Through the optimization and adjustment of combustion and other measures, the coal-fired generating units can reduce the emission of air pollutants and tap the potential, and then use a series of advanced pollutant control technologies to achieve ultra-clean emissions such as SO2, NOX, soot, aerosols and mercury, Emissions control at gas turbine standards.
This paper discusses the overall idea of ​​achieving ultra-clean emission of coal-fired units by considering the mutual synergy and impact of various environmental protection equipment, optimizing the design of low nitrogen combustion and flue gas denitrification, the utilization of flue gas waste heat and low temperature electrostatic precipitating, wet desulfurization and wet ESP, fan and flue to optimize the layout, etc., to achieve organic energy-saving and environmental protection.
A series of double towers double cycle desulfurization
At present, limestone-gypsum wet flue gas desulfurization process commonly used in China generally has a desulfurization efficiency of 90% -95%. When the sulfur content of coal is high, the increasingly stringent SO2 emission standard can not be met. Currently, the highly efficient wet France desulfurization synergistic technologies include: increase the spray layer and the pool volume, holding liquid sieve wet flue gas desulfurization, two series of flue gas desulfurization.
The single-tower coal-fired low-sulfur coal-fired units (SO2 <1000mg / Nm3) can meet the ultra-clean emission requirements. The desulfurization efficiency of 98.8% for the FGD inlet (SO2 <3000mg / Nm3) Or enhance the gas-liquid mass transfer measures, single-tower technology can also meet the 35mg / Nm3; FGD inlet (SO2> 3000mg / Nm3) Desulfurization efficiency and stable operation of more than 99.2%, can be used in tandem double towers double loop technology, The desulfurization efficiency is 80-90%, the SO2 concentration at the outlet of the first-stage absorption tower is controlled to 500-700mg / Nm3, and the second-stage absorption tower with the desulfurization efficiency of about 95% is used to control the SO2 emission concentration below 35mg / Nm3.
2 efficient and full load denitration
2.1 Low-NOx combustion and SCR coupled denitration
The boiler uses low-NOx combustion technology, including NOX burner (LNB), air-staged combustion (SAS), fuel fractionated combustion (OFA), flue gas in circulation (FGR) and other effective control of the boiler NOX emission concentration. For the bituminous coal boiler, NOX is controlled from the source to below 500mg / Nm3 by using low-nitrogen combustion technology, and then reduced to less than 50mg / Nm3 by SCR; for a "W" boiler burning anthracite, a two-stage SCR reaction 50mg / Nm3 emission standards, but the actual modification there is more difficult.
At present, many coal-fired power plants have implemented the technology of low-NOx combustion and SCR flue gas denitrification. This technology has matured without technical risks and fully meets the requirements of low-nitrogen emission.
2.2 full load denitration
At present, the applicable temperature of the flue gas denitration catalyst used in coal-fired units is mostly between 300 and 420 DEG C, and the effect of the catalyst will be affected if the flue gas temperature is too high or too low. When the unit is under a low load, the temperature of flue gas entering the SCR inlet will be lower than 300 ° C. In order to improve the denitrification efficiency, the temperature window of the catalyst must be improved so that it can still play a catalytic role between 250 ° C and 420 ° C.
In order to meet the flue gas temperature of denitrification inlet under the low load condition of the unit, the economizer feedwater temperature can be raised through technological innovation or the economizer can be arranged in stages. After increasing the inlet temperature of the denitrification device, in order to ensure the thermal efficiency of the unit does not decline, it can be taken to increase the heat exchanger area of ​​the air preheater and set up the technical scheme of waste heat utilization of the flue gas.
Under the 30% BMCR load condition, the third stage of Waigaoqiao achieved the outlet flue gas temperature at the economizer and the inlet water temperature at the economizer respectively 307 ° C and 228 ° C, realizing the operation of denitration at the lowest steady-state load, that is, full-load denitrification technology.
3 smoke and dust synergistic discharge control
Through the setting of low temperature electric dust removal (with flue gas waste heat utilization) + high efficiency wet desulfurization unit + wet electrostatic precipitator program to achieve synergistic removal of ultra-clean soot emissions, the existing electrostatic precipitator reduces the concentration of soot to 30-40 mg / Nm3, After efficient wet desulfurization device and then reduced to 15 ~ 20mg / Nm3, and ultimately through the wet electrostatic precipitator to meet the emission requirements of 5mg / Nm3. As the wet desulfurization device dust removal principle, we all know, this section focuses on electrostatic precipitator with flue gas heat recovery system and low temperature dust collector technology.
3.1 electric precipitator with flue gas waste heat utilization system
The general design of power plant boiler exhaust temperature of 120 ~ 140 ℃, the loss of heat up to 3% -8% of the total input fuel heat plant, and the actual operation of the unit exhaust temperature is generally higher than the design value of the exhaust gas caused by high boiler temperature The efficiency is reduced, ESP dust removal efficiency is reduced, desulfurization water consumption increases. Through the use of low temperature dust collector with flue gas waste heat is an effective solution to this hazard.
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