Section 3: Anaerobic digestion
Published 9 December 2021
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Certain types of organic waste and purpose-grown crops can be used to produce bioenergy through the process of anaerobic digestion. Anaerobic digestion (AD) is a natural process in which plant and animal materials are converted into useful products by micro-organisms in the absence of air. The process releases biogas, (mainly a mixture of around 60% methane and 40% carbon dioxide) which can be used directly to provide heat, power or transport fuel. Biogas can also be purified by removal of the carbon dioxide to produce biomethane, which can be fed directly into the public natural gas grid in the same way as natural gas, or used as a vehicle fuel. The types of materials suitable for AD include food waste, slurry and manure, crops and crop residues.
3.1 Energy produced from anaerobic digestion
Data from the Department for Business, Energy and Industrial Strategy (BEIS) in the Digest of UK Energy Statistics (DUKES) publication show the use of AD in the UK for energy, with the figures representing the energy content of the fuel used. These figures cover both electricity and heat produced from anaerobic digestion of crops and non-water waste feedstocks, in tonnes of oil equivalent (Figure 3.1). While this dataset does not separate energy contributions from crop and waste feedstocks, it can be used to indicate general trends the in use of anaerobic digestion.
Figure 3.1: Volume of energy produced from anaerobic digestion in the UK 2015 - 2020 (‘000 tonnes of oil equivalent)
| Type | AD for heat | AD for electricity | Total |
|---|---|---|---|
| 2015 | 119 | 487 | |
| 2016 | 53 | 708 | |
| 2017 | 62 | 863 | |
| 2018 | 65 | 918 | |
| 2019 | 68 | 947 | |
| 2020 | 68 | 953 |
Source: Digest of UK Energy Statistics
In 2020, total energy produced from anaerobic digestion was 1,021 thousand tonnes of oil equivalent, which was almost equal to the amount in 2019 at 1,015 thousand tonnes. Energy production from AD followed a rapidly growing trend from 2009-2016, a result of rapidly expanding capacity for AD in the UK over this time. This was supported in part by schemes such as the Renewables Obligation (RO), which was a major support mechanism for large-scale renewables projects. Following RO closure to new generating capacity in March 2017 growth slowed and continued this trend up to 2020.
Electricity produced by AD made the most significant contribution to total energy production in 2020 at 953 thousand tonnes of oil equivalent, a marginal increase on 2019 (947 thousand tonnes). At 68 thousand tonnes of oil equivalent, heat produced by AD makes up approximately 6.7% of the total energy contribution in 2020, the same proportion as in 2019.
Much of the significant increase in energy produced from AD over recent years is attributed to several support mechanisms designed to provide financial incentives to bridge the cost gap between conventional and renewable energy sources such as AD. One such example is the non-domestic Renewable Heat Incentive (RHI), which provides payments to encourage renewable heat production. Applications to this scheme are recorded in a monthly deployment dataset, along with the amount of energy produced. This data can be used to examine heat-energy produced by AD covered by this scheme in detail, including yearly production and proportions of heat produced from biogas and biomethane injected to the gas grid (figure 3.2).
Figure 3.2 Energy produced from anaerobic digestion in the UK under the RHI scheme 2015 - 2020
| Gas | Biomethane(a) | Biogas | Total |
|---|---|---|---|
| 2015 | 626 | 27 | |
| 2016 | 1,453 | 134 | |
| 2017 | 2,175 | 420 | |
| 2018 | 2,324 | 890 | |
| 2019 | 3,202 | 973 | |
| 2020 | 3,960 | 1,254 |
Source: RHI Deployment data
(a) Biomethane is equivalent heat generated by biomethane injected into the gas grid.
By the end of 2020 there were 95 registered producers of biomethane in the non-domestic RHI, producing over 13,470 GWh of heat from AD between 2011-2020.
Biomethane generated from AD is generally fed into the mains gas grid rather than being burned directly to produce heat, and as a result is given as equivalent heat generated when this gas is burned. As in 2014-2019, this contribution makes up the most significant proportion of heat generated from AD, totalling 3.960 GWh in 2020. This comprises 23% of the total heat produced under the non-domestic RHI for 2020, and is an increase on the quantity produced in 2019 (3,202 GWh).
Heat produced directly by biogas combustion, typically through a combined heat and power (CHP) system or a biogas boiler, makes up a much smaller proportion of the heat generated from AD for all years recorded, totalling 1,254 GWh in 2020. This is 8% of the total heat produced under the non-domestic RHI for 2020 and an increase of 29% from 2019 (973 GWh).
Changes to the tariffs available under the RHI scheme introduced in 2017 are likely to have had impacts on heat-energy production from AD, and also on feedstocks used. One of the most significant changes is that from May 2018, all AD plants producing biomethane or biogas were subject to feedstock restrictions, with payments limited where more than 50% of feedstocks come from crops or other non-waste sources. [footnote 1]
3.2 Crops as feedstocks for AD
Anaerobic digestion uses both organic waste and crop feedstocks to produce biogas and biomethane. The annual NNFCC Anaerobic Digestion Deployment in the United Kingdom report [footnote 2] records the annual usage of feedstocks in operational plants and estimates for proposed developments in the UK (figure 3.3). These figures include a breakdown into a number of farm and non-water waste feedstocks in tonnes per annum, including contributions from crops, and also from crop wastes; which includes both crop residues (such as straw) and waste crops (such as vegetable outgrades).
Figure 3.3: Use of feedstocks in operational AD plants by feedstock tonnage per annum, 2018/19 and 2019/20
| Year | Manure/slurry | Crops | Food waste | Crop waste | Other waste | Total |
|---|---|---|---|---|---|---|
| 2018/19 | 16% | 31% | 31% | 4% | 19% | |
| 2019/20 | 17% | 30% | 29% | 4% | 20% |
Source: NNFCC Anaerobic Digestion Deployment in the UK
Figure 3.4: Use of feedstocks in planned AD plants by feedstock tonnage per annum, 2018/19 and 2019/20
| Year | Manure/slurry | Crops | Food waste | Crop waste | Other waste | Total |
|---|---|---|---|---|---|---|
| 2018/19 | 19% | 25% | 31% | 4% | 20% | |
| 2019/20 | 17% | 22% | 28% | 4% | 28% |
Source: NNFCC Anaerobic Digestion Deployment in the UK
In the UK in 2019/20 there were 579 operational plants, with a cumulative installed capacity of 466 MWe. Of these, 418 were farm fed (cumulative installed capacity 222 MWe) with the remainder being waste fed. A further 331 AD projects were under development in this time, with a proposed cumulative installed capacity of 269 MWe. Of these, 228 were farm fed (cumulative installed capacity 118 MWe) with the remainder being waste fed.
Of the feedstocks used in operational plants in 2019/2020, 34% were crop derived (approximately 4.7 million tonnes). The vast majority of this (4.2 million tonnes, 30% of total feedstocks) was crops purpose grown for AD, while crop waste made up 525 thousand tonnes (3.8% of total). The remaining 66% of feedstocks tonnage comprised non-crop wastes.
It is estimated that crop feedstocks required a cropping area of 93 thousand hectares in the UK in 2019/20.
A similar distribution is expected for AD plants under development in 2019/2020. 26% of feedstocks are crop derived (2.3 million tonnes), made up of 1.9 million tonnes of purpose-grown crops (22% of the total) and 347 thousand tonnes of crop waste (4% of the total). It is estimated that these feedstocks would require a cropping area of 44 thousand hectares in the UK.
3.3 Types of crops used as feedstocks
Types of crops suitable for use as feedstocks include maize, grass and oilseeds. Official statistics on the amount and type of crops grown used for AD are currently limited to maize. We will be exploring possible ways to gather crop feedstock data in the future.
Figure 3.5: Maize by intended usage, England
| year | Grain maize | Fodder maize | Maize for AD | |
|---|---|---|---|---|
| 2015 | 8 | 132 | 34 | |
| 2016 | 8 | 122 | 52 | |
| 2017 | 8 | 118 | 57 | |
| 2018 | 10 | 140 | 57 | |
| 2019 | 9 | 136 | 67 | |
| 2020 | 12 | 136 | 75 |
Currently, no data is available for the UK-wide area of maize produced for AD, however, the June Survey of Agricultural and Horticulture asked farmers in England to specify the end purpose of their maize for the first time in 2014.
In June 2020 the area of maize being grown for AD was 75 thousand hectares. This is an increase of 12% compared to 2019 and equates to 34% of the total maize area in 2020 and 1.3% of the total arable area.