Environmental and experimental evaluation of producing chemicals from CO2 using bioelectrochemical systems

Abstract

Microbial electrosynthesis (MES) which uses microbes and electricity to generate high grade chemicals could contribute to the reduction of greenhouse emissions as it uses CO2 in the process. The implementation of this technology on an industrial scale could be on the horizon. Currently, little is known about the environmental loads associated with the successful scale up of the technology with regards to global warming potential and other environmental burdens. Such knowledge is needed in order for relatively new bioprocesses like MES to be sustainably scaled up and industrially applied. This research conducted an empirical and environmental investigation of MES for the synthesis of chemicals from CO2. Experimentally, MES for bio production of chemicals from CO2 was investigated using mixed culture as biocatalyst. CO2 introduced into H-shaped bioelectrochemical systems produced methane, formic, acetic and propionic acids more readily however under some conditions isobutyric acid and ethanol were synthesized. Different polarizations (-0.8V, -1.0V, -1.2V and -1.4V vs Ag/AgCl) and temperatures (27oC and 40oC) were used revealing that bioproduction was affected by changes to these parameters. Biofilm growth and gradual acclimation to CO2 achieved a maximum production rate of 3677μM/day at -1.4V vs Ag/AgCl and 40oC. However an average decline of 18 percent in the coulombic efficiency was observed when the potential was reduced by 0.2V.This showed that there may be energy and environmental risks associated with products synthesized at lower potentials needing confirmation by an environmental analysis. The environmental impacts of products synthesized through MES were examined by modelling a simulated industrial plant (1000 tonnes/year). Environmental analyses were used to reveal the main products to target for MES. Different MES plants generating a range of biochemicals were modelled considering two sources of energy (natural gas and UK national grid), one at a time. This gave specific and detailed scenarios that allowed comparison of the environmental impacts. Results shows that the synthesis of acetic acid, propionic acid, ethanol and methanol released more carbon dioxide than it used for both natural gas and the UK national grid. However, formic acid (-3,421 tonnes CO2 eqv) was found to be the only product having a negative global warming potential using natural gas and comparatively low environmental impacts in other environmental categories. It was concluded that formic acid synthesis through MES is a more suitable product than the other biochemicals analysed in terms of energy efficiency, global warming potential and other potentially harmful environmental impact categories.