The SCR method uses NH3 (also urea, H2, HC, and CO, etc.) as a reducing agent to reduce NOx to N2 and H2O. NH3 selectively reacts only with NO but does not react with O2 in the flue gas, and O2 can promote the reaction of NH3 with NO.
The SCR denitrification device mainly includes an SCR reactor, an auxiliary system, an ammonia storage and treatment system, and an ammonia injection system. The core of SCR is SCR denitrification catalyst, which is usually made into honeycomb, plate or corrugated type.
The SCR catalyst is divided into high temperature (345-590°C), medium temperature (260-380°C) and low temperature (80-300°C). The suitable reaction temperature of different catalysts is different. The activity temperature window of vanadium-tungsten-titanium-based catalyst is 320-420°C. The optimal reaction temperature window is concentrated at 340-380°C.
The catalyst supports include TiO2, TiO2/SiO2, TiO2/silicate, Al2O3/SiO2, and activated carbon, and the support may be single-component or multi-component; its catalytically active component elements are oxides of W, Mo, and V. The development of composite oxides containing elements such as Fe, Ce, Mn, Bi, and Cu, as well as catalysts such as zeolite molecular sieves, carbon-based catalysts, and metal oxides.
SCR denitration technology is currently the most widely used flue gas denitration technology in the world. Its advantages are no by-products, no secondary pollution, simple device structure, mature technology, high denitration efficiency, reliable operation, and easy maintenance. The disadvantage is that the catalyst is deactivated. Residues of NH3 in the exhaust gas and under aerobic conditions, SO3 reacts with excess NH3 to produce corrosive and viscous NH4HSO4, which can cause damage to the tail flue equipment. The average life of the SCR catalyst is about 3a.
The deactivation mechanism of SCR catalysts is complex. Different coal species lead to different flue gas compositions and different SCR denitration catalyst formulations and molding processes, causing different SCR catalyst deactivation mechanisms. In general, the deactivation of the SCR catalyst includes alkali metal (Na, K) poisoning, ammonium salt poisoning, catalyst clogging, and mechanical wear. Fly ash flushes the surface of the catalyst to cause loss of active components, and thermal sintering results in transformation of the supported TiO2 crystal form.
Regeneration methods include: efficient cleaning, such as vacuum or compressed air soot blowing, ultrasonic cleaning and cleaning solution soaking or spraying; active ingredient supplements, such as re-impregnation and baking of active ingredients. Deactivated spent SCR denitration catalysts that have lost their recycling value need to be detoxified or utilized. The feasible approach is: to recover the precious metal or heavy metal in the used SCR denitration catalyst, such as V and/or W, and to recover the noble metal or heavy metal, the harmless used SCR denitration catalyst can be used as raw material for producing SCR denitration catalyst, or For making building materials.