The environmental impact of the heavy metals contained in the combustion product ash depends on the speciation of the heavy metals and the size distributions of the heavy metals in the ash. Therefore, the behavior of cadmium, lead, copper, and zinc was studied experimentally during circulating fluidized bed combustion (CFBC) of Swedish forest residue. The size distributions and concentrations of the heavy metals in the fly ash particles and in the gas phase were determined by low-pressure impactors and filters upstream of the convective back pass at 830 °C. Downstream of the convective back pass at T ) 150 °C, the size distributions were determined. The fly ash from CFBC was found to contain two separate particle classes. Fine particles (D p < 0.5 µm) consisted mainly of KCl, and coarse particles (D p > 0.5 µm) contained as major elements Ca and Si. Major fraction of all the studied heavy metals were found in the coarse fly ash particles at location 1 at 830 °C; 7-26% of Pb, 24-27% of Cu, 1-8% of Cd, and less than 1% of Zn were found in the gas phase. The gas-to-particle conversion route for Cd, Pb, and Cu was found by chemical surface reaction, probably with silicates. None of the studied heavy metals were enriched in the fine particles at the inlet of the electrostatic precipitator.

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Ash formation was studied experimentally during combustion of forest residue and willow in a 35 MW circulating fluidized-bed co-generation plant. Ash particles and vapors were sampled downstream of the process cyclone at flue gas temperatures of 810-850 °C. Inorganic vapors and fly ash particles were collected separately on filters. Size-classified fly ash particle samples were collected with an impactor. Elemental analysis methods and electron microscopy were used for ash characterization. During combustion of forest residue, about 30-40% of the total ash and a similar percentage of Ca and P was attached on the bed-material particles and were eventually removed from the bed along with bottom ash. K was retained with a higher efficiency (about 50%). S and Cl were volatilized in the furnace and were not retained in the bed. During combustion of forest residue, 80% of S had already reacted with species present in supermicrometer ash particles when detected after the process cyclone at 810-850 °C. When willow was combusted, about one-half of the sulfur remained as SO 2 . More than one-half of the particlephase S was present as alkali sulfate fine particles that were almost nonexistent during forest residue combustion. Cl was present as gaseous species (KCl and presumably HCl) at 810-850 °C with both fuels.

Fly ash deposition on heat-exchanger surfaces during fluidized-bed combustion of biomass causes operational problems such as reduced heat transfer and corrosion of superheater tubes. Ash deposition and alkali vapor condensation were studied during circulating fluidized-bed combustion of forest residue in a 35 MW co-generation plant. A 70 ± 10% amount of fly ash was deposited on the heat-exchanger surfaces in the convective back pass between soot-blowing periods. Practically all the largest ash particles (d > 10 μm) but only a small fraction of particles d < 3 μm were deposited. The deposition efficiency of particles with a given size was not correlated with their elemental composition. About 50% of alkali chloride vapors (KCl and NaCl) were condensed in the convective back pass on fine-mode particles (d < 0.6 μm) and the other 50% on the coarse-mode particles. Alkali chlorides were not effectively deposited since they were depleted in the largest (d > 10 μm) ash particles.