At present, selective catalytic reduction (SCR), one of the mainstream technologies of flue gas denitration, is widely used and selected by a large number of power plants under construction. The denitration efficiency of SCR denitration device can reach more than 85%, which can meet strict environmental emission standards. If the NOx content in the flue gas generated after the combustion of the designed coal of the boiler is high, the SCR Flue gas denitration device can ensure the flue gas denitration efficiency and greatly reduce the emission of flue gas NOx.
1. Introduction to denitration device
The flue gas denitration transformation project of coal-fired units adopts selective catalytic reduction (SCR) denitration device, and each boiler is equipped with 2 SCR reactors. The catalyst layer in the reactor is designed as two layers and one layer is reserved. The type of catalyst adopts honeycomb type. The installation position of the flue gas system is the connecting flue from the outlet at the lower part of the low-temperature economizer at the tail of the boiler to the inlet of the SCR reactor body, and from the outlet of the SCR reactor body to the inlet of the rotary air preheater. Ammonia gas (anhydrous liquid ammonia) is used as denitration reducing agent. An ammonia station is built for the project, including liquid ammonia unloading, storage, evaporation and supply system to meet the denitration needs of two units.
SCR denitration principle
The main chemical reactions of "selective catalyst reduction flue gas denitration" technology are as follows:
The reaction products are water and nitrogen that are harmless to the environment, but the reaction speed can only be sufficient under the condition of more than 800 ℃. In industrial application, the catalyst for relevant reaction must be installed. Under the action of the catalyst, the reaction temperature drops to about 400 ℃. The working temperature of the catalyst in SCR denitration system has a certain range. When the temperature is too high (> 450 ℃), the catalyst will accelerate aging; When the temperature is about 300 ℃, another side reaction will also occur under the action of the same catalyst:
That is, ammonia salt is generated, which is viscous and easy to adhere to the catalyst and the heating surface at the tail of the boiler, affecting the operation of the boiler. Therefore, it is allowed to spray ammonia for denitration only when the flue gas temperature in the catalyst environment is between 300-420 ℃.
3. Impact of SCR system on boiler during operation
After long-term operation, it is found that the current of the forced draft fan and induced draft fan of the unit increases, the power consumption increases, the pressure difference at the inlet and outlet of the secondary air and flue gas of the air preheater increases, from normal 0.5-1.1kpa to 1.5-2.2kpa under 50% - 100% load conditions, and the flue gas temperature also increases. After the installation of SCR denitration device, it also has other effects on the boiler system in actual operation. According to the actual parameters and operation, it is summarized as follows:
3.1 influence on flue resistance.
The SCR denitration device increases the flue gas resistance by about 800pa and blocks the volume of honeycomb catalyst. With the increase of operation time, the more serious the catalyst blockage is, which will increase the power consumption of suction fan. The SCR denitration system currently designed is not designed with bypass system. If the catalyst blockage is serious, it will directly affect the safe and stable operation of boiler.
3.2 impact of SCR denitration on boiler efficiency.
(1) After installing the SCR denitration system, the heat loss of the boiler is mainly that the flue gas temperature will be reduced by about 6 ℃ after passing through the denitration system, which will have a certain impact on the boiler efficiency.
(2) The heat exchange element of the air preheater is blocked, the smoke exhaust temperature increases, and the power consumption of the induced draft fan increases. Ammonia reacts with sulfur trioxide to produce ammonia hydrogen sulfate. Ammonia hydrogen sulfate is a viscous substance in the environment with a temperature of 180 ~ 200 ℃, so the dust in the flue gas in the high-temperature section and low-temperature section of the air preheater is easy to adhere to the heat exchange surface of the air preheater together with ammonia hydrogen sulfate, which makes the heat exchange elements of the air preheater dirty. The heat exchange effect of the air preheater is the main reason for the increase of exhaust gas temperature and the decrease of boiler efficiency.
(3) The air leakage rate of air preheater increases. After the flue gas passes through the SCR denitration system, the pressure drop will increase by about 800pa. In order to balance the pressure inside the furnace, the output of the suction fan will increase, which will reduce the flue gas pressure inside the air preheater, increase the air / flue gas pressure difference of the air preheater, increase the air leakage rate of the air preheater and reduce the boiler efficiency.
3.3 impact on air preheater.
The impact of SCR denitration device on air preheater is more prominent. The main reason is the corrosivity and adhesion of ammonia bisulfate. Ammonia hydrogen sulfate adheres to the heat exchange element of the air preheater together with dust, which will not only reduce the heat exchange effect, but also produce low-temperature corrosion in the low-temperature section of the air preheater and cause ash deposition of the air preheater. The ammonia escape rate of SCR denitration device is generally designed to be no more than 3ppm. When the escape rate exceeds the design value, a large amount of hydrogen sulfate ammonia will be generated, resulting in serious blockage of air preheater, which will increase the power consumption of suction fan and large pressure fluctuation of primary fan main pipe. In addition, when burning high sulfur coal, the SO3 content in the flue gas is high, and the sulfuric acid solution may condense on the heat exchange element in the low-temperature section of the air preheater, resulting in low-temperature corrosion of the air preheater.
4. Targeted measures taken
In view of the above situation, in order to reduce the negative impact of SCR denitration on boilers, we have summarized some experience and learned from the technical measures of other plants. In actual operation, the following measures are taken:
4.1 heat transfer element and flushing transformation of air preheater
The air preheater installed with SCR denitration process needs special design in preventing low-temperature section corrosion, ash blockage and cleaning. In order to prevent the reduction of heat transfer effect due to the dirt of the air preheater or the forced shutdown caused by the blockage of the air preheater, the heat transfer element in the low-temperature section of the air preheater shall be enamel surface heat transfer element. In order to avoid forced shutdown due to serious blockage of air preheater during boiler operation, dual medium soot blower (steam and water) may be considered in the selection of air preheater blower to realize on-line water flushing of air preheater. During normal operation, steam is used for regular soot blowing, and high-pressure water is used for flushing when the air preheater is seriously blocked. If the heat transfer element of air preheater is not made of enamel, it will be seriously blocked after being put into SCR denitration system for half a year. Therefore, it is generally necessary to replace the low-temperature section of the heat transfer element of the air preheater with enamel tube air preheater.
4.2 reduce the ash deposition of SCR denitration catalyst.
The content of dust in flue gas is closely related to the ash content of coal and combustion adjustment, but the factors affecting the ash deposition of denitration catalyst are also related to the operation of ash drainage system of economizer and soot blower installed in denitration device. If the ash drainage system of economizer fails to work normally, a large amount of dust will be brought into the upper catalyst of denitration. Even if the soot blowing of denitration system is strengthened, the blockage of honeycomb catalyst can not be avoided. The soot blower used for denitration device has two ways: acoustic soot blower and steam soot blower. The acoustic soot blower has obvious effect when the amount of ash is small, and can completely blow away the accumulated ash at the corners. However, when the ash content is large, the rake steam soot blower can play a good role.
4.3 strengthen soot blowing and regularly wash and maintain the air preheater
During the operation of SCR denitration system, it is inevitable that the catalyst and air preheater are blocked by ash deposition. It is necessary to strengthen the soot blowing in the SCR reactor area and air preheater, especially in the low-temperature section of air preheater. When the flue resistance increases, clean the catalyst in time. If the differential pressure between the inlet and outlet of the air preheater increases, strengthen the air preheater or water flushing in time. To fundamentally solve the problem, replace the enamel tube air preheater.
4.4 control ammonia escape rate
In order to reduce the influence of denitration unit on boiler, it is particularly important to control the generation of ammonia bisulfate. The reaction rate of producing ammonia bisulfate is mainly related to temperature, ammonia, SO3 and water content in flue gas. For the actual operation of thermal power units, the content of SO3 and water in boiler flue gas can not be controlled. Therefore, the escape rate of ammonia must be strictly controlled.
(1) Strictly control the injection amount of ammonia to prevent ammonia escape caused by excessive ammonia. Under normal circumstances, the ammonia escape rate should be controlled to exceed 3ppm to ensure the correct full-time input of metering equipment and strengthen maintenance.
(2) Maintain the activity of the catalyst. The service life of SCR denitration catalyst is generally 5 ~ 6 years. The design of SCR denitration device is 2 + 1 mode. When the denitration efficiency cannot reach the design value or meet the national environmental protection emission requirements, in order to ensure the safe operation of the boiler, the catalyst must be cleaned or the standby layer catalyst must be installed.
(3) Strengthen the monitoring of the inlet and outlet differential pressure of the air preheater. When the inlet and outlet differential pressure of the air preheater increases, reduce the amount of ammonia injection in time and increase the number of soot blowing in the low-temperature section of the air preheater.
Using SCR denitration device, the contamination, blockage and corrosion of heating surface at the tail of boiler (mainly air preheater) can be controlled by limiting ammonia escape and SO2 / SO3 oxidation rate of catalyst; The influence on boiler thermal efficiency can be controlled by reducing the length of smoke and reducing the heat dissipation area; The influence on induced draft fan (i.e. the increase of boiler flue gas resistance) can be controlled by reasonably designing flue shape, reasonably selecting flue gas flow rate, installing guide device, shortening flue length, etc.
Due to the increasingly stringent environmental emission standards in recent years, the investment of denitration equipment is necessary for all plants in the future, and the problems caused by denitration are inevitable. We should make use of the existing operation experience to summarize the technology, strengthen treatment and maintenance, ensure the emission up to environmental standards, reduce the negative impact of denitration equipment operation on the boiler and increase economic benefits.
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