Biogas an all-rounder

Cultivation of Energy Crops

Today, energy crops are the most important raw material used in the generation of biogas. They offer the following advantages: they can be cultivated locally, produce a new crop every year and generate energy which is virtually CO₂-neutral. At the same time, fermentation residue from the plant serves as valuable fertiliser, thus creating a closed and sustainable utilisation cycle. The nutrients contained in the fermentation product are available in mineral form, meaning that they are more readily available for absorption by plants.

In addition, the odour of fermentation residue is not as unpleasant as that of liquid manure and, as organic acids in the material are decomposed by bacteria, it is less aggressive. In returning the fermentation product to the soil, an environmentally viable cycle is closed. A wide variety of energy crops can be used in a biogas plant – with wood the only material that the bacteria are unable to decompose. As a result, the cultivation of renewable raw materials brings greater diversity to the fields and thus improves soil quality.

To produce biogas, farmers cultivate special energy crops or use existing grassland. Almost any plant and any plant component can be fermented in a biogas plant. The cultivation of energy crops therefore supports sustainable agriculture adapted to the local environment. Scientists, institutes and companies are continuously researching new crops and cultivation methods. The utilisation of biogas thus contributes to biodiversity and soil quality.

Currently, renewable raw materials are cultivated on some two million hectares in Germany. At 940,000 hectares, the largest area is covered by rapeseed for biodiesel and vegetable oil. In 2010, farmers cultivated energy crops for biogas on around 650,000 hectares; this corresponds to less than four percent of the country’s total arable land – an area of 17 million hectares.

The Wuppertal Institute for Climate, Environment and Energy forecasts that 1.1 million hectares will be used for the cultivation of biogas energy crops by 2020 – without compromising food production. The Institute also considers yield increases of two percent per year possible.

Plant mixes to suit all needs

Currently, the most important energy crop with the highest output in biogas plants is maize silage. An average harvest of 50 to 60 metric tons from one hectare of maize silage produces some 10,000 cubic metres of biogas. Modern plants are able to convert this biogas into around 20,000 kilowatt hours of electricity.

However, the energy crop maize silage does not grow everywhere. This is why many farmers in different regions develop their own crop mixes for energy production. The ensuing crop rotation preserves soil fertility and supports the farmer in adhering to laws such as the “Cross Compliance” Regulation. According to this regulation, farmers only receive EU subsidies if they adhere to certain rules on the preservation of human, animal and plant health.

Wide range of crops available

A popular alternative to maize is whole crop silage (WCS). Here, farmers sow winter crops in the autumn, such as rye, which are then mowed and ensiled while still green in April and May.

This still leaves time to sow another crop on the same field. Depending on the site, this could be maize silage or a catch crop, such as oil radish, mustard or rape. Sugar beet, sunflowers and other crops are also suitable for biogas production.

In addition to cultivated biomass, grasses from permanent pastures also play an increasingly important role. According to a study, even today around 17 percent of the biomass generated from energy crops could be produced from grass. Yielding 172 cubic metres of biogas per metric ton of fresh mass, grass is almost as productive as maize. Moreover, the fact that many biogas producers receive late crops at low cost makes grass a valuable supplement to field crops in several regions.