Why is it necessary to control ammonia escape below 3ppm?

2023-01-11 10:46

1、The reason of high ammonia escape

Ammonia escape is an important parameter that affects the operation of SCR system. In the actual production process, more ammonia than the theoretical amount usually reaches the reactor, and the excess ammonia in the downstream of the flue gas after the reaction is called ammonia escape, which is expressed by the ammonia content per unit volume. In order to meet environmental requirements, a certain excess of ammonia is often required, so it also corresponds to there will be a suitable ammonia escape value, the value is designed to be no greater than 3ppm, but often the actual operation is large, mainly due to the following factors.

1) uneven distribution of ammonia flow rate for each ammonia lance, uneven local distribution of ammonia in the flue gas, uneven flue gas flow rate, large differences in the amount of ammonia sprayed at the outlet of each lance, and relatively high ammonia escape at high concentrations. 2) 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, but the reaction temperature is too high, and ammonia will additionally generate NO, if the temperature 3) Catalyst blockage, denitrification efficiency decreases, in order to keep the environmental protection parameters do not exceed the standard, will spray more ammonia, which will cause a vicious circle, the catalyst local blockage, aging performance, resulting in different catalytic efficiency in various parts of the catalyst, in order to control the export parameters, can only increase the amount of ammonia spray, which leads to local ammonia escape higher. 4) Atomization air volume is small, the gun 5) ammonia concentration, ammonia concentration configuration, the concentration of high and low can not be controlled, based on the sense of configuration, as far as the current boiler, basically ammonia concentration is high, ammonia regulating valve open too small, bad atomization easy to self-closing, resulting in high ammonia escape, operation is difficult. 6) combustion fluctuations, SNCR inlet flue gas NOX concentration significantly fluctuations, often increase the amount of ammonia spraying, mechanical to achieve "standard emissions", excessive ammonia, ammonia can lead to increased ammonia escape, directly endangering the safe operation of equipment and systems after the furnace.

2, the control of ammonia escape

1) For the deviation of ammonia escape caused by uneven distribution of ammonia spraying flow, it can be controlled by adjusting the ball valve in front of the ammonia lance, making the rotating lance head face down as much as possible in normal operation, increasing the reaction time, and evenly distributing ammonia spraying for each lance (its operation depends on the pressure drop), fully reacting NH₃ with NO, reducing the NH₃/NO molar ratio, thus reducing ammonia escape and achieving a balance between denitrification efficiency and operation cost. Ammonia fugitive concentration increase is also closely related to the ammonia lance nozzle, when the ammonia lance nozzle is clogged will intensify the generation of fugitive ammonia, should check the ammonia lance during boiler operation, timely unclogging or replacement, to ensure that the ammonia lance is put into operation normally.

2) 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, and at low load, the flue gas temperature drops, and the local flue gas temperature is too low, which will cause the catalyst activity to drop, thus causing the ammonia escape to rise. The catalyst selected for this denitrification is in the range of 315~380℃ is good, so the flue gas temperature should be maintained in the range under the conditions of boiler load and combustion. When coal powder is burned, the SCR reactor temperature reaches about 345℃, which can meet the reaction conditions of NOx and ammonia well, and the SCR reactor reaction efficiency is improved, the NOx and ammonia escape concentration at the SCR reactor outlet is low, the average NOx concentration reaches 60mg/m3, and the average ammonia escape concentration reaches 2.8ppm; when gas is mixed, the SCR reactor temperature is only about 300℃. At this time, the SCR reactor outlet NOx and ammonia fugitive concentration can be reduced by increasing the flame center position of the boiler through boiler air distribution adjustment or increasing the SCR reactor temperature by increasing the gas volume of the upper gas gun.

3) The catalyst has a service life, once the use of too long aging, the catalytic effect will become worse, denitrification reaction will also become worse, in order to ensure environmental protection qualified in the case of a large number of ammonia will cause ammonia escape increased, so when the catalyst is aging to be replaced in a timely manner when the furnace repair, to ensure that the ammonia escape qualified at the same time, but also to do a better job of environmental protection.

4) In coal-fired boilers, the denitrification reaction zone is in a high dust area, which will accumulate dust in the reaction zone, and the accumulation of dust will make the reaction worse and increase the ammonia escape. During the operation of the boiler, the SCR reactor is blown at least once a week to remove the accumulated dust in the SCR reactor to improve the efficiency of the SCR reactor and reduce the concentration of ammonia escape.

5) Atomization wind for denitrification reaction is obvious, but also directly determines the ammonia escape, and whether the ammonia can be fully atomized and air volume is proportional to the relationship, in order to improve the ammonia gun atomization effect, need to improve the compressed air pressure at 350kpa or more.

6) When the boiler combustion disturbance should be timely according to the NOx content of the denitration reactor inlet to adjust the ammonia water distribution, to prevent ammonia escape too large or large deviation on both sides, or even because the adjustment is not in place to bring about the problem of environmental protection exceedance. Boiler load changes 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 NOx content in the flue gas decreases making the flow rate in the SCR reactor decrease and the residence time of flue gas on the catalyst increase, which improves the denitrification efficiency and thus reduces the ammonia escape concentration.

7) Other influencing factors and prevention of boiler flue gas in the SCR reactor residence time of 0.1 to 0.2s, in order to make the residual ammonia in the boiler flue gas and flue gas nitrogen oxides in the catalyst under the action of sufficient reaction time, reduce the boiler SCR reactor outlet nitrogen oxides, ammonia escape concentration, usually choose to reduce the negative pressure of the boiler chamber, the boiler operation process of the boiler chamber negative pressure control in - During the operation of the boiler, the negative pressure of the boiler chamber is controlled between -30~-50Pa, the boiler combustion is stable, and the concentration of ammonia escape can be effectively controlled under the premise of the SCR reactor outlet NOx emission standard. When ammonia escape is too large and not properly controlled, ammonium hydrogen sulfate will be generated, which will not only cause catalyst layer failure and air preheater blockage, but also cause more serious problems and reduce the life of corrosive equipment.

In conclusion, reasonable control of ammonia fugitive concentration at boiler SCR outlet can effectively prevent boiler air preheater blockage and reduce the corrosion of downstream equipment by ammonia, so ammonia fugitive should be paid great attention during the operation of SCR denitrification plant. In view of this, it is necessary to strengthen the scientific control of SNCR and SCR operation stage, so as to control the ammonia escape rate of SCR unit to about 3ppm or even below, and reduce the impact of ammonium sulfate or ammonium hydrogen sulfate generation on the equipment after the furnace after the ammonia escape.


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