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1、外文出处： A. A. Shatil , N. S. K. A. A. S., & V. Kudryavtsev, A. (2008). Controllingthe furnace process in coal-fired boilers. Thermal Engineering, 55, 1, 72-77.Controlling the Furnace Process in Coal-Fired BoilersThe unstable trends that exist in the market of fuel supplied to thermal power plants and

2、the situati ons in which the parameters of their operati on n eed to be cha nged (or preserved), as well as the tendency toward the econo mical and en vir onmen tal requireme nts placed on them beco ming more stri ngent, are factors that make the problem of con troll ing the combusti on and heat tra

3、n sfer processes in furn ace devices very urge nt. The solutio n to this problem has two aspects. The first invo Ives developme nt of a combusti on tech no logy an d,accordi ngly, the design of a furnace device when new installations are designed. The second invoIves modernization of already existin

4、g equipment. In both cases,thetechnical solutions being adopted must be properly substantiated with the use of both experimental and calculati on studies.The experienee Central Boiler-Turbine Institute Research and Production Association (TsKTI) and ZiO specialists gained from operati on of boilers

5、and experime ntal inv estigati ons they carried out on models allowed them to propose several new desig ns of multifuel and man euverable in other words, con trollable furn ace devices that had bee n put in operati on at power stations for several years. Along with this, an approximate zero-one-dime

6、nsional, zon ewise calculati on model of the furn ace process in boilers had bee n developed at the TsKTI, which allowed TsKTI specialists to carry out engin eeri ng calculati ons of the mai n parameters of this process and calculate studies of furn aces emplo ying differe nt tech no logies of firin

7、g and combusti on modes .Naturally, furn ace process adjustme nt methods like cha nging the air excess factor, stack gas recirculation fraction, and distribution of fuel and air among the tiers of burners, as well as other operations written in the boiler operational chart, are used during boiler op

8、eration.However, the effect they have on the process is limited in nature. On the other hand, con trol of the fur nace process in a boiler implies the possibility of making substa ntial cha nges in the conditions under which the combustion and heat transfer proceed in order to considerably expand th

9、e range of loads, minimize heat losses, reduce the extent to which the furnace is contaminated with slag, decrease the emissions of harmful substances, and shift to another fuel. Such a control can be obtained by making use of the following three main factors:(i) the flows of oxidizer and gases bein

10、g set to move in the flame in a desired aerod yn amic manner;(ii) the method used to supply fuel into the furn ace and the place at which it is admitted thereto;(iii) the finen ess to which the fuel is milled.The latter case implies that a flame-bed method is used along with the flame method for com

11、busti ng fuel.The bed combusti on method can be impleme nted in three desig n versi ons: mecha ni cal grates with a dense bed, fluidized-bed furn aces, and spouted-bed fur naces.As will be show n below, the first factor can be made to work by sett ing up bulky vortices transferring large volumes of

12、air and combustion products across and along the furnace device. If fuel is fired in a flame, the optimal method of feeding it to the furnace is to admit it to the zones n ear the cen ters of circulat ing vortices, a situati on especially typical of highly intense furnace devices. The combustion pro

13、cess in these zones features a low air excess factor ( a 1) and a long local time for which the components dwell in them, factors that help make the combusti on process more stable and reduce the emissi on of n itroge n oxides .Also important for the control of a furnace process when solid fuel is f

14、ired is the fineness to which it is milled; if we wish to mi ni mize in complete combusti on, the degree to which fuel is milled should be harm oni zed with the locatio n at which the fuel is admitted in to the furn ace and the method for suppl ying it there, for the occurre nee of unburned carb on

15、may be due not on ly to in complete combusti on of large-size fuel fracti ons, but also due to fine ones faili ng to ign ite (especially whe n the content of volatiles Vdaf 20%).Owi ng to the possibility of pictorially dem on strati ng the motio n of flows, furn ace aerod yn amics is attract ing a g

16、reat deal of atte nti on of researchers and desig ners who develop and improve furnace devices. At the same time, furnace aerodynamics lies at the heart of mixi ng (mass tran sfer), a process the qua ntitative parameters of which can be estimated only in directly or by special measureme nts. The qua

17、lity with which comp onents are mixed in the furn ace chamber proper depe nds on the nu mber, layout, and mome nta of the jets flow ing out from in dividual burners or no zzles, as well as on their in teracti on with the flow of flue gases, with one ano ther, or with the wall.It was suggested that t

18、he gas-jet throw dista nee be used as a parameter determ ining the degree to which fuel is mixed with air in the gas burner channel. Such an approach to estimating how efficient the mixing is may to a certain degree be used in analyzing the furnace as a mixing apparatus. Obviously, the greater the j

19、et length (and its momentum), the Ion ger the time duri ng which the velocity gradie nt it creates in the furn ace will persist there, a parameter that determines how completely the flows are mixed in it. Note that the higher the degree to which a jet is turbulized at the outlet from a nozzle or bur

20、ner, the shorter the dista nee which it covers, and, accord in gly, the less completely the comp onents are mixed in the furn ace volume. Once through bur ners have adva ntages over swirl ones in this respect.It is was proposed that the exte nt to which once through jets are mixed as they pen etrate

21、 with velo city w2 and density p 2 into a transverse (drift) flow moving with velocity w1 and hav ing den sity p 1 be correlated with the relative jet throw dista nee in the followi ng wayWhere ks is a proportionality factor that depends on the“ pitch s”ks)etween the jet a1.5-.8).The results of an e

22、xperime ntal in vestigati on in which the mixi ng of gas with air in a burner and the n in a furn ace was studied using the in complete ness of mixing as a parameter are reported in 5.A round once through jet is inten sively mixed with the surro unding medium in a furn ace within its initial section

23、, where the flow velocity at the jet axis is still equal to the velocity w2 at the nozzle orifice of radius rO.The velocity of the jet blown into the furnace drops very rapidly bey ond the confines of the in itial sect ion, and the axis it has in the case of wall-m oun ted bur ners bends toward the

24、outlet from the furn ace.One may con sider that there are three theoretical models for an alyz ing the mixi ng of jets with flowrate G2 that enter into a stream with flowrate G1. The first model is for the case when jets flow into a“free ” space (G1= 0),the second model is for the case when jets flo

25、winto a transverse (drift) current with flowrate G1 - G2, and the third model is for the case whe n jets flow into a drift stream with flowrate G1G2. The sec ond model represe nts mixi ng in the cha nnel of a gas bur ner, and the third model represe nts mix ing in a fur nace chamber. We assume that

26、the mixing pattern we have in a furnace is closer to the first model than it is to the second one, since 0 G1/G2 1, and we will assume that the throw distance h of the jet being drifted is equal to the length SO of the“free ” jet s initial section. The ejectionthe jet being drifted the n rema ins th

27、e same as that of the“ free ” jet, and the len gth ofsection can be determined using the well-known empirical formula of GN. Abramovich 6: S0= 0.67r0/a, (2)where a is the jet structure factor and rO is the nozzle radius.At a = 0.07, the len gth of the round jet s in itial sect ion is equal to 10 rO

28、and the radius tjet has at the tran siti on secti on (at the end of the in itial sect ion) is equal to 3.3 r0. The mass flowrate in the jet is doubled in this case. The corresponding minimum furnace cross-sectional area Ff for a round once through bur ner with the outlet cross-secti onal area Fb wil

29、l the n be equal to and the ratio Ff/Fb 20This value is close to the actual values found in furnaces equipped with once through burners. In furnaces equipped with swirl burners, a= 0.14 and Ff/Fb 10. In both cases, the interval between the burners is equal to the jet diameter in the transition secti

30、on d tr , which differs little from the value that has been established in practice and recomme nded in 7.The method traditi on ally used to con trol the furn ace process in large boilers con sists of equipping them with a large number of burners arranged in several tiers. Obviously, if the dista nc

31、e betwee n the tiers is relatively small, operati ons on disc connecting or connecting them affect the entire process only slightly. A furnace design employing large flat-flame burners equipped with means for controlling the flame core position using the aerodynamic principle is a step forward. Addi

32、tional possibilities for controlling the process in TPE-214 and TPE-215 boilers with a steam output of 670 t/h were obtained through the use of flat-flame burners arran ged in two tiers with a large dista nce betwee n the tiers; this made it possible not only to raise or lower the flame, but also to

33、 concen trate or disperse the release of heat in it 1. A very tangible effect was obtained from installing multifuel (operating on coal and open-hearth, coke, and natural gases) flat-flame burners in the boilers of cogeneration stations at metallurgical pla nts in Ukraine and Russia.Unfortunately, w

34、e have to state that, even at present, those in charge of selecting the type, quantity, and layout of burners in a furnace sometimes adopt technical solutions that are far from being optimal. This problem should therefore be con sidered in more detail.If we in crease the nu mber of burners nb in a f

35、urn ace while retai ning their total cross-secti onal area (X Fb=idem) and the total flowrate of air through them, their equivale nt diameters deq will become smaller, as willthe jet momentums Gbwb, resulting in a corresp onding decrease in the jet throw dista nce hb and the mass they eject. The spa

36、ce with high velocity gradients also becomes smaller, resulting in poorer mixing in the furnace as a whole. This factor becomes especially important when the emissions of NOx and CO are suppressed rightinside the furnace using staged combustion (at b 1) underahe conditions of a fortiori nonuniform d

37、istribution of fuel among the burners.In 1, a qua ntitative relati on ship was established betwee n the parameters characteriz ing the quality with which once through jets mix with one another as they flow into a limited space with the geometrical parameter of concentration = with nb = idem and Gb =

38、 idem. By decreas ing this parameter we improve the mass tran sfer in the furn ace; however, this en tails an in crease in the flow velocity and the expe nditure of en ergy (pressure drop) in the burners with the same Fb. At the same time, we know from experienceand calculations that good mixing in

39、a furnace can be obtained without increasing the head loss if we resort to large Ion g-ra nge jets. This allows a much less stri ngent requireme nt to be placed on the degree of uniformity with which fuel must be distributed among the burners. Moreover, fuel may in this case be fed to the furn ace l

40、ocati on where it is required from process con trol con siderati ons.For illustrati on purposes, we will estimate the effect the nu mber of bur ners has on the mix ing in a furn ace at = = idem. schematically shows the pla n views of two furn ace chambersdifferi ng in the nu mber of once through rou

41、nd no zzles (two and four) placed in a tier (on one side of the furnace). The furnaces have the same total outlet cross-sectional areas of the nozzles ( Fb) and the same jet velocities related to these areas (wb). The well-known swirl furn ace of the TsKTI has a desig n close to the furn ace arran g

42、eme nt un der con sideratio n. According to the data of 1, the air fraction 田ir that characterizes the mixing and enters through once through bur ners into the furn ace volume ben eath them can be estimated using the formula B air= 1 -(3) which has been verified in the range = 0.03-0.06 for a furnac

43、e chamber equipped with two fron tal once through burn ers. Obviously, if we in crease the nu mber of burners by a factor of 2, their equivale nt diameter, the len gth of the in itial sect ion of jets S0 and the areahey “ serve ” will reduce by a factor of Then, for example, at = 0.05, the fraction

44、airvill decrease from 0.75 to 0.65. Thus, Eq. (3) may be written in the following form for approximately assess ing the effect of once through bur ners on the quality of mixi ng in a furnace:B air =315f nb,where is the number of burners (or air nozzles) on one wallwhe n they are arran ged in one tie

45、r both in on esided and opposite mann ers.The nu mber of burners may be ten tatively related to the furn ace depth af (at the same = idem) using the expressi on (5)It should be no ted that the axes of two large opposite air no zzles ( = 1a n arran geme nt impleme nted in an inv erted furn ace had to

46、 be in cli ned dow nward by more tha n 50 8.One well-known example of a furnace device in which once through jets are used to create a large vortex coveri ng a con siderable part of its volume is a furn ace with tangen tially arran ged burn ers. Such furn aces have received especially wide use in co

47、mb in ati on with pulveriz ing fans. However, burners with cha nn els hav ing a small equivale nt diameter are frequently used for firing low-calorific brown coals with high content of moisture. As a result, the jets of air-dust mixture and sec on dary air that go out from their cha nn els at differ

48、e nt velocities(w2/w1 = 2 ) become turbulized and lose the ability to be thrown a long distanee; as a con seque nee, the flame comes closer to the waterwalls and the latter are con tam in ated with slag. One method by which the tangen tial combusti on scheme can be improved con sists of organizing t

49、he so-called concentric admission of large jets of air-dust mixture and sec on dary air with the fuel and air no zzles spaced apart from one ano ther over the furn ace perimeter, accompanied by intensifying the ventilation of mills 9, 10. Despite the fact that thetemperature level in the flame decre

50、ases,the combusti on does not become less stable because the fuel mixes with air in a stepwise manner in a horiz on tal pla ne.Vortex furn ace desig ns with large vortices the rotati on axes of which are arran ged tran sversely with respect to the mai n direct ion of gas flow have wide possibilities

51、 in terms of con trolli ng the furn ace process. In 1, four furn ace schemes with a con trollable flame are described, which employ the principle of large jets colliding with one another; three of these schemes have been implemented. A boiler with a steam capacity of 230 t/h has been retrofitted in

52、accorda nee with one of these schemes (with an inv erted furn ace) . Tests of this boiler, during which air-dust mixture was fed at a velocity of 25 430 m/s from the boiler front using a highc oncen trati on dust system, showed that the temperature of gases at the outlet from the furn ace had a fair

53、ly uniform distributio n both along the fur nace width and depth . A simple method of shift ing the flame core over the furn ace height was checked duri ng the operati on of this boiler, which consisted of changing the ratio of air flowrates through the front and rear nozzles;this allowed a shift to

54、 be made from running the furnace in a dry-bottom mode to a slag-tap mode and vice versa. A bottom-blast furn ace scheme has received rather wide use in boilers equipped with different types of burners and mills. Boilers with steam capacities ranging from 50 to 1650 t/h with such an aerod yn amic sc

55、heme of furn aces manu factured by ZiO and Sibe nergomash have bee n in stalled at a few power statio ns in Russia and abroad . We have to point out that, so far as the efficiency of furnace process control is concerned, a comb in atio n of the follow ing two aerod yn amic schemesis of special in te

56、rest: the inv erted scheme and the bottom-blast one. The flow patter n and a calculati on an alysis of the furn ace process in such a fur nace duri ng the combustio n of lea n coal are prese nted in 13.Below, two other tech niq ues for con trolli ng the furn ace process are con sidered. Boilers with

57、 flame -stoker furn aces have gained accepta nee in in dustrial power engin eeri ng, devices that can be regarded to certa in degree as con trollable ones owing to the prese nee of two zones in them . Very different kinds of fuel can be jointly combusted in these furnaces rather easily. An example o

58、f calculating such a furnace device is given in 2. As for boilers of larger capacity, work on develop ing con trollable two-z one furn aces is progress ing slowly . The developme nt of a furn ace device using the so-called VIR tech no logy (the tran sliterated abbreviation of the Russian introductio

59、n, innovation, and retrofitting) can be considered as holdi ng promise in this respect. Those invo Ived in bringing this tech no logy to the state of in dustry sta ndard en coun tered difficulties of an operati on al n ature (the con trol of the process also presented certain difficulties). In our opinion, these difficulties are due to the fact that the distribution of fuel over fractions can be optimized to a limited extent and that the flow in