Anaerobic Digestion Plant - Teagasc Grange
- Introduction and background
- Teagasc Grange Biogas Plant
- Environmental sustainability
- Economic viability of anaerobic digestion
- Optimization of anaerobic digestion
- The European Commission recently launched the REPowerEU plan which aims to diversify gas supplies, speed up the deployment of renewable gases and replace gas in heating and power generation.
- Biogas and biomethane are renewable energy sources which can be used in any of the three energy vectors, electricity, transport and heat.
- In Ireland, the anaerobic digestion (AD) sector is relatively underdeveloped despite a lot of studies indicating its potential due to the abundance of grassland and cattle slurry, which can used to provide feedstock for AD.
- Teagasc Grange is the site for a pilot-scale AD plant which is currently under construction and due to be commissioned in 2022.
- Research at Teagasc Grange is exploring the environmental and economic sustainability of biomethane from a range of grass silages derived from swards differing in species composition and in the rates of nitrogen fertiliser received.
- This research highlights that the fertiliser inputs and sward type used in silage production are key determinants in the sustainability of biogas production
Introduction and background
Geopolitical events in first quarter of 2022 illustrated the challenges European energy policy faces in terms of affordability, reliability of supply, and environmental sustainability, and these events led to the announcement by the European Commission of the REPowerEU plan. The plan seeks to diversify gas supplies, speed-up the deployment of renewable gases and replace natural gas, which is a fossil fuel, in heating and power generation. The European Union (EU) currently imports 90% of its gas consumption, (European Commission, 2022) demonstrating the need to accelerate the clean energy transition in Europe.
Anaerobic digestion (AD) is a multi-step process whereby organic waste and residues are converted into biogas by a group of microorganisms in an anaerobic environment. While biogas could be used as any of the three energy vectors, in Ireland it would be most useful if used for renewable heat or transport. Biogas refers to the gas prior to upgrading, which contains approximately 55% methane (CH4), while biomethane refers to the upgraded gas, containing approximately 97% CH4. Anaerobic digestion plants can be fed a wide range of organic feedstocks. There are many suitable feedstocks for biogas production from the agricultural sector, including crops such as maize specifically cultivated for biogas production, animal slurry and manures, as well as waste and by-products from agro-industries.
The biogas industry in Ireland is relatively underdeveloped despite a lot of studies indicating its potential. As of 2019 there were 12 agricultural biogas plants in Ireland, with a further 6 under construction. The potential for a biogas industry derives largely from the abundance of grassland, which can used to grow feedstock for AD, and the significant number of livestock and hence slurry that can be co-digested with grass and grass silage in the AD process. The ultimate goal is to not only increase renewable energy resources but also to promote sustainable development in rural areas, reduce energy costs for farmers and provide the opportunity to increase farm incomes.
Teagasc Grange Biogas Plant
Teagasc Grange is the site of a biogas plant which is currently under construction and due to be commissioned in 2022. The two main components of the plant are:
The digester is a 1,625m3 capacity concrete pre-cast panel (pre-stressed and post-tensioned) tank – 18.63 m internal diameter and 6 m high. The tank wall is fitted with external insulation protected by steel cladding. A dual membrane biogas collection system is fitted on top of the tank. The outer visible dome is made of PVC coated reinforced polyester fabric, whereas the inner gas-proof membrane is made of LDPE. Two submersible propeller agitators are used for mixing the digester contents and are mounted on vertical guiderails. The equipment can be accessed through hatches fitted in stainless steel cantilevered pedestals fixed to the top of the tank’s concrete wall panels without the need to de-commission the digester or remove the gas collection system.
The digestate vessel is of similar structure and capacity to the digester vessel with the exception that there is no insulation or cover. The construction allows that a cover can be added at a future date. The digestate tank is mixed by a fixed propeller agitator in the tank wall.
Gas cleaning removes non-desirable gases. Initially sulphur is removed by air addition to the digester vessel headspace. The gas is de-humidified by passing it through a chiller to bring the temperature below its dew point. An activated carbon filter is used for the further removal of sulphur/hydrogen sulphide and carbon dioxide will be subsequently removed to produce bio-methane (98% methane).
Liquid and solid feed stocks are pre-mixed, macerated and homogenised before being fed into the digester vessel. Nominal feedstock mix per day is 10 tonnes (t) of grass silage at 25% dry matter (DM) and 10 t of slurry at 8% DM. The quantities of each will depend on silage ‘quality’ primarily in respect of dry matter digestibility. A pumped feedstock recirculating loop is used for heating the material through an external heat exchanger. The digester can operate at mesophilic (35-40°C) or thermophilic (55-60°C) conditions.
When fully-operational, expected nominal gas production will be 70 m3/hour. The bio-methane produced will be pressurised for transport by road tanker to the national gas grid at the injection point in Nurney, Co. Kildare. Alternatively, the bio-methane can be used for natural gas powered trucks with refuelling on site. In addition, the option of using a natural gas tractor on the research farm at Grange will be explored.
The environmental sustainability of biogas and biomethane derived from grass silage is an important consideration. The Renewable Energy Directive (RED) is the legal framework for the development of renewable energy across all sectors of the EU economy. Energy crops, i.e. crops that are grown solely for energy production, have been shown to fare poorly in sustainability assessments as they often compete with animal feed or human food and induce land use change. The RED also requires that renewable heat and transport fuel have emissions savings of 80% and 65%, respectively, versus their fossil fuel comparators.
Research was conducted at Teagasc Grange to determine the environmental sustainability, in respect of greenhouse gas (GHG) emissions, of bio-methane gas production. The comparative performance of a range of grass silages derived from swards differing in species composition and the rates of nitrogen fertiliser received was assessed with the aim being to identify the sward type with the lowest quantity of GHG emissions per unit of energy produced. The five sward types, yields obtained and the GHG emissions produced in the production of biomethane from each sward are described in Table 1.
Table 1. Sward types, species included, rates of inorganic nitrogen applied and annual dry matter (DM) yield
|Species of forage||Nitrogen - kg/ha||Annual DM yield - kg/ha||GHG emissions - g CO2eq/MJ|
|Perennial ryegrass & red clover||0||10,771||23.1|
|Multi-species sward - Timothy, perennial ryegrass, red clover, ribworth plantain, chicory||0||11,697||23.7|
|Multi-species sward - Timothy, perennial ryegrass, red clover, ribworth plantain, chicory||120||12,171||38.4|
The lowest GHG emissions per unit of energy generated (g CO2e/MJ) was from the perennial ryegrass/red clover sward. Regardless of sward type, the stage in the AD process which contributed most to overall emissions was ‘fugitive’ methane losses from biogas production, which accounted for 35% of total emissions. This methane is lost to the environment due to ‘leakage’ in the AD process and is, therefore, not available for energy production. This was followed by agro-chemical inputs (20%) and field nitrous oxide (N2O) emissions (15%). Transport, biogas generation and upgrading accounted for the remaining 30% of emissions.
When considering the ability of grass silage to meet the minimum emissions savings under the RED, this research highlights that the fertilizer inputs and sward type used in silage production are key determinants in the sustainability of biogas production. Silage should be produced with minimal inorganic nitrogen to meet the necessary emissions savings. This can be achieved through the incorporation of legumes, such as red clover, or through the use of multi-species swards.
Economic viability of anaerobic digestion
While there is much interest in biogas as a mitigation technology in Ireland, another issue that remains to be addressed is the financial returns. There is limited research on the economics of grass silage and cattle slurry co-digestion for grid injection. Current research at Teagasc Grange is seeking to determine the breakeven price for biogas based on the annual costs and incomes for a biogas plant. The AD system in this study is based on the biogas located at Teagasc, Grange, Dunsany, Co. Meath.
Optimization of anaerobic digestion
Research at Grange is exploring alternative combinations of feedstock in AD with the overall aim being to optimize biogas production. Co-digestion of manure with grass silage provides many advantages in optimizing the AD process. The main ones are an increase in the biogas yield and process stability, increasing the biogas plant’s economic viability. Mathematical modelling of the AD process is a powerful tool allowing changes in operating conditions to be simulated and determining which variables are most influencing the biogas and methane yields. For instance, the mix ratio between the feedstocks (grass silage: cattle slurry ratio), the hydraulic retention time (HRT) and the reactor temperature are some of the variables that affect biogas production.
Paul Crosson1, Ciara Beausang1, Sofia Tisocco1,2 and JJ Lenehan1
1Teagasc, Grange Animal & Grassland Research and Innovation Centre, Dunsany, Co. Meath
2College of Science and Engineering, National University of Ireland, Galway
Funding from MaREI, the Science Foundation Ireland (SFI) Research Centre for Energy, Climate and Marine received by Dr. Ciara Beausang is gratefully acknowledged.