Biomass ash and heavy metals

Burning of biomass in power plants is one way of utilisation of solar energy for power generation. The carbon dioxide emitted during combustion of biomass has been consumed in the previous production of the biomass by photosynthesis. Therefore, power generation from biomass is considered as a carbon dioxide neutral process. When also the ash from the combustion is recycled to the ground where the biomass is produced, a sustainable material cycle is established. Due to volatilisation in the combustion zone and subsequent condensation during cooling of the flue gas the more volatile heavy metals are accumulate in the fly ash. Depending on the concentration of heavy metals in the biomass, the concentrations of some heavy metals are above the limits for utilization of the ash according “Der sachgerechte Einsatz von Pflanzenaschen im Wald (1997)” and “Der sachgerechte Einsatz von Pflanzenaschen im Acker- und Grünland (1998)“, both issued by “Bundesministerium für Land- und Forstwirtschaft, Fachbeirat für Bodenfruchtbarkeit und Bodenschutz” As condensation takes place especially on the surface of the flue gas dust, the concentration of heavy metals is expected to be higher in the finer fraction of the dust, whose specific surface area is higher. Fly ash from the biomass power plant (9 MWe) of Linz Strom GmbH was separated into size fractions by a laboratory classifier. For the different size fractions the particle size distribution and the heavy metal concentration was measured. In figure 1 the results are shown as values measured for the different size fractions relative to the value measured for the whole sample. As to be expected the relative value of the calculated specific surface area (calculated from the particle size) decreases with the median (x50,3) of the size of the samples, the exponent in the regression function is close to - 1 ( theoretical value). The concentration of non volatile elements (Ca, Mg) was independent from the particle size. The exponent is close to zero. Zn was found to be the most volatile Element with an exponent of -0,94. Figure 1: Distribution of heavy metals In a further test, ash samples where separated into two fractions operating the laboratory classifier at different parameters. With an average Cd concentration of 26,4 mg/kgTS in the fly ash and a limit of 8 mg/kgTS for utilization of the ash (for the investigated plant Cd is usually the limiting element for utilization of the fly ash) a coarse fraction with approximately 1/3 of the mass of the fly ash can be produced for utilization by classification.