Combustion Characteristics of Fine- and Micro-pulverized Coal in the

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Combustion Characteristics of Fine- and Micro-pulverized Coal in the Mixture of O2/CO2 Xiangyong Huang, Xiumin Jiang,* Xiangxin Han, and Hui Wang Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed June 9, 2008. ReVised Manuscript ReceiVed August 7, 2008

The effects of oxygen concentration, particle size, and heating rate on the coal combustion characteristics under an O2/CO2 atmosphere were investigated. The results indicated that the oxygen concentration played the most important role. As the oxygen concentration increases, the ignition and burnout temperatures decrease and the comprehensive combustion property index S increases. Moreover, the improvement of the oxygen concentration intensified the effects of the other factors. The ignition mechanism changes from heterohomogeneous type to homogeneous type as the oxygen concentration increases. The ignition and burnout temperatures decrease slightly as the mean particle size decreases, and the index S increases measurably as the mean particle size decreases. The heating rate has different effects on the ignition temperature, burnout temperature, and index S at different oxygen concentrations.

1. Introduction Special attention has been paid to NOx and SOx emissions from coal-fired combustors for a long time,1 and in recent years, the international society and governments have become aware of the significant greenhouse problem brought by CO2 coming from combustion of fossil fuels, especially coal.2,3 Many scholars have appealed that it is urgent to develop feasible and economic CO2 reduction technologies. O2/CO2 combustion, using a mixture of pure O2 and recycled flue gas as an oxidant during coal combustion, is one of the developing technologies for CO2 sequestration. The combustion of coal in the O2/CO2 atmosphere will result in a much higher CO2 concentration in flue gas than in conventional coal combustion (using air as an oxidant), which can greatly reduce the cost of CO2 capture. Moreover, the technology can also result in lower NOx and SOx emissions.4-6 Therefore, the technology is being considered as a promising and effective technology in CO2 reduction as well as in other pollutant emissions control. However, there still exist many difficulties on the way to implementation. Previous results have demonstrated that the substitution of CO2 for N2 retarded coal particle ignition and reduced flame stability and flame propagation velocity.7-9 It was also found that coal combustion in the CO2/O2 atmosphere showed a lower flame temperature.6 These unexpected features * To whom correspondence should be addressed. Telephone: +86-2134205681. Fax: +86-21-34205681. E-mail: [email protected]. (1) Li, Z.; Yang, L.; Qiu, P.; Sun, R.; Chen, L.; Sun, S. Int. J. Energy Res. 2004, 28 (5), 511–520. (2) Wall, T. F. Proc. Combust. Inst. 2007, 31, 31–47. (3) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (4) Croiset, E.; Thambimuthu, K. V. Fuel 2001, 80 (14), 2117–2121. (5) Okazaki, K.; Ando, T. Energy 1997, 22 (2-3), 207–215. (6) Nozaki, T.; Takano, S.; Kiga, T.; Omata, K.; Kimura, N. Energy 1997, 22 (2-3), 199–205. (7) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31 (2), 1905– 1912. (8) Suda, T.; Masuko, K.; Sato, J.; Yamamoto, A.; Okazaki, K. Fuel 2007, 86 (12-13), 2008–2015. (9) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84 (7-8), 833–840.

are unfavorable to its application, and many experts and engineers are seeking solutions to overcome the difficulties. For example, increasing the oxygen concentration is considered one of feasible ways to increase combustion stability. However, it will raise the operation cost and may bring some new issues in practical operation, such as decreasing the flexibility of load adjustment. These challenges originate from the inadequate understanding of the combustion process under the O2/CO2 atmosphere. Coal ignition and burnout are important processes during coal combustion and essential factors to be considered in boiler design, which have significant influence on the other combustion characteristics, such as flame stability, pollutant formation and emission, and flame extinction. However, little research has been conducted under the O2/CO2 atmosphere,7,10,11 and more effort is needed to further study and understand the complex processes. Moreover, little previous literature is found to evaluate the effect of the particle size of coal, and not enough information focuses on the effect of the oxygen concentration and heating rate. According to our previous studies in a conventional combustion atmosphere,12-14 these factors have noticeable effects on the combustion characteristics. This paper mainly aims to clarify the effect of the particle size and provide more information on the effects of the oxygen concentration and heating rate on thermal gravimetric (TG)/differential thermal gravimetric (DTG) curves and ignition and burnout characteristics. Furthermore, a comprehensive combustion property index (S) is introduced to assess the combined influences of the three factors. Although the results cannot be applied directly to pulverized coal (10) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kato, M. Energy ConVers. Manage. 1997, 38, 129–134. (11) Arias, B.; Pevida, C.; Rubiera, F.; Pis, J. J. Fuel 2008, 87 (12), 2753–2759. (12) Jiang, X.; Zheng, C.; Qiu, J.; Li, J.; Liu, D. Energy Fuels 2001, 15, 1100–1102. (13) Zhang, C.; Jiang, X.; Wei, L.; Wang, H. Energy ConVers. Manage. 2007, 48, 797–802. (14) Jiang, X.; Li, J.; Qiu, J. Chin. Soc. Electr. Eng. 2000, 20 (6), 71– 74, 78 (in Chinese).

10.1021/ef800444c CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

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Table 1. Proximate and Ultimate Analyses of TieFa Bituminous Coal proximate analysis (wt %) (ad) moisture (mass %) volatile (mass %) ash (mass %) fixed carbon (mass %)

5.82 30.30 22.65 41.23

ultimate analysis (wt %) (ad) C H O N S

55.69 3.88 10.62 0.75 0.59

combustion, they will provide useful information on the effects of O2/CO2 atmospheres. 2. Experimental Section 2.1. Material and Test Methods. TieFa (Tf) bituminous coal was chosen for the experiments. The properties of the coal are presented in Table 1. The coal samples were pulverized into three different mean particle sizes using a jet mill. As analyzed by the Malvern MAM5004 Laser Mastersizer made in the U.K., the resulting mean particle sizes of the samples were 11.34, 18.95, and 33.68 µm. Temperature-programmed combustion experiments were carried out in a DTG-60H thermal analyzer made by Shimadzu Company that combines TG and differential thermal analysis (DTA) measurements. A coal sample of ∼10 mg was placed in a platinum crucible and heated at different heating rates (10, 30, and 50 °C min-1) in an O2/CO2 mixture atmosphere from room temperature to 1200 °C. Four types of O2/CO2 mixtures with volumetric mixture ratios of O2/CO2 ) 1:9, 1:4, 1:1, and 4:1, respectively, were employed, which covered the range of the oxygen concentration in practice. The flow rate of the mixture gas was 80 mL min-1. 2.2. Determinations of Ignition and Burnout Temperatures. There are several methods to determine the ignition and burnout temperatures according to the TG/DTG curves.15-18 The values obtained from different methods are slightly different, but as long as a consistent definition of these quantities is used, then they can be compared quantitatively. In this work, a widely adopted determination of the ignition and burnout temperatures was used to characterize the ignition and burnout processes. As shown in Figure 1, the determination of the ignition temperature was described as follows:15 A vertical line (dash) is drawn through the DTG peak point to intersect the corresponding TG curve at point A. Then, at point A, a line (solid oblique line) tangent to the TG curve is introduced. As expected, the tangent line will cut the horizontal line (dash dot) starting from the point of initial weight loss on the TG curve at point B. The temperature corresponding to the point B is defined as the ignition temperature. The burnout temperature is defined as the temperature at which the rate of combustion at the end of major combustion diminishes to 1% min-1.18

3. Results and Discussion 3.1. Oxygen Concentration Effects. Figure 2 shows the TG/ DTG curves of coal samples with a mean particle size of 11.34 µm at the heating rate of 10 °C min-1 and various oxygen concentrations (10, 20, 50, and 80%). From the TG/DTG curves, it can be concluded that the moisture of the samples is lost in a range of 100-140 °C, followed by the primary devolatilization and char combustion above 240 °C. It is easily observed that the oxygen concentration has a significant effect on the coal particle combustion. The (15) Ma, B.; Li, X.; Xu, L.; Wang, K.; Wang, X. Thermochim. Acta 2006, 445, 19–22. (16) Pranda, P.; Prandova, K.; Hlavacek, V. Fuel Process. Technol. 1999, 61, 211–221. (17) Sun, C. L.; Zhang, M. Y. Combust. Flame 1998, 115, 267–274. (18) Choudhury, N.; Biswas, S.; Sarkar, P.; Kumar, M.; Ghosal, S.; Mitra, T.; Mukherjee, A.; Choudhury, A. Int. J. Coal Geol. 2008, 74, 145– 153.

Figure 1. Ignition temperature definition sketch.

profiles of the curves at lower oxygen concentrations (10 and 20%) are greatly different from those at higher oxygen concentrations (50 and 80%). Under the oxygen-lean conditions, from 240 to about 600 °C, the TG curves have a monotonically decreasing slope and the corresponding DTG curves show a single peak or two overlapped peaks. However, under the oxygen-rich conditions, the TG curves are separated by turning points into two-segment curves and their corresponding DTG curves give two completely separated peaks. The difference indicates that the devolatilization and char combustion stages are partially or completely overlapped at the lower oxygen concentrations, while the two stages are clearly separated at the higher oxygen concentrations. This is mainly due to the different behaviors of devolatilization and volatile matter at different oxygen concentrations. Under the oxygen-lean conditions, the low-oxygen atmosphere retards the ignition of both the volatile matter and the char, especially that of volatile matter. The effect of oxygen is to change the ignition mechanism of coal particles: homogeneous or heterogeneous ignition. When the sample is ignited homogeneously, two separated exothermic peaks appear on the DTG curve. While for heterogeneous ignition, only one exothermic peak can be found on the DTG curve.19 Considering the DTA curves in Figure 3, it can be concluded that a heterohomogeneous (O2: 10%) or homogeneous ignition (O2: 20%) occurs at a relatively higher temperature, which results in an overlap of the two stages; thus, the mass loss contribution of the two stages cannot be determined easily. However, it is different at the higher oxygen concentrations (50 and 80%), far from the low-oxygen atmosphere, which is expedient for the ignition of both volatile matter and coal char. The volatile matter released from the coal particles can be ignited at about 250 °C. The heat from the volatile combustion then accelerates the heating process of the coal particles and advances the release of the volatile matter; thus, more heat is released. The selfacceleration process at the higher oxygen concentration greatly increases the devolatilization rate and results in the sharp peaks around 250 °C in the DTG curves. Moreover, two exothermic peaks in the DTA curves (see Figure 3) verify the two corresponding exothermic stages, indicating that the volatile matter ignited first and then the char began to burn. Therefore, the ignition at the higher oxygen concentrations is concluded to be via a homogeneous ignition pathway.19 It is interesting that the two peaks in DTG curves are completely separated and there exists a temperature difference of about 150 °C between them, because of the following reasons: (1) access of oxygen to the char is impeded by the combustion of the volatile matter, and thus, the ignition of the char is delayed; (2) the combustion rate of the char is small when it is (19) Chen, Y.; Mort, S.; Pan, W. P. Thermochim. Acta 1996, 275, 149– 158.

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Figure 2. TG/DTG curves of the coal sample (particle size, 11.34 µm; heating rate, 10 °C/min).

Figure 3. DTA curves of the coal sample (particle size, 11.34 µm; heating rate, 10 °C/min). Table 2. Parameters of the TG/DTG Curves (11.34 µm and 10 °C/min) oxygen fraction (%)

turning point (°C)

mass loss fractiona

50 80

284.36 266.70

60.24/39.76 75.69/24.31

a

DTG peaks (°C) first second peak peak 266.4 255.3

456.5 462.1

Devolatilization/char combustion mass loss fraction.

just ignited. For simplicity, the turning points in the TG curves can be regarded as the ending temperatures of devolatilization (but not the initial temperatures of char combustion). As a result, the mass loss fractions of devolatilization and char combustion can be figured out easily according to the locations of the turning points. Table 2 gives some specifics of these features, showing that the mass loss fraction of devolatilization at an oxygen concentration of 50% is less than that at an oxygen concentration of 80%, because the comparatively higher oxygen concentration makes the combustion of volatile matter occur more easily and rapidly, amplifying the self-acceleration effect. The greater combustion rate of volatile matter consumes more oxygen, and thus, enhances reduce the access of oxygen to the char surface, as mentioned above. At this condition, the time for devolatilization is prolonged and more organic matter is converted into volatile matter than that at the comparatively lower oxygen concentration. However, the peaks in the DTG curves indicate that the oxygen concentration does not seem to have a significant effect on the reaction temperature ranges of the two stages, with differences of only several centigrade (the first peak, 266 versus 255 °C; the second peak, 456 versus 462 °C).

Figure 4a shows that the ignition temperature decreases as the oxygen concentration increases, varying from 440-448 °C at an oxygen concentration of 10% to 273-280 °C at an oxygen concentration of 80%, which is consistent with the results of Molina et al.7 There is significant difference between ignition temperatures at different oxygen concentrations. The data clearly indicate that the oxygen concentration has a remarkable effect on the ignition temperature. The reasons for this phenomenon are that the reaction rates of both devolatilization and char combustion increase as the oxygen concentration increases; moreover, the low CO2 concentration in the oxidant stream inhibits the char surface reaction less. Under this favorable condition, the total reaction rate is enhanced obviously, which decreases the ignition temperature noticeably. It can be observed that the differences among ignition temperatures at different heating rates and with different particle sizes are relatively small when the oxygen concentration is high (g50%) but more significant when the oxygen concentration is below 50%. The maximum difference among the five cases is up to 98 °C (O2: 10%) and 239 °C (O2: 20%) at oxygen concentrations of 10 and 20%, respectively. Whereas at oxygen concentrations of 50 and 80%, it is only 6 °C (O2: 50%) and 14 °C (O2: 80%). It can be explained that the heating rate and particle size may have a greater effect on diffusion properties of coal particles than kinetics properties at low temperature. As a result, it has a greater influence on the ignition temperature at lower oxygen concentrations (diffusion control conditions) than that at higher oxygen concentrations (kinetics control conditions). Figure 4b shows that the effect of the oxygen concentration on the burnout temperature is similar to that of the ignition temperature, which can be explained as that of the ignition temperature. From the discussion, the ignition and burnout temperatures decrease as the oxygen concentration increases, which means that the mass loss shifts to a lower and narrower temperature range as the oxygen concentration becomes higher, owing to the promoting effect of oxygen on coal combustion. 3.2. Heating Rate Effects. Figure 5 shows the TG/DTG curves of a coal sample with a mean particle size of 11.34 µm at oxygen concentrations of 20 and 50%. It can be seen that at different oxygen concentrations the heating rate plays different roles. At lower oxygen concentrations (10 and 20%), the TG/DTG curves are affected obviously by the heating rate. As the heating rate decreases, the mass loss profiles shift to a higher temperature range. The reason may be the time lag because of conductive heat transfer at the higher

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Figure 4. Effect of the oxygen concentration on the ignition and burnout temperatures.

Figure 5. TG/DTG curves of a coal sample with mean particle size of 11.34 µm.

heating rate. At a small heating rate, the temperature distribution within the coal particles is more homogeneous. Consequently, the reaction can proceed more sufficiently, and the loss of mass by the coal sample occurs over a more narrow temperature range. However, at the higher heating rate, the temperature climbed to a higher level before the reaction proceeds sufficiently. At the higher oxygen concentrations (50 and 80%), a slight effect can be observed. All of the curves have almost the same temperature range of mass loss, from 210 to 550 °C. The main differences exist in the turning points. They appear earlier as the heating rate decreases. Unlike the smooth reaction at the lower oxygen concentrations, the reaction around the turning point behaves more intensely at the higher oxygen concentrations, which may be affected by the thermal lag induced by the higher heating rate. Figure 6 shows the ignition and burnout temperatures of coal samples with the mean particle size of 11.34 µm at four different oxygen concentrations. The data show that the heating rate has a greater effect on ignition and burnout temperatures at lower oxygen concentration than at higher oxygen concentration, as mentioned above. As the heating rate increases, the ignition temperature increases under the oxygen-lean conditions but decreases slightly under the oxygen-rich conditions. However, the behavior of the burnout temperature is more complex than that of the ignition temperature. The effect of the heating rate

on the burnout temperature is ambiguous, and clear causative factors are not evident. Considering the errors in experiments and data processing, it appears that the heating rate has an obvious effect on the ignition and burnout temperatures at lower oxygen concentrations and little effect at higher oxygen concentrations. 3.3. Particle Size Effects. Figure 7 shows the TG/DTG curves of coal samples with different mean particle sizes at the heating rate of 30 °C min-1. It can be seen that the particle size has a certain effect on the mass loss profiles. The mass loss profiles are similar in shape but shift to the higher temperature as the particle size increases. This is due to the specific area increases, and the increase in the overall reactivity of coal char becomes better as the mean particle size decreases.12 The changes increase the rates of both devolatilization and char combustion at the same temperature range. From Figure 8a, it can be seen that the ignition temperature decreases as the mean particle size decreases and the reduction in the ignition temperature varies with the oxygen concentration. It is decreased by 8, 22, 11, and 7 °C at oxygen concentrations of 10, 20, 50, and 80%, respectively: the value first rose and then fell, reaching the maximum at 20% oxygen. The observations can be explained as follows: at lower oxygen concentra-

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Figure 6. Effect of the heating rate on the ignition and burnout temperatures (particle size, 11.34 µm).

Figure 7. TG/DTG curves of coal samples with different mean particle sizes (heating rate, 30 °C/min).

Figure 8. Effects of the particle size on the ignition and burnout temperatures (heating rate, 30 °C/min).

tions, the volatile matter and the char are hard to ignite. As a result, the rate of chemical reactions is relatively small. Given the small rate of the chemical reaction, the magnitude of the improvement brought by the particle size is significant. However, at high oxygen concentrations, the volatile matter is easily ignited and this makes the mass loss rate larger. Under the condition of high oxygen, the effect of the particle size is

negligible in comparison to the significant effect of the oxygen concentration. For the same reason, a similar trend can be observed for the burnout temperature from Figure 8b. It is noteworthy that the effect of the particle size here is not as significant as that observed by previous research in conventional combustion.12,13 There are two reasons that might explain this difference. On one hand, the size range is not large enough

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Figure 9. Comprehensive evaluation with the index S.

as that used in previous research. On the other hand, the heattransfer properties (e.g., thermal conductivity and heat-transfer coefficient) of the O2/CO2 mixture are better than that of the O2/N2 mixture,3 which offset the impact of the decreasing particle size. 3.4. Comprehensive Evaluation. A comprehensive combustion property index S is introduced to evaluate the combustion characteristics is defined as follows:12 dW dW ( dt ) ( dt ) S) c

c

max Ti2Th

mean

(1)

where (dW/dt)cmax and (dW/dt)cmean are the maximum and mean burning rates, respectively. Ti is the ignition temperature, and Th is the burnout temperature. It is deduced that the higher the S, the better the combustion property of the coal. The inclusion of several important parameters such as ignition temperature, burnout temperature, or mean burning rate in the index gives a comparatively comprehensive evaluation and can be regarded as a reference for practical operation. 3.4.1. Oxygen Concentration Effects. From Figure 9a, it could be concluded that the oxygen concentration played a very important role during the coal combustion process. The richer the oxygen in the ambient gas, the greater the index. The index S increases from about 1-15 × 10-7 at the lower oxygen concentrations (10 and 20%) to 15-141 × 10-7 at the higher oxygen concentrations (50 and 80%), increasing roughly by an order of magnitude. However, the magnitude of improvement does not stay constant but changes with the oxygen concentration. The index S increases by about 3-12 × 10-7 when the oxygen concentration is less than or equal to 20% and about 9-57 × 10-7 when the oxygen concentration is greater than or equal to 50%. The result shows that the high oxygen concentration is more favorable to coal combustion than the low oxygen concentration. 3.4.2. Heating Rate Effects. Figure 9b shows that the index S increases as the heating rate increases. However, the heating

rate has different effects on the index at different oxygen concentrations. In general, the promotion effect is obvious under oxygen-rich conditions and unnoticeable under oxygen-lean conditions. It is interesting to note that the ignition or burnout temperature has different trends with the oxygen concentration. This is attributed to the great impacts of the two burning rates on the index, the maximum and mean burning rates. The rates at high oxygen concentrations are at least several times that at low oxygen concentrations. On the other hand, the drop in the ignition or burnout temperature at high oxygen concentrations is limited as compared to the ignition or burnout temperature at low oxygen concentrations. As a result, the impacts of the rates greatly exceed that of the two temperatures and determine the trend of the index. 3.4.3. Particle Size Effects. Figure 9c shows the comprehensive combustion property index S at different particle sizes with a heating rate of 30 °C min-1. The particle size has a recognizable effect on the index S. The index increases as the particle size decreases, which is intensified as the oxygen concentration increases, similar to the trend of the index at different heating rates. It increases by 1 × 10-7, 7 × 10-7, 23 × 10-7, and 36 × 10-7 at oxygen concentrations of 10, 20, 50, and 80%, respectively. That is to say, the combustion property index becomes better as the particle size decreases, especially at oxygen-rich conditions. The reasons are analyzed above in Heating Rate Effects. 4. Conclusions On the basis of the experiments, the following conclusions are drawn: (1) Among the three factors (oxygen concentration, mean particle size, and heating rate), the oxygen concentration is the most important factor for combustion characteristics, while next is the heating rate and then the mean particle size. (2) The elevation of the oxygen concentration cannot only decrease the ignition or burnout temperature and enhance the coal combustion property but also intensify the effects of the particle size and heating rate on

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the comprehensive combustion property index. On the contrary, the effects of the two factors on the ignition or burnout temperature are weakened at high oxygen concentrations. (3) The ignition mechanism changes from the heterohomogeneous type at an oxygen concentration of 10% to the homogeneous type at an oxygen concentration of greater than or equal to 20%. (4) The heating rate and particle size have measurable effects at low oxygen concentrations but unnoticeable effects at high oxygen concentrations. As the

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particle size decreases, the ignition and burnout temperatures decrease, while the comprehensive combustion index S increases. As the heating rate increases, the ignition and burnout temperatures increase when the oxygen concentration is less than or equal to 50%. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50876060). EF800444C