The influence of heat treatment and weathering on the gasification reactivity of Montana lignite

Abstract

Thermogravimetric analysis and temperature programmed desorption have been used to assess the influence of annealing and of char weathering on the gasification reactivity of a Montana lignite. To this end, lignite samples were heat treated at 900°C for different times in a nitrogen atmosphere. Samples either weathered under ambient conditions for six months or nonweathered were tested to assess their reactivity toward oxygen and carbon dioxide and their propensity to chemisorb either reactants. Reactivity and chemisorption capacity of heat-treated chars were found to be closely related to each other. Annealing mainly affects the availability of sites that are active toward oxygen chemisorption, a property that is directly reflected by char oxyreactivity. Also, the effect of weathering is strongly emphasized by the previous temperature history of the chars. On the contrary, the ability of chars to chemisorb carbon dioxide and related gasification reactivity are negligibly affected by heat treatment. Results are analyzed considering possible relationships between the reactivity, chemisorption capacity, and chemical and microstructural modifications induced by heat treatment in low-rank coals. It is believed that modifications of the organic structure, rather than mineral matter transformations, are responsible for the observed phenomena.