CFD MODELLING OF THE FLOW AND REACTIONS IN A FLASH FURNACE SMELTER REACTION SHAFT

Transcription

1 Third Internatinal Cnference n CFD in the Minerals and Prcess Industries CSIRO, Melburne, Australia - December CFD MODELLING OF THE FLOW AND REACTIONS IN A FLASH FURNACE SMELTER REACTION SHAFT Christpher B. SOLNORDAL, Frank R. A. JORGENSEN, Peter T. L. KOH, and Arthur HUNT CSIRO Minerals, Claytn, Victria 9, AUSTRALIA WMC Resurces Ltd Olympic Dam Operatins, Suth Australia, AUSTRALIA ABSTRACT The perfrmance f flash furnace burners can be evaluated quickly and efficiently using CFD mdelling. Gas flws are mdelled using the cnventinal Eulerian apprach, while Lagrangian particle tracking is used t mdel the flw f slid feed thrugh the burner and int the reactin shaft. A Cmpsite Particle Mdel has been develped that cnsiders the slid feed t be made up f single particles cntaining apprpriate quantities f cncentrate, flux and dust. Slid fuels (such as cal) can als be included in the cmpsite particle. Reactins between the slids and gas are then mdelled using standard heat and mass transfer relatinships. Results frm the mdelling prcess are shwn fr WMC s Olympic Dam cpper flash smelter with the burner that was used frm 98-. Flw patterns, temperature and gas cmpsitin distributins, particle dispersin and residence time, and verall extent f sulphur remval are predicted and used t evaluate furnace perfrmance. Hwever, results are sensitive t the assumed size f the cmpsite particles, and plant measurements are required t determine the apprpriate cmpsite particle size t predict quantitative data. INTRODUCTION T accurately mdel the cmbustin prcesses taking place within a flash smelter it was necessary t develp a separate mdel f flash cncentrate cmbustin that culd be incrprated int the CFD packages in use at CSIRO Minerals. Initial develpment tk place within the PHOENICS envirnment fr nickel flash smelting (Kh and Jrgensen, 99). CSIRO prted the mdel t CFX- and mdified it t investigate zinc flash smelting (Kh et al., 997). The current wrk was undertaken in t investigate the capability f a new cncentrate burner in WMC s Olympic Dam Operatins (ODO) cpper flash smelting shaft ver a range f perating cnditins. Thus the sub-mdel was further mdified t simulate the cmbustin in the ODO cpper flash smelting shaft. The burner mdelled in this wrk was in peratin frm 998 t early. It has since been replaced by a new technlgy burner that perates at ver 7 tph cncentrate. In this paper the cmbustin sub-mdel is presented, tgether with sme results f the ODO smelter investigatins, in particular the perfrmance f the smelter under high flw rate and turn dwn cnditins. MODEL GEOMETRY The mdelling wrk was perfrmed using the cnfiguratin f the burners as it was in 999 (Figure ) Cncentrate, dust, flux and xygen enriched air enter the reactin shaft thrugh a central burner. Additinal heat fr cmbustin is prvided by three il burners psitined arund the cncentrate burner. il burners.8 m Cncentrate burner.8 m Reactin shaft Figure : Schematic diagram f gemetry.. m. m settler exit The slids react with the xygen in the reactin shaft, liquid metal and slag are depsited in the bath and gases and dust exit thrugh the settler. The gemetry was symmetrical abut its lngitudinal axis, and was mdelled as a symmetric half-slice. MATHEMATICAL MODEL FORMULATION The prblem was slved using CFX-.. Eulerian simulatin was used fr the gas phase, while Lagrangian particle tracking was used fr cncentrate particles and il drplets. Mdel features included:- Steady state slutin fr mmentum, energy and turbulence; Standard k-ε turbulence mdel (Launder and Spalding, 97); Radiatin using the Shah technique (Lckwd and Shah, 98); Oil drplets were simulated using a ttal f particle tracks; Cncentrate particles were represented by a ttal f particle tracks. Cncentrate Cmbustin Submdel CFX- did nt allw the simultaneus tracking f cncentrate particles and il drplets, s individual tracking rutines were develped by CSIRO Minerals. The cncentrate cmbustin sub-mdel assumed all slid material (cncentrate, flux and dust) entered the flw dmain in single cmpsite particles cntaining all cmpnents. The cmpsite particle wuld then underg a series f reactins, the speed f which depended n the temperature and cmpsitin f the particle, and the lcal gas cmpsitin. The sub-mdel was tailred t the Cpyright CSIRO Australia

2 chemistry f the ODO flash furnace feed (see Table and Table ) In summary:- Particles heat up t C withut reactin; Cvellite (CuS), chalcpyrite (CuFeS ) and pyrite (FeS ) decmpses at C; - Labile sulphur frm the decmpsitin reactins cmbusts with xygen t frm SO at sme distance frm the particle, releasing heat t the gas phase, and particles ignite; - Sme heat is transferred back t the particles. Since the reactins are endthermic, the decmpsitin rate is limited by this heat transfer; - The temperature remains cnstant at C until all CuS, CuFeS and FeS has decmpsed; Particles cntinue t heat up t 7 C; At 7 C the brnite (Cu FeS ), digenite (Cu.7 S) and CuSO, and CuFe O frm the dust are assumed t decmpse. Again liberated sulphur reacts with xygen t frm SO, and the temperature remains cnstant at 7 C until the reactins are cmplete; Particles cntinue t heat up t 8 C with n further reactin; At 8 C the remaining minerals either react r decmpse further; - In this case the reactin rate is cntrlled by xygen diffusin t the particle surface; - The verall reactin is exthermic; - FeO and Cu are prduced in the particle, with sulphur dixide entering the gas phase. Once all xygen in cnsumed, the reactins cease. As a particle underges these reactins its mass and diameter are changing. The relatinship between particle temperature and the fractin f sulphur remved frm the particle is shwn schematically in Figure. Cncentrate Dust Flux Average Lw High Cu:S Cu (wt%) S (wt%) Fe (wt%) SiO (wt%) Al O (wt%)... CaO (wt%)... U O 8 (g/t) Other (wt%).. Table : Nminal feed cmpsitins. Cncentrate (wt%) Average Lw High Chalcpyrite CuFeS Brnite Cu FeS Chalccite Cu S.. 7. Cvellite CuS.8.. Pyrite FeS Hematite Fe O... Silica SiO... Cpper Cu... Lime CaO... Alumina Al O... Table : Mineralgical Cmpsitin f Cncentrate. Oil Cmbustin Submdel The cding f the il cmbustin submdel utilised the eddy break-up mdel resident in CFX, but allwed simultaneus calculatin f bth il and cncentrate cmbustin (Kh 99). Significant pints t nte are :- Temperature (deg C) 8 7 Decmpsitin at 7 C Decmpsitin at C Reactins abve 8 C Time (r distance frm burner) Temperature Extent f sulphur remval Figure : Idealised bahaviur f the cmpsite particle during cmbustin. The il vapur reactin rate is determined by the chemical kinetics in cnjunctin with the eddy-breakup cmbustin mdel; The frward reactin is mdelled; The rate cnstant is determined frm the Arrhenius equatin; All heat frm the reactin is released t the furnace in the prducts f cmbustin and by radiatin and cnvectin. Bundary Cnditins Inlets The cncentrate and il burners were mdelled as Dirichlet bundaries. Velcity and temperature cnditins are specified in Table. Turbulence quantities were estimated at the inlets using empirical functins f the inlet hydraulic diameter. Such estimates are valid fr small inlets int a large flw dmain. Input stream Inlet Flw Rate O cnc. Temp. ( C) Cncentrate Cnc chute 7 kg/hr - Dust Cnc chute 7 kg/hr - Flux Cnc chute kg/hr - Chute gas Cnc chute. Cmbustin gas Cmb entry 8 Nm /hr. Oil Oil burner 8 l/hr/burner - Oil cmbustin gas Oil burner. Nm /hr/burner.. Table : Nminal inlet cnditins t main flw streams. Walls All slid surfaces such as the shaft wall and rf were mdelled as n-slip wall bundaries. The bath surface was als mdelled as a wall bundary, since the mtin f the bath was nt significant cmpared t the gas flw. The shaft rf temperature was specified as C, while the shaft and settler walls and bath surface had a temperature f C. Sme surfaces f the cncentrate burner had a zer heat flux bundary cnditin, while the cled cmpnents were held at a cnstant temperature f C. Extent f sulphur remval

3 Outlet Only ne utlet bundary was present in the gemetry, which crrespnded t the utflw thrugh the settler. A cnstant pressure bundary cnditin was specified s that all gradients perpendicular t the bundary were zer. RESULTS The cnditins under which simulatins were perfrmed are summarised in Table. Runs and use the same cnditins summarised in Table, while Runs and use prprtinately lwer and higher rates f flw. xygen in the gas stream reacts with the cncentrate and generates SO. Sufficient xygen is supplied t the furnace t allw sulphur in the cncentrate t cmpletely cmbust. The far field xygen cncentratin is zer indicating that all xygen has been cnsumed and all sulphur has therefre been remved frm the cncentrate (Figure (f)). R u n Slids rate (tph) cns/flux/ dust Cns Tt O flw rate (Nrmal m /hr) Oil rate (l/hr/ burner) Ptcle size (µm) 7/7/ Ave 7 8 7/7/ Ave 7 8 // High 887 //9 Lw 7 Table : Inlet cnditins fr the CFD runs. Nminal Flw Cnditins: Run Reactin Shaft Flw Patterns Figure shws the velcity vectr plt fr Run in the symmetry plane. The central jet is shwn t spread nly marginally as it travels dwn the reactin shaft, and recirculatin znes are established n bth sides f the jet. There is a slight displacement f the jet twards the settler, due t the flw exiting the shaft in that directin. The vectr plt als shws the lcatin f the il burner in the symmetry plane, which jins the central jet f cncentrate apprximately - m dwn the reactin shaft. Figure shws a single slice thrugh the reactin shaft,.8 m belw the shaft rf. In this figure the hrizntal and cmpnent f velcity is shwn using vectrs, while regins f rising and descending flw are shwn using clured cnturs. At this level in the shaft gases rise up the wall ppsite the setter (left), then flw acrss the upper shaft regin (black vectrs) and descend slightly n the shaft wall nearest the settler (right). The flames frm the tw ff-centre il burners have been pushed in the directin f the settler by the bulk gas in the shaft, leading t the asymmetric lcatin f the plume. Figure shws an issurface f cnstant velcity in the reactin shaft that further demnstrates this behaviur. Temperature and Gas Cmpsitin Distributins Figure shws the distributins f temperature and O in the symmetry plane, as well as the predicted particulate behaviur. Figure (a) shws the temperature distributin in the furnace, and bth the cncentrate plume and il jet is well defined. The il flame is the httest part f the furnace (8 C) while the centre f the cncentrate jet remains relatively cl until it appraches the bttm f the reactin shaft. Cncentrate therefre heats up (and reacts) frm the utside f the jet. The cl regin narrws as the bath surface is apprached. Hwever, it is predicted that temperatures belw C prevail dwn the reactin shaft. The vlume cncentratin f O is shwn in Figure (b). At the gas inlet the xygen vlume fractin is.. The Figure : Velcity field in symmetry plane fr Run. Vertical velcity Oil burner > m/s - - Red regins are - ascending flw -8 < - m/s Hrizntal Velcity represented by vectrs Oil burner Figure : Full slice thrugh the reactin shaft.8 m belw the shaft rf, Run. Oil burner flames C 8 C Directin f settler Figure : Issurface f cnstant speed (7 m/s) clured by temperature. Run. Figure (c) shws f the cncentrate particle tracks, clur-cded with the degree f sulphur remved frm the particle. The cncentrate particles nly start t react as they mve ut f the cl regin f the central

4 plume. Als, all f the particles end up cmpletely reacted (red). Several f the particle tracks cntinue ut the settler (as dust), while sme recirculate within the reactin shaft. Calculatin f the particle tracks tk the greatest amunt f cmputatinal time, and hence a time limit f. s was impsed n each track. Therefre, much f the particle recirculatin in the shaft is nt shwn. (a) Temperature ( C) (b) Vlume fractin f O (c) Particle tracks, shwing sulphur remval (a) Temperature ( C), µ m particles (Run ) C 8 C (d) Particle Residence Time....8 (e) Particle Temperature 8 Particle Temperature ( C) (f) Overall Degree f Sulphur Remved Sulphur Remved (-) Figure : Temperature, and O distributins fr Run, tgether with cncentrate particle characteristics. C 8 C µ m particles (Run ) (b) Overall Degree f Sulphur Remved µ m particles (Run ) Sulphur Remved (-) Figure 7: Temperature distributins and degree f sulphur remval fr Runs and. Figure (d) and (e) shw characteristics f a selectin f particle tracks as they travel dwn the reactin shaft. On each graph there are tw hrizntal dashed lines. The line at m frm the shaft rf represents the shaft rf itself, while the line at.8 m frm the shaft rf represents the settler rf. The graphs end.8 m frm the shaft rf, which is the level f the bath surface. Figure (d) shws the particle elapsed time in the reactin shaft. Mst particles have reached the bath surface within. and. s. Figure (e) shws the temperature f particles as they travel dwn the reactin shaft. Each particle heats up relatively quickly nce it starts t cmbust, and individual particles are shwn t cmpletely react within abut m nce they cmmence reactin. Hwever, the distributin f particle temperatures further indicates that it is the utside particles that react and nt until they have reacted d the innermst particles get the pprtunity t react. Figure (f) shws the average remval f sulphur frm all cncentrate particles. The ttal sulphur remved is predicted t be greater than 99%. Hwever, this predictin is based n the assumptin f cncentrate particles having a cnstant initial diameter f µm. The presence f larger particles will reduce the degree f reactin. Cnversely, smaller particles will react faster than predicted. Variatin in Assumed Cmpsite Particle Size: Run The nminal cnditins mdelled in Run assumed the cmpsite particle t have a cnstant size f µm. Little is knwn abut the true particle size distributin f the slids feed and it is believed that much f the cncentrate is agglmerated within the reactin shaft as it descends (Debrincat et al., ; Debrincat, ). Hwever, since the majrity f the primary particles are less than µm in diameter, it was decided t investigate the effect f decreasing the particle size t a cnstant value f µm. The results are shwn in Figure 7. Figure 7 (a) shws the temperature distributin fr Run. The cncentrate plume is substantially shrter when using an assumed cmpsite particle size f µm (c.f. Figure (a)), suggesting that the smaller particles are reacting faster. The lw thermal mass f the smaller particles allws them t heat up and ignite quickly, while their greater surface area als increases reactin rate. The extent f reactin is shwn in Figure 7 (b) fr bth Runs and. The µm particles allw the cmplete remval f sulphur frm the cncentrate within. m f the shaft rf, which is cnsiderably faster than fr the µm particles. Since the results predicted by the mdel are dependent n cncentrate particle size and the actual size distributin is nt accurately knwn, all data must be used fr bserving variatinal trends rather than abslute predictins f burner/shaft behaviur. Variatin in Feed Rate: Runs and Runs and investigated the effect f varying the feed rate f cncentrate int the reactin shaft. As the rate f cncentrate was changed, s t was the cncentrate. At the lwer cncentrate rate a greater quantity f heat was required and cnsequently the flw rates t the il burners were increased (Table ). At the high cncentrate rate the reactin was autgenus and the il burners were turned ff. Als, the xygen flw rate thrugh the cncentrate burner was adjusted t allw cmplete cmbustin f the cncentrate. The temperature distributins in the reactin shaft fr Runs, and are shwn in Figure 8 (a) (c). The temperature plume with tph cncentrate (Run, Figure 8 (a)) is substantially smaller than fr 7 tph cncentrate (Run, Figure 8 (b)). Als, the il flame is htter and lnger, and the reactin shaft is slightly higher in temperature. The temperature prfile fr tph cncentrate (Run, Figure 8 (c)) is cnsiderably cler and the cncentrate plume jets rapidly t the bath surface.

5 The mdel predicts the cmbustin gas t jet twards the bath surface withut heating up abve C. Such behaviur may cause an accumulatin f slid cncentrate n the bath surface, and even lcal freezing f the bath. (a) tph (Run ) (b) 7 tph (Run ) (d) tph (Run ) (e) 7 tph (Run ) (f) tph (Run )....8 (c) tph (Run ) Figure 8: Temperature distributins and particle residence times fr Runs, and. tph (Run ) 7 tph (Run ) tph (Run )....8 Sulphur Remved (-) C 8 C 8 C Figure 9: Extent f reactin fr Runs, and. The extent f reactin as a functin f distance belw the shaft rf is shwn in Figure 9 fr the three cncentrate feed rates, and the pr extent f reactin at tph is shwn. At tph cncentrate rate the entire feed is fully reacted by apprximately m belw the shaft rf. Fr the nminal flw rate f 7 tph the cncentrate can nearly fully react by the time the cncentrate reaches the bath surface. Hwever, at tph cncentrate less than % f the cncentrate has reacted befre hitting the bath surface. Figure 8 (d) - (f) shws the residence time f particles under the three cnditins f cncentrate feed rate. There is nly a small difference in residence time between the tph and 7 tph feed rates, each being apprximately.. s. Hwever, at tph the residence time f mst particles is significantly smaller at.. s. This result suggests that the initial velcity f the cmbustin gas, as well as the flw rate f gas and slids, cntrls the residence time f the cncentrate particles. DISCUSSION Variatin in Cncentrate Particle Size The cncentrate particle size has a significant effect n the rate f reactin ccurring in the reactin shaft. This is because the reactin rate is dependent n the surface area available fr heat transfer and reactin between gas and slid, and small particles have a significantly greater surface area per unit mass f cncentrate. Althugh particle size distributins can be measured fr a given sample f cncentrate, the distributin will depend greatly n the cnditins under which it is measured. Furthermre, the presence f flux and dust in the feed will alter the distributin, especially flux which has an average particle size ne rder f magnitude greater than the cncentrate and dust. Jrgensen (987) has shwn that agglmerates may exist in the shaft due t pr dispersin f cncentrate, while Debrincat et al. () and Debrincat () have demnstrated the high prpensity f flash furnace cncentrate t resist dispersin even in a highly turbulent dispersing envirnment. Thus it is difficult t give an estimate f the particle size distributin f the feed stream entering the reactin shaft, r indeed specify hw that distributin may vary in the shaft. Anther unknwn in the estimatin f particle size distributin is the effect f the assumed distributin n the destinatin f the cncentrate particles. If all particles are assumed t be extremely small then they will likely be carried ut f the furnace in the ff-gases, even if they are cmpletely reacted. At this stage the distributin f cncentrate particle size in the reactin shaft it is prbably the biggest unknwn in the flash furnace mdelling prblem, and given the number f variables gverning its value, a cnstant value fr cmpsite particle size f µm was cnsidered the best apprach. By decreasing this t µm it was pssible t gauge the amunt f change the assumed particle size has n the predicted results. As shwn in Figure 7, the size f the cmbustin regin beneath the burner was significantly reduced when a particle size f µm was assumed. Als, the same quantity f cncentrate was fully reacted apprximately. m further up the reactin shaft when assuming the smaller particle size. These results give an indicatin f hw sensitive the predicted furnace behaviur is t the assumed cmpsite particle size, and it is recmmended that plant measurements be taken in rder t tune this parameter s that quantitative results can be btained. Variatin in Cncentrate Feed Rate Runs and demnstrated the effect f varying the cncentrate feed rate. At the lw feed rate it was pssible fr all the sulphur t be remved frm the cncentrate within the reactin shaft (Figure 9). The results indicate that cncentrate is nt spread mre efficiently at lwer

6 flw rates and the gas-slid cntact is apprximately the same fr all three runs. Hwever, the residence time f cncentrate is greater at the lwer feed rates (Figure 8), which wuld allw particles mre time t fully react. The predicted behaviur fr Run suggests that the furnace wuld nt run at feed rates f tph. Furthermre, at 7 tph cncentrate feed rate there is nly just enugh height in the shaft t allw full sulphur remval, suggesting that any increase in feed rate will result in decreased efficiency. One aspect f the high feed rate case (Run ) that may limit perfrmance is that the gas velcity thrugh the burner had t be increased t m/s (frm m/s) t allw enugh gas int the furnace. Figure 8 (f) shws that the high velcity reduces the residence time f the cncentrate, which cnsequently limits the extent f reactin. Mdel Limitatins As with all mathematical mdels, the predictins f the mdel are nly as gd as the assumptins made in mdelling the prcess. In the current wrk several limitatins were identified with the methds used, the mst significant f which is the assumed cmpsite particle size distributin (which has already been discussed). Other pints t cnsider are: Wall Bundary Cnditins. All furnace walls were assumed t have cnstant temperatures. In sme simulatins (Figure 8 (c), fr example) the furnace and settler walls are clearly at a higher temperature than the gas, and the assumptin f cnstant temperature walls is invalid. A better mdelling apprach wuld be t use furnace jacket heat lsses t specify a heat flux bundary cnditin at the wall. Bath Surface Bundary Cnditins. Similar prblems ccurred frm specifying the bath surface t be at a cnstant temperature f C. Again Figure 8 (c) prvides an extreme example where cl gases (apprximately C) frm the cncentrate burner are predicted t jet nt the bath surface, but the gas temperature artificially elevates as it appraches the bath surface. An accurate mdel f the transfer f heat away frm this lcalised surface wuld be cmplex. High Slids Lading Arund the Burner. The slid particles are tracked thrugh the gas using a Lagrangian apprach. One assumptin f this apprach is that the cncentratin f slids is negligible. Hwever, in the high slids regin and arund the slids chute this assumptin is nt valid. It was therefre nt pssible t simulate the flw within the slids chute, and the initial velcity f slids at the exit was determined experimentally. CONCULSIONS The CSIRO flash furnace cncentrate cmbustin submdel has been successfully applied t the cpper flash furnace at ODO. The flw field and reactins ccurring within the reactin shaft have been simulated. The fllwing bservatins were made: Flw patterns under nminal perating cnditins f 7 tph cncentrate are nt axisymmetric in the reactin shaft. The central jet is displaced tward the settler exit, as are the flames frm the il burners. The regin f cncentrate cmbustin is cnfined t the central jet f cmbustin gas, and is kept well away frm shaft walls. The predicted reactin behaviur is strngly affected by the slids particle size that is assumed. Assuming a cmpsite particle size is cnstant at µm: Slids feed rate greatly affects the perfrmance f the furnace, bth in terms f degree f sulphur remval and furnace temperature. At a cncentrate feed rate f tph all sulphur is predicted t be remved frm the cncentrate, whereas at tph nly % f the sulphur is remved and plume temperatures drp. Large quantities f unreacted cncentrate and cl gas jet nt the surface f the bath. The furnace height is just adequate t allw full remval f sulphur at a feed rate f 7 tph, and any increase in feed rate will lead t decreased burner/shaft perfrmance. ACKNOWLEDGMENTS The assistance f Phil Schwarz (CSIRO Minerals), and Greg Hill, Simn Cmrlec and Ilija Sutal (ODO) is gratefully acknwledged. Financial supprt fr this wrk was prvided by WMC Resurces Ltd. Permissin t publish this wrk was granted by CSIRO Minerals and ODO, and is gratefully acknwledged. REFERENCES DEBRINCAT, D.P.,. Disintegratin f Pwder Agglmerates in a Flash Furnace Shaft, PhD Thesis, University f Melburne, Melburne, Vic., Australia. DEBRINCAT, D.P., SOLNORDAL, C.B., VAN DEVENTER, J., JORGENSEN, F.R.A. and KOH, P.T.L.,. Twards Understanding the in Situ Agglmeratin f Nickel Cncentrate Pwder During Flash Furnace Injectin, Prceedings st Asian Particle Technlgy Sympsium, Bangkk, Thailand, - December. JORGENSEN, F.R.A., 987. Investigatin f Shaft Reactins in Nickel Flash Smelting at. kg/min Scale, Pyrmetallurgy 87, - Sept. 987, Institutin f Mining and Metallurgy, Lndn, UK, pp KOH, P.T.L. and JORGENSEN, F.R.A, (99), Mdelling Particulate Flw and Cmbustin in a Flash Smelter, In CHEMECA 9, Prceedings f the nd Australian Chemical Engineering Cnference, Perth, WA, Australia, pp KOH, P.T.L., NGUYEN, T.V. and JORGENSEN, F.R.A, (997), Numerical Mdelling f Cmbustin in a Zinc Flash Smelter, In Prc. Int. Cnf. On CFD in Mineral and Metal Prcessing and Pwer Generatin, Melburne, Vic., Australia, - July 997, pp. -. LAUNDER, B.E. and SPALDING, D.B., 97. The Numerical Cmputatin f Turbulent Flws. Cmp. Meths. Appl. Mech. Engng.,, pp LOCKWOOD, F.C. and SHAH, N.G., 98. A New Radiatin Slutin Methd fr Incrpratin in General Cmbustin Predictins Prcedures. Prceedings f the 8 th Sympsium (Int.) n Cmbustin, The Cmbustin Institute, pp. -.