Unburned carbon from coal combustion ash: An overview

Vast quantities of ash are produced in coal combustion power stations annually. Including aluminosilicate matrix, all ashes also contain unburned carbon (UC) of varying amounts; in some ashes it can exceed 10% (or even 45% in stoker boilers). High UC levels in ash does not only constitute an energy loss during combustion, it can also hinder technological utilization of such ashes (e.g., in cement industry). Thus, effective technological utilization of UC (e.g., in adsorption processes) will result in multiple benefits - it will not only convert the waste material into a valuable product, but it will also facilitate utilization of ash fractions, from which UC has been separated (which consequently help to solve ash disposal problem). For this reason, the main aim of this paper is to provide an overview of research related to UC from coal combustion ashes. It reviews factors affecting UC content in ash, methods of UC determination, UC separation techniques, different approaches in studying UC, and so on. Particular attention has been paid to feasible utilization options of this material in reference to its properties. Further research in the related areas is discussed as well.