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Energy & Fuels 1996,9, 290-294
Evaluation of Vapor-Phase Reactivity of Primary Tar Produced by Flash Pyrolysis of Coal Jun-ichiro Hayashi, Shinobu Amamoto, Katsuki Kusakabe, and Shigeharu Morooka" Department of Chemical Science and Technology, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812, Japan Received July 13, 1994. Revised Manuscript Received January 13, 1995@
Flash pyrolysis of Morwell brown coal and Wandoan subbituminous coal was performed with three types of pyrolyzers: a Curie-point pyrolyzer in which the secondary pyrolysis of volatiles was suppressed, an entrained-flow reactor in which the primary and the secondary pyrolysis proceeded at the same temperature, and a fluidized-bed reactor that was divided into dense bed and freeboard regions of the primary and the secondary pyrolysis, respectively. The temperature in the dense bed and freeboard regions of the fluidized bed reactor was independently controlled. The yield of mono- and diaromatic products, the most abundant in tar, was adopted as the measure for the reactivity of initial tar generated by the primary pyrolysis, and the role of temperature in the primary pyrolysis zone on the reactivity of the initial tar in the vapor phase was examined. The yield of mono- and diaromatic products for Wandoan coal was not affected by the primary pyrolysis temperature above 600 "C but that for Morwell coal was greatly influenced.
Introduction Flash pyrolysis is an attractive coal conversion process for obtaining liquid products such as BTX and polycyclic aromatic hydrocarbons. Normally, it consists of two reactions in series: primary pyrolysis of the macromolecular network of coal, and secondary pyrolysis of volatiles in the gas phase. Useful aromatics are mainly formed by the latter reaction, while the former reaction produces their precursors. In this article, the primary tar is defined as the tar that has not undergone vapor-phase reactions, Le., extraparticle secondary reactions. Then the term "primary pyrolysis" includes the intraparticle secondary reactions. In our previous study1s2we systematically investigated the change in molecular composition of flash pyrolysis tar by the secondary reaction on the basis of the yield of PAHs classified into homologous series with specific ring structure and specified aliphatic substituents. However, the role of the precursor-forming primary pyrolysis has not yet been clarified. Several research groups investigated the effect of the peak temperature on tar yield by using wire-mesh pyrolyzer^,^-^ in which the vapor-phase secondary reactions were minimized. They reported that mass yield of tar was nearly constant at above 600-700 "C under heating rates of 103-104 "C/s. This indicates that mass yield of the primary tar is apparently not affected by the pyrolysis temperature in this temperature region. * To whom all correspondence should be addressed. Abstract published in Advance ACS Abstracts, February 15, 1995. (1)Hayashi, J.-i.; Taniguchi, T.; Kusakabe, K.; Morooka, S.; Yumura, M. Energy Fuels 1993,7, 58. (2) Hayashi, J.-i.; Nakagawa, K.; Kusakabe, K.; Morooka, S. Fuel Processes Technol. 1992,30, 237. (3) Unger, P. E.; Suuberg, E. M. Fuel 1984,63,606. (4) Oh, M. S.; Peters, W. A.; Howard, J. B. AIChE J . 1989,35,775. (5) Freihaut, J. D.; Proscia, W. M. Energy Fuels 1989,3,625. (6) Gonec, Z. S.; Gibbins, J. R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990,69, 383. (7) Li, C.-Z.: Bartle, K. D.; Kandiyoti, R. Fuel 1993,72,3.
0887-0624/95/2509-0290$09.00/0
Curie-point pyrolyzers were also employed, and quite similar results were obtained on the temperature dependence of mass yield of tar.8-10 Tar is a complicated mixture of coal fragments having widely distributed chemical structure. Structural parameters such as WC atomic ratio, carbonhydrogen aromaticity,11J2average molecular mass, and molecular mass distribution (MMD)3,4,13-16 have been investigated for flash pyrolysis tars obtained with the wire-mesh pyrolyzers. Suuberg et al.13 and Unger and Suuberg3 reported that the MMD for tars from bituminous coals was not changed by temperature at above 600 "C. On the other hand, Li et al.15 investigated the MMD for tars from three maceral concentrates of a bituminous coal and found that the MMD of the liptinite tar shifted to the smaller mass side with an increase in peak temperature above 600 "C. Furthermore, Suuberg et al.14 and Yun et observed that the MMD was changed during the tar releasing period. These results imply that the molecular composition of flash pyrolysis tar is affected by pyrolysis temperature even under minimized contribution of the secondary reaction, while its total mass is unchanged. Previous studies on the change in molecular composition of flash pyrolysis tar at above 600 "C are, however, not sufficient to predict the vaporphase reactivity of the primary tar. In this article, we propose a technique to evaluate the thermal reactivity (8) Xu, W.-C.; A. Tomita, A. Fuel 1987,66, 627. (9) Xu, W.-C.; A. Tomita, A. Fuel 1987,66, 632. (10) Miura, K.: Mae, K.; Asaoka, S.; Hashimoto, K. Energy Fuels 1991,5,340. (11)Khan, M. R. Fuel 1989,68, 1522. (12) Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980,59, 479. (13) Suuberg, E. M.; Unger, P. E.; Lilly, W. D. Fuel 1985,64, 956. (14)Suuberg, E. M.; Unger, P. E.; Larsen, J . W. Energy Fuels 1987, 1, 305. (15) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993,72, 3. (16)Yun, Y.: Mauzelaar, H. L. C.; Simmleit, N.; Shulten, H.-R. Energy Fuels 1991,5,22.
0 1995 American Chemical Society
Vapor-Phase Reactivity of Primary Tar
of flash pyrolysis tar. Flash pyrolysis of two low-rank coals was conducted by using three kinds of pyrolyzers: a Curie-point pyrolyzer (CPP), a fluidized bed reactor (FBP), and an entrained-flow pyrolyzer (EFP). The effect of the primary pyrolysis temperature on the nature of initial tar as precursor of light aromatics was examined on the basis of the yield of mono- and diaromatic compounds by varying the combination of temperature profiles in the primary and secondary reaction zones.
Experimental Section Coal Samples. Monvell brown coal (C = 65, H = 4.7 wt %, daf) and Wandoan subbituminous coal (C = 76, H = 5.2 wt %, daf) were employed for pyrolysis experiments. The coal samples were pulverized, sized to 0.074-0.125 mm, and dried in vacuo at 100 "C for 24 h. Curie-PointPyrolysis. Flash pyrolysis was carried out using a Curie-point pyrolyzer (JHP-2, Japan Analytical Industry) connected to an FID and a TCD gas chromatograph. A 1-3 mg coal sample was tightly wrapped in a ferromagnetic foil whose Curie point was 590, 670, 764, or 920 "C. The foil was set in a quartz tube and inductively heated for 5.0 s. The time period required for heating up t o the Curie-point temperatures was approximately 0.2 s. Gaseous products were introduced directly into a TCD or an FID-GC, while tar was condensed in quartz wool packed down-stream from the pyrofoil. The sweeping velocity of the carrier gas was 0.060.07 d s . Guell et al.17pyrolyzed a bituminous coal in a wiremesh reactor and reported that the tar yield was constant in a range of the sweep gas velocity, 0.02-0.12 d s . In the present study, the mass yields of total volatile matter and tar were independent of the heating period in a range of 1.0-10.0 s. We carefully checked coking of tar on the surface of the pyrofoil with a microbalance and found that there was no weight change of the pyrofoil after the pyrolysis. This indicates that the extraparticle secondary reaction of tar did not proceed to an extent where solid deposits were formed, but we cannot say that the vapor-phase secondary reaction was completely negligible in the CPP. However, the pyrolysis with the CPP is not to recover the primary tar itself but to confirm that the mass yield of tar is nearly constant above 600 "C when the vapor-phase secondary reactions is minimized and t o check the completeness of the devolatilization in the other two practical pyrolyzers by comparing the char yield. Fluidized-BedPyrolysis. A fluidized-bed pyrolyzer (FBP hereafter) was divided into two zones: a dense bed for the primary pyrolysis and a freeboard for the secondary pyrolysis. The temperatures in the dense bed (Tp)and freeboard (T,) were independently controlled. A schematic of the pyrolyzer is shown in Figure 1. Details of the FBP were reported elsewhere.' Coal particles were continuously fed into the dense bed (TP= 600 "C), and the primary pyrolysis was completed there.' Volatiles generated in the dense bed were transferred into the freeboard, where the residence time of tar vapor was 2.5-3.5 s. T,was varied in the range of 600-900 "C. Char particles entrained were collected with a cyclone, and tar was completely collected with a cold trap kept at -70 "C. The tar was analyzed with an FID-GC with a capillary column. Noncondensable products were collected in an impermeable bag and subjected t o GC analysis. Entrained-Flow Pyrolysis. Coal samples were also pyrolyzed in an entrained-flow pyrolyzer (EFP) of 1.0 m length and 0.0157 m i.d. As schematically shown in Figure 1, the primary and secondary pyrolysis in the EFP proceeded at the same temperature Tps (=Tp= Ts),which was varied in the range of 700-900 "C. The residence time of coal particles was (17)Guell, A. J.;Kandiyoti, R.Energy Fuels 1993,7, 943.
Energy & Fuels, Vol. 9, No. 2, 1995 291 Char and votatiles
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Figure 1. Schematics of fluidized-bed and entrained-flow pyrolyzers. estimated as 1.5-2.0 s. The procedures of recovery and analysis of liquid and gaseous products were the same as with the FBP.
Results and Discussion Figure 2 shows the temperature dependence of the yield of tar and benzene in the CPP pyrolysis. When TPwas higher than 600 "C, the tar yield was almost constant at ca. 26 wt % for Wandoan coal and ca. 17 wt % for Morwell coal. The tar yield slightly decreased above 764 "C. These results are basically consistect - ~ the other with those of the previous s t ~ d i e s . ~On hand, benzene yield was significantly influenced by Tp above 700 "C for both coals. The slightly decreasing tar yield and increasing benzene yield might be due to the secondary reaction of the tar. However, it is more reasonable to consider that the change in the benzene yield reflects the change in molecular composition of the primary tar. Actually, Ofosu-Asante et a1.18 pyrolyzed the original and chemically modified Illinois No. 6 coals in a wire mesh reactor and found that the benzene yield was increased with increasing peak temperature while that of tar was not changed. The previous pyrolysis studies8-10 with CPP also revealed that the yield of benzene and other monoaromatics was significantiv increased above 600 "C.
Hayashi et al.
292 Energy & Fuels, Vol. 9, No. 2, 1995 30
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I
2 I
Unlike in the primary pyrolysis, the temperature for the secondary reaction, Ts,affected both mass yield and molecular composition of flash pyrolysis tar. The effect of Ts were examined with the FBP under the same T p of 600 "C. Figure 3 shows the effect of Ts on the yield of individual aromatic homologues having the same number of double bonds per molecule, db, for the FBP tars from Wandoan coal. Each db homologue includes unsubstituted and alkyl-substituted compounds. The yield of 3-5 db homologues was determined by GC analysis, while that of 5-12 db homologues was found by FI-MS analysis followed by the multistep separation of tar by solvent extraction, column chromatography, and high-performance liquid chromatography. The details are reported e1sewhere.l The aromatic fraction shown in Figure 3 accounted for 15 and 60 w t % of whole tar at TS= 600 and 900 "C, respectively. All of the yield do not include heterocyclic and hydroxylic compounds.
6 I
Regardless of ring size, the yield of each homologue was simply increased with increasing Ts. The secondary reaction at any Ts gave a similar ring-size distribution. The average number of aromatic carbons per molecule, C,,, was calculated for homologues obtained at each temperature by the following equation. n
n
where Cn and Y,,are respectively the number of aromatic carbons per molecule and the molar yield of each homologue having n double bonds. The value of Car was 10.8-11.0 at Ts = 600-900 "C. The result indicates that the secondary reaction hardly changed the ringsize distribution and that the distribution in the tar was decided by the coal structure and the primary pyrolysis that produced the primary tar. At any Ts,the 3 db
Energy & Fuels, Vol. 9, No. 2, 1995 293
Vapor-Phase Reactivity of Primary Tar %I, , ,,
70
60
:
700 "C. For Wandoan coal, the BTX yield in the EFP was more than twice that in the FBP at 800 "C, and the PCX yield showed a maximum (0.4 wt %) at TS = 700 "C both in the EFP and FBP. Meanwhile, the PCX yield of Morwell coal reached a maximum (0.6 wt %) at 800 "C in the FFP. The temperature dependence of the yield of intermediate products such as PCX was decided by relative rates between formation and decomposition. The result for the PCX yield obtained in the EFP suggests that the formation rate of PCX and its precursors was faster than their cracking rate of PCX and its precursors was faster than their cracking rate in the range of 700-800 "C and that the change in the primary reaction was responsible for the increase in the PCX formation rate. The remarkable increase in BTX and PCX yields in this range in comparison with those in the FBP should be ascribed t o the nature of the primary tar from Morwell coal. In the primary pyrolysis step, tar is generated by bond-breaking of the macromolecular network of coal, followed by stabilization of fragment radicals. Cross-
linking reactions14J9occurring in the primary pyrolysis suppress tar formation. Cross-linking associated with decomposition of oxygen-containing groups such as phenolic -OH and -COOH is significant in low-rank coals. The tar yield and its molecular composition should be affected by relative reaction rates of bondbreaking and cross-linking, having different activation energies. Then the relative rates are changed by changing the time-temperature profiles of p y r o l y s i ~ . ~ ~ The pyrolysis of Morwell coal in the present study strongly suggests that the relative rates between bondbreaking and cross-linking were greatly changed at 700-800 "C. The change in thermal reactivity of the primary tar also affects the yield of diaromatic products in the range of TS and Tps > 800 "C. The yield of naphthalene and indene at Tps = 900 "C in the FFP was about twice that in the EBP at the same Ts. The selectivity of monoand diaromatic compounds in tar is probably not affected by the secondary reaction, according to the result of Figure 3. In the case of Morwell coal, therefore, the molecular composition and thermal reactivity of the primary tar are dependent on the temperature of the primary reaction, even in the range higher than 600 "C.
Conclusions The effect of the temperature of the primary pyrolysis on the thermal reactivity of the primary tar was investigated by using the CPP, the FBP and the EFP. Based on the pyrolysis results, the following conclusions can be drawn. 1. Yields of aromatic hydrocarbons without heteroatoms were simply increased with increasing Ts. However, their ring size distributions hardly varied. 2. The nature of the primary tar as the precursor of light aromatics such as BTX was not affected by Tp and Tps of 600-900 "C for Wandoan coal, but it was drastically changed for Morwell coal above 700 "C. EF940140S (18) Ofosu-Asante, K.; Stock, L. M.; Zabransky, R. F. Fuel 1989,68,
567. (19) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E . Energy Fuels 1990, 4 , 42.
