Farm-based anaerobic digestion plants can improve efficiency from 10% to 30%, by taking a proactive approach in three key areas; feedstock, mixing and monitoring, according to industry consultant and Privilege Finance’s technical director, Phil Hobbs.
With a bumper maize harvest currently underway across the country, Dr Hobbs says that feedstock quality is the biggest factor affecting digestibility and gas yields, and offers the greatest scope for improvement.
“The key to any good harvest is ensuring that optimal dry matter (DM) is reached, and for maize silage that is 33% to 35% DM. However, good silage clamp management is equally important to ensure that DM quality is not compromised by rain or air during storage, which can have a huge impact on digestibility.
“Once matured, silage crops typically take 40 to 100 days to be digested depending on, principally, digestibility, but there are pre-treatment technologies available to help improve biogas yield and reduce retention times. Although these require an additional capital outlay, gas yield uplifts of 10% to 15% are possible.
“Reducing chop length can also help to improve biogas yield. However, operators need to consider the additional cost associated with increased forager fuel.”
Poor mixing of digestate within the fermentation vessel can also have a big impact on feedstock performance. “The first sign of a problem is a floating or sinking layer, which can result in a crust formation. This can cause expensive down-time especially if the digester needs to be emptied and restarted.”
A proactive approach with regular monitoring and maintenance is vital to ensure that the fermentation process is working to its full potential. “An AD site runs every day of the week, therefore careful monitoring is needed to ensure that any warning signs aren’t missed.
“Regular analysis of digester substrates within the monitoring is important for identifying potential issues that reduce performance. This usually costs around £500 every three or four months, but when the loss of revenue associated with a breakdown is considered, it is much more cost effective to take this proactive approach.”