A study of Indian limestones for sulfur capture in FBC plants: Particle size sensitivity of sulfation behavior

• Indian limestones were characterized for sulfation for the first time.
• Sulfation behavior was found to be more particle size sensitive for Indian limestones.
• Calcium carbonate content did not correlate with sulfur capture behavior.
• Particle size sensitivity was due to the formation of thermally activated fractures (TIFs).

India generates about 59% of its electricity from coal (EIA, 2014) [1]. Most of the coal available in India is of low quality, high ash and low calorific value. Fluidized bed combustion is an appropriate technology to utilize these low quality coals. Currently there are no emission regulations for SO2 emissions from power plants. However, SO2 control is becoming a requirement for some projects funded by International agencies to control air emissions. For the first time, five limestone samples from various parts of India that are currently being used in fluidized bed power plants are characterized as sorbents for sulfur capture behavior in a TGA. These limestones were identified as Sorbents A–E, and were characterized for their physical properties (porosity and pore size distribution, surface area), chemical composition analysis, petrographic characteristics, and mineral structural features using XRD. Analytical results indicated that the particle size of the sorbents plays a critical role in the sulfation behavior of the sorbents with some sorbents are more sensitive to particle size than the others. Hot Stage Microscopy observations indicated that this sensitivity was due to the formation of thermally activated fractures (TIFs) at fluidized bed temperatures. BET surface area and BJH pore diameter tests performed on the five sorbents indicated that Sorbent A had the highest surface area of all the limestones and large also the pore diameter. After comparing the BET data with the TGA sulfation data, it appears that Sorbent B might be subjected to the pore-plugging effect due to its small pore diameter. Sorbent C and Sorbent E had low BET surface area and high BJH pore diameter suggesting that although they may not experience much diffusion resistance, due to the low surface area, their sulfur utilization might be limited for coarse particle sizes. The sulfation behavior of these Indian limestones is compared with that of some US limestones with similar physical, chemical and petrographical properties.