Energy 8z Fuels 1993, 7, 57-66
57
Control of Molecular Composition of Tar by Secondary Reaction in Fluidized-Bed Pyrolysis of a Subbituminous Coal Jun-ichiro Hayashi,* Tsutomu Kawakami, Tomohiro Taniguchi, Kabuki Kusakabe, and Shigeharu Morooka Department of Chemical Science and Technology, Kyushu University, Fukuoka 812, Japan
Morio Yumura National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan Received June 4, 1992. Revised Manuscript Received September 22, 1992
Changes in the molecular structure of tar are investigated in a fluidized-bed reactor divided into two regions: a dense bed for the primary reaction and a freeboard for the secondary reaction in the gas phase. The temperature in the dense bed is kept at 600 "C and that in the freeboard is varied in the range of 600-900 "C. A pulverized subbituminous coal (C = 76 wt % 1 is continuously fed into the dense bed, and tar is recovered at the reactor outlet. The tar is separated by solvent extraction into preasphaltene and asphaltene. The asphaltene is further classified by column chromatography into saturates, aromatics, phenolicethers, nitrogen-containingcompounds, and hydroxyliccompounds. The aromatics are fractionated by HPLC on the basis of the number of double bonds per molecule (db)intodicyclic (5db), dicyclic(6db), tricyclic (7db),tetracyclic (8db),tetracyclic (9db), pentacyclic (10 db), and pentacyclic and greater (+11 db) PAHs (polycyclic aromatic hydrocarbons). Each homologue is characterized by FIMS and 'H NMR to determine molar yield,number-based distribution j3-,and y-carbons per molecule. of aliphatic carbonsper molecule,and number-baseddistribution of a-, At freeboard temperatures of 600-700 "C, detachment of aliphatic substituents proceeds with bond cleavagea t aryl-a positions rather than a-j3 and remote positions. Decomposition of a-methyl groups is not observed below 700 "C. At a freeboard temperature of 600 "C, PAHs with three to four aliphatic carbons per molecule are most abundant, and the mole fraction of unsubstituted PAHs is only ca. 5% for each homologue. At 900 "C, however, the fraction of unsubstituted PAHs is more than 50%. The structural distribution in pyrolysis products is thus controlled by changing the freeboard temperature.
Introduction
Pyrolyzers. Polycyclic aromatichydrocarbons (PAHs) derived from coal tar and pitch are very useful as starting materials for various specialty chemicals. To produce PAHs, flash pyrolysis of coal is attractive, because its tar yield is much higher than that of other carbonization processes under slow heating.' As Khan2noted, however, the quality (e.g., H/C ratio) of flash pyrolysis tar is inferior to that of slow pyrolysis tar. Coal pyrolysis consists of two reactions in series: the primary reaction of the macromolecular network of coal to generate tar,and the secondary reaction of tar in the gas phasea3Solomon et alq4 studied the pyrolysis of a lowrank coal by minimizing the secondary reaction. The yield of tar increased with increasing heating rate, but its molecular weight became higher. It was difficult to obtain light products in high yield by controlling the primary reaction alone. Thus the secondary reaction should be controlled as well. Many researchers have investigated the effects of the * Address all correspondence to this author.
F A X : 81-92-651-5606. (1) Peters, W.; Bertling, H. Fuel 1965, 44, 317. (2) Khan, M.R. Fuel 1989,68, 1522. (3) Serio, M.A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, I , 138. (4) Solomon, P. R.;Serio, M. A.; Despande, G.V.; Kroo, E. Energy Fuels 1990, 4 , 42.
0887-0624/93/2507-0057$04.00/0
reaction time and temperature of flash pyrolysis on the yield and molecular composition of tar in practical reactors such as entrained-flowreactors,516fluidizedbed reactom,7-"J and a spouted bed reactorall However, the behavior of flash pyrolysis tar cannot be analyzed in these pyrolyzers, in which the primary and secondary reactions proceed simultaneously. Division of reaction zones is indispensable for quantitative examination of time- and temperaturedependent changes in molecular composition of the tar. Xu et al.12 and Serio et al.13 separated these reaction zones by using two-stage reactors comprising a fixed bed of coal and a tubular reactor connected downstream. The primary reaction zone was heated at a rate of 60 "C/min12 and 5 "C/min.13 They investigated the yield and carbon aromaticity of the tar evolved from bituminous coals over a wide range of temperatures and residence times1* and (5) Bissett, L. A. Energy Fuels 1988,2, 827. (6) Wornat, M.J.; Sarofim, A. F.; Longwell, J. P. Energy -. Fuels 1987, 1, 431. (7) Gonenc, 2.S.;Gibbins, J. R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69,383. (8)Tyler, R. J. Fuel 1980, 59,218. (9) Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980, 59,479. (10) Katheklakis, I. E.:Shi-Lin. L.: Bartle, K. D.: Kandivoti. R. Fuel 1990, 69,172. (11) Teo, K. C.; Watkinson, A. P. Fuel 1986, 65, 949. (12) Xu, W.-C.; Tomita, A. Fuel Process. Technol. 1989, 21, 25. (13) Serio, M. A.; Peters, W. A,; Howard, J. B. Ind. Eng. Chem. Res. 1987,26, 1831.
0 1993 American Chemical Society
Hayashi et al.
58 Energy & Fuels, Vol. 7, No. 1, 1993
-r
& T ,ar;
gas and char
Secondary reaction
Freeboard
1
Residence time 4-59 Temperature 600 - WOmC
Fluidized bed (Silica sand) $ Temperature 600°C
Primary reaction
Y Coal particles
__I
Figure 1. Separation of primary and secondary reactions in fluidized-bed pyrolyzer. 1,Fluidized-bed coal feeder; 2, pyrolysis reactor; 3 and 4, electric heaters for freeboard and dense bed, respectively; 5, thermocouples; 6, tar trap; 7, cold trap at -40 O C to collect light hydrocarbons and water; 8, needle valve; 9, rotary pump; 10, gas bags.
reported that the yield of specificcompounds such as BTX and hydrocarbon gases were influenced by the secondary rea~ti0n.l~ The yield of benzene, for instance, increased with the residence time of tar vapor, reaching a constant value characteristic of temperatures between 500 and 900 "C when the residence time was longer than several seconds. The effect of temperature on molecular structure of the tar was more important than that of residence time because there were variousprecursors having a wide variety of activation energies. We can apply the two-stage concept of Xu et al.12and Serio et al.13 to the flash pyrolysis using wire-mesh14and Curie-point pyrolyzers.ls When a certain amount of tar is necessary for detailed characterization, however, these reactors are not adequate. Hayashi et al.16 therefore developed a fluidized-bed reactor divided into two zones: a dense bed for the primary reaction and a freeboard above the dense bed for the secondary reaction. Figure 1 is a schematic illustration of the reactor. A pulverized coal is continuously fed into the dense bed and is pyrolyzed. The residence time of coal particles in the dense bed should be longer than that required for tar evolution. When coal particles are heated at a rate of around 1000 "Us, tar evolution period is ca. 0.2 sal7Fletcher et a1.18investigated the effects of final temperature and hold time on the tar yield from Pittsburgh No. 8 coal at the heating rate of 3000 "C/S and reported that the tar yield reached an ultimate value irrespective of the holding time when the final temperature exceeded 600 "C. The tar transferred from the coal particles is introduced into the freeboard and is processed there for several seconds. In fluidized-bed pyrolysis, secondary reactions are known to occur in the dense bed to some extent.lg However, Serio et al.13 reported that no change in average molecular structure of tar was observed within a residence time of 1.1s at a temperature below 600 "C. To minimize the extent of the secondary reaction in the dense bed, the dense-bed temperature should be at 600 "C for the primary (14) Howard, J. B. In Chemistry of Coal Utilization,2nd. Suppl. Vol.; Elliott, M. A., Ed.; Wiley: New York, 1981; p 665. (15) McClennen, W. H.; Meuzelaar, H. L. C.; Metcalf, G. S.; Hill, G. R. Fuel 1983. 62. - * 1422. (16) Hayashi, J.-i.; Nakagawa,K.; Kusakabe, K.; Morooka, S.; Yumura, M. Fuel Process. Technol., in press. (17) Freihaut, J. D.; Proscia, W. M. Energy Fuels 1989, 3, 625. (18) Yeboah, Y. D.; Longwell, J. P.; Howard, J. B.; Peters, W. A. Ind. Eng. Chem. Process Des. Diu. 1980,19, 646. (19) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54. ~~
~
~
-
-
reaction. The yield of tar is generally maximized at a bed temperature of ca. 600 OC for fluidized-bedpyrolysis.8Jo*2021 Structural Analysis of Tar. Tar is a complicated mixture of poly- or monocyclic aromatic ring compounds that carry functional groups such as alkyl chains and hydroxylic groups as substituents for ring hydrogen. The behavior of tar molecules under pyrolytic condition depends on the ring system and its substituents. Their distributions are determined by fractionating the tar by column chromatography that can distinguish the polarity of molecules. Schiller et al.,22using a neutral alumina column, separated a benzene-soluble portion of coalderived liquids into six categories: saturates, aromatic hydrocarbons, etheric compounds, nitrogen compounds, hydroxylic compounds, and compounds with higher molecular weight. Major structural elements commonlyseen in the above categories other than saturates are aromatic rings and substituted alkyl chains. Teo et aL23 characterized the flash pyrolysis tar of Canadiancoalsusing the GC-MStechnique combined with solvent extraction and high-performance liquid chromatography (HPLC). They determined yields of aromatic homologueshaving not more than three aliphatic carbons. To determinethe distribution of aromaticring structure, aromatic hydrocarbons should be separated on the basis of ring structure prior to mass spectrometry. Boduszynski et al.24325 classified nonpolar compounds in a cola liquefaction oil by HPLC with a Micro-Bondpack NH2 column and the mobile phase of n-heptane. The attractive point of this HPLC technique is that the elution volume of aromatic compounds is determined by the number of double bonds per molecule (db)alone and is hardly affected by alkyl substituents.26 After aromatic hydrocarbons are fractionated by ring structure, the aliphatic distribution in each homologue having the same ring structure is analyzed by field-ionization mass spectrometry (FIMS), which forms molecular ions and not fragmentsSz7By this procedure, the yield of a group of molecules having a specificring structure with specifiedaliphatic substituents is found. In the present study, flash pyrolysis of a subbituminous coal is conducted in a fluidized-bed reactor that is divided into two regions. Tar is processed in the freeboard at a temperature different from that in the dense bed, and is characterized by FIMS and lH NMR after separation based on polarity and ring structure of the molecule. The features determined are contribution of etheric compounds, hydroxylic compounds,and preasphaltene as the source of PAHs; molar yield of homologues;number based distribution of aliphatic carbonsper molecule;and number based distribution of cy,a-, cy-, @-,and y-carbons per molecule. Experimental Section Flash Pyrolysis. Wandoan subbituminuous coal (C 76.2, H 5.77, N 1.03 in w t %, daf) was employed for the experiment. The (20) Tyler, R. J. Fuel 1979, 58, 680. (21) Takeuchi, M.; Berkowitz, N. Fuel 1989,68, 1311. (22) Schiller, J. E.; Mathiason, D. R. Anal. Chem. 1977,49, 1225. (23) Teo, K. C.; Watkinson, A. P. Fuel 1987,66,1123. (24) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983,55,225. (25) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Fuel 1985, 64, 242. (26) Wise, S. A.; Cheder, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E.Anal. Chem. 1977,49, 2306. (27) Scheppele, S. E.; Grizzle,P. L.;Greenwood, G. J.;Marriott,T. D.; Perreira, N. B. Anal. Chem. 1976,48, 2105.
Control of Molecular Composition of Tar
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Energy & Fuels, Vol. 7, No. 1,1993 69
Flash-pyrolysis Tar I
I
Extraction with benzene
Inwluble component
Soluble comportmi
Column c h r o m o l ~ r a p h with y neutral alumina
Benzene
I
Chloroform
I
&
t
I
80% THF
20910 methanol
I
++
Preasphaltenel
Figure 3. Separation scheme for tar. Figure 2. Schematic diagram of experimental apparatus. coal sample was pulverized, sized to 0.037-0.074 mm, and dried in vacuo at 100 OC. A schematic diagram of the pyrolyzer is shown in Figure 2. The fluidized-bed reactor was made of a SUS 304 stainless steel tube of 37 mm inner diameter. Silica of size 0.0744.125 mm was fluidized by nitrogen, the settled-bed height being 150 mm. The dense and freeboard zones were heated separately with external furnaces whose temperatures were independently controlled. The temperature was set in the range of 600-900 OC for the freeboard and fixed at 600 OC for the dense bed. Coal particles in a gas-tight hopper were entrained by nitrogen gas flowing at 25 mL/s and were introduced at a constant rate of 4.2 mg/s into the dense bed through a stainless steel tube of 2 mm i.d. and fixed at the bottom. The total gas flow rate was 33.3 mL/s at 20 OC. The total residence time of tar vapor was 4.0-4.5 8. The tar trap was fitted with a glass-fiber thimble that acted as a filter. Most of the tar was collected in the tar trap, while that deposited on the walls of the reactor head and the tubings was washed out with tetrahydrofuran and collectedby evaporating the solvent. Water and light hydrocarbons were collected in a cold trap kept at -40 OC. The outlet gas passing through the tar trap (CI-Ca hydrocarbons, CO and COZ)was collected in sampling bags and analyzed with a gas chromatograph equipped with a catalyst column for methanation of CO and COz and a flameionization detector. The mass balance was attained within 9699% in the present experiments. Separation and Characterization of Tar Samples. The tarwas separated into benzene-soluble(asphaltene)and -insoluble (preasphaltene) portions. The asphaltene was further fractionated by the following method of Schiller and Mathiason:22A 0.4-0.5-g aliquot of the asphaltene was dissolved in 5 mL of tetrahydrofuran and mixed with pre-heat-treated neutral alumina (activity I grade, ICN Biomedicals). The alumina particles adsorbingthe asphaltene were dried and packed in a glasscolumn. Then the asphaltene was eluted successively with benzene, chloroform,and a 4 1 mixture of tetrahydrofuran-methanol. The component not eluted by the mixed solvent was considered as preasphaltene, and its yield was added to that of the benzeneinsolubleportion. The scheme of the separation and the notation of samples are shown in Figure 3. The fractions other than F1 were subjected to FIMS analysis with a mass spectrometer (JEOL MS-FDOS). Compounds were identified from molecular ions selectively formed by field ionization, and the relative abundance was calculated from the ion intensity spectra. The F1 fraction, containing no heteroatoms, was further separated into saturates and aromatics at 20 "C with a semipreparative NH2 column (Hiber Fertigsaule RT Pre-Packed Column RT 250-10, Merck) with n-heptane as the mobile phase at a flow rate 2.0-3.0 mL/min. A differential refractometer (TOSO, HLC-803A) and a UV spectrometer (JASCO, UVDEC 100-IV)connected in series were used as detectors. About 15mg of the F1 component dissolved in 1.0 mL of n-heptane was repeatedly injected, and ca. 100 mg of organics was obtained for each sample. Unsubstituted PAHs, naphthalene (5db), biphenyl (6 db), fluorene (6 db), phenanthrene (7 db), pyrene (8 db),
20
I
c a
z c temperature [ "C ]
Figure 4. Effect of freeboard temperature on tar yield. fluoranthene (8 db), 1.2-benzofluorene (8 db), chrysene (9 db), benzo[a]pyrene (10 db), perylene (10 db), and 1,2:5,6-dibenzanthracene (11db), were used as standard compounds. By this method, aromatics were classified by number of double bonds per molecule (db) into 3 (monocyclic),5 (dicyclic), 6 (dicyclic), 7 (tricyclic), 8 (tetracyclic), 9 tetracyclic), 10 (pentacyclic), and 11 and 12 (pentacyclic and greater PAHs). The double-bond fractions obtained from the HPLC were analyzed by lH NMR (GSX400 MHz, JEOL). The spectrum was divided into four regions based on type of proton: aromatic (H-, 6.0-9.5 ppm); ring-joining methylene (H,,, 3.2-4.5 ppm); a position to aromatic ring (H,, 2.0-3.2 ppm); fi position to aromatic ring (Ho, 1.8-1.0 ppm); and y position and remote to aromaticring (H7,0.5-1.0 ppm). Signalsattributedtofi-hydrogen of hydroaromatic structure were hardly observed. That structure was therefore neglected in the present data analysis.
Results and Discussion Temperature Dependence of Yield and Chemical Composition of Tar. Figure 4 shows the effect of freeboard temperature on the tar yield. When fine coal particles were fed into the dense bed, most of particles were entrained by the gas flow. When coal particles were in the size range of 0.21-0.42 mm; however, entrainment hardly occurred. Tar yield by the pyrolysis of 0.21-0.42mm particles was virtually identical to that shown in Figure 4 for 0.037-0.074-mm particles. The agreement was also obtained when only the dense bed was heated. These results indicate that the tar evolution was completed in the dense bed even in the case that the entrainment occurred, because the residence time of particles was much longer than that of gas. The total volatile yield is determined by the primary reaction. Therefore, when all particles resided in the dense bed with no entrainment, the total volatile yield was 42 wt %, daf and was independent of the freeboard temperature. Figure 5 shows the FIMS spectra of the F2 component obtained at various freeboard temperatures. The lines appearing at intervals of 14mass units (-CHr) correspond
Energy & Fuels, Vol. 7, No.1, 1993 600°C
o.20r n
Hayashi et al. 900°C
1 1 9 155 167 1 7 9 703 105 2 2 9 2 4 1 753 1 7 7 2 7 9
0.20
800°C
-
0.10
n
1
0.00 1 2 9 1 5 5 167 179 203 705 2 7 9 2 C 1
253 2 7 7 7 7 9
Molecular weight
Figure 6. Relative yield of homologues of nitrogen-containing compounds. Molecularweightsare of unsubstitutedcompounds in each homologue: 129, quinoline; 155, phenylpyridine; 167, carbazole; 179, benzoquinoline;203, azapyrene (including benzocarbazole (217) homologue); 205, phenylquinoline or naphthylpyridine; 229, azachrysene. 70
60 50
40
900°C
30 20 10
1
0 600
Molecular mass (m/z)
700
aoo
900
Temperature [ "C ]
Figure 5. FIMS spectra of F2 component.
Figure 7. Temperature dependence of tar composition.
to each homologue having the same aromatic ring and 2 number. The spectra are composed of signals of odd and even mass numbers. The odd mass number is attributed to nitrogen-containing compounds (N compounds) and the even number to aromatic ethers. Schiller and Mathiason= found the same tendency in a chloroform elute of tars, As indicated in Figure 5, the molecularmass distribution of the F2 component shifts in the direction of smaller m/z number with increasingfreeboard temperature, while the number of signals,i.e., the number of compounds, becomes fewer. At a freeboard temperature of 600 "C,the signal intensitiesof aromaticethers are almostequivalentto those of N compounds. A t higher temperatures, however, the intensity of aromatic ethers decreases and N compounds become dominant in the F2 component. Figure 6 showsthe relativeabundance of 11 homologues in N compounds. In the homologues obtained at 800 and 900 "C, only methyl substituents are dominant. Most aromatic ethers other than dibenzofuran homologue diminish above 800 "C. Then homologues of carbonyl compounds, fluorenone (Mw= 180)and benzofluorenone (Mw= 230), are as abundant as the carbazole homologue, which is a major one in N compounds. The FIMS spectra of the F3 and F4 components reveal that both are composed of aromatics having one hydroxyl
group per molecule (denoted as hydroxyls hereafter) and that they hardly contain N compounds. The F3 component includes components of lower molecular weight than does the F4. With increasing freeboard temperature the NMR spectra shift to the smaller m/z side and become simpler,as seen with the F2 component. The homologous series prevailing in the F3 component are those of hydroxylic compounds with aromatic rings of benzene, naphthalene,acenaphthylene,fluorene,and biphenyl. The homologue of benzene is much more abundant than are the others. The hydroxylic homologous series with rings of naphthalene, biphenyl,dibenzofuran,phenanthrene (or anthracene), pyrene (or fluoranthrene), and phenylnaphthalene are important in the F4 component. The FIMS analysis of the F1 component shows that aliphatic hydrocarbons and polycyclic aromatic hydrocarbons without heteroatoms are dominant. Alkane alkene pairs are major constituents of the aliphatic hydrocarbons, and the pair of Ca compounds is the most abundant. Figure 7 showsyields of componentsnormalized by total tar yield as a function of freeboard temperature. The yield of hydroxyls is the sum of the yield of the F3 and F4 components. There are three pattems of temperature dependence. The yield of PAHs and N compounds increases with increasing temperature and then attains ca. 80 w t % of the total tar yield at 900 "C, while that of saturates and preasphaltene decreases monotonically
(28) Schiller, J.
E.Anal. Chem. 1977,49,2292.
Energy & Fuels, Vol. 7, No. 1,1993 61
Control of Molecular Composition of Tar
L
.
a
1
300(1,
A CO A COZ
ly
0-0
1m
I
m . e co+coz
154
0 Tar
1
0
200
Tomperrture I 'C
380
250
488
350
450
b
1
Figure 8. Amount of oxygen released from tar. 78
above 600OC. Ethers and hydroxyls show their maximum yield at 700 "C. The F3 component,which contains lower molecular weight hydroxyls, reaches maximum yield at 800 OC. It is clear that the increase in the yield of PAHs and N compounds is compensated by the decrease of the other components. Roles as precursors of PAHs and N compounds vary by component. Preasphaltene molecules are considered to have polyhydroxylic and oligomeric structures. This is supported by their showing the most sparse solubility in benzene and the highest polarity. The decrease in preasphaltene yield at 600-700 OC can be attributed to the degradation of oligomer-sized molecules to monomeric products which increase the yield of monomeric components other than saturates. The evolution of inorganic gases involves the decomposition of hydroxyl groups and contributes to the production of PAHs. Figure 8 shows the amount of oxygen released from tar during the secondary reaction. The data denoted by 0 are calculated from the difference in oxygen content in tars, generated under the reference condition that the freeboard is kept at 600"C and under the actual condition at higher temperatures. The plots indicated by 0 are calculated from the difference of the total yield of CO and C02 under the reference and actual conditions. As shown in Figure 8, these agree well. In the present experiment, the yield of water was ca. 6 wt % and was independent of the freeboard temperature. Thus the decomposition of hydroxyl groups leads to the evolution of CO and CO2, and the formation of water by condensation between two hydroxyl groups is not a major pathway. From Figures 7 and 8, hydroxyl groups that are decomposed below 700 "C originate from preasphaltene, which is the only decomposable unsaturated component in this temperature region. ,Cypres et al.29studied direct postcracking of volatile8 evolved from the pyrolysis of a coal under hydrogen pressure of 1 MPa. The decomposition of hydroxyl groups proceeded above 600OC and was responsible for the formation of aromatic compounds such as BTX. Yield and Structural Distribution of PAH Component. The FIMS analysis reveals that a PAH molecule has 6-12 double bonds. Figure 9 shows the FIMS spectra of the 6,7,and 8 db fractions separated from tar at 800 "C. The spectra shown in Figure 9a consist of two series of signals appearing at the mass number 152 + 14n and 154 + 14n (n is an integer). The former is attributed to the homologue of acenaphthylene(Mw= 152)and fluorene (Mw = 166)and the latter to that of biphenyl (Mw= 154). These homologues have the same double-bondnumber of (29) Cypres,
R.;Furfai, 5.Fuel
1986,64, 33.
C
I
178
216
'/ I r1
'T"
I
I
zm 2 u 258
,
6. Figure 9, b and c, shows that the 7 and 8 db fractions are composed of the homologues of phenanthrene (or anthracene) and pyrene (or fluoranthene), respectively. Each fraction is a mixture of compounds having the same number of aromatic rings and a different number of methylene units. Strictly, a fraction includes homologues of different db numbers. For instance, the 8 db fraction is a mixture of pyrene of fluoranthene homologues, but containe compounds of molecular maas 190 + 14n,ordinarily categorized as the cyclopentaphenanthrene homologue of 7 db. The coexistence of this homologue is confiied by the lH NMR spectrum, indicating the signal ascribed to ring-joining methylene. Even if fractions contain homologues of different db, the yield of each homologue is calculated by solving a set of linear equations for the yield of each fraction and the relative abundance of homologues in the fraction. Peaks with the smallest mass in the FIMS spectrum of each homologue are assigned to the PAHs listed in Table
I.
All the PAH homologues are composed of an unaubstituted PAH and its alkyl derivatives. Since alkyl substituentsare sensitive to the reaction temperature, the total mass of compounds is not a reliable indicator of the yield of homologue. Therefore, the mass of a substituted PAH having n aliphatic carbons per molecule, Y(subPAH), is converted to that of an unsubstituted PAH, Y(PAH). Y(PAH) M,(PAH)Y(sub-PAH)/M,(PAH) + 14n (1) where M,(PAH) is the molecular weight of the unsub-
Hayashi et al.
62 Energy & Fuels, Vol. 7, No.1, 1993 Table I. Molecular Mass of Initial Peaks in FIMS Spectra of Each Double Bond (db) Homologue molecular mass db 128 (naphthalene) 5 152 (acenaphthylene),154 (biphenyl) 6 178 (anthracene, phenanthrene) 7 202 (pyrene and fluorenthene) 8 226,228 (chrysene) 9 252 (benzo[a]pyrene,perylene), 10 254 (phenylphenenthrene) 276,278(picene, dibenzanthracene) 11 300 (coronene),302 (dibenzpyrene), 12 304 (phenanthrenoanthracene) 0 Compounds indicated in parentheses are PAHs to which the peake are assigned.
I
* -A-
8d.b. 9 d.b.
-0-
10d.b.
-m-
11 d.b.
+ 12 d.b.
0.0
I
600
I 700
M)o
900
Tmpomture [ "C 1
Figure 10. Yield of individual double-bond homologues converted to that of unsubstituted PAH. stituted PAH. The total mass yield of a homologue is evaluated by summing Y(PAH) for all componentsin the homologue. Figure 10 shows the yield of the 6-12 db homologues calculated from eq 1. No quantitative data could be obtained for the yield of the 5 db homologue of the naphthalene ring, because its high volatility caused a loss during solvent removal and FIMS analysisunder vacuum. The yields of the unsubstituted aromatic rings increase monotonically with increasingtemperature. This suggests that the formation of PAHs below 900 "C is predominant over the soot formation that causes a decrease in the PAH yield. The homologues of 6,7,and 8db are most abundant. Each comprises more than 1.0 w t % of coal on a daf basis at 900 OC, equivalent to the amount of BTX (3 db homologue), reported by Xu and Tomita.I2 Figure 11 showsthe relationship between the molar yield of each homologue and the reaction temperature. The temperature range, where the yield increase is most significant, shifts toward the high-temperature side with the increase in db number. The yields of the 11 and 12 db, homologues increase remarkably above 700 "C in accord with the evolution of hydrogen gas. In this temperature range, a part of the 11 and 12 db homologues is formed via condensation of smaller aromatic rings, and hydrogen gas is produced as a result. In the region of 600-700OC, however, the increase in yield of the 10, 11, and 12 db homologues is much smaller than that of the 6,7,and 8 db homologues, suggesting only a small chance of condensation.
Figure 12shows the number-averaged molecular weight of the aromatic ring, M I , calculated by
where Y,, and M,,, are respectively the molar yield of an n-db homologue and the molecular mass of PAH corresponding to n db. The value of M,decreases at 600-700 OC and increases at 700-900 OC. The decrease can be ascribed to the degradation of molecules containing more than two ring systems (oligomer) linked with methylene groups such as ethylene bridge.mJ1 The degradation of an oligomer forms monomeric molecules. Then, for instance, a molecule consisting of one naphthalene (5 db) ring and one phenanthrene (7 db) ring is a possible structure of 12 db molecules. However, the FIMS cannot distinguish between methyldibenzpyrenes and naphthylphenanthrylmethanes, which have the same db number of 12 and molecular mass of 316. Aliphatic Carbons Attached to Aromatic Ring. The number-based distribution of aliphatic carbons(methylene units) per molecule is evaluated in Figure 13 for the 6,7, 8, and 9 db homologues. The FIMS analysis combined with the HPLC separation allows the determination of the yield of molecules composed of a specific aromatic ring and a specific number of aliphatic carbons. The number of aliphatic carbons per molecule is distributed over the range of 0-9 at 600 OC, where aliphatic carbons are most abundant. Teo and Watkinaonllpyrolyzed coals in a spouted bed and characterized the molecular composition of tar by HPLC and GC-MS. They found that the number of aliphatic carbons was less than three for aromatic homologues with 1-4 rings. Our results, on the other hand, reveal that PAHs bearing more than four or five aliphatic carbons are abundant particularly in the low-temperature tars. These PAHs were not identified with GC-MS as used by Teo and Watkinson. They also investigated the effect of reaction temperature on the yield of individual homologues in the range of 440-690 OC but found no consistent tendency. This is apparently due to their experimental conditions, in which the primary and secondary reactions were not independently controlled. In the present study, molecules carrying three or four aliphatic carbons are dominant in each homologue obtained at 600 OC. However, the distribution moves toward the small-numberaide with increasing secondary reaction temperature. The mole fraction of uneubstituted PAHs is only ca. 5% in the homologues obtained at 600 OC but is ca. 50% at 900 OC. The temperature dependenceof the yield of substituted molecules varies with the number of aliphatic carbons. In the case of the 7 db homologue, the yield of C1 and CZ derivatives shows a maximum at 800 OC and that of C3 and Cq derivatives at 700 OC. Molecules having more than five aliphatic carbons decrease monotonically over the temperature range tested. Figure 14 shows the effect of the temperature on the yield of unsubstitutsd PAHs, normalized by that obtained at 900 OC. Despite the difference in ring structure, the normalized yields change similarly. Formation of unsub stituted PAHs proceeds significantly in the range of 800900 OC. The yield at 900 OC is 30-100 times that at 600 (30)Squire, K.R.;Solomon, P.R.;Caragelo, R. M.;DiTarato, M. 1986,65,833. (31)Solomon, P.R.;Hamblen, R.M.;Serio, M.A.; Deahpande,G.V. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987,32, 83.
B.Fuel
Energy & Fuels, Vol. 7, No. 1,1993 63
Control of Molecular Composition of Tar
m
700
m
900m
7w
m
900m
1 Figure 11. Molar yield of double-bond homologues and hydrogen. Temperature [
"c ]
Temperature [ "C
OC, indicating that aromatic ring units without alkyl substituents hardly exist in the original coal used. Classificationof Aliphatic Carbons. The NMRdata of each db fraction are analyzed by assuming the existence of only methyl, ethyl, and propyl chains. This is reasonable because the @-hydrogencontent is at most half that of a-hydrogen even at 600 OC, where the ratio of 8- to a-hydrogenis the highest for all the fractions. Contrarily, Collin et al.S2 reported that @-hydrogenwas more abundant than a-hydrogen in a tar obtained by fluidized-bed pyrolysis of Australian coals below 700 "C. This discrepancy is explained by the nature of the samples analyzed fractionated PAHs in our study vs unseparated tar by Collin et al. In general, tars contain a substantial amount of nonaromatic compounds such as long-chain paraffins, of which virtually all hydrogen is in 8 position. In the present experiment, @-hydrogen was as abundant as a-hydrogen when tar at 600 OC was analyzed without separation. The originalS3and modified Brown-Ladner methods have been employed to describe averaged chemical structure of tar. Parameters such as ring size and degree of substitution are derived on the assumption that all the alkyl chains are attached to aromatic rings. Therefore, nonaromatic compounds must be removed from tar samples. The number of chains is given by the followingequations
N(methy1) = [N(H,) - W(ethy1) - 2N(propyl)l/3 (5) N(a,a-methylene) = N(Ha,,)/2
(6)
where N(H,), N(H6), and N(H,) are respectively the number-based content of a-, @-, and y-hydrogen normalized by the total hydrogen content. The total number of aliphatic carbons is
C, = N(methy1) + 2N(ethyl) + 3N(propyl) + N(a,a-methylene) (7) The sum of the numbers of protonated and alkylated aromatic carbons is (32)Collin, P.J.; Tyler, €2. J.; Wileon, M.A. Fuel 1980,69,479. (33) Brown, J. K.;Ladner, W.R. Fuel 1960,39,87.
7w
m
900
Temperature [ "C ]
Temperature
[ "C
I
202 (8d.b.)
..............
185
J
600
7W
178 (7d.b.)
8W
Temperature [ 'C
900
1
Figure 12. Number-averagemolecular weight of aromatic ring for 6-12 db homologues.
C,, = N(methy1) + N(ethy1) + N(propy1) + N(H,)
+
W(a,a-methylene) (8) where N(H,) is the number-based content of aromatic protons. The total number of aromatic carbons is given as (9) = cpa +cb where cb is the number of bridgehead carbons. The ratio of Cpa to c b is determnied from the ring structure. The value of CJC, is calculated from the composition of PAHs by the FIMS analysis in all the fractions of different db and is for instance 10/14 for the 7 db homologue. The averaged number of aliphatic carbons per molecule is
c,
where ARCis the averaged number of aromatic carbons per molecule. The value ofN(CH2)can also be determined by the FIMS data as shown in Figure 13. The values of N(CH2)calculated from eq 10 are compared with those by the FIMS analysis in Figure 15. They are in agreement within an error of 10%. Figure 16 exhibits the composition of aliphatic carbons in the fractions of 7,8,9, and 10 db. The number of a-, @-, and ycarbons decreases with increasing temperature. The number of a,a-carbons is independent of temperature for the 6,7, and 8 db fractions and increases slightly in the range of 600-800 "C. Above 800 OC it decreases for the 6 1 0 db fractions. The temperature dependence of the yield of the phenanthrene and anthracene homologues and the cyclopentaphenanthrenehomologue are compared in Figure 17. The latter homolgue has one ring-joining methylene per molecule, and its yield decreases with increasing temperature above 800 "C, which supports the result shown in Figure 16. Figure 18 shows the number of methyl and ethyl chains per molecule for the 6,7,8, and 9 db fractions. The number
64 Energy & Fuels, Vol. 7, No.1, 1993
Hayashi et al.
201
1
10
0
6 d.b.. 800°C
7 d.b. , 800°C
l o ~ ~ ~ , n l .
.
.
.
.
,
0
1
40 30 20 10 1
0
3
2
5
4
6
0
8
7
9 1 0 1 1
0
Number of aliphatic carbons per molecule 21
1
3
2
4
6
5
7
8
9 1 0 1 1
Number of aliphatic carbons per molecule
1
I
8 d.b., 600°C
9 d.b.. 600°C
I
0.8 1 0.4
0.0
."0
8 d.b., 700°C
10 5
0
9 d.b., 800°C
8 d.b., 800°C 10
.r-l--.
0
.
.
.
.
201
I
8 d.b., 900°C
10 0
1
2
3
4
5
6
7
8
9 1 0 1 1
Number of aliphatic carbons per molecule
0
1
2
3
5
4
6
7
8
9 1 0 1 1
Number of aliphatic carbons per molecule
Figure 13. Distribution of number of aliphatic carbons per molecule for 6, 7,8,and 9 db homologues.
.
p'
700°C
A wo0c
3t 0
600
700
800
900
Temperature [ "C ]
// --
1
2
3
4
5
Number of aliphatic cartxms per molecule from NMR
900 "C.
Figure 15. Comparison of number of aliphatic carbons per molecule determined by NMR analysis with that by FIMS.
of ethyl chains decreases by 6585% in the range of 600700 "C. That of methyl chains hardly changes for the 8
and 9 db fractions and decreases by ca. 10% for the 6 and 7 db fractions in the temperature range 600-700 OC. The
Figure 14. Yield of unsubstituted PAHs normalized by yield at
Energy & Fuels, Vol. 7, No. 1,1993 66
Control of Molecular Composition of Tar 8 d.b.
BDE value of the a-and &bonds in ethylbenzene are 97 and 76 kcal/mol,3arespectively. If 0 scission and subse-
G"
0 Beta
0
E
Alpha Alpha-alpha
2
I I
0 Q
O
?
l
0
n
o
L
m
Alpha-alpha
600
700
ROO
900
0
9 d.b.
-:
I
I -
I
GOO
700
Roo
900
ROO
700
800
900
Gamma
0 Alpha Alpha-alpha
) .
0
3
L
Q)
n
5
2
z
l
0 600
700
aoo
Temperature [ "C ]
goo
Temperature [ "C ]
Figure 16. Composition of aliphatic carbons in 7,8,9, and 10 db fractions.
quent stabilizationof benzylic radicals occurredselectively, the number of methyl chains would increase. As can be seen in Figure 18,however, the number of methyl chains hardly changes for the 8 and 9 db fractions even if ethyl chains decompose significantly in the range 600-700 O C . This result strongly suggests selective a-bond scission. A slight decrease in number of methyl chains for the 7 db fraction indicates that a-bond scission of methyl chains also occurs, as confiimed by the evolution of methane from tar in this temperature range. Several research group^^^^^ investigated the pyrolysis of long-chain alkylbenzenes and reported that major products were styrene, ethylbenzene, and toluene. When dodecylbenzene was pyrolyzed at 800 "C,benzene formed via a-bond scission was one of the major products, but its yield was lower than that of the above-mentioned products.35 Smith et al.= conducted the pyrolysis of longchain alkylated PAHs in the temperature range of 375425 "C and proposed two pathways: a- and @-bond scissions. The selectivity of each scission was strongly dependent on the structure of the initial compounds and was well correlated by their localization energy (LE),that is, the difference in ?r-electron binding energy between the initialand transition states in the substitutionreaction. The LE value is given as the Dewar reactivity number, ND.~' LE = 2B(a-or + a-os) = BND
6 0 0 7 0 0 8 0 0 9 0 0
Temperature[ "C ]
Figure 17. Yield of phenanthrene (and anthracene)homologue and cyclopentaphenanthrene homologue.
reduction in the number of methyl chains is more pronounced as the temperature increases from 800to 900
"C. Collin et al.,32who analyzed flash pyrolysis tarswithout separation, reported that a-carbons were stable at 600700 "C based on their result that the a-proton fraction to the totalprotons increased in this temperature range, while they also reported that the degree of substitution of aromatic rings decreased from 600to 700 O C . This means that detachment of a-carbons occurred, although a-carbons were stabler than 8- and ycarbons. In the present study, the a-proton fraction is decreased by 15-20% for each db fraction in the range 600-700 "C. When the whole tar was analyzed, however, the fraction slightly increased in this temperature range. This result is in agreement with that by Collin et al. The difference between the results from the whole tar and the db fractions is due to the removal of aliphaticcompounds in the separationstep. They are abundant of 0-protons and are decomposed in the range 600-700 O C . Ethyl chains attached to an aromatic carbon have aand @-positionC-C bonds. When homolytic bond cleavage is predominant in the decomposition of alkyl chains, @-bondscission must proceed faster than that of a-bond, because the bond dissociation energy (BDE)of the former is much higher than that of the latter. For example, the
(11)
where B is the resonance integral, and a-or and a-osare the nonbonding molecular orbital (NBMO)coefficients at the position of substitution. Then the Dewar number is calculated from the position of the aromatic carbon and is not directly affected by the aromatic ring structure itself. Figure 19showsthe Dewar number of typical PAHs. Smith et al.38 reported that a-bond cleavage was dominantwhen the Dewar number of alkyl-substituted PAH was smaller than 1.81, and that /%scissionprevailed for compounds with larger Dewar numbers. Savage et aL40 carried out the pyrolysis of l-dodecylpyrene (DDP, N = 1.51)at 375-425 "C and found that the product selectivity of pyrene formed by the a-bond scission was less than that of other major products such as l-methylpyrene at an early stage of the pyrolysis. They also found that the selectivity increased with increasing reaction time and reached about 10 times that of 1methylpyrene when the DDP was converted completely. This time- and conversion-dependent selectivity implies that a cleavage via hydrogen-transfer mechanism is responsible for the pyrene formation. McMillen et al.41,42 proposed a radical hydrogen-transfer mechanism indi(34) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,33, 493.
(35)Billaud, F.; Chaverot, P.;Berthelin, M.; Freund, E. I d . Eng. Chem. Res. 1988,27, 1529. (36) Blouri. B.: Hamdan.~. F.; Herault. D. I d . E M-. Chem.Process Des. Diu. 1985,24,.30.' (37) Mushrush. G. W.: Hazlett. R. N. I d . E M-. Chem. Fundam. 1984. 23, '288.
(38) Smith, C. M.; Savage, P. E. AIChE J. 1991,37,1613. (39) Dewar, M. J. S. J. Am. Chem. SOC.1952, 74,3357. (40) Savaee. P. E.: Jacobs. G. E.: Javanmardian. M. I d . E M- . Chem. Res. 1989,2&645. (41) McMillen, D. F.; Malhotra, R.; Chang,S.; Ogier, W. C.; Nigenda, S. E.; Fleming. R. H. Fuel 1987.66, 1611. (42) McMiilen, D. F.; Malhotra, R.; Hum, G. P.;Chang S . Energy Fuels 1987, I , 193.
66 Energy & Fuels, Vol. 7, No.1, 1993
Hayashi et al. 2.04
1.2 6 d.b. 0 MahY'
0
0.8
Ethyl
0.4 2.31
0.0
1.26
\
\
\
1.89
@.%
7 d.b.
0
MahY' Ethyl
1157
1.33
\
1.F
0.5 0.0
Figure 19. Dewar reactivity number of typical PAHs. Conclusions
1.2
9 d.b. 0 Mahyl
0.8
0
Ethyl
0.4
0.0 600
700
800
900
Temperature [ "C ]
Figure 18. Number of methyl and ethyl chains per molecule for 6, 7, 8, and 9 db fractions.
cating direct hydrogen transfer to the ipso positions of alkyl-substituted aromatic compounds. Based on the above discussion, both the cleavage of a-bond of ethyl groups at 600-700 "C and the detachment of methyl groups at higher temperatures seem to be controlled by hydrogen-transfer steps. According to the results in Figure 7, possible mechanisms are as follows: (1)Above 600 "C, cleavageof 0-H bond of hydroxylgroup contained in preasphaltene molecules. (2) At 600-700 "C, decompositionof long-chainparaffins and olefiis behaving as hydrogen d0nors.~3(3) Above 700 "C, cleavage of 0-H bond of hydroxylic compounds. (4) Above 700 "C, condensation between aromatic ringsto produce hydrogen and methyl radicals. (43) Ofusu-A., K.;Stock, L. M.;Zabranaki, R. F.Fuel 1989,68,567.
Flash pyrolysis of a subbituminouscoal was conducted in a fluidized-bedreactor that was divided into two regions. The weight fraction of aromatic hydrocarbons and nitrogen-containingcompounds increased monotonicallywith increasing secondary reaction temperature. Polar molecular components of preasphaltene and hydroxylic compoundswere important precursors of thermallystable components. The separationof polycyclic aromatichydrocarbonsinto homologues having individualring structure was achieved by HPLC. The doublebond number of homologues ranged from 6 to 12, and the molar yield of each homologue increased monotonically with increasing temperature. The temperature range in which the yield of a homologue increased steeplywas dependent on ring size and was 700900 "C for the PAH homologues of 10, 11,and 12 db. This was explained by the evolution of hydrogen due to condensation between aromatic rings. A t 600 "C the number of aliphatic carbons per molecule was widely distributed from 0 to 9, and molecules carrying three or four aliphatic carbons were most abundant. The distribution curve shifted to the smaller side with increasing temperature. The mole fraction of unsubstituted PAH moleculeswas more than 50% for most homologuesformed at 900 "C. The number of aliphatic carbons at a-,@-, and y-positions decreased monotonicallywith temperature, while a,a-carbons were stable up to 800 OC. The number of ethyl groups decreased significantly in the range of 600700 "C, due to the cleavage of the aryl-a bond. Decomposition of methyl groups proceeded above 700 "C.
