Nitrogen oxide (NO x) emissions are the main pollutant from gas turbines. Nitrogen oxides include nitrogen dioxide (NO 2) and nitrogen oxide (NO). 90% to 95% of the nitrogen oxides that form from a combustion process are in the form of NO, but nitrogen oxide will, later photochemically react to form NO 2. Nitrous oxide (N2 O) can also form from NO x control processes and is sometimes a concern. Nitrogen oxide, nitrogen dioxide and sometimes nitrous oxide are produced in high enough concentrations to be considered pollutants.
The combustor is the source of NO x emissions for gas turbines, but cogeneration and dual cycle power plants usually have a duct burner which is an extra source of NO x emissions in the heat recovery steam generator (HRSG). The quantity of NO x emissions from the duct burner is independent of the combustor operation.
The main method for the control of NO x emissions from gas turbines include diluent injection, selective catalytic reduction, and low NO x burners. NO x control systems can be categorized by being a “front end” control or a “back end” control. A front end control is an attempt to control NOx emissions by preventing them from forming during combustion. A back end NO x control is an attempt to convert NO x emissions back to N2, the natural form of nitrogen in the atmosphere. The amount of NO x emissions produced is highly dependent on the fuel, the ambient conditions, the design of the combustor, and the percentage of the rated full power output that the engine is operating.
Diluent injection, also known as wet controls, water injection, or steam injection, is a popular method used to decrease NO x emissions. It involves injecting water or steam into the combustor of a gas turbine in order to quench the flame. This lowers the combustion zone temperature inside the combustor. As stated earlier, the formation of NO x compounds is a function of the temperature inside the combustion chamber and lower combustion temperatures will produce less NOx. The temperature of combustion is the main factor affecting NO x formation.
Older gas turbines primarily made use of water injection, but today’s modern gas turbines mostly use steam injection. Other factors such as the availability and cost of steam, impacts on maintenance and performance, and the availability of water or steam injection nozzles help determine the type of injection.
Catalytic combustion involves using a catalyst bed to oxidize a lean air/fuel mixture within a combustor instead of burning with a flame as a blow torch in a conventional combustor. In a catalytic combustor, the air/fuel mixture oxidizes at lower temperatures, producing less NO x.
One manufacturer currently has available a catalytic combustion system known as XononTM flameless combustor that is currently demonstratable with NO x emissions below 3ppm and carbon monoxide and unburned hydrocarbon emissions below 10 ppm with no other emission controls. This system will start undergoing official testing in a Kawasaki gas turbine in July of 1996 and will be commercially available for Kawasaki engines in 1997. This new combustor employs a “chemical thermostat” that prevents the catalyst from getting too hot.
