Effects of Coal Structures on Denitrogenation during Flash Pyrolysis

Yallourn, Taiheiyo, South Banko, Miike, and Hunter Valley coals were pyrolyzed ... raw Yallourn coal were partially changed by hydrogenation over a Ru...
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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

Energy Fuels 1999.13:934-940. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 01/25/19. For personal use only.

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: smorotcf@ 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. Ha¨ma¨la¨inen 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.; Ha¨ma¨la¨inen, 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) Ha¨ma¨la¨inen, 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

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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

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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) Ha¨ma¨la¨inen, 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).