Mass spectroscopic characterization of tars from the gasification of low-rank coals

Mass spectroscopic characterization of tars from the gasification of low-rank coals David J. Miller,

Jacquelyn

Grand Forks Energy Technology 58202, USA (Received 23 June 1980)

K. Olson and Harold Center,

US Department

H. Schobert

of Energy,

Grand

Forks,

ND

Mass spectroscopic methods were developed for characterization of gasifier tars. The tars studied were produced during pilot plant operation of the GFETC slagging fixed-bed gasifier. Nine samples were selected, representing four low-rank coals and a range of gasifier conditions. The tars were analysed by gas chromatography-mass spectrometry, high-resolution mass spectrometry, and low-voltage mass spectrometry. Whole samples and fractions separated by column chromatography were analysed by conventional g.c.-m.s. techniques for identification of major compounds. Carbon number analysis was accomplished using h.r.m..s. for mass identification and 1.v.m.s. for quantitative analysis. The chemical nature of the coal gasified was shown to be important in determining the composition of the tar. Approximately twenty constituents varied markedly in tars produced from different coals gasified under identical conditions. The important operating parameters affecting tar composition are the amount of hydrogen in the gas phase and the coal residence time. Neither operating pressure nor oxygen rate appear to be related to the composition of the tar.

The Grand Forks Energy Technology Center is operating a slagging, fixed-bed coal gasification pilot plant. The 900 kg h-l unit is the only gasifier of its type in the United States. The pilot plant was installed in 1958 and was first operated until 1965. Details of design and construction of the pilot plant and results of the 1958-1965 tests have Renewed interest in the production of been published’.‘. synthetic fuels from coals and concern for the potential environmental problems of coal led to resumption of pilot plant tests in 1976. The GFETC gasifier operates at pressures of 6892758 kPa, oxygen feed rate of 113.3-169.9 m3 h-‘, and oxygen/steam ratios of 0.9-1.1. The coal is reacted with oxygen and steam to produce a gas consisting primarily of carbon monoxide and hydrogen in a 2:l ratio. Recent tests have used low-rank coals, lignite and subbituminous, as fuels. Results of these tests have been published previously3.‘. Mass spectrometry is a very important method for characterization of the tars produced during gasification. The composition of the tar was studied to determine how it would be affected by variations in operating parameters. The mass spectral analyses of tars produced from nine tests using four coals and a range of conditions were correlated with the conditions in which they were produced and the type of coal gasified. Tar and water vapours, condensed from the exiting gas stream, accumulate in a spray washer where the liquids are sampled periodically. The analyses considered in this paper were obtained from tar samples collected at the end of the run. Detailed sampling procedures have been described in a previous publication5. EXPERIMENTAL Sample preparation Samples of as-received gasifier tar were distilled at reduced pressure to remove water and volatile light oils. 0016-2361/81/050370~05$2.00 @ 1981 IPC Business Press 370

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Saturate determinations were then performed by column chromatographic elution with hexane from neutral alumina. Fractions for high-resolution mass spectroscopic analysis were prepared by a method previously described6. Mass spectrometry

(h.r.m.s. and 1.v.m.s.)

The instrument used was an AEI MS-30* single-beam, high-resolution mass spectrometer interfaced with a DS50 data system. Approximately 1 mg of gross sample or of column chromatography fractions was introduced into the mass spectrometer using an all-glass heated inlet system operating at 300°C. Source temperature was 3OO”C, the ion source p*ssure was 2.0-2.7 x 10e4 Pa, and the ionizing voltage was 70 eV. A high-resolution spectrum (800&9000 resolving power) was obtained for each fraction and for the gross sample for molecular formula assignment. Quantitative data was obtained by lowering the ionization potential to 10 eV ’ and decreasing the resolution to 1500. A minimum of six scans were averaged and calibration with known hydrocarbons, oxygen compounds, and nitrogen compounds was used to calculate concentrations from the low-voltage intensity data. Gas chromatograophy-mass

spectrometry

(g.c.-m.s.)

Materials were analysed by g.c.-m.s. using a Varian 2740 gas chromatograph* coupled with a DuPont 21491B* mass spectrometer. A 1.2 m x 2 mm id. glass column packed with 3% OV-17 on SO/l00 Supelcoport was used for compound separation. The helium carrier gas flow rate was 30 ml min- ‘, and the column temperature was programmed from 7&300°C at 6°C min- ‘. G.c. peak areas were determined using a Spectra Physics System l* computing integrator; response factors were * Reference to specific brand names or models is done to facilitate understanding and neither constitutes Department of Energy

nor implies endorsement

by the

Mass spectroscopic measured with appropriate pure standards, The ion source was at 250°C and ionizing voltage was 70 eV. RESULTS

AND

DISCUSSION

Runno. RA-36 -40 -39 -47 -52 -37 -56 -66 -55

of gasifier operating

conditions

Operating pressure

Coal type

Oxygen

(kPa)

Indianhead Lignite Indianhead Lignite Indianhead Lignite Indianhead Lignite Indianhead Lignite Indianhead Lignite Gascoyne Lignite Baukol-Noonan Lignite Rosebud Subbituminous

Tab/e 2 Low-voltage

Slagging operation

rate (m3 h-l)

(h)

862

113

6.78

1379

113

9.42

2758

113

8.03

2068

142

5.75

2068

170

24.45

2758

170

6.92

2068

170

4.63

2068

170

8.05

2068

170

4.05

mass spectrometric

analysis of RA-52

of tars: D. J. Mdfer et al.

the values shown here for RA-52 are of the same order of magnitude as those for all nine analyses. It can be seen that the tar is composed of a large number of compounds, none of which occurs in high concentrations. The compound types which predominate are phenols, pyridines, naphthalenes, and some high-molecular-weight aromatics. The results reported in this paper provide an interesting comparison with a recent investigation by Hayatsu and co-workers’. They reported results for the gas chromatographic analysis of the hydrocarbon-rich fraction of the 3: 1 benzene-methanol extraction of Decker lignite. Of the 23 aromatic compounds identified in their gas chromatogram, 15 belong to compound types observed in our mass spectrometric analyses. The major difference between the solvent-extracted aromatics and the SFBG tar is that the latter does not contain some highly alkylated compounds, e.g. C,, or C,, tetralins (Z No.= -8. MW=l32, 146) found in the solvent extract. It was aIso suggested by those investigators that pyridine (Z No. = - 5N, MW = 79) would be the only heteroaromatic, other than oxygen compounds, in lignite. Pyridines are indeed the predominant non-oxygen heteroaromatics observed in SFBG tar, although quinolines (Z No. = - 11N. MW = 129) are also observed. The operating parameters in the SFBG are the pressure, oxygen feed rate, and oxygen/steam molar ratio. In the current SFBG test series, the ratio was varied only from 0.9 to 1.1; the nine tests covered in this paper all used a ratio of 1.0. The oxygen/steam molar ratio is therefore not considered a variable in the context of this paper. The data from the tests with Indianhead lignite were used to investigate the effects of pressure and rate on tar composition. Runs 36,39, and 40 (Tub/e 3) provide a series of runs at constant oxygen rate and three different pressures. The principal effect of changing pressure is to change the residence time of the gas phaseg. The residence time of the coal (taken as the average time required to gasify each charge of coal during a run) changed only slightly, dropping from 60 min at 862 kPa to 57 min at 2758 kPa.

End-of-run tar samples from nine SFBG runs were analysed. The coals used and the gasification conditions in each run are shown in Table 1. The runs shown with Indianhead lignite represent virtually the full range of conditions over which the SFBG was operated. The other runs provide a means of comparing results for four lowrank coals tested under the same gasification conditions. The complete low-voltage mass spectrometric analysis for run RA-52 end-of-run tar with water and distillable light oils removed is given in Table 2. This was by far the longest and most successful run in the 1976-78 test series. Although the concentrations of individual species vary somewhat with operating conditions or coal feedstock,

Tab/e 7 Summary

characterization

end-of-run

tar (wt %I

Probable Structural Types

ZNo.

6

7

8

9

10

Benzenes

-6

0.04

0.17

0.22

0.40

0.00

0.07 0.00

0.71 0.32 3.00

1.27 0.00 2.16

1.05

0.69

0.40

1.34

0.98

0.87

0.66

0.47

0.83

0.62

0.48

0.37

0.24

0.16

0.11

2.01

2.86

0.69

0.43

0.39

0.25

0.21

0.18

2.28

0.91

0.61

0.33

0.31

0.21

0.13

4.78

1.81

0.58

0.25 0.59

0.16 0.23

0.08 0.14 0.93

2.88 0.96 1.19 7.56 2.83 12.99 3.37 6.40 1.98

I ndanes/ tetralins I ndenes Naphthalenes Acenaphthenel biphenyls Fluorene/acenaphthalenes Phenanthrenel anthracenes Pyrene/f luoranthanes Chrysenes Benzopyrenes Pyridines Quinolines Phenols lndanols Naphthols Dibenzofurans

-8 -10 -12

11

12

13

-18

0.26

4.31

2.13

1.65

4.04

3.78

0.56 0.71 2.79 0.67

0.22 0.61 0.65 0.76 2.75

15

16

17

18

19

20

21

Total 0.83

-16

-22 -24 -28 -5N -1lN -60 -80 -120 -160

14

0.08 0.72 0.08 0.72 1.58

0.55

0.19

0.05

0.39 1.12 0.62

0.35 0.51 0.48

0.48 0.26 0.37

4.19 0.32

0.18 0.24

0.16

0.31

0.24

10.03 2.81 7.02

0.26

0.11

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Mass spectroscopic

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No correlation of tar composition with pressure was found for these three tests. However, a good correlation exists between tar composition and the amount of hydrogen in the product gas. These results are shown for selected compounds in Table 3. For all of the components shown in Tub/e 3, the correlation coefficient for tar composition uersus hydrogen content is equal to or greater than 0.8513. The oxygen feed rate is the major factor controlling the gas production rates in the gasilier’. This parameter has a

Table 3 Correlation

of tar compositions

with hydrogen

in product

gas RUfl

Operating (kPa) Hydrogen

content

-8 -12

-14 -16

-18 -22 -60

-80

-120 -160

40

862

1379

2758

29.28

31.02

25.21

(%) Correlation coefficient

Mass 174 188 128 156 170 184 212 210 152 194 208 222 220 244 122 136 150 148 162 176 190 186 200 210

Tab/e 4 Correlation

1.09 0.58 2.95 1.61 1.22 1.09 0.61 0.54 0.98 0.51 0.41 0.29 0.45 0.12 5.89 4.80 1 .32 1.00 1.06 0.73 0.42 0.89 0.44 0.54

1.15 0.72 1.58 1.65 1.37 1.21 0.65 0.73 1.32 0.63 0.52 0.36 0.58 0.18 6.28 5.35 1.63 1.06 1.17 0.90 0.61 0.98 0.50 0.73

of tar composition

Run Actual hydrogen content of gas (%I Average coal residence time fmin) Tar component (%I

Z No.

0.82 0.47 3.49 1.42 1.02 0.83 0.46 0.49 0.74 0.38 0.32 0.21 0.42 0.00 5.61 4.53 1.25 0.84 0.74 0.53 0.34 0.59 0.26 0.49

(corrected 37 28.46 39

0.992 0.956 0.890 0.991 0.989 1.000 0.996 0.851 0.947 0.979 0.960 0.982 0.840 0.999 0.949 0.912 0.838 1 .ooo 0.999 0.984 0.897 0.997 0.999 0.851

to 26.90%

Hz) with residence time

39 25.21 57

47 26.04 44

52 27.89 41 Correlation Coefficient

Mass 120 174 188 156 170 184 212 226 166 178 202 107 94 204 159 172

-6 -8 -12

-16 -18 -22 -5N -60 -80 -120

372

39

of

gas,(%) Tar components ZNo.

36 pressure

large effect on the coal residence time. For example, an increase in oxygen rate from 113.3 m3 h _ ’ to 169.9 m3 h- ’ (at 2758 kPa) reduced the average coal residence time from 57 to 39 min. No direct correlation between tar composition and oxygen rate, or coal residence time, was observed. To correct for any effects that may have been caused by variations in the hydrogen content of the gas, a multiple linear regression analysis was performed to relate the concentration of each component to both hydrogen in the gas and coal residence time. An average hydrogen content of 26.90”/, (the average of runs 37, 39, 47, and 52) was substituted into the equation obtained from the regression analysis to calculate a hydrogen-corrected concentration for each component. This procedure is the same as the methods used for removal of the effect of the most influential factor when performing a fractional factorial experiment lo For a number of compounds the hydrogen-corrected concentration correlates well with residence time. These results are shown in Table 4. The results for phenol (Z No. = - 60, MW =94) are of particular interest because the phenol concentration is important in considerations of effluent handling and treatment and of potential recovery of commercially valuable compounds. The effects of coal feedstock on tar composition may be seen by comparing results for the gasification of BaukolNoonan, Gascoyne, and Indianhead lignites and Rosebud subbituminous coal at identical pressures and oxygen rates. The analyses of these coals are given in Tub/e 5. Approximately 20 compounds were found to vary sharply as a function of the coal gasified. These variations are shown in the data given in Table 6. Development of an understanding of the effect of the molecular frameworks in coal and the likely mechanisms of reactions occurring in the carbonization zone of the SFBG on the composition of gasifier effluents is a major objective of gasification research at GFETC. Several preliminary correlations between coal type and tar composition may be developed. The average molecular weight of the oxygen, nitrogen, and sulphur compounds shows virtually no change when

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0.05 1.19 0.68 2.06 1.70 1.34 0.7 1 0.01 3.54 2.79 1.65 1.71 2.20 0.35 1.78 1.56

0.84 0.35 0.23 0.59 0.26 0.36 0.25 0.63 0.83 1.08 1 .oo 5.22 7.38 0.64 0.68 0.45

0.27 0.96 0.56 1.65 1.30

1.07 0.58 0.18 2.79 2.31 1.47 2.68 3.64 0.43 1.48 1.25

0.14 1.10 0.63 1.90 1.54 1.23 0.66 0.08 3.24 2.60 1.58 2.10 2.78 0.38 1.66 1.44

0.985 0.873 0.890 0.805 0.894 0.965 0.850 0.815 0.979 0.936 0.832 0.880 0.590 0.883 0.938 0.873

Mass spectroscopic Table 5 Comparative

characterization

of tars: 0. J. Miller et al.

analyses of coals tested in the SFBG

Coal

Indianhead

Gascoyne

Baukol-Noonan

Rosebud

Rank

Lignite

Lignite

Lignite

Subbituminous

Mine location

Indianhead Mine North American Coal Corp. Mercer Co. North Dakota

Gascoyne Mine Knife River Coal Mining Co. Bowman Co. North Dakota

Center Mine Baukol-Noonan Oliver Co. North Dakota

41.5 25.4 26.9

32.6 26.5 34.6

Proximate

Analysis fwt %, as-received)

Moisture= Volatile matter Fixed carbon

29.1 28.0 34.7

Ash Ultimate

Analysis

19.8 28.8 38.6

8.2

6.2

6.3

12.8

4.8 70.8 0.9 1.8 21.7

2.4 69.9 1 .o 1.4 25.3

2.7 73.7 0.9 0.6 22.1

2.9 73.3 1.3 2.3 20.3

(wt %, maf)

H C N S 0 (by difference) a Moisture

Rosebud Mine Rosebud Coal Sales Co. Carbon Co. Wyoming

Inc.

values are given for coals as tested in the gasifier.

These values do not represent as-mined moisture because of partial drying during

storage and handling

Tab/e 6 Variation

of tar composition

Indianhead 41 27.89

Coal Residence time fmin) Hydrogen content f%) Tar component

Z No.

with coal type Baukol-Noonan 41 28.29

Rosebud

Gascovne 36 30.81

56 25.52

f%) Mass

-12

128 142 170 184

3.00 2.15 0.97 0.87

0.58 1.22 1.09 1.17

0.00 0.33 1.62 1.67

0.00 0.69 2.27 2.21

-16

152 166 180

2.00 2.84 0.69

1.75 3.01 0.91

1.50 4.96 1.54

0.93 3.67 1.25

-18

178 192

2.27 0.91

3.21 1.23

5.08 2.00

5.01 1.81

208

0.61

0.89

1.27

1.29

202

3.31

2.76

1.80

2.44

-22 -5N

107

4.29

2.30

0.37

0.34

121

2.13

1.89

0.61

0.64

-60

94 108 136 150

1.64 4.02 2.78 0.65

0.27 0.84 2.17 0.70

0.20 0.12 0.57 0.38

0.07 0.08 0.77 0.44

-120

134 148 162

2.75 1.58 1.12

2.52 1.70 1.37

1.34 1.12 1.15

2.59 2.00 1.63

comparing the tars from the four coals. For naphthols (Z No. = - 120, MW= 144) for example, the average molecular weight varies from 159 in tar from Indianhead lignite to 168 for Gascoyne tar. The only correlation which exists is that the average molecular weight of the pyridines and quinolines correlates with the maf nitrogen content of the coals. For the pyridines the correlation coefficient is 0.745. The extent of alkylation of the aromatics does not change from one tar to another except for the naphthalene (Z No. = - 12, MW= 128), fluorene (Z No. = - 16, MW = 166), and chrysene (Z No. = -24, MW=228) derivatives. In those cases the degree of alkylation, expressed

as average number of methyl substituents, correlates directly with the maf carbon to hydrogen ratio in the coals. The correlation coefficients range from 0.847 for the fluorenes to 0.969 for the naphthalenes. The totals of naphthalenes and 3-ring aromatics show an inverse correlation with maf hydrogen content of the coal. The correlation coefficients are - 0.868 and - 0.979, respectively. The ratio of the total nitrogen compounds to oxygen shows an inverse correlation with maf nitrogen in the coal, with a correlation coefficient of -0.917. These results show that the chemical nature of the coal being gasified is important in determining the com-

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position of the effluent tar. As research continues, the respective importance of coal structure and carbonization zone reaction mechanisms in determining tar composition will be further clarified.

CONCLUSIONS An important finding of this study is that distinct differences are observed in effluent tar composition when different coals are gasified under comparable conditions. This finding emphasizes the need for operation of the SFBG with a variety of coals to develop a data base on effluent composition for the gasification of iow-rank coals. The mass spectrometric techniques described here represent a powerful procedure for the analysis and characterization of gasification tars. However, as only certain compounds show marked variation with changing gasification conditions or coal feedstock, additional research is desirable to develop new separation procedures for selective concentration of those compounds. Results of the operation of the SFBG on the same coal but at varying gasification conditions show that the major effects on tar composition are the hydrogen content of the gas stream and the residence time of the coal in the

374

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gasifier. This conclusion is based on the application of the statistical techniques mentioned previously to the data base given in the accompanying Tables. The amounts of some specific compounds or compound classes can be correlated with chemical properties of the coal gasified. As research progresses more understanding will be gained of the effect of coal structure on effluent composition. REFERENCES I 2 3 4 5

10

Gronhovd, G. H., Harak, A. E., Kube, W. R. and Oppelt, W. H. US Bureau of Mines RI 1962,6085 Gronhovd, G. H., Harak, A. E., Fegley, M. M. and Severson, D. E. US Bureau of Mines RI 1970, 7408 Ellman. R. C. et al. US Department of Energy, GFERC/IC-77-I, 1977 Ellman, R. C., Johnson, B. C., and Fegley, M. M. Tenth Synthetic Pipeline Gas Symposium, 1978 Paulson, L. E., Schobert, H. H. and Ellman, R. C. Am. Gem. SW. Dir. Fuel Chem. Preprints 1978, 23(2), 107 Schiller, J. E. and Mathiason, D. M. And. Chem. 1977,49(8), 1225 Johnson, B. H. and Aczel, T. Anal. Chem. 1967, 39, 682 Hayatsu. R. er crl. Fuel 1978, 57, 541 Johnson, B. C.. Schobert, H. H. and Fegley, M. M. Coul Proc. Tech., Am. Inst. Chem. Eng. 1978, Vol. IV Lipson, C. and Sheth, N. J. ‘Statistical Design and Analysis of Engineering Experiments’, McGraw-Hill Book Co., 1973