Best practice for making silage for AD plants

For crop-fed AD plants, quality of feedstock is the key to efficient biogas production throughout the year. Tim Elsome, General Manager of FM BioEnergy, explains the importance of creating good silage to ensure you get the highest gas output from your feedstock.

- Tim Elsome

Between 25-40% of dry matter is estimated to be lost in silage clamps, with up to a third of these losses due to poor practices when the clamp is being unloaded. It, therefore, makes sense to adopt best practice at all stages of the ensiling process.

We’ve highlighted the top eight areas where dry matter losses can be prevented and provided our top tips for best practice at each stage; from harvesting the crop to removing feedstock from the clamp and feeding the digester.

• Harvest period

As well as choosing a crop and variety that has been shown to produce good biogas yields, the crop should be cut at the ideal stage of growth so that energy content is maximised in order to optimise the dry matter yield.

For a typical maize crop, this will be when the whole plant dry matter is between 32-36%. Harvesting earlier will sacrifice yield potential and greener crops will lose more fluid in the clamp. Later harvested crops with more fibre content may be less digestible subsequently in the digester. If in doubt, it is better to harvest a little early rather than too late.

• Chop length

Maize for use in AD plants is generally chopped smaller than for forage use; 4-6 mm is optimal. The largest possible surface area is best for biogas production, as it aids rapid bacterial breakdown in the digester. However, shorter chop lengths can lead to difficulties in clamping and compaction, and operators need to achieve a balance.

Chop length will also depend on crop maturity. For maize, the suitability of the chop length can be tested by squeezing a handful of the harvested crop. If a lot of moisture runs out, you should delay harvest or increase the chop length. However, if the material does not stay compressed after squeezing, you should shorten the chop length.

• Silage additives