NOX contributes to acid deposition and eutrophication which in turn can lead to potential changes occurring in soil and water quality. The subsequent impacts of acid deposition can be significant, including adverse effects on aquatic ecosystems in rivers and lakes and damage to forests, crops and other vegetation. Eutrophication can lead to severe reductions in water quality with subsequent impacts including decreased biodiversity, changes in species composition and dominance, and toxicity effects. In many cases, the deposition of acidifying and eutrophying substances still exceeds the critical loads of the ecosystems. It is NO2 that is associated with adverse effects on human health, as at high concentrations it can cause inflammation of the airways. NO2 also contributes to the formation of secondary particulate aerosols and tropospheric ozone in the atmosphere both are important air pollutants due to their adverse impacts on human health. NOx is therefore linked both directly and indirectly to effects on human health. There are many ways to significantly reduce NOx emissions such as SCR technology.
Selective catalytic reduction of NOx employs a system that injects a reductant, also known as diesel exhaust fluid (DEF), into the exhaust stream where it reacts with a catalyst to convert NOx emissions to N2 (nitrogen gas) and oxygen. The catalytic reaction requires certain temperature criteria for NOx reduction to occur. Selective catalytic reduction of NOx by nitrogen compounds, such as ammonia or urea has been developed for and well proven in industrial stationary applications. The SCR technology was first applied in thermal power plants in Japan in the late 1970s, followed by widespread application in Europe since the mid-1980s. In the USA, SCR systems were introduced for gas turbines in the 1990s, with increasing potential for NOx control from coal-fired power plants. In addition to coal-fired cogeneration plants and gas turbines, SCR applications also include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, as well as municipal waste plants and incinerators.
In an SCR system, exhaust gas from the combustion process is cooled and ammonia-containing air is injected into the flue gas prior to it passing over a catalyst. The catalyst promotes a reaction between NOx and ammonia to form nitrogen and water vapor. When properly designed and operated, SCR systems commonly provide NOx reductions in the range of 50% to 90%.