Numerical study of effect of hydrogen content on the structure of syngas diffusion flame

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1 HEFAT th Internatonal Conference on Heat Transfer, Flud echancs and Thermodynamcs July 2014 Orlando, Florda umercal study of effect of hydrogen content on the structure of syngas dffuson flame Pryanka Sarnkar*, Akhlesh Kumar Sahu and T. Sundararajan Department of echancal Engneerng, Indan Insttute of Technology adras, Chenna, 60006, Inda *Correspondng author: ABSTRACT Syngas, produced durng gasfcaton reactons, prmarly s composed of carbon-monoxde (CO) and hydrogen (H 2 ), along th other gases lke carbondoxde (CO 2 ), ntrogen ( 2 ) and methane (CH 4 ). The composton of these gases produced durng gasfcaton vares th the type of fuel, oxdzer concentraton, operatonal pressure and temperature. For nstance, the amount of CO produced durng the endothermc reactons depends strongly on the temperature. The flame structure for dfferent syngas compostons ould be an mportant nput for desgnng a good syngas burner. Ths paper s an attempt to numercally study the effect of varyng the hydrogen concentraton n syngas, on the resultng flame structure. A straght cylndrcal tube of 4 mm dameter has been consdered as the burner tube. Length has been chosen such that flo completely develops thn the tube. The jet emergng at the ext of the tube entrans ar naturally and the flo s lamnar. An ax-symmetrc model th smplfed Davs mechansm (detaled chemstry mechansm consstng of fourteen speces) has been used. A detaled examnaton of the reacton zone ndcates that along the radal drecton, CO oxdaton occurs folloed by the formaton of ater vapor. The hgher speces dffusvty and knetc rate coeffcent values for the hydrogen- oxygen reacton over those of CO play a crucal role n determnng the overall syngas flame characterstcs. KEYWORDS Syngas; flame structure; non-premxed flame; numercal smulaton; detaled chemcal knetcs ITRODUCTIO Due to the ncreased demand and depleton of fossl fuels, there s a need for alternatve fuels. Varous researchers have focused ther attenton toards sold fuels such as coal and bomass, hch are abundantly avalable orldde. In order to provde a clean energy source and to reduce carbon foot prnt, Integrated Gasfcaton Combned Cycle (IGCC) technology has been ntroduced to acheve hgher effcency and loer emssons. The IGCC poer plants allo the gasfcaton of de range of lqud and sold fuels and even aste-derved materals to convert nto fuel gas mxtures that can be used n the gas turbne to generate electrcty [1]. In an IGCC poer plant, the sold and lqud fuels are gasfed and the evolved gas s called syngas. The syngas s composed of many gaseous consttuents, chef ones beng CO and H 2, along th other gases lke H 2 O, CO 2, 2 and CH 4 [2]. The composton of syngas vares largely, dependng on the fuel resource (bomass and dfferent types of coal), type of gasfcaton and processng condtons. Hydrogen mole fracton n syngas vares from 9% to 40% n a typcal IGCC plant fueled by coal, bomass or sold aste []. Also the mole fracton of hydrogen n a coal-based IGCC poer plant for typcal processng condtons as reported to vary from 25% to 70% [4]. The varaton of syngas composton has great nfluence on the combuston process, and hence s a challenge for combustor desgn. Several research orks have been reported on the fundamental combuston characterstcs of syngas gnton delay [7, 8] and flame speed [9, 10], and also on the development of detaled chemcal reacton mechansms [11, 12]. Varous studes on the combuston propertes such as adabatc flame temperature and flammablty lmts have also been carred out. easurement of lamnar burnng velocty hch s an mportant characterstc of the fuel mxture and hch governs the structure and stablty of premxed flames, has also been reported [2, 5, 6]. The presence of Hydrogen n the fuel mxture affects the flame speed and structure, because of ts hgh lamnar burnng velocty as compared to the other gaseous fuels [1, 14, and 15]. Consderable amount of data s avalable n lterature on the effects of fuel propertes on flame speed. Hoever, the research ork avalable s lmted th respect to the flame structure of lo calorfc value mult-component fuels. Partcularly, the effects of dluents such as CO 2 and 2 on the combuston process of lo calorfc 704

2 Wall 40 mm 240 mm Pressure nlet p= 1 atm Axs value syngas are not addressed. Therefore, n ths paper, numercal study of the behavor of dffuson flame over a de range of H 2 /CO ratos s presented. The changes n flame structure due to varatons n the H 2 concentraton are nvestgated, to be of use n the desgn of syngas burners. OECLATURE D Bnary mass dffuson coeffcent g Gravtatonal acceleraton vector (m/s 2 ) h f Enthalpy of formaton (J/kg) I Unt tensor J Dffusve mass flux of speces (kg/ms 2 ) olecular eght of mxture (kg/mol) Total number of speces p Statc pressure (/m 2 ) q R Radaton heat flux /m R et mass rate of producton of speces (kg/m s) r * Dmensonless radal dstance scaled by nternal radus of the burner X ole fracton Y ass fracton z * Dmensonless axal dstance scaled by nternal radus of the burner olecular vscosty Densty (kg/m ) Stress tensor (/m 2 ) Subscrpt Speces COPUTATIOAL DOAI An ax-symmetrc, unsteady, pressure based solver has been used to smulate lamnar dffuson flames of syngas. Commercal CFD softare FLUET 1 has been used for ths purpose. The detaled and optmzed chemcal mechansm of Davs et al. [1] havng 14 speces and 8 reacton steps has been used as the chemstry sub-model. The convergence crteron s set as 10 - to 10-6 for non-dmensonal resdues, and overall mass balance has been mantaned to less than 1% of the fuel flo rate. The computatonal doman along th the boundary condtons s shon n Fg. 1. A computatonal doman extendng to 60 dameters (240 mm) n the axal drecton and 10 dameters (40 mm) n the radal drecton has been consdered, for a burner of 4 mm outer dameter. Proper doman ndependence study has been carred out ntally to fx the doman extent. Pressure outlet p = 1 atm r 2 mm Pressure nlet p = 1 atm 2 mm Fgure 1: Computatonal doman and boundary condtons GRIDS USED EAR BURER The grd has been created usng the commercal softare of GABIT A non-unform rectangular mesh th more than 100,000 cells has been generated and the fne grd regon near the burner ext s shon n Fg. 2. Burner all 40 mm Fgure 2: Typcal non-unform grd used near burner ext g V nlet = 2 m/s T nlet = 00K z 0 r 0 V r Axs Y mx 705

3 The grd sze near the ext of the burner s kept as 2*10-4 m to accurately represent the hgh gradent regons n the dffuson flame. ODEL PARAETERS A stanless steel tube burner of 4 mm nternal dameter and 8 mm external dameter havng a length of 40 mm has been consdered. The basc syngas composton employed n ths study has been taken from the ork of Iyengar and Haque [16], ho have nvestgated the gasfcaton of Indan coals. In order to study the effect of ncreasng H 2 concentraton on the flame structure of syngas, volume percentage of H 2 has been vared from approxmately 16% to 26%, keepng the percentages of CO and CO 2 n the syngas constant. Wth ncrease n H 2 fracton, the ntrogen content n the fuel mxture s correspondngly reduced. Volumetrc composton of the fuel mxture for the three cases consdered n ths study s reported n Table 1. Intally the cold flo has been solved and after ts convergence, a hgh temperature zone s patched n a small regon near the burner ext to ntate combuston. After obtanng convergence th ths, radaton model s turned on usng a User Defned Functon (UDF) rtten n C language. Second order dscretzaton methods have been employed to obtan ether a converged solutons. Table 1: Syngas composton used n the study Cases Case 1 Case 2 Case Syngas composton % by volume and mass fracton s gven n brackets H 2 CO CO (0.012) 21 (0.017) 26 (0.02) 15. (0.168) 15. (0.177) 15. (0.187) 10.7 (0.184) 10.7 (0.19) 10.7 (0.204) 58 (0.628) 5 (0.609) 48 (0.584) GOVERIG EQUATIOS The governng equatons for the problem are the conservaton of mass, momentum, speces and energy as gven belo. Contnuty equaton ( V ) 0 t omentum equaton ( V ).( VV ) p. g t Where 2 T [ V V. VI ] Speces conservaton equatons ( Y ).( VY). J R S t Conservaton equatons have been solved for -1 actve speces. The mass fracton of 2 hch s an nert speces, s obtaned from the dentty Y =1. Full mult-component dffuson s consdered. Speces velocty s calculated usng the axell Stefan equaton. J 1 j1 Where D j A A B j D Y j 1 [ D] [ A] [ B] X D 1 X D j j X,,, j X D j1, j1 j, 1 D 1 X j,, B X, j, Here, [A] and [B] are (-1) (-1) matrces. Energy equaton The energy conservaton equaton consstng of enthalpy transport by speces and radatve heat flux losses, s gven by t C T. VC T. KT p R h f, 1 1 p, DjC p TY q, R 706

4 RESULTS AD DISCUSSIO Fgure shos the axal temperature varaton for three cases th dfferent hydrogen and ntrogen contents n the syngas. fuel. At hgher fracton of hydrogen (case ), the reacton zone ncreases n dth due to larger leakage of hydrogen. Fgure : Temperature varaton along the axs from the burner ext It s clear from Fg. that the maxmum temperature values attaned are almost the same for all the three cases. Follong lterature, the flame heght s taken as the axal locaton at hch the maxmum temperature s reached. The flame heght can be estmated from the hgh temperature zone seen n the sothermal contour plots (Fg. 4). It s apparent that as the hydrogen content n the fuel s ncreased from 16% to 21% and then to 26%, the heght of the flame ncreases and flame dth also ncreases radally outards. Ths can be attrbuted to the hgher dffusvty and flame speed values for hydrogen. Radal profles of the temperature at an axal locaton of around (z*=15) from the burner ext hch s the mdplane of the overall flame extent n the axal drecton, are shon n Fg. 5. Here also, t s clear that th the ncrease n hydrogen percentage (from 16%, 21% and 26%) there s a shft n the maxmum temperature value from around r* value of 1.6 to 1.9. The maxmum temperature for case 1 s approxmately 200 K less than that for case and the maxmum value of case 2 les n beteen those of case 2 and case. Fgure 4: Contours of temperature T (K) The radal profles of mass fractons of hydrogen and oxygen at an axal locaton of (z*=15) from the burner ext are shon n Fg. 6. It s clear that the hydrogen dffuson zone extends approxmately from the r* value of 1.4 to 2.2 at that locaton. Leakage of both fuel and oxygen occur beyond the reacton zone, hch s expected n the case of a mult-component Fgure 5: Radal temperature profles at z*=15 In Fgs. 7.a and 7.b, the mass fracton contours and net reacton rate of H 2 are plotted. It s evdent that H 2 combuston reactons are completed n an axal dstance of about 7-8 dameters from the burner ext. 707

5 Fgure 6: Varatons of H 2 and O 2 mass fractons along the radal drecton at z* = 15 At a hgher hydrogen fracton (case ), the reacton zone extends to a larger axal extent. From Fg 7.b, t s observed that H 2 gets consumed to form H 2 O and OH follong the reactons O + H 2 = H + OH, OH + H 2 = H + H 2 O, H + H +H 2 = H 2 + H 2 close to the burner tp. Some dstance aay from the burner ext hydrogen producton occurs due to the ater gas shft reacton, especally at hgher nput of H 2 %. Fgure 7 (b) kg m s Fgure 7: Contours of (a) mass fracton and (b) net reacton rate of H 2 The radal profles of mass fractons of CO and oxygen at the axal locaton of z*=15 from the burner ext are shon n Fg. 8. Fgure 7 (a) Fgure 8: Varatons of mass fractons of CO and O 2 along the radal drecton at z* =

6 The CO reacton zone s slghtly der than that of H 2 n the radal drecton. The CO reacton zone also expands (z*~ 25) to a larger axal extent than hydrogen (see Fg. 11.a). These can be attrbuted to the loer flame speed of CO n comparson th hydrogen. If flame speed s less, then a relatvely larger flame surface area ll be needed, hch can be acheved th larger radal and axal extents for the reacton zone. Radal profles of CO 2 and H 2 O at the md-plane of the flame (z*=15 from the burner ext) are shon n Fg. 9. The maxmum value of CO 2 mass fracton occurs closer to the axs at a radal dstance of around r*=1.6, as a consequence both formaton reactons and the presence of CO 2 as a dluent n the fuel mxture. Water vapor s formed slghtly aay from the axs at around r*=1.95. Ths s due to the hgher dffusvty value for hydrogen and also for OH hch plays an ntermedary role n the formaton of H 2 O. The ater vapor mass fracton ncreases th ncreasng hydrogen content of the fuel; hoever, the maxmum locaton remans approxmately the same. contrbuton of the CO n terms of ts mass fracton ncreases (Fg. 11.a). Fgure 10: Varatons of mass fracton of OH along the radal drecton at z*=15 Fgure 9: Varatons of mass fractons of CO 2 and H 2 O along the radal drecton at z* = 15 Fgure 10 shos the radal profles of OH for the three cases at the axal locaton of z*=15. In general, the mass fracton of OH ncreases th ncrease n the H 2 fracton of the fuel. The maxmum OH locaton les n the range 1.8 < r*< 2.2, closely follong the zones of hgh H 2 O concentraton as ell as temperature. In Fgs. 11.a and 11.b, the mass fracton contours and the net reacton rates of CO are shon. Although ncrease n the volume percentage of hydrogen from case 1 to case, does not sgnfcantly affect the overall features of the CO reacton zone, subtle changes are seen beteen the three cases. Frstly, at a larger hydrogen volume percentage, the relatve Fgure 11 (a) Also, the net reacton rate of CO s mostly negatve n magntude, mplyng the occurrence of prmarly oxdaton reactons for CO. Hoever n the hgh temperature zones, CO producton occurs due to CO 2 dssocaton (see Fg. 1. b). The consumpton of CO (Fg 11.b.) and producton of CO 2 (Fg 1.b.) occurs 709

7 at the burner rm and gradually moves radally nards at larger axal dstance. Ths s because of the reactons nvolved n the system, gven as CO + O + (+) = CO 2 (+), CO + O 2 = CO 2 + O, CO + HO 2 = CO 2 + OH. kg m s Fgure 12 (a) Fgure 11 (b) Fgure 11: Contours of (a) mass fracton and (b) net reacton rate of CO Flame zone expands radally as H 2 mass fracton n the nput mxture s ncreased. Both the O 2 mass fracton contours and the net reacton rates of O 2 shon n Fg. 12.a and 12.b confrm ths trend as the percentage of H 2 changes from 16% to 26%. The reactons H + O 2 = O + OH and H2 + O 2 = HO 2 + H cause the consumpton of O 2 to form O, OH, HO 2, H. The net reacton rate contours of oxygen (Fg. 12. b) also ndcate dssocaton reactons to be occurrng n the hgh temperature zones, gvng rse to postve O 2 reacton rates. In Fgs. 1.a and 1.b, the mass fracton contours and net reacton rates of CO 2 are plotted. The reacton rate contours of CO 2 closely follo that of CO, ndcatng that oxdaton of CO s the prmary route for the formaton of CO 2. The mass fracton contours of CO 2 are governed by the amount of CO 2 present as a dluent n the ntal mxture, formaton reactons for CO 2 and the convectve and dffusve transport kg m s Fgure 12 (b) Fgure 12: Contours of (a) mass fracton and (b) net reacton rate of O 2 710

8 of CO 2 n the ambent. The trends seen for the major speces can be summed up as follos: It s apparent (Fg. 7 (a)) that H 2 havng lo mass fracton s consumed to form both H and OH, prmarly through the reacton, H 2 +O=OH+H (chan branchng). The OH reactons lead to the formaton of H 2 O through the reactons, O+H 2 = OH + H, OH+H 2 = H 2 O+H (chan propagaton) and OH + HO 2 = H 2 O+O 2 (chan termnaton). These occur near the burner ext. CO forms CO 2 th the help of OH, hch s an mportant reacton n CO-H 2 system, gven as, CO + OH = CO 2 + H. Hoever, CO 2 (Fg 1.b) formaton occurs aay from the burner ext but close to the axs, manly for to reasons; H 2 O reactons are faster, H 2 mass s lo and more quantty (by mass) of CO s present, hch s bascally transported by convecton n the axal drecton. Thus, hydrogen spreads both axally as ell as radally due to ts lghtness and hgher dffusvty. It may also be noted that reverse reactons nvolvng H 2 O+H and CO 2 +H are much sloer than the correspondng forard reactons OH+H 2 and CO+OH, manly because fnal stable products are seen n the reverse reactons, and they occur near the maxmum temperature zone. Fgure 1 (a) Fgure 1 (b) kg m s Fgure 1: Contours of (a) mass fracton and (b) net reacton rate of CO 2 The mass fracton contours and net reacton rates of OH are seen n Fgs. 14.(a) and (b). The mass fracton of OH s maxmum n the hgh temperature zone near the burner rm. Here also the smlar trend s observed here OH spreads radally outards (spreads der) and axally aay from the burner ext th the ncrease n composton of H 2 mass fracton. The follong reactons partcpate to form OH along th the other speces H + O 2 = O + OH, HO 2 + H = OH + OH, HO 2 + O = OH + O 2, H 2 O 2 + O = OH + HO 2, CO + HO 2 = CO 2 + OH, HCO + O = CO + OH. Both zones of net postve and negatve reacton rates are seen hch sgnfy zones of OH producton and consumpton. Atomc hydrogen s a hghly actve radcal and t s capable of dffusng nsde the flame zone also. It partcpates n reactons nvolvng OH outsde the flame and n reactons nvolvng CO 2 and CO nsde the flame, thus gettng dstrbuted n the hgh reacton zone around the flame. Also the producton zone of H becomes thcker (seen n radal drecton) and slghtly moves aay from the burner ext (n axal drecton) th ncrease n percentage of hydrogen mass fracton. It s also seen from Fg 15, 711

9 Fgure 14 (b) Fgure 14: Contours of (a) mass fracton and (b) net reacton rate of OH Fgure 14 (a) Fgure 15: ass fracton of H that the maxmum producton of atomc hydrogen occurs at the burner tp. Thus, the reacton nvolvng CO+OH = CO 2 + H could be the man step for the producton of atomc hydrogen. kg m s COCLUSIOS Syngas combuston n jet dffuson flame mode has been numercally smulated. Basc composton of the syngas has been taken from the product of gasfcaton of Indan coals reported n lterature. An axsymmetrc model th smplfed Davs mechansm [1] has been used to smulate jet dffuson flames fueled by syngas. The hydrogen content n the fuel mxture has been ncreased by keepng the fractons of CO 2 and CO the same, by decreasng the ntrogen n the fuel. Three cases have been smulated. A detaled examnaton of the reacton zone ndcates that along the radal drecton, CO oxdaton follos the formaton of ater vapor by hydrogen oxdaton. The hgher speces dffusvty and knetc rate coeffcent values for the hydrogen-oxygen reacton over those of CO play a crucal role n determnng the overall syngas 712

10 flame characterstcs. The structure of the jet dffuson flame and the hgh temperature locatons can be vared by varyng the hydrogen content n the fuel mxture. Knoledge of these changes n the flame structure could be of use hle desgnng syngas burners for dfferent gas compostons. REFERECES [1] Longell JP, Rubn ES, Wlson J. Coal: energy for the future. Prog Energy Combust. Sc, 21, 1995, [2] atarajan J, Kochar Y, Leuen T, Setzman J. Pressure and preheat dependence of lamnar flame speeds of H 2 /CO/CO 2 /O 2 /He mxtures. Proc Combust Inst, 2, 2009, [] Hannemann F, Koestln B, Zmmermann G, Haupt G. Hydrogen and syngas combuston: pre-condton for IGCC and ZEIGCC. Semens AG poer generaton, W8I, 2005, G2. [4] Brdar R, Jones R. GE IGCC technology and experence th advanced gas turbnes. GE poer systems, Schenectady, Y, 2000 GER [5] Sun, H., Yang SI, Jomaas G, La CK. Hghpressure lamnar flame speeds and knetc modelng of carbon monoxde/ hydrogen combuston. Proc Combust Inst, 1, 2007, [6] Herzler, J., aumann, C., Shock tube study of the gnton of lean CO/H 2 fuel blends at ntermedate temperatures and hgh pressure. Combust. Sc. Tech., 180, 2008, [7] Dryer FL, Chaos. Ignton of syngas/ar and hydrogen/ar mxtures at lo temperatures and hgh pressures: expermental data nterpretaton and knetc modelng mplcatons. Combust Flame, 152, 2008, [8] Ichkaa Y, Otaara Y, Kobayash H, Ogam Y, Kudo T, Okuyama, et al. Flame structure and radaton characterstcs of CO/H 2 /CO 2 /ar turbulent premxed flames at hgh pressure. Proc Combust Inst,, 2011, [9] Danele S., Jansohn P, antzaras J, Boulouchos K. Turbulent flame speed for syngas at gas turbne relevant condtons. Proc Combust Inst,, 2011, [10] Frassoldat A, Faravell T, Ranz E. The gnton, combuston and flame structure of carbon monoxde/hydrogen mxtures. ote 1: detaled knetc modelng of syngas combuston also n presence of ntrogen compounds. Int J. Hydrogen Energy, 2, 2007, [11] Sung CJ, La CK. Fundamental combuston propertes of H 2 /CO mxtures: gnton and flame propagaton at elevated pressures. Combust. Sc. Technol., 180, 2008, [12] Chaos, Dryer FL. Syngas combuston knetcs and applcatons. Combust Sc Technol, Vol. 180, 2008, pp [1] Davs SG, Josh AV, Wang H, Egolfopoulos F. An optmzed knetc model of H 2 /CO combuston. Proc Combust Inst, 0, 2005, [14] La CK, Jomaas G, Bechtold JK. Cellular nstabltes of expandng hydrogen/propane sphercal flames at elevated pressures: theory and experment. Proc. Combust. Inst., 0, 2005, [15] Tahtouh T, Halter F, Samson E, ounam- Rousselle C. Effects of hydrogen addton and ntrogen dluton on the lamnar flame characterstcs of premxed methane-ar flames. Int. J. Hydrogen Energy, 4, 2009, [16] R. K. Iyengar and R. Haque. Gasfcaton of hgh-ash Indan coals for poer generaton. Fuel Processng Technol., 27, 1991,