Zinc nanoparticle formation and physicochemical properties in wood combustion – Experiments with zinc-doped pellets in a small-scale boiler

Abstract Fine particles are the most important type of pollutant affecting urban air quality. Recent studies have highlighted the relevance for health effects of the zinc component of these particles. Zinc is traditionally associated with industrial and waste combustion plant emissions, although not covered by current regulations (e.g. the EU Waste Incineration Directive). However, pure wood combustion also produces substantial amounts of zinc particles. In this study, pure wood pellet fuels doped with three doses of Zn powder were combusted in a small grate boiler. The emissions were then analysed by a broad array of techniques to shed light on the health-related properties of particles originating from Zn-rich fuel combustion. In addition, reference pellets without Zn doping (during efficient and poor combustion conditions) were studied. Zinc was found to be efficiently released from the fuel and enriched in the fine particle fraction, a trend supported also by thermodynamic equilibrium calculations. The enrichment was systematically observed as changes in the size, mass, chemical composition, and shape of the particles. The growth of the particles was mainly due to the coagulation and growth of the pure crystalline zinc oxide (ZnO) cores. With high Zn doping ZnO nanorods were clearly formed, whereas with a low Zn content in the fuel other ash-forming species defined the particle morphology better. The ZnO formation process was found to be thermodynamically similar to the production of engineered nanomaterials. This study suggests that more attention should be paid to the zinc content of biomass fuels with regards to emission legislation, for example when wood bark is utilised in energy production. This concern, in particular, the small and medium scale (below 1 MW) power plants as efficient particle removal techniques are generally utilised in large scale power plants.