Effects of Pressure and Oxygen Concentration on the Combustion of

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Energy & Fuels 1999, 13, 130-145

Effects of Pressure and Oxygen Concentration on the Combustion of Different Coals Timo Joutsenoja,*,† Jaakko Saastamoinen,‡ Martti Aho,‡ and Rolf Hernberg† Tampere University of Technology, Department of Physics, Plasma Technology Laboratory, P.O. Box 692, FIN-33101 Tampere, Finland, and VTT Energy, P.O. Box 1603, FIN-40101 Jyva¨ skyla¨ , Finland Received June 24, 1998

A pyrometric method was used for simultaneous in situ measurement of the temperature and size of individual coal particles in a pressurized entrained flow reactor. Several series of measurements were made in the gas temperature range 1150-1270 K to study the effects of pressure (0.2-1.0 MPa) and oxygen volume fraction (3-30 vol %) on the particle temperature and size distributions and on the mean degree of burnoff at well-defined residence times. The fuels used in the experiments varied strongly in their reactivity: lignite from France (Gardanne), high-volatile bituminous (hvb) coals from Germany (Westerholt, Go¨ttelborn) and Poland (mixture), and anthracite from Germany (Niederberg). Milled fuel was sieved into nominal-size fractions in the range from 75 to 180 µm. The strongest increase in combustion rate at increased pressure occurred for anthracite, which was the least reactive among the fuels studied. This is shown by increasing the mass-loss rate and increasing particle temperatures. Pressure had no effect on the combustion of the lignite sample, the most reactive fuel studied. Some evidence of swelling of hvb coal particles was observed during the early stages of combustion. The particle size remained roughly unchanged until the degree of burnoff exceeded 90%, whereafter the particle size started to decrease due to fragmentation. Experimental results from particle temperature measurements at various oxygen concentrations are compared with literature data. Results from in situ particle measurements including size and temperature recording have not been presented previously at elevated pressures for such a wide range of coals. Measurements on combustion rates, particle temperatures and particle sizes were analyzed with a single-particle combustion model. The effect of pressure on the surface reaction kinetics for an anthracite was found to be small compared to the oxygen content and temperature.

Introduction The major advantages of pressurized combustion of solid fuels are that the net efficiency of electricity production is increased and CO2 emissions per unit of produced electrical energy is decreased at the same time. Additional advantages result from reduced furnace size and a presumable long-term decrease of the production costs of electricity. Several coal-fired pressurized units based on a combined cycle already exist for power production on pilot and small commercial scale. These can be divided into two categories: pressurized fluidized-bed combined cycles (PFBC) and integrated gasification combined cycles (IGCC).1,2 Direct coal-fired combined cycles have been studied on a pilot scale. This technique would provide the highest net efficiency and lowest CO2 emissions of all known coal combustion concepts. However, the calculated difference in net efficiency between this technique and modern steam power plants has decreased due to †

Tampere University of Technology. VTT Energy. (1) Nilsson, C.; Clarke, L. IEACR73/30; IEA Coal Research: London, 1994. (2) Sloss, L. IEAPER/30; IEA Coal Research: London, 1996. ‡

recent improvements of the materials of superheaters. Combined cycles cannot effectively utilize the improved materials for superheater tubes because the gas temperature at the outlet of the gas turbine is rather low, and therefore, the steam cannot be heated to supercritical conditions. One option for the furnace of a direct coal-fired combined cycle process is the slagging-type combustor, which has also been studied on a pilot scale. However, the flue gas has been found to contain too many alkalis for a gas turbine and the conditions for sulfur capture were not optimal.3-5 Up to now, no method has been found to decrease the concentrations of alkali vapors in the gas turbine inlet to the allowed level ( B(Tp) must hold. It is also seen that the equation predicts the existence of two possible particle sizes corresponding to one temperature. Only one particle size is associated with one temperature in the diffusion-limited case (1/k f 0). Theoretical Maximum Particle Temperature. By differentiating the particle temperature T in eq 6 with respect to the particle diameter and by solving the equation when dT/dDp ) 0, it can be seen that the maximum particle temperature is reached when [A(Tp)]2 ) B(Tp). We obtain for the maximum possible temperature of a particle the expression

∆Tmax ) Tp,max - Tg ) fShφmFgDO2 ( ∆HYO2,∞ - xq/k)2 (7) Nuφhλ x It should be noted that this is not an explicit expression, since q and k depend on Tp, but the maximum particle temperature can be solved by iteration. The maximum temperature increases with increasing ∆H and the oxygen mass fraction. The critical particle diameter at which the maximum particle temperature is reached is

Dp,cr ) A(Tp,max) ) fShφmFgDO2(x∆HYO2,∞/kq - 1/k) (8) It is seen that the critical particle size increases at higher mass-transfer rates (large values of Sh and DO2). When the ratio of CO/CO2 produced on the particle surface and ∆H increase, the critical particle size increases. However, there is no clear experimental evidence for this theoretically obtained maximum particle temperature in the char combustion stage; either this particle size and temperature are beyond the (25) Mitchell, R. E.; Madsen, O. H. 21st Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; p 173. (26) Wall, T. F.; Tate, A. G.; Bailey, J. G.; Jenness, L. G.; Mitchell, R. E.; Hurt, R. H. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 1207. (27) Hurt, R. H. Energy Fuels 1993, 7, 721. (28) Joutsenoja, T.; Hernberg, R. Appl. Opt. 1998, 37, 3487. (29) Reichelt, T.; Joutsenoja T.; Spliethoff, H.; Hein, K. R. G.; Hernberg, R. 27th Symposium (International) on Combustion, Boulder, Colorado, August 2-7, 1998.

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Figure 5. Effect of pressure at constant O2 volume fraction (10 vol %) on the average particle temperatures of Polish hvb coal (Pol), Gardanne lignite (Gar), Go¨ttelborn hvb coal (Go¨t), and Niederberg anthracite (Nie A, Nie B). The residence time corresponding to the measurement position and the gas temperature are given in the legend. The initial nominal particle size was 100-125 µm for Nie B and 140-180 µm for the others.

Figure 4. Time required for 90% burnoff as a function of pressure. Oxygen volume fraction was fixed to 10 vol %. (a) Two size fractions of Niederberg anthracite at Tg ) 1270 K. (b) Gardanne lignite, Go¨ttelborn, and Polish hvb coal at Tg ) 1150-1170 K

detection limit or such a maximum temperature does not exist in reality. This means that the shrinking particle model cannot describe the combustion behavior of particles of a wide size range with the same apparent kinetic constants. Results Effect of Pressure on Burning Rate, Particle Temperatures, and Particle Sizes. The coals, process conditions, and results of the pyrometric particle measurements are summarized in Table 2. The residence time and degree of burnoff corresponding to the position of the pyrometric measurement are also given. The oxygen volume fraction was 10 vol % in all of the experiments in Table 2. The gas temperature was about 100 K higher in the experiments with anthracite than with other coals. The strongest increase of the burning rate at an increase of pressure at constant O2 volume fraction occurred for anthracite (Figures 2 and 4), the reactivity of which is the lowest among the fuels studied. The pressure effect was strongest with the smaller size fraction of anthracite particles (size B), although the observed effect was also clear for the larger size fraction (size A). For both size fractions of anthracite the effect of pressure clearly becomes stronger when 0.1 MPa is approached. The effect is weak or negligible at pressures above 0.6 MPa. The differences that were observed between two size fractions in the burnoff measurements of anthracite (Figure 4a) cannot be verified by particle temperature measurement (Figure 5). The reason for this is technical;

the differences occurred almost solely at pressures below 0.4 MPa, and unfortunately the temperatures at these pressures were below the detection limit of the pyrometer. Also, the average temperature of Polish and Go¨ttelborn coals decreased with decreasing pressure (Figure 5). Particle temperatures of Go¨ttelborn coal increase more strongly with pressure than those of Polish coal, although the combustion rate of Polish coal shows a stronger pressure dependence (cf. Figure 4). The average temperature of Gardanne lignite particles remained constant with pressure, indicating diffusion control (Figure 5). It was the highest average temperature among the measured fuels. The difference of 35 K between pressure ranges 0.2-0.4 and 0.6-0.9 MPa is due to differences in residence times, i.e., different degree of burnoff (the effect of burnoff is discussed below). Furthermore, the difference is mainly induced by the changes in the cold-end tail of the temperature distribution, and thus, the difference in the position of the peak of the distribution is smaller, only 13 K (cf. Table 2). High temperatures of lignite particles and their independence of pressure are in good agreement with Figure 4, showing the highest combustion rate and no pressure influence with this fuel. The temperature and size distributions of anthracite particles were wider than with the other fuels studied. Thus, the detection limit also played a more important role and had, in some cases, a significant effect on the average values and standard deviations obtained from the measured anthracite data. Figure 6 clearly illustrates how particle temperatures decrease with decreasing pressure and how at 0.2 MPa the most dense part of the detected particle population becomes sharply cut by the detection limit. Only the cases where the effect of the detection limit is negligible or small are included Table 2. Figure 7 shows the effect of pressure on the temperature distribution of Go¨ttelborn hvb coal particles. The temperature at the peak of the distribution slightly decreased with decreasing pressure, and the low-temperature tail of the distribution grew when pressure decreased. Thus, the standard deviation of particle

Combustion of Different Coals

Energy & Fuels, Vol. 13, No. 1, 1999 137

Figure 6. Particle temperature and size of individual Niederberg anthracite (size A) particles at four pressures and three residence times (Tg ) 1270 K, c[O2] ) 10 vol %). The dashed curves indicate calculated particle temperatures for diffusion-controlled conversion into CO2 (upper) and CO (lower). The solid curve indicates the detection limit of the pyrometric measurement.

temperatures also increased with decreasing pressure. Due to the variation of the shape of the temperature distribution, the use of average particle temperatures leads to a slight overestimation of the effect of pressure on the particle temperature. A similar behavior was found also for Polish hvb coal. Dashed curves in Figures 6 and 7 indicate the calculated particle temperature in cases of diffusioncontrolled conversion into CO and CO2 at the corresponding gas temperature and O2 volume fraction. The

calculated temperatures for the measured average particle size are also given in Table 2. Monson et al.7 found that the gas-particle temperature difference of Utah hvb coal char slightly increases with increasing pressure. The magnitude and the trend of the effect of pressure on the particle temperature is in good agreement with the results presented here. Effect of Burnoff on Particle Temperature and Size Distributions. The results suggested some swelling of hvb coals during the early stage of combustion.


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Figure 7. Left: Particle temperature and size of individual Go¨ttelborn hvb coal particles at four pressures (Tg ) 1160 K, c[O2] ) 10 vol %). The dashed lines indicate calculated particle temperatures for diffusion-controlled conversion into CO2 (upper) and CO (lower). The solid curve indicates the detection limit of the pyrometric measurement. Right: Temperature distributions of particles in the size range 100-200 µm (limits shown in the plot on left by vertical lines).

This was best shown with Go¨ttelborn coal, where a remarkable number of particles 200-300 µm in size appeared at 80 ms, although their number in the feedstock was low (Figure 7). A decrease of particle size due to fragmentation30,31 was clearly seen when the

degree of burnoff exceeded 90%. The best examples of this are the decreased particle size of Go¨ttelborn char sampled at 0.8 MPa and 300 ms (burnoff 98.8%, Table 2) and for anthracite (size A) sampled at 0.8 MPa and 650 ms (burnoff 97.8%, Figure 6).

(30) Baxter, L. L. Combust. Flame 1992, 90, 174. (31) Mitchell, R. E.; Akanetuk, A. E. J. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3137.

From Figure 6 it can be seen that the average temperature of anthracite decreases from a residence time of 100 ms to 370 ms and particles smaller than 70



Figure 8. Effect of the stage of burnoff on the average particle temperatures of Go¨ttelborn hvb coal (Go¨t, Tg ) 1170 K) and two size fractions of Niederberg anthracite (Nie A, Nie B, Tg ) 1270 K). The oxygen volume fraction was 10 vol % in all of the experiments. The pressure is as given in the legend. The initial nominal particle size was 100-125 µm for Nie B and 140-180 µm for the others.

µm have burned out while the peak of the size distribution has not changed. At a residence time of 650 ms we can also see that the temperature has not decreased more and the size distribution has shifted toward smaller particle sizes. The observed variation in the average particle temperature during char combustion is between 60 and 120 K (cf. Tables 2 and 3). The reactivity and particle temperature have been found to decrease at greater conversions.32,33 The decrease of the temperatures at a later stage (Figure 8) can be partly due to the ash layer formed on the particle, which decreases the rate of diffusion of oxygen to the carbon surface. Ash vaporization can lower the particle temperatures. The mass release curve of Gardanne lignite in oxidation has a peak due to the thermal decomposition of CaCO3.34 In the present reactor, the particles experience much higher heating rates. It can be assumed, however, that the decomposition is complete at the time of the temperature measurements. Correlation between Particle Temperature and Size. The results of the regression analysis of all fuels except anthracite, for which a clear correlation is not present, are given in Tables 2 and 3. For some of the experiments the results of the analysis are not comparable due to the effect of the detection limit or a too low number of particles. These results are not included in the tables or the figures. In all of the cases the particle temperature decreases with increasing particle size or remains constant. The slope of the trendline becomes steeper with increasing oxygen concentration for Gardanne lignite (Figure 9), which was found for other coals as well. However, the magnitude of the Dp-Tp dependence varied greatly between different coals, being the strongest for Gardanne lignite and the weakest for Go¨ttelborn and Westerholt hvb coals. The effect of pressure on the slope of the trendline was negligible for Gardanne lignite and for Go¨ttelborn hvb coal, but for Polish hvb coal the slope became steeper with decreasing pressure. Despite the clear Dp-Tp dependencies that (32) Hurt, R. H.; Davis, K. A. 25th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 561. (33) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471. (34) Va´rhegyi, G.; Szabo´, P.; Jakab, E.; Till, F.; Richard, J.-R. Energy Fuels 1996, 10, 1208.

Figure 9. Particle temperature and size of individual Gardanne lignite particles at six oxygen volume fractions (Tg ) 1150 K, p ) 1.0 MPa, tr ) 130 ms). The dashed curves indicate calculated particle temperatures for diffusion-controlled conversion into CO2 at O2 volume fractions 5, 15, and 30 vol %. Dotted curves indicate the corresponding conversion into CO. The solid curve indicates the detection limit of the pyrometric measurement.

were found, it should be noticed that in nearly all of the cases the changes in particle temperature within a size fraction of 100 µm width are smaller than the standard deviation of the particle temperature. This indicates that the variation of individual particle temperatures is induced by other nonhomogeneous properties of the particles. The literature shows a wide scatter in the observed dependence of Tp and Dp.7,21-29 Comparison of the results is very difficult due to the large variation in process conditions, stage of combustion, and rank of fuel. However, it can be noticed that near zero and even slightly positive slopes have been observed7,24-26 as well as clearly negative slopes.21-24,27-29 According to eq 1 the ash content of the particle has no direct effect on the particle temperature when the possible diffusion resistance of the ash formed around the particle is ignored. The ash may naturally catalyze the reactions. If it is assumed that the ash formed on the particle surface is swept away, the rate of diffusion of oxygen from the bulk to a spherical surface of the particle is not affected by the material of the particle. However, there may be some effect reducing the consumption of carbon in oxidation, since an oxygen molecule may come into contact with an inert ash particle of the surface instead of carbon; this is accounted for in the apparent char reactivity parameters, but this phenomenon is also relevant in diffusion-limited


Joutsenoja et al.

Figure 10. Measured particle excess temperatures of various fuels as a function the oxygen volume fraction. The fuel, gas temperature and source of data are given in the legend. The open symbols indicate data of the present work. The pressure is 1.0 MPa for the present work and Utah coal and atmospheric pressure for the others.

combustion, reducing the combustion rate. Also, the density of the char particles have no direct influence, but coal particles with lower density may contain more voids. The surface voids of a particle decrease the combustion rate in diffusion-limited combustion.35 The existence of two particle sizes corresponding to one temperature (cf. eq 6) is not well detected in our measurements. On one hand, when the particle temperature is high, B(Tp) is small and Dp becomes small and falls beyond the detection limit of our measurements. On the other hand, at low temperatures, the detection limit changes so that only larger particles can be detected. A third explanation is that the simplified shrinking particle model with a constant apparent kinetic constant does not describe the real combustion behavior of particles with different sizes well enough. Effect of Oxygen Concentration on Particle Temperature. Measurements were made with Westerholt hvb coal, Polish hvb coal, and Gardanne lignite to study the effect of the oxygen volume fraction (3-30 vol % O2) on the particle temperature. The experimental parameters and the results of the pyrometric measurements are summarized in Table 3. The oxygen volume fraction has a strong effect on the fuel particle temperature, and thus, the particle populations measured at the different O2 volume fractions are clearly separated (Figure 9). The particle-size distribution measured at a fixed residence time shifts toward smaller diameters with increasing O2 volume fraction. At 3 vol % O2, the particles larger than 200 µm show some evidence of swelling. A clear decrease in particle size is observed when the O2 volume fraction increases from 3 to 9 vol %. From 9 to 30 vol % O2 the decrease of particle size is slower but still observable. At 21 and 30 vol % O2, fragmentation of particles due to the high degree of burnoff (>90%; estimated on the basis of flame length and other experiments) can be observed. However, combustion at 21-30 vol % O2 is still very intensive at the position corresponding to 130 ms. The results of the experiments with Westerholt and Polish hvb coals were similar to the ones of Gardanne lignite (cf. Table 3). The particle temperatures obtained at high O2 volume fractions are very high. The average (35) Bayless, D. J.; Schroeder, A. R.; Peters, J. E.; Buckius, R. O. Combust. Flame 1997, 108, 187.

temperatures were very close to each other for all three fuels at O2 volume fractions of 15-30 vol %. At the O2 volume fractions of 5-10 vol %, small differences were observed, Gardanne lignite having the highest and Westerholt coal the lowest average temperatures. Curves indicating calculated particle temperature in cases of diffusion-controlled conversion into CO and CO2 at the corresponding process conditions (5, 15, and 30 vol % O2) are also included in Figure 9. It can be clearly seen that when the oxygen volume fraction and particle temperature increases, the measured population shifts from the CO2 curve toward CO curve. In Table 3, the calculated temperatures for the measured average particle size are also given. The existing literature shows differences in particle temperatures measured at nearly comparable conditions. Some results of the measurements of other research groups are shown in Figure 10. The combustion conditions in the measurements carried out by Monson et al.7 are closest to the present ones. The other experiments were made at atmospheric pressure and a different gas temperature. The gas temperature in the measurements of Young et al.23 was about 150 K lower than in the present case, while in the measurements of Schroeder et al.36 and Mitchell and McLean37 it was about 150 K higher. In the measurements made by Timothy et al.,38 Hurt,27 and Waters et al.24 it was 300550 K higher. The comparison shows that the gasparticle temperature difference measured in the present experiments agrees well with the results of Monson et al.7 and Young et al.23 at the oxygen volume fractions 3-10 vol %, while the other groups27,36-38 have measured excess temperatures for particles, which are lower by 150-300 K. There are several explanations for the wide scatter of the results. One explanation of the higher temperatures in the present experiments is the elevated pressure. It can explain the difference between 100 and 300 (36) Schroeder, A. R.; Thompson, D. M.; Daves, G. G.; Buckius, R. O.; Krier, H.; Peters, J. E. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 1161. (37) Mitchell, R. E.; McLean, W. J. 19th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1113. (38) Timothy, L. D.; Sarofirm, A. F.; Be´er, J. M. 19th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1123.


K compared to the experiments in atmospheric pressure with hvb coals27,36,38 and carbon.24 Also, a higher gas temperature24,27,36-38 leads to a smaller gas-particle temperature difference. One reason for this is that with increasing temperature the product at the particle surface preferentially becomes CO with a lower heat of combustion compared to CO2, the proportion of which is reducing.7,39 Heat loss by radiation also increases at higher temperatures, reducing the temperature difference. At very high temperatures, the dissociation reaction reduces the heat generation in combustion of CO in the vicinity of the particle. The temperature history of the particle and the phase of combustion are significant to the measured temperature. In some cases the type24 or pretreatment7,23,27,37 of fuels was also different. The radiative heat transfer between the particle and surrounding walls is totally different in reactors with quartz walls that are not externally heated.23,24,27,37 At higher oxygen volume fractions, the scatter increases. In this case the excess temperatures of the present measurements are the highest ones reported. Standard Deviation of Temperature Distribution. The standard deviation of temperatures of individual fuel particles, σT, at fixed process conditions varied from about 40 to 130 K (cf. Tables 2 and 3). This shows that the variation of the temperatures of individual fuel particles is not negligible, even in wellcontrolled process conditions. The effect of the oxygen volume fraction on the standard deviation was relatively weak. For Gardanne lignite, the σT clearly increases (from 28-83 K) with an increasing O2 volume fraction, which is partially caused by the increased Dp-Tp dependence (Figure 9). For Polish hvb coal, the effect is the opposite but weaker (σT ) 45-73 K). For Westerholt hvb coal, σT remains nearly constant (52-57 K) except for a higher σT (86 K) at 30 vol %, which is caused by particles burning at a significantly lower temperature than the main population of particles. Table 2 shows that σT of Polish and Go¨ttelborn hvb coals and Niederberg anthracite increases with decreasing pressure. For Gardanne lignite, σT was not affected by pressure but there is a clear difference between values measured at different residence times. Also in the experiments with anthracite it can be seen that during a rapid combustion phase σT is somewhat higher than during final burnout. Since the pressure and O2 volume fraction may even have opposite effects on σT of different fuels and since the effect of residence time also varies with fuel, it is difficult to make a general comparison between σT of different fuels. The magnitude of the fluctuation of process conditions may also vary between experiments, and the preparation and preservation of fuel samples may have an effect on the measured values of σT. Thus, strong conclusions based on σT of measured particle populations should not be drawn. However, it is clear from visual observation of Dp-Tp plots (Figures 6, 7, and 9) of all the experiments that the widths of the temperature distributions of Niederberg anthracite are distinctly wider than the ones of other fuels measured in the present experiments. Hurt et al.40 have recently made a comparison be(39) Hayhurst, A. N.; Parmar, M. S. Chem. Eng. Sci. 1998, 53, 427.


tween the σT of coals of various rank. They showed that σT is larger for low-reactivity coals than for highreactivity coals. This agrees well with the results of our measurements. They also showed that the σT of bituminous coals is larger at higher oxygen concentrations. This is not supported by our experiments with bituminous coals, whereas a similar behavior was observed for Gardanne lignite. Pulverized coal is a mixture of heterogeneous particles.41 The chemical and physical properties of coal, such as H/C ratio, ash content, internal surface area and sulfur content, may depend on the initial particle size,42 but particle-to-particle differences between particles of equal size exist. In addition to the effect of unburned carbon in fly-ash, this scatter has an effect on the temperature and gas concentrations fields in a furnace. There is a clear trend of increasing temperature with decreasing particle size, which can be predicted by modeling, but great particle-to-particle differences are also seen between particles of equal size. The particleto-particle differences in chemical reactivity properties are probably the most important reason for the difference in temperatures of particles of equal size. The more homogeneous a coal is, the smaller is the temperature difference between particles of equal size and the more predictable the combustion behavior in a furnace. Distribution in the char reactivity is required to improve models for predicting unburned carbon loss from pulverized fuel furnaces.43,44 It is possible to describe the char reactivity of different coals by a single activation energy and coal-related frequency factor.45 Also particleto-particle variation in the reactivity of a specific coal can be accounted for by using a single activation energy but distributed frequency factor.40,46-48 It has a significant influence on the carbon loss and abundance of small particles in the unburned carbon size distribution.49 Some of the differences can be explained by the particle-to-particle variation in mineral matter, especially Gardanne lignite, which contains 33% ash of which a great part is Ca that is known50 to promote the production of CO2. The differences in the ratio of CO/ CO2 formed on the particle surface can greatly affect the particle temperatures. This ratio may differ from one fuel to the other or from one particle to the other. All correlations presented in the literature39 predict that (40) Hurt, R. H.; Lunden, M. M.; Brehob, E. G.; Maloney, D. J. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3169. (41) Gibbins, J.; Beeley, T.; Williamson, J. Fundamentals of Pulverised Coal Heterogeneity with Particular application to burn-out prediction; ETSU-COAL-R-098; Energy Technology Support Unit: Harwell, U.K., 1997. (42) Palmer, A. D.; Cheng, M.; Goulet, J.-C.; Furimsky, E. Fuel 1990, 69, 183. (43) Walsh, P. M.; Xie, J.; Douglas, R. E.; Battista, J. J.; Zawadzki, E. A. Fuel 1994, 73, 1074. (44) Williams, A.; Pourkashanian, M., Jones, J. M.; Rowlands, L. J. Inst. Energy 1997, 70, 102. (45) Fu, W. B.; Zhang, B. L.; Zheng, S. M. Combust. Flame 1997, 109, 587. (46) Sahu, R.; Northrop, P. S.; Flagan, R. C.; Gavalas, G. R. Combust. Sci. Technol. 1988, 60, 215. (47) Hurt, R. H. Coal Sci. Technol. 1995, 24, 611. (48) Beeley, T.; Crelling, J.; Gibbins, J.; Hurt, R.; Lunden, M.; Man, C.; Williamson, J.; Yang, N. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3103 (49) Walsh, P. M. Energy Fuels 1997, 11, 965. (50) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1991, 5, 214.


the amount of CO in relation to CO2 is increased at a higher particle temperature. At high temperatures CO2 may become the effective product due to rapid oxidation of CO in the vicinity of the particle surface, especially at high pressures. Also, the reverse dissociation reaction CO2 f CO may take place. The temperature differences are partly due to differences in the particle shape. More compact particles have been found to combust at higher temperatures than large particles with irregular shapes.36 Some shapes promote increased heat and mass transfer between the particle and gas more than other ones. Particle-to-particle variation in the emissivity of combusting particles of equal size partly accounts for the differences of the measured particle temperatures. The ratio of radiation and conduction heat transfer between the particle and its surroundings is hh/hm ≈ σTp3Dp/ Nuφhλ. Thus, the effect of emissivity on the particle temperature becomes enhanced with increasing particle temperature and size. For example, for Dp ) 100 µm and Tg ) Tw ) 1150 K, the calculated temperature elevations resulting from a drop in emissivity from 0.9 used in the calculations to 0.7 are 18, 56, and 100 K at particle temperatures of 1500, 2000, and 2500 K, respectively. Thus, the particle-to-particle scatter in emissivity can partly explain the observed temperature scatter. This could also be a reason for the few temperature points for larger diameters that are located above the curve of diffusion-limited combustion with CO2 production. For a larger particle size, Dp ) 200 µm in the same situation, the corresponding temperature elevations are 30, 80, and 132 K, respectively. Equation 5 may even predict two steady-state solutions for the particle temperature in some range of gas temperatures. The prevailing state then depends on the particle temperature history. In this case the particle density and specific heat may also have an influence on the particle temperature. If the particle temperature after the devolatilization stage is low, then k may remain low, but if initially the temperature is high, then k may become large enough and the particle temperature remains well above the gas temperature. In addition, small changes in gas temperature or particle reactivity can bring about relatively large particle temperature changes under certain conditions.27 It is difficult to develop a model for the reactivity distribution relying solely on measured particle temperatures; a high temperature of an individual particle does not necessarily correspond to high reactivity, since particle-to-particle variations in the ratio of CO/CO2 production, particle shape, and emissivity affect the particle temperature as well as the reactivity. The separate effects of physical and chemical properties on the scatter in the particle temperatures has been studied by Hurt.27 Comparison of Burnoff of Different Fuels. Various ways to correlate the reactivity of coal char with different easily measured properties of coals have been presented.51-55 According to the correlation which predicts char reactivity to decrease with increasing carbon content of the parent coal,51 the coals studied can be (51) Hurt, R. H.; Mitchell, R. E. 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 1243 (52) Liu, G.; Erbar, R. C. Fuel Sci. Technol. Int. 1993, 11, 463.

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grouped in ascending order with respect to reactivity as follows: Niederberg anthracite, Westerholt hvb coal, Polish and Go¨ttelborn hvb coal, Gardanne lignite. The distribution of mineral matter in pulverized coal particles in relation to burnoff behavior has been studied,56 but no clear dependence was found. When comparing burning times and reactivities of different coal chars, it should be noted that in addition to the kinetic reactivity parameters, the density and the ash content of the char particle affect the combustion time. The density of the char particle is related to the density of the parent coal particle F0 by, Fp ) (1 - ν) × F0/S, where S is the volumetric swelling ratio and v is the actual volatile mass fraction that exceeds the volatile matter of the proximate analysis due to higher peak temperature and heating rate. It is seen from eq 1 that the combustion time is proportional to (1 - a)Fp. The initial density of the particle also has an influence on the reaction mode of porous carbon particles,57 but here analysis is restricted to the shrinking particle model. The time to reach a specific conversion of char or the burning time (mc/mc0 ) 0) can be obtained by integrating eq 1

(

Dp02 (1 - a)Fp tc ) [1 - (mc/mc0)2/3] + YO2,∞ 4fShφmFgDO2 Dp0 2k

)

[1 - (mc/mc0)1/3] (9)

with respect to the particle diameter, when the particle temperature is approximated as constant. The burning time of a char with higher chemical reactivity can even be longer than that of a less reactive one due to differences in char density Fp and ash content, as shown by eq 9. The organic char mass/initial char mass is mc/ mc0 ) (Dp/Dp0)3. The char density will be smaller for coals having a greater fraction of volatiles. The yield of volatiles and, subsequently, char density are also affected by the maximum temperature reached by the particle during devolatilization. The actual volatile matter released during devolatilization was estimated in the calculations here by a formula presented in the literature58 by using the calculated particle temperatures. However, the maximum temperature reached during devolatilization may greatly exceed the temperatures attained during char combustion. This depends on the temperature and oxygen content of the gas and the particle size;15 in addition, pressure may have some effect. The normalized mass of the char particle is related to the measured normalized total mass by mc/mc0 ) [(1 - a)/(1 - a - v)]m/m0. The char particle having a less combustible material will burn in a shorter time, if other conditions are the same, but naturally a coal having a higher chemical reactivity gives a higher volumetric combustion power in the furnace, since the surface mass flux from particles is higher. The effect of pressure is minor for reactive coals burning close to the diffusion limit, since diffusion is (53) Hampartsoumian, E.; Pourkashanian, M.; Williams, A. J. Inst. Energy 1989, 62, 48. (54) Cloke, M.; Lester, E. Fuel 1994, 73, 315. (55) Cloke, M.; Lester, E.; Gibb. W. Fuel 1997, 76, 1257. (56) Wigley, F.; Williamson, J.; Gibb, W. H. Fuel 1997, 73, 1283. (57) Essenhigh, R. H. Combust. Flame 1994, 99, 269. (58) Zhang, Y. P.; Mou, J. M.; Fu, W. B. Fuel 1990, 69, 401.


Figure 11. Effect of pressure at constant O2 volume fraction (10 vol %) on the ratio of organic mass, m, and initial mass, m0, of particles of Polish hvb coal (Pol), Gardanne lignite (Gar), Go¨ttelborn hvb coal (Go¨t), and Niederberg anthracite (Nie A, Nie B). The residence time corresponding to the measurement position and the gas temperature are given in the legend. The initial nominal particle size was 100-125 µm for Nie B and 140-180 µm for the others.

not affected by pressure and is higher for less reactive coals.8 This trend is clearly shown by our measurements here. The increase of pressure is clearly shown to increase the combustion rate of the Polish coal (Figure 11), which has also been shown earlier8 in a different situation. This is probably not due to an effect of pressure on the burning rate of char but the increase of pressure may affect the intensity of gas-phase reactions and energy feedback around the particle. The effect of pressure is shown to increase the mass loss in a short residence time of 100 ms (Figure 11), which supports this conclusion. It has been found using measurements and modeling15 that this coal is quite sensitive to the combustion atmosphere. It has even been found that the combustion time of a larger particle can be shorter than that of a smaller one, if the conditions are suitable.15 The burning time of Gardanne lignite and Go¨ttelborn hvb coal are not so much affected by the increase in the pressure (Figures 4 and 11) as the Polish coal. It is seen that Gardanne lignite and Go¨ttelborn hvb coal are the most reactive. However, there is some consistent difference in the combustion behavior. The burnoff of Gardanne lignite is quite insensitive to pressure, but the burnoff of Go¨ttelborn hvb coal is constant in the pressure range 0.2-0.5 MPa and then increases slowly with increasing pressure in the range 0.5-0.8 MPa. The increase of pressure has the strongest effect on the combustion rate of the Niederberg anthracite (Figures 4 and 11). Anthracite contains a low amount of volatiles. Thus, the increase of the burning rate at higher pressures is due to the higher chemical reaction rate of char with oxygen. The combustion rate of Niederberg anthracite is clearly under the diffusion level. At low conversion, when devolatilization may still take place and under 20% of the organic mass is released (cf. Table 2), the increase in the pressure has a slight effect in the pressure range 0.3-0.4 MPa but no effect in the range 0.5-0.8 MPa. At higher conversions and smaller particle size, the combustion rate increases with increasing pressure more clearly. Determination of Pressure Dependency of Ki-


Figure 12. Chemical surface reaction rate coefficient k. The oxygen content in the gas (in vol %) is attached for the cases denoted by filled symbols. For other cases, the gas oxygen content is 10 vol %.

netic Reactivity Parameters. The influence of pressure on the apparent kinetic parameters,7 kinetics,59 and on the relative importance of adsorption, desorption, and boundary-layer diffusion60,61 has been studied recently. The measured burnoff is used to determine the kinetic parameters. The reaction rate coefficient k can be solved from eq 9

k)

2YO2,∞tc (1 - a)FpDp0

-

1 - (m/m0)1/3 Dp0 2fShφmFgDO2

(10) 2/3

[1 - (m/m0) ]

By using this equation, the reaction rate coefficient corresponding to each measured burnoff can be evaluated. The results based on measured mass losses at a given time are presented in Figure 12 for Niederberg anthracite. The char burning stage was roughly estimated to start after 65 and 130 ms for 100-125 and 140-160 µm particles, respectively. The average particle size of the range was used as the initial particle size. The average particle temperatures were estimated using eq 5 and particle diameters corresponding to average conversion 0.5(1 + mc/mc0). All the points for cases where the oxygen content of the atmosphere is 10 vol % (open symbols) but total pressure varies are located practically on a single line, indicating an Arrhenius-type relation between k and temperature. No significant effect of pressure can be seen here. Gas oxygen content has a much greater effect on the reaction rate coefficient. However, a clear increase of burning rate at higher pressures has been shown earlier (Figures 4 and 11). It may be that the temperature here is such that a substantially small increase in the reaction rate due to pressure has a favorable net effect on the burning rate. Summary and Conclusions A pyrometric method capable of simultaneous in situ measurement of the temperature and size of individual fuel particles was used to study the combustion of (59) Croiset, E.; Mallet, C.; Rouan, J.-P.; Richard, J.-R. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3095. (60) Essenhigh, R. H.; Mescher, A. M. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3085. (61) Essenhigh, R. H.; Mescher, A. M. Combust. Flame 1997, 111, 350.


particles in a pressurized entrained flow reactor. Several series of measurements were carried out in order to study the effects of the pressure and oxygen volume fraction on the particle temperatures of various coals. The effect of total pressure on particle temperatures was found to vary between different fuels. The average temperature of Gardanne lignite remained nearly constant, and the average temperature of Polish and Go¨ttelborn hvb coals increased slightly with increasing pressure. However these changes were small compared to the particle-to-particle standard deviation. The strongest effect of pressure on the average particle temperature was found for Niederberg anthracite. On the basis of an interpretation of burnoff measurements by a simple model, pressure was found to have no significant effect on the apparent chemical reaction rate coefficient in the shrinking-particle burning model compared to the effects of temperature and oxygen content. The previous conclusion8 that pressure has the greatest effect on such chars which burn farthest from the diffusion limit (the least reactive chars) is clearly reflected by the measurements. The oxygen volume fraction was found to have a very strong effect on particle temperatures, which can also be predicted by the model. The particle excess temperatures measured in the present study were higher than those reported earlier in the literature. At high oxygen levels (c[O2] > 15 vol %), the difference from the previous measurements was especially significant. The model predicts that there is a maximum particle temperature with a specific size. However, due to the scatter in particle temperatures caused by the coal heterogeneity and due to the detection limit of the temperature measurements, this theoretical conclusion could not be validated experimentally. Nomenclature a ) ash content b ) boundary-layer coefficient c ) specific heat, J kg-1 K-1 DO2 ) diffusivity of oxygen in gas, m2 s-1 Dp ) particle diameter, m f ) mass ratio of carbon and oxidizer ∆H ) heat of reaction, J kg-1 h ) heat-transfer coefficient, W m-2 K-1 hm ) mass-transfer coefficient, m s-1 k ) effective reaction coefficient, m s-1 M ) molecular weight, kg kmol-1 m ) organic mass, kg m ˘ ′′ ) mass flux, kg m-2 s-1 Nu ) Nusselt number p ) pressure, Pa q ) radiation flux, W m-2 Re ) Reynolds number Rg ) ideal gas constant, 8.314 kJ kmol-1 K-1 r ) radial coordinate, m Sh ) Sherwood number S ) volumetric swelling ratio T ) temperature, K t ) time, s Y ) mass fraction v ) volatile mass fraction a ) temperature exponent for heat conductivity β ) temperature exponent for diffusivity φh ) coefficient for reduction of heat transfer φm ) coefficient for reduction of mass transfer

Joutsenoja et al. ) emissivity λ ) heat conductivity of gas, W m-1 K-1 F ) density, concentration (mass/volume) σ ) Stefan-Boltzmann’s constant, 5.67 × 10-8 W m-2 K-4 Indexes B ) at-boundary layer temperature b ) 90% burnoff c ) char g ) gas r ) radiation O2 ) oxygen p ) particle surface ref ) reference s ) gas at particle surface 0 ) initial state ∞ ) gas far from particle

Appendix Temperature-Dependent Gas Properties. Usually the transport properties are evaluated without further justification at the average boundary layer temperature Tb ) 0.5(Ts + T∞), which is just an assumption. It is shown here that this estimate, indeed, gives a good approximation. The temperature gradient near a combusting particle is steep, and the transport properties and gas density depend strongly on temperature (λ ≈ T3/4, DO2 ≈ T7/4/p, Fg ≈ p/T). It is assumed here that no CO oxidation takes place in the boundary layer of a small particle,16 but in extreme conditions of high temperature and pressure and large particle size, this assumption may not be valid. The conservation equations for energy and oxygen in a stagnant boundary layer around a small particle (Re ) 0) are

(

)

(

)

dYO2 dTg d d λr2 ) 0, FgDO2r2 )0 dr dr dr dr

(A1)

with boundary conditions Tg(R) ) Ts, Tg(∞) ) T∞, YO2(R) ) YO2,s and YO2(∞) ) YO2,∞. The equation for energy can be integrated twice and the following relationship between gas temperature and radial location around the particle is obtained

G(Tg) - G(Ts) G(T∞) - G(Ts)

) 1 - R/r, where G(x) )

∫λ dx

(A2)

The heat-transfer coefficient h ) λNu/Dp between gas and particle is defined as

( )

h(T∞ - Ts) ) λs

dTg dr

r)R

)

λs G(T∞) - G(Ts) R G′(Ts)

(A3)

where G’(Ts) ) F(Ts) ) λs. The heat-transfer coefficient is h ) λb/R. The boundary-layer temperature Tb is defined so that Nu ) hDp/λb ) 2, where λb ) λ(Tb). The boundary-layer coefficient b in the relation Tb ) bTs + (1 - b)T∞ is defined as b ) (Tb - T∞)/(Ts - T∞). In the following the heat conductivity and the product of density and diffusivity are expressed in the forms

λ ) λref(Tg/Tref)R, FgDO2 ) Fg,refDO2,ref(Tg/Tref)β (A4) where the values at reference temperature are known. The temperature distribution around the particle be-



comes

Tg ) [Ts1+R + (T∞1+R - Ts1+R)(1 - R/r)]1/(1+R)

(A5)

The boundary-layer temperature is

(

Tb ) F-1

) (

)

1+R G(T∞) - G(Ts) - Ts1+R 1/R 1 T∞ ) T∞ - Ts 1+R T∞ - Ts (A6)

where F-1(x) is the inverse function of F(x). The equation for the conservation of oxygen in the boundary layer can be integrated

YO2 - YO2,s YO2,∞ - YO2,s [T∞1+R

-

)

Figure A1. Boundary-layer coefficient b for temperaturedependent heat conductivity and diffusivity, when R ) β ) 3/4.

∫Rr F Ddr r2/∫R∞F Ddr r2 ) g

O2

g

O2

- Ts1+R)R/r]1-β/(1+R) T∞1+R-β - Ts1+R-β

(T∞1+R

- Ts1+R-β

(A7)

where the last form is obtained by using eqs A4 and A5. The mass-transfer coefficient, when Sh ) 2, is

Fbhm )

FbDO2,b R

)

FsDO2,s

( ) (∫ dYO2

YO2,∞ - YO2,s dr

)

r)R 2 ∞ R R

)

dr FgDO2r2

-1

(A8)

The boundary-layer temperature Tb at which FgD ) FbDb can be solved

Tb )

(

1+R - Ts1+R 1 + R - β T∞ 1 + R T 1+R-β - T 1+R-β ∞

s

)

1/β

(A9)

In the special case when R ) β, which is the case here, the boundary-layer temperature for diffusion is equal to that for heat conduction defined by eq A6. The gas heat conductivity and diffusivity of oxygen have a significant effect on the particle temperature. They themselves depend on temperature which varies greatly in the vicinity of the particle. It can be seen (Figure A1) that using the average boundary-layer temperature corresponding to the boundary-layer coefficient b ) 0.5 gives a good approximation, since the coefficient varies between 0.48 and 0.52. EF980139J

Effects of Pressure and Oxygen Concentration on the Combustion of

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