A N um eric al S tu dy on Com bu s tion Ch aract eri s tic s of Hig h - T em perature Cat aly tic Com bu s t or 200 1 2 ( )
A N um eric al S tu dy on Com bu s tion Ch aract eri s tic s of Hig h - T em perature Cat aly tic Com bu s t or 200 1 2 ( )
200 1 2
I (Pt ) T hermal (Hybrid). 2, GRI 2.11, Chou. Site density.,. T hermal ( ) CO NOx.., CH 4, T hermal,.,,,,,. T hermal
II, NOx N 2 +O(+M ) N 2O(+M ) NO, N 2O. N 2O NO, T hermal NO Prompt NO. CO NOx 1ppm 6 ppm.
III A B S T RA CT T he objectiv e of this paper w as to inv estigat e combustion characteristics of the hybrid catalytic combu stor w hich con sist ed of a catalytic combu stor and a therm al combustor with a lean m ethane- air mixtur e on platinum cataly st. Num erical investigations ar e performed by a tw o- dimen sion boundary layer model and detailed reaction m echanism. F or gaseou s chemistry GRI 2.11 w as employed and for surface chemistry the r eaction m echanism by Chou w as u sed. At catalytic combu stor, for the m or e accurat e calculation s the actual site den sity of catalytic surface w hich w as w ashcoated support s w as decided by the comparison with experimental data. T he effects of gaseou s r eaction on catalytic combu stion inv estigated by comparison of t w o cases w hich both surface and gaseou s chemistry w er e con sidered and surface chemistry w as con sider ed only. T he effect s of oper ation condition s at the entrance w er e evaluat ed. At thermal combustor, the production char acteristics of CO and NOx w hen an addition fuel w as injected at entr ance of the thermal combustor entr ance or not. T he result s w er e as follow s : At cat alytic combu stor, the effect s of gaseou s r eaction w ere too w eek t o alt er the macroscopic view point such as light off location and CH4 conv er sion r ate. But in therm al combu stor, gaseou s r eaction had a great effect s on interm ediates
IV formation at dow n str eam of the catalytic combu st or and combu stion characteristics at entrance of the thermal combustor. T herefor e both surface and gaseou s chemistry mu st be incorpor ated to estim ate combu stion characteristics accurately. T he num erical simulation with these result s enable u s to predict the effect s of oper ation condition s r easonably such as equiv alence ratio, temperature, v elocity, channel diam eter and pr essure. It w ill also pr ovide beneficial information s to decide design factor s and operating condition s of cat alytic combu stor. In case additional fuel w as not injected at therm al combu stor, it w as show n that the production of N2O w as m or e dominant than that of NO due to the relatively important reaction N 2 +O(+M ) N 2O(+M ). How ever, pr oduction of NO becomes more incr eased than that of N2O, and Prompt NO mechanism becomes mor e dominant than T hermal NO m echanism with increasing the am ount of additional fuel injection. Finally w e could identify that the emissions of CO and NOx w er e v ery low about 1 ppm and 6 ppm respectiv ely at an exit.
V 1. 1 1.1 1 1.2 2 1.3 5 2. 6 2.1 6 2.2 6 2.3 11 2.4 13 3. 16 3.1 16 3.1.1 Site density 16 3.1.2 18 3.1.3 20 3.1.4 23
VI 3.2 T hermal 40 3.2.1 (Catalytic Stabilized) 40 3.2.2 (Hybrid) 42 4. 47 5. 49
VII N OM EN CLA T U RE c p : Mixture heat capacity c p k : Specific heat capacity of kth species C k : Creation rate of species k D k : Destruction rate of species k D kj : Multicomponent diffusion coefficient D T k : T hermal diffusion coefficient g : Acceleration of gr avity h k : Specific enthalpy of species k k f i : Forw ard reaction rate constant k ri : Rever se reaction rate constant K g : T otal number of gas- phase species K s : T otal number of surface species M : Mass flux M l : Mass loss rate at the lower boundary p : T hermodynamics pr essur e r : Radial distance s k : Rate of production of kth species by surface reaction
VIII T : T emperature u : Axial direction velocity v : Stefan velocity V k r : Diffusion velocity of species k W k : Molecular weight of species k W : Mixture m ean molecular w eight x : Axial distance X k : Mole fraction of species k Y k : Mass fraction of species k : T hermal conductivity of mixture : Viscosity of moxture : Stream function : Density i : Density at the reactor inlet : Normalized stream function k : Rate of production of species k ki' : Stoichiometric coefficient of reactant k ki' ' : Stoichiom etric coefficient of pr oduct k Z k : Site fraction of the kth species : Site density for surface phase
IX LIS T OF F IGU RE S Fig. 1 Fig. 2 Fig. 3 Description of r eaction s in the monolith Surface r eaction mechanism of methane oxidation Schem atic of the hybrid catalytic combu stor and computational domain Fig. 4 Profiles of mean temperature and CH4 m ole fraction w ith site densities Fig. 5 Structur e of catalytic reaction in the m onolith Fig. 6 Radical pr ofiles of temperature and CH4 m ole fraction at selected axial distance Fig. 7 Fig. 8 Contour s of CO and OH mole fr action for Surf and Both Profiles of CO and NOx mole fr action for Surf and Both at therm al combu stor Fig. 9 T emper atur e and CH4 mole fr action isopleths for varying inlet equivalence r atio Fig. 10 Streamwise w all and mean t emperature and CH4 conver sion rate at exit for v arying inlet equiv alence ratio Fig. 11 T emper atur e and CH4 mole fr action isopleths for varying inlet temper ature Fig. 12 Streamwise w all and mean t emperature and CH4 conver sion
X rate at exit for v arying inlet t emperature Fig. 13 T emper atur e and CH4 mole fr action isopleths for varying inlet velocity Fig. 14 Streamwise w all and mean t emperature and CH4 conver sion rate at exit for v arying inlet v elocity Fig. 15 T emper atur e and CH4 mole fr action isopleths for varying diameter of monolith channel Fig. 16 Streamwise w all and mean t emperature and CH4 conver sion rate at exit for v arying diameter of monolith channel Fig. 17 T emper atur e and CH4 mole fr action isopleths for varying inlet pressure Fig. 18 Streamwise w all and mean t emperature and CH4 conver sion rate at exit for v arying inlet pressur e Fig. 19 Profiles of mean temperature, r adicals (OH, O, H ), CO and NOx mole fr action at therm al combu stor Fig. 20 Profiles of N2O pr oduction r ate by each r eaction at center line Fig. 21 Profiles of mean temperature and NOx when 25% and 50% of initial CH4 w ere added Fig. 22 Profiles of mean temperature, NOx and CO mole fr action at the exit w hen CH4 w as added
XI LIS T OF T A B LE S T able 1. Calculation Condition s at inlet of the cat alytic combu st or T able 2. Surface r eaction m echanism of CH4 oxidation on platinum taken fr om C. P. Chou
1 1.1 1.,, CO2, (NOx ),,.,.,. (Flame combustion ), (NOx ), (CO).. [1] (Flameless combustion),., NOx, CO
2 (UHC) [2,3].,,, [4,5,6 ]. Fig. 1 (Homogeneous), (Heterogeneous). Fig. 2,,,.. 1.2. 1200. (Pd) (Pt ), [7] (Hexaaluminates) [8,9,10]. (Supports ),.
3 Fig. 1 Description of reactions in the monolith (a) Adsorption reactions (b) Surface reactions and desorption reactions Fig. 2 Surface reaction mechanism of methane oxidation
4, (Catalytically stabilized) (Hybrid), [11,12,13]., 1,, 2. [14] Raja (Parabolic) (E - lliptic) 2,. [15 ],. Hickman [16] Deutschmann [17], Chou [18 ] CH 4 CH 4., Schlegel [19,20 ] CH 4 C3H 8 NOx,. Dalla Betta [2 1] CO,
5..,, T hermal,. T hermal. 1.3 (CH4 ) (Pt ) T hermal..,,, Monolith. T hermal ( ) (Catalytically Stabilized) (Hybrid), CO NOx.
6 2.1 2. Fig. 3 (a) Monolith., Fig. 3 (b) T hermal., 2. [2 2 ] 0.15 cm, T hermal 8 cm 80 cm. 2.2 (Steady ) 2, Von Mises x M (1) (4). [2 3 ]
7 (a) Schematic of the hybrid catalytic combustor (b) Schematic and dimensions of the computational domain Fig. 3 Schematic of the hybrid catalytic combustor and computational domain
8 = u u x - u M ( dm dx - dm l u ) dx + dp dx u M 2 ( u r 2 u ) + g ( i - ) ( 1) u Y k x - = k W k - dm l dx Y k u M ( dm - ) dx u M ( r Y k V k r ) ( k = 1,, K g ) (2) = u c p T x - u c p M ( dm dx - dm l ) dx T u M 2 ( u r 2 T ) - K g k = 1 k W k h k - 2 u r M K g k = 1 Y k V k r c p k T ( 3) p = R T W (4) x. r, u, Y k k, T, c p, p,,, W, k, V k (5).
9 V k r = u r X k WM K g j k W jd kj X j - D T k Y k ur TM T (5) X k k, W j, D kj, D T k. k (6). k = C k - D k (6), C k D k k, (7), (8). C k = I i = 1 ' ki k ri K j = 1 [ X j ] ' ' j i + I i = 1 ' ' ki k f i K j = 1 [ X j ] ' j i ( 7) D k = I i = 1 K ' ki k [ X f i j ] j = 1 ' j i + I i = 1 K ' ' k i k [ X ri j ] j = 1 ' ' j i ( 8) ' k i i k, ' ' k i i k, k f i k ri.
10 (9) (10 12). Site fraction Z k d t = s k = 0 ( k = K g + 1,..., K g + K s ) (9) Site fraction, Z k k (Surface coverage), Site density, s k. (J k + v Y k) n = s k W k, ( k = 1 K g ) ( 10), J k, n, v (Stefan ), (11). v = 1 ( K g k = 1 s k W k ) n ( 11)
11 { T K r - g (J k + Y k v) h k n k = 1 }g = K g + K s k = K g + 1 s k W k h k ( 12). CRESLAF Code [2 3 ], CHEMKIN- [2 4] SURFACE CHEMKIN (Ver 4.0) [25 ] T RANFIT Package [2 6 ]. 2.3.,. 70,.. Hybrid T hermal. T able 1 Bond [2 7]
12 T ab le 1. Calculation Condition s at inlet of the catalytic combu stor 0.39 867 [K] 730 [cm/s] 1 [atm],. T hermal.,.,., Fig. 3 (a) T hermal,, T hermal.
13 2.4 CH 4 Chou [18 ], T able 2. H, OH, O, H 2O, CO, C, CH, CH2, CH3, Pt Bulk Pt (B) 11. 7, 11 5 23. Arrhenius. Motz Wise [28 ] Sticking coefficient, Sticking coefficient 1. 49 279 GRI 2.11 [29 ]. C1, C2 - Chemistry, NOx T hermal NO, Prompt NO, N2O NO NO2.. T hermal GRI 2.11, NOx T hermal NO Nishioka [30 ].
14 GRI NOx Zeldovich T hermal NO, NOx T hermal NO N 2O Prompt NOx T hermal T hermal NO. T hermal CH 4. C on v ers ion [ % ] = CO 2 CO 2
15 Surface reaction mechanism of CH4 Table 2 oxidation on platinum taken from C. P. Chou ADSORPTION REA CTION A [cm,mole,sec] S Ea [kj/mole] A01 : O2+2PT (*) => 2O(*)+2PT (B) 0.003 0.0 0.00 A02 : CH4+2PT (*) => CH3(*)+H (*)+2PT (B) 0.150 0.0 27.00 A03 : CH4+O(*)+PT (*) => CH3(*)+OH (*)+PT (B) 0.430 0.0 59.20 A04 : CO+PT (*) => CO(*)+PT (B) 0.840 0.0 0.00 A05 : H2+2PT (*) => H (*)+H (*)+2PT (B) 0.046 0.0 0.00 A06 : OH +PT (*) => OH (*)+PT (B) 1.000 0.0 0.00 A07 : H2O+PT (*) => H2O(*)+PT (B) 0.500 0.0 0.00 SURFA CE REA CTION S01 : CH3(*)+PT (*) => CH2(*)+H (*)+PT (B) 1.0E +21 0.0 20.00 S02 : CH2(*)+PT (*) => CH (*)+H (*)+PT (B) 1.0E +21 0.0 20.00 S03 : CH (*)+PT (*) => C(*)+H (*)+PT (B) 1.0E +21 0.0 20.00 S04 : H (*)+O(*)+PT (B) => OH (*)+PT (*) 1.0E +20 0.0 10.50 S05 : OH (*)+PT (*) => H (*)+O(*)+PT (B) 1.0E +12 0.0 20.80 S06 : H (*)+OH (*)+PT (B) => H2O(*)+PT (*) 1.0E +21 0.0 62.50 S07 : 2OH (*) => H2O(*)+O(*) 1.0E +20 0.0 51.25 S08 : H2O(*)+PT (*) => OH (*)+H (*)+PT (B) 1.8E +13 0.0 54.20 S09 : C(*)+O(*)+PT (B) => CO(*)+PT (*) 5.0E +20 0.0 62.50 S10 : CO(*)+PT (*) => C(*)+O(*)+PT (B) 1.0E +13 0.0 156.50 S11 : CO(*)+O(*)+2PT (B) => CO2+2PT (*) 4.0E +20 0.0 49.14 DESORP TION D01 : 2O(*)+2PT (B) => O2+2PT (*) 1.0E +21 0.0 216.00 D02 : CO(*)+PT (B) => CO+PT (*) 8.5E +12 0.0 152.50 D03 : 2H (*)+2PT (B) => H2+2PT (*) 5.0E +20 0.0 67.40 D04 : OH (*)+PT (B) => OH+PT (*) 1.5E +13 0.0 192.80 D05 : H2O(*)+PT (B) => H2O+PT (*) 1.0E +13 0.0 45.00 S: Stick coefficient, (*): surface species, PT(*): Free surface site, PT(B): Bulk species site The kinetic constant of the ith reaction is expressed as k f i = A T exp ( - E a R u T ) Case of adsorption process, k f i = ( S 1 - S/ 2 ) 1 m R T 2 W
16 3.1 3. 3.1.1 S ite den s ity Site Density ( SD ),, SD.,, SD. 2.707E - 09 m ole / cm 2 [17]. Bond [2 7] SD,,. Fig. 4 Bond SD CH 4. Fig. 4 (a) SD 2.707E - 09 m ole / cm 2,. SD 1.757E - 10 m ole/ cm 2,
17 (a) Temperature Fig. 4 b) CH4 Conversion rate Profiles of mean temperature and CH4 mole fraction with site densities
18. Fig. 4 (b ) CH 4 SD, SD 1.757E - 10 m ole / cm 2. CO. Bond SD 1.757E - 10 m ole/ cm 2. 3.1.2 Fig. 5,. 2.7 cm, (Light off) (Surface ignition ).,, (Kinetic control region) (Mass transfer control region ).,. [3 1].
19 Fig. 5 Structure of catalytic reaction in the monolith
20,. 3.1.3, Fig. 6 ( Surf ) ( Both ) CH 4.,., CH 4. Fig. 7 OH CO. Fig. 7 (a) Surf, OH, Both, Surf,. OH, OH. Fig. 7 (b ) CO, Both Surf..
21 (a) Radial profiles of temperature (b) Radial profiles of CH4 Fig. 6 Radial profiles of temperature and CH4 mole fraction at selected axial distance
22 (a) OH mole fraction (b) CO mole fraction Fig. 7 Contours of CO and OH mole fraction for Surf and Both
23 Fig. 8 T hermal CO NOx. Surf Both T hermal, T hermal., Both CO NOx Surf. CO, Both, NOx.,, CH 4,, T hermal. T hermal. 3.1.4,. Bond [27 ]
24 Fig. 8 Profiles of CO and NOx mole fraction for Surf and Both at thermal combustor
25 0.39, 867 K, 730 cm/ s, 1 atm. Fig. 9 0.35 0.39 CH 4. 0.35,,, CH4.,. 0.39, 2.7 cm,. CH4.,. Fig. 10 CH 4. 0.35 CH 4,. 0.39, 1300 K CH 4 90 %. T hermal 0.375
26 (a) Temperature (b) CH4 mole fraction Fig. 9 Temperature and CH4 mole fraction isopleths for varying inlet equivalence ratio
27 Fig. 10 Streamwise wall and mean temperature and CH4 conversion rate at exit for varying inlet equivalence ratio
28. Fig. 11 840 K 850 K CH 4,. 840 K, 900 K, CH4. 850 K 10 K 6 cm, CH 4.. Fig. 12 CH 4. 850 K, 1200 K, CH 4 70%. Griffin [3 2 ] 0.4 857 K. 1300 K 870 K. CH 4. Fig. 13 5, 15 m/ s CH 4
29 (a) Temperature (b) CH4 mole fraction Fig. 11 Temperature and CH4 mole fraction isopleths for varying inlet temperature
30 Fig. 12 Streamwise wall and mean temperature and CH4 conversion rate at exit for varying inlet temperature
31. 5 m/ s 2 cm 15 m/ s 6 cm.. CH4, 15 m/ s CH 4. Fig. 14 CH 4.,. CH 4,. Fig. 15 Monolith 0.1, 0.3 cm, CH 4, Monolith. 0.3 cm, 0.1 cm., 0.1 cm
32 (a) Temperature (b) CH4 mole fraction Fig. 13 Temperature and CH4 mole fraction isopleths for varying inlet velocity
33 Fig. 14 Streamwise wall and mean temperature and CH4 conversion rate at exit for varying inlet velocity
34, 0.3 cm.. CH4. Fig. 16 Monolith. Ratio, 0.3 cm, CH 4.,.,. Fig. 17 1, 20 atm, CH4. 10 atm,.,. 20 atm, 1 atm,
35 (a) Temperature (b) CH4 mole fraction Fig. 15 Temperature and CH4 mole fraction isopleths for varying diameter of monolith channel
36 Fig. 16 Streamwise wall and mean temperature and CH4 conversion rate at exit for varying diameter of monolith channel
37. CH 4, CH 4. Fig. 18 CH 4. CH 4, 10 atm. CH 4,. CH 4,,,.
38 (a) Temperature (b) CH4 mole fraction Fig. 17 Temperature and CH4 mole fraction isopleths for varying inlet pressure
39 Fig. 18 Streamwise wall and mean temperature and CH4 conversion rate at exit for varying inlet pressure
40 3.2 T herm al 3.2.1 (Cat aly tic St abilized ) Fig. 19 T hermal, OH, O, H, CO NOx. Fig. 19 (a) 5 cm 1400 K, OH, O, H,. Fig. 19 (b) CO, CO2. NOx 0.4 ppm, N 2O NO NO2. NOx NO NO2, N 2O CH 4 NOx [3 3,3 4 ]., Fig. 20 N2O (R1 R7) ( ), R7 0.1. N 2O R6, N 2 O(+M ) N 2O R7. N 2O H, O N2 R3, R1, R7
41 (a) Mean temperature and radicals (b) CO and NOx Fig. 19 Profiles of mean temperature, radicals(oh, O, H), CO and NOx mole fraction at thermal combustor
42., 1400 K N2O NOx. 3.2.2 (Hy brid ) T hermal, CH4 (%). Fig. 21 (a), (b) 25% 50% NOx, T hermal NO NO T heraml NO Prompt NO. 25%, 5 cm 1600 K. NOx, NO N 2O N 2 N 2O, NOx. NOx NO, NO Prompt NO, T hermal NO. 50%, 1800 K. NOx N2O NO, T hermal NO NOx. NOx NO, NO T hermal NO
43 Fig. 20 Profiles of N2O production rate by each reaction at center line
44 Prompt NO. Fig. 22 T hermal, CO, NOx T hermal NO, N2O. 1400 1800 K, CO NOx 25%. T hermal NO 1600 K, 50% 1800 K NOx Prompt NO NO. 1600 K, CO NOx 5.9 ppm, 1.1 ppm.
45 (a) 25% CH4 addition (b) 50% CH4 addition Fig. 21 Profiles of mean temperature and NOx when 25% and 50% of initial CH4 were added
46 Fig. 22 Profiles of mean temperature, NOx and CO mole fraction at the exit when CH4 was added
47 4., T hermal CO NOx. 1., CH 4, T hermal,. 2.,,,,. 3. T hermal, NOx NO N 2O.. N 2 +O(+M ) = N 2O(+M ) 4. T hermal N2O
48 NO, T hermal NO Prompt NO. 5. 1600 K, NOx CO 1.1 ppm 5.9 ppm.
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