Energy Fuels 2010, 24, 165–172 Published on Web 08/13/2009
: DOI:10.1021/ef9005324
Numerical Simulation of the Partial Oxidation of Hot Coke Oven Gas with a Detailed Chemical Kinetic Model† Koyo Norinaga* and Jun-ichiro Hayashi Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received May 25, 2009. Revised Manuscript Received July 16, 2009
The non-catalytic partial oxidation of hot coke oven gas (HCOG) was numerically simulated using a detailed chemical kinetic model and a completely mixed batch reactor model. The kinetic model was primarily based on that developed by Richter and Howard [Phys. Chem. Chem. Phys. 2002, 4 (11), 20382055], including more than 200 chemical species and more than 2000 elementary-step-like reactions. The HCOG was modeled as a multi-component gas mixture involving H2, CO, CO2, CH4, C2 hydrocarbons, H2O, and 31 aromatic hydrocarbons, such as benzene and toluene, as well as polycyclic aromatic hydrocarbons up to coronene, to represent the HCOG tar. The effect of oxygen addition in the inlet gas mixture was investigated at oxygen concentrations from 0 to 15 vol %. The simulations indicated that oxygen was consumed almost completely for the combustion of reactive light gases, such as H2 and CO, and light hydrocarbons, such as CH4 and C2H6, within a reaction time of a few tens of milliseconds when the inlet gas temperature was 1173 K. In reforming the tar involved in HCOG, the primary role of oxygen should be to induce temperature increases of the reacting gas by combustion, thereby accelerating the subsequent reforming of the tar by steam. A reaction pathway analysis suggested that the co-existence of oxygen and aromatic hydrocarbon radicals was necessary to decompose aromatic compounds into compounds of lower molecular mass.
aromatic hydrocarbons (PAHs). Kirton and Crisp analyzed in detail the condensable components of coke oven emissions by gas chromatography.2 The chromatogram could be resolved into 183 peaks, 99 of which were identified and quantitated. This reveals the high complexity of HCOGreforming chemistry. In developing an efficient reforming process for HCOG, it is necessary to understand the chemistry and kinetics of HCOG reforming. Jess studied the kinetics of the thermal conversion of aromatic hydrocarbons, such as naphthalene, benzene, and toluene, in the presence of hydrogen and steam.10 The primary product from the aromatic hydrocarbons was found to be soot. Neither the soot nor the organic cracking products, such as methane, was converted completely by steam to CO and H2, even at the highest temperature investigated (1673 K). Miura et al. conducted pyrolysis and catalytic steam reforming of tar from an actual COG, aiming to convert the tar in HCOG into synthesis gas. Here, soot (they called it coke) was also the major product of both pyrolysis and catalytic reforming. Other experimental efforts have been made to clarify the effects of operating variables, such as temperature, residence time, catalyst, and gas-phase composition, on the product distributions in COG tar reforming.4,6-9,11-13 However, mechanistic interpretations of the observed phenomena are scarcely found in the literature. A detailed chemical kinetic approach to develop a reaction mechanism consisting of hundreds or thousands of
1. Introduction In producing metallurgical cokes, a coke oven gas (COG) is given off at a level of 300-350 N m3 per ton of coal.1 The COG consists primarily of hydrogen and methane and also involves carbon monoxide, carbon dioxide, and tar.2 In the conventional process, the hot COG (HCOG) exhausted from coke ovens is quenched to recover the tar, which is further distilled to obtain marketable feed stocks for aromatic chemicals and pitch cokes. The tar-free COG is refined through a desulfurization facility and used as fuels for coke ovens, fired heaters in ironworks, and power stations.1 Despite the current established system of COG processing, the production of clean gas by reforming HCOG has been studied to provide an efficient use of HCOG heat.3-9 HCOG is a multi-component gas mixture. In particular, the tar comprising about 30 wt % of the HCOG contains various aromatic compounds, including monoring aromatics, such as benzene, toluene, and xylene, as well as various polycyclic † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: norinaga@ cm.kyushu-u.ac.jp. (1) Aramaki, T. J. Jpn. Inst. Energy 2006, 85 (5), 342–347. (2) Kirton, P. J.; Ellis, J.; Crisp, P. T. Fuel 1991, 70 (12), 1383–1389. (3) Jess, A. Stahl Eisen 1995, 115 (3), 115–119. (4) Jess, A. Chem. Eng. Process. 1996, 35 (6), 487–494. (5) Miura, K.; Kawase, M.; Nakagawa, H.; Ashida, R.; Nakai, T.; Ishikawa, T. J. Chem. Eng. Jpn. 2003, 36 (7), 735–741. (6) Onozaki, M.; Watanabe, K.; Hashimoto, T.; Saegusa, H.; Katayama, Y. Fuel 2006, 85 (2), 143–149. (7) Li, L. Y.; Morishita, K.; Takarada, T. J. Chem. Eng. Jpn. 2006, 39 (4), 461–468. (8) Guo, J. Z.; Hou, Z. Y.; Gao, J.; Zheng, X. M. Energy Fuels 2008, 22 (3), 1444–1448. (9) Cheng, H. W.; Zhang, Y. W.; Lu, X. G.; Ding, W. Z.; Li, Q. Energy Fuels 2009, 23 (1), 414–421.
r 2009 American Chemical Society
(10) Jess, A. Fuel 1996, 75 (12), 1441–1448. (11) BraekmanDanheux, C.; Fontana, A.; Laurent, P.; Lolivier, P. Fuel 1996, 75 (5), 579–584. (12) Shen, J.; Wang, Z. Z. Energy Fuels 2007, 21 (6), 3588–3592. (13) Zhang, Y. W.; Li, Q.; Shen, P. J.; Liu, Y.; Yang, Z. B.; Ding, W. Z.; Lu, X. G. Int. J. Hydrogen Energy 2008, 33 (13), 3311–3319.
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: DOI:10.1021/ef9005324
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elementary-like reaction steps is a promising method for elucidating an accurate description of the phenomena that occur in the gas phase.14 To date, numerical simulations with detailed chemical kinetic models have been performed to predict the chemistry and kinetics of combustion, as well as the pyrolysis of hydrocarbons.15-23 However, few studies have reported on the partial oxidation as well as the steam reforming of this multi-component gas mixture. In the current study, a numerical simulation of the partial oxidation of HCOG was conducted by employing a detailed chemical kinetic model to understand the reforming chemistry and kinetics at the mechanistic level.
Table 1. Composition of the Simulated HCOG for the Numerical Simulation
2. Numerical Simulation 2.1. Computation. A reaction mechanism for hydrocarbon combustion proposed by Richter and Howard18 was used as the primary basis for the kinetic model to simulate HCOG reforming. Oxidation reactions of phenyl, naphthyl, and acenaphthyl radicals with oxygen and oxidation reactions of benzene, naphthalene, and acenaphthylene with OH radicals provided by Sheng and Dean19 were added to the mechanism to account for the CO2 formation observed experimentally in the steam reforming of naphthalene.24 The mechanism used in this study consisted of 2235 reactions, including 257 chemical species from the smallest species of hydrogen radical to the largest molecule of coronene. Simulations were conducted using the HOMREA software package that was designed for computational analysis of time-dependent homogeneous reaction systems.14 In this program, the input includes the forward reaction rate parameters and thermodynamic polynomials for all of the participating species, in addition to temperature, pressure, and reactant concentrations. The program calculates the rate constant of the backward reaction for each reversible reaction given. Computations were performed under isobaric and adiabatic conditions. Here, the rate constants of the chemical reactions taken into account shall be considered as parameters of the system. The solutions of the differential equation system were numerically evaluated using a code written in the Fortran 77 at given initial conditions, such as initial concentrations, pressure, temperature, etc. A detailed description of the numerical simulation is reported elsewhere.21 The numerical simulations were executed at various oxygen concentrations. Four different HCOG/O2 volumetric mixing ratios (100:0, 95:5, 90:10, and 85:15) were tested. The inlet gas temperature was set at 1173 K, which is a temperature typical of the actual HCOG emitted from a coke oven.6 2.2. HCOG Composition. The composition of the COG was estimated on the basis of the literature (see Table 1).1,2 Using the detailed analysis of organic matter in COG by Kirton et al.,2 the COG tar was represented by a mixture of 31 aromatic
compound
mole fraction
H2 CH4 C2H2 C2H4 C2H6 CO CO2 N2 O2 H2O benzene toluene ethylbenzene styrene phenol indene naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene cyclopenta[def]phenanthrene 2-phenylnaphthalene fluoranthene acephnanthrylene aceanthrylene pyrene benzo[a]fluorene cyclopenta[cd]pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene benzo[ghi]perylene anthanthrene coronene
0.433000 0.253000 0.009857 0.009857 0.003286 0.054000 0.016000 0.021000 0.000000 0.186000 0.007065 0.001581 0.000977 0.000137 0.000301 0.000395 0.002197 0.000192 0.000086 0.000151 0.000031 0.000095 0.000308 0.000078 0.000012 0.000006 0.000124 0.000005 0.000004 0.000085 0.000007 0.000002 0.000034 0.000036 0.000027 0.000021 0.000022 0.000006 0.000009 0.000004 0.000001
hydrocarbons, including benzene, toluene, and PAHs up to coronene. The chemical structures of the compounds considered to model the tar from HCOG are shown in Figure 1. Although the tar also involved aromatic compounds with hetero atoms, such as benzofuran, indole, quinoline, benzothiophene, carbazole, and their derivatives, these heterocyclic compounds were excluded from the simulated COG because little information is available on the elementary reactions and kinetic parameters for their thermochemistries.
3. Results and Discussion
(14) Warnatz, J.; Maas, U.; Dibble, R. W. Combustion, 3rd ed.; Springer-Verlag: Heidelberg, NY, 2000. (15) Dean, A. M. J. Phys. Chem. 1990, 94 (4), 1432–1439. (16) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Castaldi, M. J.; Senkan, S. M. Combust. Sci. Technol. 1996, 116 (1-6), 211–287. (17) Wang, H.; Frenklach, M. Combust. Flame 1997, 110 (1-2), 173– 221. (18) Richter, H.; Howard, J. B. Phys. Chem. Chem. Phys. 2002, 4 (11), 2038–2055. (19) Sheng, C. Y.; Dean, A. M. J. Phys. Chem. A 2004, 108 (17), 3772– 3783. (20) Ziegler, I.; Fournet, R.; Marquaire, P. M. J. Anal. Appl. Pyrolysis 2005, 73 (2), 212–230. (21) Norinaga, K.; Deutschmann, O. Ind. Eng. Chem. Res. 2007, 46 (11), 3547–3557. (22) Khan, R. U.; Bajohr, S.; Buchholz, D.; Reimert, R.; Minh, H. D.; Norinaga, K.; Janardhanan, V. M.; Tischer, S.; Deutschmann, O. J. Anal. Appl. Pyrolysis 2008, 81 (2), 148–156. (23) Norinaga, K.; Janardhanan, V. M.; Deutschmann, O. Int. J. Chem. Kinet. 2008, 40 (4), 199–208. (24) Garcia, X. A.; Huttinger, K. J. Fuel 1989, 68 (10), 1300–1310.
3.1. Predictive Capability of the Kinetic Model. Prior to simulation of the HCOG partial oxidation, the predictive capability of the present reaction model was tested through comparisons of numerical simulation outputs to experimental results. We simulated the experiments of Garcia and Huttinger,24 who studied the pyrolysis and steam reforming of naphthalene in a flow reactor. Hydrogen yields as a function of the residence time are depicted in Figure 2. Naphthalene was pyrolyzed in a flow reactor at temperatures ranging from 1023 to 1173 K. The initial naphthalene mole fraction was 0.028; the remainder was argon. The hydrogen yield increased with the residence time and temperature. The simulation results are also given in Figure 2. While an over-prediction after 30 s at 1173 K and an under-prediction before 30 s at 1123 K were observed, 166
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: DOI:10.1021/ef9005324
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Figure 1. Chemical structures of aromatic species contained in the simulated COG.
better agreement was found at other residence times at 1123 and 1173 K and over the entire residence time at 1023 and 1073 K. Shown in Figure 3 is a comparison between simulations and experiments for steam reforming of naphthalene. The initial mole fractions of naphthalene and H2O were 0.01 and 0.2, respectively, and the remainder was argon. CO2 is a major products in naphthalene steam reforming. Despite the under-predictions at 1123 and 1173 K, the kinetic model reproduced the experiments fairly. Considering that this is a first attempt to use a detailed chemical kinetic model to predict pyrolysis and steam reforming of naphthalene and that no adjustable parameter was used in the numerical
simulations, the reasonable agreement between computations and experiments should be encouraging evidence for further investigation to simulate the reforming reactions of HCOG. Nevertheless, it is necessary to validate more persistently the applicability of the reaction mechanism. Comparisons to available experimental data obtained at well-defined conditions10 are currently underway and will be reported in the future. 3.2. Reaction Characteristics of Simulated HCOG. The time courses of the reacting gas temperature in the pyrolysis and partial oxidation of the modeled HCOG are illustrated in Figure 4. When oxygen was added to HCOG, the temperature rose very quickly. The temperature increments 167
Energy Fuels 2010, 24, 165–172
: DOI:10.1021/ef9005324
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Figure 2. Hydrogen yield as a function of the residence time during pyrolysis of naphthalene at various temperatures. Initial naphthalene mole fraction = 0.028. nH2/nnapthalene represents the molar ratio of H2 to the initial amount of naphthalene. Symbols are the experiments, and lines are the numerical simulations.
Figure 4. Numerically simulated temperatures of the reacting gas in pyrolysis and partial oxidation of the model COG as a function of the reaction time. The upper panel shows an enlarged view of the results at residence times from 0 to 0.05 s.
PAHs through the hydrogen abstraction and acetylene addition mechanism.17,25 The numerical simulation results for the aromatic hydrocarbons that represent the tar involved in HCOG are given in Figure 7. In the absence of oxygen, the conversion of aromatic hydrocarbons (except indene) is rather difficult. Most aromatic hydrocarbons are converted by the addition of oxygen, whereas formations are more dominant for pyrene and benzo[a]pyrene. The enhanced formation of pyrene by oxygen addition is a challenge to interpret simply and was attributed to the thermal stability of pyrene and the mole fraction increase of acetylene that added to phenanthrene, forming pyrene. The conversion of these refractory compounds became remarkable at 15% of oxygen. The mole fraction of coronene increased over time, regardless of the oxygen concentration, because it was the largest PAH considered in the reaction sequences. In the actual HCOGreforming process, such heavy PAHs should be converted into soot. Also presented in Figure 7 are the total mole fractions of 34 PAHs ranging from indene to coronene. For the net decomposition of the PAHs, it was necessary to elevate the temperature beyond 1800 K, which required more than 15% oxygen. The water consumption corresponded well to the formation of hydrogen and carbon monoxide, as well as the consumption of the PAHs, indicating that the steam reforming proceeded slowly following the rapid increase in temperature because of combustion. 3.3. Reaction Pathway Analysis. Shown in Figure 8 is a reaction flux diagram for the important pathways of naphthalene conversion in HCOG reforming at 10% oxygen and a reaction time of 0.5 s. The arrow thickness represents the degree of the contribution of the reaction for the consumption of the compound at the starting point of the arrow.
Figure 3. Carbon dioxide yield as a function of the residence time during steam reforming of naphthalene at various temperatures. Initial naphthalene and H2O mole fractions are 0.01 and 0.2, respectively. nCO2/nnapthalene represents the molar ratio of CO2 to the initial amount of naphthalene. Symbols are the experiments, and lines are the numerical simulations.
became extensive with increasing oxygen. The maximum temperatures were 1400, 1700, and 1800 K for oxygen contents of 5, 10, and 15%, respectively. After the peaks were exhibited at very early stages of the reaction, the temperatures decreased gradually over time and reached almost constant values. The rapid temperature increases in the HCOG partial oxidations accompanied rapid decreases in the mole fractions of oxygen, hydrogen, methane, C2H6, and C2H4 and the rapid increases in mole fractions of carbon monoxide, carbon dioxide, and water (Figures 5 and 6). These data indicate that the temperature increases of the reacting gas in the HCOG partial oxidation at the very early stages were induced mainly by the combustions of hydrogen and light hydrocarbons. The subsequent gradual decreases in temperature accompanied gradual decreases in the mole fractions of water, suggesting that endothermic steam reforming dominated after the depletion of oxygen. Unlike C2H6 and C2H4, C2H2 was refractory and its mole fraction actually increased in the early stages of the reactions. Consecutive dehydrogenations of C2H6 and C2H4 should correspond to the formation of C2H2. The gradual decrease in the C2H2 mole fraction was attributed to decomposition by steam reforming and consumption for the growth of
(25) Wang, H.; Frenklach, M. J. Phys. Chem. 1994, 98 (44), 11465– 11489.
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Figure 5. Numerically simulated mole fractions of inorganic gases in pyrolysis and partial oxidation of the model COG as a function of the reaction time. The upper panels for hydrogen, oxygen, and water show enlarged views of the results at residence times from 0 to 0.05 s.
reactions during steam gasification of naphthalene.24 They also found naphthol, indene, styrene, benzene, acenaphtylene, and phenanthrene in the products. Beside the reaction pathways leading to small molecules, there also existed opposing pathways, such as the combination of cyclopentadienyl and indenyl radicals to form phenanthrene. Such radical combinations are recognized as an important route in growing PAHs/soot.16,26,27 The interpretation of the reaction pathways of naphthalene, which should be a typical compound involved in HCOG tar at a mechanistic level, revealed that many elementary-step-like reactions were involved in the conversion of tar to small molecules. It is clear that the addition of oxygen is necessary to decompose naphthalene. However, the reactivity of oxygen is extremely high, and the oxygen is almost completely consumed for the combustion of hydrogen and lower hydrocarbons
For example, 23% of naphthalene was consumed by the reaction with H and OH radicals and converted to 1-naphhtyl radical. Naphthalene likely decomposed through three steps: the first was naphthyl radical formation by the attacks of H and OH radicals; the second was the oxygen attack and subsequent addition of oxygen to form naphthoxy radical; and the third was the detachment of carbon monoxide converting to a compound with lower molecular mass. When similar processes were repeated, naphthalene containing 10 carbon atoms converted to indene with 9 carbon atoms as a result of the carbon monoxide release. The indene subsequently converted to the stiryl radical with 8 carbon atoms. The styril radical decomposed into the phenyl radical and acetylene. Oxygen attacked the phenyl radical to form the phenoxy radical, which further decomposed into carbon monoxide and the cyclopentadienyl radical. The cyclopentadienyl radical thermally cracked into acetylene and the propargyl radical to eventually accomplish dearomatization. This reaction sequence derived from the reaction flux analysis is more or less similar to the scheme of chemical
(26) Mulholland, J. A.; Lu, M.; Kim, D. H. Proc. Combust. Inst. 2000, 28, 2593–2599. (27) Lu, M. M.; Mulholland, J. A. Chemosphere 2001, 42 (5-7), 625–633.
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Figure 6. Numerically simulated mole fractions of lower hydrocarbons in pyrolysis and partial oxidation of the model COG as a function of the reaction time. The upper panels show an enlarged view of the results at residence times from 0 to 0.05 s.
tar and a secondary oxygen injection downstream supplies the oxygen necessary to decompose aromatic radicals, would be one option to improve the selectivity of tar reforming in the partial oxidation of HCOG.
within a few tens of milliseconds. In such a situation, it is difficult to convert naphthalene to smaller species, even if it converts to a radical, because the concentration of oxygen is very low, owing to the rapid consumption for the combustion of reactive compounds. The HCOG is a multi-component mixture that involves compounds of varying reactivity. The selective reforming of tar in HCOG is desirable to avoid decreasing the calorific value of the reformed gas and soot formation, but this is a very difficult task because oxygen prefers to react with more reactive species, such as hydrogen and lower hydrocarbons involved in the HCOG. The mechanistic interpretations may suggest a guiding principle to increase the selectivity of tar reforming, where there would be sufficient oxygen present to attack/add to aromatic radicals when the aromatic compounds become radicals upon H abstraction by H and OH radicals. Therefore, multi-staged injections of oxygen to the HCOG reformer, where a primary oxygen injection induces a sufficient increase in temperature to reform
4. Conclusions A detailed chemical kinetic modeling approach was first applied to simulate the partial oxidation of HCOG. The kinetic model proposed by Richter and Howard18 is an established model, of which the predictive capabilities have been critically evaluated for the combustion of light hydrocarbons. Because the application of this kinetic model to the prediction of the reforming chemistry of the HCOG-containing PAHs is new to this work, its predictive capabilities were examined through comparisons of numerical simulation outputs with existing experimental results for the pyrolysis and steam reforming of naphthalene reported by Garcia and Huttinger.24 The kinetic model could reasonably predict the 170
Energy Fuels 2010, 24, 165–172
: DOI:10.1021/ef9005324
Norinaga and Hayashi
Figure 7. Numerically simulated mole fractions of aromatic hydrocarbons in pyrolysis and partial oxidation of the model COG as a function of the reaction time.
Figure 8. Result of the reaction pathway analysis for the naphthalene conversion in partial oxidation of the model COG at 1173 K and COG/O2 = 90:10.
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formation of hydrogen in naphthalene pyrolysis and the formation of carbon dioxide in naphthalene steam reforming. HCOG was modeled as a multi-component gas mixture involving H2, CO, CO2, CH4, C2 hydrocarbons, H2O, and 31 aromatic hydrocarbons, such as benzene, toluene, and PAHs up to coronene, to represent the HCOG tar. The numerical simulations of the model HCOG indicate that oxygen was consumed almost completely for the combustion of reactive light gases, such as hydrogen, CO, CH4, and C2 hydrocarbons, within a reaction time of several milliseconds when the inlet gas temperature was 1173 K. The major role of oxygen should be to induce temperature increases of the reacting gas by such combustions, thereby accelerating the subsequent reforming of tar by steam. The use of detailed chemical kinetic modeling is certainly valuable to obtain a deeper insight into the chemical processes involved. Our reaction pathway analysis demonstrated that naphthalene likely decomposed via the three steps: naphthyl radical formation by the attacks of H and OH radicals, oxygen attack and subsequent addition of oxygen to form the naphthoxy radical, and finally, detachment of carbon monoxide, converting to a compound of lower molecular mass.
Further, we found that the addition of oxygen to the naphthyl radical was a necessary step to decompose naphthalene. The mechanistic interpretations may suggest a guiding principle to increase selectivity of tar reforming to create a reaction circumstance where sufficient oxygen is available to attack/add to aromatic radicals when the aromatic compounds become radicals upon H abstraction by H and OH radicals. This demonstrates how the injection of oxygen to the HCOG reformers, including factors such as the position of the injectors as well as timing and multi-staged oxygen injections, could be crucial in a reactor design to improve the selectivity of tar reforming in the partial oxidation of HCOG. Acknowledgment. The authors acknowledge the use of the computer program HOMREA by Prof. J. Warnatz (University of Heidelberg). K.N. also acknowledges support from the Ministry of Education, Science, Sports, and Culture Grant-in-Aid for Young Scientists (20760518). The authors are also grateful to New Energy and Industrial Technology Development Organization (NEDO), JFE 21st Century Foundation, and Steel Industry Foundation for the Advancement of Environmental Protection Technology for their financial supports.
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