Experimental Investigation of the Combustion of Bituminous Coal in

Aug 31, 2010 - and Yoshihiko Ninomiya§. †Department of Chemical Engineering, Monash University, GPO Box 36, Clayton Campus, Victoria 3800, Australi...
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Energy Fuels 2010, 24, 4803–4811 Published on Web 08/31/2010

: DOI:10.1021/ef100314k

Experimental Investigation of the Combustion of Bituminous Coal in Air and O2/CO2 Mixtures: 1. Particle Imaging of the Combustion of Coal and Char Lian Zhang,*,† Eleanor Binner,† Luguang Chen,† Yu Qiao,† Chun-Zhu Li,†,‡ Sankar Bhattacharya,† and Yoshihiko Ninomiya§ ‡

† Department of Chemical Engineering, Monash University, GPO Box 36, Clayton Campus, Victoria 3800, Australia, Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, WA 6102, GPO Box U1987, Perth, WA 6845, Australia, and §Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, 487-8501, Kasugai, Aichi, Japan

Received January 14, 2010. Revised Manuscript Received August 18, 2010

Combustion of a low-volatile bituminous coal in air versus two O2/CO2 mixtures (21/79 and 27/73, v/v) was conducted at two furnace temperatures of 800 and 1000 °C in a lab-scale drop tube furnace (DTF). Through in situ photographic observation and measurement of overall coal burnout rate, CO emission profile, and unburnt char properties, a variety of distinct phenomena relating to oxy-fuel combustion has been revealed. Consistent with the literature, the significant thermal effect of CO2 due to its large product of CpF (specific heat capacity and density) relative to that of N2 retarded volatile ignition in the two O2/CO2 mixtures. As a result, the volatiles released in O2/CO2 remained as a thick protective sheath on char surface for a relatively long duration, which mainly converted into CO through partial oxidation in 21% O2/79% CO2. Increasing the O2 fraction to 27% in CO2 triggered the ignition/oxidation of the unburnt volatiles once their concentrations were critically accumulated on char surface in a relatively low position in the DTF. Char oxidation behavior in the late stages of the DTF was also greatly changed under oxy-fuel conditions. Due to an insufficient O2 in char particle vicinity, the partial oxidation and even gasification of char to CO were favored during oxy-firing, which yielded less enthalpy heat and hence lowered char particle temperature substantially. Char consumption rate was, however, affected little or even slightly increased. A detailed mathematical modeling is required to quantitatively clarify the oxidation behavior of coal char in the presence of the abundant CO2 in the DTF.

delayed in O2/CO2 in comparison to in O2/N2 with the same O2 concentration. To match the flame/particle temperature in air, a large amount of O2 in CO2, typically around 30%, is required.1 Coal conversion rate, char properties, and reactivity are also affected by the replacement of air with an O2/CO2 mixture. The influence of bulk gas, however, varies greatly with coal property and combustion facility/condition. At a given O2 concentration, coal burnout rate in O2/CO2 is slower than in O2/N2.5,6 This is not unexpected as a lower particle/ flame temperature exists in O2/CO2. A slow transfer of O2 in CO2 (20% less than in N2) also greatly retards the char-O2 oxidation reaction on the condition that this reaction is controlled by O2 diffusion through an external gas boundary layer.7 The endothermic char-CO2 gasification reaction, as most likely occurring at high temperatures,8,9 further makes oxy-fuel combustion complex. Knowledge for oxy-fuel combustion is still scarce. One major reason is that coal combustion is a very complex process governed by transient phenomena and a series of chemical

Introduction Coal combustion is one of the major sources for power generation, providing approximately 37% of the electricity requirement in the world.1 Its greenhouse gas emissions, particularly of carbon dioxide (CO2), however, have been facing stringent regulations with respect to the climate change. Efforts must be made to reduce and eventually eliminate CO2 emission in the short/medium term, thereby maintaining a sustainable utilization of coal in the carbon-constrained future. Oxy-fuel combustion is a process of burning coal in a gas stream of oxygen (O2) mixed with recycled flue gas (RFG), generating a CO2-rich flue gas that is potentially subjected to direct sequestration/storage with minimal treatment.1,2 Extensive studies in both pilot-plant and lab scales have pointed out the pronounced influence of gas composition (air versus O2/CO2) on coal combustion performance. The heat transfer and temperature distribution in a furnace are greatly affected by the large specific heat capacity of CO2.1,3,4 Coal ignition is *To whom correspondence should be addressed. Phone: þ61-3-99052592. Fax: þ61-3-9905-5685. E-mail: [email protected]. (1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31 (4), 283–307. (2) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31, 1905– 1912. (3) Kakaras, E.; Koumanakos, A.; Doukelis, A.; Giannakopoulos, D.; Vorrias, I. Fuel 2007, 86, 2144–2150. (4) Khare, S. P.; Wall, T. F.; Farida, A. Z.; Liu, Y.; Moghtaderi, B.; Gupta, R. P. Fuel 2008, 87, 1042–1049. r 2010 American Chemical Society

(5) Bejarano, P. A.; Levendis, Y. A. Combust. Flame 2000, 153, 270– 287. (6) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833–840. (7) Shaddix, C. R.; Molina, A. Proceeding of the 5th joint meeting of the US sections of the Combustion Institute, San Diego, CA, USA, March 25-28, 2007; Paper G24. (8) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Fuel Process. Technol. 2009, 90, 797–802. (9) Shaddix, C. R.; Murphy, J. J. Proceedings of the 20th Pittsburgh Coal Conference, Pittsburgh, USA, Sept 15-19, 2003; CD-ROM.

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reactions occurring at each subprocess. The role of CO2 in each process has not been clarified yet. Most of the works to date mainly focused on heat transfer,4,10,11 global conversion rate of coal/char,6,12 and pollutant emissions.6,13 Little is known about the details of each subprocess. One can imagine that, once the ignition of the volatiles is delayed with shifting bulk gas from air to an O2/CO2 mixture, all the subsequent subprocesses would be delayed and altered accordingly. In this regard, a state-of-the-art high-speed camera with a maximum shutter speed of 2000 frames per second (fps) was employed to in situ record the dynamic oxidation of coal in a transparent quartz DTF. In addition to the previous investigation of particle velocity with the use of high-speed camera,14 this study focused on photographing transient phenomena to examine in situ the oxidation dynamics of the burning of volatiles and char particles. Compared to the imaging system used in the literature,15,16 the high-speed camera employed here is advanced enough ensuring the resolution of coal combustion sequence down to 2 ms. The luminosity (i.e., brightness) of discernible spots in the camera’s field of view (FOV) was used as a sign of the oxidation intensity of volatiles and char. Measurement of coal conversion rate, char properties, and CO emissions was also carried out to clarify coal combustion behavior in O2/CO2. By augmenting a companion paper on ash formation in air versus O2/CO2,17 this study aims to provide further evidence to promote the understanding on the role of CO2 on the combustion of bituminous coal and hence shed new lights into the retrofitting of existing power generation plants with oxy-firing technology.

Table 1. Properties of the Coal Sample Tested Here Proximate Analysis, wt %, As Received moisture volatile matter (VM) fixed carbon (FC) ash

3.8 24.4 44.0 27.8

Ultimate analysis, wt % daf carbon hydrogen nitrogen oxygen þ sulfur (by diff.)

76.0 5.2 1.2 17.6

SiO2 Al2O3 Fe2O3 CaO MgO TiO2 Na2O K2O SO3 P2O5 Cl ZnO

Ash Composition, wt %

41.3 30.4 5.7 2.4 0.5 2.4 0.1 0.8 15.3 0.4 0.2 0.02

coal and primary gas at the top of the inner tube of the reactor. Due to this unique configuration, a very uniform gas temperature is guaranteed in most of the whole reactor.14 The same temperature profile was confirmed between air and the two O2/CO2 mixtures at a given furnace temperature. Three water-cooled coal injectors with a length of 0, 600, and 1200 mm protruding into the inner chamber of the quartz reactor were employed for coal combustion at a reactor distance of 1800, 1200, and 600 mm, respectively. Coal particle residence time for a reaction distance of 600 mm is estimated to be ∼1 s.14 Therefore, the longest residence time for coal particle in DTF is ∼3 s, which is similar to the industrial scale. A flask and a thimble filter with a cutoff size of ∼0.5 μm were installed at about 100 mm downstream the reactor to collect particles, which were also continuously quenched by water and dry ice at an estimated quench rate of ∼5000 °C 3 s-1. This rate is comparable to that usually achieved by a water-cooled nitrogen-quenched suction probe in the conventional DTF reaction facility.19,20 It is high enough to prevent any secondary reactions of char and ashes. This sampling method is advantageous in terms of sample collection and coal conversion determination. Conventionally, only a portion of coal combustion products is iso-kinetically sucked by a sampling probe, which is then used for coal burnout calculation based on the ash-tracer method.21 The accuracy of this method, however, depends on a number of assumptions such as that the particles collected are representative and that ash recovery is 100% when compared with the original mineral matter content in raw coal. These assumptions are unrealistic when the vaporisation of metals is prominent, especially during low-rank coal combustion. The unburnt char yield was calculated on the carbon balance basis: Mchar - Mash - in - char Char yieldð%, dafÞ ¼  100 ð1Þ Mcoal - Mash - in - coal

Experimental Section Coal Property. A bituminous coal from China was tested for the combustion experiments. It was pulverized to 106-153 μm and air-dried prior to use. As shown in Table 1, the coal sample tested contains 24.4 wt % volatile matter (VM) and a large quantity of ash (27.8% on dry mass basis). The char generated from coal pyrolysis in N2 at the furnace temperature of 1000 °C was also tested. Coal pyrolysis was conducted in a lab-scale DTF, as will be explained in detail later. All the volatile matter was removed during coal pyrolysis, resulting in a char yield of 55.2 wt % on the dry-and-ash-free (daf) basis. Coal Combustion Facility. An electrically heated DTF coupled with a transparent quartz reactor with a length of 2000 mm and two cylindrical chambers was employed for coal combustion.14,18 Coal at a feeding rate of ∼0.5 g/min was entrained by 1.0 L/min cold primary gas into the top of the inner chamber (50 mm in diameter) of the quartz reactor. The majority of the gas used for combustion, namely secondary gas, was introduced at 9.0 L/min from the bottom of the outer chamber (80 mm in diameter). It was heated to furnace temperature before passing through a quartz frit and mixing with (10) Andersson, K.; Johansson, R.; Hj€arstam, S.; Johnsson, F.; Leckner, B. Exp. Thermal Fluid Sci. 2008, 33, 67–76. (11) Bejarano, P. A.; Levendis, Y. A. Combust. Flame 2008, 153, 270– 287. (12) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 2109–2115. (13) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2008, 47 (6), 1835–1845. (14) Zhang, L.; Binner, E.; Qiao, Y.; Li, C.-Z. Energy Fuels 2010, 24, 29–37. (15) Shaddix, C. R.; Molina, A. Proc. Comb. Inst. 2009, 32, 2091– 2098. (16) McLean, W. J.; Hardesty, D. R.; Pohl, A. J. H. Proc. Comb. Inst. 1981, 8, 1239–1248. (17) Zhang, L.; Jiao, F.; Chen, L.; Binner, E.; Bhattacharya, S.; Ninomiya, Y; Li, C.-Z. Fuel, 2009, submitted. (18) Zhang, L.; Binner, E.; Qiao, Y.; Li, C.-Z. Fuel 2010, 89, 2703– 2712.

The symbols Mchar and Mcoal denote the mass of char collected and coal fed during an experiment, respectively. Mash-in-char and Mash-in-coal are the mass of ash in the char and coal respectively, which were determined by burning char and coal at a heating rate of 21% O2/79% CO2 was observed at the first stage. Moreover, it is further confirmed that char oxidation rate at the middle stage is the largest in 21% O2/79% CO2. Irrespective of gas environment, the majority of the char was consumed before 1200 mm, thus little of it remained unburnt at the last stage. Coal Combustion Sequence. Oxidation of volatiles played an important role at the initial stage (0-600 mm) during coal combustion. In this regard, the combustion sequence of raw coal before 600 mm was recorded by the high-speed camera at several distances, including coal pyrolysis, volatile ignition, and oxidation. Typical photographs for coal combustion in air are shown in Figure 3. When coal particles were injected into hot air at the furnace temperature of 800 °C (panel a), they initially underwent heating before 200 mm, thus most of the particles were invisible or weakly luminous in the camera’s FOV. Round luminous spots were observed with the reaction length extending to 300 mm, indicative of the release and ignition of volatiles. The round shape was attributed to the ejection of both gaseous and tarry volatiles out of coal particles. Particularly, the tarry volatiles are dominant in the products of bituminous coal pyrolysis, which preferentially remained on particle surface due to the viscous properties.22,23 Upon meeting oxygen, the evolved volatiles rapidly ignited from 300 mm onward, the oxidation of which progressed steadily and was intensified with the reactor length down to 400 mm. Accordingly, a large quantity of luminous spots attached with volatile flame were observed in camera’s FOV. The volatile oxidation was nearly complete at 600 mm, hence only a few of sparkling spots were observed at a long distance. Irrespective of reaction distance, coal combustion intensity is greatly improved with furnace temperature increasing to 1000 °C, as expected. As illustrated in panel b of Figure 3, coal ignition in air occurred before 150 mm, relative to 300 mm for 800 °C, yielding a large quantity of strongly incandescent spots. The oxidation of volatiles was further intensified at 250-300 mm, as evidenced by the presence of vast luminous spots with long volatile trails in the camera’s FOV. The oxidation of volatiles was complete by 400 mm, leaving abundant elongated spots with weak luminosity. Char oxidation should commence at this step or between 300 and 400 mm, as the volatile tails disappeared. Most of the char was also consumed before 600 mm, thus leaving very limited discernible spots at 600 mm, in agreement with a low char yield (∼10% in Figure 1b) achieved at this stage. A Similar coal combustion sequence was observed in the two O2/CO2 mixtures, that is, coal pyrolysis occurred initially,

Figure 2. Char conversion as a function of reaction distance. Panels a and b depict the furnace temperatures of 800 and 1000 °C, respectively.

In this regard, it is reliable to conclude that a 27% O2 fraction in CO2 is sufficient to exceed air in terms of coal conversion. The gap of coal conversion among three gases became smaller and even negligible with the increase in reaction distance (i.e., coal residence time). Irrespective of bulk gas composition, the overall coal combustion rate was significantly improved when the furnace temperature was increased to 1000 °C. The difference of coal conversion among three bulk gases was also greatly reduced. As evidenced in Figure 1b, an unburnt char yield of ∼10% was achieved at 600 mm in air, demonstrating an extremely intense oxidation of both volatiles and char. Replacing air by 21% O2/79% CO2 still played a negative role in coal conversion, as evidenced by the ∼20% unburnt coal at 600 mm in this gas atmosphere. Increasing the O2 fraction in CO2 to 27% reduced the unburnt coal content to ∼5 wt % at 600 mm, further demonstrating the intense coal combustion in this gas atmosphere. With the reactor distance increasing to 1200 mm, nearly all the carbonaceous materials in coal were burnt out irrespective of bulk gas composition. Since the oxidation of char is the principal phenomenon occurring from 600 mm onward, char conversion rates at three distances/stages, that is, 0-600, 600-1200, and 12001800 mm, were further extracted from Figure 1. Char conversion at the first stage was determined by Char conversion, % ¼

FC - UC600  100 FC

ð2Þ

Where the symbol UC600 denotes the mass of the unburnt carbon collected at 600 mm, with a unit of wt % on the daf basis of raw coal. Regarding char conversion in the latter two stages, it was determined by Char conversion, % ¼

UCi - UCi þ 600  100 FC

ð3Þ

Where the subscript “i” denotes 600 or 1200 mm, and “i þ 600” denotes a distance 600 mm downward of i. As illustrated in Figure 2, at a low furnace temperature of 800 °C, char conversion in air progressed steadily with

(22) Chen, J. C.; Taniguchi, M.; Ito, K. Fuel 1995, 74 (3), 323–330. (23) Ponzio, A.; Senthoorselvan, S.; Yang, W.; Blasiak, W.; Eriksson, O. Fuel 2008, 87, 974–987.

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Figure 3. Air combustion sequences at 800 °C (a) and 1000 °C (b). Photos 1-4 in panel a, for 800 °C, were taken at the distances of 200, 300, 400, and 600 mm, respectively. Photos 1-4 in panel b, for 1000 °C, were taken at 150, 300, 400, and 600 mm, respectively, cited from ref 14.

steadily due to the thermal feedback from volatiles oxidation. The luminous spot in camera’s FOV thus gradually grew in size. Coal ignition was greatly delayed with the bulk gas shifting from air to 21% O2/79% CO2. As indicated, the volatiles evolved were mostly accumulated on char surface, which were even difficult to ignite in 12 ms before leaving camera’s FOV. This is attributed to the distinct properties of CO2. Physically, the combination of heat capacity (Cp on molar basis) and density (F) of a gas gives a measure of the thermal sink for any heat that is chemically released.2 Therefore, the ignition of volatiles was delayed, and the oxidation of volatiles was elongated in this gas, as the product of CpF of CO2 is around 1.7 times that of N2. Due to this reason, the unburnt volatiles were accumulated as thick cloud as a protective sheath on char particle surface, which significantly blurred the spots in camera’s FOV, and also doubled the average size of the discernible spots from 6.0 pixel numbers in air to 13.1 in 21% O2/79% CO2 (as suggested by statistically measuring through Image-Pro 6.1, data not shown here). Increasing O2 fraction in CO2 to 27% is beneficial for volatile ignition, as the product of CpF of the bulk gas is reduced. It is however still insufficient to match the product of CpF of air. As suggested by the spots in circle for 27% O2/ 73% CO2 in Figure 4, the volatiles released were only partly ignited in 4 ms, most of which still remained unburnt (and thus gray in the camera’s FOV) until 12 ms and greatly blurred char particles in the camera’s FOV. Apparently, improving the O2 fraction in CO2 to a higher level such as 30-35% is essential.5 The optical intensities of the discernible spots at three different stages for the furnace temperature of 1000 °C were further statistically processed and are shown in Figure 5. Note that 300-1000 individual spots at a distance were counted for the average optical intensity and standard deviation. The error bar denotes twice the standard deviation. In a given gas atmosphere the largest intensity was observed at 250 mm, responding to intense volatiles oxidation providing vast thermal feedback to coal particles. The low intensities at 150 and 600 mm account for coal devolatilsation/ignition and char oxidation, respectively. In both cases fewer of the evolved volatiles were oxidized. This measurement is consistent with the photographs illustrated in Figures 3 and 4.

Figure 4. Dynamic information for the release of volatiles and ignition at the reactor distance of 150 mm and the furnace temperature of 1000 °C.

followed by volatiles ignition, volatiles oxidation, and char oxidation. Moreover, due to a prior heat-up of the secondary gas and a similar thermal conductivity between N2 and CO2,2,20 the initial heat-up profile of coal before ignition was plausibly the same in the three bulk gases.2 A similar onset time for coal devolatilization is also expected in the three bulk gases. However, due to the distinct properties of CO2, the distinguishable ignition and oxidation behaviors in O2/CO2 were revealed by high-speed camera photography. As indicated by the typical multiple photographs for a short distance of 150 mm in Figure 4, the round spot highlighted by a circle in air was weakly incandescent at the beginning of photography. It ignited quickly in 4 ms, and consequently emitted strong radiation around particles in the next photograph. The release of volatiles and their ignition continued 4807

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Figure 5. Statistic comparison of the optical intensity of luminous spots observed at different stages in three bulk gases at the furnace temperature of 1000 °C.

The bulk gas composition affected coal particle luminosity differently at a different step. For both coal pyrolysis at 150 mm and char oxidation at 600 mm, the average intensity of coal particles in the two O2/CO2 mixtures was reduced to less than half of those in air, indicating the substantially large thermal effect of CO2 at these two stages. However, at 250 mm for an intense oxidation of volatiles, the optical intensity of coal particles decreased in a sequence of 27% O2/73% CO2 > air > 21% O2/79% CO2, which is inconsistent with other distances, but in agreement with the sequence of coal burnout rate in Figures 1 and 2. Furthermore, as the luminosity of coal particles in 27% O2/73% CO2 was increased by nearly 20% compared to air (according to the difference of the average grayness between 27% O2/73% CO2 and air, 175 versus 150), it is indicative that the volatiles evolved during coal pyrolysis was accumulated and oxidized until 250 mm in 27% O2/73% CO2, relative to the intense ignition and oxidation of volatiles in 150-200 mm in air. Volatiles were rarely ignited/oxidized before 250 mm in 21% O2/79% CO2, although more of the unburnt volatiles were accumulated on the char surface. This should be attributed to the lower O2 concentration on coal particle surface. The slow diffusion of O2 in CO2 greatly restricted its local quantity on coal particle surface. Conversion of the unburnt volatiles before 600 mm in O2/CO2 is noteworthy. As few of them were ignited at the low furnace temperature, 800 °C, the volatiles evolved could decompose into gaseous species and/or light hydrocarbons through partial oxidation. The emissions of CO as a function of reaction distance in Figure 6 partly proved this hypothesis. As illustrated in panel a, for each gas atmosphere, a large amount of CO was emitted at 600 mm. At the furnace temperature of 800 °C, approximately 8.0 wt % daf of CO was emitted at 600 mm in 21% O2/79% CO2, which was entirely contributed from volatiles decomposition, as little char was oxidized at this distance. Increasing the O2 fraction in CO2 enhanced the diffusion rate of O2. Therefore, the amount of unburnt CO dropped to less than 4 wt % daf at 600 mm. Less CO was emitted at 600 mm in air, suggesting its rapid mass transfer and oxidation into CO2 in nitrogen. This reaction might happen concurrently with coal oxidation to CO, rather than slowly in the oxy-fuel cases. Moreover, it is clear that, irrespective of the bulk gas composition, the CO

Figure 6. CO emission versus particle residence time at the furnace temperatures of 800 °C (a) and 1000 °C (b).

generated from volatiles oxidation before 600 mm was rapidly oxidized into CO2 with the reactor distance increasing to 1200 mm onward. Increasing furnace temperature to 1000 °C further increased the oxidation rate of CO into CO2. As shown in panel b, the CO content emitted at 600 mm in 21% O2/79% CO2 only accounts for 0.9 wt % daf, relative to 0.7 and 0.4 wt % daf emitted in the 27% O2/ 73% CO2 and air, respectively. Char Oxidation in Air versus O2/CO2 Mixtures. Char oxidation is the principal phenomenon occurring from 600 mm onward. Its observation with high-speed camera was not made in the raw coal case due to the unavailability of coal injectors to access any distance between 600 mm and 1200 mm. In contrast, a char sample generated by coal pyrolysis in N2 at 600 mm and 1000 °C was fed into DTF for photography. High-speed camera observations were made at several distances before 600 mm, which was anticipated to provide information about the combustion of raw coal between 600 and 1200 mm. The furnace temperature of 1000 °C and two bulk gases (air vs 21% O2/79% CO2) were tested. Char ignition occurred slowly compared to volatiles. There are two major types of luminous spots observed during char ignition, as illustrated in Figure 7 for a reaction distance of 200 mm from coal injector in air. The round spot (see panel a) denotes porous char particles formed from coal swelling in inert N2, which was dim at the beginning of photography, gradually turning bright due to ignition in 24 ms. With time further increasing to 34 ms, the round spot became smaller and dimer again, indicating the completion of char oxidation. The rod-like spot (see panel b) indicates 4808

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another type of char derived from coal pyrolysis. It might be formed from the inertinite macerals in coal that underwent little change in shape during pyrolysis. These particles further fragmented once injected into the oxidizing atmosphere. The resulting small pieces ignited in 24 ms. Their oxidation was also complete in 34 ms. These findings are same as that observed for the swollen particles. The ignition and oxidation of char particles are intensified with the reaction distance increasing to 300 mm, see Figure 8a. The luminous spots, either in round or elongated shape, are also larger than those in Figure 7, indicative of the intense release of radiation heat at this stage. Moreover, it was observed that the individual char particles as shown in circle even collided with one another to merge into larger particles

during their oxidation. Such a phenomenon is influential in terms of the evolution of mineral species, the agglomeration of which could be promoted by char particle collision. Char oxidation is nearly complete at 600 mm (see panel b), and hence, most of the char particles are very dim and even invisible in the camera’s FOV. Moreover, the luminous spots observed possessed large size compared to those observed at 200 and 300 mm, evidencing the difficulty for large coal particles and those containing high mineral species to burn, as expected. Change in bulk gas composition also played a remarkable role in the properties of luminous spots formed during char oxidation. As demonstrated quantitatively in Figure 9, at 100 mm near coal injector the optical intensity for luminous char particles observed in 21% O2/79% CO2 is ∼30% less than in air. Their difference was further increased to approximately 60% at 300 mm onward. Such a gap is even larger than that observed for volatile oxidation at 250 mm in Figure 5. Apparently, due to the absence of volatiles, the char in oxyfuel mode was more difficult to ignite. Here again, the large thermal effect of the abundant CO2 can explain this phenomenon. Nevertheless, as the char conversion rate still remained relatively high during oxy-firing in the middle stages from 600 to 1200 mm (see Figure 2), it is clear that the char mass consumption did not cease at this stage, although its temperature was rather lower in the two O2/CO2 mixtures. One plausible explanation is a partial oxidation of char to CO rather than directly to CO2 during oxy-fuel combustion, same as that observed for volatiles. Consequently, less heat was generated than that from the direct oxidation of char to CO2, according to ð4Þ C þ O2 f CO2 ΔH° ¼ - 393 KJ=mol

Figure 7. Two major types (a and b) for char ignition in air at 200 mm from coal injector at the furnace temperature of 1000 °C.

C þ O2 f CO

ΔH° ¼ - 110 KJ=mol

Figure 8. Char oxidation in air at 300 mm (a) and 600 mm (b) from coal injector at the furnace temperature of 1000 °C.

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affect the emission of other air pollutants such as CO, which are apparently of importance during oxy-firing of coal in an O2-lean environment, for example, in an integrated coalfired power plant process with membrane-based O2 supply.27 Conclusions A bituminous coal sample was combusted in air and two O2/CO2 mixtures (21/79 and 27/73) at two furnace/gas temperatures of 800 and 1000 °C in a lab-scale DTF. Through in situ photography and measurement of overall coal burnout rate, CO emission, and char properties, distinct behavior for the oxidation of volatiles and char during oxy-firing has been explored. The volatiles evolved in O2/CO2 mixtures preferentially remained in the vicinity of char particles in a long duration to form a thick protective sheath. Due to the large thermal effect of CO2 and the low diffusivity of O2, the ignition and oxidation of evolved volatiles occurred in a relatively low position in the DTF. The unburnt volatiles mainly converted to CO through partial oxidation in 21% O2 diluted in CO2, forming a detached and weak flame. Increasing the O2 fraction in CO2 to 27% promoted the O2 diffusion in CO2, and hence, triggered the oxidation of the volatiles accumulated on char surface. Accordingly, vast thermal feedback was generated for a continuous oxidation of char particles. Char oxidation behavior in the later stages of DTF was also distinct in the O2/CO2 mixtures. Apart from the partial oxidation, char-CO2 gasification reaction on char surface and/or inside could occur, which greatly offset the negative influence of the thermal effect of CO2 and improved coal mass reduction rate. A quantitative analysis of the importance of this reaction through mathematical modeling is being considered.

Figure 9. Optical intensity of char particles as a function of reaction distance in air and 21% O2/79% CO2 at the furnace temperature of 1000 °C. Table 2. CO2 Surface Area of the Unburnt Chars Collected at 600 mm, m2/g daf

air 21% O2/79% CO2 27% O2/73% CO2

800 °C

1000 °C

65 135 135

41 95 65

Such a reason is very likely in the late stages of coal combustion where the O2 is sufficiently lean and char oxidation is under O2 transfer control.24 The abundant CO2 greatly restricted O2 diffusion to char surface. The endothermic char-CO2 gasification reaction, as has been mentioned in the literature,8,9,25 is another probable reason causing significant mass reduction on the char surface where the local O2 content is extremely lowered by the diffusion resistance of CO2 boundary layer and the volatile cloud. Moreover, compared to O2, which is mostly consumed on the surface, the abundant CO2 could even penetrate the interior of particles,26 thereby triggering the char-CO2 gasification inside and creating a larger specific surface area for the oxy-fuel char samples. This is supported by the results for the surface areas of the 600 mm chars in Table 2. As can be seen, irrespective of furnace temperature, the char obtained in air shows the smallest surface area. Apparently, the char-CO2 gasification reaction on char surface competed favorably with the char-O2 reaction in the overall char mass loss rate. Nevertheless, to quantitatively clarify the importance of char-CO2 gasification, a detailed mathematical modeling similar to that has been done elsewhere9,25 must be carried out, taking every distinct phenomena observed in this study into account. Particularly, apart from partial oxidation and CO2-gasification reactions, the presence of a volatile cloud on the char surface and its resistance against O2 diffusion needs to be considered during modeling. Delay on the ignition and oxidation of the volatile cloud could also cause the formation of tarry species and

Acknowledgment. This study is financially supported by the Victorian State Government under its Energy Technology Innovation Strategy (ETIS) program and the Latrobe valley generators including International Power Hazelwood, International Power Loy Yang B, Loy Yang Power, and TRUenergy. The HRL group was also thanked for their assistance in the project. Professor Minghou Xu at the State Key Laboratory of Coal Combustion in Huazhong University of Science and Technology, China, is thanked for kindly providing the coal sample tested in this study. Mr. Yong Sun in the Department of Chemical Engineering of Monash University is also thanked for char surface area analysis.

Nomenclature daf = dry-and-ash-free basis dtf = drop-tube furnace fps = frames per second FC = fixed carbon in raw coal, wt % FOV = field of view of high-speed camera or two-color pyrometer Mcoal = mass of raw coal fed in a run, g Mchar = mass of char collected in a run, g Mash-in-coal = mass of ash in the raw coal fed into a run, g Mash-in-char = mass of ash in the char collected in a run, g RFG = recycled flue gas UC600 = unburnt carbon collected at a reaction distance of 600 mm in DTF, wt %-daf on the basis of raw coal.

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: DOI:10.1021/ef100314k

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UCi = unburnt carbon collected at a reaction distance of 600 mm or 1200 mm, wt % daf on the basis of raw coal. UCiþ600 = unburnt carbon collected at a reaction distance 600 mm downstream of the distance i, 1200 mm or 1200 mm. wt % daf on the basis of raw coal.

VM = Volatile matter in raw coal, wt % Greek Symbols Cp = Specific heat capacity of gas, kJ/kg 3 K F = Gas density, kg/m3

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