Ash characteristics of high alkali sawdust and sanderdust biomass fuels

Suspension firing of sawdust and sanderdust fuels is often used in the wood product's industry to raise steam and provide a heat source for drying and curing operations. The unusually high alkali content of these fuels can give rise to a number of problems that affect the operation of downstream plant systems. The research reported here focuses on the physical and chemical properties of the ash generated by these unique and important biomass fuels and uses this information to identify the mechanisms that control mineral-to-ash transformations.

Four sanderdust and sawdust fuels, typical of those used to fire industrial-scale suspension burners, are fired in a laboratory-scale tunnel furnace. Size distribution, morphology, and size versus composition are obtained for particles between 0.0075 and 10 μm by combining a cascade impactor and an electrical aerosol analyzer (EAA). Each of the fuels showed a dominant mode of calcium-rich skeletal particles of size ≥8.3 μm that are the residue from char burnout. A second, minor mode that seems to be the result of fragmentation appears at 1–3 μm. This consists of fluxed particles that, while still predominantly calcium, also contain Fe, Al, Mn, and Si. Another minor mode at 0.4 μm also appears to be the result of fragmentation. Much of the alkali mineral matter becomes submicron aerosol via the vaporization, condensation, coagulation mechanism. This large yield of aerosol (of the order of 30% of the total ash mass) appears as chlorides in high chlorine fuels and as sulfates and carbonates otherwise. In general, only a small fraction of the alkali metals are captured by the residual ash, and no metals other than Na and K are generally detected with the aerosol. The aerosol size varies between 0.01 and 0.1 μm depending on experimental conditions.