Ph.D. Student Institute of Environmental and Occupational Health Sciences, College of Public Health, National Taiwan University TAIPEI, Taipei, Taiwan (Republic of China)
The USEPA has promulgated major Maximum Achievable Control Technology standards, requiring many plants to continuously measure particulate matter (PM) emission in stack gas flow. Particulate matter continuous emission monitoring system (PM CEMS) is an equipment that used to provide mass concentration data in these stationary pollution sources. PM CEMS could be divided into two major parts, extractive type and in-situ type. Extractive type PM CEMS are gradually equipped to replace in-situ type PM CEMS due to their lower detection limit and less dependent on the characteristics of particles and environment. However, extractive type PM CEMS needs a long sampling train to transport samples from the chimney to the analytical instrument. In a previous study, the sampling train transmission efficiency of a commercially available PM CEMS had been evaluated. The study showed that only about 5% of 10 μm particles can pass through the sampling train, even 2.5 μm particles have a loss of up to 15%. And, most of the particle losses occurred in the straight tube because of the turbulent flow. The purpose of PM sampling is to obtain representative data that can show the concentration and size distribution of aerosol particles in a specific environment. Particle losses would change the size distribution of samples and affect the accuracy of the measurements. There are several studies that have discussed the transmission efficiency of aerosol particles in horizontal straight tubes and developed related empirical formulas to calculate transmission efficiency. Nevertheless, the calculation results cannot fit the experimental data effectively. The model shows that the transmission efficiency increases with the increase of sampling rate under turbulent flow conditions. Moreover, the trend of transmission efficiency of our experimental data is contrary to that calculated by the model. In this work, the conduit dimensions and sampling rate in the horizontal straight tube are varied and optimized. A new empirical model is also developed.
In an aerosol penetration test apparatus, an ultrasonic atomizer (model 8700-120, Sonotek Inc., Highland, NY) is used to generate micrometer-sized challenging NaCl particles. The aerosol output is then introduced into the test chamber through a radioactive source, 10 mCi Am-241, to neutralize aerosols to the Boltzmann charge equilibrium, before diluted with dried filtered air. An Aerodynamic Particle Sizer (APS, Model 3321, TSI Inc., St. Paul, MN, USA) is employed to measure the aerosol size distributions and number concentrations upstream and downstream of the long sampling probe. The length of the sampling duct is 160 cm. Three inner diameters are set, 4.5 mm, 7.7 mm and 10 mm. The Reynolds number (Re) ranging from 500-5000 is employed to study the Re number dependency.
The results show that the aerosol transmission efficiency is the highest when the Re of sampling flow near 2100. The transmission efficiency increases with the increase of Re under laminar flow (Re4000) condition. Therefore, the Re number of the sampling flow should be set to about 2100 to optimize the sampling efficiency, when conducting a sampling operation.
