Application of an Existing Detailed Chemical Kinetic Model to a

Jun 24, 2010 - A test plant was installed on a platform of an operating coke oven. HCOG was fed into a tubular reactor (0.6 m i.d. and 3.2 m long) at ...
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Ind. Eng. Chem. Res. 2010, 49, 10565–10571

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Application of an Existing Detailed Chemical Kinetic Model to a Practical System of Hot Coke Oven Gas Reforming by Noncatalytic Partial Oxidation Koyo Norinaga,*,† Hiroshi Yatabe,‡ Masahiro Matsuoka,§ and Jun-ichiro Hayashi† Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Fukuoka 816-8580, Japan, Coal Gasification System Center, Kure DiVision, Babcock-Hitachi K.K., Kure 737-8508, Japan, and R&D Center, Nippon Coke & Engineering Co., Ltd., Kitakyushu 808-0021, Japan

For more efficient utilization of coke oven gas (COG), a byproduct from the production of metallurgical cokes, a reforming technology of hot COG (HCOG) was developed to obtain material gases suitable for methanol production. A test plant was installed on a platform of an operating coke oven. HCOG was fed into a tubular reactor (0.6 m i.d. and 3.2 m long) at flow rates from 28 to 103 Nm3/h and was partially oxidized by injecting O2 (from 12 to 30 Nm3/h) from nozzles near the inlet. Exhaustive test runs identified the appropriate reforming conditions required to achieve more than 2.2-fold syngas amplifications, and the optimum product gas composition for methanol synthesis. Numerical simulations using detailed chemical kinetics coupled with a plug-flow reactor model were also conducted. The kinetic model developed by Richter and Howard [Phys. Chem. Chem. Phys. 2002, 4, 2038-2055] including 257 chemical species and 2216 elementary steplike reactions was used. HCOG was modeled as a multicomponent 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. Satisfactory agreement was observed in comparisons between the predictions from the numerical simulations and the data measured from the 20 test runs, indicating that the model can be a promising tool toward designing a demonstration/commercial HCOG reforming plant. 1. Introduction In the dry distillation of coal, that is, the metallurgical coke making process, a coke oven gas (COG) is emitted as a byproduct at a level of 300-350 Nm3 per ton of coal.1 The COG consists primarily of hydrogen and methane, and also involves carbon monoxide, carbon dioxide, and tar.2 The hot COG (HCOG) exhausted from coke ovens is conventionally quenched to recover the tar, which is distilled further to obtain marketable feed stocks for aromatic chemicals and pitch cokes. The tar-free COG is refined through a desulfurization facility and utilized 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 (without a cooling process) has been studied to provide an efficient utilization of the HCOG heat.3-18 Catalytic and noncatalytic approaches to reforming HCOG have been reported. Nickel is used frequently in catalytic HCOG reforming with steam, O2, and CO2.4,10,13-15 The deactivation of the catalyst by coking as well as sulfur poisoning due to H2S contained in the HCOG at a level of 4-7 g/Nm3 are likely to be unavoidable issues.19,20 Catalysts that are tolerant to the severe conditions of HCOG reforming are under development. Noncatalytic methods are also being studied, aiming at a more robust HCOG reforming technology.5,7-9,18 Most studies on the catalytic and noncatalytic reforming of HCOG employ a simulated HCOG to evaluate the catalyst activity and reaction characteristics. Little information is available on the reforming characteristics of real HCOG, which is a multicomponent gas mixture. In particular, the tar, comprising about 30 wt % of HCOG, contains * To whom correspondence should be addressed. Tel:. Fax: +8111-726-0731. E-mail: [email protected]. † Kyushu University. ‡ Babcock-Hitachi K.K. § Nippon Coke & Engineering Co., Ltd.

various aromatic compounds, including mono-ring aromatics such as benzene, toluene, and xylene, as well as various polycyclic aromatic hydrocarbons.2 This reveals the high complexity of HCOG reforming chemistries. To develop an efficient reforming process for HCOG, a deeper understanding of the chemistry and kinetics of HCOG reforming is essential. Onozaki et al.9 studied the noncatalytic partial oxidation and steam reforming of HCOG using gas evolved from a coal dry distillation in a small-scale coke oven that had been charged with 80 kg of coal. They succeeded in converting more than 98% of the carbon in the gas introduced to the reformer and increased the syngas volume by 2- to 3-fold. Feasibility studies have shown that hydrogen can be produced at a cost of about 9-10 Japanese yen/Nm3, which corresponds to 30% less than the cost of hydrogen obtained through quenching and purification of COG followed by pressure swing adsorption. According to the results obtained from bench-scale HCOG reforming tests, research toward developing a HCOG noncatalytic reforming technology has been conducted in collaboration between a coke manufacturer (Nippon Coke & Engineering Co., Ltd.) and an industrial plant developer (Babcock-Hitachi K.K.) in Japan since 2006. In this project, pilot-scale test facilities were installed on an actual operating coke oven for demonstration and commercialization. The performance of the reforming reactor was studied by measuring gas compositions at both the inlet and outlet of the reformer. This report presents part of the outputs from the pilot-scale tests. In addition, numerical simulations were conducted to achieve a better understanding of the HCOG reforming chemistry and kinetics, as well as of the precise prediction of reformer performance. Detailed chemical kinetic modeling coupled with a reactor flow model was employed for this purpose, since this modeling approach is state of the art for characterizing chemically reacting flows observed in industrially important reaction systems such as combustion,21-23 partial oxidation,24-26 and pyrolysis.27-29 The predictive capabilities of the developed

10.1021/ie100506v  2010 American Chemical Society Published on Web 06/24/2010

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Figure 1. Schematic representation of the pilot equipment installed on the platform of the operating coke oven for studying HCOG reforming by noncatalytic partial oxidation.

model were evaluated critically by exhaustive comparisons of the simulative results with the data measured from a total of 20 runs of the pilot-scale HCOG reforming tests. In turn, if good agreements are obtained between the test results and the numerical simulations, it is possible to support that the pilot tests results are certainly reliable since the detailed chemical kinetic model includes no adjustable parameters and was proved capable to predict other reaction system such as hydrocarbon combustion. It is also possible to interpret what is going on inside the COG reformer based on the model at mechanistic level. 2. Pilot-Scale HCOG Reforming Figure 1 shows a schematic diagram of the pilot equipment for HCOG reforming, which was installed on a platform of fullscale coke ovens of Kitakyushu Coking Works of Nippon Coke & Engineering Co., Ltd. The HCOG was collected from three coke chambers and introduced to the reformer together with O2 injected from four nozzles near the inlet. Position and angle of the nozzles were optimized based on preliminary CFD calculations so that mixing of O2 with the crude COG was

completed as quickly as possible. The flow rates of the HCOG introduced to the reformer were controlled by sliding dampers inserted into the extraction pipes and ranged from 28 to 103 Nm3/h, which correspond to roughly 7-25% of the HCOG flow rate from one coke chamber (400 Nm3/h), whereas the O2 flow rates were changed from 12 to 30 Nm3/h in response to transient HCOG flow rates. The operations were performed under atmospheric pressure. Gas compositions at both the inlet and outlet of the reformer were measured using online GC (Round Science, Automatic GC, AG-1). In gas samplings, gas was drawn through a condenser and a mist trap at a rate of 600-1000 mL/min by using a diaphragm pump. The sampled gas was distributed simultaneously into two different GC columns (Molecular Sieve 13X and Porapak Q) for separations and detected using a TCD and FID. Lower limits of the detection are 0.05% and 10 ppm for TCD and FID, respectively. Accuracies in determining gas concentrations by the online GC measurements ranged within 5%. Argon, of which flow rate was strictly controlled by using a mass flow controller, was continuously fed with gas emitted from the coke oven. Volume flow rates at reactor inlet were determined based on argon

Figure 2. Two-dimensional illustrations of the shape and dimension of the real HCOG reformer (left) and the idealized reactor for a plug-flow simulation (right). The plots present the measured temperature profile inside the real HCOG reformer (left) and the corresponding temperature profile in the idealized reactor (right), in which a fit curve approximated with a 6th-order polynomial function for the DETCHEM input is indicated.

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 1. Reaction Conditions Used for the Plug-Flow Reactor Simulation

pressure, kPa gas temperature at the idealized reactor inlet, K linear velocity at the idealized reactor inlet, m/s

run 1

run 2

100 1408 0.28

100 1494 0.47

concentrations measured by online GC. The concentrations of condensing products such as water and tar were also determined by off-line samplings. The tar composition was analyzed qualitatively by GC-MS as reported previously.30 Temperature profiles were measured continuously using R-type thermocouples inserted vertically at different positions inside the reformer. 3. Numerical Simulation with a Detailed Chemical Kinetic Model For a better understanding of the chemical reactions involved in HCOG reforming, numerical simulations were conducted using a detailed chemical kinetic model. Shown in Figure 2, the real reactor was idealized as a tube reactor with a constant inner diameter for one-dimensional plug-flow simulation such that the gas introduced experiences the same temperature history as it does in the real reactor. The reaction mechanism used in this study was proposed by Richter and Howard23 and consisted of 2216 reactions, including 257 chemical species from the smallest species of hydrogen radical to the largest molecule of coronene. The thermodynamic data for the species involved in the mechanism as well as the rate constants were used without modification. The calculations were performed with the PLUG code in the DETCHEM program package (DETCHEMPLUG).31 Boundary conditions necessary for the calculations such as pressure, linear velocity, and temperature of the feed gas at the reactor inlet were determined directly from the test conditions (see Table 1 for representative conditions). Other required inputs included composition of the feed gas and temperature profiles along the reactor length; the latter was given as a polynomial

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function fit to the temperature data obtained from each test run, which exhibited slightly different profiles among individual tests. For the determination of feed compositions, an assumption was made that HCOG and O2 were completely mixed at the reactor inlet. The initial compositions of the compounds detectable by GC as well as water were determined easily. However, numerous compounds are present in HCOG tar, as depicted in the GC-MS total ion chromatogram of the tar sampled during the operation (Figure 3). Because it is currently impossible to take into account all of the compounds that comprise HCOG tar, the tar was assumed to be a mixture of approximately 31 aromatic hydrocarbons based on the quantitative analysis of coke over tar by Kirton.2 Table 2 lists two examples of inlet gas compositions for the plug-flow reactor simulations. Detailed descriptions of the numerical simulation and the approximation of the feed gas compositions can be found elsewhere.18,32 4. Results and Discussion Table 3 lists exemplarily the chemical compositions of the HCOG and the product gas obtained in selected runs. The current reforming via noncatalytic partial oxidation yielded increases in H2 and CO concentrations, while the concentrations of hydrocarbons, particularly methane, decreased significantly. More than 2.2-fold syngas amplification ratios were obtained from these runs. Soot concentration was also estimated based on weight of soot collected at a cyclone separator downstream of a gas cooler (see Figure 1) and ranged from 10 to 17 g/Nm3. The product gas compositions were evaluated in terms of the R-parameter, which is defined as R ) ([H2] - [CO2])/([CO] + [CO2]), where [H2], [CO2], and [CO] represent the moles of each gas fed in at the methanol synthesis stage. To optimize the process, this parameter needs to be equal to or slightly greater than two.33 The R value was 2.0 for the selected runs, indicating that HCOG reforming by noncatalytic partial oxidation is a promising method for supplying material gas to the methanol synthesis process.

Figure 3. Total ion chromatogram from GC-MS analysis of the tar sampled from the operating coke oven at Kitakyushu Coking Works of Nippon Coke & Engineering Co., Ltd. Identified compounds (from left to right) are benzene, toluene, xylenes, phenylacetylene, phenol, indene, naphthalene, 2-methylnaphthalene, 1-methylnaphthalene, biphenyl, biphenylene, dimethylnaphthalene, acenaphthylene, acenaphthene, dibenzofuran, fluorene, methyl-acenaphtylene, methyldibenzofuran, phenanthrene, anthracene, acridine, carbazole, methyl-anthracene, methyl-phenanthrene, 4H-cyclopenta[def]phenanthrene, phenylnaphthalene, fluoranthene, acephenanthrylene, methyl-pyrene, benz[a]anthracene, chrysene, triphenylene, benzo[k]fluoranthene, benzo[j]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene, o-phenylenepyrene, and benzo[ghi]perylene.

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Table 2. Inlet Gas Compositions for the Plug-Flow Reactor Simulations mole fraction, compound

run 1

run 2

H2 CH4 C2H4 C2H6 CO CO2 N2 O2 H2O benzene toluene 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

0.265958 0.158296 0.002378 0.000530 0.028439 0.009215 0.024673 0.215430 0.276934 0.005782 0.000109 0.000382 0.000835 0.001097 0.006096 0.000533 0.000238 0.000420 0.000086 0.000264 0.000856 0.000215 0.000034 0.000016 0.000345 0.000014 0.000011 0.000237 0.000019 0.000006 0.000094 0.000100 0.000076 0.000058 0.000062 0.000016 0.000025 0.000010 0.000003

0.283812 0.158407 0.003905 0.000633 0.033822 0.012747 0.006439 0.217270 0.264115 0.006301 0.000135 0.000375 0.000821 0.001079 0.005994 0.000524 0.000234 0.000413 0.000084 0.000259 0.000841 0.000212 0.000033 0.000015 0.000340 0.000013 0.000011 0.000233 0.000019 0.000006 0.000092 0.000098 0.000075 0.000057 0.000061 0.000016 0.000024 0.000010 0.000003

Figure 4 shows an example of the outputs from the DETCHEMPLUG simulations and includes a temperature profile (top), mole fraction profiles for the major species (middle), and a total mole fraction profile of aromatic hydrocarbons from

Figure 4. DETCHEMPLUG simulation results for one of the 20 runs of HCOG reforming (run 15). A temperature profile along the reactor flow direction (top), mole fraction profiles for the major gas phase constituents (middle), and total mole fraction of the aromatic hydrocarbons (bottom) are shown.

benzene to coronene (bottom), which represents the tar concentration. With respect to these compounds, changes in the mole fractions with reactor position are remarkable before 1.0 m; beyond that, the little change observed is likely due to approaching chemical equilibrium and decreasing temperature. The residence time of the HCOG and O2 mixture in the reactor calculated by the DETCHEMPLUG simulations ranged from 5 to 13 s. Figure 5 compares computationally predicted flow rates at the rector outlet (position ) 2.97 m) with measured flow rates in the HCOG reformer. The comparisons were made for the results of 20 test runs. Although the simulation tends to overpredict the outlet flow rate, fair agreements were observed. The ratio of the outlet flow rates predicted numerically to those measured ranged from 0.9 to 1.4.

Table 3. Chemical Compositions of the HCOG and the Reformed Gas Obtained from the Pilot-Scale Test Plant Equipped on an Operating Coke Oven run 1

flow rate, Nm3/h O2 flow rate, Nm3/h

dry gas composition, vol%

concentration of condensing compounds, g/Nm3 gas amplification ratio a, Rb, -

H2 N2 CH4 CO CO2 C2H4 C2H6 H2S benzene toluene water tar

run 2

HCOG

reformed gas

HCOG

reformed gas

28.0 12.0 53.7 5.0 32.0 5.7 1.9 0.48 0.11 0.02 1.17 0.02 449 153

43.0 65.4 4.7 0.8 21.2 7.9 0.02 0.00 0.00 0.00 0.00 n.d. -

44.0 19.0 56.0 1.3 31.3 6.7 2.5 0.77 0.12 0.09 1.24 0.03 419 147

69.0 68.0 0.5 0.8 22.7 8.0 0.01 0.00 0.01 0.00 0.00 n.d. -

2.2 2.0

2.3 2.0

a Gas amplification ratio ) A/B, where A is the gas volume of H2 and CO under the reformed gas at standard conditions and B is the gas volume of H2 and CO in the inlet crude COG under standard conditions. b R-parameter is defined as an index following equations for the optimum gas composition of the product gases for methanol synthesis calculated by R ) ([H2] - [CO2])/([CO] + [CO2]), where [H2], [CO2], and [CO] are the moles of each gas in the reformed gas.

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Figure 5. Gas flow rates at the reformer outlet predicted by the DETCHEMPLUG simulation versus those measured by the pilot-scale tests for HCOG reforming by noncatalytic partial oxidation. Figure 7. Enlarged view of the data presented in Figure 4 for the reactor position from 0 to 0.8 m. The major events in chemical reactions are also indicated.

Figure 6. Major gas compositions at the reformer outlet predicted by the DETCHEMPLUG simulation versus those measured by the pilot-scale tests for HCOG reforming by noncatalytic partial oxidation.

As illustrated in Figure 6, computational predictions at the reactor outlet were compared with the test run data for major compounds such as H2, CO, CO2, and CH4. In general, the predictions agreed fairly well with the test results, considering that no adjustable parameters for matching the measured data were used in the kinetic model. The ratios of the volume % predicted numerically compared to those measured ranged 0.9-1.0, 1.1-1.2, 0.6-1.4, and 0.3-2.0 for H2, CO, CO2, and CH4, respectively. These fair predictions in the H2 and CO concentrations should support deriving an optimum condition to convert HCOG as quantitatively as possible to syngas by the numerical simulations. Most of the H2 fraction data, however, were underpredicted because the current kinetic model contains no H2 yielding gas-solid reactions such as carbon deposition, as well as soot formation mechanisms. The model predicts for some cases higher exit flow rates than measured, but the major gas component (H2) is under-predicted. This seems a contradictory result. Many factors are supposed to be involved in this contradiction. Transient phenomenon like an accumulation of coke (carbon) on reactor wall is likely to be one of the

factors. The carbon deposition reduces gas volume but increases H2 concentration. Nevertheless, not only chemistry aspects, but other factors such as adopting an oversimplified flow model, would contribute to the disagreements. Studies on coupling the detailed chemical kinetic model to more realistic flow models are ongoing and will improve the precision of the predictions. The mole fraction profiles at the initial part of the reactor are extracted in Figure 7. This enlarged figure reveals the dominating reaction at different positions along the HCOG reformer. Specifically, the first dominant reaction was the combustion of H2, one of the major components of the HCOG. The partial oxidation of CH4 overlapped with H2 combustion, leading to an increase in CO concentration. Exothermic reactions are dominated in the initial part of the reactor. These behaviors should depend on the degree of mixing between fuel and oxidant gases. It needs further research to examine our assumption of premixed condition of fuel and oxidant. Future study will include CFD coupled with chemical kinetic model. The injected O2 was almost completely consumed at a reactor position of around 0.1 m, at which the mole fractions of H2 and CO2 started to increase due to the water-gas shift reaction. Decreases in H2O and CH4 mole fractions and increases in CO and H2 are clear indicators for the occurrence of steam reforming of hydrocarbons after the depletion of O2. Beyond a reactor position of around 0.3 m, the concentration profiles of major species changed little, indicating that the major events in the chemical reactions of HCOG reforming by partial oxidation were nearly complete. Indeed, this information on the chemical kinetics is useful for designing reactor systems and optimizing operational variables toward the development of demonstration/ commercial plants for HCOG reforming. Equilibrium calculations were also conducted to evaluate whether equilibrium is reached or how far the system is from thermodynamic equilibrium. Figure 8 compares mole fraction profiles along with the reactor axis obtained kinetically with those obtained from equilibrium calculations. DETCHEMEQUILI 31 was used for the calculations. At a given pressure (1 bar) and an inlet chemical composition, gas phase composition at thermodynamic equilibrium depends solely on temperature. Since temperature is known along with the reformer, the composition at equilibrium can also be drawn as a function of reactor

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Figure 8. Comparison of mole fraction profiles along with the reactor axis calculated kinetically (lines, DETCHEMPLUG) with those calculated based on thermodynamic equilibrium (dotted lines, DETCHEMEQUILI) for one of the 20 runs of HCOG reforming (run 15).

position. It is evident that there are always gaps between the mole fractions predicted kinetically and those from equilibrium calculations, indicating that the current system is far from equilibrium and kinetically controlled. 5. Conclusions A pilot-scale test plant for HCOG reforming by noncatalytic partial oxidation was installed on a platform of an operating coke oven. The HCOG was fed into a tubular reactor (0.6 m i.d. and 3.2 m long) at flow rates varying from 28 to 103 Nm3/h and partially oxidized by injecting O2 from 12 to 30 Nm3/h from nozzles near the inlet. Exhaustive test runs identified the most suitable reforming conditions, which obtained more than 2.2-fold syngas amplifications, and optimum product gas compositions for methanol synthesis. Numerical simulations were also conducted using detailed kinetics coupled with a plug-flow reactor model. The HCOG was modeled as a multicomponent gas mixture containing H2, CO, CO2, CH4, C2 hydrocarbons, H2O, and 31 aromatic hydrocarbons to represent the HCOG tar. Satisfactory agreements were observed in comparisons between the predictions from the numerical simulations and the data measured from the 20 test runs for the major observables, such as product gas flow rate and the compositions of H2, CO, CO2, and CH4, demonstrating that the model can be a promising tool toward developing a demonstration/ commercial HCOG reforming plant. Acknowledgment This research and development were partially supported by Coal division of Agency for Natural Resources and Energy, the New Energy and industrial Development Organization, the Institute of Applied Energy, and Japan Coal Energy Center. K.N. also acknowledges financial support from Steel Industry Foundation for the Advancement of Environmental Protection Technology. Literature Cited (1) Aramaki, T. Status of the coke oven gas in Japan. J. Jpn. Inst. Energy 2006, 85 (5), 342–347.

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ReceiVed for reView March 6, 2010 ReVised manuscript receiVed June 8, 2010 Accepted June 10, 2010 IE100506V