by Michael Cheshire
Anaerobic digestion has become a significant technology in UK for the treatment of waste materials such as food waste, sewage sludge and manure, and for the processing of crops. It is currently mostly being applied at a large scale because the injection of biomethane to the grid is subsidised by the UK government. The technology for biogas upgrading and addition of propane for grid entry is only economic at large scale. In rural areas, there are limited suitable locations for grid entry.
With the UK’s commitment to net zero and with increasing energy prices, the moment has come for a reassessment of the role of small-scale on-farm biogas, which can be carbon-negative. A couple of early reports which are relevant to the discussion about the greenhouse gas mitigation credentials of the anaerobic digestion of manure include:
- In 2015, Bangor University and REA produced a report sponsored by the BBSRC AD Network: ‘Evaluating cost effective greenhouse gas abatement by small scale anaerobic digestion’. This concluded that each tonne of dry matter of cattle manure processed through anaerobic digestion results in the avoidance of 1449 kg CO2e. This figure includes the other greenhouse gas benefits of energy production and fertiliser reduction.
- In 2012, a brief report (by the author and not peer-reviewed): ‘Greenhouse gas mitigation from anaerobic digestion’ suggested that each tonne of dry matter of cattle manure processed through anaerobic digestion results in the avoidance of 900 kg CO2e.
These reports use different assumptions for the greenhouse gas mitigation credentials of the anaerobic digestion of manure, but all point to a very positive mitigation of greenhouse gas. The numbers may vary, but it is important to conclude that the production of biogas from manure results in negative emissions of greenhouse gases. In this, biogas is unusual because most renewable energy technologies have small positive emissions of greenhouse gas.
Other significant benefits include reducing the costs of farming and sheltering the industry from energy and fertiliser price inflation. Biogas from manure was pioneered in 1970’s in the wake of the Middle East oil crises and high fossil fuel prices. In 1980 the price of oil, inflation adjusted, was US $124 per barrel; during 2021 it varied between US $46 and US $66 per barrel, and has risen to circa US $100 per barrel in 2022.
Until the price of energy increases to a level which makes biogas economic, farms should be incentivised to install on-farm biogas plants, probably through capital grants or loans. This can be justified to achieve the decarbonisation and environmental benefits required from agriculture. The advantage of tariff-based subsidies is that they help encourage good performance, but they can be expensive for government to monitor.
Power to gas
Future options for biogas
Anaerobic digesters are part of the circular bioeconomy. As a tool within the carbon cycle they use engineered systems to capture and utilise fugitive emissions from bio-degradation, making them a low carbon system.
Due to multiple factors – not least the number of domestic boilers that will need replacement – the gas grid is unlikely to become close to 100% hydrogen during the asset life of current biomethane plants. However, the first plants built under the Feed-In Tariff (FiT) will lose their tariff support in 2030 (2031 for the first of the RHI plants) and some policy consideration will be needed to ensure that these do not become stranded assets.
Sites near a gas grid may be able to raise the capital to expand their feedstocks, AD plant and land base to make it economic to do grid injection. For others, smaller scale biomethane upgrading for fuel use is a possibility.
Virtual pipelines
One option may be a virtual pipeline where several plants upgrade their biogas on-site, and compress the gas into tanks which are transported to a central gas injection point. For small plants, researchers have explored the possibility and costs of a mobile biogas upgrading vehicle as part of a virtual pipeline. A further variation on this theme for both smaller plants and those FiT plants coming to the end of their life is described in the CNG Services case study.
Biomethanisation
A further option is ‘in-situ’ biomethanisation (or ‘power-to-gas’), where electrolytic hydrogen from renewables is injected into a digester and resultant biogas is produced at circa 95% methane content, as the carbon (C) in the carbon dioxide (CO2) combines with the hydrogen (H2) to produce methane (CH4). The process can also be carried out biologically at larger scale in a separate reactor, using electrolytic hydrogen and a large carbon source (e.g. CO2 from cement production).
However, in-situ technology is better suited to farm-scale. Technically, this requires sufficient excess renewables and an electrolyser of a suitable size, such as those being developed by Enapter. The economics of such an undertaking are complex and will be determined by factors including carbon pricing, energy prices, hydrogen policy and grid composition.
Using nutrients and CO2 from AD
Producing local protein sources for both animals and humans is a major challenge for agriculture and it is possible to take excess waste nutrients produced from AD of food and farm waste to cultivate algal biomass, which could be used as a protein source for animal feed and other products of value.
The carbon dioxide produced during the biogas upgrading process is an important product for industry and is a concentrated source which is helpful in terms of its economic usage. Companies such as Biocarbonics are seeking to utilise ‘green CO2’ from a number of AD plants to provide a local continuous supply for food, beverage and greenhouse use.
In another business model entirely, Future Biogas have partnered with the Northern Lights Project to build a port facility on the Humber Estuary for CO2 from AD biomethane plants to be transported for storage under the North Sea. As this is permanent geological storage (carbon capture and storage-CCS), the company plans to sell carbon offsets to corporate buyers who wish to offset their emissions.
Case study
Integrating Renewable Technologies and AD in a Circular Economy
Integrating renewable technologies with AD for biomethane production
CNG Services is planning to construct a distributed network of anaerobic digesters in Cheshire to feed dry biogas through underground pipelines into a central upgrading hub. At the hub, biogas will be upgraded into biomethane and injected into the gas grid (National Transmission System, NTS). All CO2 produced from the membrane-based upgrading process will be captured and taken to carbon capture and storage (CCS) facilities. Revenue will be generated from a combination of the Green Gas Support Scheme (GGSS) and Renewable Transport Fuel Certificates (RTFCs).
The project will be a good example of integrating renewable energy systems, since the digester heating will be provided by ground/water source heat pumps. Electricity will be supplied using a private electricity network (SP Networks) which includes ~25 MW solar generation and batteries.
The hub will include a bio-CNG ‘Mother Station’ supplied from the NTS to allow farm tractors and trucks in the area to be supplied with biomethane. The aim is for no diesel vehicles on the farms from 2025. The hub, which will have no flare, will be designed with extra capacity to allow future AD’s to be added to the network. There is a target of 10 AD plants by 2025, likely requiring 50 miles of dry biogas pipelines.
The initial AD plant will fund the hub upgrading, NTS injection and bio-CNG infrastructure. To join, smaller AD plants will only require dehydration and H2S removal to condition the biogas before it is sent into the pipeline, thus making smaller mainly manure-based AD plants more viable. This is further improved by eliminating the need for a CHP.
It is envisaged that the heat pumps will be between 50 kWe and 120 kWe, depending upon the size of the digester and heat will be supplemented through digestate heat recovery. The heat pumps will provide sufficient chilling for condensate removal from the biogas and, at one site, will also be used to chill milk at the main dairy.
The British AD technology is capable of handling slurry from cows bedded on sand and it incorporates low energy mixing technology that further reduces energy demand.
In addition to biomethane injection at the hub and use on the farm, the bio-CNG will be exported by trailer and available for use by logistics companies in the area, including those involved in food distribution.
Due to its combination of highly efficient heat pump technology, solar PV, battery storage, manure AD feedstocks, digestate production as a replacement for fossil fertiliser, biomethane tractors/heavy goods vehicles and grid use, the whole project will have an extremely low carbon footprint which will only improve as the electricity grid decarbonises further.