Energy & Fuels 1995,9, 1028-1034
1028
Role of Iron Catalyst Impregnated by Solvent Swelling Method in Pyrolytic Removal of Coal Nitrogen Jun-ichiro Hayashi," Katsuki Kusakabe, and Shigeharu Morooka Department of Chemical Science and Technology, Kyushu University, 6-10-1,Hakozaki, Higashi-ku, Fukuoka 812-81, Japan
Michael Nielsen and Edward Furimsky Energy Research Laboratories, Canada Centre for Mineral and Energy Technology (CANMET), 555 Booth Street, Ottawa, Ontario, K1A OGl Canada Received June 1, 1995. Revised Manuscript Received August 18, 1995@
Organometallic iron precursors, ferrocene and ferric acetate, were impregnated into Illinois No. 6 (IL), Wyoming (WY), and Yallourn (YL) coals by solvent swelling technique in THF, ethanol, and a THF/ethanol binary solvent. Then iron-impregnated coals were pyrolyzed in a flow of helium a t atmospheric pressure in a fixed bed and a thermobalance. Conversion of coal nitrogen to N2 was 20, 38, and 30%, respectively, for original IL, WY, and YL coals. Iron formed from both precursors lowered the onset temperature of N2 evolution by 20-100 "C. When ferrocene was impregnated in coals a t a concentration of 1.7-1.8 wt % as Fe, nitrogen conversion was increased to 52, 71, and 68% for IL, WY, and YL coals, respectively. Ferric acetate impregnated into IL coal from THF/ethanol solution increased the nitrogen conversion much more than that from ethanol solution. The expansion of microporous coal structure by the swelling was essential for better dispersion of the catalyst precursor. The evolution of HCN as well as NH3 was effectively suppressed above 600 "C by the presence of iron but not influenced significantly by combinations of catalyst precursors and solvents. The increase in Nz yield was compensated by the decrease in nitrogen emitted as HCN and NH3 and in tar and char. The increase in CO evolution from the iron-impregnated IL coal at 600-800 "C was explained by catalytic rearrangement of aromatic structure of char, accompanying the removal of nitrogen as N2. In a range of 600-750 "C, the evolution of CO as well as N2 from the other coals increased remarkably with a significant decrease in C02 evolution, which was caused by iron-catalyzed C02 gasification in char micropores.
Introduction Chemical Forms and Thermal Behavior of Coal Nitrogen. Nitrogen is normally incorporated in coal at a concentration of 0.5-2 % wt and is emitted as nitrogen oxides (NO,) by combustion.l The initial stage of combustion is pyrolysis occurring in the coal matrix. Since nitrogen in pyrolysates is the major source of fuel NO, in combustion,1,2coal nitrogen as much as possible should have to be converted to N2 in the pyrolysis stage. The fate of coal nitrogen in pyrolysis depends on the distribution of nitrogen in N-containing functional groups, structures of which have been investigated by X-ray photoelectron spectroscopy (XPS).3-10 Kambara et a1.8 conducted rapid pyrolysis of 20 coals and investigated the relationship between the yield of N-contain* To whom all correspondence should be addressed.
Abstract published in Advance ACS Abstracts, October 1, 1995. (1)Pershing, D. W.; Wendt, J . 0. L. Ind. Eng. Chem. Process Des. Dev. 1979,18, 60. (2) Chen, S. L.; Heap, M. L.; Pershing, D. W. Fuel 1982,61, 1218. (3) Jones, R. B.; McCourt, C. B.; Swift, P. Proc. Int. Conf. Coal Science, Diisseldorf. (4) Perry, D. L.; Grint, A. Fuel 1983,62,1029. (5) Bartle, K. D.; Wallace, S. Fuel Process Technol. 1987,15, 351. (6)Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989,68, 1450. (7) Burchill, P.; Welch, L. S. Fuel 1989,68, 100. (8)Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993,7,1013. (9) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8,896. (10) Chen, J. C.; Niksa, S. Energy Fuels 1992,6, 264.
0887-0624/95/2509-1028$09.00/0
ing gases and the nitrogen functionality in coals. Quaternary nitrogen forms up to 15%of total nitrogen and was minor compared t o pyrrole and pyridine nitrogen which formed about 60 and 30% of total nitrogen, respectively. The amount of nitrogen converted to NH3 agreed well with that of quaternary nitrogen for bituminous coals. On the other hand, HCN, the most abundant nitrogen product in their experiment, originated from pyrrole and pyridine nitrogen. Nitrogen-containing compounds in rapid pyrolysis tar are derivatives of pyrrole, pyridine, and benz~nitrile.ll-~~ They are thermally less stable than PAHs having no heteroatoms but are much more stable than hydroxyls and ethers.13 Solomon et al.14found that the nitrogen content in rapid pyrolysis tar was equivalent to that in the parent coal. However, only 5% or less of coal nitrogen was converted to N2 by rapid pyrolysis of 20 coal samples at 1200 "C.8 The heating rate influences the nitrogen distribution in volatiles more strongly than overall conversion of coal nitrogen. Bassilakis et al.15 investigated the evolution of NH3 and HCN in a slow-heating pyrolysis (30 "C/min) (11)Nelson, P. F.; Kelly, M. D.; Warnat, M. J . Fuel 1991,70, 403. (12) Hayashi, J.-i.; Nakagawa, K.; Kusakabe, K.; Morooka, S. Fuel Process Technol. 1992,30,237. (13)Hayashi, J.-i.; Kawakami, K.; Taniguchi, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993,7,57. (14) Solomon, P. R.; Colket, M. B.; Carangelo, R. M. Fuel 1978,57, 749.
0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 6, 1995 1029
Pyrolytic Removal of Coal Nitrogen
of Argonne Premium coals using TG-FTIR technique. Nitrogen evolved as HCN was less than 5% of total nitrogen in the initial coals. On the other hand, NH3 yield was comparable to the quaternary nitrogen content in the original coal, as seen in the rapid pyrolysis result.8 Kelemen et al.9 reported that quaternary nitrogen was decomposed selectively t o other nitrogens in a mild pyrolysis at about 400 "C. The effect of heating rate was strong as observed for the Nz evolution. Stanczyk et a1.16 pyrolyzed a subbituminous coal at a heating rate of 30 "C/min. The N2 evolution began at 600-650 "C and exhibited two maxima at 750 and 1200 "C. The final yield of Nz reached 50%of total nitrogen. The yield of HCN was lower, and the yield of Nz was higher, in slow pyrolysis than in rapid pyrolysis. Therefore, in slow pyrolysis, pyrrole and pyridine nitrogens were reincorporated into more condensed and thermally stable aromatic compounds, and HCN was formed directly by the scission of the C-N bond. Catalytic Effect of Iron on Nitrogen Removal in Pyrolysis. Ohtsuka et al.17J8 precipitated ultrafine FeOOH particles onto a brown coal by adding Ca(0H)z in an aqueous solution of FeC13 and pyrolyzed the coal under a medium heating rate of 10 "C/s. The N2 yield of the iron-loaded coal at 900 "C was about 50%, which was much higher than 3% for the original coal, although the total volatile yield from both coals was equivalent. The selective increase in N2 yield was balanced by the decrease in nitrogen fixed in HCN, tar, and char. The enhanced Nz evolution cannot be explained only by the secondary conversion of N-containing volatiles t o N2. The direct formation of N2 from coal or char must be accelerated by the presence of iron. In another experiment,lg Ohtsuka and Furimsky prepared ultrafine iron particles beforehand and loaded them onto a subbituminous coal in acetone under ultrasonic irradiation. The ultrafine iron enhanced the nitrogen conversion during a C02 gasification. However, in pyrolysis under a helium atmosphere, the nitrogen conversion into Nz was not so pronounced, although the temperature where the N2 evolution began was lowered from 700 to 600 "C. A possible explanation of these apparently contradicting results is as follows: Brown coals contain much carboxyl groups, which act as cation exchange sites. Fe3+ ions introduced by the ion exchange are molecularly dispersed in the coal matrix. Meanwhile, ultrafine iron particles whose size was ca. 15 nm19 did not enhance the conversion of coal or char nitrogen to Nz significantly. It may be because the micropores in coal or char where pyrolysis reactions proceed are less than 1 nm. The above discussion leads to a concept that iron catalyst o r precursor particles should be located in micropores prior to pyrolysis to assure the contact between catalyst and coal a t molecular level. However, it is difficult t o apply the ion-exchange method in aqueous media to subbituminous and bituminous coals which are less microporous than are brown coals and have much less carboxyl groups. Recently, several research groups impregnated or(15) Bassilakis, R.; Zhao, Y.; Solomon, P. R. Energy Fuels 1993,7, 710. (16) Stanczyk, K.; Boudou, J. P. Fuel 1994,73, 940. (17) Ohtsuka, Y.; Mori, H.; Nonaka, K.; Watanabe, T.; Asami, K. Energy Fuels 1993,7, 1095. (18) Ohtsuka, Y.; Mori, H.; Watanabe, T.;Asami, K. Fuel 1994,73, 1093. (19) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995,9, 141.
Table 1. Elemental Composition of Coals" coal C H N O+Sb Illinois No. 6 (IL) 75.6 5.3 1.40 17.7 Wyoming (WY) 73.8 5.1 1.0 20.1 Yallourn (YL) 65.9 4.9 0.65 28.6
ash 13 6 2
Elements in wt % daf basis; ash in wt % dry basis. By difference. [I
ganometallic compounds as precursors of liquefaction catalyst into coals swollen in good solvents such as tetrahydrofuran (THF).20-22 However, the effect of swelling on the catalyst activity was not fully understood. In the present study, iron precursors, ferric acetate and ferrocene, were impregnated into three coals of different ranks swollen in organic solvents. The ironloaded coals as well as the raw ones were subjected t o slow pyrolysis of 10 "C/min t o 1000 "C under atmospheric pressure. The evolution rate of nitrogencontaining gases was continuously measured, and the effect of the incorporated iron species on their evolution rate was examined.
Experimental Section Coal Samples and Catalyst Precursors. Illinois No. 6 coal (IL), Wyoming subbituminous coal (WY), and Yallourn brown coal (YL) were pulverized, sized to 0.074-0.125 mm, and dried in a vacuum oven at 80 "C. Elemental compositions of these coals are listed in Table 1. Ferric acetate, Fe(0H)(CH&00)2, and ferrocene, Fe(CsH&, were used as the iron catalyst precursors. Ferric acetate was dissolved in ethanol or ethanobTHF ( 1 : l by weight) whereas ferrocene was dissolved in THF. Ferric chloride, FeC13, was also used t o load Fe3+into the YL coal by the ion-exchange method. Fe(NO& ~ ~ -it~ ~ was often used as iron catalyst for coal g a s i f i c a t i ~ n , but was not employed in this study to avoid incorporation of nitrogen originated from the nitrate. Catalyst Impregnation. About 3-4 g of coal sample was added to the solution containing a prescribed amount of the respective iron precursor. The solventkoal weight ratio was 3.0 for ethanol and 4.0 for THF and ethanol/THF mixed solvent. The mixture was quiescently kept in a flask for 12 h at 30 "C. The solvent was then slowly evaporated at 70 "C for 5 h in the flow of nitrogen at atmospheric pressure, followed by vacuum drying at 40 "C for 12 h. The amount of solvent remaining in the iron-impregnated coal was determined by the weight difference between the iron-impregnated coal and the total weight of initial coal and iron presursor, assuming that the iron precursor was not evaporated during the drying stage. Ferric acetate impregnated from ethanol and ethanobTHF solutions is respectively abbreviated as FeAc(Et0H) and FeAC(Et0WTHF) hereafter. Table 2 indicates the amounts of iron loaded into coals by the different methods. The initial mass of iron impregnated from ferrocene in THF was 3.7-3.8% of dry coal, and about 50% of ferrocene was desorbed during the pyrolysis below 300 "C. A thermogravimetric analysis of ferrocene, carried out separately with a heating rate of 10 "C/min, revealed that ferrocene was completely sublimated before the temperature reached 200 "C. This means that ferrocene, when introduced (20) Joseph, T. Fuel 1991,70, 139. (21)Artok, L.; Davis, A.; Mitchell, G. D.; Schobert, H. H. Energy Fuels 1993,7, 63. (22) Song,C.; Parfitt, D. S.; Schobert, H. H. Energy Fuels 1994,8, A1 2 ---.
(23) Furimsky, E.; Sears, P.; Suzuki, T.Energy Fuels 1988,2,634. (24) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.; Tomita, A. Energy Fuels 1989,3, 686. (25)Yamashita, H.; Yoshida, S.; Tomita, A. Energy Fuels 1991,5, 52.
1030 Energy & Fuels, Vol. 9, No. 6, 1995
Hayashi et al.
Table 2. Combinations of Iron Precursor and Solvent amount of iron loaded coal iron precursor solvent (wt % on dry basis) IL FeAc" EtOH 4.17 FeAc EtOWTHF 1.95 1.75 ferrocene THF WY FeAc EtOH 3.77 ferrocene THF 1.74 YL Fe3+(ion exchange) HzO 1.96b FeAc EtOH 3.86 2.03 FeAc EtOWTHF ferrocene THF 1.80 a Ferric acetate, Fe(OH)(CH3COO)z. Incorporated Fe3+ was completely removed from the coal in 5 N HC1. The amount of Fe3+ leached was determined by ICP-AES.
Temperature ["C] m 4 0 0 6 0 0 8 0 0 1 M X )
0.02
-
0.05
,ri,
t
Ferrocene
0.04
into the coal, was tightly trapped and molecularly dispersed in the coal network. The swelling ratios of the raw coals in the solvents were measured at 30 "C by the method of Green et aLZ6 Swelling time was 48 h. Fixed-Bed Pyrolysis. The raw and iron-impregnated coals were pyrolyzed in a quartz tube fixed-bed reactor of 10 " mm i.d. set in a vertical position. The reactor was externally heated using an electric furnace. A coal sample of ca. 0.6 g was supported on a tightly packing quartz wool plug. The pyrolysis was carried out in an atmospheric flow of helium (purity 99.9999%)at a rate of 0.6 Umin and a heating rate of 10 "C/min from 20 to 1000 "C. The temperature was held at 1000 "C for 40 min. F'yrolysis gas was continuously introduced into an FT-IR gas analyzer (B&K Model 13011, and the concentration of H20, CO, C02, NH3, and HCN was determined. The pyrolysis gas was also sampled at 2.5 or 3 min intervals and sent t o micro-TCD gas chromatograph (MTI Model 200). Then the concentration of Nz, CO, and COZwas determined with Molecular Sieve 5A and Hayesep A columns connected in series. The total conversion of coal nitrogen into volatiles was calculated from char yield and nitrogen content in the char. The concentration of N2 and NH3 in pyrolysis gas was typically in the range 0-200 ppm, while that of HCN was at most 10-30 ppm. Thermogravimetry. Raw and iron-impregnated coals were subjected to thermogravimetric analysis (TGA)in helium. Samples were heated to 1000 "C at a rate of 10 "C/min. The gas flow rate was fixed at 0.03 L/min. Nishiyama et al.24 reported that iron incorporated into a brown coal by the ionexchange method promoted the carbonization above 400 "C. In the present study, the catalytic activity of iron impregnated by the swelling method was evaluated from the degree of coal volatilization that would be suppressed if iron was allowed to access the internal surface of coal. The TGA in COZ atmosphere was also carried out. The degree of coal gasification with C02 as an oxidant was used to evaluated the catalytic activity and dispersivity of iron catalyst in the organic matrix of coal.
Results and Discussion N2 Evolution. Figure la-c shows evolution profiles of Nz from raw and iron-impregnated coals. As indicated in Figure l a , the N2 evolution from IL coal was obviously promoted by iron impregnated from ferrocene and ferric acetate. The temperature where the N2 formation started was about 600 "C for the ironimpregnated coals and was lowered by 100 "C compared with that for the raw coal. However, the impregnation hardly changed the temperature at which the evolution rate became a maximum, 5". The impregnation of ferrocene promoted the N2 evolution most significantly,
(26) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935.
1MX)
1
...
00
Ferrocene
d O.OlO[
e
Raw.
L
O.ooO0
_ _ . ) . . . . . . . . . I . .
20
40
Eo
80
100
120
140
Time [min]
Figure 1. Evolution profiles of Nz from raw and ironimpregnated coals: (a) IL, (b) WY, (c) YL.
and the final NZ yield was 0.72 w t % which was 2.6 times higher than that of the raw coal. On the other hand, the impregnation from ferrice acetate in ethanol was far less effective than that from ferrocene in THF, although the initial amount of iron loaded from ferric acetate was 2.4 times that of ferrocene as shown in Table 2. Ferrocene and ferric acetate were decomposed at 250-400 and 250-350 "C, respectively, as confirmed by the evolution of cyclopentadiene from the former and acetic acid from the latter. When N2 evolution begins at 600 "C, iron is considered to exist as oxides and a metal.17J8 Fe hydroxide may be present in case of ferric acetate. However, the difference in the catalytic activity may be due to degree of dispersion rather than species of iron. Actually, the reactivity of iron catalyst formed from ferric nitrate was significantly changed by its degree of dispersion in a brown coal.25 The swelling ratio of IL coal in ethanol was 1.1at 30 "C, while that in THF was 1.7 at the same temperature. The swelling ratio is a measure for the expansion of microporous coal structure and has a strong relationship with the penetration of catalyst precursors into the coal structure.20-22 The size and shape of precursor molecules should be also taken into account. Since the swelling ratio of IL coal was 1.5 in the mixed solvent, the diffusion of ferric acetate into the coal matrix was much increased than in ethanol. As indicated in Figure l a , the Nz evolution was actually enhanced. Table 3 shows that the iron impregnation increased the N2 evolution rate. To compensate for the amount of iron, the increase in molar yield of N2 is normalized
Pyrolytic Removal of Coal Nitrogen
Energy & Fuels, Vol. 9,No. 6, 1995 1031
Table 3. Final Yields of Nitrogen- or Oxygen-Containing Inorganic Gases ( w t % Raw Coal on daf Basis) coal precursor (solvent) IL none FeAdEtOH) 0.53 FeAc(EtOH!THF) ferrocene w none FeAc(Et0H) ferrocene n none Fe3+(ion-exchange) FeAc(Et0mHF) ferrocene a Yield
N2 0.28 0.006 0.67 0.73 0.38 0.60 0.71 0.20 0.32 0.34 0.44
HCN" CO 0.024 5.76 0.009 0.008 0.018 0.015 0.013 0.033 0.021 0.016 0.020
4.14 3.07 6.30 6.38 8.89 10.3 10.0 13.6 15.6 14.5 15.2
Temperature ['C]
m 400 600 e00 loo0 1Mx 0.003 3
C02 1.47
n 2.09 2.01 6.47 7.49 6.94 10.8 11.6 11.6 11.5
of nitrogen evolved as HCN.
to moles of iron loaded. The ratio of the normalized N2 evolution for the FeAc(Et0H)-, FeAc(EtOH/THF)-, and ferrocene-impregnated IL coals was 0.24,0.80, and 1.0, respectively. Thus the overall activity of iron from FeAc(Et0WTHF) was about 3.3 times higher than that from FeAc(Et0H). The impregnation of ferrocene and FeAc promoted the N2 formation also from the WY coal as indicated in Figure lb. For the raw WY coal, the N2 evolution started a t 640 "C and passed two peaks at 730 and 940 "C. For the iron-impregnated WY coals, the N2 evolution began a t 580 "C and reached the first peak at 640 "C. The highest Tm, appeared at 840,880, and 920 "C for the FeAc-impregnated, ferrocene-impregnated, and raw WY coals, respectively. Figure ICexhibits the N2 evolution profiles of YL coal. Nitrogen evolution suddenly began at about 580 "C, and the evolution rate quickly increased toward the first maximum at 620-630 "C. The evolution profile was more complex than that of IL and WY coals. The value of Tm, was different for each coal sample prepared differently. The introduction of Fe3+ions by ion exchange was as effective as the impregnation of FeAc(Et0H) and FeAc(EtOWTHF) for promoting the N:! evolution and was less effective than that of ferrocene. A n equivalent activity between the precursors Fe3+and FeAc suggests that FeAc is at least partially incorporated with the YL coal matrix via ion exchange, which is feasible in ethanol. The evolution rate of N2 from the ferroceneimpregnated YL coal was larger than that from any other YL coals at the temperature range of 580-900 "C. For the ferrocene-impregnated YL coal, three peaks were observed at 620, 740, and 900 "C. For other YL coals only two peaks appeared at 600-800 "C. The result that the ferrocene-impregnated YL coal showed the highest N2 yield is not explainable a t the present moment, but it is clear that iron species formed from ferrocene behave differently in the N2 formation process above 800 "C from those initially bound to carboxyl groups. Perhaps ferrocene interacted more extensively with organic matter of coal because of its low polarity. On the other hand, FeAc and Fe3+ could also interact with mineral matter. The ferrocene-impregnated YL and WY coals gave similar N2 evolution profiles. This suggests that chemical forms of iron derived from ferrocene were similar in these coals. The final yields of N2, HCN, CO, and C02 from all coals tested were summarized in Table 3. HCN Evolution. Figure 2a-c shows evolution profiles of HCN. For the IL coal, the impregnation of iron
.E
4L
n
0.0015 Ferrocen
d
/Raw
.,; .
,FeAc(EtOH)
0.0010
0.0000
FeAc(THFE1OH)
0
20
40
€0
Bo
100
120
140
Time [min]
Figure 2. Evolution profiles of HCN from raw and ironimpregnated coals: (a) IL, (b) WY, (c) YL.
somewhat promoted the HCN evolution below 600 "C but considerably suppressed it above 600 "C. The temperature region where the HCN evolution was suppressed coincided with that where the N2 evolution was accelerated. This indicates that the generation of HCN as well as that of N2 was influenced by the presence of iron. Two different explanations are possible for the reduction of HCN yield. One is that iron catalyst promotes the secondary reaction of HCN t o N2 while HCN diffises through the porous structure of coal. The other is that the HCN formation itself is suppressed by iron. The HCN evolution from the iron-impregnated coals was not affected by combinations of precursor and solvent. The secondary reaction of HCN in the micropore system is therefore more likely. However, the conversion of HCN precursor to N2 little contributes to the increase in Nz yield in the presence of iron, because nitrogen evolved as HCN from the IL raw coal was 1.7% of total nitrogen and was quite smaller than N2 evolution which was 20% of total nitrogen. The suppression of HCN evolution was also observed for the WL and YL coals. The evolution rate of HCN from the raw W coal showed two peaks a t 450-600 and 700-900 "C. The peak appearing at the lower temperature was much wider than that at the higher temperature. The formation of HCN above 700 "C was effectively suppressed in the presence of iron, but that below 600 "C remained unchanged. Impregnated iron activated the N2 evolution at about 600 "C, which was higher than the low-temperature peak of HCN evolution. This can be a main reason for less significant
1032 Energy & Fuels, Vol. 9, No. 6,1995
Hayashi et al. IL Raw
-.-
. f
FeAc(EtOH)
E
FeAc(EtOHflHF)
Y
a,
c
9
Ferrocene
0.01
C
0 .c
3 0
0
1 .o
0.5
1.5
w 0.00
FeAc(EtOH) FerroceneFHF) 0
1\
/Raw
0.2
0.6
0.4
0.8
1.2
1.0
YL Raw
0
&
Ion Exchange n nn
"-0
20
40
60
80
100
120
140
Time [min]
FeAc(Et0HfrHF) Ferrocene(THF)
Figure 3. Evolution profiles of NH3 from raw and ironimpregnated coals: (a) IL, (b) W.
reduction of HCN yield for the WY coal than that for the IL coal, from which the HCN evolution began at 600 "C. However, it is also found that iron suppressed the HCN formation from the YL coal to some extent even at 300-450 "C. This indicates that the impregnated iron influenced the pyrolysis behavior of the coal in that temperature region where tar formation was prevailing. The extent of HCN reduction in the pyrolysis of YL and WY coals was little affected by impregnation solvents as seen with IL coal. N H 3 Evolution. Figure 3a,b exhibits evolution profiles of NH3 from IL and WY coals. Gaseous NH3 has two IR absorption peaks at 965 and 930 cm-l. In the present experiment, the concentration of NH3 in pyrolysis gas was determined from the absorption at 941 and 981 cm-l. Unfortunately, this band included some absorption due t o some species, probably such as dienes and acetate, were evolved in the temperature range of 400-550 "C. Thus, quantitative discussion is only possible on the NH3 evolution above 600 "C,where the reliability of the FT-IR measurement was confirmed. The profiles evidently show that iron derived from both precursors suppressed the NH3 evolution in this temperature range. Nitrogen Distribution in Pyrolysis Products. Figure 4 shows the nitrogen distribution in pyrolysates of the raw IL, WY, and YL coals. The higher the N2 yield, the lower the nitrogen content in char. The increase in N2 yield due to the impregnation of iron was compensated by the decrease in nitrogen emitted as HCN and NH3 and in tar and char. This suggests that the N2 yield in the presence of iron catalyst is promoted by (1) secondary conversion of HCN, NH3, and Ncontaining tar to N2 in coal micropores, (2) conversion of HCN, NH3, and N-containing tar precursors to N2forming precursors, and (3)additional conversion of char nitrogen to N2. At present, we cannot fully specify the mechanism. As a future work, chemical forms of nitrogen as well as iron species should be determined in each reaction stage from coal to char. To understand the role of iron in the pyrolysis, however, evolution rates
0
0.2
0.8
06
0.4
Nitrogen content [wt%, daf raw coal]
Figure 4. Nitrogen distribution in pyrolysis products. 16
12
a
4
0 100
300
500
700
900
700
900
Temperature ["C]
100
300
500 Temperature ["C]
Figure 5. Difference in char yield between raw coal and ironimpregnated coals a t 100-900 "C in thermogravimetric analysis a t a heating rate of 10 "C/min under helium. Char yield from IL raw coals a t 900 "C are 49.5 and 63.5 wt % on dry basis, respectively.
of tar and inorganic gases give useful information as stated below. Thermogravimetry. Figure 5 indicates the difference in char yield in pyrolysis between iron-impregnated
Pyrolytic Removal of Coal Nitrogen
Energy & Fuels, Vol. 9,No. 6, 1995 1033 Temperature ['C] 2-30
BM) 800 1003 1000 0.32 4M)
1 : IL
-.-
-.f
m
e .
-
,
,
f
pl
0.8l""""''""'"....l-.l----r?----.
FeAc(Et0H)
co
o.6
I
22
e
..
0.4
C
.P
.2
s
Lu
0.2
w 0.0
20
40
60
80
100
120
140
Time [min]
0.0 0
x)
40
60
80
100
120
140
Time [min]
Figure 6. Evolution profiles of C O and C 0 2 from raw and iron-impregnated IL coals.
Figure 7. Evolution profiles of C O and C 0 2 from raw and iron-impregnated WY coals.
coal and raw coal.
continues to a higher temperature range. Cleavage of aromatic C-C, C-H, and C-0 bonds in char enhances rearrangement of aromatic structure and leads to the CO formation. The N2 'evolution may be induced by catalytic carbonization that accompanies the removal of heteroatoms. This mechanism is valid if there is no specific interaction between iron and char nitrogen. The increase in CO evolution was more pronounced for the ferrocene-impregnated IL coal than that for the FeAcimpregnated IL coal in spite of the absence of oxygen in ferrocene. This implies that ferrocene is more effective as the iron precursor than FeAc for removing heteroatoms such as oxygen and nitrogen during carbonization. The promotion of the CO generation was also observed in the pyrolysis of YL and WY coals as exhibited in Figures 7 and 8. The evolution profiles were quite different from those of IL coal. Iron impregnated into the YL coal little affected the CO evolution rate below 580 "C but increased it remarkably at 580-700 "C as shown in Figure 8. The same effect was observed in the TGA experiment. The Tma of CO evolution from the iron-impregnated YL coals precisely agreed with the first Tm, of Nz evolution. The C02 evolution rate quickly decreased at the same temperature. The discontinuous change in the C02 formation suggests the secondary conversion of C02 in micropores of char. The role of iron catalyst in the C 0 2 gasification is expressed by the following redox reaction^.^'-^^
AYchar= (char yield from iron-impregnated coal) (char yield from the raw coal) The char yield is expressed in the unit of kg per 100 kg of raw coal, which is taken as the base because ironimpregnated coal is heavier than raw coal. For instance, the IL coal impregnated with FeAc(Et0H) contained 14.2 kg of FeAc and 0.7 kg of adsorbed ethanol, and the initial mass was 114.9 kg per 100 kg of raw coal. As shown in Figure 5a, all precursors suppressed the weight loss of the IL coal in a range of 350-420 "C where the major volatile product was tar. This effect could not be observed if iron precursors were located on the external surface of coal. This result supports the significant reduction of tar nitrogen revealed in Figure 4. FeActEtOHPTHF)suppressed the tar formation more significantly than did the FeAc(Et0H) despite that the amount of iron loaded from FeActEtOHPTHF) was less than half that from FeAc(Et0H). This indicates that iron was dispersed more finely when it was loaded in the binary solvent than in ethanol. On the other hand, well-dispersed iron promoted the weight loss of coal above 600 "C. This is due t o the promotion of the CO evolution, which will be discussed in the next section. In the pyrolysis of the YL coal, all iron and precursors decreased the weight loss rate in a wide temperature range of 300-600 "C that included the tar formation period. At 600-650 "C, the AYchar was quickly decreased due to the rapid increase in CO evolution. Iron loaded from ferrocene partly suppressed the overall weight loss above 700 "C,while FeAc and ion-exchanged Fe3+ showed very little effect. Evolution Behavior of Inorganic Gases. As shown in Figure 6, iron formed from FeAc and ferrocene enhanced the evolution of CO and N2 from the IL coal in a tfmperature range of 600-800 "C. In general, tar formation is almost complete by 600 "C and carbonization caused by condensation among aromatic rings
+ C = F e + CO Fe + C 0 2 = FeO + CO FeO
(1) (2)
The rapid change in evolution rates of N2, CO, and C02 (27)Mckee, D.W.Carbon 1974,12,453. (28)Huttinger, K.J. Fuel 1983,62,166. (29)Ohtsuka, Y.; Tomita, A.; Tamai, Y. Energy Fuels 1987,I , 32. (30)Kasaoka, S.;Sakata, Y.; Yamashita, H.; Nishio, T. Int. Chem. Eng. 1981,21,419. (31)Inui, T.;Otowa, T.; Okazumi, F. Carbon 1986,23,193. (32)Harmann, G.;Huttinger, K. Carbon 1986,24,429.
Hayashi et al.
1034 Energy & Fuels, Vol. 9, No. 6, 1995 Temperature ["C] 200
0
800
600
4M)
loo0
0.6
10
.-
fZ
1 YL .
Ferrocene
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U 06-
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.
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-
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2
,
" Y
0
10
20
30
80
50
40
70
80
90
1
100
Time [min]
Figure 8. Evolution profiles of CO and COz from raw and iron-impregnated YL coals.
tion at 500-900 "C was investigated by pyrolyzing the raw and ferrocene-impregnated YL coals. Iron enhanced the Hz formation above 700 "C, and the H2 yield from the raw YL coal was 0.8 wt % a t 800 "C and 1.3 wt % at 900 "C. In the presence of iron, however, the yield increased to 1.2 wt % at 800 "C and 1.9 wt % at 900 "C. This suggests that iron from ferrocene promoted the condensation reactions associated with removal of heteroatoms. C02 Gasification Test. Figure 9 exhibits the rate of weight loss in COSfor the IL coals. Each profile was in a good agreement with that obtained in the TGA in He below 600 "C. The C 0 2 gasification began at about 700 "C for the iron-impregnated coals and a t 850 "C for the raw coal. The gasification was most effectively catalyzed by iron formed from ferrocene. Iron from FeAc(Et0WTHF)was less active, but T,,, 920 "C, was the same as that of iron from FeAc(Et0H). The gasification rate of the FeAC(THF)coal increased even above 1000 "C. Therefore, the difference in reactivity of FeAc(THF/EtOH) and FeAc(Et0H) coals is attributed to the variation in dispersion of iron. The C 0 2 gasification was also carried out for the YL coal. The conversion of raw coal was finished a t 950 "C, and the gasification of the YL coal impregnated from ferrocene or FeAc was completed at 880 and 920 "C, respectively. The role of iron in the CO2 gasification of the YL coal was less significant than that for the IL coal.
Conclusions FeAc(THF/EtOH)
i,
5
FeAC(Et0
2
.-
e ? O
y"..
...
.../ . ./'
Raw
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I'
,-..*,'
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n
700
200
300
400
5W
600
700
BOO
900
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Temperature YC]
Figure 9. Conversion rate of raw and iron-impregnated IL coals heated a t a rate of 10 "C/min in COz.
in a narrow temperature range may be explained as follows: C 0 2 generated by pyrolysis is converted to CO, oxidizing iron catalyst. Then, iron oxide gasifies the neighboring carbon atoms to CO. Nitrogen atoms are converted to N2 during the gasification. Ohtsuka and Furimskylg showed that char nitrogen is quantitatively converted t o N2 in C 0 2 gasification above 600 " C . Similar rapid changes in CO, COZ and NO evolution rates were also observed in the pyrolysis of the raw YL coal, but not so strongly. The YL coal originally contains Ca2+and Mg2+bound to carboxyl groups. These alkaline earth metal cations are active for C02/H20 gasification and enhance the COz gasification in micropores of the YL raw coal. The above discussion partially explains the promotion of N2 evolution in the presence of iron catalyst. The Nz evolution rate above 750 "C from the ferrocene-impregnated YL coal was more than that from any other YL coals. Thus the promotion of N2 evolution above 700 "C is explained by the increase in other inorganic gases. Similarly to CO, hydrogen is a major volatile in high-temperature pyrolysis and is formed by the condensation among aromatic rings. The H2 evolu-
Ferrocene and FeAc were impregnated into IL, WY, and YL coals in THF, EtOH, and THF/EtOH solutions. Iron derived from the precursors enhanced the N2 evolution from coals and lowered the reaction temperature of rapid pyrolysis by 20-100 "C. The Nz yield of the ferrocene-impregnated IL, WY, and YL coals was respectively 2.6, 1.9, and 2.1 times higher than that of the original coals. The N2 yield from IL coal impregnated with FeAc was much increased when the solvent was replaced from EtOH to THF/EtOH. This is explained by the expansion of the microporous coal structure and degree of catalyst dispersion. The evolution of HCN as well as NH3 was effectively suppressed above 600 "C, irrespective of combination of catalyst precursors and solvents. The increase in N2 yield was canceled out by the decrease in nitrogen emitted as HCN and NH3 and in tar and char. The promoted evolution of CO from the IL by iron at 600-800 "C indicates the enhancement of the rearrangement of aromatic structure of the char that proceeded with the removal of nitrogen as N2. The CO evolution as well as N2 evolution from the YL and WY coals was significantly promoted in the presence of iron at 600-750 "C, where the C02 evolution was remarkably suppressed. Thus the C02 gasification was enhanced by the iron catalyst dispersed in micropores of char, and carbon and nitrogen in char were converted to CO and N2, respectively.
Acknowledgment. This study was conducted under the Japan-Canada Joint Research Program "Developing Advanced Coal Utilization Processes to Minimize Environmental Impact" (Grant No. 04044086) by the Ministry of Education, Science and Culture, Japan. EF9501020
