Common problems in practical application of SCR denitrification catalyst for biomass boilers

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1. Biomass boiler flue gas characteristics

(1) The concentration of sulfur dioxide and nitrogen oxide is low and fluctuates greatly; when burning pure biomass, the concentration of SO2 and NOx fluctuates between 120-250mg/m3, such as template, wood, and bark in the fuel, and the concentration of SO2 and NOx in the flue gas It fluctuates from 250 to 600 mg/m3.

(2) The hydrogen content in biomass is relatively high, and the moisture content in flue gas is also high, reaching 15% to 30%.

(3) The mass fraction of alkali metal in biomass fume is relatively high, which can reach over 8%.

(4) The SNCR denitrification efficiency of biomass grate furnace is 10%-25%, and the SNCR denitrification efficiency of biomass circulating fluidized bed is 40%-60%. The denitrification efficiency is unstable and cannot meet the ultra-low emission requirements stably. Below 50mg/Nm3.

Due to the low denitrification efficiency of SNCR, it is necessary to combine SCR denitrification technology to achieve higher denitrification efficiency. However, the biomass fuel itself contains alkaline substances such as K, Na, and Ca. After combustion, fly ash enters the SCR system, adsorbs on the surface of the SCR catalyst or blocks the catalyst pores, and reacts with the active components on the catalyst surface, causing the catalyst to be poisoned and deactivated.

Secondly, due to the low temperature and high moisture content of the flue gas at the tail of the biomass boiler. If a catalyst is installed at the tail of the boiler, water will be adsorbed on the active sites of the catalyst surface, reducing the adsorption sites of ammonia, thereby reducing the denitrification reaction speed and denitrification activity; under low temperature conditions, a larger moisture content will make fly ash more likely to stick. Attached to the surface of the catalyst, resulting in more rapid alkali poisoning.

At present, in order to avoid the influence of alkali metals on the catalyst, biomass boilers often adopt dust removal and desulfurization followed by denitrification; the flue gas after dust removal and desulfurization is increased in temperature by means of GGH, hot blast stove or steam heater, and then conventional SCR is used. The catalyst is used for denitration. This method, investment and operating costs are very high. If SCR denitration technology is used before the flue gas dust removal of biomass boilers, the cost and energy consumption of biomass denitration can be effectively reduced, but the problems of catalyst poisoning, deactivation, blockage and abrasion must be solved.

2. The influence of alkali metals on the denitration catalyst and the treatment plan

2.1 The influence of soluble alkali metal salts such as sodium and potassium on denitration catalysts

Alkali metals are the most toxic element to catalysts, and their toxicity strength is proportional to their alkalinity. Catalyst poisoning caused by alkali metals includes physical poisoning and chemical poisoning. Physical poisoning: Alkali metal usually does not exist in liquid form. Its salt particles are only deposited on the surface of the catalyst or block some of the pores of the catalyst, hindering the diffusion of NO and NH3 into the catalyst, thereby making the catalyst poisoning and deactivating. If water vapor condenses on the catalyst, alkali metals will cause chemical poisoning.

The active material of the SCR catalyst is V2O5, which has both B acid sites (V-OH) and L acid sites (V=O). The catalyst activity is proportional to the number of B acid sites. The presence of alkali metal ions will reduce the number of B acid sites of the catalyst and generate inactive KVO3; it will also reduce the stability of B acid sites and reduce the catalytic reduction ability of vanadium and tungsten/molybdenum. The decrease in the number of B acid sites and the decrease in stability will directly lead to a decrease in NH3 and surface oxygen adsorption, thereby reducing the activity of the catalyst.

Soluble alkali metal salts such as sodium and potassium are more basic than NH3, and the alkali metal reacts with the active sites of the catalyst, causing catalyst poisoning. In a humid environment, the impact of alkali metals on the catalyst is more serious.

2.2 The influence of calcium oxide on denitration catalyst

The catalyst will gradually deactivate after long-term operation in the flue gas containing high calcium fly ash. Several possible reasons for the deactivation are: CaO causes the blockage of micropores, the alkalinity of CaO reduces the acidity of the catalyst and the generated CaSO4 reduces the activity.

(1) Calcium oxide causes blockage of micropores

The fly ash contains high CaO content and high viscosity; and the fly ash has a small particle size, most of which are below 10μm. When the fly ash is in contact with the catalyst, it is very easy to adsorb on the surface of the catalyst and block the micropores of the catalyst, resulting in a decrease in the activity of the catalyst. However, the affinity of CaO relative to other components in fly ash with catalyst components is not particularly prominent, and it is not a component that is particularly easy to diffuse into the catalyst. In addition, relative to chemical effects, physical effects are generally reversible. The fly ash deposited on the catalyst surface can be removed in time by periodic soot blowing, so the blockage of the catalyst micropores by CaO is not the main reason for the decrease in activity.

(2) The alkalinity of calcium oxide reduces the acidity of the catalyst

Since CaO itself is an alkaline substance, and the active sites in the currently used V2O5-based catalysts are acidic, the CaO deposited on the catalyst surface will neutralize the acid sites on the catalyst surface and block the occurrence of the catalytic reaction. Since the reaction between CaO and the acid sites on the catalyst surface is a solid-solid reaction, the reaction speed is slow; and fly ash often contains more basic K2O and Na2O, it is generally believed that CaO is the secondary cause of alkali poisoning of the catalyst. However, in view of the high concentration of fly ash and the high calcium content in fly ash, the effect of the alkalinity of CaO on the catalyst should be paid attention to.

(3) CaSO4 generated makes the activity decrease

Due to the CaSO4 formed by the reaction of CaO deposited on the surface of the catalyst and SO3 in the flue gas, the blockage of the catalyst micropores is the main reason for the degradation of catalyst performance. The mechanism of CaO poisoning includes four steps.

Step 1-CaO attaches to the macroscopic pores on the surface of the catalyst.

Step 2-SO3 leaks the air film around the CaO particles.

Step 3-SO3 diffuses into the CaO particles.

Step 4-As SO3 diffuses into the CaO particles, it reacts with CaO to form CaSO4.

In the process of CaO poisoning, CaO is first deposited on the surface of the catalyst, and the deposition rate is relatively slow. The reaction between CaO deposited on the catalyst surface and SO3 in the flue gas is a gas-solid reaction. Because there are active materials on the catalyst surface to catalyze the oxidation of SO2 to generate SO3, the concentration of SO3 is relatively high, and the reaction speed is fast. The volume of CaSO4 generated by the rapid reaction will expand by about 14%, which will cover the active sites of the reaction, block the surface of the catalyst, and affect the diffusion of the reactants in the microporous structure of the catalyst. In the CaO poisoning mechanism, the relatively slow deposition rate of CaO is the key to control. Reducing the amount of CaO deposited on the catalyst surface is an effective means to slow down the catalyst poisoning.

2.3 Anti-alkali metal poisoning treatment plan

In view of the characteristics of biomass boiler flue gas, Huadian Guangda has made performance improvements on the basis of conventional catalysts: developed a denitration catalyst with high efficiency and resistance to alkali metals, which has excellent activity and long use in high alkali metal content fly ash flue gas life.

Through theoretical analysis, performance evaluation and characterization of the catalyst, analysis of the relationship between the occurrence of various elements in the catalyst and the performance of the catalyst, an in-depth understanding of the principle of catalyst poisoning, and cutting into the reduction of chemical and physical poisoning of the catalyst, successfully developed the anti- Alkali metal poisoning SCR denitration catalyst.

(1) Increase the acidity of the catalyst surface

Alkali metals and alkaline earth metals poison the catalyst, mainly by reacting with the acid site of the active center (V). Occupying the acid site causes ammonia to be unable to be adsorbed on the acid site, resulting in reduced catalyst activity. On this basis, the transition metal elements of VIB, IB, VIII subgroups and rare earth metals can increase the acid site of the catalyst. Through a series of work such as screening compounding, optimization of processing methods, and adjustment of proportions, a suitable promoter was successfully selected, which increased the overall acid site of the denitrification catalyst, and increased the adsorption sites of ammonia and alkali metal resistance. Thereby increasing the capacity of alkali metals.

(2) Add anti-alkali metal additives

Alkali metals and alkaline earth metals interact with the acid sites of the active center of the catalyst. In order to avoid catalyst deactivation or reduce the rate of catalyst deactivation, it is necessary to reduce the contact between alkali metals and alkaline earth metals and the acid sites of the active center, that is, to protect the active center of the catalyst. Increase the energy barrier for alkali metals and alkaline earth metals to contact the active center and use additives with higher activity and easier reaction with alkali metals. Starting from these two aspects, we first selected an auxiliary agent that has good dispersibility on the titanium dioxide and a certain steric hindrance, so that the alkali metals and alkaline earth metals are not easy to react with the catalyst.

(3) Adjust the catalyst formula

Due to the high content of alkaline earth metals in the flue gas fly ash, the ratio of active substances and co-catalysts and the processing method have been adjusted, and the overall catalyst activity has been improved through the compounding of additives and the improvement of processing technology. Through the impregnation pre-poisoning activity test and the observation of the apparent morphology, the catalyst has good alkali metal resistance.