Abstract Limestones have been used to capture SO2 emitted during the combustion of coal for decades. However, due to the complexities of the variables involved in this process, many issues are still being studied. Some tests were performed in a fluidized bed reactor, which was used to evaluate the performance of the limestones in the bed for flue gas desulfurization, burning low sulfur coal. Metamorphic dolomitic limestones of low cost were evaluated considering the impurity level and need for beneficiation. They were characterized chemically, physically and microstructurally (X-ray fluorescence, X-ray diffraction, laser particle size analyzer, scanning electronic microscope). The data obtained were compared to a calcitic limestone of elevated purity and higher market value. The results showed little effect of the major impurities present in the dolomitic limestones in the calcination and desulfurization processes. Microcrystalline impurities showed a marked effect promoting grain sintering in calcination conditions of TGA/DTA tests in different atmospheres, as shown in SEM images. Dolomitic limestones generate more solid waste and capture less SO2 per kcal (a slightly decrease in the thermal efficiency of the reactor) compared to calcitic limestones. However, due to the lower cost, they were considered to be attractive for the FBC process (2 mg SO2/kcal at Ca/S=3).

Calcários são muito utilizados na captura das emissões gasosas sulfurosas emitidas durante a queima do carvão na geração de energia, por meio de sua injeção em caldeiras e fornalhas. A caracterização técnica deste material torna-se necessária à correta seleção e utilização de uma rocha calcária. Neste trabalho, foi realizada a análise técnica e microestrutural dessa matéria-prima, incluindo-se a caracterização dos parâmetros físico-químicos, texturais e microestruturais do calcário in natura, calcinado e sulfatado. Um calcário dolomítico foi avaliado quanto às propriedades físico-químicas, distribuição de tamanhos de partículas, área específica, volume de poros e análise em microscópio eletrônico de varredura. Por apresentar origem metamórfica, o calcário apresentou-se compacto e pouco poroso, porém após a calcinação a textura muda drasticamente, com a diminuição da densidade, aumento da superfície reativa e desenvolvimento de porosidade aberta. Estes são parâmetros fundamentais a um bom sorvente de óxido de enxofre. A fratura de partículas é gerada pela pressão interna durante a conversão dos carbonatos em óxido de cálcio e liberação do CO2 durante a calcinação. O óxido de cálcio nascente reage com óxido de enxofre, formando uma camada compacta de sulfato de cálcio, a qual reduz a área específica pelo bloqueio dos poros, causando o aumento de granulometria média e relativa, na etapa de sulfatação. A caracterização tecnológica realizada mostrou aspectos relevantes quanto à microestrutura dos calcários calcinados e sulfatados. A pressão gerada pelo CO2 durante a sulfatação, resultado da calcinação incompleta do calcário, é capaz de romper a fina camada de CaSO4, deixando poros arredondados.

Limestones are widely used to capture sulfur gas emissions emitted during the burning of coal in power generation through its injection in boilers and furnaces. The technical characterization of this material becomes necessary to correct selection and use of a limestone rock. In this work, the technical and microstructural analysis of this raw material, including the characterization of the limestone in nature, calcined, and sulfated was performed. A limestone was evaluated in respect to physicochemical properties, particle size distribution, surface area, pore volume and SEM analysis. The metamorphic origin of the limestone explains its compact and little porous appearance, but after calcination the texture changes dramatically, with decreasing density, increased reactive surface and development of open porosity. These are key parameters for a good sulfur oxide sorbent. The fracture of particles is generated by the internal pressure during conversion of the carbonate into calcium oxide and release of CO2 during calcination. The calcium oxide reacts with sulfur oxide forming a compact layer of calcium sulphate, which reduces the specific area by blocking the pores, causing an increase in average and relative particle size during sulfation stage. The technological characterization performed revealed important aspects about the microstructure of calcined and sulfated limestone. The pressure generated by CO2 during sulfation, resulting from incomplete calcination of the limestone, is capable of breaking the thin layer of CaSO4 leaving rounded pores therein.