Particles from biomass combustion -Characteristics and influence of additives
Doctoral thesis, 2008
High particle concentration in the air is a health risk, and particles from domestic biomass combustion are a significant particle source. In larger boilers, particles are an important element in the build-up of deposits on heat transfer surfaces, causing increased operation and maintenance costs. The objective of this thesis is to present new emission data for particles from biomass combustion in domestic devices and grate-fired district heating boilers (downstream of multicyclones), to investigate particles in a circulating fluidised bed (CFB) boiler, and to illustrate how particles can be influenced by additives. Finally, the two main approaches used in domestic combustion are compared, and a probe for sampling at high flue gas temperature is designed.
The mass concentration of particles from domestic combustion of woody fuels was measured to be 12-120 mg/MJ, except for two extreme cases. Combustion of wood pellets showed the lowest emissions, comparable with the mass concentrations from district heating boilers. Domestic oat combustion gave 114 mg/MJ. The use of limestone as an additive for oat combustion did not affect the emitted particle mass, while the addition of kaolin resulted in lower concentration of particles.
The number concentration of particles was 1•1012-1•1014 1/MJ for domestic combustion, and lower from district heating boilers. Flue gas from domestic devices and district heating boilers showed particle size distributions dominated by fine particles (< 1 µm), both in terms of mass and number (measured with a low-pressure impactor and an electrical low-pressure impactor). Under poor combustion conditions, concentrations were higher, with the fine particle maximum in the size distribution being displaced towards a larger size. Efficient combustion of wood pellets resulted in particles mainly containing K2SO4, while wood/straw pellets led to KCl particles. Potassium and phosphorus dominated the particles from oat combustion.
The mass size distributions in the CFB boiler had maxima for coarse and fine particles respectively. The addition of kaolin to the combustion or co-combustion with sewage sludge lowered the concentration of fine particles, but increased the concentration of coarse particles.
For domestic wood combustion, the value of particle mass concentration measured in cooled flue gas was higher (2-10 times) than the values measured in the chimney.