Reasons for high ammonia escape

Ammonia escape is an impactSCRAn important parameter for system operation, in actual production processes, more than the theoretical amount of ammonia usually reaches the reactor. The excess ammonia downstream of the flue gas after the reaction is called ammonia escape, which is represented by the ammonia content per unit volume.

In order to meet environmental protection requirements, a certain excess of ammonia is often required, so there will also be a suitable ammonia escape value, which is designed to be no greater than5ppmHowever, in actual operation, it often tends to be larger due to the following factors:

（1）Each ammonia spray gun has an uneven distribution of ammonia flow rate, and there is uneven distribution of ammonia water in the flue gas. The flow velocity of the flue gas is also uneven, and there is a significant difference in the amount of ammonia sprayed at the outlet of each spray gun. In areas with high concentration, ammonia escape is relatively higher.

（2）The flue gas temperature and reaction temperature are too low,NOxThe decrease in reaction rate with ammonia will causeNHA large amount of escape occurs, but if the reaction temperature is too high, additional ammonia will be generatedNOIf the temperature is too high or too low to achieve the reaction effect, it will inevitably increase ammonia escape.

（3）Catalyst blockage leads to a decrease in denitrification efficiency. In order to maintain environmental parameters within the standard, more ammonia will be sprayed, which will cause a vicious cycle. Local blockage and performance aging of the catalyst will result in different catalytic efficiencies at various locations. In order to control the outlet parameters, the amount of ammonia sprayed can only be increased, leading to an increase in local ammonia escape.

（4）The atomization air volume is too small, the spray gun atomization is not good, and the ammonia water and smoke cannot be fully mixed, resulting in a large amount of ammonia escaping.

（5）The concentration of ammonia water, the configuration of ammonia water concentration, and the level of concentration cannot be controlled. Based on intuition, for current boilers, the concentration of ammonia water is generally high. If the ammonia water regulating valve is opened too small, the atomization is difficult and prone to self closing, resulting in high ammonia escape and difficult operation.

（6）During combustion fluctuations,SNCRIn the smoke at the entranceNOXSignificant fluctuations in concentration often lead to an increase in ammonia injection volume, which is mechanically achieved“compliance discharge”Excessive ammonia water can lead to increased ammonia escape, directly endangering the safe operation of equipment and systems behind the furnace.

Control of ammonia escape

（1）For the deviation of ammonia escape caused by uneven distribution of ammonia injection flow rate, it can be controlled by adjusting the ball valve in front of the ammonia spray gun. During normal operation, try to make the rotating spray gun head face down as much as possible to increase reaction time, and ensure that each gun sprays ammonia evenly (depending on the pressure drop during operation),NHAlong withNOFully respond and reduceNH₃/NOThe molar ratio is used to reduce ammonia escape and achieve a balance between denitrification efficiency and operating costs. The increase in ammonia escape concentration is closely related to the nozzle of the ammonia spray gun. When the nozzle of the ammonia spray gun is blocked, it will exacerbate the production of ammonia escape. The ammonia spray gun should be checked during boiler operation, promptly unblocked or replaced to ensure its normal operation.

（2）The temperature of the flue gas determinesSNCRandSCRThe reaction effect affects the size of ammonia escape. The temperature variation of flue gas is large. At low loads, the flue gas temperature decreases. If the local flue gas temperature is too low, it will cause a decrease in catalyst activity, resulting in an increase in ammonia escape. The catalyst selected for this denitrification is315~380℃The range is, so it is necessary to maintain the flue gas temperature within the range according to the boiler load and combustion conditions under the conditions that are met. When coal powder is burned specifically,SCRThe reactor temperature has reached345℃Left and right, it can well meet the reaction conditions of nitrogen oxides and ammonia water,SCRThe efficiency of the reactor reaction is improved,SCRThe escape concentration of nitrogen oxides and ammonia at the outlet of the reactor is low, and the average concentration of nitrogen oxides reaches60mg/m³The average concentration of ammonia escape reached2.8ppmWhen gas is co fired,SCRThe reactor temperature is only300℃Left and right, at this time, the center position of the boiler flame can be increased by adjusting the boiler air distribution or by increasing the amount of upper combustion gasSCRMethod of reducing reactor temperatureSCRThe concentration of nitrogen oxides and ammonia escape at the outlet of the reactor.

（3）Catalysts have a lifespan, and once used for too long and aged, the catalytic effect will deteriorate, and the denitrification reaction will also deteriorate. In order to ensure environmental compliance, injecting a large amount of ammonia will increase ammonia escape. Therefore, when the catalyst ages, it should be replaced in a timely manner during shutdown and major repairs to ensure that ammonia escape is qualified while also doing a better job in environmental protection.

（4）Coal fired boilers, the denitrification reaction zone is located in a high dust area, which will accumulate dust in the reaction zone. Dust accumulation will worsen the reaction and increase ammonia escape. During the operation of the boilerSCRThe reactor should be blown and cleaned at least once a weekSCRIncreased ash accumulation in the reactorSCRReactor efficiency, reducing ammonia escape concentration.

（5）The atomization wind has a significant effect on denitrification reaction and directly determines the escape of ammonia. The sufficient atomization of ammonia water is proportional to the air volume. To improve the atomization effect of ammonia gun, it is necessary to increase the compressed air pressure350kpaThe above.

（6）When the boiler combustion is disturbed, it is necessary to promptly follow the inlet of the denitrification reactorNOxAdjust the distribution of ammonia water based on its content to prevent excessive ammonia escape or large deviations on both sides, and even environmental issues caused by inadequate adjustment. Changes in boiler load can lead to variations in boiler flue gas volume, flue gas temperature, andSCRChanges in inlet concentration. When the boiler load decreases, the amount of flue gas decreases, and the nitrogen oxide content in the flue gas decreasesSCRThe flow rate inside the reactor decreases, and the residence time of flue gas on the catalyst increases, which improves the denitrification efficiency and reduces the concentration of ammonia escape.

（7）Other influencing factors and prevention measures

Boiler flue gasSCRThe residence time of the reactor is0.1～0.2sIn order to ensure sufficient reaction time between residual ammonia water in boiler flue gas and nitrogen oxides in flue gas under the action of catalyst, and to reduce the boilerSCRThe concentration of nitrogen oxides and ammonia escaping from the reactor outlet is usually reduced by reducing the negative pressure of the boiler furnace. During the operation of the boiler, the negative pressure of the boiler furnace is controlled at-30～-50PaBetween them, the boiler combustion is stableSCRUnder the premise of meeting the emission standards for nitrogen oxides at the reactor outlet, the concentration of ammonia escape can be effectively controlled. If the ammonia escape is not properly controlled, ammonium bisulfate will be generated, which will not only cause the failure of the catalyst layer and blockage of the air preheater, but also lead to more serious problems, such as corrosion and reduced lifespan of the equipment.

In short, reasonable control of boilersSCRThe concentration of ammonia escape from the outlet can effectively prevent blockage of the boiler air preheater and reduce the corrosion of downstream equipment by ammonia water,SCRHigh attention should be paid to ammonia escape during the operation of denitrification devices. Given this, it is necessary to strengthenSNCRTheSCRScientific regulation during the operational phase willSCRControl the ammonia escape rate of the device to3ppmTo reduce the impact of ammonium sulfate or ammonium bisulfate generation on the equipment behind the furnace after ammonia escape.