K Raghunathan

Date of Graduation


Document Type



Kinetics of steam gasification of North Dakota lignite were studied in a unique high-temperature TGA in the temperature range 800-1200{dollar}\\sp\\circ{dollar}C at steam partial pressures 0.25 to 0.75 atm. A significant feature of this work is that chars were prepared "in-situ"; coal was devolatilized in steam and gasification of the char thus produced was allowed to proceed continuously without interruption. Evolution of microstructural properties and surface area with char conversion are studied at different temperatures using CO{dollar}\\sb2{dollar} adsorption techniques. Evolution of reactive surface area is measured via oxygen chemisorption. A microscopic model is developed, assuming the void phase to be composed of overlapping spherical cavities. Using the concepts of geometric probability and population balance, the model is solved analytically. The model specifically addresses the effect of reactive sites and describes the evolution of reactivity and surface area with reaction separately. The model suggests that initial reactive site concentration and subsequent deactivation have a significant effect on reactivity and surface area. For non-catalytic gasification, good agreement is obtained between the model and literature data. Based on qualitative discussions, the model appears to be valid for catalytic gasification as well. Various research groups have reported that their char conversion versus time data from different experiments can be unified into a single curve when conversion is plotted against normalized time, t/t{dollar}\\sb{lcub}1/2{rcub}{dollar}, where t{dollar}\\sb{lcub}1/2{rcub}{dollar} is the half-life of the reaction, or time taken for 50% conversion of char. The rationale behind this observation is explained. With the aid of correlations reported in literature for the unified experimental data, a master curve is derived to approximate conversion-time data from most gasification systems. Consequently, it is shown that for steam and CO{dollar}\\sb2{dollar} gasification, the product of half-life and average reactivity is nearly a constant with a value of 0.38. It is quantitatively shown that half-life can be directly used as a reactivity index for characterizing the char-gas reaction. If the half-life of the reaction is known at a few temperatures, activation energy of the reaction can be estimated. Char conversion can be predicted in the temperature range up to about 70% conversion without requiring additional data.