Parallel aerosol sampling with SMPS and LPI in biomass combustion

1. Parallel aerosol sampling with SMPS and LPI in biomass combustion B. Stojkova, M. Hagström, J. B. C. Pettersson, Department of Chemistry, Physical Chemistry, Göteborg University, GÖTEBORG, SWEDEN Introduction This work is motivated by the need for understanding processes leading to alkali-induced superheater corrosion. This is a severe problem in industrial-scale biomass combustion due to the relatively high alkali content of biomass fuels compared to fossil fuels. Alkali metals in combustion facilities are often present as aerosols, either newly formed submicron particles or on the surface of coarse fly ash particles. We have constructed a low pressure impactor (LPI), designed to collect particles in 12 size classes from 12 m down to 0.02 m diameter. In conjuction with these measurements submicron size distributions have been obtained with a scanning mobility particle sizer (SMPS). Results Particle collection with the Low Pressure Impactor (LPI) Figure 3 shows size distributions based on the mass of collected particles. There is little significant variation between different wood fuel mixing ratios, with the exception of the accumulation mode particles (diameter 0.3-0.7 m). The collected particles show some variation in colour and water solubility. Experimental The tests were run in a 12-MWth CFB boiler at Chalmers University of Technology. The boiler load was maintained at about 9 MWth during all tests. A variety of fuel mixes from coal, wood chips and sawdust pellets, as well as additions of gaseous HCl and SO2 have provided the operating conditions. Figure 3. LPI size distributions based on collected particle mass. Particle size distributions obtained with SMPS Measurements on respective sides of the textile filter (baghouse) shown in Figure 4 indicates that the collection efficiency of the filter is very high. Figure 1. Setup of the LPI during measurements at Chalmers CFB. Abbreviations: T1,T2 = thermocouples; P = Pressure gauge; DMA = Differential Mobility Analyzer; CPC = Condensation particle counter Figure 4. Particle size distributions at respective sides of the textile filter. Low Pressure Impactor (LPI) An LPI setup designed for sampling combustion aerosols has been constructed and is shown in figure 2. The particles are collected on steel plates prepared with vacuum grease dissolved in toluene. The samples presented in this work were obtained at a location after the primary cyclone, at a temperature of approximately 350 °C. Some measurements were also made at 850°C, in the vicinity of the heat exchanging surfaces. Chemical analysis of the collected particles will be carried out with TXRF and ICP-MS. Outlook These measurement techniques are under development, and various methods for improved sampling and dilution of aerosols are being investigated. The combination of SMPS and LPI will be employed in further field measurements including small scale biomass-fueled district heating plants as well as domestic boilers. Acknowledgments: This work is supported by the Swedish Energy Administration (STEM). We would like to thank Mr. Benny Lönn for support and construction work. Figure 2. Cross-section of the Low Pressure Impactor (LPI).