Research progress on the recycling and reuse of SCR denitrification catalysts
Publishdate：2021-04-09 Views：17
The nitrogen oxides (NOx) emitted by coal-fired power plants are one of the main atmospheric pollutants, as well as the main substances that form photochemical smog, acid rain pollution, and damage the ozone layer. How to effectively control NOx emissions has become an important issue of concern in current environmental protection. Among numerous denitrification technologies, Selective Catalytic Reduction (SCR) has become the most widely used flue gas denitrification technology in the world due to its advantages of no by-products, simple equipment, high removal efficiency (up to 90% or more), reliable operation, and easy maintenance.
The key issue in adopting SCR technology is to choose excellent catalysts, whose performance directly affects the overall denitrification effect of the SCR system. After years of industrial practice and verification, metal oxide catalysts are widely used, with rutile type titanium dioxide as the carrier and vanadium oxide as the active substance, supplemented by tungsten oxide or molybdenum oxide as co catalysts.
At present, the operating temperature range of vanadium based catalysts used for flue gas denitrification in coal-fired power plants is 310~430 ℃, which is equivalent to the flue gas temperature at the outlet of the boiler economizer. Therefore, the SCR denitrification reactor is directly installed between the boiler economizer and the air preheater, which is known as the high-level layout method [2]. Although the catalyst activity is beneficial for the reaction under this arrangement, the high concentration of dust in the flue gas in this arrangement can wash away the catalyst and cause it to be poisoned. At the same time, the high temperature of the flue gas can cause sintering and deactivation of the catalyst, shortening its lifespan.
When the activity of the catalyst decreases and its performance deteriorates to a certain extent, the catalyst needs to be replaced. In addition to the consumption of ammonia, the replacement of the catalyst accounts for the majority of the operating costs. Catalysts with reversible poisoning and catalysts with reduced activity can be reused through regeneration, with a regeneration cost of only 20-30% of the replacement cost, while activity can be restored to 90% -100% of its original value, or even higher [3].
In addition, waste SCR denitrification catalysts that cannot be regenerated contain valuable metals such as vanadium, which can cause environmental pollution if discarded directly. Among them, vanadium is a rare metal that is dispersed but not concentrated in nature, and there are not many enriched vanadium ores, making extraction and separation difficult. In recent years, with the development of technology, the demand for vanadium has increased by about 5% annually, leading to a continuous increase in vanadium prices. Therefore, recovering V2O5 from discarded SCR denitrification catalysts can avoid environmental pollution and save valuable resources.
Deactivation mechanism of 2 SCR catalysts
The main reasons for the decrease or even deactivation of SCR catalyst activity during the operation of the SCR system are as follows [4,5].
2.1 Sintering and volatilization of active components caused by high temperature
Temperature has a significant impact on the activity of SCR catalysts, and there have been related studies on the thermal deactivation of V2O5-WO3/TiO2 catalysts. Prolonged exposure to high temperature environments above 450 ℃ can cause sintering of the active site (surface) of the catalyst, directly leading to an increase in catalyst particles, a decrease in surface area, and volatilization loss of some active components, thereby reducing catalyst activity.
Reiche et al. [6] studied the activity changes of V2O5/TiO2 at different temperatures and found that the catalyst would be severely deactivated when the temperature exceeded 500 ℃. The research results of Moradi et al. [7] indicate that external environmental temperature is an important parameter during catalyst deactivation. When the external environment temperature increases, the role of submicroscopic particles in catalyst deactivation will be strengthened.
2.2 Catalyst poisoning caused by alkali metals and alkali earth metal oxides
The soluble alkali metals in fly ash mainly include two substances, Na and K. In the ionic state of aqueous solution, they can penetrate deep into the catalyst and directly react with the active particles of the catalyst, causing acid poisoning to reduce its adsorption capacity and activity for NH3, thereby reducing catalytic activity [8].
Alkali metal elements are considered to be a type of element with great toxicity to catalysts, therefore the essence of alkali metal poisoning has become the focus of exploration. Kamata et al. [9] confirmed through denitrification activity experiments that as the surface K2O content of the catalyst increases, the NO conversion rate sharply decreases. When the K2O mass fraction reaches 1%, the catalyst activity is almost completely lost.
They also used DRFIT and other methods to analyze the mechanism of catalyst potassium poisoning: the presence of K2O greatly reduces the number of Bronsted acidic sites, one of the active sites of the SCR catalyst, and also weakens the acidity of the Bronsted acidic sites. However, as the surface K2O content of the SCR catalyst increases, the number of Lewis acidic sites, another active site, hardly changes. This indicates that after potassium poisoning of the SCR catalyst, The decrease in activity is caused by a change in the Bronsted acidic site.
In addition, the increase of alkali metal substances will increase the pH value of the carrier oxide. Under high temperature sintering, it will change the crystal structure of the catalyst, cause structural collapse, block the inner pores, and lead to a decrease in activity. Therefore, if the content of K2O and Na2O in the flue gas increases, the deactivation of the catalyst becomes more severe [10]. Zhu Chongbing et al. [11,12] poisoned V2O5-WO3/TiO2 catalyst using simulated poisoning method. By detecting the denitrification activity of the catalyst after poisoning, the deactivation degree of the catalyst under the same molar ratio of alkali metal oxide poisoning conditions was compared, and the following conclusions were obtained:
The combination of alkali metal oxides and V species on the catalyst surface generates some alkali metal salts (such as KVO3, NaVO3), which changes the surface structure of the catalyst and greatly reduces the number of effective active sites in the catalyst, leading to a decrease in catalyst activity. The toxicity order of two alkali metal oxides on the catalyst is K2O>Na2O. The influence of alkaline earth metal elements (Ca, Mg) on SCR catalysts is mainly manifested in the deposition of oxides on the catalyst surface and further reaction, resulting in pore structure blockage.
The detection results of XRD on the catalyst surface by Benson et al. [13] showed that the alkaline earth metal compounds deposited on the catalyst surface were mainly CaSO4, while the rest were Ca3Mg (SiO4) 2 and CaCO3. CaSO4 and CaCO3 were obtained by reacting CaO with SO3 and CO2, respectively. Nicosia et al. [14] confirmed through NH3-TPD and DRFIT measurements that Ca can also affect the adsorption of NH3 on the Bronsted acidic site and V5+O, just like K, while having almost no effect on the Lewis acidic site. However, at the same molar fraction, the effect of Ca is smaller than that of K.
The nitrogen oxides (NOx) emitted by coal-fired power plants are one of the main atmospheric pollutants, as well as the main substances that form photochemical smog, acid rain pollution, and damage the ozone layer. How to effectively control NOx emissions has become an important issue of concern in current environmental protection. Among numerous denitrification technologies, Selective Catalytic Reduction (SCR) has become the most widely used flue gas denitrification technology in the world due to its advantages of no by-products, simple equipment, high removal efficiency (up to 90% or more), reliable operation, and easy maintenance.
The key issue in adopting SCR technology is to choose excellent catalysts, whose performance directly affects the overall denitrification effect of the SCR system. After years of industrial practice and verification, metal oxide catalysts are widely used, with rutile type titanium dioxide as the carrier and vanadium oxide as the active substance, supplemented by tungsten oxide or molybdenum oxide as co catalysts.
At present, the operating temperature range of vanadium based catalysts used for flue gas denitrification in coal-fired power plants is 310~430 ℃, which is equivalent to the flue gas temperature at the outlet of the boiler economizer. Therefore, the SCR denitrification reactor is directly installed between the boiler economizer and the air preheater, which is known as the high-level layout method [2]. Although the catalyst activity is beneficial for the reaction under this arrangement, the high concentration of dust in the flue gas in this arrangement can wash away the catalyst and cause it to be poisoned. At the same time, the high temperature of the flue gas can cause sintering and deactivation of the catalyst, shortening its lifespan.
When the activity of the catalyst decreases and its performance deteriorates to a certain extent, the catalyst needs to be replaced. In addition to the consumption of ammonia, the replacement of the catalyst accounts for the majority of the operating costs. Catalysts with reversible poisoning and catalysts with reduced activity can be reused through regeneration, with a regeneration cost of only 20-30% of the replacement cost, while activity can be restored to 90% -100% of its original value, or even higher [3].
In addition, waste SCR denitrification catalysts that cannot be regenerated contain valuable metals such as vanadium, which can cause environmental pollution if discarded directly. Among them, vanadium is a rare metal that is dispersed but not concentrated in nature, and there are not many enriched vanadium ores, making extraction and separation difficult. In recent years, with the development of technology, the demand for vanadium has increased by about 5% annually, leading to a continuous increase in vanadium prices. Therefore, recovering V2O5 from discarded SCR denitrification catalysts can avoid environmental pollution and save valuable resources.
Deactivation mechanism of 2 SCR catalysts
The main reasons for the decrease or even deactivation of SCR catalyst activity during the operation of the SCR system are as follows [4,5].
2.1 Sintering and volatilization of active components caused by high temperature
Temperature has a significant impact on the activity of SCR catalysts, and there have been related studies on the thermal deactivation of V2O5-WO3/TiO2 catalysts. Prolonged exposure to high temperature environments above 450 ℃ can cause sintering of the active site (surface) of the catalyst, directly leading to an increase in catalyst particles, a decrease in surface area, and volatilization loss of some active components, thereby reducing catalyst activity.
Reiche et al. [6] studied the activity changes of V2O5/TiO2 at different temperatures and found that the catalyst would be severely deactivated when the temperature exceeded 500 ℃. The research results of Moradi et al. [7] indicate that external environmental temperature is an important parameter during catalyst deactivation. When the external environment temperature increases, the role of submicroscopic particles in catalyst deactivation will be strengthened.
2.2 Catalyst poisoning caused by alkali metals and alkali earth metal oxides
The soluble alkali metals in fly ash mainly include two substances, Na and K. In the ionic state of aqueous solution, they can penetrate deep into the catalyst and directly react with the active particles of the catalyst, causing acid poisoning to reduce its adsorption capacity and activity for NH3, thereby reducing catalytic activity [8].
Alkali metal elements are considered to be a type of element with great toxicity to catalysts, therefore the essence of alkali metal poisoning has become the focus of exploration. Kamata et al. [9] confirmed through denitrification activity experiments that as the surface K2O content of the catalyst increases, the NO conversion rate sharply decreases. When the K2O mass fraction reaches 1%, the catalyst activity is almost completely lost.
They also used DRFIT and other methods to analyze the mechanism of catalyst potassium poisoning: the presence of K2O greatly reduces the number of Bronsted acidic sites, one of the active sites of the SCR catalyst, and also weakens the acidity of the Bronsted acidic sites. However, as the surface K2O content of the SCR catalyst increases, the number of Lewis acidic sites, another active site, hardly changes. This indicates that after potassium poisoning of the SCR catalyst, The decrease in activity is caused by a change in the Bronsted acidic site.
In addition, the increase of alkali metal substances will increase the pH value of the carrier oxide. Under high temperature sintering, it will change the crystal structure of the catalyst, cause structural collapse, block the inner pores, and lead to a decrease in activity. Therefore, if the content of K2O and Na2O in the flue gas increases, the deactivation of the catalyst becomes more severe [10]. Zhu Chongbing et al. [11,12] poisoned V2O5-WO3/TiO2 catalyst using simulated poisoning method. By detecting the denitrification activity of the catalyst after poisoning, the deactivation degree of the catalyst under the same molar ratio of alkali metal oxide poisoning conditions was compared, and the following conclusions were obtained:
The combination of alkali metal oxides and V species on the catalyst surface generates some alkali metal salts (such as KVO3, NaVO3), which changes the surface structure of the catalyst and greatly reduces the number of effective active sites in the catalyst, leading to a decrease in catalyst activity. The toxicity order of two alkali metal oxides on the catalyst is K2O>Na2O. The influence of alkaline earth metal elements (Ca, Mg) on SCR catalysts is mainly manifested in the deposition of oxides on the catalyst surface and further reaction, resulting in pore structure blockage.
The detection results of XRD on the catalyst surface by Benson et al. [13] showed that the alkaline earth metal compounds deposited on the catalyst surface were mainly CaSO4, while the rest were Ca3Mg (SiO4) 2 and CaCO3. CaSO4 and CaCO3 were obtained by reacting CaO with SO3 and CO2, respectively. Nicosia et al. [14] confirmed through NH3-TPD and DRFIT measurements that Ca can also affect the adsorption of NH3 on the Bronsted acidic site and V5+O, just like K, while having almost no effect on the Lewis acidic site. However, at the same molar fraction, the effect of Ca is smaller than that of K.
