Concentration of Heavy Metals in Fly Ash from Empty Fruit Bunches Combustion by Particle Size

Empty fruit bunches (EFB) are produced during the production of palm oil. This residue is used as fuel in biomass combustion plants. Beside other ways of utilization, the fly ash from off-gas de-dusting can be utilized as soil conditioner. However, to prevent build-up of heavy metals in the soil the heavy metal content of the ash should be low. Yet, in literature only few data on the heavy metal content of EFB combustion fly ash are available. In this study, the concentration of various heavy metals in a fly ash as well as their distribution in the different size fractions was investigated. A cyclone fly ash sample from a grate-fired combustion plant was split into six particle size classes using a laboratory air classifier. The concentration of various metals was analysed by ICP-OES and IC after aqua regia digestion of the sample. The results show that the concentration of all heavy metals investigated was quite low in all size fractions. For several components a considerable dependence of the concentration on the particle size was observed. Significantly increased concentrations in the fine size fractions were found for the heavy metals Cd, Cu and Pb. The concentrations in the finest size fractions were up to six times higher compared to the original fly ash. The heavy metal contamination of the cyclone fly ash was below the limit concentrations for utilization of various European countries. Although some heavy metals are significantly enriched in the finest size fractions their concentrations are still comparatively low.