Resarch and Development of Hot Primary Air Heater for Coal-Fired Boilersin Power Plant

LECTRICITYResearchandDeuelopmentdR山 man ThEaterCoakilredBoilersinMg吗Abstract: The reasons of introducing cold air into pulverizer are ana/zed for boilers with large capacity and high parametershe temperature rises of the exhaust gas are calculated when varying the amount of the cold air. The hot priman air heatera new technology, is developed to eliminate the cold air from the pulverized coal system. The applications, advantages anddisadvantages are introduced in detail for the new device and sy'stem. It is concluded that introducing cold air into pulverizer isone of the major factors that causes the exhaust gas temperature of boilers with large capacity to be high. The amount of the coldair could be reduced significantly, even to zero in some cases by adopting the hot primary air heater, whichobs the erof the boiler efectively. Thheatercould play part roles of the steam-air heater or thehot air recirculation system, could also be used to adjust the exhaust gas temperature within the range of 20 C by controlling theflow rate of the cooling medium. Moreover, the startup period of the steam-air heater or the hot air recirculation system will beshortened, which is a unique advantage of the hot primary air heater among the measures to drop the exhaust gas temperatureKeywords: hot primary air heater, exhaust gas temperature; pulverized coal system, hot air recirculation, steam-air heatercold primary air being introduced into the pulverized coalIntroductionsystem, which reduces the hot primary air induced into thefumace for combustion, and decreases the heat exchangeThe exhaust gas temperature is usually higher than efficiency of air heaters with less heat absorption It'sthe designed values in most coal-fired boilers, especially common in many coal-fired power plants and is the mainin the ultra-supercritical boilers. It is one of the serious reason that causes the exhaust gas temperature to beproblems in power plants at the present time. In some higherboilers, it is even 20C higher than the designed value atleast, which could increase the net coal consumption byDropping the exhaust gas temperature is one of themore than 3 g/kwhmportant medure the net coal consumptionfor coal-fired中国煤化工 case when theA good many factors could contribute to the over-high temperatureCNMH G value. Thoughexhaust gas temperature. One of them is the excessive many technical measures have been adopted to decrease2011.3 ELECTRICITY 35SEthe exhaust gas temperature of the boiler, studies aiming it's a universal phenomenon to introduce the cold primaryat the excessive cold primary air have seldom been made, air into the pulverized coal system for large boilers inand few relevant solutions and technical measures have domestic coal-fired power plantsbeen published or reported by now. The hot primary airheater is developed to reduce, even cancel the cold air The cold primary air introduced into the pulverizedfor the pulverized coal system, which could eventually coal system accounts for about 10% to 30% of the totallower the exhaust gas temperature or maintain it within a primary air in most cases. Correspondingly, the requiredreasonable scope.hot primary air through the air heater is decreased byabout 10% to 30%Function of cold primary air in thepulverized coal systemThough the inherent ability of heat exchange isdetermined by the design capacity for the air heater, afterFor coal-fired boilers, when the coal, boiler output and the boiler is put into operation, it will be impacted by theoxygen are fixed, the required air in the fumace and the ratio of air to gas through the air heater(called air-gasgas flow rate through the air heaters are certain. The air ratio for short, similarly hereinafter ); and it is similar toleakage of the furmace and the cold air into the pulverized the exhaust gas temperature. Based on the arrangement ofcoal system play the main roles in influencing the air the air-gas system in existing boilers, more heat will bequantity through the air heaters under such working absorbed by the cooling medium( that is, the air) and moreconditions. The latter, in particular, will make greater heat will be released by the hot medium(that is, the gasInpacwhen increasing the air-gas ratio of the air heater, whichcould result in lower exhaust gas temperature. ContrarilyGenerally, the temperature of the hot primary air lowering the air-gas ratio will lead to higher exhaust gasis over 300C for large power units. According to the temperature. According to the estimated calculation, if thesafety requirements in the operation instructions for cold primary air accounts for 10% in the pulverized coathe pulverized coal system, there is a strict limit for the system, the exhaust gas temperature will rise about 10 Ctemperature of the air-coal mixture at the outlet of the and if it accounts for 30%, the exhaust gas temperaturepulverizer, which requires that the air temperature at the will rise about 20C. Such significant impact should notinlet of the pulverizer should be limited under 280 C. be neglectedTherefore, the cold primary air system is designed for theair-gas system in the existing boilers, which is a part of It is obvious that sending less cold primary air into thethe primary air while not heated by the air heater. In other pulverized coal system is a technical means feasible andwords, this part of primary air is guided directly from effective to drop the exhaust gas temperature of the boilerthe outlet of the primary air fan to the inlet duct of the However, it is in fact a way that destroys the economicspulverizer. When the temperature of the air-coal mixture of boiler performances. If the cold primary is decreasedat the outlet of the pulverizer is higher than the set value, or eliminated, the running safety of the pulverized coit will be lowered by inducing the cold primary air to system will be inevitably affected. Consequently, newguarantee the safe running of the pulverized coal system. methods must be sought to solve this problem completely,which means that the current primary air system must beIt should be noticed that there are some uncertain improvedfactors which will influence the amount of the coalprimary air after the boiler being put into operation. For Hot primary air hearterexample, the difference between the actual coal andthe design coal will lead the actual flow rate of the cold In 2009, a new technology named the hot primaryprimary air to increase, larger than the designed value. air heater was developed by the Xi,an Thermal PoweResearch Institute to avoid the cold air in the pulverizedInfluence of cold primary aircoal system. Fig. I shows the sketch of the whole systemThe air temperature at the inlet of the pulverizer is As shown in Fig. 1, the hot primary air is the heatingusually under 280C for the running boilers with high source of the heater, and the heated working substance iscapacity. Sometimes it's lower than 250 C or even lower the main conde中国煤化工 erative systemthan 210 C. From this point of view, the cold primary air of the boilerCNMHG will return tois necessary for the pulverized coal system. In addition, the regenerative systerm alter absorbing some of the heat36-ELECTRICITY 2011.3LECTRICITYeah小/primary air can be controlled below 3%o in such a case.Hot primary air ductLet's take the following case as an example. SupposeOutlet duct of hot primary air heaterthe temperatures of the cold primary air, the hot primaryir at the outlet of the air heater and the primary air atInlet duct of hot primary air heaterthe inlet of the pulverizer are30℃,330℃and280℃,Adjusting valve for flow raterespectively, then the proportion of the cold primaryair to the total primary air will be as high as 16%. If thetemperature of the hot primary air drops to 280C byAir heateradding the hot primary air heater the quantity of thecold primary air will be zero and the proportion of thehot primary air through the air heater will increase byabout 16%. Finally, the exhaust gas temperature will dropnearly 10 C for the boiler. Taking the different workingconditions for different pulverizers into account, supposeFig. 1 Systemic sketch of hot primary air heaterthe temperature of the hot primary air might be loweredfrom the hot primary air, and then the temperature of the by far to 300 C, the proportion of the cold primary air tohot primary air will drop. Therefore, the air-coal mixture the total primary air is decreased by about 6%and that oftemperature at the pulverizer outlet is lowered, which the hot primary air through the air heater is increased bycould keep the safe running of the pulverized coal system. about 10%. The exhaust gas temperature will still dropnearly6℃insuch a caseThe adjusting valve at the inlet of the hot primaryair heater is used to control the flow rate of the main At present, the temperature of the hot primary air is ascondensate water passing through it, thus the temperature high as 350C to 390 C while the primary air temperatureof the hot primary air at the pulverizer inlet can be at the inlet of the pulverizer is usually under 280 C forcontrollable and adjustable. Meanwhile the flow rate of the boilers with high capacity and parameters. It's obviousthe hot primary air can also be controlled by setting the that there is greater potential for these boilers to drop theadjusting valves of the hot and the cold primary airexhaust gas temperature by adopting the hot primary airheater. Under the circumstance of the hot primary airIn the new system the temperature and flow rate of temperature being 390 C at the outlet of the air heaterthe hot primary air are controlled by the absorbed heat of and other parameters remaining equal to those of thethe hot primary air heater and by adjusting the valves of former case, the proportion of the cold primary air to thethe hot and the cold primary air. The flow rate increases total primary air is increased to be 30%. If the temperatureand the temperature decreasesfor the hot primary air, of the hot primary air drops to 280 C, the proportion ofwhich ensures that they are remained the same as those of the hot primary air through the air heater will increasethe mixed primary air(that is, the cold primary air being by about 30% and the exhaust gas temperature will dropintroduced into the hot primary air in the original system) about 20Cinto the pulverizer in the original system. In a brief, lesscold primary air is sent into the pulverizer successfullyTo sum up, it is theoretically feasible to add a hoton the basis of making no change about the pulverizer's primary air heater to the hot primary air duct, andworking condition in the new system. Theoretically, the practically the quantity of the cold primary air could beflow rate of the cold primary air could be zero when all decreased significantly with the exhaust gas temperaturethe pulverizers are running under equal conditions. Due to dropping obviously on the basis of no other changes beingthe variability and instability of the actual coals, however, made to the pulverizer's runningthe working condition of each pulverizer might be ratherdifferent from those of others in fact. In order to meet the Advantages and disadvantages of hotrequirements of the pulverizer system with the demand of primary air heatermaximum drying heat, the temperature of the hot primaryair after the hot primary air heater must be high enough toAddheater or a low-pressuremeet the drying requirement of the pulverizer. Therefore, economizer i中国煤化工 cal measures toa small quantity of cold primary air is necessary to other drop the exhatCNMH Ge hot primary airpulverizers. It's promising that the proportion of the cold heater and theeconomizer are heat recovery372011.3 ELECTRICITYSEEdevices, which will exert impacts on the regenerative4)According to the working principle of thesystem of the boiler. Compared to the low-pressure regenerative system, the cooling medium with highereconomizer, the hot primary air heater has the following temperature and pressure will exert less impact on theadvantagesregenerative system for both the hot primary air heaterand the low-pressure economizer. Contrarily, the lowerThe efficiency of heat exchange is higher The gas parameters will have greater impact. Limited by thetemperature is between 130 C and 160 C at the inlet flue gas temperature the cooling medium of the lowof the low-pressure economizer and the temperature pressure economizer can only be taken from the low-of the cooling medium from the regenerative system is pressure side of the condensate circuit of the boiler'sbetween 60C and 100C. Consequently, the temperature regenerative system, which leads to the economizer to bedifference between the gas and the cooling medium is a low-pressure type while for the hot primary air heater,less than 50C usually, which means a larger area for the the cooling medium could be either the condensate waterheat exchanger is required with the flow resistance and from the high-pressure side of the regenerative systemthe original investment also being increased. However, or the high-temperature high-pressure condensate waterthe temperature difference between the hot primary air from the low-pressure side, which has less impactand the cooling medium is over 150C when the hot the regenerative system than that of the low-pressureprimary air heater is adopted, because the temperature of economizer. The hot primary air heater could be a high-the hot primary air is higher than 300C and that of the preessuretype when the hot primary air temperature iscooling medium from the regenerative system is between high enough(for example, over 350 C)and the feedC and 150C usually. Therefore, the heat exchange water temperature for the boiler is under 290 Cefficiency is higher, the area of the heat exchanger is this time, the hot primary air heater has no impact onsmaller, and the flow resistance and the investment are the regenerative system, but it plays the role as a highalso lowerpressure economizer dropping the boiler' s exhaust gastemperature directly other than recovering the waste heatDue to little dust in the primary air, the exchanger from the fiue gassurface will not be contaminated for the hot primaryair heater. In addition, the low temperature corrosion 5) The resistance of the hot primary air heater will bewill not happen on the exchanger surface because smaller than that of the low-pressure economizer if the hotthere are no reducing gases in the air. Therefore, it's primary air heater is designed to be a low-pressure oneunnecessary to give the exchanger surface any anti The flow resistances of the hot air duct downstream thecorrosion pretreatment, which could reduce the cost hot primary air heater will drop for both the temperaturefurther. Comparatively speaking, the low-pressure and the flow rate of the hot air decrease. In additioneconomizer is installed in the flue gas duct where the dust the flow resistance of the cold primary air duct will alsoconcentration is high and the economizer surface is prone drop for the flow rate of the cold air decreasesto be contaminated, which could reduce the efficiency of resistance of the air heater will rise. Suppose the airthe heat exchange. Moreover, the reducing gases such as flow rate through the air heater increases by about 10%,so, in the flue gas are likely to lead to low-temperature it's calculated that the resistance of the air heater willcorrosion on the exchanger surface, so its necessary increase by about 50 Pa and the total resistance of the hotto give some pretreatment for anti-corrosion and anti- primary air duct downstream the hot primary air heaterclogging, which could increase the cost further.and the cold air duct will drop by about 50 Pa, whichcancels out each other. Therefore, the added resistance3)The flow resistance caused by the hot primary of the system is only the resistance of the low-pressureir heater and its related system can be compensated hot primary air heater itself. On the other hand, the flowby increasing the outlet pressure of the primary air resistance of the low-pressure economizer is large, butfan without any additional power. Similarly, the flow it drops the resistance of the downstream economizerresistance caused by the low-pressure economizer can be and also the output of the induced draft fan, which willcompensated by raising the pressure of the induced draft somewhat compensate the resistance of the low-pressurefan without any additional power. However, there is a economizer. From the viewpoint of resistance, it's hard towide margin for the output of the primary air fan while tell which is better between the hot primary air heater andthe output of the induced draft fan is usually insufficient. the low-pressur中国煤化工Therefore, the plan of adopting the hot primary air heaterCNMHGis more feasible in practice6) Similar to the low-pressure economizer, the main38ELECTRICITY 2011.3LECTRICITYdevice( that is, the hot primary air heater) of the system be either a low-pressure heater or a high-pressure heater,dopting the hot primary air heater is a steam-water or even a heater to heat the feed water of the boiler. Thereheat exchanger, which is a technically matured product. are also many choices for the parameters of the workingMoreover, the system is quite simple with only one steam- medium(condensate water)in the low-pressure heaterwater heat exchanger being installed in the hot primary Different types or different parameters mean differentair duct as shown in Fig. I and the cooling medium being advantages and disadvantages. In general, the higher thetaken from the condensate water system, which makes no temperature difference between the cooling medium andchange to the entire system and is easy to run.the hot primary air, the greater impact on the regenerativesystem of the boiler with lower efficiency in heat recoveryThere are some limitations of the hot primary air when the drop of flue gas temperature is fixed, but theheater though its advantages are obvious. The most major manufacturing cost and investment will be less and thelimitation is that this kind of system can only be applied to flow resistance of the system will be smaller at the samethe coal-fired boiler in which the proportion of the cold air time. The smaller temperature difference will lead to theintroduced into the pulverized coal system is over 5%. If opposite result. Therefore, when reconstructing a boiler,the proportion of the cold air is less than 5%, the exhaust the technologies and economics should be analyzed forgas temperature could drop only about 3C by adopting making the best choice.the hot primary air heaterAnother application of the hot primaryAnother limitation is the decreasing extent of the air heaterexhaust gas temperature. Currently, the maximalproportion of the cold air to the total primary air is about The exhaust gas temperature of the boiler will always30% for a coal-fired boiler Based on this figure, the drop change due to the variability of many factors such asof the exhaust gas temperature will be less than 30 C by the coal, unit load and atmospheric temperature. Takeadopting the hot primary air heateraccount of both the running economics of the boiler andrunning safety of the air heater, the boiler exhaust gasIn spite of the disadvantages mentioned above, there temperature should not drop blindly but be controlledare good prospects for the hot primary air heater because strictly. For example, the exhaust gas temperature couldthe cold air is usually sent into the pulverizer for coal-fired be near or under the dew-point temperature at low loadboilers, especially for the boilers with high capacity and if it is only 10C to 20 C higher than the designed tem-high parameters in thermal power plants with the exhaust perature or the dew-point temperature at rated loadgas temperature of usually 30 C higher than the designed Instead of being dropped (or the low-temperaturecorrosion and clogging might happen on the air heater),the exhaust gas temperature should be raised slightlyIt should be noted that the hot primary air heater could under such conditions. That is the reason why the steam-air heater or the hot air recirculation system is set to raisethe exhaust gas temperature for over 95% coal-firedboilers. It means that the unit's safety is compensated byIts economIcsDue to the low exhaust gas temperature, thetemperature difference for heat transfer is smaller atlow load for the low-pressure economizer. Sometimesthe temperature difference is so small that the lowpressure economizer cannot be put into service, and thelow temperature corrosion and clogging will be serioustoo. Under the circumstances, the temperature differencefor heat transfer will still be large enough and the heatexchange efficiency will remain rather high for the hotprimary airMore important. the exhaust gastemperature中国煤化工 the flow rate ofthe coolingCNMH Gthe hot primary( To be continued on P46)2011.3 ELECTRICITY-S9SESecond, the security assessment on wind power or Pv up the upgrading of grids, improve the grid structure,power integration, the management on the compensation and perform feasibility studies on connecting systemfor ancillary services and the management on wind project to select the connection plan on merit order. Bypower or PV power dispatch shall be carried out in order proactively providing grid connection services for theto systematically improve the regulation on the grid wind power or PV power, the grid enterprises can henceconnection and operation of wind power and PV power. ensure the timely on-grid operation of the generationproject. In addition, on the premise that the power systemThe cooperation between generation enterprises and security and stability is ensured, grid enterprises shallrid enterprises in wind power and Pv power integration strictly comply with the requirements of the Renewableand accommodation shall be strengthened. The generation Energy Act and other laws and regulations to purchaseenterprises shall actively collaborate with grid enterprises the on-grid electricity at the feed-in price approved by theby putting forward feasible delivery and accommodation state's relevant department of price administration. Theyschemes and submitting them to the authorized also have to strictly implement the price policy for feed-departments of the government for confirmation. On the in tariff for wind power and PV power, do a better job inother hand, when the grid enterprises start to process the distribution of additional electricity price and achievethe application for grid connection from a wind power sufficient and timely electricity settlement.or a PV power project, they shall prepare carefully thetransmission line, grid structure and terminal for theproject according to the requirements of grid developmentTranslated by Zhang Sufangplans and wind power or Pv power project plans, sperEdited by Zhang Yan(Continued from P39)of the cold air into the pulverizer could be under 3%orair heater which can play a role of the steam-air heater even be zero and the exhaust gas temperature of the boileror the hot air recirculation system, shortening the startup drops successfullyperiod for the steam-air heater or the hot air recirculationsystem consequently and improving the economics of the 2 ) The hot primary air heater could be used to adjustthe exhaust gas temperature of the boiler. Though the hotprimary air heater cannot replace the steam-air heater orThe temperature of the hot primary air can be raised the hot air recirculation system completely because of theby reducing the flow rate of the cooling medium passing limited adjustment, it could effectively shorten the startingthrough the hot primary air heater if the exhaust gas period of the steam-air heater or the hot air recirculationtemperature drops under the dew-point temperature for system.the running unit with some cold air being mixed into thepulverized coal system in such case, which could rise the 3)The hot primary air heater could be a high-pressureexhaust gas temperature to a certain extent. Although the heater, a low-pressure heater or just an economizer. Eachmaximum rise of the exhaust gas temperature is less than type has its own advantages and disadvantages and will be20C, it is enough to keep the exhaust gas temperature suitable for some certain external conditions. Therefore,over the dew-point temperature when the atmospheric its application depends on the practical situation whentemperature and the unit load change, which could reconstructing a certain boiler. The technologies andshorten the startup period for the steam-air heater or the economics should be compared and analyzed for makinghot air recirculation system. This application is a unique the best choice.advantage of the hot primary air heater to drop the exhaustReferencegas temperature.[1] Wang CC. The instruction leaflet of the patent on hotConclusionsprimary air heater and its related system for coal-firedboiler in power plant [R]. Shaanxi: Xi'an Thermal1)The hot primary air heater is developed to reducePower Research Institute Co. Ltd 2009the amount of the cold air into the pulverized coal systemIt can lower the temperature of the hot primary air at the中国煤化工inlet of the pulverizer effectively and recover some heatCNMHG Zhu Xianrafrom the hot primary air. Consequently, the proportionEdited by Zhang YanELECTRICITY 2011.3