Sulfur dioxide-limestone reaction

The combustion of sulfur-rich char is accompanied by the production of an undesirable reaction product, viz., sulfur dioxide. However, most of the sulfur dioxide should be removed from the combustion gases before they leave the combustor. This may be accomplished by the introduction into the combustor of suitable additives which can absorb sulfur dioxide. Limestone is such an additive. The limestone reacts with sulfur dioxide in the presence of oxygen to form calcium sulfate, which is a solid product and can be easily removed from the reactor. In this work, a model is proposed for the prediction of sulfur dioxide removal from the combustion gases, based on knowledge of gas-solid reactions taking place on a single pellet. 

Formation of emissions from fluidised-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all soHds are removed from the system as flyash. SO2 and HCl are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CO2, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. SO2 and HCl capture rates of 70—90% are readily achieved with fluidi2ed beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

It (1 ) deals with the derivation of relations giving the particle size distribution in the bed, overflow, and carryover streams and their respective weights. This theory will be extended to include the effects of particle growth or shrinkage (Z>1 or Z<1). For typical combustion of char containing sulfur followed by sulfur dioxide absorption by limestone, relations will be derived to determine the extent of sulfur retention. The reaction, carryover, and overflow rates will be evaluated with particular attention to their dependence on Z. 

Single Pellet One Reaction. The sulfation reaction which is considered here for calcium carbonate is given by Eq. 3, and the temperature and concentration profiles of a typical growing limestone particle are shown in Figure 2. The rate of disappearance of sulfur dioxide is assumed to be the first order and is given by... 

Modelling of the Limestone-Sulfur Dioxide Reaction in a Fluidized-Bed Combustor" Fuel 1973, 121-127. 

Koppel, L. "A Model for Predicting the Extent of Reaction of Limestone and Sulfur Dioxide During Fluidized-Bed Combustion of Coal" Appendix C, pp. 60-77, in Jonke, A. A. "Reduction of Atmospheric Pollution by the Application of Fluidized Bed Combustion" Argonne National Laboratory Annual Report, ANL/ES-CEN-1002, July 1969-June 1970. 

The controlling chemical reactions for the lime/limestone wet scrubbing SO2 removal systems have been established. In both the lime and limestone systems, the principal absorption reaction is calcium sulfite plus sulfur dioxide to form calcium bisulfite. Methods of preventing both calcium sulfite and calcium sulfate scaling are presented. 

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

FIG. 1—Reactions of sulfur dioxide with limestone to produce flue gas gypsum. 

When the calcium carbonate, CaC03, in limestone is heated to a high temperature, it decomposes into calcium oxide (called lime or quick lime) and carbon dioxide. Lime was used by tbe early Romans, Greeks, and Egyptians to make cement and is used today to make over 150 different chemicals. In another reaction, calcium oxide and water form calcium hydroxide, Ca(OH)2 (called slaked lime), used to remove the sulfur dioxide from smoke stacks above power plants burning bigb-sulflir coal. The equations for all these reactions are below. Determine the oxidation number for each atom in the equation and identify whether the reactions are redox reaction or not. For each redox reaction, identify what is oxidized and what is reduced. 

This work shows that oxygen-free sulfite in lime/limestone slurries, exposed to sulfur dioxide, slowly decomposes under process conditions. In fact, auto-redox reactions of sulfur oxyacids can occur in all coal desulfurization systems, including coal-gasification systems and impurities present in commercial flue gas systems are capable of catalyzing the reaction under process conditions. Our experiments indicate that any large-scale coal utilization will depend on appropriate control of the autoredox reactions of sulfur species. 

The reaction systems of interest in this study include both research samples containing reagent grade solution, which are carefully purged of oxygen, as well as samples drawn from pilot scale sulfur dioxide scrubbers using commercial grade lime and limestone slurries exposed to air. 

Dry deposition refers to sulfation, the direct gas/solid reaction between sulfur dioxide and limestone. This reaction does require alteration of the sulfur dioxide to sulfur trioxide in the atmosphere as well as the presence of a high humidity level (80% or more) as a catalyst for the reaction. The chemical reaction involved is ... 

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