Development of an aerosol sampler for pressurized systems and its application to investigate the effect of residence time on PM1 formation in a 15 bar oxy-coal combustor

Many sophisticated instruments are available for obtaining detailed information about aerosol particle size, shape and composition, however, most of these devices require that the aerosol is at atmospheric pressure. At the same time, many important industrial processes involving aerosols operate at elevated pressure. Thus, a sampling system is needed that can reduce the pressure of a sample stream to atmospheric pressure without sample bias due to particle losses or generation. In this work, we present the design and testing of such a device, a pressurized particle sampler (PPS) for sampling pressurized aerosols. To obtain a representative sample, the PPS utilizes: 1) two-stage dilution to minimize particle generation in the sampling probe, 2) a pressurized PM10 cyclone to remove large particles, 3) a sonic nozzle connected to an axisymmetric expansion chamber to depressurize the aerosol sampling stream with minimum impaction losses on the walls, and 4) an isokinetic sampler after the expansion to extract the aerosol to the instrumentation at atmospheric pressure. The performance of the PPS is compared with that of a simple sonic nozzle, which has been used in several previous studies to investigate aerosol formation in pressurized systems. Two approaches were employed for this comparative test. First, polydisperse NaCl particles were generated at 15 bar using a pressurized nebulizer. It was found that the pressure reduction through a sonic nozzle can lead to an underestimation of the aerosol formation due to the impaction losses on the walls during the stream expansion. In a second comparison test using ash particles, the results showed that the amount of particle loss to the walls downstream of the sonic nozzle increased with pressure. The performance of the PPS was evaluated and found to be superior to that of a sonic nozzle, and then the system was employed in a pressurized oxy-coal combustor to understand the effect of coal particle residence time on PM1 formation at 15 bar. The number particle size distribution of PM1 was found to exhibit two distinct modes, an ultrafine and an intermediate mode; however, with increasing residence time, the peak in the ultrafine mode decreased, and the peak in the intermediate mode declined steadily, and eventually becoming independent of residence time.