The valuable component of biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases. The proportion of methane depends on the feedstock and the efficiency of the process, with the range for methane content being 40% to 70%. Biogas is saturated and contains H2S, and the simplest use is in a boiler to produce hot water or steam.
The most common use is where the biogas fuels an internal combustion gas engine in a Combined Heat and Power (CHP) unit to produce electricity and heat. In Sweden the compressed gas is used as a vehicle fuel and there are a number of biogas filling stations for cars and buses. The gas can also be upgraded and used in gas supply networks. The use of biogas in solid oxide fuel cells is also being researched.
Biogas can be combusted directly to produce heat. In this case, there is no need to scrub the hydrogen sulphide in the biogas. Usually the process utilize dual-fuel burner and the conversion efficiency is 80 to 90%. The main components of the system are anaerobic digester, biogas holder, pressure switch, booster fan, solenoid valve, dual fuel burner and combustion air blower.
The most common method for utilization of biogas in developing countries is for cooking and lighting. Conventional gas burners and gas lamps can easily be adjusted to biogas by changing the air to gas ratio. In more industrialized countries boilers are present only in a small number of plants where biogas is used as fuel only without additional CHP. In a number of industrial applications biogas is used for steam production.
Burning biogas in a boiler is an established and reliable technology. Low demands are set on the biogas quality for this application. Pressure usually has to be around 8 to 25 mbar. Furthermore it is recommended to reduce the level of hydrogen sulphide to below 1 000 ppm, this allows to maintain the dew point around 150 °C.
CHP Applications
Biogas is the ideal fuel for generation of electric power or combined heat and power. A number of different technologies are available and applied. The most common technology for power generation is internal combustion. Engines are available in sizes from a few kilowatts up to several megawatts. Gas engines can either be SI-engines (spark ignition) or dual fuel engines. Dual fuel engines with injection of diesel (10% and up) or sometimes plant oil are very popular in smaller scales because they have good electric efficiencies up to guaranteed 43%.
The biogas pressure is turbo-charged and after-cooled and has a high compression ratio in the gas engines. The cooling tower provides cooling water for the gas engines. The main component of the system required for utilizing the technology are anaerobic digester, moisture remover, flame arrester, waste gas burner, scrubber, compressor, storage, receiver, regulator, pressure switch and switch board.
Gas turbines are an established technology in sizes above 500 kW. In recent years also small scale engines, so called micro-turbines in the range of 25 to 100kW have been successfully introduced in biogas applications. They have efficiencies comparable to small SI-engines with low emissions and allow recovery of low pressure steam which is interesting for industrial applications. Micro turbines are small, high-speed, integrated power plants that include a turbine, compressor, generator and power electronics to produce power.
New Trends
The benefit of the anaerobic treatment will depend on the improvement of the process regarding a higher biogas yield per m3 of biomass and an increase in the degree of degradation. Furthermore, the benefit of the process can be multiplied by the conversion of the effluent from the process into a valuable product. In order to improve the economical benefit of biogas production, the future trend will go to integrated concepts of different conversion processes, where biogas production will still be a significant part. In a so-called biorefinery concept, close to 100% of the biomass is converted into energy or valuable by-products, making the whole concept more economically profitable and increasing the value in terms of sustainability.
One example of such biorefinery concept is the Danish Bioethanol Concept that combines the production of bioethanol from lignocellulosic biomass with biogas production of the residue stream. Another example is the combination of biogas production from manure with manure separation into a liquid and a solid fraction for separation of nutrients. One of the most promising concepts is the treatment of the liquid fraction on the farm-site in a UASB reactor while the solid fraction is transported to the centralized biogas plant where wet-oxidation can be implemented to increase the biogas yield of the fiber fraction. Integration of the wet oxidation pre-treatment of the solid fraction leads to a high degradation efficiency of the lignocellulosic solid fraction.
