Published June 2024
The conventional fossil-derived sources of energy have resulted in substantial global environmental issues and are hindrance to the path of carbon-neutral scenario. Hence renewable energy sources are gaining impetus and the technologies for conversion of biomass to energy have gained popularity, including conversion of waste biomass to biogas through anaerobic digestion. The biogas is inherently renewable, contrary to fossil fuels, because it is generated from biomass, which is practically a reserve of the solar energy via plant photosynthesis process.
The production of biogas, by anaerobic digestion of biomass/organic matter, is a mature technology, used primarily for the generation of renewable energy and for the valorization of organic residues. In anaerobic digestion process different microorganisms follow diverse metabolic pathways to decompose the organic matter under anaerobic conditions. The biogas is composed of mainly methane (50%-70%) and carbon dioxide (25%-50%), with minor fractions of other components, like hydrogen sulfide, ammonia, water, etc. The biogas can be purified and upgraded to biomethane by separating methane from rest of the components. The purified biomethane (more than 95%), hence obtained, can be used either for generating electricity, in thermal applications or it can be further compressed to be used as fuel, called as bio-compressed natural gas (bio-CNG) or renewable natural gas (RNG).
Only 2% of the globally available organic wastes and feedstocks are now being used to produce biogas. Agriculture is the second-largest sector responsible for greenhouse gas (GHG) emissions, with farms accounting for 12% of the total global GHG emissions. Approximately 20% of these emissions are caused by animal manure wastes and continue to grow by around 1% per year, while the synthetic fertilizers account for 13% of the agricultural emissions. Through anaerobic digestion of animal manure, biomethane can be produced and the leftover digestate can replace synthetic fertilizer use, reducing costs and associated emissions.
This review sheds light on the industry status, process chemistry and technology landscape for biomass-to-biogas conversion technology. The techno-economic assessment for a farm-based biomass-to-biogas plant in the US Gulf Coast (USGC) region, producing 8.13 MMlb/y (396.38 Mscf/d) of bio-CNG from anaerobic digestion of cow manure, is covered. On-site, the raw biogas produced is cleaned and upgraded to remove hydrogen sulfide, water, and carbon dioxide. The cleaned biogas is used as the fuel to generate electricity and heat via a combined heat and power (CHP) unit, to run all the plant operations, while the upgraded biogas is eventually compressed to produce vehicle-grade bio-CNG. Along with the process technology, this review includes the process flow diagram, material balance, major equipment list with specifications, capital, and operating expenditures, and the overall production cost for producing bio-CNG from anaerobic digestion of cow manure. The economic evaluation presented is Process Economics Program’s (PEP) independent interpretation of the commercial process based on information presented in the open literature, such as patents, technical articles, etc. It may not reflect, in whole or in part, the actual plant configuration.