1. Reasons for high ammonia escape
Ammonia slip is an important parameter affecting the operation of the SCR system. In the actual production process, more than the theoretical amount of ammonia usually reaches the reactor. After the reaction, the excess ammonia in downstream of the flue gas is called ammonia slip. Indicated by the ammonia content.
In order to meet the requirements of environmental protection, a certain amount of ammonia is often required, so there will be a suitable ammonia escape value, which is designed to be no more than 3ppm, but it is often too large in actual operation, mainly due to the following factors:
(1) The flow distribution of ammonia sprayed by each ammonia spray gun is uneven, there is an uneven local distribution of ammonia water in the flue gas, the flue gas flow rate is uneven, and the amount of ammonia sprayed at the outlet of each spray gun varies greatly, and ammonia escape is relatively high in places with high concentrations.
(2) Flue gas temperature. If the reaction temperature is too low, the reaction rate of NOx and ammonia will decrease, which will cause a large amount of NH₃ to escape. However, if the reaction temperature is too high, ammonia will generate additional NO. If the temperature is too high or too low, it will not reach The reaction effect and is bound to increase ammonia escape.
(3) The catalyst is clogged and the denitrification efficiency is reduced. In order to keep the environmental protection parameters within the standard, more ammonia will be sprayed, which will cause a vicious cycle. The catalyst is partially clogged and its performance is aging, resulting in different catalytic efficiencies of the catalyst. In order to control the outlet parameters, Can only increase the amount of ammonia injection, resulting in increased local ammonia slip.
(4) The atomization air volume is too small, the atomization of the spray gun is not good, the ammonia water and the flue gas cannot be fully mixed, and a large amount of ammonia will escape.
(5) Concentration of ammonia water, the configuration of ammonia water concentration, the concentration cannot be controlled, and the configuration is based on feeling. As far as the current boiler is concerned, the concentration of ammonia water is basically high, the opening of the ammonia water regulating valve is too small, the atomization is not good and it is easy to close automatically, resulting in Ammonia escape is high and operation is difficult.
(6) When the combustion fluctuates, the concentration of NOX in the flue gas at the inlet of the SNCR fluctuates greatly, which often increases the amount of ammonia sprayed to mechanically achieve “standard discharge”. Excessive ammonia water can lead to increased ammonia escape, which directly endangers the equipment behind the furnace and The system operates safely.
2. Control of ammonia escape
(1) For the deviation of ammonia escape caused by the uneven distribution of ammonia injection flow, it can be controlled by adjusting the ball valve in front of the ammonia water spray gun. Uniform (the operation depends on the pressure drop), NH₃ and NO fully react, reducing the NH₃/NO molar ratio, thereby reducing ammonia escape, and achieving a balance between denitrification efficiency and operating costs. The increase in ammonia escape concentration is also closely related to the nozzle of the ammonia water spray gun. When the ammonia water spray gun nozzle is blocked, the generation of escaped ammonia will be aggravated. The ammonia water spray gun should be checked during the operation of the boiler, and it should be dredged or replaced in time to ensure the normal operation of the ammonia water spray gun.
(2) Flue gas temperature determines the reaction effect of SNCR and SCR and then affects the size of ammonia escape. The flue gas temperature varies greatly. At low loads, the flue gas temperature drops. If the local flue temperature is too low, the activity of the catalyst will decrease, which will lead to the increase of ammonia escape. The catalyst selected for this denitrification is the best in the range of 315~380°C, so it is necessary to maintain the flue gas temperature in the optimal range under the satisfied conditions according to the boiler load and combustion conditions. When pulverized coal is exclusively burned, the temperature of the SCR reactor reaches about 345°C, which can well meet the reaction conditions of nitrogen oxides and ammonia water. The reaction efficiency of the SCR reactor is improved, and the escape concentration of nitrogen oxides and ammonia at the outlet of the SCR reactor is low. The average ammonia concentration reaches 60mg/m3, and the average ammonia escape concentration reaches 2.8 ppm; when the gas is co-fired, the temperature of the SCR reactor is only about 300°C. The method of increasing the temperature of the SCR reactor reduces the escape concentration of nitrogen oxides and ammonia at the outlet of the SCR reactor.
(3) The catalyst has a service life. Once it is used for a long time and ages, the catalytic effect will deteriorate, and the denitrification reaction will also deteriorate. In order to ensure environmental protection, a large amount of ammonia injection will cause ammonia to escape increasingly. Therefore, when the catalyst When is aging, it should be replaced in time when the furnace is shut down for major repairs, so as to ensure that the ammonia escape is qualified, and it can also better protect the environment.
(4) For coal-fired boilers, the denitrification reaction zone is located in a high-dust area, and dust will accumulate in the reaction zone. Ash accumulation will make the reaction worse and ammonia escape will increase. During the operation of the boiler, the SCR reactor should be soot blown at least once a week to remove the soot accumulated in the SCR reactor to improve the efficiency of the SCR reactor and reduce the ammonia escape concentration.
(5) The atomizing wind has an obvious denitrification reaction, which also directly determines the escape of ammonia, and whether the ammonia water can be fully atomized is directly proportional to the air volume. In order to improve the atomization effect of the ammonia gun, it is necessary to increase the compressed air pressure above 350kpa.
(6) When the boiler combustion is disturbed, it is necessary to adjust the distribution of ammonia water according to the NOx content at the inlet of the denitrification reactor in time to prevent excessive ammonia escape or large deviation on both sides, and even environmental protection problems caused by the improper adjustment. Changes in boiler load will lead to changes in boiler flue gas volume, flue gas temperature and SCR inlet concentration. When the boiler load decreases, the amount of flue gas decreases, the nitrogen oxide content in the flue gas decreases, the flow rate in the SCR reactor decreases, the residence time of the flue gas on the catalyst increases, the denitrification efficiency is improved, and the ammonia slip concentration is reduced.
(7) Other influencing factors and precautions
The residence time of the boiler flue gas in the SCR reactor is 0.1-0.2s. In order to make the residual ammonia water in the boiler flue gas and the nitrogen oxides in the flue gas have enough reaction time under the action of the catalyst, reduce the nitrogen oxides and nitrogen oxides at the outlet of the boiler SCR reactor. Ammonia escape concentration is usually carried out by reducing the negative pressure of the boiler furnace. During the operation of the boiler, the negative pressure of the boiler furnace is controlled between -30～-50Pa, and the boiler combustion is stable. Under the premise that the discharge of nitrogen oxides at the outlet of the SCR reactor meets the standard, Ammonia escape concentration can be effectively controlled. When the ammonia escape is too large and not properly controlled, the ammonium bisulfate will be generated, which will not only cause the failure of the catalyst layer and the clogging of the air preheater but also cause more serious problems, corrode the equipment and reduce the service life.
In short, reasonable control of the ammonia escape concentration at the boiler SCR outlet can effectively prevent the blockage of the boiler air preheater and reduce the corrosion of ammonia water on downstream equipment. The SCR denitrification device should pay great attention to ammonia escape during operation. In view of this, it is necessary to strengthen the scientific regulation of the SNCR and SCR operation stages, control the ammonia escape rate of the SCR device to about 3ppm or even below, and reduce the impact of ammonium sulfate or ammonium bisulfate formation on the equipment behind the furnace after ammonia escape.