Fine particle and trace element emissions from waste combustion: Comparison of fluidized bed and grate firing

Waste-to-energy applications are increasingly being used to simultaneously reduce the amount of waste and produce electricity and heat. As waste materials typically contain high concentrations of impurities that are transformed during combustion, the flue gases need to be cleaned efficiently in order to avoid harmful emissions to the environment. In this investigation, we determined experimentally fly ash particle characteristics – particle size, composition and concentration – as well as particle and trace element emissions during waste combustion in two full-scale plants using different combustion technologies but similar gas cleaning technology.
The two combustion plants were a grate-fired boiler, and a circulating fluidized bed (CFB) boiler. Both boilers used selective non-catalytic reduction (SNCR) for NOx control together with novel integrated flue gas desulfurization (NID) for flue gas cleaning. The trace elements included in the investigation were As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, and V.

Fine fly ash particles were formed from the gas phase ash-forming species by nucleation and condensation. Concentration of particles smaller than 1.0 μm by diameter (PM1.0) at grate-fired plant was 1.0–1.4 g/Nm3, approximately four times the PM1.0 concentration at CFB combustion, 0.25–0.31 g/Nm3, as determined upstream of the flue gas cleaning system NID.
The average total fly ash mass concentration was higher at CFB combustion than at grate firing with 4.6 g/m3 and 1.4 g/m3, respectively. Particle and trace element emissions were very low from both grate-fired and fluidized bed plants. Fabric filter particle collection efficiency was 99.99% by mass at both plants. All the measured emissions were clearly below the limit values set by European Waste Incineration Directive (WID).