SCR (Selective Catalytic Reduction) is a selective catalytic reduction technology. At present, ammonia catalytic reduction method is the most widely used technology. It has no by-products, no secondary pollution, simple device structure, high removal efficiency (up to 90%), reliable operation, and easy maintenance. Selectivity means that in the presence of oxygen and in the presence of oxygen, NH3 preferentially undergoes a reduction and removal reaction with NOx to form nitrogen and water, without oxidizing with the oxygen in the flue gas.