Analysis on the development status, application status and molding process of SCR denitration cataly

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Selective catalytic reduction (SCR) technology is the most critical technology to control nitrogen oxide (NOx) emissions. It is widely used in industrial flue gas denitrification such as thermal power plants and incineration plants, as well as diesel vehicle exhaust purification. The technology uses NH3 produced by urea, ammonia or liquid ammonia as the reducing agent, and the core is a denitration catalyst with good catalytic activity, high selectivity, high mechanical strength and stable operation. From the initial popularization and application of traditional vanadium-titanium catalysts in the power denitration industry to the extensive research on low-temperature catalysts used in non-electric industries such as steel and glass, SCR catalysts have made breakthroughs in their development and application. The development of traditional vanadium-titanium catalysts has been relatively mature, but the scope of application is narrow and the conditions are harsh; low-temperature catalysts have problems such as easy poisoning, low life, and applicability of working conditions that need to be resolved urgently. The SCR catalyst molding process is the key to its application and industrial promotion. China has achieved comprehensive popularization and promotion of traditional catalyst molding technology, but its application effect is not good compared with foreign catalysts. In recent years, research on low-temperature SCR catalysts has made breakthroughs. The results, application and promotion are subject to engineering verification. Therefore, through in-depth research on catalyst production technology and molding process, research and development of catalysts with independent intellectual property rights that can withstand the test of actual engineering is an important link in the development of SCR technology in the future.

1. The development history of traditional SCR denitration catalysts

1.1 Application of SCR catalyst abroad

The American Engelhard Company successfully developed the SCR catalyst for the first time in 1957. It is composed of precious metals such as Pt, Rh and Pb. It has high catalytic activity, but it is expensive, has a narrow temperature range, is easily poisoned, and is not suitable for industrial applications. The V2O5(WO3)/TiO2 (vanadium-titanium series) catalyst produced by Hitachi, Mitsubishi Heavy Industries, etc. has realized commercial application earlier. In the 1970s and 1980s, Japan, Europe and the United States successively built multiple denitration systems. The commercial application of vanadium-titanium-based SCR catalysts became mature, and they were mainly used for flue gas pollution control in the power industry. SCR catalysts have made certain progress in research and application in the past 30 years. So far, the traditional SCR catalyst production and application technology has been popularized, but the core technology is mastered by several large foreign companies, such as Corning in the United States, Lurgi in Germany, and BHK in Japan.

1.2 Development status of domestic SCR catalysts

China's environmental protection industry started late, and the application of SCR catalysts lags behind Western countries. In 1999, the mainland first introduced the SCR denitrification catalyst for flue gas treatment in the thermal power industry, and continued to promote and popularize it for the following 10 years. As of 2012, the domestic flue gas denitrification unit capacity has been put into operation at 120GW, and it has been increasing in the past five years. In 2016, the proportion of thermal power denitrification units was as high as 91.7% (see Table 1 for details). During the "Eleventh Five-Year Plan" period, China’s total NOx emissions have shown an upward trend year by year, reaching 24.05 million tons in 2011 (industrial sources accounted for 71.9%); and the strict implementation of the "Emission Standards for Air Pollutants for Thermal Power Plants" (GB 13223-2011) And the SCR denitrification unit has been put into operation at full load, and the total NOx emissions have been reduced year by year. In 2017, the emissions have dropped to about 17 million tons.

In recent years, the penetration rate of SCR catalysts in the power industry is close to saturation. In the face of increasingly severe environmental pressures, with the power industry's limited emission reduction capabilities, NOx reduction in non-electric industries (steel, coking, cement, glass) will become the focus. China's denitrification market will continue to expand, and the demand gap for SCR catalysts will also expand. Before 2006, domestic catalyst supply basically relied on foreign countries. With the advancement of the denitration industry, domestic companies established corresponding catalyst production bases to continuously meet the increasing demand. Table 2 shows the main SCR denitration catalyst manufacturers in China and their catalyst information. At present, the core technology (active formula and molding process) of domestic catalyst companies mainly originates from abroad. For the development of traditional vanadium-titanium-based catalysts, the first task is to realize fully independent localization as soon as possible, save production costs, and increase market competitiveness; at the same time, speed up the modification research of traditional catalysts, broaden the scope of application, and extend the service life.

2. Research and application status of low temperature SCR catalyst

In recent years, the proportion of NOx emissions from industrial furnaces and kilns in the non-electric industry has continued to rise, and it has become an important source of air pollution. During the 13th Five-Year Plan period, planning measures such as "ultra-low emissions" and "blue sky defense" have been implemented successively, and stricter requirements have been imposed on industrial flue gas pollution emissions. In view of the low flue gas temperature characteristics of non-electric power industries such as steel and glass building materials (such as steel sintering/pellet flue temperature 120~180℃, daily glass furnace flue gas temperature 180~240℃), traditional SCR denitration technology works with catalysts The disadvantages of high temperature, no suitable heat source, and high heating operation cost are not suitable for direct use. The catalyst must be improved in a targeted manner to improve its applicability in the field of low-temperature flue gas denitration.

2.1 Research and exploration of low temperature SCR catalyst

At present, research on low-temperature SCR catalysts at home and abroad is mainly focused on vanadium-based (V), manganese-based (Mn) and other metal oxides (such as Fe, Ce), etc., and certain progress has been made through related engineering exploration.

Studies have shown that traditional vanadium-titanium catalysts can broaden the low-temperature performance of the catalyst to a certain extent by doping transition metals or optimizing the support structure. At the same time, the catalyst with MnOx as the main component is the focus of current research. Because MnOx contains a large amount of free O, it can complete a good catalytic cycle in the catalytic process, which is the main reason for its low-temperature activity. However, the presence of H2O and SO2 in the actual flue gas is continuous and unavoidable, which has a significant inhibitory effect on the SCR reaction of the MnOx catalyst. In order to solve the problem of catalyst resistance, Gao et al. prepared a MnOx-CeOx-MeOx three-way catalyst by a co-precipitation method. The experimental results show that the Co/Ni doping improves the anti-poisoning ability of the two-component MnOx-CeOx catalyst. The activity remained at about 78% after the concentration of 400mg/m3 SO for 21h, which was 10% higher than other samples. In addition, the performance of catalysts is closely related to morphology and structure. The development of SCR catalysts with special morphologies is an important development direction in the future. Researchers often use advanced material synthesis techniques to prepare catalysts with more perfect structure and crystal form. Guo et al. prepared CeOx@MnOx catalyst with a core-shell structure and used it for the catalytic oxidation of NO. The results show that the CeOx@MnOx catalyst has higher NOx catalytic activity than the CeMnOx catalyst prepared by the traditional method (citric acid method).

2.2 Current status of low-temperature SCR catalyst engineering application

At present, there are still some problems in the industrial application of low-temperature catalysts: Mn-based catalysts have poor water and sulfur resistance; other types of catalysts are less commercialized due to their complicated manufacturing processes. However, several foreign companies (Shell of the Netherlands, Topsoe of Denmark, etc.) have successfully applied low-temperature SCR catalysts to actual production.

In recent years, domestic research and engineering exploration of low-temperature SCR catalysts have also achieved certain results. Table 3 lists the production of low-temperature SCR catalysts by major domestic enterprises and their engineering applications. In addition, the new Mn catalyst developed by this research group has also achieved excellent test results in a pilot test of a steel company in Hebei. It has been continuously operated for 720 hours at 150°C and airspeed in the range of 4000~6000h-1, and its activity has always been maintained at more than 90%.

Low temperature SCR catalyst is an important direction for the future development of denitration field. While improving the resistance and stability of the catalyst, it is the development of this field to continue to develop new materials, new configurations, improve efficiency, reduce costs, and surpass foreign countries in technology. An important step.

3. Catalyst molding technology

SCR can be divided into 3 categories according to its shape: honeycomb type, plate type and corrugated type. These three categories are all integrated catalysts suitable for large industrial flue gas flow and high dust content. The three types of catalysts have practical applications in domestic and foreign markets, but the characteristics, scope of application and molding process of different types of catalysts have led to a wide gap in their share of domestic and foreign markets. Among them, the honeycomb type SCR catalyst market accounted for more than 60%, followed by the plate type catalyst, and the corrugated plate type accounted for only a small part. The characteristics and application scope of these three types of catalysts are shown in Table 4.

At present, China's universities and institutes have carried out some exploratory studies on the composition, reaction mechanism and catalyst poisoning of catalyst active components, but there are few patents and documents concerning the preparation and molding process of the overall catalyst. In this context, how to solve the bottleneck problems that restrict the development of China's denitrification market, such as dependence on imports and high prices, and realize the localization and large-scale production of SCR catalysts, and finally form the production technology of flue gas denitration catalysts with independent intellectual property rights. The top priority for the development of China's denitrification process.

3.1 Honeycomb catalyst molding process

The honeycomb type catalyst is currently the most widely used type of catalyst, and its molding method can be divided into extrusion molding type and coating type.

In the dry mixing and wet mixing steps in the extrusion molding process, active component precursors, carriers, structural additives (binders, pore formers, structural enhancers), water, etc. should be added in sequence to form a plastic catalyst slurry, which is dried. The firing and other links are finally formed. The prepared catalyst can be adjusted in size according to requirements. Because the active groups of the catalyst are dispersed throughout the substrate, this type of catalyst has a long service life, excellent wear resistance, and can be used in complex soot conditions. In the molding process, the process conditions and molding aids are extremely critical to the molding process. Forzatti et al. found that in the process of catalyst extrusion, the structure and performance of the catalyst can be changed by controlling the extrusion pressure and speed, and the relationship among them can be found, which is of great significance for guiding the production of catalysts. Sun Ke from Zhejiang University investigated the influence of forming aids on the performance of the Ce-Mn/TiO2 catalyst system, and focused on the influence of structural aids (glass fibers) on the activity and mechanical properties of the catalyst.

Another forming method of honeycomb catalyst is honeycomb ceramic coating technology, which uses ready-made honeycomb ceramic material as a carrier and coats a layer of catalytically active slurry on the surface. This technology greatly reduces the amount of active components, saves costs, and can ensure that the mechanical strength of the catalyst meets the requirements of industrial production. The choice of carrier is extremely important for the molding process. Honeycomb cordierite is currently recognized as one of the most suitable carriers for denitration catalysts. It has the advantages of good thermal stability and high mechanical strength [24], but it needs pretreatment to change its surface properties. . The loading method is also important for the molding process. The active component needs to be mixed with a binder or dispersant in advance to form a slurry, and then attached to the surface of the carrier by dipping or spraying. After drying and roasting, the overall catalyst is obtained. The biggest drawback of this method is The active component has poor adhesion to the substrate and is easy to fall off, which is not suitable for working conditions with large air volume and high smoke and dust [25]. In order to prevent the catalyst surface from falling off, Popovych et al. loaded an aluminum coating on the cordierite surface and coated the active component solution on the aluminum coating. The test found that the aluminum coating not only reduced the catalyst falling rate, but also coated Larger surface area is also conducive to the development of catalytic activity.

3.2 Overview of the molding process of plate and corrugated plate catalysts

As another widely-used catalyst, plate-type catalysts have continuously increased their domestic market share in recent years, maintaining around 30%. The raw materials containing carrier (TiO2, Al2O3) and active components (V2O5, WO3, MoO3) are fully kneaded in the kneading machine, and the uniformly kneaded mud is coated on the metal mesh, and the catalyst veneer is made by means such as drying and roasting. . The flat-plate catalyst uses a stainless steel sieve plate as the structural framework, which has high mechanical strength, will not cause the overall catalyst to collapse, and is safe and stable in operation. The main energy consumption of the denitration system operation comes from the fan power consumption caused by the fan resistance. During the assembly process of the plate catalyst, the plate spacing can be adjusted according to the flue gas conditions to reduce the bed resistance and reduce the denitrification energy consumption. Similar to the shortcomings of other coated and shaped monolithic catalysts, plate-type catalysts are also easy to wear and have a low life span. To solve this problem, it is usually necessary to improve the adhesion ability of the slurry on the surface of the carrier.

Gu Dongliang’s research found that the addition of different additives to the slurry will affect the activity of the catalyst and the mechanical strength after forming. A self-designed roller press was used to prepare a plate catalyst with a low shedding rate and less prone to cracking. Related experimental verification.

Compared with the first two types of catalysts, the market share of corrugated plate catalysts is very low. Only a few manufacturers such as Topsoe and Hitachi Shipbuilding can produce them in the world. Most of the domestic products are imported. The molding process is similar to that of the plate catalyst, except that the carrier is replaced with a corrugated ceramic/glass fiber board. The ceramic fiber boards are superimposed on each other, and the triangular or trapezoidal pore structure formed constitutes the basic pattern of the catalyst. The use of new carrier materials greatly reduces the density of the catalyst (40%-50% lighter than the honeycomb catalyst of the same volume) and is easy to assemble and disassemble. However, the wave-shaped structure design increases the contact area with the flue gas and also causes fly ash deposition and is extremely easy to wear, which limits its industrial application. In order to improve the corrugated plate defects and speed up market promotion, researchers continue to study and improve. He Yafei et al. hardened the ends of the corrugated catalyst to greatly reduce the wear rate of the catalyst. At the same time, it is found that the service life of this type of catalyst can be further extended through a reasonable arrangement. At present, domestic and foreign experts and scholars are constantly improving the performance of this type of catalyst through research, and will be more applied to actual production in the future.