TRENDS IN ELEMENTAL LEACHING AND PETROGRAPHY FROM COAL COMBUSTION BY-PRODUCTS FROM TWO STOKER BOILERS BEFORE AND AFTER MODERNIZATION OF THE ASH HANDLING SYSTEM

Feed coal and coal combustion by-products (CCBs) were sampled from two stoker boilers before and after the ash handling system was modernized. In general, the feed coal has not changed significantly in the five years between sampling events, however the December 2010 feed coal was anomalous. This relative stability provided a unique opportunity to examine the changing optical and chemical character, as well as leaching behavior, of the ash in relation to system modification. Prior to retrofit, emission controls on the systems consisted of multicyclone dust collectors, as well as a single baghouse. Following modernization, the boilers are operating somewhat closer to peak efficiency and have additional pollution controls: three diatomaceous-earth lined baghouses that together remove virtually all particulate matter from the combustion gas stream. CCBs were sampled from multiple points, including bottom ash, sidestream ash, multicyclone ash, and, where present, baghouse CCB. Bulk chemistry, optical petrography, and chemical leaching tests were completed on samples acquired following modernization and compared to pre-existing data. Changes are seen in CCB size distributions and carbon contents, especially in the multicyclone and baghouse CCBs, where fine carbons are now prevalent. Samples of each CCB type were shaken for 16 hours in 2% nitric acid to find total leachability. This method was chosen over other methodologies due to the acidity of the local groundwater and lack of results in pilot studies using batch leaching and groundwater leaching techniques. Results indicate that in terms of ash stability, ashes produced prior to the retrofit have greater total leachability than ashes produced after the retrofit.