A Proposed Mechanism for Ash Fouling Burning Northern Great Plains Lignite

Fouling of boiler heat transfer surfaces by lignite ash continues to be a problem that reduces boiler efficiency and availability. The present study evaluates the fouling mechanism in a microscopic sense. It endeavors to establish a relationship between the identity and mode of occurrence of mineral matter in lignite and the resultant ash deposits. Severe fouling conditions are believed caused by the formation of a melted phase binding material within the deposit. Sodium, volatilized from coal and reacted in the flue gas to form sulfate or carbonate, reacts with a fly ash subspecies (termed matrix parent) to form the integrative matrix within ash deposits. The reactive subspecies (matrix parent) is derived from the inherent ash present in each burning coal particle. However, only inherent ash having elemental compositions lying within a given range is fluxable by sodium to form a sticky, deposit-integrating material. The major elements comprising the matrix are Na, Ca, Al, and Si. A chemical solution to the lignite fouling problem must alter the fluxing sodium or the fluxable matrix parent.