The anaerobic digestion process produces biogas and digestate from a series of biological processes in which microorganisms break down organic feedstock (biomass) in a digester in the absence of oxygen. The resulting digestate by-product can be used as a fertiliser. The produced biogas contains around 55% methane, mostly combined with CO₂. Biogas cannot directly be injected into the gas grid. To enable injection into the gas grid, biogas needs to be upgraded to biomethane with a defined methane content by removing CO₂ and other contaminants.9 Purification (removing pollutants) is also required prior to injection into the gas grid to adhere to European specifications for grid injection.10
Thermal gasification, or pyrogasification, uses woody and lignocellulose biomass (forestry residues) to produce biomethane (or solid organic waste in more general). A benefit of this technology is that it allows the use of additional biomass types (forestry residues) for biomethane production as compared with anaerobic digestion. Thermal gasification produces a mixture of CO, hydrogen, and CO₂ (syngas) through a complete thermal breakdown of the feedstock in a gasifier in the presence of a controlled amount of oxygen and steam at high temperatures.11 Biomethane is produced at high pressures of ~40 bar.2
After the gasification process, a gas cleaning unit removes pollutants like sulphur and chlorides from the syngas.7 The cleaned syngas is then converted to biomethane (methanation) in a catalytic reactor using nickel catalysts or a biological reactor. The methanation process converts the cleaned gas into a mix of biomethane, CO₂, and water; a gas upgrading unit removes in a next step this CO₂ and water. The resulting biomethane meets the standards for injection into the gas grid.
Hydrothermal gasification, or SWC, enables treatment and gas conversion of raw liquid or wet biomass. The hydrothermal gasification process uses the specific properties of water in the supercritical phase (>374°C and >221 bar), where water becomes a reactive solvent. The wet biomass is increased in pressure and temperature until reaching the supercritical phase. In this phase, carbon from the organic dry biomass reacts with hydrogen from water molecules and produces a high pressure, methane-rich syngas also containing hydrogen and CO₂. After gas cleaning, which mainly removes CO₂, the resulting biomethane can be injected into the gas grid. Two hydrothermal gasification technology families exist with or without the use of a catalyst.
Anaerobic digestion is widely adopted to produce biogas and biomethane for almost all biomethane production in Europe today. Biomass to biomethane yield has a wide range around 0.36 m³ of biomethane per kg of feedstock—for example, 0.21 m³/kg for manure, 0.36 m³/kg for maize, and 0.40 m³/kg for biowaste.9, 12
Gasification is a less mature technology than anaerobic digestion but is able to produce biomethane at a larger scale.9 Thermal gasification is only in an early commercial stage, with several large demonstration plants across Europe.2, 13 In addition, thermal gasification has a higher yield in energy output than anaerobic digestion with about 0.55 m³ of biomethane per kg feedstock.7 Large-scale early commercial projects exist, among others, in Germany (<1 GWh biomethane production through gasification in 2018).14 In the Netherlands, Torrgas is working with Gasunie to construct a 25 MW gasification-to-methane plant in Delfzijl.15 Hydrothermal gasification is in the demonstration or pilot stage.16 In the Netherlands, Gasunie and SCW Systems are working together to upscale the first industrial demonstration plant in Alkmaar, increasing production from an initial 1.8 MWth to 18.6 MWth in 2021.16 Gasification technologies are less mature than commercial anaerobic digestion but are expected to further scale-up starting in the mid-2020s. Therefore, the remainder of this report focuses on biomethane production from anaerobic digestion unless otherwise indicated.