Energy Fuels 2009, 23, 4278–4285 Published on Web 07/28/2009
: DOI:10.1021/ef9002928
Combustion of Coal Chars in O2/CO2 and O2/N2 Mixtures: A Comparative Study with Non-isothermal Thermogravimetric Analyzer (TGA) Tests Hao Liu* School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom Received April 5, 2009. Revised Manuscript Received July 8, 2009
Two chars prepared from parent coals of a high-volatile bituminous coal and an anthracite coal were subjected to non-isothermal combustion tests in a thermogravimetric analyzer (TGA). The chars were burned in mixtures of O2/CO2 and O2/N2 with O2 concentrations of 3, 6, 10, 21, and 30%. A range of non-isothermal combustion tests of each char were conducted with linear heating rates of 2.5, 5, 7.5, 10, and 12.5 K/min. Detailed comparisons of measured char combustion rates show that replacing the inert nitrogen gas in the oxidizer with CO2 has very little influence on the measured combustion rates of coal chars at any char conversion level under the conditions of the experiments. Four different modelfree isoconversion methods using the experimental data were applied to determine the activation energies of the combustion of chars in O2/CO2 mixtures. The results show that the activation energy of each char, determined by any of the four methods, decreases with the increase of the char conversion level. The effect of diffusion, which becomes more pronounced at higher char conversion levels, is believed to be the main reason for the above observation. At a low char conversion level (e.g., 20%), when the effect of diffusion can be neglected, the activation energy of each char determined by the most accurate method under the conditions of this study was found to be in good agreement with literature data: for the anthracite char, it was 138.03 kJ/mol, and for the bituminous coal char, it was 127.83 kJ/mol. The measured combustion rate of each char was found to be approximately first-order to the concentration of O2 in the O2/CO2 mixtures.
already progressed from laboratory research (e.g., refs 5-7) to large-scale demonstrations (e.g., ref 8). To control the combustion temperature of an oxy-coal combustion process, the oxygen supplied by an air separation unit, which has typical purity of ca. 95%, is mixed with the recycled flue gas (mainly CO2). Combustion of coal/char in the O2/CO2 mixtures of an oxy-coal combustion boiler is expected to be different from that in the O2/N2 mixtures of a conventional coal-air combustion boiler because CO2 has a larger specific heat than N2 and coal/char can be gasified by CO2. Previous investigations of oxy-coal combustion5-7 have concluded that to achieve similar temperature levels in combustion furnaces, the initial oxygen concentration in the O2/CO2 mixture has to be “enriched” to the order of 30% when the O2/CO2 mixture replaces air as the oxidizer. This indicates that, with oxy-coal combustion, coal/char is partly burned under oxygen-enriched environments. Char combustion is an important part of the combustion of a coal particle and warrants separate investigations from its parent coal. Indeed, there are numerous studies concentrating on the combustion kinetics of coal chars by use of various types of reactors, including entrained flow, fixed bed, fluidized bed, isothermal thermogravimetric analyzer (TGA) and
1. Introduction Carbon capture and storage (CCS) is now widely considered as one of the real options to reduce CO2 emissions from fossil fuel use, particularly from power plants.1 As one of three main CO2 capture approaches, oxy-fuel combustion has received a lot of attention over the past decade because of its potential to produce highly concentrated CO2 stream, which is almost sequestration-ready, from fossil-fuel-fired plants.1,2 The carbon dioxides in the conventional coal-air combustion flue gas, which are in low concentration (ca. 1415% by volume), have to be captured by a postcombustion method using an organic sorbent, such as monoethanolamine (MEA). Postcombustion CO2 capture can incur significant economic and energy penalties to the coal-fired power plants.3 Oxy-coal combustion uses oxygen instead of air for the combustion of coal to produce a flue gas that is mainly CO2 (greater than 80% by volume) and water vapor. The water vapor can be easily removed by condensation, and the remaining CO2 can be purified relatively inexpensively.4 As a method of CO2 capture for power plants, oxy-coal combustion has *To whom correspondence should be addressed. Telephone: þ44115-8467674. Fax: þ44-115-9513159. E-mail:
[email protected]. uk. (1) Intergovernmental Panel on Climate Change (IPCC). 2005: IPCC Special Report on Carbon Dioxide Capture and Storage; Metz, B., Davidson, O., de Coninck, H. C., Loos, M., Meyer, L. A., Eds.; Cambridge University Press: Cambridge, U.K., 2005. (2) Wall, T. F. Proc. Combust. Inst. 2007, 31, 31–47. (3) Hadjipaschalis, I.; Christou, C.; Poullikkas, A. Int. J. Emerging Electr. Power Syst. 2008, 9, 1, article 5. (4) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Int. J. Greenhouse Gas Control 2008, 2, 9–20. r 2009 American Chemical Society
(5) Croiset, E.; Thambimuthu, K.; Palmer, A. D. Can. J. Chem. Eng. 2000, 78, 402–407. (6) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833–840. (7) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 2109–2115. (8) Fitzgerald, F. D. Current oxyfuel combustion activities at Doosan Babcock Energy Limited. Coal Research Forum 19th Annual Meeting, University of Nottingham, Nottingham, U.K., April 10, 2008 (available at http://www.coalresearchforum.org/20080410_fitzgerald.pdf).
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Energy Fuels 2009, 23, 4278–4285
: DOI:10.1021/ef9002928
Liu
Table 1. Analyses of Parent Coals (% as received) and BET Surface Areas (m2/g) of the Chars proximate analysis
ultimate analysis
coal/char
M
VMa
ash
FCa
C
H
N
O þ Sb
U.K. anthracite U.K. bituminous
4.2 2.6
7.0 29.4
17.4 11.4
71.4 56.6
70.8 70.6
1.6 4.6
1.0 1.6
5.0 9.2
BET surface area (m2/g) of chars anthracite char (A char) a
3.4
bituminous char (B char)
2.0
Determined according to BS 1016-104.3:1998.25 b By difference.
non-isothermal TGA (e.g., refs 9-22). The majority of previous investigations on the combustion kinetics of coal chars were conducted with O2/N2 mixtures, although there have been numerous investigations concentrating on the CO2 gasification kinetics of coal chars (e.g., refs 20, 23, and 24). V arhegyi et al.14 compared the char reactivity in O2/Ar and O2/CO2 mixtures by means of a non-isothermal thermobalance and concluded that the char reaction rate was proportional to the O2 concentration of the gas mixture and was not influenced by the presence of high amounts of CO2. The main objectives of the present research were (1) to compare char combustion rates during the whole char conversion stages in O2/N2 and O2/CO2 mixtures, which have identical O2 concentrations by means of non-isothermal TGA tests, and (2) to obtain two important parameters of the char combustion kinetics, namely, the activation energy and the reaction order with respect to the O2 concentration in O2/CO2 mixtures by use of model-free isoconversion methods. To achieve these objectives, a number of non-isothermal combustion tests with linear heating rates of the two chars prepared from parent coals of a high-volatile bituminous coal and an anthracite coal had been conducted with a TGA and four model-free isoconversion methods had been used to interpret the experimental results.
used with the previous investigations of the author on oxycoal combustion.6,7 The chars were produced by following the procedures of BS 1016-104.3:199825 for volatile matter analysis. First, 1 g of pulverized coal particles supplied by a commercial coal-fired power plant with a mean diameter of 70 μm was placed in a tall cylindrical ceramic crucible; second, the crucible with its lid on was placed in a preheated (to 1123 K) volatile analysis oven for 7 min; after 7 min in the oven, the crucible with its lid kept on was removed from the oven to a metal slab for several minutes and then to a glass vessel filled with desiccant for cooling to room temperature; finally, the char in the crucible was ground, if necessary, and sieved to the required size range (1.5%, up to ∼6%), whereas the KAS/Vyazovkin linear
where y=E/(RT) and yf =E/(RTf). The temperature integral, p(y), defined in eq 10, has been approximated by various researchers (e.g., refs 41-43). Z ¥ expð-yÞ dy ð10Þ pðyÞ ¼ y2 yf Starink27 summarized several approximations of p(y) as the following general form: expð-Ay þ BÞ ð11Þ pðyÞ = yd where A, B, and d are specific values associated with each approximation. (40) 195. (41) (42) (43)
β E ¼ -A þ C1 RTf Tf d
(44) Flynn, J. H.; Wall, L. A. Polym. Lett. 1966, 4, 323–328. (45) Ozawa, T. Bull. Chem. Soc. Jpn. 1965, 38, 1881–1886. (46) Kissinger, H. E. J. Res. Nat. Bur. Stand. 1956, 57, 217–221. (47) Kissinger, H. E. Anal. Chem. 1957, 29, 1702–1706. (48) Akahira, T.; Sunose, T. Transactions of the Joint Convention of Four Electrical Institutes, Research Report of Chiba Institute of Technology, 1971; Paper 246.
Friedman, H. L J. Polym. Sci., Part C: Polym. Lett. 1963, 6, 183– Doyle, C. D. J. Appl. Polym. Sci. 1962, 6, 639–642. Murray, P.; White, J. Trans. Br. Ceram. Soc. 1955, 54, 204–237. Senum, G. I.; Yang, R. T. J. Therm. Anal. 1977, 16, 445–447.
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Energy Fuels 2009, 23, 4278–4285
: DOI:10.1021/ef9002928
Liu
Table 2. Comparison of Activation Energies (kJ/mol) of A Char (Anthracite Coal) Calculated from Different Isoconversion Methods conversion/methods
Friedman
Ozawa-Flynn-Wall
KAS/Vyazovkin linear
Starink
X
range of Tf (K)
E
R2
E
R2
E
R2
E
R2
0.1 0.2 0.3 0.4 0.5
805.4-866.3 827.1-891.7 841.2-908.6 851.4-922.1 860.1-933.9
143.82 138.44 128.68 121.97 113.88
0.9953 0.9965 0.9939 0.9909 0.9904
142.76 147.22 142.22 140.11 136.90
0.9982 0.9984 0.9982 0.9976 0.9972
136.28 137.58 135.06 132.64 129.10
0.9978 0.9980 0.9977 0.9969 0.9964
136.73 138.03 135.53 133.13 129.59
0.9979 0.9980 0.9977 0.9969 0.9964
Table 3. Comparison of Activation Energies (kJ/mol) of B Char (U.K. Bituminous Coal) Calculated from Different Isoconversion Methods conversion/methods
Friedman
Ozawa-Flynn-Wall
KAS/Vyazovkin linear
Starink
X
range of Tf (K)
E
R2
E
R2
E
R2
E
R2
0.1 0.2 0.3 0.4 0.5
780.2-840.9 796.9-861.3 806.6-875.7 813.8-888.6 821.3-901.0
132.62 114.78 96.04 83.42 80.82
0.9795 0.9791 0.9759 0.9802 0.9784
136.54 134.20 129.03 121.79 116.78
0.9987 0.9962 0.9943 0.9937 0.9922
130.15 127.39 121.74 113.96 108.53
0.9983 0.9952 0.9927 0.9918 0.9896
130.69 127.83 122.30 114.43 109.02
0.9983 0.9953 0.9928 0.9918 0.9898
Figure 4. Correlations of KAS/Vyyazovkin linear and Friedman methods of A char.
Figure 5. Correlations of Ozawa-Flynn-Wall and Starink methods of B char.
and Starink methods predict slightly lower values (
