Monday, April 30, 2007

Hydrogen production in a trickle bed reactor using extremely thermophilic bacteria

Johan W. Van Groenestijn1, Koen P.H. Meesters1, and Pieternal A.M. Claassen2. (1) TNO, P.O. Box 360, Zeist, 3700 AJ, Netherlands, (2) Wageningen UR, Agrotechnology and Food Sciences Group

Extreme thermophiles generally have higher hydrogen/glucose yield factors than mesophilic fermentative hydrogen producing bacteria. In thermophilic organisms metabolic pathways result in, in theory, 4 mol H2/mol glucose. However, hydrogen production by the species used, is inhibited at a high hydrogen pressure. To decrease the hydrogen concentration, nitrogen gas was used as a stripping gas in a trickle bed reactor with a continuous gas phase. In this reactor a biofilm grows on a packing material with a high surface area and a solution of sugar trickles down the bed. The bacteria in the biofilm take up the sugars and the hydrogen is easily transferred to the gas phase.

A 400 liter trickle bed bioreactor with 190 liter packing material was inoculated with a pure culture of Caldicellulosiruptor saccharolyticus, fed with a solution of sucrose and nutrients and operated for 2.5 months at a temperature of 73 degrees centrigrade and a pH of 6.5. It was able to produce 530 mol H2/m3 packing/day with a stoichiometry of 2.5 to 3 mol H2 per mol hexose converted. At high concentrations of sucrose, about equal amounts (weight) of acetic acid and lactic acid were produced, while considerably lower amounts of lactic acid were produced under substrate (sucrose) limitation. Microscopic observations indicated that the inoculated species was still dominant after 2.5 months.

A gas stream containing 4% H2 and 2% CO2 (v/v) in a bulk of nitrogen gas was produced, and methane was absent. In the project methods to use and/or purify this gas stream were developed.