Wheat straw ash and its use as a silica source

The overall aim of this project was to study the reutilisation of the major ash components, predominantly silica and potassium, in wheat straw combustion residues to form alkali silicate binders for novel inorganic composite boards.

Wheat straw of one variety and year was studied in detail on a lab-scale. Studies on the effects of combustion time and temperature show, for the first time, that the inherent alkali in the wheat straw is sufficient to solubilise up to 25% of the silica in the ash at room temperature. Combustion at temperatures below 600 °C gives the highest silica extraction by producing the most alkaline solutions, preventing calcium solubility and maintaining silica in a more reactive form. The first detailed comparisons of the effect of wheat straw variety and growth year on the properties of its combustion residues show that their mineralogical composition, leaching of inorganic components, silica structure and alkalinity can be correlated to the initial elemental composition.

Pretreatment of wheat straw by ethanol extraction indicates that this can reduce the potassium and chloride content similar to that achieved by water washing, potentially producing a less problematic material for combustion, without significantly reducing the alkalinity of the combustion residues. This is believed to be the first reported observation of this effect.

Pyrolysis of the wheat straw followed by char washing prior to combustion demonstrates improved extractability of the silica present utilising the inherent alkalinity, compared to extraction of the ashes. The leached chars display a lower ash content, lower potassium content, lower chloride content and higher calorific value, making them more suitable for combustion than the initial straw.

Finally, the formation of alkali silicate solutions from commercial biomass combustion residues is demonstrated. Biosilicate solutions comparable to commercial potassium silicate solutions are formed by extraction of the bottom ashes. These show potential as binders for inorganic composite boards.