ADVANCED COMBUSTION ENGINEERING RESEARCH CENTER

Char Oxidation Laboratory: High and Low Temperature Char Oxidation Kinetics and Physical Properties

FACILITY × Brigham Young University × Fall 1995

OBJECTIVE AND SCOPE

This laboratory is able to measure char oxidation rates and char oxidation kinetics at both high (1200-2000K) and low temperatures (500-900K). Several physical property measurements can also be done, including a variety of surface area and density measurements. If the char for a given coal is unavailable, we can prepare char from coal in a rapid heating-rate flame environment. The lab also has an excellent semi-empirical model which predicts char oxidation behavior at combustion conditions.

APPARATUS/EXPERIMENTAL TECHNIQUES AVAILABLE

Flat-flame methane burner for char production from most coals.
€ Can produce chars as function of pyrolysis severity.

Drop-tube reactor with 2-color pyrometer to measure high-temperature char oxidation kinetics (1200-2000K).

TGA to measure low-temperature char oxidation kinetics (500-900K) using isothermal technique.

TGA technique to measure CaO surface area.

Surface area analyzer for N2 BET and CO2 D-P surface areas.

Pycnometer to measure densities.

BCOM Char Oxidation Model.
€ Predicts high-temperature global rate and particle temperature as function of burnout, temperature, and oxygen partial pressure.
€ Uses low-temperature intrinsic kinetics and structural and Deff models.
€ Predicts behavior in all 3 zones of combustion.
€ Eliminates need to determine surface areas.

EXAMPLES OF POSSIBLE STUDIES

High-Temperature Rate Measurements.
€ Determined for 3 series of Zap chars as function of burnout, CaO surface area and temperature.
€ Used to test and validate char oxidation submodel.

Comparison of Calculated High-Temperature To Measured Low- Temperature Intrinsic Kinetics.
€ Gave insight into validity of pore models.

Effects of High-Temperature Burnout Level.
€ Global and intrinsic char oxidation rates.
€ Physical and chemical properties of chars.

Correlation of Rates with Physical and Chemical Properties.
€ CaO surface areas found to correlate very well with low-temperature rates for Zap and Dietz chars; N2 and CO2 surface areas do not.

Effects of Char Preparation and Pyrolysis Conditions.
€ Oxidation rate decreases with increasing flat-flame burner residence time at constant mass loss.
€ Studied effects of devolatilization reactor type, heating rate, and gas composition.

APPLICATIONS

Char oxidation is an important issue in energy utilization. The required burnout time of coal/char particles is important in applications such as conversion of oil-fired boilers to coal, the development of new clean-coal technologies, and the development of entrained-flow gasifiers. Unburned carbon in boiler fly- ash, which represents a loss of efficiency and creates a disposal problem for utilities, is an important current concern as combustion zones are modified for NOx control. There is a great need to characterize the oxidation reactivity of coals and chars. The BYU char laboratory has the capabilities necessary to do a complete kinetic and physical properties characterization.

POSSIBLE SCENARIOS

Customer needs high temperature char oxidation kinetics.

1. Customer provides char.
We obtain preexponential function in our TGA and input it to BCOM to predict high-temperature kinetics (rates, oxygen order, activation energy, preexponential factor) over a range of temperatures, oxygen concentrations, and burnout levels. We also provide a complete set of physical properties including N2 and CO2 surface areas and true and particle densities.

2. Customer provides only coal.
We produce char in our flat-flame burner and then proceed as in (1) above.

3. Customer wants experimental validation (high temperature).
We proceed as in (2) above and also obtain high-temperature kinetic measurements using our drop tube reactor system.

FURTHER INFORMATION

For further information contact:

Professor William C. Hecker
Chemical Engineering Dept.
Brigham Young University
Provo, UT 84602
Phone: (801) 378-6235
Fax: (801) 378-7799
E-mail: heckerw@et.byu.edu

ACKNOWLEDGMENTS

The measurement capabilities and techniques of the Char Laboratory have been developed by William C. Hecker and a long list of excellent students. Calvin H. Bar tholomew has also provided valuable assistance. Financial support from NSF, the State of Utah, and 40 industrial participants through ACERC is gratefully acknowledged.

RECENT PUBLICATIONS

W.C. Reade, K.W. Morris, and W.C. Hecker, "Modeling the Effects of Burnout on High Temperature Char Oxidation," Coal Science (ed. J.A. Pajares and J.M.D. Tascon), Coal Science and Technology 24, Elsevier Science B.V., Amsterdam, pp. 639, September 1995.

R.F. Cope, C.B. Arrington, and W.C. Hecker, "Effect of CaO Sur-face Area on Intrinsic Char Oxidation Rates for Beulah Zap Chars," Energy &;Fuels 8 (5), 1095 (Sept 1994).

R.F. Cope, C.R. Monson, G.J. Germane, and W.C. Hecker, "Improved Diameter, Velocity, and Temperature Measurements for Char Particles in Drop Tube Reactors." Energy &;Fuels 8 (4), 925 (July/Aug 1994).

W.C. Hecker, K.M. McDonald, W. Reade, M. Swenson, and R.F. Cope, "Effects of Burnout on Char Oxidation Kinetics," Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp.1225-1231 (1992).

K. M. McDonald, W. D. Hyde, and W. C. Hecker, "Low Temperture Char Oxidation Kinetics: Effect of Preparation Method." Fuel 71, 319-323 (1992).

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