Green guard: a powerful way to clean up rare earth based catalysts

"Our SCR flue gas denitrification unit USES a high dust type process and the reactor is located between the economizer and the air preheater. There are 2 SCR reactors for each boiler, and the center line of the boiler is arranged symmetrically. 4 layers of catalysts are arranged in the SCR reactor, 18 holes for the catalysts in the first and fourth layers, 15 holes for the catalysts in the second and third layers, 5×9 modules are arranged in each layer, and 72 products in each module. After the test, the denitrification efficiency and ammonia escape, SO2/SO3 conversion rate, the overall flue gas resistance of the reactor, ammonia consumption and other indicators are all qualified." Wang zhimin, general manager of Inner Mongolia Seagate environmental protection technology co., LTD. (hereinafter referred to as Seagate environmental protection) explained to me.

It is reported that the denitration catalyst of the project has been running for more than 24,000 hours since it was put into operation in July 2014, but it is still in good operation and is still in use.

Future trend of denitrification catalyst

SO2 and NOx, as major pollutants in the atmosphere, have attracted wide attention. China's NOx emissions are forecast to reach 35.4mt by 2030 under baseline conditions. "The emissions from fixed sources (thermal power plants, cement plants, boilers, etc.) account for about 50 percent, which is the key to reducing emissions. The key to denitrification of pollutants discharged by thermal power plants and other enterprises is the catalyst, whose catalytic performance directly affects the overall denitrification efficiency of the denitrification system. "Wang said.

The traditional denitrification catalyst is mainly vanadium group denitrification catalyst, which has been put into the market earlier, the technology is mature, and the application is wide. The core technology of vanadium titanium denitrification catalyst is in the hands of a few developed countries, and there are still many properties to be optimized. After years of efforts, vanadium and titanium catalyst technology is becoming mature, domestic production rate is very high. "However, there is no independent intellectual property right, and vanadium and titanium denitrification catalyst has toxicity and environmental pollution, so it is of great practical significance to study SCR denitrification catalyst with independent intellectual property right and high efficiency and low cost." Nanjing university of technology professor zhu shemin told me.

In 2014, the environmental protection department issued "on strengthening the waste flue gas denitration catalyst supervision notice, specify the waste flue gas denitration catalyst (vanadium and titanium) leaching toxicity hazardous characteristics, it is classified as" national hazardous waste list "in" HW49 other waste ", the use of denitration catalyst management puts forward the more strict requirements. At the same time, the list of substitutes for toxic and harmful raw materials (products) encouraged by the state (2016 edition) issued by the ministry of industry and information technology, the ministry of science and technology and the ministry of environmental protection on December 14, 2016 clearly listed rare-earth denitrification catalysts as substitutes for vanadium denitrification catalysts.

"In fact, vanadium pentoxide (V2O5), the main active ingredient used in vanadium and titanium denitrate catalysts, is a highly toxic substance, which is highly hazardous according to the hazard level of occupational exposure (GBZ 230-2010) and can cause serious damage to human respiratory system and skin. Zhu said it was imperative to replace it.

Technical characteristics of rare earth SCR catalyst for flue gas denitrification

In recent years, how to make full use of the unique 4f orbital structure and excellent REDOX properties of rare earth materials in catalysts has become the focus of research.

Rare earth base of denitration catalyst active component is composed of lanthanum, cerium, yttrium oxide and other rare earth elements, such as transition metal oxides, the titanium base ceramic as the carrier, titanium zirconium compound metal oxide as the second carrier, using rare earth elements is not full of electronic 4 f orbit as a catalyst active ingredients shown when using the electronic properties of "memory", effectively stored with oxygen vacancy formation of free electrons, then very good promote the adsorption and activation of molecular oxygen, can effectively solve the cerium oxide as an active substance when the problem of weak acid. The product got rid of vanadium pentoxide and optimized the performance of vanadium base denitrification catalyst.

"For example, in the process of exhaust from power plants, the flue gas changes from turbulent flow to laminar flow in the catalyst. The ash particles (irregular shapes) do not follow the laminar flow pattern. The ash particles tend to flip over and hit the inner wall of the catalyst over the length of the channel, so the wear is uniform from top to bottom. Thin-walled catalysts with small volume densities don't last very long." Zhi-min wang continued to the author, Seagate environmental protection in high strength, high specific surface of denitration catalyst preparation technology, controllable pore, pore forming agent gamma Al2O3 and ion doping has realized the crystal structure, grain size, hole quantity, structure, distribution regulation and increase the specific surface and catalytic activity, solve the highly active and the contradictions between the high strength, high wear resistance and effectively extend the service life of the catalyst.

"At present, vanadium and titanium catalysts have a fatal problem, that is, when the flue gas temperature is below 300℃, the catalytic performance decreases and the activity decreases. Zhou guanghe, chief engineer of Seagate environmental protection, explained that we have broken through two core technologies and further optimized the performance of rare-earth SCR flue gas denitrification catalyst.

The structure and defect regulation were carried out through the doping of Zr4+ and other auxiliary catalytic ions. It was found for the first time that the material structure defect would lead to the change of the acidic potential on the surface of the catalyst. The active temperature window is widened. The theory of structure-assisted catalysis is proposed to improve REDOX performance and stability of catalysts. Breaking through the concept that the traditional supporting body is limited to the skeleton function, al2tio5-tio2-sio2 multiphase honeycomb ceramic supporting body was invented to facilitate the catalytic denitration function. Using domestic TiO2, SiO2 and Al2O3 as raw materials through chemical modification, nano modification and introduction of non-stoichiometry, the surface structure defect of the supporting body oxygen vacancy increased, surface acidity increased, specific surface area increased, denitrification activity increased, and the strength of the supporting body increased and the thermal expansion coefficient greatly decreased.

"The service life of catalyst under the condition of existing technology for three years (24000 h), according to the calculation, since 2015, our country each year to replace the toxic denitration catalyst containing vanadium and titanium will be up to 30-400000 cubic meters, a large number of toxic waste catalyst such as custody or improper handling, extremely easy contaminated soil and groundwater, the secondary pollution to cause our country environment huge hidden trouble." Wang is anxious in his heart.

The necessity of using rare earth SCR catalyst for denitrification of flue gas

The substituted vanadium titanium series toxic denitrification catalyst, with the current technical level, the hazardous waste treatment cost of vanadium titanium series denitrification catalyst is up to 6000--7000 yuan /m3, which increases the burden of using enterprises.

"Moreover, the environmental protection tax law stipulates that the environmental protection tax of 1,000 yuan per ton is levied on waste vanadium, titanium denitrification catalysts and similar hazardous wastes, which is 40 times that of ordinary solid wastes such as rare earth denitrification catalysts." "Wang said.

Seagate environmental protection depends on the materials of nanjing university of technology state key laboratory of chemical engineering technology, the national natural science foundation of China, the national "863 project", national science and technology support plan and the special investment funds to support successful development, production with independent intellectual property rights in our country, conform to the national standard (GB/T34700-2017) of rare earth type selective catalytic reduction (SCR) denitration catalyst, filled the gap of the flue gas denitration catalyst technology at home and abroad, the technical performance reached the international advanced level.

Using the patented technology with an annual output of 50,000 cubic meters of rare earth SCR flue gas denitrification catalyst, the products are suitable for the treatment of nitrogen oxide pollutants discharged by coal fired power plants, industrial coal fired boiler (kiln) furnaces, cement plants, coking plants, biological power plants, petroleum coke burning enterprises, glass factories and other industries. The first phase of the project was officially completed and put into production in 2018. The production process adopts advanced honeycomb extrusion process and firing scheme, with high automation, advanced technical level and reliable operation.

"Our products have been successfully applied in more than 100 denitrification projects since they were launched in 2012, covering coal fired power generation, glass, coking, smelting, cement, ceramics and chemical industries. All the projects have reached the national environmental protection emission standard and achieved good use effect. Some denitrification catalysts have been running for more than 24,000 hours, but they are still working well and are still in use." Wang is full of confidence in the future.

According to wang zhimin's calculation, there are about 850,000 m3 catalysts in operation in China during the 13th five-year plan period, and it is expected that the waste vanadium and titanium denitrification catalyst will be replaced annually from 2018 up to 300,000 m3.If all the catalysts are replaced with rare earth denitrification catalysts, the annual hazardous waste will be reduced by 300,000 m3. Rare earth denitrification catalyst can balance the utilization of lanthanum and cerium light rare earth resources rich in rare earth in north China and promote the healthy development of rare earth industry.