MECHANICAL ENGINEERING—ENGINEERING MECHANICS
Proudly Presents:
Dr. Reginald Mitchell
Professor—Stanford University
Reginald E. Mitchell is a professor in the Mechanical Engineering Department at Stanford University in
Palo Alto, California and the current Director of the High Temperature Gasdynamics Laboratory at Stanford. After leaving the Massachusetts Institute of Technology in 1975 with a Doctorate of Science degree
in chemical engineering, he joined the research staff at Sandia National Laboratories in Livermore, CA.
While at Sandia, he was awarded the title “Distinguished Member of the Technical Staff” for his research
efforts in coal combustion science. After 16 years at Sandia, he joined the faculty at Stanford where his
research activities expanded to include topics on biomass as well as on the development of coal and biomass conversion technologies that facilitate CO2 capture. Besides ongoing studies on understanding and
predicting coal and biomass combustion and gasification conversion phenomena, his current studies involve developing fuel cells that run on coal and biomass. Professor Mitchell is the author of over 100
technical articles and he has given numerous talks about his research at universities, conferences, symposia, and other professional meetings. He has received the AWU-DOE Laboratory Distinguished Lectureship Award for his professional achievement and commitment to science education from the United States Department of Energy and the Tau Beta Pi
Award for excellence in undergraduate teaching from the School of Engineering at Stanford. He is the current Secretary of the
International Combustion Institute.
Thursday,January21,2016
4:00 pm — 103 EERC
The Impact of Co-firing Coal and Biomass on Mixed Char
Reactivity under Gasification Conditions
This work focuses on the gasification behaviors of the chars formed during devolatilization when co-firing coal/biomass blends into reducing
environments. In this experimental and modeling investigation, Wyodak coal, a sub-bituminous coal from the Powder River Basin, was the
coal employed and corn stover, the residue stalk, leaves, husk, and cob remaining after the harvest of corn, was the biomass. Torrefied corn
stover was also used in selected tests. Chars were produced by injecting pulverized fuel particles into a laboratory-scale entrained flow reactor
in which a reducing environment was established at ~1550 K and 1 atm. The injected fuel was either pure Wyodak coal or pure corn stover
or a pre-mixed blend of the two. Char particles were extracted from the reactor just subsequent to devolatilization and subjected to tests performed in a TGA to determine specific surface areas and reactivities to CO2, H2O and O2 under kinetics-limited conditions. The results indicated that the specific surface areas of the mixed chars were lower than the specific surface areas of the chars produced from the pure fuels.
In addition, the mixed char reactivities were outside the bounds of the reactivities of the chars produced from the pure fuels, being lower in
CO2-containng environments and higher in environments containing H2O. The cause of this impact of the fraction of biomass in the coal/
biomass blend on mixed char specific surface area and reactivity is due, in part, to coal particle devolatilization rates being altered when cofiring. Biomass particles release their volatile matter earlier than coal particles, and the endothermic reactions that occur between biomass volatiles and the hot reactor gases lower the gas temperatures in the vicinity of the devolatilizing coal particles, affecting their devolatilization
rates. The affected coal char particles have properties that differ from the char particles formed in the absence of the devolatilizing biomass
particles.
In this presentation, the experimental procedures and data are discussed. Mixed char specific surface area and reactivity models are also presented that are capable of accurately predicting these mixed char properties for all coal/biomass blends. The models require information not
only on the specific surface areas and reactivities of the chars produced from the pure fuels but also information on the specific surface area
and reactivity of the mixed char produced from the fuel blend that has the lowest specific surface area. For blends having less biomass, the
specific surface area and reactivity of the blend are bound by the specific surface areas and reactivities of the pure coal char and this mixed
char and for blends having more biomass, the specific surface area and reactivity of the blend are bound by the specific surface areas and reactivities of this mixed char and the pure biomass char.