Zinc nanoparticle formation and physicochemical properties in wood combustion – Experiments with zinc-doped pellets in a small-scale boiler J. Tissari a,⇑ , O. Sippula a , T. Torvela a , H. Lamberg a , J. Leskinen a , T. Karhunen a , S. Paukkunen b , M.-R. Hirvonen c , J. Jokiniemi a a Fine Particle and Aerosol Technology Laboratory, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland b Karelia University of Applied Sciences, Centre For Bioeconomy, Sirkkalantie 12 A, FI-80100 Joensuu, Finland c Inhalation Toxicology Laboratory, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland highlights  Pure pellet fuels were doped with different doses of Zn powder.  Zn from the fuel efficiently enriched the fine particle fraction of the emissions.  Structure and state of the zinc particles was highly dependent on fuel Zn content.  With high Zn content in the fuel, ZnO nanorods were formed.  Biofuel zinc requires more attention in emissions legislation. article info Article history: Received 20 July 2014 Received in revised form 20 November 2014 Accepted 24 November 2014 Available online 5 December 2014 Keywords: Wood pellet combustion Fuel Zinc behaviour Particle emissions 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 reg- ulations (e.g. the EU Waste Incineration Directive). However, pure wood combustion also produces sub- stantial 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 com- bustion. In addition, reference pellets without Zn doping (during efficient and poor combustion condi- tions) 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, http://dx.doi.org/10.1016/j.fuel.2014.11.076 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved. Abbreviations: BC, black carbon; DGI, Dekati Gravimetric Impactor; DLPI, Dekati Low Pressure Impactor. Measure particle mass size distribution; DR, dilution ratio; EC, elemental carbon; ED, ejector diluter; EDS, energy dispersive X-ray spectrometry; ELPI, Electrical Low Pressure Impactor. Measure particle number size distribution; GMD, geometric mean diameter of particles; MMD, Mass median diameter; OC, organic carbon; OM, organic matter; PAH, Polycyclic Aromatic Hydrocarbons; PM, Particulate Mass; PM 1 , Particle mass below aerodynamic diameter of 1 lm; PRD, porous tube diluter; REF, experiment case without Zn doping of fuel; RWC, residential wood combustion; SMPS, Scanning Mobility Particle Sizer. Measures particle number size distribution; TEM, transmission electron microscopy; TEOM, Tapered Element Oscillating Microbalance. Real time particle mass monitor; XRD, X-ray diffraction; Zn, zinc as an elemental form; zinc, zinc as a general form; Zn170, experiment case with doping of 170 mg Zn/kg fuel; Zn480, experiment case with doping of 480 mg Zn/kg fuel; Zn2300, experiment case with doping of 2300 mg Zn/kg fuel. ⇑ Corresponding author at: Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland. Tel.: +358 403553237; fax: +358 17163098. E-mail address: jarkko.tissari@uef.fi (J. Tissari). Fuel 143 (2015) 404–413 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel