The chemical value of wheat straw combustion residues

Abstract

Considerable mineralogical changes occur during the combustion of biomass such as wheat straw, which lead to variation in the physical, chemical and textural properties of the ashes formed. Mineralogical and chemical changes occur above combustion temperatures of 500 °C causing reductions in the solubility of potassium, chloride, silica and sulfate. This can be directly correlated to the evaporation of KCl, dissociation of K₂SO₄, softening of silica and formation of alkali silicates. Calcium extraction increased above combustion temperatures of 700 °C due to the decomposition of CaCO₃. We have discovered, for the first time, that the inherent alkali in unleached wheat straw is sufficient to solubilise up to 30% of the silica in the ash at room temperature. This could be used to produce potassium silicate solution as a new valuable by-product of wheat straw combustion. It was also found that incomplete combustion at high temperatures, following leaching of ashes can lead to the formation of porous activated carbons and inorganic materials, demonstrating yet another valuable use for this waste material.