FACT SHEET Upgrading Biogas to Biomethane
On-farm digester model source: FNR On farm Biogas plant Introduction to Anaerobic Digestion (AD) Anaerobic Digestion (AD) is the process whereby organic matter is broken down by bacteria and enzymes in an oxygen-free environment. The organic matter is released as biogas; this is a mixture of the combustible gas methane (50-75%), carbon dioxide (25-45%), small amounts of water (2-7%) and trace gases. This process occurs in bogs, landfill and in the stomachs of animals. The type of feedstock used by anaerobic digesters varies; it can include pig or cattle slurry, food waste, energy crops (grass silage, maize-silage, grain), municipal solid waste from households and organic solid waste from industry. Materials with high lignin content, e.g. any kind of wood, are not suitable for biogas production. Feedstock is pumped into a closed vessel (digester) which has been inoculated with suitable bacteria. Anaerobic (0% oxygen) conditions are then maintained in the vessel and the temperature is held at a constant value (typically 40 C). The produced biogas can be upgraded to natural gas (fossil) quality and injected into the gas grid or used as a vehicle fuel but is normally used on site to generate heat and electricity in a Combined Heat and Power unit (CHP). The biogas yield depends on the composition of the feedstock and on the ambient conditions in the digester (e.g. temperature, retention time). It is possible that the same feedstock could have different gas yields. From one cubic meter biogas approximately 2 kwh electricity and 2 kwh heat can be produced depending on the CHP unit and gas composition (e.g. 55% CH4 content in the biogas, 20 mega joule (MJ)/ m 3, 38% electrical and thermal efficiency CHP unit). The residue or digestate of the AD process can be separated into a liquid and fibrous fraction. The liquid can be returned to the land as a high value fertiliser and the solid fibre used as a soil conditioner. Fermentation improves the quality of manure as nutrients are more available for plants and pathogens and weed seeds are killed. Furthermore, as odours are broken down and neutralised during the fermentation process the development of odours during liquid manure storage and spreading is greatly reduced. Organic products from industry which are used to produce biogas provide interesting agricultural opportunities. By using organic residues such as distiller s pulp, grease or food wastes, the natural material cycles (carbon and nitrogen) is closed and provides a recirculation of the nutrients into agriculture. Biogas technologies contribute to environmental protection by releasing no carbon dioxide (CO 2 ) in comparison to fossil fuels. Energy from biogas is largely CO 2 neutral because the CO 2 released from burning biogas was already removed from the atmosphere through photosynthesis. The fermentation of manure also reduces emissions from methane, a gas that would have far more devastating effects on the climate than CO 2 if it escaped uncontrolled from raw liquid manure. Biomethane Upgrading of biogas has gained increased attention due to rising oil and natural gas prices and increasing targets for renewable fuel quotas
in many countries. New plants are continually being built. According to the International Energy Agency Task 37 (Energy from Biogas) the number of upgrading plants was around 100 in 2009. Source: IEA bioenergy Task 37: Energy from Biogas Biogas can be upgraded to biomethane and injected into the natural gas grid to substitute natural gas or can be compressed and fuelled via a pumping station at the place of production. The advantage of injection and gas utilisation for example by a CHP in a village, is that both electricity and heat can be fully used. In addition, the medium or high pressure gas grid can serve as storage because pressure can vary. Injected biomethane can be used at any ratio with natural gas as vehicle fuel. Biomethane as vehicle fuel Utilisation of biogas in the transport sector is a technology with great potential and with important socio-economic benefits. Biogas is already used as vehicle fuel in countries like Sweden, Germany and Switzerland. The number of private cars, public transportation vehicles and trucks driven on biogas (biomethane) is increasing. Biomethane can be used as fuel in the same way and by the same vehicles like the natural gas. An increasing number of European cities are exchanging their diesel buses with biomethane driven ones. Many biogas driven private cars are converted vehicles which have been retro-fitted with a compressed gas tank and a gas supply system, in addition to the fossil fuel system. There are also specially built biogas vehicles, which are optimised for better efficiency and more convenient placement of gas cylinders, without losing luggage space. The biogas is stored at 200 to 250 bars, in pressure vessels, made of steel or aluminium composite materials. Today, more than 50 manufacturers worldwide offer some 250 models of commuter, light and heavy duty gas driven vehicles. Heavy duty vehicles can be converted to run on methane gas only, but in some cases also dual fuel engines are used. A dual fuel engine uses a diesel injection system and the gas is ignited by injection of a small amount of diesel oil. Dual fuel engines require less engine development and maintain the same driveability as a diesel vehicle. Biomethane vehicles have substantial overall advantages compared to vehicles equipped with petrol or diesel engines. The overall carbon dioxide emissions are drastically reduced, depending on the feedstock substrate and origin of electricity (fossil or renewable) used for gas
upgrading and compressing. Emission of NOx and Non Methane Hydrocarbons (NMHC), particles and soot are also reduced, even compared with very modern diesel engines, equipped with particle filters. At the end of 2010 there were more than 1.4 million natural gas vehicles (NGV) running in Europe including 145,000 buses and 108,000 trucks. In total, there were 2,600 public and 1,100 private fuelling stations available. Most of the private stations are city owned to refuel public transport buses and waste trucks. Sweden is leading the way in biomethane with a 55% share of all gas used in vehicles followed by Switzerland with 22%. So far nine countries in Europe allow injection of biomethane. Most of the gas driven light duty vehicles are retro-fitted by specialized workshops. Some others are factory built like from Fiat, Opel or Volkswagen. Biomethane for grid injection Biomethane can be injected and distributed through the natural gas grid, after it has been compressed to the pipeline pressure. In many EU countries, the access to the gas grid is guaranteed for all biogas suppliers. There are several advantages of using the gas grid for distribution of biomethane. One important advantage is that the grid connects the production site of biomethane, which is usually in rural areas, with more densely populated areas. This enables the gas to reach new customers. It is also possible to increase the biogas production at a remote site, without concerns about utilisation of heat excess. Grid injection means that the biogas plant only needs a small CHP unit for the process energy or a biogas burner. Sweden, Switzerland, Germany and France have introduced standards (certification systems) for injecting biogas into the natural gas grid. The standards, prescribing the limits for components like sulphur, oxygen, particles and water dew point, have the aim of avoiding contamination of the gas grid or the end users. The Wobbe index was introduced, to avoid influence on gas measurements and end use. The standards are in most cases easily achievable through current upgrading processes. For this kind of application, landfill gas can be difficult to upgrade to acceptable quality due to its high nitrogen content. In Europe, biogas feed plants are in operation in Sweden, Germany, Biogas pipes Austria, the Netherlands, Switzerland and France. The main barriers for biomethane injection are the high costs of upgrading and grid connection. Grid injection is also limited by location of suitable biomethane production and upgrading sites, which have to be close to the natural gas grid. Cleaning of biogas for upgrading Biogas can be distributed through the existing natural gas networks and used for the same purposes as natural gas or it can be compressed and used as renewable vehicle fuel. Prior to injection into the natural gas grid or to utilisation as vehicle fuel, biogas must undergo an upgrading process, where all contaminants as well as carbon dioxide are removed and the content of methane must be increased from the usual 50-75% to more than 95%. The upgraded biogas is often named biomethane. Various technologies can be applied for removal of contaminants. Removal of carbon dioxide is done in order to reach the required Wobbe index of gas. When removing carbon dioxide from biogas, small amounts of methane (CH 4 ) are also removed. As methane has
a 23-fold stronger greenhouse gas effect than CO 2, it is important to keep methane losses low, for both economic and environmental reasons. Two common methods of removing carbon dioxide from biogas are absorption (water scrubbing, organic solvent scrubbing) and adsorption (pressure swing adsorption, PSA). Less frequently used are membrane separation, cryogenic separation and process internal upgrading, which is a relatively new method, currently under development. Apart from methane and carbon dioxide, biogas can also contain water, hydrogen sulphide, nitrogen, oxygen, ammonia, siloxanes and particles. The concentrations of these impurities are dependent on the composition of the substrate from which the gas was produced. In those upgrading technologies where carbon dioxide is separated from the biogas, some of the other unwanted compounds are also separated. However, to prevent corrosion and mechanical wear of the upgrading equipment itself, it can be advantageous to clean the gas before the upgrading. Several techniques for biogas upgrading exist today and they are continually being improved. In parallel, new techniques are under development. These new developments, both for new and more traditional techniques, can lower investment costs and operational costs. The developments can also lead to other advantages such as lower methane emission which is important from both an economical and environmental perspective. Upgrading technologies Upgrading of biogas or landfill gas to Biomethane is defined as removal of carbon dioxide from the biogas. This will result in an increased energy density since the concentration of methane is increased to up to 95%. Several technologies for biogas upgrading are commercially available and others are at the pilot or demonstration level. For more detailed information please visit www.iea-biogas.net Pressure Swing Adsorption (PSA) l Carbon dioxide adsorption by activated carbon or zeolites under elevated pressure l Regeneration by decrease in pressure l Four to nine upgrading vessels in parallel l Water and hydrogen sulphide have first to be removed Water Scrubbing l Most common upgrading technology l Carbon dioxide dissolves in water (more soluble that methane) l Methane dissolves to a much lower extend l Increase concentration of methane in the gas l Dissolved methane recovered in flash tank l Water regenerated in desorption column (Carbon dioxide is released) Organic physical scrubbing l Similar to water scrubbing l Carbon dioxide is absorbed in an organic solvent such as polyethylene glycol l Organic solution is regenerating by heating Chemical scrubbing l Chemical scrubbers use amine solution l Carbon dioxide binds chemically to liquid but also reacts chemically with the amine (e.g. MEA, Monoethanolamine) l Selective reaction l Low methane losses l Regeneration by heating of the solution Other technologies are currently under development such as separation by membranes. For more information about biogas and Biomethane please visit: www.seai.ie/bioenergy/ www.iea-biogas.net Biogas Verstaerker, Ronnenberg Sustainable Energy Authority of Ireland Wilton Park House Wilton Place Dublin 2 Ireland t +353 1 808 2100 f +353 1 808 2002 e info@seai.ie w www.seai.ie The Sustainable Energy Authority of Ireland is partly financed by Ireland s EU Structural Funds Programme cofunded by the Irish Government and the European Union