigate the effects of reduced pressure conditions on fermentative hydrogen production in order to find the optimal pressure to enhance the efficiency of hydrogen production.
First, a continuous flow stirred tank reactor (CSTR) was used to explore the effect of substrate concentration (16.8, 20, and 25 g COD/L) on hydrogen production under the condition of HRT (hydraulic retention time) 4 h. The results showed that the amount of substrate utilization is very close for three tests, being about 15.9-17.5 g COD/L. Although the best H2 yield (HY, 2.73 mol H2/mol hexose) occurred at sucrose concentration 25 g COD/L, the substrate utilization efficiency was only 69.5%. It implied that increasing cell concentration could more improve hydrogen production performance at low HRT.
An AnGSB system was used to conduct the fermentational hydrogen tesy at various pressure (760-190 mmHg) under the conditions of sucrose concentration 25 g COD/L and HRT 4 h. The results show that the hydrogen production performance increased with pressure decrease; however, the granular sludge was washout at 190 mmHg due to rapid vacuum pumping, resulting in hydrogen production rate (HPR) gradually decrease. Therefore, It suggested that 380 mmHg was appropriate operation pressure. The HPR, HY, and overall hydrogen production efficiency (HPE) was 1.35 mol/L/d, 1.98 mol H2/mol hexose, and 43.0%, respectively. Relative to the test at normal pressure (760 mmHg) operation, the HPE raised only 7%. Therefore, the hydrogen fermentation at sucrose concentration 20 g COD/L, HRT 4 h and 380 mmHg was conducted. The results showed that HY and HPE was 2.42 mol H2/mol hexose and 52.2%, respectively; relative to the reduced test of sucrose concentration 25 g COD/L, the hydrogen production performance increased about 21%.
Under both sucrose concentration 20 g COD/L and 380 mmHg, the fermentation tests with AnGSB system at different HRT (4-0.75 h) were carried out. The results showed that the best hydrogen production performance were occurred at HRT 4-2 h, obtaining a HPE of 49-53%. Relative to the test of atmospheric pressure (760 mmHg), the HPE increased 12-21%. On the other hand, lactic acid content in soluble microbial products (SMP) at HRT 1-0.75 h was clearly high (approximately 37.0%). This implied that the metabolic pathways shifted from butyric acid to lactic acid, resulting in hydrogen production performance decrerse. In addition, we found that granular sludge formation usually accompanied generation of high concentration EPS (extracellular polysaccharides) and was unfavorable to hydrogen production; the EPS/VSS ratio was up to 0.92 g EPS/g VSS. This problem could be improved by proper stirring or reflux to reduce EPS generation.
An agitated granular sludge bed system (AGSB) installed stirring blades was used to explore the effect of stirring rate (30, 45, 100, and 125 rpm) on hydrogen production at sucrose concentration 20 g COD/L, HRT 4 h, and 760 mmHg. The results showed that lower stirring rate favored formation of granular sludge, but accumulation of mass granular sludge promoted EPS production, resulting in change of metabolic pathways and decrease of hydrogen production. The best hydrogen production performance occurred at a stirring rate of 125 rpm, the HPR and HPE was 0.808 mol/L/d and 32.3%, respectively.
Finally, using upflow reactor (UFR) explored the effect of reflux ratio (5, 10, 20, 30 and 1990) on hydrogen production under the condutions of sucrose concentration 20 g COD/L, HRT 4 h, and 760 mmHg. The results showed that the hydrogen production performance was poor when reflux ratio was 5-30, and the HPE was 14-21%. However, the HPE was 43.2% when reflux ratio was 1990. The major reson that high reflux ratio could avoid EPS formation and metabolic pathway shifting from butyric acid fermention to ethanol fermentation.
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