Capture of Potassium and Cadmium by Kaolin in Oxidizing and Reducing Atmospheres

Abstract

Nutrient elements including alkali metals in biomass ash need to be returned to the forest by ash recycling. It is, however, difficult to recycle the total biomass ash due to the high concentrations of heavy metals in some ash fractions. During fluidized bed combustion of biomass, alkali metals tend to attach on bed particles or deposit on heat transfer surfaces, causing bed agglomeration, slagging, and fouling problems, while heavy metals are found enriched in the fly ash. Therefore, it is important to control both alkali and heavy metals during combustion of biomass. In this study, the competition between potassium and cadmium adsorption on kaolin was investigated experimentally in a fixed bed reactor equipped with an alkali detector. The experiments were made at 850 °C under both oxidizing and reducing conditions, which are relevant to fluidized bed combustion of biomass. The oxidizing gas was air, while the reducing gas was composed of 80% N2, 10% CO, and 10% CO2. Fine aerosols of droplets containing the metals in water were produced from their aqueous solutions by means of an atomizer and subsequently vaporized in the reactor. Atomic absorption spectrometry (AAS) and X-ray powder diffraction (XRD) techniques were used for quantitative and qualitative analyses of the products. The results indicate that potassium is captured by kaolin in both oxidizing and reducing atmospheres. Kaolin captures cadmium in air, but not under reducing conditions. In addition, reducing conditions appear to promote potassium adsorption on kaolin. The copresence of potassium and cadmium compounds in the gas phase promotes the capture of both metals by kaolin. This promotion was more pronounced for potassium than for cadmium, indicating a preference of kaolin for potassium capture over cadmium capture. In air, the total amount of potassium captured by kaolin increased when cadmium was added, but the water-soluble fraction decreased slightly. Moreover, kaolin captures KCl slightly more effectively than KOH in air, but less effectively than KOH under reducing conditions.