Reactions in Brown Coal Pyrolysis Responsible for Heating Rate

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Energy & Fuels 2000, 14, 400-408

Reactions in Brown Coal Pyrolysis Responsible for Heating Rate Effect on Tar Yield Jun-ichiro Hayashi,* Hiroshi Takahashi, Satoshi Doi, Haruo Kumagai, and Tadatoshi Chiba Center for Advanced Research of Energy Technology, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan

Tadashi Yoshida Hokkaido National Industrial Research Institute, 2-17-2-1, Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan

Atsushi Tsutsumi Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received July 13, 1999

This study was carried out for the purpose of experimentally establishing the variation with heating rate of the extent of bridge breaking relative to that of cross-linking, which is a reasonable explanation of the heating rate effect on the tar yield in pyrolysis of low rank coals. A brown coal was pyrolyzed slowly at a heating rate of 0.167 K s-1 and rapidly at 2-3 × 103 K s-1. The yield of tar in the rapid pyrolysis increased with temperature and leveled off at 923 K and 26 mol-C per 100 mol-C in the coal, while 723 K and 15 mol-C in the slow pyrolysis. The loss of aliphatic carbon (∆Cal) due to aromatization was employed as the measure for the extent of bridge breaking, assuming that the loss is indispensable to supplying donatable hydrogen to cap radicals formed by cleavage of bridges connecting aromatic clusters. The extent of cross-linking was elucidated from the yields of H2O and CO2 that are the major and plausible products of condensation among hydroxylic and carboxylic groups. The rapid pyrolysis was found to give the yield of H2O smaller than that in the slow pyrolysis at every ∆Cal, indicative of the activation energy for H2O formation smaller than that for the loss of aliphatic carbon. The larger ceiling yield of tar with higher heating rate was thus consistent with relatively enhanced bridge breaking and suppressed crosslinking such as dehydration condensation. Unlike the yield of H2O, that of CO2 as a function of ∆Cal little depended on the heating rate, suggesting that the CO2 formation is not responsible for the observed heating rate effect on the ceiling yield.

Introduction Tar is a major product in coal pyrolysis unless its secondary reactions in the gas-phase proceed extensively. Upon heating coal, the evolution of tar commences at ca. 600 K and the yield of tar increases with temperature and levels off with a highest attainable value at a certain temperature. The highest attainable yield, hereafter referred to as the ceiling yield, is known to depend more or less on the heating rate 1-9 as well * Author to whom correspondence should be addressed at Center for Advanced Research of Energy Technology, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan. Phone: +81-11-706-6850. Fax: +81-11-726-0731. E-mail: [email protected]. (1) Stompel, Z. J.; Bartle, K. D.; Frere, B. Fuel 1982, 61, 817. (2) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 632. (3) Kahn, M. R. Fuel 1989, 68, 1522. (4) Gonenc, Z. S.; Gibbins, J. R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69, 383. (5) Solomon, P. R.; Serio, M. A.; Deshpande, G. V.; Kroo, E. Energy Fuels 1992, 4, 42. (6) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (7) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 3.

as other operational conditions such as pressure and coal type. The heating-rate dependency of the ceiling yield has been found in pyrolysis of coals ranging from lignite to bituminous ranks. For bituminous coals, the ceiling yield is prone to increase as the heating rate increases while the yields of noncondensable gases remain unchanged or change slightly.4-7 The tar evolution from lignites and brown coals is influenced more significantly than that from higher rank coals.8,9 In wire-mesh pyrolysis of a brown coal,8 the ceiling yield obtained by heating at 1000 K s-1 is 2-3 times greater on a mass basis than that at 1 K s-1. The enhanced evolution of tar accompanied suppressed evolution of noncondensable gases.8 Similar characteristics were found in the pyrolysis of another brown coal,9 in which a higher heating rate resulted in selectivities being (8) Sathe, C.; Pang, Y.; Li, C.-Z. Proc. 15th Pittsburgh Coal Conf. 1998. (9) Matsuo, Y.; Hayashi, J.-i.; Kusakabe, K.; Morooka, S. Coal Sci. Technol. 1995, 24, 929.

10.1021/ef9901490 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000

Reactions in Brown Coal Pyrolysis and Tar Yield

larger to tar and smaller to dominant inorganic gases such as H2O, CO, CO2, and H2. In general, tar is evolved from coal through the formation of low-molecular-mass compounds by thermal degradation of networked macromolecules and their mass transport that involves intraparticle diffusion/ convection and evaporation. Of these consecutive processes, the former chemical process consists mainly of the breaking of inter-aromatic-cluster bridges and crosslinking. The bridge breaking involves formation of radicals by homolytic cleavage of labile C-C and C-O bonds and stabilization of a portion of the radicals by donatable hydrogen, and leads to depolymerization. On the other hand, the cross-linking produces bridges and causes polymerization. The chemical process is therefore governed by the rate of bridge breaking relative to that of cross-linking, and a larger relative rate of bridge breaking would result in a greater yield of tar as fragments of the macromolecular network. If these reactions obey the Arrhenius law and the activation energy for the bridge breaking is larger than that for the cross-linking, the increase in the heating rate should increase the relative rate of the former reaction, leading to an increase in the ceiling yield of tar. Thus, the experimental definition of the extents of these key reactions may enable to evaluate the responsibility of the chemical processes for the heating rate effect on tar yield. Competing bridge breaking and cross-linking are considered in a recent model of coal pyrolysis, the FGDVC model (functional groups - decomposition, vaporization, and Cross-linking model),10,11 which describes the chemical structure of coal as a mixture of a networked macromolecule and low-mass compounds trapped in the network. The bridge breaking in the model requires homolytic cleavage of a labile bond forming free radicals and the consumption of donatable hydrogen to cap the radicals. The major sources of donatable hydrogen are hydroaromatic rings and ethylene-type bridges.10,11 The supply of donatable hydrogen from these structures is inevitably accompanied by the aliphatic-to-aromatic carbon conversion, and the amount of released hydrogen would correspond to that of aliphatic carbon converted into aromatic carbon. Thus, the loss of bridges due to the bridge breaking may be estimated from that of aliphatic carbon, which can be determined by analyzing carbon type distributions of the initial coal and the pyrolysis products.12,13 The estimation is expected to be reasonable unless a considerable portion of donatable hydrogen is consumed for the formation of gaseous hydrocarbons, the sources of which are not bridges but peripheral alkyl chains carried by aromatic clusters. The FG-DVC model also assumes that cross-links are formed by thermal decomposition of specific functional groups in coal into corresponding gases. So far, efforts have been made to define the cross-linking occurring (10) Solomon, P. R.; Hamblen, D. G.; Yu, Z.-Z.; Serio, M. A. Fuel 1990, 69, 754. (11) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405. (12) Vassallo, A. M.; Wilson, M. A.; Edwards, J. H. Fuel 1987, 66, 623. (13) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643.

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in pyrolyzing coal. Suuberg et al.14,15 estimated the extent of cross-linking in coals during pyrolysis from the change in the swelling ratio of char with pyridine. The reduction of the ratio for lignites occurred at lower temperatures than that for bituminous coals, and the extent of the reduction corresponded well with the amount of CO2 released. Solomon et al.16 and Ibarra et al.17 also found for low-rank coals that the difference in the swelling ratio between the initial coals and their chars was closely related to the amount of CO2 or H2O, and these inorganic gases are evolved from brown coals and lignites even at temperatures from 400 to 600 K where tar formation is negligible.18,19 This suggests smaller activation energy for the formation of CO2 and H2O than that for tar evolution as taken in the FG-DVC model.10,11 The low rank coals contain quite a number of carboxyls and hydroxyls, most of which are associated via hydrogen bonds. Hydrogen bonds can destabilize O-H bonds and thereby induce condensation among the groups that would form H2O, CO2, and cross-links with chemical structures such as -O-, -CO-, and -COO-. If H2O and CO2 are the products of major cross-linking reactions, their yields are the quantitative measures of the amount of cross-links formed during the pyrolysis. The primary objective of this work is to experimentally examine the variation with the heating rate of the relationship between the loss of aliphatic carbon and the yields of gaseous products, expecting their correspondence to the amounts of lost bridges and formed cross-links, respectively. The second objective of this study is to examine the effect of the carboxylate removal on the formation of tar and inorganic gases as well as the loss of aliphatic carbon. Removal and addition of ion-exchanged metallic cations considerably increases and decreases the ceiling yield of tar from lignites and brown coals, respectively.20-25 These low rank coals more or less contain organically bound Na+, Ca2+, Mg2+, Fe2+, and Fe3+ in forms of carboxylates that are easily replaced by protons through ion-exchange reactions. Some studies 22-24 have so far been carried out to examine the roles of the species and have shown that their removal increases the tar yield considerably under a variety of heating conditions. Although roles of such metallic species in so-called retrograde reactions are not well understood even for model compounds systems,26,27 a most authentic (14) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Fuel 1985, 64, 1668. (15) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305. (16) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Fuel 1986, 65, 182. (17) Ibarra, J. V.; Moliner, R.; Gavilan, M. P. Fuel 1991, 70, 408. (18) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Gravel, D. Energy Fuels 1990, 4, 319. (19) Hayashi, J-i.; Matsuo, Y.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 284. (20) Franklin, D. H.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 39. (21) Hippo, E. J.; Jenkins, R. D.; Walker, P. L., Jr. Fuel 1979, 58, 338. (22) Jones, J. C. In The Science of Victorian Brown Coal; Durie, R. A., Ed.; Butterworth-Heinemann: Oxford, U.K., 1991; Chapter 9. (23) Tyler, R. J.; Schafer, H. N. S. Fuel 1980, 59, 487. (24) Wornat, M. J.; Nelson, P. F. Energy Fuels 1992, 6, 136. (25) Hayashi, J.-i.; Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1996, 10, 1099. (26) Manion, J. A.; McMillen, D. F.; Malhotra, R. Energy Fuels 1996, 10, 776. (27) Britt, P. F.; Buchanan, A. C., III; Hoenigmen, R. L. Coal Sci. Technol. 1995, 24, 577.

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Table 1. Properties of Coal Samples [wt % on dry coal basis]

[eq/100 mol-C]

sample

C

H

N

S

Ca

Mg

Fe

Na

Al

fal [mol-C/100 mol-C]

YL YLA

64.4 64.7

5.0 5.0

0.57 0.45

0.23 0.25

0.29

0.70

0.32

0.12

0.13

0.39 0.38

explanation for the increased tar yield is that Ca, Mg, and Fe carboxylates behave as cross-links, which are removable by ion exchange in acid treatments. Thus, the removal should lead to the decrease in the initial cross-link density. In addition to this, since the ionexchange converts carboxylates into carboxyls that might be cross-link precursors as described above, the conversion may bring about some changes in the characteristics of the formation of H2O and CO2 and also remove catalytic effects of the metallic species. This paper reports the results of the above-described experimental examinations for the pyrolysis of a brown coal with slow heating at a rate of 0.167 K s-1 and that with a rapid heating at rates in the order of 2-3 × 103 K s-1. As described in the following section, the slow pyrolysis was carried out in a fixed-bed reactor, and the rapid pyrolysis was performed in a Curie-point reactor and a drop-tube reactor. The Curie-point reactor was employed to determine the distribution of pyrolysates that are nearly free from the vapor-phase secondary reactions, while the drop-tube reactor was used to obtain sufficient amounts of tar and char to be subjected to the analysis of distribution of carbon types. Experimental Section Coal Samples. Yallourn brown coal was used for the experiments. The raw coal was dried at 353 K for 12 h under vacuum, and was stored in glass ampules prior to pyrolysis. The dried coal, hereafter referred to as YL, was also acidtreated in order to remove organically bound and ionexchangeable metal cations from the coal. A 20 g sample of YL was suspended in 500 mL of 5 N HCl at 298 K. The suspension was stirred for 72 h and then filtered. An inductively coupled plasma emission spectrometry (ICP-ES) analysis of the filtrate confirmed that Ca, Mg, Fe, Na, and Al were leached from YL. Subsequently, the solid residue was washed repeatedly with deionized water until no chlorine ions were detected in the washing. The residue was finally dried in the same manner as above. This sample is denoted by YLA. Table 1 lists the elemental compositions, contents of ion-exchangeable metallic species, and carbon aliphaticity (fal) of YL and YLA. Rapid Pyrolysis in a Curie-Point Reactor (CPR). Samples prepared as above were pyrolyzed in a Curie-point pyrolyzer (CPR; Japan Analytical Industry, model JHP-2) in an atmospheric flow of nitrogen or helium as a carrier gas. Typically, about 2 mg of the sample was tightly wrapped in a ferromagnetic foil and placed in the reactor tube made of quartz glass. After the reaction zone was purged with the carrier gas, the foil was heated inductively up to its Curie point temperature of 631, 718, 863, 943, 1037, or 1193 K within ca. 0.2 s (manufacturer-guaranteed). The pyrolysis temperatures were defined as the Curie-point temperatures of the corresponding ferromagnetic foils. The total pyrolysis time was fixed at 5.0 s, which was long enough for complete release of volatiles from the sample by the pyrolysis.28 All the volatiles were quickly purged from the vicinity of the foil to minimize the extent of the secondary pyrolysis in the gas phase. Gaseous (28) Hayashi, J.-i.; Norinaga, K.; Yamashita, T.; Chiba, T. Energy Fuels 1999, 13, 611.

Figure 1. Schematic diagram of apparatus for drop-tube pyrolysis. products were introduced directly into a TCD-GC (Shimadzu, model GC-8A) that was equipped with a column of Gasukuropak 54 (GL Science inc.), and the yields of inorganic gases (H2, H2O, CO, and CO2) and gaseous hydrocarbons (CH4, C2H4, C2H6, C3H6, C3H8, C4H8, C4H10, C5H10, and C5H12; referred to as GHC) were determined. A major portion of the liquid (defined as volatiles heavier than C7 hydrocarbons) condensed in a bed of quartz wool packed just downstream from the ferromagnetic foil, while lighter liquids were introduced into the gas-separation column. No structural analysis was made for the liquid products due to their small amounts. The pyrolysis was repeated 5-6 times under the same conditions in order to ensure the reliability of the results. The reproducibility of the total volatile yield was obtained with the average deviation smaller than 2% on the basis of the initial sample mass. Rapid Pyrolysis in a Drop-Tube Reactor (DTR). The coal particles were continuously fed into a vertically placed drop-tube reactor (DTR) that was made of a SUS 304 stainless steel tube with i.d. of 16 mm, and pyrolyzed at temperatures ranging from 631 to 923 K. The apparatus is schematically shown in Figure 1. The lengths of heated and isothermal zones of the reactor were 1200 and 800 mm, respectively. Particles were fed together with atmospheric nitrogen gas flowing at a rate of 500 mL-STP min-1 from a vibration feeder into DTR through a nozzle of 6.5 mm-i.d. and 8.0 mm-o.d.. Atmospheric nitrogen gas was also continuously fed from a thin side tube at a rate of 500 mL-STP min-1. The feed rate of the particles was in a range of 0.05-0.1 g min-1. Char particles were trapped in a collector at the reactor bottom or a cyclone downstream. Heavy tar was trapped with a glass fiber filter thimble at 393 K while light tar and water with a fixed bed of glass beads in a liquid nitrogen/methanol bath of 193 K. Water

Reactions in Brown Coal Pyrolysis and Tar Yield collected in the trap was dissolved in methanol together with the light tar, and its amount was measured by means of CarlFischer titrimetry using a titrator (Kyoto Electric, model MKC210). The light tar was analyzed with an FID-GC (HewlettPackard, model 6890) and was found to consist mainly of monoaromatics; benzene, toluene, styrene, xylenes, and phenol. The total yield of these mono-aromatic compounds was 0.5% at most on the carbon basis at pyrolysis temperatures lower than 923 K while as large as 2% above 1073 K. The solution of methanol containing the light tar was merged into the heavy tar solution of tetrahydrofuran (THF). The solvents of the mixed solutions were evaporated, and the residue (hereafter referred to as tar) was recovered and weighed. The evaporation caused the loss of the above-described mono-aromatic compounds. GHC, carbon oxides (CO and CO2), and hydrogen passing through the cold trap were collected in an aluminumcoated impermeable gas bag and analyzed using an FID-GC (GL Science, model GC-380) equipped with a reactor to convert carbon oxides into CH4 between the separation column and detector. Slow Pyrolysis in a Fixed-Bed Reactor (FBR). About 0.5 g of YL was placed in a vertically placed quartz glass tube (i.d. of 8.5 mm) and pyrolyzed in atmospheric nitrogen gas flowing at a rate of 340 mL-STP min-1. The sample was heated at a rate of 10 K min-1 up to a temperature ranging from 573 to 823 K, and then cooled at 200 K min-1 to ambient temperature without holding the temperature. The tar and gaseous products were collected and analyzed in the same manner as were those from DTR pyrolysis. Analysis of Tar and Char. The tar samples obtained by the pyrolysis in DTR and FBR were analyzed by 13C NMR. Each sample was dissolved in CDCl3 (100% atom-D) containing Cr(III)(acac)3 as the relaxation promoter and subjected to the analysis by a gated decoupling method on a spectrometer (JEOL, model JNM-a500). The fractions of aromatic and aliphatic carbons were determined from integrated relative intensity of signals appearing at chemical shifts from tetramethylsilane in the ranges 90-200 ppm and 0-90 ppm, respectively. The char samples from DTR and FBR experiments were also analyzed CP/MAS 13C NMR according to the method of Yoshida et al.29,30 All the CP/MAS spectra were obtained on a spectrometer (JEOL, model GX-270) with a 13C frequency of 67.80 MHz, employing a pulse width of 4.0 µs, a pulse repetition time of 7 s, and a contact time of 2 ms. Chemical shifts were first calibrated against external adamantane (a low-field peak at 38.3 ppm) and then recalculated to give a shift from tetramethylsilane. The rotor was spun at 10 kHz, a rate high enough to sweep spinning sidebands away from the range of -20 to 280 ppm. In each analysis, pulses were accumulated 5000 times. The fractions of aromatic carbon, including carbonylic and carboxylic carbons, and aliphatic carbon were determined from integrated relative intensity of signals appearing at chemical shifts from tetramethylsilane in the ranges 90-200 ppm and 0-90 ppm, respectively.


Figure 2. Char yields from YL for pyrolysis in DTR and CPR as a function of temperature.

volatiles in the gas phase occur concurrently with the primary reactions in the coal matrix that form the volatiles, while in CPR32,33 the secondary reactions of volatiles are sufficiently suppressed. As shown in the figure, the yields can be expressed as nearly the identical functions of temperature. Thus, the primary reactions that determine the coal conversion into volatiles complete in the both reactors at the individual temperatures, and the difference in the yields of volatiles between DTR and CPR can therefore be attributed to the secondary reactions of volatiles that proceed exclusively in DTR. Figure 3 plots the observed yields of CO2, H2O, and H2 against the temperature ranging from 631 to 943 K. In the figure Yx/CP and Yx/DT denote the yields of product x for the pyrolysis in CPR and DTR, respectively. The yields of the individual gases for CPR and DTR coincide well with each other, and hence these gases are formed by the secondary reactions to negligible extents. Figure 4 shows the observed yields of CO and GHC. YCO/DT and YGHC/DT are greater than the corresponding yields for the pyrolysis in CPR at temperatures above 723 and 823 K, respectively. This result indicates the decomposition of phenolic OH and alkyl chains of tar in the gas phase.34,35 Here, the differences in the yields of CO or GHC at a temperature between DTR and CPR pyrolyses are, respectively, defined as ∆YCO, or ∆YGHC on a carbon basis as

∆YCO ) YCO/DT - YCO/CP or ∆YGHC ) YGHC/DT YGHC/CP (1)

Results and Discussion Effect of Heating Rate on Ceiling Yield of Tar from YL. Figure 2 compares the char yields from YL for the pyrolysis in CPR and DTR on a coal mass basis. In the both reactors, the heating rates of coal particles31 were in the same orders of magnitude, (2-3) × 103 K s-1. In DTR, the secondary (extra-particle) reactions of

Since the pyrolysis temperatures in CPR did not coincide with those in DTR, the yield at a desired temperature in CPR was interpolated describing the observed yield by a polynomial function of temperature. Considering that GHC and CO are the gaseous products of the secondary reactions of tar at temperatures lower than 943 K, the yield of the initial tar that

(29) Yoshida, T.; Nakata, Y.; Yoshida, R.; Ueda, S.; Kanda, N.; Maekawa, Y. Fuel 1982, 61, 824. (30) Hayashi, J.-i.; Aizawa, S.; Kumagai, H.; Yoshida, T.; Chiba, T. Energy Fuels 1999, 13, 69. (31) Hayashi, J.-i.; Takahashi, H.; Iwatsuki, M.; Essaki, K.; Kuramoto, K.; Tsutsumi, A.; Chiba, T. Fuel, in press.

(32) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 627. (33) Hayashi, J.-i.; Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1995, 9, 290. (34) Hayashi, J.-i.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S.; Yumura, M. Energy Fuels 1993, 7, 63. (35) Cypres, R.; Furfai, S. Fuel 1985, 64, 33.

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Figure 4. Yields of CO2, H2O, and H2 from YL as a function of pyrolysis temperature.

Figure 3. Yields of CO and GHC from YL as a function of pyrolysis temperature. Yx/DT, Yx/CP and ∆Yx are the yield of product x for pyrolysis in DTR and CPR, and their difference between, respectively.

may be formed in DTR (referred to as Ytar/DT,i) can be estimated by

Ytar/DT,i ) Ytar/DT + ∆YCO + ∆YGHC

(2)

The yield of initial tar can also be determined only from the product yields in the pyrolysis in CPR as Ytar/CP,d.

Ytar/CP,d ) 100 - (Ychar/CP + YCO/CP + YCO2/CP + YGHC/CP) (3) Figure 5 shows Ytar/CP,d and Ytar/DT,i as a function of temperature. The yields are in good agreement with each other, and this demonstrates the reasonableness

Figure 5. Observed and calculated yields of initial tar from YL for slow and rapid pyrolyses.

of eqs 1-3 and the procedure for the product recovery. The initial tar yields level off around 26 mol-C per 100 mol-C in YL at 923 K, where the tar evolution is judged to be complete. The figure also shows the tar yields in the fixed-bed pyrolysis. Ytar/FB is the observed tar yield while Ytar/FB,d is given by difference calculated in the same manner as Ytar/CP,d.

Ytar/FB,d ) 100 - (Ychar/FB + YCO/FB + YCO2/FB + YGHC/FB) where Yx/FB is the yield of the product x. Ytar/FB and Ytar/FB,d agree well with one another, and reach the ceiling values of about 15 mol-C at 723 K, which is appreciably lower than that in the case of the rapid pyrolysis, i.e., 923 K. It should also be noted that the



Figure 6. Yields of volatile products from YL, (a) tar, (b) CO, (c) CO2, and (d) GHC as a function of total carbon conversion into volatiles.

ceiling yield of tar in the fixed-bed pyrolysis is fairly lower than that in the rapid pyrolysis. Thus, the present experimental conditions give rise to the effect of the heating rate on the ceiling yield. Selectivity to Tar and Gaseous Products on Carbon Basis. Figure 6 shows the yields of the products as a function of the total carbon conversion into volatiles, referred to as Xc, to examine the heating rate effects on the selectivity to the individual volatile products. As shown in Figure 6a, Ytar/DT,i and Ytar/FB can be represented by the same function of Xc at temperatures lower than those where the tar yield levels offs 923 K for the rapid pyrolysis and 723 K for the slow pyrolysis. Hence, the heating rate is not a factor affecting the selectivity to tar when its evolution continues, although the rate influences the ceiling yield. Similar tendencies are noticed for the yields of the gaseous products, CO, CO2, and GHC, as illustrated in Figure 6b-d, respectively. Much different from the carbon-containing gaseous products, the yield of H2O as a function of Xc depended on the heating rate. Figure 7a makes it evident that YH2O/DT as a function of Xc is smaller than YH2O/FB. It can be said from Figure 6 that the rapid and slow pyrolyses give almost the same selectivities to the individual carbon-containing volatiles unless the tar evolution is completed. No significant variation of the selectivity with the heating rate suggests that the heating rate little influences the rate of tar evolution relative to those of CO, CO2, and GHC formation, even if these gases are formed by the crosslinking. On the other hand, as shown in Figure 7b, the higher heating rate results in the lower yield of H2O relative to tar yield. The heating rate effect on the chemical composition of tar was also examined on the basis of its carbon aromaticity and aliphaticity. As described in the Experimental Section, the tar samples were analyzed by 13C NMR, and their carbon aromaticity (f ar-tar) and aliphaticity (fal-tar) were determined. The amounts of

Figure 7. Yield of H2O from YL as a function of (a) total carbon conversion into volatiles and (b) tar yield.

these different types of carbon are given as follows in the unit of mol-C/100 mol-C in YL.

Car-tar/DT ) Ytar/DT ‚ far-tar/DT or Car-tar/FB ) Ytar/FB ‚ far-tar/FB Cal-tar/DT ) Ytar/DT ‚ fal-tar/DT or Cal-tar/FB ) Ytar/FB ‚ fal-tar/FB where Car-tar and Cal-tar are the amounts of aromatic and aliphatic carbons contained in the tar, and their sum equals to Ytar/DT or Ytar/FB since far + fal ) 1. For the tar obtained in the DTR pyrolysis, Car-tar/DT and Cal-tar/DT are smaller than the amounts of aromatic and aliphatic carbons that may be contained in the initial tar, respectively, due to the decomposition of phenolic groups and alkyl chains forming CO and GHC. The loss of aromatic carbon from the initial tar can be estimated as ∆YCO while that of aliphatic carbon as ∆YGHC ‚

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Ychar/DT and Ychar/FB were determined from their respective mass yields and carbon contents. The total amounts of aliphatic carbon involved in the products, Cal/DT and Cal/FB, are then calculated as follows:

Cal/DT ) Cal-char/DT + Cal-tar/DT,i + YGHC/CP Cal/FB ) Cal-char/FB + Cal-tar/FB + YGHC/FB The loss of aliphatic carbon is here defined as ∆Cal and is calculated from the difference in its amount between the initial coal and product.

∆Cal ) (100‚fal,coal - Cal/DT) or (100‚fal,coal - Cal/FB)

Figure 8. Relationship between yield of tar and amount of aromatic carbon in tar. Table 2. Carbon Aliphaticity of Chars DTR pyrolysis

FBR pyrolysis

temperature [K]

fal,char

temperature [K]

fal,char

630 723 773 823 873 923

0.388 0.273 0.229 0.211 0.166 0.110

573 623 673 723 773

0.378 0.379 0.299 0.213 0.115

Car-tar/DT and Cal-tar/DT are then corrected respectively to Car-tar/DT,i and Cal-tar/DT,i as

Car-tar/DT,i ) Car-tar/DT + ∆YCO Cal-tar/DT,i ) Cal-tar/DT + ∆YGHC and accordingly,

Car-tar/DT,i + Cal-tar/DT,i ) Ytar/DT,i Figure 8 shows the relationships of Car-tar/DT,i and Car-tar/FB with the corresponding tar yields, Ytar/DT,i and Ytar/FB, respectively. The close agreement between Car-tar/DT,i and Car-tar/FB suggests no significant effect of the heating rate on the chemical composition of tar, although no other chemical properties of tars such as molecular mass distribution and oxygen functionality were investigated. Relationship between Loss of Aliphatic Carbon and Yield of Gaseous Product. In this section, the loss of aliphatic carbon by the pyrolysis is determined and used as a measure for the loss of bridges. The aliphaticity of the char samples from the pyrolyses in DTR and FBR was determined by NMR and are referred to as fal-char/DT or fal-char/FB, respectively. Table 2 lists the aliphaticity of the char samples. The amounts of aliphatic carbon contained in the chars from DTR and FBR pyrolyses are given in the unit of mol-C per 100 mol-C in YL by

Cal-char/DT ) Ychar/DT ‚ fal-char Cal-char/FB ) Ychar/FB ‚ fal-char

where fal,coal is the aliphaticity of the initial coal, shown in Table 1. ∆Cal was found to increase with an increase in the temperature up to 21 mol-C at 773 K in FBR pyrolysis and up to 18 mol-C at 923 K in DTR pyrolysis. Figure 9 shows the yields of the gaseous products as a function of ∆Cal. In Figure 9a the relationship between the yield of H2O and ∆Cal is seen to depend on the heating rate. The FBR pyrolysis with the lower heating rate gives a yield of H2O greater than that in DTR pyrolysis with the higher rate. This result clearly demonstrates that the heating rate influences the relative rates of the key reactions, namely H2O formation and aliphatic-to-aromatic carbon conversion, and as a result affects the relationship between the extents of these reactions. The activation energy for the aromatization larger than that for the H2O formation is a most authentic explanation for the larger relative rate and extent of the former reaction with the higher heating rate. Unlike the yield of H2O, those of CO, CO2, and GHC depend little on the heating rate as shown in Figures 9b-d. Hence it is implausible that the formation of CO2, CO, and GHC are the key reactions to explain the heating rate dependence of the yield of tar although their formation might accompany cross-linking. Regarding the formation of H2O, further consideration may be needed. As mentioned above, the major reaction to form H2O is the condensation between -OH groups that is expected to produce cross-links such as biphenyl or biaryl ethers. When the increase in the heating rate results in enhanced tar evolution due to a larger rate of bridge breaking relative to that of cross-linking, more hydroxylic groups could be released from the pyrolyzing coal matrix without being decomposed by the condensation, and then survive as hydroxyls of tar. The release of hydroxylic groups would reduce their concentration in the coal matrix and contribute to the reduction of the rate of condensation. Taking the above into account, the heating rate effect on the H2O yield shown in Figure 9a may be due not only to its activation energy lower than that for bridge breaking but also enhanced evolution of tar that carries hydroxylic groups. Effect of Removal of Metal Cations on Product Yields and Extent of Aromatization. Figure 10 compares the yields of tar and gaseous products and ∆Cal for the pyrolysis of YL and YLA in DTR. The yield of product x from YLA is suffixed by x - A. As seen in Figure 10a, Ytar-A/DT,i is larger by about 5 mol-C than Ytar/DT,i at all temperatures in the range from 723 to 923 K. Figure 10b-f show no significant difference in ∆Cal and the yields of the gaseous products between YL and



Figure 9. Yields of H2O, CO, CO2, and GHC as a function of ∆Cal.

by the metal cation removal is a major factor of the enhanced tar evolution under the present rapid pyrolysis conditions. However, the effects of the metal cation removal cannot readily be generalized for other pyrolysis conditions. Serio et al.36 found that a dimineralization of causes a great decrease in the yields of inorganic gases and an increase in that of tar in pyrolysis with a slow heating rate of 0.5 K s-1. Roles of carboxylates in the cross-linking reactions may vary with the heating rate. Conclusions

Figure 10. Product yields and ∆Cal for YL and YLA as a function of temperature. b, YL; O, YLA. Table 3. Yields of Tar and Inorganic Gases from BZ and BZA sample

Ytar/CP,d

YH2O/CP

YCO/CP

YCO2/CP

BZ BZA

20 29

11.0 11.3

2.6 2.9

3.5 2.7

YLA. This result suggests that the removal of metal cations enhances the tar evolution influencing neither the rates of bridge breaking nor cross-linking under the present experimental conditions. The effect of the metal cation removal was also examined for another coal, the APCS Beulah Zap lignite (BZ). The conditions of the HCl treatment and drying are described elsewhere.28 Table 3 shows the yields of inorganic gases and tar from dried Beulah Zap (BZ) and HCl-treated/dried BZ (BZA) for the pyrolysis in CPP at 1037 K. The metal cation removal results in a considerable increase in the tar yield while only slight changes in the yields of CO, CO2, and H2O. Thus, the decrease in the cross-link density

Yallourn brown coal was pyrolyzed at a slow heating rate of 0.167 K s-1 and rapid heating rates of (2-3) × 103 K s-1. The rapid pyrolysis gave a ceiling yield of tar at 26 mol-C per 100 mol-C of the coal, much greater than the slow pyrolysis, 15 mol-C. Such a heating rate effect on the tar yield is supposed to be induced by the bridge breaking and cross-linking reaction with different activation energies, and hence variations with the heating rate of the relationship between the gaseous product yields and loss of aliphatic carbon were examined, expecting their correspondence to the extents of bridge breaking and cross-linking, respectively. Within the experimental conditions, the following conclusions can be drawn. (1) The heating rate little affects the selectivity to tar, CO, CO2, and gaseous hydrocarbons (GHC) on the carbon basis. (2) The relationship between the loss of aliphatic carbon (∆Cal) and the yield of H2O is influenced by the heating rate. The rapid heating results in smaller yield of H2O as a function of ∆Cal than the slow heating. This is explained by the reaction to form H2O with activation energy smaller than that for aliphatic-to-aromatic carbon conversion. Considering the correspondence of the aliphatic carbon loss and H2O formation to the extents of bridge breaking and cross-linking, respectively, the relatively enhanced loss of aliphatic carbon and suppressed H2O formation by rapid heating can be a chemical explanation of the heating-rate effect on the ceiling yield of tar. (36) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1993, 38, 577.

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(3) The relationships of ∆Cal with the yields of CO2, CO, and GHC hardly depend on the heating rate, and therefore none of the reactions forming these gases are key reactions to explain the heating rate effect on the ceiling yield of tar. (4) The removal of metal cations by the acid treatment increases the tar yield in DTR pyrolysis by about 5 molC/100 mol-C of initial coal, while leaving the aliphatic-

Hayashi et al.

to-aromatic carbon conversion and the rates of H2O, CO, CO2, and GHC formation nearly unchanged. Acknowledgment. This work was supported by a “Research for the Future Project” grant from The Japan Society for the Promotion of Science (JSPS). EF9901490

Reactions in Brown Coal Pyrolysis Responsible for Heating Rate

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