CFD MODELLING OF AIR-FIRED AND OXY-FUEL COMBUSTION IN A 100 KW UNIT FIRING PROPANE

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1 Proceedngs of the Internatonal Conference on Mechancal Engneerng 011 (ICME011) 18-0 December 011, Dhaa, Bangladesh ICME11-. CFD MODELLING OF AIR-FIRED AND OXY-FUEL COMBUSTION IN A 100 KW UNIT FIRING PROPANE Auda Hussen Al-Abbas and Jamal Naser Facult of Engneerng and Industral Scence, Swnburne Unverst of Technolog, Australa. ABSTRACT A computatonal flud dnamcs (CFD) modellng stud s undertaen ntegratng the ar-fred and ox-el combuston cases for chemcal reactons, radatve heat transfer, and gas compostons nto a 3-D hbrd unstructured grd CFD code. A swrl necton sstem s used to acheve the flame stablt of the turbulent non-premxed combustble gases. An Edd Breaup (EBU) combuston model wth approprate emprcal cocents s emploed for ths stud. Valdaton and comparson of both combuston cases wth the expermental data, whch conducted on a 100 W faclt unt, were made b comparng the temperature dstrbuton levels and speces concentraton levels. The ox-el combuston case showed that the flame s obvousl concentrated n the central regon, and t s not spread nsde the rnace compared to the ar-fred flame. The swrl ect s certanl used to enhance the turbulent mxng and to acheve the nternal recrculaton of flames. B swtchng to ox-el fred combuston, results show that the carbon doxde concentraton ncreases from around 15 % (g/g) to 76 % (g/g) under wet bass n flue gas. Under the same operatng condtons, combuston dela s clearl observed n ox-el combuston case compared to ar-fred case due to ncomplete consumpton of oxgen at the near-burner regon. Ths CFD model, after valdaton aganst expermental data, s expected to provde a strong confdence on the combuston characterstcs of both combuston cases, partcularl at the challengng locatons of the rnace. Kewords: Non-Premxed Flames, Ox-Fuel Combuston, CO Capture, Swrl Effect, and CFD. 1. INTRODUCTION Over the past ears, envronmental concerns about anthropogenc emssons of greenhouse gases (GHG) are sgnfcantl leadng to global clmate change [1]. The most mportant resource of these anthropogenc GHG emssons s carbon doxde emsson. Recentl, fossl els are stll provdng around 85% of the world s demand of electrcal energ []. The most ectve technque that can acheve the hghest reducton n GHG emssons s to capture carbon doxde from the conventonal power generatons. The electrct power plants are commonl represented as the largest source of CO amongst other commercal ndustres [3]. Therefore, the exstng power plants have to be precsel nvestgated n order to overcome the extra cost of swtchng to CO capture plants and to mprove ther performance at smlar operatng condtons to those of non- CO capture plants. Several technques to capture carbon doxde are beng ncreasngl developed. The three man technques that are developed for the capture of CO are pre-combuston capture, post-combuston capture, and ox-el combuston. Ox-el combuston technolog has been wdel consdered as a vable opton to control and reduce several tpes of gaseous emssons from PC power plants [1]. The ox-el combuston s to burn fossl el wth pure oxgen (produced n crogenc ar separaton unts) and reccled flue gas (RFG) to produce a hgh concentraton of CO n the flue gas. Ths technque wll lead to mae ts separaton and compresson processes easer and more cost-ectve. Usng pure oxgen and RFG nstead of ar to burn el was frstl used b Abraham [4]. The purpose was to ncrease CO concentratons, whch can be economcall used for enhanced ol recover b nectng CO underground for permanent storage. In the last few ears, man comparsons have been expermentall conducted on the sold el combuston n ar and n mxtures of O / CO [5-7]. Oxgen concentratons n feed gas, heat transfer and flame characterstcs n ox-el combuston should be precsel studed to acheve smlar combuston characterstcs to the conventonal combuston case. The numercal calculaton methods can extensvel ICME011 1

2 provde a wde range of cent nformaton n rnace and burner s desgn before mang an expensve and tme-consumng expermental studes. To our nowledge, there s stll lttle research wor conducted on the ox-el combuston n numercal modellng. Therefore, ths stud s focused on the numercal soluton to gve a good nsght n the phscal and chemcal mechansms of ox-el combuston technque. In the present stud, temperature dstrbutons, speces concentratons, and veloct components are numercall calculated under ar-fred and ox-el combuston envronments for a 100 W down-fred rnace. Therefore, the purpose of ths wor s to valdate the CFD model, and to compare the above-mentoned varables for both the combuston cases. The same volumetrc flow rates of el (propane) and reactants (O /N ) or (O /CO ) are used n feed gases based on the measurements set up.. MATHEMATICAL MODELS The commercal CFD, AVL Fre ver.008. code s emploed to analze the computatonal doman sstem that ncludes chemcal reacton, radatve heat transfer, flud flow feld, and turbulent models. It s mportant to accuratel calculate and predct the temperature levels, turbulent flow felds, and speces concentratons from combuston sstems. In order to llustrate the applcatons of CFD on the turbulent non-premxed gaseous combuston, t s necessar to defne all the mathematcal models assocated wth combuston phenomenon..1. Flow Feld Model To llustrate the turbulent non-premxed flame characterstcs of propane (C 3 H 8 ) combuston n ar and O /CO mxture, the three-dmensonal governng equatons of mass conservaton, momentum, and energ transport equatons n the transent condtons have been solved n the Cartesan tensor form: Mass conservaton equaton ρ + ( ρu ) 0 (1) - Momentum conservaton equaton P ' ' ( ρu ) + ( ρu u ) + ( τ ) + ( ρu u ) () -Energ transport equaton T ( ρ E) u ( ρ E P) u τ + S (3) [ ] ( ) φ where φ denotes enthalp and concentraton of speces, whle term S φ represents the approprate source of the varablesφ. The local denst of the mxture s dependent on pressure, reactants, products concentratons, and temperature and determned through the equaton of state: sp ( MW ) ICME011 ρ RT P Y (4) Whle the temperature values can be calculated from the enthalp: sp Y h h T (5) c p The ectve stress tensor and thermal conductvt are gven b: u u u ( τ ) µ + µ δ (6) x x 3 x c p µ t K K + (7) σ t Fuel and feed oxdzer gases (ar or O / CO ) are non-premxed before enterng nto the rnace accordng to the burner s desgn. In ths numercal stud, t s assumed that the swrl created b the prmar and secondar regsters. Ths can ensure better mxng condtons for reactants n order to avod an external recrculaton n the flame structure and to eep t n stable form durng burnng reactants... Speces Transport Model In order to reduce the number of equatons to be solved, dmensonal quanttes are ntroduced to express the reactve sstem. The mass fracton, resdual gas mass fracton, and mxture fracton are gven b [8]: m, u (8) mtot mrg g (9) moxd m, u + m, b f (10) mtot The soluton of transport equatons for the denst weghted mean quanttes, f, and g s llustrated n the followng equatons: ( ρ ) + ( ρu ) + S t x x Γ x (11). Where S ρ. r, ths term wll brefl explan n the combuston model secton. f ( ) ( ) ρ f + ρu f Γ (1) f g ( ) ( ) ρ g + ρu g Γ (13) g The chemcal reacton sstem n ths stud conssts of fve speces: C 3 H 8, O, CO, H O, and N. Wth the followng algebrac expressons, the mass fracton of the above-mentoned speces can be calculated n terms of

3 the total mxture mass: a o N pr co HO [( 1 f ) S( f )] 1 ( 1 a )( 1 f ) 1 a a 3 1 pr pr o N (14) The parameters a are the mass fractons of th speces: O, CO, and H O, S s stochometrc ar/ el rato, whle n and m are representng the number of carbon and hdrogen atoms n the hdrocarbon el (C n H m C 3 H 8 )..3. Combuston Model The numercal smulaton of the mean chemcal reacton rates consders a man problem n the determnaton of chemcal netc processes. Ths complcated calculaton s due to non-lnear nctons of the local values of temperature and speces concentratons. A sngle step rreversble reacton of a hdrocarbon el (C 3 H 8 ) wth ar or O /CO mxture s consdered n ths stud. Accordng to the expermental operatng condtons of ths gaseous combuston, the chemcal reacton equatons were wth 15% excess ar: Ar-fred combuston chemcal equaton C 3H ( O N ) 3 CO + 4HO O N (15) -Ox-el fred combuston chemcal equaton C 3H O 3CO + 4H O O (16) Turbulence controlled combuston model, Edd Breaup (EBU) model, s used for these combuston smulatons. The EBU model s a popular and an cent model n combuston calculaton, whch was frstl proposed b Spaldng [9] and modfed later b Magnussen and Hertager [10]. The mean reacton rate can be wrtten accordng to [10]: C C Ox pr pr ρ r ρ mn,, (17) τ R S 1+ S The rate of consumpton of el s specfed as a ncton of local flow propertes, thus t s dependent upon the turbulent tme scale ( τ R ). The frst two terms of the mnmum value of operator smpl verf f el or oxgen s present n lmtng quantt, and the thrd term s used for a reacton possblt. C and C pr are emprcal cocents, and the exact values for these cocents are dependent on the el and the detaled structure of the turbulent flow feld. More than twent smulaton tests were carred out to adust and select the perfect values accordng to avalable expermental data [11]. The best results found out that b ncreasng the values of these cocents lead to an ntensfcaton of the turbulent reacton rate. Therefore, under these specal condtons of ths stud, C and C pr are set to 6.0 and 0.5 for ar-fred case, and 3.0 and 0.5 for ox-el fred. Detaled nformaton related to the mathematcal models and computatonal method descrptons used n ths stud can be found n the prevous prelmnar CFD stud [1]. 3. BOUNDARY CONDITIONS AND MESH The nlets of el and other oxdzers n prmar and secondar regsters are located n the central poston of the top-wall of rnace. The ntalzaton nformaton s ept constant for both combuston cases such as an ntal temperature (98.15 K) and ntal pressure (10135 Pa). The veloctes of prmar and secondar regsters of oxdzers (ar or O /CO ) are affected b the fn angles 45 0 (normal veloct V n 8.46 m/s) and 15 0 (normal veloct V n 3.3 m/s), respectvel n order to stablze the flame structure of combuston. For precse modellng, the wall of rnace s dvded nto two parts: top-wall and vertcal-wall. For both parts of wall, no-slp condton (u, v, and w 0) and wall emssvt ( ε 0.41 for ar-fred case and ε 0.5 for ox-el frng case) are appled, but the temperature value was onl constant for top-wall (95 K) and changeable for vertcal-wall. Two second-degree polnomal nctons are used to calculate the temperature dstrbuton along the vertcal-wall n order to match the real values of expermental data n that regon of the rnace wall. A hgher mesh concentraton s used along the centerlne axs of rnace, whle the mesh sze s graduall ncreased awa nearb the wall and outlet boundares as shown n Fg.1. Three dfferent non-unform structured grd sstems are used for grd ndependence test wth 4000, , and cells. The grd ndependence test ndcated that the grd number (480000) satsfes more accurate results amongst the other grd sstems. It made a good balance between the computatonal accurac and the computng cost. Fg 1. Grd sstem of computatonal doman ICME011 3

4 4. RESULTS AND DISCUSSION The composton of feed gases for both prmar and secondar regsters of ox-el combuston case was composed of 1% vol. of O (smlar to that of ar-fred case) and 79% vol. of CO. Ths change n combuston meda led to a sgnfcant mpact on the combuston characterstcs and flame structure. Fg. shows temperature dstrbutons of ar-fred flame and ox-el flame at the vertcal cut of the rnace. The ar-fred flame exhbts a more brght aspect than the ox-el flame. Ths feature of flame lumnous appearance can hghl be attrbuted to the capablt of CO to absorb radaton n the latter flame, whch has the hgher concentraton n the flue gas compared to ar-fred case. On the other hand, the ox-el flame, whch has the same oxgen concentraton n the feed gases, showed that the flame was obvousl concentrated n the central poston, and t was not spread nsde the rnace compared to the ar-fred flame. Fg. Temperature dstrbutons (K) for the ar-fred flame (left) and the ox-el flame (rght) at the vertcal cut (X - Y plane) of rnace. Fg 3. Temperature dstrbuton at 553 mm from burner ext for ar-fred and ox-el combuston cases. In addton, the pea flame temperature level of combuston was drastcall decreased from 1853 K for the ar-fred case to 1518 for the ox-el case. Ths drop n the flame temperature s sgnfcantl affected the combuston dela n ox-el case. These numercal results of combuston dela n the ox-flame were extremel consstent wth the recent stud of Ref. [13]. However, these dfferences n the flame shapes between ar-fred flame and ox-el fred flame are prncpall due to the dfferences n the thermodnamcs propertes between ntrogen and carbon doxde, and radatve propertes of gas mxture. Fg. 3 llustrates the valdaton and comparson of the temperature profles at 553 mm from burner ext plane for the ar-fred and the ox-el combuston cases. The overall agreement was acheved n ths challengng locaton except for ox-el case. Pea temperature was occurred between central pont of the rnace and 0.1 m n the radal drecton. Ths can be frstl explaned due to the ntensve regon of combustble gases; secondl because of the concentraton of flame n ths radal dstance of the rnace that led to a slght ncrease n the flame temperature level. These numercal results revealed that the mxture fractons were closer to Stochometrc (theoretcal ξ ξst ) characterstcs. Ths means that the entre oxgen and el were completel consumed, and as a result the flame temperature was at a maxmum value. Fg. 4 shows the veloct vectors of the prmar and secondar swrl regsters of the burner at the nlet of the rnace. Ths swrl ensures that the el s completel burnt n the closest regon to the burner ext, and therefore elmnates undesrable gases such as carbon monoxde (CO), smoe, soot, etc. In Fg. 5, the ect of swrl numbers (S) on the stablt of flame n three dfferent recrculaton zones s showed. The recrculaton zones of the flow feld were dvded nto three dfferent separate zones: nternal recrculaton zone, reacton zone, and external recrculaton zone. The swrl numbers were 0.79 and 0.1 for the prmar and secondar regsters, respectvel. These swrls are used to enhance mxng zone between the el and oxdzer (ar or O /CO ) streams. Due to ths swrl ect, well-mxng condtons were acheved n the closest regon to the burner ext, and ICME011 4

5 therefore t mght avod the gases travel downstream of the rnace to a well-mxng pont. In addton to the well-mxng condtons, the aerodnamc ect of swrl can shorten the flame length. At the most ntense combuston locaton (port 1), Fg. 6 shows a comparson between the expermental data and the predcted results for the mass fracton dstrbuton of carbon doxde for both combuston cases n wet bass. For the ar-fred case, over-predcton was notced; whle the ox-el case showed relatvel better predctons except from the central pont of the rnace to ~0.08 m n the radal drecton. However, ths locaton of the rnace s obvousl showed ncreasng carbon doxde concentratons for the ox-el combuston case. Fg 5. Inlet veloct vectors (m/sec) of three dfferent recrculaton zones. Fg 4. Veloct vectors (m/sec) of prmar (A) and secondar (B) swrl regsters at the burner ext plane. As the ntrogen s replaced b CO n the feed gases, carbon doxde concentraton s hghl ncreased from around 15 % (g/g) to 76 % (g/g) under wet bass n flue gas. Ths result reveals that the carbon doxde concentraton was ncreased b approxmatel 5 tmes n wet bass. Fg 6. Mass fracton dstrbuton of carbon doxde at 384 (mm) from burner ext plane for the ar-fred and the ox-el combuston cases n wet bass. ICME011 5

6 5. SUMMARY AND CONCLUSIONS A three-dmensonal Computatonal Flud Dnamcs (CFD) model has been developed to smulate the ar-fred and ox-el combuston cases. The temperature dstrbuton levels, speces concentraton, and veloct dstrbutons were nvestgated at the most ntense combuston locatons of the rnace. An cent combuston model (EBU) wth the most approprate emprcal cocents s used n ths stud. Prmar and secondar swrl regsters are emploed to enhance an nternal recrculaton mechansm of flames and to ensure a well-mxng condton for turbulent non-premxed gaseous reactants. Slght over-predctons of temperature levels and speces concentraton levels are notced compared to the measurements. These over-predct ma be occurred because of neglectng the mult-step chemcal reactons, whch lead to dssocate carbon doxde and water vapour n the combustble products nto several ntermedate speces. The numercal results showed that the flame s clearl concentrated n the central poston n ox-el case, and t s not spread nsde the rnace compared to the ar-fred flame. The combuston dela n ox-el case s evdent. Ths dela s lel happened due to nsuffcent oxgen consumpton at the near-burner regon compared to the reference case. The resdence tme of combustble reactants s consderabl ncreased due to adoptng the swrl necton sstem that leads to enhance flame stablt. The carbon doxde concentraton rses up b ~ 5 tmes for ox-el compared to ar-fred case under wet bass. Fnall, even though there are some dscrepances between the measured data and numercal results, the CFD results are consderabl showed the same qualtatve trend as the experments. However, ths stud represents the second stage of the entre smulaton of ox-el combuston. The next step of ths proect wll comprehensvel focus on the brown coal combuston n the ll-scale tangentall-fred rnace under ox-el combuston technque. 6. ACKNOWLEDGEMENTS The authors wsh to than the mnstr of hgher educaton and scentfc research (MOHESR) n Iraq Government for the fnancal support durng the scholarshp perod (009-01) n Australa. 7. REFERENCES 1. T. Wall, Y. Lu, C. Spero, L. Ellott, S. Khare, R. Rathnam, F. Zeenathal, B. Moghtader, B. Buhre, C. Sheng, R. Gupta, T. Yamada, K. Mano, J. Yu, 009, An overvew on oxel coal combuston--state of the art research and technolog development, Chemcal Engneerng Research and Desgn, J. Davson, 007, Performance and costs of power plants wth capture and storage of CO, Energ, H. Lu, R. Zalan, B.M. Gbbs, 005, Comparsons of pulverzed coal combuston n ar and n mxtures of O /CO, Fuel, 84, B.M. Abraham, 198, Coal-oxgen process provdes CO for enhanced recover, Ol Gas J.; (Unted States); Journal Volume: 80:11, Pages: 68-70, H. Lu, R. Zalan, B.M. Gbbs, 005, Comparsons of pulverzed coal combuston n ar and n mxtures of O /CO, Fuel, 84, Y. Tan, E. Croset, M.A. Douglas, K.V. Thambmuthu, 006, Combuston characterstcs of coal n a mxture of oxgen and reccled flue gas, Fuel, 85, T. Suda, K. Masuo, J. Sato, A. Yamamoto, K. Oaza, 007, Effect of carbon doxde on flame propagaton of pulverzed coal clouds n CO /O combuston, Fuel, 86, Anonmous: 008, AVL Fre CFD Solver v 8.5 manual, A-800 Gras, Austra, 9. D.B. Spaldng, 1971, Mxng and chemcal reacton n stead confned turbulent flames, Smposum (Internatonal) on Combuston, B.F. Magnussen, B.H. Hertager, 1977, On mathematcal modelng of turbulent combuston wth specal emphass on soot formaton and combuston, Smposum (Internatonal) on Combuston, 16, K. Andersson, 007, Characterzaton of ox-el flames - ther composton, temperature and radaton, Chalmers Unverst of Technolog, Thess, Göteborg. 1. A. H. Al-Abbas, J. Naser, D. Dodds, 011, CFD modellng of ar-fred and ox-el combuston of lgnte n a 100 W rnace, Fuel, 90, J. D. Moore and K. K. Kuo, 008, Effect of swtchng methane/oxgen reactants n a coaxal nector on the stablt of non-premxed flame, Combuston Scence and Technolog, 180, MAILING ADRESS Jamal Naser Facult of Engneerng and Industral Scence, Swnburne Unverst of Technolog, Hawthorn, Vctora 31, Australa. ICME011 6