Optimum sulfation temperature for sorbents used in fluidized bed coal combustion

Optimum sulfation temperature for sorbents used in fluidized bed coal combustion

Fuel Processing Technology, 25 (1990) 227-240 227 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands Optimum Sulfation Temp...

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Fuel Processing Technology, 25 (1990) 227-240


Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Optimum Sulfation Temperature for Sorbents Used in Fluidized Bed Coal Combustion MOHD ZAKI HAJI-SULAIMAN*, ALAN W. SCARONI and SAVASH YAVUZKURT

The Combustion Laboratory, The Pennsylvania State University, University Park, PA 16802 (U.S.A.) (Received September 26th, 1989; accepted in revised form February 9th, 1990)

ABSTRACT An investigation was performed to identify the factors responsible for the existence of a characteristic optimum sulfation temperature for limestones and dolomites used as sorbents during the fluidized bed combustion of coal. The optimum sulfation temperature for a particular sorbent was found to be a function of the operating conditions, and ranged between 800 and 900°C. A reduction in the level of sulfur uptake by the sorbents at high temperatures was found to be a consequence of pore blockage by product formation. As the sulfation reaction rate increased with increasing temperature, the CaS04 produced accumulated at the external surface of the sorbent particles, blocking access to the porous interior. The parameters which most affected the optimum sulfation temperature were the physical structure (pore size and surface area) of the sorbent, and the operating temperature and residence time. For fixed operating conditions it was necessary to have a proper combination of surface area and porosity to ensure high utilization. For short particle residence times, the optimum temperature for high sulfur retention by the sorbent was several hundred degrees higher than when the particles were allowed to react for longer periods of time. This was attributed to kinetic rather than equilibrium constraints.


In recent fluidized combustion has become accepted as a versatile and practical clean coal combustion technology. By using a suitable sorbent as the bed material, high sulfur coal can be burned while maintaining low SO2 emissions without substantial penalty in combustion efficiency [1]. From numerous studies which have been conducted [2,3] it is generally agreed that under fluidized bed combustion conditions the appropriate operating temperature for optimum sulfur retention is around 800-900 ° C. The reasons for the existence of an optimum sulfation temperature are still not clear and several factors have been suggested in the literature. *Present address: Department of Chemical Engineering, University of Malaya, 59100 Kuala Lumpur, Malaysia.


© 1990 Elsevier Science Publishers B.V.

228 Moss [4 ] proposed that sulfation in fluidized bed combustors occurs via two routes; one involving S02 and the other S03 Route 1: CaO ÷ S02-~ CaSOa


and CaSO3 + ½02-* CaS04


Route 2: SO2 + ½02-* SO3


and S03 + CaO-* CaS04


Moss suggested that both routes operate below 850 ° C but at higher temperatures CaS03, being unstable, does not form. In other words, above about 850 ° C the reactions in Route 1 involving CaS03 as an intermediate are not significant. According to Jonke et al. [5] a fluidized bed combustor contains both oxidizing regions where sulfation occurs and reducing regions where there is CO present. The regeneration of CaO can take place in the presence of CO by the reaction CaS04 + CO-* CaO + C02 + S02


It was suggested that the competition between sulfation and regeneration was the cause of the observed reduction in S02 retention at high temperatures. Dennis et al. [6] and Dennis and Hayhurst [7] furnished some evidence for the role of burning coal particles, and particularly the volatiles, in reducing sulfur uptake by sorbents at high temperatures. From several experiments they concluded that, as a result of radical quenching in a fluidized bed containing sand, coal volatiles can coexist with oxygen in the particulate phase at temperatures below 900 ° C. Above this temperature the 02 concentration in the particulate phase drops to a low level. As such, the fuel sulfur released predominantly as H2S may be oxidized to S02, but because of the low 02 concentration, will react very slowly with CaO at these temperatures. There are others [8] who argue that, at high temperatures, changes occur in the structure of calcines primarily due to sintering. As a result there is a reduction in the concentration of active sites and, therefore, the reactivity towards S02 is decreased. The optimum sulfation temperature also is a function of the system employed. In the dry limestone injection system this optimum temperature has been reported to be about 1200 °C [9 ]. A satisfactory explanation has not yet been offered for this, although it is likely that kinetic rather than equilibrium considerations prevail. It may also be that sulfation occurs in a lower temperature region of the system than that reported. It has been proposed that the


relatively slow rate of calcination for large sorbent particles which are commonly employed in fluidized bed combustion may contribute to the lower optimum sulfation temperature. However, relatively similar optimum temperatures have been observed for precalcined and raw sorbents [10]. The present study was conducted to improve understanding of the effect of temperature on the sulfation of sorbents under well-defined fluidized bed conditions. The experimental results provided a clear indication that the reduction in the sulfur retention by the sorbents at high temperature was due to pore blockage as a result of CaS04 formation. For short particle residence times, the temperature for optimum sulfur retention was higher than when longer reaction time was available. Thus, the existence of different optimum sulfation temperatures in the different systems can be explained in terms of differences in particle residence times, that is, in terms of kinetic rather than equilibrium considerations. EXPERIMENTAL

A schematic diagram of the experimental apparatus is given in Fig. 1. It consisted of a quartz fluidized bed reactor (3.8 cm ID) with a gas preheating section below the distributor which is positioned centrally within an enclosing electric tube furnace. The system was designed for batch operation at atmospheric pressure. The temperature in the bed was controlled by an imbedded chromel-alumel thermocouple connected to a furnace temperature controller. The fluidizing gas was a blend of C02, 02, N2 and S02, simulating the dry products of coal combustion. The gases were mixed in the desired proportion using calibrated mass flow controllers. A portion of the exhaust gas from the reactor was fed to a series of on-line gas analyzers which were linked directly to a computerized data acquisition system. Prior to conducting a sulfation experiment, sized sorbent particles (1.0-1.2 m m diameter) were calcined under preselected conditions in the fluidized bed reactor. Sulfation of the precalcined sorbent was then accomplished by two different techniques. The first involved a known amount of sorbent which was charged into a bed of silica sand fluidized by the reactant gas mixture. The S02 concentration in the exhaust gas was continuously monitored. The extent of sulfation was calculated from a sulfur balance on the inlet and exhaust gas streams. This is expressed by the following equation: X


= - ~ - J (1 - Yo)dt 0

where X is the conversion (S/Ca mole ratio), G the molar flowrate of gas entering the bed, Yi the mole fraction of SO2 in the fluidizing gas, Yo the ratio of the outlet SO2 concentration at time t to that at t = 0 and M the number of























hi h

Nfi I





Fig. 1. Schematic diagram of the experimental apparatus TABLE 1 Chemical and physical properties of sorbents

CaC03 (wt.%) MgC03 (wt.%) I"(wt.%) Vpo(cm3/g) So (m2/g) Po(g/c m3)

High calcium limestone


96.4 1.40 1.3 0.006 1.0 2.47

52.0 43.8 4.2 0.014 1.6 2.86

"Other components.

moles of calcium in the batch. In selected experiments partially sulfated sorbent was sampled for analysis. Separating the sorbent from the inert bed material was found to be difficult, so a second technique was employed. A similar amount of sorbent was charged into the reactor and heated to the reaction temperature in N2 before switching to the reaction gas. The extent of sulfation was calculated from the total sulfur content of the sample (as determined using a Leco Sulfur Analyzer). In the method the sample was combusted in 02, during which the sulfur was released as SOe. The SO2 was titrated using HC1, KI and a small amount of KIO3. It was found [ 11 ] that for a given amount of sorbent, the extent of conversion as


measured by the two techniques under identical operating conditions agreed to within 2%. In this work, narrowly sized particles in the range 1.0-1.2 m m diameter of a high calcium limestone and a dolomite were used as the sorbents. Their physical and chemical properties are given in Table 1. The reported sulfation experiments were performed in a gaseous mixture containing 0.49% SO2. RESULTS


Figure 2 shows the variation in sulfur uptake with temperature for three dolomite calcines prepared at calcination temperatures of 850, 900 and 935 °C for a residence time of 20 minutes. The calcined samples were characterized with respect to their pore size and surface area as indicated in Fig. 3 and Table 2, respectively.Each sample had a different conversion/temperature relationship producing a characteristic optimum sulfation temperature. The possibility of desulfurization at high temperatures was minimized by maintaining oxidizing conditions in the bed. A concentration of 4 vol.% 02 was continuously added to the reaction gas before itentered the bed. As confirmed by the 02 analysis of the exhaust gas stream, there was no measurable change in the 02 concentration due to sulfation. According to Turkdogan and Rice [12 ], who conducted an in depth study of the desulfurization of lime during 0.45 C A L C I N A T I O N T E M P (°C) • 935 r'l 9 0 0 A 850




0.30 700










Bed Temperature (°C)

Fig. 2. Variation in sulfur uptake with temperature for dolomite calcines prepared at different calcination temperatures. Particleresidence time is 20 rain (20 vol.% C02 in the sulfatinggas).

232 1.5


CALCINATION TEMP (°C) ,:, 935°C D 900°C



]o It

-z~ 850°C

0.9 "Z <




~ --




100 Pore






Fig. 3. Effect of calcination temperature on the pore size distribution of dolomite calcines. TABLE2 V~iationinsu~ace~eawithcalcining~mperature Temperature (°C)

BET surface area (m2/g)

850 900 935

15.5 14.3 12.7

and subsequent to calcination, the decomposition of C a S O 4 is inhibited in oxidizing conditions.. In air they did not observe desulfurization at temperatures up to 1200°C. Therefore, the observed drop in sulfur retention at high temperature in the current work was not attributed to desulfurization. As shown in Fig. 3 the calcination temperature affected the physical structure of the calcines. The calcine produced at 850 oC had a bimodal pore distribution, whereas the calcines produced at 900 and 935 oC had unimodal distributions. There was a general trend of increasing average pore radius with increasing calcination temperature. These structural differences may affect kinetics of sulfation but should not affect the thermodynamics. If desulfurization had occurred, a single optimum sulfation temperature would have been observed, since all experiments were performed under identical conditions using calcines originating from the same raw sorbent. Further evidence to support the conclusion that desulfurization did not oc-

233 0.15 TEMPERATURE (°C) A 650 × 850 •



o E 8 A


0.00 0







T 960

T 1200


Time (sec)

Fig. 4. Effect of temperature on the kinetics of sulfur uptake of high calcium limestone calcines

prepared at 950 °C. (No C02 in the sulfating gas.)

cur iscontained in Fig. 4, which shows the sulfationbehavior of the precalcined high calcium limestone as a function of time at differenttemperatures. Even though the experimental conditions employed were somewhat differentthan those used in Fig. 2, the general conclusion is the same. In Fig. 2, 20 vol.% CO2 was added to the reaction gas which is the equilibrium partialpressure of CO2 at a temperature of 789 °C [13]. From the Loss-on-lgnition test ( A S T M C25) itwas confirmed that the parallelrecarbonation reaction of C a O with CO2 had occurred simultaneously with sulfationat 750 °C. It isvery likelythat the lower conversion achieved at this temperature was due to the competition between these two reactions. The data in Fig. 4 were obtained without C02 in the reaction gas, so that a wider temperature range could be studied. As shown in this figure the curves obtained at all temperatures continue to increase throughout the duration investigated,even though the testperformed at 650 °C exhibited the highest sulfur uptake at the longest residence time. A continuation of sulfuruptake is an indication that equilibrium had not been reached for the time/temperatures used in this study. The dependence of the optimum sulfationtemperature on the sorbent structure also eliminates the need for invoking a contribution of reaction chemistry as proposed by Moss [4]. Indeed, a change in the reaction chemistry to route


2 at temperatures above 850 °C has been dismissed by several investigators. Kocafe et al. [14] compared the sulfation rates of reagent grade CaO using S02-02-He mixture and a S03-He mixture as the reaction gas. No significant difference in the rates of sulfation was observed, suggesting that the reaction mechanism involving SO3 as the intermediate product did not pertain. Dennis and Hayhurst [7] estimated sulfation rates in the absence of significant external mass transfer resistance and did not find a systematic dependence on the 02 concentration. If reaction route 2 were significant, which involves the formation of S03, the rate should be dependent on the 02 concentration. The results of the present investigation support the conclusion that the reduction in sulfur retention at high temperatures is due to changes in the structure of the reacting solid. When experiments were conducted at low temperatures, changes in the solid structure were mainly due to the formation of CaS04 which completely filled or blocked the pore network. At high temperatures sintering can become significant and can cause additional changes to the physical properties of the reacting solid. Figure 5 shows the variation in the surface area with reaction temperature of the dolomite calcines, partially sulfated to approximately the same extent. These calcines were prepared by calcination in a mixture of 20% CO2 in N2 at 850°C. The sulfation performances as a function of temperature are given in Figure 2 (the curve indicated by the triangles). The dimensionless surface area used is defined as the ratio of the BET* surface area at conversion X, Sx, expressed on a unit weight of calcine 1.0





x = 0.40

< • u

x = 0.38 0.6

1: U')


_~ c .2

x = 0.38



x = 0.34 N


0.0 700

i 750

i 800

i 850

i 900


Temperature (°C) Fig. 5. V a r i a t i o n in d i m e n s i o n l e s s surface area o f partially sulfated d o l o m i t e calcines w i t h sulfation t e m p e r a t u r e . Soc = 15.5 m 2 g-1. * B E T s t a n d s for B r u n a u e r - E m m e t t - T e l l e r .


basis, to that at zero conversion. Beyond 800 ° C there is a drop in the measured surface area with increasing temperature. The contribution to the drop in the surface area due to sintering by increasing the temperature from 800 to 850 ° C should be minimal, since the original sample was prepared at 850 ° C. This was confirmed experimentally by surface area measurements made on the unreacted sample which had been exposed for an additional 20 minutes at 800 ° C to the same gaseous atmosphere but without S02 addition. There was essentially no difference between the surface area of this sample and that of the original sample. The reduction in the surface area between 800-850 oC is not a result of CaS04 filling the available pores, since there is a slight decrease in conversion from 0.38 to 0.34) with increasing temperature. The most likely explanation is that a larger fraction of the pores within the reacting solid has been blocked at the higher temperature. The surface area within the closed pores is not accessible to N2 and therefore is not reflected in the measured total surface area. Since the conversion at 850°C is slightly lower than that at 800°C, it is suggested that at the higher temperature CaSO4 formed rapidly near the external surface of the particle and subsequently blocked pore entrances. As a result of this blockage further reaction of SO2 with reactive CaO in the particle interior was hindered. Due to a lower rate of reaction at 800 ° C, S02 was able to diffuse deeper into the particle, hence, more CaO participated in the reaction to give a higher overall calcium utilization. This observation provides initial, but strong, evidence that the low conversion achieved at high temperatures is due to early pore blockage close to the external surface of the particle. The further drop in the surface area with increasing temperatures up to 935 ° C, as shown in Fig. 5, was attributed to both sintering and pore blockage, the relative contribution of each process was not determined, however. The sample reacted at 750 °C had a slightly lower surface area than that reacted at 800 ° C. As discussed earlier, for the C02 concentration employed in the experiments, the reaction between CaO and CO2 occurred in parallel with the sulfation reaction. In this case some loss in surface area due to the recarbonation process can be expected since the solid volume of CaCO3 is larger than that of CaO. The effect of temperature on the sulfation rate is highlighted in Fig. 6. The observed reaction rates during sulfation are plotted against the fractional conversion at operating temperatures of 650, 850 and 950 ° C. As shown, the initial rate of sulfation increases with increasing temperature. At 950 ° C the sulfation rate is about a factor of two higher than the initial rate at 650 ° C. As reaction progresses the rate measured at 950°C drops rapidly and is essentially zero at a conversion of 5.5%. The decrease in the reaction rate with conversion is more gradual at 650 ° C. The rapid drop in the reaction rate with conversion at high temperatures is consistent with early blockage of pore entrances. As mentioned previously, due to the rapid initial sulfur uptake at high temperatures, a substantial amount of CaSO4 accumulates at the external surface of the re-

236 14




8 >c 13¢


4 0


0 0.00

i 0.02

i 0.04

i 0.06

~ 0.08

1 0.10


Final Conversion, x

Fig. 6. Dependence of sulfation rate on conversion for high calcium limestones calcines. acting particle. This argument is consistent with the data in Fig. 4, where the curve at 950 ° C levels off within a very short residence time. At low operating temperature, the rate of diffusion of SO2 into the solid matrix is faster than the rate of chemical reaction. Sulfur dioxide will penetrate deeper into the particle and react more uniformly. At this temperature pore blockage does not occur rapidly and the reaction proceeds at a higher rate even at relatively high conversions. Hence, a more uniform sulfur distribution would be expected in a particle which had been reacted at low temperature compared to the higher temperature case. Additional evidence to support the conclusion of early pore blockage at high temperatures can be taken from the surface area measurements shown in Fig. 7. Calcines of the high calcium limestone prepared at conditions similar to those in Fig. 3 were used in this series of experiments. No CO2 was added to the reaction gas. The dimensionless surface area used in the plot is the same as that defined for Fig. 5. T h e drop in the surface area with conversion is more pronounced at 950 ° C than at 670 ° C. At 10% conversion the sample reacted at 950 ° C had about a 15% smaller surface area than the one prepared at 670 ° C, even though the extent of conversion achieved was the same. An explanation for the observed trend is that at 950°C much of the surface area within the solid has been isolated because of pore entrance blockage. Loss in surface area due to sintering is always possible. However, both experiments used the same calcine, which had been prepared at 950 ° C. An additional 1-hour exposure of

237 1.0

0.9 ~

d <~



2, w


RE 0.6

0.5 0.00


T E M P E R A T U R E (°C) [3 6 7 0 •

950 ,






Fractional Conversion, x

Fig. 7. Effect of sulfation temperature on the variation in the dimensionless surface area of partially sulfated high calcium limestone calcines with conversion. No C02 in the sulfation gas. (Sot= 13.3 m2 g- 1.) the calcine to the same temperature (950°C) gave a 3% reduction in the surface area. Therefore, loss of surface area due to sintering alone cannot explain the results shown in Fig. 7. The optimum sulfation temperature for sorbents reported in the literature varies as a function of the operating conditions used. From the results reported by Ulerich et al. [ 15 ], the optimum temperature appeared to correspond closely to the calcination temperature at which the samples were prepared. Those prepared at 815 and 900 ° C gave optimum sulfur uptake at round 815 and 900 ° C, respectively. These experiments were performed on 1 mm sorbent particles at a residence time of 1 hour. There is not a simple correlation between optimum sulfation temperature and calcination temperature in the current work as shown in Fig. 2. The calcine prepared at 850 ° C performed best at a sulfation temperature of 800 ° C, while the one prepared at 900 °C exhibited the highest sulfur uptake at 900 ° C. The sample precalcined at 935 °C had an optimum sulfation temperature at about 870 ° C and did not perform as well as the other two. The drop in the conversion at temperatures below 800 °C is partly due to the competition between recarbonation and sulfation as discussed earlier. Indeed, for calcines of high calcium limestone, the sulfur uptake at 650 ° C is higher than at 850°C when the C02 in the reaction gas is removed, as shown in Fig. 4. However, it is important to realize that further reduction in the operating temperature will eventually result in a lower sulfur uptake, since the rates of the


chemical reactions are slow at these temperatures and equilibrium becomes important. With respect to the physical properties of the sorbent, the two most significant parameters are pore size and surface area. According to Fig. 3, the average pore size increased from 50 to 85/~m by increasing the calcination temperature from 850 to 935 ° C. As expected the B E T surface area decreased monotonically within the temperature range investigated (Table 2). Among the three samples, the one prepared at 900°C had the best combination of surface area and pore size to give the highest sulfur uptake at 900 ° C. The importance of a proper balance between these two parameters can be argued in terms of pore blockage which is delayed by the presence of large pores and active site concentration which increases with increasing surface area. Borgwardt [16] reported a continuous increase in calcium utilization with increasing temperature over the range 650-980 ° C. In other words, no optimum temperature was observed during the four seconds residence time investigated. Silcox et al. [9], who simulated the conditions used in dry limestone injection systems, found an optim u m temperature around 1200 ° C. The sorbent residence time in the furnace was of the order of a few milliseconds. By referring to Fig. 4 it can be concluded that particle residence time is an important variable in the determination of the optimum sulfation temperature. For a residence time of 20 seconds, 950 °C was to be the best temperature for achieving high utilization. At a slightly longer residence time of 120 seconds the optimum temperature was 850 ° C. On the other hand, when the residence time was greater than 400 seconds, operating at 650 ° C produced the highest sulfur retention. In fluidized bed combustion the optimum sulfation temperature appears to be about 200-300°C lower than that for limestone injection into pulverized coal combustors. There is a significant difference in the sorbent residence time in these two systems. In pulverized coal combustion sorbent particles are exposed to SO2 for only a few seconds, while those in fluidized bed combustion can react in the bed for several minutes. The trend of increasing optimum temperature with decreasing residence time is in agreement with the observation presented in Fig. 4. CONCLUSIONS

This study has provided some experimental evidence for the role of sorbent physical structure and pore blockage in limiting calcium utilization at high temperature. Due to the rapid rate of sulfation at these temperatures, the formation of CaSO4 quickly blocks pore entrances which prevents further reaction. At lower operating temperatures SO2 penetrates deeper into the particle interior, since the rate of surface reaction is much slower. As a result there is a delay in pore mouth closure to give a more uniform conversion throughout the particle and generally a higher overall calcium utilization. For different


samples, the optimum sulfation temperature depends largely on the physical structure developed upon calcination. In order to obtain a high utilization an optimum combination of pore size and surface area is necessary. When the particle residence time is short the temperature for optimum sulfur retention is generally higher than for those particles which are allowed to react for longer times. ACKNOWLEDGEMENTS

Financial support for this work was provided by the Ben Franklin Partnership Program of the Commonwealth of Pennsylvania and the Pennsylvania Energy Development Authority (PEDA). Financial assistance by the Government of Malaysia to one of the authors (MZHS) is also acknowledged. The samples were provided by SME Limestone Co., Lowellville, Ohio; Greet Limestone Co., Morgantown, West Virginia; Con Lime Co., Bellefonte, Pennsylvania, and Duff and Sons, Huntsville, Ohio. NOTATION

G M So Soc

yo X

Yi Yo Po

molar flowrate of gas entering the bed moles of calcium in the batch BET surface area of raw sorbent BET surface area of calcine pore volume of raw sorbent fractional conversion of CaO to CaS04 mole fraction of SO2 in the fluidizing gas ratio of outlet S02 concentration at time t to that at t= 0 particle density of raw sorbent

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240 7 Dennis, J.S. and Hayhurst, A.N., 1984. 20th. Int. Symp. on Combustion. The Combustion Institute, Pittsburgh, PA, pp. 1347-1355. 8 Turkdogan, E.T., 1980. Physical Chemistry of High Temperature Technology.Academic Press, London. 9 Silcox, G.O., Slaughter, D.M. and Pershing, D.W., 1984. 20th Int. Symp. on Combustion. The Combustion Institute, Pittsburgh, PA, pp. 1357-1364. 10 O'Neill, E.P., Keairns, D.C. and Kittle, W.F., 1976. Thermochimica Acta, 14: 209. 11 Haji Sulaiman, M.Z., 1988. Ph.D. Thesis, The Pennsylvania State University, University Park, PA. 12 Turkdogan, E.T. and Rice, B.B., 1973. Am. Inst. Min. Eng. Trans., 254: 29. 13 Turkdogan, E.T., Olsson, R.G., Wriedt, H.A. and Drken, L.S., 1973. Trans. Am. Inst. Mett. Eng., 254: 19. 14 Kocafe, D., Karman, D. and Steward, F.R., 1985. Can. J. Chem. Eng., 63: 971. 15 Ulerich, N.H., O'Neill, E.P. and Keairns, D.L., 1978. Thermochimica Acta, 26: 269. 16 Borgwardt, R.H., 1970. Environ. Sci. Technol., 4: 59.