An overview on conversion technologies to produce value added products from CH4 and CO2 as major biogas constituents

Anaerobic digestion of organic wastes produces valuable biogas. The main constituents of biogas, methane (CH4) and carbon dioxide (CO2), although being greenhouse gases (GHGs) are versatile feedstock for production of valuable fuels and chemicals. This paper provided an overview on conversion technologies to produce syngas and methanol from CH4 and CO2. Two conversion routes are considered: (1) reforming of methane which is used to produce syngas from biogas and (2) CO2 hydrogenation which directly converts CO2 to methanol. An overview of the most significant factors to attain high conversion efficiency in both conversion processes is presented. The effect of operating parameters (including reaction temperature, gas hourly space velocity (GHSV), reduction temperature, etc.), catalyst preparation method and use of promoters in catalyst on conversion efficiency and product yield is reviewed and discussed. An overview on techno-economic analysis of biogas constituent conversion is presented based on the available literature. This survey of literature indicates that despite the high potential of biogas constituents to be converted to value added products, many problems in terms of technology maturity and economic feasibility need to be addressed before the process could be exploited at industrial scale. Issues such as catalyst deactivation, biogas purification and H2S removal from biogas need to be obviated. With further improvement of the process and also with the involvement of policy makers and private sectors, the conversion technologies can help to mitigate the GHG release and the products obtained from the conversion can be used as sources of renewable fuel and energy.