Denitration Catalyst Temperature Window: The "Golden Range" for Clean Combustion

In industrial fields such as coal-fired power plants and iron and steel metallurgy, nitrogen oxides (NOx) are among the primary atmospheric pollutants. Selective Catalytic Reduction (SCR) technology, as the most effective denitration method currently available, is centered on catalysts. The activity of catalysts is closely related to a key parameter — reaction temperature. This temperature is not set arbitrarily; it is strictly confined within a specific "window", and this range directly determines the efficiency, safety and economy of the denitration system.

Core Temperature Window: The "Active Range" of Catalysts

Currently widely used medium-temperature vanadium-titanium catalysts typically have a designed optimal temperature window ranging from 200℃ to 420℃. This is a "golden range" verified through long-term practical applications.

When the flue gas temperature is near the lower limit of this window (e.g., 300-320℃), the catalyst activity begins to fully manifest, effectively catalyzing the reaction between ammonia and NOx. As the temperature rises, the reaction kinetic conditions improve, and the denitration efficiency increases significantly, peaking in the middle of the window. However, when the temperature approaches or exceeds the upper limit of the window (above about 400℃), side reactions intensify, especially the direct oxidation of ammonia, which not only leads to the waste of reducing agents but also may cause permanent deactivation of the catalyst due to long-term high-temperature thermal sintering. If the temperature is too low (usually below 280℃), the reaction rate is too slow, and unreacted ammonia will escape, resulting in secondary pollution and corrosion of downstream equipment.

Dual Challenges of Temperature "Out-of-Bounds"

In actual operation, the problems caused by temperature deviating from the window are complex and severe.

On the high-temperature side, in addition to the sintering deactivation of catalysts, the migration of trace impurities such as arsenic and alkali metals in flue gas intensifies at high temperatures, which will block the catalyst pores or cover its active sites more quickly. Furthermore, excessively high temperatures pose a severe test to the reactor and its internal support structures.

On the low-temperature side, the challenges are equally formidable. The most intractable problem is the formation and deposition of ammonium bisulfate (ABS). When the temperature is below the sulfuric acid dew point and ammonia escape exists, SO₃ in the flue gas reacts with ammonia and water vapor to form ABS. This viscous liquid substance will firmly adhere to the micropores of the catalyst and the heat exchange elements of the downstream air preheater, causing catalyst blockage and deactivation and a sharp rise in the pressure difference of the air preheater, which seriously affects the safe operation of the unit.

Therefore, the design of the SCR system must arrange the catalyst between the boiler economizer and the air preheater to ensure that the flue gas temperature stably falls within this critical window. Accurate flue gas temperature measurement, reliable ammonia injection control, and operation strategies to cope with load fluctuations are all necessary means to safeguard this temperature range.

Expanding Boundaries: Exploration of Low-Temperature and High-Temperature Catalysts

To adapt to more complex industrial scenarios, the research and development of catalyst technology are constantly expanding the boundaries of temperature.

Low-temperature catalysts (with an operating temperature as low as 180-300℃) are a current research hotspot. They usually use transition metal oxides such as manganese and cerium as active components, aiming to arrange the SCR reactor after the electrostatic precipitator and desulfurization system, thereby avoiding abrasion, blockage and poisoning of the catalyst by fly ash in the flue gas, and can use low-cost steel to make the reactor to reduce costs. However, their sulfur and water resistance stability and long-term activity retention are still key difficulties in industrial application.

On the other hand, high-temperature catalysts (capable of withstanding temperatures above 450℃) are oriented to special high-temperature flue gas environments such as biomass boilers and gas turbines, or used to cope with abnormal boiler operating conditions. Such catalysts must have extremely excellent thermal stability and anti-sintering ability.

The temperature window of denitration catalysts is an accurate scale where scientific principles intersect with engineering practices. It is not only the result of catalyst formulation, but also one of the core bases for the design of the entire flue gas purification system. In-depth understanding and precise control of this "golden range" are the foundation for achieving efficient, stable and long-life denitration operation. In the future, with the progress of materials science, catalysts with wider temperature ranges and stronger resistance will continue to emerge, but the core principle of "conducting efficient reactions at appropriate temperatures" will never change. The ability to control the temperature range is always a key benchmark for measuring the level of denitration technology.