1. Ammonia escapehighthe reason
Ammonia escape is an important parameter that affects the operation of SCR systems. In actual production processes, more than the theoretical amount of ammonia usually reaches the reactor, and the excess ammonia downstream of the flue gas after reaction is called ammonia escape. Ammonia escape is represented by the ammonia content per unit volume.In order to meet environmental protection requirements, a certain excess of ammonia is often required, which corresponds to a suitable ammonia escape value. This value is designed to be no more than 3ppm, but it is often larger in actual operation 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) If the flue gas temperature and reaction temperature are too low, the reaction rate between NOx and ammonia will decrease, causing 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 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) When the combustion fluctuates, the concentration of NOX in the flue gas at the inlet of SNCR fluctuates significantly, which often increases the amount of ammonia injection and mechanically achieves "standard emissions". Excessive ammonia water can lead to increased ammonia escape, directly endangering the safe operation of equipment and systems behind the furnace.2. Control of ammonia escape1) 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, the rotating spray gun head should be turned downwards as much as possible to increase the reaction time. Each gun should spray ammonia evenly (depending on the pressure drop during operation), and NH3 and NO should fully react to reduce 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 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 flue gas temperature determines the reaction effect of SNCR and SCR, which in turn affects the size of ammonia escape. The temperature of the flue gas varies greatly. 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 is in the range of 315-380 ℃, so the flue gas temperature should be maintained within the range according to the boiler load and combustion conditions. When coal powder is burned, the temperature of the SCR reactor reaches around 345 ℃, which can well meet the reaction conditions of nitrogen oxides and ammonia water. The reaction efficiency of the SCR reactor is improved, and the nitrogen oxide and ammonia escape concentrations at the outlet of the SCR reactor are low, with an average nitrogen oxide concentration of 60mg/m3 and an average ammonia escape concentration of 2.8ppm; When coal gas is co fired, the temperature of the SCR reactor is only around 300 ℃. At this time, by adjusting the boiler air distribution to increase the center position of the boiler flame or by increasing the amount of upper combustion gas to increase the temperature of the SCR reactor, the concentration of nitrogen oxides and ammonia escaping from the outlet of the SCR reactor can be reduced.
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 boiler, the SCR reactor should be blown with ash at least once a week to remove the accumulated ash and improve the efficiency of the SCR reactor, while reducing the concentration of ammonia escape.5) The atomization wind has a significant effect on denitrification reaction and directly determines ammonia escape. The sufficient atomization of ammonia water is directly proportional to the air volume. To improve the atomization effect of ammonia gun, the compressed air pressure needs to be increased above 350kPa.6) When the boiler combustion is disturbed, it is necessary to adjust the distribution of ammonia water in a timely manner according to the NOx content at the inlet of the denitrification reactor, to prevent excessive ammonia escape or large deviation on both sides, and even environmental problems caused by inadequate adjustment. Changes in boiler load can cause variations in boiler flue gas volume, flue gas temperature, and SCR inlet concentration. When the boiler load decreases, the amount of flue gas decreases, and the nitrogen oxide content in the flue gas decreases, resulting in a decrease in the flow rate in the SCR reactor. The residence time of the flue gas on the catalyst increases, which improves the denitrification efficiency and reduces the concentration of ammonia escape.7) Other influencing factors and prevention measuresThe residence time of boiler flue gas in the SCR reactor is 0.1-0.2 seconds. In order to ensure sufficient reaction time between residual ammonia water in the boiler flue gas and nitrogen oxides in the flue gas under the action of the catalyst, and to reduce the concentration of nitrogen oxides and ammonia escape at the outlet of the boiler SCR reactor, the method of reducing the negative pressure of the boiler furnace is usually chosen. During the operation of the boiler, the negative pressure of the boiler furnace is controlled between -30 and -50 Pa, and the boiler combustion is stable. Under the premise of meeting the standard emission of nitrogen oxides at the outlet of the SCR reactor, 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 the ammonia escape concentration at the outlet of the boiler SCR can effectively prevent blockage of the boiler air preheater and reduce the corrosion of downstream equipment by ammonia water. The SCR denitrification device should attach great importance to ammonia escape during operation. In view of this, it is necessary to strengthen the scientific regulation of SNCR and SCR operation stages, control the ammonia escape rate of SCR devices to around 3ppm or even below, and reduce the impact of ammonium sulfate or ammonium bisulfate generation on furnace equipment after ammonia escape.