Development and Performance Evaluation of a Porous Tube Dilutor for Real-time Measurements of Fine Particles in High-humidity Environments

Received: August 29, 2018
Revised: August 29, 2018
Accepted: October 8, 2018

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2018.08.0319  

Cite this article:

Woo, C.G., Hong, K.J., Kim, H.J., Kim, Y.J., Han, B., An, J., Kang, S.J. and Chun, S.N. (2019). Development and Performance Evaluation of a Porous Tube Dilutor for Real-time Measurements of Fine Particles in High-humidity Environments. Aerosol Air Qual. Res. 19: 461-466. https://doi.org/10.4209/aaqr.2018.08.0319

HIGHLIGHTS

• For measuring fine particles in the exhaust gas, porous tube dilutor was developed.
• Through dilution in the sampling process, condensable particles were excluded.
• Particles larger than 3 µm in diameter were successfully measured.
• The dilutor can be used to continuous real-time monitoring PM₁₀ and PM_2.5.

Keywords: Sampling; PM10; PM2.5; Dilution; Condensable.

INTRODUCTION

Health concerns arise on days with high PM10 or PM2.5 levels (Health Effects Institute, 2017). To reduce PM10 or PM2.5 levels, precise measurements at the source of PM10 and PM2.5 are required so that the causes of generation can be analyzed and appropriate reduction technologies applied. Currently, fine particle measurements at fixed sources are performed manually via sampling and mass measuring; this takes at least 2–3 days and, potentially, over a week (Environmental Protection Agency, 1998). Thus, it is difficult to analyze fine particle emission status and control the concentrations thereof in real time.

Most environments emitting fine particles are highly humid. When particles are measured using existing real- time instruments, it is impossible to precisely measure fine particles because of the presence of condensable particles generated during the sampling process (Lipsky et al., 2005). When measuring car emissions, a hot dilution method is used to suppress the generation of condensable particles (hot air is injected). Ejector and rotating disk dilutors using hot-dilution methods are commercially available (Lyyränen et al., 2004; Burtscher, 2005; Li et al., 2011) (Fig. 1). However, particle loss, caused by air-mixing and cavity transport, occurs during dilution; it is difficult to precisely measure PM10 or PM2.5 levels (Lipsky et al., 2002).

Fig. 1. Commercial dilutors: (a) an ejector dilutor (Dekati® Dilutor) and (b) a rotating disk dilutor (MD19-2E; Matter Engineering, Kirchberg, Switzerland).

In this study, we developed a sampler with a porous tube dilutor (Deuerling et al., 2010) exhibiting low-level particle loss, and suppressed condensable particle generation. We compared our dilutor with a commercial ejector-type dilutor (DEED; Dekati Ltd., Kangasala, Finland) in terms of measurement of particles emitted by a car diesel engine and a coal-fired boiler. Condensable particle suppression and the extent of large particle loss were compared. Condensable particle reduction characteristics were compared using a test chamber featuring high-level humidity.

METHODS

Fig. 2 shows the porous tube dilutor. The first-dilution air (heated to 200°C) was introduced via a high-velocity ejector nozzle. The negative pressure created by the airflow allowed air containing fine particles to enter the sampling probe. The second-dilution air, at ambient temperature, was injected into the end of the ejector through a metallic, porous cylindrical tube filled with 20-μm-diameter beads. Particle loss at the inner wall during mixing at the end of the ejector was minimized by the porous structure.

Fig. 2. The porous tube dilutor used in this work.

First, to measure fine particles in diesel exhaust gas of high humidity and high temperature, a diesel engine was connected to a dynamo system. The displacement of the commercial car engine used (Santa Fé; Hyundai Motor Company, Seoul, Korea) was 1,991 mL, and the maximum output was 126 PS (92.7 kW) at 4,000 rpm. The engine oil employed was SAE 10W/30 (Hyundai Mobis, Seoul, Korea).

Most experiments were performed at 2,000 rpm, with a torque of 18 kg m–1 and an exhaust gas temperature of 530°C. Using the DEED and our new diluting sampler, exhaust gas was sampled before passage through the diesel particulate filter (DPF) and analyzed using the particle analyzer of the Scanning Mobility Particle Sizer (SMPS+C; Grimm Aerosol Technik Ainring GmbH & Co. KG, Ainring, Germany). The dilution ratios were fixed at 100 for the DEED and 10–20 for our new diluting sampler.

Fine particles were prepared using a 100,000 kcal h coal-fired boiler (GV-10; Global Village Grand Vision Co., Ltd, Siheung-Si, Korea). The coal, Vietnamese anthracite,  was supplied to the boiler at 20 kg h^-1. The temperature of hot water from the boiler was 70°C and that of the exhaust gas was 100–150°C during testing. The size distributions of fine particles from the coal boiler were measured (0.01– 10 μm) using the SMPS+C device and an Optical Particle Counter (OPC) (PAS 1.109; Grimm Aerosol Technik Ainring GmbH & Co. KG, Ainring, Germany). We compared the DEED and our new dilutor.

When removing condensable particles from highly humid environments during dilution, changes in the levels of particles created by a humidifier (AOS-2055; Air-O-Swiss, Widnau, Switzerland) and an air purifier (FU-550; Sharp, Osaka, Japan) in a 30-m3 test chamber (4.0 × 3.1 × 2.4 m) were measured using our dilutor. Test particles were prepared by atomizing KCl (1% w/v) solution using an atomizer (3076; TSI Inc., Shoreview, Minnesota, USA) combined with an aerosol neutralizer (3012; TSI Inc., Shoreview, Minnesota, USA) and diffusion drier; the humidifier was used to generate condensable particles. The air purifier was employed to control KCl particle concentration. Changes in particle levels were measured by the OPC; the 0.25- and 0.35-μm channel particles were averaged to yield the levels of 0.3-μm particles per minute.

RESULTS AND DISCUSSION

Measurements of Fine Particles in Diesel Engine and Coal-fired Boiler Exhaust

Fig. 3 shows the particle size distributions of diesel engine exhaust flowing through our dilutor at different first-dilution airflow rates. The temperature of the first- dilution air was 200°C and the flow rate of ambient-dilution (second-dilution) air was 37 L min^-1. At 20 L min^–1 of first-dilution air, peaks at 12 and 70 nm were evident. These were, respectively, the nuclei mode formed by saturation of condensable components (water or soluble organics), and the accumulation mode from particle accumulation. The particle numbers decreased slightly when the flow rates of first-dilution air increased from 20 to 22 and 24 L min^-1. However, the condensable nucleus decreased. When the first-dilution airflow rate exceeded 26 L min^-1, that nucleus disappeared but the particles associated with the 70-nm peak remained. The dilution air was of high temperature and low relative humidity. As the flow rate of the dilution air increased, condensable nucleation was suppressed; the particle numbers in the nucleus decreased. Thus, when sufficient hot air was supplied to the porous tube dilutor, particulate matter could be reliably measured in the absence of condensable particles.

Fig. 3. The size distributions of diesel particles at different first-dilution airflow rates at 200°C.

Fig. 4. The size distributions of diesel particles diluted by the ejector and porous tube dilutors.

Fig. 4 shows the particle size distributions in diesel engine exhaust as measured by both the DEED and our new dilutor. The temperature of the first-dilution air for the DEED was 100°C. The nuclei mode particles were prominent when the dilution air temperature was too low. The size distribution of particles was similar to that of our porous tube dilutor, but the level was only 0.1-fold that of our new dilutor. Therefore, our dilutor suppressed condensable particle generation via a lower dilution airflow rate.

Fig. 5. The size distributions of particles from a coal-fired boiler diluted using the ejector and porous tube dilutors.

Fig. 5 shows the particle size distributions in coal-fired boiler exhaust using the DEED and our new dilutor. The particle concentration was normalized to the total particle number to exclude the effect of different dilution ratios. The size distributions of accumulated particles were nearly- identical using the two kinds of dilutors. However, a condensable nucleus at about 0.01 μm was evident, and fine particles over 3 μm were absent, when the DEED was used. Thus, particles 3–10 μm in diameter were not measured by the DEED, but were measured by our dilutor. Thus, our dilutor affords a low dilution ratio, no condensable particles, and minimal loss of large particles during mixing at the ejector.

Measurement of Fine Particles in the Test Chamber

Fig. 6 shows changes in the levels of particles between 0.25 and 0.35 μm in diameter, as revealed by the OPC when KCl test particles were supplied to the test chamber by the atomizer when the humidifier was on. When the background concentration had attained a steady-state of 2–3 particles cm^–3 for 10 min, KCl particles were injected into the chamber over 10 min to 660 particles cm^-3, and the concentration was then held constant for the next 10 min. When the humidifier was switched on, the levels of 0.3-μm-diameter particles increased slowly for 12 min, and then rapidly. Finally, the
concentration attained 1,170 particles cm^-3. In other words, the OPC could not distinguish water particles from test particles.

Fig. 6. Changes in the levels of particles 0.25–0.35 μm in size after operation of the humidifier.

Fig. 7 shows particle measurements in the 30-m³ test chamber under various sampling conditions. The air purifier was operated in the low flow rate mode after injection of KCl particles. Sampling was performed with and without the dilutor, as dictated by the Korean air purifier test standard (SPS-KACA002-132). Then, dilutor sampling was performed once more during humidifier operation. The clean air delivery rate (CADR) of the air purifier can be calculated by multiplying 30 m³ (the test chamber volume) by the decay constant k in the following equation: C/C₀ = exp(–kt). For the “without dilutor” condition, the CADR was 2.43 m³ min^-1 . For the “with dilutor” and “with dilutor during humidifier-on” conditions, the values ranged 2.40–2.43 m³ min-¹. Thus, even if condensed water droplets are present, solid particles can be precisely measured by drying condensable particles in our new dilutor.

Fig. 7. Relative concentrations of 0.3-μm-diameter KCl particles over time after air cleaner operation under different dilution conditions.

CONCLUSIONS

We developed a porous-tube dilutor featuring two-stage (hot and cold) dilution that suppresses and removes condensable particles. Diesel engine exhaust particles exhibited a bi-modal size distribution: a nuclei mode for condensable particles and an accumulation mode for solid particles. By increasing the flow rate of the first-dilution hot air into the porous tube dilutor, the condensable nucleus was suppressed, and the accumulated solid particles were thus selectively measured. The accumulated solid particles in coal-fired boiler exhaust were also selectively measured. The loss rate of particles larger than 3 μm in diameter was lower than that of the ejector method (the DEED). To separate and measure solid particles in a 30-m3 test chamber when they were mixed with water particles generated by a humidifier, our new dilutor eliminated the latter particles via drying.

ACKNOWLEDGMENTS

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea (Grant No. 20161110100140).

Aerosol Air Qual. Res. 19 :461 -466 . https://doi.org/10.4209/aaqr.2018.08.0319