Evaluation of ash /slag heavy metal characteristics and potassium recovery of four biomass boilers

The utilization of biomass combustion for electricity generation has grown an importance and popularity. However, effective management of ash produced during biomass combustion is a major challenge. To analyze the ecological risk of heavy metals and ash characteristics in biomass ash, an inductive coupled plasma emission spectrometer (ICP) and X-ray diffraction (XRD) were used. The effects of temperature (20 °C, 40 °C, 60 °C), stirrer time (1h, 2h, 3h), and water-ash ratio (1:1, 2:1, 3:1) on potassium extraction were studied. The results show that the ecological risk index (RI) values of the fly ash and slag samples are very high, and all samples have very high risk (VHR) mainly due to Cd higher toxicity than other heavy metals. The main constituents of slag are aluminosilicate and SiO₂, whereas fly ash contains CaSO₄, KCl, SiO₂, and KAlSi₃O₈. The fly ash from straw-fired biomass boiler contains more potassium, making it the most suitable raw material for the actual potassium extraction. In addition, temperature and stirrer time have little effect on potassium solubility while the effect of water-ash ratio is significant. Therefore, the most cost-effective potassium extraction method is water washing at room temperature with a stirrer time of about 1 h, a water-ash ratio of about 2:1, and leaching fly ash multiple times with incorporating the water recycling.