Measurement and numerical simulation of ultrafine particle size distribution in the early stage of high-sodium lignite combustion

The formation of ultrafine particulate matter in the early stage of high-sodium lignite combustion was quantitatively investigated in a downward Hencken flat-flame burner under two ambiences of 1200 K and 1500 K. Spatially resolved measurement of the ultrafine particle size distributions (PSDs) was made by using a scanning mobility particle sizer with a high sensitivity in the 4.45-156.8 nm size range. The experimental results indicate that, during the residence time of coal particles from ∼20 ms to ∼40 ms, the number PSDs under 1200 K exhibit bi-modal while those under 1500 K keep uni-modal. As time is evolved, the detected peak of ultrafine particles moves from 10.62 nm to 80.54 nm under 1200 K, while that from 10.76 nm to 38.46 nm under 1500 K. Then, a physico-chemical mechanism responsible for the incipient formation of ultrafine particles during high-sodium lignite combustion was developed and computed by solving a discrete-sectional population balance model. The number PSDs of ultrafine particles and the dynamic behavior of Na release measured in experiments are consistent with the simulation results. It is further divulged that the intrinsic cause of the PSD transition between different ambient temperatures is the high concentration of newly formed particles, instead of the enhanced collision frequency.