Article pubs.acs.org/est
Size Distribution, Chemical Composition, and Hygroscopicity of Fine Particles Emitted from an Oil-Fired Heating Plant Matti Happonen,† Fanni Myllar̈ i,† Panu Karjalainen,† Anna Frey,‡ Sanna Saarikoski,‡ Samara Carbone,‡ Risto Hillamo,‡ Liisa Pirjola,¶ Anna Haÿ rinen,§ Jorma Kytömak̈ i,§ Jarkko V. Niemi,∥ Jorma Keskinen,† and Topi Rönkkö*,† †
Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland Air Quality, Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland ¶ Department of Technology, Metropolia University of Applied Sciences, P.O. Box 4000, FI-00180, Helsinki, Finland § Helsingin Energia Plc, FI-00090 Helen, Helsinki, Finland ∥ Helsinki Region Environmental Services Authority (HSY), P.O. Box 100, FI-00066 HSY, Finland ‡
ABSTRACT: Heavy fuel oil (HFO) is a commonly used fuel in industrial heating and power generation and for large marine vessels. In this study, the fine particle emissions of a 47 MW oil-fired boiler were studied at 30 MW power and with three different fuels. The studied fuels were HFO, water emulsion of HFO, and water emulsion of HFO mixed with light fuel oil (LFO). With all the fuels, the boiler emitted considerable amounts of particles smaller than 200 nm in diameter. Further, these small particles were quite hygroscopic even as fresh and, in the case of HFO+LFO emulsion, the hygroscopic growth of the particles was dependent on particle size. The use of emulsions and the addition of LFO to the fuel had a reducing effect on the hygroscopic growth of particles. The use of emulsions lowered the sulfate content of the smallest particles but did not affect significantly the sulfate content of particles larger than 42 nm and, further, the addition of LFO considerably increased the black carbon content of particulate matter. The results indicate that even the fine particles emitted from HFO based combustion can have a significant effect on cloud formation, visibility, and air quality.
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INTRODUCTION Heavy fuel oil (HFO) is a petroleum product that is left when all distillates have been separated from crude oil. HFO includes much impurities such as sulfur, asphaltenes, and vanadium. The main uses of HFO are in industrial heating and power generation and in the transport sector, where it is used as fuel for large marine vessels. HFO is generally preheated before combustion to lower its viscosity. The combustion of HFO is known to produce considerable amounts of sulfur oxides (SOx) and particulate matter (PM). Particulate matter emitted in HFO combustion has high metallic ash (Ni, V, Fe, Cr, Na)1 and sulfur content. From these, the vanadium content has especially been of interest due to its adverse effects on human health.2,3 Because of the high PM and sulfur emissions associated with HFO, meeting the emission legislation practically means either efficient exhaust after-treatment, influencing the fuel composition to achieve cleaner combustion, or both. Techniques that can be used to modify the fuel in oil-fired burners are mixing a portion of light fuel oil (LFO) into HFO and creating water emulsions of the fuel oil. Mixing LFO into HFO mainly makes the fuel a higher grade of fuel oil, that is, the sulfur and ash contents of the resulting blend are lower than © 2013 American Chemical Society
those of HFO. The advantage of using oil−water emulsion is the occurrence of the so-called microexplosions that take place in the emulsion prior to its combustion.4,5 Microexplosions enable a secondary atomization of fuel droplets prior to the combustion. When the water−oil emulsion is sprayed into the combustion chamber, the small water droplets enclosed in oil droplets start to expand at a greater rate than the oil. The rapid evaporation of the water then results in disintegration of the oil droplets to smaller ones before the combustion of the oil.6 The smaller droplets at the combustion then enable more efficient combustion of the fuel leading to a reduction in PM while the cooling effect of introducing water to the combustion reduces NOx formation.7 Size distribution of particles emitted in HFO combustion has typically been observed to be bimodal.1,8 The larger particles (diameter typically >1 μm) consist of the char residues of oil droplets and particulates re-entrained to the exhaust from walls. The smaller particles (diameter
