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Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle
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Proceedings of the
Proceedings of the Combustion Institute 32 (2009) 2099–2106
Combustion Institute www.elsevier.com/locate/proci
Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle P.J. van Eyk a,*, P.J. Ashman a, Z.T. Alwahabi a, G.J. Nathan b a
Cooperative Research Centre for Clean Power from Lignite, School of Chemical Engineering, The University of Adelaide, SA 5005, Australia b School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia
Abstract The temporal history of the release of volatile alkali species during coal combustion is a significant, but poorly understood factor in the fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. We present new results of the simultaneous measurement of particle temperature, particle size and the atomic sodium concentration in the plume of a burning coal particle. During the char phase, the sodium concentration in the plume was found to be linearly dependent on the inverse of particle diameter, but during the ash phase the sodium concentration was found to decay exponentially with decreasing particle temperature. The centreline decay of Na within the plume above the burning particle consists of one region controlled by a first order chemical reaction and a second region controlled by diffusion. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Sodium; Coal; LIF; Fouling
1. Introduction The release of impurities within coal during combustion is significant to fouling and corrosion of heat transfer surfaces within industrial coalfired boilers [1]. In particular, sodium (Na) species emitted from coal combustion have been shown to form the initial deposit which accommodates the subsequent build up of other ash particles. Elemental sodium in low-rank coal may occur in several forms. It may be water-bound (Na salts in solution), organically-bound (e.g. attached to car-
*
Corresponding author. Fax: +61 8 8303 4373. E-mail address: [email protected] (P.J. van Eyk).
boxylic acid groups), or bound within clay minerals. Sodium associated with carboxylic acid groups has been shown to decompose early in the devolatilisation stage of coal combustion [2– 4], to form sodium carbonate [5]. It has also been shown that sodium chloride decomposes in the early stages of coal combustion, with the chlorine forming HCl [6,7] and the sodium postulated to form sodium carbonate [8]. The clay bound sodium will not vaporise due to the low activity of silica melts [9]. Equilibrium calculations predict that atomic sodium is the favoured species within a flame environment, except when very large amounts of chlorine are also present [10]. In the post-flame gases, sodium chloride and sodium hydroxide are the principle sodium species [9]. However, the temporal history of these processes
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through the various stages of combustion remains poorly understood. Previous workers [11–13], have studied sodium release during coal combustion by analysing the bulk composition of fly ash and the various ash deposits from both full-scale and laboratory-scale combustors. Other studies have measured vapourphase species in the post-combustion gases in a large variety of systems using on-line measurements of the molecular sodium species in the flue gases (eg. NaOH and NaCl). Methods include both optical methods (absorption, spontaneous emission and induced fluorescence) as well as mass spectrometry [14]. While these techniques give insight into the final forms in the gaseous and deposit phases of the sodium released from the coal, it is not possible to infer from them the temporal history of sodium release directly from individual particles, or to identify the nature or concentration of the sodium intermediates. Other studies have utilised laser techniques to determine the concentration of sodium atoms within flame environments using saturated laser-induced fluorescence [15,16]. Although quantitative data can be obtained using saturated laser-induced fluorescence, the large laser power requirements only allow point measurements in the flame. An experimental method has been established in our laboratory [17] to allow the quantitative planar measurement of the release of sodium atoms during the combustion of a single coal particle. The laser-induced fluorescence (LIF) measurement is calibrated using laser absorption to provide an overall accuracy of ±5.7%. The sensitivity of these measurements was 0.04 ppb for a signal-to-noise ratio of 2 and 0.2 ppb for a signal-to-noise ratio of 10. The temporal response, set by the rep-rate of the laser and camera, was 5 Hz for that system. From this previous work [17], the release of Na was found to vary with particle burn time, sb, with the peak occurring near to the end of the char combustion phase. Also, the axial distribution of atomic Na concentration through the plume was found to decay due primarily to either diffusion or chemical reaction, depending on the proximity to the particle. However, that investigation did not relate the sodium measurements to char burn-out properties of the fuel. To accurately model the char burn-out of a given fuel, the char temperature, local oxygen concentrations, residence times within the reaction zone and time/temperature histories of each particle must be known [18]. To compute the release of alkali in a computationally efficient manner, it is highly desirable to scale it against other parameters that are modelled directly, such as particle diameter, Dp (and hence burn-out) and particle temperature. Furthermore, since each coal particle is different, the measurement of those parameters simultaneously with the sodium con-
centration is required for the development and validation of such ‘‘sub-models”. Given the limitations of previous measurements, this work aims to provide, for the first time, spatially- and temporally-resolved simultaneous measurements of three important combustion parameters for a single coal particle. These are the quantitative concentration of atomic sodium, the surface temperature of the coal particle and the coal particle size. While other quantitative measurements of Na have been performed, they have been at a single point, and have not been conducted simultaneously with these other parameters. Although care needs to be taken in extrapolating the results of a single experiment, it is inevitable that very detailed measurements can only be performed for limited number of cases. Also, there is substantial value in identifying what does happen in at least one case, and in demonstrating an approach by which such detailed data can be provided for other assessments. 2. Experimental methods Sodium atoms were measured in the plume of a single coal particle burning in the large, uniform region above a flat flame burner. The burner design is shown in Fig. 1. It contains an upper chamber into which air is introduced via side tubes. This passes through a packed bed and stainless steel honeycomb. Natural gas is fed through the bottom of the lower chamber into a manifold of 384 hypodermic tubes. These direct the fuel to the outlet of the burner through a grid of 17 17 fuel ports. The honeycomb is arranged so that each 1 mm diameter stainless steel fuel port is surrounded by six hexagonal air ports. A small blue cone is produced above each fuel port. The overall air to fuel ratio is fuel lean in this
Fig. 1. Schematic diagram of the flat flame burner (all lengths in mm).
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experiment (1.3 stoichiometric air requirement), and the excess air in the flame was used to burn the coal particle. A shroud of air around the central burner was utilised to stabilise the flame and prevent puffing. A tuneable dye laser (Lambda Physics Scanmate), pumped by a Nd:YAG laser (Coherent Brilliant B), was used to excite fluorescence. Figure 2 shows the laser and optical configuration. The dye laser was scanned around 588–590 nm to excite the D1 and D2 Na lines at 589.59 and 589.0 nm, respectively. Measurements were performed using the D1 line (589.59 nm) because strong beam absorption was observed when using the D2 line. The laser-induced fluorescence signal from the Na atoms was collected using a gated intensified CCD camera (Princeton Instruments ICCD576), which was aligned orthogonal to the laser sheet. Instantaneous images were collected at 2 Hz while the coal particle burned. Because of the very short gate width of the camera (10 ns), no spectral interferences were observed while collecting Na fluorescence. Thus no interference filters were used for the camera. A polarizing filter was used to reduce scattering of laser light from soot particles which occurred during the release of volatiles from the coal during the first stage of combustion. No other scattering from particles was observed after this stage. From our calibration experiments [17], it was determined that signal trapping was not significant with our experimental system. Similarly, saturation will only be reached for our system at considerably higher laser fluences than those used in our experiment. Hence, the trapping and saturation effects were neglected in our analysis. The flame was operated with natural gas and air flow rates of 5.0 and 60.6 L/min (STP), respectively. The shroud gas flow rate was adjusted to eliminate visible unsteadiness in the primary flame. A coal particle, weighing approximately 52.5 mg and 6.2 mm in diameter, was suspended 10 mm above the burner surface on a loop made from platinum wire of 0.5 mm diameter. The coal used for this test was Loy Yang, a Victorian
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Table 1 Loy Yang brown coal chemical analysis [19] % (dry ash free basis) C H N S O (by difference) % (dry basis) Ash Na Mg Ca Al Fe
68.2 4.8 0.63 0.27 26.1 0.8 0.09 0.08 0.04 0.02 0.07
brown coal. The sample of coal used in this study was obtained from the same sample as was used in a previous study by Hulston et al. [19], and the chemical analysis of this coal is given in Table 1. The coal particle used in this experiment was air-dried in an oven at 50 °C overnight. A parallel laser sheet was established by using cylindrical lenses (CL1 and CL2 in Fig. 2). The incident laser fluence for each image was determined by passing the sheet through a calibrated water cell. Each image provides a 2D array of fluorescence values in the plume above the particle, and was converted to absolute atomic sodium concentrations by using the calibration technique reported by van Eyk et al. [17]. The bottom of the imaged region was 7.5 mm above the wire, a limit set to prevent interference from the direct scattering from the particle, and the top of the imaged region was 41.5 mm above the wire. The coal particle temperature was measured using the well established two colour pyrometry technique [20,21]. Two SLR cameras (Canon 400D) were used, each fitted with a narrow bandwidth optical filter of 515 and 532.5 nm, respectively. The cameras were located according to Fig. 2. Recording from the two SLR cameras was started simultaneously as the flat flame was ignited below the suspended coal particle. This provided a measure of both temperature and diameter of the coal particle throughout the entire combustion process. Images were recorded in greyscale every 10 s to give 2D images of particle radiation at the two wavelengths. The surface temperature of the particle, Tp was determined by comparison of the intensity at two wavelengths (E515 and E532.5) at a location on the particle, using Wien’s equation: hc 1 k12 kT k1 ð1Þ T p ¼ k5 Ek1 2 ln eek1 5 =E k2 k k2 1
Fig. 2. Laser and detector set-up for measurement of atomic sodium, particle temperature and particle diameter.
where, Eki is the emissive power of the emitter at wavelength ki eki is the emissivity of the solid surface at wavelength ki, h is Planck’s constant, c is
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the speed of light and k is the Boltzmann constant. The measurement location, close to the bottom of the particle, was selected on a frame-by-frame basis, since the particle shape changes as the particle burns, to provide a feature that could be definitively identified in each of the two images. This makes the measurement a time-consuming process, which restricts the number of particles that can be realistically assessed in this way. The emissivity of the solid surface was assumed to be the same at both wavelengths, because the wavelengths chosen were close together, thus cancelling the ek1/ek2 term from Eq. (1). The intensities collected by each camera depend on the response of each CCD array to wavelength, and the transmission through each optical filter, so a calibration of the ratio E515/ E532.5 versus temperature was required. This calibration was achieved by measuring E515/E532.5 from the bead of a thermocouple suitably positioned at appropriate locations within the flame, while also directly measuring the thermocouple’s temperature. This was undertaken for temperatures from 1300–1500 K to obtain a linear correction for the value of E515/E532.5 from Eq. (1). The SLR images from one camera were also used to determine Dp as a function of combustion time. Here Dp was defined as the geometric mean of the diameters of the particle in the x and y directions. This was further used to estimate the particle volume, Vp. Since the particle chosen for investigation was approximately spherical, this approximation is reasonable. 3. Results and discussion Figure 3 presents a typical image of the atomic sodium concentration field in the plume of the burning particle. This image corresponds to an average of ±10 s of data on either side of the time of peak atomic sodium concentration. The differences between the instantaneous and averaged images are relatively small, due to the generally steady nature of the laminar flat flame, but averaging reduces these ‘‘short-term” (relative to the particle burn time) variations in release rate. It is clearly evident that the concentration decays axially with distance from the wire, z, and radially with distance from the axis, r. The general shape is consistent with the gases being evolved radially from the particle surface, but then being convected upward by buoyancy, following which the concentration decays by the combined influences of dilution and chemical reaction. The variation of atomic Na at a reference location in the plume, Tp and Vp are plotted against sb in Fig. 4. The reference location for the concentration of atomic Na was chosen near to the peak value on the axis at z = 10 mm. Each point in the figure is averaged from 10 s of 2 Hz instanta-
Fig. 3. 2D image of atomic sodium concentration, [Na], in the plume above a burning coal particle during peak sodium emission, where r is the radial distance from the particle and z is the axial distance from the wire; region: 7.5 mm < z < 41.5 mm above the wire and 10 mm < r < 10 mm from the wire.
neous data. As can be seen from Fig. 4, a short initial peak in the measured signal (100 ppb) was observed at sb 10 s. The signal during this period has previously been attributed mainly to interference caused by scattering from soot particles present in the flame during coal devolatilisation, which typically dominates over fluorescence [17]. However, even if the signal from Na fluorescence during this period is non-negligible, the short duration of this peak means that its contribution to total Na release is small. Following this, the sodium concentration increases gradually from several ppb to a large peak (60 ppb) at approximately 910 s. Over the same period, Tp increases gradually from around 1200 to 1600 K,
Fig. 4. Variation of particle temperature, Tp, particle volume, Vp and atomic sodium concentration, [Na] 10 mm directly above the wire, with sb.
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while Vp shrinks from around 60 to 1.5 mm3. Importantly the peak Na emission coincides with the peak temperature and the instant when the coal particle stops shrinking. Significantly also, the rapid rise in Na concentration (sb 800 s), also coincides with a rapid rise in Tp and a comparatively fast decrease in Vp. After the peak, both [Na] and Tp are observed to decay, while Vp remains constant. This latter stage obviously corresponds to the ash phase, since the low and almost constant Dp implies that char burn-out is completed. The decay in Tp asymptotes to that of the flat flame (1450 K), giving further confidence in the temperature measurement technique, and [Na] asymptotes to zero. It is clear that the presence of the wire and coal particle in the flat flame has an influence on the flame temperature, Tff, in the vicinity of the particle, and hence on Tp. Separate thermocouple measurements of the effect on Tff of inert objects placed at approximately the same location in the flame as the coal particle are presented in Fig. 5. With no objects in the flame, Tff was measured to be 1446 K, but reduces to 1413 K when the wire is also placed within the flame. Placing stainless steel balls bearings, of size 3.2 and 6.4 mm onto the wire, further reduces Tff to 1387 and 1349 K, respectively. This suggests the particle is cooled by radiation and in turn convectively cools the surrounding gases, thus lowering Tff in the plume. Therefore, the gradual increase in Tp shown in Fig. 4 could be due, at least in part, to variations in Tff as the coal particle shrinks during combustion. However, the relatively small temperature increase (64 K) when comparing Tff for the wire only to the 6.4 mm stainless steel ball bearing does not account for the large increase in Tp of 400 K. Thus, a substantial contribution to the increase in Tp may also be attributed to an increasing reaction rate at the particle surface during the latter stages of particle burn-out. The finding that Tp and sodium concentration, [Na], peak at the same time (910 s) indicates a strong temperature dependence on the release of sodium from the burning particle, which in turn
Fig. 6. Variation of (a) d[Na]/dsb 10 mm directly above the wire, (b) dDp/dsb and (c) dTp/dsb with combustion time.
depends on burning rate, and so Dp. These relationships are assessed by analysing the derivatives d[Na]/dt, dDp/dt and dTp/dt in Fig. 6. These derivatives were determined from the average slope of every 50 s of data. In Stage I (char phase), there is a strong relationship between all three derivatives. This suggests that as the particle burns (and Dp decreases), the rate of combustion increases, therefore causing Tp to rise and more Na is released. In Stage II (end of char phase), Dp decreases rapidly, corresponding to a rapid increase in Tp and release of Na. The external surface area per volume of a spherical particle is inversely proportional to Dp. Thus, the sharp increase in temperature and [Na] occurs at the time where there is a sharp increase in the external surface area per volume of the particle. In Stage III (the ash phase), a strong relationship between d[Na]/dt and dTp/dt is evident. The drop in Tp during this stage is quite slow. This may be caused by weak exothermic reactions in the ash. The small non-zero derivative of diameter during this phase supports the deduction that some weak reactions are still occurring. The slow decrease in temperature during this phase leads to a slow decrease in the rate of Na release, but Tp is sufficient for significant release of sodium after the char combustion is complete. From this data we derive the following empirical correlations. For the char phase, [Na] correlates with Dp alone (Eq. (2)), while for the ash stage [Na] correlates with Tp alone (Eq. (3)). ½Nachar ¼ ð126:5=Dp Þ 20:42 R2 ¼ 0:95 10
½Naash ¼ 2:34 10 Fig. 5. Temperature measurement at a point 10 mm above the wire for the cases: no wire, wire only, 3.2 mm stainless steel ball bearing on the wire loop and 6.4 mm stainless steel ball bearing on the wire loop.
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R2 ¼ 0:95
ð2Þ
expð0:016 T p Þ ð3Þ
where [Na] is in ppb, Dp is in mm and Tp is in K. The Eqs. (2) and (3) have interesting conse-
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quences. Firstly, during the char combustion phase, the sodium release is proportional to the inverse Dp, and therefore the ratio of surface area, Ap to Vp (as noted earlier). During this stage the form of sodium in the coal is most likely sodium carbonate [5,8]. Hence this finding is consistent with the conversion of this form of sodium to gas phase atomic sodium being limited by Ap/ Vp. In addition Tp also increases as Ap/Vp increases. Hence, the controlling factor in the release of sodium during this stage is availability of the particle surface to the gaseous environment. Secondly, during the ash phase, the sodium release is exponentially dependent on Tp alone. That is, if the burned particle remains in a flame environment, sodium will continue to be released for as long as the particle remains in this environment and there is residual sodium in the ash (presumably of non-silicate form). In fact, van Eyk et al. [17] showed in a separate experiment that almost three times as much sodium was released during the ash stage as from the char combustion stage for a particle suspended infinitely in the flame. This implies that minimising the release of sodium during coal combustion requires removing particles from the flame as soon as combustion is completed. Next we seek to assess the regions in the plume over which the decay in [Na] with z is governed by chemical reactions, diffusion, or both. If a region exists in which the decay in [Na] along the centreline of the plume is governed by reactions, the following relationships will hold to first order: ln½Naz ¼ ln½Nap k R ðz Dp Þ
ð4Þ 1
where kR is the net scalar decay rate (mm ) due to the net effect of all chemical reactions that result in the overall decay of atomic sodium within the flame, [Na]z is the sodium concentration at axial height z above the wire, and [Na]p is the extrapolated concentration at the particle surface. This equation assumes that diffusion effects are negligible. In this way, using Eq. (4) and plotting ln[Na]z against z Dp (Fig. 7), enables the coefficients kR and [Na]p to be determined for every time-step (over z Dp < 20 mm, where diffusion effects are negligible). It is well established that the centreline concentration decay, by diffusion, of a non-reacting round jet or plume, scales with 1/z, (eg. Mi et al. [22]). Scaling the results in this way therefore allows any regions in which the decay of the plume is diffusion-controlled to be determined. Figure 7 presents both methods of scaling to identify the regions in the plume where either reactions or diffusion are dominant, at time, sb = 300 s. Figure 7(a) shows that for z Dp < 21 mm, a linear relationship exists between ln[Na]z and z Dp, over which kR=0.0607 mm1. Figure 7(b) presents [Na]p/[Na]z against z Dp at sb = 300 s. It is evident that, for z Dp > 21 mm, there is a linear decay in [Na]p/
Fig. 7. Plot of (a) ln[Na]z and (b) [Na]p/[Na]z versus z Dp for sb = 300 s, showing the regions controlled by chemical reactions and diffusion.
[Na]z. The combination of these results shows that there are two regions in the plume above this burning particle: a chemical reaction-dominated region and a diffusion-dominated region. This trend was found to be consistently true for the entire data set, although the transition height between the reaction-dominated and the diffusion-dominated regions, depends on [Na]z (ie. higher values of [Na]z cause higher transition values of z Dp). By analysing each centreline for sb = 50–910 s (ie. during char combustion) it is apparent that only the highest [Na] values lead to good predictions using the reaction-only model. In the regions with the lower [Na] during this time, the departure from this model suggests a significant role of diffusion. Nevertheless, as for the case at sb = 300 s a good fit to the experimental data was found close to the coal particle (z Dp < 20 mm) for all cases in the range sb = 50–910 s. Figure 8 allows us to assess how kR and [Na]p, as determined from the centreline data, vary with combustion time. By developing a simple kinetic model of this system, we have previously shown [17] that within the region where diffusion is negligible, the decay of atomic sodium is controlled by reactions to form NaOH, NaO and NaO2 alone. Thus determining the variation of kR with burn time allows the conversion of atomic Na to these other forms to be estimated. As can be seen, kR decreases slowly from 0.065 to 0.060 mm1 during most of the char combustion phase (50–800 s), but decreases more rapidly at the same time as there is a sharp increase in Tp (from 800–910 s). It reaches a minimum of 0.054 mm1 at the time where char combustion is complete (910 s). After this
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ticle can be characterised by two regimes. In the region close to the particle, the decay is characterised by first order chemical reaction, and beyond this by diffusion. The transition height between the regions depends on [Na] in the plume, but the extent of the reaction controlled region was at least 20 mm above the particle throughout the experiment, corresponding to about six times the initial particle diameter.
Acknowledgments
Fig. 8. Parameters kR and [Na]p as a function of sb.
time, kR is observed to increase and stabilise to around 0.060 mm1. The small increase in Tp with sb (as discussed earlier) accounts for the slight decrease in the value of kR with time, and indicates that the decay in [Na] in the plume decreases with temperature. The negative spike in kR at 800–910 s is postulated to be due to the additional influence of particle reactions in heating the plume (since the particle temperature was high at this time, Fig. 4). Subsequently, the value of kR stabilises due to the particle no longer heating the plume (ie. there is only ash left). 4. Conclusions The concentration of Na in the plume of a burning coal particle has been measured simultaneously with particle surface temperature, Tp, and particle diameter, Dp, for the first time. This is shown to yield internally consistent results and to provide powerful insights, even though the data is restricted to a single particle (due to the time required to perform the investigation). Firstly, it is clear that the emission of Na during the volatile stage is very much less than that released during the char and ash phases. That released from the hot ash can also be significant if Tp remains sufficiently high. Indeed, for this particle, Na release continued for 500 s after char combustion was complete. The release rate of Na is not constant through these phases, but depends both on the Tp and the rate of combustion of the particle. During the char phase, [Na] in the plume was found to be inversely dependent on Dp, while during the ash phase it was found to depend exponentially on Tp. This implies that the residence time of coal particles in a flame after the end of char combustion has a dramatic influence on the quantity of sodium released from a coal flame. Secondly, it is apparent that the centreline decay of [Na] in the plume above this burning par-
The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre for Clean Power from Lignite which was established under the Australian Government’s Cooperative Research Centres program. The project also received partial support from the Australian Research Council Discovery scheme. The paper has also been strengthened by addressing the insightful comments of the anonymous reviewers of the paper, for which the authors are also very grateful. References [1] R. Bryers, Prog. Energ. Combust. Sci. 22 (1996) 29– 120. [2] J.B. Murray, D.G. Evans, Fuel 51 (1972) 290, 1972. [3] H.N.S. Schafer, Fuel 58 (1979) 667–672. [4] M.E. Morgan, R.G. Jenkins, in: H.H. Schobert (Ed.), The Chemistry of Low Rank Coals, American Chemical Society, Washington, DC, 1984, pp. 213– 226. [5] J.B. Murray, R.C. Ledger, State Electricity Commission of Victoria, Planning and Investigations Dept. Scientific Div. Report No. 255, 1972. [6] G.N. Daybell, W.J.S. Pringle, Fuel 37 (1958) 283. [7] K.H. Brimsmead, R.W. Kear, Fuel 35 (1956) 84. [8] E.R. Lindner, PhD Thesis, The University of Newcastle, Australia, 1988. [9] L.J. Wibberley, T.F. Wall, Fuel 61 (1982) 87–92. [10] S. Srinivasachar, J.J. Helble, D.O. Ham, G. Domazetis, Prog. Energ. Combust. Sci. 16 (1990) 303–309. [11] G.R. Markowski, D.S. Ensor, R.G. Hopper, R.C. Carr, Environ. Sci. Technol. 14 (11) (1980) 1400– 1402. [12] D. Ounsted, J. Schoen, J. Inst. Fuel 33 (1960) 199– 206. [13] R.J. Quann, M. Neville, M. Janghorbani, C.A. Mims, A.F. Sarofim, Environ. Sci. Tech. 16 (11) (1982) 776–781. [14] P. Monkhouse, Prog. Energ. Combust. Sci. 28 (2002) 331–381. [15] B. Smith, J.D. Winefordner, N. Omenetto, J. Appl. Phys. 48 (7) (1977). [16] A.J. Hynes, M. Steinberg, K. Schofield, J. Chem. Phys. 80 (6) (1984) 2585–2597. [17] P.J. van Eyk, P.J. Ashman, Z.T. Alwahabi, G.J. Nathan, Combust. Flame (2008), doi: 10.1016/ j.combustflame.2008.05.012.
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[18] A. Williams, M. Paurkashanian, J.M. Jones, N. Skorupska, Combustion and Gasification of Coal, Taylor & Francis, New York, USA, 2000, p. 101. [19] J. Hulston, G. Favas, A.L. Chaffee, Fuel 84 (2005) 1940–1948.
[20] L.D. Timothy, A.F. Sarofim, J.M. Beer, Proc. Combust. Inst. 19 (1982) 1123–1130. [21] D.A. Tichenor, R.E. Mitchell, K.R. Henchken, S. Niksa, Proc. Combust. Inst. 20 (1984) 1213–1221. [22] J. Mi, D.S. Nobes, G.J. Nathan, J. Fluid Mech. 432 (2001) 91–125.
Proceedings of the
Proceedings of the Combustion Institute 32 (2009) 2099–2106
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Simultaneous measurements of the release of atomic sodium, particle diameter and particle temperature for a single burning coal particle P.J. van Eyk a,*, P.J. Ashman a, Z.T. Alwahabi a, G.J. Nathan b a
Cooperative Research Centre for Clean Power from Lignite, School of Chemical Engineering, The University of Adelaide, SA 5005, Australia b School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia
Abstract The temporal history of the release of volatile alkali species during coal combustion is a significant, but poorly understood factor in the fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. We present new results of the simultaneous measurement of particle temperature, particle size and the atomic sodium concentration in the plume of a burning coal particle. During the char phase, the sodium concentration in the plume was found to be linearly dependent on the inverse of particle diameter, but during the ash phase the sodium concentration was found to decay exponentially with decreasing particle temperature. The centreline decay of Na within the plume above the burning particle consists of one region controlled by a first order chemical reaction and a second region controlled by diffusion. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Sodium; Coal; LIF; Fouling
1. Introduction The release of impurities within coal during combustion is significant to fouling and corrosion of heat transfer surfaces within industrial coalfired boilers [1]. In particular, sodium (Na) species emitted from coal combustion have been shown to form the initial deposit which accommodates the subsequent build up of other ash particles. Elemental sodium in low-rank coal may occur in several forms. It may be water-bound (Na salts in solution), organically-bound (e.g. attached to car-
*
Corresponding author. Fax: +61 8 8303 4373. E-mail address: [email protected] (P.J. van Eyk).
boxylic acid groups), or bound within clay minerals. Sodium associated with carboxylic acid groups has been shown to decompose early in the devolatilisation stage of coal combustion [2– 4], to form sodium carbonate [5]. It has also been shown that sodium chloride decomposes in the early stages of coal combustion, with the chlorine forming HCl [6,7] and the sodium postulated to form sodium carbonate [8]. The clay bound sodium will not vaporise due to the low activity of silica melts [9]. Equilibrium calculations predict that atomic sodium is the favoured species within a flame environment, except when very large amounts of chlorine are also present [10]. In the post-flame gases, sodium chloride and sodium hydroxide are the principle sodium species [9]. However, the temporal history of these processes
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.07.038
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through the various stages of combustion remains poorly understood. Previous workers [11–13], have studied sodium release during coal combustion by analysing the bulk composition of fly ash and the various ash deposits from both full-scale and laboratory-scale combustors. Other studies have measured vapourphase species in the post-combustion gases in a large variety of systems using on-line measurements of the molecular sodium species in the flue gases (eg. NaOH and NaCl). Methods include both optical methods (absorption, spontaneous emission and induced fluorescence) as well as mass spectrometry [14]. While these techniques give insight into the final forms in the gaseous and deposit phases of the sodium released from the coal, it is not possible to infer from them the temporal history of sodium release directly from individual particles, or to identify the nature or concentration of the sodium intermediates. Other studies have utilised laser techniques to determine the concentration of sodium atoms within flame environments using saturated laser-induced fluorescence [15,16]. Although quantitative data can be obtained using saturated laser-induced fluorescence, the large laser power requirements only allow point measurements in the flame. An experimental method has been established in our laboratory [17] to allow the quantitative planar measurement of the release of sodium atoms during the combustion of a single coal particle. The laser-induced fluorescence (LIF) measurement is calibrated using laser absorption to provide an overall accuracy of ±5.7%. The sensitivity of these measurements was 0.04 ppb for a signal-to-noise ratio of 2 and 0.2 ppb for a signal-to-noise ratio of 10. The temporal response, set by the rep-rate of the laser and camera, was 5 Hz for that system. From this previous work [17], the release of Na was found to vary with particle burn time, sb, with the peak occurring near to the end of the char combustion phase. Also, the axial distribution of atomic Na concentration through the plume was found to decay due primarily to either diffusion or chemical reaction, depending on the proximity to the particle. However, that investigation did not relate the sodium measurements to char burn-out properties of the fuel. To accurately model the char burn-out of a given fuel, the char temperature, local oxygen concentrations, residence times within the reaction zone and time/temperature histories of each particle must be known [18]. To compute the release of alkali in a computationally efficient manner, it is highly desirable to scale it against other parameters that are modelled directly, such as particle diameter, Dp (and hence burn-out) and particle temperature. Furthermore, since each coal particle is different, the measurement of those parameters simultaneously with the sodium con-
centration is required for the development and validation of such ‘‘sub-models”. Given the limitations of previous measurements, this work aims to provide, for the first time, spatially- and temporally-resolved simultaneous measurements of three important combustion parameters for a single coal particle. These are the quantitative concentration of atomic sodium, the surface temperature of the coal particle and the coal particle size. While other quantitative measurements of Na have been performed, they have been at a single point, and have not been conducted simultaneously with these other parameters. Although care needs to be taken in extrapolating the results of a single experiment, it is inevitable that very detailed measurements can only be performed for limited number of cases. Also, there is substantial value in identifying what does happen in at least one case, and in demonstrating an approach by which such detailed data can be provided for other assessments. 2. Experimental methods Sodium atoms were measured in the plume of a single coal particle burning in the large, uniform region above a flat flame burner. The burner design is shown in Fig. 1. It contains an upper chamber into which air is introduced via side tubes. This passes through a packed bed and stainless steel honeycomb. Natural gas is fed through the bottom of the lower chamber into a manifold of 384 hypodermic tubes. These direct the fuel to the outlet of the burner through a grid of 17 17 fuel ports. The honeycomb is arranged so that each 1 mm diameter stainless steel fuel port is surrounded by six hexagonal air ports. A small blue cone is produced above each fuel port. The overall air to fuel ratio is fuel lean in this
Fig. 1. Schematic diagram of the flat flame burner (all lengths in mm).
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experiment (1.3 stoichiometric air requirement), and the excess air in the flame was used to burn the coal particle. A shroud of air around the central burner was utilised to stabilise the flame and prevent puffing. A tuneable dye laser (Lambda Physics Scanmate), pumped by a Nd:YAG laser (Coherent Brilliant B), was used to excite fluorescence. Figure 2 shows the laser and optical configuration. The dye laser was scanned around 588–590 nm to excite the D1 and D2 Na lines at 589.59 and 589.0 nm, respectively. Measurements were performed using the D1 line (589.59 nm) because strong beam absorption was observed when using the D2 line. The laser-induced fluorescence signal from the Na atoms was collected using a gated intensified CCD camera (Princeton Instruments ICCD576), which was aligned orthogonal to the laser sheet. Instantaneous images were collected at 2 Hz while the coal particle burned. Because of the very short gate width of the camera (10 ns), no spectral interferences were observed while collecting Na fluorescence. Thus no interference filters were used for the camera. A polarizing filter was used to reduce scattering of laser light from soot particles which occurred during the release of volatiles from the coal during the first stage of combustion. No other scattering from particles was observed after this stage. From our calibration experiments [17], it was determined that signal trapping was not significant with our experimental system. Similarly, saturation will only be reached for our system at considerably higher laser fluences than those used in our experiment. Hence, the trapping and saturation effects were neglected in our analysis. The flame was operated with natural gas and air flow rates of 5.0 and 60.6 L/min (STP), respectively. The shroud gas flow rate was adjusted to eliminate visible unsteadiness in the primary flame. A coal particle, weighing approximately 52.5 mg and 6.2 mm in diameter, was suspended 10 mm above the burner surface on a loop made from platinum wire of 0.5 mm diameter. The coal used for this test was Loy Yang, a Victorian
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Table 1 Loy Yang brown coal chemical analysis [19] % (dry ash free basis) C H N S O (by difference) % (dry basis) Ash Na Mg Ca Al Fe
68.2 4.8 0.63 0.27 26.1 0.8 0.09 0.08 0.04 0.02 0.07
brown coal. The sample of coal used in this study was obtained from the same sample as was used in a previous study by Hulston et al. [19], and the chemical analysis of this coal is given in Table 1. The coal particle used in this experiment was air-dried in an oven at 50 °C overnight. A parallel laser sheet was established by using cylindrical lenses (CL1 and CL2 in Fig. 2). The incident laser fluence for each image was determined by passing the sheet through a calibrated water cell. Each image provides a 2D array of fluorescence values in the plume above the particle, and was converted to absolute atomic sodium concentrations by using the calibration technique reported by van Eyk et al. [17]. The bottom of the imaged region was 7.5 mm above the wire, a limit set to prevent interference from the direct scattering from the particle, and the top of the imaged region was 41.5 mm above the wire. The coal particle temperature was measured using the well established two colour pyrometry technique [20,21]. Two SLR cameras (Canon 400D) were used, each fitted with a narrow bandwidth optical filter of 515 and 532.5 nm, respectively. The cameras were located according to Fig. 2. Recording from the two SLR cameras was started simultaneously as the flat flame was ignited below the suspended coal particle. This provided a measure of both temperature and diameter of the coal particle throughout the entire combustion process. Images were recorded in greyscale every 10 s to give 2D images of particle radiation at the two wavelengths. The surface temperature of the particle, Tp was determined by comparison of the intensity at two wavelengths (E515 and E532.5) at a location on the particle, using Wien’s equation: hc 1 k12 kT k1 ð1Þ T p ¼ k5 Ek1 2 ln eek1 5 =E k2 k k2 1
Fig. 2. Laser and detector set-up for measurement of atomic sodium, particle temperature and particle diameter.
where, Eki is the emissive power of the emitter at wavelength ki eki is the emissivity of the solid surface at wavelength ki, h is Planck’s constant, c is
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the speed of light and k is the Boltzmann constant. The measurement location, close to the bottom of the particle, was selected on a frame-by-frame basis, since the particle shape changes as the particle burns, to provide a feature that could be definitively identified in each of the two images. This makes the measurement a time-consuming process, which restricts the number of particles that can be realistically assessed in this way. The emissivity of the solid surface was assumed to be the same at both wavelengths, because the wavelengths chosen were close together, thus cancelling the ek1/ek2 term from Eq. (1). The intensities collected by each camera depend on the response of each CCD array to wavelength, and the transmission through each optical filter, so a calibration of the ratio E515/ E532.5 versus temperature was required. This calibration was achieved by measuring E515/E532.5 from the bead of a thermocouple suitably positioned at appropriate locations within the flame, while also directly measuring the thermocouple’s temperature. This was undertaken for temperatures from 1300–1500 K to obtain a linear correction for the value of E515/E532.5 from Eq. (1). The SLR images from one camera were also used to determine Dp as a function of combustion time. Here Dp was defined as the geometric mean of the diameters of the particle in the x and y directions. This was further used to estimate the particle volume, Vp. Since the particle chosen for investigation was approximately spherical, this approximation is reasonable. 3. Results and discussion Figure 3 presents a typical image of the atomic sodium concentration field in the plume of the burning particle. This image corresponds to an average of ±10 s of data on either side of the time of peak atomic sodium concentration. The differences between the instantaneous and averaged images are relatively small, due to the generally steady nature of the laminar flat flame, but averaging reduces these ‘‘short-term” (relative to the particle burn time) variations in release rate. It is clearly evident that the concentration decays axially with distance from the wire, z, and radially with distance from the axis, r. The general shape is consistent with the gases being evolved radially from the particle surface, but then being convected upward by buoyancy, following which the concentration decays by the combined influences of dilution and chemical reaction. The variation of atomic Na at a reference location in the plume, Tp and Vp are plotted against sb in Fig. 4. The reference location for the concentration of atomic Na was chosen near to the peak value on the axis at z = 10 mm. Each point in the figure is averaged from 10 s of 2 Hz instanta-
Fig. 3. 2D image of atomic sodium concentration, [Na], in the plume above a burning coal particle during peak sodium emission, where r is the radial distance from the particle and z is the axial distance from the wire; region: 7.5 mm < z < 41.5 mm above the wire and 10 mm < r < 10 mm from the wire.
neous data. As can be seen from Fig. 4, a short initial peak in the measured signal (100 ppb) was observed at sb 10 s. The signal during this period has previously been attributed mainly to interference caused by scattering from soot particles present in the flame during coal devolatilisation, which typically dominates over fluorescence [17]. However, even if the signal from Na fluorescence during this period is non-negligible, the short duration of this peak means that its contribution to total Na release is small. Following this, the sodium concentration increases gradually from several ppb to a large peak (60 ppb) at approximately 910 s. Over the same period, Tp increases gradually from around 1200 to 1600 K,
Fig. 4. Variation of particle temperature, Tp, particle volume, Vp and atomic sodium concentration, [Na] 10 mm directly above the wire, with sb.
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while Vp shrinks from around 60 to 1.5 mm3. Importantly the peak Na emission coincides with the peak temperature and the instant when the coal particle stops shrinking. Significantly also, the rapid rise in Na concentration (sb 800 s), also coincides with a rapid rise in Tp and a comparatively fast decrease in Vp. After the peak, both [Na] and Tp are observed to decay, while Vp remains constant. This latter stage obviously corresponds to the ash phase, since the low and almost constant Dp implies that char burn-out is completed. The decay in Tp asymptotes to that of the flat flame (1450 K), giving further confidence in the temperature measurement technique, and [Na] asymptotes to zero. It is clear that the presence of the wire and coal particle in the flat flame has an influence on the flame temperature, Tff, in the vicinity of the particle, and hence on Tp. Separate thermocouple measurements of the effect on Tff of inert objects placed at approximately the same location in the flame as the coal particle are presented in Fig. 5. With no objects in the flame, Tff was measured to be 1446 K, but reduces to 1413 K when the wire is also placed within the flame. Placing stainless steel balls bearings, of size 3.2 and 6.4 mm onto the wire, further reduces Tff to 1387 and 1349 K, respectively. This suggests the particle is cooled by radiation and in turn convectively cools the surrounding gases, thus lowering Tff in the plume. Therefore, the gradual increase in Tp shown in Fig. 4 could be due, at least in part, to variations in Tff as the coal particle shrinks during combustion. However, the relatively small temperature increase (64 K) when comparing Tff for the wire only to the 6.4 mm stainless steel ball bearing does not account for the large increase in Tp of 400 K. Thus, a substantial contribution to the increase in Tp may also be attributed to an increasing reaction rate at the particle surface during the latter stages of particle burn-out. The finding that Tp and sodium concentration, [Na], peak at the same time (910 s) indicates a strong temperature dependence on the release of sodium from the burning particle, which in turn
Fig. 6. Variation of (a) d[Na]/dsb 10 mm directly above the wire, (b) dDp/dsb and (c) dTp/dsb with combustion time.
depends on burning rate, and so Dp. These relationships are assessed by analysing the derivatives d[Na]/dt, dDp/dt and dTp/dt in Fig. 6. These derivatives were determined from the average slope of every 50 s of data. In Stage I (char phase), there is a strong relationship between all three derivatives. This suggests that as the particle burns (and Dp decreases), the rate of combustion increases, therefore causing Tp to rise and more Na is released. In Stage II (end of char phase), Dp decreases rapidly, corresponding to a rapid increase in Tp and release of Na. The external surface area per volume of a spherical particle is inversely proportional to Dp. Thus, the sharp increase in temperature and [Na] occurs at the time where there is a sharp increase in the external surface area per volume of the particle. In Stage III (the ash phase), a strong relationship between d[Na]/dt and dTp/dt is evident. The drop in Tp during this stage is quite slow. This may be caused by weak exothermic reactions in the ash. The small non-zero derivative of diameter during this phase supports the deduction that some weak reactions are still occurring. The slow decrease in temperature during this phase leads to a slow decrease in the rate of Na release, but Tp is sufficient for significant release of sodium after the char combustion is complete. From this data we derive the following empirical correlations. For the char phase, [Na] correlates with Dp alone (Eq. (2)), while for the ash stage [Na] correlates with Tp alone (Eq. (3)). ½Nachar ¼ ð126:5=Dp Þ 20:42 R2 ¼ 0:95 10
½Naash ¼ 2:34 10 Fig. 5. Temperature measurement at a point 10 mm above the wire for the cases: no wire, wire only, 3.2 mm stainless steel ball bearing on the wire loop and 6.4 mm stainless steel ball bearing on the wire loop.
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R2 ¼ 0:95
ð2Þ
expð0:016 T p Þ ð3Þ
where [Na] is in ppb, Dp is in mm and Tp is in K. The Eqs. (2) and (3) have interesting conse-
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quences. Firstly, during the char combustion phase, the sodium release is proportional to the inverse Dp, and therefore the ratio of surface area, Ap to Vp (as noted earlier). During this stage the form of sodium in the coal is most likely sodium carbonate [5,8]. Hence this finding is consistent with the conversion of this form of sodium to gas phase atomic sodium being limited by Ap/ Vp. In addition Tp also increases as Ap/Vp increases. Hence, the controlling factor in the release of sodium during this stage is availability of the particle surface to the gaseous environment. Secondly, during the ash phase, the sodium release is exponentially dependent on Tp alone. That is, if the burned particle remains in a flame environment, sodium will continue to be released for as long as the particle remains in this environment and there is residual sodium in the ash (presumably of non-silicate form). In fact, van Eyk et al. [17] showed in a separate experiment that almost three times as much sodium was released during the ash stage as from the char combustion stage for a particle suspended infinitely in the flame. This implies that minimising the release of sodium during coal combustion requires removing particles from the flame as soon as combustion is completed. Next we seek to assess the regions in the plume over which the decay in [Na] with z is governed by chemical reactions, diffusion, or both. If a region exists in which the decay in [Na] along the centreline of the plume is governed by reactions, the following relationships will hold to first order: ln½Naz ¼ ln½Nap k R ðz Dp Þ
ð4Þ 1
where kR is the net scalar decay rate (mm ) due to the net effect of all chemical reactions that result in the overall decay of atomic sodium within the flame, [Na]z is the sodium concentration at axial height z above the wire, and [Na]p is the extrapolated concentration at the particle surface. This equation assumes that diffusion effects are negligible. In this way, using Eq. (4) and plotting ln[Na]z against z Dp (Fig. 7), enables the coefficients kR and [Na]p to be determined for every time-step (over z Dp < 20 mm, where diffusion effects are negligible). It is well established that the centreline concentration decay, by diffusion, of a non-reacting round jet or plume, scales with 1/z, (eg. Mi et al. [22]). Scaling the results in this way therefore allows any regions in which the decay of the plume is diffusion-controlled to be determined. Figure 7 presents both methods of scaling to identify the regions in the plume where either reactions or diffusion are dominant, at time, sb = 300 s. Figure 7(a) shows that for z Dp < 21 mm, a linear relationship exists between ln[Na]z and z Dp, over which kR=0.0607 mm1. Figure 7(b) presents [Na]p/[Na]z against z Dp at sb = 300 s. It is evident that, for z Dp > 21 mm, there is a linear decay in [Na]p/
Fig. 7. Plot of (a) ln[Na]z and (b) [Na]p/[Na]z versus z Dp for sb = 300 s, showing the regions controlled by chemical reactions and diffusion.
[Na]z. The combination of these results shows that there are two regions in the plume above this burning particle: a chemical reaction-dominated region and a diffusion-dominated region. This trend was found to be consistently true for the entire data set, although the transition height between the reaction-dominated and the diffusion-dominated regions, depends on [Na]z (ie. higher values of [Na]z cause higher transition values of z Dp). By analysing each centreline for sb = 50–910 s (ie. during char combustion) it is apparent that only the highest [Na] values lead to good predictions using the reaction-only model. In the regions with the lower [Na] during this time, the departure from this model suggests a significant role of diffusion. Nevertheless, as for the case at sb = 300 s a good fit to the experimental data was found close to the coal particle (z Dp < 20 mm) for all cases in the range sb = 50–910 s. Figure 8 allows us to assess how kR and [Na]p, as determined from the centreline data, vary with combustion time. By developing a simple kinetic model of this system, we have previously shown [17] that within the region where diffusion is negligible, the decay of atomic sodium is controlled by reactions to form NaOH, NaO and NaO2 alone. Thus determining the variation of kR with burn time allows the conversion of atomic Na to these other forms to be estimated. As can be seen, kR decreases slowly from 0.065 to 0.060 mm1 during most of the char combustion phase (50–800 s), but decreases more rapidly at the same time as there is a sharp increase in Tp (from 800–910 s). It reaches a minimum of 0.054 mm1 at the time where char combustion is complete (910 s). After this
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ticle can be characterised by two regimes. In the region close to the particle, the decay is characterised by first order chemical reaction, and beyond this by diffusion. The transition height between the regions depends on [Na] in the plume, but the extent of the reaction controlled region was at least 20 mm above the particle throughout the experiment, corresponding to about six times the initial particle diameter.
Acknowledgments
Fig. 8. Parameters kR and [Na]p as a function of sb.
time, kR is observed to increase and stabilise to around 0.060 mm1. The small increase in Tp with sb (as discussed earlier) accounts for the slight decrease in the value of kR with time, and indicates that the decay in [Na] in the plume decreases with temperature. The negative spike in kR at 800–910 s is postulated to be due to the additional influence of particle reactions in heating the plume (since the particle temperature was high at this time, Fig. 4). Subsequently, the value of kR stabilises due to the particle no longer heating the plume (ie. there is only ash left). 4. Conclusions The concentration of Na in the plume of a burning coal particle has been measured simultaneously with particle surface temperature, Tp, and particle diameter, Dp, for the first time. This is shown to yield internally consistent results and to provide powerful insights, even though the data is restricted to a single particle (due to the time required to perform the investigation). Firstly, it is clear that the emission of Na during the volatile stage is very much less than that released during the char and ash phases. That released from the hot ash can also be significant if Tp remains sufficiently high. Indeed, for this particle, Na release continued for 500 s after char combustion was complete. The release rate of Na is not constant through these phases, but depends both on the Tp and the rate of combustion of the particle. During the char phase, [Na] in the plume was found to be inversely dependent on Dp, while during the ash phase it was found to depend exponentially on Tp. This implies that the residence time of coal particles in a flame after the end of char combustion has a dramatic influence on the quantity of sodium released from a coal flame. Secondly, it is apparent that the centreline decay of [Na] in the plume above this burning par-
The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre for Clean Power from Lignite which was established under the Australian Government’s Cooperative Research Centres program. The project also received partial support from the Australian Research Council Discovery scheme. The paper has also been strengthened by addressing the insightful comments of the anonymous reviewers of the paper, for which the authors are also very grateful. References [1] R. Bryers, Prog. Energ. Combust. Sci. 22 (1996) 29– 120. [2] J.B. Murray, D.G. Evans, Fuel 51 (1972) 290, 1972. [3] H.N.S. Schafer, Fuel 58 (1979) 667–672. [4] M.E. Morgan, R.G. Jenkins, in: H.H. Schobert (Ed.), The Chemistry of Low Rank Coals, American Chemical Society, Washington, DC, 1984, pp. 213– 226. [5] J.B. Murray, R.C. Ledger, State Electricity Commission of Victoria, Planning and Investigations Dept. Scientific Div. Report No. 255, 1972. [6] G.N. Daybell, W.J.S. Pringle, Fuel 37 (1958) 283. [7] K.H. Brimsmead, R.W. Kear, Fuel 35 (1956) 84. [8] E.R. Lindner, PhD Thesis, The University of Newcastle, Australia, 1988. [9] L.J. Wibberley, T.F. Wall, Fuel 61 (1982) 87–92. [10] S. Srinivasachar, J.J. Helble, D.O. Ham, G. Domazetis, Prog. Energ. Combust. Sci. 16 (1990) 303–309. [11] G.R. Markowski, D.S. Ensor, R.G. Hopper, R.C. Carr, Environ. Sci. Technol. 14 (11) (1980) 1400– 1402. [12] D. Ounsted, J. Schoen, J. Inst. Fuel 33 (1960) 199– 206. [13] R.J. Quann, M. Neville, M. Janghorbani, C.A. Mims, A.F. Sarofim, Environ. Sci. Tech. 16 (11) (1982) 776–781. [14] P. Monkhouse, Prog. Energ. Combust. Sci. 28 (2002) 331–381. [15] B. Smith, J.D. Winefordner, N. Omenetto, J. Appl. Phys. 48 (7) (1977). [16] A.J. Hynes, M. Steinberg, K. Schofield, J. Chem. Phys. 80 (6) (1984) 2585–2597. [17] P.J. van Eyk, P.J. Ashman, Z.T. Alwahabi, G.J. Nathan, Combust. Flame (2008), doi: 10.1016/ j.combustflame.2008.05.012.
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