- Email: [email protected]
Evolution prediction of coal-nitrogen in high pressure pyrolysis processes
Fuel 81 (2002) 2317–2324 www.fuelfirst.com
Evolution prediction of coal-nitrogen in high pressure pyrolysis processesq Yukihiko Okumuraa,*, Yuriko Sugiyamab, Ken Okazakib,* b
a Department of Control Engineering, Maizuru National College of Technology, Shiroya 234, Maizuru 625-8511, Japan Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan
Received 19 November 2001; accepted 26 April 2002; available online 11 July 2002
Abstract A pyrolysis model which can describe the effects of pressure on the evolution of coal-nitrogen has been constructed based on the FLASHCHAINw model in order to relate the gas release mechanism under high pressure conditions to the polymer reactions in coal. Various kinds of nitrogen-containing gaseous species in the evolved volatiles and their secondary decomposition and coupling processes have been also clarified by considering the elementary reactions of pyrrole-type nitrogen as the primary type of bound nitrogen in the first evoluted heavy species (tar vapor). The results show that the recombination reactions of metaplast are activated in a coal by the increase in pressure, resulting in a lesser amount of tar vapor and more intermediate chars. Thus, the conversion ratio of coal-nitrogen to gaseous volatile-nitrogen increases with the increase of pressure and N-gas converted from the fuel-N is much larger than the tar-N, and becomes more significant in high pressure conditions. Due to the chemical kinetics of the gas phase reactions, a shift in the distributions of tar-N vapor and gas-N with the increase of pressure can be predicted, and larger amounts of H2CCHCN and bipyrrole gases are rapidly formed through three-body reactions, while HCN gas decreases greatly compared with the reaction at normal pressures. The changes of gas composition are in close agreement with the experimental results. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal pyrolysis; Pressurized entrained flow gasification; Pressurized fluid bed combustion; Chemical reaction
1. Introduction Research on pressurized entrained flow gasification (PEFG) and pressurized fluid bed combustion (PFBC) has energetically advanced with the aim of utilizing coal more cleanly and efficiently. It has been reported that coal’s evolution, i.e. its devolatilization with heterogeneous reactions and gas phase reactions, at rapid heating under high pressures of 20 – 30 atm differs greatly from the case of normal pressure [1,2]. However, the effect of pressure on the evolution processes has neither been investigated nor clarified in detail in spite of its importance in the further development of combustion technology. Several investigations [3 – 9] of coal-N evolution have been conducted. It has been observed that the minerals which are included in low-rank coals activate mainly N2 evolution [3,4] and that both the heating rate and the temperature influence the evolution of coal-nitrogen [5 – 7]. However, almost all of these experiments [3 –7] for fuel-N have been carried out under normal pressure conditions, * Corresponding authors. Tel.: þ 81-773-62-8954; fax: þ 81-773-625558. E-mail addresses: [email protected] (Y. Okumura), [email protected] (K. Okazaki). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
leaving a great lack of pyrolysis data for coal-N under high pressure conditions. In addition, an evolution model which can be realistically incorporated into actual systems for both gasification and combustion has also not been developed [10,11], because of the complicated phenomena involved in the actual operating conditions as they change from normal pressure to higher pressure, or higher excess air ratio in PFBC to lower that in PEFG. In light of this, a pyrolysis model has been constructed based on the FLASHCHAINw model [12 – 14] that includes a quantitative estimation of the volatilized gases and released tar vapor at high pressures. Simultaneously, various kinds of nitrogen-containing gaseous species (N-gas) in the evolved volatiles and their secondary decomposition and coupling processes have been clarified by considering the elementary reactions for pyrrole-type nitrogen as a main type of bound-nitrogen in first evoluted heavy species. The weight fractions for the volatilized gas, the released tar vapor, and the residual char are compared in detail with the experimental results.
2. Theoretical analysis
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 1 7 0 - 9
The FLASHCHAINw model has been developed by
2318
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Niksa in 1991 [12]. This theory involves coal’s chemical constitution, a four-step reaction mechanism, chain statistics, and the flash distillation analogy to explain the devolatilization of various coal types. It has now applied actually to the computational fluid dynamic simulator of the entrained flow gasifiers and the pressurized fluid bed in the program of Basic Research Associate for Innovative Coal Utilization (BRAIN-C program, Japan) [10,11]. It has been compared with experimental data, and proved and confirmed that the predicted yields of non-condensible gas and tar are in agreement with measured distributions over the wide ranges of heating rate [13] and temperature etc [14]. This model was selected for this application from the above facts, in which the FLSCHAINw model can predict the pyrolysis process for many type of coals [11,14] and can estimate polymer chain reactions under high pressure conditions. In this study, the consideration was developed by relating gas release mechanism under high pressure condition to the polymer reactions in the coal.
2.1. Modeling of coal pyrolysis under high pressure conditions In this section, a model which can describe the Nevolution and the effect of pressure is proposed on the basis of the FLASHCHAINw model [12 – 14], and the theoretical analysis of pyrolysis under high pressure conditions are performed including full chemical kinetics for the N-gas formation. The discussion will focus on the chain and scission reactions in the coal under high pressure and the reaction mechanisms of decomposition and coupling processes for the nitrogen-containing matters in the gas phase. Fig. 1 shows the conceptual scheme of the bridge reaction processes in coal. The fragments are classified according to their size, as they are called metaplast (small size), intermediate (middle size), and reactant (large size). Only the metaplast, which is the lightest fragment group, can be released to the outside of coal as tar vapor. In this model, the bridges are distinguished as char links, labile bridges, and peripheral groups in terms of the molecule combination-forms. The pyrolysis reaction progresses simultaneously via the next four reactions.
ðR2Þ 3. Peripheral group elimination
ðR3Þ
4. Recombination
ðR4Þ
The release form from coal-nitrogen to nitrogen-containing matters is modeled on the basis that nitrogen is contained only in the aromatic nucleus. It is confirmed by X-ray photoelectron spectroscopy [16,17] that the main types of bound nitrogen in coal are pyrrole and pyridine types. The vapor –liquid phase equilibrium is also included in the present model as playing a role in the pressure effect. The amount of released tar vapor can be calculated by the ratio of saturated vapor pressure of the metaplast to the surrounding gas pressure. The partial pressure of the tar vapor formed from j-mer is expressed by Raoult’s law in Eq. (1). The j shows the number of combinations of the monomer, which consisted of an aromatic nuclear and a bridge, and the molecular weight is 250 –400. The molecule weight of j-mer increases with an increased number of j. Jp X
pj ¼ HP0 ¼
j¼1
Jp X
Xmj Psat ðT; MWtj Þ
ð1Þ
j¼1
ðR1Þ
where pj is the partial pressure of the tar vapor formed from j-mer, P0 is the surrounding gas pressure, Psat ðT; MWtj ) is the saturated vapor pressure of the metaplasts, H is the instantaneous mole fraction of tar vapor within the coal, and Xmj is the mole fraction of j-mer. Assuming that the yield rate of released tar vapor is proportional to production rate of the volatilized gas, the following Eq. (2) can be derived. tj H 1 dY £ G Gj ¼ p ð2Þ J G dt X tj
2. Condensation of bridge (carbonization by the condensation reaction)
where Gj is the yield rate of released tar vapor, tj is the amount of the tar vapor, G is the instantaneous mole fraction of gas within the coal, YG is the mole fraction of volatilized
1. Bridge scission of the segments
j¼1
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2319
Table 2 Reaction rates for calculation
Bridge scission Recombination Peripheral group elimination Volatile N release
A (1/s)
E (kJ/mol)
2 £ 1011 6 £ 1016 1 £ 1015 1.29 £ 104
167 217 230 71.7
chemical species for gases and tar, requiring a complicated process. Therefore, the formative nitrogen-containing gaseous species and their secondary decomposition and coupling processes have been clarified by considering elementary reactions (Table 3) [15] for pyrrole-type nitrogen as a main type of bound nitrogen in tar [16,17].
3. Theoretical results and discussion 3.1. Effect of pressure on the pyrolysis process
Fig. 1. Evolution mechanism based on FLASHCHAINw model. (a) Schematic diagram of evolution mechanism and (b) devolatilization diagram of the coal-N.
gas and subscript j is the number of the aromatic nuclei ðJ p ¼ 5Þ: The partial pressure of the tar vapor is calculated using Eq. (1), and the yield rate of tar vapor can then be obtained from Eq. (2). The calculation condition and coal properties are shown in Tables 1 and 2, respectively. The rate constants for pyrolysis which is recommended by Niksa [12] and Kambara are used in the present calculation. The FLASHCHAINw model has an advantage and a disadvantage. The effects of heating rate on gas, tar, and char formations can be shown over a wide range. However, the model cannot easily evaluate the various kinds of
The effect of pressure on volatilized gas, released tar, and char formations are shown in Fig. 2, with the solid and broken lines showing calculation results. It can be found that the amount of released tar vapor decreases abruptly with increase of the surrounding gas pressure, while the released gas slightly increases. The total weight fraction of the released volatile matter decreases under increasing pressure. This agrees well with experimental results [1]. It can be explained by Eq. (1) that the mole fraction of tar vapor within the coal decreases with the increase of pressure P0. Consequently, the tar vapor formation is suppressed by pressure, and the recombination reaction that creates intermediate char is more strongly activated. Fig. 3 shows the gas formation with the reaction route, in which the amount of gas at 10 atm is determined to be the standard value ( ¼ 1.0). The number in Fig. 3 corresponds to the route shown in Fig. 1(a). It can be seen that the recombination reactions of the metaplast (route (4) in Fig. 1(a)) is activated and the gas formation from the condensation and peripheral group elimination reactions in
Table 1 Calculation and experimental condition for N-release High volatile bituminous coal Pressure Maximum temperature Heating rate
1.0–15.0 atm (absolute) 1038 K 1000 K/s þ 10 s holding
Ultimate analysis of coal (d.a.f) C H 80.1 5.9
O 12.1
N 1.4
Proximate analysis of coal (wt%) VM FC 41.8 48.6
Ash 9.6
Moisture 1.16
S 0.55
Fig. 2. Effect of pressure on released volatile matters and char (1000 K/s, maximum temperature: 1038 K).
2320
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Table 3 Gas reactions of pyrrole C4H5N ¼ PYRLNE PYRLNE ¼ C3H4P þ HCN PYRLNE ¼ C3H5CN PYRLNE ¼ ALLYLCN PYRLNE ¼ C2H2 þ CH3CN H þ C3H5CN ¼ AC3H4CN þ H2 H þ C3H5CN ¼ CH3 þ H2CCHCN H þ C3H5CN ¼ C3H5 þ HCN H þ ALLYLCN ¼ AC3H4CN þ H2 H þ ALLYLCN ¼ C2H4 þ CH2CN H þ ALLYLCN ¼ C3H5 þ HCN CH3 þ C3H5CN ¼ AC3H4CN þ CH4 CH3 þ ALLYLCN ¼ AC3H4CN þ CH4 H þ HCCHCN ¼ H2CCHCN AC3H4CN ¼ CH3CCCN þ H C3H5 ¼ C3H4P þ H CH3 þ H2CCHCN ¼ HCCHCN þ CH4 H þ H2CCHCN ¼ HCCHCN þ H2 HCCHCN þ M ¼ HCCCN þ H þ M H þ H2CCHCN ¼ HCN þ C2H3 H þ CH3CN ¼ CH3 þ HCN H þ C2H2 ¼ C2Hp3 H þ C2H4 ¼ C2H3 þ H2 CH3 þ H2 ¼ CH4 þ H CH3 þ CH3 ¼ C2Hp6 C2H5CN ¼ CH3 þ CH2CN C2H5CN þ H ¼ HCN þ C2H5 C2H5CN þ H ¼ CH3CHCN þ H2 CH3CHCN ¼ H2CCHCN þ H H þ C3H5 ¼ C3H6 H þ C3H5 ¼ C3H4P þ H2 CH3 þ HCCHCN ¼ C3H5CN C2H3 þ CH2CN ¼ ALLYLCN C2H3 þ C2H3 ¼ C4H6 C2H3 þ C2H3 ¼ C2H4 þ C2H2 C3H5 þ CH2CN ¼ C3H4P þ CH3CN C3H5 þ CH2CN ¼ C4H6 þ HCN AC3H4CN þ CH3 ¼ C4H6 þ HCN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
H þ AC3H4CN ¼ ALLYLCN H þ AC3H4CN ¼ C3H5CN C2H3 þ CH2CN ¼ C2H2 þ CH3CN H þ CH2CN ¼ CH3CN CH3 þ C3H5CN ¼ C3H4CN þ CH4 C3H4CN ¼ HCCCN þ CH3 CH3 þ C2H3 ¼ CH4 þ C2H2 C3H5CN ¼ C3H4P þ HCN ALLYLCN ¼ C3H4P þ HCN CH2CN þ ALLYLCN ¼ C3H4CN þ CH3CN C3H4CN ¼ C2H2 þ CH2CN AC3H4CN ¼ HCCCN þ CH3 C2H4 þ M ¼ C2H2 þ H2 þ Mp C4H6 ¼ C2H2 þ C2H4 CH3CN þ C2H3 ¼ H2CCHCN þ CH3 H þ C3H4P ¼ CH3 þ C2H2 CH3 þ CH3CN ¼ CH2CN þ CH4 CH2CN þ CH2CN ¼ H2CCHCN þ HCN C4H5N ¼ PYRLYL þ H PYRLYL þ PYRLYL ¼ BIPRL C2H3 þ C2H4 ¼ C4H6 þ H H þ C4H5N ¼ PYRLYL þ H2 CH3 þ C4H5N ¼ PYRLYL þ CH4 C3H4P ¼ C3H3 þ H CH3 þ C3H4P ¼ C3H3 þ CH4 H þ C3H4P ¼ C3H3 þ H2 C3H3 þ C3H3 ¼ L -C6Hp6 L -C6H6 ¼ C6H6 H þ C2H6 ¼ C2H5 þ H2 C2H5 ¼ C2H4 þ Hp C2H2 þ C2H2 ¼ C4H4 C4H4 ¼ H2 þ C4H2 C2H3 þ C2H2 ¼ N-C4H5 N–C4H5 ¼ C4H4 þ H C2H þ H2 ¼ H þ C2H2 C2H þ C2H2 ¼ C4H2 þ H C2H þ C2H3 ¼ C4H4
Species identification [25] C4H5N: pyrrole; PYRLNE: pyrrolenine(2H-pyrrole); C3H4P: propyne; ALLYLCN: allyl cyanide; AC3H4CN: cyanoallylradical; alkeneyne isomer of benzene etc.
L -C6H6:
the intermediate fragments group is also enhanced (route (2) in Fig. 1(a)). It is concluded that this model including the effect of vapor – liquid equilibrium is capable of qualitatively accounting for the weight loss of coal in high pressure
Fig. 3. Gas formation with the reaction route (1000 K/s, maximum temperature: 1038 K).
conditions. In addition, it is noted that the applicability of this mechanism is restricted to this rank of coal, i.e. high volatile bituminous coals in which substantial amounts of the metaplast are generated in coals and the recombinations in the condensed phase are important. Hereafter, the temperature-dependent profile of weight loss and the product profiles of evolved gases at high pressure are discussed in terms of the data shown in Fig. 4. It should be noted that the experimental conditions (1 K/s) expressed in Fig. 4 differ from those of Table 1 in order to obtain and illustrate the release process of gases and the weight loss of coal in detail. In Fig. 4, each gas composition is formed at A – E temperature ranges, respectively. The temperature for an initial pyrolysis at 10 atm rises more and the high pressure pyrolysis is suppressed at the final stage over 600 8C; subsequently, the evolution of volatile matter is completed more quickly than under normal pressure. It can also be seen that at an initial stage of the evolution process mainly carbon dioxide is evolved, while at the
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2321
Fig. 4. Temperature dependent profiles of weight loss and gas composition with pyrolysis process (experiment by author using the pressurized thermobalance).
middle stage CO, CH4, and H2 gases, and at the last stage mainly H2 is released. It has been observed that CO2, CO, CH4, or H2 are formed from the carboxyl group (– COOH), the ether group (– O – ), the methyl group ( –CH3), and H, respectively [18]. Since the carboxyl groups combined with coal’s structure are especially weak, it can be inferred that the scission process of the carboxyl group can progress primarily at an initial stage of the pyrolysis process. It is found from an analysis of the released gas composition after pyrolysis that the volume proportion of H2 in light fuel gas (CH4 þ H2) decreases with the increase of pressure. It is considered that the H2 evolution is affected by both the greater degree of hydrogen enrichment in tars [19] and the decrease in H2 at the last stage of the evolution process (see Fig. 4). 3.2. Effect of pressure on volatile-N The relationship between pressure and the fuel-N conversion is shown in Fig. 5. The experiment (Kjeldahl method) was carried out by Kambara [1]. It is indicated by calculation that the amount of released nitrogen-containing gases (N-gas) increase with the increase of pressure, and
Fig. 5. Comparison of volatile-N yield with experimental results (1000 K/s, maximum temperature: 1038 K).
that the amount of N-gas converted from the fuel-N is much larger than the tar-N, and becomes more significant in high pressure conditions [20]. The reason the gasification reaction is activated is closely similar to the description provided in Section 3.1, i.e. the tar precursor called ‘metaplast’ remaining in the coal enhances the condensation and peripheral group elimination reactions in the intermediate fragment groups and the N-gas formation. It can be inferred that the actual gas-N formation is further increased due to the following two reasons. First, the trace elements (Fe, Ca) in coal under high pressures, in which the releases of Fe and Ca are also suppressed by the pressure, would enhance the evolution of nitrogen gas. Second, the influence of radicals would appear at high pressures. The decyclization reactions for both the pyrrole and pyridine types of nitrogen are actually activated by radicals such as alkyl radical or H, which are remarkably formed from the secondary decomposition of the tar [21]. At high pressures, the radical with the tar precursor becomes more highly activated in the coal. Consequently it has been observed from experiment [21] that the HCN gas formation from actual coal proceeds more easily than that from pure pyrrole. However, the present model cannot directly illustrate why the release of coal-N is enhanced by elevated pressure [1], and the tendency of released nitrogencontaining gases to increase agrees well with the experimental results only. Here, we discussed why release of coal-N increased under high pressure during the experimental procedures [16]. It has been reported that the coal-N release from the pressurized entrained flow systems [10] and from the wire grid tests [9] of several coals is not promoted by elevated pressures. It was found during this experimental procedures of Kambara [11] that the secondary decomposition of tar-N readily occurs due to a consistently maintained high temperature (10 s, 1038 K) [1], and to the coal sample being enclosed with glass wool at both ends of the quartz glass [1] (i.e. reaction zone, i.d 1.3 mm, length:
2322
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Fig. 6. Pyrolysis and volatilization schemes for coal-N.
30 mm). It is believed that a large amount of tar-N, which can evolve mainly at a high temperature from 800 K [16], would be difficult to release in this experimental apparatus at the last stage of pyrolysis, and that only gas-N would be released. The released tar-N at the last pyrolysis stage cannot be separated completely from the coal itself, thus these phenomena may cause enhanced nitrogen release. 3.2.1. Secondary decomposition mechanism of volatile-N In this section the decomposition and coupling reaction mechanisms for nitrogen containing gaseous species are further discussed by combining the gas phase reaction model with the FLASHCHAINw model. Based on recent research [22 –24], the release scheme of coal-N in the pyrolysis and devolatilization processes have been suggested as shown in Fig. 6. In the primary stage of decomposition, the various kinds of heavy nitrogencontaining matter, which has mainly a polycyclic structure, are evolved, while simultaneously NH3 and HCN gases are directly released from the coal to the gas phase (6 –8% of fuel-N) [16]. In the secondary stage of decomposition, most of heavy nitrogen matter is converted to NH3 and HCN
gases and soot. These decomposition reactions would be more important when the experiment is performed at rapid heating (1000 K/s). Thus, the reaction processes of secondary cracking must be calculated even at the high pressure of 15 atm. The influence of pressure on the elemental reactions can be included into the model by the three-body effects for the thermal dissociation and recombination reactions (see p mark in Table 3), which strongly depend on pressure [15]. This elementary reaction should be applied only to tar vapor and gas, in which case we can predict a shift in the distribution of tar-N and gas-N. Fig. 7 shows the gas phase reactions [25] of pyrrole type nitrogen under high pressure and normal conditions. It can be seen from the upper figures that the pyrrole reactions are enhanced with the increase of pressure through three-body reactions, and that larger amounts of H2CCHCN and bipyrrole gases are rapidly formed as compared with the case at 1 atm. Those tendencies at lower temperatures of approximately 1000 K appear strongly in comparison with those seen at higher temperatures of approximately 2000 K. It is also confirmed that the decomposition and coupling reactions for pyrrole at high pressures become more active than those of pyridine. These facts are consistent with the experimental results (Fig. 8). The comparison the results of calculation with the experimental results in regard to N-gas composition [1] is shown in Fig. 9. The decrease of HCN and the increase of other N-gases under increasing pressure agree well with experimental results. The amount of NH3 gas increases with the increase in pressure, and it has been reported that the NH3 gas is formed from amine-type nitrogen [1]. Thereby in order to elucidate the NH3 formation mechanisms, the decomposition
Fig. 7. Gas phase reactions for pyrrole type nitrogen under high pressure conditions.
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2323
process of amine type nitrogen must be investigated. The three-body reactions in the gas phase and the effects of remaining metaplast in coal must be included in the pyrolysis process for coal-N at high pressures.
4. Conclusion The theoretical analysis of thermal pyrolysis at high pressures is performed including the full chemical kinetics of N-gas formation. The study obtained the following results: Fig. 8. Effect of pressure on decomposed fraction of nitrogen obtained by Kambara [1]. (a) Pyrrolic-N and (b) pyridinic-N.
4.1. Effect of pressure 1. Tar vapor formation is suppressed by pressure, and the recombination reactions of the remaining metaplast are then activated, resulting in more intermediate chars. 2. The model, which includes the effect of vapor – liquid phase equilibrium, is capable of accounting for the weight fractions of char, tar, and gas at high pressures, as demonstrated by comparison with high-pressure pyrolysis experiments. 3. The volume proportion of H 2 in light fuel gas (CH4 þ H2) shows a tendency to decrease with increasing pressure. This is due mainly to the suppression of H2 evolution at the last stage of the pyrolysis process. 4.2. Release mechanism of coal-N 1. The released nitrogen-containing gases (N-gas) increase with the increase of pressure. This is mainly due to large amount of metaplast remaining in coal. Specifically, the recombination and peripheral group elimination reactions in the intermediate fragment group are activated by the increase of pressure, consequently resulting in more N-gas formation. 2. Due to the chemical kinetics of gas phase reactions, pyrrole reactions are also enhanced by the increase of pressure through three body reactions. HCN gas is primarily released at 1 atm, while at higher pressures a larger amount of H2CCHCN and bipyrrole gases can be formed. The tendency of HCN to decrease and the tendency of the amounts of other N-gases to increase at increasing pressures agree well with experimental results.
Acknowledgments
Fig. 9. Comparison of predicted N-gas composition with experimental results. (a) Effect of pressure on HCN conversion ratio and (b) effect of pressure on fuel-N conversion ratio for nitrogen-containing gases.
This research was financially supported in part by Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Program, sponsored by the New Energy and Industrial Development Organization (NEDO, Japan) and by Electrical Power Development Co. Ltd. The authors expresses appreciation to these organizations.
2324
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
References [1] Kambara S. Symposium series 48 of chemical engineering association (in Japanese). Japan: The Society of Chemical Engineers; 1995. p. 151. [2] Okumura Y, Sugiyama Y, Okazaki K. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):141. [3] Wu Z, Otsuka Y. Energy Fuels 1997;11:902. [4] Zhiheng W, Sugimoto Y, Kawashima H. J Jpn Inst Energy 2001;80: 97–104. [5] Chen JC, Castagnoli C, Niksa S. Energy Fuels 1992;6:264. [6] Bassilakis R, Zhao Y, Solomon PR, Serio MA. Energy Fuels 1993;7: 710–20. [7] Wu Z, Otsuka Y. Energy Fuels 1997;11:477. [8] Cai HY, Guell AJ, Dugwell DR, Kandiyoti R. Fuel 1993;72(3): 321–7. [9] Tomita A. No. 9703TM5021. Tohoku University, Japan, 1997. [10] Reports of Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Project (Pyrolysis and Gasification Group), NEDO-C9939. The New Energy and Industrial Development Organization (in Japanese), 2000. [11] Reports of Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Project (Pyrolysis and Gasification Group), NEDO-C0022. The New Energy and Industrial Development Organization (in Japanese), 2001.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Niksa S, Kerstein AR. Energy Fuels 1991;5:647–65. Niksa S. Energy Fuels 1991;5:665–73. Niksa S, Kerstein AR. Energy Fuels 1991;5:673–83. Kee RJ, Ruply FM, Meeks E, Miller JA. Sandia Report, SAND968216, UC-405, 1996. Kambara S, Takarada T, Yamamoto Y, Kato K. Energy Fuels 1993;7: 1013–20. Davidson RM. Nitrogen Coal (IEA Coal Res) 1994;6. Solomon PR, Hamblen DG, Carangelo RM, Serio MA, Deshpande GV. Energy Fuels 1988;2:405 –22. Niksa S. Rapid coal devolatilization at elevated pressures. The 11th International Conference on Coal Science, San Francisco, 2001. Xu WC, Kumagai M. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):145. Ledesma EB, Li C-Z, Nelson PF, Mackie JC. Energy Fuels 1998;12: 536 –41. Wendt JOL. Combust Sci Technol 1995;108:323. Nelson PF, Nicholls PM, Ledesma EL. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):154. Leppalahti J, Koljonen T. Fuel Process Technol 1995;43:1. Mackie JC, Colket MB, Nelson PF, Esler M. Int J Chem Kinet 1991; 23:733–60.
Evolution prediction of coal-nitrogen in high pressure pyrolysis processesq Yukihiko Okumuraa,*, Yuriko Sugiyamab, Ken Okazakib,* b
a Department of Control Engineering, Maizuru National College of Technology, Shiroya 234, Maizuru 625-8511, Japan Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan
Received 19 November 2001; accepted 26 April 2002; available online 11 July 2002
Abstract A pyrolysis model which can describe the effects of pressure on the evolution of coal-nitrogen has been constructed based on the FLASHCHAINw model in order to relate the gas release mechanism under high pressure conditions to the polymer reactions in coal. Various kinds of nitrogen-containing gaseous species in the evolved volatiles and their secondary decomposition and coupling processes have been also clarified by considering the elementary reactions of pyrrole-type nitrogen as the primary type of bound nitrogen in the first evoluted heavy species (tar vapor). The results show that the recombination reactions of metaplast are activated in a coal by the increase in pressure, resulting in a lesser amount of tar vapor and more intermediate chars. Thus, the conversion ratio of coal-nitrogen to gaseous volatile-nitrogen increases with the increase of pressure and N-gas converted from the fuel-N is much larger than the tar-N, and becomes more significant in high pressure conditions. Due to the chemical kinetics of the gas phase reactions, a shift in the distributions of tar-N vapor and gas-N with the increase of pressure can be predicted, and larger amounts of H2CCHCN and bipyrrole gases are rapidly formed through three-body reactions, while HCN gas decreases greatly compared with the reaction at normal pressures. The changes of gas composition are in close agreement with the experimental results. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal pyrolysis; Pressurized entrained flow gasification; Pressurized fluid bed combustion; Chemical reaction
1. Introduction Research on pressurized entrained flow gasification (PEFG) and pressurized fluid bed combustion (PFBC) has energetically advanced with the aim of utilizing coal more cleanly and efficiently. It has been reported that coal’s evolution, i.e. its devolatilization with heterogeneous reactions and gas phase reactions, at rapid heating under high pressures of 20 – 30 atm differs greatly from the case of normal pressure [1,2]. However, the effect of pressure on the evolution processes has neither been investigated nor clarified in detail in spite of its importance in the further development of combustion technology. Several investigations [3 – 9] of coal-N evolution have been conducted. It has been observed that the minerals which are included in low-rank coals activate mainly N2 evolution [3,4] and that both the heating rate and the temperature influence the evolution of coal-nitrogen [5 – 7]. However, almost all of these experiments [3 –7] for fuel-N have been carried out under normal pressure conditions, * Corresponding authors. Tel.: þ 81-773-62-8954; fax: þ 81-773-625558. E-mail addresses: [email protected] (Y. Okumura), [email protected] (K. Okazaki). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
leaving a great lack of pyrolysis data for coal-N under high pressure conditions. In addition, an evolution model which can be realistically incorporated into actual systems for both gasification and combustion has also not been developed [10,11], because of the complicated phenomena involved in the actual operating conditions as they change from normal pressure to higher pressure, or higher excess air ratio in PFBC to lower that in PEFG. In light of this, a pyrolysis model has been constructed based on the FLASHCHAINw model [12 – 14] that includes a quantitative estimation of the volatilized gases and released tar vapor at high pressures. Simultaneously, various kinds of nitrogen-containing gaseous species (N-gas) in the evolved volatiles and their secondary decomposition and coupling processes have been clarified by considering the elementary reactions for pyrrole-type nitrogen as a main type of bound-nitrogen in first evoluted heavy species. The weight fractions for the volatilized gas, the released tar vapor, and the residual char are compared in detail with the experimental results.
2. Theoretical analysis
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 1 7 0 - 9
The FLASHCHAINw model has been developed by
2318
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Niksa in 1991 [12]. This theory involves coal’s chemical constitution, a four-step reaction mechanism, chain statistics, and the flash distillation analogy to explain the devolatilization of various coal types. It has now applied actually to the computational fluid dynamic simulator of the entrained flow gasifiers and the pressurized fluid bed in the program of Basic Research Associate for Innovative Coal Utilization (BRAIN-C program, Japan) [10,11]. It has been compared with experimental data, and proved and confirmed that the predicted yields of non-condensible gas and tar are in agreement with measured distributions over the wide ranges of heating rate [13] and temperature etc [14]. This model was selected for this application from the above facts, in which the FLSCHAINw model can predict the pyrolysis process for many type of coals [11,14] and can estimate polymer chain reactions under high pressure conditions. In this study, the consideration was developed by relating gas release mechanism under high pressure condition to the polymer reactions in the coal.
2.1. Modeling of coal pyrolysis under high pressure conditions In this section, a model which can describe the Nevolution and the effect of pressure is proposed on the basis of the FLASHCHAINw model [12 – 14], and the theoretical analysis of pyrolysis under high pressure conditions are performed including full chemical kinetics for the N-gas formation. The discussion will focus on the chain and scission reactions in the coal under high pressure and the reaction mechanisms of decomposition and coupling processes for the nitrogen-containing matters in the gas phase. Fig. 1 shows the conceptual scheme of the bridge reaction processes in coal. The fragments are classified according to their size, as they are called metaplast (small size), intermediate (middle size), and reactant (large size). Only the metaplast, which is the lightest fragment group, can be released to the outside of coal as tar vapor. In this model, the bridges are distinguished as char links, labile bridges, and peripheral groups in terms of the molecule combination-forms. The pyrolysis reaction progresses simultaneously via the next four reactions.
ðR2Þ 3. Peripheral group elimination
ðR3Þ
4. Recombination
ðR4Þ
The release form from coal-nitrogen to nitrogen-containing matters is modeled on the basis that nitrogen is contained only in the aromatic nucleus. It is confirmed by X-ray photoelectron spectroscopy [16,17] that the main types of bound nitrogen in coal are pyrrole and pyridine types. The vapor –liquid phase equilibrium is also included in the present model as playing a role in the pressure effect. The amount of released tar vapor can be calculated by the ratio of saturated vapor pressure of the metaplast to the surrounding gas pressure. The partial pressure of the tar vapor formed from j-mer is expressed by Raoult’s law in Eq. (1). The j shows the number of combinations of the monomer, which consisted of an aromatic nuclear and a bridge, and the molecular weight is 250 –400. The molecule weight of j-mer increases with an increased number of j. Jp X
pj ¼ HP0 ¼
j¼1
Jp X
Xmj Psat ðT; MWtj Þ
ð1Þ
j¼1
ðR1Þ
where pj is the partial pressure of the tar vapor formed from j-mer, P0 is the surrounding gas pressure, Psat ðT; MWtj ) is the saturated vapor pressure of the metaplasts, H is the instantaneous mole fraction of tar vapor within the coal, and Xmj is the mole fraction of j-mer. Assuming that the yield rate of released tar vapor is proportional to production rate of the volatilized gas, the following Eq. (2) can be derived. tj H 1 dY £ G Gj ¼ p ð2Þ J G dt X tj
2. Condensation of bridge (carbonization by the condensation reaction)
where Gj is the yield rate of released tar vapor, tj is the amount of the tar vapor, G is the instantaneous mole fraction of gas within the coal, YG is the mole fraction of volatilized
1. Bridge scission of the segments
j¼1
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2319
Table 2 Reaction rates for calculation
Bridge scission Recombination Peripheral group elimination Volatile N release
A (1/s)
E (kJ/mol)
2 £ 1011 6 £ 1016 1 £ 1015 1.29 £ 104
167 217 230 71.7
chemical species for gases and tar, requiring a complicated process. Therefore, the formative nitrogen-containing gaseous species and their secondary decomposition and coupling processes have been clarified by considering elementary reactions (Table 3) [15] for pyrrole-type nitrogen as a main type of bound nitrogen in tar [16,17].
3. Theoretical results and discussion 3.1. Effect of pressure on the pyrolysis process
Fig. 1. Evolution mechanism based on FLASHCHAINw model. (a) Schematic diagram of evolution mechanism and (b) devolatilization diagram of the coal-N.
gas and subscript j is the number of the aromatic nuclei ðJ p ¼ 5Þ: The partial pressure of the tar vapor is calculated using Eq. (1), and the yield rate of tar vapor can then be obtained from Eq. (2). The calculation condition and coal properties are shown in Tables 1 and 2, respectively. The rate constants for pyrolysis which is recommended by Niksa [12] and Kambara are used in the present calculation. The FLASHCHAINw model has an advantage and a disadvantage. The effects of heating rate on gas, tar, and char formations can be shown over a wide range. However, the model cannot easily evaluate the various kinds of
The effect of pressure on volatilized gas, released tar, and char formations are shown in Fig. 2, with the solid and broken lines showing calculation results. It can be found that the amount of released tar vapor decreases abruptly with increase of the surrounding gas pressure, while the released gas slightly increases. The total weight fraction of the released volatile matter decreases under increasing pressure. This agrees well with experimental results [1]. It can be explained by Eq. (1) that the mole fraction of tar vapor within the coal decreases with the increase of pressure P0. Consequently, the tar vapor formation is suppressed by pressure, and the recombination reaction that creates intermediate char is more strongly activated. Fig. 3 shows the gas formation with the reaction route, in which the amount of gas at 10 atm is determined to be the standard value ( ¼ 1.0). The number in Fig. 3 corresponds to the route shown in Fig. 1(a). It can be seen that the recombination reactions of the metaplast (route (4) in Fig. 1(a)) is activated and the gas formation from the condensation and peripheral group elimination reactions in
Table 1 Calculation and experimental condition for N-release High volatile bituminous coal Pressure Maximum temperature Heating rate
1.0–15.0 atm (absolute) 1038 K 1000 K/s þ 10 s holding
Ultimate analysis of coal (d.a.f) C H 80.1 5.9
O 12.1
N 1.4
Proximate analysis of coal (wt%) VM FC 41.8 48.6
Ash 9.6
Moisture 1.16
S 0.55
Fig. 2. Effect of pressure on released volatile matters and char (1000 K/s, maximum temperature: 1038 K).
2320
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Table 3 Gas reactions of pyrrole C4H5N ¼ PYRLNE PYRLNE ¼ C3H4P þ HCN PYRLNE ¼ C3H5CN PYRLNE ¼ ALLYLCN PYRLNE ¼ C2H2 þ CH3CN H þ C3H5CN ¼ AC3H4CN þ H2 H þ C3H5CN ¼ CH3 þ H2CCHCN H þ C3H5CN ¼ C3H5 þ HCN H þ ALLYLCN ¼ AC3H4CN þ H2 H þ ALLYLCN ¼ C2H4 þ CH2CN H þ ALLYLCN ¼ C3H5 þ HCN CH3 þ C3H5CN ¼ AC3H4CN þ CH4 CH3 þ ALLYLCN ¼ AC3H4CN þ CH4 H þ HCCHCN ¼ H2CCHCN AC3H4CN ¼ CH3CCCN þ H C3H5 ¼ C3H4P þ H CH3 þ H2CCHCN ¼ HCCHCN þ CH4 H þ H2CCHCN ¼ HCCHCN þ H2 HCCHCN þ M ¼ HCCCN þ H þ M H þ H2CCHCN ¼ HCN þ C2H3 H þ CH3CN ¼ CH3 þ HCN H þ C2H2 ¼ C2Hp3 H þ C2H4 ¼ C2H3 þ H2 CH3 þ H2 ¼ CH4 þ H CH3 þ CH3 ¼ C2Hp6 C2H5CN ¼ CH3 þ CH2CN C2H5CN þ H ¼ HCN þ C2H5 C2H5CN þ H ¼ CH3CHCN þ H2 CH3CHCN ¼ H2CCHCN þ H H þ C3H5 ¼ C3H6 H þ C3H5 ¼ C3H4P þ H2 CH3 þ HCCHCN ¼ C3H5CN C2H3 þ CH2CN ¼ ALLYLCN C2H3 þ C2H3 ¼ C4H6 C2H3 þ C2H3 ¼ C2H4 þ C2H2 C3H5 þ CH2CN ¼ C3H4P þ CH3CN C3H5 þ CH2CN ¼ C4H6 þ HCN AC3H4CN þ CH3 ¼ C4H6 þ HCN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
H þ AC3H4CN ¼ ALLYLCN H þ AC3H4CN ¼ C3H5CN C2H3 þ CH2CN ¼ C2H2 þ CH3CN H þ CH2CN ¼ CH3CN CH3 þ C3H5CN ¼ C3H4CN þ CH4 C3H4CN ¼ HCCCN þ CH3 CH3 þ C2H3 ¼ CH4 þ C2H2 C3H5CN ¼ C3H4P þ HCN ALLYLCN ¼ C3H4P þ HCN CH2CN þ ALLYLCN ¼ C3H4CN þ CH3CN C3H4CN ¼ C2H2 þ CH2CN AC3H4CN ¼ HCCCN þ CH3 C2H4 þ M ¼ C2H2 þ H2 þ Mp C4H6 ¼ C2H2 þ C2H4 CH3CN þ C2H3 ¼ H2CCHCN þ CH3 H þ C3H4P ¼ CH3 þ C2H2 CH3 þ CH3CN ¼ CH2CN þ CH4 CH2CN þ CH2CN ¼ H2CCHCN þ HCN C4H5N ¼ PYRLYL þ H PYRLYL þ PYRLYL ¼ BIPRL C2H3 þ C2H4 ¼ C4H6 þ H H þ C4H5N ¼ PYRLYL þ H2 CH3 þ C4H5N ¼ PYRLYL þ CH4 C3H4P ¼ C3H3 þ H CH3 þ C3H4P ¼ C3H3 þ CH4 H þ C3H4P ¼ C3H3 þ H2 C3H3 þ C3H3 ¼ L -C6Hp6 L -C6H6 ¼ C6H6 H þ C2H6 ¼ C2H5 þ H2 C2H5 ¼ C2H4 þ Hp C2H2 þ C2H2 ¼ C4H4 C4H4 ¼ H2 þ C4H2 C2H3 þ C2H2 ¼ N-C4H5 N–C4H5 ¼ C4H4 þ H C2H þ H2 ¼ H þ C2H2 C2H þ C2H2 ¼ C4H2 þ H C2H þ C2H3 ¼ C4H4
Species identification [25] C4H5N: pyrrole; PYRLNE: pyrrolenine(2H-pyrrole); C3H4P: propyne; ALLYLCN: allyl cyanide; AC3H4CN: cyanoallylradical; alkeneyne isomer of benzene etc.
L -C6H6:
the intermediate fragments group is also enhanced (route (2) in Fig. 1(a)). It is concluded that this model including the effect of vapor – liquid equilibrium is capable of qualitatively accounting for the weight loss of coal in high pressure
Fig. 3. Gas formation with the reaction route (1000 K/s, maximum temperature: 1038 K).
conditions. In addition, it is noted that the applicability of this mechanism is restricted to this rank of coal, i.e. high volatile bituminous coals in which substantial amounts of the metaplast are generated in coals and the recombinations in the condensed phase are important. Hereafter, the temperature-dependent profile of weight loss and the product profiles of evolved gases at high pressure are discussed in terms of the data shown in Fig. 4. It should be noted that the experimental conditions (1 K/s) expressed in Fig. 4 differ from those of Table 1 in order to obtain and illustrate the release process of gases and the weight loss of coal in detail. In Fig. 4, each gas composition is formed at A – E temperature ranges, respectively. The temperature for an initial pyrolysis at 10 atm rises more and the high pressure pyrolysis is suppressed at the final stage over 600 8C; subsequently, the evolution of volatile matter is completed more quickly than under normal pressure. It can also be seen that at an initial stage of the evolution process mainly carbon dioxide is evolved, while at the
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2321
Fig. 4. Temperature dependent profiles of weight loss and gas composition with pyrolysis process (experiment by author using the pressurized thermobalance).
middle stage CO, CH4, and H2 gases, and at the last stage mainly H2 is released. It has been observed that CO2, CO, CH4, or H2 are formed from the carboxyl group (– COOH), the ether group (– O – ), the methyl group ( –CH3), and H, respectively [18]. Since the carboxyl groups combined with coal’s structure are especially weak, it can be inferred that the scission process of the carboxyl group can progress primarily at an initial stage of the pyrolysis process. It is found from an analysis of the released gas composition after pyrolysis that the volume proportion of H2 in light fuel gas (CH4 þ H2) decreases with the increase of pressure. It is considered that the H2 evolution is affected by both the greater degree of hydrogen enrichment in tars [19] and the decrease in H2 at the last stage of the evolution process (see Fig. 4). 3.2. Effect of pressure on volatile-N The relationship between pressure and the fuel-N conversion is shown in Fig. 5. The experiment (Kjeldahl method) was carried out by Kambara [1]. It is indicated by calculation that the amount of released nitrogen-containing gases (N-gas) increase with the increase of pressure, and
Fig. 5. Comparison of volatile-N yield with experimental results (1000 K/s, maximum temperature: 1038 K).
that the amount of N-gas converted from the fuel-N is much larger than the tar-N, and becomes more significant in high pressure conditions [20]. The reason the gasification reaction is activated is closely similar to the description provided in Section 3.1, i.e. the tar precursor called ‘metaplast’ remaining in the coal enhances the condensation and peripheral group elimination reactions in the intermediate fragment groups and the N-gas formation. It can be inferred that the actual gas-N formation is further increased due to the following two reasons. First, the trace elements (Fe, Ca) in coal under high pressures, in which the releases of Fe and Ca are also suppressed by the pressure, would enhance the evolution of nitrogen gas. Second, the influence of radicals would appear at high pressures. The decyclization reactions for both the pyrrole and pyridine types of nitrogen are actually activated by radicals such as alkyl radical or H, which are remarkably formed from the secondary decomposition of the tar [21]. At high pressures, the radical with the tar precursor becomes more highly activated in the coal. Consequently it has been observed from experiment [21] that the HCN gas formation from actual coal proceeds more easily than that from pure pyrrole. However, the present model cannot directly illustrate why the release of coal-N is enhanced by elevated pressure [1], and the tendency of released nitrogencontaining gases to increase agrees well with the experimental results only. Here, we discussed why release of coal-N increased under high pressure during the experimental procedures [16]. It has been reported that the coal-N release from the pressurized entrained flow systems [10] and from the wire grid tests [9] of several coals is not promoted by elevated pressures. It was found during this experimental procedures of Kambara [11] that the secondary decomposition of tar-N readily occurs due to a consistently maintained high temperature (10 s, 1038 K) [1], and to the coal sample being enclosed with glass wool at both ends of the quartz glass [1] (i.e. reaction zone, i.d 1.3 mm, length:
2322
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
Fig. 6. Pyrolysis and volatilization schemes for coal-N.
30 mm). It is believed that a large amount of tar-N, which can evolve mainly at a high temperature from 800 K [16], would be difficult to release in this experimental apparatus at the last stage of pyrolysis, and that only gas-N would be released. The released tar-N at the last pyrolysis stage cannot be separated completely from the coal itself, thus these phenomena may cause enhanced nitrogen release. 3.2.1. Secondary decomposition mechanism of volatile-N In this section the decomposition and coupling reaction mechanisms for nitrogen containing gaseous species are further discussed by combining the gas phase reaction model with the FLASHCHAINw model. Based on recent research [22 –24], the release scheme of coal-N in the pyrolysis and devolatilization processes have been suggested as shown in Fig. 6. In the primary stage of decomposition, the various kinds of heavy nitrogencontaining matter, which has mainly a polycyclic structure, are evolved, while simultaneously NH3 and HCN gases are directly released from the coal to the gas phase (6 –8% of fuel-N) [16]. In the secondary stage of decomposition, most of heavy nitrogen matter is converted to NH3 and HCN
gases and soot. These decomposition reactions would be more important when the experiment is performed at rapid heating (1000 K/s). Thus, the reaction processes of secondary cracking must be calculated even at the high pressure of 15 atm. The influence of pressure on the elemental reactions can be included into the model by the three-body effects for the thermal dissociation and recombination reactions (see p mark in Table 3), which strongly depend on pressure [15]. This elementary reaction should be applied only to tar vapor and gas, in which case we can predict a shift in the distribution of tar-N and gas-N. Fig. 7 shows the gas phase reactions [25] of pyrrole type nitrogen under high pressure and normal conditions. It can be seen from the upper figures that the pyrrole reactions are enhanced with the increase of pressure through three-body reactions, and that larger amounts of H2CCHCN and bipyrrole gases are rapidly formed as compared with the case at 1 atm. Those tendencies at lower temperatures of approximately 1000 K appear strongly in comparison with those seen at higher temperatures of approximately 2000 K. It is also confirmed that the decomposition and coupling reactions for pyrrole at high pressures become more active than those of pyridine. These facts are consistent with the experimental results (Fig. 8). The comparison the results of calculation with the experimental results in regard to N-gas composition [1] is shown in Fig. 9. The decrease of HCN and the increase of other N-gases under increasing pressure agree well with experimental results. The amount of NH3 gas increases with the increase in pressure, and it has been reported that the NH3 gas is formed from amine-type nitrogen [1]. Thereby in order to elucidate the NH3 formation mechanisms, the decomposition
Fig. 7. Gas phase reactions for pyrrole type nitrogen under high pressure conditions.
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
2323
process of amine type nitrogen must be investigated. The three-body reactions in the gas phase and the effects of remaining metaplast in coal must be included in the pyrolysis process for coal-N at high pressures.
4. Conclusion The theoretical analysis of thermal pyrolysis at high pressures is performed including the full chemical kinetics of N-gas formation. The study obtained the following results: Fig. 8. Effect of pressure on decomposed fraction of nitrogen obtained by Kambara [1]. (a) Pyrrolic-N and (b) pyridinic-N.
4.1. Effect of pressure 1. Tar vapor formation is suppressed by pressure, and the recombination reactions of the remaining metaplast are then activated, resulting in more intermediate chars. 2. The model, which includes the effect of vapor – liquid phase equilibrium, is capable of accounting for the weight fractions of char, tar, and gas at high pressures, as demonstrated by comparison with high-pressure pyrolysis experiments. 3. The volume proportion of H 2 in light fuel gas (CH4 þ H2) shows a tendency to decrease with increasing pressure. This is due mainly to the suppression of H2 evolution at the last stage of the pyrolysis process. 4.2. Release mechanism of coal-N 1. The released nitrogen-containing gases (N-gas) increase with the increase of pressure. This is mainly due to large amount of metaplast remaining in coal. Specifically, the recombination and peripheral group elimination reactions in the intermediate fragment group are activated by the increase of pressure, consequently resulting in more N-gas formation. 2. Due to the chemical kinetics of gas phase reactions, pyrrole reactions are also enhanced by the increase of pressure through three body reactions. HCN gas is primarily released at 1 atm, while at higher pressures a larger amount of H2CCHCN and bipyrrole gases can be formed. The tendency of HCN to decrease and the tendency of the amounts of other N-gases to increase at increasing pressures agree well with experimental results.
Acknowledgments
Fig. 9. Comparison of predicted N-gas composition with experimental results. (a) Effect of pressure on HCN conversion ratio and (b) effect of pressure on fuel-N conversion ratio for nitrogen-containing gases.
This research was financially supported in part by Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Program, sponsored by the New Energy and Industrial Development Organization (NEDO, Japan) and by Electrical Power Development Co. Ltd. The authors expresses appreciation to these organizations.
2324
Y. Okumura et al. / Fuel 81 (2002) 2317–2324
References [1] Kambara S. Symposium series 48 of chemical engineering association (in Japanese). Japan: The Society of Chemical Engineers; 1995. p. 151. [2] Okumura Y, Sugiyama Y, Okazaki K. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):141. [3] Wu Z, Otsuka Y. Energy Fuels 1997;11:902. [4] Zhiheng W, Sugimoto Y, Kawashima H. J Jpn Inst Energy 2001;80: 97–104. [5] Chen JC, Castagnoli C, Niksa S. Energy Fuels 1992;6:264. [6] Bassilakis R, Zhao Y, Solomon PR, Serio MA. Energy Fuels 1993;7: 710–20. [7] Wu Z, Otsuka Y. Energy Fuels 1997;11:477. [8] Cai HY, Guell AJ, Dugwell DR, Kandiyoti R. Fuel 1993;72(3): 321–7. [9] Tomita A. No. 9703TM5021. Tohoku University, Japan, 1997. [10] Reports of Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Project (Pyrolysis and Gasification Group), NEDO-C9939. The New Energy and Industrial Development Organization (in Japanese), 2000. [11] Reports of Basic Research Associate for Innovative Coal Utilization (BRAIN-C) Project (Pyrolysis and Gasification Group), NEDO-C0022. The New Energy and Industrial Development Organization (in Japanese), 2001.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Niksa S, Kerstein AR. Energy Fuels 1991;5:647–65. Niksa S. Energy Fuels 1991;5:665–73. Niksa S, Kerstein AR. Energy Fuels 1991;5:673–83. Kee RJ, Ruply FM, Meeks E, Miller JA. Sandia Report, SAND968216, UC-405, 1996. Kambara S, Takarada T, Yamamoto Y, Kato K. Energy Fuels 1993;7: 1013–20. Davidson RM. Nitrogen Coal (IEA Coal Res) 1994;6. Solomon PR, Hamblen DG, Carangelo RM, Serio MA, Deshpande GV. Energy Fuels 1988;2:405 –22. Niksa S. Rapid coal devolatilization at elevated pressures. The 11th International Conference on Coal Science, San Francisco, 2001. Xu WC, Kumagai M. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):145. Ledesma EB, Li C-Z, Nelson PF, Mackie JC. Energy Fuels 1998;12: 536 –41. Wendt JOL. Combust Sci Technol 1995;108:323. Nelson PF, Nicholls PM, Ledesma EL. 221st ACS Natl Meet (Am Chem Soc Div Fuel Chem) 2001;46(1):154. Leppalahti J, Koljonen T. Fuel Process Technol 1995;43:1. Mackie JC, Colket MB, Nelson PF, Esler M. Int J Chem Kinet 1991; 23:733–60.