Biogas an all-rounder

Indispensable for Climate Protection

Biogas can make an essential contribution to climate protection. This is why the increasing generation and utilisation of biogas will play a vital role in achieving the national minimum objectives. The lower the emissions from biogas production itself are kept, and the more efficiently biogas is used, the greater the benefits for the climate. One important aspect in terms of the carbon footprint of biogas is that it closes cycles. For example, agriculture can save on mineral fertiliser, the production of which is very energy-intensive.

A wide variety of energy crops can be used in biogas plants. Currently, the most important energy crop is maize. It is able to convert water and nutrients efficiently into high biomass yields, meaning that fewer energy crops must be grown. The carbon dioxide produced as a result of generating energy with biogas is approximately equivalent to the amount absorbed by the plant during growth. The plant residue obtained during fermentation is then returned to the agricultural land. This reduces the need for mineral fertiliser, which pollutes the atmosphere with nitrous oxide emissions.

The nutrient cycle is closed

Organic waste such as garden cuttings or left-over food can be fermented in a biogas plant. This benefits the climate in two respects. Firstly, it prevents methane, a product of the decomposition of organic matter, from escaping into the atmosphere. Methane is 23 times more detrimental to the climate than carbon dioxide. Secondly, the biogas plant uses methane to generate energy. Using liquid or solid manure from livestock in a biogas plant is particularly good for the climate. It constitutes a pre-processing step, as it makes the livestock manure more tolerable for plants and reduces methane emissions during storage and spreading on fields or pastures. The neighbourhood will also be grateful, because the odour emissions of fermented products are considerably lower than those of “normal” liquid manure. Surveys have shown that biogas plants can reduce the emissions of cattle farming by up to 25 percent. By introducing the “manure bonus”, the law has promoted close cooperation between biogas producers and cattle farmers. According to the regulation, additional remuneration of four cents per kilowatt hour for plants with capacities of up to 150 kilowatts is paid for the use of 30 percent liquid manure or more in the fermenter. Cattle farms also benefit from the residual heat of power generation, which can be used to heat cowsheds.

Fewer emissions by replacing fossil energy

The higher the proportion of liquid manure used in the biogas plant and the better the use made of the resulting heat, the more the climate will benefit. In an ideal scenario, up to 100 percent of fossil energy could be replaced. In 2009, eleven million metric tons of carbon dioxide emissions were avoided thanks to the use of biogas plants. Taking into account the potential savings on mineral fertiliser and the losses from storing liquid manure, the provision of substrate causes relatively few emissions. Even energy crops ultimately produce four to six times as much energy as is required for their cultivation. One kilowatt hour of biogas electricity from grass or maize requires 80 to 90 percent less energy than that needed to produce one kilowatt hour of electricity from coal. If waste is used as a feedstock and the waste heat is used to replace fossil energy sources for heating, this will further improve the carbon footprint.

To avoid methane emissions during biogas generation, the storage containers for the fermented residual material are sealed. Applying the liquid fermentation residue close to plant roots using a trailing hose, or by working it into the ground quickly if the field has not been planted, can considerably reduce ammonia emissions. The plant technology itself has also been steadily improved over the past few years: motors in cogeneration power plants, where the biogas is burned in order to generate electricity, are increasingly equipped with catalytic converters for flue gas purification.