HIGH PRESSURE CONTROLLED-PROFILE DROP-TUBE REACTOR (HPCP)
The HPCP drop tube furnace1 was designed specifically to determine pyrolysis and oxidation rates and kinetics of solid fuels at both atmospheric and high pressure. It is a laminar flow furnace with a computer-controlled wall temperature profile to create isothermal conditions for reactivity tests. Solids and gaseous products are separated aerodynamically and collected for analysis.
A schematic of the HPCP system is shown in Fig. 1. Particles are fed with the primary gas through a water-cooled injector, which is moveable to vary particle residence times. The secondary gas flows into a preheater prior to entering the reactor. Wall heaters maintain an isothermal temperature profile. Four optical access windows are located near the bottom of the reactor. The collection probe collects the entire mass flow and quenches the particle reaction just below the diagnostic volume. A virtual impactor aerodynamically separates the particles from most of the gas flow and tars which are produced. Particles are captured in a cyclone, tars collected in filters, and the gases are either saved for analysis or vented from the system.
The collection probe is water-cooled with gas quench jets in the probe tip. A permeable liner inside the main probe tube allows quench gas to be injected radially along the length of the probe to reduce particle and tar deposition inside the probe. A virtual impactor follows in-line with the collection probe to aerodynamically separate the gases from the heavier particles. A cyclone separates char particles from tars and aerosols. Tars and aerosols from both legs of the virtual impactor are eventually collected on filters.
This reactor has been used for (a) char oxidation research as a function of pressure for both small particles (~50 mm diameter)1 and large particles (~5 mm diameter)2; and (b) devolatilization experiments at atmospheric pressure as a function of coal type, heating rate, and oxidation environment.3-5 The HPCP is often used to perform moderate temperature experiments (800 to 1200 K) to provide char and tar samples as a function of residence time during devolatilization. Pyrolysis experiments generally do not utilize the optics for measuring particle temperature since such measurements are not possible when particle temperatures are lower than wall temperatures.
METHANE FLAT FLAME BURNER SYSTEM (FFB)
A schematic of the flat-flame flow reactor system is shown in Fig. 2. It consists of a Hencken flat flame burner, similar to that used at Sandia6,7, and several designs of towers to confine the flame. The air, methane and hydrogen provide a high-temperature flame environment for coal pyrolysis. The outlet of the burner is a 2" by 2" square. The flow rates of air, methane and hydrogen are adjusted to obtain a horizontally uniform flame. The velocity of the hot gas above the burner is approximately 2 m/s (i.e., laminar flow). The coal particles are fed into the burner by a syringe particle feeder, driven by a stepping motor. The pulse signal used for driving the stepping motor is generated by a computer. The feed rate of coal particles can be adjusted by changing the frequency of the pulse signal (i.e., the stepping rate of the motor). The coal particles are entrained by a stream of carrier nitrogen gas. The hot combustion products from the methane/hydrogen/air flame heat the coal particles which are injected along the centerline of the laminar flow reactor. The flat flame can be operated under either fuel-rich or lean conditions so that the post-flame gases provide a reducing or oxidizing atmosphere for coal devolatilization. The flame temperature can be adjusted by changing the flow rates of inert gas, fuel and oxidizer. Flame temperatures along the height of the tower are measured in the absence of particles using a fine-wire silica-coated type B thermocouple, and then correcting the thermocouple reading for radiation heat loss.
The FFB is equipped with a water-cooled, gas-quench probe with porous liner for reduced deposition, and the probe is followed by a virtual impactor, cyclone, and filter system patterned after the collection system described above in the HPCP. The FFB experiments provide char and soot samples from a high temperature, high heating rate environment with products of methane combustion present, which is closer to industrial combustor environments than conventional drop tube furnaces. Experiments in the FFB give data on completion of devolatilization (i.e., the fraction of nitrogen released during devolatilization).
CHEMICAL ANALYSIS
TECHNIQUES
Proximate and Ultimate Analysis. Proximate analysis are performed at BYU for all coal and char samples. Ultimate analysis will be performed at BYU for all samples of coal, char, tar, extract, and extract residue.
ICP Analysis. Inductively coupled plasma (ICP) atomic emission spectroscopy is used to determine Ti content in coal and char samples. Ti is then used as a tracer to determine the extent of mass release due to devolatilization. The mass release determined from the Ti-tracer technique is compared to the ash tracer technique (from proximate analysis) and to the mass balance (weigh coal input to reactor and char collected).
Soxhlet Extraction Apparatus. Solvent extractions of coals and partially-devolatilized chars are performed
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| Figure 1.Schematic of the high pressure controlled-profile drop-tube reactor. | Figure
2.Schematic of the methane-air flat flame burner (FFB). |
NMR Analyses. Standard solid-state 15NMR spectroscopic techniques are employed initially to examine coal, char, tar, extract, and extract residue samples. This analysis is generally performed at the University of Utah under the direction of Professor Pugmire. This technique is relatively well-established, and is hampered by the low concentration of nitrogen in coals (<2 wt.%).
High Resolution Chromatography Experiments. High resolution capillary gas chromatography of coal tars and char extracts are performed at BYU under the direction of Prof. Lee, Dept. of Chemistry. As an ACERC partici-pant, Dr. Lee employs advanced high resolution chromatographic techniques to analyze ACERC-generated samples relating to coal devolatilization and combustion. Sample chromatograms of nitrogen-containing polyaromatic com-pounds, using a nitrogen-phosphorous detector, have been reported by Chang and coworkers.9
REFERENCES
1. Monson, C. R., and G. J. Germane, Energy and Fuels, 7, 928-936 (1993).
2. Bateman, K. J., G. J. Germane, L. D. Smoot, and A.U. Blackham, Brigham Young University, in preparation (1994).
3. Gale, T. K., C. H. Bartholomew, and T. H. Fletcher, Combustion and Flame, 100, 94-100 (1995).
4. Gale, T. K., T. H. Fletcher, and C. H. Bartholomew, Energy and Fuels, 9, 513-524 (1995).
5. Fletcher, T. H., S. Bai, J. Ma, S. Woods, M. S. Solum, R. J. Pugmire, and D. M. Grant, International
Conference on Coal Science; Conference Pro-ceedings, Banff, Alberta, Canada, 293-296 (1993).
6. McLean, W. M., D. R. Hardesty, and J. H. Pohl, 18th Symp. (Int.) on Comb., The Combustion Institute, Pittsburgh, PA, 1239 (1980).
7. Mitchell, R. E., R. H. Hurt, L. L. Baxter, and D. R. Hardesty Sandia Report SAND 92-8208, available NTIS (1992).
8. Fletcher, T. H., S. Bai, R. J. Pugmire, M. S. Solum, S. Wood, and D. M. Grant, Energy and Fuels, 7, 734-742 (1993).
9. Chang, H. K., K. D. Bartle, K. E. Markides, and M. L. Lee, "Structural Comparison of Low-Molecular-Weight Extractable Compounds in Different Rank Coals Using Capillary Column Gas Chromatography," In Advances in Coal Spectroscopy, H. L. C. Meuzelaar, Ed., Plenum, New York, pp. 141-164 (1992).
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