The Process of Selective Catalytic Reduction for Reducing NOx Emissions

The method of converting the harmful nitrogen oxide emissions (NOx) into harmless nitrogen gas and water, with the help of a catalytic reaction is called Selective Catalytic Reduction or SCR. The process of SCR involves the catalytic reaction of ammonia steam or ammonia air injected into the exhaust pipes containing NOx gases. The gases mix thoroughly and on passing through a catalytic bed, the NOx is reduced. The catalyst only promotes the reduction but is not affected by it. When urea is used as a reducing agent, the general by product of such a reaction is carbon dioxide.

The commercial viability of SCR technique is visible as it is found in large utility boilers, solid waste boilers and industrial boilers, diesel engines used in large ships, gas turbines, diesel locomotives, automobiles and power plants. The SCR catalyst technology is impressive as it is known to reduce the NOx emissions by nearly 70-95%.

Working of the SCR catalyst in power plants
Power plants emit huge volumes of NOx as they derive their power from the combustion of fuels. With the new technique of SCR catalyst, these emissions can be reduced considerably. The mechanical process of SCR involves a reactor chamber with a catalyst bed containing catalyst modules and an ammonia injection grid system.

The ammonia air or steam is injected through the exhaust pipes containing the obnoxious NOx gases and on passing through the catalytic bed reduces the oxides of nitrogen into water and benign nitrogen gas. The reductants used by the SCR catalyst technique are aqueous ammonia, anhydrous ammonia or urea. All three have their limitations, but urea is the most commonly used reluctant. Thus the Selective Catalytic Reduction technique is known to reduce the harmful nitrogen oxides by nearly ten fold.

Limitations of SCR catalyst in power plants
The most common limitation of SCR in power plants is the ammonia slip. This refers to the release of ammonia that has not been used in the chemical reaction of SCR. This happens when the required temperature levels necessary for the chemical reaction are not maintained or when there is too much ammonia injected into the system. Another common problem faced by the SCR catalyst technology in power plants is the emission of ammonium sulfate and ammonium bisulfate. With high requirements of ammonia to be injected for the chemical process, the excess ammonia combines with the sulfates in the air to form ammonium sulfate. This ammonium sulfate is known to cause shortness of breath, respiratory inflammation and coughing.

How much NOx is removed by SCR catalyst?
It has been seen that with the use of proper SCR catalysts and system design, effective levels of more than 90% of NOx removal can be achieved. Several coal-fired boilers in Virginia and New Hampshire having the retrofit SCR catalyst systems are known to reduce the NOx levels by 60 to 65% during severe load timings. This figure is likely to be bettered to 85%. Even gas-fired utility boilers are known to be benefited by the SCR technique, where the emission levels achieved are more than 90%, thus not limiting the facility to large coal-fired boilers only. The SCR technique has known to reduce the emissions by up to 95% in gas turbines and power plants.