EVC4: Anaerobic digestion to biogas and biomethane
Typical feedstocks for biogas production are agricultural substrates (e.g. corn, specifically grown energy crops or cover crops), manure, wet waste fractions from the agriculture and food industry sector as well as sludges from e.g. water treatment works in both cities and industries and other organic wastes from households and municipalities. A special case is recovery of landfill gas from waste landfills to prevent release of the methane formed over decades into the atmosphere.
Pre-treatment of the material to be fed into the digester depends on the nature of the feedstock but may involve removal of non-digestable materials (plastics, metals, glass, grit), washing, milling, screening and pressing depending on the feedstock. All biomass fractions, with the exception of lignin, can be degraded by anaerobic microbes. For high fraction of or dedicated lignocellulosic feeds, e.g. agricultural residues, a pre-treatment is in most cases used to make the cellulose and hemi-cellulose better available for the bacterial degradation. The pre-treated feedstock is fed to the digester, a sealed container, where it undergoes decomposition in the absence of oxygen over a period of several days. This process can take place at different operating temperatures, most commonly at 35 – 40 °C (mesophilic) but also at higher temperatures, 55 – 60 °C (thermophilic), thereby increasing the rate of digestion. Based on the constituents and consistency of the feedstock treated, an anaerobic digester can be designed as a ‘wet’, ‘dry’, ‘liquid’ or ‘co-digestion’ system while there are many types of reaction systems depending on the capacity and the nature of the feedstock. During the digestion, the bacterial population in the digester decomposes organic compounds in several steps (hydrolysis, acidogenesis, methanogenesis) to a mixture of almost equal parts of methane and CO2 with some trace gases, mainly nitrogen and hydrogen sulphide, the “biogas”, which is collected in storage tanks or inflatable domes.
For use as vehicle fuel or for grid injection, the CH4 content of the biogas must be increased (> 97 % CH4) by removing most of the CO2 from the biogas. Furthermore, the gas has to be dried and different trace gases (H2S, siloxanes) removed. The upgraded gas is referred to as biomethane and can be stored and used in form of compressed natural gas (CNG) or liquefied natural gas (LNG) in internal combustion engines in vehicles, or simply injected into the grid for further use There is a variety of commercially available upgrading technologies (e.g. PSA or membrane separation, amine or pressurized water scrubbing).
The residues and waters after digestion contain dissolved organics and inorganics as well as non-digested solids. Depending of the feed, these residues can have a value as e.g. fertilizers or require other treatments prior to their disposal.
Upgraded biogas, i.e. biogas for transport, can in the EU, subject to meeting the GHG reduction threshold, be or not be, eligible for double counting, depending on the feedstock used and both categories can be produced in the same plant.
There is a total of close to 50 million of micro-scale digesters operating around the globe, predominantly in China (84 %) and India (10 %), directly fuelling stoves and small furnaces. In addition, there are an estimated 132 000 small, medium or large-scale digesters (i.e. an approximate range of 0.5 to 20 MW gas output), again predominantly in China (83 %) but also in Europe (13 %), USA, India and Canada, and there is a rapid growth in these numbers. The main application of the biogas is power and CHP using IC engines, 88 TWhel was generated in 2016 (indicating a global biogas production of the order of over 300 TWh).
In recent years the upgrading of biogas to biomethane for use as transport fuel or for grid injection has become a proven technology in larger scale installations (2-20 MW gas output). Globally some 700 plants upgrade biogas to biomethane predominantly in Europe, 77%, mainly in Germany but also in the UK, Sweden, France and the Netherlands, a few also producing liquefied bio-methane), USA (7 %), China (4 %), Canada (3 %) and a few in Japan, South Korea, Brazil and India. The European production of bio-methane amounted to 17 TWh(1.5 Mtoe) while in the USA it is around 4 TWh. So in general this technology is at TRL 9, but when using agricultural wastes (straw) as the sole feed, there is one plant by Verbio in operation and another one in construction in Germany, who also bought the former Dupont cellulosic ethanol plant, and plan to convert this to bio-methane.
Acknowledgement: Large parts of the texts were taken from Lars Waldheim´s contribution to the report “The Contribution of Advanced Renewable Transport Fuels to Transport Decarbonisation in 2030 and beyond”
 This pre-treatment might be enzymatic, chemical or physical and for dedicated lignocellulosic feeds fairly comparable to the pre-treatment of lignocellulosic material for alcohol production.
 Global Potential of Biogas. World Biogas Association, June 2019