Boiler flue gas waste heat recovery for power system
The exhaust gas temperature of power station boilers in our country is more than the design value, and reducing the exhaust gas temperature is limited by various factors such as low temperature corrosion, space layout, and ash plugging on the heating surface. The production of power systems cannot be separated from power plant boilers. The important indicators of boiler design include the flue gas temperature of the power plant boiler, which affects the production cost of the boiler, as well as the thermal efficiency, low-temperature corrosion of flue gas at the heating surface of the boiler tail, flue resistance, flue gas Condensation caused by condensation on the heated surface of the tail caused by condensation. Shanghai Qihuang Analysis In today's increasingly strict energy-saving and emission-reduction policies, the recovery and utilization of waste heat from flue gas of power station boilers is particularly important.
Characteristics of various flue gas waste heat recovery systems:
Flue gas waste heat recovery technology has been gradually promoted and used in power station boilers, and has become one of the means to improve unit efficiency, reduce energy consumption and reduce pollutant emissions. The flue gas waste heat recovery system is to set up one or more stage heat exchangers at the end of the boiler, use the heat in the exhaust gas to heat the feed water or replace other heaters, etc., in order to fully recover the heat in the exhaust gas of the boiler, further improve efficiency and reduce Energy consumption. After several years of development, the flue gas waste heat recovery system has developed a variety of methods. The heat loss of exhaust gas seriously affects the efficiency of the boiler, and also affects the efficiency of the unit. It is estimated that every increase or decrease in the temperature of the exhaust gas of the boiler by 1 ℃ will reduce or increase the efficiency of the boiler by about 0.05%. The flue gas temperature of ordinary bituminous coal boiler is about 120 ～ 130 ℃, and the flue gas temperature of lignite boiler is about 140 ～ 150 ℃. If the final exhaust gas temperature is reduced to 100 ℃, the boiler efficiency can be improved by about 12.5%, and the unit efficiency will also be significantly improved.
The heat exchanger with waste heat recovery is installed at the entrance of the air preheater, which is used to heat the high-pressure feed water after the deaerator to reduce the steam extraction of the steam turbine and reduce the heat consumption of the steam turbine. Strong corrosion resistance. It also reduces the temperature of the hot air at the outlet of the preheater, which has a certain effect on the operation of the pulverizing system and the combustion organization of the boiler, because the system reduces the temperature of the flue gas at the inlet of the preheater. It is generally used in places where the fuel has good ignition and stable combustion characteristics and does not require high hot air temperature. For example, for high-moisture brown coal boilers that use a fan milling system, due to the good ignition characteristics of the coal, the milling system uses hot air + hot furnace smoke (cold furnace smoke) as the desiccant. high. After adopting the system, the temperature of the flue gas at the inlet of the preheater can be effectively reduced, thereby reducing the temperature of the flue gas at the outlet of the preheater, so as to recover the heat of the flue gas and reduce the loss of smoke exhaust.
Use flue gas bypass
The flue gas passes through the preheater, and another 20-30% of the bypass is used to exchange heat with the residual heat arranged in the bypass flue using a heat exchanger, and is mixed with the main flue gas at the outlet of the preheater. The amount of air is 100%, and the amount of flue gas passing through the preheater is only 70 to 80%. The cooling capacity of the air to the flue gas is enhanced. The flue gas at the outlet of the preheater can be controlled under the condition that the temperature of the air at the outlet of the preheater is unchanged. The temperature drops to a lower level. The bypass flue gas has a high temperature and a large temperature difference, and the choice of the heated medium is more flexible.
Way of installing heat exchanger downstream of preheater
This method generally uses a pretend heat exchanger, using water or air to absorb flue gas heat and recycle it. Since the flue gas temperature is already low here, in order to maintain a certain heat exchange temperature difference, the working medium to be heated is usually low-temperature feed water before the deaerator, cold air, or other industrial water. Some power plants use this system to reduce the flue gas temperature below 100 ° C and directly enter the desulfurization tower. The main feature of the system is that the heat exchanger is installed downstream of the air preheater, which has no effect on the heat exchange conditions of the air preheater, and the temperature of the hot air at the outlet of the preheater can be kept unchanged. However, due to the low flue gas temperature, the heat transfer temperature difference of the heat exchanger is small, the heat exchange efficiency is low, and a large heat exchange area is required. The finned tube heat exchanger is usually used. In addition, because the temperature of the flue gas and the working medium are low, and the temperature of the heat exchanger tube wall is low, good anti-corrosion measures must be adopted.
The implementation of the boiler exhaust heat recovery and utilization has an impact on other systems. Since the boiler exhaust heat utilization is mostly a retrofit project, it will have more or less impact on other projects of the unit. The following will analyze these effects.
Effect on steam turbine exhaust pressure
Since the saturation temperature in the condenser determines the size of the turbine exhaust pressure and the vacuum degree of the condenser, the saturation temperature of the condenser is kept constant by increasing the circulating water of the unit, the vacuum degree in the condenser and the turbine The exhaust pressure will remain at the rated value, and the power of the steam turbine will not change, but the increase in the amount of circulating water leads to an increase in energy consumption of the circulating water pump and an increase in power consumption.
Impact on induced draft fan or booster fan
The power consumption of the induced draft fan or booster fan will be affected, because the resistance element of the flue gas heat exchanger is added in the flue gas system. The effect of fan current can be calculated as follows.
Among them, Q3 fan outlet volume flow m3 / s, △ P fan pipeline pressure change, η fan efficiency 0.8, μ power factor 0.75, if the flue gas heat exchanger is arranged between the dust collector and the induced fan, the increased flue gas resistance makes the The current of the fan increases, but the volume flow of flue gas at the outlet of the induced draft fan becomes smaller due to the change of smoke temperature, so that the current of the induced draft fan becomes smaller. When considering the efficiency of the flue gas heat exchanger, the two aspects should be considered together. Fans are also suitable.
Impact on condensate pump
After the heating and low-pressure feed water technology is used to transform and recover the waste heat of the flue gas of the boiler, the regenerative system of the unit will increase the flow of condensate, thereby increasing the energy consumption of the condensate pump and reducing the steam extraction of the turbine.
Efficiency Analysis of Recovery and Utilization of Boiler Exhaust Heat
The analysis of boiler exhaust heat recovery and utilization efficiency should mainly consider the heat used by the system, the change of the turbine vacuum, the increase in the energy consumption of each fan caused by the resistance of the flue gas heat exchanger, and the change in the water consumption of the desulfurization process.
Effect on water consumption of wet desulfurization process
After using the technology of boiler waste heat recovery and utilization, the temperature of the flue gas entering the wet desulfurization absorption tower is greatly reduced, which makes the evaporation of water in the desulfurization system much reduced, and the process water consumption of the wet desulfurization system is reduced accordingly. It is estimated that the water consumption of the desulfurization process can be reduced by about 40t / h after the boiler exhaust heat recovery technology is generally used.
Impact on blower
The technical scheme of heating the air with the waste heat of the flue gas of the boiler increases the resistance of the air supply side, and the influence of this part of resistance should be deducted when calculating the recovery efficiency of the waste heat of the flue gas.
Among the various heat losses of power plant boilers, the exhaust gas loss accounts for the largest proportion of the total heat loss of the boiler. The performance is more obvious in high-parameter power plant boilers, and the exhaust gas loss accounts for 40% to 50% of the total heat loss of the boiler, or even higher. Therefore, reducing the exhaust gas temperature and improving the thermal efficiency of the boiler are of great significance to the energy saving and emission reduction of the power plant.
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