Effects of Coal Structures on Denitrogenation during Flash Pyrolysis

934
Energy & Fuels 1999, 13, 934-940
Effects of Coal Structures on Denitrogenation during
Flash Pyrolysis
Hideyuki Takagi, Takaaki Isoda, Katsuki Kusakabe, and Shigeharu Morooka*
Department of Materials Physics and Chemistry, Graduate School of Engineering,
Kyushu University, Fukuoka 812-8581, Japan
Received January 20, 1999. Revised Manuscript Received March 18, 1999
Yallourn, Taiheiyo, South Banko, Miike, and Hunter Valley coals were pyrolyzed using a Curiepoint pyrolyzer at 500-1040 °C. Product distributions were determined by GC-TCD and GCFTD, and N2 evolved in the gas phase was analyzed using a quadrupole mass spectrometer.
Samples were also prepared by mildly oxidizing Yallourn coal and hydrogenating the resulting
ethanol soluble over a Ru/Al2O3 catalyst. Quinoline and hydrogenated quinolines were also used
as models in order to better understand the role of the heterocyclic structure in denitrogenation
during pyrolysis. More than 90 mol % of the nitrogen atoms in the coals remained in the char
and tar as a result of the pyrolysis of the raw coals. However, the heterocyclic structures of the
raw Yallourn coal were partially changed by hydrogenation over a Ru/Al2O3 catalyst, and this
alteration enhanced the cleavage of those structures during pyrolysis, decreased the nitrogen
content of the char, and increased the yield of nitrogen compounds in the gas phase.
Introduction
Although nitrogen is a minor component of coal,
typically found in a range of 0.5-2.5 wt %, it represents
the major source of fuel NOx during combustion.1
Pyrolysis, the first step in coal conversion processes, is
a determinant of the distribution of nitrogen-containing
compounds in the final products. Nitrogen exists in coal
as pyrroles, pyridines, and quaternary compounds, and
has been characterized by X-ray photoelectron spectroscopy.2 Kelemen et al.3 investigated nitrogen functionalities in coals, the carbon content of which was 75-93
wt %, and determined the content of each nitrogen type
as follows: pyrrolic nitrogen, 59-65 mol %; pyridinic
nitrogen, 25-33 mol %; and quaternary nitrogen, 3-16
mol %. The content of pyrrolic and pyridinic nitrogen
increased, while that of quaternary nitrogen decreased,
with increasing coal rank. Mullins et al.4 determined
nitrogen species in coal by X-ray absorption near-edge
spectroscopy. Pyrrolic nitrogen was a major component,
of which content was 50-66 mol %.
A number of studies have been reported on the
formation of N2O and NOx during combustion of coal.5-11
* Author to whom correspondence should be addressed.
Telephone: +81-92-642-3551. Fax: +81-92-651-5606. E-mail: [email protected]
mbox.nc.kyushu-u.ac.jp.
(1) Pershing, D. W.; Wendt, J. O. L. Ind. Eng. Chem. Process Des.
Dev. 1979, 18, 60.
(2) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100.
(3) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994,
8, 896.
(4) Mullins, O. C.; Mitra-Kirtley, S.; van Elp, J.; Cramer, S. Appl.
Spectrosc. 1993, 47, 1268.
(5) Kilpinen, P.; Hupa, M. Combust. Flame 1991, 85, 94.
(6) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Fuel
1982, 61, 1218.
(7) Kambara, S.; Takarada, T.; Toyoshima, M.; Kato, K. Fuel 1995,
74, 1247.
(8) Lazaro, M. J.; Ibarra, J. V.; Moliner, R.; Andres, A. G.; Thomas,
K. M. Fuel 1996, 75, 1014.
In the initial pyrolysis stage, nitrogen atoms which are
originally contained in the coal structure are transformed to volatile matter and char. The nitrogen
compounds in the volatiles are further decomposed to
N-containing gases, largely HCN and NH3.12 These
N-containing gases are converted to N2O and NOx in
the presence of H2O, and radicals such as OH and O.
Thus, a relationship exists between the formation of
N2O and NOx and the formation of HCN and NH3.
Nelson et al.13 pyrolyzed three coals in a fluidized bed
reactor, which was capable of rapidly heating coal
samples. The rate of formation of HCN was related to
the rates of decomposition of heterocyclic compounds,
such as pyridine and pyrrole. Kambara et al.14 investigated the relationship between the yield of gaseous
nitrogen compounds and the nitrogen functionalities in
coals. Pyridine and pyrrole nitrogen was converted to
HCN, and the amount of produced NH3 was in good
agreement with the amount of quaternary nitrogen in
the coals. Hämäläinen et al.15 proposed a pathway by
which volatile nitrogen compounds could be converted
to HCN in the initial reaction and to NH3 in the
secondary reaction. Hirama et al.16 burnt coals mixed
with five model compounds, including pyridine, pyrrole,
and aniline, in a fluidized bed reactor, and investigated
the effects of nitrogen functionalities in the coals on N2O
and NOx emission. No remarkable differences in N2O
(9) Nelson, P. F.; Li, C.-Z.; Ledesma, E. Energy Fuels 1996, 10, 264
(10) Aho, M. J.; Hämäläinen, J. P.; Tummavuori, J. L. Fuel 1993,
72, 837.
(11) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy
Fuels 1996, 10, 188.
(12) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749.
(13) Nelson, P. F.; Kelly, M. D.; Wornat, M. J. Fuel 1991, 70, 403.
(14) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy
Fuels 1993, 7, 1013.
(15) Hämäläinen, J. P.; Aho, M. J. Fuel 1995, 74, 1922.
(16) Hirama, T.; Hosoda, H.; Sasaki, M.; Harada, M.; Suzuki, Y.;
Moritomi, H. J. Jpn. Inst. Energy 1995, 74, 213 (in Japanese).
10.1021/ef990010p CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/06/1999
Coal Structures and Denitrogenation in Flash Pyrolysis
Energy & Fuels, Vol. 13, No. 4, 1999 935
Table 1. Elemental Analyses of Coals
wt % (daf)
coal
abbreviation
C
H
N
(O + S)a
ash (wt %)
H/C
O/C
Yallourn
South Banko
Taiheiyo
Miike
Hunter Valley
oxidized YL
hydrogenated YL
YL
SB
TH
MI
HV
YL-O
YL-H
62.0
68.5
78.7
79.9
83.2
60.6
61.8
5.0
5.4
6.2
6.1
5.4
5.4
6.6
0.6
1.4
1.2
1.2
2.1
0.5
0.4
32.4
24.7
13.9
12.8
9.3
33.5
31.2
1.2
2.0
9.9
16.1
9.5
1.0
2.1
0.97
0.95
0.95
0.92
0.78
1.07
1.28
0.39
0.27
0.13
0.12
0.08
0.41
0.38
a
Determined by difference.
and NOx emission levels were found among the model
compounds which possessed different nitrogen functionalities. Wojtowicz et al.17 reported that NO and N2O
were released from pyrrolic and pyridinic groups at the
same proportion of approximately 5:1. These results
suggest that nitrogen functionalities in the heterocyclic
structure of coal are associated with the formation of
N-containing compounds during pyrolysis.
The rate of denitrogenation during coal pyrolysis is
often influenced by minerals. Ohtsuka et al.18 pyrolyzed
sixteen coals at temperatures below 1000 °C with a
heating rate of 400 °C/s. Two coals, which contained
minerals such as Ca2MgFe2O6 and CaMgAlFeO6, gave
higher nitrogen conversions of 50-60 mol %, based on
moles of nitrogen in the raw coals. These investigators
impregnated FeOOH particles in a brown coal using an
aqueous solution of Ca(OH)2 and FeCl3 and pyrolyzed
the treated coal at 900 °C in an atmosphere of He.19,20
The yield of N2 increased to 50-60 mol % from 4 mol %
of nitrogen atoms in the raw coal. Hayashi et al.21
impregnated iron precursors, such as ferrocene and
ferric acetate, into coals by a solvent swelling technique
using a mixture of THF and ethanol, and the resulting
Fe-impregnated coals were pyrolyzed at 1000 °C. The
N2 yield was increased from 30 mol % of the original
nitrogen atoms in the raw coal to 68 mol % for Feimpregnated Yallourn coal.
Since more than 80 mol % of the nitrogen in coal
exists in the form of heterocyclic rings,3 the denitrogenation reactivity of the heterocyclic ring structure plays
an important role in the formation of nitrogen-containing compounds during pyrolysis. We recently reported
that Yallourn coal, which had been oxidized under mild
conditions with aqueous H2O2 in the presence of 1-propanol, was solubilized in alcohol at a yield of 80 wt %
on the basis of the dry raw coal mass.22 Ethanol solubles
were further hydrogenated using a Ru/Al2O3 catalyst
in a mixed solvent of ethanol and acetic acid at 120 °C
for 12-72 h at a hydrogen pressure of 10 MPa,23 to give
yellowish white solids (hydrogenated white coal). Structural analyses of the hydrogenated coals suggest that
aromatic rings were converted into saturated rings, i.e.,
that the sp2 bonding structure of the raw coal was
largely converted to an sp3 bonding structure. The
nitrogen content was decreased to 0.44 wt % from 0.61
(17) Wojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1995, 74, 507.
(18) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477.
(19) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 1022.
(20) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141.
(21) Hayashi, J.-i.; Kusakabe, K.; Morooka, S.; Furimsky, E. Energy
Fuels 1995, 9, 1028.
(22) Isoda, T.; Tomita, H.; Kusakabe, K.; Morooka, S.; Hayashi, J.i. Proc. Int. Conf. Coal Sci. 1997, 581.
(23) Isoda, T.; Takagi, H.; Kusakabe, K.; Morooka, S. Energy Fuels
1998, 12, 503.
wt % of the raw coal. Thus the oxidation and hydrogenation of coal alter the heterocyclic structure of nitrogen
compounds and increase denitrogenation. However,
research on the chemical constitution of products is not
sufficient for a complete evaluation of reactions during
pyrolysis.
The objective of the present study is to investigate
the role of the heterocyclic structure of coal in denitrogenation during flash pyrolysis. Yallourn, South Banko,
Taiheiyo, Miike, and Hunter Valley, as well as oxidized
and hydrogenated Yallourn coals, were pyrolyzed using
a Curie-point pyrolyzer (CPP). Secondary pyrolysis was
negligible in the CPP, since volatile matter was entrained from the reaction zone by the sweep gas.
Oxidized and hydrogenated Yallourn coals, as well as
quinoline and hydrogenated quinolines, were also used
as the samples.
Experimental Section
The following coalssYallourn (YL), South Banko (SB),
Taiheiyo (TH), Miike (MI), and Hunter Valley (HV) as well as
modified YL coalsswere used in this study. Table 1 summarizes the elemental analyses of the coals. All the coals were
pulverized to particles which were 74-125 µm in size and then
dried in vacuo at 70 °C for 6 h. The modification of YL coal
was carried out as follows: YL coal was oxidized in a solution
consisting of aqueous H2O2 (30 wt %; Wako Chemical Ind.)
and 1-propanol at 70 °C for 6 h under atmospheric pressure.
The products were stirred in ethanol under ultrasonic irradiation at room temperature, to give ethanol-soluble (ES) and
ethanol-insoluble (EI) fractions. The ES fraction was evaporated and then subjected to hydrogenation. The EI fraction
was dried at 70 °C for 12 h in vacuo and then weighed. The
ES fraction was hydrogenated in a batch-autoclave at 120 °C
for 72 h in a H2 atmosphere of 10 MPa, using an aluminasupported ruthenium (powdered Ru/Al2O3; Wako Chemical
Ind., ruthenium content ) 5 wt %) catalyst in the presence of
acetic acid and ethanol. The product was recovered, and the
catalyst was removed by centrifugation. After removal of the
solvent, the products were extracted with water to give a
water-soluble fraction (WSO) and a residue (hydrogenated
ethanol-soluble fraction; H-ES). The oxidized and hydrogenated coal fractions are defined as (ES + EI) and (H-ES + EI
+ WSO), and are referred to as YL-O and YL-H coals,
respectively. Minerals in coals were analyzed by X-ray diffraction (XRD, Rigaku RINT-2500 KS). Solid products were
characterized by Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer, Paragon 1000).
Pyrolysis. The raw, oxidized, and hydrogenated coals were
pyrolyzed using CPP (Japan Analytical Industry, JHP-22). A
1-2 mg sample was tightly wrapped in a ferromagnetic foil
and heated at 764 °C. The heating rate was about 3000 °C/s,
and the total pyrolysis time was 5.0 s. Volatiles were quickly
transported from the reaction zone by flowing inert gas in
order to minimize secondary reactions in the gas phase. Yields
of inorganic gases (IOG; H2 + H2O + CO + CO2) and
936 Energy & Fuels, Vol. 13, No. 4, 1999
Takagi et al.
Figure 1. Schematic diagram for analysis of N2 produced by
pyrolysis of coal.
hydrocarbon gases (HCG; C1-C5) were determined using gas
chromatographs which were directly interfaced with the
pyrolyzer. Tar was defined as liquid products containing more
than 6 carbon atoms and was calculated by difference between
the initial mass of the coal sample and the yields of char, HCG,
and IOG. Yields were expressed in wt % based on the dry raw
coal mass.
Nitrogen molecules released to the gas phase during pyrolysis were detected using a quadrupole mass spectrometer
(H-MS; Anelva), and HCN and NH3 were detected using a GCFTD (flame thermionic detector, GC-14B; Shimadzu), which
was interfaced with the pyrolyzer. Figure 1 shows a schematic
diagram for the analysis of N2 produced by the pyrolysis.
Gaseous species which were evolved in CPP were diluted into
a flow of helium and introduced in a buffer chamber. The gas
in the chamber was then sampled to the high-vacuum chamber
through a fused silica capillary sheathed with a stainless steel
tube. Air leaking from the outside was minimized by introducing helium into the space between the fused silica capillary
and the sheath. To minimize the contamination, the introduction part (tubing and buffer chamber) was heated at 120 °C
using an electric heater. Pure N2 was used as a standard and
was introduced at the top of the CPP.
HCN and NH3 produced by the flash pyrolysis were detected
using a GC-FTD equipped with a glass column. With the
exception of N2, nitrogen-containing molecules can be detected
by FTD. A dioxane solution of NH3 (0.5 mol %) was used as a
calibration standard. Acetone cyanohydrin, which was quantitatively decomposed to HCN by pyrolysis in the CPP, was
used as an HCN standard. The yield of nitrogen in the tar
was determined by the difference between the total nitrogen
in the coal and the total nitrogen yield in the char, HCN, NH3,
and N2. Yields of nitrogen compounds were calculated as mol
% on the basis of the total nitrogen atoms in each raw coal.
Pyrolysis and Hydrogenation of Model Compounds.
Quinoline and hydrogenated quinolines were used as models
for heterocyclic compounds. Three grams of quinoline were
hydrogenated in a batch-autoclave at 120 °C for 0.5-6 h in a
H2 atmosphere of 6 MPa, using 0.5 g of an alumina-supported
ruthenium catalyst. Quinoline and the hydrogenated quinolines were then pyrolyzed using the CPP at 1040 °C for 9 s in
a helium atmosphere. Products were analyzed using GC-FTD.
Results
Pyrolysis Reactivity of Coals. Figure 2 shows the
product yields of YL coal as a function of pyrolysis
temperature. The yield of char was 65 wt % at 500 °C
Figure 2. Product yields of YL coal as a function of pyrolysis
temperature.
Table 2. Product Yields of SB, TH, MI, and HV Coals as a
Function of Pyrolysis Temperature
product yield (wt % raw coal)
coal
temp (°C)
char
tar
CO2
CO
H2O
HCG
SB
500
590
764
920
1040
74.7
64.2
50.7
42.4
42.5
13.0
23.2
26.4
31.7
31.0
4.7
4.7
7.0
6.7
7.5
1.0
1.1
4.7
8.0
7.0
5.4
5.1
7.3
6.9
7.1
1.2
1.7
3.4
4.3
4.7
TH
500
590
764
920
1040
78.5
60.2
50.8
45.7
46.2
15.7
25.8
30.1
35.7
36.9
1.6
4.9
3.2
3.0
2.9
0.5
1.2
2.6
4.0
5.2
2.6
5.3
8.7
7.7
5.6
1.1
2.5
4.3
3.8
3.0
MI
500
590
764
920
1040
89.9
72.1
58.5
51.3
52.2
7.4
19.4
31.3
33.7
32.7
0.4
1.9
1.0
2.1
1.6
0.2
0.9
1.0
3.0
1.9
1.1
3.0
2.6
3.9
3.3
0.9
2.7
5.3
5.5
5.3
HV
500
590
764
920
1040
91.0
87.8
66.4
57.7
57.7
6.5
6.7
21.8
29.7
28.3
0.7
0.6
1.2
1.6
1.3
0.2
0.3
2.0
3.5
4.0
0.8
2.1
3.3
3.5
2.6
0.8
2.5
5.0
3.9
5.9
and decreased with increasing pyrolysis temperature
above 900 °C. The yield of char decreased slightly above
900 °C, finally reaching 43 wt % at 1040 °C. The yield
of CO increased with increasing pyrolysis temperature,
but the yields of H2O and CO2 were not greatly temperature dependent. The yield of hydrocarbon gas
(HCG) remained unchanged at the pyrolysis temperatures tested. Table 2 shows product yields of the SB,
TH, MI, and HV coals as a function of pyrolysis
temperature. The yield of char decreased with increasing carbon content of the raw coals. The char yield of
YL raw coal, whose carbon content was 62 wt %, was
48 wt % at 764 °C. The HV coal, the carbon content of
which was 83 wt %, was 66 wt % at 764 °C. The yield of
CO2 decreased with increasing carbon content of coals.
Coal Structures and Denitrogenation in Flash Pyrolysis
Energy & Fuels, Vol. 13, No. 4, 1999 937
Table 3. Minerals in Raw Coals
coal
ash (wt %)
mineralsa
YL
SB
TH
MI
HV
1.2
2.0
9.9
16.1
9.5
quartz, kaolinite
quartz, kaolinite
quartz, kaolinite, calcite
quartz, kaolinite, calcite, pyrite
quartz, kaolinite, pyrite
a Quartz, SiO ; kaolinite, Al Si O (OH) ; calcite, CaCO ; pyrite,
2
2 2 5
4
3
FeS2.
Figure 3. Product yields of YL-O coal as a function of
pyrolysis temperature.
Figure 4. Product yields of YL-H coal as a function of
pyrolysis temperature.
Figure 3 shows product yields of the oxidized Yallourn
(YL-O) coal as a function of pyrolysis temperature. The
char yield at 1040 °C decreased from 43 wt % for the
raw YL coal to 34 wt % for YL-O, while the tar yield
increased as the result of oxidation. The yield of CO2
was increased, that of CO was decreased, and that of
H2O remained unchanged, by the oxidation. Figure 4
shows product yields of hydrogenated Yallourn coal (YLH) as a function of pyrolysis temperature. The char yield
of YL-H was further decreased by hydrogenation, and
was 24 wt % at 1040 °C. At all temperatures tested,
the tar yield of the hydrogenated coal was more than
double that of the raw YL coal. The yield of CO2
decreased to the level of that obtained for the raw coals,
while the CO and H2O yields became less than those of
the raw YL coal, by the hydrogenation.
Denitrogenation Reactivity. Figure 5 shows the
nitrogen distributions in pyrolysates as a function of
pyrolysis temperature. The fraction of nitrogen atoms
which remained in the char (hereafter, referred to as
the fraction of char-N) was 50-90 mol % on the basis
of the total nitrogen atoms of each raw coal samples.
The fraction of nitrogen atoms recovered in tar, HCN
and NH3, calculated by subtracting the nitrogen atoms
determined as char-N and gaseous N2 from the total
nitrogen atoms, was 20-40 mol %. The fraction of
nitrogen atoms recovered as N2 (hereafter, referred to
as the fraction of N2-N) was 0-3 mol %. The distribution
of nitrogen was dependent on the coal type, and the
fraction of char-N increased with increasing carbon
content of the coals. For YL coal, the total nitrogen
content decreased to 0.55 wt % after the oxidation and
to 0.44 wt % after the hydrogenation, from 0.61 wt % of
the raw coal. At 1040 °C, the fraction of char-N for YL-O
was 45 mol % of the initial value, and was approximately the same as for the raw YL coal. However, the
fraction of char-N for YL-H coal was 28 mol % of the
initial value at the same temperature.
Figure 6 shows the relationship between the char
yield and the fraction of char-N, based on the initial
nitrogen atoms, after pyrolysis at temperatures of 5001040 °C. The fraction of char-N decreased with decreasing char yield for all coal samples including YL-O and
YL-H, and was larger than the char yield, suggesting
that nitrogen was concentrated in the char. As shown
in Figure 7, the fraction of N2-N was 0-3.5 mol % for
the raw coals at all temperatures tested. The fraction
of N2-N was 2-4 mol % for the YL-O coal pyrolyzed at
500-800 °C, and was slightly higher than that of raw
YL coal at temperatures higher than 900 °C. However,
the fraction of N2-N for YL-H was 6.8 mol % after
pyrolysis at 1040 °C. Figure 8 shows the relationship
between the fraction of N2-N and char yield after
pyrolysis at temperatures of 500-1040 °C. The fraction
of N2-N decreased with decreasing char yield.
Table 3 shows minerals in the raw coals, as determined by XRD. The ash contents of the raw YL and SB
coals were less than 2 wt %, consisting largely of quartz
(SiO2) and kaolinite (Al2Si2O5(OH)4). The amount of ash
for the TH, MI, and HV coals was 9-16 wt %. Calcite
(CaCO3) and pyrite (FeS2) were also identified in these
coal samples.
Figure 9 shows the distribution of nitrogen as a result
of flash pyrolysis at 1040 °C. The fraction of nitrogen
atoms recovered as NH3 (hereafter, referred to as the
fraction of NH3-N) was 0.3-1.8 mol % and was the
938 Energy & Fuels, Vol. 13, No. 4, 1999
Takagi et al.
Figure 5. Nitrogen distributions in pyrolysates as a function of pyrolysis temperature.
largest and smallest for YL and HV coals, respectively.
The fraction of NH3-N was slightly increased as a result
of the oxidation and hydrogenation procedures. The
fraction of nitrogen atoms recovered as HCN (hereafter,
referred to as the fraction of HCN-N) was 0.6-5.8 mol
% and was larger than the fraction of NH3-N. The
fraction of HCN-N was approximately 2 mol % for the
YL and YL-O coals, and 5.8 mol % for the YL-H coal.
Figure 10 shows the fractions of HCN-N and NH3-N
produced by pyrolysis of hydrogenated quinolines. Quinoline was completely hydrogenated to decahydroquinoline by reaction at 120 °C for 6 h over the Ru/Al2O3
catalyst. The fraction of HCN-N increased with increasing degree of hydrogenation of quinoline and was 44 mol
% for decahydroquinoline. However, the fraction of
NH3-N remained at the level of 2-4 mol %. The yield
of N2 was negligible (less than 0.1 wt %).
Discussion
to char and tar at temperatures of 500-1040 °C.
However, the fractions of HCN-N, NH3-N, and N2-N
were less than 10 mol % of the raw coals. The fraction
of char-N decreased with decreasing char yield. Thus,
the denitrogenation during flash pyrolysis depends on
the pyrolysis reactivity of coals, and the amount of
nitrogen contained by the char can be decreased by
improving the pyrolysis reactivity as a result of altering
the coal structure. Nitrogen atoms are concentrated in
char since heterocyclic nitrogen compounds are chemically stable.3
The H2O2 oxidation of YL coal results in an increase
in the yields of volatile matter and CO2. The increase
in the yield of volatile matter can be attributed to
degradation and cleavage of the bridge structure. The
CO2 yield is related to the decomposition of carboxy
groups and the formation of cross-links, while the CO
and H2O yields are related to the decomposition of
phenolic hydroxy groups.24-26 The experimental results
Pyrolysis and Denitrogenation Reactivity of
Coals. During flash pyrolysis, more than 90 mol % of
the nitrogen atoms in the raw coals were transferred
(24) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy
Fuels 1990, 4, 42.
(25) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 1668.
(26) Schafer, H. N. S. Fuel 1979, 58, 667.
Coal Structures and Denitrogenation in Flash Pyrolysis
Energy & Fuels, Vol. 13, No. 4, 1999 939
Figure 8. Relationship between fraction of N2-N and char
yield.
Figure 6. Relationship between fraction of char-N and char
yield.
Figure 7. Fraction of N2-N as a function of pyrolysis temperature.
Figure 9. Nitrogen distributions by pyrolysis at 1040 °C.
obtained in this study suggest that the major reaction
during the H2O2 oxidation is the introduction of carboxy
groups into the coal structure. Hydrogenation over the
Ru/Al2O3 catalyst further improves pyrolysis reactivity
by partially converting the sp3 structures in coal to the
sp2 structures. FT-IR analysis indicates that the decomposition of carboxy groups in the oxidized coals
during hydrogenation is negligible.23 The decrease in
CO2 yield suggests that the formation of cross-links
produced via decarboxylation during pyrolysis is suppressed.
Kambara et al.14 pyrolyzed twenty coals at 1215 °C
with a heating rate of 7500 °C/s. The fractions of HCNN, NH3-N, and N2-N in the products were 21-66, 2-13,
and 0-4 mol %, respectively. Since the volatile matter
was largely decomposed (tar yield ) 0-5 wt %), the high
yields of HCN and NH3 can be attributed to secondary
reactions in the gas phase. In the present study,
however, secondary reactions of volatile matter were
negligible. Fractions of HCN-N, NH3-N, and N2-N were
0.5-6, 0-2, and 0-6 mol %, respectively, and the tar
yield was 10-50 wt %.
Effect of Heterocyclic Structure on Denitrogenation Reactivity. The ruthenium complex catalyzes
the hydrogenation of nitrogen-containing heterocyclics.27,28 We reported that CdC bonds in the coal
structure were converted to C-C bonds during hydrogenation over a Ru catalyst.23 In the present study,
quinoline was easily hydrogenated over the Ru/Al2O3
catalyst. This suggests that heterocyclic nitrogencontaining structures in the coals were partially hydrogenated by this catalyst.
After H2O2 oxidation, the nitrogen content in the raw
YL coal was decreased from 0.61 to 0.55 wt % on the
(27) Muroi, T. Catalyst 1994, 36, 531 (in Japanese).
(28) Laine, R. M.; Thomas, D. W.; Cary, L. W. J. Am. Chem. Soc.
1982, 104, 1763.
940 Energy & Fuels, Vol. 13, No. 4, 1999
Figure 10. Fractions of HCN-N and NH3-N produced by
pyrolysis of hydrogenated quinolines.
basis of dry coal mass. Bridges between aromatic rings
in the coal structure were largely decomposed by the
H2O2 oxidation, but the amount of heterocyclics remained largely unchanged. By hydrogenation after the
H2O2 oxidation, the nitrogen content was further decreased to 0.44 wt %, which was 72 mol % of the total
nitrogen in the raw YL coal. This may be explained as
follows. During the hydrogenation, 5-10 wt % of the
ethanol soluble fraction was deposited on the catalyst.
The residues contained 0.65 wt % of nitrogen based on
elemental analysis, while the YL-H coal contained 0.44
wt % nitrogen. Thus, the nitrogen content in the
residues was larger than that in the YL-H coal. It was
also found that hydrogenation of quinoline over the Ru/
Al2O3 catalyst for 2 h decreased the nitrogen content
by approximately 2 mol %. The decrease in nitrogen
content during hydrogenation can be attributed to (1)
deposition of residues on the catalyst, and (2) decomposition of the heterocyclic structure.
It has been reported that the distribution of Ncontaining gases was affected by oxygen-containing
gases, such as CO and CO2.29 Kilpinen and Hupa5
proposed reactions between HCN and oxygen-containing
radicals
Takagi et al.
introduced carboxy groups without cleaving the heterocyclic structures in YL.
The fraction of HCN-N was increased, while that of
NH3-N remained unchanged, by hydrogenation over the
Ru/Al2O2 catalyst. Figure 10 shows that the fraction of
HCN-N by pyrolysis of quinolines increased with increasing degree of hydrogenation, and that the fraction
of NH3-N was small. Thus, the heterocyclic structure
in the coals was hydrogenated over the Ru/Al2O3
catalyst, leading to the production of HCN. As a result,
the fraction of char-N decreased.
It has been reported that an Fe catalyst favors the
production of N2.18 Calcite, which was decomposed to
CaO, also acted as a catalyst for the hydrolysis of HCN
to NH3 as well as the decomposition of NH3 to N2.30 In
this study, species of minerals contained in the coals
did not appear to affect the fractions of HCN-N, NH3N, and N2-N. However, the fraction of N2-N, one of the
final products in the conversion reaction of N-containing
compounds, was roughly the same as that of NH3 and
HCN for the coal samples. Since the pyrolysis of the
model compounds did not give rise to N2, it is possible
that the conversion of HCN to N2 was accelerated by
minerals in the coals.
Conclusions
Yallourn, South Banko, Taiheiyo, Miike, and Hunter
Valley coals, as well as oxidized and hydrogenated
Yallourn coals, were pyrolyzed using a CPP at 5001040 °C. Quinoline and hydrogenated quinolines were
also used as model compounds.
(1) More than 90 mol % of the nitrogen atoms in the
raw coals were transferred to char and tar during the
primary pyrolysis step. The yield of gaseous products,
such as HCN, NH3, and N2, was less than 10 mol %.
(2) The heterocyclic structures in the coals were
partially hydrogenated by the Ru/Al2O3 catalyst. The
hydrogenation allowed for a more facile cleavage of the
heterocyclic structure during pyrolysis. Thus, the fraction of nitrogen atoms which remained in the char was
decreased, and that of the nitrogen atoms released as
gaseous compounds was increased.
(3) The yield of HCN-N during pyrolysis was increased, while that of NH3-N remained unchanged, by
hydrogenation. The formation of HCN was related to
the decomposition of heterocyclic structures such as
pyridines. The increase in HCN yield was attributed to
the conversion of heterocyclic rings into saturated rings
by hydrogenation.
OH, H2O, O
HCN 98 NH3
Acknowledgment. This study was financially supported by the Basic Research Association for Innovated
Coal Utilization (BRAIN-C) Program and the International Joint Research Program, sponsored by New
Energy and Industrial Technology Development Organization, Japan (NEDO). We also thank the Center of
Coal Utilization, Japan, and Japan Institute of Energy.
Useful discussions with Professor Masakatsu Nomura
of Osaka University are deeply acknowledged.
As shown in Figure 9, the fractions of HCN-N, NH3N, and N2-N remained unchanged as a result of H2O2
oxidization of the raw YL coal, while the yield of CO2
was increased. When heterocyclic compounds which
contained O-containing functional groups were pyrolyzed, the ratio HCN/NH3 was decreased by phenolic
hydroxy groups and was unchanged by carboxy groups.29
This is in agreement with the present result that the
H2O2 oxidation in the presence of 1-propanol mainly
EF990010P
(29) Hämäläinen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994,
73, 1894.
(30) Kasaoka, S.; Sasaoka, E.; Ozaki, A. J. Fuel Soc. Jpn. 1982, 61,
1086 (in Japanese).