COMPUTER COMBUSTION-RADIA TION MODEL OF RDF BOILERS A T COLUMBUS, OHIO

Transcription

1 COMPUTER COMBUSTIONRADIA TION MODEL OF RDF BOILERS A T COLUMBUS, OHIO DONALD A. KADUNC Alden E. Stilsn & Assciates Clumbus, Ohi ROBERT H. ESSENHIGH The Ohi State University Clumbus, Ohi ABSTRACT A cmputer mdel based n the Clumbus, Ohi RDF furnace system has been develped. The mdel simulates cmbusin and radiatin heat transfers in the furnace. Variables evaluated were: Arrhenius cmbustin factrs, turbulen t mixing factr, radiatin Qlackening factr, airfuel rati, and Bragg stirred reactr height. Results prvide design evaluatin criteria. SUMMARY A cmputer prgram was develped t mdel the cmbustin behavir in stker fired bilers [l]. The mdel is applicable t cal, bark, and refuse furnaces. The mdel cnsists f a Bragg stirred reactr vlume [2] fllwed by a plug flw vlume. The latter vlume is represented by a series f slices. Cmbustin was mdeled as a secnd rder reactin prprtinal t fuel and xygen cncentratins. An Arrhenius rate cnstant [3] was included. The effects f frequency factr and activatin energy were studied. The airfuel rati was varied. A bypass factr was therized; this factr accunted fr fuel residence time distributin and mixing behavir within vlumes. The size f the stirred reactr vlume was a variable investigated. The radiatin heat transfer was mdeled as a functin f cncentratins f H and CO2, and f particulates (flame blackening). Results shwed that it was nt pssible t distinguish the effects between frequency factr and activatin energy. High frequency factr r lw activatin energy prduced rapid burning, while ppsite values retarded and in sme cases stpped burning. The cmbustin appears t be highly insensitive t these factrs individually; any attempt t experimentally islate them will require extreme care. Increasing the airfuel rati increased the cmbustin efficiency, but reduced the furnace temperatures and biler efficiency. As the stirred reactr vlume was increased, the cmbustin efficiency, peak furnace temperature, and biler efficiency were reduced. In additin, the peak temperature shifted t higher in the furnace. Increasing bypass prduced results similar t thse due t larger stirred reactr vlumes, except that peak furnace temperature was nt necessarily reduced. It was pstulated that a relatinship between stirred reactr vlume and bypass existed. It was assumed that as the vlume increased, the bypass wuld decrease. A functin f these variables was develped and tested. As the vlume was increased and bypass reduced, the peak temperature drpped and shifted higher in the furnace. The effects n cmbustin and biler efficiency were incnclusive, but results indicate the tw variables cancel each ther (prducing nearly cnstant values). Blackening reduces cmbustin efficiency and temperature, but increases biler efficiency. N peak temperature shift ccurred. 469

2 BURNOUT ZONE STIRRED REACTOR ZONE DELAY AREAS BYPASS ZONE elva PERFECTY STIRRED ZONE L.I ' FIG. 1 PHYSICAL MODEL COMPUTER MODEL A cmputer prgram was develped frm a rather extensive mathematical mdel [1] f a stker fired biler. Figure 1 illustrates the mdel. A Bragg mdel [2] was used; this cnsists f a stirred reactr vlume fllwed by a plug flw vlume. A stirred reactr is a regin f highly turbulent flw, which in mst bilers is prduced thrugh the intrductin f high velcity verfire jets. This vlume has unifrm prperties (Le., cnstant temperature, pressure, cmpsitin). Delay areas were added t accunt fr premixing in the bed and in the verfire jets. A plug flw vlume is characterized as a fairly quiescent regin with prperties varying in Hie directin f flw. It is mdeled as a series f thin slices with cnstant prperties. The fuel is an idealized hydrcarbn mixture cntaining Carbn, Hydrgen, Oxygen, water and ash. Nitrgen, Chlrine and Sulphur cmpunds may be included if pllutin r crrsin studies pllutin r crrsin studies are desired. In this paper the fuel cmpsitin is held cnstant and is patterned after nrmal municipal refuse. Cmbustin is assumed t be a secnd rder reactin prprtinal t the prduct f xygen and fuel, where k=aeeart sec1 is the Arrhenius equatin fr the temperature (T, R *) reactin rate cnstant (k, secl) [3]. The frequency factr (A, sec 1) is a measure f the rate f mlecular cllisins and the activatin energy (Ea' Btulb) indicates the critical energy that mlecules must acquire befre they can react. R(BtulbF) is the gas cnstant. Radiatin heat transfer is.dependent n the lcal emissivities f CO2 and 'H in the gas stream. Httel [4] prvides values fr these quantities dependent n temperature and cncentratin. A blackening factr, ala Beuters [4] is added t accunt fr ther gaseus and particulate emissins. The radiatin frm each slice interacts with all ther slices and the walls f the furnace. Attenuatin is dependent n the absrptivity f each slice, which is,a functin f its temperature, and CO2H cncentratin. The absrptivity f the walls is dependent n their surface prperties. Cntained in the prgram are functinal representatins f all pertinent thermdynamic and heat transfer parameters. Mst are temperature dependent. Als, in the prgram is a menu f design parameters (Le., biler dimensins, fuel cmpsitin, preheat, exhaust gas recirculatin, perating pressures, etc.) that may be changed t study their effects. All such parameters were assigned values in this study t represent a Clumbus Ohi Municipal Electric Plant biler perating at its full lad design pint. The slutin requires slving a set f temperature dependent, nnlinear equatins using a NewtnRaphsn iterative technique. RESULTS AND DISCUSSION Six parameters were studied t determine their effect n the cmbustin prcess within th furnace. The parameters were frequency factr, activatin energy, airfuel rati, bypass factr, stirred reactr height, and blackening factr. Fur f the parameters deal directly with the cmbustin prcess. The frequency factr and activatin energy determine the reactinrate velcity cnstant. The airfuel rati enters int the rate equatin as infrmatin t calculate the air and fuel cncentratins within that equatin. The bypass factr enters the rate equatin as a stirring cefficient. The stirred reactr height prvides a parameter t study the verall effect f gemetry n the cmbustin prcess. The blackening factr prvides a 470

3 parameter t study the influence f gas emissivity n heat transfer. INPUT DATA The dimensins f the furnace used were 18x 18x ft (S.SxS.SxI2 m) high which clsely represent thse f the Clumbus bilers. Fuel cmpsitin was a simplified representatin f refuse. Carbn Hydrgen Oxygen H Ash Heat f Cmbustin Fuel I nput Rate Fuel Characteristics 26 percent by weight 4 percent percent percent percent 4,500 Btulb (3000 calkg) 50,000 Ibhr (22,700 kghrl The air and cmbustin gases were mdeled as high temperature air with variable parameters f specific heat, cnductivity, Prandtl number, and Reynlds number. The range f frequency factrs and activatin energies was chsen t include the exprimental values determined by Biswas [6.06 x 106 sec 1, 29,700 Btuhr (16.S kcal)] 7 and Shieh [O.ISS x 106 sec1, 3S,000 Btuhr (.1 kcal)] 8. These are the nly data available which apprximates refuse fuel. The bypass factr range is between zer and 1.0 with a base value f 0.3. N theretical r experimental values have been established at this time which wuld limit this range. The range f study f the blackening factr was frm 1.0 t 3.0. Bueters was able t determine an experimental value fr natural gas f abut 1.2 and fr il f abut 1.S. The fllwing base line figures were used during analysis. Activatin Energy Frequency Factr Excess Air Bypass Stirred Reactr Ht. Blackening Base Line Values 32,000 Btuhr (17,800 cal) 4X 1 06 sec' 50 percent 30 percent 13 ft (4 m) 2.0 RESULTSGENERAL CHARACTERISTICS The aspects f mst general interest required frm the mdel, were the predictins f temperature and reactantprduct prflles alng the flw path f the furnace. The results btained, which will be described in detail in later sectins f this paper, are generally in accrdance with what ne wuld expect in a furnace f this type. The temperature prflles match expectatins; the fuel buqlout and reactin prducts prflles are reasnable; hwever, the cmbustiqn efficiency appears t be slightly higher than expected. The thermal efficiency, nevertheless, is in the range that wuld be expected in such a biler. Figure 2 illustrates typical predictins; characteristically, tw different types f temperature prflles are generated. Curve A shws a regin f cnstant temperature thrugh the stirred reactr regin fllwed by a cntinuusly decreasing temperature prfile. In Curve B the stirred reactr cnstant temperature regin is fllwed by a peaking temperature prflle (in each case the cnstant temperature is an artifact f the stirred reactr assumptin that the prperties thrughut its vlume are unifrm). The data als revealed tw types f cmbustin prflles. As shwn by Curve C f Fig. 3 ne type f cmbustin was characterized by a step functin. This was due t cmplete cmbustin ccurring within a single slice. The secnd type f cmbustin prfile is shwn by Curve D. In this case cmbustin in the furnace takes place within a number f slices. The types f prfiles are due t differences in reactin rates. Tw patterns f heat absrptin ccurred within the furnace. Curve E f Figure 3 shws a cntinu HEIGHT.. FIG. 2 TYPICAL TEMPERATURE PROF ILES 100% r ' CURVE C _ COMBUSTION (curve 0 80 I J (FT.) BOILER HEIGHT FIG. 3 COMBUSTION & ABSORPT ION PROFILES 471

4 usly increasing functin with a cntinuusly decreasing slpe. Curve F begins with a functin f cnstant slpe within the stirred reactr fllwed by a functin similar t Curve E. The difference is due t the missin f a stirred reactr in the furnace, shwn as Curve E. FREQUENCY FACTOR AND ACTIVATION ENERGY Generally, (see Fig. 4) high activatin energy and lw frequency factr, cmbined, have the effect f generating pr cmbustin, and lw activatin energy cmbined with a high frequency factr has the effect f prmting gd cmbustin. Very different cmbinatins f frequency factrs and activatin energies culd all prduce nearly identical results which implies th at the cmbustin is smewhat insensitive t the reactin 'u 3 ACTIVATION ENERGY x 10 (BTOlB) \ fr.o" "". ( I'" V V 2 " V IXIO. V IJ 6 II) t II: e 1 u 8 c( 5 IL 4. 5 IxlO. V.,,... \ v. FIG. 4 FREQUENCY FACTOR VS ACTIVATION ENERGY O r (R) 2800 EXCESS AIR 20 F =:25:...: % 20 t= 5"'0::...: %=.J % 00 L FIG. 5 EXCESS AIR TEMPERATURE PROFILES 38 rates; evidently, it must be dminated by the heat transfer. It als means that determinatin f kinetic cnstants (Ea and A) frm experimental data must be dne with great care r the values can be subject t substantial errr. EFFECT OF AIRFUEL RATIO Figure 5 shws the temperature prfiles in the furnace fr fully develped cmbustin with differing excess air ratis. Increasing excess air substantially reduces the maximum furnace temperature. The temperature prfile is als flattened ut as the excess air is increased. The effect f airfuel rati n cmbustin efficiency is illustrated in Fig. 6. One hundred percent air prduced mre cmplete cmbustin than 50 percent excess air. The rate equatin is gverned in part by the xygen cncentratin and temperature f the gas. This higher cncentratin due t increased air wuld prduce faster cmbustin ther things being equal. Hwever, the lwer temperature ffsets the xygen differences and the tw ppsite effects d nt ttally cancel; the effect f higher xygen cncentratin slightly verrides the effect due t lwer temperature, and the run with the 100 percent excess air burns mre cmpletely than the run with 50 percent. When runs f 100 percent and 25 percent are cmpared, hwever, the effects in the lwer prtin f the furnace are reversed; the effect f temperature dminates ver that f xygen cncentratin, and the run with 25 percent excess air burns faster than the run with 100 percent excess air. Nevertheless, in the upper prtin f the furnace the effect f xygen cncentratin again dminates ver the effect f temperature and the run with the higher percentage f excess air burns faster and has a mre cmplete burnut prfile. Frm an envirnmental standpint high excess air is beneficial. Figure 6 als illustrates that as the excess air increases in a biler the heat absrptin rate decreases significantly. T increase the biler perating efficiency, excess air shuld be minimized. EFFECT OF BYPASS It was felt that mixing and residence time distributin were verriding influences n cmbustin. A bypass factr was added t the cmbustin equatin which frced cmbustin t take place ver a large vlume within the furnace. With 472

5 """ COMBUSTION BOILER HE IGHT 0 0 ' _ A8S0RPTION 30 1FT.) FIG.6 EXCESS AIR COMBUSTION & ABSORPTION PROFILES 3000 BOILER HEIGHT (FT.) ( " R) BYPASS ,.. 10 % 50% FIG.7 BYPASS FACTOR TEMPERATURE PROFILES ut bypass, all the cmbustin takes place within the stirred reactr. As is shwn in Fig. 8, when bypass is increased, cmbustin takes place ver a larger vlume f the furnace. The cmbustin rate is slwed as bypass is increased further. At 0.9, cmbustin in the furnace stps. This trend shws that pr mixing, as mdeled by high bypass, slws the cmbustin rate and can even prevent cmbustin frm taking place. Figure 7 shws the effect f bypass n the temperature distributin within the furnace. Withut bypass, the temperature declines as the gases mve up the furnace. At lw bypass (0.1) the temperature als declines a.s the gases mve up the furnace, bu t the slpe f the curve is less than the curve withut bypass. The maximum temperature, als, decreases. As the bypass is increased t 0.3, the temperature prfile n lnger is declining. A hump develps in the prfile. As the bypass is increased further t 0.5, the hump becmes mre prnunced and the temperature peaks higher up in the furnace. The figure als shws that increases in bypas prduce a reductin in cmbustin efficiency. This effect f reduced cmbustin efficiency is thrughut the furnace even thugh bypass nly directly effects the first few slices. I 80 BYPASS e. r._e ,. 30% I ", ' I. I' O% : COMBUSTION. 70"...., _... 30% _.50% _... ' _e ABSORPTION (FT.) BOILER HEIGHT FIG.8 BYPASS COMBUSTION & ABSORPTION PROFILES OXYGEN ;.. 14 FUEL ACTUAL II)..J J '" BYPASS '" z 8 ALONE 0... '" 6 )( (FT.) BOILER HEIGHT FIG.9 BURNUP PROFILES In additin t reducing cmbustin efficiency bypass has a detrimental effect n the heat absrptin rate within the furnace. This can be clearly seen in Fig. 8 where the heat absrptin rate drps significantly as the bypass increases. It is thught that bypass is t sme extent a cntrllable parameter. Overfire air can be intrduced int the furnace at a sufficient velcity and in such a pattern as t prmte better mixing within the furnace. This leads directly t a stirred reactr design where vilent mixing is prmted. The vilent mixing speeds cmbustin and reduces bypass. INTERACTIONS BETWEEN BYPASS, FREQUENCY FACTOR AND ACTIVATION ENERGY A further analysis shwed that the bypass nly has an effect n the first few slices. After these slices, the frequency factr and activatin energy retard the maximum burning rate in a slice as is shwn by the higher fuel cncentratin in the latter slices than wuld be expected by bypass alne. Figure 9 shws a typical break pint between the bypass and reactivity effects. The curve fr bypass alne was cmputed n the basis f the expected gemetrical decay pattern f fuel in the biler. The ther curve was the actual result f a cmputer run shwing the amunt f fuel in the biler 473

6 BOILER HEIGHT (FT.l 3000,,""'"T'"r..,."'" (R) :::._:'...:=_...J 00 L....;;;,, ' BOILER HEIGHT (FT.) FIG. 10 STI RRED REACTOR HEIGHT TEMPERATURE PROF ILES FIG. 11 STIRRED REACTOR HEIGHT COMBUST ION AND ABSORPTION PROFILES at each slice. The difference in the tw curves demnstrated that the burnut was affected by the frequency factr and activiatin energy. The fuel burnup was similar in all runs. The fuel and xygen prfiles fr a typical run are shwn in the figure. It is seen that the xygen prfile is nearly flat. The slightly decreasing prfile f the xygen will asympttically apprach the value crrespnding t the amunt f excess air present in the system. At the same time the fuel will asympttically apprach zer as the furnace height is increased. The xygen prfile can be cnsidered cnstant. This leads t the cnclusin that the fuel burnup is a pseudfirstrder reactin. This can be taken as a quite general statement when a significant amunt f excess air is present. EFFECT OF STIRRED REACTOR HEIGHT The biler stirred reactr height was varied frm zer t ft (12 m). Figure 10 shws the change in furnace temperature prfile as the stirred reactr height is increased. Increases in reactr height cause the heat t be generated in a larger vlume. This leads t lwer furnace temperatures. The temperature peak mves up the furnace as the stirred reactr height is increased. Figure 11 als shws the effect the stirred reactr height has n cmbustin. When studied as a separate variable, increasing the stirred reactr height causes the cmbustin t be slwed. Larger stirred reactr vlumes cause the furnace temperature t drp and these lwer temperatures lead t the slwer burning rates. Figure 11 illustrates that larger stirred reactr vlumes reduce the furnace heat absrptin rate. Again, the larger stirred reactr vlumes prduce lwer verall furnace temperatures. These lwer furnace temperatures reduce the net amunt f heat transfer t the walls, which lwers the heat absrptin rate. INTERACTION BETWEEN BYPASS AND STIRRED REACTOR HEIGHT Increasing the stirred reactr height while hlding bypass cnstant reduces the heat absrptin rate. Increasing the bypass while hlding the stirred reactr height cnstant reduces the heat absrptin rate. One culd intuitively reasn that bypass will be large in shrter stirred reactr znes and apprach zer as the stirred reactr height is increased t infinity. An attempt was made t study the cmbined effect f bypass and stirred reactr height. An arbitrary relatin was made between bypass and stirred reactr height. This is shwn in the table. TABLE 1 HEAT UTILIZATION RATE VS BYPASS AND STIRRED REACTOR HE IGHT Stirred Reactr Heat Utilizatin Bypass Height, ft (m) Rate 0.7 7(2) (4) (6) (9) (12) 0.3 N presumptin is made that the crrelatin is realistic. It can be seen frm the data that the effect f small bypass n the heat utilizatin rate appears t partially reverse the effect f large stirred reactr heights. Evidently, the tw parameters tend t cancel each ther ut. EFFECT OF BLACKENING FACTOR The principal effects f blackening are t change the temperature prfile in the furnace and t change the heat utilizatin rate. 474

7 '' (R) F===== 20 FIG. 12 BLACKENING FACTOR TEMPERATURE PROF ILES 100 ')(,L:_i3;SiJ.E.Ei:. E:. :;:"U!l 80 COMBUST ION 3.0 BLACKENIN i.9... :.!..,' t! ':' 1.0 ABSORPTION (FT.) BOILER HEIGHT FIG. 13 BLACKENING FACTOR COMBUSTION & ABSORPT ION PROFILES By increasing the emissivity, blackening causes better heat transfer between the gas and the walls even thugh an increase in blackening causes a decrease in peak furnace temperature, as is seen in Fig. 12. It als causes a steeper decay in the temperature prfile. Figure 13 shws the effect f the blackening factr n the cmbustin prfiles. Blackening has little if any effect n cmbustin prfiles. Hwever, as is seen in the figure blackening has a large effect n the heat absrptin ability f the furnace. High values f blackening prduce much higher heat absrptin rates than lw values f blackening. It shuld be nted that increases in blackening cause reduced furnace temperatures as is shwn in Fig. 12. This reduced temperature wuld reduce heat transfer t the walls causing lwer heat absrptin rates, but the ppsite is actually ccuring. S, the increase in emissivity caused by blackening greatly vershadws the reductin in furnace temperatures due t blackening, and this prduces the large heat absrptin rate. EFFECT OF TEMPERATURE The cmbustin prcess is highly dependent n temperature. The temperature f the gas enters the cmbustin equatin as part f the expnen TEMPERAT URE (0 R) rrr.,r..,...;.;,.;..;;.! 80 i= &.J ::;) III :I 8... ;;;:;;;... = :::;;;;=...J FIG. 14 PROF ILES OF COMBUSTION VS TEMPERATURE tial term f the Arrhenius rate cnstant. In rder t study the effect f temperature n the cmbustin efficiency, transient cmputer data prfiles were analyzed. Transient cmbustin prfiles f slices 5" 18 ft (5.5 m); 6,19.5 ft (6.0 m); 7,21 ft (6.5 m); and 8,22.5 ft (7.0 m) are shwn in Fig. 14. The cmputer prgram was adjusted t induce cmbustin in the upper part f the furnace (as with a suspensin fired biler). Cmbustin began at abut 18 ft (5.5 m) ff the grate. Slice seven shws the typical S shaped plt f cmbustin efficiency versus temperature. Slices 5 and 6 shw the beginnings f such a curve. Hwever, peak cmbustin rates were nt btained in these slices and the curves are nt fully develped. Nte the ffset f the curves t the right as the slice number increases. This is because less fuel and xygen are available in the later slices causing the cmbustin rate t slw. The cmbustin rate is prprtinal t bth fuel and xygen. As seen in Figure 14, slice 8 shws the effect f fuel starvatin n the cmbustin prcess. As slice 7 uses up all the fuel there is such a small quantity f fuel present in slice 8 that the cmbustin rate slws dramatically in this slice. In this case, the cmbustin rate actually drps with time even thugh the temperature is increasing. In slice 8 the effect f the lack f sufficient fuel verrides the effect f increasing temperature and the cmbustin rate slws. CONVERSION FACTORS K kg m cal = (R) (lb) (ft) (Btulb) 475

8 REFERENCES [1) Kadunc, D. A., Dissertatin: "Cmputer Mdel f Cmbustin and Radiatin Prcesses in Refuse Derived Fuel Fired Stker Bilers," The Ohi State University, 1981; Available at: University Micrfilms Internatinal, 300 N. Zeeb Rad, Ann Arbr, Michigan [2) Bragg, S. L., "Applicatin f Reactin Rate Thery t Cmbustin Chamber Analysis," Aernautical Research Cuncil Paper N , C.F. 272, September [3) Glassman, I., Cmbustin, Academic Press, New Yrk, N.Y., [4) Httel, H. C. and Sarfim, A. F., Radiative Transfer, McGrawHili, New Yrk, [5) Bueters, "Cmbustin Prducts Emissivity by F c Operatr," Cmbustin, March [6) Bueters, K. A., Cgli, J. G., and Habelt, W. W., "Perfrmance Predictin f Tangentially Fired Utility Furnaces by Cmputer Mdel," 15th Internatinal Sympsium n Cmbustin, Tky, Japan, August 2531, (7) Biswas, B. K. and Essenhigh, R. H., "The Prblem f Smke Frmatin and Cntrl," Paper N. 39f presented at A.1. Chem.E. 70th Natinal Meeting, Atlantic City, AugSept., [8) Shieh, W. and Essenhigh, R. H., "Cmbustin f Cmputer Cards in a Cntinuus Test Incineratr: A Cmparisn f Thery and Experiment," Prceedings f 1972 Natinal Incineratr Cnference, ASME' New Yrk, 1972, pp Key Wrds Biler Cal Clumbus 'Cmbustin Pllutin RefuseDerived Fuel Stker 476