After low nitrogen burner, denitration system and the corresponding auxiliary after modification, in the actual operation of NOx emissions can meet the national requirements of environmental protection, for the normal operation of the unit in the future created prerequisites, at the same time also bring some negative effects on unit.
1. The minimum technical output of the unit is reduced and its peak regulating capacity is poor
Most of the power plant of denitration adopt SCR denitration reactor, catalyst design temperature is 300 ℃ ~ 420 ℃. After denitration system run operation unit, in addition to the synchronous production unit of denitration system, due to early boiler design did not consider the operating temperature of the denitration system, most of the unit after transforming the low load running, the boiler economizer outlet smoke temperature can meet the phenomenon of denitration system normal operation temperature, and minimum technology before the transform of output has the varying degree to rise, the peakload ability decline. According to the above situation, choose the typical nine power plant in Shanxi Province has carried on the investigation and research, comprehensive nine power plant, the minimum technical output, after transforming unit, the minimum technical output between 50% and 77%. According to weighted average, the minimum technical output of boiler is 59%, which is 19% higher than the minimum stable combustion load of 40% before reforming. The average minimum technical output of the boiler was 62% when the unit was modified, and the minimum technical output of the boiler was 22% higher than before the reform. Among them, the original low-nitrogen combustion and denitrification system has the best effect, while the low-nitrogen and denitrification system has the worst effect.
2. Poor regulatory performance of AGC
Unit after low nitrogen burner alteration, furnace combustion mode has changed significantly, in the case of using the original coordinate methods, unit in AGC control, big deviation main steam pressure, steam drum water level fluctuation; When the load adjustment rate is relatively fast, it is easy to exceed the standard of NOx emission in a short period of time due to the lag of denitrification ammonia injection reaction, which seriously affects the load adjustment rate. The unit that carries out the synchronous fan modification has not carried out the fan adjustment characteristic test after the modification. When the load is low, the pressure in the furnace fluctuates greatly under the original control mode.
3. Poor operation economy
Most of the units that have completed the transformation of low nitrogen burners and denitrification have the phenomenon of economic reduction. Mainly reflected in:
1. The carbon content of fly ash increases
After the modification of the low-nitrogen burner of the unit, due to the change of combustion mode, if the ratio of air powder is not good, the oxygen-deficient combustion in the main combustion area and the low-temperature combustion in the embers area are not conducive to the coal combustion. In addition, after the modification, the combustion adjustment of the unit is more inclined to reduce the adjustment of NOx emission, which leads to the increase of carbon content of fly ash and the decrease of the unit economy.
2. The heat transfer effect of air preheater decreases, and the exhaust temperature increases
After denitration system put into operation, denitration by-product ammonium hydrogen sulfate under the lower than the dew point temperature, easy bonding on air preheater of the heating surface, and highly viscous, flying through the air preheater in the flue gas ash deposits on air preheater heating surface easily, cause air preheater heat transfer effect, exhaust temperature, boiler efficiency is reduced.
3. The reheat steam temperature is low
Reconstruction based on different situation, some power plants after modification, reheat steam temperature to reduce the 5 ℃ ~ 20 ℃, seriously affect the economy of the unit. According to estimate every 10 ℃ below the reheat steam temperature, affecting the unit coal consumption unit about 1 g of standard coal.
The economy of most denitrification and denitrification reconstruction units is affected to a certain extent. Through investigation, the efficiency of the unit after transformation is affected by about 1%. To a single 600 mw unit, load rate of 80%, annual operating hours 5500 hours, BiaoMei unit price 600 yuan, boiler efficiency by 1%, the unit energy consumption increased about 3 g standard coal estimates that consumption BiaoMei 7920 tons a year, equivalent to 4.75 million yuan, the unit cost of electricity increased by 0.18 points.
4.Plug and corrosion of boiler auxiliaries
When the boiler is under low load, the production of NH3HSO3, a byproduct of denitrification, is increased due to the decrease of temperature at the denitrification inlet, the decrease of the activity of denitrification catalyst and the increase of ammonia escape rate. NH3HSO3 has strong adhesion, high dew point temperature and certain corrosion, which is easy to block air preheater, leading to increased differential pressure of air preheater, poor heat transfer effect and cold end corrosion.
In addition, if the ammonia escape rate is not well controlled and the NH3HSO4 production is large, it will also cause corrosion of the cloth bag duster and the fan blade. Through investigation, it is found that some power plants have corrosion phenomenon of cloth bag duster and fan during unit maintenance.
1. Safe operation of liquid ammonia station
Liquid ammonia, also known as anhydrous ammonia, is a colorless liquid, volatile, corrosive, irritating smell. The liquid ammonia volatilizes into ammonia gas, belongs to inflammable and explosive gas, which can cause poisoning after leakage, irritate eyes and mucous membrane, and is in danger of burns. There are safety hazards such as leakage, electrostatic fire, explosion and poisoning in transportation, loading, unloading, storage and maintenance of liquid ammonia. Using liquid ammonia as a reducing agent of power plant, potential safety hazard, especially near the urban areas of power plant, using liquid ammonia denitration, the transportation, storage, operation risk bigger, greatly increases the risk of unit operation.
2. Denitrification and transformation, high operating cost
Denitration modification increases the resistance of the original specific systems, denitration by-products have blockage and corrosion of air preheater, dust collector, induced draft fan and other equipment, to form a complete set of air preheater and induced draft fan renovation, a large range transform, high cost.