What is biogas
Whether it is in bogs and swamps or in the ruminants' stomachs: biogas forms anywhere that organic material is decomposing in a moist environment in the absence of oxygen. A variety of bacteria, including methane bacteria, do the main work here. A biogas plant reproduces this process by technical means. The output and composition of the biogas vary depending on the composition of the input material and also the process technology. Ultimately the energy content of the biogas is directly dependent on the methane content. Thus one cubic meter (m3) of methane has an energy content of approx. 10 kilowatt hours (9.97 kWh).
Biogas can be sourced from numerous organic starting materials. In agricultural biogas plants these are mostly energy crops grown specifically for this purpose, and also animal excrement (slurry and manure), that serve as substrates. The use of slurry and other farm fertilisers is of major significance, not solely from the perspective of climate protection (reduction of emissions); it also plays a part in stabilising the process. Crops for use as renewable resources include maize, grain, grasses, and sugar beets, among many others; however, maize currently accounts for the largest share, as a crop with high mass and gas yield, as well as offering the lowest specific costs. In some regions, however, the large amount of maize cultivation can have adverse effects on soil fertility and on biodiversity. As public discussion of this topic gains momentum, a lot of pressure is being applied to the search for alternatives. The aim is to structure the cultivation of energy crops in a way which is as sustainable and as environmentally friendly as possible. Accordingly, new energy crops such as cup plant (silphium perfoliatum), sorghum, wild flowers or special grasses are now the focus of attention.
As the graphic to the left shows, the various substrates achieve very different biogas yields; the methane-content levels of the respective biogas also differ. As a result, depending on the composition of the input substrate, the gas output and the methane content also fluctuate.
Aside from renewable resources, agricultural by-products and residues, there are also certain non-agricultural substrates suitable for biogas production: these include residual material from the food industry (e. g. pomace, stillage, residues from grease traps), vegetable waste from wholesale markets, food waste, grass cuttings, material from landscape conservation, or organic waste from municipal waste disposal.
For biogas production a diverse range of installation concepts is applied. They differ according to the process characteristics, such as the dry matter content, the way in which material is fed in or the number of process phases. Accordingly, depending on the dry matter content, a distinction is made between dry and wet anaerobic digestion. Almost all agricultural biogas plants work in a wet-anaerobic-digestion procedure, at an operating temperature in the mesophilic range (32–42 °C), with the familiar round containers and gas hoods. When using slurry, only wet anaerobic digestion is an option; the solid biomass which is introduced needs to be well broken down and has to make pumpable and stirrable.
By contrast, dry anaerobic digestion is particularly of interest for farms without either slurry or any other liquid base substrates at their disposal. Unlike with wet anaerobic digestion, in the case of dry anaerobic digestion it is not possible either to pump the material to be anaerobically digested or to make it flow; nor is it constantly being stirred or mixed up. However, as is the case with wet anaerobic digestion, a moist medium is necessary for the biological anaerobic-digestion process. This is produced by mixing the material with process fluid before the anaerobic digestion or by constantly spraying it with fermentation fluid during the process of anaerobic digestion.
The scheme shows how an agricultural biogas plant operates and how the basic elements are arranged – preliminary tank/substrate input, the digester with the stirring unit, the gas storage, the post-digester and the utilisation of the biogas (options: biogas upgrading, CHP unit, or other). In the preliminary tank, the substrates are stored on an interim basis, chopped, thinned and mixed; from there, they are then sent into the insulated and heated digester. This is the core element of the plant: it has to be water-tight, gas-tight, and impermeable to light. Appropriate stirring technology guarantees the homogeneity of the substrate and supports the gas production. The biogas comes into the gas storage, while the digested substrate is transported into the digestate storage tank, the latter usually also serves as a secondary digestion container.
If the mix is also anaerobically digesting any substrates that justify concern in terms of disease prevention – e. g. slaughterhouse waste or other food waste – the material must be sanitised and heated to over 70 °C for at least one hour, to kill off germs.
The fluid or solid residue of the process is characterised as (wet or dry) digestate or biogas slurry; farmers mostly use it as organic fertilizer, because of its high nutrient content. In relation to raw slurry, digestates have essential advantages, e. g. reduced intensity of smell and a reduced corrosivity to crops. In terms of nutrients, the composition fluctuates depending on the input substrates used.
Upgrading and utilisation
Biogas offers many options for use. It can be used both for generating electricity and heat and also as a fuel and a natural gas substitute. Biogas is storeable and can be transported via the natural gas grid; by this it is available at any time, independent of the place of its origin. Energy production from biogas is not subject to any fluctuations according to the time of the day or year, or weather factors; thus it can take place continuously in accordance with demand.
Thanks to fixed remuneration rates for conversion into electricity, the generation of electricity and heat, in close proximity to the biogas plant, is currently the primary way in which biogas is used in Germany. The energy is converted in combined heat and power (CHP) units: electricity and heat are produced simultaneously. The CHP unit consists of an internal combustion engine powered by biogas, driving a generator for the production of electrical energy.
Apart from the electrical energy, the CHP unit generates heating as a coupled product. The biogas plant itself uses 20–40 percent of the waste heat for heating the digester, depending on the type of installation and the time of year. From the environmental viewpoint, and for economically viable operation of the unit, it is imperative that the remaining heat generated is used purposefully. One option is to use the heat of the CHP unit for heating domestic and farm buildings. If it is not possible to use the heat in the immediate area of the biogas plant, it can be brought to the consumers with the help of decentralised heating networks. That way, besides supplying residential buildings, one can also supply municipal facilities such as swimming pools or hospitals, and business enterprises. In the case of larger distances, the biogas itself can be transported via gas pipelines to a so-called “satellite” CHP plant, producing electricity and heat at the place of demand.
In principle, biogas is also suitable as an energy source for fuel cells, Stirling engines and micro gas turbines. At present, the advantages of these technologies – such as greater effciency or lower operating costs – are still outweighed by the higher investment costs. A further possibility for effcient use is offered by ORC technology. This produces additional energy from waste heat.
Another option is to upgrade biogas to natural gas quality and feed it into the natural gas grid. Upgrading technologies currently used for biomethane are: pressurised water scrubbing, pressure swing adsorption, and physical and chemical scrubbing processes, as well as membrane technology. These separate the biogas, dividing off the desired methane from the other accompanying gases. In this way, by separating off carbon dioxide, and other trace gases (where applicable), the proportion of methane in the biogas is raised from around 50 percent to the level necessary for the respective gas grid, namely 85–98 percent.
The upgraded biogas, now called “biomethane” and in effect identical with natural gas in chemical terms, can be transported by the available infrastructure of the natural gas grid, across any distance, to locations with high demand for heating all year round. The gas grid commands a huge transport and storage potential and is thus able to decouple the generation of energy from the demand for it. At the same time, the use of the gas grid reduces the need to expand the high-voltage power grids.
Because of the higher investment costs and operating costs involved, it is primarily for larger enterprises that it is worthwhile to make the upgrade and feed-in, but technical progress is allowing smaller and smaller installations to take part in this market directly.
Biomethane is also used as a fuel in natural gas vehicles. Natural gas burns comparatively cleanly but as a fossil fuel it does emit additional CO2 ; by contrast, due to being plant-based, biomethane provides a high potential for saving CO2 , one that also compares extremely well among the options for biofuels. Accordingly, a 25% share of biomethane in natural gas reduces the CO2 emissions by 20%.