Available at: https://digitalcommons.calpoly.edu/theses/1516
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Anaerobic digestion can be used to decrease the mass of organic wastes to be disposed of while producing useful biogas (CH4 and CO2) 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 H2 gas with CH4 at a volume ratio of 15:85 H2:CH4, which allows burning at ultra-lean air-to-fuel ratios. For biogas systems, a potential low-cost NOx control strategy is to produce H2-CH4 mixtures through two-phase anaerobic digestion, where two digester tanks are operated in series, with the first one producing a majority H2 and the second CH4. The resulting mixture of H2, CH4, and CO2 should combust with low NOx emissions. Furthermore, in theory, if the biogas from the second-phase is sparged through the first-phase, H2 would be stripped from the first-phase liquid medium, and H2 production would be more thermodynamically favored, possibly increasing H2 production.
Laboratory experiments were used to determine the optimal conditions to generate biogas with a 15:85 H2:CH4 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 H2, (2) determine the sparging rate required to achieve 15:85 H2:CH4 in the biogas, and (3) operate the first and second-phases together with second-phase biogas being sparged through first-phase medium to achieve 15:85 H2:CH4. The results from each of these objectives are described below.
(1) The optimal conditions for H2 production in the first-phase were an organic loading rate of 22.9 g COD/L-day (chemical oxygen demand) and a hydraulic residence time of 12 hours. The resulting pH in the first-phase was 6.11 when operated under these conditions. Optimized hydrogen production in the first phase resulted in the generation of 1.02 ± 0.13 L H2/Ldigester-day, which can also be expressed as 0.61 ± 0.10 mol H2/mol glucoseconsumed, 0.42 ± 0.06 mol H2/mol glucoseintroduced, 1.06 ± 0.16 mol H2/mol CODdestroyed, and 0.06 ± 0.01 mol H2/mol CODintroduced.
(2) Initial sparging experiments were conducted using nitrogen (N2) to represent second-phase biogas. The rates tested ranged from 1- 30 L N2/Lfirst-phase digester-hr. A 1.1 L gas/L-hr sparging rate was projected to result in a 15:85 H2:CH4 ratio. The projection was made using a power regression model (R2 = 0.99) of sparging rate vs. hydrogen content results, assuming the sparged N2 was replaced with typical biogas (60% CH4 and 40% CO2).
(3) When both phases were integrated, the second-phase produced enough gas to sparge at only 0.28 L gas/Lfirst-phase digester-hr, which was far less than the optimal 1.1 L gas/Lfirst-phase digester-hr sparging rate. A non-optimal H2:CH4 ratio of 15:12 was obtained at the 0.28 L gas/L-hr sparging rate. Insufficient CH4 was generated due to the low organic loading provided to the second-phase.
Although the 1.1 L gas/L-hr sparging rate was not tested in an integrated system, the results obtained from the 0.28 L gas/L-hr sparging rate differed from what was predicted by the nitrogen sparging model by only 14%. Therefore, the model was fairly accurate (at least at a low flow rate of 0.28 L gas/L-hr) and could still be valid for the predicted optimal flow rate of 1.1 L gas/L-hr.
For future two-phase digestion studies, biogas production from the second-phase can be increased by adding more substrate to the second-phase or by using fixed-film digesters to possibly increase the number density of methanogens. It is also recommended to digest practical waste feedstocks, and possibly digest different feedstocks in the first and second-phases. Also, the effects of carbon dioxide on the combustion characteristics and NOx emissions of hydrogen-methane mixtures in biogas need to be researched.