Hydrogen Energy Recovery and Environmental Sustainability on Renewable Biomass System
Clean Technology 2008

Hydrogen Energy Recovery and Environmental Sustainability on Renewable Biomass System

R. Saravanane, B. Radjaram
Pondicherry Engineering College, IN

hydrogen, biomass, sustainability

Hydrogen is regarded as the fuel of the future. Conversion of hydrogen in fuel cells has high energetic conversion efficiency and water is the sole end-product. Presently, hydrogen is produced from fossil reserves with the concomitant release of anthropogenic carbon-dioxide. Therefore, new hydrogen production technologies are being developed, based on the utilization of renewable resources. Biological hydrogen production, i.e. the production of hydrogen from biomass by micro-organisms, is a common phenomenon. In swamps, in rumina of cows, and also in the well-known biogas installations, hydrogen is produced from biomass under anoxic circumstances. However, this hydrogen is being consumed as soon as it is being produced by methanogenic bacteria and methane is the end-product which becomes available. In this project, methane production is uncoupled from hydrogen production by installing a two-stage bioprocess. In the first stage hydrogen and acetic acid are produced from biomass by thermophilic bacteria. In the second stage, the acetic acid in the effluent is converted to hydrogen by purple non-sulfur bacteria. The main issues are (i) Conversion of biomass from an energy crop or an organic waste stream to fermentablefeedstock; Selection of thermophilic and photo-heterotrophic micro-organisms and design of optimal growth and production conditions; Design of an integrated bioprocess;Development of recovery and purification methods for upgrading the gas to fuel cell specifications. A large variety of thermophilic bacteria allow efficient hydrogen production from biomass. The metabolic pathways have been partially elucidated to enable careful scrutiny of the fermentation by monitoring simple parameters. Thermophilic fermentations have been done with pure substrates and with hydrolysates of energy crops and agro-industrial waste streams to establish the future applicability of the bioprocess. The off-gas from a thermophilic fermentation gas been tested, successfully, in a PEM fuel cell.The liquid effluent from the thermophilic fermentation has been fed to a photo-heterotrophic fermentation and was completely converted to hydrogen and carbon dioxide. The efficiency needs further optimisation.A packed bed reactor with a working volume of 150 L has been built for the thermophilic fermentation. A new hydrogen recovery system has been designed which has enabled continuous operation for several months. Wastewaters were obtained from four different food-processing industries that had chemical oxygen demands of 9 g/L (apple processing), 21 g/L (potato processing), and 0.6 and 20 g/L (confectioners A and B). Biogas produced from all four food processing wastewaters consistently contained 60% hydrogen, with the balance as carbon dioxide. Chemical oxygen demand (COD) removals as a result of hydrogen gas production were generally in the range of 511%. Overall hydrogen gas conversions were 0.70.9 L-H2/L-wastewater for the apple wastewater, 0.1 L/L for Confectioner-A, 0.42.0 L/L for Confectioner B, and 2.12.8 L/L for the potato wastewater. When nutrients were added to samples, there was a good correlation between hydrogen production and COD removal, with an average of 0.1 to 0.01 LH2 /g . However, hydrogen production could not be correlated to COD removal in the absence of nutrients or in more extensive in-plant tests at the potato processing facility. Gas produced by a domestic wastewater sample (concentrated 25) contained only 238% hydrogen, resulting in an estimated maximum production of only 0.01 L/L for the original, non-diluted wastewater. Based on an observed hydrogen production yield from the effluent of the potato processing plant of 1.0 L-H2/L, and annual flows at the potato processing plant, it was estimated that if hydrogen gas was produced at this site it could be worth as much as $61,000/year.