Two-Phase Anaerobic Digestion to Reduce the NOx Emission Potential of Biogas

Abstract

Anaerobic digestion can be used to decrease the mass of organic wastes to be disposed of while producing useful biogas (CH 4 and CO 2) for heat or power production, but in air basins with strict emissions limits, biogas combustion is difficult to implement due to the high costs of controlling NOx emissions. NOx production can be minimized by blending H 2 gas with CH 4 at a volume ratio of 15:85 H 2 :CH 4 , which allows burning at ultra-lean air-to-fuel ratios. For biogas systems, a potential low-cost NOx control strategy is to produce H 2-CH 4 mixtures through two-phase anaerobic digestion, where two digester tanks are operated in series, with the first one producing a majority H 2 and the second CH 4. The resulting mixture of H 2 , CH 4 , and CO 2 should combust with low NOx emissions. Furthermore, in theory, if the biogas from the second-phase is sparged through the first-phase, H 2 would be stripped from the first-phase liquid medium, and H 2 production would be more thermodynamically favored, possibly increasing H 2 production. Laboratory experiments were used to determine the optimal conditions to generate biogas with a 15:85 H 2 :CH 4 ratio using two phase digestion with glucose as the substrate. Specifically, the objectives of this thesis were to (1) determine the optimal conditions for operating the first-phase to produce H 2 , (2)