Understand the cost of selective catalytic reduction (SCR) is primarily from the point of: what its primary function is; and what technological features it has and what applications it has.? SCR is a process of reducing nitrogen oxides (NOx) in exhaust gas from diesel engines. It functions through the use of a liquid-reduction agent, usually urea, injected into the exhaust stream. This urea will then decompose to form ammonia. When ammonia contacts with NOx (nitrogen oxides), also over a catalyst in this case its reaction turns nitrogen and water - avoiding harmful emissions from combustion products altogether. Because SCR is a totally dry process,no additional water is needed to carry out these reactions beyond that naturally present in the exhaust. Technological features of SCR include use of advanced catalysts, precise control systems to optimize the reduction process, etc. SCR systems are used widely in fields as various as automotive, marine and electric power generation systems, where they perform a critical role in meeting US and EU emission regulations. Investment in SCR technology is costly, but it also brings long-term environmental and economic benefits.