2.1.1 Increasing plant size and shift in upgrading technique

Almost all biomethane in Europe is produced by anaerobic digestion today. Thermal gasification is only in an early commercial stage, and hydrothermal gasification is at an industrial demonstration stage.

Biogas and biomethane are produced from organic feedstock. Two main biomethane production technologies exist: anaerobic digestion by upgrading biogas and gasification (Figure 2.2). Gasification includes thermal gasification (i.e. pyrogasification), which converts dry woody biomass, and hydrothermal gasification, also known as supercritical water gasification (SCW), which converts raw liquid and wet biomass by upgrading syngas.

Biomethane combustion emissions have a short carbon cycle and count as zero-emission following the IPPC guidelines.6 Over the life cycle of biomethane production, including feedstock cultivation, processing, and transport, a minimum of 80%-85% GHG emissions reduction can be achieved compared to natural gas.7 Even negative emissions can be achieved by capturing emissions from the biogas upgrading and combustion processes. Biomethane has the unique property of being similar to natural gas and can, with the right purity levels (~97% methane content depending on applicable gas grid requirements8), be injected into the natural gas grid without major system adaptations.

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.

In Europe, digesters for biomethane production range in size from about 100 Nm³/hr (~0.2 MWel biogas output) to over 3,000 Nm³/hr (~6.2 MWel biogas output) for large industrial plants. The average digester size in the EU went up by approximately 4% between 2017 and 2018.

For biomethane production through anaerobic digestion, organic feedstock is fed into a digestion tank for fermentation. Digesters are designed based on the type of feedstock (wet/dry/solid),¹⁷ the required process mode (continuous/batch),18 and production characteristics. Biomethane production installations can come in various sizes, ranging from local to industrial scale following the output flow rates of the biogas units:9

Small: 100 Normal cubic metres per hour [Nm³/hr]
– 250 Nm³/hr (~0.2-~0.50 MWel19)
Medium: 250 Nm³/hr-750 Nm³/hr (~0.50 MWel-1.54 MWel)
Large: >750 Nm³/hr (or >1.54 MWel)
Very large: >3,000 Nm³/hr (or >~6.17 MWel). From 2015 onwards, the size of biomethane production installation has been increasing with very large units of several thousands of m³/h.

The average size of biogas plants in the EU27 ranged from 0.2 MWel to approximately 1.8 MWel in biogas output in 2018 (Figure 2.3).5 Differences occur due to the variation in applications of biogas, scale of production (local vs. large scale), and differences in feedstock. The electricity generated by these plants amounted to 65 TWh in 2018. With average electrical efficiency of 38%, this means an input of 171 TWh of biogas (16 bcm in natural gas equivalent).2 On average, the countries with the largest number of biogas installations have smaller size installations and vice versa.

Within Europe, there is a trend towards larger digester sizes for biogas and biomethane production.2 In 2018, the European average stands at 0.61 MWel, or 296 Nm³/hr, up approximately 4% from 0.59 MWel, or 279 Nm³/hr in 2017.2, 5 Some countries have larger average digester sizes. For example, the average biogas digester size in the UK is 2.4 MWel (1,300 Nm³/hr) and is 1.8 MWel (870 Nm³/hr) in Ireland. Germany, Austria, Switzerland, Denmark, and Estonia have average digester sizes below the European average.2, 5 The European average is expected to increase, especially in industrial areas where waste streams can be combined, enabling cost reductions through economies of scale. Biodigesters can also be adopted at industrial scale by adapting existing wastewater treatment plants to process municipal sludge (order of ~1,000 m³/h), for example in Amsterdam, or by using landfill gas recovery systems to recover biogas produced from closed landfill sites (order of ~2,000 m³/h), for example in Sinsheim.9 Selected projects are highlighted in the showcase projects. The average digester size is expected to increase from 296 Nm³/hr to at least 500 Nm³/hr in 2050.2

The most common anaerobic digestion biogas to biomethane upgrading techniques in the EU are membrane separation, and water and chemical scrubbing. Over the last decade, membrane separation is increasingly being adopted—it is now the most common upgrading technique with a market share of approximately 34% of cumulative installations in 2019.

In Europe, almost 12% of biogas was upgraded to biomethane in the anaerobic digestion production process in 2018.20 Various upgrading technologies exist based on the different chemical and physical behaviours of methane and CO₂.21 Upgrading technologies can be categorised based on their separation mechanism—for example, adsorption or absorption (physical and chemical):21

Pressurised swing adsorption (PSA) separates CO₂ by using its connection behaviour to a surface under elevated pressures. PSA is a complex process that requires pretreatment and requires low energy use.22 In addition, PSA results in the most (low) methane losses of all upgrading techniques. PSW, however, only has a low energy demand.

Chemical scrubbing, physical scrubbing or water scrubbing (absorption) use different types of liquid to dissolve gas to remove CO₂ (e.g. chemical solvents, water, or an organic physical material). Chemical scrubbing requires high amounts of energy for steam production, pretreatment, and chemical inputs. However, it results in a faster upgrading process thanphysical scrubbing and almost no methane loss.22 In addition, chemical scrubbing leads to almost no methane losses under specific circumstances and when surplus high heat abundant, the process can be well integrated. Physical scrubbing is a simpler process with low operational cost and maintenance, but it requires large amounts of water, energy, and an external heat source.22

Membrane separation uses a permeable membrane to separate CO₂ and methane molecules based on their different physical characteristics. Membrane separation is a simple process with low costs and energy use, but requires pre-treatment. Due to the use of a physical barrier rather than a liquid or elevated pressure, it is more environmentally friendly. However, compared to chemical scrubbing, membrane separation has relative higher methane loss.22

The most common upgrading techniques in the EU are membrane separation (34% of total biomethane plants in 2019), and water and chemical scrubbing, with a combined share of approximately 46% of total biomethane plants in 2019 (Figure 2.4).23 Membrane separation has been increasingly adopted in the EU over the last decade due to its specific advantages (Figure 2.5). It is now the most common upgrading technique, leading to a rapid increase in market share.

The relative use of biogas-biomethane upgrading techniques varies by country. Figure 2.6 indicates the distribution of upgrading techniques by country for selected EU countries. The countries with the largest number of biomethane plants use a mix of upgrading technologies. In 2018, Germany had 200 biomethane upgrading plants; with main upgrading techniques a mix of water and chemical scrubbing and pressure swing adsorption. In contrast, in 2018, Italy had five upgrading plants and Belgium had one upgrading plant; both countries use membrane separation as the upgrading technique.24

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