ASH INTERACTIONS DURING THE COFIRING OF BIOMASS WITH FOSSIL FUELS

摘要

Cofiring of renewable biomass fuels in conventional coal-fired utilities is seen as a lowest-cost option to achieve reductions in greenhouse gas emissions. The Energy & Environmental Research Center has undertaken a fundamental study to address the viability of cofiring biomass with coal in a pulverized coal-fired boiler for power production. The objectives were to perform advanced analyses of biomass inorganic content to allow prediction of fireside performance and bench-scale combustion testing to determine ash formation and deposition processes with emphasis on the interactions between coal and biomass inorganic components. Wheat straw, alfalfa stems, and hybrid poplar were selected as candidate biomass materials for blending at a 20 wt% level with an Illinois bituminous coal and a Wyoming subbituminous coal. Combustion testing was performed to obtain deposits typical of boiler fouling and slagging conditions along with fly ash. Analysis methods using computer-controlled scanning electron microscopy and chemical fractionation were applied to determine the composition and association of inorganic materials in the biomass samples and coals, as well as the supermicron size distribution and particle compositions of the fly ash. Scanning electron microscopy point count analysis was used to determine composition and infer mineral interactions in the deposits. The results show that significant interaction of the coal and biomass inorganic components is occurring in the blend deposits, particularly with the incorporation of biomass-derived calcium and potassium into the deposits. This has the physical effect of producing blend deposits which are substantially weaker than deposits formed from the pure parent biomass or coal. Such weak deposits, although growing rapidly, are expected to be easily removable. For the fly ash, it is known that combustion of pure biomass typically results in a large submicron particle fraction due to condensation of volatilized minerals, primarily potassium and sodium sulfates and chlorides. This generation of submicron particulate is a potentially important issue from a health standpoint. On-line particle-size measurements were obtained as part of conversion and environmental process simulator combustion testing with hybrid poplar, Wyoming coal, and an 80%-20% Wyoming coal-hybrid poplar blend. These measurements indicated that there is a moderate shift in the submicron fraction to larger particle sizes and, possibly, a small shift to larger particle sizes in the supermicron particle-size range on blending. Surprisingly, the coal appeared to have more fine submicron ash than either the hybrid poplar or the blend. This work is ongoing with particle-size measurements planned as part of combustion tests with the other biomass fuels and coal-biomass blends planned to further investigate this issue.