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Activation Energy Distribution of Thermal Annealing of a Bituminous Coal Bo Feng*,† Anker Jensen,‡ Suresh K. Bhatia,§ and Kim Dam-Johansen‡ Centre for Fuels and Energy, Curtin University of Technology, Brodie-Hall Building, 1 Turner Avenue, Technology Park, Australia, Department of Chemical Engineering, Technical University of Denmark, Kgs. Lyngby, 2800 Denmark, and Department of Chemical Engineering, The University of Queensland, St Lucia, Qld 4072, Australia Received May 14, 2002
A bituminous coal was pyrolyzed in a nitrogen stream in an entrained flow reactor at various temperatures from 700 to 1475 °C. Char samples were collected at different positions along the reactor. Each collected sample was oxidized nonisothermally in a TGA for reactivity determination. The reactivity of the coal char was found to decrease rapidly with residence time until 0.5 s, after which it decreased only slightly. On the bases of the reactivity data at various temperatures, a new approach was utilized to obtaining the “true” activation energy distribution function for thermal annealing without the assumption of any distribution function form or a constant preexponential factor. It appears that the “true” activation energy distribution function consists of two separate parts corresponding to different temperature ranges, suggesting different mechanisms in different temperature ranges. Partially burnt coal chars were also collected along the reactor when the coal was oxidized in air at various temperatures from 700 to 1475 °C. The collected samples were analyzed for the residual carbon content and the specific reaction rate was estimated. The characteristic time of thermal deactivation was compared with that of oxidation under realistic conditions. The characteristic times were found to be close to each other, indicating the importance of thermal deactivation during combustion of the coal studied.
Introduction Thermal deactivation of various coals at high temperatures has been reported by many investigators,1-6 as summarized in Figure 1. It is observed that the reactivity of various fuels could decrease from tens to thousands of times, largely depending on fuel type. Low rank coals and biomass fuels show a much faster decrease in reactivity with increase of temperature than high rank coals. It also appears that the peak heat treatment temperature is more important than the residence time in affecting the subsequent reactivity of the heat treated fuel.5 Therefore, a potentially thermal deactivation could be very important in gasification processes, in which the reaction temperature is high, provided the characteristic time of thermal deactivation * Corresponding author. E-mail:
[email protected]. Phone: 61 8 9266 1137. Fax: 61 8 9266 1138. † Centre for Fuels and Energy. § Department of Chemical Engineering. ‡ Department of Chemical Engineering. (1) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288-293. (2) Beeley, T.; Crelling J.; Gibbins, J.; Hurt, R.; Lunden, M.; Man, C.; Williamson, J.; Yang, N. In Proceedings of the 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996. (3) Cai, H. Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J. Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15-24. (4) Zolin, A.; Jensen, A.; Dam-Johansen, K. In Proceedings of the 28th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 2000. (5) Shim, H. S.; Hurt, R. H. Energy Fuels 2000, 14, 340-348. (6) Russell, N. V.; Gibbins, J. R.; Man, C. K.; Williamson, J. Energy Fuels 2000, 14, 883-888.
is comparable to that of the gasification reactions. However, there is little experimental work comparing the characteristic time of thermal deactivation with that of the gasification reactions. On the basis of the NagleStrickland-Constable model,7 Senneca et al.8 found that the characteristic time of thermal deactivation is indeed comparable with that of the CO2 gasification reaction at a wide range of temperature, but is much larger than that of the oxidation reaction. The present paper attempts to obtain the characteristic times of thermal deactivation and oxidation of a Columbian bituminous coal at various temperatures. So far the activation energy distribution model9 has been used successfully for thermal deactivation of coal.10 In this model an activation energy distribution function is assumed, for example to be a normal distribution,9 a log-normal distribution,10 or a Gamma distribution function,4 and the unknown parameters in the distribution functions are fitted with the experimental data. In the present paper, a new approach,11 which avoids the assumption of a specific distribution function, based on the original work of Miura and Maki,12 is used to obtain (7) Nagle, J.; Strickland-Constable, R. F. In Fifth Conference on Carbon; Macmillan: New York, 1962. (8) Senneca, O.; Russo, P.; Salatino, P.; Masi, S. Carbon 1997, 35, 141-151. (9) Suuberg, E. M.; Wojtowicz, M.; Calo, J. M. Carbon 1989, 27, 431440. (10) Hurt, R.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113, 181-197. (11) Feng, B.; Bhatia, S. K. Chem. Eng. Sci. 2000, 55, 6187-6196. (12) Miura, K.; Maki, T. Energy Fuels 1998, 12, 864-869.
10.1021/ef020108v CCC: $25.00 © 2003 American Chemical Society Published on Web 02/01/2003
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Feng et al. Table 1. Experimental Conditionsa
atmosphere
temp (°C)
preheater gas flow (NL/min)
fuel feeder gas flow (NL/min)
estimated total residence time (s)
N2 N2 N2 N2 N2 N2 N2 air air air air
700 900 1000 1100 1200 1300 1475 700 900 1200 1475
60 20 60 62 40 40 40 60 60 60 60
5 2.7 5 5 3.3 5 5 5 5 5 5
1.35 3.00 1.03 0.96 1.30 1.20 1.00 1.35 1.12 0.89 0.75
a Reactor: entrained flow reactor in the Technical University of Denmark. Fuel: Cerrejon bituminous coal. Fuel feeding rate: 0.2 kg/h. Sample collection points (distance from the inlet of the reactor, mm): 250, 305, 371, 452, 551, 671, 817, 996, 1213, 1478, and 1800.
Figure 1. Effect of heat treatment temperature on the reactivity of carbonaceous materials. R1000 is the reaction rate of the corresponding coal char heat treated at 1000 °C. (b) Beeley et al.2 Heating conditions: high-temperature wire-mesh reactor, heating rate 104 K/s, holding time 2s. Burning conditions: nonisothermal in TGA, from 673 to 1173 K in 7% O2 at 15 K/min. Rate: A0 at 50% conversion with E being 130 kJ/mol. (O) Cai et al.3 Heating conditions: high-temperature wire-mesh reactor, heating rate 103 K/s, holding time 2 s. Burning conditions: Isothermal burning in TGA at 773 K in air. Rate: maximum rate. (1) Jenkins et al.1 Heating conditions: TGA, heating rate 10 K/min, holding time 2 h. Burning conditions: isothermal in TGA at 773 K in air. Rate: maximum rate. (∇) Zolin et al.13 Heating conditions: TGA, heating rate 45 K/min, holding time 15 min. Burning conditions: nonisothermal in TGA, from 473 to 1273 K at 5 or 20 K/min in 10% O2. Rate: A0 at 20% conversion with E being 135 kJ/ mol. (9) Shim and Hurt.5 Heating conditions: transient heat treatment device, heating rate 103 k/s, holding time 2 s. Burning conditions: nonisothermal in TGA, to 1223 K at 7 K/min in air. Rate: A0 at 20% conversion with E being 35 kcal/ mol. (0) Russell et al.6 Heating conditions: high-temperature wire-mesh reactor, heating rate 104 K/s, holding time 2 s. Burning conditions: nonisothermal in TGA, from 673 to 1173 K in 6.3% O2 at 15 K/min. Rate: A0 at 50% conversion with E being 130 kJ/mol. ([) This paper. Heating conditions: entrained flow reactor, residence time about 1 s. Burning conditions: nonisothermal in TGA, from 473 to 1273 K at 20 K/min in 21% O2. Rate: A0 at 20% conversion with E being 135 kJ/mol.
the “true” activation energy distribution function. This ”true” activation energy distribution function is compared with the above distribution functions. (The true activation energy distribution function is difficult to obtain experimentally. In many cases only part of the distribution can be obtained. For example it requires high-resolution experiments to obtain the distribution function in the low activation energy part.) Experimental Section Char Preparation. A Colombian bituminous coal, Cerrejon, was used, with volatile content of 37.4% and ash content of 6.8% (dry basis). The ultimate analysis results of this coal are as follows (dry basis): carbon 76.2%, hydrogen 4.8%, oxygen 11.1%, nitrogen 1.7%, and sulfur 0.7%. The coal was crushed and sieved to within the range of 90-225 µm.
The coal was pyrolyzed and partially oxidized in an entrained flow reactor in the Technical University of Denmark as shown in Figure 2. Details about the reactor can be found elsewhere.13 Briefly the reactor has a length of 2 m and an inner diameter of 80 mm. It is electrically heated by multiple heating elements so that the axial temperature profile is uniform through the reactor. During experiments the pressure near the inlet of the reactor was controlled to be slightly higher than the atmospheric pressure. The coal was fed into the reactor from the top at a feed rate of 0.2 kg/h through a watercooled probe with an inner diameter of 8 mm. The resulting char was collected at various positions along the reactor by a water cooled extraction probe with an inner diameter of 39 mm. The experimental conditions are summarized in Table 1. The coal was pyrolyzed and oxidized in the reactor under various conditions. The concentrations of CO, CO2 and O2 in the exaust gas were monitored during the experiments. The oxygen concentration was nearly zero in the pyrolysis experiments, while for the oxidation experiments it was around 14% at 700 °C and 900 °C and 11% at 1200 °C and 1475 °C, respectively. More than 100 samples were collected, sieved to the range of 90-125 µm and analyzed for the reactivity and ash content subsequently. Reactivity. The reactivity of all the samples was measured using two thermogravimetric analyzers (TGA) (Netzsch STA409A and Cahn TG-121) nonisothermally. Around 3 mg of the sample was heated to 200 °C from room temperature at a heating rate of 20 °C/min in nitrogen, and was held at 200 °C for 20 min before the nitrogen stream was switched to air with a total flow rate of 200 mL/min. The sample was heated again in air from 200 to 1000 °C at a heating rate of 20 °C/ min and held at 1000 °C for 30 min. It was found in the preliminary experiments that a minimum oxygen feed rate of 30 mL/min was required to supply sufficient oxygen for the oxidation reaction and avoid starvation effects. The effect of particle size was found to be negligible, indicating chemical control in the nonisothermal burning. Following Beeley et al.,2 Shim and Hurt5 and Zolin et al.,4 the reactivity is defined as the preexponential factor A0 in the equation, rc ) A0 exp(-Ec/RTc), with Ec being the activation energy of the oxidation reaction, assumed to be constant at 135 kJ/mol during combustion;13 rc here is the TGA reaction rate at 20% conversion level, and Tc is the reaction temperature. This is an approximation designed to put the rates of a common basis for comparison. The relative reactivity is the ratio of the reactivity of any char to that of the char heat treated at 700 °C for about 1 s. (13) Zolin, A. Reactivity of Solid Fuels. In Department of Chemical Engineering; Technical University of Denmark: Lyngby, 2000; p 148.
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Figure 2. Schematic diagram of the entrained flow reactor in the Technical University of Denmark. Characteristic Times. The residence time of the coal particles was calculated by the relationship, t ) L/V, where L is the distance from the inlet of the reactor to the top of the extraction probe, and V is the average velocity of the gas in the center of the reactor over the cross section area of the collection probe. This average velocity was estimated from the volumetric flow rate by assuming fully developed laminar flow in the reactor. It was assumed that the coal particles achieved the temperature of the surronding gas instantly to simplify the computations. This will be discussed later in another section. The characteristic time of thermal deactivation τtd, is defined as τtd ) 1/rtd, with rtd being the specific reaction rate of thermal deactivation, which is obtained from the equation rtd ) d(A/ A0)/(A/A0)dt, where (A/A0) is the relative reactivity at a certain residence time t. Similarly, the characteristic time of oxidation τox is defined as τox ) 1/rox, with rox being the specific reaction rate of combustion, which is calculated as rox ) dx/dt/(1 - x), where x is the conversion at time t. Approach To Obtaining the True Activation Energy Distribution Function. A new approach was used to obtain the true activation energy distribution function without the assumpton of a known distribution function. This approach has been used for the analysis of oxygen chemisorption of several carbons.11 We assume that coal char comprises active sites that are converted into much less reactive sites during
heat treatment, with each active site having an activation energy for this conversion. The initial distribution of sites with an activation energy E for thermal annealing N0(E) is related to the initial total number of sites M0 as follows:
N0(E) ) M0fa(E)
(1)
where fa(E) is the activation energy distribution function that is sought. Assuming that the sites are deactivated according to a first-order reaction, we can have
dN(E,t) E N(E,t) ) -k0 exp dt RT
(
)
(2)
where t and T are the heat treatment time and temperature correspondingly. Integration of the above equation results in the variation of N(E) with t
(
(
N ) N0 exp -k0 exp -
E t ) N0g(E,t) RT
))
(3)
Therefore the total number of remaining active sites is
M)
∫ N(E)dE ) ∫ N g(E,t)dE ) ∫ M f (E)g(E,t)dE ∞
0
∞
0
∞
0
0
0 a
(4)
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The double exponential function g(E,t) changes rapidly from 1 to 0 within a narrow range of E. Therefore a step function approximation is used for g(E,t)14 and this leads to
M ) M0
∫
E*
0
fa(E)dE
(5)
in which E* is the characteristic activation energy, which can be obtained from
E* g(E*,t) ) exp -k0 exp t ) e-1 RT
(
(
))
(6)
Here e-1 was used for simplicity following Du et al.15 although other values such as e-0.5 or 0.58 were also used. This has been discussed in detail in Feng and Bhatia.11 Equation 5 shows that the number of remaining active sites is only a function of E* if fa(E) is characteristic of the carbon. Thus, at constant M, the value E* is unchanged, and eq 6 leads to
ln t )
E* - ln k0 RT
(7)
Therefore, if the combination of heating temperature and time which leads to the same number of remaining active sites is experimentally determined, eq 7 can be used to simultaneouly obtain E* and the corresponding k0. Subsequently, eq 5 can be used to obtain the activation energy distribution function, which is the first derivative of M/M0 with respect to E*. There is no widely accepted experimental method available to obtain the number of remaining active sites directly.16 Here the relative reactivity A/A0 is used to approximate the fraction of remaining active sites M/M0 with the following assumptions: (i) M0 and A0 are the initial number of active sites and TGA reactivity for the coal char heat treated at 700 °C for 1.0 s in the entrained flow reactor. (ii) The reactivity of coal char is proportional to the number of remaining active sites, implying the converted site is essentially nonreactive compared with a fresh site and all uncovered sites are equally reactive. For comparison, a shifted Gamma distribution function (eq 8) and a log-normal distribution function (eq 9) listed below were assumed to fit the experimental data using eq 4.
fa(E) )
fa(E) )
(E - δ)R-1 Γ(R)βR
(
exp -
{
)
(E - δ) β
}
(8)
2
[ln(E/E0)] 1 exp 2m2 x m 2πE
(9)
In the fits, the preexponential factor k0 was assumed to be constant, and the values of k0, R, β, and δ were fitted for the Gamma distribution, while the values of k0, m, and E0 were fitted for the log-normal distribution method.
Results True Activation Energy Distribution. Figure 3 shows the relative reactivity of Cerrejon in the TGA after heat treatment in the entrained flow reactor for various times and at various temperatures. It can be seen that the relative reactivity remains roughly constant at the heat treatment temperature of 700 °C. It can be also observed that the reactivity of the coal char decreases fast initially and slowly afterward. It appears (14) Suuberg, E. M. Combust. Flame 1983, 50, 243-249. (15) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1990, 4, 296-323. (16) Suuberg, E. M. Thermally Induced Changes in Reactivity of Carbons. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Norwell, MA, 1991.
Figure 3. Reactivity of the Cerrejon coal chars after pyrolysis in the entrained flow reactor at various temperatures for various residence times. The relative reactivity is defined as the ratio of A0 (see text) of any char to that of the char pyrolyzed at 700 °C for about 1 s. Points are experimental data and solid lines are fittings. The pyrolysis temperatures are (b) 700 °C. (O) 900 °C. (1) 1000 °C. (3) 1100 °C. (9) 1200 °C. (0) 1300 °C. ([) 1475 °C.
that after only 0.5 s the reactivity does not change significantly with the residence time at all the temperatures from 900 to 1475 °C. This indicates that after 0.5 s of heat treatment the heat treatment temperature is more important than the residence time affecting the reactivity of the coal char, supporting the findings of Shim and Hurt.5 The reactivity of the coal decreases to only one-third (at 1000 °C) or one-tenth (at 1475 °C) of that of the fresh char very quickly, suggesting strongly the potential importance of thermal deactivation. However, the active sites could be also oxidized when oxygen is present instead of being converted or depleted thermally. Only when the rate of site conversion is comparable to or faster than that of oxidation is thermal deactivation important. This will be discussed in detail later. Equation 7 was used to obtain E* and the corresponding k0 at a series of M/M0 values based on the experimental data on Figure 3. Equation 5 was then used to obtain the activation energy distribution function at E*, and the results are shown in Figure 4. A shifted Gamma distribution function and a log-normal distribution function were also fitted to the experimental data using eq 4. The best fitted Gamma ditribution function is also displayed in the figure. The parameters for the shifted Gamma distribution function are as follows: R ) 2.257, β ) 58.71 kJ/mol, δ ) 0.1345 kJ/ mol, k0 ) 4.048 × 1011 s-1, Edm(mean activation energy) ) 132.5 kJ/mol, σ (standard deviation) ) 88.2 kJ/mol. Clearly the assumed Gamma distribution is biased from the “true” activation energy distribution, with the latter being shown in detail in the inset window. The Gamma distribution function obtained is on the side of higher activation energy. The fitted parameters for the lognormal distribution method are as follows: m ) 0.1938, E0 ) 299.7 kJ/mol, and k0 ) 4.164 × 1011 s-1. The mean activation energy is even higher. The reason the activation energy distributions functions are different from the “true” activation energy distribution function could be the assumption of a constant k0 in the former case. With the present theory both the preexponential factor
Thermal Annealing of a Bituminous Coal
Figure 4. Activation energy distribution function of thermal deactivation of the Cerrejon coal in the entrained flow reactor. Points are the function values obtained using the approach discussed in the text, and the solid line is the shifted Gamma distribution function obtained by fitting the experimental data in Figure 2. The inset shows the “true” activation energy distribution in more detail.
Figure 5. Correlation of the preexponential factor k0 with the characteristic activation energy E* for thermal annealing of Cerrejon.
and the activation energy are allowed to vary and the distribution is calculated from the experimental data. It was found that the logarithm of the preexponential factor varied almost linearly with the activation energy (as shown in Figure 5). In addition the fittings for both the Gamma distribution and the log-normal distribution were not very satisfactory. It is interesting to notice that the “true” activation energy distribution appears to consist of two distinct distributions. The activation energy distribution function on the higher energy side, curve 2 in Figure 4, was obtained from the data of relative reactivty above 0.4 in Figure 3 (thus at temperatures of 700, 900, 1000, 1100, and 1200 °C), while that on the lower energy side, curve 1 in Figure 4, was obtained from the data of relative reactivity below 0.4 in Figure 3 (thus at temperatures of 1100, 1200, 1300, and 1475 °C). This suggests different mechanisms of thermal deactivation at different temperature ranges. As we mentioned before, the complete distribution is difficult to obtain experimentally. It would appear that Figure 4 shows
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Figure 6. The fraction of unburnt carbon in the coal char (ash free basis) as a function of oxidation temperature and residence time. Points are experimental data and lines are fittings.
only a part of the complete distribution, and more experimental data are needed to obtain a complete one. The mechanisms of thermal deactivation are still to be clarified for the individual coals, but it is always associated with the loss of active sites for subsequent oxidation, regardless of pathway. At lower temperatures, loss of functional groups, heteroatoms, and disordered carbon is probably the major cause of the removal of the active sites. This is a temperature sensitive or high activation energy process. At higher temperatures, catalyzed graphitization comes into play which is a low activation energy process.17 This process probably involves a phase transition and was found to occur at temperatures higher than 950 °C for an anthracite.17 This is consistent with the experimental findings of Zolin et al.13 who found two peaks on the conversion-time curve for the same bituminous coal after heat treatment at high temperatures (>1200 °C) for 15 min and only a single peak for the coal after heat treatment at lower temperatures. The two peaks may come from the formation of significant amount of less reactive carbon at high temperatures that is enhanced by mineral matter in the coal. Comparison of the Characteristic Times. Thermal deactivation is considered important if the following two conditions are fulfillled: (1) The specific reaction rate of thermal annealing rtd is large, i.e., rtd > 0.1, or more than 10% of the active sites can be removed in 1 s, the typical residence time for coal in a combustor, and (2) the characteristic time of thermal deactivation is comparable to or less than that of oxidation, i.e., τtd < 10τox. Physically, τtd indicates the time it takes for the active sites to be removed completely by thermal annealing and τox represents the time it takes for the remaining sites to be 100% oxidized. The fraction of unburnt carbon in the coal char after oxidation in the entrained flow reactor at various gas temperatures is shown as a function of the residence time in Figure 6. At 900 °C the coal char appears to be oxidized gradually while at 1200 °C and 1475 °C the coal char is burnt rapidly within 0.7 s. The coal was also oxidized at 700 °C and it was observed that the (17) Feng, B.; Bhatia, S. K.; Barry, J. C. Carbon 2002, 40, 481496.
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ment temperature, thermal deactivation is even more important than shown in Figure 7. However, the characteristic time of oxidation in Figure 7 was calculated from the overall reaction rate, which itself includes the effect of thermal deactivation. This effect is expected to be not important to the value of the characteristic time of oxidation because the weight loss caused by thermal annealing is negligibly less than that by oxidation. It is concluded, therefore, that thermal deactivation is important throughout the combustion process of Cerrejon, and it is very likely that this coal is difficult to be burnt off completely because of the greater importance of thermal deactivation at the late stage of combustion. Also the higher the gas temperature at which the coal is oxidized, the relatively more important thermal deactivation is at the late stage of combustion. Figure 7. The characteristic time of thermal deactivation τtd and that of oxidation τox as a function of reaction temperature and residence time. The temperature for thermal deactivation is the gas temperature at which the coal particles were heat treated while the temperature for oxidation is the gas temperature at which the coal particles were burnt.
oxidation rate at 700 °C is faster than that at 900 °C. This unusual reverse reactivity order between 700 and 900 °C would reflect an ignition phenomenon with the coal at 700 °C. The characteristic time of thermal deactivation (obtained from the data shown in Figure 3 based on the specific rate of site removal approximated by that of reactivity reduction after heat treatment) is compared in Figure 7 with that of oxidation (obtained from the data in Figure 6 based on the specific rate of carbon removal by oxidation). The characteristic times increase with the increase of the residence time at all the temperatures. At 900 °C, both characteristic times increase slightly while at 1200 and 1475 °C they increase sharply. It is interesting to notice that the characteristic time of thermal deactivation is very close to that of oxidation at the gas temperature of 900 °C. In other words, the rate of the removal of active sites by thermal annealing is as fast as that by oxygen attack at 900 °C. Clearly thermal deactivation is very important in this case. At the gas temperatures of 1200 and 1475 °C, the thermal deactivation is initially important, becomes relatively less important in the middle of combustion, and afterward becomes more and more important with the increase of the residence time. At the late stage of combustion, the characteristic time of oxidation is even longer than that of thermal deactivation, indicating that thermal deactivation is important also here. It should be pointed out that the particle temperature will be significantly higher than the gas temperature in the initial stage of combustion, and approaches the gas temperature at the late stage of combustion. Since the characteristic time is reduced at higher heat treat-
Conclusions A bituminous coal was pyrolyzed and burnt in an entrained flow reactor and samples with various residence times at various temperatures were collected along the reactor. The collected samples were analyzed for the reactivity and the activation energy distribution for thermal deactivation was determined as well as the characteristic times of thermal deactivation and oxidation. The “true” activation energy distribution function appears to consists of two separate parts although neither function is fully determined by the current data. The function in the low activation energy part was obatined by the higher temperature data, while the one in the high energy part was determined using the lower temperature data. It suggests the existence of different mechanisms for thermal annealing in the two different temperature ranges. The characteristic time of thermal deactivation is as short as that of oxidation at 900 °C, comparably longer at 1200 and 1475 °C at the initial stage of combustion, while becoming shorter at the late stage of combustion. The results indicate that thermal deactivation is important throughout the combustion process of the coal studied. The higher the temperature at which the coal is burnt, the more important is thermal annealing at the late stage combustion. Acknowledgment. One of the authors (B.F.) thanks the Graduate School, the University of Queensland (UQ), for a travel award which made this collabarative work possible when he was a PhD student at UQ. He is also grateful to the Combustion and Harmful Emission Control (CHEC) research program in the Department of Chemical Engineering, the Technical University of Denmark, for supporting the research work and providing necessary facilities. B.F. thanks the staff in the CHEC group, particularly Peter Arendt Jensen and Jørn Hansen, for their help in the experiments. Interesting discussions with Dr. Zolin are also acknowledged. EF020108V
