Ind. Eng. Chem. Process Des. Dev. 1904, 23, 742-748
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Reaction of H,S and Sulfur with Limestone Particles Robert H. Borgwardt” Industrial Environmental Research Laboratoty, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
Nancy F. Roache Northrop Environmental Services, Inc., Research Triangle Park, North Carolina 27709
The directdisplacement reaction of limestone wlth H2S was studied over the temperature range 570-850 O C in a differential reactor. Measurementswith particles ranging in size from 1.6 to 100 pm showed an activation energy of 42 kcal/mol and a rate constant of 0.66 (L cm)/(g-mol of H,S min) at 750 O C . The reaction is inhiblted by H, and CaCI,. Sintering of CaCO,, dissociation of H2S, and product layer diffusion limit the rate of Cas formation at temperatures above 750 OC. Limestone also reacts with elemental sulfur, but the actlvation energy and rate are significantly lower than those of the H,S reaction.
Introduction The first large-scale effort to develop flue gas desulfurization technology for electric power plants in the 1960s was focused on the dry limestone injection process. A common recognition of the practical and economic advantages of controlling sulfur emissions by capture during coal combustion prompted extensive research on that process in Germany, Japan, the U.S., and other countries spanning a period of 10 years. Those efforts were set aside in favor of limestone scrubbers when it became clear that insufficient sulfur capture could be obtained a t practical limestone injection rates, given the unfavorable time/ temperature characteristics of utility boilers equipped with conventional burners. Recent development of multistage burners for control of NO, emissions has rekindled the earlier interest in limestone injection (Case et al., 1982) because the design features responsible for lower NO, formation in the new burners are expected also to be more favorable for sulfur capture. One of these features is a reducing zone in the first stage wherein sulfur capture might occur by reaction of H2S with the limestone or CaO to form Cas. Cas is thermodynamically more stable a t higher temperatures than the CaS04 formed under oxidizing conditions, and faster reaction rates may thus be possible. Cas would also be less limited by intraparticle resistances caused by product accumulation because it has a smaller molar volume than CaS04. This work is part of an EPA program o i research now underway (Drehmel et al., 1982) to evaluate limestone injection in multistage burners as a potential means of simultaneously reducing the emission of two precursors (NO, and SO,.) of acid rain. One reaction by which sulfur capture could occur through the Cas route is CaCO, + H2S Cas + C02 + H 2 0 (1) Reaction 1was studied by Ruth et al. (1972) with calcium carbonate in the form of half-calcined dolomite, CaCO,. MgO. They reported that H2S reacts more rapidly with CaC0, than with CaO (as fully calcined dolomite) and has a higher activation energy. The reaction rate of 60-pm particles was found to be chemically controlled for conversions up to at least 90% according to the relationship
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This article not subject to
where (CaCOJ is the unreacted fraction of calcium carbonate and,C is the gas-phase concentration of hydrogen sulfide. The rate constant, K , has units of liters per gram-mole of H2S-minute when CHBS is expressed in gram-moles per liter. An Arrhenius extrapolation of the rate data reported by Ruth et al. (1972)would predict that 40% conversion might be achieved in about 120 ms at 1200 OC if the chemical reaction rate were the sole limitation. Such an extrapolation involves many assumptions but suggests that efficient sulfur capture may be achievable a t reasonable injection rates if these kinetics can be duplicated with limestone particles. The objective of this study was to determine the kinetic parameters of reaction 1 when using limestone as the source of CaC03.
Experimental Section Apparatus. Limestone particles were exposed to H2S for varying periods in a differential reactor. The reactor was fabricated of quartz glass as illustrated in Figure 1. Except where otherwise noted, the reactor feed gas consisted of 5000 ppm of H2S in 70% C02 and the balance as N2 It entered the bottom of the reactor, passed upward through a 3.0 cm i.d. X 95 cm outer shell and downward through an annular 2.0 cm i.d. X 77 cm reactor tube. The limestone sample was positioned a t the center of the reactor tube in a 9.0 mm i.d. removable holder which was sealed by a ground-glass joint to the gas exhaust tube as shown in Figure 2. The limestone particles were dispersed in a quartz wool substrate (Thermal American Fused Quartz Co.) through which the entire gas flow passed. The sample-holder/exhaust-tubeassembly was inserted at the bottom of the reactor through a ball joint which sealed it into the gas preheat section formed by the annular space between the two larger reactor tubes. The upper 60 cm of the reactor was heated by an electric furnace to temperatures that were varied from 570 to 850 O C . The total gas feed rate was 5.0 L/min a t 25 OC, providing a superficial velocity of 490 cm/s through the limestone sample a t 750 OC. The high velocity ensured that gas-side mass transfer was not a significant resistance to the gas/solid reaction. The reactor pressure averaged 40 cm water at this velocity, representing the pressure drop across the quartz wool. Procedure. For each run, 15 mg of limestone was exposed to H2S in the reactor. The reactor was flushed with
U S . Copyright. Published 1984 by the American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984
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2 min. The sample holder was removed, and the reacted limestone (with its quartz wool substrate) was ejected into 100 mL of 0.005 M buffered iodine solution for analysis of sulfide. The iodine was stirred in a sealed container for 30 min, then titrated with 0.005 M arsenite to determine the amount of iodine unconsumed by the reaction
Cas + I2 I
II G
Figure 1. Differential reactor: (A) electric furnace; (B) gas inlet; (C) solenoid valve; (D) gas preheat zone; (E) limestone sample holder; (F) ball joint; (G) gas exhaust; (H) ceramic plug; (I) thermocouple; (J) COz gas inlet; (K) manometer connection. 9 mm
w
15 mg Limestone
Quartz Wool Substrate
Exhaust Tube
Glass Indentions(3)
-u db
35/25 Ball Joint
Figure 2. Differential reactor: detail of limestone sample-Holder/ exhaust-tube assembly.
pure C02 during sample insertion, and the sample was heated for 9 min to attain reactor temperature while under 1atm of C 0 2to prevent calcination. The H2Sreaction was initiated by opening the solenoid valve controlling the flow of reactor feed gas. The feed gas always contained sufficient C02to ensure that the H2S reaction occurred with CaC03 (not CaO). A series of runs was made with H2S exposures ranging from 5 s to 30 min. Following exposure of a sample, the reactor was flushed with pure C02, and the sample holder was withdrawn to the unheated bottom section of the reactor where it was cooled under C02 for
+
Ca2++ 21-
+S
(3)
When the sulfide analysis was completed, calcium was determined in the same solution by adding NaOH and titrating with 0.5% EDTA to a murexide end point. The percent conversion of limestone to Cas was calculated from the mole ratio of S/Ca established by the titrations. The calcium recovery indicated by the EDTA analyses agreed within 3% of that expected from the weight of limestone particles placed in the reactor; entrainment loss was therefore negligible. An evaluation of the accuracy of the analytical procedure made against mixtures of limestone and reagent grade Cas showed standard deviations of f2.5% for overall conversion. Confirmatory analyses by X-ray fluorescence showed that no sulfide loss occurred by oxidation during transfer of samples from the reactor, and that no solid sulfur species were present other than sulfide. Materials. The limestone was Fredonia Valley White from Fredonia, KY (BCR 2061), consisting of 95% CaC03 and 1.3% MgC03. The gelogical characteristics of this stone have been compared with other typical carbonate rocks by Harvey and Steinmetz (1971). Two particle sizes (60 and 100 pm) were obtained by washing the pulverized stone between 230/270 mesh and 150/170 mesh sieves, respectively. Smaller particles were obtained by dry fractionation using a Donaldson Accucut Classifier which limited the particle size distribution of each fraction to a narrow range. The principal properties of the various size fractions of limestone are shown in Table I. The mean particle diameter of each fraction was determined by Coulter Counter analysis (in 0.2 M CaC12electrolyte) and by Scanning Electron Microscope (SEM) sizing. Included in Table I are the principal chemical constituents and trace elements of each fraction. SEM examination showed the large particles to be comprised of loosely consolidated 2-pm calcite grains with copious intergranular pores. The pore diameter, determined by mercury intrusion, ranged from 0.05 to 1pm with a mean diameter of 0.4 pm. The BET analyses in Table I indicate that the larger pores were destroyed as particle size was reduced to a diameter roughly equivalent to the natural grain size, whereupon new surface was generated by fracture of the calcite grains.
Results and Discussion Effect of Particle Size. Since the natural porosity of limestone is relatively small (8% for this stone, as indicated by the data of Table I), its reaction with a gas such as H2S would be expected to be confined mainly to the outside surface of the particles. In such a case, particle size will be a critical variable affecting reaction rate. Figure 3 shows the conversion vs. time responses measured for large particles (Dp1 15 pm) at 750 "C, 5000 ppm of H2S. The large particles show a high initial reaction rate which falls rapidly above 10% conversion. The high initial rate is interpreted as evidence of reaction within the internal pore structure during the early stage. This interpretation is consistent with the behavior of the reaction of SO2 with CaO (which has a porosity of 56%) that has been shown to occur throughout the pore structure of 100-pm particles (Borgwardt, 1970). As conversion increases, the available pore volume fills with reaction product, and the reaction becomes limited to the particle surface, resulting in a re-
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 50-
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