Ammonia slip is an important parameter that affects the operation of the SCR system. In the actual production process, more ammonia than the theoretical amount usually reaches the reactor. After the reaction, the excess ammonia in the flue gas downstream is called ammonia slip. Ammonia slip is expressed by the ammonia content per unit volume. In order to meet environmental protection requirements, a certain amount of excess ammonia is often required, so there is also a corresponding appropriate ammonia slip value. This value is designed to be no more than 5ppm, but it is often larger in actual operation. The main factors are as follows:
(1) The ammonia flow rate of each ammonia spray gun is unevenly distributed, there is a local uneven distribution of ammonia water in the flue gas, the flue gas flow rate is uneven, the ammonia spray amount at each spray gun outlet is quite different, and the ammonia slip is relatively high in places with high concentration.
(2) Flue gas temperature. If the reaction temperature is too low, the reaction rate of NOx and ammonia will decrease, which will cause a large amount of NH₃ to escape. However, if the reaction temperature is too high, ammonia will generate additional NO. Therefore, NH₃ has an optimal reaction temperature. The optimal reaction temperature of SNCR ammonia is 800-1100℃. The SCR reactor is a honeycomb module with WO3 and V2O5 as active components as catalysts. The reductant is the ammonia escape from the upstream SNCR system. Under the action of the catalyst, ammonia water reacts with NOx in the temperature range of 315~380℃ to generate nitrogen and water to achieve the purpose of denitrification. If the temperature is too high or too low, the reaction effect cannot be achieved, which will inevitably increase ammonia escape.
(3) Catalyst blockage, denitrification efficiency decreases. In order to keep the environmental protection parameters within the standard, more ammonia will be sprayed, which will cause a vicious cycle, local blockage of the catalyst, performance aging, resulting in different catalytic efficiencies in different parts of the catalyst. In order to control the outlet parameters, the amount of ammonia sprayed can only be increased, which will lead to an increase in local ammonia escape.
(4) The atomizing air volume is too small, the spray gun atomization is not good, the ammonia water and the flue gas cannot be fully mixed, and a large amount of ammonia will be generated.
(5) Ammonia concentration, ammonia concentration configuration, the concentration level cannot be controlled, and it is configured based on feeling. As far as the current C boiler is concerned, the ammonia concentration is basically high, the ammonia water regulating valve opening is too small, the atomization is not good and it is easy to close automatically, resulting in high ammonia escape and difficult operation.
(6) When the combustion fluctuates, the NOX concentration in the SNCR inlet flue gas fluctuates greatly, which often increases the amount of ammonia spraying to mechanically achieve "standard emission". Excessive ammonia water can lead to increased ammonia escape, which directly endangers the safe operation of the equipment and system after the furnace.
