Energy Fuels 2010, 24, 160–164 Published on Web 09/25/2009
: DOI:10.1021/ef900533d
,§,‡
Renu K. Rathnam,^ Jianglong Yu,*,§,
,^
)
Xianchun Li,
)
)
Pyrolysis and Combustion Characteristics of an Indonesian Low-Rank Coal under O2/N2 and O2/CO2 Conditions† Qi Wang,
,‡
Terry Wall,^ and Chatphol Meesri# )
‡ School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China, §School of Power and Energy Engineering, Shenyang Institute of Aeronautical Engineering, Daoyi, Shenyang 110136, China, University of Science and Technology Liaoning, Anshan 114051, Liaoning, China, ^Chemical Engineering, University of Newcastle, Callaghan, New South Wales 2308, Australia, and #Coal Technology, Banpu Public Company Limited, Bangkok 10400, Thailand
Received May 25, 2009. Revised Manuscript Received September 1, 2009
Pyrolysis and combustion characteristics of an Indonesian low-rank coal are studied under oxy-fuel (O2/ CO2) and air (O2/N2) conditions using a drop tube furnace (DTF) and a thermogravimetric analyzer (TGA). Raw coal, dried coal, and binderless briquette samples of the same coal were used in the experiments, and the effects of drying and binderless briquetting on the reactivity of the coal under different conditions were investigated. Chars were prepared in the DTF in both N2 and CO2 atmospheres in the temperature range of 800-1400 C. The reactivity of chars under oxy-fuel and air conditions was analyzed in the TGA. The coal reactivity under oxy-fuel conditions differed from that under air combustion conditions. The temperature at which significant gasification of the coal and char took place in the concentrated CO2 gas stream was also identified. Characteristics of chars from different conditions were compared. Drying and briquetting had some noticeable influences on the reactivity of the coal under oxy-fuel conditions.
as colloidal gels, whose chemical and physical properties are irreversibly altered by drying.2 For instance, when it is heated, thermal decomposition of carboxyl groups in Victorian brown coal may occur at temperatures as low as 150 C.3 During drying, as water is progressively removed, the pore structure of the coal tends to collapse and shrinkage takes place4,5 As a result of the binderless briquetting process, a significant reduction in pore volume occurs; in particular, the macropores collapse. Changes in physicochemical properties of Victorian brown coal during mechanical thermal expression (MTE) dewatering would affect the subsequent pyrolysis and gasification reactivity.6-8 The coal reactivity under oxyfuel conditions has also been studied experimentally in the literature.9 However, the understanding of coal structure change during drying and briquetting on its combustion reactivity under oxy-fuel conditions will assist the retrofit of power plants firing brown coal.
1. Introduction Carbon capture and storage (CCS) technologies have great potential to solve the global warming issue, which has been caused by the firing of fossil fuels (such as coal, oil, and natural gas) to generate power. Among different technological options for CCS, oxy-fuel combustion through the retrofit of existing power plants to produce capture-ready concentrated CO2 is promising, and the technology is now undergoing rapid development.1 This is of crucial importance to developing countries, such as China, that heavily rely on coal to supply their primary energy. There is a great possibility that south China’s power generation will intensely use low-rank coals imported from Indonesia in the future. Low-rank coals, including brown coal and subbituminous coal, are generally featured by high moisture content (25-65%), which plays critical roles during their use (e.g., combustion, gasification, and liquefaction). Burning such a low-grade fuel for power generation will lead to more than a 20% increase in the CO2 emission compared to bituminous coals. Application of oxyfuel technology in brown coal firing power plants is therefore very attractive. Drying and binderless briquetting may transform low-rank coals into a low-moisture high-grade solid fuel. As a result of drying and briquetting, the calorific value increases and the coal structures undergo alterations, leading to changes in their reactivity. It is believed that low-rank coals may be regarded
2. Experimental Section 2.1. Sample Preparation. An Indonesian low-rank coal with raw coal properties shown in Table 1 was used to prepare the dried coal and binderless briquette coal samples. The raw, dried, and briquette coals were assigned as S1, S2, and S3 samples, respectively. The properties of dried coal and briquette coal samples are also shown in Table 1. The drying kinetics of the raw coal has been studied previously by the authors.10 (2) Deevi, S. C.; Suuberg, E. M. Fuel 1986, 66, 454–460. (3) Murray, J. B.; Evans, D. G. Fuel 1972, 51, 290–296. (4) Evans, D. G. Fuel 1973, 52, 186–190. (5) Bongers, G. D. Fuel Process. Technol. 2000, 64, 13–23. (6) Zeng, C. Energy Fuels 2006, 20, 281–286. (7) Zeng, C. Energy Fuels 2007, 21, 399–404. (8) Unal, S.; Wood, D. G.; Harris, I. J. Fuel 1992, 71, 183–192. (9) Rathnam, R. K.; Eliliott, L. K.; Wall., T. F.; Liu, Y.; Moghtaderi, B. Fuel Process. Technol. 2009, 90, 797–802. (10) Li, X.; Song, H.; Wang, Q.; Meesri, C.; Wall, T.; Yu, J. J. Environ. Sci. 2009, 21 (supplement 1), S127–S130.
† Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: jianglong.yu@ syiae.edu.cn. (1) Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. Chem. Eng. Res. Des. 2009, 87, 1003– 1016.
r 2009 American Chemical Society
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Energy Fuels 2010, 24, 160–164
: DOI:10.1021/ef900533d
Li et al.
were carried out in an O2/CO2 or air flow with a flow rate of 50 mL/min, and the peak temperature was 1000 C with a heating rate of 10 C/min. The apparent activation energy was calculated by a non-isothermal method.12,13 The specific reactivity (Rc) of the char at any given time (t) was calculated from the equation9,13,17,20 1 dw Rc ¼ w dt
Table 1. Proximate and Ultimate Analyses of the Coal Samples coal sample
S1
S2
S3
moisture content ash volatile matter fixed carbon
Proximate Analysis (wt %) (ad) 24.0 9.6 2.3 4.3 37.1 43.7 36.6 42.4
10.3 6.7 42.5 40.5
carbon hydrogen oxygen nitrogen total sulfur
Ultimate Analysis (wt %) (db) 69.3 67.8 4.79 4.7 21.6 21.5 1.0 0.98 0.24 0.25
65.1 4.54 21.7 0.92 0.23
where w is the weight [dry, ash-free (daf) basis] of the char sample at any given time t.
3. Results and Discussion The volatile matter yields of the three coal samples in N2 and CO2 at different temperatures in the DTF were shown in Figure 2. As expected, the volatile yield of the three samples increased when the pyrolysis temperature increased. The R factor is greater than unity when the pyrolysis temperature is higher than 1200 C. An interesting observation from this study was that the actual volatile matter yield was higher in CO2 than in N2 at high temperatures but lower in CO2 at low temperatures. The increase in the volatile matter yield at high temperatures in CO2 was attributed to gasification of chars by CO2. At low temperatures, the gasification reaction rate was not significant. However, because of the higher heat capacity, the temperature of CO2 gas flow may be lower than N2, resulting in a lower VM yield in CO2 gas flow. In all cases, the R factor of S1 is higher than S2 and S3, indicating that pyrolysis reactivity of the coal may be reduced by drying and briquetting. Figure 3 compares the burnout of three samples as a function of the O2 partial pressure. It is obvious that the increase in the O2 concentration leads to an increase in the coal burnout under all conditions. Under air conditions at high O2 partial pressure (21% O2), the burnout of the three samples was very high. However, at low O2 partial pressure (10% O2), the burnout of S1 was higher than that of S2 and S3. Under oxy-fuel conditions, the burnout of S1 was higher than that of S2 and S3 at different O2 partial pressures. At high O2 concentration (21% O2), the burnout of S2 and S3 under air conditions is higher than that under oxy-fuel conditions, although the burnout of S1 was rather similar under the two conditions. At low O2 partial pressure (10% O2), the burnout of S2 and S3 was similar. The reactivity of chars from the three coal samples prepared in N2 at 1400 C in the DTF was analyzed in the TGA under both oxy-fuel and air conditions. The char reactivity in oxyfuel and air conditions at a temperature of 500 C is shown in Figure 4. It can be seen the char reactivity of the S2 and S3 samples is lower than that of the S1 sample. The reactivity of the S2 and S3 samples was similar. For the same coal, the char reactivity was slightly higher under air conditions than that under oxy-fuel conditions. This was consistent with the results of R factor and burnout tests in DTF. Table 2 shows kinetic parameters of the three chars under oxy-fuel and air conditions. The kinetic parameters were
To prepare the binderless briquette coal samples, raw coal was dried in a rotary drum drier and used to produce binderless briquettes using a lab-scale briquetting machine. The three samples were then milled and sieved to obtain samples of 63-90 μm size fraction. The samples were then used for drop tube furnace (DTF) and thermogravimetric analyzer (TGA) experiments. 2.2. DTF Experiments. The schematics of the DTF (Model Astro 1000A) used for this study are shown in Figure 1. Pyrolysis experiments were conducted at temperatures of 800, 1000, 1200, and 1400 C. The feed coal and gas mixture (N2, CO2, O2/ N2, or O2/CO2) were fed from the top of the furnace through a water-cooled feeder. The bottom of the feeder was inserted into the furnace. The coal feed rate of 5 g/h was used. The secondary gas of 3.5 L/min was preheated and introduced into the furnace through the annulus between the central tube and the Kaowool shield for the feeder. A water-cooled gas-quenched sampling probe was used to collect char from the bottom of the furnace. The residence time was about 0.4-0.6 s. The ash content of each char sample was measured according to which burnout of the coal was calculated. Volatile matter yield during pyrolysis and burnout during combustion was determined using an ash tracer method according to the following equation:11,15,16 1 - ðAcoal =Achar Þ volatile yield ðburnoutÞ ¼ 100 1 - Acoal where Acoal and Achar are the ash content of the feed coal and char, respectively. The difference between the actual volatile yield and that from the proximate volatile yield may be reflected by the R factor, which was defined in the literature as14 actual volatile yield ðV Þ R factor ¼ proximate volatile matter content ðVMÞ Char samples from DTF were then subject to further TGA and scanning electron microscopy (SEM) analysis. An EMXRay JY794 SEM was used to examine both cross-sectional and morphological characteristics of the coal and char samples following the methods described in the literature.18,19,21,22 2.3. TGA. Combustion kinetic measurements of chars from different samples were studied using a Setaram-Setsys Evolution 1200 model TGA.12,13 The apparent reactivity is measured isothermally under oxy-fuel and air conditions at 500 C. In each test, about 5 mg of sample was put into the crucible. The N2 flow was switched to oxy-fuel or air conditions, and the weight loss was monitored continuously. Non-isothermal experiments
(17) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23–30. (18) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D. J. Anal. Appl. Pyrolysis 2001, 59, 887–909. (19) Yu, J.; Lucas, J.; Strezov, V.; Wall, T. Energy Fuels 2003, 17, 1160–1174. (20) Benfell, K. E.; et al. Proc. Combust. Inst. 2000, 28, 2233–2241. (21) Yu, J.; Lucas, J.; Wall, T. Prog. Energy Combust. Sci. 2007, 33, 135–170. (22) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D. Fuel Process. Technol. 2001, 69, 257–272.
(11) Artos, V.; Scaroni, A. W. Fuel 1993, 72, 927–933. (12) Kizgut, S.; Yilmaz, S. Fuel Process. Technol. 2003, 85, 103–111. (13) Russell, N. V.; Beeley, T. J. Fuel Process. Technol. 1998, 57, 113– 130. (14) Kimber, G. M.; Gray, M. D. Combust. Flame 1967, 11, 360–362. (15) Borrego, A. G.; Os orio, E.; Casal, M. D.; Vilela, A. C. F. Fuel Process. Technol. 2008, 89, 1017–1024. (16) Biswas, S.; Choudhury, N.; Sarkar, P.; Mukherjee, A.; Sahu, S. G.; Boral, P.; Choudhury, A. Fuel Process. Technol. 2006, 87, 191–199.
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: DOI:10.1021/ef900533d
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Figure 1. Schematic diagram of the DTF used for pyrolysis and burnout tests.
Figure 2. R factor of three coal samples in (a) N2 and (b) CO2 atmospheres in DTF at different temperatures. Table 2. Kinetic Parameters of the Three Chars under Oxy-fuel and Air Conditions oxy-fuel
air -1
coal Ea (kJ/mol) ln A (s ) S1 S2 S3
82.46 88.22 84.91
10.99 11.07 12.19
2
R
0.9977 0.9975 0.9952
Ea (kJ/mol) ln A (s-1) 74.66 80.71 81.62
7.66 8.17 9.98
R2 0.9962 0.9913 0.9986
negligible effects on the apparent activation energy. The apparent activation energy of the same char under air conditions was slightly lower than that under oxy-fuel conditions. The different behavior of the coal under oxy-fuel conditions from that under air combustion conditions was attributed to the following reasons: The lower diffusivity of O2 in CO2 compared to the diffusivity in N2 affects the transport of O2 to the surface of the particles, leading to the reduced char combustion rates in regime II. The combustion of volatile matter released from the coal particle also depends upon the diffusivity of O2. Therefore, the combustion of char as well as the volatile matter may be slower during oxy-fuel combustion under DTF conditions. At higher oxygen partial pressure, the
Figure 3. Comparison of the burnout of three coal samples in different O2 partial pressures in air and oxy-fuel atmospheres in DTF at 1400 C.
determined by a non-isothermal method, which assumes that the reaction is first-order.13 The heating rate was 10 C/min, and the peak temperature was 1000 C. The results indicate that the apparent activation energy of the three chars is similar under the same conditions. The changes in physical and chemical structures of the Indonesian low-rank coal may have 162
Energy Fuels 2010, 24, 160–164
: DOI:10.1021/ef900533d
Li et al.
Figure 4. Reactivity of the three chars in (a) oxy-fuel and (b) air atmospheres in TGA at 500 C. The char was prepared in N2 in DTF at 1400 C.
Figure 5. SEM images of the cross-section of char samples of (a and b) S1 and (c and d) S3 pyrolyzed in DTF at (a and c) 800 and (b and d) 1400 C in a N2 atmosphere.
Figure 6. SEM images of morphology of chars from (a) S1 and (b) S3 samples pyrolyzed in DTF at 1400 C in a N2 atmosphere.
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Energy Fuels 2010, 24, 160–164
: DOI:10.1021/ef900533d
Li et al.
Figure 7. SEM images of the morphology of chars from (a and b) S1 and (c and d) S3 pyrolyzed in DTF at (a and c) 800 and (b and d) 1400 C in a CO2 atmosphere.
burnout in both cases was very high. It is therefore difficult to compare their difference. The reactivity of the dried coal and binderless-briquetted coal samples was lower than that of the raw coal, and the difference increased at lower O2 partial pressure under oxy-fuel conditions. Cross-sectional SEM images of chars are shown in Figure 5. Most of particles have irregular shape, indicating that little fluidity developed in these coals during heating. The char structures have been classified into three major groups, according to Benfell et al.23 In this classification system, the group I chars are porous (porosity > 80%) with a single void and a thin wall in each char particle. The group II chars have a medium porosity (50-80%) and wall thickness. The group III chars have a relatively solid structure with a low porosity (
