Alkali transformation during single pellet combustion of soft wood and wheat straw

Controlling slag and deposit formation during thermochemical fuel conversion requires a fundamental understanding about ash transformation. In this work, a macro-TGA reactor was used to determine the release of ash forming elements during devolatilization and char combustion of single pellets. Soft wood and wheat straw were combusted at two temperatures (700 °C and 1000 °C) and the residual ashes were collected and analyzed for morphology, elemental and phase composition. The results showed that the single pellet combustion exhibit similar release character as in grate boilers. The temporal release was found to be both temperature and fuel dependent. For wood, the release of potassium occurred mostly during char combustion regardless of furnace temperature. Similar results were found for straw at 700 °C, but the temperature increase to 1000 °C implied that the release occurred already during devolatilization. The differences are presumably explained by different fuel phase compositions. The residual ash were composed of three different categories of phases; crystalline compounds, molten ash (glass) and char, and the work concludes that K was captured by crystalline K/Ca-carbonates as well as in amorphous glassy silicates for wood, and by almost fully molten ash of glassy silicates for straw. The fuel conversion processes occurring on a grate influence the fuel combustibility in terms of e.g. burnout, slag formation and release of fine particle and deposit forming matter, and the present work has given novel insights into the specific alkali behavior during biomass fuel conversion.