Denitration mainly occurs in the coal-fired industry. Coal combustion will produce a large number of air pollutants, such as fine particles, SO2, nitrogen oxides (NOx), etc., in which NOx is usually removed by denitration catalyst.
According to statistics, China's total energy consumption in 2019 was about 4.86 billion tons of standard coal, including 2.37 billion tons of coal consumed by the power industry, and about half of the coal consumed by key non power industries such as metallurgy, building materials and chemical industry. At present, coal-fired power generation is one of the main ways of power production in China. China attaches great importance to the ultra-low emission of coal-fired power plants. The NOx emission limit of coal-fired power plants is 50mg · m-3. In January 2017, the Ministry of environmental protection issued the technical policy for pollution prevention and control of thermal power plants, which standardized the coal-fired pollution prevention and control scheme. At the end of 2017, the National Standardization Administration Committee issued the technical specification for flue gas denitration catalyst regeneration, which further promoted the development of denitration catalyst regeneration technology and coal-fired industry in China. By the end of 2019, the installed capacity of thermal power in China has reached 1.19055 billion kw, and 86% of coal-fired power units have achieved ultra-low emission. China has built the world's largest ultra-low emission clean coal-fired power supply system.
Non electric coal-fired industries, such as steel, cement, metallurgy, coking, coal chemical industry, industrial boilers and industrial kilns, are the areas with the largest coal consumption except the power industry, but their emission standards and treatment level are far lower than that of coal-fired power plant industry, and NOx emissions account for more than 3 / 4 of the country. With the update of technology and the popularization of control measures for industrial boilers and cement production, NOx emissions gradually stopped rising in 2011. After 2017, the national and local governments have successively raised NOx emission standards for non power industries and strengthened emission management. In 2019, the crude steel output of China's iron and steel industry reached 996 million tons, and the capacity of ultra-low emission transformation has reached 62.6% of the total capacity. According to statistics, among the proposed and newly-built ultra-low emission projects in the iron and steel industry in 2019, there were 47 denitration projects, including 27 selective catalytic reduction (SCR) denitration projects, accounting for about 60%. It can be seen that SCR denitration technology is gradually popularized in non electric coal industry.
At present, the methods of flue gas denitration at home and abroad include SCR, non selective catalytic reduction (NSCR), selective non catalytic reduction (SNCR), catalytic oxidation, electron beam (EBA), adsorption and microbial methods. SCR technology was first applied to shimoneski power plant in Japan in 1975 and expanded to developed countries such as Europe and the United States and some developing countries. This method has the characteristics of high air purification rate (90%), low reaction temperature (300 ~ 400 ℃), compact treatment equipment and reliable operation. It is considered to be the best fixed source denitration technology.
Honeycomb denitration catalyst is the most widely used denitration catalyst. Due to the service life of honeycomb denitration catalyst, it is generally replaced in 3 ~ 5 years. It is expected that 150000 m3 of waste denitration catalyst will be produced every year in China in the future. Waste denitration catalyst is not only a pollutant (containing harmful components of heavy metals such as vanadium and tungsten), but also a resource, which can be reused through regeneration. On the one hand, regeneration technology is related to the regeneration quality of deactivated denitration catalyst, on the other hand, it is related to the regeneration benefit. The industrial application of denitration catalyst regeneration technology has not been reported in the literature. Therefore, the author summarizes the application and deactivation of honeycomb denitration catalyst and the research progress of honeycomb denitration catalyst regeneration technology, in order to provide help for the development of denitration catalyst.
Application and deactivation of honeycomb denitration catalyst
1.1 application of denitration catalyst
SCR technology is that under the action of metal catalyst, reducing agent (NH3, urea) selectively reacts with NOx to produce N2 and H2O, rather than being oxidized by O2. SCR technology with NH3 as reducing agent is widely used because of its excellent denitration performance. As the core of SCR system, catalyst needs to have high denitration efficiency, wide reaction temperature window and strong sulfur resistance.
The commonly used denitration catalyst is V2O5-WO3 (MoO3) / TiO2 series (TiO2 is the main carrier, V2O5 is the main active component, WO3 or MoO3 is the promoter). V2O5-WO3 / TiO2 catalyst has good denitration performance at 300 ~ 400 ℃, which is the mainstream of commercial denitration catalysts at present. Denitration catalysts can be divided into three types: plate, honeycomb and corrugated plate.
Due to the deactivation of denitration catalyst in the process of use, the regeneration of plate and corrugated plate denitration catalyst is difficult, so it is difficult to be widely used. Honeycomb denitration catalyst is made by mixing the catalyst components evenly and extrusion equipment, with a cross section of 150 mm × The catalyst is assembled into standard modules with different lengths of 150 mm. Honeycomb denitration catalyst has been widely used because of its strong durability, high corrosion resistance, high reliability, high reuse rate, low pressure drop and renewable characteristics.
At present, commercial denitration catalysts have the problems of high denitration temperature and low denitration efficiency. Improving the catalyst structure and strengthening the denitration performance of low-temperature nh3-scr are the research direction of denitration catalysts. Bian Xue et al. Obtained xceo2-ywo3 / TiO2 denitration catalyst by coprecipitation method. When the ratio of CE ∶ w = 30 ∶ 4, the denitration efficiency can be increased to 90% ~ 95%. Hu et al. Prepared Co Mn / TiO2 denitration catalyst by impregnation method. When the atomic ratio of CO to Ti was 0.05, the reaction temperature window of the catalyst was reduced to 80 ~ 180 ℃, and the denitration efficiency of the catalyst reached 94.05%. This is because oxides such as Mn3O4 and Mn2O3 are generated in the reaction process, which reduces the reduction temperature of the catalyst and improves the adsorption capacity of NH3. Therefore, the denitration efficiency increases. Gao Yanchun and others proposed to use coal gasification slag (CGS) as the carrier to prepare V / CGS low-temperature nh3-scr denitration catalyst by equal volume impregnation method. It is pre oxidized at 250 ℃ and calcined at 500 ℃, and the denitration efficiency is as high as 98%. The advantage of the catalyst is that the presence of pentavalent vanadium and sulfur dioxide can improve the performance of the catalyst, but the impurities such as Ca and Si in CGS affect the activity of the catalyst and easily lead to the deactivation of the catalyst. Liu et al. Prepared a TiO2 carrier with large specific surface area. Its BET specific surface area is 380.5 M2 · g-1. The specific surface area of active component vanadium is further increased by special heat treatment process. The reaction temperature window is 100 ℃ wider than the traditional one, and the conversion rate of no is 84%.
1.2 deactivation of honeycomb denitration catalyst
The main causes of deactivation of vanadium titanium denitration catalyst are: physical coverage, chemical poisoning (alkali metals, alkaline earth metals, arsenic, phosphorus, etc.), sintering, wear, loss of active components, etc.
Physical coverage occurs when the fly ash passes through the denitration catalyst bed. Fine fly ash particles enter the denitration catalyst bed and cover the catalyst surface or enter the channel to form blockage, resulting in the deactivation of some active sites of denitration catalyst. The deactivation of this catalyst is short-lived, and the catalyst activity can be restored by high-pressure water cleaning. When the high-temperature gas passes through the denitration catalyst bed, the pressure difference between the section center and the edge causes the fly ash particles to deposit on the outer surface of the catalyst and the inner surface of the channel at the section center first, so that the section center of the catalyst is relatively seriously inactivated due to the physical coverage of the particles. The inner surface coverage is mainly due to the blockage caused by the direct entry of small particles of fly ash into the catalyst hole, and the outer surface coverage is formed by the adsorption of fly ash particles on the surface when they enter the catalyst bed.
Chemical poisoning can be divided into alkali metal (such as K, Na) poisoning, alkaline earth metal poisoning (CA, Mg), non-metallic (P, Si, as) poisoning, etc. Alkali metal poisoning is due to the neutralization reaction between potassium ions and sodium ions and the acidic active site of the catalyst, resulting in the reduction of the number of active sites of solid acid, reducing the number of NH3 molecules adsorbed on the active site of the catalyst and reducing the denitration efficiency of the catalyst. The mechanism of alkaline earth metal poisoning is similar to that of alkali metal poisoning. When as is poisoned, the gaseous arsenic oxide As2O3 is directly adsorbed on the surface of the catalyst and then oxidized to As2O5 by the catalyst to form an as coating, which reduces the specific surface area of the catalyst, the number of active digits and the activity of the catalyst. When p is poisoned, on the one hand, because P replaces W and V in the catalyst to generate P-OH, the catalyst can only provide weak acid active sites, which reduces the adsorption capacity of the catalyst for NH3; On the other hand, P reacts with the active substance V on the catalyst to form vopo4, which occupies part of the active site, resulting in the decrease of catalyst activity. In the low-temperature SCR reaction, SO2 reacts with reactants (NH3 and O2) to produce ammonium sulfate [such as NH4HSO4, (NH4) 2SO4] and other sulfates, which will be adsorbed on the active site and aggravate the deactivation of the catalyst.
The grains of denitration catalyst will be sintered and grow up under long-term high temperature, resulting in the sintering deactivation of the catalyst. The higher the use temperature, the more serious the sintering deactivation. Sintering can be divided into the sintering of support TiO2 and active component V2O5. When vanadium titanium SCR catalyst operates at high temperature, the aggregation between support TiO2 particles leads to the increase of TiO2 crystal size. In serious cases, the crystal structure of TiO2 changes from anatase type to rutile type. The sintering of the support reduces the specific surface area of the catalyst, thus reducing the catalytic activity. The melting point of pure V2O5 is 670 ℃. High temperature operation will also cause V2O5 particles to sinter and finally reduce the catalytic activity.
The wear of denitration catalyst is also one of the reasons. When the fly ash in the flue gas quickly passes through the catalytic bed with the flue gas, it will scour the catalyst surface, cause wear over time, and reduce the activity of the catalyst due to the loss of some active components. Due to the pressure distribution of flue gas in the radial direction through the catalytic bed, the catalyst surface at the center is more seriously worn than that at the edge. Wear not only reduces the activity of catalyst, but also reduces the mechanical strength of catalyst honeycomb components, and finally reduces the denitration efficiency.
Regeneration technology of honeycomb denitration catalyst
The deactivation denitration catalyst of coal-fired power plant needs to be replaced regularly, which has a direct impact on the operation cost of SCR system. In addition, the waste of a large number of deactivated denitration catalysts will lead to environmental pollution. The results show that in most cases, the activity of the deactivated denitration catalyst can be restored to 90% ~ 105% of the original catalyst through regeneration. According to the different deactivation mechanism of denitration catalyst, its regeneration methods mainly include: physical cleaning, chemical cleaning, active component supplement, etc.
2.1 physical cleaning
Physical cleaning is to wash the deactivated denitration catalyst with water to remove the fly ash covering the catalyst surface, so as to restore the activity of some physically deactivated catalyst surfaces. Cao and others washed the deactivated denitration catalyst with high-pressure water gun. After washing, the mass fraction of Al2O3 in the catalyst decreased from 1% to 0.49%, and the mass fraction of SO3 decreased from about 0.7% to 0.54%, indicating that high-pressure water washing can effectively remove the fly ash physically adsorbed on the surface of denitration catalyst. Yu Yuexi and others used ultrasonic water to clean the deactivated denitration catalyst. Through TPR analysis, the contents of Ca, s and K decreased from 12.91%, 14.23% and 3.08% to 1.83%, 1.20% and 0.01% respectively, indicating that ultrasonic water cleaning can also remove part of the fly ash physically adsorbed on the surface of denitration catalyst to a certain extent. After ultrasonic cleaning of the deactivated denitration catalyst at 40 Hz for 45 min, Li Jian analyzed it by XRF. It was found that the mass fraction of Al2O3 in the catalyst decreased from 1.77% to 1.42%, and the mass fraction of SiO2 decreased from 4.86% to 4.25%. The research results showed that ultrasonic water cleaning improved the pore volume and specific surface area of the denitration catalyst.
2.2 chemical cleaning
Physical cleaning can only remove part of the fly ash physically adsorbed on the surface of denitration catalyst, but can not remove the fly ash chemically adsorbed on the surface of denitration catalyst. According to the different acidity and alkalinity of oxides in fly ash adsorbed on the surface of denitration catalyst, chemical cleaning can be divided into alkali cleaning and acid cleaning.
2.2.1 alkaline cleaning
Alkaline cleaning is to immerse the physically cleaned denitration catalyst in a certain concentration of NaOH, Na2CO3 and other alkaline solutions to remove the acidic substances in the fly ash adsorbed on the surface of the catalyst. Yu et al. Cleaned the deactivated denitration catalyst with 0.2 mol · L-1 NaOH solution at 30 ℃. It was found that the mass fraction of Al in the catalyst decreased from 42% to 28%, and the mass fraction of s decreased from 52% to 24%. The effect of removing Al and s was obvious. Fan Meiling and others cleaned the as poisoned denitration catalyst with 1.0 mol · L-1 Na2CO3 solution at room temperature. It was found that the content of As2O3 in the cleaned denitration catalyst decreased from 1.27% to 0.44%, and the removal rate of as reached 66%. Duan Qiutong and others treated the deactivated denitration catalyst with 0.05 mol · L-1 dilute NaOH solution for 60 min, and then immersed it in 0.5 mol · L-1 dilute H2SO4 solution for 60 min. The Na content decreased to 1.69% and the removal rates of K, Ca and Mg reached 100%.
2.2.2 acid cleaning
Alkaline cleaning can only remove the acidic substances in fly ash, and some alkaline substances need to be removed by acid cleaning. Xiao Yuting and others cleaned the deactivated denitration catalyst with 2% HNO3 solution. The results showed that the content of K in the catalyst increased from 732.2 × 10-6 to 202.5 × 10-6, Na content from 559.4 × 10-6 to 114.6 × 10-6, the content of s also decreased from 2.20% to 0.59%. The removal effect of K and Na is obvious, and the removal effect of S is also good. Zheng et al. Cleaned the deactivated denitration catalyst with 0.5 mol · L-1 H2SO4 solution for 20 min, the content of K element decreased from the initial 1 mg · g-1 to 0. After the activity test at 250 ~ 350 ℃, the activity of denitration catalyst recovered about 50% ~ 72%. Wang Le and others cleaned the deactivated denitration catalyst with 0.5mol · L-1 H2SO4 solution, and took the acid washing solution every 5 minutes to test its content. After acid washing, the content of As2O3 in the catalyst decreased from 0.040% to 0.013%, the content of K2O decreased from 0.022% to 0.018%, and the content of Cao decreased from 0.984% to 0.842%, indicating that the acid washing has an obvious effect on the removal of alkaline substances.
2.3 active component supplement
The use of denitration catalyst will lead to the loss of active components. In the regeneration process, although acid pickling and alkali washing will restore the activity of poisoned active sites on the catalyst, some catalyst surfactant will dissolve in the cleaning solution, resulting in a certain loss. Therefore, the lost active sites in the above two cases need to be supplemented. Impregnation is usually used to supplement the active components. Cui Liwen and others used one-step impregnation method to load and supplement the active components, and carried out one-step impregnation with the impregnation solution mixed with 1% ammonium metavanadate and 5% ammonium paratungstate solution. After roasting, the V content on the catalyst increased from 0.25% after cleaning to 1.13%, and the W content increased from 1.62% after cleaning to 4.83%. The activity of denitration catalyst was significantly restored. Wang Denghui and others used the step-by-step impregnation method to supplement the active components. First, the regenerated catalyst was impregnated with ammonium tungstate, and then impregnated into the ammonium metavanadate solution. The mass ratio of ammonium metavanadate to ammonium tungstate was 1 ∶ 6. The denitration efficiency of the catalyst could reach 87.7% at 300 ℃. Zhu Heng and others prepared v-mo / TiO2 cordierite denitration catalyst by simultaneous impregnation of ammonium metavanadate and ammonium molybdate tetrahydrate (mass ratio: 3 ∶ 10). The denitration efficiency of the catalyst reached 98.8% at 340 ℃.
Denitration technology is the key technology for clean emission of coal-fired power plants and other nitrogen-containing high-temperature tail gas. Due to the problem of limited service life of denitration catalyst, the denitration catalyst needs to be replaced after deactivation. If the inactivated denitration catalyst is discarded, it will cause heavy metal pollution to the environment. Therefore, the regeneration of inactivated denitration catalyst has become the development trend of denitration industry. At present, the regeneration of denitration catalyst is to restore the activity of the inactivated active site and supplement the active components to complete the regeneration process without destroying its structure. Limited by the number of times of reuse of regenerated denitration catalyst, the current repair denitration catalyst regeneration method can not meet the development needs of denitration industry. There is an urgent need to develop new recycling denitration catalyst regeneration technology.