Almost 6% of the green electricity produced in Northern Ireland last year was generated by anaerobic digestion (AD) operations.
Delegates attending a conference, hosted by Fane Valley in Portadown earlier this week, were told that the potential to grow this figure for the future is extremely significant.
Currently, the biogas produced by AD operations in Northern Ireland contains 58% biomethane.
However, given the scope of existing technologies, there is now a realistic opportunity to boost this figure up to 65% – and possibly beyond. Making this happen will require a root and branch review of how an AD plant is managed.
AD operations
There are almost 80 AD plants generating electricity in Northern Ireland at the present time.
Approximately 75% of the operations in question were represented at the Fane Valley event.
Many people may be of the view that the physical functioning of an AD plant and that of a bovine animal’s rumen are very similar.
In some ways this is true. But, at another level altogether, there are more than significant differences in the way that the two ‘fermentation vessels’ operate.
Firstly, silage remains within a bovine for 24 hours; in the case of an AD plant, we are talking up to 30 days.
Grass silage
Lallemand’s Frank Kuchenmeister told the conference that AD operators should opt to make grass silages with characteristics that are fermentation-friendly.
These specific traits relate to chop length, ammonia content and aerobic stability.
According to Kuchenmeister, the average forage crop length for an AD plant should be 45mm, with dry matters in the region of 30%.
The shorter chop length will make the incorporation of the forage in the digester much more energy efficient.
Free ammonia kills-off the microbes that are involved in the fermentation process. But Kuchenmeister also recognises the need to cut grass when it is highly digestible.
So, it comes down to a question of balance – protein content relative to the energy content of the grass at cutting.
The Lallemande representative stressed the need for high levels of anaerobic stability within silages that are used in AD operations.
He also confirmed a series of ongoing trials, investigating the impact of mycotoxins on AD operations.
Ensuring that the correct ratio of aerobic and anaerobic microbes is operating within a fermentation system is another critical aspect of all AD operations.
Mr. Gho, from Florida-based Ecological Laboratories Inc., indicated that the microbial breakdown within a fermentation process is critical in determining both the amount of biogas produced and the percentage of methane within it.
Fane Valley is about to initiate a trial in Northern Ireland with Ecological Laboratories Inc., to specifically identify which populations of microbes will act to produce the highest quality biogas from feed substrates containing large quantities of grass silage.