Energy & Fuels 1996,9, 1038-1042
1038
Steam Gasification of Coals with Calcium Hydroxide Yasuo Ohtsuka" and Kenji Asami Research Center for Carbonaceous Resources, Institute for Chemical Reaction Institute, Tohoku University, Sendai 980-77, Japan Received June 7, 1995@
Steam gasification of 16 coals with different ranks and sulfur contents was carried out isothermally at 873-973 K in the presence of Ca(OH)2with a thermobalance. The catalyst at 5 wt % Ca promotes the gasification of all the coals. The calcium on low-rank coals giving moderate reactivities without catalyst shows large rate enhancement, which leads to complete gasification within 30 min at 973 K and lowers the gasification temperature by 110-150 K. The finely dispersed Ca species is formed on the chars after devolatilization of low-rank coals with Ca(OH)2,and responsible for high catalytic activity. The calcium catalyst is less effective with high sulfur coals, and some of the sulfur evolved on coal devolatilization is captured by the calcium, but Cas loaded onto brown coal is still active in the gasification. Apparent activation energies are unchanged or slightly decreased by Ca(OH)2 addition in most cases. The catalytic effect is discussed in terms of catalyst dispersion, sulfur poisoning, and inherent minerals.
Introduction Many studies have been carried out on the Cacatalyzed gasification of coal with steam and C02. In most cases, water-soluble calcium compounds, such as the acetate, nitrate, and chloride, are used as catalyst precursors because of availability to the addition method of impregnation and ion exchange. Since inexpensiveness and abundance are indispensable requirements for catalyst sources from a practical point of view, however, the use of CaO, Ca(OH)2,and CaC03 would be suitable. When these compounds are physically mixed with coal, the gasification activities are generally quite 10w,l,~ though improved somewhat a t high temperatures of %1200 K and at elevated pressure.' It has been reported that, when air-oxidized coal samples are soaked in saturated Ca(OH)2solution, the incorporated Ca is active in steam.3 In the previous study, we have found that the Ca2+ ions in saturated Ca(OH)2 solution can readily be exchanged with COOH groups when simply mixed with brown coal at room t e m ~ e r a t u r eand , ~ the Ca-exchanged coal exhibits large reactivity in steam at low temperatures of ~ 9 5 0K.4,5 We have also shown that the effectiveness of Ca(OH)2 in the steam gasification is the same as those of calcium nitrate and a ~ e t a t e Further, .~ it is noteworthy that the exchanged Ca prepared from Ca(OH12 shows the same catalytic effect as ion-exchanged Na in the pressurized fluidized bed gasification of brown coal with steam.6 Ca(OH12 would thus be promising as a catalyst raw material. Abstract published in Aduance ACS Abstracts, October 15, 1995. (1)Haynes, W. P.; Gasior, S.J.; Forney, A. J. in Coal Gasification;
@
Massey, L. G., Ed.; Adv. Chem. Ser. 131; American Chemical Society: Washington, DC, 1974; pp. 179-202. (2) Johnson, J. L. Catal. Rev. Sci. Eng. 1976,14, 131-152. (3) Lang, R. J.; Neavel, R. C. Fuel 1982,61, 620-626. (4) Nabatame, T.; Ohtsuka, Y.; Takarada, T.; Tomita, A. J . Fuel SOC. Jpn. 1986,65,53-58. (5) Ohtsuka, Y.; Tomita, A.Fuel 1986,65,1653-1657. (6) Takarada, T.; Ohtsuka, Y.; Tomita, A. J . Fuel SOC.Jpn. 1988, 67,683-692.
The present work therefore focuses on examining the catalytic effect of Ca(OH)2on the steam gasification of many coals with different ranks and sulfur contents, and on making clear the influences of calcium dispersion, sulfur poisoning, and inherent minerals on the rate increase by catalyst addition.
Experimental Section Coal Sample. Sixteen coals with different ranks from different countries were used in the present study. All the coal samples were air-dried, ground, and sieved to the particles with size fraction 1-2 mm. The ultimate and proximate analyses are shown in Table 1, where the carbon and sulfur contents range 67-84 and 0.3-5.1 w t % (daf), respectively, and each coal is designated by a code name. Catalyst Addition. Powdery Ca(0H)Z was of research grade from J. T. Baker Chemical Co., the assay being 97%. About 10 g of coal particles was first added into an aqueous slurry of Ca(OH)Z, and then they were homogenized by kneading for 30 min. The resulting slurry was dried at 380 K in a stream of Nz. The dried sample was pelletized and then cut down to the particles with the size of 1-2 mm for convenience of gasification experiments. Ca loading was nominally 5.0 wt % as metal. The atomic absorption spectroscopy (AAS)after leaching of the metal with hot HC1 revealed that the actual loadings in all the Ca-loaded samples were 4.3-5.0 wt %. The amounts of the calcium inherently present in the original coals were also determined by the same method. In order to examine the effect of sulfur on Ca-catalyzed gasification, Cas of >99% pure in place of Ca(0H)Z was added onto YL coal with the lowest sulfur content in the same manner as above. The AAS analysis showed the actual Ca loading of 4.8 wt %. However, the total sulfur content in the Cas-loaded coal, 2.2 wt %, was lower than the calculated amount from Ca loading. The observation indicates the transformation of some C a s to Ca(0H)Z by hydrolysis in the catalyst addition step. Steam Gasification. The reactivities of the original and Ca-loaded coals were isothermally measured by use of a thermobalance (Shinku-Riko, TGD-5000) attached with infrared lamps. About 20 mg of the coal sample mounted on a quartz cell was heated at 300 Wmin under a flow of steam (66 kPa) diluted with high pure Nz, and soaked for 120 min
0887-0624/95/2509-1038$09.00/00 1995 American Chemical Society
Energy & Fuels, Vol. 9,No. 6, 1995 1039
Steam Gasification of Coals with Ca(0H)Z
Table 1. Ultimate and Proximate Analyses of Coals coal Leigh Creek Yallourn Shori Lower Wilcox South Beulah Colowyo King Cannel Illinois No. 6 Kentucky No. 12 Leopold Miller Blend Grose Valley Blair Athol Stigler Pittsburgh Miike
code LC
n SH LW SB
cw
KC IL KT LP MB GV BA SG PB MK
count@ AUS AUS CHI USA USA USA USA USA USA GER AUS AUS AUS USA USA JPN
ultimate analysis, wt % (daf) H N S
C 66.5 67.1 67.8 68.8 71.6 74.0 74.2 77.0 78.2 79.9 81.1 81.7 81.9 82.0 83.4 83.6
5.3 4.8 4.5 5.1 4.8 5.0 7.5 5.2 5.3 5.8 5.2 5.1 4.6 5.0 5.5 6.6
0.4 0.8 0.8 1.3 1.4 1.9 1.8 1.5 1.8 1.4 1.8 1.4 2.2 1.9 1.5 1.2
3.3 0.3 1.0 1.0 2.9 0.4 1.9 3.6 2.6 1.2 1.3 0.4 0.4 5.1 2.4 3.7
0 24.5 27.0 25.9 23.8 19.3 18.7 14.6 12.7 12.1 11.7 10.6 11.4 10.9 6.0 7.2 4.9
proximate analysis, wt % (db) ash VM FC 8.7 0.9 11.1 11.0 13.7 6.3 22.2 10.9 13.6 3.3 15.6 17.2 6.5 11.1 6.2 21.4
50.1 55.2 40.3 45.5 38.6 36.6 52.1 38.9 34.1 40.0 30.1 30.7 27.9 27.8 35.8 35.3
41.2 43.9 48.6 43.5 47.7 57.4 25.7 50.2 52.3 56.7 54.3 52.1 65.6 61.1 58.0 43.3
"AUS, Australia; CHI, China; USA, United States of America; GER, Germany; JPN, Japan. 100
75
50
8 25
6 0
30
60
120
Reaction time, min
Figure 1. Steam gasification profiles at 973 K for several coals without catalyst.
at a constant temperature (873-1073 K). The coal was first devolatilized on heating, and after the completion within a few minutes the resulting char was gasified with steam. The reactivity in the step of char gasification will be described throughout this paper. Two indexes of char conversion and specific rate are used to describe the reactivity of char. Char conversion is expressed as weight percent on a dry ash-free, catalyst-free basis. Specific rate is defined as the gasification rate per unit weight of remaining char and given as the average value in the conversion range of 530 wt %. The rate will be denoted as Rca or R,,,, with or without catalyst, respectively. X-ray Diffraction. X-ray diffraction (XRD)measurements of Ca-bearing chars were made with Ni-filtered Cu K a radiation to clarify the crystalline form and dispersion state of calcium catalyst on coal devolatilization preceding char gasification. The samples for XRD were prepared with a thermobalance in the same manner as above; the Ca-loaded coals were heated at 300 Wmin up to 973 K i n a stream of pure Nz, and after soaking for 10 min the resulting chars were quenched to room temperature. Sulfur Analysis. In order to clarify the degree of sulfur capture by Ca catalyst on coal devolatilization, the total sulfur contents in the original and Ca-bearing chars were determined by the conventional combustion method with an on-line infrared spectrometer. The chars were prepared by pyrolysis at 973 K under a flow of pure Nz with a quartz-made fixed bed reactor. The sulfur retained in char is calculated from char yield and the total sulfur content before and after pyrolysis. The atomic ratio of the sulfur captured to the calcium added is also calculated by using both the sulfur and calcium contents in the Ca-loaded coal and the sulfur retention in the original and Ca-bearing char.
Reaction time, min
Figure 2. Steam gasification profiles at 973 K for Ca(0H)zloaded coals.
Results Char Conversion. Char conversion at 973 K of several coals without catalyst is plotted as a function of reaction time in Figure 1. LC, LW, and SB coal showed higher conversions, whereas the coals with carbon contents of '75 wt %, for example, IL, MB, and BA coal, gave lower reactivities. Figure 2 shows the change in char conversion for Cacatalyzed gasification with reaction time. The catalyst promoted the gasification of all the coals used in the present study. However, the catalytic effect depended strongly on the coal type; the comparison of Figures 1 and 2 revealed that the reactivity sequence among the coals was quite different in the uncatalyzed and Cacatalyzed gasification. As is seen in Figure 2, the calcium on KC, YL, or SH coal giving moderate reactivity without catalyst showed larger catalytic effect, which achieved complete gasification of the char within 30 min. Gasification Rate. The relationship between Rnon, or Rca at 973 K and C % in parent coal is illustrated in Figure 3, where the specific rate for YL coal with Cas instead of Ca(OH12 is also given. In the absence of the catalyst, lower rank coals tended to have larger reaction rates, but there was no linear relationship between R,, and C % of < 75 w t %. Larger reactivities were observed for LC and LW coal with higher contents of inherent Ca of 1.0 and 1.4wt %, respectively. It is well-known that alkali and alkaline earth metal cations naturally present in low-rank coals can determine the gasification reactivitie~.~88
1040 Energy & Fuels, Vol. 9, No. 6, 1995 0
Ohtsuka and Asami
0 No catalyst 0 Ca(OH)2
A CaS
f a'-,
0
;I $
0
0.
0 0
4
0 0
Carbon in coal, wt%(daf)
Figure 3. Relationship between specific r a t e at 973 K a n d carbon content in coal.
1
10
G
I
OKC
1/T
yL
lo3, 1/K
Figure 5. Arrhenius plots for uncatalyzed a n d Ca-catalyzed
I
I
X
gasification of t h r e e coals.
Table 2. Apparent Activation Energies and the Lowering in Gasification Temperature by Calcium Addition
AE: kJ/mol
Total sulfur in coal, wi%(daf)
~
~~
~~
(7) Hippo, E. J.; Walker, P. L., Jr. Fuel 1976, 54, 245-250. ( 8 ) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1986, 64, 1438-1442.
no catalyst
5wt%Ca
AT,bK
n
160
155
150 145
150
SH LW SB
cw
Figure 4. Difference in specific r a t e at 973 K w i t h a n d without catalyst as a function of total sulfur content in coal.
As is seen in Figure 3, Rca for Ca(OH)2 was larger than R,,,, in the whole range of C %. When Rca was compared among Ca(OH)z-loaded coals, it was much higher for lower rank coals with C % of (75 wt %; KC coal with 74 wt % C gave the highest rate followed by YL coal with 67 wt % C. YL coal showed the largest ratio of Rca/Rnone of 25. When CaS in place of Ca(OH)2 was used with YL coal, the rate for CaS dropped to approximately half of that for Ca(OH)2,but it was still much higher than R,,,,. Figure 4 shows the difference in the specific rate a t 973 K with and without catalyst, Rca - Rnone, as a function of the total sulfur content in coal. The difference means the catalyst effectiveness of the calcium. KC coal gave the exceptionally high effectiveness, which might be related with the considerably high hydrogen content of 7.5 wt %, though the reason is not clear at present. When the catalyst effectivenesswas compared among the coals with almost the same S %, the calcium on low-rank coals showed larger effectiveness; for example, there was the effectiveness sequence of YL > CW > BA L GV for the first group with 0.3-0.4 wt % S, SH > LW > LP MB for the second one with 1.01.3 wt % S, and SB > KT L PB for the third one with 2.4-2.9 wt % S. Temperature Dependence of Rate. Figure 5 illustrates typical examples for the Arrhenius plots with and without Ca(OH12. The apparent activation energies calculated for eight coals are summarized in Table 2.
coal
KC IL MK a
215 165
185 170 130
135
135 135 135 160 130 135
95 60 65 150 110
160 110
Activation energy. Lowering in gasification temperature.
The energies in the uncatalyzed gasification ranged 130-215 kJ/mol. The use of Ca(OH)2 either had no effect on the activation energy or lowered it slightly in most cases. Such the phenomena have been reported by several resear~hers,~-ll who suggest that the catalysis of calcium is due to the increased number of active sites. The lowering in gasification temperature by catalyst addition, denoted as AT, can also be evaluated from Figure 5; for example, with SH coal, the temperature required to attain the specific rate of 0.5 h-l was 945 and 850 K without and with 5 w t % Ca, respectively, AT being 95 K. The values of AT are also summarized in Table 2. YL, CW, and IL coal showed larger AT of 150-160 K, whereas LW and SB coal gave smaller AT of 60-65 K. Since the contents of inherent Ca in Y L and CW coal, 0.06-0.2 w t %, were much lower than those (1.3-1.4 wt %) in LW and SB coal, the amount of inherent Ca may be one of the factors influencing AT. Sulfur Retention. The sulfur retained in the original and Ca-bearing chars prepared at 973 K is compared in Figure 6, where seven coals with 0.3-3.7 wt % S are used. The sulfur retention was always larger in the Ca(9) Otto, K.; Bartosiewicz, L.; Shelef, M. Fuel 1979, 58, 565-572. (10) Freund, H. Fuel 1986, 65, 63-66. (11)Ohtsuka, Y.; Tomita, A. In Calcium Magnesium Acetate: An
Emerging Bulk Chemical for Environmental Applications; Wise, D. L.; Levendis, Y. A,, Metghalchi, M., Eds.; Elsevier: Amsterdam, 1991; pp 253-271.
Energy &Fuels, Vol. 9, No. 6, 1995 1041
Steam Gasification of Coals with Ca(OH)2 I 0.4
1001
li
e 0
0 0 C
I-
0
50 -
?!
char
n n
0 00
A
0.2 O
j
f
8
u)
Total sulfur in coal, wt%(daf)
Figure 6. Sulfur retention at 973 K and the ratio of sulfur captured to calcium added as a function of total sulfur content in coal: 0 and 0, sulfur retained in original and Ca-bearing chars, respectively; A , ratio. L; CaO 0; Cas
P; FeS
Q; Si02
J
Table 3. Calcium Species after Pyrolysis at 973 K and Average Crystalline Size of CaO
.$ C 0) +
.-C C
.-0 I
2i5
b
SH SB
cw
KC IL MB BA PB
calcium speciesa none Cas (w) none Cas (w) CaO (w) Cas (w) CaO (m), Cas (m) CaO (m) CaO (m) CaO (m), Cas (s)
size of CaO, nm
18
26 40 36 43
a Identified by XRD;w, weak; m, medium; s, strong. Addition of Cas instead of Ca(OH)2.
of CaO determined by the Debye-Shorrer method also showed the same tendency. It is evident from these results that, when Ca(0H)z is loaded on high-rank coals, the dispersion of CaO is decreased. When Cas in place of Ca(OH)zwas used with YL coal, CaS alone appeared on the char, in contrast with no XRD lines of CaO on the char from the Ca(0H)z-loaded coal. The appearance of CaS suggests easier crystallization of CaS than CaO. CaS was the only species with the chars (SB and KC) from low-rank coals with high sulfur contents. As the rank of high-sulfur coals increased, not only Cas but also CaO was observed on the chars (IL and PB). The diffraction intensities of CaS also increased with coal rank.
Discussion
deg. Figure 7. X-ray diffraction profiles for Ca-bearing chars prepared from three coals at 973 K. 2 8 (Cu-K a ) ,
bearing char irrespective of S %. The observation indicates the sulfur capture by the externally-added calcium upon coal devolatilization. Figure 6 also shows the atomic ratio of the sulfur captured to the calcium added. The ratio increased with increasing total sulfur, but it seemed to level off beyond approximately 2 wt % S. The ratios for all the samples were '0.2, which means that '80% of the calcium added is free from contamination by the sulfur evolved on devolatilization. Calcium Species. Figure 7 shows typical XRD profiles for Ca-bearing chars prepared at 973 K. CaO was the only Ca species with the chars (CW and BA) derived from low-sulfur coals, whereas Cas as well as CaO was observed with the chars from high-sulfur coals, as with IL char. The presence of Cas corresponded to the sulfur capture by the calcium added as mentioned above. Table 3 summarizes Ca species identified by XRD for different Ca-bearing chars. No diffraction lines attributable to Ca species were detectable with YL and SH char. This means that the catalyst particles are too fine to be detected by XRD. As the rank of parent coals increased, CaO appeared and the diffraction intensities increased with coal rank. The average crystalline size
Sulfur Poisoning. The XRD results indicated the transformation of Ca(OH)z as a catalyst precursor into CaO and CaS on devolatilization (Figure 7 and Table 3). Cas is formed mainly by the reaction of CaO with HzS evolved12 and is inactive for subsequent char gasification when this form is still kept in the working state.g The sulfur capture by the calcium occurred with any coal (Figure 6 ) . Further, the effectiveness of the calcium catalyst on low-rank coals, such as YL, SH, and LC coal with almost the same C %, decreased with increasing S % (Figure 4). These observations show the catalyst deactivation by sulfur. However, it is probable from several reasons that the sulfur poisoning is less severe under the present conditions. First, the atomic ratio of the sulfur captured to the calcium added was as low as
