Why should ammonia escape be kept below 3 ppm?
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1. Reasons for high ammonia escape: Ammonia escape is an important parameter that affects the operation of SCR systems. In actual production, more ammonia than the theoretical amount reaches the reactor; the excess ammonia in the flue gas downstream of the reaction is referred to as ammonia escape. Ammonia escape is expressed as the amount of ammonia per unit volume. To meet environmental regulations, a certain excess amount of ammonia is often required; consequently, there is an appropriate value for ammonia escape, which is set to be no more than 3 ppm. However, in actual operation this value tends to be higher, due to the following factors: 1) The ammonia flow rate from each ammonia injector is uneven, resulting in uneven distribution of ammonia vapor in the flue gas as well as uneven flue gas flow rates. There are significant differences in the amount of ammonia emitted from each injector, and areas with higher concentrations experience relatively higher ammonia escape. 2) A low flue gas temperature reduces the reaction rate between NOx and ammonia, resulting in significant escape of NH3. However, if the reaction temperature is too high, ammonia will produce additional NO. If the temperature is either too high or too low to facilitate the reaction, it will inevitably lead to increased ammonia escape. 3) Catalyst clogging leads to a decrease in denitrification efficiency. To keep environmental parameters within acceptable limits, more ammonia is injected, which creates a vicious cycle: partial clogging of the catalyst and deterioration of its performance result in varying catalytic efficiencies across different areas of the catalyst. To control the outlet parameters, it is necessary to increase the amount of ammonia injected, which in turn leads to increased ammonia leakage in those areas. 4) The spray gun does not atomize properly, preventing adequate mixing between ammonia water and flue gas, which results in a large amount of ammonia escaping. 5) During combustion fluctuations, the NOX concentration in the flue gas at the SCR inlet varies significantly, which often leads to an increased amount of ammonia being injected in order to mechanically achieve compliant emissions. Excessive ammonia injection can result in increased ammonia leakage, thereby posing a direct threat to the safe operation of the equipment and systems downstream of the furnace. 2. Control of ammonia slip1) The flue gas temperature determines the reaction efficiency of SCR, which in turn affects the amount of ammonia slip. The flue gas temperature varies significantly; at low loads, the temperature drops, and excessively low local temperatures can lead to a decrease in catalyst activity, which in turn results in increased ammonia escape. The catalysts used in SCR systems perform best within the range of 315–380°C. Therefore, it is necessary to maintain the flue gas temperature within this optimal range, taking into account the boiler load and combustion conditions. It can effectively meet the reaction conditions for nitrogen oxides and ammonia vapor, thereby improving the reaction efficiency of the SCR reactor. 2) Catalysts have a limited service life; once they age due to prolonged use, their catalytic efficiency declines, as does the efficiency of the denitration process. To ensure compliance with environmental regulations, excessive ammonia injection can lead to increased ammonia leakage. Therefore, when the catalysts age, it is necessary to replace them during shutdowns for major repairs, so as to maintain acceptable levels of ammonia leakage and improve environmental performance. 3) In coal-fired boilers, the denitration reaction zone is located in an area with high dust levels, which causes dust to accumulate in that zone. This accumulation of dust reduces the efficiency of the reaction and increases ammonia emissions. During the operation of the boiler, the SCR reactor should be soot-blowed at least once a week to remove the accumulated soot, thereby improving the efficiency of the SCR reactor and reducing the ammonia escape concentration. In summary, properly controlling the ammonia escape concentration at the outlet of the boiler’s SCR system can effectively prevent blockages in the boiler’s air preheater and reduce the corrosion caused by ammonia on downstream equipment; therefore, due attention should be paid to ammonia escape during the operation of the SCR denitration system. By controlling the ammonia escape rate of the SCR device to below 3 ppm, the impact of ammonium sulfate or ammonium bisulfate resulting from ammonia escape on the equipment downstream of the furnace is reduced. 1. Reasons for high ammonia escape: Ammonia escape is an important parameter that affects the operation of SCR systems. In actual production, more ammonia than the theoretical amount reaches the reactor; the excess ammonia in the flue gas downstream of the reaction is referred to as ammonia escape. Ammonia escape is expressed as the amount of ammonia per unit volume. To meet environmental regulations, a certain excess amount of ammonia is often required; consequently, there is an appropriate value for ammonia escape, which is set to be no more than 3 ppm. However, in actual operation this value tends to be higher, due to the following factors: 1) The ammonia flow rate from each ammonia injector is uneven, resulting in uneven distribution of ammonia vapor in the flue gas as well as uneven flue gas flow rates. There are significant differences in the amount of ammonia emitted from each injector, and areas with higher concentrations experience relatively higher ammonia escape. 2) A low flue gas temperature reduces the reaction rate between NOx and ammonia, resulting in significant escape of NH3. However, if the reaction temperature is too high, ammonia will produce additional NO. If the temperature is either too high or too low to facilitate the reaction, it will inevitably lead to increased ammonia escape. 3) Catalyst clogging leads to a decrease in denitrification efficiency. To keep environmental parameters within acceptable limits, more ammonia is injected, which creates a vicious cycle: partial clogging of the catalyst and deterioration of its performance result in varying catalytic efficiencies across different areas of the catalyst. To control the outlet parameters, it is necessary to increase the amount of ammonia injected, which in turn leads to increased ammonia leakage in those areas. 4) The spray gun does not atomize properly, preventing adequate mixing between ammonia water and flue gas, which results in a large amount of ammonia escaping. 5) During combustion fluctuations, the NOX concentration in the flue gas at the SCR inlet varies significantly, which often leads to an increased amount of ammonia being injected in order to mechanically achieve compliant emissions. Excessive ammonia injection can result in increased ammonia leakage, thereby posing a direct threat to the safe operation of the equipment and systems downstream of the furnace. 2. Control of ammonia slip
1) The flue gas temperature determines the reaction efficiency of SCR, which in turn affects the amount of ammonia slip. The flue gas temperature varies significantly; at low loads, the temperature drops, and excessively low local temperatures can lead to a decrease in catalyst activity, which in turn results in increased ammonia escape. The catalysts used in SCR systems perform best within the range of 315–380°C. Therefore, it is necessary to maintain the flue gas temperature within this optimal range, taking into account the boiler load and combustion conditions. It can effectively meet the reaction conditions for nitrogen oxides and ammonia vapor, thereby improving the reaction efficiency of the SCR reactor. 2) Catalysts have a limited service life; once they age due to prolonged use, their catalytic efficiency declines, as does the efficiency of the denitration process. To ensure compliance with environmental regulations, excessive ammonia injection can lead to increased ammonia leakage. Therefore, when the catalysts age, it is necessary to replace them during shutdowns for major repairs, so as to maintain acceptable levels of ammonia leakage and improve environmental performance. 3) In coal-fired boilers, the denitration reaction zone is located in an area with high dust levels, which causes dust to accumulate in that zone. This accumulation of dust reduces the efficiency of the reaction and increases ammonia emissions. During the operation of the boiler, the SCR reactor should be soot-blowed at least once a week to remove the accumulated soot, thereby improving the efficiency of the SCR reactor and reducing the ammonia escape concentration. In summary, properly controlling the ammonia escape concentration at the outlet of the boiler’s SCR system can effectively prevent blockages in the boiler’s air preheater and reduce the corrosion caused by ammonia on downstream equipment; therefore, due attention should be paid to ammonia escape during the operation of the SCR denitration system. By controlling the ammonia escape rate of the SCR device to below 3 ppm, the impact of ammonium sulfate or ammonium bisulfate resulting from ammonia escape on the equipment downstream of the furnace is reduced.
Numerically, this can only increase the ammonia injection rate, thereby leading to an increase in local ammonia escape. 4) The spray gun does not atomize properly, preventing adequate mixing between ammonia water and flue gas, which results in a large amount of ammonia escaping. 5) During combustion fluctuations, the NOX concentration in the flue gas at the SCR inlet varies significantly, which often leads to an increased amount of ammonia being injected in order to mechanically achieve compliant emissions. Excessive ammonia injection can result in increased ammonia leakage, thereby posing a direct threat to the safe operation of the equipment and systems downstream of the furnace. 2. Control of ammonia slip
1) The flue gas temperature determines the reaction efficiency of SCR, which in turn affects the amount of ammonia slip. The flue gas temperature varies significantly; at low loads, the temperature drops, and excessively low local temperatures can lead to a decrease in catalyst activity, which in turn results in increased ammonia escape. The catalysts used in SCR systems perform best within the range of 315–380°C. Therefore, it is necessary to maintain the flue gas temperature within this optimal range, taking into account the boiler load and combustion conditions. It can effectively meet the reaction conditions for nitrogen oxides and ammonia vapor, thereby improving the reaction efficiency of the SCR reactor. 2) Catalysts have a limited service life; once they age due to prolonged use, their catalytic efficiency declines, as does the efficiency of the denitration process. To ensure compliance with environmental regulations, excessive ammonia injection can lead to increased ammonia leakage. Therefore, when the catalysts age, it is necessary to replace them during shutdowns for major repairs, so as to maintain acceptable levels of ammonia leakage and improve environmental performance. 3) In coal-fired boilers, the denitration reaction zone is located in an area with high dust levels, which causes dust to accumulate in that zone. This accumulation of dust reduces the efficiency of the reaction and increases ammonia emissions. During the operation of the boiler, the SCR reactor should be soot-blowed at least once a week to remove the accumulated soot, thereby improving the efficiency of the SCR reactor and reducing the ammonia escape concentration. In summary, properly controlling the ammonia escape concentration at the outlet of the boiler’s SCR system can effectively prevent blockages in the boiler’s air preheater and reduce the corrosion caused by ammonia on downstream equipment; therefore, due attention should be paid to ammonia escape during the operation of the SCR denitration system. By controlling the ammonia escape rate of the SCR device to below 3 ppm, the impact of ammonium sulfate or ammonium bisulfate resulting from ammonia escape on the equipment downstream of the furnace is reduced. .
1) The flue gas temperature determines the reaction efficiency of SCR, which in turn affects the amount of ammonia slip. The flue gas temperature varies significantly; at low loads, the temperature drops, and excessively low local temperatures can lead to a decrease in catalyst activity, which in turn results in increased ammonia escape. The catalysts used in SCR systems perform best within the range of 315–380°C. Therefore, it is necessary to maintain the flue gas temperature within this optimal range, taking into account the boiler load and combustion conditions. It can effectively meet the reaction conditions for nitrogen oxides and ammonia vapor, thereby improving the reaction efficiency of the SCR reactor. 2) Catalysts have a limited service life; once they age due to prolonged use, their catalytic efficiency declines, as does the efficiency of the denitration process. To ensure compliance with environmental regulations, excessive ammonia injection can lead to increased ammonia leakage. Therefore, when the catalysts age, it is necessary to replace them during shutdowns for major repairs, so as to maintain acceptable levels of ammonia leakage and improve environmental performance. 3) In coal-fired boilers, the denitration reaction zone is located in an area with high dust levels, which causes dust to accumulate in that zone. This accumulation of dust reduces the efficiency of the reaction and increases ammonia emissions. During the operation of the boiler, the SCR reactor should be soot-blowed at least once a week to remove the accumulated soot, thereby improving the efficiency of the SCR reactor and reducing the ammonia escape concentration. In summary, properly controlling the ammonia escape concentration at the outlet of the boiler’s SCR system can effectively prevent blockages in the boiler’s air preheater and reduce the corrosion caused by ammonia on downstream equipment; therefore, due attention should be paid to ammonia escape during the operation of the SCR denitration system. By controlling the ammonia escape rate of the SCR device to below 3 ppm, the impact of ammonium sulfate or ammonium bisulfate resulting from ammonia escape on the equipment downstream of the furnace is reduced. .
The ammonia escape rate is an important parameter affecting the operation of SCR systems. In actual production, more ammonia than the theoretical amount is usually injected into the reactor. The excess ammonia downstream of the flue gas after the reaction is referred to as the ammonia escape rate, which is expressed as the ammonia content per unit volume. To meet environmental regulations, a certain amount of ammonia is often required; therefore, there is an appropriate ammonia escape rate designed to be no more than 3 ppm, but in practice it tends to be too high. Due to the incomplete reaction of ammonia during the denitration process, ammonia escape is inevitable in SCR denitration, and the ammonia escape rate varies over time. The main factors and control methods are as follows: (1) The flow rate distribution of the injected ammonia is uneven. The resulting deviation in the escape rate can be controlled by adjusting the control valves at the outlets of the pyrolyzer or hydrolyzer to direct ammonia to the reactors on both sides, or by manually adjusting the manual doors of each reactor on-site to correct the uneven ammonia flow; however, the latter method is less commonly used and more cumbersome, while the former yields more significant results. (2) Low flue gas temperature. Flue gas temperature determines the effectiveness of the catalyst, which in turn affects the reaction efficiency and determines the escape rate. The catalyst used in the SCR denitration process performs best in the range of 300 to 420; therefore, the flue gas temperature should be maintained within this optimal range depending on the boiler load and combustion conditions. When load issues or accidents occur, timely intervention should be taken to maintain the flue gas temperature, unless it reaches the tripping value set by the denitration protection logic. (3) Catalyst aging, even reaching the upper limit of its lifespan. This catalyst has a long service life. Once it ages due to prolonged use, its catalytic efficiency declines, as does the denitration reaction. Spraying large amounts of urea to ensure environmental protection will lead to an increased ammonia escape rate. Therefore, as the catalyst ages, it should be replaced promptly during shutdown maintenance to ensure that the escape rate remains within acceptable levels and to better protect the environment. Furthermore, the number of catalyst layers is too low and should be increased to the design value. (4) Blockage in the denitration reaction zone. The denitration reaction zone of coal-fired boilers is located in a high-dust area, where dust inevitably accumulates within the zone, worsening the reaction and increasing the escape rate. However, soot blowing of the furnace body usually does not yield good results; therefore, acoustic soot blowers can be added to the reaction zone and placed within the catalyst layer. However, if the soot blowing interval is long or the compressed air pressure of the instrument is low, the effectiveness of the acoustic soot blower will be poor. Therefore, the soot blowing effect can be improved by increasing the number of soot blowing operations and the pressure of the soot blowing gas. (5) Amount of urea solution. Since the concentration of the urea solution used in the design has been determined, the operator should adjust the amount of urea solution without changing its concentration, in order to ensure that the urea can be fully pyrolyzed or hydrolyzed, thereby preventing excessive escape rates once it enters the reaction zone or ensuring an adequate denitration efficiency due to an insufficient amount of urea. (6) Combustion fluctuations. When there are disturbances in boiler combustion, the amount of urea should be adjusted and distributed in a timely manner based on the NOx concentration at the inlet of the denitration reactor, so as to prevent excessive ammonia slip or significant discrepancies between both sides; otherwise, it may even lead to non-compliance with environmental regulations due to improper adjustments. (7) Clogging of the ammonia injection grid nozzles. Nozzle clogging exacerbates ammonia slip. The nozzle direction of the ammonia injection grid (direction of ammonia injection) is generally the same as the direction of the flue gas. The advantage of this design is that it prevents high-dust flue gas from directly hitting the nozzle and blocking it with high-concentration flue gas. However, the nozzle also has some weaknesses in the direction of the flue gas. Firstly, ammonia has a short residence time during the mixing stage, resulting in a significantly weaker mixing effect with the flue gas compared to when it is injected in the opposite direction to the flue gas flow ； Second, nozzle clogging cannot be completely avoided by arranging them in the same direction. During the long-term operation of the unit, the steel supports and baffles in the flue above the ammonia injection grid gradually accumulate dust to a certain thickness, after which they fall off and block the nozzles directly below them. Therefore, the nozzles can be improved by installing a \"small umbrella\" above each one. Under the effect of the “small umbrella”, ammonia gas is ejected in the same direction rather than in the opposite direction. This prolongs the residence time of ammonia gas during the mixing phase, enhances its mixing with flue gas, and prevents ash deposition from clogging the nozzles. (8) The flow field distribution is uneven. The flow field distribution is a core aspect of the deaeration system design. Flow field design needs to address two issues: one is to ensure that the flue gas passes evenly through the catalyst layer, and the other is to enable ammonia to mix evenly with the flue gas as quickly as possible; the second issue is more important. The velocity of the flue gas passing through the catalyst is generally 4 to 6 m/s, and the height of each layer of catalyst is about 1 m; therefore, the reaction time for each layer of catalyst is only around 0.2 seconds. Therefore, if the ammonia concentration in certain areas of the catalyst layer is too high, exceeding what is necessary for denitration, ammonia will escape. The approach to flow field design could be: (1) enhance turbulence in front of the catalyst box, without splitting the flue gas ； Try to leave enough space above the catalyst box so that the flue gas can pass evenly through the catalyst layer in that area, allowing for even mixing of ammonia and the flue gas. (9) Human factors. Strengthen training and learning regarding the processes of the denitration system, so that operators are familiar with the methods for adjusting the denitration process, can identify problems promptly, address them specifically, and avoid mistakes that could lead to high ammonia escape rates and violations of environmental regulations.