Coal flash pyrolysis 2. Polymethylene pyrolysis tars William
H. Calkins
compounds
in low temperature
flash
and Ralph J. Tyler*
Central Research and Development Department, El du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19898, USA *Division of Fossil Fuels, CSIRO, PO Box 736, North Ryde, NS W, Australia 2113 (Received 8 August 1983; revised 2 November 1983)
Pyrolysis of coals at low temperatures (~600°C) produces tars containing the precursors of the low molecularweight aliphatic hydrocarbons, such asethyleneand propylene, observed on flash pyrolysisof the coals at higher temperatures (700-800”C). This is shown by further pyrolysis of these low temperature tars at high temperatures. Various methods, including isolation by h.p.1.c. were used to confirm the presence of straight chain paraffin and olefin pairs (C,,-C,, and above) in the low temperature tars. Pyrolysis of pure paraffins and olefins in this molecular weight range at temperatures > 700°C produce ethylene, propylene and other cracking products similar to those obtained on flash pyrolysis of coal. (Keywords: coal; flash pyrolysis;
polymethylene
compounds;
Previous investigators 1-3 have shown that flash pyrolysis of various coals at temperatures > 600°C produces varying quantities of light hydrocarbons including methane, ethylene, ethane, propylene, propane, butadiene, butenes, n-butane, benzene, toluene and xylene. In a previous paper4, the formation of ethylene, propylene and butadiene were shown to be related, presumably by pyrolysis of the same precursor in the coal. Production of methane, benzene, toluene and xylene, however, do not correlate with production of ethylene, and presumably these compounds are principally derived from different precursors in the coal. Using 13C n.m.r. (with cross polarization and magic angle spinning) on the coals this earlier work also showed the presence of long methylene chains whose concentrations correlate with the ethylene, propylene and butadiene yields from the coals, suggesting that these polymethylene chains are the precursors of the olefins. In this Paper further evidence is presented that polymethylene moieties are the principal precursors of the major oletinic and paraffrnic flash pyrolysis products in coal. EXPERIMENTAL Pyrolysis experiments Coal pyrolysis. Tars were prepared in two flash pyro-
lysers; one a laboratory scale reactor operating at l3 g h-r and the second a process development unit (PDU) capable of processing 20 kg coal h-l. The operating principles of both pyrolysers were similar and have been described in detail elsewhere’T4*’l. Briefly, coal particles of c 200 pm in a stream of N, were injected directly into a heated bed of sand fluidized by N,. Pyrolysis products from the reactor were handled differently in the two units. In the laboratory unit the tar and char were removed together in a Soxhlet thimble in a 0°C cold trap. The tar was separated from the char by 0016-2361/84/081119A%%3.00 @ 1984 Butteworth & Co. (Publishen) Ltd
flash pyrolysis
tars)
extraction with methylene chloride and methanol successively. In the PDU, char was removed in a cyclone at a temperature equal to or higher than that of the reactor, and the tar recovered in cold traps or by use of an electrostatic precipitator. Product gases were analysed by g.c. for H,, CO, CO,, Cl-C6 hydrocarbons and light aromatics. Coal particle heating rates were of the order 104”Cs-‘. Gas residence times were ~0.54.7s. Coal particle residence times depended on whether they were retained in the fluid bed by agglomeration with the sand or were elutriated. The minimum residence time of the particles, however, was the same as that of the gas. Secondary cracking of tar oapours. These experiments were conducted to establish the contribution of secondary cracking reactions of tar vapours to the production of light hydrocarbons, particularly olefins. Product vapours and gases from the small-scale pyrolyser were passed through a heated quartz wool plug to remove entrained char, then passed directly to a cracking reactor. This reactor consisted of a quartz tube heated by a furnace capable of operating at temperatures up to 1100°C. Products from the cracker passed to traps for tar recovery and were then analysed for hydrocarbons. Calculated gas residence times in the cracking reactor, assuming plug flow, were l-2 s. Pyrolysis of model compounds. Liquid model compounds were fed to the pyrolysis reactor with a microinjector positive displacement pump with 5ml capacity and pumping rate range O.Ol-5.0ml h-‘. The pump discharged into a 1.2 ml vaporizer packed with 3 mm quartz beads into which nitrogen was introduced. The nitrogen containing the vaporized compound was then conveyed to the reactor injector tube. The entire feed system was heated with heating tapes powered from powerstats and equipped with thermocouples to detect over- or under-heating of the system.
FUEL,
1984,
Vol63,
August
1119
Coal flash pyrolysis. 2: W. H. Calkins and R. J. Tyler Instrumental measurements 13C n.m.r. studies. Samples of coal pyrolysis tars were dissolved in CDCl, (z 10% solutions) and run in a
Brucker WM400 FT NMR spectrometer. Generally, samples were completely soluble in CDCl,, having already dissolved in methylene chloride during the extraction process. ‘H n.m.r. studies. For most of the assays, lo-50 mg of tar (or model compound) were weighed into a 1 ml volumetric flask and made up to volume with CDCl,. (Most of the tars studied were completely soluble in CDCl,.) For quantitative measurements, a known weight of Dow Corning DC200 silicone fluid (8.06% hydrogen) was added to the solution. Spectra were obtained on an IBM NR 80 FT NMR spectrometer. FT-i.r. Tar or tar fraction samples were prepared by spreading the undiluted sample on the surface of a salt plate. The spectra were determined on a Nicolet FTIR 7000 instrument.
G.c.-m.s. studies. Samples were run on a Varian 3700 gas chromatograph with DB-5 capillary column programmed from 60-230°C at 6°C min- ‘. Output was coupled to a Micromass 16F mass spectrometer. The tar was usually dissolved in a small amount of methylene chloride or n-hexane for injection into the capillary column, Tar fractionation by liquid phase chromatography
To isolate the olefin precursor, a separation of Millmerran 600°C pyrolysis tars (soft) was performed by preparative l.c. A preliminary separation was made in a gravity feed column containing 80g silica gel which had been equilibrated with n-butyl chloride. The tar (872.3mg) dissolved in methylene chloride (10 ml) was charged to the column which was then extracted successively with butyl chloride, methylene chloride and acetonitrile. The butyl chloride-eluted material (330 mg) was further fractionated by h.p.1.c. on a silica gel column by successive elutions with water/methanol/methylene chloride (1:3 : 1) and methanol/methylene chloride (1:4) mixtures, and finally methylene chloride alone. The fractions were examined by FT-i.r., ‘H n.m.r., osmotic molecular weight, elemental analysis, and other techniques.
result to a major extent from the cracking of tar vapours released during the initial coal pyrolysis. Secondary cracking of tar vapours
Tar vapours produced by the pyrolysis of Millmerran coal at 600°C were passed to the cracking reactor operating at 600-l 100°C. Results are shown in Figure 1, where yields are expressed as a percentage of the tar produced at 600°C (29% w/w daf coal basis) and have been corrected where necessary for hydrocarbons produced initially during coal pyrolysis, i.e. Figure 1 gives yields for secondary cracking reactions alone. The dominant product is C,H,, accounting for 19% of the tar at 900°C. Smaller yields of C,H, and butene-1 were also observed at the lower temperatures. At llOo”C, a substantial proportion of the hydrocarbons themselves crack with C,H, as the major product. These data are very similar to results obtained by the direct pyrolysis of the parent coal over the same temperature range’ and confirm that olefins, and other hydrocarbons, arise from secondary cracking of tar vapours. As a consequence, tars recovered from low temperature (<6OO”C) pyrolysis must contain the olefin precursors, and their examination should allow identification of these components. Analysis of tars produced at 600°C
Examination oftars produced at 600°C should show the presence of the precursor which previous work4 would suggest contains polymethylene chains. Samples of tars were therefore produced from a number of different coals by pyrolysis at 600°C in the small-scale laboratory coal pyrolysis unit4 for further analysis. Table 1 gives the properties of the coals from which they were derived and the ethylene yields produced on pyrolysis under standardized conditions at 850°C. Since production of tars was laborious and time consuming, larger samples were also produced from Millmerran coal at 600°C using the 20 kg h - ’ coal pyrolysis PDU of CSIRO. These tars had separated into two immiscible fractions -a hard and a soft tar in a 1:0.12 ratio. 13C n.m.r. showed the soft tar to be principally aliphatic (56% aliphatic carbon) and analysis
Chemicals and model compounds used CDCl,
n-dodecane n-hexadecane n-octadecane n-decene- 1 n-hexadecene- 1 n-octadecene- 1
Aldrich-gold label 99 atom% deuterium Lachat Cm1 Inc. 99.5% Aldrich 99x, b.p. 287°C Lachat Cm1 Inc. 99.5% Aldrich 96% Aldrich 94x, b.p. 274°C Aldrich 90%
RESULTS AND DISCUSSION As reported in previous work4, pyrolysis of coals at <6OO”C produces methane, water, CO and sometimes CO* (in low rank coals) but very little of other hydrocarbon gases. Above 6OO”C, increasing amounts of aliphatic and aromatic hydrocarbon gases are evolved as temperature increases. The following experiments were carried out to demonstrate that these hydrocarbon gases
1120
FUEL, 1984,
Vol63,
August
600
700
800
900
Cracker temperature,
1000
1100
‘C
Figure 1 Product yields from cracking of tar vapours. 0, CH,; 0, C2H4; A. C,H,:L butene-1; q i, C,H,; n . C,H,
Coal flash pyrolysis.
2: W. H. Calkins and R. J. Tyler
Table 1 Properties of coals studied
Proximate
Millmerran Texas lignite Emery PSOC 435a PSOC 124 b
1%)
Ultimate
Ethylene yield at
(% daf) Vitrinite
85O’C
Volatiles
Ash
C
H
0
N
sc
(%)
(% daf)
40.9 40.7 41.3 50.6 53.9
16.9 20.9 8.99 4.37 9.33
78.4 71.5 81.2 75.7 83.6
6.4 5.61 5.42 7.03 7.0
13.0 20.3 11.2 15.4 7.3
1.2 1.3 1.2 1.4 1.4
0.98 1.2 1 .o 0.46 0.74
88 _ 84 16
7.0 3.6 5.0 5.0 13.1
d Coal from Utah b Cannel coal c Organic sulphur
7
Chemical
Figure 2
shift (ppm)
13C n.m.r. of Millmerran I
I
I
I
soft tar I
29.5 ppm, attributed to long chain methylene groups. The soft tar, however, showed a substantially greater methylene content than the hard tar. Quantitative proton n.m.r. spectra were obtained on the same tars using a Dow Corning silicone DC200 fluid (“/,H = 8.06) internal standard. Figure 3 shows the ‘H n.m.r. spectrum for the Millmerran 600°C (soft) tar. The ‘H n.m.r. spectra indicated 33.6 and 12.3% (CH,),, in the soft and hard tar, respectively. Figure 4 shows a ‘H n.m.r. spectrum for a Texas lignite tar (600°C pyrolysis) which indicated a (CH,), content of 13%. To quantify the assignment of the 1.2 ppm peak in the proton n.m.r. spectrum, a series of model compounds having long methylene chains (> 10 CH, units) were dissolved in CDCl, containing the silicone internal standard. In all cases the 1.2-1.3 ppm peak was clearly detectable. The wt% (CH,), was calculated for each compound (Table 2). As can be seen from the table, not all methylene groups appear in the ‘H n.m.r. spectrum at 1.21.3 ppm. Only those (CH,),, groups shown in brackets in the structural formulae show up at this region. Those CH, groups conjugated with or close to carboxyl groups or benzene rings appear at higher ppm, and methyl groups appear at 0.74.8 ppm. This identifies the 1.2-1.3ppm region as due to (CH,),, but at the same time indicates that any analysis based on this method will not include all of the (CH,) groups in coal. I
I
I
I
1
I
4.0
3.0
2.0
-r
I
I
-1.0
-,
Chemical shift (ppm)
Figure 3
Proton
n.m.r. of Millmerran
soft tar
results gave the proportions: C, 83.5; H, 10.1; 0,5.4; N, 0.5; OH, OSmeqg-‘. The hard tar was somewhat less aliphatic (50% aliphatic carbon) with proportions: C, 79.4; H, 7.3; 0,8.3; N, 1.1; OH, 7.6meqg-‘. The higher aromaticity and high phenolic content probably explain the incompatibility of the hard tar with the more aliphatic and less polar soft tar. The 13C n.m.r. spectrum for a solution (x10% in CDCl,) of the Millmerran soft tar is shown in Figure 2. Both hard and soft samples showed very strong peaks at
5.0
7.0
1.0
0
-1.0
-2.0
Chemical shift (ppm)
Figure 4
Proton
n.m.r. of 6Oo’C pyrolysis tar from Texas lignite
FUEL,
1984,
Vol 63, August
1121
Coal flash pyrolysis. 2: W. H. Catkins and R. J. Tyler Table 3 shows the distribution of the protons by bonding types in pyrolysis tars (600°C) from four different coals which give widely varying yields of low molecular weight olefins and paraffins on pyrolysis. The table shows that up to 35% of protons in the tar are polymethylene protons, the rest being divided between methyl groups, hydroaromatic and benzylic hydrogens, hydrogens attached to aromatic carbons, and a small number of olefinic and some unidentified protons. Table 4 shows the distribution of types of protons in tar produced from Texas Lignite by pyrolysis at various temperatures from 600 to 920°C. This clearly shows that as the pyrolysis temperature is increased, the polymethylene content in the tar decreases as do the hydroaromatic and benzylic groups. The aromatic species, however, increase rapidly to become the predominant species. This result is supported by CSIRO studies on tars5. Analysis for polymethylene by ‘H n.m.r. in the pyrolysis tar thus provides a convenient measure of the extent of secondary cracking. Capillary g.c. coupled with m.s. is a useful technique for identification of individual constituents of pyrolysis tars. A pentane extract was prepared of a 600°C tar from a high ethylene producing cannel coal (PSOC 124) and analysed by g.c.-m.s. ( z 27% of the tar was extracted.) The g.c.-m.s. pattern of this tar is shown in Figure 5 with identification Table 2 (CH2),,
analysis of known
compounds
(proton
.< 60 z z E 40
Retenhon
Figure 5
Peak number
CH3[(CHzI CH3[(CH2)
101CH3
131 CH3 CH3[(CH2),7lCH3 CH3[(CH,),,l CH3 CH3
CH3[KH,),,l
CH, CH, [KH,
1
Table 3 Distribution
77.7 81.8 84.6 91 .J 92.5
pyrolysis tars
5.2
7.2
Hydroaromatic or benzylic
Olefinic
Aromatic
0.05
0.26
0.04
0.14
0.33 0.35
0.06 0.17
0.26
0.02
0.23
0.19
0.01
0.18
0.22
0.11
0.27
0.03
0.17
bm
0.8 CH3
1.3 (CH&,
1.67-l
H type
0.11
0.32
0.10 0.11 0.11
.J 1 .J-3.0
Coal type Texas lignite Emery PSOC 124 PSOC 435
Table 4 Pyrolysis tars - distribution
Pm
of H atoms.
0.3-0.5
I-Iwe
0.6-l
.O
1.2-l KH21n
600
0.11
0.32
700 825 850 920
0.10 0.07
0.25
1122
pyrolysis
C13Hzs C14H28
17 18
C14"30 Cl+'30
19
20 21 22 23 9: 26
Compound C19'-'40 C20H40 C2OH42 C21H42 C21"44 C2ZH44 C23H48 C24H48 C24hO C2s"so C25'-'52 C26H54 C27H56
Texas lignite pyrolysis vs. temperature
C'-'3
Pyrolysis temperature
of 600°C
of the main peaks given in Table 5. The majority of the peaks are due to straight aliphatic chains (Cl,--&), many of them in olefin/paraffin pairs. Material of higher molecular weight was also present in the tar but did not get through the capillary column. Between these aliphatic peaks there is evidence of possible branched aliphatic hydrocarbons and aryl alkyl compounds. To isolate and purify larger quantities of the olelin precursors, a separation was made of the Millmerran soft tar by preparative l.c. A preliminary separation was made by charging the tar to a preparative scale silica gel column and extracting successively with n-butyl chloride, methylene chloride and acetonitrile. The butyl chloride extract represented 60% of the sample charged. Further fractionation was performed on the butyl chloride extract by elution with water/methanol/methylene chloride mixtures and produced 23 fractions. A number of the fractions across the chromatogram were analysed by FT-i.r. and ‘H
49.1
of H atoms in 600%
14 15 16
C13H16
11 12 13
62.6
lp I CH,
Peak number
Compound
2 3 4 5 6 7 8 9 10
Actual
64.0 82.4 85.8 88.8 91.1 91.5
CH2CH2-C-OH
extract
124
Table 5 Identification of peaks in Figure 5 (g.c./m.s. pattern of pentane extract of pyrolysis tar PSOC 124,SOO”C. 0.5 s contact time)
:: CH3[KZH,)141
G.c.-m.s. pattern of pentane
tar from PSOC
n.m.r.1
Theoretical
tmw (mid
.4
1 .J-3.0
5.2
6.5-8.0
Hydroaromatic
Olefinic
Aromatic
0.05
0.26
0.04
0.14
0.04 0.03
0.26 0.21
0.05 0.05
0.11 0.12
0.16
0.01
0.25 0.41 0.57
0.07
1.6-l
.J
(%I
0.01 0.03
FUEL, 1984, Vol 63, August
0.04 0.02
0.19 0.08 0.10
0.58
Coal flash pyrolysis. 2: W. H. Calkins and R. J. Tyler I
n.m.r. and fraction 21, (representing 63% of the sample or 38% of the original tar) was found to be high in (CH,)” (concentration 68%). Figure 6 shows this ‘H n.m.r. spectrum. An i.r. spectrum of this fraction (Figure 7) shows a strong absorption peak at 720cm- ‘, a position known to be characteristic of long methylene chains. A g.c.-m.s. pattern of fraction 21 (Figure 8 with peak identifications in Table 6) showed the familiar oletin/paraffin pairs of peaks from C,, to C,,, here somewhat less contaminated with branched and aryl alkyl compounds than the unfractionated tars. The osmotic molecular weight was 306. The g.c.-m.s. pattern revealed that almost the entire fraction was long chain olefins and paraffins, though the portion which did not go through the capillary column and into the mass spectrometer could not be estimated. I
I
,
I
I
I
1
I
I
I
I
I
I
3600
I
3200
Figure 7
FT-i.r.
I
1
I
t
I
1
I
I
I
2800
2400 2000 1800 Wavenumbers
pattern
of l.c. fraction
1200
I
800
21 of Millmerran
4
soft tar
I
I
1
I
I
I
I
5
100
80
E g
40
20
2
1
E 600 :1
Figure 8
Retention time
(min)
G.c.-m.s. pattern of l.c. fraction
21 of Millmerran
soft
tar
I, 8.0
7.0
Figure 6
I
6.0
Proton
5.0
Pyrolysis of several paraffins and olefins > C, 2 in the coal pyrolysis reactor4 under conditions giving appreciable yields of hydrocarbon gases from coal (SSOC, 0.5 s contact time) gave the same products as observed with coal and 600°C tar pyrolysis (Table 7). Based on model compound pyrolysis yields and the ethylene yields obtained from the cracking of tar from the pyrolysis at 600°C of Millmerran coal suggests a long chain olefin and paraffin content of 3&40x (Figure I).
I
4.0 3.0 20 1.0 Chemical shift fppm)
n.m.r. pattern
of l.c. fraction
0
-1.0
-2.0
21 of Millmerran
soft tar
Table 6 Identification of peaks in Figure 8 [g.s.-m.s. pattern of cut 21, l.c. fractionation Millmerran 600°C pyrolysis tar (soft)] Peak number
Compound
1
C17H34
Compound C21H44
C20H40
10 11 12 13 14 15 16
C20H42
17
C2sHso
C21H42
18
C2sHs2
2
C17H36
3 4 5 6 7 8
C1SH36
9
Peak number
Cd'3s C19'-'38 C19"40
Table 7 Pyrolysis of model compounds
(85O’C.
CONCLUSIONS
C22"44
Further pyrolysis of tars produced at low temperature (600°C) at 700-11OO“C produces the hydrocarbon products characteristic of those obtained on high temperature pyrolysis of coal itself, showing that at least some of the oletin precursors in coal are transferred in some form to the tars. Examination of these tars support the hypothesis that these precursor(s) contain long poly-
C2ZH46 C23H46 C23H48 C24H48 C24HSO
0.5 s contact time) Product yield (wt%)
Compound
Methane
Ethylene
Ethane
Propylene
Eutadiene
Benzene
Dodecane Hexadecane Octadecane
4.8 5.0 4.7
68.0 58.8 54.7
0.9 4.2 3.7
11.2 10.7 10.2
2.0 2.7 2.7
1.9 3.6 -
Decene Hexadecene Octadecene
4.8 4.9 5.0
46.6 51.4 51.4
3.2 3.1 3.1
12.0 11.7 11.7
3.9 4.4 4.4
3.4 3.7 4.4
FUEL, 1984, Vol63,
August
1123
Coal flash pyrolysis. 2: W. H. Calkins and R. J. Tyler
methylene chains (C,,C,, and probably higher). The form of these polymethylene compounds in the coal itself is not known. They may derive from aryl alkyl compounds, fatty acids, alcohols or other structures in the coal; however, only a small fraction are solvent extractable. VahrmarP*‘, Bartle8s9, Denor’ and others have shown the presence of smaller amounts of polymethylene compounds in coal by solvent extraction and other methods. Further studies to determine the polymethylene concentration in specific coals will be reported in a later paper.
graphic separations and K. G. Kitson for the g.c.-m.s. data is gratefully acknowledged.
ACKNOWLEDGEMENTS
10
The assistance of A. J. Mica1 in the liquid chromato-
11
1124
FUEL, 1984, Vol63,
August
REFERENCES 1
2 3
Tyler, R. J. Fuel 1979, 58, 630 Tyler, R. J. Fuel 1980, 59, 218 Suuberg, E. M., Peters, W. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37 Calkins, W. H., Hagaman, E. and Zeldes, H. Fuel 1984,63,1113 Collin, P. J., Tyler, R. J. and Wilson, M. A. Fuel 1980, 59, 479 Vahrman, M. Chem. Br. 1972,8, 16 Vahrman, M. Fuel 1970,49, 5 Bartle, K. D., MartinT. G. and Williams, D. F. Fuel 197554,226 Bartle, K. D., Ladner, W. R., Martin, T. G., Snape, C. E. and Williams, D. F. Fuel 1979, 58, 413 Deno, N. C., Curry, K. W., Jones, A. D., Keegan, K. R., Rakitsky, W. G., Richter, C. A. and Minard, R. D. Fuel 1981.60, 210 Edwards, J. H. and Smith, I. W. Fuel 1980,59,674