Composition and Formation of Sinter and Slag in Residential Pellet Ash

Ash from burning biomass is considered to be a nuisance in combustion systems. It is potentially corrosive, and under certain conditions will sinter and fuse into larger particles of ash commonly referred to as slag, which may as a result of interfering with heat transfer reduce the overall efficiency of the combustion system. Testing was conducted to examine the various characteristics of ash and how it relates to variation in combustion temperature and formation of ash sinter. The methods used to analyze ash were SEM microscopy, ICP-OES, pycnometer analysis, morphological analysis, sorption potential, and electrical resistivity measurements. Additionally, various standards used to determine the ash content of biomass fuels were assessed for procedural accuracy. Of the four ash content determination methods assessed, it was found that testing procedure EN 14775, Solid Biofuels - Determination of Ash Content, showed the lowest average variation in repeatability, and that testing procedure ASTM D1102, Standard Test Method for Ash in Wood, showed the largest variation. A novel test dubbed Elevated Temperature Crucible Weight (ETCW), which utilized weighing the samples at slightly elevated temperatures as opposed to weighing after cooling in a desiccator, produced statistically comparable results to both testing procedures EN14775 and ASTM E1755, Standard Test Method for Ash in Biomass. Using TGA, it was found that that the mass of the ash may decrease by as much as 25 percent when increasing the temperature of the ash from 525°C to 900°C. The temperatures reached in a residential wood pellet stove may exceed 1100°C, therefore, increasing the maximum temperature reached during ash determination test will yield a more pragmatic representation of the material. A novel test utilizing electrical resistivity was developed to examine the degree of sinter in wood pellet ash. The electrical resistivity of the wood pellet ash samples showed a curvilinear increase in response to temperature, with statistically significant differences seen when comparing the samples collected at low and high temperatures. This indicates that the onset of sinter may be assessed by examining the ash’s electrical resistivity profile. The ICP-OES analysis results varied with increasing temperature and showed a substantial decrease in the percentage of carbon and potassium at elevated temperatures. Smaller changes were found in the percentages of a number of other elements. Calcium was the most abundant constituent in all tests. The SEM images showed geometric changes in some of the ash ranging from irregular through spheroidal and to crystalline. The changes in composition along with the ash structure are thought to be responsible for the variability in resistivity as the temperature of ash collection changed. The density of the wood pellet ash samples showed a somewhat curvilinear increase in response to temperature, with statistically significant differences seen when comparing the samples collected at different temperatures. The morphological analysis showed an increase in the average particle size of the samples. Finally, the results from the sorption tests suggested that ash that has been exposed to high temperatures has a decreased affinity for moisture sorption than ash collected at lower temperature levels