Ind. Eng. Chem. Res. 2002, 41, 6209
6209
Evaluation of Comments on Our Paper “Proposal for a Regenerative High-Temperature Process for Coal Gas Cleanup with Calcined Limestone” by M. Hartman and O. Trnka A. G. J. van der Ham* University of Twente, Department Chemical Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands
Sir: Hartman and Trnka discuss the disadvantage of CaS produced by high-temperature desulfurization. As stated, CaS is chemically unstable and tends to release H2S. The main advantage of our proposed regenerative desulfurization process is that it produces no CaS but rather elemental sulfur. The CaS is regenerated to CaO that is reused for another absorptionoxidation-regeneration cycle; see Figure 1 of our paper.1 Small-scale TGA experiments have shown that this approach works fine without capacity loss for at least eight absorption-oxidation-regeneration cycles (see Figure 9 of ref 1), and therefore, we expect that it will last much longer. The correlations presented in Table 2 of our paper1 are based on the data reported by Barin and Knacke in 1973.2 The translation of these data into a correlation was rechecked and is correct. However, the data published in 19933 and later include updated thermodynamic data and clearly yield another correlation. I correlated the data published by Barin et al. in 1993,3 which gives the correlation shown in Table 1, and the Table 1. New Equilibrium Constants for Reactions Investigateda,b reaction equation
equilibrium constant
formula
CaO + H2S T CaS + H2O K1 ) PH2O/PH2S ln K1 ) 0.2490 + 7140.7/T (R1) ln K3 ) 18.300 - 21170/T CaCO3 T CaO + CO2 K3 ) PCO2 (R3) a
b
Correlations based on the data reported by Barin et al.2 Pressure P in bar.
results clearly show the same outcome as reported by Hartman and Trnka. A CO2 partial pressure (PCO2) of 0.60 bar for the calcinations of CaCO3 (reaction R3 in ref 1) corresponds to a temperature of 852 °C, which is 37 °C higher than previously reported. The value of PH2S * E-mail:
[email protected].
(of reaction R11) for PH2O ) 0.60 bar is valid at 799 °C and not at 845 °C, as previously reported in the paper.1 The results also show that no operating window is left for a coal gas containing 2% CO2 (PCO2 ) 0.6 bar) and 2% H2O (PH2O ) 0.6 bar) combined with a reduction of the H2S level to 20 ppm (60 Pa). Because recarbonization of the CaO should be avoided, the temperature must be above 852 °C, which means that the H2S can be reduced to at most 82 Pa ()27 ppm). Will this make the process infeasible for its purpose? No, it simply means that the suggested process is still suitable for the bulk removal of H2S (>99%) and that, for the reduction to 20 ppm H2S, an additional polishing desulfurization step is needed for the finishing touch. Concerning the amount of CaS that can be oxidized to CaSO4, my recalculations give the same outcome (about 50%, based on molar volumes valid at ambient temperatures) as reported by Hartman and Trnka. Unfortunately, I was not able to regain my old values that were the basis for the value of 65% reported in the original paper. Fortunately for the process, the experiments (Figures 6, 8b, and 9a1) show that 55-60% of the CaS can be oxidized to CaSO4 at 700 °C. Acknowledgment I thank the authors Hartman and Trnka for their critical evaluation of the correlations and calculations. Literature Cited (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) Barin, I.; Knacke, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1973. (3) Barin, I. Thermochemical Data of Pure Substances, 2nd ed.; VCH: Weinheim, Germany, 1993 (in collaboration with Sauert, F.; Schultze-Rhonhof, E.; Shu Sheng, W.).
IE020613F
10.1021/ie020613f CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
