Biogas production and use

Biogas production is a microbiological fermentation process in which complex molecules of organic matter are broken down into simpler molecules such as alcohol, CO2, CH4, volatile acids etc. under the absence of oxygen. This natural reaction, also called anaerobic digestion, is used in a variety of applications such as the fermenting of bread or the brewing of beer. The biogas obtained is an energy-containing gas, like natural gas, but with lower caloric value. The biogas is generally valorised energetically by combustion in a combined heat and power engine (CHP) or by upgrading to natural gas quality and injection into the gas grid.

Input

Mainly organic residual flows are used as feedstock for biogas production. These can be energy crops, plant by-products, animal by-products, biowaste from households or industrial & commercial organic waste. Additionally, biogas can be extracted from wastewater streams or landfills.

Output

Biogas has an energy value of 23.4 MJ/ Nm³. This is slightly lower than its fossil natural gas counterpart with a gross combustion value of 35 MJ/ Nm³ (L gas) and 43 MJ/ Nm³ (H gas). Biogas consists of 45 – 85 vol% methane and 25 – 50 vol% carbon dioxide. Carbon dioxide is a non-flammable gas and therefore does not contribute to the caloric value of biogas. Thus, the caloric value depends entirely on the methane content.
Biogas as an energy carrier has a competitive advantage compared to instantaneous energy generation by sun and wind. After all, the energy is stored in a chemical form and can be stored over long periods of time. Biogas can offer a solution for various types of energy demands.

Electricity & heat

Combined heat and power gas engine (CHP) is the most common practice valorisation route for biogas in Europe. The idea behind CHP is that the co-generation of electrical and thermal energy is more efficient than generating them separately. Depending on the design of the biogas plant, part of the heat and electricity from the CHP is used for the fermentation process itself. For example, the biogas reactors themselves have a heat demand to stay at the right temperature. Depending on the process, a temperature of 37-38 ° C (mesophilic) or 55-57 ° C (thermophilic) will be maintained. Drying and / or evaporation is also often used for the treatment of digestate, the residual product after fermentation, whereby most of the heat is consumed.

Biomethane & biofuel for transport

Biomethane is a purified form of raw biogas from anaerobic digestion that can be used as a natural gas substitute. Specifically, this means CO2, H2O, H2S and other impurities are removed, leaving a high-caloric and pure gas. Biomethane or green gas is therefore a sustainable and renewable form of (fossil) natural gas. It can be used for all natural gas end-user applications. This means it can also be used as biofuel for transport in the form of a CNG substitute, called bio-CNG.

Digestate

Anaerobic digestion can potentially provide a company with large volumes of biogas, but only 5 – 10% of the biomass is actually converted into biogas. The residual mass that remains, the digestate, must be further processed or sold to farmers.
Digestate is a stable form of very slow or unbreakable organic material, enriched with the nutrients that were released from the biomass during the fermentation process. Digestate has a typical dry matter content of 6 to 10%. The organic nitrogen in the feedstock is converted to the ammoniacal form in the digestate, a plant-available form. Therefore, digestate can be considered as a more valuable fertilizer than the biomass which initially composed it.

Carbon dioxide

After purification of biogas to biomethane, carbon dioxide is left as a side-product. The carbon dioxide stream can be valorised in food industry or can be used to maximize photosynthesis potential in greenhouses.

Definitions

Anaerobic Digestion (AD)
Is a biological process in which microorganisms break down biodegradable material in the absence of oxygen creating two important products: biogas and digestate. AD makes the best use of organic materials by producing biogas for the generation of renewable heat, electricity, fuel and fertilizer while closing the nutrients cycle and reducing greenhouse gas emissions.
Thermal gasification (Gasification)
Is a physico-chemical oxygen depleted process in which the carbon containing components of the biomass break down to syngas instead of being completely combusted. It is a complementary technology to anaerobic digestion and greatly amplifies the potential of renewable energy in the form of heat, electricity and vehicle fuel.
Feedstock
AD can process almost any biogenic material including solid and liquid manure, energy crops, catch crops, agricultural waste and residues, industrial food and beverage waste, sewage sludge and the organic fraction of municipal solid waste. Gasification can theoretically process any carbon containing material and is a complementary technology to Anaerobic Digestion (AD), since it can treat high-solids feedstock with low anaerobic biodegradability; these include lignocellulosic feedstocks such as wood chips, and non-recyclable waste fractions of biomass origin currently landfilled or incinerated for energy recovery.
Biogas
The primary product of AD is a methane-rich renewable gas composed of 45 – 85 vol% methane and 25 – 50 vol% carbon dioxide.
Digestate
Remaining part of organic matter treated by AD, rich in nutrients and nitrogen, commonly used as an organic fertilizer in agriculture.
Syngas
The primary product of gasification is a mixture of carbon monoxide and hydrogen, with traces of methane and carbon dioxide. It may be used directly for electricity generation, or further transformed to increase its share of methane.
Biomethanation
Besides methane formed spontaneously during gasification, syngas can be transformed into methane through two catalyst aided reactions: the water-shift reaction (hydrogen and carbon dioxide formed from carbon monoxide and water) and the Sabatier reaction (methane formed from carbon dioxide and hydrogen).
Biomethane
When carbon dioxide and trace gases in biogas are removed, a methane rich renewable natural gas substitute is left in the form of biomethane. Biomethane can be injected into the gas grid, used as a vehicle fuel or used for combined heat and electricity generation.

Documents

Biogas Basics