Type Selection of Burning Equipment for Slagging-Coal Boilers

Type Selection ofBurning equipment forSlagging Coal boilersBy Li Weidong, Zhou Hongguang Zhang Jingwu, Xu Chuankai1. Huaneng Power Intermational Inc2. Xi'an Thermal Power Research Institute Co. Ltd3. Northeast Electric Power Research Institute Co, Ltd.Abstract: The type selection of burning equipment for boilers is affected significantly by the slaggingcharacteristics of coal. Based on the engineering statistics method, the designed furnace parameters are obtainedfrom the 600-MW and 1 000-MW boilers with tangential firing and wall firing. The power and arrangement of theburners are analyzed. Their impacts on slagging on heating surfaces and the carbon contents in the ash and cindersare also discussed. Thermal parameters of furnace are recommended for boilers of 600 MW and 1 000 MW firingslagging coal in the design. The static or rotary classifier should be the first choice for the pulverizing systemKeywords: burning equipment; slagging coal; type selectionheating surfaces are quite different when the furnacesIntroductionand the burners are designed differently. The followingthermal characteristic parameters and geometrySevere slagging on furnace heating surfaces in coal- dimensions are critical to the boiler fumace designfired power plants has great impact on the safety andeconomics of the boilers running. Serious slaggingFurnace volume heat release ratemight appear when the design and arrangements of the The furnace volume heat release rate of the boilerfurmace and the burners cannot meet the requirements of firing the same coal will decrease with the increase of thethe slagging characters of the designed coal or when the boilers capacity. Lower furnace volume heat release rateboiler fires heavy slagging coal instead of the designed has been adopted to reduce slagging. In recent years, lowcoal. With the bulk exploitation of Shenhua Mine and the NOx combustion with air/coal staging has been widelycombustion of Shenhua Jurassic coal with heavy slagging used. Meanwhile, both in China and in other countries, theharacters in coastal power plants in China, many fumace volume has been enlarged to decrease the furnacepractical experiences have been accumulated in long-term volume heat release rate, which could lower the slaggingoperation. To ensure the safe and economic operation of level and reduce the heat loss caused by the unburnedslagging coal-fired boilers, it is most important to the type carbon in fly ashselection of burning equipment to avoid serious slaggingin boilers. The experience of design, type selection and Furnace cross-section heat release rate qrunning is summarized for the type selection of burning The furnace cross-section heat release rate reflectsequipment in the futurethe average fow velocity of combustion materials on thehorizontal cross-section of the furnace. The lower isImpacts of slagging characters onthe smaller the average flow velocity on the cross-sectiontype selectionwill be. With the same furnace volume heat release ratetheFurnace designsection heat中国煤化工由hedtlower the flrCNMH Gace outlet; at theFor a certain slagging coal, its slagging levels on same time the temperature in the combustion zone will beELECTRICITY 2012.1sale -ECTRICITYMiraised, which is propitious for ignition but increasesburner should be a little smaller and the space betweenpossibility of slagging on the water walls in this zoneburners should be a little largerthe quantity of NOThe expanded angle at the outlet of swirl burnersGenerally, the furmace cross-section heat release rate should not be too large with the swirl intensity not tooincreases slightly along with the increase of the boiler high. In this way it could avoid slagging caused by fullycapacity. However, it tends to be constant when the boiler diffused flames or direct flame impingement to watercapacity is more than 500 MWI. The furnace cross- walls(the side wall). If the burners adopt front-wallsection heat release rate has been lowered to reduce the arrangement, the direct-flow component of the outletquantity of NOx in the past few yearscoal powder flow should not be too strong except that thpower of each single burmer is small, which could preventBurner zone wall heat release rate qBthe rear wall from slagging due to the washout on the rearTogether with the furnace cross-section heat release wallrate. the burner zone wall heat release rate also refects theflame temperature level in the centre of the combustion The following points should be paid special attentionzone in the boiler. The smaller gn is, the lower the to for direct-flow burners with four-comer arrangementtemperature of the combustion core will be, which ishelpful to reduce the possibility of water-wall slaggingo The tangential circles of the first and the second airsand the emissions of NOxshould not be too large for the direct-flow burners in orderto maintain the oxidizing atmosphere near the water-wallThe burner zone wall heat release rate is closely and guarantee the full mixing of air and coal powder inrelated to coal characters rather than the boiler capacity. time, effectively utilize the furnace space and keep theIn a word, lower 4y, ls and qF will be helpful to prevent burning coal powder in the fumace long enough.The air velocities must be reasonable. The first airHeight of the burning-out zone h,velocity should not be too low, or the burner spout willThe height of the burning-out zone is defined as the be too close to the ignition spot, which could cause thevertical distance from the centre line of the top burner burner spout to be burnt out and to be prone to slaggingnozzle to the lower edge of the platen superheater. Itrepresents the travel length of the flame. A higher height It is better to divide the burners into two or threeindicates that the burning-down period of coal powder is sections with certain space between the sections. Thisprolonged, which could lower the flue gas temperature could improve the conditions for makeup air from bothat the furnace outlet and prevent slagging on the heating sides, reduce the pressure difference between the twosurfaces at the furnace outlet Particularly, when the sides of the burner jet and prevent the air flow fromseparated over fire air(SOFA)is adopted, the burning-out serious flow deflection or wall attachment.zone height should be increased due to the postponementArrangement of burnersThe height between the centers of the adjacent firstLaying of refractory belt for water-wallsair spouts hFor hard firing coals with a low volatile content, theThis height is related to the vertical distance betweenfractory belt for water-wall surfaces could maintain the burners and the arrangement of burners(integrativethe flame temperature in the burning zone, guarantee the or segmental). It directly affects the height of the burnerignition and stabilize the combustion of coal powder. zone. With higher h,, the burner zone wall heat releaseHowever, slagging might be more prone to appear on rate will be lower and the distribution of the burner zonethe surface of the refractory dressing. Consequently, for wall heat release rate will be more dispersive along theslagging coals with a low volatile content, the laying height, which is not good for the ignition and stablemode, laying location and laying area of the refractory belt combustion of coal powder, but good for the alleviation ofin the fumace need more consideration. Heavy slagging slagging and reduction of NOx emissions.will happen if the refractory belt is not laid properlyThe height from the center of the bottom firstDesign and arrangement of burnersair spout to the upper inflection point of the furnace中国煤化工Design of burnersThis paramCNMHGhe combustibleFor heavy slagging coals, the power of each single content in theo slagging on2012.1 ELECTRICITY 3water-walls in the furnace hopper zone. If the tilting the combustion efficiency and causes direct flameburner is adopted in the furnace with four-corner impingement on the water-wall or the flame center to shifttangential firing, h, must be high enough to avoid upward, resulting in slagginglagging in the furnace hopper when the burner tiltsdown, which fits for heavy slagging coal in particular. Often limited by the actual output of pulverizers, coalHowever, if the furnace height remains unchanged, powder has to be cruder to guarantee the capacity ofincreasing of h, will lower the height of combustion the whole unit, which postpones combustion and makesflames, make the flame centre elevated and the flue gas the flame shift upward. As a result, slagging appearstemperature at the furnace outlet increased, resulting on the heating surfaces. Therefore, in type selectionin slagging on the high-temperature heating surfaces an adequate margin should be left for the output ofat the furnace outlet. The practical operation indicates pulverizers. If permitted it is better to adopt static orthat for boilers with a capacity not less than 600 Mw, rotary classifiers5.5 m is a suitable height for h,. When swirl burners areadopted for opposed wall firing, h, can be smaller. A Even distribution of air and coal powder in burnersheight of over 2.5 m will be fine.The coal powder should be ignited in time and shouldbe mixed with the secondary air in time in the furnace. IfThe distance from the center of side burners to the the distribution of air and coal powder is not uniform insidewalldifferent burners, that is, more air with less coal powderWhen the opposed wall firing with swirl burners is or less air with more coal in one or several burners, theadopted, the burned or unburned coal powder might be coal might not be ignited well, the combustion might beblown to the sidewall of the furnace due to the expansion postponed, the flame might impinge on the water-wall or aof the swirl flow at the burner outlet, which could lead to reducing atmosphere might exist in some zones, and thenslagging or high-temperature corrosion on water-walls. slagging happens When opposed wall firing is adoptedThe performance of the burner is one of the factors that slagging might happen on the side wall if the side burmercause slagging on the water-walls, therefore, the distance is over-loaded because of the high concentration of coalfrom the center of the side burners to the sidewall must be powder The local heating load will be over high in thelarge enough, and it should be larger when using heavy zone where the coal concentration of a certain burnerslagging coal or high-sulfer content coal. For example, for is over high, which causes slagging on the water-walla 300-MW boiler, it is 3. 3 m generally, but it should be surface around this burmer even on the water-wall surfaceenlarged to 3.5-3. 8 m when using heavy slagging coal or of the furnace hopper. When four-coner tangential firinghigh-sulfer content coalis adopted, more coal powder might isolate or the localtemperature over the furnace hopper might increase ifDirect burners adopting wall tangential firing can also the bottom burners are over-loaded, which could resultalleviate flow deflection or wall attachment effectively, in slagging on the water-wall surface in the zone over thewhich could avoid slagging caused by the direct flame fumace hopper.impingement to the water-wallThe uneven distribution of air and coal amonMatching properties for auxiliariesdifferent burners is not good for coal burn-out, slaggingcontrol and NO, emission control, which should be paidThe pulverized coal-fineness Ro and the uniformity special attention to when designing the air and the coalindex nducts. Feasible measures should be taken to guaranteeThe pulverized coal-fineness Ron and the uniformity the uniformity well. In general, the error of the firstindex n exert crucial influence on combustion For air flow rates for different burners should be less thanslagging coal, the pulverized coal-fineness Roo and the # +5%, and that of the coal powder should be less thanuniformity index n should be considered simultaneously +10%. Classifiers with high performance should bewith the selection of pulverizers and pulverizing systems: equipped at the outlet of medium-speed pulverizers. TheRoo should be a little smaller and the uniformity index n symmetry of the piping structure and the uniformity ofshould be as high as possible. If the coal powder is finer, the flow resistance should be taken into consideration init is more easily to burn out. If the coal powder is cruder, coal piping designing and regulating devices should beit is harder to burn out, which might cause direct flame equipped if necessary.impingement on the water-wall or the flame center to shiftupward,resulting in slagging. If the uniformity index n Type seled中国煤化工is smaller, there will be more crude particles in the coal In practiceCNMH GIgn are smaller,flow and it is more difficult to burn out, which decreases severe air sholuagtwu waav wc unit runs at theELECTRICITY 2012.1ECTRICITrated load, which could cause heavy slagging in the heavy slagging coal.fumaceConclusionsDesign and arrangement of soot blowersFor heavy slagging coals, enough soot blowers witI1)For heavy slagging coal, the power of each singleood performance should be equipped in the boiler burner should be a little lower with larger space betweenfumace and at the outlet of the furnace and should also the burnersbe equipped in the zone below the burners and around theplaten heating furnace when necessary2)The expanded angle at the swirl burner outlet shouldnot be too large and the swirl intensity should not be tooSlag disposal systemhigh to avoid slagging caused by fully diffused flames orFor heavy slagging coals and fumaces with refractory the direct flame impingement to the water-wall(the sidebelt, the width of the short side of the slag notch at the wall)bottom of the furnace hopper should be increased over1.3 m with slag breakers installed in the slag disposal 3)For direct-flow burners in four-corner arrangermensystem to ensure smooth slag disposalshould not be over large. It should not only maintain theRecommended values for furnaceoxidizing atmosphere near the water-wall, but also ensureperformancethe air and the coal powder to mix in time, use the fumacespace sufficiently and ensure the burning coal powder toFor boilers firing slagging coal or firing heavy slagging stay in the furnace long enough. The air velocities mustoal solely, lower parameters should be adopted with be reasonable. The first air velocity should not be overhigher burning-out zone height h,. For boilers of 600-Mw low, or the ignition of coal powder will be too close to theand 1 000-MW classes, the recommended values are outlet of the burner spout, which makes the burner spoutlisted in Table 1easy to be burnt out and prone to slagging4)The h, of 5.5 m is suitable for boilers with a capacityTable 1 Recommended values of furnaceof no less than 600 mw. when swirl burners withcharacteristics for boilers of 600 Mw and 1 000 Mwopposed wall firing are adopted, h, could be lower, that is(BMCRover 2.5 m600MW1000MW5)Generally, the distance from the center of sideFumace volng co376-907085heat release raburners to the sidewall should be 3.3 m. When firingHeavy slagging coal 68-80 65-78 heavy slagging coal or high-sulfur coal, it should beincreased to 35-3. 8 mFumace cross-sectionagging coa4.3-4.6444.7heat release rate9. (MW/m)Heavy slagging coal6)A good margin for the pulverizer output should beeft when selecting the devices. If permitted, a static orBumer zone wallSlagging coal 1.3-1.6 1.4-1.7 rotary classifier could be adoptedheat release ratea ( W/m)Heavy slagging coal 1.0-1.4 1.I7) For boilers of 600-MW and 1 000-Mw classesSlagging coalHeight of burning-out20-24 23-27 the recommended values for the fumace performance arezone A, (m)listed in the paper.Heavy slagging coal24Notice: For tower boilers, the lower furnace volume heat release rateReferenceshould be used and the higher burming-out zone height h, should be used.For boilers firing heavy slagging coal solely, the [l] He P A, Zhao ZH,Qin Y k. Design and Running ofmanufacturing cost and the installation cost will increasebecause the furnace must be enlarged in the primaryPress, 1987design. However, it is impossible to burn the coal overa long period. Therefore, the necessity of enlarging thefurnace should be studied when the design coal is chosen,中国煤化工 by Zhu Xiaoranit might be more reasonable to adopt mix-burning forCNMHGuby Zhang Yan2012.1 ELECTRICITY35