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Air pollution is a serious ecological environment problem. The NOx emitted from power station boilers reduces air quality and is also prone to acid rain. The SCR denitration device has the advantages of good denitration effect and advanced technology, and is widely used in power station boilers. In the process of installing and operating SCR, it is necessary to pay attention to the efficiency of flue gas denitration, and also to pay attention to the adverse effects of this device on the economy and safety of the boiler. Under the current circumstances, in the process of applying SCR denitration in China's thermal power plants, it is necessary to continuously accumulate experience to improve the safety and operational efficiency of the equipment.
1 Influence of installing SCR denitration system on boiler body and countermeasures
The SCR denitration catalyst has specific temperature requirements for the flue gas entering the SCR reactor during the reaction. The reaction temperature not only determines the reaction rate of the reactants, but also determines the reactivity of the catalyst. The suitable temperature is between 300-400 ° C. If the flue gas temperature is higher than 400 ° C, the catalyst sintering problem is easy to occur and the activity is lost. If the temperature of the flue gas is less than 300 ° C, the ammonium bisulfate level is easily increased. The next will block the corrosion boiler.
Under different load conditions, the required fuel quantity and furnace temperature are different, so the flue gas temperature will fluctuate accordingly. In order to meet the boiler variable load operation, the flue gas temperature always meets the requirements of SCR for efficient removal of NOx from flue gas. It is necessary to reform the heating surface and flue of the boiler tail to ensure that the boiler thermal efficiency does not decrease and the flue gas temperature is consistent. The requirements for the normal operation of the SCR system. The specific scheme for transforming the heating surface of the boiler tail can be divided into two types, one is to transform the economizer, and the other is to install a flue gas bypass.
(1) Reconstruction of the economizer. According to the temperature change of the tail flue in the boiler operation, the thermal calculation of the tail flue is carried out, the heat absorption is reasonably determined, and the installation position of the economizer and the number of bundles are re-arranged. The economizer can process the heat exchange pipeline without changing the pipeline specifications and materials. It is convenient to determine the heat absorption of the economizer according to the specific situation of the load change, and the purpose of controlling the temperature of the flue gas by such a route.
(2) Install a flue gas bypass, and install a flapper door and a bypass flue between the inlet of the SCR reactor and the upper part of the economizer. After the transformation, when the temperature of the flue gas is too low to meet the requirements of the catalyst operation and affect the normal denitration reaction, the bypass flue and the baffle of the original flue can be adjusted to mix the high-temperature flue gas into the low-temperature flue gas. By adjusting the proportion of different temperature flue gases at the inlet of the SCR reactor, the temperature of the flue gas entering the SCR can be effectively adjusted. This transformation is often applied to the initial stage of boiler start-up and low-load operation, because it will increase the boiler exhaust temperature and reduce the boiler thermal efficiency. Therefore, when the economizer outlet flue gas temperature reaches the standard, the flue gas bypass should be exited in time.
Influence of 2SCR Denitration System on Boiler Steel Structure and Countermeasures
After the application of the SCR system, the flue gas direction will be affected. Therefore, the flue between the air preheater and the economizer needs to be removed, and the flue gas enters the air preheater through the SCR reactor. The new flue is vertically up and connected to the SCR inlet flue and then to the boiler direction through the SCR exit flue.
(1) SCR reactor steel frame. Under normal conditions, the SCR needs to add the reactor to the tail of the boiler. The SCR reactor is supported by a new steel frame from the upper part of the blower. Because the boiler tail structure is not the same, the steel frame supporting the SCR reactor may be connected to the air preheater support frame, connected to the boiler steel frame, the dust collector support frame is connected, and the air preheater outlet flue support frame is connected. For the design of the supporting reactor structure, the effects on the boiler and other structures should be considered under certain conditions, and the steel structure needs to be recalculated.
(2) Brace of steel frame of boiler. Some units lack the conditions for applying the denitration system, and there is a diagonal bracing in the flue of the boiler tail, which makes it impossible to set the SCR flue. Therefore, it is necessary to reset the original bracing to change the steel structure of the boiler, which requires recalculating the steel structure.
Influence of 3SCR Denitration System on Air Preheater and Countermeasures
The SCR denitration system is placed in front of the air preheater, and the flue gas enters the flue gas side of the air preheater and exchanges heat with the air after passing through the SCR. The flue and denitration reactor are additional equipment, and the denitration system is under negative pressure during operation. The added SCR system will increase the resistance of the induced draft fan, and will also cause the air leakage problem of the air preheater to increase.
The main purpose of SCR denitration is to minimize the oxidation rate of SO2 SO3 to less than 1%, but the SO content in the flue gas at the outlet of the reactor will increase after the addition of the SCR denitration system, thereby increasing the ammonium sulfate and The amount of ammonium hydrogen sulfate produced. Ammonium hydrogen sulfate is a viscous substance, and when the temperature is 146-207 ° C, ammonium hydrogen sulfate becomes a viscous liquid. The temperature of the lower part of the temperature section of the air preheater and the cold section are close to this temperature. When the boiler flue gas stream containing high ash passes through the air preheater, ammonium hydrogen sulfate tends to adhere to the dust precipitator and adhere to the air preheater heat exchanger element, which causes the air preheater to reduce the heat exchange efficiency and increase the flue flow resistance. In severe cases, low temperature corrosion and clogging problems are induced. Even after unburned pulverized coal or oil mist deposition, the air preheater is installed and the SCR denitration system is installed. The air preheater needs to be modified. The specific measures are as follows:
(1) The air preheater heat exchanger element needs to be changed into two-stage arrangement. This solution covers the formation temperature range of liquid ammonium hydrogen sulfate by increasing the cold end height, and avoids the adhesion of ammonium hydrogen sulfate to the heating surface of the air preheater. Caused serious ash blocking problems.
(2) The corrugated board is used for the cold section material of the air preheater. The flue gas flow in the corrugated board is straight through, and the flue gas flow passes through a smooth waveform. During the operation, soot, slag and ammonium hydrogen sulfate are not prone to adhesion problems. Once the adhesion occurs, it can also be removed in a short time by applying the air preheater soot blower, so that the heated surface is not blocked.
(3) Enamel plated steel plate. This technology enhances the anti-adhesion effect of the cold pre-cooled components of the air preheater. The enamel element can prevent the occurrence of low temperature corrosion to the utmost extent, and the enamel surface is relatively smooth, and the heated element is not easily stained, which is also good for cleaning.
(4) AIG ammonia reduction optimization and performance evaluation tests were carried out on the SCR denitration equipment on a regular basis. Taking a 300 MW unit in a factory as an example, at full load, when the denitration efficiency is 87.8%, the outlet ammonia escaping concentration is 2.28 μL/L, and the outlet NOx concentration is 30 mg/m3. After optimization by AIG ammonia injection, the CV values of the NOx concentration at the outlet section of the A and B side reactors were reduced from 62.1%, 52.2% before adjustment to 24.5% after adjustment, 28.1%. The distribution of nitrogen oxides at the SCR outlet was even. Significantly improved, the peak of local ammonia escape decreased significantly, significantly reducing the formation of ammonium bisulfate.
In summary, the application of the SCR denitration system will cause some adverse effects, such as affecting the safe operation of the boiler and reducing the economics of the boiler operation. This requires comprehensive consideration in the process of designing the boiler so that it meets safety regulations. To minimize economic decline.
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