COMPARISON OF UCG CAVITY GROWTH WITH CFD MODEL PREDICTIONS

Seveth Iteratioal Coferece o CFD i the Mierals ad Process Idustries CSIRO, Melboure, Australia 9-11 Deceber 9 COMPARISON OF UCG CAVITY GROWTH WITH CFD MODEL PREDICTIONS Yoggag LUO *, Margaretha COERTZEN ad Stephe DUMBLE Lic Eergy Ltd, Brisbae, Queeslad 41, AUSTRALIA *Correspodig author, E-ail address: yoggagluo@liceergycoau ABSTRACT Udergroud coal gasificatio (UCG) is a eergig eergy source where coal is gasified i-situ by ijectio of a suitable oxidat (air or oxyge) for the productio of sythetic gas I this paper, a two-diesioal atheatical odel of cavity growth for i-situ UCG is preseted that takes ito accout the effects of atural ad forced covectio drivig forces o heat ad ass trasfer i cavity Based o this D cavity odel, assuig that cavity growth is uiforly expaded towards side wall ad roof wall, a three-diesioal cavity shape was cocluded The UCG process odelled was progressed by forward cobustio through a iitial liked coectio betwee ijectio ad productio poits A CFD software package (FLUENT 636) was used to siulate cobustio ad gasificatio reactios o the iterface betwee coal sea ad cavity The teperature distributio ad heat ad ass trasfer rates for the UCG process were also obtaied The predictios fro the odel icluded the cavity shape, teperature profile ad coal cosuptio These paraeters showed reasoable agreeet with the data that were obtaied ad calculated fro trial work at Lic Eergy s Chichilla site The odel was also used to copare published data fro other UCG trials with the cavity shape, diesios ad coal cosuptio as calculated by the odel The odel proved to have a high degree of correlatio with the reported easureets NOMENCLATURE A cross-sectioal area of cavity A pre-expoetial factor E activate eergy H cross sectio radius to cavity boudary k kietic rate for reactio ṁ ass flow rate reactio uber p pressure r radius i cylidrical coordiates R gas costat T teperature u local velocity U velocity of uifor strea V volue of cavity x distace alog axis fro ijectio well through lik zoe y cross sectio radius of cavity α diesioless power θ agle betwee x axis ad radius i cavity ψ strea fuctio φ velocity potetial ρ desity of blast blast regio outside of cavity boudary layer INTRODUCTION Sice the first experietal work o UCG bega i 191 i Durha, the Uited Kigdo (website lik, http://ewikipediaorg/wiki/udergroud_coal_gasificati o), curretly, various coutries have attepted to develop UCG techology with the objective of deostratig its coercial applicatio These coutries iclude the USA, Australia, Chia, South Africa ad Idia (BURTON, 4) I Australia, four eergy copaies, of which Lic Eergy is oe, are explorig UCG techology with the ai of deostratig its coercial feasibility ad applicatio The iteded ed uses for the gas geerated through UCG iclude productio of ultra clea fuel (diesel), electrical power ad cheicals UCG is regarded as a preferred coal coversio ethod, due to its ehaced eergy efficiecy whe used i cobied cycle power geeratio ad due to its lower capital ivestet whe copared to covetioal iig ad above-groud gasificatio UCG also has a reduced eviroetal ipact as it avoids groud disturbace associated with coal iig ad provides opportuity for carbo capture ad storage particularly as i soe istaces, UCG suitable coals are ofte located i sedietary basis that also have potetial for CO sequestratio UCG ivolves the gasificatio of coal i the sea, utilisig a well to iject the oxidat ad stea if required (ijectio well) ad aother well to extract the product gas (productio well) The two wells are liked usig oe of several prove techiques Lic Eergy relies o horizotally drilled coectios The coal is iitially igited ad a cavity develops as the coal is gasified Coal udergoes heatig, dryig, devolatilisatio, char reactio, gas reactio, gas ixig ad particle dispersio ad ass ad heat trasfer o the iterface betwee overburde ad cavity UCG is aily cotrolled by these cheical ad physical reactios, but the iteractio betwee geological factors ad physicocheical effects is coplicated ad caot be odelled i a detailed aer that reflects the fudaetals of these processes The cavity growth directly ipacts o the coal resource recovery ad eergy efficiecy ad therefore the ecooic feasibility Cavity growth is also related to other potetial desig cosideratios icludig avoidig surface subsidece ad groudwater cotaiatio More iportatly perhaps, Copyright 9 CSIRO Australia 1

cavity growth odellig assists i developig a uderstadig of what is happeig udergroud durig gasificatio for process cotrol ad operatioal optiisatio I this paper, a two-diesioal atheatical odel of cavity growth for UCG is preseted, cosiderig the effects of atural ad forced covectio drivig forces o the heat ad ass trasfer i cavity This fudaetally derived cavity odel ca be used to predict the geerator s life, its perforace ad the developet of the cavity shape Paraeters i the odel ca be chaged icludig coal properties, coal sea thickess ad air/oxyge erichet ijectio rate The UCG process was assued to progress by forward cobustio through soe for of direct likage A CFD software, package (FLUENT 636) was used to siulate cobustio ad gasificatio reactios o the iterface betwee coal sea ad cavity The teperature distributio ad heat ad ass trasfer for UCG process as applied i site were also obtaied The iterface teperature betwee the coal sea ad cavity is iportat for sub-odels of dryig, devolatisatio ad spallig sub-odel Teperature profile o the iterface was siulated to deterie the effects whe the cavity develops fro the coal sea ito the overburde The odel was also used to copare cavity shape ad diesios ad the coal cosued agaist published data fro other UCG trials The odel ca also be used to predict re-igitio requireets, cavity shape, coal cosuptio rate ad gas quality for ay assued gas productio rate MODEL DESCRIPTION UCG cavity flow The odel preseted here is based upo the odellig approach by SCHWARTZ (1978) For irrotatioal fluid flow, the strea fuctio ad velocity potetial i a uifor flow were defied as Equatios (1) ad () (KUNDU, 4) Figure 1: UCG cavity flow as a irrotatioal flow passig a two-diesioal half-body Strea fuctio Ur ψ = cos( π θ + + si ( π θ ) 4π (1) Velocity potetial φ = + Ur cos( π θ ) () 4πr UCG cavity flow is regarded as flow passig a seiifiite body with a sooth ose, geerally called a halfbody The strealie is plotted i Figure 1 Cavity volue is defied by Equatio (3): V = x A( x) dx (3) x Where, A( x) = πy y = r siθ x = r cosθ For a specific coal sea thickess, the half-width of the cavity is defied as equatio (4): H = (4) 4 U The pressure distributio i the cavity ca be foud fro Beroulli s equatio as (5): 1 p + 1 ρu = p + ρu (5) If the ijectio flow rate ad pressure are fixed for a uifor strea, the right side of equatio (5) is costat, so that the pressure i the cavity is a fuctio of the local velocity The half-body volue is calculated fro equatio (6), which is the result of itegratig equatio (3) betwee Л ad θ ad cobiig equatios (1), () ad (4): 3 πh 3 θ θ θ V = [ si si + csc ] (6) 3 3 The cavity growth siulatio is based o equatio (6), substitutig differet positios alog the lik tuel betwee the ijectio ad productio well at differet stages of cavity developet Assuptios ad solutio UCG processes iclude coal heatig, dryig, devolatilisatio, cobustio ad gasificatio ad spallig of aterial fro the cavity roof The dryig process is related to oisture release ad water iflux fro overburde Moisture is released ito the gas strea, reducig the gas teperature Devolatilisatio produces char, volatile atter ad tars I reality, all these processes are ivolved i the UCG cavity expasio However, i this odel, the cobustio ad gasificatio reactios are assued to be the ajor factors to cause cavity growth The axiu cavity growth is regarded to be aroud the ijectio well, as a result of the well-ixed ad developed ature of air (oxidat) flow aroud the ijectio well I the cobustio zoe, oxyge is cosued, producig high teperature gas The very fast, exotheric cobustio reactios i the oxidatio zoe provide eergy for the edotheric gasificatio reactios The coal/char gasificatio zoe cosequetly develops ext to the cobustio zoe, typically with soe overlap betwee the cobustio ad the gasificatio zoes I order to ifer the size of the gasificatio zoe, the rate of water cosuptio assued to be related to the coal cosuptio by way of the i-situ water cotet of the coal Other assuptios iclude the followig: Copyright 9 CSIRO Australia

(a) Oxyge cocetratio alog the lik directio fro ijectio well to productio well at a costat recessio rate (b) Cavity growth is uiforly expaded towards side wall ad roof wall usig syetric two-diesioal geoetry (c) The rate of cavity growth is govered by the rate at which oxyge diffuses towards the cavity wall (d) Due to the high teperature ad pressure iside the cavity spalled coal blocks are iediately gasified (e) Oxyge diffusio alog the lik directio ad towards the overburde is cotrolled by atural ad forced diffusive covectios The volue of sectio HB (separated by blue dash lie) was calculated at a specific oet i the geerator life, as show i Figure 1 It was assued that all coal located i this volue was cosued i reactio with oxyge With a costat cocessio rate of oxyge alog the lik directio, the cylidrical zoe ext to sectio of HB was further calculated by cosiderig that coicidece of coal cobustio ad coal gasificatio The oxyge diffusio rate was calculated by atural ad forced covective heat ad ass coefficiet CFD siulatio Whe the cavity size was siulated based o the cobustio ad gasificatio reactios at a certai tie, the coal cosuptio rate could be obtaied I these series of cavity doai fro tie to tie, the siulatio of UCG process usig FLUENT 636 was explored Both heterogeous (char reactio o the roof ad side walls) ad hoogeeous reactios (gas reactio i cavity) were cosidered Heterogeous cheical reactios o the surface of wall 1 C + O k CO (7) k C + H CH 4 (8) k3 C + H O CO + H (9) k4 C + CO CO (1) Hoogeeous cheical reactio i the space of cavity k5 H + O H O (11) k6 CO + O CO (1) k7 CH 4 + O CO + H O (13) k8 CO + H O CO + H (14) The kietic rate of reactios k r fro (7) to (14) is give by α ( E / RT ) k = AT e (15) Usig the stadard equatios of coservatio of oetu, ass ad eergy, which were built usig FLUENT 636 software, the flow ad teperature distributios at the wall ad i the cavity were siulated The stadard k-ε odel for the trasport of turbulet kietic eergy ad dissipatio was used to quatify the turbulet itesity The pressure outside the boudary of cavity ( P ) was assued to be 8 bar The reactio rate Copyright 9 CSIRO Australia 3 was odelled as a kietics/diffusio cotrolled process The echais for hoogeeous cheical reactios was guided by usig a fiite rate/eddy dissipatio odel Surface reactio was applied for coal/char reactio The kietic paraeters used for equatio (15) are listed i Table 1 Reactio A E α k 1 5 1 17 1794 1 K 337 1-6 15 1 K 3 8593 31 5 K 4 8593 11 5 K 5 5 1 18 1674-1 K 6 398 1 19 1674 K 7 44 1 15 155 K 8 78 16 Table 1 Kietic paraeters for the siulatio of UCG process by PERKINS (8) For the Chichilla UCG trial, a 1 thick coal sea located at 14 below the groud was gasified Operatig pressures were betwee 65 bar ad 85 bar Air was typically ijected at rates of betwee 4 3 /h ad 8 3 /h Coal properties are show i Table Proxiate aalysis (ar) (wt%) Moisture Fixed carbo Volatile atter Ash 7 383 374 43 Ultiate aalysis (ad) (wt%) C H O N S 585 47 459 64 3 Gross calorific value (MJ/kg) 183 Table Coal properties as aalysed for UCG i Chichilla RESULTS AND DISCUSSION Coal cosuptio Figure shows the coal cosuptio as deteried fro the odel ad trial field data The odel result is slightly higher for the first 1 days of the trial tha what was deteried fro the field data This is assued to be because part of uburt char or ucollected tar stayed i the cavity ad was ot icluded i the gas copositio However, the cavity odel assued that all coal i the cavity was burt or gasified by oxyge or water shift reactio based o the diffusio rate of oxyge ad water aout up to 3% total aout of coal coal aout (to) 1 1 8 6 4 D cavity shape trial data odelig siulatio 4 6 8 1 1 Tie (day) Figure Coal cosuptios for odelig ad trial data

The cavity size was predicted as show i Figure 3 I this graph, I represets the ijectio well ad P is the productio well Igitio was started at the botto of ijectio well ad gasificatio proceeded alog the horizotal lik directio The cavity was expaded uiforly toward both sides aroud the lik tuel The cavity size ad coal cosued are both fuctios of tie For exaple, after 84 days, the aout of coal cosued was 114 tos ad the cavity width was 5 as show i Figure 3(a) After 649 days, however, the cavity width was 45 ad aout of coal cosued was 6397 tos as show i Figure 3(b) 6 4 (b) (a) 84 day, 114 to ad 5 (b) 649 day, 6397 to ad 45 (a) Haa II Cavity width () - I (a) P -4-6 -5 5 1 15 Lik legth () Figure 3 D cavity growth predictios for Chichilla trial (b) Haa III Figure 4 Copariso of sweep cavity geoetry with Haa II ad III Validatio The D odel was validated by Chichilla trial outcoes as etioed above Further validatio was doe usig data fro the Haa II ad III UCG trials BRANDENBURG (1978) reported that 5 tos of coal was gasified durig the Haa II trial which lasted 5 days ad 4 tos of coal was gasified durig the Haa III i 38 days Figure 4 (a) ad (b) shows results fro the D odel The odel predicts a coal cosuptio of 4366 tos coal i 5 days usig the Haa II coditios ad 41393 tos i 38 days whe the Haa III coditios are used There is less tha 5% error betwee the results geerated with the odel ad reported data (a) At 5 days, the cavity touched the overburde (b) 5 days cavity size ad shape (c) 1 days cavity size ad shape Figure 5 Cavity growth for 4 eters coal sea thickess i 1 days at 1 3 /h gas productio Copyright 9 CSIRO Australia 4

3D Cavity growth A 3D cavity growth was fored based o the D cavity growth odel The 3D cavity is uiforly expaded towards side wall ad roof wall at a syetric twodiesioal geoetry UCG process i a 4 thickess of coal sea was siulated at 1 3 /h gas productio flow rate It was estiated that the cavity would develop ito the overburde after 5 days as show i Figure 5 (a) The estiated cavity after 5 days ad 1 days are show i Figures 5 (b) ad (c) The cavity expads aroud the ijectio well, but oce the cavity develops ito the overburde, the cavity oly expads i the horizotal directio The odel could be used to predict the UCG geerator s effective life based o predicted gas quality which will see a icreasig cocetratio of CO as the cobustio zoe oves towards the productio well ad gasificatio ad pyrolysis zoes becoig saller Teperature distributio Teperature i the cavity was predicted usig a FLUENT-based odel Figure 6 shows the teperature profile i the cavity, especially o the boudary of the cavity at differet stages of geerator developet Figure 6 (a) shows the teperature whe the cavity reaches the overburde at 5 days The teperatures were i the rage of 3 K to 154 K Figure 6 (b) shows the teperature distributio i the cavity at 5 days (3 K to 148 K) Figure 6 (c) shows the teperature profile i the cavity at 1 days (3 K to 145 K) The overall teperature after the cavity reaches the overburde decreases This is oe of the sigals that the geerator has reached the ed of its useful life as the gas quality will deteriorate gradually fro this poit Whe the cavity develops ito the overburde, the cobustio zoe area starts to decrease ad oves forward towards the productio well Cosequetly, the gasificatio ad pyrolysis zoes ove towards the productio well as well Figure 5 shows how all three zoes are expected to ove towards the productio well Figure 6 shows how the teperature decreases over tie Figure 6 Teperature distributios i the boudary of cavity (a) (b) (c) CONCLUSION A -D odel for UCG cavity growth was developed ad applied to predict the size of a UCG cavity i 3D ad the aout of coal cosued at various stages of geerator developet (days) Based o the assuptio of strea fuctio i fluid dyaic, the shape ad volue of the cavity are obtaied Based o the coal sea thickess ad the legth betwee the ijectio well ad productio well, the life of geerator ca be predicted based o relatioships derived betwee cavity shape, geerator teperature ad gas quality The odel is sesitive to chages of site-specific paraeters like the coal properties (ultiate aalysis, desity, theral coductivity, etc) The -D odel ca be adapted to predict the cavity growth of a UCG geerator at ay site Results obtaied with the odel after chagig process paraeters like the air ijectio rate ad oxyge cocetratio i the oxidat were copared It was foud that the odel is suitable to predict the cavity developet over a rage of process coditios The odel was validated usig Chichilla ad Haa II ad III trial data The error i coal cosuptio predictio is less tha 5% Teperature profiles i the cavity were calculated over the lifetie of the geerator This will allow the calculatio of coal sea surface teperatures at differet positios at differet ties over the life of the geerator This data ca be used to calculate the heat trasfer rate Whe the teperature distributio ad heatig rate i coal sea are kow, the total tar aout produced durig pyrolysis ca also be estiated REFERENCES BRANDENBURG, CF, FISCHER, DD, BOYD, RM, KING, SB ad HUMPHREY, AE, A review of LERC s i situ coal gasificatio project, Proceedig 3rd Aual UCC Syposiu, Falle Leaf Lake, CA, Jue 1977, p66-76 BURTON, E, FRIEDMANN, J, UPADHYE, R, Best practices i udergroud coal, Lawrece Liverore Natioal Laboratory, https://colllgov/pdf/best PracticesiUCG-draftpdf http://ewikipediaorg/wiki/udergroud_coal_gasifica tio KUNDU, PK, COHEN, IM, Fluid Mechaics, Third Editio, Elsevier Acadeic Press, 4, p164-165 PERKINS, G, ad SAHAJWALLA, V, Steady-state odel for estiatig gas productio fro udergroud coal gasificatio, Eergy & Fuels 8,, 39-3914 SCHWARTZ, SY, EDDY, TL, MEHTA, KH, LUTZ, SA, ad BINAIE-KONDOLOJY, MB, Cavity growth echaiss i UCG with side wall bur gasificatio, SPE Aual Fall Techical Coferece ad Exhibitio, 1-3 October 1978, Housto, Texas ACKNOWLEDEMENT The authors are grateful to Lic Eergy Ltd givig the fiacial support to preset this work i the coferece Copyright 9 CSIRO Australia 5