Factors Affecting the CO2 Emission Feature in Synthesis Gas Fermentation: Influence of Initial Syngas Ratio and Supplementation of CO2-coupler

Abstract

Development of CO2-neutral process is urgently required due to the climate crisis caused by greenhouse gas emissions represented by CO2 emission. Syngas fermentation process is a waste gas upcycling process that enables both production of useful liquid chemicals and treatment of industrial off-gas through biological gas-liquid conversion. Although the advantages of flexibility on syngas ratio and mild operative conditions, synthesis gas fermentation process is characterized by its CO2 emitting feature. The purpose of this study is to identify the dominant factors affecting the CO2 emission feature of synthesis gas fermentation in order to convert CO2-positive syngas fermentation to CO2-neutral or -negative one. Simulations were constructed to investigate key factors affecting the CO2 emission feature in syngas fermentation. In particular, the effect of initial synthesis gas ratio and supplementation of methanol/hydrogen as CO2-couplers on CO2 emissions were identified through theoretical calculation, simulation and case study. As a result, the factors significantly affecting the CO2-emitting feature of batch-mode synthesis gas fermentation were determined as the initial syngas ratio (CO:H2:CO2) and the type/concentration of CO2-couplers supplemented. According to the prediction, when supplementing methanol (50mM to 150mM) in the initial syngas with volume of 1.67 L and composition of CO:H2:CO2:N2=0.24:0.065:0.21:0.48, 0%-100% of the initial headspace CO2 was removed. Second, when supplementing hydrogen gas of 0.6atm-1.6atm in the initial syngas with the same volume and composition ratio, the CO2 emission was reduced in the range of 10%-100%. In the case study, it was calculated that 3.8% of annual CO2 emission from the steel mill could be reduced via supplementation of methanol, and 9.9% of CO2 via supplementation of hydrogen gas. The decisive factors that have been investigated in this study can be used in further achievement of optimization of carbon-negative synthesis gas fermentation.