Industrial Combustion Journal of the International Flame Research Foundation Article Number , May 2012 ISSN

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1 Industrial Combustion Journal of the International Flame Research Foundation Article Number , May 2012 ISSN Coal char combustion in O 2 /N 2 and O 2 /CO 2 conditions in a drop tube reactor: an optical study Milena Rodríguez Avila *, Markus Honkanen, Risto Raiko and Antti Oksanen Department of Energy and Process Engineering, Tampere University of Technology, P.O. Box 589, FIN-33101, Tampere, Finland. Corresponding author: Milena Rodríguez Avila Tampere University of Technology Department of Energy and Process Engineering Tel.: Fax: address: milena.rodriguez@tut.fi

2 Abstract This article presents how the combustion of coal char was studied optically in a specially designed drop tube reactor at 1123 K under varying oxygen concentrations and residence times. The char particles were produced in a drop tube reactor (at 1123 K) with nitrogen flow from pulverized coal that was sieved to a size fraction of µm. The oxygen concentrations were set to 3, 12, and 30 vol-% in N 2, and 30 vol-% in CO 2. The drop tube reactor was equipped with movable feeding and collecting probes, and the sample particles were quenched in nitrogen flow. A two-color pyrometer was used to measure the temperature, size, and velocity of the particles, and a charge-coupled device camera was used to measure particle size and velocity. The results of the experiments show that an increase in the oxygen concentration causes an increase in the char surface temperature and a decrease in the reaction time. Carbon dioxide in turn reduces the surface temperature of the particles significantly. By replacing N 2 with CO 2 at the same O 2 concentration from the atmosphere inside the reactor, the average particle surface temperature shows a decrease of approximately 300 K. This result is notable for boiler design in the future because it shows that the combustion temperature inside the boiler can be moderated. Keywords: Drop tube reactor, Oxygen, Carbon dioxide, Particle temperature, Particle size, Combustion. International Flame Research Foundation,

3 INTRODUCTION Coal is the most widely used natural fuel in the world. Its world consumption is expected to increase 56% from 2007 to 2035 [1]. In 2006, 41% of all electricity was produced from coal [2]. Thus, coal is still an important energy source, even in the future, despite the fact that the carbon dioxide released in the combustion process is one of the most significant greenhouse gases. In order to meet stricter climate policies and regulations, conventional coal combustion systems have to be modified. The main goal of developing new coal conversion and combustion technologies is to reduce both fuel consumption and the harmful emissions. To reduce the greenhouse gases that coal fired power plants emit, various options can be considered. These include enhancing the efficiency of power plants, increasing the thermal efficiencies by introducing a combined cycle such as fired or integrated gasification combined cycle (IGCC), substituting fossil fuels with renewable energy resources, and capturing and storing the carbon dioxide (CCS) from conventional plants. The so-called oxy-fuel combustion is one of the various options available for capturing and storing carbon dioxide from coal combustion, and it is considered to be one of the most promising techniques for both pulverized coal combustion and fluidized bed combustion. Oxy-fuel combustion increases energy efficiency, improves flame stability, and decreases the flue gas volume [3]. Wall et al. [4] indicate that the still developing oxyfuel technology with recycled flue gas (RFG) combustion could be installed to existing retrofitted power plants or it could be used as a base technology in future power plants. However, these power plants could not be optimized appropriately due to the lack of data regarding coal behavior under oxy-fuel combustion conditions. Therefore, experimental studies regarding coal characterization are needed. The present study is an overview of flue gas recirculation effects on coal char combustion. The fuel characteristics are studied by using two optical methods: a two-color pyrometer and a charge-coupled device (CCD) camera. Thermogravimetric analysis (TGA) and drop tube reactor (DTR) tests are techniques for examining the phenomena taking place during solid fuel particle combustion. These methods have been used for studying solid fuels, and to determine the rate of devolatilization and carbon consumption, i.e. the combustion reactivity of solid fuels under different experimental conditions, which simulate different kinds of boilers. The DTR is becoming a widely used device in these types of studies on account of its high heating rate, control of high temperatures, and control of atmospheric combustion [5, 6, 7, 8, 9, 10]. International Flame Research Foundation,

4 The particle combustion in a DTR may be studied either by internal or by external methods. The first refers to using probes to gather samples. The probes, however, alter the atmosphere with their physical presence. The external methods refer to the optical techniques that give the results in situ [11]. Various imaging techniques have been used for the characterization and analysis of the char, which forms in coal combustion. One of these techniques is scanning electron microscopy (SEM), which is used to study the morphology of chars [12, 13, 14, 15, 16]. Images acquired at a low accelerating voltage using a Field Emission Scanning Electron Microscope (FESEM) represent the surface structure and morphology of the char whereas the images obtained at high accelerating voltages typically reflect the internal structure of char [17]. The CCD camera is used for determining the velocity and size of the particles, but Cloke et al. [18] used a camera to study the form parameters of char particles, including wall-thickness, char porosity, particle size and pore count. All these imaging techniques have been applied to understand the coal combustion process and its behavior. Two-color pyrometer has already been used in other coal combustion studies [7, 10, 19, 20, 21, 22, 23] to determine the particle surface temperature at different conditions. None of them, however, used simultaneously a CCD camera. This study focuses on the high-temperature combustion of pulverized coal char under different oxygen concentrations. The goal of this work was to produce data on the size, velocity, and surface temperature changes of burning char particles in a DTR under different oxygen concentrations and varying residence times. The aforementioned variables were measured with a two-color pyrometer and a CCD camera. To define the optimum parameters for coal combustion equipment, the kinetic parameters of the char combustion process could be determined at different stages by combining these measurements with carbon conversion. MATERIAL AND METHODS DTR with Movable Probes A specially designed drop tube reactor with two movable probes and an optical window was used in the experiments (Figure 1). The reactor consists of an austenitic Cr-Ni stainless steel tube with an inner diameter of 26.3 mm. The reactor was heated electrically with a Kanthal AF resistance wire that formed four separate 150 mm heating elements. The International Flame Research Foundation,

5 reaction zone can be heated continuously up to 1373 K, and the temperature is measured with K-type thermocouples. Figure 1: Scheme of the drop tube reactor. A quartz glass window with a diameter of 29 mm was located in the middle of the reactor so that particle velocity, size, and particle surface temperature could be measured with the CCD camera and the two-color pyrometer. The gas volume flow inside the reactor was 1.6 Nl/min, which is the sum of the flow through the feeding probe and the flow around the feeder probe. This value was determined by using the ideal gas equation in a gas temperature of 1123 K and a mean gas velocity of 0.2 m/s. The gas Reynolds numbers for 3, 12 and 30 vol-% of oxygen were 36.83, 36.65, and 36.30, which means that the flow was laminar. The particle Reynolds numbers for the particle terminal velocity of 0.6 m/s at 3, 12, and 30 vol-% of oxygen were , , and , respectively. International Flame Research Foundation,

6 The laminar gas flow guarantees that the particles travel in the centerline of the reactor, which is crucial in this study since it stabilizes the particle movement during the measurements. Movement along the centerline is required because the CCD camera and the two-color pyrometer must get the particles into their field of view (FOV) in order to detect and measure them. The pictures confirmed that the particles were in fact on the centerline. Particle residence time can vary as a function of the distance between the probes: from 60 mm to 420 mm. In all cases, the gas temperature profile had isothermal values consistent with the distance between the probes, and the temperature deviation was less than 5 degrees K, i.e. gas temperature of 1123 K ±5 K. Before each measurement, the probes were placed in the desired heating zone. The constant mixture of O 2 /N 2 or O 2 /CO 2 gas flow injection was regulated by a mass flow controller. A feeding device consisting of a nitrogen-filled container sent the fuel into the hot heating zone with the mixture of gases flowing downwards through the vertical reactor tube. After this, the particles were quenched in a cold nitrogen flow in order to stop the reaction. Finally, the unburned char particles and ash matter were collected on a microfiber filter and the particles were weighed. Because of the high ratio of the gas flow to the coal char flow, the gas concentrations inside the reactor were assumed to be constant. The DTR has two limitations: (a) it cannot be used for turbulent flows because the reactor s small diameter would cause the particles to collide with the reactor walls; (b) the maximum gas temperature that can be used is 1300 K. Char Particles Preparation and Characterization Bituminous American coal was crushed and screened in order to obtain particles in the size fraction of µm. A cumulative particle size distribution shows that 50% of the particles are smaller than 125 µm, all of the particles are smaller than 300 µm, and the mean size is 118 µm. According to Jüntgen [24] a size fraction below 100 µm is used in pulverized coal combustion. The slightly larger size fraction was chosen in this study to allow enough time to observe the phenomenon. With small particles, combustion takes place so rapidly that it is difficult to observe. However, to determine the kinetic parameters, char combustion kinetic studies were conducted with a size fraction of µm [22]. Table 1: Proximate and ultimate analyses of coal (ar = as received) Proximate Analysis (wt% dry) Moisture (ar) Ash Volatile Matter Fixed Carbon Coal International Flame Research Foundation,

7 Ultimate Analysis (wt% dry) C H O N S Ash Coal The ultimate and proximate analyses of the coal are shown in Table 1. The char was produced from the parent coal in a 60 cm -long heated tube in nitrogen atmosphere at 1173 K in a gas flow of 1.6 Nl/min. The devolatilization residence time was calculated to be 1 s. During the char production, the volatile matter was measured as being equal to 50 wt%. This result differs from the laboratory analysis. The reason for this difference could be due to the method used to measure the volatile matter: in the laboratory analysis the volatile matter was measured at a low heating rate (TGA) at 1123 K whereas with the DTR it was measured at the same temperature, but at a high heating rate. Optical system Two-color pyrometer: A two-color pyrometer was used to measure the temperature, velocity and the size of the individual burning char particles through an optical window. The method for measuring the surface temperature T p of combusting fuel particles is based on detecting the thermal radiation emitted by the particle and Planck s law of radiation. Figure 2 depicts the mirror arrangement of the two-color pyrometer. The object s radiation is measured in two narrow wavelength bands and the temperature is determined from the ratio of these measurements. The wavelength bands in this study were 1.0 and 1.6 µm for the main signals, and 1.25 µm for the reference signal. Several factors affected the selection of these wavelengths. The most significant factors are the need for a) a sufficiently high spectral radiant exitance from the particle and b) no absorption in the gases present. There has to be enough spectral radiant excitance at the selected wavelengths and at the temperatures concerned, (for temperatures between 1000 and 2000 K, the strongest radiation occurs in the near infra-red range). The absorption of the radiation into the gases present has to be minimized. International Flame Research Foundation,

8 Figure 2: Two-color pyrometer. The atmospheric gases, as well as the gases in the reactor and the gases that form in the burning process, all have to be taken into account. Of all the gases present, water vapor is the only one which could absorb in the potential and actual wavelength range, but the wavelengths chosen have no significant absorption from water vapor. The optical fiber in the two-color pyrometer consists of a primary fiber with a diameter of 1 mm and reference fibers of 0.1 mm diameter, coaxially surrounding the primary fiber. The pyrometer was calibrated against a blackbody radiator in the temperature range of K; with an accuracy of 3 K for this calibration. For temperatures higher than 1443 K the accuracy was 2 K. The signal is measured at two different wavelengths. The ratio of the signals is [7] (R-R 01 )/(R 2 -R 02 ) = (F 1 (T p )-R 01 )/(F 2 (T p )-R 02 ), where R i is the signal with wavelength λ 1 when a particle is in the field of view (FOV); (R i -R 0i ) is the pulse height at wavelength band λ i (i = 1, 2); R 0i is the system response when no particles are in the FOV, (in others words it is the background signal with wavelength λ i ) and F i (T) is the system response calibrated against a blackbody radiator at the temperature T. In a two-color pyrometer, the particles are assumed to be gray bodies. Any possible errors caused by non-grayness have been analyzed in detail by Joutsenoja et al. [7]. Once a particle s temperature has been measured, its size and emissivity can be defined. Using the proportionality between the pyrometric response signal and the particle cross-section area in the FOV, the cross-section area of the particle A p can be found by International Flame Research Foundation,

9 using the equation; ε p A p /A 0 = (R i -R 0i )/ (F 1 (T p )-R 01 ), where A 0 is the cross-sectional area of the FOV in the focal plane and ε p is the emissivity of the particle, which determines it as 0.9 [25, 26]. The particle emissivity is unknown, and this creates uncertainty in the result [7]. Nevertheless, the approach used by Davis [25], and Grosshandler and Monteiro [26] offers a good estimate for the emissivity of coal and char particles. They also showed that the error in emissivity is no greater than 10%, and thus the error in particle diameter will not exceed 6%. The diameter d p of an equivalent spherical particle is calculated from A p [27]. More details of the two-color pyrometer are described in Joutsenoja et al. [7]. The diameter determined from the cross-section of the particle is the so called pyrometric size of a particle. The pyrometric size is the color size of a particle. It is the size of a black particle which would radiate as strongly at the wavelength considered as the real particle actually does. The link between pyrometric particle size and real particle size is given by: =(1 ) The CCD camera: A charge-coupled device camera was used to measure the velocity and size of the coal char particles. The values obtained were compared to the two-color pyrometer results. The settling particles were photographed with a digital CCD camera (AVT Marlin F145B2) and a pulsing LED light. An SVTEC55 semi-telecentric macro-lens with a 2x extender lens was used to obtain image magnification of 0.5 with a working distance of 220 mm. Figure 3: CCD camera setup. Doubly exposed images of the particle shadows are obtained by placing the pulsing LED array opposite the camera (see Figure 3). The white-light LED array is pulsed at 700 International Flame Research Foundation,

10 Hz frequency, which gives a time delay of 1.4 ms between each exposure. The shutter time of the camera is set to 2.8 ms so that there are two light pulses for each image. The computer analyzes them and measures the size, shape, and velocity of the settling particles. A locally adaptive, two-step grayscale segmentation algorithm [28] is employed to detect the particle outlines at about one-pixel accuracy. The particle diameter is estimated from the particle image as: =2 /, where A is the projected area of the particle. A pyser-sgi PS20 calibration plate is used to define the sizing accuracy of the imaging system. The mean sizing error of the digital imaging system is 0.01 mm, which corresponds to one image pixel. This error is clearly systematic. RESULTS AND DISCUSSION A significant number of particles was measured under each process condition. A typical sample size for the two-color pyrometer was particles, and 200 images per measurement for the CCD camera. These high numbers minimize the risk of random error, as the average error decreases with the square root of the number of particles. The CCD camera and the two-color pyrometer measurements show that pulverized char particles moved along with the laminar gas flow and were generally concentrated in the center line of the drop tube reactor, thus reducing possible temperature deviations between the fuel particles. Figures 4a and 5a show sample images from the CCD camera with a time delay of 1.4 ms. Figures 4b and 5b show the analysed images of the samples, where the detected particle outlines are highlighted and the arrows represent the particle displacement measured between the two exposures. These images from the experiment confirm that all the particles travel vertically, parallel with the flow, in the near vicinity of the reactor centerline. Figure 4 corresponds to 3 vol-% of O 2 /N 2 and Figure 5 corresponds to 12 vol-% of O 2 /N 2. Higher oxygen concentration leads to stronger glow formation, which hinders the image analysis. International Flame Research Foundation,

11 (a) Sample (b) Analysis Figure 4: Particle images from CCD camera at 3% O 2. (a) Sample (b) Analysis Figure 5: Particle images from CCD camera at 12% O 2. The measured values for particle size and temperature are presented in Figure 6. Figure 6 shows four separate groups of particles, which were detected with the two-color International Flame Research Foundation,

12 pyrometer. The results are for 14 cm heating zone, corresponding to residence times of 0.225, 0.245, and seconds for 3, 12, 30 vol-% O 2 /N 2 and 30 vol-% O 2 /CO 2 respectively. Each group presents the particle surface temperature at different oxygen concentrations in nitrogen and in carbon dioxide. Figure 6: Particle surface temperature T p and diameter d p of individual coal chars at different oxygen concentrations. Detection limit of pyrometer device (solid curve) and estimated gas temperature (dashed line) are included, 14 cm combustion zone. (dp 0 = µm). The figure also shows the detection limit of the size measurements. The limit f detection, defined by Joutsenoja et al. [7] as the minimum signal-to-noise ratio at which a particle can be detected, is achieved through the optimization of the optics, wavelength bands, detectors, and signal amplifications for the object of the measurements i:e: the particle size, temperature, and velocity. With the oxygen concentration of 3 vol-%, the detection limit cuts off particles from 60 to 210 µm. An increase in oxygen concentration leads to an increase in the particle surface temperature, which reaches its maximum values of 2400 K when the oxygen concentration is 30% in nitrogen. The lowest particle temperatures of about 1000 K correspond to the lowest oxygen concentration of 3% i.e. the particles detected with 3 vol-% of oxygen had temperatures just above the detection limit. At the same time, there is a visible tendency for International Flame Research Foundation,

13 the particle size to decrease as the oxygen level increases. This has also been demonstrated by Bejarano and Levendis [19], Joutsenoja et al. [7], and Smoot [29]. There is some deviation in the temperature and size of the particles due to their different shape, structure and morphology. For 3, 12, and 30 vol-% of O 2 /N 2 and 30 vol-% of O 2 /CO 2, the standard deviations of the char surface temperatures are 63.43, , , and K. For particle size, the standard deviations are 32.91, 29.92, 19.52, and µm. A possible reason for the temperature variation in particles of the same size could be the differences in their chemical reactivity (properties). When the coal is homogeneous, the temperature difference between particles of the same size decreases and the combustion behavior tends to be more predictable. Figure 7: Average particle size at different oxygen concentrations. The particle diameter is measured with a CCD camera and a two-color pyrometer for each separate measurement. Figure 7 shows the average particle size values detected by the two-color pyrometer and the CCD camera at different oxygen concentrations and 22 cm heating zone, corresponding to residence times of 0.374, 0.418, and seconds for 3, 12, and 30 vol-% O 2 /N 2 respectively. The particle diameter values from the two-color pyrometer are higher than the results from the CCD camera. This can be explained by the uncertainty of the emissivity value. In this study, the emissivity was assumed to be 0.9. At a lower O2 concentration, from 3 vol- % to 12 vol-%, the combusting particles are slightly larger than the expected size. International Flame Research Foundation,

14 The results of the particle size, velocity, and surface temperature measurements introduced later in this article include vertical bars that represent a 98% confidence statistical error (98% CSE). The average particle size profile at different oxygen concentrations captured with the CCD camera is shown in Figure 8. In all cases, the particle size tends to decrease, when the oxygen concentration increases. With a higher oxygen concentration, the diameter of the particle decreased faster at first but it leveled out after a certain point. A possible explanation for this is the ash layer that forms on the surface of the particle, thus slowing down the combustion. Figure 8: Average particle diameter d p as a function of residence time at different oxygen concentrations, 98% CSE. The large size of the particles at the beginning of the combustion can be explained by their varying shape. It is apparent that the sieving method that was used only separates the particles according to their two smaller dimensions. Because some of the particles were cylindrical and even had sharp edges, their terminal velocity was not the same as for the spherical particles and this leads to corresponding fluctuations in the measured values. Microscopic pictures of the coal and char were taken to see their shape and size. Figures 9 and 10 show the crushed and screened coal sample in a size fraction of µm, and the coal char after devolatilization at 1123 K. The images show that some particles do not have a spherical shape; instead they seem to be more cylindrical. The vibration used in the screening could have caused this. When the particles were screened, during the International Flame Research Foundation,

15 vibration process they had time to orientate in such a way that the longest dimension of the particles was normal to the mesh but small particles could pass through it. Figure 9: Crushed and screened coal within size range of µm. During devolatilization, the coal experiences a 50% weight loss, but there is no significant change in the shape, as can be seen from Figures 9 and 10. Joutsenoja et al. [7] attributed the large size to two factors: a) the size distribution tails the distribution both toward and beyond the nominal sieving cut size, and b) swelling. Murphy and Shaddix [10], attributed the large size compared to the original fraction to the thermal expansion of the hot char. In this study, the hypothesis is that the particle size distribution is the main explanation for the large size, either by the screening method or the orientation of the particle when it was detected by the optical devices. International Flame Research Foundation,

16 Figure 10: Coal char µm. When the temperature of the burning particle is at its maximum and the glow is the largest, the glow disturbs the measurement, especially with high oxygen concentrations. It is worth emphasizing that the results for particle size are smaller from the CCD camera than from the two-color pyrometer. However, the results for particle velocity are approximately similar. An analogous conclusion was explained in detail by Paananen [22]. The data from the two-color pyrometer is useful since it shows the change in the particle size. The results for velocity and size from the CCD camera are more accurate and can thus be used in the modeling. The varying diameter results could also be attributed to the lack of knowledge regarding the emissivity of coal char. Particle velocity measurement was one of the most valuable statistics because of its importance in determining the residence time. The residence time of the particle inside the reactor was determined by fitting the average velocity profile from the camera results with a polynomial curve as function of the reactor length. After this, the inverse function of the particle velocity was integrated over the reactor length. In all the oxygen concentrations, the velocity of the particle decreases as the combustion is taking place, until the velocity is nearly the same as the mean gas velocity. Figure 11 shows that there is a significant decrease in the velocity of the particles when they move from the feed probe to the collection probe. The decrease in the particle s weight during combustion is the reason for the decrease in their velocity. International Flame Research Foundation,

17 The particle velocity was measured with both a CCD camera and a two-color pyrometer. Thus comparisons could be made, and the data accuracy increased. An advantage of the CCD camera method is that this measurement system seems to be quick and easy to use. The results shown in Figure 11 are from the CCD camera. Figure 11: Average particle velocity v p as a function of residence time at different oxygen concentrations, 98% CSE. In all the oxygen concentrations, the velocity of the particle decreases as the combustion is taking place, until the velocity is nearly the same as the mean gas velocity. Figure 11 shows that there is a significant decrease in the velocity of the particles when they move from the feed probe to the collection probe. The decrease in the particle s weight during combustion is the reason for the decrease in their velocity. The particle velocity was measured with both a CCD camera and a two-color pyrometer. Thus comparisons could be made, and the data accuracy increased. An advantage of the CCD camera method is that this measurement system seems to be quick and easy to use. The results shown in Figure 11 are from the CCD camera. Particle surface temperature was measured during each measurement run, together with particle velocity and size. The particle surface temperature can be plotted as a function of the residence time. In the beginning, the particle surface temperature increased regardless of the oxygen concentration. Figure 12 shows that after achieving the maximum International Flame Research Foundation,

18 value the temperature started to decrease. However, oxygen has a great influence on the particle surface temperature. The maximum temperature was observed between 0.15 and 0.35 s depending on the oxygen concentration. At 30 vol-% oxygen concentration, the maximum temperature is reached at approximately 0.05 seconds. The particle surface temperature in nitrogen atmosphere is about 300 K higher than in carbon dioxide. This result was also obtained by Bejarano and Levendis [20], Murphy and Shaddix [10]. Figure 12: Correlation between char particle surface temperature and residence time at different oxygen concentrations, 98% CSE. Particles at a 3% oxygen concentration have a low surface temperature and their diameter does not change drastically. At 12 and 30% oxygen concentrations, the particle surface temperature is high and the change in diameter is more significant. As Suda et al. [30] have already observed carbon dioxide has a cooling effect on the burning temperature. This can be easily explained by the following reactions: C + O 2 CO 2 (exothermic) (1) C + CO 2 2CO (endothermic) (2) When carbon reacts with oxygen, Equation 1, it releases energy, and as a consequence, the particle surface temperature rises. In contrast, when carbon reacts with International Flame Research Foundation,

19 carbon dioxide, Equation 2, the gasification reaction consumes energy and the particle surface temperature decreases. Other possible reasons for the low temperature are: a) the higher specific heat of CO 2 compared with that of N 2, b) the adsorption of oxygen or carbon dioxide in the outer layer of the particle, c) when combustion occurs with O 2 /N 2, the only reactant gas is oxygen, which enhances the combustion, whereas with a mixture of O 2 /CO 2, both gases react with carbon, which leads to temperature reduction, d) when using CO 2, the surface temperature of the particle is affected by heat transfer from its surface (mainly radiation) and combustion or gasification reactions on the surface. Consequently, gasification reactions for CO 2 occur on the particle surface. The high heat capacity of carbon dioxide has only minor effects on the heat balance in the particle, because high gas temperatures only occur in the thin boundary layers of the particle. CONCLUSIONS In this study, an adjustable drop tube reactor (DTR) for coal and char combustion measurements was successfully developed and tested. Movable feeding and collecting probes enable changing the length of the reaction zone, thus allowing variations in residence times and particle velocities in different gas atmospheres. Hence the conditions for industrial reactors and boilers can be simulated and the optimum combustion parameters can be defined. The main objective was to obtain particle surface temperature profiles as a function of residence time. The required data was produced with a two-color pyrometer. An increase in the oxygen concentration causes an increase in the particle surface temperature. With a 30% oxygen concentration, the maximum values for the char temperatures were 2400 K, while the minimum values for the char temperature, at about 1000 K, were measured with the minimum oxygen concentration of 3%. At the same time, there is a clear tendency for the particle size to decrease as the oxygen concentration increases. The most important result obtained was that by using O 2 /CO 2 instead of O 2 /N 2, the particle surface temperature was reduced by about 300 K. This piece of information is valuable for boiler design in the future. A CCD camera was used to measure the char particle velocity and size. The velocity results from the CCD camera match the two-color pyrometer results, indicating both results are reliable. However, the particle sizes from the camera measurements are smaller than those from the two-color pyrometer, but the results were obtained with a precision and International Flame Research Foundation,

20 reproducibility, which suggests that the CCD camera can be recommended for similar studies. Nevertheless, the image quality would be improved if the background LED-light would be replaced with a laser light instead. Experiments with the adjustable DTR developed for this study allow the selection of optimum conditions for char combustion at different temperatures. This is extremely useful when designing new types of boilers. Furthermore, the measurement data obtained enables the creation of theoretical combustion models for char oxidation in laminar gas flows, a topic which will be covered in a separate article by the same author. ACKNOWLEDGMENTS We would like to thank the National Technology Agency of Finland (TEKES), Academy of Finland, Fortum Oyj, Foster Wheeler Energia Oy, PVO Oy and Metso Power Oy for their financial support for this work under the CLIMBUS programme and for their permission to publish this article. The authors acknowledge the help of MSc. Henrik Tolvanen and Ms. Taru Siitonen, a MSc. student, with the experiments, and Mr. Matti Savela, a laboratory technician, for help with the reactor construction. The coal used in this study was provided by Metso Power Oy. References [1] IEA, International Energy Outlook [2] Association World Coal, Access date: May 13, [3] C. Baukal, Oxygen-enhanced combustion, chap. Basic Principles, Air Products, 1 45, [4] T. Wall, Y. Liu, C. Spero, L. Elliot, S. Khare, R. Rathnam, F. Zeenathal, B. Moghtaderi, B. Buhre, C. Sheng, R. Gupta, T. Yamada, K. Makino, J. Yu, An overview on oxygen coal combustion - State of the art research and technology development, Chemical Engineering Research and Design 87 (2009) [5] M. Aho, K. Paakkinen, P. Pirkonen, The effect of pf pressure, oxygen partial pressure and temperature on the formation of N 2 O, NO and NO 2 from pulverised coal, Combustion and Flame 102 (1995) [6] W. Chen, A simplified model of predicting coal reaction in a partial oxidation environment, International Communication in Heat and Mass Transfer 34 (2007) [7] T. Joutsenoja, T. Stenberg, R. Hernberg, M. Aho, Pyrometric measurement of the temperature and sizing of individual combusting fuel particles, Applied Optics 36 (1998) International Flame Research Foundation,

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22 [27] T. Joutsenoja, Pyrometric thermometry and sizing of fuel particles in combustion., Ph.D. thesis, Tampere University of Technology, Finland, [28] M. Honkanen, D. Elcock, C.-J. Kuo, M. Amitay, Y. Peles, Lagrangian tracking of bubbles interacting with pin-fins in a microchannel, Experiments in Fluids 50 (2010) [29] L. Smoot, Fossil fuel combustion: A source book, chap. Coal and char combustion, John Wiley & Sons, Inc., , [30] T. Suda, K. Masuko, J. Sato, A. Yamamoto, K. Okazaki, Effect of carbon dioxide on flame propagation of pulverized coal clouds in CO 2 /O 2 combustion, Fuel 86 (2007) International Flame Research Foundation,