Article pubs.acs.org/EF
Synthesis of Primary-Particle-Size-Tuned Soot Particles by Controlled Pyrolysis of Hydrocarbon Fuels Sanghwan Cho, Seunghoon Lee, Wonnam Lee, and Sunho Park* Department of Mechanical Engineering, Dankook University, Yongin 16890, South Korea S Supporting Information *
ABSTRACT: We have developed a pyrolysis-based soot-generating system, which is able to control the primary soot particle size. The system is clean and portable and runs on diverse hydrocarbon fuels of interest. To evaluate the performance of the system, soot was generated from n-hexane and propylene with various conditions of the temperature, fuel mole fraction, and residence time in the heating zone. The results showed that the primary soot particle size was controllable within the range of 20−60 nm, and the soot yield as a function of the residence time followed a logistic curve of different shape depending upon the fuel mole fraction and heating temperature. The system that we developed can be used as a reliable soot-generating source for diverse laboratories to meet the growing demands for fundamental research on soot characteristics and soot formation mechanisms as well as the assessment of health and environmental effects of soot from various sources.
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INTRODUCTION With increasing awareness of the risk of exposure to soot and the impact of soot particles on climate change, a significant amount of research has been conducted to find the health and environmental effects of soot particles originated from combustion-based sources.1−13 The soot content is mostly unburned hydrocarbon generated when fuel is pyrolyzed under oxygen-deficient burning conditions.14 Soot particles are one of the major components of particulate matter (PM) in the atmosphere, which are categorized into PM 10, PM 2.5, or PM 0.1, mainly by their aerodynamic size in micrometer scale.2 It has been known that fine dust (PM 2.5) and ultrafine dust (PM 0.1) in busy cities consist mostly of soot particles and tire dust from transportation-related sources.15 The household indoor PM concentration also is greatly affected by cooking and heating devices running without enough ventilation.16 Typical soot particles are random agglomerates of small individual spherules that are called primary soot particles. Their sizes are about tens of nanometers in general. The size of soot agglomerates, which is the size of PM or the secondary soot particle size, can be as large as some micrometers.17−19 Chemical contents and physical characteristics of soot particles, such as the primary particle size and structure of the aggregates, highly depend upon the types of fuel and combustor from which the soot particles originate. For example, the secondary size of the soot particles from automotive diesel engines range from nanometers to sub-micrometers,20−23 while that of soot from large ship engines is up to a few micrometers and has multi-peaks in its size distribution.11 Soot from ship engines contains relatively higher sulfate contents, owing to a high sulfur concentration in ordinary ship fuels. Similarly, mineral contents are found in soot from coal-fired power plants.24,25 Recent research results have shown that the toxicity of inhaled soot is related not only to the chemical substances of soot but also to the primary and secondary sizes of the soot particles.8−10 In comparison to larger dust, ultrafine soot particles (∼PM 0.1) more easily reach pulmonary endothelial © 2016 American Chemical Society
cells without blockage. Once absorbed by a human body, the substances of soot are translocated to other organs, such as the brain, liver, and heart, and eventually, they raise the risk of some fatal diseases, such as heart diseases, cancers, and strokes.8,10,12 Given the greater toxicity of smaller soot particles, recently enforced Euro VI not only limits the mass of exhausted PM but also the number of exhausted particles.6 The environmental effects of soot particles are also becoming of great concern. Soot particles are light-absorbing aerosols in the atmosphere, and therefore, they are believed to affect global warming. It has been known that some of the atmospheric soot particles and PM migrate to arctic areas and make deposition on snow and glaciers, which results in reduction of albedo and long-term climate change.1,5,7 Light absorption and scattering by soot particles are strong functions of primary and secondary particle size, wavelength of incident light, and soot refractive index.14,26,27 Refractive index of soot itself is also a function of wavelength as well as the substances of soot.28 Most commercially available soot concentration measuring devices actually measure the intensity of absorbed or scattered light by soot particles.29−31 Therefore, it is important to know the exact size and materials of soot particles to determine the optical properties of the particles. Regulations on soot emission must be established through systematic and rigorous assessments of health and environmental effects of soot particles. It is necessary to conduct not only epidemiological surveys and field measurement of air quality but also intensive lab experiments of controlled animal exposure and investigation on determining optical behavior of soot particles of various sizes and contents from diverse sources. However, most real soot sources, including automotive engines and industrial combustors, are not appropriate for indoor lab experiments, in that they do not satisfy sanitization Received: June 2, 2016 Revised: July 24, 2016 Published: July 26, 2016 6614
DOI: 10.1021/acs.energyfuels.6b01346 Energy Fuels 2016, 30, 6614−6619
Article
Energy & Fuels
rates of all gas flows are controlled by mass flow controllers. The mixture gas of fuel and nitrogen exiting from the fuel-evaporating/ mixing module is then delivered to the soot generator and heated to 1400 °C. Fuel is pyrolyzed under the oxygen-free condition and goes through the soot inception and growth process until it exits the soot generator. The exhausted soot and nitrogen mixture is further diluted with nitrogen to adjust the soot concentration to be suitable for the smoke-meter (AVL, 415SE) for soot yield measurement. Emitted soot particles were also collected on sample grids (Ted Pella, 01883-F), and transmission electron microscopy (TEM, JEOL, JEM-2100F) images were taken to analyze the structure of the soot particles. Figure 2a shows the soot generator in detail. The mixture gas flows through the 1 m long high-purity (>99.5%, Kilntech) alumina tube
and safety guidelines of typical laboratories, and their physical dimensions easily exceed lab spaces allowed for most cases. Moreover, they only run on a single type of fuel, and the size of emitted soot particles is unregulated. For example, size distribution of soot particles from a diesel engine alters significantly with load and rotational speed.22,23 Carbon black has been used as a replacement for soot as a result of the difficulty of on-site generation of soot particles; however, chemical substances and morphology of real soot particles are very diverse and much different from carbon black as indicated in the literature.32 An alternative soot source, such as a burner, may be adopted to steadily generate soot particles as shown in a recent animal exposure test.9 In this test, soot particles were emitted from an ethylene flame formed by a coaxial burner and were delivered to the chamber where rats were bred. This kind of experiment, however, requires a separate lab to house burner-related facilities because the existence of a flame is prohibited in most bio-related laboratories. To address the issues relating to soot generation and utilization in laboratories, we have developed a flame-free portable soot generator, which runs on diverse fuels to generate primary-particle-size-controlled soot.33,34 The soot generator is designed to pyrolyze fuel at a very high temperature up to 1400 °C by electrical power and is clean and small enough to fit into a typical ventilation hood. Soot particles generated from the device have a very regulated and consistent primary particle size, which is tunable by controlling fuel and nitrogen flow rates in the pyrolyzing furnace at different temperatures. The aim of developing the device has been to readily support diverse animal tests, elucidating the dependence of soot toxicity on particle size and chemical contents, and to perform fundamental research on soot formation mechanisms and soot optical properties, by supplying fuel-and-size-controlled soot particles on site.
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Figure 2. (a) Schematic of the soot generator. Nitrogen and fuel mixture gas passes through the alumina tube heated by SiC heaters. (b) Photos of the soot-generating system (left) and the soot emission from the outlet of the soot generator (right). installed in the soot generator. The alumina tube resists very high temperatures up to 1600 °C. The inner and outer diameters of the tube are 1 and 1.5 cm, respectively. The temperature of the alumina tube is controlled by a silicon carbide (SiC) heater, R-type thermocouple, and temperature controller. The entire heater and tube structure is covered with ceramic insulation and housing for safety. A quartz window is installed at the inlet of the alumina tube for observation. The photo of the entire instrumentation of the sootgenerating system is shown in the left-hand side of Figure 2b. The soot generator is approximately 1.2 m long, such that it easily fits into a typical fume hood. The components of the fuel-evaporating/mixing module, including controllers, are all packed into a customized housing shown in the photo. The photo on the right-hand side of Figure 2b shows soot-containing exhaust gas at the outlet of the alumina tube. Experimental conditions for soot generation are given in Table 1. Liquid n-hexane (C6H14, Sigma-Aldrich, 0.659 g/mL at 25 °C, with MW = 86.18) and gaseous propylene (C3H6, a local supplier) were chosen as fuel. Nitrogen flow rates were 1, 2, or 4 standard liters per minute (slpm). Flow rates for liquid fuel in cubic centimeters per minute (ccm) and gaseous fuel in standard cubic centimeters per minute (sccm) are given in Table 1. The fuel mole fraction (Xf) in fuel and nitrogen mixture gas for each case was calculated and listed. Xf is almost proportional to fuel flow rate and to the inverse of the nitrogen flow rate because the flow rate of nitrogen is much greater than that of fuel.
EXPERIMENTAL SECTION
Figure 1 shows the schematic of the soot-generating system developed for this study. It mainly consists of the fuel-evaporating/mixing
Figure 1. Schematic of the soot-generating system. module and the soot generator. Liquid fuel is supplied to the heated evaporator by a microsyringe pump (Harvard Apparatus, PHD 2000), which precisely controls the volumetric flow rate. Liquid fuel is then evaporated into heated nitrogen, which is called carrier gas, sent from heater 2. The working temperature of the evaporator is carefully chosen to keep the fuel partial pressure in the mixture of nitrogen and fuel well below the saturation vapor pressure of the fuel at that temperature. When gaseous fuel is used, the fuel heated with heater 2 and carrier nitrogen heated with heater 1 are simply mixed together outside the evaporator. All heaters in the fuel-evaporating/mixing module run on domestic alternating current (AC) 220 V electrical power and are equipped with K-type thermocouples and proportional−integral−derivative (PID) temperature controllers. The flow
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RESULTS AND DISCUSSION The temperature of the gas flowing inside the tube changes with the setting temperature of the SiC heater, position inside the tube, and flow rate of the gas. The temperature distribution in the tube was numerically simulated, and the results were 6615
DOI: 10.1021/acs.energyfuels.6b01346 Energy Fuels 2016, 30, 6614−6619
Article
Energy & Fuels Table 1. Experimental Conditions for Soot Generation by Pyrolysis of Fuel fuel flow rate
temperature (°C) 1200, 1300, and 1400
carrier N2 flow rate (slpm) 1 2 4 1 2 4 4 2 4
n-hexane (ccm)
propylene (sccm)
fuel mole fraction, Xf (%)
29.5 29.5
1.5 1.5 1.5 0.75 0.75 0.75 0.38 1.5 0.75
0.087 0.174 0.348 0.087 0.174 0.348 0.087
compared to the measured values. Details about the simulation and measurement results have been described in the Supporting Information. The nitrogen flow rate in this work was limited up to 4 slpm because a considerable temperature deviation from the heater setting temperature was observed at higher flow rates. The flow rate of the gas mixture determines the average time that the gas stays inside the alumina tube. Because only a 0.6 m portion of the middle of the alumina is covered with the SiC heater and the remaining part is thermally insulated, the residence time of gas in the furnace (tres) has been defined in this paper as 0.6 m over the average velocity of the gas flow inside the tube. For example, tres is as low as 0.12 s for 4 slpm nitrogen flow at 1400 °C, while tres is 0.52 s for 1 slpm and 1200 °C conditions. Refer to the Supporting Information for further details. It is important to control tres for the experiments because it determines the degree of completion of the pyrolysis process.35,36 The degree of soot formation was first checked by collecting soot on slide glasses at the outlet of the soot generator for 5 s. Figure 3a shows the photos of soot samples generated with different heater setting temperatures and fuel mole fraction conditions for the fixed nitrogen flow rate of 4 slpm. When the setting temperature or fuel mole fraction (Xf) was low, a lower amount of soot was formed and its color was browner. This means that the collected samples contain relatively more organic substances than just black carbon, in that the organic fraction of soot absorbs more light of short wavelengths than the carbonized fraction of soot, in general. This observation is in accordance with the result from the previous research on pyrolysis of diesel fuel with a different soot-generating system, which showed the measured elemental C/H ratios to be as low as ∼2 for brown soot.35 To measure the soot yield, which is defined as the mass of generated soot over the mass of fuel input, exhaust gas from the soot generator was delivered to the smoke-meter by suction and filtered by the genuine paper filter, which is claimed to capture more than 95% of PM 0.1 by the smoke-meter manufacturer. The smoke-meter measures the degree of reduction in light reflection from the filter, owing to deposited soot, and relates the values to the total mass of the collected soot on the filter. Soot yields were measured for fuel mole fractions of 0.38, 0.75, and 1.5% with the highest nitrogen flow rate, 4 slpm, and were plotted in Figure 3b. At this low tres, soot was rarely formed at 1200 °C, regardless of Xf values. More percentage of fuel was converted to soot at 1300 and 1400 °C. It has been known that carbonization of soot precursors is
Figure 3. (a) Photos of soot samples generated with n-hexane and collected on slide glasses. Various heater setting temperatures (1200− 1400 °C) and fuel mole fractions (Xf = 0.5−1.5%) at the nitrogen flow rate of 4 slpm. (b) Soot yield measured with a smoke-meter for Xf = 0.38, 0.75 and 1.5%, with the heater setting temperatures of 1200, 1300, and 1400 °C, for the nitrogen flow rate of 4 slpm.
generally accelerated when the temperature increases.37 In general, a greater soot yield was observed when Xf was higher. Concentrations of soot particle precursors and nucleated particles are apparently high when fuel species are abundant and so are the rates of soot formation kinetics.38 To examine the effect of varying tres on soot yield, each soot yield value was normalized with the maximum soot yield observed at each given Xf, which was 32% at Xf = 0.75% and 41% at Xf = 1.5%, having happened at 1300 °C and the highest tres conditions. The normalized soot yield (Y*) was plotted as a function of tres for each heater setting temperature (=1200, 1300, or 1400 °C) and Xf (=0.75 or 1.5%) in Figure 4a. Each plot in the graphs seems to be a logistic curve given below A1 − A 2 Y * = A2 + p t 1 + t res
( ) 0.5
where A1 = 0 and A2 = 1 for this experiment. This equation allows Y* to be 0 at tres = 0 and to be 1 when tres goes to infinity. The fitted curves in Figure 4a are the results from the data regression, where the residence time required for Y* to reach 0.5, named t0.5, and the exponent p are given in Figure 4b. The value of p determines how much change in Y* is concentrated around tres = t0.5. Figure 4a shows that most of the soot formation process was concentrated around the beginning of the pyrolysis reaction when the setting temperature was high (1400 °C). At this temperature, a rapid increase in the soot yield was even more concentrated at low tres when Xf was higher (1.5%). When the heater setting temperature was 1300 °C, the soot formation process was slow at the beginning of pyrolysis but became faster around t0.5 and slowed again for longer tres. The trend of t0.5 in Figure 4b shows that the time scale required for soot formation by pyrolysis was longer when Xf and the setting temperature were lower. The value of p was the largest at 1300 °C when Xf = 0.75%. 6616
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of soot formation.19,24,38 The primary particles then form much larger agglomerates called secondary soot particles, which have fractal-like structures by molecular bindings.17 When tres is low, the mixture gas may exit the soot generator before the process is complete, so that the emitted particles have more unpyrolyzed hydrocarbon contents and are browner, as seen from some of the conditions in Figure 3a, which were observed for the highest nitrogen flow rate (=4 slpm) that gives the lowest tres values. The soot yield was high when the temperature was high as a result of fast carbonization at the early stage of the soot formation reaction, as explained earlier. When tres is sufficiently high, the fuel and nitrogen mixture gas goes through the entire soot formation process in the soot generator. In this case, the maximum soot yield was achieved at 1300 °C from our soot generator. It has been known from shock-tube experiments that there exists a temperature for the maximum yield of soot depending upon fuel type and operating pressure,41 although the effect of the temperature on the soot yield was exhibited differently for other experiments.42 With the findings suggesting the influence of the heater setting temperature and nitrogen flow rate on the soot formation process in the soot generator, soot particles were generated with various conditions and TEM images of those particles were taken and listed in Figure 6. For the 1 slpm
Figure 4. (a) Normalized soot yield (Y*) as a function of the residence time (tres) for fuel mole fractions (Xf) of 0.75% (top) and 1.5% (bottom). Heater setting temperatures were 1200 °C (■), 1300 °C (●), or 1400 °C (▲). (b) Parameters found by fitting the data in panel a to logistic curves. The residence time required to make Y* be 0.5 (t0.5, left) and the exponent (p, right). Xf = 0.75% (open columns) and 1.5% (solid columns). Error bars denote the standard errors from the data fittings.
Logistic curves are popularly used for autocatalytic reactions, whose products are also reactants. For example, aggregation of proteins to larger molecules usually follows a logistic curve.39 The yield of soot from the pyrolysis reaction may exhibit similar behavior, in that the reactions involve several binding reactions between soot precursors, polyaromatic hydrocarbons (PAHs), and incepted particles.38 Application of a logistic curve for a regression of the soot volume fraction in product gas was conducted in previous research on a shock-tube experiment, although its time scale of the overall reaction was only in the order of milliseconds.40 The soot formation process in the soot generator is illustrated in Figure 5. In typical soot formation processes, Figure 6. TEM images of soot particles generated with n-hexane at Xf = 1.5% condition. Temperature (1200−1400 °C) and nitrogen flow rate (1 or 2 slpm) conditions are tabulated at the upper right corner. Scale bars in the images denote 100 nm.
nitrogen flow rate condition giving the highest tres values, the primary soot particles in panels a, b, and d of Figure 6 became smaller as the setting temperature became higher for the given Xf = 1.5%. The primary particles were also found to be smaller when the nitrogen flow rate was higher or tres was smaller, as the comparison of panels b and c of Figure 6 shows. It is hypothesized that nucleation of soot particles is favored by higher temperatures; therefore, the growth of individual primary particles is limited by their large population in the soot generator. It is also believed that each individual primary particle can grow larger when it stays longer in the soot generator as a result of a lower nitrogen flow rate. The primary particle size (Dp) of each soot sample was measured with ImageJ software,43 and its average and standard deviation were plotted in Figure 7. More than 200 individual primary particles
Figure 5. Illustration of the soot formation process in the soot generator. Emitted soot is at a different stage of the process depending upon the residence time (tres).
pyrolyzed fuel substances, mainly C2H2- and/or C3H3-related radicals, form PAHs, which bind together to nucleate very small soot particles.38 The size of the incepted particles keeps growing by continued addition of PAHs onto the surface. Those individual spherules are called primary soot particles, and their typical sizes are from a few nanometers to several tens of nanometers depending upon the thermophysical conditions 6617
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CONCLUSION We have developed a clean and portable soot-generating system that can be accommodated into typical laboratories. The system can control the primary particle size and the substance of soot by pyrolysis of diverse hydrocarbon fuels at various conditions. Fuel and carrier nitrogen flow rates determine fuel mole fraction (Xf) and residence time (tres) of the mixture gas in the pyrolyzing furnace. Soot was generated with n-hexane, and the measurement results showed that the normalized soot yield (Y*) as a function of tres followed a logistic curve. When tres was low, more soot was yielded at a higher temperature as a result of fast carbonization. When sufficient tres was allowed, the soot yield trend at a given Xf became a more typical logistic curve at a certain temperature (∼1300 °C) and the maximum soot yield was achieved at that temperature. The primary particle size of soot generated from n-hexane and propylene was controllable in the range of 20−60 nm, depending upon the temperature, Xf, and tres. The developed soot-generating system can be used for not only fundamental research on soot characteristics and formation mechanisms but also assessment of health and environmental effects of soot with particular conditions.
Figure 7. Primary particle size (Dp) of soot samples generated with nhexane of Xf = 1.5% with different heater setting temperatures (1200− 1400 °C) for a nitrogen flow rate of 1 slpm (■) or 2 slpm (□). More than 200 individual primary particles from multiple TEM images were measured with ImageJ43 for each case. Each error bar denotes the standard deviation of the measured size values.
were randomly chosen from multiple images for each case. The graph shows that Dp varies in the range of 25−60 nm, which is quite similar to the size range of primary soot particles exhausted from ordinary auto engines.18,20,22−24 It has therefore been confirmed that the soot generator that we developed can control the primary particle size of its soot emission by choosing its operation conditions accordingly. Another advantage of the soot-generating system is the flexibility for the choice of fuel. Figure 8 shows TEM images
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01346. Heat-transfer analysis on the gas temperature distribution in the soot generator (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): The concept of soot generation described in this work is covered by Korean Patent 1015938060000, which is the property of Dankook University.
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ACKNOWLEDGMENTS The authors thank the financial support from the Ministry of Oceans and Fisheries of Korea through the project “Quantitative Assessment for PM & BC to Climate Change and Development of Reduction Technology for PM, BC from Ships”. The authors also thank the National Research Foundation of Korea (NRF-2015R1C1A1A01052961) for funding this research.
Figure 8. TEM images of soot particles generated with propylene for (a) fuel mole fraction (Xf) of 1.5%, nitrogen flow rate of 2 slpm, and heater setting temperature of 1200 °C conditions and (b) fuel mole fraction (Xf) of 0.75%, nitrogen flow rate of 4 slpm, and heater setting temperature of 1400 °C conditions. (c) Primary particle size (Dp) of soot samples for different heater setting temperatures (1200−1400 °C) at Xf = 1.5% and nitrogen flow rate of 2 slpm (■) or Xf = 0.75% and nitrogen flow rate of 4 slpm (□). More than 200 individual primary particles from multiple TEM images were measured with ImageJ43 for each case. Each error bar denotes the standard deviation of the measured size values.
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