During the operation of the air preheater of the steam boiler, the large amount of ammonia escaping and the low temperature of the cold end result in the deposition and ash blocking of the heat storage element ammonium bisulfate, and the resistance of the flue gas side rises with the extension of the operation time, which affects the safe and economic operation of the unit. Moreover, the air preheater cannot be handled after the shutdown inspection.
The general management plan of "incineration system optimization innovation + SCR ammonia radiation system innovation + new air preheater hot air flushing and blocking prevention system" is adopted, that is, firstly, the NOx concentration at the furnace outlet is optimized by incineration, and the ammonia injection volume is reduced as a whole, then the SCR system operation is subject to low-level ammonia escape, and finally, the new air preheater hot air flushing and assembly are carried out, and the heat storage elements are connected The cyanamide sulfate produced on the machine is eliminated in real time. Experience from the three aspects of flue gas flow to carry out general management, to reach the goal of air preheater anti blocking.
1 project overview
The boiler model is DG1025 / 18.3-ii5, and the boiler model is subcritical pressure, primary central reheating, natural circulation, single furnace, full suspension and open air plug; the boiler has average ventilation, double flue at the tail, coal-fired drum furnace; four corner tangential burning, damper temperature regulation, and put into operation in December 1995. The single boiler of the steam boiler is equipped with two air preheaters with three compartments and vertical heating surface inversion.
The air preheater is installed under the ash hopper of the economizer, and the flue gas and the atmosphere exchange heat by the countercurrent method. In order to cooperate with denitration innovation, denitration air preheater innovation was carried out for steam boiler division, all heat storage elements were changed, and rotor layout was adjusted. After innovation, steam soot blower is installed at the cold and hot ends of the air preheater. In order to prevent the air preheater from blocking, the air preheater and its associated system were innovated.
2 innovation necessity
At present, the flue gas side resistance of the air preheater of the steam boiler is gradually raised in a period of time after the boiler is started, and it reaches 2800pa at a very high load of 330MW, 1800pa higher than the planned value.
The heat exchange element of air preheater was excavated during shutdown search and inspection. The heat exchange element at cold end of air preheater produced ash blocking sign of ammonium hydrogen sulfate at a certain height. The ash scale was closely connected with the appearance of heat exchange element, and could not be removed by soot blowing during hot operation, so it could only be handled temporarily when the flushing method was accepted during shutdown.
However, the resistance of the air preheater rises again after a period of time of start-up and operation after shutdown and flushing. The disposal method of analysis shutdown and flushing can only remove the ash scale of the previous infarction. In this project, the problem that ammonium hydrogen sulfate causes the infarction of the heat exchange element of the air preheater cannot be completely handled by the method of inspection and flushing of the heat exchange element.
The air preheater produces ash blocking, which on the one hand affects the safe operation of the air and smoke system of the steam boiler, and leads to the increase of the power consumption of the fan building, on the other hand, it also affects the compliance of the air preheater to acquire the exhaust heat energy. Because of the increase of the resistance on the air and smoke side of the air preheater, the differential pressure of the primary air, the secondary air and the smoke bin increases, and the total air leakage of the air preheater also increases relatively.
In view of the doubt that the flue gas side resistance of the air preheater of the steam boiler rises with the extension of the operation time due to the ash blockage at present stage, and the tension once affected the safe and economic operation of the unit, it is considered that the doubt still occurs frequently after the unit stops the boiler to inspect the flushing heat exchange elements, so as to implement the innovation of air preheater anti blocking, ease the signs of air preheater infarction, and lower the air preheater Running resistance is imperative.
3. Innovation criteria
The innovation criteria for blocking prevention of boiler atmosphere preheater are as follows:
(1) the innovation plan tries to make the technological process and the building installation simple and reasonable.
(2) fully consider the impact of NOx emission from upstream furnace outlet and denitration system operation environment on the air preheater, and make targeted plans for air preheater anti blocking innovation based on the current operation reality and coal environment.
(3) sufficient consideration shall be given to the general temperature control of cold end of air preheater under low load and winter operation conditions.
(4) after the innovation, the flue gas side resistance of the air preheater shall not be higher than the original planned value, and the unit will never find signs of resistance rise and heat exchange element block in the era of continuous operation.
4. Technological innovation
Through the analysis of the recent situation of air preheater building and the premise of operation limit, one of the reasons why the air preheater of steam boiler has ammonium bisulfate infarction is the high escape amount of ammonia in the whole and part of SCR denitration system, and the poor uniformity of ammonia injection; the other reason is that the air preheater has a low temperature environment at the cold end under the winter operation conditions, especially under the medium and low loads.
Under the combination of large amount of ammonia escape and low general temperature of cold end, the deposition rate of ammonium bisulfate ash scale on the heat exchange element of cold end of air preheater increases rapidly, crossing the upper limit of clean talent of soot blowing system of air preheater, so that the ash blockage of heat exchange element of cold end is from less to more, and the resistance of flue gas side of air preheater is found to be raised after the boiler is started.
Technical approach: optimization and innovation of incineration system + Innovation of SCR ammonia radiation system + new air preheater hot air flushing and blocking prevention system, the innovation includes:
(1) optimization and innovation of incineration system. Through adjusting the incident angle of the primary and secondary air, further delay the primary and secondary air inclusion, increase the useful pause time of the incineration products in the recovery area, and reduce the NOx emission at the furnace outlet. Under the same emission target of the chimney outlet, the NOx emission can be reduced to comply with the set value, which helps to reduce the ammonia injection pressure of the denitrification system.
——Author：Polaris environmental protection network