Biogas an all-rounder

The Production of Power and Heat from Biogas

Biogas is a natural product produced by anaerobic bacteria wherever no oxygen is present. Manufacturers of biogas plants exploit this natural principle in their power stations. There, energy crops and biogenic waste materials are converted into a versatile energy source via fermentation.

Although biogas is a renewable energy source, it is by no means new. For the methane-containing gas is formed in nature anywhere where biomass is decomposed in the absence of oxygen. Microbes called archaea are responsible for this process. They are related to the earliest forms of life, from primitive times when conditions on earth were very different. Today they live in marshes, stagnant water and in the guts of animals and humans – wherever it is damp and there is a lack of oxygen. Biogas plants capitalise on the unique characteristics of the archaea in order to generate climate-neutral, cost-effective heat and electricity.

The biogas plant simulates the natural process

Unlike in nature, the fermentation process in a biogas plant is controlled and particularly efficient. Biology and technology are perfectly attuned to one another. This makes it possible to gain top yields of up to 25,000 kilowatt hours of electricity from one hectare of maize (10,000 square metres), and thus supply around seven households for the entire year. The efficiency of modern plants is impressive: together with heat, one hectare of maize delivers five times more energy than is required for crop cultivation, harvest and conversion into biogas.

Digesters at the heart of the system

Although biogas plants comprise many individual components, in most turnkey plants they are precisely coordinated, and today are largely automated. Each agitator, pump and tank can be monitored and controlled on a computer screen.

The bacteria themselves inhabit the digesters. These are large, air-tight containers made of steel or concrete in which the biogas is produced. They are loaded with organic feedstock. In the past, farmers predominantly used liquid or solid manure from their livestock, but these materials yield comparatively little biogas. Today they are therefore supplemented with food production waste, biological waste and energy crops.

The precision with which the systems are attuned is demonstrated by the example of substrate loading: specially developed batching systems feed biomass into the digester to the kilogramme. The proportioning devices can be controlled such that they automatically load small amounts several times per hour. These quantities are recorded by a weighing system and stored electronically. The operator is therefore able to identify precisely how much biogas is produced from the raw materials.

The conditions in the digester are exactly those under which the bacteria will work best. Accordingly, the thermally-insulated containers are heated to between 35 and 40 degrees Celsius. The carbon present in the maize, grass, liquid manure and other raw materials is converted into biogas by the bacteria within a few days. Durable agitators ensure that the content of the digester is mixed in an energy-efficient manner, and that the gas formed can escape. The biogas is then carried to a room housing the CHP plant via a gas pipe.

Biogas is 50 percent methane

Before combustion, the biogas produced must be desulphurised and dried, as the presence of hydrogen sulphides or water vapours in the gas could damage the motor. Fifty to 60 percent of the biogas is combustible methane. The remainder primarily consists of carbon dioxide and small percentages of oxygen and nitrogen. The methane burns in the motor while the carbon dioxide escapes into the atmosphere. However, as the quantity of carbon dioxide released is only equivalent to that absorbed by the plants during their growth cycle, in contrast to conventional natural gas the combustion of biogas is climate-neutral.

The motors are robust, high-tech machines which respond flexibly to varying methane content in the biogas thanks to corresponding regulators. Their efficiency has doubled over the past decade, and now lies at around 40 percent. This means that over 40 percent of the energy used is converted into electricity, though for the most part the remainder can also be used.

The combustion motor drives a generator which produces the desired electricity. This is commonly fed into the grid. The motor itself must be cooled with water. Both this cooling water, which is heated to between 80 and 90 degrees Celsius, and heat from the flue gas flow can be used efficiently via heat exchangers, for instance to heat swimming pools, greenhouses, barns or residential buildings.

Biogas manufacturing – a high-tech industry

The development of modern measurement and process technology, intensive research in the field of biochemistry and creative engineering solutions in mechanical engineering are crucial to the success of German plant technology. These things alone enable operators to produce high-quality biogas from renewable raw materials. Measurement devices for use in biogas plants must remain affordable but still capable of reporting even the smallest changes in the gas composition. Research is currently being conducted into temperature, moisture and the coordination of plant content which creates the best conditions for the micro organisms. The mini power stations are constantly adjusted to yield top performance, and must be able to cope with fluctuations in the gas composition without allowing the power to drop.