Enhancement of hydrogen production from glucose by nitrogen gas sparging

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Abstract

The effect on hydrogen yield of N2 sparging was investigated in non-sterile conditions using a hydrogen-producing mixed culture previously enriched from soya bean meal. A continuous stirred-tank reactor (CSTR) at 35°C and pH 6.0 was operated on a mineral salts-glucose (10 g l−1) medium at a hydraulic retention time (HRT) of 8.5 h, and organic loading rate of 27.02 g glucose litre reactor−1 day−1. Results are reported from an 8 week period of continuous operation, and the enrichment culture gave stable results over an extended period. A hydrogen yield of 0.85 moles H2/mole glucose consumed was obtained after 5 HRT, the gas produced being 53.4% H2. With N2 sparging at a flow rate approximately 15 times the hydrogen production rate, the hydrogen yield was 1.43 moles H2/mole glucose consumed. The specific hydrogen production rate increased from 1.446 ml hydrogen min−1g−1 biomass to 3.131 ml hydrogen min−1 g−1 biomass under sparging conditions. It is suggested that hydrogen partial pressure in the liquid phase was an important factor affecting hydrogen yield. Energy could be recovered as hydrogen from processes generating volatile fatty acids for fine chemicals and liquid bio-fuels or from acidification reactors preceding normal anaerobic biological treatment of sugary wastewaters.

Introduction

Hydrogen gas is a clean fuel, producing H2O as its only by-product as it burns, not contributing CO2, NOx, sulphur or particulates to global atmospheric pollution. It has a high energy content per unit weight (122 kJ g−1) and thus would have considerable possibilities as a fuel if the cost were low enough. With the development of storage technologies (e.g. as metal hydrides) it can be a multipurpose fuel. For nations such as Japan which import petroleum-based fuels, research on hydrogen production is particularly significant, and has become a focus of governmental support Benemann, 1996, Ueno et al., 1996, Lay et al., 1999.

Hydrogen can be generated in a number of ways, for example through fossil fuel processing, or by electrolysis using solar power. However, these processes are energy intensive and therefore expensive. Biological production of hydrogen however is potentially more attractive, especially if wastewater or other biomass could be used as the raw material. The two biological routes to hydrogen production, photosynthetic and fermentative, have been extensively reviewed by Nandi and Sengupta (1998) and Benemann (1996). These and other workers (e.g. Yokoi et al., 1995) point to the advantages of the anaerobic fermentative route since H2 can be generated using less complex plant from a large number of sources such as refuse or waste products. Glucose or other carbohydrates are the preferred carbon source for the fermentations, which give rise to acetic and butyric acids together with hydrogen gas:C6H12O6+2H2O→2CH3COOH+2CO2+4H2C6H12O6CH3CH2CH2COOH+2CO2+2H2The effluent from fermentative hydrogen generation, rich in organic acids, could be further exploited; e.g. by methanogenesis.

From the ratios of acetic and butyric acids often formed, a hydrogen yield of approximately 2.5 mol H2/mol hexose degraded can be expected, or approximately 0.3 m3 kg−1 carbohydrate utilised, with about 60% H2 in the off-gas. However it is difficult to establish a high hydrogen yield because the amount of fermentation products is significantly influenced by various factors such as nutrient levels Bahl and Gottschalk, 1984, Dabrock et al., 1992, stirring (Lamed et al., 1988), and levels of carbon dioxide (Tanisho et al., 1998).

Hydrogen production by a wide range of bacterial species, both pure and mixed, defined, cultures grown on sterile medium, and undefined enrichment cultures grown in non-sterile conditions, has been reviewed by Nandi and Sengupta (1998). From an engineering point of view a process using a stable enrichment culture yielding hydrogen from non-sterile organic wastes is required. Roychowdhury et al. (1988) demonstrated hydrogen production by enrichment cultures from cane juice, corn pulp and saccharified cellulose, but not in continuous culture. Mizuno et al. (1997) reported hydrogen production from tofu manufacturing waste in batch culture from the culture used in the experiment reported here. Kalia et al. (1994) studied hydrogen production from damaged wheat grains by Bacillus licheniformis in continuous culture over a 40 day period, though low yields were reported The experiments of Ueno et al. (1996) on hydrogen production from sugar factory wastewater by a mixed microflora in chemostat culture are the most successful known to the authors. Operation at a hydraulic retention time (HRT) of 0.5 days for 20 days is reported, giving a good hydrogen yield.

Hydrogen partial pressure in the liquid phase is one of the key factors affecting hydrogen production. Many controversial observations regarding the influence of hydrogen gas on the anaerobic breakdown of saccharides have been reported (Ruzicka, 1996). Tanisho et al. (1998) on sparging with argon obtained an increase in residual NADH, which might be expected to give an increased hydrogen production, although the hydrogen production was not actually measured. These authors found the same effect on NADH when sparging with hydrogen, and attributed this to CO2 removal.

In this study we examined the effect of nitrogen sparging on hydrogen yield in a continuous culture of mixed anaerobic microflora operating on a glucose-mineral salts non-sterile medium.

Section snippets

Inoculum

The anaerobic microflora (predominantly Clostridium sp.) was obtained from fermented soybean-meal ESPRIT, 1989, Lay et al., 1999 and maintained in the laboratories of Tohoku University on a sucrose mineral salts medium in continuous culture at 35°C and a 10 h HRT at an uncontrolled pH of between 4.7 and 5.0, stirred by gas recirculation. After one week in transit to the UK at ambient temperature, the 50 ml culture was used to inoculate 500 ml glucose-mineral salts medium in a stoppered 1 l

Results and discussion

The culture obtained from Japan was easily grown up to form an inoculum despite 7 days in transit at ambient temperature. It recovered from a period of accidental washout in continuous culture, after which the reactor was re-inoculated from growth deposited in the exit U bend tube, recovering steady state biomass levels within 4 days. The culture operated at an 8 h retention time and was white in colour. Where biomass was stagnant anaerobically, a black colour appeared. While the culture is

Conclusions

The hydrogen-producing enrichment culture was extremely stable when grown at pH 6.0 and a HRT of 8.5 h, re-growing well after dormancy and accidental washout. A flocculant biomass of concentration 1.5 g l−1 dry weight was obtained in the CSTR without sparging, a non-flocculant biomass of a lower concentration resulting during sparging.

Hydrogen yields of 0.85 and 1.43 mol/mol glucose were obtained under non-sparging and sparging conditions, respectively; sparging with nitrogen resulted in a 68%

Acknowledgments

The authors wish to thank Dr. A.J. Guwy and Miss H. Forsey for their expert technical assistance, and the UK EPSRC for an equipment grant to DLH and FRH (GR/M38346).

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