| The method of converting the harmful nitrogen oxide | | | | commonly used reluctant. Thus the Selective Catalytic |
| emissions (NOx) into harmless nitrogen gas and water, | | | | Reduction technique is known to reduce the harmful |
| with the help of a catalytic reaction is called Selective | | | | nitrogen oxides by nearly ten fold. |
| Catalytic Reduction or SCR. The process of SCR | | | | Limitations of SCR catalyst in power plants |
| involves the catalytic reaction of ammonia steam or | | | | The most common limitation of SCR in power plants is |
| ammonia air injected into the exhaust pipes containing | | | | the ammonia slip. This refers to the release of |
| NOx gases. The gases mix thoroughly and on passing | | | | ammonia that has not been used in the chemical |
| through a catalytic bed, the NOx is reduced. The | | | | reaction of SCR. This happens when the required |
| catalyst only promotes the reduction but is not | | | | temperature levels necessary for the chemical |
| affected by it. When urea is used as a reducing agent, | | | | reaction are not maintained or when there is too much |
| the general by product of such a reaction is carbon | | | | ammonia injected into the system. Another common |
| dioxide. | | | | problem faced by the SCR catalyst technology in |
| The commercial viability of SCR technique is visible as | | | | power plants is the emission of ammonium sulfate and |
| it is found in large utility boilers, solid waste boilers and | | | | ammonium bisulfate. With high requirements of |
| industrial boilers, diesel engines used in large ships, gas | | | | ammonia to be injected for the chemical process, the |
| turbines, diesel locomotives, automobiles and power | | | | excess ammonia combines with the sulfates in the air |
| plants. The SCR catalyst technology is impressive as it | | | | to form ammonium sulfate. This ammonium sulfate is |
| is known to reduce the NOx emissions by nearly | | | | known to cause shortness of breath, respiratory |
| 70-95%. | | | | inflammation and coughing. |
| Working of the SCR catalyst in power plants | | | | How much NOx is removed by SCR catalyst? |
| Power plants emit huge volumes of NOx as they | | | | It has been seen that with the use of proper SCR |
| derive their power from the combustion of fuels. With | | | | catalysts and system design, effective levels of more |
| the new technique of SCR catalyst, these emissions | | | | than 90% of NOx removal can be achieved. Several |
| can be reduced considerably. The mechanical process | | | | coal-fired boilers in Virginia and New Hampshire having |
| of SCR involves a reactor chamber with a catalyst | | | | the retrofit SCR catalyst systems are known to |
| bed containing catalyst modules and an ammonia | | | | reduce the NOx levels by 60 to 65% during severe |
| injection grid system. | | | | load timings. This figure is likely to be bettered to 85%. |
| The ammonia air or steam is injected through the | | | | Even gas-fired utility boilers are known to be benefited |
| exhaust pipes containing the obnoxious NOx gases | | | | by the SCR technique, where the emission levels |
| and on passing through the catalytic bed reduces the | | | | achieved are more than 90%, thus not limiting the |
| oxides of nitrogen into water and benign nitrogen gas. | | | | facility to large coal-fired boilers only. The SCR |
| The reductants used by the SCR catalyst technique | | | | technique has known to reduce the emissions by up to |
| are aqueous ammonia, anhydrous ammonia or urea. All | | | | 95% in gas turbines and power plants. |
| three have their limitations, but urea is the most | | | | |