Processing bagasse fly ash in the Australian industry: a review

All mills produce boiler fly ash and need to process this into a suitable waste product in a cost-effective manner. Ash systems also provide a clean water supply for wet scrubbers and the quality of the water is important to the performance of the scrubbers. Boiler ash water can also have significant impacts to the health of activated sludge wastewater treatment plants, so close management of the plant is required to prevent ingress of ash water into effluent to maintain effective wastewater treatment performance. As such, the boiler ash system has become increasingly important for sites to meet their environmental obligations for both boiler flue gas particulate emissions and for effective wastewater treatment. However, there is little information available on modern processing technology and expected equipment performance when processing bagasse fly ash. We found there was a wide range of total (suspended + dissolved) loadings in the raw slurry supplied to ash systems across the industry, which is consistent with earlier studies. There was a much narrower range of total solids loadings in the water supplied to wet scrubbers across the industry and most of these total solids loadings were less than 1.0 %. The conductivities of the water supplied to the wet scrubbers were generally quite high, which indicates that wet ash systems are relatively ineffective at removing dissolved solids. Three-stage wet ash systems produced better quality scrubber water than vacuum belt only ash systems. Settling tests showed that flocculants significantly increased settling rates in all cases. Spreadsheet modelling predicted that, if the ash system collection efficiency is only 50%, the mass flow of ash in the water supplied to the wet scrubbers is comparable to the mass flow of ash from combustion in the boilers. This will significantly increase boiler emissions and boiler operational problems.