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Ind. Eng. Chem. Res. 1999, 38, 2954-2962
Removal of H2S by Spray-Calcined Calcium Acetate William Nimmo,* Jonathan Agnew, Edward Hampartsoumian, and Jenny M. Jones Department of Fuel and Energy, School of Process, Environmental and Materials Engineering, University of Leeds, U.K.
The effectiveness of wet-spraying calcium acetate as an alternative to limestone and dolomite for the desulfurization of flue gases (in particular, H2S removal from coal gas) has been investigated by experimental studies using drop tube (DTR) and fixed-bed flow reactors (FBR). Calcium acetate solution was spray-calcined in the DTR at temperatures of 1073 and 1323 K. At the lower temperature, conversions approaching 80% were found at the longest residence time studied, 0.8 s. On the other hand, the higher temperature condition initially showed a much greater rate of calcination, indicated by greater conversion at shorter residence times, but was then followed by a much slower rate beyond 0.4 s. The final degree of conversion was in the region of 70%. Batches of spray-calcined calcium acetate (SCA), limestone, and dolomite, prepared in the DTR at 1323 K, to 70% conversion, were sulfided in the FBR at temperatures of 873 and 1173 K to assess their relative sulfur capture reactivities. Significantly higher conversions were achieved by the spray-calcined material, especially at the higher FBR temperature (1173 K) where a difference in conversion of more than 40% was observed. The physical structure of the particles formed from wet-spray calcination were determined by electron microscopy and showed highly porous cenospheres with large internal voids and an outside surface characterized by “blowholes” of between 1 and 10 µm in diameter. As a consequence, the available surface area for reaction with H2S is greater than with limestone or dolomite, producing a 4-fold difference in the level of particle conversion. Introduction The implementation of measures to control the emission of SO2 from fossil fuel combustion systems is an essential part in limiting the impact of acid rain on the environment. In particular the retrofitting of large, coalfired power stations with flue gas desulfurization (FGD) units has been the focus of the strategy in the U.K. for the reduction of sulfur emissions. Current technologies involve the use of pulverized limestone as an agent for sulfur absorption which is injected into the flue gas at the appropriate conditions of temperature and Ca/S ratios. It is proposed that similar technologies using limestone may be applied to the removal of sulfur from product gas1-5 as part of the development of the next generation of clean coal gasification systems. Recently, the use of other materials has been suggested as alternatives to limestone injection, namely the carboxylic acetate salts of Mg and Ca (CMA), but studies have been mainly concerned with SO2 capture. This paper is concerned with the study of the suitability of calcium acetate (CA) as an agent for H2S removal from coal gas. Spray pyrolysis/calcination was performed using a drop tube reactor (DTR), and the calcined material was then sulfided under controlled conditions of temperature and gas composition in a fixed bed reactor (FBR). The sulfidation efficiency was compared with that of similarly prepared limestone and dolomite. Background Carboxylic salts of Ca and Mg [e.g., Ca(CH3COO)2], whether utilized dry or from solution, have shown promising potential as agents for the removal of SO2/ * E-mail:
[email protected]. Fax: 0113 2440572. Telephone: 0113 2332513.
H2S from coal-fired combustion/gasification systems.6-12 Furthermore, it has been reported that twice the removal efficiency is achieved by spraying fine mists of solution when compared to dry injection.11 Cenospheres are formed under wet-spray calcination at high temperature; these formations are highly macroporous and can be popcornlike in structure, which can lead to increased sulfur removal efficiency. It has also been reported that there are additional advantages to be obtained by using a mixture of Ca and Mg acetate salts. Both agents remove SO2 at temperatures above 1223 K, whereas at lower temperatures, only MgO reacts to any significant degree. This increases the residence time for SO2 removal and widens the effective temperature band for emission control. Therefore, the use of aqueous solutions of Ca and Mg acetate salts has the potential to be a more efficient method of capturing sulfur (SO2 or H2S) than either dry limestone or dolomite injection. The lower volume of spent material produced using sorbents such as CMA should incur lower costs for disposal. The use of carboxylic calcium salts for SO2 removal can also reduce the concentration of NOx due to the effect of the organic portion of the agent behaving like a reburn fuel. The type of salt, therefore, may be important and may depend on the amount of hydrocarbon radicals that can be made available on pyrolysis. Previous work8 has shown that in a comparison between benzoate, acetate, formate, and propionate salts of calcium, the propionate proved to be the most efficient for both NOx and SO2 removal, achieving over 80% reductions. This material has found favor recently as an alternative to common salt as a road deicer since it is less corrosive to metals and concrete and has a lower impact on living systems. Economically and environmentally
10.1021/ie9900622 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/10/1999
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2955
Figure 3. Temperature profiles in the DTR with a common profile in evaporation zone up to 1073 K but differing profiles in the calcination zones to final temperatures of 1073 and 1323 K.
Figure 1. Schematic diagram of the drop tube reactor (DTR).
Figure 2. Calcium acetate injection system consisting of a pressurized solution reservoir and a water-cooled injection probe with a twin-fluid atomizing nozzle.
attractive methods of commercially preparing acetic acid13 (a principal feedstock in CMA manufacture) have been developed using biomass, sewage sludge, municipal solid waste, and other industrial wastes as feedstocks.14-17 Recently published works have shown that acetic acid can be synthesized from CH4 and CO2 using transition-metal catalysts.18-20 It is clear that if large quantities of CMA were required for NOx/SOx control then bulk preparation at low cost is technically feasible. Experimental Equipment and Analytical Methods Drop Tube Reactor (DTR). A schematic diagram of the drop tube furnace (2000 mm × 40 mm i.d.) is shown in Figure 1. The furnace consisted of six heated sections rated at 0.5 kW each which were linked to provide three independently controllable temperature zones. The calcium acetate solutions were sprayed into the reactor through a water-cooled spray injection system (Figure 2) utilizing an internally mixing, twinfluid atomizer and a constant-pressure feed system to
ensure a steady flow of solution to the nozzle of about 10 mL/min. Atomizing air was fed at a rate of about 3 L/min, and carrier air was fed at up to 40 L/min. Calcium acetate (A. C. S. reagent) was dissolved in deionized water to obtain a concentration of 0.11 M for the calcination experiments. The flow of liquid was varied with the carrier gas flow to produce different residence times at each port in the reactor. Quenching, due to the evaporation of water from the spray at the top of the DTR, meant that calcination conditions (>973 K) prevailed only in the bottom two-thirds of the tube, which was controlled to give the desired range of reaction temperatures. The time the particles spent in this section was taken as the residence time, up to 0.8 s. The low temperature at the top also meant that the reactor could not be operated with a uniform temperature throughout its length; therefore a fixed profile (Figure 3) was used, with the temperature varied in the last section only. Solid particle sampling from the DTR was performed using a sampling probe which was inserted into the gas flow, and a portion of the flow was directed through a two-stage sample recovery system. First, while still at temperatures >400 K, particles greater than 3-4 µm were removed by a cyclone separator with a heated catchpot (353 K). Then, the remaining fine material was trapped just downstream by a poly(tetrafluoroethylene) filter. Fixed Bed Reactor. Sulfidation experiments were performed in a fixed bed reactor (FBR) operating under differential conditions. Calcination, sulfidation, and sintering experiments have been reported using benchscale, laboratory flow reactors,2,21 where small batches of sample,
