Limestone composition and reactivity are critical factors that determine the performance of limestone-based wet flue gas desulfurization systems. Limestone quality affects sulfur dioxide (SO2) removal, reaction tank sizing, limestone consumption rate, and composition of the gypsum product and waste streams. Reactivity is a direct measure of how readily a limestone will provide alkalinity to neutralize the acid resulting from SO2 dissolution in water. In this article we review your limestone analytic measurement options and discuss their relative accuracy and limitations.
Limestone is a commonly occurring sedimentary rock, predominantly composed of calcium and magnesium carbonate (MgCO3) with various amounts of silicates, metal oxides, and other impurities. Mineralogically, limestone may be regarded as calcite, magnesian calcite, dolomite, or an aggregate with varying proportions of each of these minerals.
Pure calcite exists as crystals of calcium carbonate (CaCO3) with a rhombohedral unit cell. A limestone becomes dolomitized as magnesium ions are integrated within the calcite matrix, magnesium substituting for calcium on a one-to-one atomic basis. Limited integration of magnesium produces magnesian calcite. Complete dolomitization occurs when an equimolar ratio of CaCO3 and MgCO3 exists within the lattice.
Substitution of MgCO3 into the crystal matrix is an exothermic reaction. As a result, the dolomite and, to a lesser extent, magnesian calcite, are more thermodynamically stable structures than pure calcite. Therefore, the lattice strength is increased as magnesium is incorporated. Like calcite, the basic crystal structure of dolomite is rhombohedral, with magnesium occupying every other position from calcium on the cation plane.
The convention within industry has been to assume that all of the MgCO3 in limestone is dolomitic, with an equimolar molar ratio of MgCO3 and CaCO3. Actually, the molar ratio of these species can vary considerably from crystal to crystal, and the reactivity of each crystal varies in relation to the fraction of MgCO3 in the crystal. Most limestones selected for wet flue gas desulfurization (WFGD) application are highly calcitic, containing less than 5% MgCO3.
Limestone for Sulfur Removal
Accurate determination of limestone composition and reactivity are crucial in the selection of reagents for WFGD applications, especially for systems producing commercial grade gypsum products. (See Seminole Generating Station story, p. 52.) Although systems can be constructed to utilize a wide range of limestone composition and reactivity, limestone quality must be agreed upon early during system design because this parameter affects unit sizing, product quality, wastewater treatment, limestone consumption rate, and WFGD system performance.
Mineral species of interest are calcite, magnesian calcite, and dolomite. The magnesian calcite or substituted calcites range from 6% to 13% magnesium replacement of calcium in the crystal structure. Also of concern are the silicates and metal oxides, which are inert in the WFGD process but affect gypsum purity; concentrations of these inerts in the gypsum are often limited to specific levels for commercial grade gypsum product.
Though the use of lower-quality limestone may provide an advantage in system operating cost, the benefit of these savings should be weighed against impacts on system performance. Limestone of acceptable quality is necessary for achieving design-level performance.
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