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.
Behavior of chlorine and sulfur is critical on the formation of fine particles (here particles smaller than one micrometer, PM1.0) during combustion. In this investigation, we studied experimentally fine particle formation in a pilot-scale circulating fluidized bed reactor during combustion of bark and pulp and paper mill sludge. The effect of chlorine and sulfur on fine particle formation was investigated by adding HCl and SO 2 into the reactor. Fine fly ash particles were formed from alkali species that were released from the fuel to the gas phase. In low-HCl conditions, alkali species reacted readily with silicates, and therefore, a large fraction of alkalis was retained in the bottom ash. Consequently, fine particle concentrations in the flue gas were relatively low. In this case, fine particles were both alkali metal chlorides and sulfates. HCl addition increased the concentration of fine particles considerably due to gas phase reactions between alkali metal species and HCl. Alkali metal chlorides that were formed condensed producing large mass concentrations of fine alkali metal chloride particles. Due to extensive alkali metal chloride formation, significantly less alkalis reacted with silicates and ended up in the bottom ash than when HCl concentration was low. Further SO 2 addition transformed some of the chlorides into sulfates in the fine particle mode. At the same time, the total fine particle concentration decreased, possibly due to formation of coarse mixed K-Ca-sulfate ash particles.
Particle and trace element emissions from energy production have continuously been subject to tightening regulations. At the same time, not enough is known on the effect of different combustion processes and different fuels and fuel mixtures on the particle characteristics and particle removal device operation. In this investigation, electrostatic precipitator fractional collection efficiency and trace metal emissions were determined experimentally at a 66 MW biomass-fueled bubbling fluidized-bed combustion plant. The measurements were carried out at the inlet and outlet of the two-field electrostatic precipitator (ESP) at the flue gas temperature of 130-150 degrees C. Two fuel mixtures were investigated: biomass fuel containing 70% wood residue and 30% peat and biomass with recovered fuel containing 70% wood residue, 18% peat, and 12% recovered fuel. The particle mass concentration at the ESP inlet was 510-1400 mg/Nm3. Particle emission at the ESP outlet was 2.3-6.4 mg/Nm3. Total ESP collection efficiency was 99.2-99.8%. Collection efficiency had a minimum in particle size range of 0.1-2 microm. In this size range, collection efficiency was 96-97%. The emission of the trace metals As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Tl, and V was well below the regulation values set by EU directive for waste incineration and co-incineration.
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