Ireland’s ability to grow grass is set to form the cornerstone of our biogas industry. While our climate is particularly well suited to growing high grass yields at relatively low cost, the general consensus is that Ireland’s average grass yields could be driven further.
A recent IrBEA and Cré study quoted Teagasc figures that show the average grass yield could be increased by at least 50%. University College Cork (UCC) research also found that underutilised grassland could be brought into production.
Increasing average grass yields and bringing this underutilised land, particularly in the west and midlands, into full production is expected to provide the main organic feedstock in Ireland’s anaerobic digestion (AD) plants.
When co-digested with animal slurries, grass will meet the new sustainability criteria contained in the EU Renewable Energy Directive II (RED II), due to come into force by 2021.
Volumes
The design of Ireland’s biogas sector is likely to create new markets for grass silage. For example, an on-farm 20GWh/yr anaerobic digestion plant will require upwards of 22,000t of silage or dedicated organic feedstock annually. This requirement will increase to 44,000t for a non-farm-based plant, 40GWh/yr in size.
One tonne of grass silage (25% DM) will produce between 160m3 and 180m3 of biogas. However, grass-fed digesters bring their own set of challenges. The Irish Farmers Journal spoke recently to Dr Melanie Hecht of Schaumann BioEnergy, who explained that while grass is a good organic feedstock for biogas production, grass-fed digesters are not overly common across Europe.
Grass will be a key feedstock in Ireland's biogas industry.
Challenges
Dr Hecht explained that high levels of grass used in a feedstock mix can lead to issues with ammonia. Ammonia (NH3) is produced during the degradation of nitrogen-rich feedstocks. It is required for cell growth, but is also a cytotoxin and inhibits bacteria and archaea involved in biogas production.
Plant owners must ensure that ammonia levels don’t exceed 4-5g/l, as levels higher than this will cause ammonia inhibition, ultimately decreasing biogas yields.
Grass silage can also be low in water content and in trace elements, although much of this will likely be supplied from co-digested slurries.
Grass is high in fibre, meaning it requires a high hydraulic retention time to achieve maximum biogas yields. Hydraulic retention time describes the average number of days that a feedstock stays in a digester and is related to digester capacity and the digestibility of feedstocks.
Retention times of 80 days are not uncommon for grass-fed digesters, added Dr Hecht. In laboratory batch testing, it took 93 days to achieve 95% gas yield from grass silage. If retention times are too low, then higher volumes of silage are required to achieve desired biogas yields. This will lead to a high fibre loading, which will in turn increase the viscosity of the contents in the digester. Higher viscosity means that the digester contents becomes thicker, and semi-fluid in consistency, due to internal friction. This can lead to the formation to dead zones within a digester. Grass has a velcro-like effect and sticks together if not managed correctly. This can lead to the creation of balls/wads in the digester’s tanks. These may be difficult to remove and end up damaging the plant’s stirrers and decrease biogas yields.
Equally, if grass isn’t stirred well, it may lead to sedimentation or the formation of swimming layers. This can be a big problem as it can form a thick layer at the top of the digester. Dr Hecht explained that plants have had to be opened in the past in order to remove this swimming layer.
However, if the problems which are causing this issue are not addressed, this will likely happen again.
This becomes more likely when digester temperatures are below 40°C (mesophilic).
Finally, Dr Hecht explains that poorly ensiled silage, or silage that has been spoiled due to mould, can cause a significant problem for biology in a digester and its use should be avoided.
Read more
Biogas training course in Fermanagh
Listen: will the tillage industry miss out on Ireland’s biogas boom?
Ireland’s ability to grow grass is set to form the cornerstone of our biogas industry. While our climate is particularly well suited to growing high grass yields at relatively low cost, the general consensus is that Ireland’s average grass yields could be driven further.
A recent IrBEA and Cré study quoted Teagasc figures that show the average grass yield could be increased by at least 50%. University College Cork (UCC) research also found that underutilised grassland could be brought into production.
Increasing average grass yields and bringing this underutilised land, particularly in the west and midlands, into full production is expected to provide the main organic feedstock in Ireland’s anaerobic digestion (AD) plants.
When co-digested with animal slurries, grass will meet the new sustainability criteria contained in the EU Renewable Energy Directive II (RED II), due to come into force by 2021.
Volumes
The design of Ireland’s biogas sector is likely to create new markets for grass silage. For example, an on-farm 20GWh/yr anaerobic digestion plant will require upwards of 22,000t of silage or dedicated organic feedstock annually. This requirement will increase to 44,000t for a non-farm-based plant, 40GWh/yr in size.
One tonne of grass silage (25% DM) will produce between 160m3 and 180m3 of biogas. However, grass-fed digesters bring their own set of challenges. The Irish Farmers Journal spoke recently to Dr Melanie Hecht of Schaumann BioEnergy, who explained that while grass is a good organic feedstock for biogas production, grass-fed digesters are not overly common across Europe.
Grass will be a key feedstock in Ireland's biogas industry.
Challenges
Dr Hecht explained that high levels of grass used in a feedstock mix can lead to issues with ammonia. Ammonia (NH3) is produced during the degradation of nitrogen-rich feedstocks. It is required for cell growth, but is also a cytotoxin and inhibits bacteria and archaea involved in biogas production.
Plant owners must ensure that ammonia levels don’t exceed 4-5g/l, as levels higher than this will cause ammonia inhibition, ultimately decreasing biogas yields.
Grass silage can also be low in water content and in trace elements, although much of this will likely be supplied from co-digested slurries.
Grass is high in fibre, meaning it requires a high hydraulic retention time to achieve maximum biogas yields. Hydraulic retention time describes the average number of days that a feedstock stays in a digester and is related to digester capacity and the digestibility of feedstocks.
Retention times of 80 days are not uncommon for grass-fed digesters, added Dr Hecht. In laboratory batch testing, it took 93 days to achieve 95% gas yield from grass silage. If retention times are too low, then higher volumes of silage are required to achieve desired biogas yields. This will lead to a high fibre loading, which will in turn increase the viscosity of the contents in the digester. Higher viscosity means that the digester contents becomes thicker, and semi-fluid in consistency, due to internal friction. This can lead to the formation to dead zones within a digester. Grass has a velcro-like effect and sticks together if not managed correctly. This can lead to the creation of balls/wads in the digester’s tanks. These may be difficult to remove and end up damaging the plant’s stirrers and decrease biogas yields.
Equally, if grass isn’t stirred well, it may lead to sedimentation or the formation of swimming layers. This can be a big problem as it can form a thick layer at the top of the digester. Dr Hecht explained that plants have had to be opened in the past in order to remove this swimming layer.
However, if the problems which are causing this issue are not addressed, this will likely happen again.
This becomes more likely when digester temperatures are below 40°C (mesophilic).
Finally, Dr Hecht explains that poorly ensiled silage, or silage that has been spoiled due to mould, can cause a significant problem for biology in a digester and its use should be avoided.
Read more
Biogas training course in Fermanagh
Listen: will the tillage industry miss out on Ireland’s biogas boom?
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