Anaerobic digestion (AD) has become a common method of producing renewable energy in Northern Ireland during the past five years, partly helped by the financial incentives paid for producing green energy.
Many farmers have diversified their business to incorporate AD to supplement an existing farming enterprise, or as an alternative to livestock production.
According to government statistics, there were 89 AD plants in operation across NI at the end of 2018, up from 43 plants in 2017. Five years ago, there were 12 AD plants in NI.
Indications are the number of AD plants could rise to more than 100 plants over the next couple of years, should planning be granted to applications which are under review.
However, while planning may be approved, there is no guarantee that all of these will be built as securing finance for a project and connecting to the power grid can be problematic. Construction of a new AD plant on farm can cost between £1m and £2m.
What is AD?
AD uses the energy in organic matter (mainly grass silage in NI) to produce biogas which is a combination of methane and carbon dioxide.
The waste product produced, called digestate, can be used as a fertiliser and returned to farmland.
Organic matter is placed inside sealed tanks, or digesters, where micro-organisms break this down to methane gas, which is harvested for energy.
The process is similar to the digestive system of a cow. But instead of using the organic matter to produce milk or meat, methane is passed through a generator plant and burned to produce heat and electricity.
AD plants
AD plants can utilise energy from most organic matters, but some have a higher calorific value than others.
Slurry has a lower biogas yield than silage, as it is a waste product – animals have already digested most of the energy available.
Grass silage has a higher biogas yield, with maize silage even higher. Typical biogas yields of the three main fuels are:
Cattle slurry (10% DM) – 15m3 to 25m3/t biogasGrass silage (33% DM) – 160m3 to 200m3/t biogasMaize silage (28% DM) – 200m3 to 220m3/t biogasDry matter of the grass silage is important, as lower dry matter forages produce less gas. Silage quality is also important as the plant is basically digesting the energy available from grass.
A rule of thumb is that one acre of land is needed to produce grass silage for every 1kWh the plant produces.
A 500kWh plant is capable of utilising 12,000t of grass silage per annum, or 30t of grass silage per day fresh-weight.
Industry experts predict that around 100 AD plants would require 1.4m tonnes of feed stuff annually, most of which would be in the form of grass silage, or whole crop forages.
Other fuels
As AD plants can use slurry as a fuel source, they offer a partial solution to dealing with farm waste produced from the intensive livestock sectors.
By burning animal waste, this reduces the risks of pollution from inappropriate use of slurry and farmyard manures.
Fuelling AD plants has become somewhat of a contentious issue in NI and competition for conacre has become increasingly competitive. Land rent for top-quality silage ground capable of yielding three to four cuts per annum has exceeded £400/ac (€465/ac) with reports of £300/ac (€350/ac) being common place.
As such, intense competition for conacre has seen many livestock farmers unable to retain rented land as they cannot compete with AD plants on price.
Read more
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Anaerobic digestion (AD) has become a common method of producing renewable energy in Northern Ireland during the past five years, partly helped by the financial incentives paid for producing green energy.
Many farmers have diversified their business to incorporate AD to supplement an existing farming enterprise, or as an alternative to livestock production.
According to government statistics, there were 89 AD plants in operation across NI at the end of 2018, up from 43 plants in 2017. Five years ago, there were 12 AD plants in NI.
Indications are the number of AD plants could rise to more than 100 plants over the next couple of years, should planning be granted to applications which are under review.
However, while planning may be approved, there is no guarantee that all of these will be built as securing finance for a project and connecting to the power grid can be problematic. Construction of a new AD plant on farm can cost between £1m and £2m.
What is AD?
AD uses the energy in organic matter (mainly grass silage in NI) to produce biogas which is a combination of methane and carbon dioxide.
The waste product produced, called digestate, can be used as a fertiliser and returned to farmland.
Organic matter is placed inside sealed tanks, or digesters, where micro-organisms break this down to methane gas, which is harvested for energy.
The process is similar to the digestive system of a cow. But instead of using the organic matter to produce milk or meat, methane is passed through a generator plant and burned to produce heat and electricity.
AD plants
AD plants can utilise energy from most organic matters, but some have a higher calorific value than others.
Slurry has a lower biogas yield than silage, as it is a waste product – animals have already digested most of the energy available.
Grass silage has a higher biogas yield, with maize silage even higher. Typical biogas yields of the three main fuels are:
Cattle slurry (10% DM) – 15m3 to 25m3/t biogasGrass silage (33% DM) – 160m3 to 200m3/t biogasMaize silage (28% DM) – 200m3 to 220m3/t biogasDry matter of the grass silage is important, as lower dry matter forages produce less gas. Silage quality is also important as the plant is basically digesting the energy available from grass.
A rule of thumb is that one acre of land is needed to produce grass silage for every 1kWh the plant produces.
A 500kWh plant is capable of utilising 12,000t of grass silage per annum, or 30t of grass silage per day fresh-weight.
Industry experts predict that around 100 AD plants would require 1.4m tonnes of feed stuff annually, most of which would be in the form of grass silage, or whole crop forages.
Other fuels
As AD plants can use slurry as a fuel source, they offer a partial solution to dealing with farm waste produced from the intensive livestock sectors.
By burning animal waste, this reduces the risks of pollution from inappropriate use of slurry and farmyard manures.
Fuelling AD plants has become somewhat of a contentious issue in NI and competition for conacre has become increasingly competitive. Land rent for top-quality silage ground capable of yielding three to four cuts per annum has exceeded £400/ac (€465/ac) with reports of £300/ac (€350/ac) being common place.
As such, intense competition for conacre has seen many livestock farmers unable to retain rented land as they cannot compete with AD plants on price.
Read more
Farmer Writes: a bee sting and fertiliser spreading
Dairy management: time to put away the calendar
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