Investigation of Heat Transfer and Gasification of Two Different Fuel Injectors in an Entrained Flow Gasifier

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1 Proceedngs of the 25th Internatonal Pttsburgh Coal Conference, Pttsburgh, USA, September 29-October 2, 2008 Investgaton of Heat Transfer and Gasfcaton of Two Dfferent Fuel Inectors n an Entraned Flow Gasfer Tng Wang and Armn Slaen Energy Converson & Conservaton Center Unversty of New Orleans New Orleans, Lousana, USA Heng-Wen Hsu and Cheng-Hsen Shen Energy and Resources Laboratory Industral Technology Research Insttute Tawan, R.O.C. ASTRACT One of the problems frequently encountered n a coal gasfer operaton s fuel nector falure. Operatng n etreme hgh pressure and hgh temperature, the typcal fuel nector lfe span s 6 to 12 months. Numercal smulatons are performed to study the flow and temperature felds n the vcnty of the nector tp and the metal temperature of two dfferent fuel nector desgns -- one wth concal-nozzle tp and the other wth a blunt tp -- n a dry-fed, entraned-flow coal gasfer. The complete 3-D Naver-Stokes equatons are solved. The smulaton models the gasfcaton process wth three global heterogeneous reactons and three homogeneous reactons, ncludng volatle combuston. The results show the two dfferent nectors gve very dfferent temperature and speces dstrbutons nsde the gasfer. In the gasfer wth the concal nector tp, the hghest temperature nsde the gasfer occurs at the center of the gasfer; whereas, n the gasfer wth the blunt-tp nector, the hghest temperature occurs near the wall. There s a potental of flash back combuston n the nozzle at the tp of the concal nector due to ts premng feature of fuel and odant n the nozzle. The hghest temperatures on both nectors are the same, whch s around 1600K. However, the hghest temperature on the concal-tp nector s concentrated at one locaton wth an etended regon of 30 mm between 1600K and 1100K; whereas on the blunt-tp nector hot spots scattered and the hot regon (1600K K) only etends about 3 mm. Epermental results support smulated results and has demonstrated a short lfe of the concal-tp fuel nector and much etended lfe for the blunt-tp fuel nector. 1.0 INTRODUCTION Fuel nectors n a gasfer operate n a very harsh envronment. Etreme hgh pressure and hot temperatures often cause the nectors to fal n a short perod of tme, typcally n 6 to 12 months. Each occurrence of a faled nector has caused an unwanted operaton nterrupton that results n undesred epenses and reduced proft margn. Improvement of the operatng lfe of fuel nectors has been dentfed as one of the most mportant research goals n the gasfer ndustry. ITRI's demonstraton gasfer has faced ths smlar problem and has modfed ts fuel nectors n an attempt to lengthen the nector lfe epectancy and mnmze the fuel nector falure problems. The obectves of ths study are to (a) use CFD to analyze the flow and temperature felds n the vcnty of the nector tp and the metal temperature of two dfferent nectors and (b) conduct eperments to verfy the CFD results. Fgures 1 and 2 show the schematcs of fuel nectors before and after modfcaton. The concal nector conssts of two concentrc ppes. Fuel (dry pulverzed coal) along wth the transportng medum, ntrogen, s nected through the center ppe, whle odant (95% O 2 and 5% N 2 ) s nected through the outer ppe. The concal-nozzle desgn of the nector allows the fuel and odant to prem before enterng the gasfer. Due to the operatonal falure of ths desgn, the concal tp of the nector was thought to be the culprt because t could become too hot and burned out; hence, on the second fuel nector the concal nozzle was removed as shown n Fg. 2. Wthout the nozzle, the modfed nector has a blunt tp. The outer ppe s sealed at the tp and small holes are drlled at the tp of the outer ppe to create a smlar pressure drop as before the modfcaton. Fuel enters the gasfer through the center hole, whle odant enters through the 8 small outer holes. In the modfed nector, the fuel and odant do not m before enterng the gasfer. CFD study has been performed to nvestgate the heat transfer on both nector desgns. In ths paper, the smulaton of the concal-nozzle nector desgn s referred as Case 1, and the smulaton of the blunt-tp nector desgn s referred as Case COMPUTATIONAL MODEL 2.1 Physcal Characterstcs of the Model and Assumptons The physcal characterstcs of the model are: 1. Three-dmensonal 2. Buoyancy force and radaton are consdered

2 3. Varyng flud propertes 4. Impermeable walls Fgure 1 Schematc of the orgnal fuel nector desgn (scale: mm). Fgure 2 Schematc of new fuel nector desgn (scale: mm). The followng general assumptons are made n ths study: 1. The flow s steady. 2. No-slp condton (zero velocty) s mposed on wall surfaces. 3. Chemcal reacton s faster than the tme scale of the turbulence eddes: eddy dsspaton model s adopted. 4. Walls are nsulated (.e. adabatc). 5. Slaggng s not consdered. 2.2 Governng Equatons The equatons for conservaton laws of mass, momentum, and energy are gven as: ( ρu ) = Sm P ( ρuu ) = ρ ( τ ρu u + ) + F (1) g (2) T ( ρc put) = ρcp ut + μφ + Sh λ (3) The stress tensor τ s gven by u u 2 u k τ = + μ δ. (4) 3 k 2.3 Turbulence Model The standard k-ε turbulence model s used n ths smulaton due to ts sutablty for a wde range of wallbound and free-shear flows. The standard k-ε turbulence model s robust, economc for computaton, and accurate for a wde range of turbulent flows. The turbulence knetc energy, k, and ts rate of dsspatons, ε, are calculated from the equatons from the paper by Lauder and Spaldng [1972]. Both buoyancy and mnor compressblty effects on the turbulence model are consdered. The turbulence models are vald for the turbulent core flows,.e. the flow n the regons not n the mmedate promty of the wall. The flow very near the walls s affected by the presence of the walls. In the vscous sublayer, where the soluton varables change most rapdly, s not solved n ths study. Instead, wall functons, whch are a collecton of sem-emprcal formulas and functons, are employed to connect the vscosty-affected regon between the wall and the fullyturbulent regon [Lauder and Spaldng, 1974]. The wall functons consst of () laws-of-the-wall for mean velocty and temperature (or other scalars) and () formulas for near-wall turbulent quanttes. The standard wall functons for velocty, temperature, and speces are employed n ths study. See Slaen and Wang [2005 and 2006] for detals. 2.4 Radaton Model The P-1 radaton model s used to calculate the flu of the radaton at the nsde walls of the gasfer. The P- 1 radaton model s the smplest case of the more general P-N radaton model that s based on the epanson of the radaton ntensty I. The P-1 model requres only a lttle CPU demand and can easly be appled to varous complcated geometres. It s sutable for applcatons where the optcal thckness al s large where a s the absorpton coeffcent, and L s the length scale of the doman. See FLUENT user gude for detals [2007]. 2

3 2.5 Gasfcaton Process and Reacton Model The global chemcal reactons of coal gasfcaton [Smoot and Smth, 1985] can be generalzed as below: Heterogeneous (sold and gas) phase: C(s) + ½ O 2 CO, ΔH R = MJ/kmol (R1.1) C(s) + CO 2 2CO, ΔH R = MJ/kmol (R1.2) (Gasfcaton,Boudouard reacton) C(s) + H 2 O(g) CO + H 2, ΔH R= MJ/kmol (Gasfcaton) (R1.3) C + 2H 2 CH 4 Homogenous gas phase: CO + ½ O 2 CO 2, ΔH o R = MJ/kmol (Drect methanaton) (R1.4) ΔH R = MJ/kmol (R1.5) CO + H2O(g) CO 2 + H 2, ΔH R = MJ/kmol (Water-shft) (R1.6) CO + 3H 2 CH 4 + H 2 O, VM + O 2 CO 2 + H 2 O (VM s assumed as CH O ) ΔH o R = MJ/kmol (Methanaton) (R1.7) (R1.8) Reactons gven n R1.1, R1.5, and R1.8 are three eothermc reactons that provde the complete energy for the gasfcaton. Based on these global reactons, appromately 22% of the stochometrc oygen s requred to provde suffcent energy for gasfcaton reactons. In real applcatons, 25~30% of the stochometrc oygen s provded to ensure hgheffcent carbon converson. Methanaton (R1.4 and R1.7) s assumed to be neglgble n ths study, so t s not ncluded n the computatonal model n ths study. The volatle matters (VM) are assumed as CH O whch has a medum heatng value. The deal operaton ntends to burn the volatle but not chars, so all the carbon could be used for gasfcaton, not for combuston. Partal combuston occurs when the VM and coal m wth oygen (R1.1 and 1.8). The energy released from (R1.1 and 1.8) also heats up any coal that has not burned. When the coal s heated wthout oygen, t undergoes pyrolyss durng whch volatles are released. At the same tme, char gasfcaton (R1.2) takes place and releases CO. If a sgnfcant amount of steam ests, gasfcaton (R1.3) and water shft reacton (R1.6) occur and release H 2. The epermental data shows methanaton s nsgnfcant n the studed gasfer, so reactons R1.6 and R1.7 are not ncluded n the smulaton. All of the speces are assumed to m n the molecular level. The chemcal reactons nsde the gasfer are modeled by calculatng the transport and mng of the chemcal speces by solvng the conservaton equatons that descrbe convecton, dffuson, and reacton of each component speces. The general form of the transport equaton for each speces s C ( ρuc ) = ρd ρuc + S. (5) In ths study, the eddy-dsspaton model s used. The assumpton n ths model s that the chemcal reacton s faster than the tme scale of the turbulence eddes. Thus, the reacton rate s determned by the turbulence mng of the speces. The sources term S n equaton (5) s calculated usng the eddy-dsspaton model based on the work of Magnussen and Hertager [1976]. The net rate of producton or destructon of speces as the result of reacton r, R,r, s gven by the smaller of the two epressons below ε Y R = ν MAρ mn (6) k ν M and ε ΣY R = ν MBρ mn (7) k Σν M where v s the stochometrc coeffcent of reactant and v s stochometrc coeffcent of product. In equatons (6) and (7), the chemcal reacton rate s governed by large-eddy mng tme scale, k/ε. The smaller of the two epressons, (6) and (7), s used because t s the lmtng value that determnes the reacton rate. In ths study, the sold partcles are assumed to be gasfed nstantaneously, so the eddydsspaton model can be appled. Ths nstantaneous sold-gasfcaton approach wll provde results between the fnte-rate reacton approach and chemcally equlbrum approach. The procedure to solve the reactons s as follows: 1. The net local producton or destructon of speces n each reacton (R1.1 to R1.5) s calculated by solvng equatons (6) and (7). 2. The smaller of these values s substtuted nto the correspondng speces transport equaton (5) to calculate the local speces mass fracton, C. 3

4 3. C s then used to calculate the net enthalpy producton of each reacton equaton. 4. The net enthalpy producton becomes the source term n energy equaton (3) that affects the temperature dstrbuton. In an endothermc process, the net enthalpy producton s negatve, whch becomes a snk term n the energy equaton. The CFD model for each of the fuel nector desgn s dvded nto two steps: (1) calculate the flud mechancs and reactons nsde the entre gasfer and (2) use the calculated flow and thermal results as the boundary condton to calculate the heat transfer around and wthn the nector tself. Other than to provde boundary condtons, the purpose of performng step (1) s to nvestgate the effect of fuel nector desgns on the gasfcaton process. Ths wll be eplaned n the net secton. Table 1 Mass flow rates and mass fractons at each nlet Center ppe Outer ppe Mass flow rate, (kg/s) Mass fracton: C O H2O H volatle N Outlet 2.6 Computatonal Domans and Boundary Condtons The meshed computatonal doman for Case 1 s shown n Fg. 3. The slag tap and the quenchng secton are not ncluded n the calculaton doman because we are nterested n the regon near the nectors. Fner meshes are created near the nectors and coarser mesh s used towards the bottom and the top (et). The computatonal doman ncludes a small part of the nector whch s outlned wth red lnes. Fgure 4 shows the computatonal doman for the nector heat transfer smulaton. Temperature dstrbutons on the nector outsde/tp walls and at the mouth of the nector hole obtaned n the entre gasfer flow computaton n step 1 s used as the boundary condtons n step 2, as shown n Fg. 4. A total of 2,266,187 mesh cells are used for gasfer smulaton and 890,556 mesh cells are used for nector heat transfer calculaton. Inlets Inector Coal + N 2 O 2 + N 2 Fgure 3 Meshed computatonal doman for entre gasfer smulaton wth the concal nectors (Case 1). The meshed computatonal domans for Case 2 are presented n Fgs. 5 and 6. Smlar to Case 1 wth the concal-tp fuel nector, the temperature dstrbutons on the blunt-tp nector's wall and at the mouth of the nector holes n step 1 are used as boundary condtons n step 2. Inector hole wall (adabatc) Constant temperature wall (from step 1) 2.7 Inlet Condtons The mass flow rates of coal and odant at each nector s shown n Table1. Coal + N 2 O 2 + N 2 Inector ppe base (adabatc wall) Outlet Fgure 4 Meshed computatonal doman for heat transfer smulaton wthn the concal fuel nector (Case 1). 4

5 Inlets Outlet O 2 + N 2 Coal + N COMPUTATIONAL RESULTS Gas temperature and speces dstrbutons nsde the gasfer at the nector level for Case 1 are presented n Fg. 7. In the temperature dstrbuton, the blue regons are located at the nsde of the nectors, where the fuel and odant m before beng nected nto the gasfer. In eddy-dsspaton combuston model, whch was used n ths study, reacton occurs as soon as fuel and odant m. Ths created a problem durng the ntal stage of ths smulaton. The ump n temperature combned wth the contracton of the flow path caused the calculaton to dverge. To resolve ths problem, the reacton calculaton nsde the nector was removed so that the reacton calculaton only takes place after the fuel and odant enter the gasfer. Ths computaton dffculty mght also reflect n the real stuaton as flash back could occur n the nector nozzle when fuel and odant m. The hgh temperature generated by the flash-back combuston nsde the nozzle could have caused earler falure of the nectors wtnessed n the actual operaton. Fgure 5 Meshed computatonal doman for flud mechancs and reactons smulaton nsde gasfer for the blunt-tp nector desgn (Case 2). The temperature dstrbuton n Fg. 7 shows that there s a core of very hgh temperature at the center of the gasfer. A close-up vew of the nector s outsde surface and ts surroundng flow temperature s presented n Fg. 8 n two opposte sdes. The temperature s observed hgher on one sde of the nector than the other sde. Ths s due to the swrlng flow created by the tangental necton, whch pushes the hot ar to penetrate deeper nto one sde of the nector passage hole. Coal + N 2 Inector hole wall (adabatc) O 2 + N 2 Inector ppe base (adabatc wall) Constant temperature wall (from step 1) Outlets The contour of fed C n Fg. 7 ndcates that C s mmedately reacted. Ths s due to the fact that the fuel and the odant have already premed n the convergng nozzle of the nector and C undergoes the eothermc reacton C + ½ O 2 CO. The CO and CO 2 dstrbutons presented n Fg. 7 show that the dstrbutons of CO and CO 2 are almost opposte to each other. CO fracton s hghest when the CO 2 fracton s lowest. There s no CO n the center of the gasfer; on the other hand, the CO 2 s hghest n the center of the gasfer. At the same tme, the temperature s hghest at the center of gasfer. Ths s due to the eothermc reacton CO + ½ O 2 CO 2. The volatles are reacted very quckly, whch wll provde energy for gasfcaton. Hydrogen s rchly produced up to appromately 40% by moles at ths stage. Fgure 6 Meshed calculaton doman for heat transfer smulaton wthn the blunt-tp fuel nector (Case 2). 5

6 Fgure 7 Temperature and speces dstrbutons on the horzontal fuel necton plane for Case 1. 6

7 nector decreases to 1100K wthn 30 mm. Ths s suffcent to melt the nector tp. Fgure 10 presents temperature dstrbuton on the outsde wall of the nector. It shows the locaton where the hghest temperature occurs on one sde of the nector. Ths hghest-temperature locaton s the sde where the hot temperature hts from the adacent nector as mentoned earler. Horzontal center plane Vertcal center plane Fgure 8 Temperature dstrbuton on the nector outsde surface and ts surroundng flow for Case 1 seen from two opposte sdes. Due to the tangental flow necton from the adacent nector, one sde has hgher temperature than the other sde. Ths s the result of the gasfcaton smulaton n the entre gasfer n step 1. The nector heat transfer study result s shown n Fg. 9. In ths smulaton, any potental flashback combuston n the nozzle s removed. The heat source s assumed to be entrely located outsde the nector as beng calculated n step 1. Temperature whch was obtaned from the step 1 of the smulaton mentoned earler s mposed on the outsde wall of the convergng part of the nector at constant values. Ths constant temperature boundary condton allows thermal energy to transfer nto step 2 computatonal doman wthout a lmt, lke a constant temperature reservor. The fuel and odant movng nsde the ppes actually act as coolants to carry away the heat conducted through the ppe walls. The gas n the narrow space between the outer ppe wall and the gasfer s almost statonary. The heat transfer n ths ar layer s domnated by heat conducton mode. Fgure 9 shows the temperature dstrbutons on the horzontal and vertcal center planes of the computatonal doman, whch nclude both the flud and sold regons. The temperature calculated n the nector shows a very hot temperature, around 1600 K, near the tp of the nector. The hot temperature on the surface of the Fgure 9 Temperature dstrbuton on the horzontal and vertcal center planes of the nector for Case 1. Ths s the result of step 2 focusng on the entre nector. Fgure 10 Temperature dstrbuton on the nector outsde wall for Case 1. 7

8 Fgure 11 Temperature and speces dstrbutons on the horzontal fuel necton plane for Case 2. 8

9 The results of blunt-tp nector (Case 2) are gven n Fg. 11. The dstrbuton of fed carbon (C) shows that, very dfferent from Case 1, C s not mmedately consumed (reacted) because t s not premed nsde the nector as n Case 1. It s noted that O 2 nected facng the center sde of the nector s mmedately consumed and produces CO, whle CO 2 s produced n the near wall regon. Judgng from the hgh temperature (near 3600K), hgh CO 2 and H 2, and low O 2 and volatles n the near wall regon, t can be concluded that two eothermc reactons, whch are gasfcaton of volatles (VM + O 2 CO + H 2 ) and CO + ½O 2 CO 2, preval n the nearwall regon and produces a lot of heat there. As soon as CO s produced by the gasfcaton of volatles, t s burned by O 2 to produce CO 2. Producton of hydrogen s not as much as n Case 1. Healthy amount of H 2 and CO are produced at ths stage wth H 2. An enlarged velocty vector plot supermposed wth temperature n color s shown n Fg. 12. Interacton of nected et flow and the tangental momentum from the adacent nector can be clearly seen n ths fgure. n Case 2 s less lkely to fal due to the etreme temperature compared to the nector n Case 1. Et gas temperature and compostons for both Cases 1 and 2 are tabulated n Table 2. Even though the temperature and speces dstrbutons at the necton level seem very dfferent, the et syngas for both cases are pretty close to each other. Fgure 13 Temperature dstrbuton on outsde wall of nector for Case 2. Horzontal center plane Fgure 12 Velocty vectors and temperature dstrbutons on the horzontal fuel necton plane for Case 2. Fg. 13 presents the temperature contour on the outsde wall of one of the nectors for Case 2. Agan, ths temperature contour was obtaned from the gasfcaton smulaton of the entre gasfer. It was then used as the wall boundary condton at the nector tp. Smlar to Case 1, the hghest temperature observed s around 1600 K. However, ths hgh temperature seems to be scatterngly dstrbuted, as opposed to concentrated at one locaton n Case 1. Furthermore, Fg. 14 shows the hgh temperature quckly decays to 1000 K wthn 2 mm (vs. 30 mm n Case 1). Ths means that the nector Vertcal center plane Fgure 14 Temperature dstrbuton on the horzontal and vertcal center planes of the nector for Case 2. 9

10 Table 2 Gasfer et gas temperature and compostons for Cases 1 and 2 Parameters Case 1 Case 2 Et temperature, K Carbon fuel converson 72% 86% effcency, % Fuel converson effcency, % Components at et: 73% 80% Mole Mole no. Mole Mole no. fracton (mole) fracton (mole) CO 41.9% % 0.30 H % % 0.23 CO % % 0.08 VM 0.7% % 0.05 H 2 O 0.0% % 0.00 N % % 0.09 C 0.0% % 0.00 Heatng value (MJ/kg) Fgure 15 Eteror wall of the gasfer usng fuel nectors wth pre-med fuel nozzles. Red spot ndcates heat penetraton through worn-out brck caused by premed flash back combuston n the nozzle. 4.0 EXPERIMENT As shown n the schematc of the orgnal fuel nector n Fg. 1, odant, transport gas and coal powder enter the gasfcaton chamber n a premed condton. Ths premed type nector has not always been stable durng operaton. In test condtons, gasfer temperature reaches over 1400 C. Durng operaton, sometmes coal partcles have blocked the annular space n the nector, preventng contnuous feed of coal nto the gasfer. Flashback n the annular space of nector can potentally damage the nector. Shown n Fg. 15 s the eteror wall of the gasfer where the red spot above the nector ndcates the heat penetrates refractory nsulaton caused by the combuston of coal partcles nsde the nector. Fgure 16 shows fuel nectors that faled due to very hgh operatng temperature. It was speculated that the hgh temperature n the nector was caused by flash back combuston due to premed combuston nsde the concal nozzle. To remove ths premed desgn, the nozzle has been cut off as shown n Fg. 17. The revsed fuel nectors have performed stably and relably for several months at 15 bars wthout any complcaton. However, after a long perod of testng, the thckness of the refractory-lner wall has been worn away and became thnner than the orgnal contour of gasfer nsde wall. The nozzle tp eventually protruded over the worn-out refractory-lner and s eposed to the etreme hgh temperature n the gasfer. As a result, the nector tps were eventually burned out as shown n Fg. 18. Based on the test eperence, t s suggested that the length of coal nector should be less than the thckness of refractory-lner by at least 10 mm. Ths wll help ensure the gasfer wll operate at a safe, contnuous and stable condton. Fgure 16 Burned out concal-tp fuel nectors. Fgure 17 Blunt-tp (non-premed) coal nector. Outer eght holes transport oygen and the nner hole transports coal powder and ntrogen. 10

11 Kuo, K. Y., 1986, Prncples of Combuston, John Wley and Sons, New York. Lauder, B.E., and Spaldng, D.B., Lectures n Mathematcal Modelng of Turbulence, Academc Press, London, England, Lauder, B.E., and Spaldng, D.B., The Numercal Computaton of Turbulent Flows, Computer Methods n Appled Mechancs and Engneerng, 3: , Magnussen, B.F., and Hertager, "On mathematcal models of turbulent combuston wth specal emphass on soot formaton and combuston," 16th Symp. (Int l) on Combuston. The Combuston Insttute, Fgure 18 The rght blunt-tp nector shows t s n mnt condton after a short servce and the left nector shows a burned-out tp after etended servce when the refractory brck has been worn away and the fuel nector eventually protruded out from the wall wthout protecton. 5.0 CONCLUSIONS The two dfferent fuel nector desgns gve very dfferent temperature and speces dstrbutons nsde the gasfer. In Case 1, the hghest temperature nsde the gasfer occurs at the center of the gasfer; whereas, n Case 2, t occurs near the wall. Case 1 produces hgh H 2 and low CO; whereas, Case 2 produces both rch H 2 and CO. There s a potental of flash back combuston n the nozzle at the tp of the concal nector due to ts premng feature of fuel and odant n the nozzle. The hghest temperatures on both nectors are the same, around 1600 K. However, the hghest temperature on the concal-tp nector (Case 1) s concentrated at one locaton wth an etended regon of 30 mm between 1600 K and 1100 K; whereas on the blunt-tp nector (Case 2), the mamum temperature dstrbuton s scattered and the hot regon (1600K K) only etends about 3 mm. Therefore, the blunt-tp nector s less lkely to fal. Epermental results support smulated results and has demonstrated a short lfe for the concaltp fuel nector and an etended lfe for the blunt-tp fuel nector. However, eventually the blunt-tp fuel nector dd burn out after etended servce when the refractory brck was worn away and the fuel nector tp protruded from the wall wthout any protecton. Patankar, S.V., Numercal Heat Transfer and Flud Flow, McGraw Hll, Smoot, D.L., and Smth, P.J., Coal Combuston and Gasfcaton, Plenum Press, Wang, T., Slaen, A., Hsu, H. W., and Shen, C. H., "Effect of Slag Tap Sze on Gasfcaton Performance and Heat Losses n a Quench-type Coal," Paper 37-4, presented at the 24th Internatonal Pttsburgh Coal-Gen Conference, Johannesburg, South Afrca, Sept , Wang,T., Slaen, A., Hsu, H. W., and Lo, M. C., " Part- Load Smulaton and Eperments of a Small Coal Gasfer," Paper 20-3, presented at the 23rd Internatonal Pttsburgh Coal-Gen Conference, Pttsburgh, Pennsylvana, Sept , REFERENCES FLUENT User s Gude, February