Reginald E. Mitchell

Associate Professor
Faculty of Thermosciences Division, Department of Mechanical Engineering

Phone: 650-725-2015 | Fax: 650-723-1748 | Email: remitche@stanford

Degrees

    B.S. University of Denver - Chemical Engineering (1968)

    M.S. New Jersey Institute of Technology - Chemical Engineering (1970)

    Sc.D. M.I.T. (1975)

Research Interests

Coal and biomass combustion and gasification, pyrite combustion, pollutant formation and destruction during combustion, hydrocarbon flame chemistry and structure.

Reginald Mitchell is the author of over 50 papers concerned with combustion phenomena. He has presented more than 50 seminars on various aspects of coal char oxidation and has given two short courses on coal combustion. He is a member of the Combustion Institute, being active in the Western States Section of the Combustion Institute. He is also a member of the National Organization of Black Chemists and Chemical Engineers, having served as Chair of its Western Region for several years. In 1997, he joined the Editorial Board of Combustion and Flame, the journal of the Combustion Institute.

Professor Mitchell is the Chair of the Thermosciences Division (TSD) of the Mechanical Engineering Department and is the current Director of the High Temperature Gasdynamics Laboratory, a research laboratory within the TSD that houses research in combustion science, pollution science, fluid mechanics, spray dynamics, plasma science, materials synthesis, and laser-based optical diagnostics. He teaches courses in radiation heat trasfer, thermodynamics, and combustion. In 1994, he received the Stanford Tau Beta Pi Award for excellence in undergraduate teaching.

Recent Publications

P. A. Campbell, R. E. Mitchell and L. Ma, "Characterization of Coal-Char and Biomass-Char Reactivities to Oxygen," Proc. Combust. Inst. 29 519-526, 2002.

R. E. Mitchell, "An Intrinsic Kinetics-Based, Particle-Population Balance Model for Char Oxidation During Pulverized Coal Combustion," Proc. Combust. Inst. 28 2261-2270, 2000.

L. L. Baxter, R. E. Mitchell, and T. H. Fletcher, "Release of Inorganic Material During Coal Devolatilization," Combustion and Flame 108(4), 494, 1997.

R. E. Mitchell and A. E. J. Akanetuk, "The Impact of Fragmentation on Char Conversion during Pulverized Coal Combustion," Proc. Combust. Inst. 26 3137-3144, 1996.

L. L. Baxter, R. E. Mitchell, T. H. Fletcher, and R. H. Hurt, "Nitrogen Release during Coal Combustion," Energy and Fuels 10(1), 189, 1996.

T. F. Wall, A. G. Tate, J. G. Bailey, L. G. Jenness, R. E. Mitchell, and R. H. Hurt, "The Temperature, Burning Rates, and Char Character of Pulverized Coal Particles Prepared from Maceral Concentrates", Proc. Combust. Inst. 24 1207, 1992.

Robert H. Hurt and Reginald E. Mitchell, "On the Combustion Kinetics of Heterogeneous Char Particle Populations", Proc. Combust. Inst. 24 1233, 1992.

Robert H. Hurt and Reginald E. Mitchell, "Unified High-Temperature Char Combustion Kinetics for a Suite of Coals of Various Rank", Proc. Combust. Inst. 24 1243, 1992.

Recent Conference Papers and Posters

R. E. Mitchell and L. Ma, "Intrinsic Reactivity-based Model for Mode of Particle Burning," 3rd Joint Meeting of the U.S. Sections of the Combustion Institute, Chicago, IL, March 16-19, 2003.

 

L. Sørum and R. E. Mitchell, "On the Reactivity of Chars from Cellulosic Wastes: The Influence of Ash Content," 3rd Joint Meeting of the U.S. Sections of the Combustion Institute, Chicago, IL, March 16-19, 2003.

P. A. Campbell and R. E. Mitchell, "Oxy-Reactivity Studies of Partially Reacted Samples of a Pulverized Coal Char," Eleventh International Conference on Coal Science, San Francisco, September 30 - October 5, 2001.

 

N. K. Tsai and R. E. Mitchell, "Measuring Changes in CO2 Surface Area with Mass Loss using a Pressurized Thermogravimetric Analyzer," Work-in-Progress Poster, Twenty-Seventh International Symposium on Combustion, Boulder, CO, August 1 - 7, 1998.

 

A. E. Jacob Akanetuk and R. E. Mitchell, "Pathway of Pyrite Oxidation to Non-Slagging Species," Paper 96F-051, Western States Section/The Combustion Institute 1996 Fall Meeting, Los Angeles, CA, October 28 - 29, 1996.

 

R. E. Mitchell and N. K. Tsai, "The Change in Char Surface Area during Gasification," Paper No. 96F-056, Western States Section/The Combustion Institute 1996 Fall Meeting, Los Angeles, CA, October 28 - 29, 1996.

 

R. E. Mitchell, R. Diaz, and G. Ramdeen, "The Effect of Porosity on Char Particle Fragmentation During Pulverized Coal Combustion," Paper 95S-116, Central States/Western States/Mexican National Sections of the Combustion Institute and American Flame Research Committee Joint Technical Meeting, San Antonio, Texas, April 23-26, 1995.

 

R. E. Mitchell and N. K. Tsai, "CO2 Gasification of Illinois #6 Coal Chars in High CO Environments," Paper 95F-179, Western States Section/The Combustion Institute 1995 Fall Meeting, Stanford, CA, October 30-31, 1995.

 

T. Kusakabe and R. E. Mitchell, "Autoignition Study for Diesel Sprays," Paper 95F-161, Western States Section/The Combustion Institute 1995 Fall Meeting, Stanford, CA, October 30-31, 1995.

 

R. E. Mitchell, R. Diaz, and G. Ramdeen, "The Effect of Porosity on Char Particle Fragmentation During Pulverized Coal Combustion," Central States/Western States/Mexican National Sections of the Combustion Institute and American Flame Research Committee Joint Technical Meeting, San Antonio, Texas, April 23-26, 1995.

Recent Final Reports

R. E. Mitchell, "Characterization of Variations in Char Reactivity and Mode of Particle Burning during Combustion of Pulverized Solids," Final Technical Report, NSF Grant #CTS-9903403 (Project Director: F. Fisher), July 2003.

R. E. Mitchell. "Mechanisms of Pyrite Oxidation to Non-Sagging Species," Final Technical Report, DOE Grant DE-FG22-94PC94205 (Project Manager: K. Das), September 2002.

R. E. Mitchell. "Utilization of Waste Renewable Fuels in Boilers with Minimization of Pollutant Emissions: Task 2.3: Kinetics of Biomass and Waste Particles Gasification/ Reburning," Final Technical Report prepared for G.E. Energy and Environment Research Corporation (Group Leader: G. Rizeq) and the California Energy Commission (Project Manager: R. Kapoor), Contract # CEC 500-98-037, December 2001.

R. E. Mitchell. "Char Particle Fragmentation and its Effect on Unburned Carbon during Pulverized Coal Combustion," Final Technical Report, DOE Grant DE-FG22-92PC92528 (Project Officer: J. Hickerson), January 1998.

R. E. Mitchell and N. Tsai, "A Preliminary Investigation of Coal Char Reactivities to CO2 and H2O in Gaseous Environments Containing Nominally 60% CO and 30% H2 at Pressures from 1 to 25 atm in the Temperature Range 900 - 1100 °C," Final Report prepared for Shell Synthetic Fuels, Inc. (Project Manager: L. Clomburg), February 1997.

R. E. Mitchell and A. E. J. Akanetuk, "Coal particle Size Distributions for LOI Models," Final Technical Report prepared for Electric Power Research Institute (Program Manager: A. Mehta), November 1996.

Summaries of Research Projects

Professor Mitchell's research interests are in the areas of combustion and gasification of solid fuels and pollutant formation during combustion. In coal and biomass related projects, his students carry out fundamental research concerned with characterizing the chemical and physical transformations that coal and biomass particles undergo during combustion and gasification processes. Experiments are performed in an entrained flow reactor and in a pressurized thermogravimetric analyzer in order to obtain information on the chemical reactivity and morphology of particles during the conversion process. The information is used to develop models that describe char-particle properties as functions of the gaseous environment the particles experience during conversion. The models can be used in comprehensive codes developed to predict the performances of coal-fired and biomass-fired boilers, furnaces, and gasifiers. Research support has been received from the United States Department of Energy, Shell Synthetic Fuels, Inc., The Electric Power Research Institute, The California Energy Commission, The National Science Foundation, and Stanford University's Global Climate and Environment Project. Recent and current studies are briefly described below.

Coal and Biomass Char Reactivity Studies

This study is concerned with characterizing the behaviors of coal and biomass char particles when exposed to hot oxidizing gases typical of the environments established in furnaces and boilers. Emphasis is placed on development and validation of a heterogeneous reaction mechanism that describes the intrinsic chemical reactivity of the chars of coal and biomass materials to oxygen. Chars are produced under various conditions in order to understand the impact of devolatilization rates on initial char chemical and physical properties. Char combustion tests performed in the entrained flow reactor provide information on overall particle burning rates that are controlled by the combined effects of intrinsic reactivity and pore diffusion limitations. Partially reacted chars are examined to determine their physical and chemical properties as functions of conversion. Combustion tests in the thermogravimetric analyzer provide the information needed to characterize char reactivity to oxygen as a function of char properties and gas composition, temperature, and pressure. The data are used to formulate and validate models that describe the intrinsic chemical reaction rates. The results permit the prediction of char particle mass loss rates and off-gas composition for specified gaseous conditions.

Model parameters are determined for essentially all coal and biomass materials used in experiments performed in our laboratory. The heterogeneous reaction mechanism is modified as data for more materials are amassed. Our goal is to obtain the information needed to understand how char reactivity is related to parent coal or biomass properties.

Elevated-Pressure, Coal and Biomass Conversion Studies

In upcoming activities, research efforts will focus on characterizing coal and biomass conversion phenomena at elevated pressures. Data needed to assess the accuracy of predictions made employing the heterogeneous oxidation model and the mode of burning model at high-pressures will be obtained in experiments performed in our entrained flow reactor, which has been enclosed in a pressure-chamber to permit oxidation tests at pressures up to 50 bar. Char reactivities to oxygen at high pressures will be measured in our pressurized thermogravimetric analyzer. Modification will be made to the models so that they accurately predict char particle morphology and reactivity during conversion under high-temperature, high-pressure conditions.

Staged-Combustion with Nitrogen-Enriched Air Studies

In a new endeavor, a study is being undertaken to assess the strategy of using nitrogen-enriched air (an O2/N2 mixture containing more than 79% nitrogen) during coal and biomass combustion in order to reduce the emissions of nitrogen oxides (NOx). Using nitrogen-enriched air instead of normal air reduces combustion temperatures for the same equivalence ratios. A two-staged combustion scheme is being tested in which coal (or biomass) and nitrogen-enriched air are burned under fuel-rich conditions in the first stage. Sufficient nitrogen-enriched air is added in the second stage to result in a burned gas containing about 3% oxygen. Our efforts are focused on quantifying the possible reductions in NOx emissions and on understanding the chemistry responsible for the reductions in NO formation rates. Key variables include the first-stage equivalence ratio, the fraction of nitrogen in the nitrogen-enriched air mixture, and the residence time at secondary air addition. The data are being used to validate models of the Staged-Combustion with Nitrogen-Enriched Air (SCNEA) process being developed at Lawrence Livermore National Laboratory.

Coal and Biomass Gasification Studies

In proposed work, the goal is to develop the ability to predict accurately coal and biomass conversion rates in gasifiers operating under conditions to produce a synthesis gas from coal and biomass gasification products. Of particular interest is determining coal and biomass conversion rates in gasifier environments optimized for hydrogen production. Towards this end, experiments are proposed aimed at characterizing the reactivities of coal and biomass chars to steam and carbon dioxide at high pressures. The information will be used to develop and validate models for predicting coal and biomass conversion rates and off-gas compositions in various gasifier configurations.

Carbon Deactivation Studies

The goal of this study is to characterize carbon deactivation due to the thermal annealing that occurs as char particles burn at high temperatures. The heterogeneous char oxidation mechanism is being modified to include chemical pathways leading to carbon deactivation. The work is done in concert with the char reactivity studies discussed above. The data obtained with all the materials tested in our laboratory are analyzed to provide information on carbon deactivation rates. Results are used to explain reasons for unburned carbon found in the ashes of coal-fired furnaces and utility boilers.

Char Fragmentation Studies

This project focused on characterizing the impact of fragmentation on char weight loss during pulverized coal combustion. Specific objectives were to identify conditions under which fragmentation events become significant, to determine if the mineral matter in coal affects fragmentation patterns, and to relate the effects of fragmentation events to unburned carbon in ash. Experiments were performed in our entrained flow reactor, which permits partially reacted chars to be extracted at selected residence times. The char samples were characterized in order to determine extents of mass loss and particle size distributions. The data were used to formulate and validate a particle population balance model that allows for both particle burning and fragmentation. The model was used to examine and quantify effects of fragmentation on char combustion.

Mode of Char Particle Burning Studies

This study was concerned with characterizing the variations in char particle apparent density and size with mass loss. The goal was to develop the ability to predict reductions in particle size and apparent density given the intrinsic chemical reactivity of the particle material and gaseous conditions. Based on measurements of the chemical and physical properties of chars at various extents of conversion, a model describing the relationship between particle size, apparent density and mass remaining was developed and validated. For specified gaseous conditions, the model yields the size and apparent density of a char particle at a given extent of conversion based on the instantaneous state of the particle and its intrinsic chemical reactivity.

Pyrite Transformation Studies

The fundamental chemical and mass transport processes that control the rate of conversion of pyrite to hematite were investigated in entrained flow reactor experiments. The objectives of the work were to identity the mechanisms of pyrite combustion and to quantify their effects on the overall oxidation rate in order to formulate a rate expression for the combustion of pyrite that accounts for coal properties as well as furnace conditions. Results of the proposed research were used to identify conditions that minimize the significance of those pyrite transformations responsible for the higher slagging propensity of staged, low-NOx pulverized coal combustor retrofits.

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Last update 3/30/2005

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