Studies of transformations of inorganic constituents in a Texas lignite during combustion

The mechanisms of coal ash formation were studied under closely controlled combustion conditions. Monticello lignite, from Titus County, Texas was combusted in a drop-tube furnace at 1500°C and fly ash was collected and aerodynamically size segregated into six stages. Short residence time chars were also produced in the drop-tube furnace. The coal was analyzed to determine both mineralogical and organically bound components using computer controlled scanning electron microscopy (CCSEM) and chemical fractionation techniques, respectively. Scanning electron microscopy/microprobe techniques were used to classify and determine the distribution of various ash particle types in each size fraction. Over 80% of the Monticello mineral content consisted of quartz and clay minerals and relatively large quantities of Ca, Mg, and possibly Fe were organically bound. Extensive reaction between the quartz and clay minerals and organically bound Ca resulted in amorphous and crystalline Ca-silicate and Ca-aluminosilicate phases in the fly ash. Finely subdivided discrete minerals or organically bound cations of Mg and Fe were concentrated in the finer fraction of the fly ash.