Characterization of sulfation behavior of Indian limestones for sulfur capture

Sulfur dioxide capture from coal combustion is becoming a requirement in India to meet the emission regulations. Circulating Fluidized Bed Combustors (CFBC) have received con- siderable attention from utilities worldwide because of their ability to capture SO₂ using limestones. In this study, five limestone samples, currently being used in power plants in various parts of India were characterized for sulfur capture for the first time. These samples were marked Sorbent A, Sorbent B, Sorbent C, Sorbent D, and Sorbent E. They were characterized based on their porosity and pore size distribution, surface area, chemical composition analysis, petrographic characteristics, and SO₂ capture performance in a Thermogravimetric Analyzer (TGA). The calcium carbonate content of the limestones varied between 70 and 95 wt. %. Three size fractions, -20+25, -80+100 and -200 +270 US mesh, were used to determine the sulfation behavior in a TGA. Some limestones were found to be more particle size sensitive towards sulfation than others. Petrographic variation explained these differences for chemically similar limestones. Hot stage microscopy data revealed that the most particle size insensitive limestone, developed a network of Thermally Induced Fractures (TIFs) that provided easier access to internal surface area compared to the rest of the limestones that did not develop TIFs. BET surface area of the raw limestone did not correlate well with the stone requirement of the limestones studied. Regardless of the calcium carbonate content, limestones that developed TIFs were found to have higher calcium utilization efficiency and were also observed to be insensitive to the particle size during sulfation. Thus smaller particle size was not a requirement to maximize sulfur capture.