Syngas Fermentation and Microbial Electrosynthesis Process Integration to Advance Biogas Production

Abstract

Norway is implementing several CO2 emissions control measures to become a low-emission society by 2050. As a part of this great vision, it strives to be the first nation to end the sales of fossil-fuelled vehicles by 2025. However, heavy transport still relies on fossil fuels that can be substantially replaced by liquified biogas. The demand-driven biogas market volume is more extensive than the current supply volume. The feedstock availability and lower methane yield are the potential bottlenecks that hinder the complete substitution of biomethane as a clean fuel. This PhD project studies syngas fermentation and microbial electrosynthesis (MES) as two sustainable technologies to advance methane yield.

Fermenting syngas into a biogas reactor is an arduous process. Inefficient gas-liquid (GL) mass transfer of H2, the slow growth rate of the microorganisms that account for the fermentation, and lack of reducing equivalents are the foremost challenges. The original contribution of this PhD project to the knowledge is published as five open-access journal articles and a conference paper. Article 1 examines the effect of H2 partial pressure on syngas fermentation to overcome the GL mass transfer limitation. Article 2 explores the possibility of using moving bed biofilm to overcome the kinetic growth limitation of the slow-growing microorganisms. Articles 3, 4, and 5 investigate the impact of MES integration as a source of reducing equivalents for syngas fermentation; herein, the lowest benchmark potential for acetate synthesis and GL mass transfer are scrutinised. In addition, preliminary modelling work was performed in Modelica as an interface between the power electronic side and the biochemical side of the MES reactor, presented in a conference (Article 6).

Digested sludge from Knarrdalstrand WWTP-AD, Porsgrunn, was pre-treated to enriched homoacetogens and utilised as the inoculum for all the experiments. In articles 1 to 4, the syngas was mimicked as H2 in the headspace and CO2 in the liquid medium as bicarbonate salt, while an industrially relevant syngas mixture (15 % CO, 15 % H2, 20 % N2 in 50 % CO2) was used in article 5. Initially, the impact of 1 to 25 bar H2 headspace pressure on the homoacetogenic medium with 0.41 volatile and total solid ratio (VS: TS) was studied. The 15 bar was identified as the optimum headspace pressure. There 47.24 mmol of H2 was consumed at an optimum rate of 6.22 mol h-1 L-1, and 3.0 g L-1 of acetic acid was synthesised. In study two, moving bed biofilm was incorporated into a 15 bar H2 pressurised reactor. The biofilm integrated batch depicted a 33 % improvement in the H2 uptake rate (200 mmol L-1 d-1) and 48 % in acetate synthesis rate (37.4 mmol L-1 d-1). The stereo microscopic images affirmed the biofilm accumulation on the carriers at an average thickness of 160 μm.

In order to study the impact of MES on syngas fermentation (article 3), the experiment was performed in three phases. The fermentation was done with the suspended medium alone in the first phase. Then electrodes were integrated into the planktonic medium, referred to as a single-chamber MES reactor; therein, electrodes were not poised with any voltage in phase two (open circuit mode, OCM), while cathode was poised with negative potential in phase three (closed-circuit mode, CCM). In contrast to the planktonic fermentation, the biofilm assimilated on the electrodes in the OCM tripled the H2 (1021 mmol) consumption. Also, the gas uptake rate (13.5 mmol L-1 d-1) and acetate synthesis (57 mmol L-1) improved by 125 % and 63 %. An optimum potential (– 0.8 V vs. SHE) for acetate synthesis was adapted from previous USN studies to perform phase three. The study hypothesised that potential could trigger the H2 GL mass transfer and acetate synthesis. However, it failed because the applied potential (- 0.8 V) could have been deprived of its biotic nature; thus, the anodic potential exceeded the acetic acid oxidation potential, resulting in a rapid drop in concentration. Therefore article 4 attempted to determine the lowest benchmark potential for acetate synthesis and improved H2 GL mass transfer.

Study 4 was performed in two phases, OCM and CCM. The CCM examined the impact of reducing power from – 25 to – 175 mV vs. Ag/AgCl (3.0 NaCl) on the acetic acid synthesis and H2 GL mass transfer. Compared to the OCM, the -175 mV enhanced the acetate synthesis rate (0.225 mmol L-1 h-1) by 26 % and was identified as the lowest benchmark potential, while there was no improvement in H2 consumption. The direct electron transfer from cathode to microorganism avoided the H2 consumption from headspace. Article 5 is quite similar to article 4. However, the distinct advancement is that instead of pure H2 in the headspace, industrial relevant syngas has been fermented to move the technology readiness level forward. The experiments were performed in both open and closed-circuit modes. During the CCM, the impact of reducing power from – 50 to – 400 mV vs. Ag/AgCl (3.0 NaCl) was examined on acetic acid synthesis, syngas utilisation and CO inhibition. The – 150 mV was figured out as the lowest benchmark potential for the optimum acetic acid synthesis rate (0.263 mmol L-1 h-1), which is 15 times higher than the OCM rate. Sixty per cent of the CO from the headspace was consumed without any noticeable inhibition.

The overall PhD study concludes that elevated syngas pressure, MBB, and MES incorporation significantly enhanced acetate synthesis that can further be converted into methane into a biogas reactor, thus advancing biogas production.