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Energy & Fuels 2009, 23, 1134–1135
Characterization of Oxy-coal Combustion by TemperatureProgrammed Desorption Wei-Yin Chen,* Guang Shi, and Shaolong Wan Department of Chemical Engineering, Anderson Hall, P.O. Box 1848, UniVersity of Mississippi, UniVersity, Mississippi 38677-9740 ReceiVed September 26, 2008. ReVised Manuscript ReceiVed January 8, 2009 Current studies on the ignition, devolatilization, and combustion kinetics of chars in oxy-coal combustion reflect the fundamental importance of oxidant-activated mechanisms in the early stage of char oxidation.1-6 Independently, temperatureprogrammed desorption (TPD) has revealed sensitive characteristics of chars oxidized by 1-21% O2.7-9 The existence of stable surface oxides that desorb between 1100 and 1650 °C is demonstrated.7,8 Oxygen from the gas phase, organic portions of the coal, and minerals in the coal have profound influences on the formation and desorption of stable surface oxides in the early stage of coal combustion.8,9 In this work, coal oxidation and the subsequent TPD are conducted in situ in a two-staged reactor/gas chromatography/ mass spectrometry (MS)/differential scanning calorimetry system.8,9 An Illinois No. 6 bituminous coal is oxidized by three different types of oxidants: O2/CO2 (30%/70%), O2/Ar (30%/70%), and CO2/Ar (70%/30%). Coal particles are fed into an oxidation reactor by a particle feeder at a steady rate.10 To obtain reasonable carbon burnouts, the gas flow rate for oxidation is set at 3.7 L/min, which corresponds to 50 ms of residence time in the 4-in. isothermal zone of a parabolic axial temperature distribution.9 The reactions in the heating and cooling periods can contribute up to about 71% of the overall conversion.11 Thus, the actual residence time of coal particles during oxidation is much longer than 50 ms and likely in the range of 150 ms. A Bunsen burner is used to measure the carbon burnout of char by assuming that the ash remaining in the crucible is the tie constituent in char. We also assume that devolatilization is essentially complete in our estimation; thus, the carbon burnout is the difference between the fixed carbon of coal determined by proximate analysis and the weight loss of char measured in the Bunsen burner procedure. In an earlier work,8 we demonstrated that devolatilization with a relatively longer residence time than the present work is essentially complete during oxidation. We are confident that the devolitization for the current * To whom correspondence should be addressed. Telephone: (662) 9155651. Fax(662) 915-7023. E-mail:
[email protected]. (1) Shaddix, C.; Molina, A. Proceedings of the 5th U.S. Combustion Meeting, San Diego, CA, 2007. (2) Murphy, J. J.; Shaddix, C. R. Combust. Flame 2006, 144, 710–729. (3) Andersson, K.; Johnsson, F. Fuel 2007, 86, 656–668. (4) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31, 1905– 1912. (5) Suda, T.; Masuko, K.; Sato, J.; Yamamoto, A.; Okazaki, K. Fuel 2007, 86, 2008–2015. (6) Khare, S. P.; Wall, T. F.; Farida, A. Z.; Liu, Y.; Moghtaderi, B.; Gupta, R. P. Fuel 2007, 87, 1042–1049. (7) Chen, W. Y.; Wan, S.; Shi, G. Energy Fuels 2007, 21, 778–792. (8) Chen, W. Y.; Shi, G.; Wan, S. Energy Fuels 2008, 22, 3724–3735. (9) Wan, S.; Chen, W. Y.; Shi, G. Energy Fuels 2009, in press. (10) Chen, W. Y.; Gowan, G. C.; Shi, G.; Wan, S. ReV. Sci. Instrum. 2008, 79, 083904. (11) Chen, W. Y. Chem. Eng. Educ. 1999, 238–243.
Table 1. Experimental Parameters and Carbon Burnout oxidation oxidant oxidant temperature, °C concentration O2/CO2 O2/Ar CO2/Ar
900
30/70 30/70 70/30
SR 1.08 1.08 ∼2.0
residence carbon time, s burnout, % 0.20
56.5 46.5 19
work is essentially complete for a few reasons. Kobayashi12 and Kobayashi et al.13 demonstrated that coal devolatilization completes in a flow reactor operated under conditions similar to ours, and the devolatilization level increases with an increase in the temperature of a flow reactor up to 2100 K and 200 ms. The particle residence times for the current oxidation experiments are in the same range. It is worth mentioning that the fixed carbon of coal is determined at 950 °C (by the Huffman Laboratory) and our oxidation experiments were conducted at 900 °C. Because these two quantities are estimated at temperatures that are sufficiently close to each other, our procedure should be reasonably good for determining the carbon burnout at 900 °C. Moreover, oxygen balance reveals that the CO emissions below 900 °C during our TPD experiments account for about 4.3% and 0.2% of the total oxygen and carbon in the char, respectively. The coal feeding rate at about 1100 mg/min for the O2/CO2 and O2/Ar runs and 70 mg/min for the CO2/Ar run correspond to stoichiometric ratio (SR) ranges from 1.08 to 2.0; see Table 1. Because of its relatively low reactivity, CO2 is not considered to be an oxidant in the O2/CO2 mixture in the calculation for SRs. Char oxidation produces flames in O2/CO2 and O2/Ar but no flame in CO2/Ar. The reactivity of young char is known to be much higher than those of chars that have been thermally treated for a long time.8,9 Therefore, it is likely that the observed carbon conversion from the CO2/Ar system, 19%, is higher than those reported for the old chars. Ar and He have nearly identical heat capacities, about 20.8 J/mol/K at 450 °C, the mean temperature. CO2 has a heat capacity of about 49.9 J/mol/K at the same temperature, which is 2.4 times those of Ar and He. On the basis of the algorithm established earlier,11 the heating rates of these gases should not vary significantly in the reactor tube. Experimental determination of the temperature distribution of the inner wall of the reactor in the axial direction by a previously established method11 also confirms this anticipated result. TPD is carried out in a SiC tube as soon as a desirable amount of char is collected on the support materials.8 Char is heated at 5 °C/min rate and in a 75 mL/min ultrahigh-purity He flow, (12) Kobayashi, H. Devolatilization of pulverized coal at high temperatures. Ph.D. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1976. (13) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1977; pp 411-425.
10.1021/ef800822h CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
Communications
Figure 1. TPD profiles of bituminous young chars oxidized at 900 °C by three different oxidants. All of these profiles are normalized to 1 mg of carbon in char before TPD. Char oxidized by O2/CO2 produces less CO than those oxidized by O2/Ar or CO2/Ar. Char from O2/CO2 has more complete pyrolysis during oxidation rather than the other two chars. Char from CO2/Ar produces more functional groups on the coal’s surface during oxidation. The peaks around 1450 °C are mainly contributed by carbon oxidation induced by mineral decomposition. The minerals’ catalytic roles are enhanced in the presence of higher CO during combustion such as that in oxy-coal combustion.
which corresponds to a gas residence time of 20 s (or Peclet number 5040) in the transfer line before the gas is analyzed by MS. The gas residence time of desorption products in the hot zone of the reactor decreases from 87 to 17 s (or Peclet number from 41.2 to 10.8) when the temperature increases from 100 to 1650 °C, respectively. After the peak temperature is reached, the reactor is maintained at 1650 °C for 90 min and then cooled at 5 °C/min. Residual oxidants in the carrier He are removed by treatment of He with Cu turning at 550 °C before entering the TPD reactor.8 Figure 1 presents the CO emissions during TPD of the three chars described above. These CO productions are normalized to 1 mg of carbon in the char prior to TPD. Char oxidized by O2/CO2 has a CO emission at about 900 °C, and chars oxidized by O2/Ar or CO2/Ar have CO emissions at about 700 °C. These CO emissions below 1000 °C are mainly from incomplete pyrolysis during oxidation. There are one or more peaks between 1100 and 1500 °C, with a distinct peak at 1270 °C, suggesting the existence and complexity of the stable surface oxide. The CO emissions at 1450 °C are mainly contributed by mineralcatalyzed carbon oxidation.9 The char oxidized by CO2/Ar has a much more complex TPD profile than two others below 1300 °C. It also produces notably higher CO over a wide temperature range. These characteristics may be the result of the more complex CO2/carbon interactions. Alternatively, the endothermic nature of the CO2/carbon reaction may lead to a lower particle temperature than that in O2.14 Moreover, the rate of the CO2/carbon reaction is much lower than that of the O2/carbon reaction at the same temperature. Lower temperature assists the retention of surface oxides that subsequently desorb at higher temperature during TPD. (14) Saastamoinen, J. J.; Aho, M. J.; Hamalainen, J. P. Energy Fuels 1996, 10, 121–133.
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Char oxidized by O2/CO2 emits the lowest amount of CO among the three chars below 1000 °C during TPD. Its TPD profile is similar to that for the O2/Ar char in the temperature range of 1100-1300 °C. The CO peak for the CO2/Ar char is slightly higher than those for the two others, which is likely due to the retention of stable surface oxides at a lower particle temperature of oxidation as discussed above. Chars oxidized by O2/CO2 and CO2/Ar have the lowest and highest CO emissions at around 1450 °C during TPD, respectively. Because this CO production is associated with carbon oxidation originating from mineral decomposition,8,9 the trend observed here suggests that minerals in the O2/CO2 char are severely reduced while the CO2/Ar char is the least reduced. It has been demonstrated that minerals in the char are reduced by gaseous CO during oxidation.15,16 It is also known that the CO2 and CO concentration profiles inside the particle in the O2/CO2 mixture, i.e., in the oxy combustion environment, are much higher than those in air combustion.17 As a result, it is not surprising to note that the O2/CO2 char produces less CO than the O2/Ar char at 1450 °C during TPD. The CO2/Ar char also has a high CO concentration profile during oxidation. Nevertheless, because of the endothermic reaction of CO2 with carbon, the extent of CO/minerals interactions is expected to be low at reduced temperatures. Thus, the observed high CO profile from the CO2/Ar char is also expected. Although the CO concentration inside the coal particle is higher in the oxy combustion environment than air-fired combustion, the CO/CO2 ratio is lower.17 The reaction equilibrium in the oxy environment tends to push the minerals toward their oxidized forms. The TPD profile in Figure 1 suggests that CO does reduce more minerals in the oxy environment than that in O2/Ar at 900 °C, and the reactions appear to be reaction-controlled. Nevertheless, mineral’s role may be limited in practical oxy flame temperatures because the mineral reduction may be thermodynamically controlled. TPD profiles of chars oxidized in different gases reveal that CO2 in oxy-coal combustion promote the oxidation through at least two different mechanisms: by forming several types of functional groups on the char surface and by inducing mineralcatalyzed carbon oxidation. Particles reach a higher temperature in O2/CO2 than that in CO2 alone, which leads to a simpler TPD profile. The presence of CO2 increases the concentrations of both CO and CO2, and the mineral reduction by CO appears to be reaction-controlled at 900 °C, thus facilitating the reduction and evaporation of refractory, oxidized minerals, resulting in less oxidized minerals remaining in the subsequent chars and a lower CO peak at 1450 °C during TPD. Acknowledgment. The authors acknowledge financial support of the National Science Foundation under Grant CTS-0122504. EF800822H (15) Quann, R. J.; Sarofim, A. F. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; pp 14291440. (16) Quann, R. J. Ash Vaporization under Simulated Pulverized Coal Combustion Conditions. Ph.D. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1982. (17) Krishnamoorthy, G.; Veranth, J. M. Energy Fuels 2003, 17, 1367– 1371.
