翻译部分
英文原文
Kinetic study of the reaction between sulfur dioxide and calcium hydroxide at low temperature in a
fixed-bed reactor
Abstract
A quantitative study of the influence of inlet sulfurdioxide concentration (600–3000 ppm),relative humidity (20–60%), reactortemperature(56–86℃)and different amounts (0–30 wt.%) ofinorganic additives(NaCl, CaCl2 andNaOH) on gas desulfurization has been carried out in acontinuous downflow fixed-bed reactor containing calcium hydroxide diluted with silica sand.Results show that the reaction rate does not depend on sulfur dioxide partial pressure (zero-order Kinetics) and that the temperature and the relative humidity have a positive influence on reactionrate. An apparent activation energy of 32 kJ/mol Ca(OH)2 has been estimated for the reaction.
An empirical reaction rate equation at 71.5℃ and 36.7% relative humidity that includes thetype and amount of additive is proposed. It has been found that
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calcium chloride is the bestadditive studied because it allows for a higher degree of sulfur dioxide removal. 2000 ElsevierScience B.V. All rights reserved.
Keywords: Desulfurization; Sulfur dioxide; Calcium hydroxide; Kinetics; Inorganic additives
1. Introduction
The increasing concern during the last few years on the protection of the environmenthas had its influence on the design and operation of power plants, especially on thereduction of sulfur dioxide and nitrogen oxide emissions from them. They are the mainpollutants from coal and fuel-oil combustion in power plants. Both gases are responsiblefor acid rain.
In USA and Europe, new power plants that use fuels with significant quantities ofsulfur have to meet severe standards to reduce these air pollutants. One of the majorproblems facing older power plants is that they were designed prior to the presentstandards for pollution control and therefore have no facilities on space to incorporatesuch controls.
The technologies to control sulfur dioxide emissions can be distributed into threegroups by considering if the treatment is done before, during or after the
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combustion. Itseems clear that the last group of technologies cited is the most advantageous, fromvarious points of view, for power stations which have been in operation for many years.These are called FGD technologies (Flue Gas Desulfurization),and among them, themost usedare: IDS (In-Duct Scrubbing, developed by General Electric); E-Sox(developed by US EPA, Babcok and Wilcox, Ohio Coal Development Office and OhioEdison), EPRIHYPAS (Hybrid Pollution Abatement System, developed by ElectricPower Research Institute), DRAVO HALT (Hydrate Addition at Low Temperature,developed by Dravo), CONSOL COOLSIDE (developed by Consolidated CoalCom-pany)and ADVACATE(developed by Acurex and US EPA). These processes are basedon the injection of a solid sorbent plus water by spraying or injecting a slurry into theduct situated between the air preheater and the particulate collection system. Calciumhydroxide or limestone are usually used as sorbents to capture sulfur dioxide and acalcium sulfiter/sulfate mixture is obtained as the reaction product.
Klingspor and Stromberg proposed a mechanism to explain the reactionbetween sulfur dioxide and calcium hydroxide or calcium carbonate in the presence ofwater vapor. According to them, when the relative humidity is low (below 20%), sulfurdioxide and water can be adsorbed on the solid surface, however, no reaction occursuntil there is at least a monolayer of water molecules adsorbed on the surface. As therelative humidity increases, less sulfur dioxide can be adsorbed on the surface becausewater adsorption on the solid occurs preferentially due to intermolecular forces. Thus,sulfur dioxide has to be absorbed on the adsorbed water, forming complexes where thesulfur atom is
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bound to the oxygen atom of water. This fact leads to the formation of apositive charged hydrogen atom that can combine with hydroxide or carbonate ions fromthe sorbent to form reaction intermediates and products. Experimental findings show thatthe reaction rates for lime and limestone are similar. Consequently, the complexformation SO2 nH O is considered to be the rate-determining step, since all further reactions are different for the two types of sorbents. The initial rate of the process isindependent of sulfur dioxide concentration when the relative humidity is below 70%.Above this value, the reaction rate becomes gradually more and more dependent on thesulfur dioxide partial pressure. This fact can be attributed to the formation of stableconfigurations of water ligands around the sulfur dioxide molecules. Also, it has beenfound that the initial reaction rate is a very weak function of temperature but increasesexponentially with relative humidity, for both hydrated lime and limestone.
Jorgensen also studied this reaction in a bench-scale sand bed reactor. Someof their conclusions point out that the calcium hydroxide conversion has a very strongdependence on relative humidity. The conversion rate is increased moderately withtemperature in agreement with activation energy of 25 kJ/mol. However, there is noclear indication of increasing conversion with increasing sulfur dioxide concentration.
Ruiz-Alsop and Rochelle found that the relative humidity is the most importantvariable affecting the reaction of sulfur dioxide and calcium hydroxide. The chemicalreaction taking place at the surface of the unreacted calcium
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hydroxide presentszero-order kinetics in sulfur dioxide. At high relative humidity and/or high SO2concentration, the chemical reaction at the surface of the unreacted calcium hydroxidesolid controls the overall reaction rate. At low relative humidity and/or low sulfurdioxide levels, diffusion of sulfur dioxide through the solid product layer becomes therate-controlling step. The reaction rate has a weak temperature dependence. Theactivation energy of the reaction was estimated to be 12 kJ/mol.
Experimental data by Krammer showed that the reaction ratedepends onthe sulfur dioxide concentration but only at low concentrations and not so obvious athigher concentrations. In contrast to other publications, they found that the influence ofSO2 concentration on the reaction rate is rather linked to the conversion than to the 2relative humidity, which has a major impact on the conversion throughout the entirereaction as usually reported in literature. But they found out that the initial reaction rateseems to be independent of relative humidity and sulfur dioxide concentration, whichhad not been reported yet. They postulated that the reaction can be divided into thefollowing foursteps. During the initial stage, a chemisorption process of the sulfurdioxide on the particle surface seems to be important and the reaction rate decreasesexponentially with increasing conversion. Simultaneously, a nucleation processdominates the formation of the consecutive product layers where the reaction rateincreases with increasing relative humidity. The rate of reaction increases untilproduct layer diffusion takes over and reaction rate decreases again with conversion. Itshould be noted that only relative humidity
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