Technical Measures and Selections for Reducing Flue Gas Heat Loss of Large Coal-Fired Boilers

6EETechnical measures and selectionsfor Reducing Flue Gas Heat Loss ofLarge coal Fired BoilersBy Wang ChunchangXi'an Thermal Power Research Institute Co LtdAbstract: The main technologies for reducing flue gas heat loss of pulverized coal-fired boilers are introduced,and the suitability of these technologies for boiler operation and the principles for selection of thesetechnologies are explored. The main conclusions are: 1)the non-equilibrium control over fue gas flow ratesat the inlet of the air heater and the reversal rotation of the air heater rotator should be popularized as regulartechnologies in large boilers; 2)increasing the area of the air heater to reduce the flue gas heat loss in pulcoal-fired boilers should be the top option and increasing the area of the economizer be the next choice; 3)low-pressure economizer technology could save energy under special conditions and should be compared with theechnology of increasing economizer area in terms of technical economics when the latter is feasible; 4)the hotprimary air heater is only suitable to the pulverizing system with a large amount of cold air mixedKeywords: exhaust flue gas temperature; air heater; economizer; fue gas waste heat recovery; hot primary airheater; thermal systemSo far, coal-fired units with a capacity not less recovery technology; 3)reducing the air leakage rate ofan 600 MW have become the mainstay in Chinese air heaters. This paper introduces the main technologiesthermal power plants. These units have drawn extensive foror reducing the flue gas heat loss. Based on theattentions for their performance on energy saving and introduction, the matching performance and selection oftion reduction In fact, most utility boilers these technologies are discussed to provide references formatching the 600-MW or above units fire bituminous coal boiler retrofitor lignite with the unburned carbon heat loss less than1.0%, and even less than 0.5% for those 1 000-MW utility Main technologies for reducing theboilers. However,the exhaust flue gas heat loss of these exhaust flue gas temperaturboilers is about 5.0%, which has some room for energysavingThe exhaust flue gas temperature could be loweredto the designed value by increasing the heating surfacesGenerally, the exhaust flue gas temperature of large in the flue duct or enhancing the heat transfer. Inpulverized coal-fired boilers is higher than the designed practice, it is feasible and more effective to increase thevalue after these boilers are put into operation. the heating surface area of the economizer or the air heater;approaches to reduce the exhaust flue gas temperature to additionally,中国煤化工 ature can also bethe designed value(or even lower)include: 1)technical lowered byhe gas-air systemmeasures to reduce flue gas heat loss; 2)flue gas heat and its equipYHCNMHG36ELECTRICITY 2012.1LECTRICITYMeReversal rotation of air heater rotatorsIncreasing the area of air heaters in the reserved spaceSo far, in large boilers put into operation with will increase the flow resistance of the gas-air system,high parameters, reversal rotation is adopted by most but the total power increase of fans accounts for onlyair heater rotators that is, the rotator rotates from the about 10% of the effects of the reduction of exhaust fluesecondary air chamber to the primary air chamber. gas temperature. Consequently, increasing the area of airHowever, in large boilers commissioned earlier, positive heaters is the preferred technique when there is needrotation is more used. When a large amount of cold air reduce the flue gas heat lossis introduced into the pulverizing system, the primaryair temperature can be lowered by 10-20C through The exhaust flue gas temperature of some of the largechanging the rotation direction from positive to reversal boilers in operation is 15C higher than the designedwith a larger temperature difference at the initial end and value, which is hard to be decreased by increasing 10% ofa smaller temperature difference at the final end of the air the area or changing materials or plate types of air heatersheater;meanwhile, the secondary air temperature can be Therefore, it is recommended that the reserved spaceincreased by 10-20C. Consequently, the cold air mixed should be increased so that the space can be fully utilizedinto the pulverizing system can be reduced by about after the boilers are put into operation.3%, that is, the cold air passing through the air heater isincreased by about 3%, which could reduce the exhaustIncreasing the area of economizersflue gas temperature by about 3CIncreasing the area of the economizer is one ofthe regular methods to reduce the exhaust flue gasAlthough the exhaust flue gas temperature cannot be temperature. Usually, the area of the economizer islowered substantially by the reversal rotation technology, enlarged by decreasing the overhaul space or bythe technology does not have negative impact on the retrofitting the economizer. However, not all the overhaulboilers original gas-air system, and it only needs some spaces can be decreasedretrofit costs, which is superior to any other technoTherefore, for boilers with a large amount of cold airGenerally, the heat transfer temperature differencemixed into the pulverizing system, when positive rotation between the working medium in the economizer tubes andis adopted by the air heater, the priority is to change the the exit flue gas exceeds 60C. From the perspective ofrotation direction from positive to reversalheat transfer. some area can be added to the economizerto reduce the exhaust flue gas temperature. Different fronIncreasing the area of air heatersincreasing the area of the air heater, when the area of theAmong the technologies for reducing the exhaust flue economizer is increased, the heat transfer of the air heateras temperature the simplest one is to increase the area of will be affected. Therefore, if the area of the economizerair heaters or apply new materials to enhance heat transfer is planned to be increased, the outlet water temperaturewith only impacts on system resistance and fan outputs of the economizer should be lower than the saturatedfor the gas-air system. This technology is not influenced temperature, otherwise the water will boil in the tubes ofby any other factorsthe economizer, which would have impacts on the safeoperation of the economizer. Meanwhile, the impactsMost large boilers adopt rotary air heater. When of the increase of the economizer area on the air heaterboilers are put into operation, the area of the air heater can should be considered.hardly be increased. Therefore, during design, 10% of theupper part of the air heater will be reserved without any The outlet flue gas temperature of the economizerheating surfaces for future's retrofit to lower the exhaust one of the major parameters to balance the heat transferfue gas temperature, which is applicable in practicetemperature difference and the heat absorption of theeconomizer and the air heater. When it decreases the heatThe heat transfer temperature difference between the absorption of the economizer will increase and that ofinlet air and the exhaust flue gas is about 100C, while the air heater will decrease. Therefore, when it is higherthe temperature difference between the outlet air and than the designed value, the designed heat exchange ofthe inlet flue gas is over 40'C. When the heating surface the air heater will not be affected if the economizer areais increased by 10%, the exhaust flue gas temperature is increased adequately. Conversely when it is not highercan be reduced by 3-5C, which is able to remain than the design中国煤化工 exchangeenough temperature difference between the fue gas and be affected if tCNMHGreased whichthe air.requires that theould be divided2012.1 ELECTRICITY 3>EEreasonably between the economizer and the air heater andwhen the actual temperature difference of the economizeror the air heater is small. the economizer area should beincreased with cautionAdopting hot primary air heaterIt is common for large boilers with high parametersthat the air heater inlet flue gas temperature is close tothe designed value while the outlet temperature is higherthan the designed value. One of the main reasons is thatthe amount of cold air mixed into the pulverizing systemis so large that the air flowing through the air heater is notenough, leading to the increase of the boiler exhaust fluegas temperature. This problem can be solved by adoptinga hot primary air heaterThe hot primary air heater is used to lower the boilerexhaust flue gas temperature. It is installed in the hot Consequently, the hot primary air heater can partly replaceprimary air pipelines to heat up the working medium the steam-air heater or the hot air circulating system(which comes from condensate systems or feedwater or reducing their operating time, which is not availablesystems )with the hot primary air, which could decrease in other technologies for reducing the exhaust flue gasthe hot primary air temperature to(or close to)the allowed temperaturevalue at the inlet of the mill. Consequently, the cold airmixed into the pulverizing system is reduced nearly to When the amount of cold air mixed into thezero, making the air flowing through the air heater to pulverizing system is more than adequate and the inletbe increased and the exhaust flue gas temperature to be flue gas temperature of the air heater is not higher thanthe designed value, increasing the economizer area willmake the temperature difference of the economizer or theThe hot primary air heater can be made as a high air heater decrease, which is not feasible Increasing thepressure type(the medium to be heated is the feedwater of air heater area will raise the primary air temperature andboilers)or a low pressure type(the medium to be heated the amount of cold air mixed into the pulverizing systemcomes from the condensate system). The high pressure will also be increased. In this situation, it is reasonable totype does not have any impact on the thermal system while adopt the hot primary air heater. When the inlet flue gasthe low pressure type has some impacts on the thermal temperature of the air heater is higher than the designedsystem. The high pressure type has narrower application value, it is feasible to increase the economizer areacope because the temperature difference between thecooling medium and the hot primary air is small, while in Low-pressure economizerthe low pressure type it is as high as 200C. Therefore, thelow pressure type has higher efficiency in heat exchange The low-pressure economizer is a type of heatwith smaller heating areas, lower flowing resistance exchanger set behind the air heater or the electrostaticith no low-temperature corrosion, no ash fouling, no precipitator to absorb the waste heat of flue gas. It hasantiseptic treatment and relatively low manufacture some impacts on the thermal system. These impactscosts. Moreover, 1)the temperature difference of the are related to the cooling medium, i.e. the water intakehot primary air heater is hardly affected by the boiler position in the condensate system. The lower the coolingexhaust flue gas temperature. Therefore, the hot primary water temperature and pressure, the greater the impacts onair heater can be used without limitations to boiler loads the thermal system will beand seasons; 2) the hot primary air heater could adjustthe hot primary air temperature by changing the cooling If the low-pressure economizer is set behind the airmedium flow rate, which can regulate the exhaust flue gas heater, the temperature difference between its workingtemperature in a certain range to overcome the exhaust medium and中国煤化工 mall, so the heatflue gas temperature changes caused by the temperature exchange isCNMH Geating areas anddifference between day and night and boiler load changes. flow resistanteunlv, uy cyuumizer should be38ELECTRICITY 2012.1LECTRICIprotected against low-temperature corrosion with stricter disappears. The non equilibrium control technology is torequirements for materials, which is more costlyincrease the metal temperature and the flue temperatureby increasing the flow rate of the flue gas flowing throughGenerally, the temperature difference of economizers the initial end and consequently to improve the resistanceis over 60C. Assuming that the working medium against low-temperature corrosion for the air heatertemperature at the inlet of the low-pressure economizer is Since the heat exchange is enhanced for theabout 90C under boiler rated load conditions in summer, the exhaust flue gas temperature can be reduced bywhen the exhaust flue gas temperature is lower than around 2C. In addition, the steam-air heater or the hot air150C, the temperature of the flue gas going through the circulating system can be removed in some boilerseconomizer will be reduced by over 20'C. In such cases,the temperature difference of low-pressure economizers is Principles for selection of technologiesaround 40C. while in winter, it will be reduced further to20'C or so. Assuming that the temperature of the flue gas 1) For newly built boilers, the non equilibriumflowing through the economizer is lowered by 20C and control over flue gas flow rate at the inlet of air heatersthe temperature difference of the economizer is over 40C, and the reversal rotation of air heater rotators should beit is better to increase the area of the economizer than popularized as conventional technologies, and meanwhileto add a low-pressure economizer. Moreover, although the reserved space in air heaters should be enlargedincreasing the economizer area has some impacts on theheat transfer of the air heater, because of the impacts of 2)For boilers in operation, the non equilibrium controllow-pressure economizers on the thermal system which over flue gas flow rates at the inlet of air heaters and theare greater than increasing the economizer area, the reversal rotation of air heater rotators could also be widelyexhaust flue gas temperature of some boilers under low used as conventional technologiesload conditions in winter will be lower than 100C. Inthese cases, the temperature difference of the low-pressure3)Among the various technologies for reducing theeconomizer will be lower than 10 C, even close to zero, flue gas heat loss, if the equipment and field conditionsunable to absorb the flue gas waste heat, but the flowing permitted increasing the air heater area is preferred, thenresistance still exists, causing the electricity consumption increasing the economizer area is recommended, andof induced draft fans to increase. Consequently, the low- finally the hot primary air heater and the low-pressupressure economizer is subject to seasonal change and economizer could be considered.boiler load changes. Under low load conditions in winterit is more reasonable to increase the economizer area than Conclusionsto set a low pressure economizer.In other countries, low-pressure economizers are flue gas flow rate control in air heaters, the reversalmainly used in lignite-fired boilers with high exhaust flue rotation technology of air heaters and the technology ofgas temperatures and seldom used in boilers firing other increasing the air heater area or the economizer area havecoals. In China, it has begun to be used in recent years few negative effects on boilers except some impacts onin boilers with the exhaust flue gas temperature all over the resistance of the flue gas-air system, therefore, these150'C and few low-temperature corrosion problems, but technologies could be selected to reduce the exhaust fluehas not been undergone long-term operationgas temperatureNon equlibrium control over flue gas2)The technologies of hot primary air heaters and low-flow rates at the inlet of air heaterspressure economizers not only increase the fiow resistanceof the flue gas-air system, but also have some effects onWhen the rotator of air heaters rotates from air the thermal system, which is subject to boiler operationide to flue gas side, the metal temperature of the heat conditions. Particularly, the temperature difference isstorage plates of its initial end is the lowest and the flue greatly affected by the boiler load and the environmentgas temperature is also the lowest under the current flue temperature, which should be paid attention to when inflow distribution, which tends to cause low-temperature practical applicationcorrosion and clogging most easily. As the temperatures of中国煤化工the heat storage plates and the flue gas rise gradually, thepossibility of low-temperature corrosion is reduced, evenCNMHGed by ZhangYan2012. 1 ELECTRICITY 39