Ind. Eng. Chem. Process Des. Dev. 1902, 21I 324-330
324
Pyrolytic Desulfurization of Coal in Fluidized Beds of Calcined Dolomite Yaw D. Yeboah,' John P. Longwell, Jack B. Howard, and Wllllam A. Peters' Energy Laboratory and Department of Chemical Engineering, Massachusetts Institute of Technobgy Cambridge, Massachusetts 02139
The effects of calcined dolomitic stones on the distribution of parent coal sulfur to char, tar, liquor, H2S, and seven vapor phase organic compounds have been determined for the fluidized-bed pyrolysis of Illinois No. 6 bituminous coal and a 5545 (w/w) mixture of Texas lignite and Illinois No. 6 bituminous coal. These studies were conducted under 1 atm of N, and over temperature and fluidizing gas velocity ranges of 700-1 144 K and 0.24-0.30 m/s, respectively. In the absence of dolomitic stones, 50-75% of the coal sulfur was accounted for in the char, 7-13% in the tar, < I % in the liquor, and about 15-30% in the gas, of which 90-95% was H2S. The dolomitic stones caused virtually total elimination of H,S from the gas and major reductions in the yields of vapor phase organic sulfur compounds. The fraction of original coal sulfur disposed to tar was also reduced by the dolomitic stones, but with the exception of high calcium quicklime, they exerted no significant effect on the absolute sulfur content
of the tar.
Introduction Knowledge of the fate of parent sulfur during thermal degradation of coal is important in determining the sulfur content of coal-derived fuels and metallurgical coke, and it is useful in predicting the extent of sulfur oxide emissions during combustion processes. Since various factors, including reactor configuration, are known to influence the pyrolytic reactions of coal (Anthony and Howard, 1976; Howard, 1981), previous investigations of pyrolytic desulfurization have been conducted in equipment that has ranged from laboratory scale packed bed reactors (Powell, 1920a,b; 1923a,b; Mangelsdorf and Broughton, 1932; Snow, 1932; Brewer and Ghosh, 1949; Ghosh and Brewer, 1950; Yergey et al., 1974; Block et al., 19751, to thermal balances and captive sample electrical screen heaters (Sinha and Walker, 1972a,b; Solomon, 1977; Solomon and Colket, 1978) and entrained flow systems (McIver, 1978; Bierl, 1979). Some studies have also reported the use of dense phase fluidized bed reactors for the thermal desulfurization of coal (Jacobs and Mirkus, 1958; Bodman et al., 1977; Kor, 1977; Haldipur and Wheelock, 1977) and of coal char (Zielke et al., 1954; Batchelor et al., 1960; Gray et al., 1970; Maa et al., 1975). The study of pyrolytic desulfurization in dense phase fluidized beds, in particular, is of interest because of the promise for utilization of this mode of fluid-solids processing in modern coal conversion and combustion processes. An example of one such processing method is the combined fluidized-bed pyrolysis-fluidized-bed combustion (FBP-FBC) scheme, with recycle of dolomitic stones between the two units. This scheme was proposed by the authors for generating storable clean fuels, electric power, and/or steam from coal and other carbonaceous feedstocks (Yeboah, 1979; Yeboah et al., 1980). The effect of the dolomitic stones on coal desulfurization in the combustor can be predicted because of the numerous studies that have been conducted on the potential use of these stones for SO2 removal. However, lack of information on the effects of dolomitic stones on pyrolysis under fluidized-bed conditions has precluded similar predictions for the pyrolyzer. As part of a research program aimed at assessing the technical and economic feasibility of the integrated General Electric Co.,
CR&D, Schenectady, NY 12301. 01 96-430518211 121-0324$01.25/0
FBP-FBC process, the distribution of parent coal sulfur to products during pyrolysis in the presence and absence of various dolomitic stones was established. Such information was acquired over ranges of temperature, fluidizing velocity, and coal types of commercial interest. This paper summarizes the results of this aspect of that work and provides answers to these questions: (1)For the pyrolysis of coal in a fluidized bed of inert solids, e.g., sand, what is the fractional distribution of parent sulfur to product char, tar,gas, and liquor as a function of temperature, coal type, and fluidizing velocity? (2) For the conditions of (l), what are the specific organic and inorganic sulfur compounds found in the pyrolysis gas? (3) How do the presence of calcined dolomitic stones affect the results in (1)and (2), and are the observed rates of reaction of H a with the solids sufficiently rapid under fluidized-bed conditions to be of commercial interest? (4) Can in situ removal of organic sulfur compounds from the gaseous and liquid products be achieved during pyrolysis in the presence of calcined dolomitic stones without external addition of hydrogen? Experimental Section Experiments were conducted over the temperature range of 700-1144 K and fluidizing gas velocities of 0.24 and 0.30 m/s of nitrogen using Illinois No. 6 bituminous coal and a Texas lignite. Microscopic analysis (Neavel, 1978) revealed the Texas lignite to be a 5545% (w/w) mixture of Texas lignite and Illinois No. 6 bituminous coal resulting from contamination. For purposes of discussion, this coal sample will be referred to as the Texas Lignite/Illinois No. 6 mixture sample. Runs were performed with and without dolomitic stones, which included two fully calcined dolomites (Warner and Flintkote), a high calcium quicklime, and the overflow bed material from Exxon's miniplant fluidized bed combustor (Hoke et al., 1976; Hoke, 1977). Analyses of the coal and stones used are shown in Tables I and 11. All experiments were carried out at 1 atm pressure with a coal and dolomite particle size range of 250-500 Fm. The experimental apparatus and procedure have been described in detail previously (Yeboah, 1979; Yeboah et al., 1980). In brief, the system consisted of a gas feed unit, a 5 cm i.d. x 2 m long 316 stainless steel reactor consisting of an approximately 1 m long upstream preheater and a downstream section of a 0.23 m long fluidized bed zone 0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 325 I
Table I. Proximate and Ultimate Analyses, Btu Content, and Fischer Assays of Coal Samples Used Texas Lignite/ Illinois No. 6 bituminous coal, basis, % as rec'd
DAF
as rec'd
DAF
39.60 49.31 2.69 8.40
44.53 o 55.47
68.38 5.02 0.09 3.94 10.41
76.91 5.95 1.2 0.10 4.43 11.71
volatiles fixed carbon moisture ash carbon hydrogen nitrogen chlorine sulfur oxygen (diff)
Ultimate Analysis 51.32 74.32 3.79 5.49 0.95 1.38 0.05 0.07 2.57 3.72 10.37 15.03
Btu/lb
BTU Content 9048 13104
12159
13675
tar and light oils gas water char ash
Fischer 11.3 8.06 17.21 50.23 12.97
11.56 6.35 7.48 64.93 8.40
14.4 7.1 6.2 72.3 _-
Assay 15.1 10.7 8.7 65.2
_-
WARNER DOLOMITE IWDI A CALCIUM QUICKLIME
I
1
nian
1033 I1400)
1144 (1600)
'F
incoi
Illinois No. 6 bituminous coal, basis, %
Proximate Analysis 35.15 50.91: 33.90 49.09 17.98 -12.97
__
I
I
0 SAND
1.07
_-
i
STONES
5 l0
__
70
700 1800)
81 1
922
(10001
( 1 200)
K
TEMPERATURE
and an about 0.46 m long freeboard zone. Other parts of the unit included a screw-driven solids feeder, a solids overflow collection unit, a liquid products collection system, and a gas sampling and analysis unit. Solids were fed continuously at a rate of about 13 g/min. The tar and liquor products were removed by product collection trains and the gaseous products were sampled periodically and analyzed with a Perkin-Elmer Model 3920B gas chromatograph equipped with thermal conductivity, flame ionization, and sulfur specific flame photometric detectors. The simultaneous use of all three detectors on each gas sample enabled the identification and quantitative characterization of about 20 gaseous products of pyrolysis, including H2S and seven vapor phase organic sulfur compounds. The yields of products, the elemental composition (determined by Huffman Laboratories, Wheat Ridge, CO) and the atomic ratios (relative to carbon) of char and tar,and the molecular composition of the gas were determined for each set of experimental conditions. Also determined were the heating values of the products, the fractional distribution of the parent coal heating value and sulfur going to products, and the extents of desulfurization and decarbonation of the gas by the dolomitic stones. Material balances in most runs were 90-95%, while sulfur and carbon balances typically ranged from 94 to 104% and 90 to 97 90, respectively.
Results and Discussion The present paper focuses on fractional distribution of parent coal sulfur to pyrolysis products as a function of operating conditions. Other results including the yields of the liquid and solid products and of other nonsulfur gases and the C , H, N, 0, and heating value analyses of product chars, tars, and liquors are reported by Yeboah (1979) and Yeboah et al. (1980). Sulfur Distribution in the Absence of Dolomitic Stones. Effect of Temperature. The effects of operating temperature on the percentages of parent sulfur distributed to product char, tar, and gas for the coal samples studied are shown in Figures 1and 2 and Tables I11
Figure 1. Distribution of parent sulfur of 55/45 (W/W) mixture of Texas Lignite/Illinois No. 6 bituminous coal to pyrolysis products at a fluidizing gas velocity of 0.24 m/s. Probable uncertainties in these data, expressed as wt ?h of parent sulfur, are: char, f3; tar, fl; and gas, k0.3. I 0 SAND
I
I
I
0 WARNER DOLOMITE (WD)
v FLUIDIZED BED COMBUSTOR STONES IFECS)
I? 1 o L
..
700 i8OOi
81 1 !lOOO)
922
1033
( 1 200)
11400)
1144 (1600)
K O F
TEMPERATURE
Figure 2. Distribution of parent sulfur of Illinois No. 6 bituminous coal to pyrolysis products at a fluidizing gas velocity of 0.24 m/s. Probable uncertainties in these data, expressed as wt 70of parent sulfur, are: char, f3; tar, kl;and gas, k0.3.
and IV. The probable uncertainties in the sulfur distribution data are given in captions to Figures 1 through 4 and in footnotes to Tables I11 and IV. This information was inferred from the differences observed in duplicate
326 Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 Table 11. Physical and Chemical Properties of Stones Used stone type Flint ko t e dolomite
property specific surface area, m'/g average pore radius, .A bulk density, g/cm3 true powder density, g/cm3
6.8 2517 0.850 3.2
Warner dolomite
quicklime
sand
7.55 1840 0.978 2.989
0.01
Physical Properties 6.3 2234 0.984 3.066
__
.4&03
CaSO, MgO CaCO, CaO Ca(OH), c0,z-
1.622 2.502
so,2-
__
__
-_
--
0.398 0.018 0.533 0.025 0.0014 0.0004
0.4003 0.0077 0.5574 0.0391 0.0077 0.0002 0.0200
SiO,
3.25 2095 1.12 2.9
__
Chemical Composition (Weight Fraction) 0.0011 0.0048 0.0052 0.00065 0.0035 0.00595 0.0033 --
acid insolubles Fe*O,
fluidized bed combustor stone
__
0.5539 0.2130 0.0113 0.0946 -_ 0.0068 0.3910
0.0078
__
__0.9519
-_
0.0080 0.0029 _-
1.00
__
Table 111.a Percentage of Parent Sulfur of Texas Lignitelnlinois No. 6 Coal t o Volatile Pyrolysis Products tar
gas
fl. vel., m/s
T, K
sand
WD
HCQ
sand
0.24 0.24 0.24 0.24 0.24 0.30 0.30 0.30 0.30
7 00 811 922 1033 1144 700 811 922 1033
9.25 9.28 10.14 7.08 5.95 9.52 9.29 9.51 9.27
6.89 8.80 7.56 7.71 nm 7.01 8.34 8.0 8.08
7.74 5.17 6.86 5.38 nm 10.13 8.73 7.13 7.06
14.22 16.49 21.78 26.66 29.38 11.77 16.05 21.11 24.52
HCQ
0.20 0.05 0.04 nm
.
T, K
sand
0.24 0.24 0.24 0.24 0.24 0.30 0.30 0.30 0.30
700 811 922 1033 1144 700 811 922 1033
0.32 0.18 0.08 0.11 0.15 0.18 0.04 0.06 0.11
WD
0.20 nm
-
0.19 0.08 0.07
0.12 0.86
H,C-S-CH,
fl. vel,, ms
cos
H2S
WD
sand
WD HCQ sand
12.53 14.86 20.38 25.18 28.43 10.40 14.47 19.84 23.23
-
nm
HCQ
0.02 nm
0.13
sand
WD
0.16 0.19 0.23 0.29 0.37 0.10 0.16 0.24 0.32
0.05 0.12 nm
0.18
*
0.03
-
nm
~
0.21
thiophene HCQ
nm
0.53 0.53 0.54 0.56 0.42 0.51 0.54 0.53 0.57
nm
CS,/C,H,S
WD
0.02
sand
WD
0.12 0.15 0.14 0.11
0.04
0.08 0.16 0.14 0.12
0.05 nm 0.06
0.12 0.09
WD HCQ
0.01 nm 0.13 0.08 0.04
nm
0.30 0.31 0.06 0.09 0.01 0.33 0.40 0.04 0.09
nm
3-methylthiophene HC&
0.04 nm
CH,SH HCQ' sand
0.01
sand 0.27 0.27 0.35 0.32 0.17 0.28 0.25 0.08
WD
HCQ
0.13 nm
nm
0.04
0.00
Parent coal sulfur content was 3.72 wt % DAF. A dash indicates a level below 0.01%; nm implies not measured. Probable uncertainties in these data, expressed as wt % of parent sulfur are tar, +1;gas, 50.3; H,S, + 0 . 2 ;other sulfur compounds, + 0.02. Table IV.a Percentage of Parent Sulfur of Illinois No. 6 Coal to Volatile Pyrolysis Products (0.24 m/s Fluidizing Gas Velocity) T,K 700 811
922 1033
-
tar
sand
WD
FBCS
gas
9.84 10.51 13.05 8.80
8.94 8.90 9.90 8.41
9.04 8.78 10.65 8.21
sand 18.26 22.72 26.14 30.82
H,3C-S-CH,
FBCS
0.05 0.10
0.25 2.00 2.59 4.16
sand WDFBCS 16.70 21.06 - 1.80 24.58 - 2.21 29.31 - 3.74
Cs,/c,H,S
T,K
sand
WD
FBCS
sand
700 811 922 1033
0.29 0.27 0.22 0.12
-
0.04 0.03 0.03 0.05
0.15 0.23 0.27 0.30
cos
H2S
WD
sand 0.47 0.49
0.50 0.51
WD 0.01 0.05
thiophene
WD
FBCS
sand 0.14
0.02 0.03
0.10 0.04 0.10 0.11
0.14 0.15
0.18
CH,SH FBCS 0.03 0.04
0.07 0.10
sand WD FBCS 0.30 0.31 0.20 0.15
-
0.02
3-methylthiophene
WD
FBCS
sand
0.01 0.01
0.04 0.03 0.08 0.07
0.21 0.22 0.23 0.26
WD
FBCS 0.02 0.06
0.02 0.01
0.09
0.08
Parent coal sulfur content was 4.43 wt % DAF. A dash indicates a level below 0.01%; nm implies not measured. Probable uncertainties in these data, expressed as wt % of parent sulfur, are tar, 2 1 ; gas, 20.3; H,S, i 0 . 2 ; other sulfur compounds, +0.02. a
I
301
TEX LlGllLL NO. 6
0 WARNER DOLOMlTEiWDI A WOH CALCIUM OUlCKLlME IHCOl
ILL.NO 6
q WARNER OOLOMCTE lWDl
0 FLUIDIZED BE0 COMBUSTOR STONES
22
-
20
-
4
8 SAND
28 -
I-
0
0 SAND
P
-
24
0.30mls
0.24 ml6
0 TEX LlGllLL NO. 6 10.24 mlrl
28
-
-
0 ILL NO. 6 10.24 mlw
wcsi
SAND
18-
16
-
14
-
A
FBCS WITH ILL NO. 6 10.24 mlrl
0 WD WITH ILL NO. 6 (0.24 AND 0.30 mi.)
12
-
0WDIO 24ANDANDnca0.30WITH TEX LIGIILL NO. 6 mill
#'
WDANDHCQ OTHER STONES A
v
700 i800l
I 81 1
I1 0001
9
I
I
922
1033 I1 4001
(12001 TEMPERATURE
I
/
1144 "K 116001 1°F)
Figure 3. Effects of dolomitic stones, temperature, coal type, and fluidizing gas velocity on the fraction of parent coal sulfur going to H2S during pyrolysis. The probable uncertainty in these data expressed as wt % of parent sulfur, is f0.2.
runs at selected experimental conditions. The uncertainty in char data results mainly from uncertainties in chemical analysis, while variations in run conditions are primarily responsible for the uncertainties in the data for gas, H2S, and other sulfur compounds. The uncertainties in tar data arise from uncertainties in both of the above factors. Char accounted for about 50-75% of the original coal sulfur with the absolute fraction decreasing monotonically as reaction temperature increased. These findings are consistent with the previous fluidized-bed studies of Jacobs and Mirkus (1958) and Haldipur and Wheelock (1977). Analyses of the product char consistently gave 4.3-4.8 w t 3'% sulfur and a heating value of 14400-15 800 Btu/lb over the ranges of temperature, coal type, stone type, and fluidizing velocities studied. The fractional amount of sulfur found in product tars, shown in Figures 1 and 2, exhibited a maximum between 840 and 922 K. Since the measured sulfur content of these tars changed little with increasing temperature (about 2.0-3.0 wt % sulfur in the tars over the range of operating conditions), the maxima are believed to reflect the corresponding behavior in the temperature dependence of the absolute yields of these tars as described previously (Yeboah, 1979; Yeboah et al., 1980). The heating values of the product tars varied from 15OOO to 17OOO Btu/lb over the range of experimental conditions studied. It is interesting to compare the present fluidized-bed data on fractional sulfur distribution to tar with those reported in the packed tube studies of Brewer and Ghosh (1949) and Ghosh and Brewer (1950). They carbonized approximately 1 5 g samples of an Illinois No. 6 coal of very similar sulfur content (3.55 wt % ,assumed as-received, vs. 3.94 wt % in the present work). The sample was heated to final temperature over a period of 1.5 to 2.5 h and then
+
0
328 Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982
scale packed bed reactors operated under similar temperature conditions. For both coal samples, hydrogen sulfide accounted for 90-95% of the sulfur in the vapor phase and the fraction of the parent coal sulfur ending up as this product increased monotonically as reaction temperature increased (Figure 3). The overall trend in Figure 4 appears to indicate that the fraction of parent sulfur found in the organic vapor-phase compounds decreases as temperature increases. The precision in the data for these low level organic vapor-phase sulfur compounds, however, makes it difficult to determine the trends in the fraction of the parent coal sulfur going to the individual products as temperature increases. The overall decline with increasing temperature may reflect augmented secondary cracking of the more labile vapor-phase organic sulfur compounds such as methyl mercaptan and methyl sulfide. The liquor product of pyrolysis, which typically represented 3 to 7 wt % of the original weight of coal, accounted for no more than a few tenths of a percent (
