Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated

Nov 15, 1995 - or dolomite to the bed and removing the sulfur from the system with the bottom ash in the form of CaS. This work describes the H2S upta...
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Ind. Eng. Chem. Res. 1996, 35, 176-183

Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 1. Sorbent Performance K. Patrik Yrjas,* Cornelis A. P. Zevenhoven, and Mikko M. Hupa Department of Chemical Engineering, Åbo Akademi University, Lemminka¨ isenkatu 14-18B, FIN-20520 Turku/Åbo, Finland

Sulfur emission control in fossil fuel gasification plants implies the removal of H2S from the product gas either inside the furnace or in the gas clean-up system. In a fluidized-bed gasifier, in-bed sulfur capture can be accomplished by adding a calcium-based sorbent such as limestone or dolomite to the bed and removing the sulfur from the system with the bottom ash in the form of CaS. This work describes the H2S uptake by a set of physically and chemically different limestones and dolomites under pressurized conditions, typically for those in a pressurized fluidized-bed gasifier (2 MPa, 950 °C). The tests were done with a pressurized thermobalance at two pCO2 levels. Thus, the sulfidation of both calcined and uncalcined sorbents could be analyzed. The effect of pH2S was also investigated for uncalcined limestones and half-calcined dolomites. The results are presented as conversion of CaCO3 or CaO to CaS vs time plots. The results are also compared with the sulfur capture performance of the same sorbents under pressurized combustion conditions. Introduction Sulfur emission control in fossil fuel gasification plants implies the removal of H2S from the product gas either inside the furnace or in the gas clean-up system. In a fluidized-bed gasifier (FBG), in-bed sulfur capture can be accomplished by adding a calcium-based sorbent such as limestone (CaCO3) or dolomite (CaCO3‚MgCO3) to the bed and removing the sulfur from the system in the form of CaS. Depending on the CO2 partial pressure and the temperature in the gasifier, the sorbents may decompose to the respective oxides.

CaCO3·MgCO3(s) S CaCO3(s) + MgO(s) + CO2 S CaO(s) + MgO(s) + 2CO2 (1) CaCO3(s) S CaO(s) + CO2

(2)

In fluidized-bed gasifiers, used in Integrated Gasification Combined Cycles (IGCC) concepts (Mojtahedi et al., 1992), the conditions are such that MgCO3 always calcines to MgO. However, the stability of CaCO3 is dependent on the CO2 partial pressure and the temperature. This dependence is illustrated in Figure 1. Figure 1 also identifies the typical ranges for atmospheric and pressurized fluidized-bed processes. Depending on the conditions, the sulfur absorbents present in the gasifier are referred to as follows: CaCO3, uncalcined limestone; CaO, calcined limestone; CaCO3‚ MgCO3, uncalcined dolomite; CaCO3 + MgO, halfcalcined dolomite; CaO + MgO, fully calcined dolomite. Thus, the two sulfidation reactions considered in this paper are:

CaCO3(s) + H2S S CaS(s) + H2O + CO2

(3)

CaO(s) + H2S S CaS(s) + H2O

(4)

If dolomite is used as an absorbent, the MgO will not react with sulfur compounds, because MgS is an unstable compound under gasification conditions. * Author to whom correspondence is addressed. Telephone: (int.+35821) 2654311. Fax: (int. +35821) 2654780. E-mail: [email protected].

0888-5885/96/2635-0176$12.00/0

Figure 1. CaCO3-CaO equilibrium as a function of temperature and pCO2, (Yrjas, 1994).

Reactions 3 and 4 are both limited by thermodynamic equilibrium, and not all H2S can be captured with Cabased sorbents in a gasification reactor (Liliedahl et al., 1992; Yrjas and Iisa, 1991; Kurkela et al., 1991; Bryan et al., 1988; Squires et al., 1971). Depending on the conditions, there will remain a H2S equilibrium concentration of 100-500 ppm. Due to this, in practical implementations a secondary desulfurization step may be needed. Generally, the sulfidation of CaO under atmospheric pressure has been found to be relatively fast and conversions up to 100% have been achieved (O’Neill et al., 1972; Attar and Dupois, 1979, Efthimiadis and Sotirchos, 1992). High conversions (80-90%) were also achieved by Illerup et al. (1993a) and Yrjas et al. (1995) for calcined limestones under pressurized conditions. Illerup et al. (1993a) reported a higher final conversion when sulfidating calcined limestones with higher initial porosity. Allen and Hayhurst (1991) used 75-94 µm particles of pure CaO to study the capture of H2S in a thermobalance. The reaction was deemed to be limited by chemical reaction kinetics. The sulfidation reaction was found not to follow first-order kinetics. Instead, it was suggested to obey a Langmuir-Hinshelwood type of mechanism, with Ca(OH)(SH) as an intermediate © 1996 American Chemical Society

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product. Activation energies between 21 kJ/mol (Westmoreland et al., 1977) and 128 kJ/mol (Borgwardt et al., 1984) have been reported for the sulfidation of CaO. However, some of the lower activation energy values were determined from experiments done with a thermogravimetric apparatus (TGA) and may have been influenced by mass-transfer effects (Yrjas et al., 1995). No significant differences between the sulfidation of CaO and CaO + MgO have been observed (Schreiber and Petrie, 1978; Attar and Dupois, 1979; Yrjas et al., 1995). However, Abbasian et al. (1990) suggest that dolomite is preferably used because, in the subsequent oxidation step, the CaS produced from CaO + MgO reacts with O2 more easily than CaS produced from CaO. The oxidation of CaS is needed to produce the stable and disposable product, CaSO4. Illerup et al. (1993a) and Yrjas et al. (1995) reported the sulfidation reaction of CaCO3 to be very slow at 1 and 2 MPa, respectively (700-950 °C). The conversions were below 20% for the tested limestones after a sulfidation time of 120 min. Yrjas et al. (1995) reported an activation energy between 90 and 120 kJ/mol (850950 °C), where product layer diffusion was shown to control the sulfidation rate. Borgwardt and Roache (1984) showed that for smaller particles (1-3 µm) over 60% conversion could be achieved (570-850 °C, 0.1 MPa, 70% CO2). They also reported an activation energy of 180 kJ/mol for the reaction between H2S and CaCO3. Krishnan and Sotirchos (1994) studied the sulfidation of CaCO3 with an atmospheric TGA at a high CO2 partial pressure (0.07 MPa) to prevent the limestone from decomposing. They obtained a conversion of about 70% for small particles (53-62 mm), while the conversion for larger particles (297-350 mm) was below 20%. Fenouil et al. (1994) attributed the poor conversion of CaCO3 to sintering of the sulfidation product, CaS. Thus, CaS would form a thin but dense product layer on the limestone particles, hindering the continuation of the reaction, although the molar volume of CaCO3 is larger than the molar volume of the product, CaS. The sintering would only take place in the presence of CO2, which they state to act as a catalyst. Ruth et al. (1972) studied the sulfidation of CaCO3 + MgO at 0.1 MPa with a high CO2 partial pressure (550800 °C). They reported that the sulfidation of CaCO3 + MgO was considerably faster than the sulfidation of CaCO3. Ruth et al. (1972) also reported the reaction between H2S and CaCO3 + MgO to be faster than that between H2S and CaO + MgO. The activation energy for the sulfidation of CaCO3 + MgO (60-65 µm) was reported to be 251 kJ/mol (550-750 °C). Yrjas et al. (1995) studied the sulfidation of three natural halfcalcined dolomites at 2 MPa and 725-950 °C. The final conversions to CaS at 950 °C varied between 60-90%. The final conversions were concluded to be directly proportional to the MgCO3/CaCO3 molar ratio, which was determined from calcination experiments. Yrjas et al. (1995) reported the activation energy to be 300-400 kJ/mol in the temperature range 725-775 °C, when the limiting resistance was shown to be chemical reaction. Efthimiadis and Sotirchos (1992) and Illerup et al. (1993a) reported that H2S capture by CaO is more efficient than the corresponding desulfurization reaction between SO2 and CaO. They concluded this behavior to be due to the differences in the molar volumes of the solid reactant (VCaCO3 ) 36.9 cm3/mol) and the products (VCaSO4 ) 46.0 cm3/mol, VCaS ) 28.9 cm3/mol). Because the molar volume of CaSO4 is larger than that for the

original material (CaCO3), pore blocking will occur, thus decreasing the reactive surface area. This is, however, not the case for the reaction between CaO and H2S where the product, CaS, has a significantly lower molar volume than the initial material, CaCO3, and the reaction can proceed to higher conversions without pore plugging. Krishnan and Sotirchos (1994) compared the direct sulfation (SO2) and direct sulfidation (H2S) of CaCO3. At 750 °C they state that the utilization of CaCO3 is higher under gasification conditions than under combustion conditions. This behavior can probably be attributed to the differences in the molar volumes of the participating solid materials. The purpose of this work was to investigate the sulfur capture abilities of different natural Ca-based absorbents under pressurized gasification conditions. In this paper (no. 1) the experimental results are presented and discussed. In paper 2, the results are being modeled in more detail. The model which is used was developed to take the change of the effective diffusivity due to conversion into account. Experimental Section The apparatus used in this work was a pressurized thermogravimetric apparatus, PTGA. The electrically heated reactor was constructed of Incoloy 800 and lined with a quartz glass tube. A more detailed description of the apparatus is given by Yrjas et al. (1993) and Iisa (1992). The sample holder was cylindrical, and the sample was placed in a shell between an inner core and an outer net. In our experiments, the thickness of the sample layer in the holder was 1 mm. The sample holder was made of platinum. The sulfidation experiments were all made at 2 MPa and at 750 or 950 °C and a gas flow of 3.33 L/min at NTP. In the tests under calcining conditions, the inlet gas composition during sulfidation was: 0.2 vol % H2S, 3.5 vol % CO2, 2 vol % H2, and the rest N2. In the tests under noncalcining conditions, the composition was: 0.2 vol % H2S, 20 vol % CO2, 10 vol % H2, and the rest N2. In the experiments under noncalcining conditions, the addition of CO2 was necessary to ensure that CaCO3 did not calcine. H2 was added to the inlet gas to prevent the dissociation of H2S to other sulfur compounds (COS, S2, SO, SO2, etc.). The combinations of H2S, H2, CO2 and N2 were chosen, because the resulting H2S equilibrium concentrations were the same at the same temperature in the gas phase. The calculations showed that, at equilibrium, at least 90% of the gas-phase sulfur would remain as H2S in all the experiments. Experimental Procedure. Eleven limestones and six dolomites were used in the experiments. The CaCO3 and MgCO3 contents were calculated from the weight loss results from PTGA experiments performed under calcining conditions. The obtained CaCO3 and MgCO3 contents by this method showed significant differences compared to the elemental analyses. This could be because in some cases the elemental analyses were performed on a different particle size fraction and different size fractions may have variations in the compositions. The differences could not be caused by incomplete calcination in the PTGA runs, because in that case the resulting Ca and Mg contents should be lower than the elemental analyses. This was, however,

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Table 1. Properties and Compositions of the Tested Limestones and Dolomites sorbent

particle size (mm)

particle density (kg/m3)

specific surface, N2 BET (m2/g)

particle porosity, Hg porosimetry

CaCO3a (wt %)

MgCO3a (wt %)

limestone 1 limestone 2 limestone 3 limestone 4 limestone 5 limestone 6 limestone 7 limestone 8 limestone 9 limestone 10 limestone 11 dolomite 12 dolomite 13 dolomite 14 dolomite 15 dolomite 16 dolomite 17

250-300 250-300 250-300 250-300 250-300 250-300 250-300 250-300 250-300 250-355 250-355 250-300 200-400 200-400 250-300 250-300 200-400

1681 2416 b b 1351 1762 b b 2786 b b b b b 2727 2855 b

2.25 1.49 b b 4.63 2.94 b b 3.74 b b b b b 0.93 0.06 b

0.178 0.075 b b 0.324 0.077 b b 0.063 b b b b b 0.022 0.010 b

98.5 99.4 99.8 98.9 97.0 82.3 99.1 90.9 86.1 100.5 101.0 61.8 44.6 60.6 67.1 56.4 56.7

3.1 2.0 2.0 2.5 2.0 2.0 1.4 4.2 8.0 1.8 0.6 45.1 19.6 38.2 29.8 40.3 39.0

a

The values are derived from calcination experiments with a pressurized TGA. bNot measured.

not always the case. The compositions obtained from the calcination experiments are used in this paper. A total of 100 mg of the absorbent was mixed with 200 mg of inert material to minimize-mass transfer limitations. After this, the sample was heated to 850 °C in 100 vol % CO2 at 0.1 MPa. During this step the MgCO3 calcined to MgO. If the desired absorbent was CaCO3, the sample was left under these conditions (100 vol % CO2, 850 °C, 0.1 MPa) until the weight stabilized. If the desired absorbent was to be in the form of CaO, the gas environment was changed to N2, and calcination was allowed to proceed. This stage was continued until the weight of the sample no longer decreased. After these steps both the temperature and pressure were changed to the specified values. The gas composition was then adjusted to the sulfidation conditions of the experiment. During the adjustment period, the gas flow was conducted through a bypass, and there was a stagnant atmosphere in the thermobalance reactor. Once the specified gas composition was obtained, the gas flow was switched back to the thermobalance reactor and the sulfidation began. The sulfidation was allowed to proceed for about 2 h. The temperature and the weight of the sample were recorded at 10-50 s intervals during the experiments. The conversion of CaO or CaCO3 to CaS was calculated from the weight change and the initial weight of the sample. Also, the initial porosities, densities, and surface areas were measured of five of the limestones and of two of the dolomites. The compositions and physical properties of the materials are shown in Table 1. A more detailed elemental analysis is presented in the appendix. Results The experiments were done mainly at 950 °C and 2 MPa, but there were also some runs at 750 °C to investigate the influence of pH2S on the sulfidation of half-calcined dolomite. The lower temperature was chosen because, at 950 °C the sulfidation limiting resistance was found to be sample bed diffusion and at temperatures equal or below 775 °C the limiting resistance was shown to be chemical reaction (Yrjas et al., 1995). As is observed from Figure 2, the conversions of the uncalcined limestones vary between 5% and 40%. The high reactivity of limestone 5 can be explained by its high initial porosity. Limestone 5 has also earlier been

Figure 2. Conversion of uncalcined limestone to CaS as a function of time at 950 °C and 2 MPa.

found to behave differently than the other limestones under both gasification (Illerup et al., 1993a) and combustion conditions (Illerup et al., 1993b; Yrjas et al., 1993). When comparing the porosity with the reactivity of the sorbents, we can see that (Figure 3), the higher the porosity, the higher the final conversion. This is in agreement with what Illerup et al. (1993a) found for the sulfidation of calcined limestones. However, limestone 9 differs slightly from this trend. This difference may have been caused by the calcination of the relatively high MgCO3 content (8.0 wt %) in this specific limestone. Due to the decomposition of MgCO3 to MgO, the porosity of the limestone was increased, thus increasing the conversion rate. Also, the effect of pH2S on the sulfidation of uncalcined limestone was investigated. In the aim to study the effect of pH2S on identical product layers, the uncalcined limestone was first exposed to a gas containing 0.2 vol % H2S for 60 min. After this initial step, the H2S

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Figure 3. Conversion of uncalcined and calcined limestone compared to the initial porosity of the limestone. Sulfidation at 950 °C and 2 MPa.

Figure 5. Conversion of half-calcined dolomite to CaS as a function of time at 950 °C and 2 MPa.

Figure 4. Conversion vs time with pH2S as a parameter for uncalcined limestone 9. The samples were first sulfided under identical conditions with 0.2% H2S (0.004 MPa) for 60 min, after which the H2S concentration was changed. Sulfidation at 950 °C and 2 MPa.

Figure 6. Conversion after 60 min as a function of the Mg/Ca molar ratio in the sorbents. Sulfidation at 950 °C and 2 MPa.

concentration was changed to 0.1, 0.2, 0.3, 0.4, or 0.5 vol % (pH2S ) 0.002-0.01 MPa) and the conversion rate was measured. From the slopes in Figure 4 it was observed that the change of pH2S did not influence the sulfidation rate. This suggests that the limiting resistance under these conditions was not dependent on H2S but on some other species involved. From Figure 5 a fast initial conversion rate of halfcalcined dolomite was observed. However, the conversion rates slowed abruptly down at different levels of conversion for the dolomites. As mentioned earlier, Yrjas et al. (1995) found a proportionality between the final conversion and the MgCO3/CaCO3 molar ratio for half-calcined dolomites. They explained this behavior by the fact that natural dolomites consist of a mixture of dolomite particles (CaCO3‚MgCO3) and limestone particles (CaCO3). In the beginning of the sulfidation it was mainly the dolomite particles that reacted, and when all dolomite particles were sulfided, the reaction continued with the limestone particles which were

shown to react very slowly (Figure 2). The ratio MgCO3/ CaCO3 can be used as an indicator of the molar fraction of dolomite particles in the natural dolomite in question. Here, of course, it was assumed that all MgCO3 derived from calcination runs was bound to the dolomite and no free MgCO3 was present. The conclusion by Yrjas et al. (1995) is supported by Figure 6 showing the conversion after 60 min vs the MgCO3/CaCO3 molar ratio, for all tested sorbents. Also, for the sulfidation of half-calcined dolomite the effect of pH2S was investigated. These pressurized experiments (2 MPa) were, as earlier mentioned, done at 750 °C. The conversion rates at four different pH2S (0.004, 0.006, 0.008, and 0.01 MPa) were measured for dolomites 14, 15, and 16. The experiments showed a very strong dependency of pH2S (Figure 7). Taking the reaction rate between 5% and 10% conversion and assuming that the H2S concentration on the sample surface was the same as that in the bulk gas, the resulting apparent reaction orders were 3.6 for dolo-

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Figure 7. Conversion vs time with pH2S as a parameter for halfcalcined dolomite 16. Sulfidation at 750 °C and 2 MPa.

Figure 9. Conversion of fully calcined dolomite to CaS as a function of time at 950 °C and 2 MPa.

Figure 10. Conversion of fully calcined and half-calcined dolomite as a function of time at 750 °C and 2MPa.

Figure 8. Conversion of calcined limestone to CaS as a function of time at 950 °C and 2 MPa.

mites 14 and 15 and 3.8 for dolomite 16. It should be noted that, with such high H2S dependencies, very small differences in the experiments will cause noticeable changes. Therefore, the dependencies of H2S may very well be of the same order, although the experimental results differ slightly. The kinetics of the sulfidation reaction of calcined limestone and fully calcined dolomite could not be investigated with the PTGA due to sample bed diffusion being the limiting resistance in the experiments (Yrjas et al., 1995). However, sample bed diffusion did not influence the final conversions given in Figures 8 and 9. Figure 8 shows the conversion of CaO to CaS of the eleven calcined limestones as a function of time. Rela-

tively high initial conversion rates were observed, with final conversions varying from 70 to 90%. The conversions of the calcined limestones follow to some degree the initial porosity (Figure 3). However, limestone 9 seems to differ. The reason for limestone 9 being different can again be attributed to its relatively high content of MgCO3, which upon calcination will increase the particle porosity. In Figure 9 the sulfidation of fully calcined dolomite is illustrated. No significant differences could be observed when comparing with the sulfidation of calcined limestone. The final conversions of the six fully calcined dolomites tested varied between 78% and 88%. The conclusion by Ruth et al. (1972), that the sulfidation of fully calcined dolomite is slower than the sulfidation of half-calcined dolomite, could not be supported (Figure 10). In Figure 10 the comparison of the sulfidation rates was made at a lower temperature (750 °C), to ensure that at least one of the sulfidation reactions was not limited by mass-transfer resistances. In this case the sulfidation of half-calcined dolomite was

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Figure 11. Comparison of sulfur capture efficiencies of limestone under pressurized combustion and pressurized gasification conditions.

limited by reaction kinetics, while the sulfidation of fully calcined dolomite was limited by sample bed diffusion (Yrjas et al., 1995), i.e., the sulfidation rate for the latter case would have been even faster if diffusion resistances could have been minimized. The sorbents used in this work have earlier been used by Yrjas et al. (1993) in an experimental series under pressurized combustion conditions, when the major sulfur species is SO2. In Figures 11 and 12 the sulfur absorption capacities of the different sorbents are compared under pressurized combustion conditions vs pressurized gasification conditions. It can be clearly seen that the conversion of uncalcined limestone was significantly higher under combustion conditions. This is not in agreement with what Krishnan and Sotirchos (1994) concluded. However, Krishnan and Sotirchos (1994) performed their experiments at 750 °C, while our experiments were done at 950 °C. Therefore, one explanation for the different behavior could be that the sintering of CaS increased strongly at the higher temperature (Fenouil et al., 1994) and therefore the sulfidation reaction was inhibited. An interesting fact is that half-calcined dolomite reached a higher final conversion under gasification conditions than under combustion conditions (Figure 12), while for uncalcined limestone it was the opposite way. In both cases CaCO3 reacted as sorbent, but the dolomite particles were, as

Figure 12. Comparison of sulfur capture efficiencies of dolomite under pressurized combustion and pressurized gasification conditions.

mentioned earlier, more porous than the limestone particles due to MgCO3 calcination. This suggests that the porosity has more influence on the sorbent utilization under gasification conditions than under combustion conditions. The large differences between the halfcalcined and fully calcined conditions for dolomites 13 and 15 were due to the low MgCO3/CaCO3 molar ratio, indicating that the dolomites contained large amounts of limestone particles. When these particles were calcined further, their behavior followed that of the limestones in Figure 8. Conclusions In this paper the sulfidation of limestone and dolomite under typical pressurized gasification conditions has been investigated and discussed. All experiments have been performed with a pressurized thermogravimetric apparatus under both calcining and uncalcining conditions. Uncalcined limestones were shown to be poor H2S sorbents. The conversions were found to be in some degree dependent on the initial porosities of the lime-

Table 2. Chemical Compositions of the Tested Sulfur Absorbents (wt %) sorbent

C

Si

Al

Fe

Mn

Ti

P

K

Na

CaCO3a

MgCO3a

CaCO3b

MgCO3b

limestone 1 limestone 2 limestone 3 limestone 4 limestone 5 limestone 6 limestone 7 limestone 8 limestone 9 limestone 10 limestone 11 dolomite 12 dolomite 13 dolomite 14 dolomite 15 dolomite 16 dolomite 17

11.9 11.8 11.9 11.9 11.9 10.9 11.3 11.0 11.3 11.8 12.0 12.9 7.17 12.6 11.9 9.40 12.1

0.21 0.26 0.11 0.10 0.17 3.89 1.60 2.46 1.56 0.28 0.03 0.04

0.053 0.058 0.021 0.026 0.037 0.206 0.38 0.704 0.672