Particle emissions from combustion of single wood pellets
Paper in proceedings, 2006
This is a time-resolved study of particles emitted during combustion of wood pellets with respect to size and composition. Two different experiments were carried out in a heated quartz glass reactor. First, real-time measurements were performed of particles, particle-bound polycyclic aromatic hydrocarbons (pPAH), carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2). The concentration of particle-bound polycyclic aromatic hydrocarbons was used as a soot indicator. Then, emission of inorganic particles was studied by calculating the change in release rate from the mass change with time for ash and metals, obtained from chemical analyses of partially burnt pellets. Particles were measured by an Electrical Low-Pressure Impactor, simultaneously with pPAH, using a Photoelectric Aerosol Sensor. To obtain time-resolved information about the metals released, 250 – 290 single pellets were burnt for various periods of time and subsequently analysed. The combustion was quenched after devolatilisation and after 25, 50, and 75 % of the char conversion (mass-basis). The original pellets and partially burnt pellets were chemically analysed for carbon, hydrogen, nitrogen, oxygen, ash and six of the most abundant metals in the fuel. Analyses of metal content were carried out for Calcium (Ca), Potassium (K), Manganese (Mn), Magnesium (Mg), Zinc (Zn), and Sodium (Na) with an ICP-OES. Fully burnt pellets were analysed for metal composition. The results indicate that soot is mostly emitted during devolatilisation. The particles emitted during char combustion are inorganic, containing calcium, potassium, manganese, and magnesium. The release of sodium was equally distributed between devolatilisation and char combustion. The zinc release took place mainly during devolatilisation. Although the main part of calcium was released during char combustion, the mass of calcium released during devolatilisation was relatively high, corresponding to about 30 % of the ash released during this period and the concentration of calcium was 10 times or higher than those of the remaining metals studied.