What are the types of catalysts used in SCR?

Catalysts are classified into honeycomb, corrugated plate, and plate types, with the choice depending on the type of dust and site conditions. Based on temperature, it is divided into high-temperature and low-temperature categories, with the boundary set at 280–320°C. At present, high-temperature applications above 320°C are well developed in China, while low-temperature applications are not yet as effective in many manufacturers’ operations. Also, when classified by composition, it’s relatively complex; for example, there are considerations such as toxicity or non-toxicity (based on the vanadium content)

There are three main types of SCR catalysts available on the market today: honeycomb, plate, and corrugated plate. Honeycomb format is used more frequently.

Commercial catalysts used in SCR systems mainly include precious metal catalysts, metal oxide catalysts, perovskite-type catalysts, molecular sieve catalysts, and carbon-based catalysts.

In industry, plate, corrugated, and honeycomb types are commonly used. They exhibit strong resistance to the deposition of fly ash and corrosion. In terms of mechanical strength and surface utilization efficiency, honeycomb and corrugated types have advantages, and they are also easy to install and replace; therefore, catalysts of this structure are often used in large vertical SCR reactors. Plate catalysts are commonly used in horizontal reactors, while granular and cylindrical catalysts are only found in equipment for processing small volumes of gas. The honeycomb catalyst unit must be designed with appropriate \"pitch\" and \"wall thickness\", and its peripheral edges should be surface-hardened to enhance its resistance to erosion wear. Honeycomb catalysts are suitable for the top-to-bottom flow of flue gas, while plate catalysts are suitable for vertical flue gas flow and can also be used for horizontal flow. In the market, the main forms of catalysts are flat-type (including corrugated) and honeycomb-type. Plate-type catalysts have the active material \"coated\" on a metal framework; the pores between the plates are large, resulting in low resistance. However, the contact surface area per unit volume is small, which requires a larger amount of catalyst. Since plate catalysts have a metal framework and high strength, their length can reach 1500 mm. To achieve the same denitration efficiency, a smaller number of catalyst layers can be used, allowing the SCR reactor to be designed to be more compact and space-saving. The greatest advantage of flat catalysts is that they are less prone to ash clogging. Since the SCR is generally installed before the air preheater, the mass concentration of fly ash in the flue gas can reach 15–20 g/m3 (under standard conditions). If the gap between the catalysts is too small, it will lead to fly ash clogging, increased wear, and higher system resistance. Generally, the flue gas velocity within the catalyst gaps is not uniform, with an average velocity of about 8 m/s. In areas where the velocity is below 3 m/s, fly ash may adhere to the catalyst, preventing it from coming into contact with the flue gas. According to a study by BHK, approximately 13% of the flow rates in the flat catalyst area are below 3 m/s, while about 22% of the flow rates in the honeycomb catalyst area are below 3 m/s. Therefore, the problem of ash clogging is more severe in honeycomb types than in flat types, and the solution is to increase the length of the catalyst cells. In coal-fired units, the cell length of honeycomb catalysts should generally be over 6 mm; the greater the length, the smaller the specific surface area of the catalyst, and thus more catalyst is required. If honeycomb catalysts are used in coal-fired units, the amount required can be reduced by about 20% compared to flat catalysts; however, their unit price is about 20% higher than that of flat catalysts, resulting in similar overall costs for both types. However, in gas and fuel-fired units, due to the extremely low ash content, the length of the honeycomb catalyst cells can be reduced to increase the specific surface area and thus save on catalyst usage. Furthermore, gas turbine units have good rapid start-up and shutdown capabilities as well as fast heating rates. The active materials in the plate catalysts attached to the metal framework are prone to deformation and eventual failure due to the effects of thermal stress. Therefore, honeycomb catalysts are widely used in gas turbine units. The cross-sectional area of a typical honeycomb catalyst unit is 150mm×150mm. Due to the large effective area per unit volume, less catalyst is required to achieve the same denitration effect compared to flat-plate types. However, since TiO2 is the carrier for the catalyst in my case, and due to limitations in overall strength, the length of conventional honeycomb catalysts can generally only reach up to 1000 mm. In practical use, the cross-sectional dimensions of the units in a honeycomb-shaped catalyst are standard, while the length is determined by the height of the bed. Due to the small gaps in honeycomb catalysts, which result in high flow resistance, a lower flue gas velocity is required compared to flat catalysts; this leads to an increased reactor cross-sectional area. Table 7-3 shows the reactor volume dimensions for a 600MW combustion unit (with one reactor) using two different types of catalysts. Some believe that the SCR catalyst with the best performance is of a corrugated structure, which uses reinforced fibrous titanium dioxide (TiO2) as a carrier, with a fine mixture of V2O5 and WO3 evenly distributed across the surface of the catalyst. This special production method gives the catalyst higher activity and a lower rate of passivation, as well as better mechanical strength and corrosion resistance. Currently, manufacturers offer 6 specifications of corrugated catalysts suitable for low-ash and high-ash operating conditions. These catalysts have relatively low porosity and V2O5 content, resulting in a low SO2 oxidation rate but high denitration activity, as demonstrated by operational experience. This catalyst product comes in two standard modules, A and H, with masses of approximately 50 kg and 27 kg respectively; it is resistant to vibration and can be moved easily. Single modules are commonly used in small processing devices, while larger systems require combined designs. In large-scale equipment, to facilitate installation, multiple individual modules are usually assembled into larger modules, which often consist of 12 to 16 individual modules. The number of modules to use is determined by the size of the system and the layout of the equipment. According to the manufacturer, this catalyst can be used in conditions of high gas flow rates, high dust levels, and high NOx concentrations; for example, the flue gas flow rate can reach 2,100,000 m3/h ； The volume fraction of NOx is 2000 ×10-6 ； The fly ash mass concentration is 25 g/m3. It is normal for catalysts to gradually lose their activity due to poisoning, sintering, clogging, wear, and aging; therefore, they need to be replaced regularly during use. Generally, the service life of catalysts designed for coal-fired flue gas, as provided by catalyst suppliers, ranges from 10,000 to 30,000 hours. When the catalyst’s activity drops to a certain level, the system exhibits a decrease in denitration efficiency or an increase in ammonia leakage. When either of these conditions fails to meet the design requirements, it is necessary to replace the catalyst. Honeycomb catalysts are often prepared as units of standard size, which are then combined to form modules placed in special frames; several such modules together constitute a catalytic bed. Typically, 3 to 4 layers of catalyst are used in each reactor, with the number determined based on actual conditions and the required denitration efficiency. Whether it is newly loaded or replaced regularly, the procedures must be followed, and specialized tools as well as a suitable working area are required.