Scalability of the Microbial Electro-acetogenesis Process for Biogas Upgradation: Performance and Technoeconomic Assessment of a Liter-Scale System
Moumita Roy, Mansi Saich, Sunil A. Patil
Abstract
Biogas is a promising sustainable alternative energy source, but its high CO 2 content reduces its energy density and calorific value, making it unsuitable for use as a fuel or for other applications. Its upgradation through CO 2 removal is thus required to increase its CH 4 content and application prospects. Microbial electrosynthesis (MES) is emerging as a promising technology for this purpose since it offers CO 2 utilization rather than a mere removal of this greenhouse gas. Though MES has been reported to be effective in upgrading biogas in small laboratory-scale reactors, its scalability and economic feasibility remain to be assessed. To this end, this study aimed to validate biogas upgradation via the microbial electro-acetogenesis process at different applied currents in liter-scale systems and assess its technoeconomic prospects. At a fixed current density of −0.5 mA/cm 2 in galvanostatically controlled MES reactors fed continuously with biogas at a 1 L/d rate, the methane content increased from an initial 56 ± 2 to 86 ± 6% in upgraded biogas. The conversion of CO 2 into acetic acid (3.6 ± 0.6 g/L) by the Acetobacterium -dominated mixed-microbial culture resulted in an efficient biogas upgradation. At an E cell of 2.6 V, the Coulombic and energy efficiencies of the process considering the acetic acid product were 82 ± 16 and 36 ± 7%, respectively. Though the low applied electric current density (e.g., −0.66 mA/cm 2 ) affected biogas upgradation, it resulted in the mixture of CH 4 and H 2 (i.e., hythane), also a potential fuel. The technoeconomic analysis considering the operational costs suggests the economic impracticability of the demonstrated process at low acetic acid titers, and capital expenses remain the major bottleneck in realizing the near future practical implementation of this technology.