Devolatilization and Ignition Characteristics of S.A Coals

Devolatilization and
Ignition Characteristics of
S.A Coals
Presenter: Sandile Peta
Academic Mentor: Prof. Walter Schmitz
Industrial Mentor: Prof. Louis Jestin
Date: 5th May 2014
Acknowledgement
This research was conducted
through the Eskom Power Plant
Engineering Institute (EPPEI)
Eskom Specialisation Centre for
Combustion Engineering at
The University of the Witwatersrand
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Introduction
• Pulverized Fuel (PF) combustion process
• Research objectives
PF Combustion Process in a Coal Boiler
The combustion of a PF particle in a coal boiler is characterized by the following stages
(Zajaç et al, 2009):
1. Rapid heating, (<10 000K/s)
2. Devolatilization reaction (up to c.a. 200ms)
3. Char combustion reaction (up to c.a. 2000 - 3000ms)
Basic model for flame propagation (Taniguchi,
M. 2012)
Ignition and flame propagation phenomena (Taniguchi et al,
2011)
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Research objectives
The aim of the present work is to determine the devolatilization and ignition
parameters of three South African coals utilized for power generation. To achieve
the objectives, the research scope included the following:
• Determine S.A. power generation coal Two-step devolatilization model
kinetic parameters (activation energies, frequency factors).
• Calculate the devolatilization profiles for the three coals using the Kobayashi
(Two-step) devolatilization model.
• Compare Two-step model calculated volatile yields for the three coals with
DTF measured volatile yields.
• Measure S.A. power generation coal ignition and flame propagation properties
(flame length and flame propagation velocity).
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Research method
• Tools
• Determination of devolatilization parameters
Tools
VM=20, Ash=44
VM=25, Ash=37
VM=21, Ash=31
VM=44, Ash=6
VM=25, Ash=16
VM=31, Ash=15
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Determination of coal devolatilization
parameters
Coal sampling
Coal chemical analysis
DTF devolatilization
calculation
Arrhenius plots
Devolatilization
kinetic parameters
(A, E, YY)
PC Coal Lab DTF calculation conditions
Gas
temperature
2-step model
devolatilization
profiles
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Results
• Drop Tube Furnace (DTF) measurements
• 2-step model devolatilization prediction
(StarCCM+ CFD code)
• Transparent Wall Reactor (TWR) measurements
ESKOM RT&D Drop Tube Furnace
ESKOM RT&D Drop Tube Furnace (DTF)
CAD model of ESKOM RT&D DTF
DTF gas temperature
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DTF measured volatile yields
50
Weight loss, daf (%),
Coal B: dry
VM =25%
Ash = 37%
Weight
loss
40
Coal C: dry
VM = 21%
Ash =31%
30
Coal A: dry
VM =20%
Ash = 44%
Volatile matter
conversion ratio
3
2
20
1
10
1400 oC wall temperature set-point
0
0
0
50
100
150
200
250
300
Residence time (ms)
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Volatile matter conversion ratio (-)
4
60
Comparison of 2-step model prediction & DTF
measurement
60
Coal B (DTF)
Coal C
Weight loss, daf (%)
50
Coal B
Coal C (DTF)
40
Coal A
30
Coal A (DTF)
20
10
0
0
50
100
150
200
Residence time (ms)
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250
Hitachi Research Laboratory Transparent
Wall Reactor (TWR)
HRL Transparent Wall Reactor schematic
Photos of HRL Transparent Wall Reactor
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TWR flame propagation video at fixed
particle concentration - 0.5kgcoal/ m3air
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Normalized flame propagation velocity, Sb (-)
TWR flame propagation velocity at fixed
particle concentration - 0.5kgcoal/ m3air
1.0
0.8
0.6
Coal D dry:
VM=44%,
Ash=6%
0.4
Coal B dry:
VM=25%, Ash=37%
Coal F dry:
VM=31%, Ash=15%
Coal C dry:
VM=21%, Ash=31%
0.2
Coal A dry:
VM=20%, Ash=44%
Coal E dry:
VM=25%, Ash=16%
0.0
0
10
15
20
30
40
Volatile matter, dry basis (%)
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Conclusions
• S.A. coal volatile yield measurements
• 2-step model devolatilization prediction
(StarCCM+ CFD code)
• S.A. coal flame length and flame velocity
Recommendations
• ESKOM Low NOx firing system retrofits
Conclusions
• The volatile yield profiles of the three coals show that
the high heating rate volatile yield of the three coals
is a function of the proximate volatile matter
content.
• The Two-step model accurately predicts ultimate
volatile
matter
yield.
The
curvature
of
devolatilization profile is however not well
predicted.
• Maximum flame lengths of S.A coals found to be
similar to Anthracite which is difficult to ignite in a
conventional Low NOx coal burner, due to
significantly lower volatile matter content.
• Flame propagation velocities of S.A. coals is low
relative to other coals.
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Recommendations
• Since ESKOM is embarking on a Low NOx technology future, it is necessary
to adapt firing system designs to combust S.A. coals by:
i. Reducing particle size
Ignition delay reduction
ii. Increasing particle residence time and
iii.Maximizing heat flux to the coal particles Volatile yield enhancement
Burner outlet
Ignition and flame propagation phenomena (Taniguchi et al,
2011)
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Low NOx PF burner flame in Babcock-Hitachi
test furnace (Ochi et al, 2009)
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Thank you

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