High-Pressure Reaction Kinetics of Hydrogen Sulfide and Uncalcined

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High-Pressure Reaction Kinetics of Hydrogen Sulfide and Uncalcined Limestone Powder Rajeev Agnihotri, Shriniwas S. Chauk, Santhosh K. Misro, and Liang-Shih Fan* Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210

High-pressure and high-temperature in-gasifier removal of H2S using CaCO3 sorbent powder is studied in a differential-bed flow-through reactor at pressures of up to 2 MPa and temperatures of up to 900 °C. The CaCO3 sulfidation kinetic data are obtained under conditions closely simulating the pressurized fluidized bed gasifier conditions of an integrated gasification combined cycle system. The effects of total pressure, H2S partial pressure (2-5 kPa), reaction temperature, CO2 and H2 partial pressures, particle size (16-200 µm), and CaCO3 surface area on the extent of H2S removal and solid conversion are determined. The gasifier pressure is found to have a negative effect on the sulfidation reaction. The sulfidation reaction is first order with respect to H2S partial pressure. The conversion data of the sulfidation reaction are analyzed using the grain model, with the overall reaction rate being controlled by chemical reaction at the interface and by diffusion through the product layer. At higher conversions, the reaction is found to be product layer diffusion controlled with an apparent activation energy of about 48.7 kcal/mol. The high-pressure sulfidation kinetic data obtained in this study would be useful in understanding and optimizing the in-gasifier H2S capture using CaCO3 sorbents. Introduction Coal is the principal source of electricity generation in the U.S. accounting for nearly 56% of the electricity produced. Currently, coal-fired power plants average about 33% efficiency (energy conversion to electricity). Advanced coal-based electric power generating processes, such as the integrated gasification combined cycle (IGCC) and the second-generation pressurized fluidized bed combustors (PFBC) are under investigation to increase the utilization of high-sulfur coal in an efficient and environmentally friendly manner. Compared to existing low-efficiency combustion-based power generation methods, new clean coal technologies are projected to greatly improve efficiency (45-55%) while reducing air pollution for the same amount of coal usage. Commercial success of these advanced coal-based power-generating systems is greatly dependent on the accompanying air pollution control systems. The IGCC technology with a projected efficiency of 52% (Clean coal technologysprogram profile, 1994; Clean power systemss program profile, 1994) is considered to be one of the potential technologies which would meet the energy and environmental demands of the future. The coal gasification process is conducted at temperatures of 800-1100 °C and pressures of 0.5-3.2 MPa, where coal is partially oxidized in the presence of steam to produce a fuel gas mixture containing hydrogen, carbon monoxide, carbon dioxide, methane, and nitrogen (if air is used for gasification). In the reducing conditions encountered in a gasifier the sulfur in coal is released primarily in the form of H2S while forming COS (carbonyl sulfide) and CS2 (carbon disulfide) as secondary sulfur species. During gasification, more than 90% of the sulfur in the gas phase has been reported to be in the form of H2S. Due to strict environmental regulations * To whom correspondence should be addressed. Phone: (614) 292-7907. Fax: (614) 292-3769. E-mail: [email protected].

and the demanding operating conditions of these advanced systems, a strict reduction of sulfur species is required. Squires et al. (1969) have shown that removing the sulfur and other contaminants from the coal gas at high temperature and pressure offers numerous advantages over other gas-cleaning processes, primarily higher thermal efficiency and reduced volume of processing vessels. Recent advances toward high-efficiency powergenerating units (PFBG/IGCC) (Frey and Rubin, 1992) propose that gasifiers should be operated under higher pressures (1-2 MPa) and temperatures (800-950 °C). In such processes, the fuel gases are cleaned prior to sending them through the gas turbine. The hot gas cleanup is necessary not only for the protection of the gas turbine hardware but also to comply with the environmental regulations and improve the overall efficiency of these processes. Coal gas desulfurization to sufficiently low levels at temperatures above 600 °C and at the gasifier pressures of 1-2 MPa is now recognized as crucial to efficient and economical coal utilization in IGCC systems. One of the strategies to achieve this goal is to remove the majority of the fuelbound sulfur in-gasifier by using calcium-based sorbents. Post-gasifier desulfurization is undertaken to polish the product gas to reduce the remaining H2S concentration to less than 50 ppm (Mojtahedi et al., 1994). Expensive novel sorbents (usually metal oxides) are used in the polishing units to reduce H2S to desired levels. The in-bed or in-gasifier removal of H2S can be carried out by using calcium-based sorbents such as lime (CaO), limestone (CaCO3), and dolomite (CaCO3-MgCO3). There exists a spatial distribution of temperature and gas-phase composition in a gasifier. Depending on the gasifier temperature and partial pressure of CO2 in the fuel gas stream, the calcium carbonate could calcine to give CaO or remain uncalcined. Both calcined and noncalcined limestone reacts with H2S, forming product

10.1021/ie990271m CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999

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CaS. The majority of the studies reported in the literature on limestone sulfidation are conducted under calcining conditions. There are only a few studies on the CaCO3-H2S reaction conducted under actual gasifier conditions. Limestone reacts with H2S under these conditions to form solid product CaS as

CaCO3 + H2S a CaS + H2O + CO2

(1)

H2S reaction with CaO above the calcination temperature of limestone is exothermic in nature whereas direct limestone sulfidation is an endothermic reaction; thus, it follows that the maximum conversion at equilibrium is obtained at the calcination temperature of limestone for a given fuel gas composition and total pressure. According to thermodynamic calculations, it is possible to reduce 98% of H2S (from 4000 ppmv to levels as low as 80 ppmv) from fuel gases by using Cabased sorbents at temperatures close to the limestone calcination temperature and thereby reducing the load on expensive post-gasifier polishing units (Fenouil et al., 1994). For a given operating pressure and CO2 and H2O content in the fuel gas, the equilibrium concentration of H2S in the outlet fuel gas stream at the limestone calcination temperature can be calculated from the equilibrium constants for limestone sulfidation and limestone calcination and is given by (Fenouil, 1995)

yH2S ) (1.75 × 10-3)yH2OyCO20.364P0.364

(2)

where P is the total pressure expressed in bars. However, there is a lack of relevant high-pressure limestone sulfidation kinetic data to support these theoretical predictions. Attar and Dupuis (1979) investigated sulfidation of CaCO3 calcite (mineral matter in coal) in a pulsed differential reactor at 400-800 °C and found that chemical reaction was the limiting step until about 78 monolayers of CaS were formed. After that the reaction was limited by diffusion through the CaS crust. They obtained an activation energy of 37 kcal/mol in the temperature range of 560-670 °C. However, a much lower activation energy of 4 kcal/mol was obtained at temperatures above 770 °C, which was attributed to the mass transfer limitations in their apparatus. Freund (1981) studied the kinetics of reaction between limestone/dolomite (∼69 µm) and H2S in the temperature range of 1065-1310 °C at 1 atm in a tubular laminar flow reactor with cocurrent flow of gas and solids. He found that sintering rates of CaCO3 were faster than sulfidation rates under the experimental conditions of study, and the sulfidation reaction was severely diffusion limited. Borgwardt and Roache (1984) investigated sulfidation of limestone particles (1.6-100 µm) in a differential bed reactor in the temperature range of 570-850 °C. They reported that the rate of reaction varied inversely with the particle size and that the initial rate was high, which leveled-off rapidly above 11% conversion for larger (>15 µm) limestone particles and attributed the poor conversion to sintering of CaCO3, which led to a loss of initial surface area and porosity. They reported the apparent activation energy of sulfidation to be 42 kcal/mol. Sulfidation studies conducted by Fenouil et al. (1994) showed negligible sintering of millimeter-sized limestone particles in the temperature range of 700-900 °C. They found no significant loss of surface area of lime-

stone particles and related the poor reactivity to sintering of the CaS product layer enveloping the CaCO3 particle, suggesting that CaS covers the limestone particles with a nonporous layer. They reported the activation energy of CaS sintering to be 48 kcal/mol for the temperature range 750-900 °C. Krishnan and Sotirchos (1994) studied different factors influencing the H2S uptake by CaCO3 in a thermogravimetric analysis system under noncalcining ambient pressure conditions. The effects of particle size, temperature, and H2S partial pressure were investigated. A very strong influence of particle size and temperature on the process was revealed by the experimental data. They obtained sulfidation conversions around 70% for smaller CaCO3 particles (53-62 µm) while the conversions for larger particles (297-350 µm) were below 15% at 750 °C. Yrjas et al. (1996) studied the H2S absorption capacities of limestone and dolomite (125-180 µm) in a pressurized thermobalance at 950 °C and 2 MPa. The calcined limestone and fully calcined dolomite showed the highest sulfidation conversions in the range of 8090%. Sulfidation of uncalcined limestone was slow, and in 2 h, only about 20% conversion was observed. They found apparent activation energies in the range of 2228.9 kcal/mol for uncalcined limestone. Lin et al. (1996) conducted a kinetic study on the high-pressure reaction of H2S with limestone and half-calcined dolomite in a thermogravimetric analyzer (TGA) at 1 MPa and in the temperature range of 750-870 °C. The reactions of both sorbents were determined to be first order with respect to H2S partial pressure. They found the activation energies for both materials to be approximately 43 kcal/ mol; however, the initial reaction rate of half-calcined dolomite (50 µm) was significantly higher than that of limestone (50 µm). This difference in initial rates was attributed to a more open pore structure of half-calcined dolomite. Zevenhoven et al. (1998) studied the uptake of H2S by limestone particles (250-300 µm) at 850950 °C and 1.5-2 MPa. They proposed a changing internal structure (CIS) model, which related the reaction kinetics and product layer diffusion to the intraparticle surface. Diego et al. (1999) conducted sulfidation experiments with limestones and half-calcined dolomite (0.4-1.6 mm) in an atmospheric pressure thermogravimetric analyzer in the temperature range of 600-850 °C. Their study revealed the effects of reaction temperature, gasphase composition, sorbent particle size, and H2S concentration on sulfidation conversion. They observed that the sulfidation rate of limestone was independent of particle size, suggesting that pore diffusion was not an important resistance affecting the overall sulfidation rate. They also analyzed the effect of coal gas composition on sulfidation and found that the sulfidation of limestone was unaffected by any gas-phase component above 800 °C; however, below 800 °C it increased with increasing (H2S + COS) partial pressure. The objective of this study is to determine the kinetic parameters of high-pressure sulfidation of CaCO3. In this work, high-pressure (0.1-2.0 MPa) and hightemperature (650-900 °C) sulfidation of CaCO3 sorbent is studied in a flow-through differential-bed reactor. The reaction between CaCO3 and H2S is carried out under typical pressurized gasifier conditions. The effects of various parameters such as total pressure, reaction temperature, H2S partial pressure, gas-phase composition, sorbent particle diameter, and initial sorbent

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Figure 1. Schematic diagram of the high-pressure and -temperature reactor (HTPR) assembly.

surface area on sulfidation characteristics of CaCO3 particles are investigated. Experimental Section Apparatus. For investigating the sulfidation kinetics at the gasifier conditions of higher pressures and temperatures, a novel reactor system is designed. This reactor has the flexibility of operating under fixed bed (differential) as well as fluidized bed mode, and can be used as an entrained flow reactor with minor modifications. The overall reactor assembly is shown in Figure 1. The schematic of the high-pressure and -temperature reactor (HPTR) vessel is shown in Figure 2. The details of the reactor assembly are discussed elsewhere (Agnihotri et al., 1998, Chauk, 1999). The pressure vessel is designed to operate at pressures up to 5.0 MPa and temperatures up to 1000 °C. The reactor involves a dual-shell design and an internal diffusion furnace. The 30 in. tall internal diffusion furnace (Thermcraft) is a vertical three-zone heater. The use of an internal diffusion furnace is necessary because of the high gas flow rates required under elevated pressures. The pressure vessel is made of alonized SS 316 and has internal insulation to avoid contact with the heater. The main reactor consists of a 2.0 in. o.d. Inconel-600 tube housed in the pressure vessel. Inconel600 is chosen due to its corrosion resistance against H2S at the temperatures and pressures of interest here. In the dual-shell type of arrangement the reactor walls need not be thick due to the pressure balance. Furthermore, this type of reactor arrangement reduces the use of expensive materials of construction. The annular space between the Inconel tube and the inner core of the internal diffusion furnace provides the preheating zone for the bulk of the reactant gases and also provides

Figure 2. Schematic of the high pressure and -temperature reactor (HTPR).

enough time for all the gas-phase reactions, particularly the water-gas-shift reaction, to equilibrate. At the bottom of the reactor, the 2.0 in. o.d. Inconel tube slides inside a 2.25 in. i.d. Inconel tube, which allows the entry of the heated bulk gas into the 2 in. tube and also allows for the thermal expansion of the Inconel tubes. The flange plates at the top and bottom and the sidewalls of the pressure vessel have openings to accommodate

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thermocouples for close monitoring of gas temperatures both above and below the sorbent bed. The reactant gases (CO, CO2, and N2) are introduced into the outer shell of the pressure vessel whereas H2 and H2S are introduced directly above the sorbent bed via a specially designed movable lance, to ensure minimal H2S contact with the alonized steel walls of the pressure vessel. A specially designed sorbent bed holder made from thick-walled 1.0 in. o.d. quartz tube is inserted into the reactor from the bottom. The quartz sorbent bed holder is designed to support the sorbent particles over a sintered ceramic fritt, which can also act as a gas distributor for the fluidized bed mode of operation. The total gas flow rate through the sorbent bed at any given pressure and temperature condition is maintained at approximately 10 lpm. The reactant gas mainly consists of 1-3 kPa of H2S, 2-5% CO, 3-20% CO2 (noncalcining conditions), 2-10% H2, and balance N2. The presence of CO is essential to prevent oxidation of CaS to CaSO4 by CO2 at elevated temperatures (Fenouil, 1995), whereas H2S thermal decomposition is reduced by the presence of H2. The combination of individual gases is chosen such that the partial pressure of H2S resulting from equilibrium gasphase interactions would be at the desired value. H2O is a major component of the fuel gas; however, direct injection of H2O (either as water or steam) into the gas stream was not carried out to limit the number of variables that could influence the reaction. It is postulated that the presence of significant amounts of CO2 and H2 would result in H2O in the gas stream according to the water-gas-shift reaction. Mass flow controllers are used to deliver the precise amount of gases against the reactor pressure to give a predetermined reactant gas composition. To eliminate pressure fluctuations, a line regulator is added downstream of the mass flow controllers. Reactant gas mixture is introduced into the pressure vessel and is heated via direct contact with the internal diffusion furnace. Provision is made to determine the inlet gas composition by on-line gas chromatography prior to sending the gas into the reactor. The exiting reactant gases from the reactor are sent through a coiled condenser to quench them, and any condensed water is removed in the water knockout pot. A backpressure regulator is used to maintain the reactor assembly at a predetermined pressure setting. Procedure. For all the experiments the ambience of the reactor is maintained such that the fugacity of CO2 in the reactor inhibits the calcination of limestone at the temperatures of interest. As an initial step, preliminary runs were performed to confirm the noncalcining conditions at a fixed CO2 partial pressure which would be used for the rest of the limestone sulfidation investigations in the reactor. The thermogravimetric analysis of the solids was performed to determine that the CO2 partial pressure used provided noncalcining conditions. For all the sulfidation studies 15 mg of limestone is sprinkled evenly on the quartz wool inside the sample holder and inserted into the reactor. During sample insertion the reactor is continuously flushed with CO2. After the sorbent bed is sealed in place, the nitrogen flow is started and the system is pressurized to the desired pressure. H2/H2S and CO gas streams are introduced into the reactor and the sorbent is exposed to the H2S laden gas stream for varying periods of time. To terminate the sulfidation reaction at any given time, the H2/H2S, CO, and N2 gas streams are shut off and

Table 1. Chemical Composition and Initial Structural Properties of CaCO3 Samples Investigated CaCO3-1a CaCO3-2b CaCO3-3c CaCO3-4c composition (wt %) CaCO3 SiO2 Al2O3 MgO Fe2O3 trace elements median particle size (d50, µm) BET surface area (m2/g) pore volume (cm3/g)

97.0 0.8 0.5 1.0 0.5 0.2 7.8

95.0 1.2 0.9 1.4 0.7 0.8 13.5

99.0 0.2 0.2 0.4 0.1 0.1 1.5

99.0 0.2 0.2 0.4 0.1 0.1 1.8

5.0

8.0

11.0

14.0

0.042

0.048

0.10

0.12

a

Linwood Mineral and Mining Co., Davenport, IA. b Germany Valley Mineral Co., Pennsylvania. c High-reactivity carbonate (Fan et al., 1998).

the reactor is purged with CO2 with the sample holder still inside the reactor. The spent sorbent is allowed to cool in this noncalcining atmosphere. It is then removed from the reactor, weighed, and mixed in 120 mL of 0.005 M buffered iodine solution for estimation of the sulfidation conversion by titrating it against standard arsenite solution. Analysis. The solids are analyzed using iodometric titration techniques. The spent sorbent is dissolved in 120 mL of 0.005 M buffered iodine solution using an ultrasonic mixer.

CaS + I2 a Ca2+ + 2I- + S

(3)

The resulting solution is titrated against 0.005 M arsenious acid, and the amount of iodine that is left over after reaction 3 is estimated.

As2O3 + 2I2 + 2H2O a As2O5 + 4HI

(4)

This titration helps in estimating the amount of sulfur and thus the amount of CaS in the spent sorbent. Sorbents. Most of the experiments were done using Linwood carbonate. Some of the experiments, to characterize the effect of sorbent structural properties, used Germany Valley carbonate and in-house synthesized high-reactivity carbonate (whose structural properties could be tailored). High-reactivity calcium carbonate particles were synthesized following a patented process (Fan et al., 1998). Table 1 shows the composition of the different sorbents used. The particle size distribution of all the CaCO3 sorbents tested in this study is obtained using Sedigraph 5100 as shown in Figure 3. The surface area, pore volume, and pore size distributions for each sorbent are characterized by conducting the low-temperature nitrogen adsorption (BET) measurements. Results and Discussion Effect of Operating Pressure and Temperature. Since gasifiers are operated at high pressures (1-3.5 MPa), this study focuses on high-pressure sulfidation of limestone. The first set of experiments included study of the effect of total pressure on the sulfidation reaction. The total pressure is varied between 0.1 and 2 MPa, and the flow rate is maintained at about 10.0 lpm through the sorbent bed, which is adjusted by changing the STP flow rate to account for the compression due to pressure elevation and thermal expansion of the gas. All experiments are conducted under a constant partial pressure of H2S (2 kPa). The gas composition is adjusted

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Figure 3. Particle size distribution of CaCO3 sorbents investigated.

Figure 4. Effect of total pressure on CaCO3 sulfidation (reaction temperature 800 °C; H2S partial pressure 2 kPa; reaction time 13 min).

to maintain noncalcining conditions inside the reactor. The reaction temperature is maintained at 800 °C and the CO2 partial pressure at 50 kPa. Figure 4 shows the effect of total pressure on sorbent conversion when Linwood carbonate is exposed to H2S (2kPa) for 13 min. An adverse effect of increasing total pressure is observed. This could be associated with the fact that there is an increase in the number of gaseous moles during the limestone sulfidation reaction. Thus, on the basis of thermodynamics, the forward reaction would be expected to be inhibited at higher pressures. After the effect of total pressure is studied, the rest of the experiments are conducted at a representative total pressure of 1 MPa.

Figure 5. Effect of reaction temperature on CaCO3 sulfidation (total pressure 1 MPa; H2S partial pressure 2 kPa; CaCO3 surface area 5.0 m2/g).

To determine the effect of operating temperature on the reaction, sulfidation of CaCO3 is studied at temperatures ranging between 650 and 900 °C. The partial pressure of the H2S in the inlet gas stream is 3.5 kPa, and the H2/H2S ratio is maintained at 5.0 for all the runs. A total pressure of 1 MPa is maintained with a CO2 partial pressure of 50 kPa to prevent calcination of CaCO3. Prior to performing sulfidation, the sorbent is presintered in an atmosphere of CO2 at 900 °C to isolate the effect of temperature on the sulfidation reaction from the effects of sintering. Figure 5 shows a positive effect of temperature on sulfidation conversion of Linwood carbonate. Effect of Gas Composition. The reactant gas composition is varied to determine the effect of various gas constituents. Experiments are carried out at 1 MPa of total pressure and 800 °C to determine the dependence of CaCO3 sulfidation on H2S, CO2, and H2 partial pressures. The effect of varying the H2S partial pressure in the gas phase on the sulfidation conversion is shown in Figure 6. As can be seen from the figure, with an increase in the partial pressure of H2S from 2 kPa (representative of low sulfur coals) to 5 kPa (typical of high sulfur coals), an increase in the sulfidation conversion and initial reaction rate is observed. From the initial rate of reaction, the order of the reaction with respect to H2S partial pressure is determined to be approximately 0.85, which is slightly less than the value of 1 reported in the literature for ambient pressure sulfidation (Borgwardt et al., 1984; Krishnan and Sotirchos, 1994; Lin et al., 1994). Figure 7 shows the data obtained by varying the partial pressure of H2 in the feed stream. The increasing partial pressure of H2 is represented as an increase in the mole ratio of H2 to H2S with the H2S partial pressure fixed at 3.5 kPa. The sorbent is exposed to the H2/H2S laden gas stream for 5 min at 800 °C and 1 MPa for all the runs. Initially the presence of hydrogen increases the rate of sulfidation of CaCO3; however, as the H2/ H2S mole ratio is increased further, the reaction is observed to be inhibited. The initial rise in the conver-

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Figure 6. Effect of H2S partial pressure on CaCO3 sulfidation showing the comparison of experimental data and product layer diffusion controlled model predictions (reaction temperature 800 °C; total pressure 1 MPa; CaCO3 surface area 5.0 m2/g).

Figure 7. Effect of the partial pressure of H2 on CaCO3 sulfidation (reaction temperature 800 °C; total pressure 1 MPa; CaCO3 surface area 5.0 m2/g; H2S partial pressure 3.5 kPa; reaction time 5 min).

sion can be explained on the basis of reduced thermal decomposition of H2S in the presence of H2. Thus, the gas stream in the presence of H2 would exert a higher partial pressure of H2S for a particular inlet gas stream composition than in its absence, which would lead to higher conversions. Inhibition of sulfidation reaction is observed upon further increasing the hydrogen concentration in the gas stream. Similar observations have also been reported in ambient pressure sulfidation studies. Attar and Dupuis (1979) have hypothesized that the desired product CaS is formed as a result of the dissociation of surface complexes. On the basis of this view, they attributed the decrease in sulfidation conver-

Figure 8. Effect of the partial pressure of CO2 on CaCO3 sulfidation (reaction temperature 800 °C; total pressure 1 MPa; CaCO3 surface area 5.0 m2/g; H2S partial pressure 3.5 kPa).

sion in the presence of H2 to the dissociation of surface complexes to form H2S instead of CaS. Borgwardt and Roache (1984) presented an alternate view based on their experimental findings. They suggested that the decrease in the sulfidation was due to the reduction in the permeability of the product layer in the presence of H2. Hence, though H2 increases the conversion when present in small quantities by suppressing the dissociation of H2S, it has an adverse effect on the conversion when present in larger quantities. Sulfidation of limestone is accompanied by evolution of CO2 as shown in reaction 1, and its partial pressure is expected to influence the sulfidation conversion adversely. The range of CO2 partial pressure studied is chosen to provide noncalcining conditions for all the experiments. The effect of CO2 partial pressure is found to be negligible in the range of 30-200 kPa, as shown in Figure 8. This can be associated with the fact that, at such high partial pressures of CO2 (compared to H2S partial pressure), the reverse of the CaCO3 sulfidation reaction can be considered to be a pseudo-zero-order reaction with respect to CO2 partial pressure, resulting in a negligible effect on the reaction. To prevent formation of CaSO4 due to the presence of CO2, CO is added to the inlet stream for all the experiments (Fenouil et al., 1994). The oxidation of CaS to CaSO4 by CO2 is dominant at higher temperatures; however, to maintain the gas composition constant under all the conditions investigated in this study, small amounts of CO are included in the reacting gas even at lower temperatures. Effect of Sorbent Particle Size (dp) and Surface Area. Sorbent particle size affects the sulfidation reaction kinetics while other sorbent physical characteristics such as surface area and pore volume also play a crucial part in determining the overall sorbent utilization and the rate of sulfidation reaction. To obtain particles with different initial dp, the sorbent, Germany Valley carbonate (average surface area of 8.0 m2/g), is crushed and sieved using sieves of different mesh sizes. For these

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Figure 9. Effect of sorbent particle size on CaCO3 sulfidation (reaction temperature 800 °C; total pressure 1 MPa; H2S partial pressure 3.5 kPa).

Figure 10. Effect of initial sorbent surface area on CaCO3 sulfidation (reaction temperature 800 °C; total pressure 1 MPa; H2S partial pressure 3.5 kPa).

experiments particle sizes ranging between less than 20 and 150-250 µm are used. Figure 9 shows that, at 800 °C, 1 MPa, and 3.5 kPa of H2S, lower particle sizes result in higher conversions and reaction rates. Experiments are also conducted under identical conditions with four sorbents possessing different initial surface areas ranging between 5 and 14 m2/g and with similar mass median particle diameter (d50) of about 1 µm. The partial pressure of H2S is maintained at 3.5 kPa at a reaction temperature of 800 °C for all the experiments. Figure 10 shows that higher initial surface area led to higher initial reaction rates and higher levels of conversion. The scanning electron micrograph (SEM) of a partially sulfided CaCO3 particle is shown in Figure 11. The SEM picture shows the outer surface of the particle to have a highly fused molten-chocolate-like texture with negligible porosity. This could be due to the formation of high molar volume CaS product, which causes pore mouth plugging and blockage. The EDS results show that in the vicinity of external particle surface the Ca to S molar ratio is close to unity, indicating that the surface layer predominantly contains CaS. This would suggest that carbonate sulfidation at elevated pressure proceeds via the unreacted shrinking core model. High-Pressure Sulfidation Kinetic Modeling. The product layer diffusion model is based on the grain theory, which assumes that the sorbent particle consists of nonporous grains of uniform size. According to this theory, the reactant gas H2S diffuses through the product layer (CaS) surrounding each grain and reacts with the CaCO3 at the interface of the product and the unreacted core. For small particles (