Maize for Biogas Production

Maize crops can be grown as a raw material (feedstock) for anaerobic digesters, in the production of biogas.

Anaerobic Digestion (AD) is the breakdown of non-woody material to produce methane by micro-organisms in the absence of oxygen. The methane is burnt in a combined heat and power engine to create electricity and heat. The income generated from an AD plant is directly determined by the methane producing capability of the feedstock, and the consequent amount of electricity and heat produced and sold to the national grid.

Research has shown that maize produces the greatest amount of biogas per hectare of crop grown (see figure below). On average, one hectare of maize can produce up to 10,000 cubic metres of biogas, more than double that produced by cereals and grass silage.

Variety Selection for Biogas use....

An AD digester runs on a similar principle to a cow’s rumen: it is the microbes that digest the organic feedstock. It follows that the feedstock should be of suitable dry matter and energy content to maximise the efficiency of this process. In addition, the input volume will influence total output, so crop yield is also important.

The maize crop should be ready for harvest early enough to avoid harvesting difficulties that could damage the soil structure. The target harvest dry matter should ideally be between 30% and 32% and the crop must achieve a minimum of 28% dry matter content in even the worst season. At 32% DM the crop has formed a mature cob and maximised both its total dry matter and energy yield.

Typically a 500KW output AD plant requires 220 hectares of maize silage, yielding an average of 50t/ha fresh matter. So, when choosing what to grow, select a range of varieties with different harvest dates, ensuring that all of them can achieve the target DM for the large area to be harvested within the harvest window.

Varieties developed for biogas production in Continental Europe should be avoided as they mature too late in UK conditions to reach the required dry matter. In general, higher yielding varieties from the ‘Late’ and ‘Early’ segment offer the best compromise between total dry matter yield potential and acceptable quality.

We recommend that growers select the highest energy producers within the correct maturity group for their area.