Economic and empirical investigation of bioelectrochemical systems for CO₂ utilization

Abstract

This thesis investigates economically and empirically the feasibility of CO2 reduction into high grade chemicals using microbial electrosynthesis (MES). An economic evaluation was initially performed for MES and anaerobic fermentation (AF) for 100 tonnes per year (t/y) acetic acid production. MES and AF incurred high investment and production costs; however, integrating MES and AF decreased investment costs, doubled production rates, and set production cost at 0.24 £/Kg which is market competitive (0.48 £/Kg). Although integrating MES and AF processes showed to be cost effective, it generated no positive return across 15 years of operation. Similar analyses were used to evaluate MES as stand-alone process for the production of acetic, formic and propionic acids, methanol, and ethanol at higher production rates (1000 t/y). High returns were evaluated for formic acid (21%) and ethanol (14%) compared to the minimum requirements of the industry (11.60%) making these products economically attractive. Experimentally, volatile fatty acid bioproduction was investigated in H-shape bioelectrochemical systems using Shewanella Oneidensis MR-1 as biocatalyst and CO2 as a substrate on polarised carbon cloth electrodes. Biofilm and mediated driven systems were used to evaluate the influence of electron transfer on bioproduction. It was found that mediated systems (0.66 mmol/L) produced more volatile fatty acids than biofilm systems (0.53 mmol/L), suggesting that the use of mediators enhances electron transfer. Different polarizations (-0.2, -0.4, -0.6 and -0.8 V) were also evaluated on biofilm driven systems, revealing that volatile fatty acid production was not affected by polarization (p=0.192) and incurred low cathode capture (13-77%) and energy (0.0009-0.6%) efficiencies which suggests a biochemical process rather than respiration. This was later confirmed using extracted proteins from Shewanella Oneidensis MR-1 cells. The effects of operating conditions (i.e. temperature and agitation) and biofilm development technique: open circuit (OCP) and closed circuit (CCP) potential, were further assessed for energy production. It was found that energy production increased with high temperature (30 oC) and slow agitation (90 rpm), as reflected by higher current generation (median = 12.05 μA), more live cells number (median=2.3×106 cells), and better electrode bacterial coverage (median=35.29%). In addition, using OCP biofilms offered further advantages by reducing the lag phase (1-2 days). The effect of OCP and CCP biofilms operating at the best operating conditions found were then examined for chemical production. OCP and CCP biofilms resulted in the synthesis of different chemicals suggesting that the bacterial metabolism is dependent on the biofilm development conditions. These findings offer insights on MR-1 performance and reveal a bright opportunity towards the use and scale-up of MES for a technically and economically viable bioprocess.