Experimental Investigation into Combustion Characteristics of Two

Energy Fuels 2010, 24, 6034–6040 Published on Web 10/13/2010

: DOI:10.1021/ef100978n

Experimental Investigation into Combustion Characteristics of Two Sub-bituminous Coals in O2/N2 and O2/CO2 Environments Yong-Gyun Kim, Jae-Dong Kim, Byoung-Hwa Lee, Ju-Hun Song, Young-June Chang, and Chung-Hwan Jeon* School of Mechanical Engineering, Pusan Clean Coal Center, Pusan National University, 30 Jangjeon-dong, Gumjeong-ku, Busan 609-735, Republic of Korea Received July 28, 2010. Revised Manuscript Received September 20, 2010

There are still arguments on the coal combustion reactivity under conventional and oxy-coal combustion atmospheres. The selection of the experimental device in such a study is crucial. A laminar entrained flow reactor system that is capable of producing a similar combustion environment of a utility furnace in terms of the high heating rate and temperature was adopted in this study. This study investigated the characteristics of sub-bituminous coal combustion in its processes of devolatilization and char oxidation by examining the structure and length of the flame. The effects of the particle size and oxygen molar fraction in N2 and CO2 diluent gases were studied. The O2 mole fraction was varied from 0 to 50%. The flame became shorter as the O2 concentration increased. It means that reactivity in both the devolatilization and char oxidation processes increased in a higher O2 concentration environment. It was barely seen that the volatile burning realm shrunk, O2 diffusion accelerated, and luminosity concentrated near the burning particle as the O2 concentration increased. The larger size coals showed thicker and longer flames, and there is a small difference in the burning time between two different sub-bituminous coals. When the O2/CO2 and O2/N2 environment effects are compared, the flame was shorter in the oxy-fuel condition than in the O2/N2 condition.

Recently, some researchers2,3 examined the combustion processes for carbon capture. Wall benchmarked five technologies, including conventional pulverized fuel technology and integrated gasification combined cycle (IGCC) and oxyfuel combustion. According to the examination,2 the oxy-fuel combustion method may be the most viable choice considering the efficiency and the cost of electricity (COE), supposing that carbon tax is over $15/ton of CO2. Several countries including Korea are now in the phase of developing or demonstrating oxy-fuel combustion on a commercial power furnace scale.4 In 2007, a 100 MWe class oxycombustion power station project was launched in Korea. According to this project, it is scheduled to complete the construction by 1015 and the demonstration before 2020. Sub-bituminous coal will mostly be used as fuel, although the facility was designed to have flexibility in the selection of coals. For the successful installation and operation of such a facility, it is essential to understand the coal combustion mechanism under unconventional conditions, in which the characteristics of PC combustion are very different from those in the air-blowing atmosphere. Wall2 pointed out the research and development necessity of combustion characterization in an O2/re-circulating flue gas (RFG) environment for oxy-fuel combustion technology. Buhre et al.5 also mentioned the need of fundamental experiments on oxy-fuel combustion through a laboratory-scale reactor. Many researchers have studied the characteristics of

Introduction Coal fuel has made great contributions to electric power generation in Korea. In 2007, it accounted for 38.4% of all energy resources consumed to generate electric power in Korea.1 This is because coal, the most abundant fossil fuel in the world, is cheaper than any other fuel. Recently, however, the electric power generation companies that use pulverized coal (PC) as fuel have faced new challenges. They include the pressures on the reduction of carbon dioxide with the awareness of global warming and the usage of low-calorific coal fuel because of the hike of the coal price in the market. There is no doubt about the fact that the reduction of greenhouse gas emission is an urgent contemporary demand. CO2 gas is the most responsible one among greenhouse gases in terms of the volume of emission. Such CO2 gas emission is mainly from power generation to fossil fuel combustion. Several possible solutions have been suggested to reduce greenhouse gas emission from fossil-fuel-fired power generation. One of them is the CO 2 capture and storage (CCS) method. This method has been thought of as the most practical solution for step-change reduction. Among the CCS technologies, oxy-fuel combustion is regarded as a promising alternative to achieve carbon dioxide capture and clean coal technology. An advantage of oxy-fuel combustion is that it can be applied to conventional PC-fired furnaces through a retrofit as well as a new PC power plant. *To whom correspondence should be addressed. Telephone: 82-51510-7324. Fax: 82-51-582-9818. E-mail: [email protected]. (1) Korea Energy Economics Institute. Year Book of Energy Statistics 2008; Ministry of Knowledge and Economy: Republic of Korea, 2008. (2) Wall, T. F. Proc. Combust. Inst. 2007, 31, 31–47. r 2010 American Chemical Society

(3) Davison, J. Energy 2007, 32, 1163–1176. (4) http://www.co2captureandstorage.info/SummerSchool/SS09% 20presentations/04_Lupion.pdf. (5) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307.

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: DOI:10.1021/ef100978n

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oxy-fuel combustion using laboratory-scale reactors, such as a thermogravimetric analyzer (TGA),6-8 drop tube furnace (DTF),8-12 and laminar entrained flow reactor (LEFR)13. They have performed the experiments in different conditions and obtained different results. Many researchers7,9-11,13 have reported that coal or char reactivity in a N2 diluent is better than in a CO2 diluent. Molina et al.13 calculated thermophysical properties for N2 and CO2 gases and found that molar heat capacity, density, and energy per volume for CO2 is higher than for N2. Such different properties could affect the characteristic of combustion in oxy-fuel combustion. Shaddix and Molina14 mentioned that the lower overall char-burning rate in O2/CO2 atmospheres compared to the burning rate in O2/N2 atmospheres was due to the lower diffusivity of O2 in CO2 compared to that in N2. However, Borrego et al.12 reported that the reactivity in O2/N2 atmospheres is similar to the reactivity in O2/CO2 atmospheres for high-rank coals. In addition, Rathnam et al.8 recently obtained better reactivity in the O2/CO2 atmosphere compared to O2/N2 atmospheres. Thus, there are still arguments on the prevalence of coal or char reactivity in the O2/CO2 and O2/N2 atmospheres. At the center of the arguments, there are major parameters, such as the diffusivity of O2 in CO2 and N2 diluents, development of meso- and micropores in CO2 and N2 diluents, and char-CO2 reaction. Although oxy-fuel combustion is a viable technology in both the present and near future, more experimental data on the characterization of oxy-fuel combustion are necessary. This study is intended as a preliminary investigation on the characteristics of sub-bituminous coal combustion under an oxy-fuel combustion atmosphere through the observation of the flame. Two sub-bituminous coals that would be main fuel in the oxy-combustion power station project in Korea were selected in this study. They are imported from Indonesia. It is meaningful to study such coal because studies using Indonesian coal are rare and the consumption of sub-bituminous coals would increase soon based on the prospect of the coal price in the market and the volume in reserves. The instantaneous behavior of a single burning particle in the combustion process under an oxy-fuel combustion atmosphere was investigated through the intensified charged coupled device (ICCD) camera equipped with a microscopic lens. The characteristics of coal combustion manifested in devolatilization and char oxidation were investigated through flame structure and length. The effects of the particle size and oxygen mole fraction on the characteristic of the flame structure in O2/CO2 and O2/N2 environments was studied. The O2 mole fraction was varied at 6, 10, 21, 30, and 50%.

Figure 1. Schematic of the experimental apparatus.

Experimental Section Reactor System Setup. A sophisticated LEFR system for this study was designed and constructed by the Pusan Clean Coal Center at the Pusan National University, Busan, Republic of Korea, after studying other laboratory-scale reactors. The experimental system apparatus was shown in Figure 1. The facility consists of several major components as follows. Flat Flame Burner and Reactor. The burner patterned after the Henken burner is responsible for making the flat flame platform that makes up the burning environment of coal particles. Stainless-steel tubes (inner diameter = 0.6 mm) are inserted into the hexagonal honeycomb cell matrix, having a cross-sectional area (41  41 mm), as shown in Figure 1. These tubes are the channels to supply gas fuel. In addition, a coal feeding tube located at the center of the matrix is also inserted into the cell matrix. The other void honeycomb cells are the channels for the oxidizer. Coal particles are injected into the diffusion flat flamelets, which are formed on the top surface of the burner. The burner is surmounted by a quartz tower, which accommodates optical access. The square quartz tower corresponds to the reactor. The tower has a function to prevent intrusion of surrounding air. Coal Feeder. The coal feeding system used in this study is shown in Figure 1. The feeder in this experiment was modified from those.15,16 Ma15 pushed coal particles in a syringe by the plunger out into a glass funnel that is connected to a feeding tube to the reactor. The moving speed of the plunger is controlled by a stepper motor. There is the possibility that a lump of coal particles suddenly drops from the syringe, and the particles dropping on the funnel could be stagnant on the inner surface of the funnel. We removed the funnel and directly connected the syringe mounting on a syringe pump to the feeing tube. In addition, the length of tube was minimized to prevent clogging the tube. Molina16 used a sample tube without a plunger filled with coal particles and mounted on a syringe pump. He directly pushed the sample tube by the syringe pump. The feeder consists of a modified enclosed syringe, which will be a vessel of bulk coal particles, a syringe pump, which pushes the syringe plunger with constant speed, corresponding to the flow rate of feeding coals,

(6) Huang, X. Y.; Jiang, X. M.; Han, X. X.; Wang, H. Energy Fuels 2008, 22, 3756–3762. (7) Li, Q. Z.; Zhao, C. S.; Chen, X. P.; Wu, W. F.; Li, Y. J. J. Anal. Appl. Pyrolysis 2009, 85, 521–528. (8) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y. H.; Moghtader, B. Fuel Process. Technol. 2009, 90, 797–802. (9) Saastamoinen, J. J.; Aho, M. J.; Hamalainen, J. P.; Hernberg, R.; Joutsenoja, T. Energy Fuels 1996, 10, 121–133. (10) Borrego, A. G.; Alvarez, D. Energy Fuels 2007, 21, 3171–3179. (11) Bejarano, P. A.; Levendis, Y. A. Combust. Flame 2008, 153, 270–287. (12) Borrego, A. G.; Alvarez, D.; Fern
andez-Domı´ nguez, I.; Ballesteros, J. C.; Men
endez, R. Proceedings of the 24th Annual International Pittsburgh Coal Conference; Johannesburg, South Africa, Sept 10-14, 2007. (13) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31, 1905– 1912. (14) Shaddix, C. R.; Molina, A. Proceedings of the 5th U.S. Combustion Meeting Organized by the Western States Section of the Combustion Institute; The University of California at San Diego, San Diego, CA, 2007.

(15) Ma, J. L. Soot formation during coal pyrolysis. Ph.D. Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, 1996. (16) Molina, A. Evolution of nitrogen during char oxidation. Ph.D. dissertation, Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT, 2002.

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Kim et al. Table 1. Species Components and Their Molar Fraction for the Environmental Conditions condition

O2

CO2

N2

H2O

Tad (K)

N06 N10 N21 N30 N50 C06 C10 C21 C30 C50

0.06 0.10 0.21 0.30 0.50 0.06 0.10 0.21 0.30 0.50

0.05 0.06 0.06 0.06 0.06 0.79 0.75 0.64 0.55 0.35

0.73 0.69 0.58 0.49 0.29 0.00 0.00 0.00 0.00 0.00

0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15

1817 1817 1814 1807 1810 1811 1818 1813 1813 1813

Figure 2. Velocity profile of the particles along the centerline of the reactor. The coal injection and flue gas velocities are the calculated initial velocities of the cold carrier gas (298 K) at the coal particle injection tip and the hot gas products at 1780 K, assuming that the particles move in the same speed with the gases.

and two metal capillary tubes, which are inserted into the enclosed syringe throughout the urethane rubber cap. One of the two tubes is for the passage of carrier gas, while the other is for that of feeding coal. Nitrogen or carbon dioxide will be used as the carrier gas depending upon the experimental condition in this study. A vibrator is attached to the urethane rubber cap to exert vibration onto the syringe. Then, the particles floating in the air are entrained through the feeding tube by the carrier gas. The coal feeding mass flow rate is checked by weighing the mass. It shows good repeatability in measuring the weight. The coal feeding rate in this study is 0.2 g/h. This flow rate is so low not to affect the gas temperature and velocity profile by particle combustion. This feeding rate is even comparable to the flow rate (0.02 g/min) in other similar experiments.13 Gas Supply System. All gases supplied to the burner are used either as an oxidizer or a fuel to produce a flat diffusion flame, and its flue gas forms the atmosphere in which coal particles burn. The gases are carefully selected to trace a similar combustion environment of an actual PC furnace in terms of temperature, oxygen molar fraction, and major diluents. The flow rate of each gas is controlled by a mass flow controller. Ethylene, hydrogen, and carbon dioxide gases as the fuel and oxygen and carbon dioxide gases as the oxidizer are selected for the O2/CO2 environment. In the same manner, methane, hydrogen, and nitrogen gases as the fuel and oxygen and nitrogen gases as the oxidizer are selected for the O2/N2 environment. Imaging System. Flame Structure and Length. Digital images of the flame are obtained with a Nikon D70 model camera under the same conditions (aperture, f/2.8; exposure time, 1 s) at all times. More than 20 images for each particular experimental condition are taken. The average flame length for the condition is determined through digital image processing. The images most representative of each condition are presented in this paper. In addition, the particle velocity was measured using a high-speed camera (Vossk€ uhler HCC-100 model) under the C30 condition. Figure 2 shows the velocity profile of the particles measured. The centerline particle velocity is accelerated by the mixing with hot flue gas, and then the flow develops in the reactor with the effect of volatile expansion. Single Burning Particle Image. Images of a single burning particle are obtained by the aid of an ICCD camera (Princeton Instruments) equipped with a microscopic lens. A total of 10 μs of exposure time is applied to each image at all times. Among the captured images for a particular condition, a representative image is selected. Experimental Conditions. Environmental Condition. The O2/ CO2 and O2/N2 environmental conditions were produced by species in flue gas of the flat flame. The species components and their molar fractions were presented in Table 1. The product species and their fraction in the table are the result calculated through a chemical equivalence equation. The first column lists

Figure 3. Gas temperature distribution along the center line of the reactor.

various conditions determined by the molar fractions of O2, CO2, N2, and H2O. Tad in the table is the adiabatic temperature. The environmental conditions with varying O2 molar fractions for the O2/N2 atmosphere were denoted as N06-N50. The numbers are the O2 molar percentages here. In the same way for the O2/CO2 atmosphere, each condition was identified from C06 to C50. To avoid the interference by different moisture contents in each environmental condition, the moisture molar fraction is equally controlled for all cases. In addition, the adiabatic temperature was also fixed at about 1817 K for each condition. The total gas flow rate flowing through the reactor for each condition was fixed in 57 standard liters per minute (slpm) to avoid the change of residence time of the burning particle because of the change of the particle speed. The flow rate of the coal particle carrier gas was controlled at 30 standard cubic centimeters per minute (sccm) for all of the times. Figure 3 shows corrected gas temperature distribution along the center line of the reactor. The z in the figure denotes the axis of a flowing coal particle from the top surface of the burner. The temperatures in the reactor were measured with a silica coated R-type thermocouple, which has a 0.9 mm junction diameter. Measurement was performed without feeding the coal particle. The measured temperatures were corrected, considering radiation loss from the thermocouple junction. A mean quartz wall temperature of 990 K was applied to the calculation based on the measurement. The temperature is pretty high in the early stage of coal burning, 6036

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Table 2. Properties of Coal Samples proximate (wt %, air dry)

ultimate [wt %, dry ash free (daf)]

coal

moisture

volatile matter (VM)

ash

fixed carbon (FC)

C

H

O

N

S

MHU Roto-South

16.14 16.31

38.27 41.16

6.93 3.69

38.7 38.9

81.81 72.67

5.31 4.79

10.38 21.18

1.65 1.30

0.86 0.05

Figure 4. Size distribution of the group 2 of MHU coal. The number in the parentheses is the number of samples found in a size interval. The number on the x axis is the center of each size interval. Each size interval is 10.9 μm. Figure 5. Scene of history of the coal combustion process through ICCD images.

where the devolatilization process happens (z=0-150 mm) in the reactor. In reference to the similar experimental conditions in terms of temperature and O2 level in the studies,17,18 the temperature in the region where char burns in the reactor may be far away from the diffusion limit, although the temperature level in the lower region where the devolatilization process occurs in our experiment is probably near the diffusion limit. Properties of the PCs. Two characteristic coal sources, namely, Roto-South and Multi Harapan Utama (MHU) coals, were used for these experiments. Each coal belongs to typical sub-bituminous coal in rank. The results of proximate and ultimate analyses of the coals were shown in Table 2. After the coal was ground, it was sieved and classified into three size categories: group 1 (32-45 μm), group 2 (45-75 μm), and group 3 (75-90 μm). We repeated the sieving 3 times for each size group. Figure 4 shows the size distribution of the group 2 of MHU coal as an example. We measured the size distribution of the sample using the shadow grapher instrument, equipping the analysis software. From the Gaussian fit, the center is 61.23 μm (arithmetic mean diameter of this sample is 60 μm), with R2 = 0.986.

char-burning processes are clearly distinguishable by the images. Each process is not divided at a particular moment but overlapping for a time. On the initial stage of volatile release and burning, a large volume of volatile is emitted with rapid heating of the coal particle. In addition, the boundary of the volatile diffusion realm extends up to about 4 times as large as the particle size. However, the realm shrinks and the shape takes the form of the corona with intensified luminosity. In the char-burning stage, particle luminosity becomes lower as time elapsed. It means that the char particle temperature becomes lower as it burns in the last stage. Size Effect on the Flame Structure and Length. Figure 6 shows the comparison of the flame structure and length according to the change of the particle size for MHU coal under the N30 condition. This flame typically takes the form of laminar flame, as seen. It shows that the overall flame structure and length are directly influenced by the change of the particle size, as expected. The flame lengths under this condition are 388 ((19), 417 ((17), and 422 ((11) mm for the size groups 1, 2, and 3, respectively. The flame layer in the devolatilization zone becomes thicker as the particle size increases. This layer looks like a bright yellow color because of the higher temperature when volatile burns. The figure also shows the effects of the particle size on the volatile- and char-burning processes as well as ignition onset. The duration for each process increases as the particle size increases. The ignition onset points for the size groups 1 and 2 are not clear in the figure because of the background of the flat flame. However, the effect of the particle size on the ignition onset for group 3 is seen, and this point is retarded in comparison to the other cases. It is thought that it takes a longer time to absorb enough energy to reach the ignition onset temperature of the particle as the diameter increases. The ignition for group 3 starts within 5.8 ms after the coal particle is injected into the reactor. This calculation is based

Results and Discussion Coal-Burning Process. Figure 5 shows the history of the coal combustion process observed through the magnified ICCD images and an image of the digital camera. These images were taken with MHU coal in the 75-90 μm size class under the C30 environmental conditions. The seven ICCD images in the figure are representatives of the instantaneous particles burning at the particular positions (z = 10, 15, 30, 50, 100, 150, and 200 mm). The positions are marked by the lines. The ICCD images were compared to the image obtained by the digital camera. Heating, volatile-burning, and (17) Murphy, J. J.; Shaddix, C. R. Combust. Flame 2006, 144, 710– 729. (18) Wall, T. F.; Liu, Y. H.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C. D.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. Chem. Eng. Res. Des. 2009, 87, 1003–1016.

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Figure 8. Comparison of the diluent effects of N2 and CO2 for two coals.

matter, oxygen, and carbon contents, as shown in Table 2. In comparison to MHU coal, Roto-South coal is 3% higher in volatile matter, 10% higher in oxygen element, but 9% lower in carbon element. These data may support the explanation why the flame length of Roto-South coal is shorter. Effect of O2/CO2 and O2/N2 Diluents. The comparison of the effect of CO2 and N2 diluents on the flame length is shown in Figure 8. The flame lengths in the CO2 diluent are 7 and 10% lower than in the N2 diluent at 21% O2 and 30% O2 levels for Roto-South coal, respectively. In addition, for MHU coal, the lengths in the CO2 diluent are also 6 and 9% lower than in the N2 diluent at 21% O2 and 30% O2 levels, respectively. It means that the reactivity of coal combustion in the CO2 diluent atmosphere is better than in the N2 diluent in this experimental condition. Surprisingly, this result is different from other reports. Others7,9-11,13 have explained that the coal combustion reactivity under the N2 diluent is better than that in the CO2 diluent at the same O2 molar fraction. Shaddix and Moilina19 explained that the decrease in the char-burning rate may be attributable to the lower rate of oxygen diffusion through the CO2-rich boundary layer. However, a similar result with this study was recently reported by Rathnam et al.8 They explained that the better char burnout in the CO2 atmosphere is attributed to the char-CO2 gasification reaction. There are still some arguments on the issues of the coal combustion reactivity under a CO2 atmosphere like this. From the literature, the contribution of a carbon dioxide bath gas to the reactivity could be summarized in two aspects, namely, positive and negative effects. The char-CO2 gasification and the much more micropore development under the CO2 atmosphere can play a positive role in enhancing the reactivity. On the other hand, the lower O2 diffusion rate through the CO2-rich boundary layer and the endothermic effect of the char-CO2 gasification reaction under a CO2 atmosphere can play a negative role in the enhancement of the reactivity. Char-CO2 Gasification Reaction. Varheyi et al.20 reported that the char-CO2 reaction started at 1073 K. Rathnam et al.8 confirmed that the char-CO2 gasification reaction occurred if the coal particle temperature is over 1030 K. Rathnam et al.8 deduced that the char-CO2 gasification effect in addition to devolatilization brought about the higher volatile yield in the CO2 atmosphere. The char-CO2 reaction is conspicuous at low O2 levels and a high temperature atmosphere under a high CO2 level atmosphere.

Figure 6. Size effect on the flame structure and length for (a) group 1, (b) group 2, and (c) group 3.

Figure 7. Comparison of the coal property effect on the flame length under the conditions of size group 2 and the O2/CO2 atmosphere.

on the particle speed of 2.6 m/s measured at z=100 by the high-speed camera images and an assumption that the coal particle speed is maintained constant during the early heating process. Two broken lines are in the figure. The bottom and top lines are the extinction point of the char and the end point of volatile burning. The char burning starts at about 32.7, 43.1, and 53.5 ms for the respective arithmetic mean diameters of 38.5, 60, and 82.5 μm under the experimental conditions. The calculated residence time of the coal particles in the reactor is about 100 ms based on the particle velocity profile in the reactor. Effect of the Coal Properties. The flame lengths as a function of the O2 molar fraction under the O2/CO2 atmosphere are compared for Roto-South and MHU coals in Figure 7. The size group 2 for two coals is taken for the comparison, and the O2 mole fraction is changed from 6, 10, 21, 30, to 50%. Roto-South coal shows better reactivity compared to MHU coal. The flame length of Roto-South coal is consistently shorter than that of MHU coal. In addition, the flame length of Roto-South coal is 5-10% lower than that of MHU coal as the O2 mole fraction is changed from 10 to 30%. However, the difference is not very big considering the deviation. The increase of the O2 mole fraction makes the difference smaller. Although two coals belong to the same coal rank as sub-bituminous coal, their combustion reactivity can be influenced by the properties of coals. It is not easy to determine what properties make the difference in reactivity. However, it might be caused by the difference in volatile

(19) Shaddix, C. R.; Molina, A. Influence of CO2 on coal char combustion kinetics in oxy-fuel applications. Proceedings of the 5th U.S. Combustion Meeting Organized by the Western States Section of the Combustion Institute; The University of California at San Diego, San Diego, CA, 2007. (20) Varheyi, G.; Szabo, P.; Jakab, E.; Till, F. Energy Fuels 1996, 10, 1208–1214.

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9

Saastamoinen et al. also mentioned that char-CO2 gasification may become comparable to the O2 diffusion rate when the temperature is high enough in the diffusion control regime. In addition, they anticipated that the char-CO2 reaction prevails over the O2 diffusion rate somewhere in the high-temperature zone of the diffusion control regime. Although Rathnam et al.8 could not see the char-CO2 gasification in the higher O2 combustion experiment in TGA because the char burnout ended before reaching the temperature where the char-CO2 gasification occurs, they anticipate the contribution of the char-CO2 gasification to carbon loss even at the higher O2 levels if the temperature of combustion is above the onset point of the char-CO2 gasification reaction. In addition, they also predicted an additional char-CO2 reaction, which increases the reactivity where the temperature is moderately high in regime 2 but the combustion rate is limited by the diffusion of O2 in regime 3. Although two groups revealed a little different opinion on the regime that the char-CO2 reaction prevails over the O2 diffusion rate, they stated the significant role of the char-CO2 reaction in the coal combustion reactivity in the CO2 atmosphere. Micropore Formation under the CO2 Atmosphere. Borrego  k et al.21 reported that micropores in the et al.10 and Zajdlı´ char particle are more developed in the CO2 atmosphere. Rathnam et al.8 also verified through the BrunauerEmmett-Teller (BET) CO2 surface area measurement that the surface area of the char collected from the drop tube furnace pyrolysis experiments in the CO2 atmosphere was larger than the surface area of the char produced in the N2 atmosphere. If much more micropore in the char particle is developed in the CO2 diluent compared to the N2 diluent during the devolatilization process, it will provide a chance to enhance the carbon loss in the following char-burning process through either the char-O2 or char-CO2 reaction because of the expanded total surface area. In other words, the total carbon loss increases as the surface increases even if the char-O2 or char-CO2 reaction rate is maintained constant. Lower Diffusion Rate in the CO2 Diluent. As mentioned earlier, Shaddix and Moilina14 deduced that the lower charburning rate in the CO2 environment is due to the lower oxygen diffusion in the CO2 diluent. Such gas properties probably cause a negative effect on the overall coal combustion rate. In addition, they13 also expected that the lower diffusivity of volatile (representing volatile as CH4) would result in a slower volatile consumption rate in the CO2 diluent. However, they reported that the duration of volatile combustion in the CO2 and N2 diluents is same in the same O2 levels and there was no measurable effect on the duration of volatile combustion because of the presence of CO2. Ironically, this report may explain the effects of the char-CO2 reaction and lower diffusivity of the volatile during the devolatilization process in the CO2 environment. The opposite effects, such as the char-CO2 reaction and lower diffusivity of the volatile during the devolatilization process in the CO2 environment, might hide the effect of the lower diffusivity of the volatile in the CO2 environment. It might be why the same duration of volatile combustion was observed.

Endothermic Effect of the Char-CO2 Reaction. Rodriguez et al.22 reported that the char particle temperature in the N2 diluent is higher than that in the CO2 diluent because charCO2 gasification is an endothermic reaction and drops the temperature of the char particle. Moilina and Shaddix23 also mentioned that the char combustion temperature is consistently lower in the CO2 diluent and deduced that this caused a lower char-burning rate. It can be evidence of the charCO2 reaction if the char particle temperature under the CO2 diluent is lower than in the N2 diluent. Of course, the char reactivity will drop if the lower char particle temperature is solely considered. However, the additional carbon loss because of the char-CO2 reaction may compensate for the effect of the temperature drop of char particles in certain conditions. Thus, it is expected that there is competition of two opposite effects on the coal combustion reactivity and the coal-burning rate depends upon the temperature and O2 level under the oxy-fuel combustion environment, which is why there are some opposite results on the coal combustion reactivity among the researchers. Therefore, the better reactivity of coal combustion under the CO2 atmosphere in this study may be explained as follows. During the devolatilization process of coal in the CO2 environment with high temperature, the char-CO2 reaction may occur and carbon is released and leaves micropore voids. Thus, the total surface area of a char particle becomes larger, and this plays a role in the enhancement of the char-burning rate. The char-CO2 reaction drops the temperature of char particles, and this plays a role in the suppression of char oxidation reactivity. Eventually, the positive effects of carbon dioxide gas to the overall coal combustion rate may prevail over the negative effects in our experimental conditions. We do not yet clearly know how much the char-CO2 reaction plays on the global reactivity of coal combustion because of its opposite roles. More detailed studies are needed for the new findings. Conclusions This preliminary study of the characteristics of oxy-fuel coal combustion was performed in O2/CO2 and O2/N2 environments through the sophisticated LEFR system. The effects of the coal particle size, diluents, and O2 mole fraction on the flame structure and length for the two sub-bituminous coals were investigated, and the results are as follows: (1) As expected, the coal particle size significantly affects each step of coal combustion. As the size increases, the volatile burning regime extends and the burning time of volatile and char becomes longer as well. (2) Roto-South coal shows a little bit better combustion reactivity in the same conditions compared to MHU coal, although they belong to the same coal rank. (3) The effect of the O2 molar fraction in the oxy-fuel combustion environment was clearly seen on the coal combustion reactivity. The increase of the O2 mole fraction is (22) Rodriguez, M.; Raiko, R. Effect of O2 and CO2 content on particle surface temperature and size of coal char during combustion. Proceedings of the Finnish-Swedish Flame Days 2009; Finland International Flame Research Foundation, Naantali, Finland, Jan 28-29, 2009. (23) Molina, A.; Shaddix, C. R. Coal particle ignition and devolatilization during oxygen-enhanced and oxygen/carbon dioxide pulverized coal combustion. Proceedings of the 2005 Fall Meeting Western States Section of the Combustion Institute; Stanford, CA, Oct 17-18, 2005; Paper 05F-20.


Remiarov
a, B. Chem. (21) Zajdlı´ k, R.; Markos, J.; Jelemensk
y, L.; Pap. 2000, 54, 467–472.

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: DOI:10.1021/ef100978n

Kim et al.

rate as a function of the temperature and oxygen level will be investigated.

inversely proportional to the flame length, as expected. (4) In the comparison of the effect of CO2 and N2 diluents under the same O2 mole fraction, the flame in the CO2 diluent is shorter. It could be explained by the effect of the char-CO2 gasification reaction, but it is not yet clear. More detailed studies are needed to find why it is the case. In particular, it is necessary to investigate the role of the char-CO2 reaction depending upon the combustion conditions, and the char-CO2 reaction

Acknowledgment. This work was supported by the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Knowledge Economy (2007-P-EP-H-ME-03-0000).

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