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

<p>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.</p> <p>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.</p> <p>(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/L_digester-day, which can also be expressed as 0.61 ± 0.10 mol H₂/mol glucose_consumed, 0.42 ± 0.06 mol H₂/mol glucose_introduced, 1.06 ± 0.16 mol H₂/mol COD_destroyed, and 0.06 ± 0.01 mol H₂/mol COD_introduced.</p> <p>(2) Initial sparging experiments were conducted using nitrogen (N₂) to represent second-phase biogas. The rates tested ranged from 1- 30 L N₂/L_{first-phase digester}-hr. A 1.1 L gas/L-hr sparging rate was projected to result in a 15:85 H₂:CH₄ratio. The projection was made using a power regression model<strong> </strong>(R² = 0.99) of sparging rate vs. hydrogen content results, assuming the sparged N₂ was replaced with typical biogas (60% CH₄ and 40% CO₂).</p> <p>(3) When both phases were integrated, the second-phase produced enough gas to sparge at only 0.28 L gas/L_{first-phase digester}-hr, which was far less than the optimal 1.1 L gas/L_{first-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.</p> <p>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.</p> <p>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.</p>

Name thesis:degree_name

MS in Civil and Environmental Engineering

Discipline thesis:degree_discipline

Civil and Environmental Engineering

Year dc:date.available

2015

Author dc:creator

• Olivas, Nathaniel Manuel

Contributors dc:contributor

• Tryg Lundquist

• anaerobic
• digestion
• two
• phase
• NOx
• biogas
• Environmental Engineering

Identifier

10.15368/theses.2015.165

OAI identifier oai:identifier

oai:digitalcommons.calpoly.edu:theses-2670