| Abstract
| - Nutrient elements including alkali metals in biomass ash need to be returned to the forest byash recycling. It is, however, difficult to recycle the total biomass ash due to the highconcentrations 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 bedagglomeration, slagging, and fouling problems, while heavy metals are found enriched in the flyash. Therefore, it is important to control both alkali and heavy metals during combustion ofbiomass. In this study, the competition between potassium and cadmium adsorption on kaolinwas investigated experimentally in a fixed bed reactor equipped with an alkali detector. Theexperiments were made at 850 °C under both oxidizing and reducing conditions, which are relevantto fluidized bed combustion of biomass. The oxidizing gas was air, while the reducing gas wascomposed of 80% N2, 10% CO, and 10% CO2. Fine aerosols of droplets containing the metals inwater were produced from their aqueous solutions by means of an atomizer and subsequentlyvaporized 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 resultsindicate that potassium is captured by kaolin in both oxidizing and reducing atmospheres. Kaolincaptures cadmium in air, but not under reducing conditions. In addition, reducing conditionsappear to promote potassium adsorption on kaolin. The copresence of potassium and cadmiumcompounds in the gas phase promotes the capture of both metals by kaolin. This promotion wasmore pronounced for potassium than for cadmium, indicating a preference of kaolin for potassiumcapture over cadmium capture. In air, the total amount of potassium captured by kaolin increasedwhen cadmium was added, but the water-soluble fraction decreased slightly. Moreover, kaolincaptures KCl slightly more effectively than KOH in air, but less effectively than KOH underreducing conditions.
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