Data Stream Processing

Data Stream Processing

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  • mémoire
Data Stream Processing Sumit Ganguly IIT Kanpur
  • finite family of functions
  • wise independence
  • lower bound for deterministic estimation of f2 alon matias szegedy
  • hash family
  • data

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COMBUSTION AND GASIFICATION OF CHARS IN OXYGEN AND
CARBON DIOXIDE AT ELEVATED PRESSURE





A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY











Liqiang Ma
August 2006












©Copyright by Liqiang Ma 2006
All Rights Reserved




ii
I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope
and quality as dissertation for the degree of Doctor of Philosophy.


_________________________________
(Reginald E. Mitchell) Principal Advisor





I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope
and quality as dissertation for the degree of Doctor of Philosophy.


_________________________________
(Craig T. Bowman)





I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope
and quality as dissertation for the degree of Doctor of Philosophy.


_________________________________
(David M. Golden)





Approved for the University Committee on Graduate Studies.





iii

iv Abstract

Coal has been an important energy resource for more than 100 years, and it will continue
stto be an important energy resource in the 21 century. Currently, the energy from coal
accounts for 23% of the energy consumed in the United States and about 70% in China.
However, the traditional schemes for utilization of coal have been criticized for their low
efficiency and high pollution characteristics compared to oil and natural gas. Some clean coal
technologies, such as Integrated Gasification Combined Cycle (IGCC) and Pressurized
Fluidized Bed Combustion (PFBC) have been identified as viable technologies to address
these problems. Fundamental understanding of the physical and chemical processes involved
in these advanced coal conversion technologies is necessary to achieve higher efficiency and
smaller impact on the environment.
Char combustion and gasification are complicated technologies, involving several
chemical and physical processes. These processes include the transport of reactive gases
across the boundary layer surrounding the char particle, the transport of gases through the
porous structure of the char particle, and the chemical reactions on the carbon surfaces within
the char particle. In order to gain an understanding of these processes and their effects on the
char conversion process, extensive experimental efforts were made to characterize the
reactivity of chars of a variety of carbonaceous materials. The experiments, conducted in a
Pressurized Thermogravimetric Analyzer (PTGA) and in a High Pressure Flow Reactor
(HPFR), cover a wide range of temperatures (773 - 1650 K), pressures (1 - 20 atm), and gas
compositions (different mixtures of O /N and CO /N ). 2 2 2 2
Under pulverized coal combustion conditions, char particles burn with reductions in both
diameter and apparent density due to the O concentration gradients established inside burning 2
particles at high temperatures. The power-law mode of burning model is used by many to
relate particle apparent density, diameter and mass loss during char oxidation. However, this
model pre-supposes that the relationship between particle diameter and apparent density is
fixed even as rate-limiting steps change during the course of burning. A single char particle
conversion model that eliminates this shortcoming was developed in this study and used to
calculate variations in particle size and apparent density when burning at high temperatures.
The results of these calculations were used to establish a relationship between the
v effectiveness factor and the Thiele modulus, which was employed in the development of the
intrinsic reactivity-based mode-of-particle-burning model. The relations allow for variations
in particle size and apparent density during conversion that depend on the instantaneous state
of the char particle. In the model, a 6-step reaction mechanism was used to describe the char
reactivity. It was demonstrated that the mode of particle burning model can predict the
burning behaviors of coal and biomass char particles undergoing oxidation in the type of
environments in real burners and furnaces.
In advanced coal conversion technologies such as IGCC and PFBC, the char conversion
process occurs at elevated pressures. The separate effects of total pressure, oxygen mole
fraction and oxygen partial pressure on the char reactivity as particles burn were examined
using the single char particle conversion model developed. At low temperature, where
chemistry controls the reaction rate, the reactivity was found to be dependent solely on the
oxygen partial pressure for fixed temperature. The calculated results at high temperatures
indicated that the reactivity decreases with increasing total pressure at constant oxygen partial
pressure. At fixed total pressure, the reactivity increases with increasing oxygen mole fraction.
These predicted trends were confirmed by the results of experiments performed in the high
pressure flow reactor. The model also explained the observations of high and low global
reaction orders at high temperatures from various other studies.
It was found in other studies that the physical structures of the char particles after
devolatilization are different and can be characterized as being cenospherical, mixed and
dense. For some coals (primarily bituminous coals), cenospherical char particle formation
increases with increasing pressure. To more accurately characterize char combustion under
elevated pressures, a char structure model was integrated with the char combustion model
previously developed to capture the different burning behaviors of char particles with different
structures. Calculations of the mass loss and apparent density agreed with the experimental
values measured from the experiments at elevated pressure. The calculated particle
temperatures were consistent with the measurements of particle temperature at comparable
burning conditions in other studies. The model adequately predicted the behaviors of char
particles burning under conditions of high temperature and elevated pressure.
In characterizing char conversion during gasification, it is necessary to investigate the
reaction between carbon and carbon dioxide for better predictions of char reactivity. A
carbon-carbon dioxide reaction mechanism was developed based on previous investigations
vi and current work. The reaction mechanism developed was evaluated using gasification rates
determined from experiments under different gasification conditions. It was demonstrated that
the mechanism developed can provide accurate predictions of char reactivity over a wide
range of temperatures, pressures, and gas compositions, which the previous models were not
capable of doing.
vii
viii Acknowledgement
I would like to thank my advisor, Professor Mitchell, for the enthusiastic support for
making it possible for me to conduct this work, and for encouraging my creative tendencies in
the research. I wish to thank my reading committee, Professor Bowman and Professor Golden,
for devoting time to this thesis and for assistance throughout my Ph.D. studies.
My thanks also go to my colleagues in the Heterogeneous Combustion Laboratory: Paul
Campbell, Lars Sørum, Illka Saarenpää, Andrew Lee and Bumjick Kim. Thank you all for
your cooperation, technical assistance and helpful discussion. I also want to gratefully
acknowledge my friends at Stanford: Xin Zhou, Shuhuai Yao, Yue Liang, Xiaojun Yu and
Jian Luo, for the help and friendship during our study together.
I want to thank my wife Hongrui. She has encouraged me and helped me with her love,
patience and understanding throughout my study. Thanks are also due to my family for their
encouragement and prayers.
Last but definitely not least, I want to express my highest praise to my Lord, Jesus Christ.
Although I just knew Him for only about half a year, I felt that He has always accompanied
me and helped me during my thesis work. May the Lord be pleased with my work and the
glory is His.










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