Elimination of nitrogen from coal in pyrolysis and hydropyrolysis: a study of coal and model chars

Elimination of nitrogen from pyrolysis and hydropyrolysis: coal and model chars Krzysztof

Staiiczyk

and Jean

coal in a study

of

P. Boudou”

Institute of Coal Chemistry, Polish Academy of Sciences, 1 Baja 62,44- 700 Gliwice, Universite’ P & M Curie, 4 Place Jussieu, Tour 16-26, 75230 Paris 5, France (Received 9 November 1992; revised 8 June 1993)

Poland

l

To determine

the changes in coal nitrogen structures during pyrolysis and hydropyrolysis, model chars from defined nitrogen compounds were prepared and subjected to further heating under pyrolysis and hydropyrolysis conditions. Elimination of nitrogen from such model chars as well as from coal chars was investigated by pyrolysis-mass spectrometry. (Keywords:

coal: structure;

reactivity)

The problem of nitrogen in coal is still not well understood because of difficulties in the examination of nitrogen functionality. A knowledge of the functionality of nitrogen in coals is vital to their utilization. It suffices to mention the environmental impact during coal combustion: in pulverized combustion -80% of the nitrogen oxides emitted is formed by reaction of nitrogen chemically bound in the fuel’,2, and in fluidized bed combustion, fuel NO, forms the whole of the nitrogen oxide emissions3. Partial control of this emission can be achieved by use of staged combustion or flue gas However, further improvement in NO, treatment4. abatement may be gained by a better understanding of the reactivity of nitrogen structures and their transformation in pyrolysis and combustion. Essentially the bulk of the available data appears to be consistent with the hypothesis that most ofthe nitrogen in coal is present in ring compounds’. From analysis of coal decomposition products it appears that most of the nitrogen in coal is present in tertiary (sp2 hybridized) N heterocycles. From the results of X-ray photoelectron spectroscopy (XPS) it is evident that secondary (sp3 hybridized) heterocycles of pyrrolic type predominate throughout the bituminous coal and anthracite regions and the proportion of pyridine-type nitrogen increases with coal rank6q7. XANES shows that aromatic pyrrolic and pyridinic nitrogen are the prevalent forms of nitrogen in coal and there is little evidence of saturated amines’. Of the polycyclic aromatic nitrogen compounds identified in tar, most contain only one nitrogen atom, but there are indications ofcompounds with two or three N atoms”. A wide range of number of rings has been detected, from one aromatic ring to nine fused rings, but most analyses have involved three- to five-ring structures of carbazole and acridine types”. In hydropyrolysis, which can be considered as a pretreatment for coal to be burned, a substantial part of the nitrogen is eliminated, more than in pyrolysis in argon. The nitrogen remaining in the residual char and 00 16-236 l/94/06/0940-05 ii;: 1994 Butterworth-Heinemann 940

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its structure is the subject of present interest. The aim is to answer the question as to the kind of nitrogen eliminated from coal in the presence of hydrogen. For this purpose, model chars from well-defined nitrogen compounds have been prepared and subjected to heating under hydropyrolysis and pyrolysis conditions. Nitrogen compounds such as acridine and carbazole were chosen as structures representative of nitrogen occurring in coal. EXPERIMENTAL Pyrolysis of a high-volatile bituminous coal (Janina mine, Poland) was carried out in a fixed-bed reactor in hydrogen or argon flow. The apparatus has been described elsewhere”. The temperature of pyrolysis was varied from 4.50 to 850°C. The hydrogen pressure, residence time, gas flow rate and heating rate were constant in all experiments: 3 MPa, 30 min, 4 1 min- ’ and 15 K min- ’ respectively. Ultimate and proximate analyses of the coal and chars were performed according to ASTM standards. The model chars were prepared in the same fixed-bed reactor by carbonization of acridine and carbazole at 600°C under 5 MPa of argon for 1 h. The nitrogen compounds were heated to 400°C at 15 K min- ’ and then to 600°C at 2 K min- I. These chars were subjected to further heating under 5 MPa of argon or hydrogen at 600°C for 15 min or heated to 800°C and soaked for 15 min in argon or hydrogen. Evolution of N, from coal, coal chars and model chars heated to 1300°C was examined by the pyrolysismass spectrometry method described elsewhere12. Gas evolution profiles were obtained by heating a sample (l-2 mg) of powdered material (< 80 pm) at 30 K min-’ in a flow of oxygen-free, high-grade helium plus 1 vol.% of neon (4 ml min- ‘, at atmospheric pressure). N,, CH, and H, were detected with a mass spectrometer after selective trapping of other volatile species. Calibration was effected with neon as an internal standard and external standards introduced in frontal or elution mode.

Elimination

RESULTS Coal chars The analyses of the coal and pyrolysis and hydropyrolysis chars are listed in Table 1, as well as the coal conversion and nitrogen elimination. Comparison of low-temperature chars (450°C) from pyrolysis and hydropyroiysis shows a negative influence of hydrogen at this temperature, i.e. the coal conversion and degree of nitrogen elimination are lower in hydrogen atmosphere. At high temperature (SSO‘C) the coal conversion and nitrogen elimination in hydrogen are higher than in argon. Figure I shows that there are two maxima in the evolution of nitrogen from the coal and the chars with increasing temperature. The first maximum is observed just below 800°C and the second below 1200°C. Table 2 gives the yields of methane, hydrogen and nitrogen from Pyyms. examination of the coal and chars and the yield of molecular nitrogen corresponding to the first and second peaks. The data show the strongest influence of hydrogen on nitrogen elimination for the 850°C char in both the first and second temperature intervals. The total yield of nitrogen expressed with respect to the yield of nitrogen from the initial coal clearly shows the degree of nitrogen elimination in pyrolysis and hydropyrolysis. Figure 2 illustrates the nitrogen yields at the first and second peaks relative to the nitrogen yield from the initial coal and shows the large difference in nitrogen emission between pyrolysis and hydropyrolysis chars at the second peak. Model chars The elemental analyses of the model chars are given in Table 3. All the chars obtained at 600°C contain similar amounts of nitrogen, but those obtained at 800°C differ in N content. Acridine-type nitrogen is much more resistant to elimination. The influence of atmosphere is observed only for carbazole char, for which hydrogen slightly promotes nitrogen elimination. Table 4 shows the yield of molecular nitrogen and confirms that acridinetype nitrogen is more resistant to elimination. Figure 3 shows the evolution of N, during pyrolysis of model chars. For carbazole char obtained at 600°C a maximum N, release occurs below 1100°C; for that obtained at 800°C the maximum shifts to - 1150°C for both pyrolysis and hydropyrolysis chars. For acridine

Table I

Analyses

of coal and pyrolysis Proximate

W Coal Pyrolysis 450°C 600°C 850°C

A

chars, conversion

analysis

FC

of nitrogen

and J. P. Boudou

chars obtained at 600 and 800°C the N, release maxima occur at 1150°C and just below 1200°C respectively. Figure 4 compares the nitrogen evolution from 600 and 800°C carbazole and acridine chars. There is little influence of hydrogen on nitrogen elimination from both acridine and carbazole chars. The influence of temperature is much stronger in the case of carbazoletype nitrogen. Nitrogen release from carbazole char obtained at 600°C is twice that from char obtained at 800°C whereas for acridine char the difference in nitrogen release is -60%. Figure 5 compares nitrogen evolution for carbazole and acridine chars prepared at 600 and 800°C. Whereas for 600°C chars elimination of nitrogen is higher for

ryrolysls

remperature

L

Figure 1 Nitrogen, hydrogen and methane evolution during Py-m.s. ofcoal and chars prepared in argon or hydrogen at 450,600 and 850°C

of coal and nitrogen

(wt%)

from coal: K. Stahczyk

elimination

Ultimate

analysis

(wt% daf) N elimination (X)

ollirr

Coal conversion (wt%)

73.8

4.6

1.8

1.6

18.2

_

_

VM

H

N

s

8.4

22.5

40.6

28.5

41.2

1.4 1.9 0.8

30.1 35.3 44.0

53.6 55.6 53.9

14.9 1.2 1.3

21.8 11.5 2.4

81.0 86.0 93.7

4.0 3.2 1.3

1.7 2.0 1.2

0.8 1.1 1.3

12.5 7.1 2.5

26.2 35.0 50.5

30.4 21.9 61.1

chars 1.3 1.1 2.2

27.0 37.4 49.6

55.2 56.6 46.1

16.5 4.9 1.5

23.0 8.0 3.1

79.1 91.5 96.0

4.1 3.3 1.9

1.7 1.5 0.9

0.7 0.6 0.3

14.4 3.1 0.9

20. I 44.6 70.5

24.6 53.8 85.4

chars

Hydropyrolysis 450°C 600°C 850°C

a (NC,,, - N<,,ar)/Ncoa,. where N = wt% N (daf)

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Elimination Table 2

of nitrogen

Yields” of methane,

from coal: K. Stariczyk hydrogen

and nitrogen

and J. P. Boudou

from Pyyms.

(one replication)

of coal and pyrolysis

chars

CH, N,(I?’ ;t% Coal Pyrolysis 450°C

C)

Nz(II)~

Nz(T)~

N,(I)

(“wt% H)

(“w:% H)

1.9

10.4

38.5

24.9

25.5

50.4

100.0

(wt% N)

N,(II)

N,(T)

(wt% initial N, yield) loo.0

loo.0

chars 1.4

9.3

44.3

31.3

31.2

62.5

96.6

93.8

95.2

600°C

0.3

2.5

52.8

19.0

33.1

52.1

60.1

102.3

81.5

850°C

0.0

0.0

80.1

12.9

54.2

67.1

19.9

81.7

51.2

chars 1.5

9.8

49.2

25.3

30.8

56.1

85.3

101.3

93.4

600°C

0.2

2.0

76.0

25.2

33.7

58.9

54.7

71.5

63.2

850°C

0.1

1.7

81.2

5.4

12.5

17.8

5.0

11.2

8.2

Hydropyrolysis 450°C

a Expressed as percentages of C, H or N content of coal or char sample, as appropriate bI refers to the yield corresponding to the first peak (3OtSlOOO”C interval) in Figure I, II to that for the second total yield ‘Yield expressed with respect to that from the coal in the same temperature interval

I*”

FIRSTPEAK

Table 3

(3w-1ooo’C)

Analyses

peak (100&1300”C),

and T to the

of the model chars (wt%) C

H

N

Total

Carbazole

86.2

5.4

8.4

100.0

Pyrolysis chars 600°C 8OO’C

86.2 88.8

3.6 1.3

7.6 3.8

97.4 93.9

86.1 86.2

3.4 1.5

7.2 3.5

96.7 91.2

87.2

5.0

7.8

100.0

85.7 89.6

3.2 1.4

7.3 5.4

96.2 96.5

83.6 87.4

2.4 1.4

7.4 5.5

93.4 94.3

Hydropyrolysis 600°C 8OO’C

chars

Acridine Pyrolysis 600°C 800°C

chars

Hydropyrolysis 600°C 8Oo’C

chars

Table 4 Yields” of methane, hydrogen and nitrogen from Py-m.s. replication) of pyrolysis chars of carbazole and acridine

(one

CH,

Carbazole

Acridine

Figure 2 Yield of nitrogen hydrogen at 450, 600 and nitrogen yield corresponding

in Py-m.s. of chars prepared in argon or 85O”C, expressed as a percentage of the to the first or second peak of the coal

carbazole char, both chars obtained with respect to nitrogen release.

at 800°C are similar

DISCUSSION Nitrogen

elimination from coal in hydrogen

In pyrolysis and hydropyrolysis part of the nitrogen is eliminated.

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(“wt% C)

Et%

Ar 600°C Ar 800°C

0.4 0.0

H, 600°C H, 800°C

c”wi% H)

:t%

2.8 0.7

75.7 71.9

56.5 55.4

0.3 0.0

2.8 0.4

77.6 68.3

56.0 63.4

Ar 600°C Ar 800°C

0.1 0.0

1.1 0.3

59.6 59.9

45.4 37.3

H, 600°C H, 800°C

0.1 0.0

1.5 0.5

85.7 79.1

44.9 38.3

“Yields expressed

H)

N)

as in Table 2

show that the presence of hydrogen promotes nitrogen elimination. The questions to be asked are what kind of nitrogen is eliminated in pyrolysis and what is the influence of the reducing atmosphere of hydrogen. The results obtained for coal and coal chars, for which two peaks of nitrogen evolution are observed, are consistent with the work of Klein and Jiintgen’“, who found a first peak of nitrogen release at -700°C during pyrolysis of coal at 1 K min- ’ and expected a second

Elimination

of nitrogen

from

coal:

K. Stabczyk

and J. P. Boudou

“C

r

-

c 100 I “C

r

-i:Ihi~~~ 60”

X0”

I”,,0

12””

‘C

Pyrolysis temperature

Figure 3 Nitrogen, hydrogen and methane evolution during Py-m.s. of model chars prepared in argon or hydrogen at 600 and 800°C from carbazole or acridine

-

-

-

:1 : I

i

jis temperature

c

-I

of chars from carbazole temperatures

-

TC

IWO

Pyrolysis temper,

Figure 5 Nitrogen evolution during Py-m.s. and 800°C: comparison of char precursors

Ire

of chars prepared

at 600

one at >lOOO”C. The present work shows that with increasing pyrolysis temperature the intensity of the first peak of nitrogen evolution decreases and that of the second increases. Part of the nitrogen is thus eliminated and part is transformed into more stable forms which decompose at 1200°C. The latter part is greater for argon pyrolysis char than for hydropyrolysis char. Such transformation towards a more stable structure is observed even for 450°C char. At 600°C such transformation is very clear and stronger in argon pyrolysis. Hydrogen prevents transformation of the first-peak nitrogen into the more energetically stable form evolved in the second peak. Most probably hydrogen prevents recombination of radicals and coking of tar on the residual char. The yield of tar is higher in hydropyrolysis, and hydropyrolysis tar also contains more nitrogen than pyrolysis tar14, so a greater part of the nitrogen in coal is eliminated in a reducing atmosphere in the form of tar compounds. The hydropyrolysis char is richer in hydrogen, as shown by the CH, and H, evolution. Hydrogen saturates the radicals and consequently prevents coking and recombination of species containing nitrogen atoms and their transformation into more stable structures. At 85O”C, under hydrogen, ~90% of the nitrogen is eliminated from coal. Thermal

Figure 4 Nitrogen evolution during Py-m.s. and acridine: comparison of char preparation

-

degradation

of pyrrolic

and pyridinic

nitrogen

To clarify which kind of nitrogen is eliminated in the first and second peaks of coal char pyrolysis was the object of examining the model chars. The difference in the amounts of nitrogen in the carbazole and acridine

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Elimination

of nitrogen

from coal: K. Stahczyk

and J. P. Boudou

chars prepared at 800°C can be explained by fission of N-H bonds at this temperature and elimination of nitrogen. This is confirmed by FT-i.r. examination, which shows that no N-H bonds remain in carbazole char obtained at 800°C whereas such bonds still exist in carbazole char obtained at 600°C. For all the model chars only one peak of nitrogen evolution was observed, in the range - 1lO(r12OO”C. The position of the peak depends on the size of the condensed aromatic structures, which explains why it is shifted towards higher temperature with increasing hydropyrolysis temperature. At the same time, with increasing temperature part of the nitrogen is eliminated. The shift in the maximum of nitrogen release for carbazole char may also be caused by transformation of secondary into tertiary nitrogen structures. For both model chars the peaks occur at the position of the second peak of nitrogen emission from the coal chars. Klein and Jiintgen13 suggest that aliphatic nitrogen is responsible for the first peak, and heterocyclic nitrogen for the second peak. Amines are decomposed at 50~670”C’5. Pyrrole, indole and carbazole start to decompose at the same temperature (10 wt% decomposition at 770°C) because of N-H bond fissioni6. From other work it is known that pyrrole is decomposed at -850°C and pyrrolic derivatives at 500-650”C17. From this it may be concluded that in the first maximum of nitrogen release from coal and coal chars, aliphatic nitrogen (amines) as well as heterocyclic nitrogen of secondary type is eliminated. This is supported by the observation that the first peak diminishes but does not disappear as the char preparation temperature increases. For argon pyrolysis, even the char obtained at 850°C still shows a distinct first peak, shifted to -900°C. Such a temperature of pyrolysis is too high for amine groups to survive. It appears that the model chars simulate the second peak of nitrogen release from coal char, but the possibility cannot be excluded that part of the pyrrolic nitrogen in less-condensed structures is eliminated from coal char in the first peak. The model chars cannot precisely simulate coal chars, because they have little functionality apart from N atoms, and they have no chains, so that it is not possible to discern any differential effect due to reactions of recombination induced by functional groups and chain-breaking during pyrolysis.

2. The presence of hydrogen inhibits transformation of nitrogen into more stable structures. 3. Model chars of acridine and carbazole type simulate the second peak of nitrogen release from coal chars. 4. An increase in carbonization temperature shifts nitrogen release towards higher temperature because of condensation of aromatic structures.

ACKNOWLEDGEMENTS This work was supported by the Polish and French Governments through the Poland-France International Programme of Scientific Cooperation in Carbochemistry (PICS). The authors wish to thank the Institut Francais du Pttrole and Jean Espitalie for their financial and scientific contributions.

REFERENCES 1 2

3 4 5 6 I 8 9 10 II 12 13 14

CONCLUSIONS 1. In inert pyrolysis and hydropyrolysis, two parallel processes are observed: elimination of nitrogen and its transformation into more stable structures.

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Pershing, D. W., Martin, G. B. and Berkan, E. AIChE Symp. Ser. 1975, 71 (148) 19 Pershing, D. W. and Wendt, J. 0. in ‘Sixteenth Symposium (International) on Combustion’, The Combustion Institute, Pittsburgh, 1977, p. 389 Gulyurtlu, L. et al. in Proceedings, 1989 International Conference on Coal Science, NEDO, Tokyo, 1989, p. 473 Morrison, G. F. ‘Nitrogen Oxides from Coal Combustion’, IEA Coal Research, London, 1980 Attar, A. and Hendrickson, G. G. in ‘Coal Structure’ (Ed. R. A. Meyers), Academic Press, New York, 1982, p. 138 Burchill, P. in Proceedings, 1987 International Conference on Coal Science, Elsevier, Amsterdam, 1987, p. 5 Burchill, P. and Welch, L. S. Fuel 1989, 68, 100 Kirtley, S. M., Mullins, 0. C., van Elp, .I. and Cramer, S. P. Am. C’hem. Sot. Div. Fuel Chem. Preprints 1992, 37, 1103 Borra, C., Wiesler, D. and Novotny, M. Anal. Chem. 1987,59,339 Ostman. C. E. and Colmsio. A. L. Fuel 1988. 67. 336 Wiatowski, M. and Fabis:G. ErdBl Kohle, Erdgas, Petrochemie 1993, 46, 74 Boudou, J. P., Bimer, J., Salbut, P. D., Cagniant, D. and Gruber, R. Fuel 1994, 73, 907 Klein, J. and Jiintgen, H. in ‘Advances in Organic Geochemistry’, Pergamon Press, Oxford, 1972, p. 647 Hershkowitz, F., Olmstead, W. N., Rhodes, R. P. and Rose, K. 0. in ‘Geochemistry and Chemistry of Oil Shales’, Symposium Series 230, American Chemical Society, Washington, DC, 1983, p. 301 Eureleus, H. J. and Jolley, L. J. J. Chem. Sot. 1935, 929 Jones, R. A. and Bean, G. P. ‘The Chemistry of Pyrroles’, Academic Press, New York, 1977 Bruinsma, 0. S. L. et al. Fuel 1988, 67, 334