Fly Ash Erosion Measurements in Utility Boilers and a Pilot Scale Facility

Abstract

Erosion in pulverised fuel fired utility boilers has been a long standing problem, exacerbated by the lack of a coherent explanation of the problem, and difficulties in mounting experiments that can directly measure erosion in the gas passages of a boiler. Recent research has examined the theoretical basis of erosion, resulting in the formulation of a mechanism which appears to have placed the problem on a firmer basis. Erosion measurements in utility boilers and a pilot scale test facility were performed using a specially designed probe which measured minute levels of erosion by means of the removal of a thin surface layer activated with a radioactive isotope. Measurements of boiler tube erosion were made in two utility boilers burning coals with widely differing erosion characteristics. Similar measurements were performed in a pilot scale Boiler Simulation Furnace (BSF) for comparison. Coal and fly ash samples collected in the course of the erosion experiments were investigated using QEM*SEM (Quantitative Evaluation of Materials by Scanning Electron Microscope), developed by CSIRO in Australia, to characterise fly ash in regard to its erosion characteristics. A count of the particle characteristics indicated that a significant number of particles were coated with porous clay-derived alumino-silicates, effectively reducing their eroding impact. Comparison of the boiler and pilot scale data showed that the lower temperature regime of the test facility produced less clay coatings on the particle, but more significant was the fact that the ashes produced in the test facility were appreciably finer than those from the power station, indicating that the pilot scale furnace was scalping the coarser fractions from the fly ash size distribution.