Biogas is obtained from anaerobic digestion process, a fermentation process, which takes place in a closed airtight digester where organic raw materials such as manure, food waste, sewage sludge and organic industrial waste are converted into biogas and digestate as products. The produced biogas is a mixture of 50-70% methane and 30-50% carbon dioxide and smaller amounts of water vapor, hydrogen sulphide and other minor components and trace elements. The wet digestate results from anaerobic digestion of the substrates, which are pumped out of the digester tank, after the extraction of biogas.
Very simple biogas digesters have been in use in China, India and many other Asian countries for many years. Industrial applications of biogas production started well over 50 years ago as a means of stabilizing sewage sludge at waste water treatment plants. The biogas industry expanded in the 1970’s and 1980’s as increased production of different organic materials (such as manure and industrial wastewater from sugar refinery and pulp mills) became more widely used. Starting in the mid 1990’s extraction of landfill gas (low quality biogas) came to the fore, along with the construction of farm-based biogas plants and anaerobic digestion of solid wastes from food industry and food waste. After 2000’s, there was an increased interest in biogas and so, construction of farm-based biogas plants took place and an industrial sector was established.
The multiple functions of biogas in circular economy:
1. Biogas: a part of the modern society’s energy supply system
Biogas, made from organic waste streams, does not add to the carbon dioxide load in the atmosphere. Carbon dioxide produced during combustion of biogas is offset by either the carbon dioxide consumed by the biomass, which is digested. Biogas is thus a “green” sustainable energy vector and has a significant role in shifting to a sustainable decarbonized society. Biogas has many uses in the sustainable society that can be utilized in a broader perspective than today. Industries, as well as households, can use biogas for heating and hot water supply. Biogas can be used to supply warm air for drying, for example, in laundries, carpentries, industrial coating facilities and other places where there is a need for fast and efficient drying. The exhausts from upgraded biogas combustion are clean and do not generate odours or particles.
2. Biogas used for heat and electricity production
The most common use of biogas is in a non-upgraded form for production of electricity and heat production. The default use of biogas is for CHP (Central Heat and Power) production, which is in fact production of renewable electricity and heat, also known as cogeneration. The heat from the CHP engine can also be used to drive an absorption chiller to give a source of cooling, resulting in a combined cooling, heat and power (CCHP), also known as trigeneration. The utilization of the renewable heat is very important, as it brings about significant additional economic and environmental benefits, on top of the utilization of biogas for renewable electricity production.
3. Upgraded biogas or ‘bio-methane’ used as vehicle fuel
Raw biogas can be upgraded in a process which removes hydrogen sulphide, water, particles and CO2 present in the gas. The process creates a gas consisting mainly of methane and thus increases its energy content. Clean upgraded biogas is used as fuel for cars, buses and trucks of various sizes. In several countries, there is a well-developed infrastructure for vehicle gas, and it is possible to fuel natural gas vehicles (NGVs) in the most densely populated areas of such countries. Today, vehicle gas like CNG, LPG is used mostly for buses, trucks and passenger cars.
4. Upgraded biogas ‘bio-methane’ injection into the gas grid
Biomethane from renewable sources is also fed into the national transmission network for natural gas in several countries.
5. Reduction of Green House Gas (GHG) emissions
One of the main reasons for a transition from fossil energy and fuel to renewable energy and fuel is the reduction in greenhouse gas (GHG) emissions. The production of biofuels and bioenergy contributes to a significant reduction of GHG emissions. In many areas around the world, organic substances, considered as waste, are still deposited in landfill sites where they decompose, releasing methane (CH4) with a global warming potential (GWP) 21 times that of CO2. When these streams of organic waste are redirected from landfill to a biogas facility, a significant reduction in methane emissions from landfills occurs.
6. Improved nutrient up-take efficiency in agriculture
Intensive agriculture is one of the major greenhouse gas sources worldwide. These emissions are associated with enteric fermentation, management of manures and production of synthetic fossil fuel based fertilizers. Anaerobic digestion systems remove the easily degradable carbon compounds in feedstocks such as slurries, and converts them to biogas. When the remaining digestate is applied as biofertiliser, the slow to degrade carbon is recycled back to soils, contributing to build up of the humus content of the soil and its long-term suitability for agriculture. Macro and micro-nutrients contained in digestate are predominately in mineral form which makes them easily accessible to the plant roots, compared with nutrients in raw manure and slurry, which are mainly organic compounds, and must be mineralized in order to be up-taken by the plants. As such digestate has a higher nutrient uptake efficiency, compared with raw manure and slurries.
Organic matter in digestate can build up the humus content in the soil; this is a benefit unique to organic fertilizers, which is particularly crucial for arid and semi-arid lands with low carbon content. The destruction of weed seeds in the AD process is another significant benefit to organic farmers.
7. Energy security
Fossil energy is still in abundant use around the world. This energy comes in the form of coal, oil and natural gas from a relatively limited geographical region and is used worldwide. Many countries are thus dependent on a few countries for energy supply. A transition to a bio-based/renewable energy production system would better balance the energy supply situation around the world; more countries and regions would be able to become energy self-sustainable.
8. Optimal utilization of resources
In a sustainable society where resources are used efficiently, what previously was considered to be waste is instead included in a production circle where organic material and nutrients such as nitrogen and phosphorus are returned to the soil to replace chemical fossil fuel sourced fertilizer. When digesting municipal and industrial food waste such as waste from super markets and restaurants or slaughterhouse waste, biogas is produced, and valuable nutrients accumulate in the digestate where they are easily used as fertilizer. One ton of digested food waste produces 1200 kWh biogas energy, which is enough fuel to drive 1900 km with a gas fueled car. The food waste from 3000 households can fuel a gas bus for a year.
Some countries already have targets for energy recovery from food waste. The Swedish government, for example, has a target that at least half of all generated food waste from households, shops and restaurants be separated and treated to recover nutrients and that 40% is treated to recover energy by 2018.
9. Generating income in rural areas
The biogas plant itself is not labor intensive but it can create new business opportunities in rural areas which otherwise suffer from depopulation. Through collaboration with different farms, the biogas plant can create different job opportunities along the process chain, such as raw material cultivation and collection. By increasing local energy production, income stays in the local area instead of going to global energy markets.
In the future bio-economy, wastes will be transformed to highvalue products and chemical building blocks, fuels, power and heating; biogas facilities will play a vital role in this development, and in the implementation of the novel production paths that arise in the transition to a bio-economy.
The future of the biogas facility is a factory where value is created from previously wasted materials. This ensures sustainability of the environment and potential for financial gain for the local community. The flexibility of the anaerobic digestion system and its ability to digest a multitude of organic feedstocks, while producing a significant range of products ensures the role of anaerobic digestion and biogas in the circular economy.