Gas Sensors used to Improve the Efficiency of “Green Energy Generation”
Growth in Biogas Electricity Generation
The generation of electricity from biogas is becoming ever more
popular, driven at least in part by the introduction of government set feed- in tariffs and other green project initiatives. A prime example of which comes from Germany, one of the first countries to introduce a “feed in
tariff” scheme around 10 years ago, resulting in a significant number of biogas plants being built.
The countries that were first in the
introduction of ‘feed in tariff’ schemes are also early adopters of biogas energy, have driven several green
projects promoting the installation of biogas plants and supported the continued growth within the biogas industry. The ‘feed in tariff’ schemes guarantee a minimum tariff payment to the generation plant owner, usually fixed for a long period, 5 to 10 years, or more, is not uncommon. The guaranteed minimum tariff is used as an incentive to support the return on investment on the large upfront costs required to purchase and install such plants.
In the last 20 years, biogas utilisation has been successful in wastewater treatment plants, industrial processing applications, landfill and the agricultural sector. The future increased use of biogas energy is a strong goal in most countries, not only because is it a renewable energy source but it will help in the reduction of greenhouse gas emissions, water pollution and soil degradation and it may influence agriculture sectors worldwide to produce green energy. Adoption of biogas production is growing and is supported by many countries looking for green alternatives to generate electricity for heating and lighting and as a fuel for motor vehicles to deliver goods and public transport.
It is important to ensure the quality and efficiency of the biogas produced as, before it can be used in the generation of electricity or in converted vehicles, it must be cleaned and converted to bio-methane.
What is Bio-methane?
Bio-methane is a cleaned version of biogas which is created from the natural processing of decomposing waste material in an anaerobic digester system. Anaerobic digestion is the natural breakdown process of any organic substances from animal or plant origin like household rubbish, food scraps from kitchens or food process plants etc.
The waste material is placed in a sealed large vessel, where anaerobic bacteria (those which function in a non-oxygen environment) decomposes the waste material producing and releasing gases like methane and carbon dioxide over a period of time. Biogas is generated from many sources of anaerobic digestion of waste, such as:
•Landfill sites •Wastewater treatment facilities •Food processing plants •Large livestock farms •Vegetable matter •Meat waste etc
Biogas production
Raw biogas produced from anaerobic digestion is flammable when mixed with air or oxygen, but is not of a high enough quality for gas resale or for the use of electricity generation. For biogas to be used to generate electricity, power cars or to be fed on to the national gas grid, it will need to be cleaned up or “Upgraded” and converted to bio-methane.
Upgrading
This process is used to remove the carbon dioxide, hydrogen sulphide and other contaminants which normally appear in biogas and which could easily damage the biogas machinery or limit the efficiency of electricity generation. The presence of hydrogen sulphide and high humidity will create corrosive acid which is strong enough, over a period of time, to corrode and damage the very expensive generation equipment.
Biogas upgrade facilities are also used to improve the quality of the biogas to meet the natural gas standards. These are the standards set by the gas industry to ensure the quality and calorific value of the gas fed in to the national gas grid. The consumer is therefore guaranteed then
Typical Contents of Raw Biogas Compound
Chemical Symbol
Methane
Carbon dioxide
Nitrogen Hydrogen
Hydrogen Sulphide
Oxygen
CH4 CO2
N2 H2 H2S O2 % Volume
50-75 25-50
0-10 0-1 0-3 0-2
amount of heat energy given off per litre of gas burnt. The upgrade system creates a purified biogas that is now called bio-methane, which can now be used to power modified vehicles or alternatively used to generate electricity. The electricity generated can be used locally or be fed back into the national grid to earn the feed in tariff payment. There are many process methods that can be used to upgrade biogas some of these include pressurised water scrubbing (PWS), pressure swing adsorption (PSA) and other chemical treatments.
Pressurised water scrubbing (PWS)
This is a physical absorption process where raw biogas is forced through a pressurised water tank. The carbon dioxide and some of the other gases like hydrogen sulphide and ammonia are absorbed by the water. This is the most common process used for upgrading biogas, especially in Europe.
This pressurised water scrubbing process can deliver 99% methane with the manufacturer guarantee of a maximum 1% methane loss in the system.
Pressure Swing Absorption (PSA)
This process cools the gas to very low temperatures, which forces the water in the biogas to condense and separate from the gas. Carbon dioxide is then removed by the use of active carbon materials to absorb the carbon dioxide and leave just the bio-methane.
Other chemical treatments
Other treatments utilises the pressure washing technique (PWS) but with the addition of a washing fluid as an additive to the water. The chemical additive increases the efficiency of removal of the carbon dioxide, hydrogen sulphide and other contaminants. A heat regeneration process is required to maintain the performance and keep the additive in the best condition. A replacement is required approximately every 10 years. Other chemical processes can involve washing the biogas with monoethanol amine (MEA) or diethanol amine (DEA) to again remove the contaminants and leave just the bio-methane.
The efficiency of the electricity generation is dependent on the quality of the bio-methane and the upgrading process. To ensure this process is effective for the efficient generation of electricity and to prevent damage to expensive equipment, measurements are required for the percent volume concentration of the methane and carbon dioxide plus the measurement of the ppm levels of corrosive gases like hydrogen sulphide. Gas generation efficiency is not mandatory for your personal generation needs but is recommended by the generator engine manufacturers to produce the best efficiency.
It is vital to check on the corrosive gas concentration in order to
IET
Annual Buyers Guide 2010
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