Optic Diagnostics on Pulverized Coal Particles Combustion Dynamics and Alkali Metal Release Behavior

164 Pages·2019·8.362 MB·English

by  Ye Yuan

Springer Theses
Recognizing Outstanding Ph.D. Research
Ye Yuan
Optic Diagnostics
on Pulverized Coal
Particles Combustion
Dynamics and
Alkali Metal Release
Behavior
Springer Theses
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Ye Yuan
Optic Diagnostics
on Pulverized Coal Particles
Combustion Dynamics
and Alkali Metal Release
Behavior
Doctoral Thesis accepted by
Tsinghua University, Beijing, China
123
Author Supervisor
Dr. YeYuan Prof. ShuiqingLi
Department ofEnergy andPower Department ofEnergy andPower
Engineering Engineering
TsinghuaUniversity TsinghuaUniversity
Beijing,China Beijing,China
HuanengCleanEnergy Research Institute
Beijing,China
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-10-4812-8 ISBN978-981-10-4813-5 (eBook)
https://doi.org/10.1007/978-981-10-4813-5
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Dedicated to my family.
’
Supervisor s Foreword
We are now facing the serious environmental and social concerns of fossil fuel
utilization in the twenty-first century. Electricity from coal is the most stable and
lowest cost compared with other kinds of power generation. Thus, clean coal
combustion technology with high efficiency and low emission, especially low
carbon emission technology, is of great significance for the future. Despite more
than a hundred years of research on coal combustion, there are still a lot of chal-
lenging issues to be addressed to meet the needs of emerging technologies.
Considering the complexity of coal combustion than any other fossil fuels, in situ
optical diagnostics plays an important role for further divulging the detailed
mechanism of pulverized coal combustion and pollutant formation.
Although optic diagnostic technology has been developed for a long time in
gas-phase combustion, the application to the coal combustion system is still rare.
This thesis focuses on developing several new optic diagnostic methods for
studying coal combustion processes, such as ignition, devolatilization, alkali metal
transformation and burnout characteristics.
The main innovative points are summarized as follows:
1. Optic diagnostics on coal particle devolatilization and ignition
This work develops a novel multi-filter technique to detect the CH* signal
during coal combustion, which can be used to characterize the volatile release
andreactionprocess.TheconsecutiveimagesthroughmultiplefiltersnearCH*
emission band, centered at 420, 430 and 440 nm, are exquisitely processed to
subtract the interferences from continuum blackbody radiation of particles and
soots in the CH* band. Then, the signal of CH* chemiluminescence is capable
ofprovidingagoodindicatorofthecoaldevolatilizationprocess.Theprevalent
devolatilization/homogeneous ignition are observed at high temperatures with
apparent CH* signals, whereas CH* emission can rarely be detected in the
initial stage of low-temperature case. The normalized visible light signal
intensity, deleting the background noise, is innovatively established to charac-
terize the ignition delay of coal particle streams. The prevalent transition from
vii
viii Supervisor’sForeword
heterogeneous ignition to hetero-homogeneous ignition due to ambient tem-
perature incensement is observed, based on the signal curve and flame visual
image.
2. Optic diagnostic on dynamic behavior of sodium release from coal combustion
By utilizing the gap between the excitation energies of the gas and particle
phases, a new low-intensity phase-selective laser-induced breakdown spec-
troscopy (PS-LIBS) is developed to distinguish the existence of sodium in the
particle or gas phase and examine the sodium release behavior along the
combustion process. For the first time, the in situ verification of the gas-phase
Nareleaseaccompanyingcoaldevolatilizationisfulfilled.Thetemperaturehasa
great effect on the sodium release behavior. It is also found that the
devolatilization time, Na transition time and the particulate matter formation
time are all strongly related.
3. RGB pyrometry and CBK model research on char combustion
Three-color pyrometry method is also built and calibrated using a digital
camera, which can be adopted to detect particle surface temperature. An
emissivity model based on the spectral emission measurement is employed to
modifytheRGBpyrometry,sincetheemissivityisakeyfactorincharburnout
temperature. The results find that the char emissivity is sensitive to the wave-
length, not to the temperature. The CBK model also well predicts the char
particle temperature history and investigates diameter, density and surface area
changingbehaviorduringcharburnout.Asub-modelbasedonreactionsiteshas
been developed to describe the inhibition effect during char burnout.
Overall, it is a systematic thesis including experimental and theoretical work,
offering a good understanding of the complex process of pulverized coal com-
bustion. The applicationofopticdiagnostictechnique incoal system fills theoptic
research blank in complex pulverized coal combustion research to a great extent.
The flat-flame burner combined with optic diagnostic methods will become a
powerfultechniqueforfutureparticlecombustionresearch,includingcoal,biomass
and metal.
Beijing, China Prof. Shuiqing Li
January 2019
Abstract
Coal combustion is a complex physical and chemical process, including hetero-
geneous reaction. Even though the coal combustion research has been conducted
for a hundred years, there still have a lot of problems to be solved to meet the
challengesofhigherefficiency.Atthemeantime,pollutantdynamicgenerationlike
SO , NO and PM also increases the difficulties in coal combustion process
x x
exploring. Therefore, this thesis develops several optic diagnostic methods for the
coal combustion research. For example, novel multi-filter array and visible light
signal methods are separately adopted to investigate the devolatilization and igni-
tion process. Low-intensity phase-selective selective laser-induced breakdown
spectroscopy and RGB color pyrometry are also used to study the alkali metal
dynamic behavior and char surface burnout temperature. In the end, transient
ignition model and carbon burnout kinetic (CBK) model offer theoretical analyses
for the devolatilization, ignition and burnout process.
Firstly,thisworkdesignsanovelmulti-filtertechniquetodetecttheCH*signal
during coal ignition, which can be used to characterize the volatile release and
reaction process. This method is found to be useful in eliminating the extra inter-
ference from the particle blackbody emission. The results imply that the tempera-
ture has a dominate effect on the volatile release. After that, on the basis of
devolatilization timescale analysis, this thesis is also assessing a study of the col-
lective ignition behaviors of dispersed coal particle streams. The prevalent transi-
tion from heterogeneous ignition to hetero-homogeneous ignition due to ambient
temperature is observed. Then, the effects of ambient temperature, oxygen mole
fraction, oxy-fuel ambient and coal rank on the characterized ignition delay are
examined.Besides,thetransientmodelisdeveloped,whichnotonlywellinterprets
the observed ignition transition phenomena, but also approximately predicts a
variation of heterogeneous ignition time as a function of oxygen fraction.
SincetheNareleasebehaviorisstronglyrelatedtothecoaldevolatilization and
ignition process, we further examine the dynamic behavior of sodium (Na) release
duringthepulverizedcoalcombustionofZhundongligniteonHenckenburner.By
utilizing the gap between the excitation energies of the gas and particle phases, a
new low-intensity laser-induced breakdown spectroscopy (PS-LIBS) is developed
ix
x Abstract
to distinguish the existence of sodium in the particle or gas phase along the com-
bustion process. For the first time, the in situ verification of the gas-phase Na
release accompanying coal devolatilization is fulfilled when the ambient tempera-
tureishighenough.Furthermore,theeffectsofambienttemperature,coalrankand
oxygen concentration are discussed. Based on the sampling results, it is founded
that the devolatilization time, Na transition time and the particulate matter forma-
tion time are all strongly related.
In fact, particle temperature plays a very important role in coal burnout process
andashparticleformation.ThelastpartofthisworkuseRGBcolorpyrometryand
CBK model to study the char particle temperature on Hencken burner. An emis-
sivitymodelbasedonthespectralemissionmeasurementisemployedtomodifythe
RGBpyrometry.RGBmethodsuccessfullydetectstheaccuratesurfacetemperature
of burning char particles. The effect of temperature, oxygen mole fraction and
oxy-fuelambientisalsoconsideredinthiswork.TheCBKmodelalsowellpredicts
thechar particle temperature history andinvestigates diameter,densityandsurface
area changing behavior during char burnout.
(cid:1) (cid:1) (cid:1)
Keywords Coalcombustion Insituopticdiagnostic CH*chemiluminescence
(cid:1) (cid:1)
Ignition Na release behavior RGB color pyrometry