Source: Thermal Control Circle
1. The reason for the high ammonia escape
Ammonia escape is an important parameter affecting the operation of the SCR system, in the actual production process, usually more than the theoretical amount of ammonia reaches the reactor, and the excess ammonia downstream of the flue gas after the reaction is called ammonia escape, and the ammonia escape is expressed by the ammonia content per unit volume.
In order to meet the requirements of environmental protection, a certain excess 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 large in actual operation, mainly due to the following factors:
1) The ammonia spray flow distribution of each ammonia spray gun is uneven, the local distribution of ammonia in the flue gas is uneven, the flue gas flow rate is uneven, the ammonia injection amount at the outlet of each spray gun is quite different, and the ammonia escape is relatively high in the place where the concentration is high.
2) The flue gas temperature, the reaction temperature is too low, the reaction rate of NOx and ammonia is reduced, which will cause a large amount of NH₃ escape, however, if the reaction temperature is too high, ammonia will additionally generate NO, if the temperature is too high and too low to achieve the reaction effect, it is bound to increase the ammonia escape.
3) The catalyst is blocked, the denitrification efficiency decreases, in order to keep the environmental protection parameters do not exceed the standard, more ammonia will be sprayed, which will cause a vicious circle, the catalyst is locally blocked, the performance is aging, resulting in different catalytic efficiency of the catalyst, in order to control the export parameters, only the amount of ammonia can be increased, resulting in the increase of local ammonia escape.
4) The atomization air volume is small, the spray gun atomization is not good, and the ammonia and flue gas cannot be fully mixed, which will produce a large amount of ammonia escape.
5) Ammonia concentration, ammonia concentration configuration, the concentration can not be controlled, with the feeling configuration, as far as the current boiler is concerned, basically the ammonia concentration is high, the ammonia valve is too small, the atomization is not good and easy to close, resulting in high ammonia escape and difficult operation.
6) When the combustion fluctuates, the NOX concentration in the flue gas at the inlet of SNCR fluctuates greatly, which often increases the amount of ammonia injected and mechanically achieves "standard discharge", and excessive ammonia can lead to an increase in ammonia escape, which directly endangers the safe operation of equipment and systems behind the furnace.
2. Control of ammonia escape
1) For the ammonia escape deviation caused by the uneven distribution of ammonia injection flow, the ball valve control before the ammonia spray gun can be adjusted, and the head of the rotating spray gun can be turned down as much as possible in the usual operation, so as to increase the reaction time, and the ammonia spraying of each gun is evenly distributed (its operation depends on the pressure drop), and NH₃ fully reacts with NO to reduce the NH₃/NO molar ratio, thereby reducing the ammonia escape and achieving the balance between denitrification efficiency and operating costs. The increase in ammonia escape concentration is also closely related to the ammonia spray gun nozzle, when the ammonia spray gun nozzle is blocked, it will aggravate the production of escaping ammonia, the ammonia spray gun should be checked during the operation of the boiler, and the ammonia spray gun should be dredged or replaced in time to ensure that the ammonia spray gun is put into operation normally.
2) The flue gas temperature determines the reaction effect of SNCR and SCR, which in turn affects the size of ammonia escape. The flue gas temperature varies greatly, at low load, the flue gas temperature drops, and the local flue gas temperature is too low, which will cause the catalyst activity to decrease, thereby causing the ammonia escape to rise, and the catalyst used in 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 conditions met according to the boiler load and combustion. When pulverized coal is burned, the temperature of the SCR reactor reaches about 345 °C, which can well meet the reaction conditions of nitrogen oxides and ammonia, the reaction efficiency of the SCR reactor is improved, the escape concentration of nitrogen oxides and ammonia at the outlet of the SCR reactor is low, and the average concentration of nitrogen oxides reaches 60mg/ m3, the average ammonia escape concentration reaches 2.8ppm, and the SCR reactor temperature is only about 300°C when the gas is mixed burning, so the nitrogen oxide and ammonia escape concentration at the outlet of the SCR reactor is reduced by increasing the central position of the boiler flame through boiler air distribution adjustment or by increasing the gas volume of the upper gas gun to increase the temperature of the SCR reactor.
3) The catalyst has a service life, once the use time is too long, the catalytic effect will be worse, the denitrification reaction will also be worse, in order to ensure that the environmental protection is qualified, a large amount of ammonia injection will cause ammonia escape to increase, so when the catalyst is aging, it should be replaced in time when the furnace is stopped for major repairs, so as to ensure that the ammonia escape is qualified, and at the same time, it can also do a better job in environmental protection.
4) For coal-fired boilers, the denitrification reaction zone is in the high dust area, which will accumulate dust in the reaction zone, and the ash accumulation will make the reaction worse and the ammonia escape will increase. During the operation of the boiler, the SCR reactor blows soot at least once a week to remove the ash accumulation of the SCR reactor, improve the efficiency of the SCR reactor, and reduce the ammonia escape concentration.
5) The atomization wind reacts obviously to denitrification, which also directly determines the escape of ammonia, and whether the ammonia can be fully atomized is 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 to more than 350kpa.
6) When the boiler combustion is disturbed, the ammonia should be adjusted and distributed according to the NOx content at the inlet of the denitrification reactor in time to prevent the ammonia from escaping too much or the deviation on both sides, or even the environmental protection exceeding the standard caused by the adjustment is not in place. Changes in boiler load will lead to changes in boiler flue gas volume, flue gas temperature and SCR inlet concentration. When the boiler load is reduced, the flue gas volume decreases, and the nitrogen oxide content in the flue gas decreases, so that the flow rate in the SCR reactor decreases, the residence time of the flue gas on the catalyst increases, and the denitrification efficiency is improved, thereby reducing the ammonia escape concentration.
7) Other influencing factors and precautions
The residence time of boiler flue gas in the SCR reactor is 0.1~0.2s, in order to make the residual ammonia in the boiler flue gas and the nitrogen oxides in the flue gas have enough reaction time under the action of the catalyst, and reduce the nitrogen oxide and ammonia escape concentration at the outlet of the SCR reactor of the boiler, the negative pressure of the boiler furnace is usually reduced, and the negative pressure of the boiler furnace is controlled between -30~-50Pa during the operation of the boiler, and the combustion of the boiler is stable. Ammonia escape concentrations can be effectively controlled. When the ammonia escape is too large and not well controlled, the ammonium bisulfate will be generated, which will not only cause the failure of the catalyst layer and the blockage of the air preheater, but also cause more serious problems, corroding the equipment and reducing the life of the equipment.
In short, reasonable control of the ammonia escape concentration at the SCR outlet of the boiler can effectively prevent the blockage of the boiler air preheater and reduce the corrosion of ammonia to the downstream equipment. In view of this, it is necessary to strengthen the scientific regulation of SNCR and SCR operation stage, control the ammonia escape rate of SCR device to about 3ppm, or even below, and reduce the impact of ammonium sulfate or ammonium bisulfate generation after ammonia escape on the equipment behind the furnace.