Slagging Characteristics during Combustion of Corn Stovers with and without Kaolin and Calcite

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Abstract

Ash-related problems have more than occasionally been observed in biomass-fired boilers and also recently in biopellet burners. These problems can lead to reduced reliability of the combustion systems as well as bad publicity for the market. When agricultural residues are used as biofuel feedstock, slagging problems will be worse. The objectives of the present work were, therefore, to examine the effects of kaolin and calcite addition on the slagging tendency of corn stover fuel when corn stover pellets are burned in a small-scale appliance and determine the slag characteristics during combustion. Pellets with an additive/fuel ratio of 3% (dry mass) were combusted in an underfed burner (50 kW) that is installed in a boiler with 90 kW output. The choice of 3% additive/fuel ratio was based on analyses of the ash melting behavior of seven fuel mixtures that combine either 0−3% kaolin or 0−3% calcite and corn stovers. The 3% kaolin and calcite addition increased the ash melting temperature (IT) by about 100−200 °C. When the 3% kaolin or calcite was added to the corn stover raw material, the severe slagging tendency of the fuel was considerably reduced. The slag quantities from burning kaolin- and calcite-added fuels were about half and one-third, respectively, of that from nonadditive pellets. The slag deposits from the burner were characterized with scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). The XRD was also used to examine the chemical composition of corresponding bottom ash in the boiler. The results indicated that the reduction of slagging when using additives can be attributed to a change from relatively low melting temperature silicates to higher melting temperature silicates. For the corn stover without additives, the low melting fractions of the slag were assumed to consist mainly of potassium calcium silicate, indirectly observed as a glass by the XRD. When kaolin was added, a depletion of potassium was observed because of the extensive formation of leucite (KAlSi2O6) and the glass became dominated by calcium, aluminum, and silicon. This process was accompanied by a considerable reduction of glass amount. In the case of CaCO3 addition, however, calcium magnesium silicates formed to an extent that the glass (low melting material) finally became dominated by potassium silicate. This process was also accompanied by a substantial reduction of the amount of glass.

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