How does it work?
With SCR technology, reduction of nitrogen oxide (more commonly known as NOx) reactions is designed to occur in an oxidizing atmosphere. It is referred to as selective because ammonia is used to reduce NOx levels inside a catalyst system. NOx is a major pollutant and ammonia, the reducing agent, is combined with NOx and converts it into a composition of water, nitrogen and a small fraction of carbon dioxide. These three by-products occur naturally in the atmosphere and are present in the air we breathe. Automotive-grade urea, or Diesel Exhaust Fluid, is typically used as a reductant source. In order to introduce the oxidizing ammonia, the diesel exhaust fluid is hydrolyzed rapidly. With the use of selective catalytic reduction technology, levels of NOx emissions may be reduced by approximately 90%.
Selective catalytic reduction of nitrogen oxides with the use of ammonia or urea may be simply called SCR, have been improved and tested in industrial stationary applications. In the late 1970s, SCR technology was first tested in Japan’s thermal power plants. By the middle of 1980s, it was already widely used in Europe. SCR came to the United States in the 1990s and was first used on gas turbines. It showed such promised that it could be used on coal-fired power plants to reduce NOx emissions. Aside from gas turbines and coal-fired power plants, SCR was also used on equipment used on chemical processing industry, coke ovens, furnaces, waste plants for municipalities and incinerators. These applications use a variety of fuels such as natural gas, industrial gases, light or heavy oil, crude oil and pulverized coal.
Because of the rise of several emission standards, SCR is the only technology that has proved to deliver high results by reducing NOx emissions level down to the required amount.
SCR systems utilized two types of ammonia, pure anhydrous ammonia and aqueous ammonia. Pure anhydrous ammonia is found to be toxic and hazardous to health. Due to high vapour pressure produced by anhydrous ammonia, it needs thick-shell, pressurized storage tanks and piping. Aqueous ammonia, on the other hand, is easier to manage and less hazardous. Its vapour pressure is almost atmospheric under normal temperature which makes it easier to transport through highways.