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Modeling NO-Char Reaction at High Temperature Juwei Zhang, Shaozeng Sun,* Xidong Hu, Rui Sun, and Yukun Qin Combustion Engineering Research Institute, School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China ReceiVed January 18, 2009. ReVised Manuscript ReceiVed February 23, 2009
Five Chinese coal chars were prepared in a flat flame flow reactor that can simulate the conditions of real pulverized coal combustion. The kinetics of the NO-char reaction at 1273, 1373, 1473, and 1573 K was studied in a drop tube furnace. A new model of the high-temperature NO-char reaction that took into account the pore diffusion and thermal annealing of char was proposed. The NO-char reaction was predicted by a combination of the model and computational fluid dynamics. Through the comparison of experimental and predicted results, the kinetic parameters for all chars were determined. The predicted results agree well with experimental results at 1273, 1373, and 1473 K, but the model largely underestimates the experimental data at 1573 K. This behavior is explained by a new mechanism of the high-temperature NO-char reaction. In addition, the model successfully normalizes the reactivity of chars with different ranks at high temperature, which is considered to be valuable in predicting the NO reduction on char surface at high temperature.
1. Introduction NO is one of the major pollutants of coal combustion. However, the detail mechanism of NO reduction on char surface is still not clearly understood, especially at high temperature (above 1273 K), and there are large variations in reaction data.1 There were large numbers of experimental studies on the NO-char reaction in the past2-15 (detailed literature review can be found in our previous paper16), and some kinetic models were obtained from these experiments. Nevertheless, these models could not describe the NO-char reaction accurately since they were oversimplified. For example, it is well-known that pore diffusion can not be ignored in the gas-solid reaction at high temperature. However, in most studies, the effectiveness factor * Corresponding author: Phone: +86-451-86412238; fax: +86-45186412528; e-mail:
[email protected]. (1) Aarna, I.; Suuberg, E. M. Fuel 1997, 76 (6), 475–491. (2) Zhong, B. J.; Zhang, H. S.; Fu, W. B. Combust. Flame 2003, 132 (3), 364–373. (3) Chambrion, P.; Suzuki, T.; Zhang, G. Z.; Kyotani, T.; Tomita, A. Energy Fuels 1997, 11 (3), 68–85. (4) Wu, S. L.; Iisa, K. Energy Fuels 1998, 12 (3), 457–463. (5) Aarna, I.; Suuberg, E. M. Energy Fuels 1999, 13 (6), 1145–1153. (6) Liu, H.; Kojima, T.; Feng, B.; Liu, D. C.; Lu, J. D. Energy Fuels 2001, 15 (3), 696–701. (7) Sorensen, C. O.; Johnsson, J. E.; Jensen, A. Energy Fuels 2001, 15 (6), 1359–1368. (8) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Energy Fuels 2003, 17 (6), 1429–1436. (9) Lopez, D.; Calo, J. Energy Fuels 2007, 21 (4), 1872–1878. (10) Dong, L.; Gao, S. Q.; Song, W. L.; Xu, G. W. Fuel Process. Technol. 2007, 88 (7), 707–715. (11) Song, Y. H.; Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1981, 25, 237–340. (12) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Beer, J. M. Eighteenth Symposium (International) on Combustion, The Combustion Institute: Pittsburgh, PA, 1981; pp 111-120. (13) Zhong, B. J.; Tang, H. Combust. Flame 2007, 149 (1-2), 234– 243. (14) Li, Y. H.; Radovic, L. R.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1999, 54 (19), 4125–4136. (15) Wang, S. B.; Slovak, V.; Haynes, B. S. Fuel Process. Technol. 2005, 86 (6), 651–660. (16) Sun, S. Z.; Zhang, J. W.; Hu, X. D.; Wu, S. H.; Yang, J. C.; Wang, Y.; Qin, Y. K. Energy Fuels 2008, 23 (1), 74–80.
that signified the extent of pore diffusion limitation was neglected13,17,18 or considered to be constant during the reaction process.19 According to the definition of the effectiveness factor, it changes with the partial pressure of gaseous species. Therefore, it is necessary to take into account the variations of the effectiveness factor. In this study, a new model of the NO-char reaction at high temperature was proposed, and the kinetic parameters were determined by a combination of experiments and computational fluid dynamics (CFD) simulation. On the basis of the results of experiments and prediction, the mechanism of the NO-char reaction at high temperature was discussed. The purpose of this study is to build a new high-temperature NO-char reaction model that can take into account the pore diffusion and thermal annealing of char and to explore the mechanism of the NO-char reaction at high temperature. 2. Experimental Section Char Preparation. Five Chinese coals, a lignite (YB coal), two bituminous coals (SH coal and SJ coal), and two anthracite (YQ coal and JC coal), were used in this study. The raw coals were milled and sieved to ensure that almost all coal particles were in the size range 53-75 µm. The chars were prepared from these coals in a flat flame flow reactor (FFR) as in the previous paper.16 Coal particles were pyrolyzed in a postflame zone of the flat flame operated under fuel-rich conditions with almost no oxygen existing in the postflame region. The maximum particle heating rate in the FFR was 105 K/s. The feeding rate of coal particles was 4-6 g/h, depending on the bulk density of coals. The particle residence time in this experiment was around 60 ms, which ensured that the devolatilization process was completed in the postflame zone in FFR.20 (17) Jones, J. M.; Patterson, P. M.; Pourkashanian, M.; Williams, A. Carbon 1999, 37 (10), 1545–552. (18) Taniguchi, M.; Yamamoto, K.; Kobayashi, H.; Kiyama, K. Fuel 2002, 81 (3), 363–371. (19) Schonenbeck, C.; Gadiou, R.; Schwartz, D. Fuel 2004, 83 (4), 443– 450. (20) Ma, J. L. Ph.D. Thesis, Brigham Young University, Utah, USA, 1996.
10.1021/ef9000488 CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
Modeling NO-Char Reaction at High Temperature
Energy & Fuels, Vol. 23, 2009 2377 Table 1. Properties of Chars Prepared from FFR SJ char
YQ char
JC char
volatiles (%) ash (%) fixed carbon (%)
Proximate Analysis (db) 14.2 11.1 9.3 31.3 10.6 56.4 54.5 78.3 34.3
5.8 11.6 82.5
6.4 6.1 87.9
carbon (%) hydrogen (%) nitrogen (%) sulfur (%)
Ultimate Analysis (daf) 92.1 92.6 93.9 3.6 2.3 2.5 1 0.9 1.5 1.5 1.4 1.6
96.1 2.2 1.3 1.8
97.1 0.9 1.2 0.5
properties
YB char
SH char
Specific Surface Area (m2 g-1) BET 217.7 244.4 70.0 26.2 2.8 Hga 18.2 11.1 6.9 2.0 1.9 2215 1586 2065 1833 2172 true density (kg m-3) porosity 0.845 0.784 0.746 0.557 0.601 mass mean 52.9 53.3 50.8 46.5 31.2 b diameter (µm) spread parameterb 1.39 1.21 1.24 1.42 1.39 a The specific surface area of pores larger than 20 nm. parameters obtained from size distribution of char particles.
Figure 1. Schematic of DTF facility.
Experimental Apparatus. Experiments were conducted in a high-temperature, laminar drop tube furnace (DTF). A schematic of the DTF is shown in Figure 1. The flow reactor is an alumina tube with an inner diameter of 50 mm and a length of 1.4 m, which was heated electrically to the required temperature before every experiment. The furnace temperatures in experiments were set to 1273, 1373, 1473, and 1573 K, respectively. Char particles were entrained by N2 (0.5 L/min, 1 atm, 273 K) into the furnace at a feeding rate of about 5-20 g/h depending upon the bulk density of chars. A gas flow (5 L/min, 1 atm, 273 K) consisting of NO and N2 was preheated to 873 K before being introduced into the hot zone of the furnace. The initial NO concentration was set to 1040 ppmv. Reaction products were sampled by an oil-cooled, nitrogenquenched probe, and the composition of exhaust gases were measured by an online Fourier transform infrared (FTIR) gas analyzer (GASMET-DX4000, Finland). The sampling points were fixed to the four positions that were 400, 500, 600, and 700 mm away from injection port, respectively. Characterization Measurements. The complete pore structure for all char samples was obtained by combining porosimetry with gas adsorption. Macropore/mesopore surface area and pore volume were measured by mercury porosimetry (AutoporeII 9220, Micromeritics, Norcross, GA). Nitrogen adsorption at 77 K was performed in an automated adsorption analyzer (ASAP 2020, Micromeritics, Norcross, GA), and the specific surface area was calculated by the Brunauer-Emmett-Teller (BET) equation. The true density was measured by a helium pycnometer (AccuPyc 1330, Micromeritics, Norcross, GA). The size distribution of char particles was estimated by a laser size analyzer (Master Min, Malvern, U.K.). The results are given in Table 1.
3. Modeling CFD Simulation. The radial mixing between char and gases is difficult because the laminar flow was used in DTF. To take into account the poor radial mixing, an axisymmetric twodimensional (2D) simulation of the reactor was used in this study. Axisymmetric simulations have also been used in other
b
The
studies to model the reactions in DTF.19,21,22 The governing equations for gas-phase fluid mechanics, heat transfer, thermal radiation, and scalar transport were solved with the commercial CFD code FLUENT, with the gravitational model, P1 radiation model, and particle radiation model being enabled. The particle trajectories were calculated using spherical drag-law, and coupled calculations of discrete and continuum phase were enabled. It was observed that the size distribution of char particles in this study agreed well with Rosin-Rammler distribution, which can be expressed as j
Yd ) e-(d/d)
n
(1)
where dj and n are input parameters of discrete phase model in FLUENT, which can be obtained through fitting the particle size data to the Rosin-Rammler function. The values of dj and n for all the chars are listed in Table 1. When trajectory calculations were performed, the complete range of sizes was divided into adequate discrete intervals and each was represented by a mean diameter. NO-Char Reaction Submodel. The NO-char reaction is described by the following overall reactions: 2NO + C f N2 + CO2
(R1)
NO + C f 1/2N2 + CO
(R2)
According to the previous studies,1,4,16 R2 was the major reaction at high temperature; therefore, three gaseous species, NO, N2, and CO were considered in the reaction model. In addition, the reaction of NO with coal char was found to be of first order with respect to NO.1,11,19 For a single char particle, the rate of the heterogeneous reaction is given as follows: -RC ) YCkCSCPNOη
(2)
where the intrinsic rate constant kC and the inner surface area of a single char particle SC are expressed as follows: (21) Brewster, B. S.; Smoot, L. D.; Barthelson, S. H. Energy Fuels 1995, 9 (5), 870–879. (22) Visona, S. P.; Stanmore, B. R. Combust. Flame 1999, 118 (1-2), 61–75.
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kC ) A exp(-E/RTP)
(3)
SC ) VPFPAi ) 1/6πd3FPAi
(4)
where Ai is the specific internal surface area of the char particle that can be measured by mercury porosimetry or nitrogen adsorption. On the basis of our previous study16 and other studies,25,26 in comparison to the BET surface area (measured by nitrogen adsorption), the Hg surface area (measured by mercury porosimetry) is a better basis for normalizing the reactivity of different coal chars since the measured values are less scattered. So the Hg surface area is used to represent the specific internal surface area of the char particle. η is the effectiveness factor, or the ratio of the actual reaction rate to the rate attainable if no pore diffusion resistance existed: 1 1 3 φ tanh φ φ
(
η)
)
(5)
where φ is the Thiele modulus: φ)
d 2
Figure 2. Gas temperature distribution along the axis of FFR.
RAiFpkCPNO DeCNO
(6)
According to eqs 5 and 6, η changes with Ai, PNO, and CNO, which are not constant during the reaction process of char particles. Assuming that the bulk and Knudsen diffusion proceed in parallel, the effectiveness diffusion coefficient De is given by: De )
(
ε 1 1 + τ D0 Dk
)
-1
(7)
The porosity of char particle ε can be obtained from ε)1-
FP Ft
(8)
The tortuosity factor τ can be obtained through the expression proposed by Wakao and Smith:23 τ ) 1/ε
(9)
The molecular diffusion coefficient D0 and Knudsen diffusion coefficient Dk can be expressed as [(TP + T∞)/2] d
0.75
D 0 ) C1 Dk )
2re 3
8RTP πMNO
(10)
(11)
where re is an empirical mean pore radius according to the pore model proposed by Satterfield,24 and it can be defined as 2ε re ) Ai FP
NO-char reaction, which can be attributed to the thermal annealing of char particles. The main mechanism of thermal annealing is the gradual graphitic ordering of the char by structural changes within the carbon matrix.27-29 Thermal annealing is a complex process, and several models have been proposed to incorporate the thermal history of chars,30-34 among which the model of Hurt et al.31 is the most suitable and simple one. In this model, the active sites were proposed to be deactivated by a first-order thermal process, which was assumed to share a common pre-exponential factor for annealing with distributed activation energies for annealing. The ratio of the total number of sites to the number of sites at the initial time (N/N0) can be calculated from
(12)
Thermal Annealing Submodel. According to our previous study,16 the surface area decreases during the high-temperature (23) Wakao, N.; Smith, J. M. Chem. Eng. Sci. 1962, 17 (11), 825–837. (24) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; MIT Press: Cambridge, MA, 1970. (25) Commandre, J. M.; Stanmore, B. R.; Salvador, S. Combust. Flame 2002, 128 (3), 211–216. (26) Salvador, S.; Commandre, J. M.; Stanmore, B. R.; Gadiou, R. Energy Fuels 2004, 18 (2), 296–301.
dFE ) -AdFE exp(-Ed /RT) dt N ) N0
∫
∞
0
FE(Ed, t)dEd
(13)
(14)
The value of N/N0 can be obtained by a numerical integration of eqs 13 and 14 with a 30-bin discretization in Ed. At the initial time, the frequency distribution function is assumed to be a normalized log-normal distribution31 in Ed or a shifted Γ-distribution.33,34 The shifted Γ-distribution was used in this study since it was proven to be able to predict well the thermal annealing of char at high temperatures.34 The shifted Γ-distribution was defined as (27) Russell, N. V.; Gibbins, J. R.; Man, C. K.; Williamson, J. Energy Fuels 1999, 14 (4), 883–888. (28) Zolin, A.; Jensen, A. D.; Jensen, P. A.; Johansen, K. D. Fuel 2002, 81 (8), 1065–1075. (29) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Michell, G. D. Fuel 1995, 74 (9), 1297–1306. (30) Senneca, O.; Russo, P.; Salatino, P.; Masi, S. Carbon 1997, 35 (1), 141–151. (31) Hurt, R. H.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113 (1-2), 181–197. (32) Salatino, P.; Senneca, O.; Masi, S. Energy Fuels 1999, 13 (6), 1154– 1159. (33) Zolin, A.; Jensen, A.; Johansen, K. D. Proc. Combust. Inst. 2000, 28, 2181–2188. (34) Zolin, A.; Jensen, A.; Johansen, K. D. Combust. Flame 2001, 125 (4), 1341–1360.
Modeling NO-Char Reaction at High Temperature
Energy & Fuels, Vol. 23, 2009 2379
Figure 3. Comparison between experimental and predicted NO conversion for all chars along the axial direction of DTF.
FE(Ed) )
(Ed - δ)(R-1) Γ(R)β
R
(
exp -
Ed - δ β
)
(15)
where δ is a shift parameter taking into account a positive activation energy at the beginning of deactivation (kJ/mol). The mean activation energy (Ed,m) and variance (δd2) are simple functions of R and β (Ed,m ) δ + Rβ and δd2 ) Rβ2). The parameters obtained by Zolin et al.33 from different chars were used in this study. To use the thermal annealing submodel in this study, it is assumed that the reactive surface area of char is proportional to the total surface area, which was verified in other studies.14,35,36 Therefore, eq 11 becomes (35) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849–856.
Ai ) Ai0
∫
∞
0
FE(Ed, t)dEd
(16)
where Ai0 is the specific internal surface area of the char particle at the beginning of thermal annealing, and Ai0 must be specified for a given type of char. The thermal annealing of char occurs not only in the process of the high-temperature reaction with NO in DTF but also in the process of preparation in FFR. Assuming that the thermal annealing of char began at the end of devolatilization (around 15 ms20) in flat flame and that the temperature of gases exceeded that of the char particles by 200 °C in flat flame,37 Ai0 could be (36) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28 (1), 7– 19. (37) Therssen, E.; Gourichon, L.; Delfosse, L. Combust. Flame 1995, 103 (1-2), 115–128.
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Table 2. Calculated Results of Thermal Annealing in FFR for All Chars Ai/Ai0 Ai0 (m2/kg)
YB char
SH char
SJ char
YQ char
JC char
0.1211 150.3
0.1211 91.7
0.1211 57.0
0.1211 16.5
0.1211 15.7
determined based on the gas temperature distribution in the axial direction of FFR (see Figure 2). The calculated results are shown in Table 2. 4. Results and Discussion Calculation of Kinetic Parameters. The simple flow and temperature field were calculated from CFD simulation before the introduction of the NO-char reaction submodel and the thermal annealing submodel. The NO-char reaction submodel needs several input parameters such as the intrinsic preexponential factor (E), the intrinsic activation energy (A), the porosity of the char particle (ε), the mean pore radius (re), and the specific internal surface area of the char particle (Ai). Among these parameters, E and A are unknown, however, they can be estimated by fitting experimental data to calculate results via minimizing the sum of square errors. This method was used in other kinetic studies of both the char combustion38,39 and the NO-char reaction.19,40 The predicted NO concentration values were compared with experimental data, which were measured at different temperatures and different sampling positions, and the deviations are calculated as
(
[NO]pre - [NO]exp δi,j ) [NO]exp
)
(17) i,j
where i is the index set of reaction temperatures, and j is the index set of the sampling positions. The error for each prediction associated to a particular pair (E, A) is calculated as
δ)
∑ ∑δ
2 i,j
i
j
imaxjmax
(18)
Then, the pair (E, A) corresponding to the minimum error δ is selected to be the best estimation of the kinetic parameters for the char tested. Analysis of Predicted Results. Figure 3 shows the comparison between experimental and predicted NO conversion along the axis of DTF. For all types of char, good agreement between the trends of predicted and experimental results is observed at 1273, 1373, and 1473 K. Although Figure 3b shows different results between the experiment and the prediction for SH char, it can be found that experimental data distribute in both sides of predicted curves at 1273, 1373, and 1473K. Therefore, the predicted results are still acceptable. However, the model obviously underestimates the experimental results at 1573 K. To find the causes of disagreement between the predicted and experimental results at 1573 K, the mechanism of the NO-char reaction at high temperature should be explored. It seems that an unknown chemical reaction mechanism exists at high temperature. Some studies40-42 on the NO heterogeneous reduction mechanism were already done with most authors (38) Ballester, J.; Jimenez, S. Combust. Flame 2005, 142 (3), 210–222. (39) Jimenez, S.; Remacha, P.; Ballesteros, J. C.; Gimenez, A.; Ballester, J. Combust. Flame 2008, 152 (4), 588–603. (40) Lopez, A. B.; Garcia, A. G. G; Suarez, J. A. C. EnViron. Sci. Technol. 2002, 36 (24), 5447–5454. (41) Chambrion, P.; Kyotani, T.; Tomita, A. Energy Fuels 1998, 12 (2), 416–421.
Table 3. The Optimal Combinations of E and A, and Associated Minimum Errors δ at 1273, 1373, and 1473 K for All Chars YB char SH char SJ char YQ char JC char
E (kJ/mol)
A (kgC m-2 s-1 Pa-1)
δ
105 130 125 120 135
0.0009 0.008 0.002 0.002 0.006
0.008 41 0.016 24 0.001 56 0.001 30 0.001 24
focusing on their work on low temperatures that were below 1273 K. The reaction mechanism at high temperatures above 1273 K was then extrapolated from results obtained at much lower temperatures. Therefore, it is doubtful that the reaction mechanism at lower temperatures is suitable for the current hightemperature conditions tested. According to previous studies,40-42 the global NO heterogeneous reduction mechanism at temperatures above 1000 K could be explained by a combination of NO attack on the char forming surface complexes (O complexes and N complexes) and the desorption of these complexes. The main mechanism at high temperature could be summarizes as 2Cac + NO f C(N) + C(O)
(R3)
C(N) + NO f N2 + C(O)
(R4)
C(O) f CO
(R5)
where Cac is a freshly formed site that is created after the desorption of carbon-oxygen complexes (by means of R5). The mechanism above 1000 K is controlled by NO attack on the char surface (by means of reactions R3 and R4), which means that the rate of NO attack is lower than the desorption rate of C(O) in this temperature range. In R3, N complex C(N) and O complex C(O) are formed, and they both play significant roles in the high-temperature NO-char reaction. It is well-known that C(N) is thermally more stable than C(O), which is well recognized in many studies.41-43 Chambrion et al.41 found that the amount of C(N) increased with temperature, whereas the amount of C(O) decreased with temperature. Therefore, it is not proper to set the C(N)/C(O) ratio (m) to be unity as in R3. Then R3 should be corrected as when m e 1 (m + 1)Cac + NO f mC(N) + C(O) + (1 - m)*- N (R6) when m > 1 (m + 1)Cac + mNO f mC(N) + C(O) + (m - 1)*-O (R7) where *-N and *-O are added to keep the elemental balance of N and O, which are perhaps the oxidized catalytic surface species.7,40 Anyway, their nature and compositions are not fully known. Combining R4 and R6 with R7, the amount of NO per amount of carbon consumed (NO/C) is (m + 1)/(m + 1) when m e 1 and 2m/(m + 1) when m > 1, so m e 1, NO/C ) (m + 1)/(m + 1) ) 1
(19)
(42) Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Fuel 2007, 86 (12), 41–49. (43) Millet, J.; Millet, J. E.; Vivare, A. J. Chim. Phys. 1963, 60, 553– 562.
Modeling NO-Char Reaction at High Temperature
Energy & Fuels, Vol. 23, 2009 2381 Table 4. Key to Figure 4
symbol
char type
reactor
temperature range (K)
activation energy kJ/mol)
surface area basis
9 O 2 3 [ tilted 4 titled 1 ] f g
lignite char (YB char) high-volatile bituminous coal char (SH char) low-volatile bituminous coal char (SJ char) anthracite char (YQ char) anthracite char (JC char) Montana lignite char Montana lignite char sub-bituminous coal char several kinds of carbons low-volatile bituminous coal char
DTF DTF DTF DTF DTF DTF DTF DTF DTF and fixed bed DTF
1273-1473 1273-1473 1273-1473 1273-1473 1273-1473 1250-1750 1250-1750 1273-1573 1073-1750 1073-1273
105 130 125 120 135 137 147 120 133 131
Hg Hg Hg Hg Hg BET (external) BET (external) BET
m > 1, 1 < NO/C ) 2m/(m + 1) e 2
(20)
Then, eqs 19 and 20 could be used to explain why the model proposed in this study, which bases on the previous reaction mechanism, failed at 1573 K. In the global reaction R2, NO/C is assumed to be 1. At 1273, 1373, and 1473 K, if the amount of C(N) is less than or equal to that of C(O), (i.e., NO/C ) 1, m e 1), then R2 can be used to describe the NO-char reaction, but at 1573 K, if the amount of C(N) is larger than that of C(O), (i.e., 1 < NO/C e 2, m > 1), then R2 can not be applied to the NO-char reaction. In this study, since NO/C > 1 at 1573 K, actually more NO was consumed per amount of carbon in comparison to R2, then the model underestimated the experimental results (see Figure 3). Assuming that there is no C(O) formed (i.e., mf +∞, NO/C ) 2) in R7 at 1573 K, the global reaction at 1573 K can be written as C + 2NO f CO + N2 + *-O
(R8)
The kinetic parameters (E and A) of R8 for all types of char were assumed to be the same as those obtained from the reactions at 1273, 1373, and 1473 K (see Table 3), then the NO conversion could be calculated. The calculated results are also presented in Figure 3. It can be seen that the predicted results for all types of char are distinctly improved, but the predicted results for YQ and JC chars still largely underestimate the NO conversions. This can be attributed to two causes. First, it is difficult to obtain the accurate value of m, and second, it is not sure whether the reaction at 1573 K (R7) has the same kinetic parameters as that at the temperatures below 1573 K (R6). However, both problems are not clearly understood at present and will be studied in the forthcoming work.
Hg
reference this study this study this study this study this study Song et al.11 Levy et al.12 Schonenbeck et al.19 Aarna and Suuberg1 Commandre et al.25
In addition, it is worth mentioning that there is a critical temperature Tc (NO/C ) 1 below Tc and NO/C > 1 above Tc) between 1473 and 1573 K under experimental conditions in DTF for all types of char. Chambrion et al.41 found that Tc was around 750 °C, which was much lower than the one in this study. However, in comparison with the small-scale fixed bed (with a bed height of 5 mm and a reactor inner diameter of 7 mm) used by Chambrion et al., DTF used in this study is more appropriate to create experimental conditions related to real pulverized coal combustion. Therefore, the value of Tc obtained in this study may be more useful in practical industrial applications of pulverized coal. The optimal combinations of E and A, and associated minimum errors δ at 1273, 1373, and 1473 K for all types of char are shown in Table 3. The scatter of E can be attributed to the catalytic effect of inherent mineral in char, since the catalytic metal matter can catalyze the NO-carbon reaction both by increasing the number of active sites and by the turnover rate on these sites as well as by significantly decreasing the activation energy.2,9,44 From Table 1, JC and SH chars have lower ash content; therefore, E and A obtained from JC and SH chars are considered as the real intrinsic kinetic parameters. Figure 4 and Table 4 summarize some first-order rate constants for the NO-char reaction. All data were obtained in DTF, except the data of Aarna and Suuberg,1 which were obtained by averaging a great deal of reported kinetic parameters. There are more than 2 orders of magnitude spread in all rate constants. The rate constants obtained in this study were all higher than others, since the Hg surface area was used as the area basis and the thermal annealing was taken into account in model. It should be noted that the rate constants obtained in this study distribute in a narrow zone, which indicates that the model proposed in this study to some extent successfully normalizes the reactivity of chars with different ranks at high temperature. It is well-known that there are large variations in the data of the NO-char reaction. Therefore, the normalized reactivity of different chars is of great use in predicting the NO reduction on char surface at high temperature. Conclusions The kinetic experimental study on the reaction of NO with five Chinese coal-chars was carried out in a DTF. A new model of the high-temperature NO-char reaction that took into account the pore diffusion and thermal annealing of char was proposed, and the kinetic parameters of reaction were determined for all types of char. Through the comparison of experimental and predicted results, some conclusions can be drawn as follows: (1) For all types of char, the predicted results agree well with experimental data at 1273, 1373, and 1473 K, but the model
Figure 4. Summary of first-order rate constants for the NO-char reaction
(44) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez, D. L. C Energy Fuels 1996, 10 (1), 158–168.
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based on previous reaction mechanism underestimates the experimental data at 1573 K. (2) On the basis of the previous study on the mechanism of the NO-char reaction, a new mechanism taking into account the variations of C(N)/C(O) was proposed, in which more NO is consumed per amount of carbon in comparison to previous mechanism, and this can be used to explain the failure of the previous model at 1573 K. (3) The critical temperature Tc (NO/C ) 1 below Tc and NO/C > 1 above Tc) is between 1473 and 1573 K under conditions related to real pulverized coal combustion. (4) The model in which the Hg surface area is used as the area basis successfully normalizes the reactivity of chars of different ranks, which is very important in predicting the NO-char reaction at high temperature. Acknowledgment. This work is sponsored by the Ministry of Science and Technology of China through the National Basic Research Program of China (contract No: 2006CB200303), the National High Technology Research and Development of China (contract No: 2007AA05Z336), Natural Science Foundation of China (contract No: 50576020), and the Program of Excellent Team in Harbin Institute of Technology. Nomenclature d ) particle diameter (m) dj ) mass mean diameter (m) Yd ) mass fraction of particles with diameter greater than d(dimensionless) n ) spread parameter (dimensionless) RC ) rate of the particle surface species (carbon) consumption (kgC/s) YC ) mass fraction of surface species (carbon) in the char particle (dimensionless) kC ) intrinsic rate constant (kgC m-2 s-1 Pa-1)
Zhang et al.
SC ) internal surface area of the char particle (m2) PNO ) bulk partial pressure of NO (Pa) η ) effectiveness factor (dimensionless) A ) intrinsic pre-exponential factor for the NO-char reaction (kgC m-2 s-1 Pa-1) E ) intrinsic activation energy for the NO-char reaction (kJ/ mol) R ) gas constant (J mol-1 K-1) TP ) temperature of the char particle (K) VP ) volume of the char particle (m3) FP ) apparent density of the char particle (kg m-3) Ai ) specific internal surface area of the char particle (m2 kg-1) φ ) Thiele modulus (dimensionless) R ) mass of NO per mass of carbon consumed (dimensionless) De ) effective diffusion coefficient in the particle pores (m2 s-1) CNO ) concentration of NO in the bulk gas (kg m-3) ε ) porosity of the char particle (dimensionless) τ ) tortuosity factor of the pores (dimensionless) D0 ) molecular diffusion coefficient (m2 s-1) Dk ) Knudsen diffusion coefficient (m2 s-1) FP ) apparent density of the char particle (kg m-3) Ft ) true density of the char particle ((kg m-3) C1 ) mass diffusion-limited rate constant (m3 s-1 K-0.75) T∞ ) temperature of bulk gas far from the char particle (K) re ) empirical mean pore radius (m) MNO ) molecular mass of NO (kg mol-1) FE ) normalized frequency distribution function for active sites (kJ-1) Ad ) pre-exponential factor for annealing (s-1) Ed ) activation energy for annealing (kJ/mol) Ai0 ) specific internal surface area of the char particle at the beginning of deactivation (m2 kg-1) [NO]pre ) predicted NO concentration (ppm) [NO]exp ) measured NO concentration (ppm) EF9000488
