Exhaust gas is emitted as a result of the combustion of fuels such as coal, diesel, gasoline, or natural gas. In most cases the exhaust gas may include a small part of toxic substances, such as carbon monoxide (CO) from incomplete combustion, and nitrogen oxides (NOx) from excessive combustion temperatures. In such cases, it is required to reduce the toxic substances from the exhaust gas before it is discharged into the atmosphere. For example, controlling NOx emissions from diesel engines is of crucial importance for the commercial application of advanced diesel engine technologies as stricter emission standards are coming into effect worldwide.
One of the widely used strategies for controlling NOx emissions from lean-burnt engines is to reduce NOx with a reductant to nitrogen over a selective catalytic reduction (SCR) catalyst. Among the various SCR technologies, the SCR of NOx by ammonia (NH3-SCR) offers some unique advantages such as high NOx reduction efficiencies over a wide range of temperatures, high selectivity to nitrogen (N2) and good catalyst stability when compared with the alternative technologies such as the hydrocarbon/urea-SCR and the lean NOx trap (LNT) technologies.
Currently, the ammonia is bought off-site and transported to the site where the exhaust stream is treated to reduce NOx. Devices for transporting and storing the ammonia, such as storage tanks are needed. The use of such additional devices increases the whole operating cost.
Thus there is a need for practical systems, which can reduce NOx in exhaust with more readily available materials.