The causes of ammonia escape formation and how to control it
Causes of high ammonia escape
Ammonia escape is an important parameter that affects the operation of SCR systems. In the actual production process, usually more ammonia than the theoretical amount reaches the reactor, and the excess ammonia 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 the environmental requirements, a certain excess ammonia is often required, so it also corresponds that there will be a suitable ammonia escape value, which is designed to be no more than 5 ppm, but is often large in actual operation, mainly due to the following factors.
(1) each ammonia lance spray ammonia flow distribution is uneven, there is uneven local distribution of ammonia in the flue gas, the flue gas flow rate is not uniform, the amount of ammonia sprayed at the outlet of each lance varies greatly, the concentration of high places ammonia escape is relatively high.
(2) Flue gas temperature, the reaction temperature is too low, the reaction rate of NOx and ammonia is reduced, which will cause a large number of NH₃ escape, however, the reaction temperature is too high, ammonia will generate additional NO, if the temperature is too high and too low to achieve the reaction effect, it is bound to increase the ammonia escape.
(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 rise.
(4) The atomization air volume is small, the gun atomization is not good, ammonia and flue gas can not be fully mixed, will produce a large number of ammonia escape.
(5) ammonia concentration, ammonia concentration configuration, the concentration of high and low can not be controlled, by the feeling 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, the operation is difficult.
(6) combustion fluctuations, SNCR inlet flue gas NOX concentration fluctuates significantly, often increasing the amount of ammonia sprayed, mechanically achieve the "standard emissions", excess ammonia, can lead to increased ammonia escape, directly endangering the safe operation of equipment and systems after the furnace.
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), so that NH₃ and NO can fully react and reduce 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 aggravate the generation of fugitive ammonia, should check the ammonia lance during the 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℃ as the best, so the flue gas temperature should be maintained in the best range under the conditions met according to the boiler load and combustion conditions. The SCR reactor temperature reaches 345℃ when coal powder is burned, which can meet the reaction conditions of nitrogen oxide and ammonia well, and the reaction efficiency of SCR reactor is improved, and the concentration of nitrogen oxide and ammonia escape from SCR reactor outlet is low, the average concentration of nitrogen oxide reaches 60mg/m³, and the average concentration of ammonia escape reaches 2.8ppm; when gas is mixed, the temperature of SCR reactor is only about 300℃. At this time, the SCR reactor outlet NOx and ammonia escape 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 used for 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 spray will cause ammonia escape increased, so when the catalyst is aging to be replaced in a timely manner when the furnace is stopped for major repairs, to ensure that ammonia escape qualified at the same time, but also better to do a good 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 determine the ammonia escape, and whether the ammonia can be fully atomized and the wind volume is proportional to the relationship, in order to improve the ammonia gun atomization effect, need to improve the compressed air pressure in more than 350kpa.
(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
Boiler flue gas in the SCR reactor residence time of 0.1 ~ 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 between -30 ~ -50Pa. Boiler combustion stability, in the SCR reactor outlet NOx emission standards, ammonia escape concentration can be effectively controlled. 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, and control the ammonia escape rate of SCR unit to about 3ppm or even below, so as to 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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