Ind. Eng. Chem. Res. 2002, 41, 6207-6208
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CORRESPONDENCE Comments on “Proposal for a Regenerative High-Temperature Process for Coal Gas Cleanup with Calcined Limestone” M. Hartman* and O. Trnka Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Suchdol, Czech Repubic
Sir: A new and needed technology has been reported for the removal of hydrogen sulfide from raw coal gas at high temperature by van der Ham et al.1 The problem of what to do with the spent sorbent (CaS) has been encountered for a long time, but it has not yet been fully resolved. Unlike CaSO4, which is relatively inert and slightly soluble in water, CaS is chemically unstable and tends to release H2S. Therefore, the sulfur-loaded sorbent must either be stabilized before disposal, i.e., oxidized to CaSO4, or posssibly regenerated, i.e., converted to CaO. A few issues in the paper of van der Ham et al.1 deserve to be put in more precise terms. A fundamental difference between the reaction of H2S with lime (CaO) and that with limestone (CaCO3) is the presence of an alkaline, highly developed porous texture in the reacting lime particles. To eliminate hindrance effects of carbon dioxide on H2S fixation, the temperature must be above the calcination temperature of CaCO3 given by the equilibrium state of the calcination reaction. Assuming the same partial pressure of CO2 as used by van der Ham et al.,1 i.e., PCO2 ) 60 kPa, we calculated the calcination temperature with the aid of the relation
ln PCO2 ) K1/T + K2
Table 1. Coefficients in Eq 1 for the Equilibrium Dissociation Pressurea of CO2 in the Reaction CaCO3 T CaO + CO2 van der Ham et al.1 -21 080.00 23.475 17 thermochemical data, Barin and Knacke3 tdecompb (°C) 844.7 tcalcc (°C) 814.5 K1 K2 based on
Hartman et al.2
Hartman et al.2
-20 985.34 22.642 52 thermochemical data, Barin and Platzki4 891.1 858.2
-20 007.43 21.686 02 experimental measurements, Johnston5 899.1 864.2
a Pressure in kPa. b Temperature at which P CO2 ) 101.325 kPa. Temperature at/above which the recarbonation of CaO does not occur when PCO2 ) 60 kPa.
c
(1)
Three different sets of the coefficients K1 and K2 in eq 1 were employed. These coefficients as well as the predicted calcination and decomposition temperatures are given in Table 1. As can be seen from the results presented in this table and as also visualized in Figure 1, the formula used by van der Ham et al.1 considerably overpredicts the equilibrium dissociation pressure of CaCO3. The correct temperature at which the dissociation pressure of CO2 above CaCO3 reaches 101.325 kPa is usually regarded in the literature6 as a value between 898 and 902 °C. This is 53-57 °C above the point estimated by the relationship of the authors.1 It also appears that the affinity between H2S and CaO is somewhat less than the authors1 presumed. As shown in Table 2 and Figure 2, the correlation of van der Ham et al.1 underpredicts the equilibrium concentrations of hydrogen sulfide. Consequently, the needed reduction of the H2S concentration to 20 ppm by volume (for PH2O * To whom correspondence should be addressed. Tel.: +420 220390254.Fax: +420220920661.E-mail:
[email protected].
Figure 1. Equilibrium dissociation pressure of CO2 (PCO2) above CaCO3 at different temperatures (t) as predicted by different authors: (1) van der Ham et al.;1 (2) Hartman et al.,2 based on the thermochemical data reported by Barin and Platzki;4 (3) Hartman et al.,2 based on the experimental measurements of Johnston.5
) 60 kPa) is not very likely at 845 °C in contrast to the authors' estimates. The authors1 state that about 65% of the CaS can be oxidized to CaSO4 because of the difference in molar volumes of CaSO4 and CaCO3. Using an approach similar to one that we employed in our previous work,9 a simple relation between the fractional porosity (e) of the reacting solid and the progress of the calcination, sulfidation, and oxidation reactions (X) can be expressed
10.1021/ie020193u CCC: $22.00 © 2002 American Chemical Society Published on Web 09/10/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002
Table 2. Coefficients in the Expressiona for the Equilibrium Constant PH2O/PH2S of the Sulfidation Reaction CaO + H2S T CaS + H2O
k1 k2 based on t20b (°C)
van der Ham et al.1
Schreiber and Petrie7
Hartman et al.8
7658.0 0.059 60 thermochemical data, Barin and Knacke3 845.1
8103.27 -0.617 09 thermochemical data
7258.68 0.103 38 thermochemical data, Barin and Platzki4 793.6
803.7
ln(PH2O/PH2S) ) k1/T + k2. Temperature in K. b Temperature at which the H2S concentration is 20 ppm in gas with 2 vol % water vapor. a
Figure 3. Changes in porosity of reacting solid in the sequence of reactions: (A) calcination of nonporous CaCO3 (CaCO3 f CaO + CO2), (B) sulfidation of CaO (CaO + H2S f CaS + H2O), (C) oxidation of CaS (with O2 or SO2) to CaSO4.
present a limiting value for the conversion of as large as 65%. Figure 3 represents the changes in porosity of the reacting solids brought about by the sequence of the calcination of CaCO3, sulfidation of CaO, and finally oxidation of CaS to CaSO4 starting with a nonporous particle of calcite. Literature Cited Figure 2. Equilibrium constant of the reaction CaO + H2S T CaS + H2O as a function of temperature: (1) van der Ham et al.;1 (2) correlation of Squires presented in Schreiber and Petrie;7 (3) Hartman et al.8
through the molar volumes of the reactants (Vreact) and products (Vprod) as
e)1-
Vreact + (Vprod - Vreact)X VCaCO3 [e(Xcalc)0) ) 0]
The symbol Vreact denotes the molar volumes of CaCO3 (36.9 cm3 mol-1, calcite), CaO (16.9 cm3 mol-1), and CaS (28.9 cm3 mol-1). The symbol Vprod denotes the molar volumes of CaO, CaS, and CaSO4 (46.0 cm3 mol-1, rhombic or monoclinic). On inserting the molar volumes in question into eq 2, we find that a particle of CaS becomes nonporous (i.e., e ) 0) when approximately 47% of the CaS is oxidized into CaSO4. In that state, the oxidation reaction effectively ceases for practical purposes. The authors1
(1) van der Ham, A. G. J.; Heesink, A. B. M.; Prins, W.; van Swaaij, W. P. M. Proposal for a Regenerative High-Temperature Process for Coal Gas Cleanup with Calcined Limestone. Ind. Eng. Chem. Res. 1996, 35, 1487-1495. (2) Hartman, M.; Svoboda, K.; Trnka, O. Effect of Water Vapor on the Equilibrium between CaO and COS in Coal Gas. Collect. Czech. Chem. Commun. 1999, 64, 157-167. (3) Barin, I.; Knacke, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1973. (4) Barin, I. Thermochemical Data of Pure Substances, 3rd ed.; VCH: Weinheim, Germany, 1995 (in collaboration with Platzki, G.). (5) Johnston, J. The Thermal Dissociation of Calcium Carbonate. J. Am. Chem. Soc. 1910, 32, 938-946. (6) Oates, J. A. H. Lime and Limestone: Chemistry and Technology, Production and Uses; Wiley-VCH: Weinheim, Germany, 1998; p 140. (7) Schreiber, C. H.; Petrie, T. W. Effect of Carbon Monoxide on the Reaction of Hydrogen Sulfide and Calcium Oxide. J. Eng. Power 1978, 100, 520-524. (8) Hartman, M.; Trnka, O.; Svoboda, K. Potential of Calcium Oxide for Removal of Hydrogen Sulfide and Carbonyl Sulfide from Coal Gas. Acta Mont. IRSM AS CR, Ser. B 1999, No. 9 (112), 5-18. (9) Hartman, M.; Pata, J.; Coughlin, R. W. Influence of Porosity of Calcium Carbonates on Their Reactivity with Sulfur Dioxide. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 411-419.
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