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The effect of potassium bromide and sodium carbonate on coal char combustion reactivity
Proceedings of the
Proceedings of the Combustion Institute 30 (2005) 2187–2195
Combustion Institute www.elsevier.com/locate/proci
The effect of potassium bromide and sodium carbonate on coal char combustion reactivity Alejandro Molina*, Jeffrey J. Murphy, Christopher R. Shaddix, Linda G. Blevins Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA
Abstract The addition of halogens, particularly iodine, to the gas during coal char oxidation has been used in previous studies to quench gas-phase chemistry, thereby allowing one to separate the effects of homogeneous and heterogeneous reactions. Halogen addition suppresses the gas-phase radicals to near-equilibrium levels. A similar effect can be expected from other compounds with high efficiency as fire suppressants, such as alkali metals. The effectiveness of the use of additives in distinguishing homogeneous and heterogeneous reactions during char oxidation relies on the assumption that radicals are suppressed while heterogeneous reactions occurring on the char surface are not affected. The present work tests this assumption for potassium bromide (KBr) and sodium carbonate (Na2CO3) reacting with a pulverized eastern bituminous coal char during oxidation. An increase in CO and a slight reduction in particle temperature were observed with the addition of KBr, consistent with known effects of halogens on gas-phase chemistry. An increase in particle size was also observed with the KBr addition. This observation and the results of model calculations suggest that there is significant incorporation of liquid KBr on the char surface under the conditions examined. With Na2CO3 addition, the particle temperature did not change, the particle size showed a slight decrease, and CO production increased. Although the mechanisms for Na interaction with radicals at combustion conditions are not well established, char oxidation modeling suggests that a decrease in OH concentration in the particle boundary layer is the cause for the observed increase in CO production. It is concluded that Na2CO3 has clear advantages over KBr for inhibiting gasphase chemistry without affecting char oxidation for the conditions investigated here. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Char; Coal; Reactivity
1. Introduction The ability of halogens to quench gas-phase radical reactions has proven to be a good tool for partitioning the effects of homogeneous and heterogeneous reactions during coal char oxida*
Corresponding author. Fax: +1 925 294 2276. E-mail address: [email protected] (A. Molina).
tion. The effect of halides during solid fuel combustion was studied as early as 1949, when Day [1] found that 2% chlorine extinguished the combustion of petroleum coke at 2000 K. More recently, Bulewicz et al. [2] described the increase in CO concentration after the addition of sodium halides and CF2ClBr during fluidized bed coal combustion. Later [3], the same researchers found that iodine was an effective additive for the inhibition of CO oxidation. The authors explained the
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.219
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increase in CO concentration by the ability of halogens to catalyze radical recombination, reducing radical levels from the superequilibrium values that occur during fluidized bed coal combustion. Based on this influence of I2 on radical recombination, Winter et al. [4] applied iodine addition in the study of N2O formation during char oxidation. In Winters experiments, the addition of I2 had a strong effect on nitrogen-containing compounds: NO concentration decreased, N2O production ceased, and HCN concentration doubled. All of these variations were explained by the ability of iodine to inhibit radical reactions [5–7]. The effect of halogens during the cofiring of coal with industrial wastes containing elevated halogen content was also studied [8]. The concern was that the presence of halogens in the coal and waste could increase CO production. Although the authors found considerable CO increase after HCl and CaBr2 Æ 1/2 H2O were added to a fluidized bed boiler, the extended residence time in the system guaranteed complete CO oxidation. Halogen reactions have been extensively studied in the field of fire suppression. The mechanism involving the quenching of gas-phase radical chemistry can be traced back to Westbrooks pioneering work [9]. Since then, several other studies of halogen fire suppression have appeared in the literature, e.g. [10]. With the concern about ozone-depletion by halogens, recent studies of flame suppression [11–13] have focused on the use of alternative materials. Sodium carbonate (Na2CO3) is one possible substitute with reported inhibitor effectiveness 10 times that of Halon 1301 (CF3Br) [12]. In fact, the ability of Na to affect radical chemistry during combustion has prompted its use in the control of nitrogen oxide [14–16] and nitrous oxide [17] emissions. Despite the proven ability of halogens and alkali metals as radical scavengers, their success as a tool for distinguishing homogeneous and heterogeneous processes depends on their ability to affect the homogeneous chemistry but not the heterogeneous reactions. In the aforementioned study of iodine addition under fluidized bed combustion conditions, it was found that devolatilization and char burnout times did not change after the halogen addition [4]. Although these observations are consistent with the hypothesis that halogen addition does not significantly affect heterogeneous chemistry, more direct evidence is desirable. Additionally, no work examining the effect of radical-scavenging additives during conditions relevant to pulverized coal combustion has been published. This paper examines the effect of the addition of a halide, potassium bromide, and an alkali metal, sodium, on homogeneous and heterogeneous reactions during oxidation of pulverized coal char. We inject coal with and without potassium bromide (KBr) and sodium carbonate (Na2CO3) into an entrained-flow reactor. These two com-
pounds have advantages over halogens such as iodine because they can be introduced in the solid phase and they have lower laboratory hazard ratings than iodine. Neither of these compounds has been examined as a radical suppressant during coal combustion previously. Studies on the use of KBr and Na2CO3 in fire suppression are limited [12,18–20]. 2. Experimental 2.1. Sample preparation Two eastern US high-volatile bituminous coals (EB1 and EB2) were used in this study. Table 1 presents the proximate and ultimate analysis of each coal and of the char produced from EB1 in the entrained-reactor described below. The char composition is used as input for modeling. EB1 was obtained from CANMET Energy Technology Center. EB2 corresponds to coal from a pulverized coal power plant. Both coals were pulverized to a commercial grind and sieved to a 106–125 lm particle size fraction. The sieving was performed at CANMET for EB1 and at Sandia for EB2. The similarity between the two coals (see Table 1) suggests that results obtained using KBr and EB1 can be compared to those obtained using Na2CO3 and EB2. KBr was ground, and the 270–400 mesh fraction (38–53 lm) was gently mixed with the EB1 in a mortar to yield a 70%w KBr mixture. Na2CO3 was also ground, and the fraction passing 140 mesh (106 lm) was gently mixed with EB2 to yield a 50%w Na2CO3 mixture. This value (compared to 70% for KBr) was used to prevent clogging of the feeding line. The mixtures were stirred until they were visually homogeneous. 2.2. Reactivity (temperature) measurements The coal combustion experiments were performed in Sandias optical entrained-flow reactor, whose general design and operation have been described in the literature [21]. A schematic of the Table 1 Fuel composition EB2
EB1 char
Ultimate (dry) (%) C 77.33 H 5.08 O 6.29 N 1.45 S 0.96
EB1
74.67 4.77 10.08 1.44 2.16
74.55 0.19 0.02 1.24 0.70
Proximate (%) Dry loss Ash Vol. matter Fixed C
1.69 8.56 34.25 55.50
0.40 22.9 7.50 69.20
0.75 8.82 34.91 55.52
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Fig. 1. Schematic of the entrained-flow reactor, collection probe, and pyrometer.
facility is shown in Fig. 1. The reactor operates at one atmosphere and uses a diffusion-flameletbased Hencken burner. A particle-sizing pyrometer is used to simultaneously measure the velocity, diameter, and temperature of individual burning char particles at selected heights within the reactor. Temperature is calculated using a Wiens law ratio of emission signals detected through 40 nm FWHM bandpass filters with center wavelengths of 550 and 700 nm. Size is determined from the geometry of the coded aperture. The particle temperature measurement uncertainty is estimated to be better than ±1% using a linearized version of Weins law coupled with a noise estimate from the recorded signal. The size measurement is subjected to a much higher measurement uncertainty, typically ±20%. Reactor oxygen concentrations of 8, 12, and 24 mol% were investigated. Higher oxygen concentrations are of interest because of their use in oxygen-enhanced combustion and also because they alter the relative importance of oxygen diffusion and kinetics [22]. Nominal gas temperatures and gas compositions for these conditions are shown in Table 2. Gas temperature within the reactor was measured with a 76 lm, type R beaded thermocouple and then corrected for radiative losses [23]. Estimated gas temperature measurement uncertainty was ±50 K.
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KBr experiments, the probe was inserted into the reactor at a height of 20.4 and 10.2 cm for the 1200 and 1400 K conditions, respectively. For the Na2CO3 experiments, the probe was located at 7.6 cm above the burner. A gas analyzer was used to draw gas at a rate of 0.4 L min1 (300 K, 101 kPa) through the probe. Char particles were removed from the gas stream using filters, and water was removed using a desiccant. The gas analyzer provides measurements of O2, CO2, and CO concentrations, using a zirconium oxide sensor, a pyroelectric element sensor, and a non-dispersive infrared spectroscopy (NDIR) analyzer, respectively. CO concentrations were normalized by the amount of carbon due to char combustion (CO plus CO2 with the background CO2 from the burner subtracted). This provided a measurement that was relatively insensitive to the rate of flow of char in the reactor. The difference in the probe location for the different experiments and the finite sample quench times within the water-cooled probe limit the use of the measured gas composition to a qualitative analysis. Figure 2 shows a typical gas concentration profile for injection of coal only into the reactor. Following the disturbances that occur after injection
2.3. Gas composition measurements Extractive gas sampling was accomplished using a water-cooled collection probe (see Fig. 1). For the
Fig. 2. Typical traces from the gas analyzer during injection of coal into the 1200 K, 12% oxygen environment.
Table 2 Summary of the test conditions showing nominal gas temperature and molar concentrations in the gases in percent Nominal T (K)
O2 (%)
CO2 (%)
H2O (%)
N2 (%)
Compound
1200 1400 1400 1400
12 12 24 8.1
1.1 2.4 2.4 3.0
13.1 14.2 14.3 14.8
73.8 71.4 59.3 74.1
KBr KBr KBr Na2CO3
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starts, steady-state conditions are obtained. The data discussed below were obtained during this steady-state period (analysis interval) in Fig. 2.
3. Model Modeling was performed with the University of Sydneys Skippy (Surface Kinetics in Porous Particles) computer program [24]. This program calculates steady-state species and temperature profiles for the reaction of a porous solid in a reacting gaseous environment. Skippy predicts species concentrations and temperature within the char pores, at the outer char surface, and within the boundary layer surrounding the char. Both heterogeneous and homogeneous reactions are considered. The reaction mechanism developed by Glarborg et al. [25] was used for the gas-phase chemistry. This mechanism includes moist CO oxidation as well as nitrogen chemistry. The bromine submechanism was based on the work from two sources: the first is from Princeton [26] regarding the effects of HCl on postflame chemistry, and the second was compiled by the National Institute of Standards and Technology (NIST) [27] to model Halon suppression of fires. The reactions in the NIST mechanism (with reactions involving fluorine-containing compounds removed) are identical to the first 55 reactions in the Princeton mechanism (with bromine replacing chlorine). Thus, these reactions, with the rates from NIST, are included in our submechanism. The Princeton mechanism contains two additional reactions involving interactions of bromine and HCO (reactions 56 and 57 in [26]); these reactions are included in our sub-mechanism with the (chlorine) rates from [26]. Reactions 84–90 from the Princeton mechanism, which describe the interactions of HCl with NO, are also included in our mechanism, again with bromine substituted for chlorine. The rates for bromine versions of these reactions were taken from various sources in the literature [28–32] where available. Otherwise, the chlorine rates from [26] were used. The potassium submechanism is the five-step mechanism proposed by Slack et al. [33], with the recommended rate coefficients from the later work of Benilov et al. [34]. Finally, the KBr production reaction studied by Husain and Lee [35] was included. The submechanism for Na2CO3 includes the mechanism described by Zamasky et al. [16], which is based on that proposed by Perry and Miller [17]. Although some of the reactions for Na have been measured as a result of their importance in atmospheric chemistry [36–39] and in some cases extended to combustion temperatures [40], some of the kinetic parameters in the Na submechanism are highly uncertain.
The heterogeneous mechanism follows that described by Molina et al. [41]. Since the experiments were carried out in a region where KBr exists in the liquid phase (KBrmp = 1007 K and KBrbp = 1708 K), the volatilization of this species was included in the model. The rate constant for this process is estimated from the reverse reaction (condensation), which is assumed to have unity accommodation coefficient and therefore proceeds at its collision rate. At the temperatures used in the experiment (1400 K), equilibrium calculations [42] show that most of the Na is present as NaOH and atomic Na. A similar conclusion was obtained by Zamanzky et al. [16]. Therefore, for the simulations including Na2CO3, the boundary condition was specified as a constant NaOH concentration. 4. Results 4.1. Gas analysis Gas analysis results are shown in Fig. 3. These results show that the addition of KBr or Na2CO3 to the system causes a marked increase in the amount of CO detected. This increase confirms that KBr and Na2CO3 are interfering with CO oxidation. 4.2. Reactivity and particle size Figure 4 shows temperature and size measurements for EB1 and KBr injected into the 1200 K, 12% oxygen environment. There is a small (40 K) but statistically significant decrease in temperature when the KBr is added. There is also a large (20–30%) increase in particle size. These
Fig. 3. Comparison of CO/carbon ratio determined from gas analysis. The uncertainty intervals show the range of one standard deviation of the measured distribution.
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Fig. 4. Comparison of measured particle temperatures and sizes for EB1 in the 1200 K, 12% O2 environment. The vertical lines represent 95% confidence limits of the mean temperatures and sizes. The points are offset from each other on the x-axis for readability.
trends were also observed for coal and KBr injected into 1400 K gas with 12% oxygen and 24% oxygen. The difference in size is smaller, however. The sizes measured for coal at 24% oxygen are about 8% larger with KBr than without. Figure 5 shows the variation in temperature and size for EB2 with Na2CO3. Contrary to the results for KBr, the variation in temperature is with-
Fig. 5. Comparison of measured particle temperatures and sizes for EB2 in the 1400 K, 8% O2 environment. The vertical lines represent 95% confidence limits of the mean temperatures and sizes. The points are offset from each other on the x-axis for readability.
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Fig. 6. Comparison of measured particle temperatures and sizes for different coals and conditions. The vertical lines represent 95% confidence limits of the mean temperatures and sizes.
in the experimental uncertainty. There is also a minor reduction in particle size that borders the experimental uncertainty. Figure 6 presents a summary of the particle size and temperature measurements for all four test conditions in Table 2. 5. Discussion The first puzzling result in Figs. 4–6 is the increase in particle size when KBr is present. Since the boiling point of KBr is 1708 K, one can expect the presence of KBr aerosols and possibly liquid KBr addition to the char particle surface. If this interaction is taking place, then it may explain the increase in particle size observed in Figs. 4 and 6. One way to verify this hypothesis is to compute the fraction of liquid KBr on the char that would increase particle size by the amount observed in Figs. 4 and 6. Using the density of KBr at its vaporization temperature (1.96 g cm3) [43], a char density of 0.7 g cm3, and measured values for the moisture and volatile content of the coal, the predicted mass fractions of KBr in the coal/ KBr mixture are 0.7, 0.5, and 0.3 for 1200 K and 12% O2, 1400 K and 12% O2, and 1400 K and 24% O2, respectively. These values are equal to or below the mass fraction of KBr in the original mixture (0.7). Furthermore, the calculated amount of KBr on the char particles decreases as the particle temperature increases. This is consistent with the hypothesis that liquid KBr exists on the char surface. As the particle temperature
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increases, the rate of KBr vaporization increases, decreasing the amount of KBr on the char surface. Figure 5 shows that the particle size measured is slightly smaller when Na2CO3 is present. This difference could be attributed to a small decrease in particle size during the mixing procedure. To completely explain the data, it is necessary to find a reason for the reduction in particle temperature when KBr is present and for the invariance of temperature when Na2CO3 is added. For KBr, we identify three possibilities. The first is that the reduction in temperature is solely due to the difference in particle size, which, in turn, is caused by some factor unrelated to KBr. The second possibility is that the particle temperature is reduced by the loss of heat due to the vaporization of KBr on the char surface. Finally, it is possible that the reduction in particle temperature is due to a decrease in the heat generated in the boundary layer surrounding the particle by the conversion of CO to CO2. The presence of bromine and potassium in the system inhibits this conversion. To examine these three possibilities, four different cases (see Table 3) were analyzed using Skippy. Case 0, the baseline case, corresponds to the combustion of char at a height of 15.2 cm with a gas temperature of 1388 K (the measured gas temperature at this height) and gas composition as described in Table 2 for the 12% O2 case. The char particle diameter was 116 lm. In Case A, the particle is assumed to contain no KBr, but the larger particle diameter measured with KBr present (135 lm) was used. In Case B, char oxidation is calculated with KBr(l) present in the particle with the mass fractions previously computed. Finally, Case C uses the same conditions as Case B, but with the homogeneous chemistry of bromine and potassium removed from the mechanism. Figure 7 shows Skippys predictions for the variations of particle temperature, OH concentration, and the ratio of the molar flux of CO to the sum of the molar fluxes of CO and CO2 with radial distance from the particle center. This last variable can be compared to the data in Fig. 3. An examination of Fig. 7 shows that although there is a reduction in particle temperature as the particle size increases from 116 to 135 lm
Table 3 Cases for simulation of char oxidation experiments in the presence of KBr Case
Diameter (lm)
KBr (%w)
Br and K homogeneous reactions
0 A B C
116 135 135 135
0 0 70 70
Present Present Present Removed
Fig. 7. Variation of temperature, OH mole fraction, and ratio of CO flux to CO + CO2 flux as predicted by Skippy for Cases 0, A, B, and C (see Table 3 for description). Gas temperature was 1400 K, with 12% O2.
(Case 0 vs. Case A), it is insignificant and not comparable to that observed in the experiment. Conversely, the presence of KBr in a char/KBr mixture (Cases B and C) reduces the particle temperature by an amount similar to that observed in the experiments. There is no difference in the predicted particle temperature between cases B and C. The simulation results therefore suggest that the difference in temperature observed in Figs. 4 and 6 can be attributed to the presence of KBr on the char surface. Figure 7 also suggests that KBr affects the CO/CO2 balance by decreasing the OH concentration. However, the model suggests that the reduction in CO oxidation has an insignificant effect on particle temperature at the conditions of the experiment. Skippy was also used to explain the increase in CO ratio obtained during Na2CO3 addition. The concentration of NaOH estimated from the solid feeding rate (0.014 g m1) and the total gas flow was 48 ppm. This value assumes complete diffusion of the chemical compounds introduced at the burner center through the reactor cross-section. The actual NaOH concentration at the reactor centerline is greater than 48 ppm due to diffusion restric-
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tions. An upper bound for NaOH concentration was estimated based on the ratio of the central core flow and reactor cross-sectional areas. The core flow diameter was determined to be 1.4 cm from a photograph showing visible chemiluminescent emission of Na*. This value gives an area ratio of 0.06 and therefore an upper bound for NaOH concentration of 760 ppm. For the present simulation, a value of 480 ppm was assumed. Following the methodology used with KBr, Skippy simulations were carried out with and without NaOH present in the background. The predicted ratio of CO/carbon decreased 11% at a radial distance of 10 times the particle radius when NaOH was present in the system. This contradicts the experimental results (Fig. 3) that show an increase in CO when Na2CO3 was injected. The predicted increase in CO oxidation is surprising, considering that the model predicts a reduction in OH mole fraction of 50% due to the presence of Na in the system. A detailed analysis of the rate of reaction shows that the two major causes of CO reduction after the oxidation with OH are: NaO2 þ CO $ NaO þ CO2
ðR:1Þ
NaO þ CO $ Na þ CO2
ðR:2Þ
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The use of a computational particle combustion model as well as analysis of the particle size change suggested that liquid KBr was incorporated on the char surface and that the presence of liquid KBr on the surface was responsible for the reduction in particle temperature. The effects of KBr on char reactivity appeared to be insignificant; however, future work with different halogen compounds that do not condense in the temperature regime of char oxidation should be conducted before drawing final conclusions. The addition of Na2CO3 to char combustion did not affect the coal char reactivity, as evaluated from changes in particle temperature. This fact and the lower laboratory safety risks of sodium carbonate compared to halides are good reasons to use Na2CO3 to quench homogeneous reactions without affecting heterogeneous reactions during pulverized coal combustion experiments. However, the uncertainty in the kinetics of Na chemistry at combustion conditions poses an obstacle for a detailed explanation of its effect on homogeneous reactions.
Acknowledgments In the mechanisms used for the simulations [16,17], R.1 and R.2 were assumed to have a pre-exponential factor of 1014 cm3 mol1 s1. If this value is reduced to 1013 cm3 mol1 s1, the model predicts an increase in the ratio of CO/carbon of 15%, which agrees with the experimental trend. It is clear that the uncertainties in the mechanism of Na interaction with radicals during combustion prevent a complete description of the process by which the addition of Na2CO3 to the system produced an increase in CO production. However, the predictions suggest that Na2CO3 reduces the radical concentration as halogens do and that it does not affect particle reactivity. Sodium carbonate also has lower health risks associated with its use than potassium bromide. Thus, while both Na2CO3 and KBr have ease-of-delivery and safety advantages over the previously used gaseous I2, Na2CO3 is better than KBr for inhibiting gas phase chemistry during coal combustion at the conditions of the present experiments. 6. Conclusions Experiments examining the effect of KBr addition on char oxidation showed an increase in the CO/carbon ratio, a decrease in particle temperature, and an increase in particle size. The effect of Na2CO3 addition also showed an increase in the CO/carbon ratio, but without significant changes in temperature or particle size. Hence, both KBr and Na2CO3 affected homogeneous chemistry by suppressing radicals.
This research was sponsored by the US Department of Energy through the National Energy Technology Laboratorys Power Systems Advanced Research Program. Professor Franz Winter (Vienna University of Technology) provided ideas and useful discussions regarding the interaction of halogens with coal char. We thank Professor Brian S. Haynes (University of Sydney, Australia) for providing us with Skippy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energys National Nuclear Security Administration under Contract DEAC04-94-AL85000. References [1] R.J. Day, Kinetics of the Carbon–Oxygen Reaction at High Temperatures, Ph.D. thesis, Pennsylvania State College, Pennsylvania, USA, 1949, p. 187. [2] E.M. Bulewicz, E. Janicka, S. Kandefer, in: International Conference on Fluidized Bed Combustion: FBC—Technology for Today. American Society of Mechanical Engineers (ASME), San Francisco, CA, USA, 1989, pp. 163–168. [3] E.J. Anthony, E.M. Bulewicz, F. Preto, L. Rubow, The effect of halogens on FBC systems, in: International Conference on Fluidized-bed Combustion. American Society of Mechanical Engineers (ASME), San Diego, CA, USA, 1993, pp. 41–52. [4] F. Winter, C. Wartha, G. Lo¨ffler, H. Hofbauer, Proc. Combust. Inst. 26 (1996) 3325–3334. [5] D.Y. Lu, E. Anthony, R. Talbot, F. Winter, G. Lo¨ffler, C. Wartha, Energy Fuels 15 (3) (2001) 533–540.
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[6] F. Winter, G. Lo¨ffler, C. Wartha, H. Hofbauer, F. Preto, E.J. Anthony, Canad. J. Chem. Eng. 77 (2) (1999) 275–283. [7] F. Winter, C. Wartha, H. Hofbauer, The relative importance of radicals on the N2O, and NO formation and destruction paths in a quartz CFBC, in: International Conference on Fluidized Bed Combustion. American Society of Mechanical Engineers (ASME), 1997, pp. 1131–1137. [8] S. Julien, C.M.H. Brereton, C.J. Lim, J.R. Grace, E.J. Anthony, Fuel 75 (14) (1996) 1655–1663. [9] C.K. Westbrook, Combust. Sci. Technol. 34 (1-6) (1983) 201–225. [10] V. Babushok, W. Tsang, A. Hamins, D.R.F. Burgess, T. Noto, Combust. Flame 107 (4) (1996) 351–367. [11] V. Babushok, W. Tsang, G. Linteris, D. Reinelt, Combust. Flame 115 (4) (1998) 551–560. [12] V. Babushok, W. Tsang, Combust. Flame 123 (4) (2000) 488–506. [13] T.M. Jayaweera, Combust. Flame, submitted for publication. [14] V.M. Zamansky, L. Ho, P.M. Maly, W. Randall Seeker, Proc. Combust. Inst. 26 (1996) 2075–2082. [15] V.M. Zamansky, M.S. Sheldon, P.M. Maly, Proc. Combust. Inst. 27 (1998) 3001–3008. [16] V. Zamansky, V. Lissianski, P. Maly, L. Ho, D. Rusli, W. Gardiner, Combust. Flame 117 (4) (1999) 821–831. [17] R.A. Perry, J.A. Miller, Int. J. Chem. Kinet. 28 (3) (1996) 217–234. [18] M. Dewitte, J. Vrebosch, A. van Tiggelen, Combust. Flame 8 (4) (1964) 257–266. [19] T. Mitani, T. Niioka, Combust. Flame 55 (1) (1984) 13–21. [20] W.M. Pitts, W. Tsang, W.G. Mallard, R.G. Gann, M.R. Nyden, Construction of an Exploratory List of Chemicals to Initiate the Search for Halon Alternatives, Technical note (Final), Report No. NIST/TN1279, National Institute of Standards and Technology, 1990. [21] R.E. Mitchell, Combust. Sci. Technol. 53 (2-3) (1987) 165–186. [22] C.R. Shaddix, J.J. Murphy, Coal char combustion reactivity in oxy-fuel applications, in: 20th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, USA, 2003. [23] C.R. Shaddix, in: National Heat Transfer Conference NHTC99 33rd. American Society of Mechanical Engineers, Albuquerque, NM, USA, 1999, p. 1150. [24] P.J. Ashman, B.S. Haynes, Improved Techniques for the Prediction of NOx Formation from Char Nitro-
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Comments Brian Haynes, University of Sydney, Australia. If your particles are coated with KBr, that should be observable under a visible microscope. Have you checked for this? In addition, a KBr cladding should have a significant effect on the optical properties needed for the two-color pyrometric technique. Could you please comment on possible errors in your measured temperature due to such effects? Reply. The evidence presented in the paper strongly suggests the incorporation of liquid KBr on the char sur-
face. SEM or optical microscope analysis of the particles would have given additional evidence of the presence of KBr on the char surface; however, we unfortunately did not collect any solid samples during the experiments. The particle-sizing technique employed in the experiments only depends on the ratio of measured intensity through two different-size apertures on a coded mask at one wavelength (700 nm) [1]. Therefore, it is independent of any changes in the optical properties due to the presence of KBr on the char surface. The accuracy of temperatures derived from the two-color pyrometry technique depends
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only on the ratio of particle emissivities at the two detection wavelengths, assumed here to be equal to one [1]. Available data on the optical properties of KBr at visible wavelengths show that its optical properties are invariant in this spectral range, suggesting that its influence on temperature measurements should be negligible.
surrounding each coal particle, resulting in diffusional transport of KBr vapor back towards the char particle, where it condenses and deposits (with thermophoresis driving the condensed particles towards the char surface).
Reference
Peter Ashman, University of Adelaide, Australia. It seems that two compounds have been selected, Na2CO3 and KBr, with the intention of suppressing the homogeneous chemistry. However, these compounds have both displayed other behavior that has masked this intended effect. Can the authors suggest other compounds that might be less problematical?
[1] D.A. Tichenor, S. Niksa, K.R. Hencken, R.E. Mitchell, Proc. Combust. Inst. 20 (1984) 1213–1221. d
Tom Fletcher, Brigham Young University, USA. I have a hard time understanding the KBr transfer mechanism. It is too low of a temperature to vaporize. If it only melts, it must collide. Usually, particle number densities were kept low in the Sandia CCL in order to avoid collisions and ensure single particle behavior. Please explain. Reply. In the sample preparation process, KBr and Na2CO3 were gently mixed with coal at a 70% and 50% weight fraction respectively until they were visually homogeneous. The KBr was ground and then sieved to a 38–53 lm size fraction before mixing with the coal (itself sieved to 106–125 lm). Therefore, some KBr powder undoubtedly coated the coal before feeding. A relatively dilute loading of particles was produced within the reactor, as you suggest, so that particle–particle interactions are minor if not negligible. However, volatilization of a portion of the KBr particles occurs in the volatile flame
d
Reply. The selection of compounds that suppress the homogeneous chemistry during char combustion but do not affect the heterogeneous chemistry is not a simple task. As the paper shows, compounds that do not convert to gaseous phase at injection conditions can affect the heterogeneous chemistry. Furthermore, some halogen-containing materials convert to toxic and corrosive halides during gas quenching. Other chemical compounds involve a complex and often not well-documented gaseous chemistry that makes it difficult to extract meaningful information from the experiments. Therefore, our laboratory is currently exploring the use of radical scavenger compounds, such as HBr, with relatively simple gas-phase chemistry and that are in gas phase at injection conditions. This approach is limited by the low concentrations of such compounds that can be easily used in sizable reactors.
Proceedings of the Combustion Institute 30 (2005) 2187–2195
Combustion Institute www.elsevier.com/locate/proci
The effect of potassium bromide and sodium carbonate on coal char combustion reactivity Alejandro Molina*, Jeffrey J. Murphy, Christopher R. Shaddix, Linda G. Blevins Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA
Abstract The addition of halogens, particularly iodine, to the gas during coal char oxidation has been used in previous studies to quench gas-phase chemistry, thereby allowing one to separate the effects of homogeneous and heterogeneous reactions. Halogen addition suppresses the gas-phase radicals to near-equilibrium levels. A similar effect can be expected from other compounds with high efficiency as fire suppressants, such as alkali metals. The effectiveness of the use of additives in distinguishing homogeneous and heterogeneous reactions during char oxidation relies on the assumption that radicals are suppressed while heterogeneous reactions occurring on the char surface are not affected. The present work tests this assumption for potassium bromide (KBr) and sodium carbonate (Na2CO3) reacting with a pulverized eastern bituminous coal char during oxidation. An increase in CO and a slight reduction in particle temperature were observed with the addition of KBr, consistent with known effects of halogens on gas-phase chemistry. An increase in particle size was also observed with the KBr addition. This observation and the results of model calculations suggest that there is significant incorporation of liquid KBr on the char surface under the conditions examined. With Na2CO3 addition, the particle temperature did not change, the particle size showed a slight decrease, and CO production increased. Although the mechanisms for Na interaction with radicals at combustion conditions are not well established, char oxidation modeling suggests that a decrease in OH concentration in the particle boundary layer is the cause for the observed increase in CO production. It is concluded that Na2CO3 has clear advantages over KBr for inhibiting gasphase chemistry without affecting char oxidation for the conditions investigated here. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Char; Coal; Reactivity
1. Introduction The ability of halogens to quench gas-phase radical reactions has proven to be a good tool for partitioning the effects of homogeneous and heterogeneous reactions during coal char oxida*
Corresponding author. Fax: +1 925 294 2276. E-mail address: [email protected] (A. Molina).
tion. The effect of halides during solid fuel combustion was studied as early as 1949, when Day [1] found that 2% chlorine extinguished the combustion of petroleum coke at 2000 K. More recently, Bulewicz et al. [2] described the increase in CO concentration after the addition of sodium halides and CF2ClBr during fluidized bed coal combustion. Later [3], the same researchers found that iodine was an effective additive for the inhibition of CO oxidation. The authors explained the
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.219
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increase in CO concentration by the ability of halogens to catalyze radical recombination, reducing radical levels from the superequilibrium values that occur during fluidized bed coal combustion. Based on this influence of I2 on radical recombination, Winter et al. [4] applied iodine addition in the study of N2O formation during char oxidation. In Winters experiments, the addition of I2 had a strong effect on nitrogen-containing compounds: NO concentration decreased, N2O production ceased, and HCN concentration doubled. All of these variations were explained by the ability of iodine to inhibit radical reactions [5–7]. The effect of halogens during the cofiring of coal with industrial wastes containing elevated halogen content was also studied [8]. The concern was that the presence of halogens in the coal and waste could increase CO production. Although the authors found considerable CO increase after HCl and CaBr2 Æ 1/2 H2O were added to a fluidized bed boiler, the extended residence time in the system guaranteed complete CO oxidation. Halogen reactions have been extensively studied in the field of fire suppression. The mechanism involving the quenching of gas-phase radical chemistry can be traced back to Westbrooks pioneering work [9]. Since then, several other studies of halogen fire suppression have appeared in the literature, e.g. [10]. With the concern about ozone-depletion by halogens, recent studies of flame suppression [11–13] have focused on the use of alternative materials. Sodium carbonate (Na2CO3) is one possible substitute with reported inhibitor effectiveness 10 times that of Halon 1301 (CF3Br) [12]. In fact, the ability of Na to affect radical chemistry during combustion has prompted its use in the control of nitrogen oxide [14–16] and nitrous oxide [17] emissions. Despite the proven ability of halogens and alkali metals as radical scavengers, their success as a tool for distinguishing homogeneous and heterogeneous processes depends on their ability to affect the homogeneous chemistry but not the heterogeneous reactions. In the aforementioned study of iodine addition under fluidized bed combustion conditions, it was found that devolatilization and char burnout times did not change after the halogen addition [4]. Although these observations are consistent with the hypothesis that halogen addition does not significantly affect heterogeneous chemistry, more direct evidence is desirable. Additionally, no work examining the effect of radical-scavenging additives during conditions relevant to pulverized coal combustion has been published. This paper examines the effect of the addition of a halide, potassium bromide, and an alkali metal, sodium, on homogeneous and heterogeneous reactions during oxidation of pulverized coal char. We inject coal with and without potassium bromide (KBr) and sodium carbonate (Na2CO3) into an entrained-flow reactor. These two com-
pounds have advantages over halogens such as iodine because they can be introduced in the solid phase and they have lower laboratory hazard ratings than iodine. Neither of these compounds has been examined as a radical suppressant during coal combustion previously. Studies on the use of KBr and Na2CO3 in fire suppression are limited [12,18–20]. 2. Experimental 2.1. Sample preparation Two eastern US high-volatile bituminous coals (EB1 and EB2) were used in this study. Table 1 presents the proximate and ultimate analysis of each coal and of the char produced from EB1 in the entrained-reactor described below. The char composition is used as input for modeling. EB1 was obtained from CANMET Energy Technology Center. EB2 corresponds to coal from a pulverized coal power plant. Both coals were pulverized to a commercial grind and sieved to a 106–125 lm particle size fraction. The sieving was performed at CANMET for EB1 and at Sandia for EB2. The similarity between the two coals (see Table 1) suggests that results obtained using KBr and EB1 can be compared to those obtained using Na2CO3 and EB2. KBr was ground, and the 270–400 mesh fraction (38–53 lm) was gently mixed with the EB1 in a mortar to yield a 70%w KBr mixture. Na2CO3 was also ground, and the fraction passing 140 mesh (106 lm) was gently mixed with EB2 to yield a 50%w Na2CO3 mixture. This value (compared to 70% for KBr) was used to prevent clogging of the feeding line. The mixtures were stirred until they were visually homogeneous. 2.2. Reactivity (temperature) measurements The coal combustion experiments were performed in Sandias optical entrained-flow reactor, whose general design and operation have been described in the literature [21]. A schematic of the Table 1 Fuel composition EB2
EB1 char
Ultimate (dry) (%) C 77.33 H 5.08 O 6.29 N 1.45 S 0.96
EB1
74.67 4.77 10.08 1.44 2.16
74.55 0.19 0.02 1.24 0.70
Proximate (%) Dry loss Ash Vol. matter Fixed C
1.69 8.56 34.25 55.50
0.40 22.9 7.50 69.20
0.75 8.82 34.91 55.52
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Fig. 1. Schematic of the entrained-flow reactor, collection probe, and pyrometer.
facility is shown in Fig. 1. The reactor operates at one atmosphere and uses a diffusion-flameletbased Hencken burner. A particle-sizing pyrometer is used to simultaneously measure the velocity, diameter, and temperature of individual burning char particles at selected heights within the reactor. Temperature is calculated using a Wiens law ratio of emission signals detected through 40 nm FWHM bandpass filters with center wavelengths of 550 and 700 nm. Size is determined from the geometry of the coded aperture. The particle temperature measurement uncertainty is estimated to be better than ±1% using a linearized version of Weins law coupled with a noise estimate from the recorded signal. The size measurement is subjected to a much higher measurement uncertainty, typically ±20%. Reactor oxygen concentrations of 8, 12, and 24 mol% were investigated. Higher oxygen concentrations are of interest because of their use in oxygen-enhanced combustion and also because they alter the relative importance of oxygen diffusion and kinetics [22]. Nominal gas temperatures and gas compositions for these conditions are shown in Table 2. Gas temperature within the reactor was measured with a 76 lm, type R beaded thermocouple and then corrected for radiative losses [23]. Estimated gas temperature measurement uncertainty was ±50 K.
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KBr experiments, the probe was inserted into the reactor at a height of 20.4 and 10.2 cm for the 1200 and 1400 K conditions, respectively. For the Na2CO3 experiments, the probe was located at 7.6 cm above the burner. A gas analyzer was used to draw gas at a rate of 0.4 L min1 (300 K, 101 kPa) through the probe. Char particles were removed from the gas stream using filters, and water was removed using a desiccant. The gas analyzer provides measurements of O2, CO2, and CO concentrations, using a zirconium oxide sensor, a pyroelectric element sensor, and a non-dispersive infrared spectroscopy (NDIR) analyzer, respectively. CO concentrations were normalized by the amount of carbon due to char combustion (CO plus CO2 with the background CO2 from the burner subtracted). This provided a measurement that was relatively insensitive to the rate of flow of char in the reactor. The difference in the probe location for the different experiments and the finite sample quench times within the water-cooled probe limit the use of the measured gas composition to a qualitative analysis. Figure 2 shows a typical gas concentration profile for injection of coal only into the reactor. Following the disturbances that occur after injection
2.3. Gas composition measurements Extractive gas sampling was accomplished using a water-cooled collection probe (see Fig. 1). For the
Fig. 2. Typical traces from the gas analyzer during injection of coal into the 1200 K, 12% oxygen environment.
Table 2 Summary of the test conditions showing nominal gas temperature and molar concentrations in the gases in percent Nominal T (K)
O2 (%)
CO2 (%)
H2O (%)
N2 (%)
Compound
1200 1400 1400 1400
12 12 24 8.1
1.1 2.4 2.4 3.0
13.1 14.2 14.3 14.8
73.8 71.4 59.3 74.1
KBr KBr KBr Na2CO3
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starts, steady-state conditions are obtained. The data discussed below were obtained during this steady-state period (analysis interval) in Fig. 2.
3. Model Modeling was performed with the University of Sydneys Skippy (Surface Kinetics in Porous Particles) computer program [24]. This program calculates steady-state species and temperature profiles for the reaction of a porous solid in a reacting gaseous environment. Skippy predicts species concentrations and temperature within the char pores, at the outer char surface, and within the boundary layer surrounding the char. Both heterogeneous and homogeneous reactions are considered. The reaction mechanism developed by Glarborg et al. [25] was used for the gas-phase chemistry. This mechanism includes moist CO oxidation as well as nitrogen chemistry. The bromine submechanism was based on the work from two sources: the first is from Princeton [26] regarding the effects of HCl on postflame chemistry, and the second was compiled by the National Institute of Standards and Technology (NIST) [27] to model Halon suppression of fires. The reactions in the NIST mechanism (with reactions involving fluorine-containing compounds removed) are identical to the first 55 reactions in the Princeton mechanism (with bromine replacing chlorine). Thus, these reactions, with the rates from NIST, are included in our submechanism. The Princeton mechanism contains two additional reactions involving interactions of bromine and HCO (reactions 56 and 57 in [26]); these reactions are included in our sub-mechanism with the (chlorine) rates from [26]. Reactions 84–90 from the Princeton mechanism, which describe the interactions of HCl with NO, are also included in our mechanism, again with bromine substituted for chlorine. The rates for bromine versions of these reactions were taken from various sources in the literature [28–32] where available. Otherwise, the chlorine rates from [26] were used. The potassium submechanism is the five-step mechanism proposed by Slack et al. [33], with the recommended rate coefficients from the later work of Benilov et al. [34]. Finally, the KBr production reaction studied by Husain and Lee [35] was included. The submechanism for Na2CO3 includes the mechanism described by Zamasky et al. [16], which is based on that proposed by Perry and Miller [17]. Although some of the reactions for Na have been measured as a result of their importance in atmospheric chemistry [36–39] and in some cases extended to combustion temperatures [40], some of the kinetic parameters in the Na submechanism are highly uncertain.
The heterogeneous mechanism follows that described by Molina et al. [41]. Since the experiments were carried out in a region where KBr exists in the liquid phase (KBrmp = 1007 K and KBrbp = 1708 K), the volatilization of this species was included in the model. The rate constant for this process is estimated from the reverse reaction (condensation), which is assumed to have unity accommodation coefficient and therefore proceeds at its collision rate. At the temperatures used in the experiment (1400 K), equilibrium calculations [42] show that most of the Na is present as NaOH and atomic Na. A similar conclusion was obtained by Zamanzky et al. [16]. Therefore, for the simulations including Na2CO3, the boundary condition was specified as a constant NaOH concentration. 4. Results 4.1. Gas analysis Gas analysis results are shown in Fig. 3. These results show that the addition of KBr or Na2CO3 to the system causes a marked increase in the amount of CO detected. This increase confirms that KBr and Na2CO3 are interfering with CO oxidation. 4.2. Reactivity and particle size Figure 4 shows temperature and size measurements for EB1 and KBr injected into the 1200 K, 12% oxygen environment. There is a small (40 K) but statistically significant decrease in temperature when the KBr is added. There is also a large (20–30%) increase in particle size. These
Fig. 3. Comparison of CO/carbon ratio determined from gas analysis. The uncertainty intervals show the range of one standard deviation of the measured distribution.
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Fig. 4. Comparison of measured particle temperatures and sizes for EB1 in the 1200 K, 12% O2 environment. The vertical lines represent 95% confidence limits of the mean temperatures and sizes. The points are offset from each other on the x-axis for readability.
trends were also observed for coal and KBr injected into 1400 K gas with 12% oxygen and 24% oxygen. The difference in size is smaller, however. The sizes measured for coal at 24% oxygen are about 8% larger with KBr than without. Figure 5 shows the variation in temperature and size for EB2 with Na2CO3. Contrary to the results for KBr, the variation in temperature is with-
Fig. 5. Comparison of measured particle temperatures and sizes for EB2 in the 1400 K, 8% O2 environment. The vertical lines represent 95% confidence limits of the mean temperatures and sizes. The points are offset from each other on the x-axis for readability.
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Fig. 6. Comparison of measured particle temperatures and sizes for different coals and conditions. The vertical lines represent 95% confidence limits of the mean temperatures and sizes.
in the experimental uncertainty. There is also a minor reduction in particle size that borders the experimental uncertainty. Figure 6 presents a summary of the particle size and temperature measurements for all four test conditions in Table 2. 5. Discussion The first puzzling result in Figs. 4–6 is the increase in particle size when KBr is present. Since the boiling point of KBr is 1708 K, one can expect the presence of KBr aerosols and possibly liquid KBr addition to the char particle surface. If this interaction is taking place, then it may explain the increase in particle size observed in Figs. 4 and 6. One way to verify this hypothesis is to compute the fraction of liquid KBr on the char that would increase particle size by the amount observed in Figs. 4 and 6. Using the density of KBr at its vaporization temperature (1.96 g cm3) [43], a char density of 0.7 g cm3, and measured values for the moisture and volatile content of the coal, the predicted mass fractions of KBr in the coal/ KBr mixture are 0.7, 0.5, and 0.3 for 1200 K and 12% O2, 1400 K and 12% O2, and 1400 K and 24% O2, respectively. These values are equal to or below the mass fraction of KBr in the original mixture (0.7). Furthermore, the calculated amount of KBr on the char particles decreases as the particle temperature increases. This is consistent with the hypothesis that liquid KBr exists on the char surface. As the particle temperature
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increases, the rate of KBr vaporization increases, decreasing the amount of KBr on the char surface. Figure 5 shows that the particle size measured is slightly smaller when Na2CO3 is present. This difference could be attributed to a small decrease in particle size during the mixing procedure. To completely explain the data, it is necessary to find a reason for the reduction in particle temperature when KBr is present and for the invariance of temperature when Na2CO3 is added. For KBr, we identify three possibilities. The first is that the reduction in temperature is solely due to the difference in particle size, which, in turn, is caused by some factor unrelated to KBr. The second possibility is that the particle temperature is reduced by the loss of heat due to the vaporization of KBr on the char surface. Finally, it is possible that the reduction in particle temperature is due to a decrease in the heat generated in the boundary layer surrounding the particle by the conversion of CO to CO2. The presence of bromine and potassium in the system inhibits this conversion. To examine these three possibilities, four different cases (see Table 3) were analyzed using Skippy. Case 0, the baseline case, corresponds to the combustion of char at a height of 15.2 cm with a gas temperature of 1388 K (the measured gas temperature at this height) and gas composition as described in Table 2 for the 12% O2 case. The char particle diameter was 116 lm. In Case A, the particle is assumed to contain no KBr, but the larger particle diameter measured with KBr present (135 lm) was used. In Case B, char oxidation is calculated with KBr(l) present in the particle with the mass fractions previously computed. Finally, Case C uses the same conditions as Case B, but with the homogeneous chemistry of bromine and potassium removed from the mechanism. Figure 7 shows Skippys predictions for the variations of particle temperature, OH concentration, and the ratio of the molar flux of CO to the sum of the molar fluxes of CO and CO2 with radial distance from the particle center. This last variable can be compared to the data in Fig. 3. An examination of Fig. 7 shows that although there is a reduction in particle temperature as the particle size increases from 116 to 135 lm
Table 3 Cases for simulation of char oxidation experiments in the presence of KBr Case
Diameter (lm)
KBr (%w)
Br and K homogeneous reactions
0 A B C
116 135 135 135
0 0 70 70
Present Present Present Removed
Fig. 7. Variation of temperature, OH mole fraction, and ratio of CO flux to CO + CO2 flux as predicted by Skippy for Cases 0, A, B, and C (see Table 3 for description). Gas temperature was 1400 K, with 12% O2.
(Case 0 vs. Case A), it is insignificant and not comparable to that observed in the experiment. Conversely, the presence of KBr in a char/KBr mixture (Cases B and C) reduces the particle temperature by an amount similar to that observed in the experiments. There is no difference in the predicted particle temperature between cases B and C. The simulation results therefore suggest that the difference in temperature observed in Figs. 4 and 6 can be attributed to the presence of KBr on the char surface. Figure 7 also suggests that KBr affects the CO/CO2 balance by decreasing the OH concentration. However, the model suggests that the reduction in CO oxidation has an insignificant effect on particle temperature at the conditions of the experiment. Skippy was also used to explain the increase in CO ratio obtained during Na2CO3 addition. The concentration of NaOH estimated from the solid feeding rate (0.014 g m1) and the total gas flow was 48 ppm. This value assumes complete diffusion of the chemical compounds introduced at the burner center through the reactor cross-section. The actual NaOH concentration at the reactor centerline is greater than 48 ppm due to diffusion restric-
A. Molina et al. / Proceedings of the Combustion Institute 30 (2005) 2187–2195
tions. An upper bound for NaOH concentration was estimated based on the ratio of the central core flow and reactor cross-sectional areas. The core flow diameter was determined to be 1.4 cm from a photograph showing visible chemiluminescent emission of Na*. This value gives an area ratio of 0.06 and therefore an upper bound for NaOH concentration of 760 ppm. For the present simulation, a value of 480 ppm was assumed. Following the methodology used with KBr, Skippy simulations were carried out with and without NaOH present in the background. The predicted ratio of CO/carbon decreased 11% at a radial distance of 10 times the particle radius when NaOH was present in the system. This contradicts the experimental results (Fig. 3) that show an increase in CO when Na2CO3 was injected. The predicted increase in CO oxidation is surprising, considering that the model predicts a reduction in OH mole fraction of 50% due to the presence of Na in the system. A detailed analysis of the rate of reaction shows that the two major causes of CO reduction after the oxidation with OH are: NaO2 þ CO $ NaO þ CO2
ðR:1Þ
NaO þ CO $ Na þ CO2
ðR:2Þ
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The use of a computational particle combustion model as well as analysis of the particle size change suggested that liquid KBr was incorporated on the char surface and that the presence of liquid KBr on the surface was responsible for the reduction in particle temperature. The effects of KBr on char reactivity appeared to be insignificant; however, future work with different halogen compounds that do not condense in the temperature regime of char oxidation should be conducted before drawing final conclusions. The addition of Na2CO3 to char combustion did not affect the coal char reactivity, as evaluated from changes in particle temperature. This fact and the lower laboratory safety risks of sodium carbonate compared to halides are good reasons to use Na2CO3 to quench homogeneous reactions without affecting heterogeneous reactions during pulverized coal combustion experiments. However, the uncertainty in the kinetics of Na chemistry at combustion conditions poses an obstacle for a detailed explanation of its effect on homogeneous reactions.
Acknowledgments In the mechanisms used for the simulations [16,17], R.1 and R.2 were assumed to have a pre-exponential factor of 1014 cm3 mol1 s1. If this value is reduced to 1013 cm3 mol1 s1, the model predicts an increase in the ratio of CO/carbon of 15%, which agrees with the experimental trend. It is clear that the uncertainties in the mechanism of Na interaction with radicals during combustion prevent a complete description of the process by which the addition of Na2CO3 to the system produced an increase in CO production. However, the predictions suggest that Na2CO3 reduces the radical concentration as halogens do and that it does not affect particle reactivity. Sodium carbonate also has lower health risks associated with its use than potassium bromide. Thus, while both Na2CO3 and KBr have ease-of-delivery and safety advantages over the previously used gaseous I2, Na2CO3 is better than KBr for inhibiting gas phase chemistry during coal combustion at the conditions of the present experiments. 6. Conclusions Experiments examining the effect of KBr addition on char oxidation showed an increase in the CO/carbon ratio, a decrease in particle temperature, and an increase in particle size. The effect of Na2CO3 addition also showed an increase in the CO/carbon ratio, but without significant changes in temperature or particle size. Hence, both KBr and Na2CO3 affected homogeneous chemistry by suppressing radicals.
This research was sponsored by the US Department of Energy through the National Energy Technology Laboratorys Power Systems Advanced Research Program. Professor Franz Winter (Vienna University of Technology) provided ideas and useful discussions regarding the interaction of halogens with coal char. We thank Professor Brian S. Haynes (University of Sydney, Australia) for providing us with Skippy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energys National Nuclear Security Administration under Contract DEAC04-94-AL85000. References [1] R.J. Day, Kinetics of the Carbon–Oxygen Reaction at High Temperatures, Ph.D. thesis, Pennsylvania State College, Pennsylvania, USA, 1949, p. 187. [2] E.M. Bulewicz, E. Janicka, S. Kandefer, in: International Conference on Fluidized Bed Combustion: FBC—Technology for Today. American Society of Mechanical Engineers (ASME), San Francisco, CA, USA, 1989, pp. 163–168. [3] E.J. Anthony, E.M. Bulewicz, F. Preto, L. Rubow, The effect of halogens on FBC systems, in: International Conference on Fluidized-bed Combustion. American Society of Mechanical Engineers (ASME), San Diego, CA, USA, 1993, pp. 41–52. [4] F. Winter, C. Wartha, G. Lo¨ffler, H. Hofbauer, Proc. Combust. Inst. 26 (1996) 3325–3334. [5] D.Y. Lu, E. Anthony, R. Talbot, F. Winter, G. Lo¨ffler, C. Wartha, Energy Fuels 15 (3) (2001) 533–540.
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Comments Brian Haynes, University of Sydney, Australia. If your particles are coated with KBr, that should be observable under a visible microscope. Have you checked for this? In addition, a KBr cladding should have a significant effect on the optical properties needed for the two-color pyrometric technique. Could you please comment on possible errors in your measured temperature due to such effects? Reply. The evidence presented in the paper strongly suggests the incorporation of liquid KBr on the char sur-
face. SEM or optical microscope analysis of the particles would have given additional evidence of the presence of KBr on the char surface; however, we unfortunately did not collect any solid samples during the experiments. The particle-sizing technique employed in the experiments only depends on the ratio of measured intensity through two different-size apertures on a coded mask at one wavelength (700 nm) [1]. Therefore, it is independent of any changes in the optical properties due to the presence of KBr on the char surface. The accuracy of temperatures derived from the two-color pyrometry technique depends
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only on the ratio of particle emissivities at the two detection wavelengths, assumed here to be equal to one [1]. Available data on the optical properties of KBr at visible wavelengths show that its optical properties are invariant in this spectral range, suggesting that its influence on temperature measurements should be negligible.
surrounding each coal particle, resulting in diffusional transport of KBr vapor back towards the char particle, where it condenses and deposits (with thermophoresis driving the condensed particles towards the char surface).
Reference
Peter Ashman, University of Adelaide, Australia. It seems that two compounds have been selected, Na2CO3 and KBr, with the intention of suppressing the homogeneous chemistry. However, these compounds have both displayed other behavior that has masked this intended effect. Can the authors suggest other compounds that might be less problematical?
[1] D.A. Tichenor, S. Niksa, K.R. Hencken, R.E. Mitchell, Proc. Combust. Inst. 20 (1984) 1213–1221. d
Tom Fletcher, Brigham Young University, USA. I have a hard time understanding the KBr transfer mechanism. It is too low of a temperature to vaporize. If it only melts, it must collide. Usually, particle number densities were kept low in the Sandia CCL in order to avoid collisions and ensure single particle behavior. Please explain. Reply. In the sample preparation process, KBr and Na2CO3 were gently mixed with coal at a 70% and 50% weight fraction respectively until they were visually homogeneous. The KBr was ground and then sieved to a 38–53 lm size fraction before mixing with the coal (itself sieved to 106–125 lm). Therefore, some KBr powder undoubtedly coated the coal before feeding. A relatively dilute loading of particles was produced within the reactor, as you suggest, so that particle–particle interactions are minor if not negligible. However, volatilization of a portion of the KBr particles occurs in the volatile flame
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Reply. The selection of compounds that suppress the homogeneous chemistry during char combustion but do not affect the heterogeneous chemistry is not a simple task. As the paper shows, compounds that do not convert to gaseous phase at injection conditions can affect the heterogeneous chemistry. Furthermore, some halogen-containing materials convert to toxic and corrosive halides during gas quenching. Other chemical compounds involve a complex and often not well-documented gaseous chemistry that makes it difficult to extract meaningful information from the experiments. Therefore, our laboratory is currently exploring the use of radical scavenger compounds, such as HBr, with relatively simple gas-phase chemistry and that are in gas phase at injection conditions. This approach is limited by the low concentrations of such compounds that can be easily used in sizable reactors.