Enhanced Control of Fine Particle Emissions from Waste Biomass Combustion Using a Hybrid Filter

This work demonstrates the feasibility of a robust and effective control of the emissions of particulate matter (PM) and heavy metals (HMs) from olive tree pruning combustion, as well as from its blends with two customary local components (a composted fraction of sewage sludge and a fraction of municipal solid waste with a high plastic content), in an approximated proportion of 50/50 (% w.t) to mimic the seasonal supply variability. The study was carried out during a 5 weeks experimental campaign in a 1MWth fluidized bed combustor. The combustor was equipped with a hybrid filtration system, consisting of an electrostatic precipitator and a bag filter module serially connected, for an enhanced control of the PM emissions when dealing with a realistic suite of fuel blends. Raw aerosol showed high concentrations in both PM and HMs, substantially dependent on the blend. However, the hybrid filter (HF) showed a good performance irrespective of these variable loads, leading to emission concentrations below 7 mg/Nm³ (max. 7% O₂, total basis), ensuring compliance with the probable emission limit values. Both mass and number filtration efficiencies ranged within 95–99%. The fractional efficiency has been measured in quasi-real time, thus enabling an assessment of the suitable operating conditions, as well as an evaluation of the reliability of HF operation. Removal efficiency for HMs was obtained and discriminated by size; results show differences among the elements considered, reflecting the differences in the chemical distribution over the size of the raw aerosol.