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Characterizing early stage sub-micron particle formation during pulverized coal combustion in a flat flame burner
Fuel 258 (2019) 115995
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Full Length Article
Characterizing early stage sub-micron particle formation during pulverized coal combustion in a flat flame burner
T
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Dishant Khatria, Akshay Gopanb, Zhiwei Yanga, , Adewale Adeosunc, Richard L. Axelbauma a
Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA Presently at Cabot Corporation, Billerica, MA 01821, USA c Presently at Intel Corporation, Ronler Acres Campus, 2501 NW 229th Ave, Hillsboro, OR, USA b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Sub-micron ash Soot Hencken burner Coal
Mineral matter evolution, soot formation, and soot oxidation are the governing processes for fine particle evolution during the early stages of pulverized coal combustion. Soot and ash can be simultaneously present in PM 0.1, and experimental results obtained to understand early stage particle formation can be ambiguous if the two are not independently evaluated. A sampling system proposed in this work differentiates between soot and mineral matter by employing a high temperature furnace downstream of the sampling system, supplied with sufficient oxygen to oxidize the soot in the sampled aerosol stream. The stream can be routed either through the furnace or bypassing it. In the bypass configuration, the SMPS measures the total PM size distribution. In the flow-through configuration, the SMPS measures the ash PM size distribution, and the difference identifies the contributions from the soot aerosols. This study has shown that there is a high probability of confounding effects in experimental measurements of early stage ultra-fine particulate matter evolution, and the effects were shown to exist at different conditions and with different coals. The results show that for PRB coal combustion, as residence time increases from 9 ms to 33 ms, the volume of total PM 0.1 decreases by 68%, and the ash PM 0.1 increases by 600%, whereas for Hongshayuan lignite coal, the volume of total PM 0.1 increases by 275%, and ash PM 0.1 increases by 939%. These results indicated that soot constitutes a major part of PM 0.1 during the early stages of fine particle evolution.
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Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.fuel.2019.115995 Received 31 May 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 12 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
elemental particle size distribution [26]. More recently, models have been developed—primarily based on the discrete-sectional population balance model—to explain the early stage PM 0.1 evolution behavior observed in experiments [27]. Though sophisticated in their analyses, it is possible that many of the previous works on early stage ash evolution suffer from ambiguities introduced due to carbonaceous material. During the early stages, the secondary reactions of tar, the oxidation of soot, and the vaporizationcondensation processes of mineral matter occur simultaneously, and particle evolution is a cumulative effect of all these mechanisms [16,17]. Neglecting any one of these mechanisms, without supporting evidence, could lead to inaccurate conclusions. For example, if applying ash formation models to understand coagulation and growth of PM 0.1, the contribution of carbonaceous matter should be clearly understood. In this work, we highlight the potential uncertainty introduced when dealing with either early stage ash or soot formation during coal combustion, if either of the two are neglected in the analysis. There are possibly conditions where one of the two can be neglected, but this should be demonstrated or justified. A new sampling and analysis system is developed in this work to differentiate between soot and mineral matter. PSD of mineral ash only and total PM 0.1 are measured. This analysis is conducted for two coals to show the applicability and importance of such analysis across different coals.
The growing demand for energy in developing countries is expected to increase fossil fuel consumption as well as the use of alternative energy resources [1]. Today, coal produces more than 27% of all energy worldwide [2], and will continue to provide reliable energy to billions of people around the globe. In the past decade, due to its low cost, abundance and broad distribution, coal contributed more to the global energy supply than any other energy source. At the same time, the benefits of coal combustion bring environmental challenges, including the emissions of particulate matter (PM) [3]. Submicron particles, especially PM 0.1, are particularly difficult to collect using conventional methods of particle removal [4], and once PM 0.1 is emitted into the environment, the particles can stay airborne for long periods of time. When inhaled into the lungs, they can enter into the interstitial spaces and diffuse into the blood stream [5]. These particles can cause tissue irritation and release the toxic chemical intermediates [5]. Inside the boilers, Zhan et al. [6] from his experimental results, have found that these fine particles form an initial sticky layer of ash deposits on boiler tubes. This results in an enhancement in the sticking probability for the larger particles, and the resulting ash deposition causes a reduction in heat transfer rates and a derating of the boiler [7]. Thus, understanding the formation and evolution of submicron particles during coal combustion is essential from both environmental and operational perspectives. In the past few decades, the formation and evolution of PM 0.1 has been studied extensively at different reactor scales–from small lab-scale reactors [8] up to MW-scale pilot plants [7,9]. The formation of PM 0.1 has been shown to be affected by coal type and rank, combustion environment, and the initial size of the coal particles [10–14]. PM 0.1 consists of both mineral matter and carbonaceous materials. Mineral matter is formed by the mechanism of volatilization-nucleation-coagulation, in which mineral matter vaporizes at high temperature and diffuses out of the coal particles, and undergoes either homogeneous nucleation or heterogeneous condensation, followed by coagulation and agglomeration. Low boiling point metals vaporize directly, whereas refractory metals are reduced to their sub-oxides before they vaporize [15]. On the other hand, carbonaceous materials are formed by the secondary gas-phase reactions of coal volatiles, in particular tar, released during particle heating [16]. Tar consists of polycyclic aromatic hydrocarbons (PAH), which undergo cracking and polymerization at high temperature, to form soot. Most studies have sampled the submicron particles at the exit of a furnace, with coal particle residence times on the order of a few seconds [16–18]. As important as these results are, such long residence times do not offer insights into the early stage processes which dominate in the first 100 ms or less. These processes, including particle heating, devolatilization, ignition, mineral matter vaporization, and formation of light gases and tar [19–21] are known to dictate char burnout, combustion efficiency, heat transfer, and PM0.1 formation [22,23] and understanding these processes can lead to efficient designs of burners, improved heat transfer characteristics, and in-flame control of particulate formation. Therefore, a controlled experimental system with the capability of sampling at short residence times, while mimicking the time-temperature history experienced by coal particles in practical furnaces, is required to understand early-stage PM 0.1 formation. Hencken burners are one of the primary systems that have such a capability. Several PM 0.1 studies have been conducted using Hencken burners to improve our understanding of the formation and evolution of ash particles during early stages. Gao et al. [24] studied the combustion of Zhundong lignite coal, and found that the competition between devolatilization of mineral matter and coalescence determines the evolution of ultrafine particles. He further showed that H2O enhances the formation of nascent Si-based PM 0.1, and CO2 diminishes PM 0.1 formation [25]. Xiao et al. found that homogeneous condensation and surface reaction are the controlling mechanisms responsible for the
2. Experimental 2.1. Hencken flat flame burner The Hencken burner used in this work is shown in Fig. 1. Here the burner is described only briefly, while a more complete description can be found in Ref. [20]. The burner is 80 mm in diameter and composed of 570 hypodermic needles, each having a 1.2 mm ID, creating an equal number of micro-diffusion flamelets. Fuel and oxidizing gases are introduced through separate chambers, and are isolated via a gasket material. The burner is unique in that it has two zones – an inner and an outer zone (see Fig. 1). Each zone can be run under globally reducing or oxidizing conditions, enabling the study of early stage coal combustion processes under a variety of conditions. A central tube with a 2 mm ID passes through the burner for coal feeding. An 80 mm diameter quartz tube isolates the combustion process from the ambient environment. A fine stainless mesh on the top of the quartz tubing minimizes external disturbances. The burner provides a stable combustion environment for coal combustion, with a heating rate of ~105 K/s. In experiments, coal particle stream was fed by entraining them in nitrogen using a micro fluidized feeder with a typical entrained gas flow rate of 60 cubic centimeters per min. A high frequency vibration source ensures a stable coal feeding rate [20]. The coal stream mass flow rate was kept at 150 mg/min during this study. Methane (CH4) diluted with nitrogen (N2) was used as the fuel, and a mixture of oxygen (O2) and nitrogen (N2) as the oxidizer. A combustion temperature of 1500 K was used in these experiments, and the post flame O2 concentration was set at 20% by adjusting flowrates of both the fuel and oxidizer streams (Fig. 2). Both the inner and outer zones were run in oxidizing conditions for this study. The temperature of the hot gas environment was measured using a B type thermocouple, corrected for radiation loss. Gas composition was measured using a Horiba gas analyzer model PG-250. The residence time of the individual coal particles was measured using high speed video of the coal particles during combustion. Video images were obtained using a NAC MEMRECAM HX- 7 high-speed camera. The camera was fitted with a SIGMA APOMACRO 180 mm lens which was used to obtain image magnification of 1:4. The camera was set to 8000 frames per second (fps), and an exposure time of 50 μs was used to resolve the motion of the single particles. The particle residence times were determined by tracking the motion of more than 40 particles and then averaging the results. The residence times of coal particles, oxygen volume percentage, and gas 2
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Fig. 1. Cross-section of the Hencken Burner (left), and photograph of coal particles burning in micro diffusion flames (right).
N2 flowing in the annulus, ensuring that the dilution N2 is heated before being introduced into the sampling probe (Fig. 3), and avoiding condensation in the probe. The temperature at the exit of the sampling probe (Thermocouple 1) is 600 K ± 50 K at different sampling heights, ensuring that minimal condensation and nucleation are expected in the probe. The particles are sampled at 1, 3, and 5 cm axial distances above the burner on the centerline. The aerosol stream from the sampling probe is diluted again in the secondary dilution step with room temperature nitrogen gas, further suppressing particle coagulation [27]. A small portion of this aerosol stream is further diluted—the rest is exhausted—to get the final dilution ratio of ~100, which yielded a particle number density that was within the measurable range of condensation particle counter (CPC). During the experiments, the non –diluted number particle size distribution (PSD) back in the flue gas did not change when the dilution ratio was further increased, indicating that the three-stage dilution process was freezing the various growth processes. After the tertiary dilution, a portion of the diluted aerosol stream flows through a cascade impactor with the final stage cut-off size of 0.7 micron. The downstream gas stream is then fed through either a bypass tube (Case I in Fig. 3), or through an electrically-heated alumina tube furnace (Case II in Fig. 3) before reaching the particle size analyzer. The alumina tube furnace is used to burn out carbonaceous particles in the stream such that only the mineral particulate matter remains, and the contribution of carbonaceous matter to the total PSD can be ascertained. The alumina tube furnace, with a 9.5 mm ID, has a constant temperature of 1373 K for a length of 25 cm, as shown in the inset of Fig. 3. To burn off carbonaceous material, O2 is uniformly mixed into the aerosol stream before entering the furnace. Nagle and StricklandConstable (NSC) soot oxidation model [28] was used to calculate time scales of 90% oxidizing of a 200 nm carbon particle at 1373 K with 50 % surrounding oxygen mole fraction. The calculated time scale is 0.35 sec. In the furnace, the residence time of the coal particles is 1.4 sec, suggesting that the residence time is high enough for all soot to oxidize. In addition, oxygen percentage in the oxidizing furnace was varied to ensure that no noticeable difference in PSD is seen when increasing oxygen percentage. Finally, an oxygen mole fraction of 50%, which was high enough to burn off all of the soot at the given oxidizing furnace temperature and residence time was used to maintain consistency in the
Fig. 2. The ambient temperature (▲), oxygen volume percentage ( ), and residence time of coal particles along the centerline of the burner at 1500 K.
temperature along the burner are shown in Fig. 2.
2.2. Sampling system The particle sampling system is shown in Fig. 3. Particle sampling is achieved with a three-stage dilution system. The Hencken burner is attached to a motor-driven slide that can control its axial position relative to the dilution sampling probe [27]. In the sampling probe, dilution N2 is supplied through a narrow annulus of the probe, and the resulting high velocity of the N2 provides a negative pressure to draw in the atmospheric pressure flue gas, and dilute the particle-laden flow. The probe samples the entire particle stream such that there is no aspiration efficiency bias during sampling (i.e., all of the particles in the Hencken environment are sampled into the sampling probe). A dilution ratio of 3–5 is maintained for the primary dilution stage, and is calculated by measuring the oxygen concentration with and without dilution using the gas analyzer. The hot flue gas heats the probe and the dilution 3
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Fig. 3. Schematic of particulate sampling system.
measurements. Before taking a set of measurements, we verified that the oxidizing furnace itself did not generate appreciable particles by running the system without coal. Finally, the number concentration and PSD of the total particles in the stream (Case I) and non-carbonaceous particles in the stream (Case II) were measured using a TSI scanning mobility particle sizer (SMPS) with a nano-differential mobility analyzer (DMA) 3085 and CPC model 3776. The sheath flow was adjusted so that the measured particle size range was between 3.46 nm and 117.6 nm, in order to capture both nucleation as well as coagulation/condensation information for the PM 0.1. Before the neutralizer, another impactor with a nozzle size of 0.0508 cm and a D50 of 735.2 nm was used to protect the SMPS.
Table 1 Properties of PRB sub bituminous coal, and Hongshayuan lignite coal. Proximate analysis (wt. %, dry basis)
Sub-bituminous Powder River Basin (PRB) coal
Hongshayuan Lignite Coal
Fixed carbon Volatile matter Ash HHV(MJ/kg)
47.63 43.33 9.04 27.45
62.28 29.8 7.92 28.53
Ultimate analysis (wt.%, dry, and ash free basis) C 74.66 H 5.45 N 1.08 O 18.24 S 0.57
2.3. Coal properties
Ash compositions (wt.%) SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
A sub-bituminous Powder River Basin (PRB) coal, and a lignite Hongshayuan coal were used in these experiments, and Table 1 lists selected coal properties. For the combustion experiments, the coal particles were dried at 378 K to ensure stable feeding and then sieved to a size range of 45–63 μm. 3. Results and discussion The PM 0.1 collected is generally composed of both carbonaceous and mineral matter. During the early stages of coal combustion, PM 0.1 particles are present in a variety of morphologies, including ash monomers, soot monomers, ash-ash agglomerates, soot-soot agglomerates and ash-soot agglomerates [26,29]. The formation of these morphologies depend on the distribution of ash and soot present in the combustion environment and also the characteristic time of various aerosol growth processes. Once the mineral matter vapors are released from the coal, they can undergo nucleation (ash monomers), coagulation (ash-ash agglomerates), coagulation with soot (ash-soot agglomerates), or heterogeneous condensation on ash or soot particles, depending on the local concentration of these species and temperature. For example, soot particles in the vicinity of a coal particle could act as seed particles for deposition of vaporized minerals, and reduce the amount of nucleated mineral matter particles by reducing the
37.44 16.50 5.15 16.20 3.05 1.06 16.90 0.40 0.65 1.21
81.91 4.38 0.85 12.31 0.56 31.05 13.32 19.14 12.72 5.02 0.65 11.65 0.63 4.44
concentration of mineral vapor. Furthermore, the interactions between soot and ash particles can affect their formation characteristics. For example, if there is a significant number density of soot particles in an ash-soot agglomerate, the ash particles in the agglomerate may not be contiguous, and thus, upon soot oxidation latter in the combustion process, the ash particles would be released independently and not yield the size of the ash agglomerate that would exist in the absence of soot. Fig. 4 shows the particle number density (PSD) of PM 0.1 generated during the early stages of coal combustion for PRB coal (both Case I and Case II) at 1500 K ambient gas temperature. The particles were sampled along the centerline at three different locations (1, 3, and 5 cm) along the vertical axial which corresponds to residence times of 9, 21, and 4
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The total PM 0.1 PSD, as can be seen in Fig. 4a, decreases in diameter with increase in the residence time of coal particles in the Hencken burner. The mode of the distribution shifts from 31 nm at 9 ms to 25 nm at 22 ms, and then to 20 nm at 33 ms. The decrease in diameter indicates the loss of sub-micron particles, signifying the effect of soot oxidation on total PM 0.1 as particles moves from 1 cm to 5 cm (i.e., 9 ms to 33 ms). At 9 ms residence time, which is just after particle ignition (ignition time found by high speed camera imaging [20] is 7 ms); the high ambient temperature and low oxygen concentration provide an excellent environment for soot to form and grow (c.f. Fig. 2). However, at longer residence times, the increase in the ambient oxygen leads to enhanced soot oxidation. It is noteworthy that for smaller sizes (PM 0.02), the particle numder density increases with increasing residence time, but for larger sizes in PM 0.1, the particle number density decreases with increasing residence time. This observation implies that the soot and soot-ash agglomerates which represents large sizes, oxidizes with increasing residence time, thereby forming small ash particles. In addition, more nucleation of ash particles is possible at higher residence times. In comparison with the PSD of the total PM 0.1, the mode of the PSD of ash particles (Fig. 4b) increase in diameter with increasing residence time, which is attributed to the conventional growth mechanisms of coagulation and condensation. The mean peak value of lognormal ash PSD increases from 11 nm at 9 ms to 14 nm at 22 ms, and then to 18 nm at 33 ms. Fig. 5 shows the total volume of soot and ash as a function of residence time per mg of coal burnt. The ash volume was obtained by multiplying each point of the ash number PSD function by the corresponding particle volume, and Dlogdp. The soot volume is obtained by subtracting the ash volume from the total volume of PM 0.1. As can be seen, the PM 0.1 soot volume decreases by almost 70% from 9 ms to 22 ms residence time, and then by 40% from 22 ms to 33 ms. This decrease is attributed to the dominance of soot oxidation over soot inception as residence time increases. Ash, as expected, increases in volume as residence time increases due to increased mineral release during devolatilization, and especially during the char oxidation stage, when the particle temperature is high [30]. Fig. 5 reveals that soot is a major contributor to total PM 0.1 during the early stages of coal combustion; therefore ignoring soot can lead to significant erroneous conclusions when comparing results obtained from PM 0.1 studies. Fig. 6 shows a comparison of PSDs for total PM 0.1 and ash PM 0.1 at three different residence times. As can be seen, the PSD of ash is significantly different from the PSD of total PM 0.1. While the difference between the PSDs is large at lower residence times (9 ms), the
Fig. 4. The evolution of the number PSDs of ultrafine PM for PRB coal combustion at 1500 K ambient gas temperature, a) without the oxidizing furnace, and b) with the oxidizing furnace. Dots represent the number PSDs and solid lines represent the lognormal fit at different residence times.
33 ms. Case I represents the total PM 0.1 PSD and Case II represents the ash PM 0.1 PSD. It is important to note that, as noted above, the ash PSD measured using this method (after burning off the soot) may not represent the ash PSD if no soot were present. The ash monomers or ash-ash agglomerates formed during burn-off of soot in the oxidizing furnace would lead to a different ash PSD than the PSD formed if only mineral vapors were present in the Hencken combustion environment. Note, however, that this ash distribution is more indicative of the actual PSD in a real system since the soot particles would undergo burnout in the downstream zones of the combustor. The PSDs for both Case I and Case II were fitted with lognormal functions and show a strong unimodal behavior in both cases (Fig. 4). The unimodal behavior of the PSDs generally indicates that coagulation/condensation is dominant over particle nucleation. These results are consistent with the findings of Gao et al. [27] who found that while a bimodal behavior of PSD is observed at low temperatures (implying a competition between particle generation and coagulation), at high temperatures, a unimodal behavior of PSD is observed (implying dominance of coagulation/condensation over particle generation).
Fig. 5. The evolution of the total volume of soot and ash in submicron range formed during combustion of per mg of PRB coal at 1500 K. 5
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difference gets narrow at larger residence times. This observation clearly suggests that the dynamics of soot oxidation, soot formation and ash formation all cumulatively affected the PSD. As discussed earlier, with the increase in residence time, the environment becomes more favorable for soot oxidation which makes the total PM 0.1 PSD more representative of ash PM 0.1 PSD. A large difference between total and ash PSD (difference between modes of the distribution = 20 nm) at 9 ms suggests that the ash formation is much slower than soot which could lead to more ash coagulation on soot seed particles than the ash to ash coagulation/condensation. This implies that at 9 ms, the majority of PM 0.1 either exists as soot-ash agglomerates or soot agglomerates. Notably, these soot-ash agglomerates may breakup, and result in a different morphology when soot burns off either at higher residence time in Hencken burner environment or in the downstream oxidizing furnace, which leads to the decrease in the difference between modes of total and ash distribution from 14 nm at 22 ms to 6 nm at 33 ms. As seen in Fig. 6, the number of particles (#/cm3) for PM 0.01 are surprisingly higher in the case of ash PSD as compared to total PSD. These extra particles might have been generated during burnout of the soot-ash agglomerates in the oxidizing furnace. Subsequently, at 22 ms and 33 ms, the generation of particles in the furnace decreases, which may be due to the formation of more ash agglomerates rather than sootash agglomerates, which stay unaffected when the aerosol stream is passed through the oxidizing furnace. Fig. 7 shows the PSD of PM 0.1 generated during the early stages of coal combustion for Hongshayuan lignite coal (both Case I and Case II) at 1500 K ambient gas temperature. In contrast to PRB coal, the mode of the unimodal lognormal distribution for total PM 0.1 (Case I) increase in diameter with increase in residence time. This increasing trend may suggest that soot is not significant in this type of coal; however, on comparison of the total PM 0.1 volume and ash PM 0.1 volume (Fig. 8), it is clear that both soot and ash are major contributors to total volume. The reverse trend of total PM 0.1 for Hongshayuan lignite coal can be attributed to the high Na vaporization during the early stages of this type of coal. Xiao et al. [31] have shown that a high Na content in coals can inhibit soot formation due to their low ionization potential [32,33] and high catalytic activity [34]. However, as can be seen in Fig. 8, as the residence time of the coal particles in the Hencken burner increases, and sodium is sufficiently vaporized from the coal, soot formation starts to proceed at a high rate. This delayed formation and growth of soot results in the inverse trends of total PM 0.1 PSD for Hongshayuan lignite coal compared to the PRB coal. To summarize, the Hongshayuan coal results clearly demonstrate the potential misinterpretation that is possible if PSD data of only total PM 0.1 is available. It is noteworthy that for Hongshayuan lignite coal, the amount of PM 0.01 after the oxidizing furnace is still higher than that without the furnace (see supplementary data, Fig. 3). Although the amount of soot is increasing with time, at a residence time of 9 ms, the relative amount of soot is significant enough for the soot-ash agglomerates to form, which breakup, and produce ash monomers in the furnace. As expected, the unimodal lognormal ash PSD in Fig. 7 increases in diameter with increase in residence time. Finally, to show the comparison of ash release rates in early-stages and large scale systems, the ash release of PRB coal and Hongshayuan coal in Hencken burner was calculated by multiplying total volume of ash by density (assumed 1.8 g/cm3 for ash). The ash release in the case of PRB coal is 4.8 mg/g-coal and for Hongshayuan coal is 6.33 mg/gcoal at 33 ms residence time in Hencken burner environment. According to Zhan et al. [6], the total ultrafine ash (PM 0.1) for PRB coal is approximately 27 mg/g-coal at the exit of a 100 kW down-fired combustor (residence time > 1 s), which means that about one fifth of the ash is produced at 33 ms residence time. This indicates the importance of early-stage processes on ash formation.
Fig. 6. The comparison of the number PSDs of ultrafine PM for PRB coal at 1500 K ambient gas temperature at residence times of (a) 9 ms, (b) 22 ms, and (c) 33 ms, without the oxidizing furnace (black) and with the oxidizing furnace (red).
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4. Conclusion The Hencken burners, due to their high level of control and close simulation of the time–temperature history of practical system, has been extensively used to study early stage ultra-fine ash evolution during coal combustion. Models have also been developed, using these experimental results and derived rates. Considering that most of these models have a large number of assumptions and fitted parameters, inaccurate interpretation of experimental results could significantly misguide model development and hence our understanding of early stage processes. The approach employed in this work has provided novel information on PM 0.1 evolution in the early stages of coal combustion. The method enables measurement of total volume of both soot and ash formed during coal combustion in the Hencken burner environment, and provides a basic understanding of the early stage morphological changes of PM 0.1. This study has shown that there is a high probability of confounding effects in experimental measurements of early stage ultra-fine particulate matter evolution for coal combustion in the Hencken burner. The ash results are confounded by the presence of soot in large quantities. Not only does soot overshadow ash in the very early stages, it also affects the PSD at later times by yielding more fine ash particles after the soot burns off. The confounding behavior was shown to exist at different conditions and with different coals. Ignoring soot when conducting ash studies—even when the mode of the PSD increases with residence time—was shown to result in inaccurate results. This could result in a misinterpretation of ash vaporization, nucleation or coagulation rates. To obtain a more accurate understanding of the early stage evolution of ultra-fine particulate matter, the experimental set up described in this work is suggested. Lastly, it should be emphasized that simply burning-off the soot in an external furnace does not give an accurate indication of the ash PSD that would be obtained if there were no soot, as the soot and ash strongly interact with each other during the evolution of PM 0.1. This also has implications for modeling early stage processes, in that accurate modelling will require consideration of both ash and soot. Acknowledgements Fig. 7. The evolution of the number PSDs of ultrafine PM for Hongshayuan lignite coal combustion at 1500 K ambient gas temperature, a) without the oxidizing furnace, and b) with the oxidizing furnace. Dots represent the number PSDs and solid lines represent the lognormal fit at different residence times.
This work was mainly funded by National Science Foundation (CBET 1705864), United States and by the Consortium for Clean Coal Utilization at Washington University in St. Louis, United States. Special thanks to Qian Huang, Tianxiang Li, Xuebin Wang for starting this work. Special thanks to Zhenghang Xiao for useful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.115995. References [1] Kienlen TW. The future of coal. Financ Anal J 2006;10:77–80. https://doi.org/10. 2469/faj.v10.n4.77. [2] IEA. Market Report Series: Coal 2018 – Executive Summary; 2018. [3] Xu M, Yu D, Yao H, Liu X, Qiao Y. Coal combustion-generated aerosols: formation and properties. Proc Combust Inst 2011;33:1681–97. https://doi.org/10.1016/j. proci.2010.09.014. [4] Zhuang Y, Biswas P. Submicrometer particle formation and control in a bench-scale pulverized coal combustor. Energy Fuels 2001;15:510–6. https://doi.org/10.1021/ ef000080s. [5] Linak WP, Yoo J-I, Wasson SJ, Zhu W, Wendt JOL, Huggins FE, et al. Ultrafine ash aerosols from coal combustion: characterization and health effects. Proc Combust Inst 2007;31:1929–37. https://doi.org/10.1016/j.proci.2006.08.086. [6] Zhan Z, Fry A, Zhang Y, Wendt JOL. Ash aerosol formation from oxy-coal combustion and its relation to ash deposit chemistry. Proc Combust Inst 2015;35:2373–80. https://doi.org/10.1016/j.proci.2014.07.001. [7] Li G, Li S, Dong M, Yao Q, Guo CY, Axelbaum RL. Comparison of particulate
Fig. 8. The evolution of the total volume of soot and ash in submicron range formed during combustion of per mg of Hongshayuan coal at 1500 K.
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[21] Baxter LL. The evolution of mineral particle size distributions during early stages of coal combustion. Prog Energy Combust Sci 1990;16:261–6. https://doi.org/10. 1016/0360-1285(90)90035-2. [22] Senior CL, Panagiotou T, Sarofim AF, Helble JJ. Formation of ultra-fine particulate matter from pulverized coal combustion; 2000. [23] Viskanta R, Mengüç MP. Radiation heat transfer in combustion systems. Prog Energy Combust Sci 1987;13:97–160. https://doi.org/10.1016/0360-1285(87) 90008-6. [24] Gao Q, Li S, Yuan Y, Zhang Y, Yao Q. Ultrafine particulate matter formation in the early stage of pulverized coal combustion of high-sodium lignite. Fuel 2015;158:224–31. https://doi.org/10.1016/j.fuel.2015.05.028. [25] Gao Q, Li S, Xu Y, Yao Q. Effect of CO2/H2O on the incipient ultrafine particulate matter formation in oxy-fuel combustion of high-sodium lignite. Energy Fuels 2018;32:4308–14. https://doi.org/10.1021/acs.energyfuels.7b03189. [26] Xiao Z, Shang T, Zhuo J, Yao Q. Study on the mechanisms of ultrafine particle formation during high-sodium coal combustion in a flat-flame burner. Fuel 2016;181:1257–64. https://doi.org/10.1016/j.fuel.2016.01.033. [27] Gao Qi, Li Shuiqing, Yang Mengmeng, Pratim Biswas QY. Measurement and numerical simulation of ultrafine particle size distribution in the early stage of highsodium lignite combustion Qi. Proc Combust Inst 2017:2083–90. https://doi.org/ 10.1016/j.amjmed.2015.10.002.This. [28] Ma J. Soot formation during coal pyrolysis. Dept Chem Eng 1996. Ph D:175. [29] Zhuo JK, Li SQ, Yao Q, Song Q. The progressive formation of submicron particulate matter in a quasi one-dimensional pulverized coal combustor. Proc Combust Inst 2009;32:2059–66. https://doi.org/10.1016/j.proci.2008.06.108. [30] Yuan Y, Li S, Zhao F, Yao Q, Long MB. Characterization on hetero-homogeneous ignition of pulverized coal particle streams using CH⁎chemiluminescence and 3 color pyrometry. Fuel 2016;184:1000–6. https://doi.org/10.1016/j.fuel.2015.11. 032. [31] Xiao Z, Tang Y, Zhuo J, Yao Q. Effect of the interaction between sodium and soot on fine particle formation in the early stage of coal combustion. Fuel 2017;206:546–54. https://doi.org/10.1016/j.fuel.2017.06.023. [32] Howard JB, Kausch WJ. Soot control by fuel additives. Prog Energy Combust Sci 1980;6:263–76. https://doi.org/10.1016/0360-1285(80)90018-0. [33] di Stasio S, LeGarrec JL, Mitchell JBA. Synchrotron radiation studies of additives in combustion, II: soot agglomerate microstructure change by alkali and alkaline-earth metal addition to a partially premixed flame. Energy Fuels 2011;25:916–25. https://doi.org/10.1021/ef1012209. [34] Bladt H, Ivleva NP, Niessner R. Internally mixed multicomponent soot: Impact of different salts on soot structure and thermo-chemical properties. J Aerosol Sci 2014;70:26–35. https://doi.org/10.1016/j.jaerosci.2013.11.007.
formation and ash deposition under oxy-fuel and conventional pulverized coal combustions. Fuel 2013;106:544–51. https://doi.org/10.1016/j.fuel.2012.10.035. Suriyawong A, Gamble M, Lee M-H, Axelbaum R, Biswas P. Submicrometer particle formation and mercury speciation under O2−CO2 coal combustion. Energy Fuels 2006;20:2357–63. https://doi.org/10.1021/ef060178s. Juul A, Frandsen J. Combustion aerosols from co-firing of coal and solid recovered fuel in a 400 mw pf- fired power plant Impacts of fuel quality, Power production and environment Publication date : 2010. Linak WP, Peterson TW. Effect of coal type and residence time on the submicron aerosol distribution from pulverized coal combustion. Aerosol Sci Technol 1984;3:77–96. https://doi.org/10.1080/02786828408958996. Liu YP, Zhong BJ, Li SH. Study on impact of coal particle ash content on bottom ash formation behavior in fluidized bed combustion; 2014. doi: 10.1109/ICIEA.2014. 6931471. Carbone F, Beretta F, D’Anna A. Multimodal ultrafine particles from pulverized coal combustion in a laboratory scale reactor. Combust Flame 2010;157:1290–7. https://doi.org/10.1016/j.combustflame.2010.03.015. Carbone F, Beretta F, D’Anna A. Factors influencing ultrafine particulate matter (PM0.1) formation under pulverized coal combustion and oxyfiring conditions. Energy Fuels 2010;24:6248–56. https://doi.org/10.1021/ef100780c. Yu D, Morris WJ, Erickson R, Wendt JOL, Fry A, Senior CL. Ash and deposit formation from oxy-coal combustion in a 100kW test furnace. Int J Greenh Gas Control 2011;5:S159–67. https://doi.org/10.1016/j.ijggc.2011.04.003. Neville M, Quann RJ, Haynes BS, Sarofim AF. Vaporization and condensation of mineral matter during pulverized coal combustion. Symp (Int) Combust 1981;18(1):1267–74. Fletcher TH, Ma J, Rigby JR, Brown AL, Webb BW. Soot in coal combustion systems. Prog Energy Combust Sci 1997;23:283–301. https://doi.org/10.1016/S03601285(97)00009-9. Morris WJ, Yu D, Wendt JOL. Soot, unburned carbon and ultrafine particle emissions from air- and oxy-coal flames. Proc Combust Inst 2011;33:3415–21. https:// doi.org/10.1016/j.proci.2010.05.059. Morris WJ, Yu D, Wendt JOL. A comparison of soot, fine particle and sodium emissions for air- and oxy-coal flames, with recycled flue gases of various compositions. Proc Combust Inst 2013;34:3453–61. https://doi.org/10.1016/j.proci. 2012.06.120. Adeosun A, Xiao Z, Yang Z, Yao Q, Axelbaum RL. The effects of particle size and reducing-to-oxidizing environment on coal stream ignition. Combust Flame 2018:1–10. https://doi.org/10.1016/j.combustflame.2018.05.003. Adeosun A. Fundamental studies of solid-fuel combustion using a two-stage flatflame burner n.d.
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Full Length Article
Characterizing early stage sub-micron particle formation during pulverized coal combustion in a flat flame burner
T
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Dishant Khatria, Akshay Gopanb, Zhiwei Yanga, , Adewale Adeosunc, Richard L. Axelbauma a
Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA Presently at Cabot Corporation, Billerica, MA 01821, USA c Presently at Intel Corporation, Ronler Acres Campus, 2501 NW 229th Ave, Hillsboro, OR, USA b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Sub-micron ash Soot Hencken burner Coal
Mineral matter evolution, soot formation, and soot oxidation are the governing processes for fine particle evolution during the early stages of pulverized coal combustion. Soot and ash can be simultaneously present in PM 0.1, and experimental results obtained to understand early stage particle formation can be ambiguous if the two are not independently evaluated. A sampling system proposed in this work differentiates between soot and mineral matter by employing a high temperature furnace downstream of the sampling system, supplied with sufficient oxygen to oxidize the soot in the sampled aerosol stream. The stream can be routed either through the furnace or bypassing it. In the bypass configuration, the SMPS measures the total PM size distribution. In the flow-through configuration, the SMPS measures the ash PM size distribution, and the difference identifies the contributions from the soot aerosols. This study has shown that there is a high probability of confounding effects in experimental measurements of early stage ultra-fine particulate matter evolution, and the effects were shown to exist at different conditions and with different coals. The results show that for PRB coal combustion, as residence time increases from 9 ms to 33 ms, the volume of total PM 0.1 decreases by 68%, and the ash PM 0.1 increases by 600%, whereas for Hongshayuan lignite coal, the volume of total PM 0.1 increases by 275%, and ash PM 0.1 increases by 939%. These results indicated that soot constitutes a major part of PM 0.1 during the early stages of fine particle evolution.
⁎
Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.fuel.2019.115995 Received 31 May 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 12 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
elemental particle size distribution [26]. More recently, models have been developed—primarily based on the discrete-sectional population balance model—to explain the early stage PM 0.1 evolution behavior observed in experiments [27]. Though sophisticated in their analyses, it is possible that many of the previous works on early stage ash evolution suffer from ambiguities introduced due to carbonaceous material. During the early stages, the secondary reactions of tar, the oxidation of soot, and the vaporizationcondensation processes of mineral matter occur simultaneously, and particle evolution is a cumulative effect of all these mechanisms [16,17]. Neglecting any one of these mechanisms, without supporting evidence, could lead to inaccurate conclusions. For example, if applying ash formation models to understand coagulation and growth of PM 0.1, the contribution of carbonaceous matter should be clearly understood. In this work, we highlight the potential uncertainty introduced when dealing with either early stage ash or soot formation during coal combustion, if either of the two are neglected in the analysis. There are possibly conditions where one of the two can be neglected, but this should be demonstrated or justified. A new sampling and analysis system is developed in this work to differentiate between soot and mineral matter. PSD of mineral ash only and total PM 0.1 are measured. This analysis is conducted for two coals to show the applicability and importance of such analysis across different coals.
The growing demand for energy in developing countries is expected to increase fossil fuel consumption as well as the use of alternative energy resources [1]. Today, coal produces more than 27% of all energy worldwide [2], and will continue to provide reliable energy to billions of people around the globe. In the past decade, due to its low cost, abundance and broad distribution, coal contributed more to the global energy supply than any other energy source. At the same time, the benefits of coal combustion bring environmental challenges, including the emissions of particulate matter (PM) [3]. Submicron particles, especially PM 0.1, are particularly difficult to collect using conventional methods of particle removal [4], and once PM 0.1 is emitted into the environment, the particles can stay airborne for long periods of time. When inhaled into the lungs, they can enter into the interstitial spaces and diffuse into the blood stream [5]. These particles can cause tissue irritation and release the toxic chemical intermediates [5]. Inside the boilers, Zhan et al. [6] from his experimental results, have found that these fine particles form an initial sticky layer of ash deposits on boiler tubes. This results in an enhancement in the sticking probability for the larger particles, and the resulting ash deposition causes a reduction in heat transfer rates and a derating of the boiler [7]. Thus, understanding the formation and evolution of submicron particles during coal combustion is essential from both environmental and operational perspectives. In the past few decades, the formation and evolution of PM 0.1 has been studied extensively at different reactor scales–from small lab-scale reactors [8] up to MW-scale pilot plants [7,9]. The formation of PM 0.1 has been shown to be affected by coal type and rank, combustion environment, and the initial size of the coal particles [10–14]. PM 0.1 consists of both mineral matter and carbonaceous materials. Mineral matter is formed by the mechanism of volatilization-nucleation-coagulation, in which mineral matter vaporizes at high temperature and diffuses out of the coal particles, and undergoes either homogeneous nucleation or heterogeneous condensation, followed by coagulation and agglomeration. Low boiling point metals vaporize directly, whereas refractory metals are reduced to their sub-oxides before they vaporize [15]. On the other hand, carbonaceous materials are formed by the secondary gas-phase reactions of coal volatiles, in particular tar, released during particle heating [16]. Tar consists of polycyclic aromatic hydrocarbons (PAH), which undergo cracking and polymerization at high temperature, to form soot. Most studies have sampled the submicron particles at the exit of a furnace, with coal particle residence times on the order of a few seconds [16–18]. As important as these results are, such long residence times do not offer insights into the early stage processes which dominate in the first 100 ms or less. These processes, including particle heating, devolatilization, ignition, mineral matter vaporization, and formation of light gases and tar [19–21] are known to dictate char burnout, combustion efficiency, heat transfer, and PM0.1 formation [22,23] and understanding these processes can lead to efficient designs of burners, improved heat transfer characteristics, and in-flame control of particulate formation. Therefore, a controlled experimental system with the capability of sampling at short residence times, while mimicking the time-temperature history experienced by coal particles in practical furnaces, is required to understand early-stage PM 0.1 formation. Hencken burners are one of the primary systems that have such a capability. Several PM 0.1 studies have been conducted using Hencken burners to improve our understanding of the formation and evolution of ash particles during early stages. Gao et al. [24] studied the combustion of Zhundong lignite coal, and found that the competition between devolatilization of mineral matter and coalescence determines the evolution of ultrafine particles. He further showed that H2O enhances the formation of nascent Si-based PM 0.1, and CO2 diminishes PM 0.1 formation [25]. Xiao et al. found that homogeneous condensation and surface reaction are the controlling mechanisms responsible for the
2. Experimental 2.1. Hencken flat flame burner The Hencken burner used in this work is shown in Fig. 1. Here the burner is described only briefly, while a more complete description can be found in Ref. [20]. The burner is 80 mm in diameter and composed of 570 hypodermic needles, each having a 1.2 mm ID, creating an equal number of micro-diffusion flamelets. Fuel and oxidizing gases are introduced through separate chambers, and are isolated via a gasket material. The burner is unique in that it has two zones – an inner and an outer zone (see Fig. 1). Each zone can be run under globally reducing or oxidizing conditions, enabling the study of early stage coal combustion processes under a variety of conditions. A central tube with a 2 mm ID passes through the burner for coal feeding. An 80 mm diameter quartz tube isolates the combustion process from the ambient environment. A fine stainless mesh on the top of the quartz tubing minimizes external disturbances. The burner provides a stable combustion environment for coal combustion, with a heating rate of ~105 K/s. In experiments, coal particle stream was fed by entraining them in nitrogen using a micro fluidized feeder with a typical entrained gas flow rate of 60 cubic centimeters per min. A high frequency vibration source ensures a stable coal feeding rate [20]. The coal stream mass flow rate was kept at 150 mg/min during this study. Methane (CH4) diluted with nitrogen (N2) was used as the fuel, and a mixture of oxygen (O2) and nitrogen (N2) as the oxidizer. A combustion temperature of 1500 K was used in these experiments, and the post flame O2 concentration was set at 20% by adjusting flowrates of both the fuel and oxidizer streams (Fig. 2). Both the inner and outer zones were run in oxidizing conditions for this study. The temperature of the hot gas environment was measured using a B type thermocouple, corrected for radiation loss. Gas composition was measured using a Horiba gas analyzer model PG-250. The residence time of the individual coal particles was measured using high speed video of the coal particles during combustion. Video images were obtained using a NAC MEMRECAM HX- 7 high-speed camera. The camera was fitted with a SIGMA APOMACRO 180 mm lens which was used to obtain image magnification of 1:4. The camera was set to 8000 frames per second (fps), and an exposure time of 50 μs was used to resolve the motion of the single particles. The particle residence times were determined by tracking the motion of more than 40 particles and then averaging the results. The residence times of coal particles, oxygen volume percentage, and gas 2
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Fig. 1. Cross-section of the Hencken Burner (left), and photograph of coal particles burning in micro diffusion flames (right).
N2 flowing in the annulus, ensuring that the dilution N2 is heated before being introduced into the sampling probe (Fig. 3), and avoiding condensation in the probe. The temperature at the exit of the sampling probe (Thermocouple 1) is 600 K ± 50 K at different sampling heights, ensuring that minimal condensation and nucleation are expected in the probe. The particles are sampled at 1, 3, and 5 cm axial distances above the burner on the centerline. The aerosol stream from the sampling probe is diluted again in the secondary dilution step with room temperature nitrogen gas, further suppressing particle coagulation [27]. A small portion of this aerosol stream is further diluted—the rest is exhausted—to get the final dilution ratio of ~100, which yielded a particle number density that was within the measurable range of condensation particle counter (CPC). During the experiments, the non –diluted number particle size distribution (PSD) back in the flue gas did not change when the dilution ratio was further increased, indicating that the three-stage dilution process was freezing the various growth processes. After the tertiary dilution, a portion of the diluted aerosol stream flows through a cascade impactor with the final stage cut-off size of 0.7 micron. The downstream gas stream is then fed through either a bypass tube (Case I in Fig. 3), or through an electrically-heated alumina tube furnace (Case II in Fig. 3) before reaching the particle size analyzer. The alumina tube furnace is used to burn out carbonaceous particles in the stream such that only the mineral particulate matter remains, and the contribution of carbonaceous matter to the total PSD can be ascertained. The alumina tube furnace, with a 9.5 mm ID, has a constant temperature of 1373 K for a length of 25 cm, as shown in the inset of Fig. 3. To burn off carbonaceous material, O2 is uniformly mixed into the aerosol stream before entering the furnace. Nagle and StricklandConstable (NSC) soot oxidation model [28] was used to calculate time scales of 90% oxidizing of a 200 nm carbon particle at 1373 K with 50 % surrounding oxygen mole fraction. The calculated time scale is 0.35 sec. In the furnace, the residence time of the coal particles is 1.4 sec, suggesting that the residence time is high enough for all soot to oxidize. In addition, oxygen percentage in the oxidizing furnace was varied to ensure that no noticeable difference in PSD is seen when increasing oxygen percentage. Finally, an oxygen mole fraction of 50%, which was high enough to burn off all of the soot at the given oxidizing furnace temperature and residence time was used to maintain consistency in the
Fig. 2. The ambient temperature (▲), oxygen volume percentage ( ), and residence time of coal particles along the centerline of the burner at 1500 K.
temperature along the burner are shown in Fig. 2.
2.2. Sampling system The particle sampling system is shown in Fig. 3. Particle sampling is achieved with a three-stage dilution system. The Hencken burner is attached to a motor-driven slide that can control its axial position relative to the dilution sampling probe [27]. In the sampling probe, dilution N2 is supplied through a narrow annulus of the probe, and the resulting high velocity of the N2 provides a negative pressure to draw in the atmospheric pressure flue gas, and dilute the particle-laden flow. The probe samples the entire particle stream such that there is no aspiration efficiency bias during sampling (i.e., all of the particles in the Hencken environment are sampled into the sampling probe). A dilution ratio of 3–5 is maintained for the primary dilution stage, and is calculated by measuring the oxygen concentration with and without dilution using the gas analyzer. The hot flue gas heats the probe and the dilution 3
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Fig. 3. Schematic of particulate sampling system.
measurements. Before taking a set of measurements, we verified that the oxidizing furnace itself did not generate appreciable particles by running the system without coal. Finally, the number concentration and PSD of the total particles in the stream (Case I) and non-carbonaceous particles in the stream (Case II) were measured using a TSI scanning mobility particle sizer (SMPS) with a nano-differential mobility analyzer (DMA) 3085 and CPC model 3776. The sheath flow was adjusted so that the measured particle size range was between 3.46 nm and 117.6 nm, in order to capture both nucleation as well as coagulation/condensation information for the PM 0.1. Before the neutralizer, another impactor with a nozzle size of 0.0508 cm and a D50 of 735.2 nm was used to protect the SMPS.
Table 1 Properties of PRB sub bituminous coal, and Hongshayuan lignite coal. Proximate analysis (wt. %, dry basis)
Sub-bituminous Powder River Basin (PRB) coal
Hongshayuan Lignite Coal
Fixed carbon Volatile matter Ash HHV(MJ/kg)
47.63 43.33 9.04 27.45
62.28 29.8 7.92 28.53
Ultimate analysis (wt.%, dry, and ash free basis) C 74.66 H 5.45 N 1.08 O 18.24 S 0.57
2.3. Coal properties
Ash compositions (wt.%) SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
A sub-bituminous Powder River Basin (PRB) coal, and a lignite Hongshayuan coal were used in these experiments, and Table 1 lists selected coal properties. For the combustion experiments, the coal particles were dried at 378 K to ensure stable feeding and then sieved to a size range of 45–63 μm. 3. Results and discussion The PM 0.1 collected is generally composed of both carbonaceous and mineral matter. During the early stages of coal combustion, PM 0.1 particles are present in a variety of morphologies, including ash monomers, soot monomers, ash-ash agglomerates, soot-soot agglomerates and ash-soot agglomerates [26,29]. The formation of these morphologies depend on the distribution of ash and soot present in the combustion environment and also the characteristic time of various aerosol growth processes. Once the mineral matter vapors are released from the coal, they can undergo nucleation (ash monomers), coagulation (ash-ash agglomerates), coagulation with soot (ash-soot agglomerates), or heterogeneous condensation on ash or soot particles, depending on the local concentration of these species and temperature. For example, soot particles in the vicinity of a coal particle could act as seed particles for deposition of vaporized minerals, and reduce the amount of nucleated mineral matter particles by reducing the
37.44 16.50 5.15 16.20 3.05 1.06 16.90 0.40 0.65 1.21
81.91 4.38 0.85 12.31 0.56 31.05 13.32 19.14 12.72 5.02 0.65 11.65 0.63 4.44
concentration of mineral vapor. Furthermore, the interactions between soot and ash particles can affect their formation characteristics. For example, if there is a significant number density of soot particles in an ash-soot agglomerate, the ash particles in the agglomerate may not be contiguous, and thus, upon soot oxidation latter in the combustion process, the ash particles would be released independently and not yield the size of the ash agglomerate that would exist in the absence of soot. Fig. 4 shows the particle number density (PSD) of PM 0.1 generated during the early stages of coal combustion for PRB coal (both Case I and Case II) at 1500 K ambient gas temperature. The particles were sampled along the centerline at three different locations (1, 3, and 5 cm) along the vertical axial which corresponds to residence times of 9, 21, and 4
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The total PM 0.1 PSD, as can be seen in Fig. 4a, decreases in diameter with increase in the residence time of coal particles in the Hencken burner. The mode of the distribution shifts from 31 nm at 9 ms to 25 nm at 22 ms, and then to 20 nm at 33 ms. The decrease in diameter indicates the loss of sub-micron particles, signifying the effect of soot oxidation on total PM 0.1 as particles moves from 1 cm to 5 cm (i.e., 9 ms to 33 ms). At 9 ms residence time, which is just after particle ignition (ignition time found by high speed camera imaging [20] is 7 ms); the high ambient temperature and low oxygen concentration provide an excellent environment for soot to form and grow (c.f. Fig. 2). However, at longer residence times, the increase in the ambient oxygen leads to enhanced soot oxidation. It is noteworthy that for smaller sizes (PM 0.02), the particle numder density increases with increasing residence time, but for larger sizes in PM 0.1, the particle number density decreases with increasing residence time. This observation implies that the soot and soot-ash agglomerates which represents large sizes, oxidizes with increasing residence time, thereby forming small ash particles. In addition, more nucleation of ash particles is possible at higher residence times. In comparison with the PSD of the total PM 0.1, the mode of the PSD of ash particles (Fig. 4b) increase in diameter with increasing residence time, which is attributed to the conventional growth mechanisms of coagulation and condensation. The mean peak value of lognormal ash PSD increases from 11 nm at 9 ms to 14 nm at 22 ms, and then to 18 nm at 33 ms. Fig. 5 shows the total volume of soot and ash as a function of residence time per mg of coal burnt. The ash volume was obtained by multiplying each point of the ash number PSD function by the corresponding particle volume, and Dlogdp. The soot volume is obtained by subtracting the ash volume from the total volume of PM 0.1. As can be seen, the PM 0.1 soot volume decreases by almost 70% from 9 ms to 22 ms residence time, and then by 40% from 22 ms to 33 ms. This decrease is attributed to the dominance of soot oxidation over soot inception as residence time increases. Ash, as expected, increases in volume as residence time increases due to increased mineral release during devolatilization, and especially during the char oxidation stage, when the particle temperature is high [30]. Fig. 5 reveals that soot is a major contributor to total PM 0.1 during the early stages of coal combustion; therefore ignoring soot can lead to significant erroneous conclusions when comparing results obtained from PM 0.1 studies. Fig. 6 shows a comparison of PSDs for total PM 0.1 and ash PM 0.1 at three different residence times. As can be seen, the PSD of ash is significantly different from the PSD of total PM 0.1. While the difference between the PSDs is large at lower residence times (9 ms), the
Fig. 4. The evolution of the number PSDs of ultrafine PM for PRB coal combustion at 1500 K ambient gas temperature, a) without the oxidizing furnace, and b) with the oxidizing furnace. Dots represent the number PSDs and solid lines represent the lognormal fit at different residence times.
33 ms. Case I represents the total PM 0.1 PSD and Case II represents the ash PM 0.1 PSD. It is important to note that, as noted above, the ash PSD measured using this method (after burning off the soot) may not represent the ash PSD if no soot were present. The ash monomers or ash-ash agglomerates formed during burn-off of soot in the oxidizing furnace would lead to a different ash PSD than the PSD formed if only mineral vapors were present in the Hencken combustion environment. Note, however, that this ash distribution is more indicative of the actual PSD in a real system since the soot particles would undergo burnout in the downstream zones of the combustor. The PSDs for both Case I and Case II were fitted with lognormal functions and show a strong unimodal behavior in both cases (Fig. 4). The unimodal behavior of the PSDs generally indicates that coagulation/condensation is dominant over particle nucleation. These results are consistent with the findings of Gao et al. [27] who found that while a bimodal behavior of PSD is observed at low temperatures (implying a competition between particle generation and coagulation), at high temperatures, a unimodal behavior of PSD is observed (implying dominance of coagulation/condensation over particle generation).
Fig. 5. The evolution of the total volume of soot and ash in submicron range formed during combustion of per mg of PRB coal at 1500 K. 5
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difference gets narrow at larger residence times. This observation clearly suggests that the dynamics of soot oxidation, soot formation and ash formation all cumulatively affected the PSD. As discussed earlier, with the increase in residence time, the environment becomes more favorable for soot oxidation which makes the total PM 0.1 PSD more representative of ash PM 0.1 PSD. A large difference between total and ash PSD (difference between modes of the distribution = 20 nm) at 9 ms suggests that the ash formation is much slower than soot which could lead to more ash coagulation on soot seed particles than the ash to ash coagulation/condensation. This implies that at 9 ms, the majority of PM 0.1 either exists as soot-ash agglomerates or soot agglomerates. Notably, these soot-ash agglomerates may breakup, and result in a different morphology when soot burns off either at higher residence time in Hencken burner environment or in the downstream oxidizing furnace, which leads to the decrease in the difference between modes of total and ash distribution from 14 nm at 22 ms to 6 nm at 33 ms. As seen in Fig. 6, the number of particles (#/cm3) for PM 0.01 are surprisingly higher in the case of ash PSD as compared to total PSD. These extra particles might have been generated during burnout of the soot-ash agglomerates in the oxidizing furnace. Subsequently, at 22 ms and 33 ms, the generation of particles in the furnace decreases, which may be due to the formation of more ash agglomerates rather than sootash agglomerates, which stay unaffected when the aerosol stream is passed through the oxidizing furnace. Fig. 7 shows the PSD of PM 0.1 generated during the early stages of coal combustion for Hongshayuan lignite coal (both Case I and Case II) at 1500 K ambient gas temperature. In contrast to PRB coal, the mode of the unimodal lognormal distribution for total PM 0.1 (Case I) increase in diameter with increase in residence time. This increasing trend may suggest that soot is not significant in this type of coal; however, on comparison of the total PM 0.1 volume and ash PM 0.1 volume (Fig. 8), it is clear that both soot and ash are major contributors to total volume. The reverse trend of total PM 0.1 for Hongshayuan lignite coal can be attributed to the high Na vaporization during the early stages of this type of coal. Xiao et al. [31] have shown that a high Na content in coals can inhibit soot formation due to their low ionization potential [32,33] and high catalytic activity [34]. However, as can be seen in Fig. 8, as the residence time of the coal particles in the Hencken burner increases, and sodium is sufficiently vaporized from the coal, soot formation starts to proceed at a high rate. This delayed formation and growth of soot results in the inverse trends of total PM 0.1 PSD for Hongshayuan lignite coal compared to the PRB coal. To summarize, the Hongshayuan coal results clearly demonstrate the potential misinterpretation that is possible if PSD data of only total PM 0.1 is available. It is noteworthy that for Hongshayuan lignite coal, the amount of PM 0.01 after the oxidizing furnace is still higher than that without the furnace (see supplementary data, Fig. 3). Although the amount of soot is increasing with time, at a residence time of 9 ms, the relative amount of soot is significant enough for the soot-ash agglomerates to form, which breakup, and produce ash monomers in the furnace. As expected, the unimodal lognormal ash PSD in Fig. 7 increases in diameter with increase in residence time. Finally, to show the comparison of ash release rates in early-stages and large scale systems, the ash release of PRB coal and Hongshayuan coal in Hencken burner was calculated by multiplying total volume of ash by density (assumed 1.8 g/cm3 for ash). The ash release in the case of PRB coal is 4.8 mg/g-coal and for Hongshayuan coal is 6.33 mg/gcoal at 33 ms residence time in Hencken burner environment. According to Zhan et al. [6], the total ultrafine ash (PM 0.1) for PRB coal is approximately 27 mg/g-coal at the exit of a 100 kW down-fired combustor (residence time > 1 s), which means that about one fifth of the ash is produced at 33 ms residence time. This indicates the importance of early-stage processes on ash formation.
Fig. 6. The comparison of the number PSDs of ultrafine PM for PRB coal at 1500 K ambient gas temperature at residence times of (a) 9 ms, (b) 22 ms, and (c) 33 ms, without the oxidizing furnace (black) and with the oxidizing furnace (red).
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4. Conclusion The Hencken burners, due to their high level of control and close simulation of the time–temperature history of practical system, has been extensively used to study early stage ultra-fine ash evolution during coal combustion. Models have also been developed, using these experimental results and derived rates. Considering that most of these models have a large number of assumptions and fitted parameters, inaccurate interpretation of experimental results could significantly misguide model development and hence our understanding of early stage processes. The approach employed in this work has provided novel information on PM 0.1 evolution in the early stages of coal combustion. The method enables measurement of total volume of both soot and ash formed during coal combustion in the Hencken burner environment, and provides a basic understanding of the early stage morphological changes of PM 0.1. This study has shown that there is a high probability of confounding effects in experimental measurements of early stage ultra-fine particulate matter evolution for coal combustion in the Hencken burner. The ash results are confounded by the presence of soot in large quantities. Not only does soot overshadow ash in the very early stages, it also affects the PSD at later times by yielding more fine ash particles after the soot burns off. The confounding behavior was shown to exist at different conditions and with different coals. Ignoring soot when conducting ash studies—even when the mode of the PSD increases with residence time—was shown to result in inaccurate results. This could result in a misinterpretation of ash vaporization, nucleation or coagulation rates. To obtain a more accurate understanding of the early stage evolution of ultra-fine particulate matter, the experimental set up described in this work is suggested. Lastly, it should be emphasized that simply burning-off the soot in an external furnace does not give an accurate indication of the ash PSD that would be obtained if there were no soot, as the soot and ash strongly interact with each other during the evolution of PM 0.1. This also has implications for modeling early stage processes, in that accurate modelling will require consideration of both ash and soot. Acknowledgements Fig. 7. The evolution of the number PSDs of ultrafine PM for Hongshayuan lignite coal combustion at 1500 K ambient gas temperature, a) without the oxidizing furnace, and b) with the oxidizing furnace. Dots represent the number PSDs and solid lines represent the lognormal fit at different residence times.
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Fig. 8. The evolution of the total volume of soot and ash in submicron range formed during combustion of per mg of Hongshayuan coal at 1500 K.
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