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Combustion Turbines MACT Standards Development
Background
Combustion turbines are fuel fired devices in which hot combustion gases pass through and turn a turbine rotor which produces shaft power. The hot exhaust gases are emitted directly to the atmosphere in some cases and in other cases (where the heat can be utilized) is passed through a heat recovery device which extracts excess heat from the exhaust gases. The efficiency for getting the heat produced during combustion into useful power is about 30% for situations where heat is not recovered, and is as high as 60% when heat is recovered. The trend is to use the higher efficiency heat recovery configuration whenever possible.
Combustion turbines are used by the electric utility industry and also by independent power producers (IPP's) to produce electricity, the gas pipeline industry to maintain pressure in gas pipelines, and at chemical and industrial plants to produce both heat and shaft power. Combustion turbines are also used in standby and emergency modes to provide electric power when the normal electric power is lost at chemical and industrial plants, at hospitals, etc.
It is estimated that there are about 8000 existing turbines in the US. The size of combustion turbines vary from 1MW to about 200MW. One MW is equivalent to about 1200 horsepower (HP) and 200MW is about 240,000HP.
Types of Fuel Used
Combustion turbines usually burn pipeline quality natural gas which is a very clean burning fuel. Number 2 diesel oil is burned in some cases mainly as a back-up for natural gas fired units (i.e. dual fuel units) and in some emergency and stand-by turbines. Very few turbines burn landfill or digester gas.
Pollutants Emitted
Even though combustion turbines are very efficient combustion devices, products of incomplete combustion lead to the formation of some hazardous air pollutants (HAPs). The following HAP's have been measured above the emission test method detection limits from combustion turbines: acetaldehyde, acrolein, benzene, ethyl benzene, formaldehyde, naphthalene, PAH, toluene, and xylene. The concentrations of these HAP's are low with many concentrations being very close to the emission test methods detection limits. A large turbine could emit about 10 ton/yr of combined HAP's, with formaldehyde accounting for about half of the HAP emissions.
Controls
There are no specific controls for HAP's on existing turbines. However, oxidation catalyst systems are installed on some turbines to control CO should also reduce HAP emissions. Some combustion turbines are designed so that they emit low CO emissions. Low CO is an indication of very good combustion and therefore HAP emissions should also be low. Very little HAP emissions data are available for these technologies and EPA may conduct emission tests to determine the emission control effectiveness of these technologies.