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A study led by experts at Queen’s University (QUB) and the Agri-Food and Biosciences Institute (AFBI) has concluded that anaerobic digestion (AD) utilising grass silage and manures could provide over 80% of NI gas network demand.
The process relies on biogas produced from AD plants being upgraded into biomethane using scrubbing technology, with this biomethane then injected into the gas grid.
According to the researchers, NI has significantly greater volumes of the renewable gas available than previously thought, with estimates coming out at over 6,000 gigawatt hours (GWh).
In the period from September 2020 to October 2021, gas energy demand in NI stood at 7,656GWh.
“There is a significant opportunity out there to decarbonise gas. There is enough resource to make a difference – there is clearly plenty there,” Professor David Rooney from QUB told attendees at an event on Monday.
Short term
In the short term out to 2030, industry representatives working alongside the researchers believe it is a realistic target to have 1,000 to 1,500 GWh of gas coming from AD.
Beyond that, with 15 small gas distribution grids across NI, a research model looked at the amount of cattle, pig and poultry manure available within a 10km radius of each grid. Assuming all this manure is directed towards AD, it could potentially produce enough biomethane to meet 67% of gas demand.
However, grass silage is also required as a feedstock alongside manure in AD, and the researchers make an assumption that there is “underutilised grass silage available” in these areas. In other words, not all land is farmed optimally, so there is potential to grow more grass, and produce more silage, without having any impact on cattle numbers.
Decarbonise
“There is a significant opportunity out there to decarbonise gas,” suggested Professor Rooney.
However, while that will benefit NI as a whole, the current accounting system means that greenhouse gas emissions attributed to residential heating would be significantly reduced, not emissions related to agriculture, which again highlights a flaw in how the data is interpreted.
But aside from that, the QUB and AFBI researchers believe that AD can help deliver other environmental benefits, especially relating to water quality.
During his presentation on Monday, Chris Johnston from AFBI pointed out that there is a significant surplus of phosphorous (P) on many farms in NI, which contributes to excess P in our rivers and lakes, and water quality below “good” status.
The vision of the researchers is that slurry and manure in highly stocked areas goes to AD, with the resultant digestate then processed and converted into bio-char in a process known as pyrolysis.
The bio-char, which is a stable carbon-rich product, is then put back on to land as a soil conditioner. Phosphorus could also be harvested off separately.
“How quickly can we turn it [water quality] around? We need a scoping study,” suggested Johnston.
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