Denitrification mainly occurs in the coal industry, where coal combustion produces a large number of air pollutants, such as fine particulate matter, SO2, nitrogen oxides (NOx), etc., of which NOx is usually removed by denitrification catalysts.

According to statistics, in 2019, our country's total energy consumption was about 4.86 billion tons of standard coal, of which the power industry consumed 2.37 billion tons of coal, and key non-power industries such as metallurgy, building materials, and chemical industry accounted for about half. At present, coal-fired power generation is one of the main ways of electricity production in our country. our country attaches great importance to the ultra-low emission of coal-fired power plants, and 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 on Pollution Prevention and Control of Thermal Power Plants", which standardizes the prevention and control plan for coal-fired pollution. At the end of 2017, the National Standardization Administration issued the "Technical Specification for the Regeneration of Flue Gas Denitrification Catalysts", which further promoted the development of denitrification catalyst regeneration technology and coal industry in our country. By the end of 2019, the installed capacity of thermal power in China was 1,190.55 million kW, and 86% of coal-fired power units had achieved ultra-low emissions, and our country had built the world's largest ultra-low-emission clean coal power supply system.

Non-electric coal-fired industries such as steel, cement, metallurgy, coking, coal chemicals, industrial boilers and industrial kilns are the areas with the largest coal consumption except the power industry, but their emission standards and governance levels are much lower than those of the coal-fired power plant industry, accounting for more than 3/4 of the country's NOx emissions. With the update of technology and the popularization of control measures at the production end of industrial boilers and cement, NOx emissions gradually stopped rising in 2011, and after 2017, the national and local governments have successively raised the NOx emission standards of non-power industries and strengthened emission management. In 2019, the crude steel output of our country's iron and steel industry reached 996 million tons, and the production capacity of ultra-low emission transformation reached 62.6% of the total production capacity. According to statistics, in 2019, there were 47 denitrification projects in the steel industry to be built and newly built, including 27 selective catalytic reduction (SCR) denitrification projects, accounting for about 60%. It can be seen that SCR denitrification technology is gradually being promoted in the non-electric coal-fired industry.

At present, the methods of flue gas denitrification 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 the Shimoneski power plant in Japan in 1975 and has been extended 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 °C), compact treatment equipment and reliable operation, and is considered to be the best fixed-source denitrification technology.

Denitrification catalysts are the most widely used in honeycomb denitrification catalysts. Honeycomb denitrification catalysts are affected by the service life, generally 3~5 years to be replaced, and it is expected that our country will produce 150,000 m3 of waste denitrification catalysts every year in the future. Waste denitrification catalysts are both a pollutant (containing harmful components of heavy metals such as vanadium and tungsten) and a resource that can be reused through recycling. On the one hand, the regeneration technology is related to the regeneration quality of the deactivated denitrification catalyst, and on the other hand, it is related to the regeneration efficiency. The industrial application of denitrification catalyst regeneration technology has not been reported in the literature. Therefore, the authors review the application and deactivation of honeycomb denitrification catalysts and the research progress of honeycomb denitrification catalyst regeneration technology, in order to help the development of denitrification catalysts.

1 Application and deactivation of honeycomb denitrification catalyst

1.1 Application of denitrification catalysts

SCR technology is a metal catalyst in which reducing agents (NH3, urea) selectively react with NOx to produce N2 and H2O, rather than being oxidized by O2. SCR technology with NH3 as reducing agent is widely used due to its excellent denitrification performance, and the catalyst, as the core of the SCR system, needs to have high denitrification efficiency, wide reaction temperature window, and strong sulfur resistance.

The commonly used denitrification catalysts are V2O5-WO3(MoO3)/TiO2 series (TiO2 is the main carrier, V2O5 is the main active ingredient, and WO3 or MoO3 is the additive). V2O5-WO3/TiO2 catalysts have good denitrification performance at 300~400 °C, which is the mainstream of commercial denitrification catalysts. Denitrification catalysts can be divided into three types: plate, honeycomb, and corrugated plate.

Due to the inactivation phenomenon of denitrification catalysts during use, it is difficult to regenerate plate and corrugated plate denitrification catalysts, so it is difficult to be widely used. Honeycomb denitrification catalysts are made by mixing the catalyst components evenly, using extrusion equipment to make catalyst elements with cross-sections of 150 mm × 150 mm and different lengths, and then assembling them into standard modules. Honeycomb denitrification catalysts are widely used due to their strong durability, high corrosion resistance, high reliability, high reusability, low pressure drop, and renewability.

At present, commercial denitrification catalysts have the problems of high denitrification temperature and low denitrification efficiency, and the research direction of denitrification catalysts is to improve the catalyst structure and enhance the denitrification performance of low-temperature NH3-SCR. Bian Xue et al. obtained xCeO2-yWO3/TiO2 denitrification catalyst by co-precipitation, and the denitrification efficiency could be increased to 90%~95% when the ratio Ce∶W=30∶4 was mixed. Hu et al. prepared a Co-Mn/TiO2 denitrification catalyst by impregnation method, and when the atomic ratio of Co to Ti was 0.05, the catalyst reaction temperature window was reduced to 80~180 °C, and the denitrification efficiency of the catalyst reached 94.05%. This is due to the formation of oxides such as Mn3O4 and Mn2O3 during the reaction, which leads to a decrease in the reduction temperature of the catalyst and an increase in the adsorption capacity of NH3, resulting in an increase in denitrification efficiency. Gao Yanchun et al. proposed to prepare V/CGS low-temperature NH3-SCR denitrification catalyst by equal volume impregnation method using coal gasification slag (CGS) as a carrier, which was pre-oxidized at 250 °C and then calcined at 500 °C, with a denitrification efficiency of up to 98%. The advantage of this 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, which can easily lead to catalyst inactivation. Liu et al. prepared a TiO2 carrier with a large specific surface area of 380.5 m2·g-1, which was further increased by special heat treatment process to further increase the specific surface area of the active component vanadium, and the reaction temperature window was wider, which was 100 °C wider than the traditional one, and the conversion rate of NO was 84%.

1.2 Deactivation of honeycomb denitrification catalysts

The main reasons for the inactivation of vanadium-titanium denitrification catalysts are: physical covering, chemical poisoning (alkali metals, alkaline earth metals, arsenic, phosphorus, etc.), sintering, wear, loss of active ingredients, etc.

Physical coverage occurs when fly ash passes through the denitrification catalyst bed, and fine fly ash particles enter the denitrification catalyst bed, covering the surface of the catalyst or entering the pores to form blockages, causing part of the active sites of the denitrification catalyst to be covered and inactivated. This catalyst inactivation is short-lived, and catalyst activity can be restored by high-pressure water cleaning. Due to the pressure difference between the center and the edge of the section when the high-temperature gas passes through the denitrification catalyst bed, fly ash particles are first deposited on the outer surface of the catalyst and the inner surface of the pore channel in the center of the section, so that the cross-sectional center of the catalyst is relatively seriously inactivated due to the physical covering of the particles. The inner surface covering is mainly due to the blockage caused by the direct entry of fly ash particles into the catalyst pores, and the outer surface covering is formed by the adsorption of fly ash particles on the surface when they enter the catalyst bed.

Chemical poisoning is divided into alkali metal (such as K, Na) poisoning, alkaline earth metal poisoning (Ca, Mg), non-metal (P, Si, As) poisoning, etc. Alkali metal poisoning is caused by the neutralization reaction of potassium and sodium ions with the acidic active sites of the catalyst, resulting in a decrease in the number of solid acid active sites, a decrease in the number of NH3 molecules adsorbed on the active site of the catalyst, and a decrease in the denitrification efficiency of the catalyst. The mechanism of metal poisoning in alkaline earth is similar to that of alkali metal poisoning. In As poisoning, because the gaseous arsenic oxide As2O3 is directly adsorbed on the surface of the catalyst, and then oxidized by the catalyst to As2O5, thus forming an As cover, the specific surface area of the catalyst decreases and the number of active sites decreases, and the catalyst activity decreases. On the one hand, P substitutes W and V in the catalyst to form P-OH, so that the catalyst can only provide a weak acid active site, 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 and causes a decrease in catalyst activity. In the low-temperature SCR reaction, SO2 reacts with reactants (NH3 and O2) to form ammonium sulfate (e.g., NH4HSO4, (NH4)2SO4) and other sulfates, which are adsorbed on the active site and exacerbate catalyst inactivation.

The denitrification catalyst grains will sinter and grow when used at long-term high temperatures, causing catalyst sintering and inactivation. Sintering can be divided into sintering of the support TiO2 and the active component V2O5, and the aggregation between the TiO2 particles occurs between the support TiO2 particles when the vanadium-titanium SCR catalyst is running at high temperature, resulting in the increase of the particle size of TiO2 crystals, and the crystal structure of TiO2 changes from anatase type to rutile type in severe cases. The sintering of the support reduces the specific surface area of the catalyst, which reduces the catalytic activity. The melting point of pure V2O5 is 670 °C, and high-temperature operation will also cause the sintering of V2O5 particles, which ultimately reduces the catalytic activity.

Wear and tear are also one of the causes of denitrification catalyst inactivation. When the fly ash in the flue gas quickly passes through the catalytic bed with the flue gas, it forms a scouring on the surface of the catalyst, causing wear and tear over time, and the loss of some active components and the decrease in catalyst activity. Due to the pressure distribution of flue gas in the radial direction through the catalytic bed, the surface of the catalyst in the center is more worn than at the edge. Wear not only reduces catalyst activity, but also reduces the mechanical strength of the catalyst honeycomb element, ultimately reducing denitrification efficiency.

2 Regeneration technology of honeycomb denitrification catalyst

Since the deactivation and denitrification catalyst in coal-fired power plants needs to be replaced regularly, it has a direct impact on the operating cost of the SCR system. In addition, the disposal of a large number of deactivated denitrification catalysts can lead to environmental pollution problems. Studies have shown that in most cases, the activity of the deactivated denitrification catalyst can be restored to 90%~105% of the original catalyst through regeneration. According to the different deactivation mechanisms of denitrification catalysts, the main regeneration methods are: physical cleaning, chemical cleaning, active component supplementation, etc.

2.1 Physical cleaning

Physical cleaning is to use water to rinse the deactivated denitrification catalyst to remove the fly ash covering the surface of the catalyst and restore the activity of the surface of the physically inactivated part of the catalyst. After rinsing, 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 flushing could effectively remove the fly ash physically adsorbed on the surface of the denitrification catalyst. Yu Yuexi et al. used ultrasonic water to clean the deactivated denitrification catalyst, and the contents of Ca, S, and K were reduced 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 the denitrification catalyst to a certain extent. After 45 min of ultrasonic cleaning of the deactivated denitrification catalyst, Li Jian performed XRF analysis on the deactivation catalyst and 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%.

2.2 Chemical cleaning

Physical cleaning can only remove part of the fly ash physically adsorbed on the surface of the denitrification catalyst, but cannot remove the fly ash chemically adsorbed on the surface of the denitrification catalyst. According to the different acidity and alkalinity of oxides in fly ash on the surface of the denitrification catalyst, chemical cleaning can be divided into alkali cleaning and acid cleaning.

2.2.1 Alkaline cleaning

Alkaline cleaning is to impregnate the denitrification catalyst after physical cleaning in a certain concentration of NaOH, Na2CO3 and other alkaline solutions to remove the acidic substances in fly ash adsorbed on the surface of the catalyst. Yu et al. with 0.2 mol· L-1 NaOH solution was washed at 30 °C to clean the deactivated denitrification catalyst, and it was found that the mass fraction of Al in the catalyst decreased from 42% to 28%, and the mass fraction of S element decreased from 52% to 24%, and the effect of removing Al and S was obvious. Fan Meiling et al. used 1.0 mol· L-1 Na2CO3 solution was used to clean the denitrification catalyst for As poisoning at room temperature, and it was found that the content of As2O3 in the cleaned denitrification catalyst decreased from 1.27% to 0.44%, and the removal rate of As reached 66%. Duan Qiutong et al. L-1 dilute NaOH solution was treated with deactivated denitrification catalyst for 60 min, and then at 0.5 mol· L-1 was impregnated in dilute H2SO4 solution for 60 min, the Na content decreased to 1.69%, and the removal rate of K, Ca, Mg, etc. reached 100%.

2.2.2 Acid cleaning

Alkaline cleaning can only remove acidic substances in fly ash, and some of these alkaline substances need to be removed by acid cleaning. Xiao Yuting et al. used 2% HNO3 solution to clean the deactivated denitrification catalyst, and the results showed that the K content in the catalyst decreased from 732.2×10-6 to 202.5×10-6, the Na content decreased from 559.4×10-6 to 114.6×10-6, and the S content also decreased from 2.20% to 0.59%. Zheng et al. with 0.5 mol· After cleaning the deactivated denitrification catalyst with L-1 H2SO4 solution for 20 min, the K content decreased from the initial 1 mg·g-1 to 0, and the activity of the denitrification catalyst was restored by about 50%~72% after the activity test at 250~350 °C. Wang Le et al. used 0.5mol· The content of As2O3 in the pickled catalyst decreased from 0.040% to 0.013%, the K2O content decreased from 0.022% to 0.018%, and the CaO content decreased from 0.984% to 0.842%, indicating that the removal effect of acid cleaning on alkaline substances was obvious.

2.3 Active component supplementation

Denitrification catalyst will lead to the loss of active components during use, and in the process of regeneration, although pickling and alkali washing treatment will restore the active site poisoned on the catalyst, some catalyst surfactants will be dissolved in the cleaning solution, causing a certain loss, so the lost active site in the above two cases needs to be replenished. The active component is usually supplemented by impregnation. Cui Liwen et al. used the one-step impregnation method to load the supplementary active components, and used the impregnation solution mixed with 1% ammonium metavanadate and 5% ammonium paratungstate solution for one-step impregnation, and the V content on the catalyst increased from 0.25% after cleaning to 1.13%, and the W content increased from 1.62% after washing to 4.83%, and the activity of the denitrification catalyst was significantly restored. Wang Denghui et al. used a step-by-step impregnation method to supplement the active components, first impregnated the regenerated catalyst with ammonium tungstate, and then impregnated in ammonium metavanadate solution, the mass ratio of ammonium metavanadate to ammonium tungstate was 1∶6, and the denitrification efficiency of the catalyst could reach 87.7% at 300 °C. Zhu Heng et al. prepared V-Mo/TiO2 cordierite denitrification catalyst with ammonium metavanadate and ammonium molybdenate tetrahydrate (mass ratio 3∶10) simultaneously, and the denitrification efficiency of the catalyst reached 98.8% at 340 °C.

3 Conclusion

Denitrification technology is the key technology for the clean emission of coal-fired power plants and other nitrogen-containing high-temperature exhaust gases, due to the problem of limited service life of denitrification catalysts, denitrification catalysts need to be replaced after deactivation, if the deactivation denitrification catalyst is abandoned will cause heavy metal pollution in the environment, therefore, the regeneration of denitrification catalysts has become the development trend of the denitrification industry. At present, denitrification catalyst regeneration is based on the reactivation of the inactivated active site without destroying its structure, and the active components are supplemented to complete the regeneration process. Due to the limitation of the number of reuses of recycled denitrification catalysts, the current restorative denitrification catalyst regeneration method cannot meet the development needs of the denitrification industry, and there is an urgent need to develop new technologies for recycling denitrification catalyst regeneration.