Rajeev Agnihotri

Ultrafast calcination and sintering of Ca(OH)2 powder : experimental and modeling

Abstract The kinetics of ultrafast calcination and sintering of Ca(OH) 2 powder is studied in the 900–1050°C temperature range in an entrained-flow reactor. Time-resolved kinetic data are obtained for 0–300 ms time scale with the residence time of the solids being measured on-line. The reaction exhibits very high initial calcination rates with sharp attenuation and subsequent virtual “die-off” at higher residence times. The interplay of calcination kinetics and sintering kinetics and the net effect on the overall surface area evolution is clearly elucidated. A modified grain model incorporating the sintering of the product CaO layer and the role of diffusion of evolved H 2 O is proposed to explain the overall phenomena. The model predictions are in good agreement with the observed experimental results.

Mechanism of CaO reaction with H2S: Diffusion through CaS product layer

Heterogeneous non-catalytic reaction between CaO and H2S to capture sulfur in coal-fired gasifier is studied in a differential bed flow-through reactor. Particles of CaO (<10 μm) are exposed to simulated fuel gas containing up to 5000 ppm H2S in the temperature range 700–900°C. The effects of reaction temperature, H2S concentration, and sorbent surface area on the extent of sulfur capture and conversion of sorbent are determined. The sulfidation reaction is analyzed using grain model with overall reaction rate controlled by chemical reaction at the interface and by diffusion through the product layer. At the later stages of the reaction, diffusion through the product layer is found to be controlling and involves migration of ions through the product layer. Direct experimental evidence is obtained to determine the migratory nature of ions and identify the mobile ionic species, by performing inert marker experiments. Scanning electron micrographs and sulfur mapping of the partially sulfided CaO tablets suggest that S2- diffuse in towards the CaO/product layer interface with a counter-diffusion of O2- towards the product layer/gas interface.

Kinetics and structural evolution of sorbents at high temperatures. Final report, September 1, 1993--August 31, 1994

The focus of this project is on furnace sorbent injection technology using dry, calcium-based sorbents for flue gas desulfurization. The goal is to provide fundamental research kinetics and the effects of sorbent properties, aimed at improving S0{sub 2} removal and increasing sorbent utilization in a cost-effective fashion. The fourth year work has been carried out in three phases: (1) structural evolution of sorbent, (2) modified sorbent studies, and (3) development of mathematical model. The results, their interpretation, and discussions are the primary focus of this report.