Biogas to methanol: A comparison of conversion processes involving direct carbon dioxide hydrogenation and via reverse water gas shift reaction

Methanol is a clean fuel and an important intermediate in chemical synthesis. In this work, the synthesis of methanol from a large-scale biogas plant utilizing biomass in the form of animal, food and dairy wastes and wastewater sludge is studied. It considers upgrading the biogas into biomethane through a separation step to remove CO2 prior to steam reforming and produces syngas for methanol synthesis. Instead of rejecting to the atmosphere, the separated CO2 is mixed with the syngas from the reformer and fed to the methanol reactor. Three process schemes are simulated (using two different catalysts) and compared on the basis of maximum CH4 conversion and methanol productivity. While one of the catalysts is a conventional catalyst preferred for a CO-rich syngas feed, the second catalyst is designed for direct hydrogenation of a CO2-rich syngas feed. The process alternatives use reverse water gas shift reaction, direct CO2 hydrogenation and intermediate water removal. From the different cases considered, the maximum possible methanol production is 2100 tons per day from a biogas feed of 1.8 x 10(6) Nm(3)/year this corresponds to a process of steam methane reforming followed by direct CO2 hydrogenation using Cu/Zn/Al/Zr catalyst (two methanol reactors in series with intermediate water removal between them.) It was also shown that all the three processes can be designed to be self-sufficient in meeting their electricity requirements without the use of any external fuel like natural gas, etc. Since we are converting waste to energy utilizing biomass and don't require any additional fossil fuels, this can be referred to as a clean and green production process. (C) 2019 Elsevier Ltd. All rights reserved.