Biogas is formed when organic matter breaks down without access to oxygen (anaerobic conditions). Some biogas is therefore formed by the breakdown of bottom sediments in lakes, in wetland areas (e.g. in marshes), and by the breakdown of feed in the rumen of livestock. Biogas consists largely of methane, CH4, carbon dioxide, CO2 , hydrogen sulfide, H2S and is saturated with water vapor (50-60% methane, and 40-50% carbon dioxide, ~1 vol % H2S). When these gases are released into the atmosphere, they contribute to the greenhouse effect. Converted to CO2 equivalents, methane has a greenhouse effect 25 times stronger than carbon dioxide.
In a biogas plant, organic waste is broken down and biogas is formed. Much of the biogas consists of methane, which can be used for energy purposes. In agriculture, manure is the most important substrate taken into a biogas plant. In addition, other substrates can be taken in, such as food waste and fish waste. This gives a greater gas yield than when using manure alone. The residue that remains after the biogas process is called bioresidue, and is used as fertilizer.
A biogas plant consists in its simplest form of a storage tank, where manure is taken in and possibly mixed with other substrates, before it is pumped into the biogas reactor. In the reactor, microorganisms break down the organic material. The microorganisms that break down the organic material depend on a certain temperature in order to exist. The substrate must therefore be heated before the biogas process can fully begin. The biogas produced in the reactor is taken out and directed to a gas storage tank. Methane is a combustible gas and can be used for heating houses, for generating electricity, and as fuel for vehicles.
Substrate is pumped in at regular intervals throughout the day, and bioresidue is also regularly removed and pumped to the final storage. The residence time of the substrate in the biogas reactor varies from plant to plant. The longer the residence time, the more biogas is produced. The residence time can vary from 7-21 days, and some plants operate with residence times of up to 40 days. It is important that the substrate is constantly stirred/moved to avoid sedimentation and to extract as much gas as possible. Some gas is produced after the substrate reaches the final storage if it is not cooled down. An alternative is to capture and direct gas in a “retention” to the gas storage.
The bioresidue formed through the biogas process becomes thin, and the nutrients, especially nitrogen, become more readily available to plants. Nutrients can also be obtained via other substrates that are mixed with manure. This makes bioresidue a valuable organic fertilizer.
Climate benefit
Biogas from livestock manure is one of the measures that can really make a difference if agriculture is to reduce its climate footprint. The biggest effect is that methane losses from manure storage, which would otherwise be released into the atmosphere, are captured and burned in one way or another (in a boiler, generator or as fuel). During combustion, CO2 will be released, but we calculate the effect of this to zero, as this has a biological origin and is bound up again in the next season it is grown for the animals. Methane is considered to have 25 times the climate impact of CO2. In the climate game, we have set the saved CO2 footprint in relation to losses from manure storage to be 55% with biogas in relation to traditional operation.
In addition, the use of biogas for heat, fuel and electricity production could replace fossil energy. In the climate game, we have calculated the total possible methane production and converted it to kWh. We have used 25% of this as electricity produced and 16% as net heat produced. The rest is losses in the plant, losses in exhaust and heat used to start the process.
Some consider the fertilizer effect of bioresidue to be greater than using manure directly. This means that saved commercial fertilizer can also be calculated into the climate accounts.
The efficiency of the biogas plant and the climate impact depend on factors such as residence time in the reactor, leaks, cooling or other treatment of bioresidue.
As with any other type of combustion plant, we must calculate a footprint to produce the equipment required.
Current raw materials, energy content and properties
In agriculture, manure is the most common substrate. The feed given to livestock is significantly broken down during digestion. The energy in a raw material for biogas comes from the dry matter. Most types of manure have a high moisture content, and therefore have a low energy content per volume.
Example of energy potential in manure:
Cattle, 8.5 % TS approx. 130 kWh per ton
Pig, 8 % TS approx. 150 kWh per ton
Chicken, 70 % TS approx. 1000 kWh per ton
(Max 20 % chicken manure)
The energy content of the gas is measured in kilowatt hours, kWh.
Biogas production increases considerably if you have access to other substrates in addition to manure. Food waste, fish sludge and slaughterhouse waste are substrates that have a much higher energy content than manure.
Example of possible energy potential in other substrates:
Sorted food waste from large households, 13 % TS approx. 750 kWh per ton
Offal, soft parts, 30% TS approx. 1950 kWh per ton
Fishing sludge, 20% TS approx. 1000 kWh per ton
Such substrates must be sanitized before they are brought into the farm, to avoid the risk of spreading diseases. There are separate regulations for the treatment of substrates brought into the facility and for the use of bioresidue from them.
Applications for biogas
- Heat production
The biogas produced in a biogas plant can be burned in a gas burner and used to heat water that goes into a water-based heating system. The heat can be used to heat houses, workshops and livestock buildings on the farm.
- Production of electric power
The biogas can also be used to power an internal combustion engine that can be connected to an electricity generator. Such a unit in a biogas plant is called CHP, «combined heat and power». The electricity produced can be used on the farm, and it is also important to collect the heat from the engine and use it to heat adjacent buildings.
- Upgrading biogas to fuel
Biogas can be used as motor fuel, and in this way replace fossil fuels. Biogas must be purified before it can be used as fuel. The methane content in the gas must be at least 98 % before it can be used as fuel. Usually, the gas is purified in larger purification plants linked to large common biogas plants. It is currently not profitable to build purification plants linked to biogas plants on farms.
What is bioresidue, and how can it best be utilized in agriculture?
Biogas is the residual product from biogas production in a biogas plant, and can be based on livestock manure, and other substrates such as food waste and slaughterhouse offal. Bioresidue is homogeneous and thin-flowing, and can be used as organic fertilizer for surface spreading and mulching. When spread on the surface, bioresidue infiltrates into the soil faster than regular liquid fertilizer. Bioresidue is a high-quality organic fertilizer, and contains the most important nutrients that plants need. Bioresidue can contain up to 4.5 kg N, 0.5 kg P and 2 kg K per m3 of fertilizer. The nutrient content will vary depending on the substrates used in the plant. Most of the nitrogen is in the ammonium form, NH4+, which in the soil is quickly converted to nitrate, NO3-. This is quickly absorbed by the plants. Biorest smells less after spreading than regular liquid fertilizer.
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