Energy & Fuels 1989, 3, 506-515
506
Intrinsic Kinetics of CaS(s) Oxidation Rowena J. Torres-Ordofiez,**tJohn P. Longwell, and Adel F. Sarofim Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 8, 1988. Revised Manuscript Received March 31, 1989
The oxidation of CaS(s) crystals in O,(g)-He(g) was investigated in a laminar flow oxidation furnace to determine the possibility of retaining sulfur in solid form as unoxidized CaS(s>during coal combustion. The retention of sulfur as CaS(s) during combustion may be a viable alternative method of controlling SO,(g) emissions. The oxidation experiments were conducted with 6.32-pm crystals at gas temperatures of 1400-1750 K in atmospheres containing 0-0.20 atm of 02(g) in He(g) for residence times of 0-0.25 s. The reaction is first order in the bulk 02(g) partial pressure for Po, I 0.20 atm at 1400 K and for Po I0.10 atm at 1500-1750 K. The oxidation of CaS(s) results in the formation of CaO(s) or both C!aO(s) and CaSO,(s) as products. For the oxidation reaction CaS(s) + 3/202(g) CaO(s) + SO,(g), application of the shrinking unreacted core model to the x - t data at 0.10 atm of O,(g) and 1400-1750 K yielded the intrinsic reaction rate constant k , = 8.2276 X lo6 exp(-20411/T) cm/s. The formation of CaS04(s),in addition to CaO(s), results in the following interesting features of the CaS(s) oxidation system: (1)the leveling off of conversion at high Po, (50.20 atm) and high temperature (21650 K) due to the formation of a CaO(s)-CaSO,(s) eutectic; (2) significant product layer diffusion at low temperature (i.e., 1400 K); and (3) S-shaped x - t curves at intermediate temperatures (1500-1550 K) as a consequence of the formation of an intermediate CaSO,(s). CaS(s) oxidation proceeds rapidly at a rate comparable to carbon oxidation to a (possible) limiting conversion due to the CaO(s)-CaSO,(s) eutectic or possible full sulfur loss for fine CaS(s).
-
Introduction The increase in the use of coal in industry and utilities in the last 20 years, particularly in heat and steam generation, has caused a simultaneous increase in concern over environmental problems related to coal combustion. One of the most pressing of these problems has been the control of sulfur oxides or SO,(g) emissions. One SO,(g) control method involves sulfur removal during combustion as solid sulfur compounds. This option, sometimes referred to as the sorbent injection alternative, presents a desirable alternative14 primarily because existing boilers may be retrofitted for this method. Moreover, by combination of sorbent injection with appropriate burner modification, control of NO,(g) and SO,(g) emissions may be ~ b t a i n e d . ~ Alkaline metals can react with the coal sulfur and retain it either as the sulfate or the sulfide, depending on reaction conditions.6 Calcium compounds, in particular, prove to be attractive sorbents because of their low cost and the stability of their sulfur forms at the high temperatures of coal combustion. These calcium-based sulfur sorbents may be coal calcium, which may be present as minerals such as calcite, as calcium ion-exchanged into the coal matrix,'-1° or as calcium stones such as limestone and dolomite, which may be injected in the postflame region. Thermodynamic calculations performed on the retention of sulfur by calcium in the products of combustion of Illinois No. 6 and Wyoming coals" indicate that the conditions favoring the two forms of the calcium solid product are (1)fuel-rich conditions (4 > 1.0) and high temperatures (T I2000 K) for CaS(s) and (2) fuel-lean conditions (4 5 1.0) and low temperatures (T 5 1500 K) for CaS04(s), where 4 = (stoichiometric fuel to oxidant ratio)/(actual fuel to oxidant ratio). The latter product is the one formed when limestone is injected in fluidized beds typically operated at about 1150
K or when limestone or a hydrated lime has been injected in the cooler portions of a furnace, as a consequence of the following reaction:
Present address: Center for Catalytic Science and Technology, Department of Chemical Engineering, Colburn Laboratory 307, University of Delaware, Newark, DE 19716.
135-153. (11) Torres-Ordoiiez, R. J. The Oxidation of CaS(s) Crystals During Simulated Coal Combustion. PhD Thesis, M.I.T., 1986; pp 78-87.
f
0887-0624/89/2503-0506$01.50/0
CaO(s) + '/02(g)
+ SO&)
-
CaSOJs)
(1)
When CaS(s) is produced from CaO(s) and sulfur species in the coal under fuel-rich conditions, it must be prevented from oxidizing when combustion is completed by the addition of supplementary oxygen, as a consequence of the following reaction:
-
CaS(s) + 3/202(g)
CaO(s) + SO&)
(2)
The survival of sulfur as CaS(s) will depend on the (1) Kokkinos, A. D.; Lewis, R. D.; Borio, D. C.; Plumley, A. L.; McElroy, M. W. Engineering and Economic Feasibility of Furnace Limestone Injection for SO2 Control. In Proceedings of the 1982 Joint Symposium on Stationary Combustion NO, Control; EPRI Dallas, TX, 1982. (2) Michelfelder, S.; Leikert, K.; Chugtai, J. Operation and Perform-
ance of the Steinmuller Low-NO, SM-Burner and ita Potential Towards Utility Boiler SO, Control via Sorbent Injection. In Proceedings of the 1982 Joint Symposium on Stationary Combustion NO, Control; EPRI Dallas, TX, 1982. (3) Giammar,R. D.; Barnes, R. H.; Weller, A. W. Limestone/Coal Fuel Pellet: A Viable Boiler Fuel. Presented at the 1982 Spring Technical Meeting Central States Section Combustion Institute; Columbus, OH, March 24-25, 1982. ( 4 ) Doyle, J. B.; Jankura, B. J. Furnace Limestone Injection with Dry Scrubbing of Exhaust Gases. Presented at the 1982 Spring Technical Meeting Central States Section Combustion Institute; Columbus, OH, March 24-25, 1982. (5) Flament, G. The Simultaneous Reduction of NO, and SO, in Coal Flames by Direct Injection of Sorbents in a Staged Mixing Burner. International Flame Research Foundation Doc. No. G 19/a/ 10; International Flame Research Foundation: IJmuiden, The Netherlands, 1981. (6) Maloney, K. L.; Engel, P. K.; Cherry, S. S. Sulfur Retention in Coal Ash. EPA-600/7-78-153B;EPA Washington, DC, 1978. (7) Schafer, H. N. S. Fuel 1970,49, 197-213. (8) Schafer, H. N. S. Fuel 1970, 49, 271-280. (9) Schafer, H. N. S. Fuel 1972,51,4-9. (10) Blom, L.; Edelhausen, L.; van Krevelen, D. W. Fuel 1957, 36,
0 1989 American Chemical Society
Kinetics of CaS(s) Oxidation
properties of the CaS(s) particle and the combustion conditions. For large CaS(s) particles formed from calcium minerals, the CaS(s) may be protected by an outer CaO(s) layer, which may form a protective crust around the unreacted CaS(s), particularly if CaO(s) deadburns to a nonporous form. In order to evaluate the feasibility of retaining the CaS(s), it is necessary to determine its kinetics of oxidation under the conditions that may be encountered in the oxidizing region downstream of the fuel-rich zone in which CaS(s) is produced. The oxidation of dense 0.76-cm pellets of CaS(s) in 02(g)-S02(g)-Ar(g) atmospheres has been previously studied by Lynch12J3at 1453-1853 K in 1-100% 02(g)and 0-20% S02(g) at gas flow rates of 0.85-3.0 L/min in a thermogravimetricanalyzer. He showed that the oxidation resulted in the formation of CaO(s) and/or CaS04(s),depending on the temperature and 02(g)partial pressure, Po2. The oxidation rates he obtained were, however, initially mass transfer controlled so that he was unable to obtain an intrinsic reaction rate expression for the CaS(s) oxidation reaction. Lyon and Freund14 studied the retention of sulfur as CaS(s) during the fuel-rich combustion of a Pittsburgh coal using limestone and ion-exchanged calcium as sulfur sorbents. They varied fuel equivalence ratios at constant residence time in their furnace to obtain carbon burnout. Their work suggests that the decrease in sulfur retention as CaS(s) as carbon conversion increases is due to the competition between char oxidation and CaS(s) oxidation and that ion-exchanged calcium retained more sulfur than an equal addition of finely calcined limestone. In the present study, small crystals of CaS(s) are oxidized with the specific objective of determining intrinsic kinetics and mechanism of the oxidation of CaS(s) under simulated coal combustion conditions.
Experimental Section Oxidation Experiments. Essentially pure (certified 99.99%) CaS(s) crystals that are commercially available (CERAC Pure, Inc.) were used in all the oxidation experiments. The reactant CaS(s) and the oxidation products were handled and stored under nitrogen in a glovebox, whenever possible. However, slight air oxidation occurred during short periods of unavoidable exposure to the atmosphere. Chemical analysis (performed by Galbraith Laboratories, Knoxville, T N ) on two CaS(s) feed samples gave 98.56 mol % Cas, 0.19 mol % CaS04(s),and 1.25 mol % CaO(s) (by difference). The size distribution of these crystals is narrow, with 81-87% of the crystals in the 4.9-7.7-pm size fraction. The CaS(s) crystals were oxidized in a laminar flow oxidation furnace for residence times up to 0.25 s, which is typical of residence times in the high-temperature zone of industrial boilers. The furnace (Figure 1)consists of a 15.0-cm (6.0-in.) central combustion zone maintained within the furnace core by an alumina muffle tube of 5.0-cm (2.0-in.) inside diameter. The main oxidizing gas enters through the top of the furnace and enters the furnace hot zone through a 2.5-cm- (1.0 in.-) thick alumina honeycomb, which surrounds the water-cooled feeder through which the CaS(s) particles are axially injected into the furnace. A small amount of the main gas (2%) enters the furnace through the feeder to entrain the particles into the furnace. On entering the furnace combustion zone, the particles are rapidly heated and ignited. Transient heatup times for the 6.32-pm particles are on the order of 10 ms compared to the 0.25-9 maximum residence time used. The particles then fall a t a velocity close to the gas velocity. Calculations show that the 2.5 cm (1.0 in.) travel through the honeycomb length is enough to establish a parabolic velocity profile with a Reynolds number of about 0.40. The time for the (12)Lynch, D.C.; Elliott, J. F. Metall. Trans. E 1978,9E,691-704. (13)Lynch, D.C.;Elliott, J. F. Metall. Trans. E 1980,IlE,415-425. (14)Lyon, R. K.;Freund, H. Combust. Flame 1982,45,191-203.
Energy & Fuels, Vol. 3, No. 4, 1989 507
c PURGE GAS-= MAIN GASl
b r F E E D TUBE
F WATER
WATER COOLED FEEDER PROSE
ATER COOLED FLANGES MUFFLE TUBE
AND WATERHONEYCOMB
ALUMINA LINER TUBE TO SUPPO HONEYCOMB
COLLECTOR
WATER COOLE FLANGES B A L L VALVE
TI
-BALL
-QUENCHING
VALVE
GAS
Figure 1. Schematic of the laminar flow drop-tube furnace. parabolic jets emanating from the honeycomb holes to coalesce into an approximate flat velocity profile corresponds to a position estimated to be 0.05 cm (0.02 in.) below the honeycomb. This flat velocity profile then develops into a parabolic velocity profile at about 2.5 cm (1.0 in.) below the honeycomb. Visual observation of coal particles falling through this furnace by Nevillels shows that the particles fall within 0.62 cm (0.24 in.) of the centerline. The main gas velocity averaged over this radius is approximately 1.94 times the main gas velocity averaged over the furnace radius. The particles, along with the gases, are withdrawn from the furnace by a water-cooled stainless-steel collection probe, which is inserted along the furnace axis from the bottom of the furnace. An argon quench rate of 2.5 X lo3 cm3/min is provided to cool the particles and gases to no more than 473 K. After passage through the probe, the particles are size classified in an Anderson impactor. Residence time is varied by adjusting the position of the collection probe. Because of the development of temperature and velocity profiles in the entrance zone, the reactions begin a t a distance of 3.8 cm (1.5 in.) below the honeycomb, which then corresponds to zero residence time in the furnace. The furnace gas temperatures ranged from 1250 to 1750 K. Measurements of the furnace wall temperature and radial and axial temperature profiies (with and without the collection probe) were performed by using optical pyrometry. The data, obtained by House in 1978, are reported in ref 11. The reaction zone, i.e., that portion of the furnace from which sampling is done (5.0-15.0 cm from the honeycomb and corresponding to 0.02-0.25 s) is isothermal for 1250 5 T 5 1750 K. The introduction of the water-cooled collection probe causes some changes in the gas temperature profile, largely due to radiation losses. However, within 0.62 cm (0.24 in.) of the centerline where the particles stay, the temperature and velocity profiles are essentially undisturbed by the probe introduction. The selection of gas compositions was based on calculations from a thermodynamic model'l which indicated that the important oxidants were O,(g), HzO(g), and CO,(g). The compositions of the oxidizing atmosphere to which the CaS(s) particles were (15)Neville, M. Formation of Inorganic Submicron Particles Under Simulated Pulverized Coal Combustion Conditions. ScD Thesis, M.I.T., 1982.
508 Energy & Fuels, Vol. 3, No. 4, 1989 6.32
m
CaS C r y s t a l s (Lot 41
PCO, = PH,O
0,100 1001,
eo
0.075
I
0.050
Torres-Ordofiez et al.
,
0.025
,
6.3211~ CaS C r y s t a l s (Lot 1)
atm 0.000
fl.7
1 1650 = 0 . 2K2 s
1
00
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9
.-YI.
-
1500 K
0
7
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al L
;
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.-0 YI L
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0 0.00
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0.10
0.15
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O2 - He 0, C0, - H,O - He ~
0.20
0.00
0.05
I
I
I
I
0.10
0.15
0.20
0.25
PO,
0.25
I
atm
X CONVERSION TO CaO (SI
v s . PO,
Figure 2. CaS(s) oxidation in 02(g)C02(p)-H20(p)-He(g) (shown as filled-in symbols with reference to the upper axis) vs oxidation in 02(g)-He (shown as open symbols with reference to the lower axis) for lot 4 crystals at 1500 and 1650 K for 0.24 and 0.22 s,
Figure 3. Percent conversion to CaO(s) vs Po for oxidation of lot 1crystals in 0 . 2 0 atm of 02(g) for 0 . 2 0 . d 5 s at 1400-1750 K.
respectively.
fuel-rich conditions, such as those present during gasification, the stable oxidizing species would be C02(g)and H,O(g), whereas under the fuel-lean conditions during combustion, the stable oxidant would be 02(g). Oxidation experiments at 1500 and 1650 K in both 02(g)-He(g) and O,(g)-CO,(g)-H,O(g)-He(g) atmospheres showed that 02(g) is the most potent oxidant (Figure 2). The filled-in symbols are for runs in which the sum of 02(g),H,O(g), and C02(g)was constant (refer to upper axis for the abscissa). The increases in value of the filled-in symbols with increases in 02(g)show the greater effectiveness of 02(g) as an oxidant than a corresponding partial pressure of an equimolal H,O(g) and C02(g)mixture. The contribution of C02(g) and H20(g) can be determined from the difference in the values for the filled-in and open symbols at a fixed 02(g)concentration. This contribution is not significant at Po, 1 0.05 atm (or Pco,, PH I0.075 atm), but can be quite appreciable at high CO,(g) and high H,O(g) pressures. For example, at 1650 K, 0.10 atm of C02(g),0.10 atm of H20(g),and no 02(g),as much as 50% of the CaS(s) is oxidized to CaO(s). One of the possible mechanisms of CaS(s) protection from oxidation is by the formation of a deadburnt CaO(s) product layer. H,O(g) and C02(g)are strong catalysts of CaO(s) sintering,16J7and their presence in combustion gas may enhance sulfur retention as CaS(s) via the protection afforded by the sintered CaO(s) layer. Under the experimental oxidation conditions used, the results suggest that sintering did not occur. In the subsequent experiments, COz(g)and H20(g)were not included, and the oxidation was conducted in 02(g)-He(g). These oxidation experiments will hopefully provide limiting levels of CaS(s) oxidation.
subjected ranged from 0 to 0.20 atm of oxidant in He&), simulating the gas atmospheres that might be encountered in a coal combustor. The bulk gas SO,(g) concentration was zero since the aim was to simulate the environment in an efficient sulfur cleanup system. Chemical Characterization. The CaS(s) and oxidation products were chemically characterized by X-ray methods. X-ray fluorescence (XRF) was used to determine the elemental (Sand Ca) composition of the CaS(s) oxidation products. The samples were slurried in a solution of amyl acetate (which eventually vaporizes) and a nitrocellulose compound (which acts as binder) and packed in 0.95-cm-diameter, 0.16-cm-deep holes drilled in plexiglass sample holders. The samples were then dried in an oven at 383 K prior to analyses on a GE Diano diffractometer. Cu or Mo K a was used as primary radiation and the samples were analyzed in a vacuum housing for a 300-s counting time. Calibration standards were known mixtures of the reactant CaS(s) and reagent grade CaO(s) powder. The XRF' method as described has a 2% precision and an average 5% accuracy, based on chemical analysis as performed by Galbraith Laboratories, Knoxville, TN. X-ray diffraction (XRD) was used to qualitatively determine the sulfur species (Le., sulfide S2- vs sulfate SO:-) in the sample. The analysis was performed on a GE Diano diffractometer using Cu K a radiation and a scan rate of 2O/min. The diffractometer had a 5 mol % detection limit. X-ray photoelectron spectroscopy (XPS) or ESCA, electron spectroscopy for chemical analysis, was used to identify the Ca, S, and 0 species present in the product layer of the CaS(s) particle. In combination with ion sputtering, the concentration profiles of these species were determined. The ESCA analyses were performed on a Perkin-Elmer Model spectrometer. The spectrometer was equipped with an argon ion gun whose sputtering rate was 20-25 A/min, based on calibration with a variety of materials.
Results Oxidation Experiments. Oxidation experiments to assess the stability of CaS(s) crystals in atmospheres simulating coal flame conditions were performed. Under
(16) Beruto, D.;Barco, L.; Searcy, A. W. J. Am. Ceram. SOC.1984,67, 512-515. (17) Anderson, P.J.; Horlock, R. F.;Avery, R. G. Proc. Br. Ceram. SOC. 1965,3,33-42.
Kinetics of CaS(s) Oxidation 6 . 3 2 rrm Cas C r y s t a l s ILOt 11
100 1
I
I
I
,
I
I
1
0
1500 K 7 = 0.24
5
1
I
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bp
0
I
1
2ov Ob
I
lot
I
/
B
0.00
Lot I
I
0.20
I
I
I
0.40 Po2
I
0.60 I
I
I
0.80
atm
Figure 4. Percent conversion to CaO(s) vs Po, for oxidation of lot 1 crystals in 0-0.80 atm of 02(g) for 0.24 s at 1500 K.
Figure 3 shows percent conversion vs Po, for oxidation carried out at 1400-1750 K for residence times varying between 0.20 and 0.25 s, which corresponds to an effective 11.43-cm (4.5-in.) travel in the furnace. At low temperature (i.e., 1400 K), the conversion is linear in bulk PO,while at the higher temperatures (1500-1750 K), the linear variation occurs at low Po, (50.10 atm), after which conversion levels off as Po, is further increased. The oxidation of CaS(s) with Oz(g)can result in the formation of CaO(s) and/or CaSO,(s) as products, with CaO(s) being the thermodynamically stable product at temperatures greater than 1473 K at a pressure of 1 atm. Only CaS(s) and CaO(s) were qualitatively detected by XRD, except at 0.20 atm of 02(g) and 1500 K, where CaSO,(s) was detected as well. Figure 4 shows conversion vs Po, for the oxidation of lot 1 crystals at 1500 K in 0-0.80 atm of 02(g) for 0.24 s. It shows that conversion levels off as Po, is increased from 0.20 to 0.80 atm. CaSO,(s) was also qualitatively detected by XRD, in addition to CaO(s) in the oxidation products, for Po, = 0.20-0.80 atm. Since CaS04(s) is thermodynamically unstable at T 1 1473 K, CaS04(s)oxidation experiments were conducted at 1500 and 1650 K. The source of the CaS04(s)crystals were CaS04.2H20(s)crystals of approximately the same size as the CaS(s) crystals. (CaS04.2H20(s)loses its first water of hydration at 401 K and the second water of hydration at 436 K. At the high heating rates in the furnace, the loss of water of hydration occurs almost instantaneously, and the dilute feed rate prevents a significant drop in particle temperature with the water loss.) Table I shows the results; the X-ray fluorescence (S/Ca) intensity ratio, which is proportional to the stability of CaS04(s),did not decrease dramatically with the severity of oxidation conditions. This indicates that there is very little, if any, decomposition of the CaSO,(s) under these oxidation conditions. Under these conditions, the bulk PSo,concentration is zero so that these experimental results suggest that the S02(g) partial pressure developed from the CaS04(s)decomposition, the extent of which may be low, is high enough to stabilize CaSO,(s) at these high temperatures.
0
I
b
I
I
Temperature,
1
Lot 1 Lot 2
I
K
Figure 5. CaSO,(s) formation vs temperature for oxidation of lot 1 CaS(s) in 0.20 atm of 02(g): (a) mole percent of CaS04(s) in the particle (from chemical analysis); (b) % [CaSO,(s)/ (CaS04(s)+ CaO(s))] ( % CaO(s) from XRF analysis).
It is to be noted that no CaS04(s) was qualitatively detected by X-ray diffraction in the oxidation products at 1400 K, which is below the decomposition temperature of CaSO,(s). Experiments were also conducted at 1250 and 1300 K in 0-0.20 atm of 02(g) in He(g) for 0-0.28 s. Chemical analyses for the sulfate concentration (performed by Galbraith Laboratories) confirmed that CaSO,(s) was present in 1 5 mol % , the diffractometer detection limit. Figure 5a shows the mole percent of CaS04(s) in the particle (as determined by chemical analyses) formed at different temperatures for oxidation of lots 1and 2 CaS(s) crystals in 0.20 atm of 02(g). Figure 5b shows the corre-
510 Energy & Fuels, Vol. 3, No. 4, 1989
Torres-Ordoiiez et al.
Table I. Oxidation of CaSOAs) Crystals" temp, K PO,,atm residence time, s 1500 1500 1500 1650 1650
0.00 0.17
0.24 0.24
0.20 0.00 0.20
0.24 0.22 0.22
S/Cab 0.1639 0.167lC 0.1566 0.1436 0.1445
"The source of the crystals was CaS04-2H20crystal feed (which loses one HzO at 401 K and the second HzO at 436 K). X-ray diffraction scans of all oxidation products reveal only CaS04. bS/Ca intensity ratio from X-ray fluorescence analysis. cThe feed for this run was the product from the oxidation at 1500 K, 0 atm of 02, and 0.50-s residence time (i.e., the previous entry in the table). 6.32
100
c
rm CaS C r y s t a l s ILOt 21 Po2 = 0.10 atm
6.32
c 1
80
1550 K 0
0
/
60
i
1
4
L o t 1 Cas C r y s t a l s and 4.14 m (shaded p o i n t s )
rm (open p o i n t s 1 Pa,
,
I
= 0.10 atm I
5
,
I
a
/ /
'
I
i
0.00
0.05
0.10
0.15
0.20
0.25
Residence time, s
Figure 6. Percent conversion vs residence time for oxidation of lot 1 6.32- and 4.14-pm CaS(s) crystals in 0.10 atm of 02(g) for 0-0.24 s a t 1500-1750 K.
sponding conversion of CaS(s) to CaSO,(s) in terms of % [CaSO,(s)/(CaO(s) + CaSO,(s))], with % CaO(s) determined from XRF analysis. Time-resolved oxidation experiments were performed in 02(g)-He(g) for 0-0.25 s in 0.10 atm of 02(g),which are conditions typical of the postflame region. Figure 6 shows percent conversion vs residence time for oxidation in 0.10 atm of 02(g) of 6.32- and 4.14-pm CaS(s) crystals at 1400-1750 K for one lot of CaS(s) crystals (lot l),and Figure 7 shows similar results for 6.32-pm crystals at 1400-1550 K for a second lot (lot 2). Only CaS(s) and CaO(s) were qualitatively detected by XRD in the oxidation products, except at 0.10 atm of 02(g)and 1550 K for lot 2. The conversion-time curves at 1500 K (Figure 6) and 1550 K (Figure 7), in particular, are strongly S-shaped. Time-resolved experiments were also performed in 02(g)-He(g) at 0.20 atm. Figure 8a shows conversion to CaO(s) (as determined by XRF) vs residence time for lot 2 crystals oxidized in 0.20 atm of Oz(g)at 1500 and 1550 K. Under these conditions, both CaO(s) and CaSO,(s) were qualitatively detected by XRD in the particle in the oxidation products. Figure 8b is a plot of mole percent of CaSO,(s) formed in the particle (as determined from chemical analyses) vs residence time for oxidation under these same conditions. At both temperatures, the amount of CaSO,(s) formed increases with time, although at 1550 K, with the scatter in the data, the CaS04(s)content could
0.05
0.10
0.15
0.20
0.25
Residence time, s
Figure 7. Percent conversion vs residence time for oxidation of lot 2 6.32-pm CaS(s) crystals in 0.10 atm of Oz(g)for 0-0.24 s a t 1400-1550
0.00
1
K.
very well level off or decrease with time after 0.13 s. ESCA Results. Figure 9 shows ESCA concentration profiles for sulfur, oxygen, and calcium for a sample that is 4% converted. The profiles are given in terms of peak intensity (which is proportional to concentration) vs ion sputter time (which is proportional to sample depth). The profiles indicate a fairly constant calcium level throughout the sample thickness analyzed. The oxygen profile decreases while the sulfur profile increases from 0-18 min of ion-sputtering into the sample; thereafter, both oxygen and sulfur profiles appear to level off. Using a sputter rate of 25 A/min and assuming that the CaS(s) core has been reached after 18 min of sputtering, one may infer a CaO(s) product layer thickness of 450 A.
Discussion The shrinking unreacted core model of Szekely and others'* is used to describe the oxidation behavior of the CaS(s) crystals in this work. This model, which assumes the reaction occurs at the sharp interface between the outer product shell and the unreacted core of the solid, is strictly valid for nonporous reactants converting to a porous product layer. The ESCA profiles for the 4 % reacted CaS(s) suggest a diffuse reaction zone, rather than a sharp reaction interface. A t 4% conversion, the product layer thickness corresponds to 1% of the particle radius so that the profiles at this extent of conversion may not be appropriate to test for the applicability of the shrinking unreacted core mechanism. Nevertheless, if constant particle size with conversion is assumed, the shrinking unreacted core model predicts a product layer thickness of 427 A for a 6.32-pm-diameter particle at 4% conversion. A product layer thickness of 450 A may be inferred from the ESCA profiles. Model Assumptions and Formulation. The following assumptions are applied (1)isothermal system (an energy balance on our system using the experimental initial rates (18) Szekely, J.; Evans, J. W.; S o h , H. Y. Gas Solid Reactions; Academic Press: New York, 1976; pp 73-88.
Energy &Fuels, Vol. 3, No. 4, 1989 511
Kinetics of CaS(s) Oxidation 4.9-7.7
,
m CaS C r y s t a l s
-
Escn PROFILE
m a i m RTOI~E
l i n t 2) 0.20 atm
/$
l
-In
0 V 0 U
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C
."In
w
-
c
/
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0 V
1500 K
t
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>
1550 K
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6
8 18 12 SPUTTER TIK 1111M.1
14
I6
I8
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Figure 9. Ca, S, and 0 ESCA concentration profiles for lot 1 CaS(s) crystals oxidized a t 1550 K with I: = 0.04.
O/
constant effective product layer diffusivity Dew The shrinking core model development with the above-outlined assumptions yields an analytical relationship between conversion ( x ) and time ( t ) . For spherical particles, the relationship is of the form ~~
~
.05 a
0.15
0.10
0.20
0.25
-Tt = g(x) + u2p(x)
Residence time, s 4.9-7.7 un Cas C r y s t a l s
-
where
( L o t 21
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,
,
Pop
0;20 atm
(3)
,
,
(4)
i '1550
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1500 K
I
J
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0.00
b
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0.10
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I
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0.15
0.20
0.25
Residence
time. s
Figure 8. Oxidation of lot 2 CaS(s) for 0 . 2 4 s in 0.20 atm of 02(g) at 1500 and 1550 K (a) percent conversion to CaO(s), and
(b)mole percent of CaS04(s) in the particle. yield temperature rises of 10, the system can be considered to be under diffusion control. Results of Model Application. The shrinking unreacted core model solution as given in eq 3-7 was applied to the experimental x - t data shown in Figure 6. The best set of parameters k, and Deftthat fit the experimental x --t data was obtained by searching on these parameters to minimize the reduced x2 by using a 10% error in the time data. The x2 minimization was achieved via a computerized iterative gradient-search routine.20 (20) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill Book Co.: New York, 1969;pp 208-222.
Torres-Ordofiez et al.
512 Energy & Fuels, Vol. 3, No. 4,1989
.... .... ,
Kinetic control
Table 11. Results of Gradient Search on x - t Lot 1 CaS(s) Crystals diam, r m temp, K k., cm/s Dem cm2/s 2 6.32 1400 3.31 4.77 X 3.660 6.32 1500 11.74 1.11X 0.056 4.14 1500 10.34 4.70 X 0.075 6.32 1650 44.46 2.47 X 0.095 4.14 1650 31.38 1.04 X 0.104 6.32 1750 72.25 2.60 X 0.148 4.14 59.10 1.90 X low2 0.107 1750
.I 1750 K
-m
0 U 0
Data for
xmjn2N" 3.0 25.0 7.0 20.0 15.0 9.0 6.0
5 9 4 10 5 7 3
"Number of data points, as shown in Figure 6. (In Figure 10a,b, only the average values of x are shown, with the error bars representing 1 standard deviation.)
*0
Cas Crystals (Lot 11 Po, = 0.10 atm
Temperature, K :
b.00
0.05
0.10
0.15
0.20
0
.
5
1750
1650
1500
0.25
Residence time 1s)
a
---I
-
4.14 yn Cas Crystals [Lot 11 PO, 0 . 1 0 atm
. ........, .. Kinetic control _ ' Mixed control
0
Y 0
C .n
in i
> C
V 0
ARRHENIUS P L O T FOR OXIDATION OF Cas (5) TO CaO (s)
2.e
Figure 11. Arrhenius plot for oxidation of lot 1 CaS(s) crystals in 0.10 atm of 02(g) at 1400-1750 K.
0 0 GO
b
0.05
0.10
0 15
Residence t i m e
0.20
0.25
Is)
Figure 10. Experimental x - t data and model fits for oxidation of (a) 6.32- and (b) 4.14-wm lot 1 CaS(s) crystals in 0.10 atm of
Odd.
Figure 10 shows the experimental x - t data for (a) 6.32and (b) 4.14-km lot 1 crystals, respectively, along with the best fits obtained by using the gradient search. Two sets of model fits were performed: (1)a fit assuming complete kinetic control, i.e., u2 = 0 (shown as the dotted curves in the figures); (2) a fit assuming mixed control (shown as the solid curves in the figures). The quality of these two sets of fits are comparable, as seen in Figure 10a,b. Table I1 shows the values of k,, D e C , a2, and minimum x 2 obtained a t each temperature. At T I 1500 K, the kinetic and mixed control fits suggest that the reaction is largely under kinetic control ( 2I0.15). This suggests that the CaO(s) product layer formed may be sufficiently porous so that the reaction is not hindered by any product layer diffusion resistance. The Deffvalues
inferred from the model fits at these temperatures are in the range (0.5-3.0) X cm2/s. These inferred values of Deffcompare favorably with values of Deffcalculated from measured structural properties of the CaO(s) product layer in a parallel study.21 At 1400 K, the assumption of mixed control gave a 4, suggesting the importance of model fit with u2 product layer diffusion, a phenomenon that usually becomes important at high rather than low temperatures. The inferred Defffrom this model fit is 5 X cm2/s, which is lo3 times smaller than the Deffof cm2/s at the higher temperatures. Although the assumption of complete kinetic control also gave an equally good model fit at 1400 K, the mixed control interpretation is used. This is because at 1400 K, the formation of CaS04(s) is possible so that the significant diffusion resistance may be attributed to the CaS04(s) instead of the CaO(s) layer. Any CaS04(s) present must be 1 5 %, the detection limit of the diffractometer used. If the values of the intrinsic rate constant k , obtained from the fits shown in Figure 10a,b (and listed in Table
-
(21) Torres-Ordofiez, R. J. Physical Transformations during CaS(s) Oxidation. Energy Fuels, in press.
Kinetics of CaS(s) Oxidation
Energy & Fuels, Vol. 3, No. 4,1989 513
11) are used, the Arrhenius rate expression for k , is (Figure 11)
k, = 8.2276 x lo6 exp
( ‘OF’) --
cm/s
rc2
(8)
The use of k , from the kinetic control fit a t 1400 K does not change the position of this end of the plot significantly. By use of the intrinsic rate expression for CaS(s) oxidation, the rate of oxidation of CaS(s) by Oz(g) may be compared to the rate of carbon oxidation to determine if it is possible to attain complete carbon burnout before losing CaS(s). Smith and Tyler2z report the following intrinsic rate constant for the noncatalyzed oxidation of semianthracite coals at 1400-2200 K in 0.10 and 0.20 atm Oz(g):
dcao =
dc,so, dcao
+
dcaso,
Deft
r p
-
rc2
= rc2 - rc1
=
.& 6,.,,, Oca0
Dcaso,
Figure 12. Shrinking unreacted core schematic for two reaction fronts rcl and rc2and two product layers CaO(s) and CaS04(s).
At 0.10 atm of Oz(g)and 1400-1750 K, the rate of CaS(s) oxidation by Oz(g) per unit surface area (2 X lo4 to 3 X g mol/(cmz s)) is sufficiently rapid so that CaS(s) is consumed ten times faster than carbon (3 X lo-’ to 5 X lo4 g mol/(cm2 s)). There may, however, be a range of surface areas for coal and CaS(s) where sulfur retention may be possible. Assuming the coal has a specific surface area of 100 m2/g while that of CaS(s) is 1 m2/g (such as the crystals used in this work), the rate of carbon conversion (3-55/s) is approximately twice as fast as that of CaS(s) oxidation (2-24/s). There is still, however, the question of maximizing carbon burnout. It is to be noted that semianthracites are among the less reactive coals so that it is possible to attain higher sulfur retention with the more reactive coals or with catalyzed carbon oxidation. Moreover, as mentioned earlier, the CaS(s) crystal size can be varied. Hence, in order to make any conclusive statements about the extent of sulfur retention attainable with CaS(s), extensive modeling of the coal conversion and CaS(s) oxidation for the specific conditions of interest will have to be performed. With the now available rate data on CaS(s) oxidation presented in this paper and literature data on carbon oxidation, such a model of simultaneous carbon conversion and sulfur retention can be developed in order to determine coal and sulfur sorbent properties and combustion conditions that will optimize carbon burnout and sulfur retention as CaS(s). Effect of CaSO, Formation. Three interesting and unique features of the CaS(s) oxidation system have not yet been addressed in great detail: (1)the leveling off of conversion a t high Po, (10.10 atm) and high temperature (21650 K); (2) the occurrence of significant product layer diffusion at low temperature (Le., 1400 K); (3) the S-shaped 3t - t curves at 1500 and 1550 K. In the following discussion, these “anomalies” are explained in terms of the formation of CaSO,(s). The oxidation results in Figure 3 showed that conversion levels off as Po is increased beyond 0.10 atm of Oz(g) at T 1 1650 K. This leveling off is not due to a temperature effect on adsorption, e.g. a Langmuir-Hinshelwood type mechanism, where the inhibition effect is seen a t low instead of high temperatures. Hence, this oxidation rate inhibition must be due to a diffusion resistance. This resistance, however, is not caused by the growth of the CaO(s) product layer, which is considerably porous21at the high conversions attained at these high temperatures, but rather is attributed to the formation of CaSO,(s). (22) Smith, I. W.; Tyler, R. J. Fuel 1972, 51, 312-321.
At these high temperatures, the equilibrium for the formation of CaSO,(s), i.e., eq 1, is given byz3 T > 1468 K log Peqso, = 1 log Po,+ log UC&O, (10) --23680 + 12.111 - T 2 where the CaO(s) activity is unity and the CaS04(s) activity is 0.6 a t 1650 K and 0.8 at 1750 K;24Po, is the interfacial Oz(g)partial pressure. At 0.10 atm, the reaction is essentially kinetically controlled so that Po = Pbo,= 0.10. This would require -so2 = 0.02 at 1650 k and 0.12 at 1750 K, which are high pressures. At 0.20 atm, these P S o 2 values are reduced to 0.01 and 0.08; they may even be lowered if the reaction is diffusion controlled, Le., Po, > Td,only CaO(s) forms as oxidation product, and its porous product layer can offer insignificant
Energy & Fuels 1989,3, 515-522
-
diffusion resistance. In regime C (Figure 13c), the intermediate temperature region where T Td, CaS04(s) (in addition to CaO(s)) forms initially to cause diffusional limitations but decomposes during some transition period since the temperature is high enough. This decomposition may also be enhanced by crack formation in the CaO(s) product layer,2l which may increase S02(g)diffusion rates out of the particle. The CaS04(s)product layer thins and diffuses outward until, in the later period, CaS04(s) decomposition is complete and CaO(s) is the sole oxidation
515
product. This formation-decomposition sequence qualitatively explains the S-shaped x - t curves in this regime.
Acknowledgment. This research was funded by the MIT-Exxon Combustion Research Program. Patient technical assistance from Anthony Modestino and assistance with the analytical techniques from Nancy Noftle are gratefully acknowledged. Registry No. Cas, 20548-54-3; C a S 0 4 , 7778-18-9; CaO, 1305-78-8; SOZ, 12624-32-7.
Limitations of Electron Spin Resonance Spectroscopy in Assessing the Role of Free Radicals in the Thermal Reactions of Coal Timothy G. Fowler? and Keith D. Bartle School of Chemistry, University of Leeds, Leeds L S 2 9 J T , U.K.
Rafael Kandiyoti" Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2BY, U.K. Received January 9, 1989. Revised Manuscript Received May 2, 1989
Numerous investigations based on electron spin resonance spectroscopy have reported observing changes in coal free-radical concentrations during pyrolysis and liquefaction. These observations constitute the main direct evidence for free-radical reactions that are thought to occur in coal during thermal treatment. As always, conclusions drawn from experimental observations depend critically on understanding exactly what parameter has been observed and on whether the observation has been carried out in a cofrect and physically significant manner. This report summarizes some recent findings pertaining to the interpretation of free-radical concentrations measured during in situ ESR-pyrolysis experiments and discusses the relationship between measured spin concentrations and the pyrolytic reactions of coal. A simple coal thermal breakdown model and numerical estimates are used to support the main conclusion that the concentration of reactive free radicals (which act as reaction intermediates) is too low to be measured with ESR and that observed changes in spin concentrations are due to relatively stable char free radicals.
Introduction Recently, considerable interest has been expressed in free-radical reactions pertaining to the pyrolysis and liquefaction of coals and related model compounds and Efforts at establishing likely reaction pathways2and estimation of kinetic parameters4 for such reactions has proceeded in parallel with experimental studies making use mainly of electron spin resonance spectroscopy (ESR). The latter technique serves to either directly observe free radicals in situ, Le., as heating of the substrate is in progress, or directly observe free radicals after the treated sample has been cooled down to ambient temperature, usually subsequent to removal from the reactor. Clear evidence of changes in concentrations of coal free radicals, as measured by ESR, when a sample of coal is subjected to thermal treatment has emerged from numerous investigations (e.g., see ref 3 and 6-8). These observations constitute the main direct evidence for 'Present address: British Gas PLC, Midlands Research Station, Wharf Lane, Solihull B91 2JW, U.K.
0887-0624/89/2503-0515$01.50/0
free-radical reactions in coal pyrolysis. It is the relationship between the free radicals observed by ESR and the pyrolytic reactions of coal that is the subject of this paper. Recently we have reported on in situ ESR measurements4'16 for coals pyrolyzed in cells of variable geometry: (1) Bockrath, B. C. In Coal Science, II; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1983; p 65. (2) Stein, S. E. In Chemistry of Coal Conuersion; Schlosberg, R. H., Ed.; Plenum: New York and London, 1985; p 13. (3) Petrakis, L.; Grandy, D. W. Free Radicals in Coals and Synthetic Fuels; Elsevier: Amsterdam, 1983. (4) Gavalas, G. R.Coal Pyrolysis; Elsevier: Amsterdam, 1982. (5) Lewis, I. C.; Singer, L. S. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P.A., Eds.; Marcel Dekker: New York and Basel, 1981, Vol. 17, p 1. (6) Smidt, J.; van Krevelen, D. W. Fuel 1959, 38, 355. (7) Ladner, W. R.;Wheatley, R. Br. Coal Util. Res. Assoc., Mon. Bull. 1965. --,-29. - , -201. -(8)Sprecher, R. F.; Retcofsky, H. L. Fuel 1983, 62, 473. (9) Fowler, T.G.; Kandiyoti, R.; Bartle, K. D. Fuel 1988, 67, 1711. (10)Fowler, T.G.; Bartle, K. D.; Kandiyoti, R. Fuel 1987, 66, 1407. (11) Fowler, T.G.;Bartle, K. D.; Kandiyoti, R. Carbon 1987,25, 709. (12) Fowler, T.G.;Bartle, K. D.; Kandiyoti, R. Fuel 1988, 67, 173.
0 1989 American Chemical Society
