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Energy & Fuels 2005, 19, 1828-1838
Effects of Pressure on Coal Pyrolysis and Char Morphology Dong Zeng and Thomas H. Fletcher* Chemical Engineering Department, 350 CB, Brigham Young University, Provo, Utah 84602 Received January 6, 2005. Revised Manuscript Received June 2, 2005
A better understanding of high-pressure coal pyrolysis is needed to design advanced coal gasification combined-cycle systems. High-temperature, high-heating-rate, and high-pressure pyrolysis experiments were conducted on three bituminous coals and a lignite, to more fully understand the effects of pressure on resultant char properties. A flat-flame burner was designed and used inside a high-pressure drop-tube reactor to provide a high-temperature, high-heatingrate environment that is representative of industrial processes. Chars were prepared at four different pressures, ranging from 0.85 atm to 15 atm. The measured total volatiles yields compared well with predictions of the Chemical Percolation Devolatilization (CPD) model. The physical properties of the char samples were analyzed, including swelling ratio, morphology, and internal surface areas. Chars produced at high pressure were determined to be in the early stage of foam structure evolution and have a higher porosity but denser skeleton. The internal surface areas of chars decreases as the pressure increases, which contributes mostly to the lower intrinsic reactivity of chars that are formed at high pressure.
Introduction Coal combustion and gasification at elevated pressures have been studied for decades, because of the potential for cleaner emissions and higher efficiencies. Coal pyrolysis usually occurs in the early stages of coal combustion, accounting for ∼50% of the mass of the coal and affecting the subsequent char reactivity. Previous coal pyrolysis experiments at elevated pressures have been performed using thermogravimetric analysis (TGA),1-3 wire-mesh reactors,4-8 and drop-tube reactors.9-12 Generally, the total volatile and tar yields decrease as the pressure increases, with tar yields being more distinctly dependent on pressure than gas yields. Therefore, the reduction in tar and total volatile yields is most significant for bituminous coals but less pronounced for lignites. The effects of pressure on tar and * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Seebauer, V. P. J.; Staudinger, G. Fuel 1997, 76, 1277-1282. (2) Sun, C. L.; Xiong, Y. Q.; Liu, Q. X.; Zhang, M. Y. Fuel 1997, 76, 639-644. (3) Arendt, P.; Heek, K. H. Fuel 1981, 60, 779-787. (4) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22, 625-656. (5) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Proc. Combust. Inst. 1978, 17, 117-130. (6) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591601. (7) Bautista, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986, 25, 536-544. (8) Cai, H. Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J. Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15-24. (9) Lee, C. W.; Scaroni, A. W.; Jenkins, R. G. Fuel 1991, 70, 957965. (10) Fatemi, M. Am. Chem. Soc., Div. Fuel Chem. Prepr. 1987, 32, 117-124. (11) Yeasmin, H.; Mathews, J. F.; Ouyang, S. Fuel 1999, 78, 1124. (12) Matsuoka, K.; Ma, Z.-X.; Akiho, H.; Zhang, Z.-G.; Tomita, A.; Fletcher, T. H.; Wojtowicz, M. A.; Niksa, S. Energy Fuels 2003, 17, 984-990.
total volatile yields seem to be more pronounced at higher temperatures. In addition, the change in tar and total volatile yields with increasing pressure seems to be most pronounced at moderate pressures (5-10 atm). The physical structure of the char that remains after pyrolysis is significantly affected by the pressure, changing char properties such as composition, porosity and internal surface area, which, in turn, affect char reactivity.13 Benfell et al.14 found that chars created at high pressure had thinner walls and a more spherical structure than chars created at lower pressures. The thinner walls made particles more susceptible to fragmentation within the furnace and during handling. Compared to the low-pressure char, the average macroporosity of high-pressure chars was higher, and the high-pressure chars contained a larger number of bubbles with smaller sizes.15 Generally, the internal surface areas for chars produced at higher pressures are lower than chars produced at atmospheric pressure. The effect of pressure on char surface area is believed to be related to the fluid behavior during devolatilization.9,14 Although the effect of pressure on coal pyrolysis has been studied extensively, entrained-flow high-pressure coal pyrolysis data are still needed at high temperatures and heating rates. TGA pyrolysis experiments suffer from being conducted at low temperatures and low heating rates (0.5 K/s) and hence have limited applicability to industrial systems. Wire-mesh reactors attain only moderate heating rates (1000 K/s) and temperatures, (13) Wall, T. F.; Gui-su, L.; Hong-wei, W.; Roberts, D. G.; Benfella, K. E.; Guptaa, S.; Lucas, J. A.; Harris, D. J. Prog. Energy Combust. Sci. 2002, 28, 405-433. (14) Benfell, K. E.; Liu, G.-S.; Roberts, D.; Harris, D. J.; Lucas, J. A.; Bailey, J. G.; Wall, T. F. Proc. Combust. Inst. 2000, 28, 2233-2241. (15) Yu, J.; Lucas, J.; Strezov, V.; Wall, T. Energy Fuels 2003, 17, 1160-1174.
10.1021/ef0500078 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
Pressure Effects on Coal Pyrolysis
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Figure 1. Schematic of the high-pressure flat-flame burner (HPFFB). Table 1. Characteristics of Coals Proximate Analysis (Wt %)
Ultimate Analysis (wt %, daf)
coal
rank
diameter (µm)
moisture
ash (dry)
volatile matter (daf)
C
H
N
S
Oa
Pittburgh #8 Kentucky #9 Illinois #6 Knife River
HvA-Bit HvB-Bit HvC-Bit Lignite
63-90 44-74 74-90 45-75
1.44 8.21 3.31 11.91
10.72 8.43 9.35 20.38
34.34 42.11 53.83 47.86
84.58 76.72 78.02 62.23
5.47 5.27 5.45 4.23
2.00 1.81 1.36 0.95
0.49 3.72 4.14 1.28
7.44 12.48 10.59 31.30
a
Balance (O ) 100 - (C + H +N +S)).
and the interaction between the coal particle and wire mesh is not clear. Many current drop-tube reactors are limited to temperatures of 1300 °C at pressures of 0.85-15 atm and particle heating rates of 105 K/s (Figure 1). Char preparation at atmospheric pressure (0.85 atm in Utah) was conducted (18) Fletcher, H. T.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54.
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Zeng and Fletcher for continuous gas temperature measurement, and a PID controller was used to control the temperature of the heating elements. An O2 analyzer was used to monitor the composition of the gases exiting the reactor. The desired flow rates of these gases were determined (a) by computing the adiabatic flame temperatures and (b) by comparing the measured temperature profiles and O2 concentrations at different pressures. The particle collection system included a water-cooled, gas-quench probe, followed by an aerodynamic separation of soot/aerosols from char.20 Coal was pyrolyzed in the HPFFB system at 1300 °C at pressures of 2.5, 6, 10, and 15 atm. The flame conditions actually used were slightly fuel-lean (∼0.4 mol % O2 in the post-flame gases), because the methane formed soot at elevated pressures under fuel-rich conditions. The particle residence times in these experiments ranged from 0.23 s to 2.0 s. Kinetic and Physical Properties Determination. Mass release resulting from pyrolysis was determined using ash, titanium, and aluminum as tracers; volatiles yields determined from all three tracers were generally in good agreement. Elemental composition was measured using a LECO CHNS instrument. Char swelling ratios were measured based on weight loss and tap density data, using the relationship21
( )( )
m F d ) m0 F0 d0
Figure 2. Flowchart of the HPFFB system. using a separate flat-flame burner. The HPFFB system uses the hot products of methane combustion to heat the particles. As shown in Figure 1, methane flows through hypodermic tubes and combusts with an oxidizer stream, which consists of either air or a mixture of air and O2. A down-fired laminarflame sheet is formed by an array of small diffusion flames. The length of the flame zone is
