Effect of coal type on the flash pyrolysis of various coals

Effect of coal type on the flash pyrolysis of various coals Wei-Chun

Xu and Akira Tomita

Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira, Sendai 980, Japan (Received 25 June 1986; revised 17 November 1986)

The effectof coal rank on the product distribution pattern was investigated in Ar at 1037K with a Curie-point pyrolyser, using seventeen coals ranging from lignite to anthracite. The yields of inorganic gases, hydrocarbon gases, light hydrocarbon liquids and tar were determined and correlated with the structural parameters of the coals. The yields of CO2 were found to have a linear relationship with the carboxylic group content of the coals. The amounts of light hydrocarbon gases correlated well with the aliphatic hydrocarbon content of the coals. (Keywords: coal type; Curie-point pyrolysis; pyrolysis product)

Coal pyrolysis studies are important since pyrolysis is the initial step in most coal conversion processes. Pyrolysis at slow heating rates has been, for many years, intensively investigated in relation to coke-making processes. Recently a new advanced technique, flash hydropyrolysis, has been developed to produce various kinds of valuable products such as BTX (benzene/toluene/xylene), oletins and SNG (substitute natural gas). In this decade, many fundamental studies on the flash pyrolysis of coal have been made, and several reviews are availablelm3. The selectivity of the pyrolysis products is determined both by the properties of the coal and the pyrolysis conditions. Pyrolysis temperature, heating rate, gas atmosphere and pressure are all important factors. Thus practical pyrolysis conditions should be determined by an optimum combination of these parameters and coal type. A combination of flash pyrolysis with an effective processing of the residualchar would become one of the most promising coal utilization processes. Although the effect of pyrolysis conditions on the product distribution have been studied in detail, few systematic studies concerning the effect of coal rank on flash pyrolysis are available. At slow heating rates, 0uchi4, Juentgen’, Furimsky6 and their coworkers have investigated the effect of coal rank on pyrolysis behaviour using several Japanese, German and Canadian coals, respectively. At fast heating rates, Tyler7 examined the effect of coal type using lOcoals with a small fluidized bed, but the coals used were limited to bituminous coals and with a fluidized bed reactor there is the disadvantage that, to an extent, some secondary reaction of volatiles takes place. Meuzelaar et aI.* have investigated the flash pyrolysis of 102 Rocky Mountain coals with a Curie-point pyrolyser-mass spectrometer system. However, their main objective was a detailed characterization of the complex organic constituents in coal. The purpose of the present study is to present the detailed distribution pattern of a wide range of pyrolysis products (both inorganics and organics) from many kinds of coals (from lignite to anthracite) upon flash pyrolysis. 0016-2361/S7/050627~05$3.00 0 1987 Butterworth & Co. (Publishers)

Ltd

Seventeen coals were flash-pyrolysed in an inert atmosphere with a Curie-point pyrolyser. The yields of gases (H,, ClC3 hydrocarbons, CO,, CO and H,O) were determined gaschromatographically. The yields of light aromatic hydrocarbon liquids were also determined. Since only small amounts of hydrocarbon liquids are produced in an inert gas atmosphere, few detailed data have been published in spite of their importance as chemical feedstocks. The pyrolysis behaviour of these 17 coals were discussed in relation to their structures. EXPERIMENTAL Coal samples

The ultimate and proximate analysis values of 17 coals are listed in Table 1. The carbon contents of these coals range between 65.4 and 93.7 wt %(daf). Raw coals were ground to under 200 mesh at ambient conditions. To characterize these coals, the diffusion reflectance spectra were recorded with a JEOL JIR-100 FTIR spectrometer. Apparatus

A Curie-point pyrolyser (Japan Analytical Industry, JHP-2) was employed in this study. The coal sample was wrapped in a ferromagnetic sheet and pyrolysed at a rate of z 3OOOK/s. No loss of coal or char powder was detected after pyrolysis. The volatile matter could escape very rapidly from the heating zone by Ar flow, and thus secondary reactions were kept to a minimum. The whole chamber and the transfer line to the gas analyser were maintained at 420K to prevent condensation of water The schematic diagram of and light hydrocarbons. the same apparatus is described by Gomi et al.9. A temperature-programmed chromatograph gas (Yanagimoto, G-3800) was connected with the pyrolyser. A thermal conductivity detector and a flame ionization detector were used to analyse the inorganic and organic gases, respectively. Two kinds of columns were employed: (1) a Porapak P column for water and light hydrocarbon liquids (benzene, toluene, xylene, phenol and cresol -

FUEL, 1987, Vol 66, May

627

Effect of coal type on coal flash pyrolysis products: Table 1

Proximate

and ultimate

analysis

W.-C. Xu and A. Tomita

of coals Proximate analysis (wt %)

Ultimate analysis (wt%,daf)

Coal

(Code)

Moisture

Ash

VM

FC

C

H

N

S

0

Yallourn Rhein Braun Morwell Velva Soyakoishi South Beulah c010wy0 Taiheiyo Millmerran Wandoan Hunter Valley Liddell Newvale Yubari Shinko Vicary Creek Keystone Hongay

W-J WI WW WI

15.0 20.3 19.6 15.8 17.7 18.1 13.2 4.4 6.6 10.4 4.4 3.7 3.3 I.1 2.2 1.6 1.6

1.0 2.3 1.6 7.4 8.2 11.2 5.5 11.7 15.3 1.4 9.0 7.1 13.9 5.1 12.1 5.1 4.4

45.3 43.7 41.4 40.2 34.4 31.6 31.5 47.0 42.8 40.9 32.2 33.3 28.3 38.0 21.2 15.7 7.2

38.7 33.7 37.4 36.6 39.7 39.1 49.8 36.9 35.3 41.3 54.4 55.3 54.5 55.8 64.5 17.8 86.8

65.4 65.8 67.4 69.1 70.2 71.8 74.0 76.0 76.9 78.5 80.3 83.5 84.2 86.9 87.8 89.4 93.7

4.9 5.5 5.0 4.8 5.2 4.7 5.0 6.5 6.6 5.8 5.0 5.4 5.0 5.6 4.7 4.4 3.3

0.6 0.8 0.5 1.4 1.8 1.4 1.9 1.2 0.5 0.9 2.0 2.1 1.4 1.9 2.1 2.2 1.2

0.3 0.3 0.3 0.6 0.2 2.9 0.4 0.3 0.6 0.4 0.4 0.6 0.5 0.3 0.4 0.8 0.8

28.8 27.6 26.8 23.9 22.4 19.2 18.6 16.0 15.4 14.4 12.2 8.4 8.9 5.2 4.9 3.1 1.3

Table 2

w

(SW (CW (W (MM) WD) WV) U-D) (NV) (W WC) (KS) WC4

Effect of holding

time (Liddell coal;

1037 K) Gas yield (wt %, daf)

Weight loss (wt %, daf)

Time (s)

HZ

co

CO,

CH,

Hz0

C,H,

CzH6

C,H,

C,Hs

0.1

32

0.15

1.61

1.47

3.96

2.17

0.57

0.67

0.51

0.25

4.0 8.0

40 42

0.48 0.50

2.81 2.87

1.77

4.29 n.m.”

3.64 3.62

0.74 0.65

0.85 0.79

0.71 0.56

0.42 0.34

1.93

’ No measurement

in this paper this group is referred to as HCl); (2) an active carbon column forinorganic gases and light hydrocarbon gases (methane, ethylene, ethane, propylene and propane ~ these gases are referred to as HCG). Procedure

The pyrolysis experiments were conducted under Ar carrier gas. Flow rate was 50ml/min(STP). The coal sample was dried at 380 K, and then z 1 mg was wrapped in a pyrofoil. The weight was determined with an electric balance (accuracy, 0.01 mg) and the foil was placed in the pyrolyser. After purging with Ar, the sample was rapidly heated to the Curie point .of the foil and held at that temperature for 4s. In preliminary experiments, it was confirmed that a holding time of 4 s was sufficiently long to obtain a maximum yield ofvolatiles (Table 2). A similar observation, on the effect of holding time in flash pyrolysis, was also recorded in the literature2. The pyrolysis temperature in this study was fixed at 1037 K. After pyrolysis the foil was removed from pyrolyser and weighed again to determine the total weight loss. Since only small amounts of sample were used in this study, tar could not be recovered. The tar yield was calculated as the difference between the total weight loss and the combined yield of the products determined by gas chromatography. The validity of this approach will be ascertained later. Pyrolysis experiments were repeated at least 4 times, and the average value is presented throughout this paper. The average yields and standard deviations for four main gases in the pyrolysis of Liddell coal are shown below as an example of reasonable reproducibility: H,, 0.48 f 0.03; C0,2.81f0.13;COz,1.77f0.15;CH,,3.64_t0.13wt%.

628

FUEL,

1987,

Vol 66, May

RESULTS Table 3 shows the total volatile matter and the yield of H,, oxygencontaining gases, CH,, hydrocarbon gases with carbon numbers of 2 and 3, and HCL for all the coals examined. The total volatile matter determined here, at 1037 K, was close to the volatile matter data determined by a proximate analysis (listed in Table I). Table 3 also suggests that the effect of coal rank on the yield of each group of volatile matter is dependent on the type of product. More details are illustrated in Figure 1. Figure la indicates a general trend that both the total gas yields and total volatile matter yields decrease with increasing carbon content. The yield of each component is plotted against the carbon content of coal in Figures lb-f. Figure lb shows that the evolution of oxygen-containing gases, CO,, CO and H,O, is considerable from low rank coals. The total amount of these three gases reaches around a quarter of the coal weight for some low rank coals. H2 evolution was rather small on the weight basis, and was marginally dependent on coal rank (see Figure lc). The yields of HCG (Cl-C3) increased with the carbon content from lignite to bituminous coal, and then decreased in the range of carbon content above 87% (Figures Zc and d). In particular TH, MM, WD and YS coals yielded large volumes of hydrocarbon gases as compared with the other coals with similar carbon contents, Interestingly the dependence of yield on carbon content was quite similar for hydrocarbon gases with carbon numbers of 2 and 3 (Figure Id). The quantities of these gases were relatively small as compared with that of methane. A comparison of Figures Ic and d reveals that the sum of C2 and C3 yields

Effect of coal type on coal flash pyrolysis Table 3

products:

W.-C. Xu and A. Tomita

Effect of coal rank on weight loss and gas yields (1037 K) Yield (wt%, daf)

Coal code YL RB MW VL SY SB cw TH MM WD HV LD NV YS vc KS HG

Weight loss (wt %, daf)

HZ

CO+CO,+H,O

51.0 52.5 55.5 48.5 49.0 47.0 41.5 53.0 51.5 52.0 38.0 39.5 35.5 38.0 24.5 17.0 6.0

0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.4 0.5 0.5 0.4

26.9 25.6 25.3 26.5 22.8 26.4 16.4 14.2 11.9 15.7 8.3 8.9 9.1 4.2 5.9 2.4 1.6

CH,

HCL

C2+C3 _~.

1.5 1.5 1.6 1.7 2.0 1.2 2.3 2.8 3.0 2.5 3.2 3.6 3.0 5.4 3.5 3.9 1.2

1.5 1.8 1.6 1.3 1.8 1.3 1.6 2.9 2.9 2.7 1.8 1.6 1.5 2.2 1.4 0.8 0.1

0.9 1.2 1.1 0.7 1.1 0.9 1.5 3.1 3.5 2.8 2.4 2.1 2.0 3.8 1.5 1.3 0.1 _._~

-.

6r

C

b 8

4-

, O-

e

0

70

80 Carbon

content

90

( %,daf )

Figure 1 Effect of coal rank on yields of various products. (a) Gas including water, (tar + HCL) and char. (b) Oxygen-containing gases. (c) Methane and hydrogen. (d) C2C3 hydrocarbons. (e) Hydrocarbon liquids: B, benzene; T, toluene; X, xylene; P, phenol; C, cresol. (f) Tar: 0, Present values; o, Literature values

is almost comparable with the Cl yield. The evolution of higher hydrocarbon with carbon numbers of 4 or more was small under the present conditions. Figure le shows the production pattern for HCL and the patterns for all the compounds are very similar to each other. There is also some resemblance with those for C2C3 gases in one sense: the yields from TH, MM, WD and YS coals are large and the yield from HG is very small. However, in another sense, the patterns for HCL and HCG are

somewhat different: the evolution of HCL from YL, RB, MW, VL and SY (carbon content 65-70x) is at the same level as HV, LD and NV (carbon content SO-85%) whereas the yields of HCG are considerably different between these two groups. The pattern for the tar yield, shown in Figure lf, is also similar to that for HCL, although the absolute quantity is very different. The tar yield amounts to about a half of the total weight loss. The small circles in Figure If indicate the values reported in

FUEL, 1987, Vol66,

May

629

Effect of coal type on coal flash pyrolysis products: W.-C. Xu and A. Tomita

the literature7,1s’2. These tar yields were directly determined with a fluidized bed reactor at a pyrolysis temperature of 873K. A good agreement with our indirect values suggests that the present approach for the estimation of tar yield is reasonable. Although the temperatures employed are considerably different in two cases, this comparison is regarded as significant, because our study on the temperature effect in a subsequent paper I3 revealed that the tar yield was almost constant above 850 K. DISCUSSION It is generally accepted that coal is an aggregate of functional groups organized into aromatic-ring clusters which are connected by weaker aliphatic and ether bridges. Pyrolysis of coal releases large fragments of the coal molecule (tar) by breaking these weak bridges. Simultaneously with the tar evolution, light gas species are evolved by the thermal decomposition of the functional groups. Thus the coal pyrolysis behaviour can be closely related to the molecular structure of coal. In this section we would like to discuss our data in relation to coal structure. First the release of oxygen-containing gases will be considered. Figure lb shows the correlation of yield with carbon content of coal, As the carbon content in coal increases the oxygen content decreases, which indicates that the yield of these gases increases with oxygen content in coal. In fact, the total amount of oxygen atoms appearing in these gases at 1037 K was around 60-80 % of the original oxygen in the coal irrespective of coal rank. When the yields of CO and H,O were plotted against the oxygen content, very good linear relationships were obtained. [CO] = 0.29 x [0] (correlation coeficient, r = 0.97)

8-

_

5-O z8cv

60

630

FUEL, 1987, Vol 66, May

0

0

4-

0 2-

@OO

Figure 2

Infrared intensity at 1760 cm-’ Relationship between COz yields and the i.r. intensity at

1760cm-’ I”

0

0

I

z

[H,O] = 0.33 x [0] (r= 0.93) where all values are presented on the wt %(daf) basis. However, in the case of CO2 evolution, the correlation was rather poor (r= 0.86), and the yield of CO, from SB and VL coals were considerably larger than those expected from their oxygen contents. It is anticipated that the CO, comes mainly from carboxyl groups, which exist abundantly in low rank coals: in order to check the quantitative correlation between them, CO2 yield was plotted against i.r. intensity at 1760cm-‘, which is assigned to the stretching vibration band of C=O in POOH. A good linear relationship (r= 0.93) was obtained and is shown in Figure 2. The variation of hydrogen content in the original coal is rather small, as can be seen in Table I, with the ratio of the largest to the smallest as little as 2, whereas that for the oxygen content is more than 20. It was initially thought that this small variation in hydrogen content in coal is the reason for the fact that H, evolution was almost invariable with coal type (Figure lc). However, the H, gas evolved is, in fact, only a part of the hydrogen present in coal (611%). The mode of hydrogen distribution is strongly dependent on the coal type. For example, in the case of YS coal, a majority of the hydrogen in the product gas appeared in HCG, whereas the largest part of hydrogen in the gas from YL coal is in H,O, because the oxygen content in YL coal is extremely high. Hydrogen from TH coal was almost equally

0

5 ae’

0

o”

O

Infrared intensity at 2850-2960

IX-I-~

Figure 3

Relationship between HCG yield and the ix. peak area at aliphatic CH stretching vibration band region

distributed between H,, H,O and HCG. These distribution patterns correspond well with the structure of coal. Thus the yield of H, gas depends not only on the hydrogen content in coal but also on other coal properties such as oxygen content. The HCG yields in Figures Ic and d show that 4 coals, TH, MM, WD and YS, are somewhat peculiar. These coals, especially the Japanese coals, have high hydrogen content, and therefore it follows that the aliphatic hydrogen content would be higher for these coals. This was confirmed by measuring the infrared intensities of C-H stretching vibrations for CH, (2870 and 2960 cm - ‘), CH, (2850 and 2925 cm-‘) and aliphatic CH (2890cm- ‘) groups. The abscissa in Figure 3 indicates the tota peak area for the above 5 absorption bands. Although the physical meaning of this parameter is not straightforward, the correlation is reasonably good

Effect of coal type on coal flash pyrolysis (I= 0.88). Thus the evolution of HCG can be explained in connection with the aliphatic structure of the original coal. Calkins et al.’ ’ suggested that all the low molecular weight olefins (C2H4, C,H, and C,H,) were derived from the same precursors, which were identified as long-chain polymethylene structures in coal; the present study in addition reveals a similarly good correlation between two olefins. The C2H, yield was almost exactly 1.2 times the C,H, yield for all coals (r=O.99), and these yields are related to the CH, group content in coal. The correlation between C,H, and C3H, yields was not so good (r = 0.92). A reasonable correlation was obtained between the tar yield and the total volatile matter.

[Tar] = 0.48 x [TVM] (r = 0.88)

products:

W. -C. Xu and A. Tomita

rank is similar to that of HCL (Figures le, f), and there is a linear relation (r = 0.92) between yields of HCL and tar as shown in Figure 4. A reason for this correlation could be that HCL and tar may be evolved from similar precursors of the coal, although their molecular weights and the absolute values of yield are totally different.

CONCLUSIONS The present study revealed the effect of coal rank on the product distribution pattern upon the flash pyrolysis of 17 coals in an inert atmosphere. The dependence of oxygen-containing gases or light hydrocarbon gases on coal rank were well correlated with the coal structure, which was determined by FT-i.r. spectroscopy.

It is noteworthy that the dependence of tar yield on coal ACKNOWLEDGEMENT

3

aI 0

The authors are indebted to Dr 0. Itoh for the FT-i.r. analysis. The financial support of the Asahi Glass Foundation for Industrial Technology is acknowledged.

0

REFERENCES

-2 5 ae‘

I 2

9 .P >

3

4 2 7 8

0 0

I

I

I

10

20

30

Tar yield Figure 4

Relationship

between

( %,daf )

HCL yield and tar yield

9 10 11 12 13

Gavalas, G. R., ‘Coal Pyrolysis’, Elsevier, Amsterdam, 1982 Howard, J. B., ‘Chemistry of Coal Utilization, Second Suppl. Vol.’ (Ed. M. A. Elliott), John Wiley, New York, 1981, p. 665 Solomon, P. R. and Hamblen, D. G., ‘Chemistry of Coal Conversion’ (Ed. R. H. Schlosberg), Plenum Press, New York, 1985, p. 121 Ouchi, K. and Honda, H. J. Fuel Sm. Jpn. 1961,40,845 Juentgen, H. Fuel Process. Technol. 1979, 2, 261 Furimsky, E., Vancea, L. and Belanger, R. Ind. Eng. Chem. Prod. Res. Deu. 1984, 23, 134 Tyler, R. J. Fuel 1980, 59, 218 MeuzeIaar, H. L.C., Harper, A. M., Hill, G. R. and Given, P. H. Fuel 1984,63, 640 Gomi, K. and Hishinuma, Y. Fuel 1982,61, 77 Cliff, D. I., Doolan, K. R., Mackie, J. C. and Tyler, R. J. Fuel 1984,63, 394 Calkins, W. H., Hagaman, E. and Zeldes, H. Fuel 1984,63,1113 Tyler, R. J. and Schafer, H. N. S. Fuel 1980, 59, 487 XII, W.-C. and Tomita, A. Fuel 1987, 66, 632

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