A Fundamental Study of Biomass Oxyfuel Combustion and Co-combustion
Timipere S. Farrow
Prof. Colin Snape: Supervisor
Review
Objectives
Experimental
Results
Conclusion
Future work
Presentation overview
1.



2.


Introduction
Carbon capture technologies
Detailed oxy-fuel combustion process
Objectives
Lab Scale Experimental techniques
Thermo gravimetric Analysis (TGA)/Horizontal Tube Furnace (HTF)
Drop Tube Furnace (DTF)
Results
I.
Combustion reactivity of Biomass Fuel under oxy-fuel and air
combustion
II.
Co-firing sawdust and coal to identify the effect of biomass on coal
char burnout
III. Co-firing in (DTF), the effect at higher temperature combustion
3.
Introduction
Overview
Objectives
Experimental
Results
Conclusion
Future work
Introduction
 The presence of CO2 and other green house gas emissions in the
atmosphere has become more problematic because of their
negative environmental impact on climate
 Stringent
environmental laws on CO₂ emissions from coal
combustion. World energy consumption is predicted to rise to 44%
and CO₂ emissions to 39% in 2030 [1]
 Increased
interest in power generation industry towards
technologies, which help to reduce CO2 emissions from fossil fuels
combustion by means of CO₂ capturing.
1. International energy outlook, 2009
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Leading Techniques for CO₂
Capture
 Biomass co-firing Presents a potential technique
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Why Biomass and co-firing?
Potential option for renewable based power
generation
 Unlike fossil fuels, biomass fuel is renewable and CO2-neutral in
the sense that the CO2 it only releases recently fixed carbon when
combusted thereby closing the carbon loop on a short time
 Partial substitution of coal for combustion
 In the UK, legislation is strong on CO₂ reduction to
meet Kyoto target and EU’s target to reduce CO₂
emissions by 20% by 2020.
 Hence the combination of oxy-fuel combustion
with biomass fuel become a CO2 sink for power
plants
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Oxy-fuel combustion Process for
cleaner fossil fuel utilisation
Fundamental studies of oxy-fuel coal combustion have
demonstrated that oxygen concentrations in the range 3040% produced temperature profiles matching those of
conventional air firing with lower NOx and SOx emissions.
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Objectives
1. To investigate the behaviour of biomass under oxy-fuel conditions in
comparison to air fired condition in terms of:
 Volatile yield
 The associated nitrogen partitioning between char and volatiles in order to
monitor NOx emissions.
 Kinetic
parameters which are useful for design of biomass oxy-fuel
combustion system.
2. To investigate how biomass will affect coal char burnout during co-firing
under oxy-fuel and air firing with particular emphasis on the catalytic effect
of biomass-contained alkali and alkaline metals on coal char burnout
Introduction
Overview
Objectives
Experimental
Results
Conclusion
Future work
Horizontal
Tube
Horizontal
Furnace
(HTF)
tube furnace
Drop tube
Furnace
Furnace(DTF)
Re-firing
Thermogravimetric
gravimetric
Thermo
Analyser (TGA)
analyser
Devolatilisation
Devolatilisation
Combustion
Devolatilisation
Sawdust
Combustion
Char
Char
Schematic diagram of experimental Approach
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Thermo gravimetric analyser (TGA)and
horizontal tube furnace (HTF) heating
rate of 150⁰C/min
TGA Heating rate is miles away from
reality
yet
give
fundamental
combustion information
TGA
Introduction
HTF, replicates TGA char production
Review
Objectives
Experimental
Results
Conclusion
Future work
Drop Tube Furnace, High heating rate, short
resident times (200-600ms) and 1600⁰C
High heating rate and high combustion temperatures, close to reality
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
What Effect does CO₂ have on
volatile yield?
100
100
N₂ 700⁰C
N₂ 900⁰C
CO₂ 900⁰C
80
80
Volatile yield (%)
Volatile yield (%)
CO₂ 700⁰C
60
40
20
60
40
20
0
0
Particle size
Particle size
100
N₂ 1100⁰C
CO₂ 1100⁰C
Volatile yield (%)
95
90
There is no particle size effect at both
conditions except for the smallest particle size at
1100⁰C
 The impact of oxy-fuel firing is pronounced at
1100⁰C due to volatile –char gasification reaction
but low at low temperatures due to Poor thermal
conductivity
85
80
c
75
Particle size
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Why do we need to maximise Nitrogen
(N₂) yield in the volatile phase?
Devolatilisation in CO₂
N₂ lost as volatiles (%)
N₂ lost as volatiles (%)
80
CO₂ 1100⁰C
CO₂ 700⁰C
60
Devolatilisation in N₂
100
100
CO₂ 900⁰C
40
20
0
80
60
N₂ 1100⁰C
40
N₂ 700⁰C
N₂ 900⁰C
20
0
Particles size
Particle size
 Char N₂ contribute to NOx formation
Beneficial to oxy-fuel due to high transformation of N₂ into the
gaseous state at high temperature
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
TGA Combustion reactivity of biomass
chars at 375⁰C
TGA in-situ char burnout in air
TGA in situ char burnout in 21% O2/79% CO2
100
100
45-63µm
45µm
80
Carbon burnout (%)
Carbon burnout (%)
45µm
45-63µm
63-75µm
75-90µm
60
90-106µm
125-250µm
40
20
0
80
63-75µm
75-90µm
90-106µm
125-250µm
60
40
20
0
0
10
20
30
40
50
60
70
80
80.00,80,
0.000
0
10
20
Time (min)
30
40
50
60
70
80
Time (min)
CO2 does not have effect on the combustion reactivity of the chars at low
temperature hence the burnout is identical with air fired condition
 Insignificant particle size effect is seen in during burnout in both conditions
except for the smallest particle size.
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Impact of low char combustion
temperature on kinetic parameters
Burnout in Air
1st order rate
90% burnout
constants at 5-95%
time
carbon burnout
(min-1)
(min)
Particle
size
(µm)
<45
45-63
63-75
75-90
90-106
125-250
0.0656
0.0515
0.0494
0.0471
0.0485
0.0492
40.80
50.50
53.20
55.30
54.20
56.20
Burnout in Oxy-fuel
1st order rate
90% burnout
constants at 5time
95% carbon
burnout (min-1)
(min)
0.0629
0.0536
0.0512
0.0487
0.0526
0.0522
45.20
52.40
55.20
56.00
53.00
53.50
Compensation Effect
Log of pre-exponential factor (A)
9.0
Air fired
oxy-fuel condition
8.5
Variation is less due to poor thermal
conductivity effect of CO2 compared with
that of N2
8.0
7.5
7.0
6.5
6.0
80
90
100
110
120
Activation Energy (E)
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Benefits of Co-firing
Samples
(TGA Analysis)
Nitrogen chars and Air combustion
CO₂ chars and 21%O₂/79% CO₂ combustion
1st order rate constants 90% burnout time
1st order rate constants
(min¯
)
(min)
)
(min)
sawdust char 700C
Kleinkopje (KK) HTF char 1000C
0.4901
0.0734
6.60
38.70
0.3114
0.0526
7.85
48.00
saw/KK char blend 50:50wt%)
0.1002
22.15
0.1089
20.65
Predicted sawKK char blend
0.0829
25.60
0.0720
31.60
Sawdust 700⁰C/coal 1000⁰C HTF nitogen char, air combustion
100
100
KK char N₂ 1000⁰C
Sawdust 700⁰C/coal 1000⁰C HTF CO₂ chars, combustion under oxy-fuel gas
KK CO₂ char 1000⁰C
Predicted 50:50 blend
80
50: 50wt% blend
Carbon burnout (w%)
Carbon burnout (wt%)
(min¯
90% burnout time
sawdust N₂ char 700⁰C
60
40
20
0
0
20
40
60
80
Time (min)
Predicted 50:50 Blend
80
50-50wt% har blend
sawdust CO₂ char 700⁰C
60
40
20
0
0
20
40
60
80
Time (min)
Improved burnout of blend but slightly more pronounced under oxy- fuel
condition
Strong synergetic effect: an indication of interactions
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Moving close to Reality, does biomass char
still affect coal char burnout?
100
kk DTF char 900C
sawkk DTF blend char 900C
Carbon burnout (%)
80
Predicted SawKK DTF 900C chars
saw DTF 900C
60
40
20
0
0
20
40
60
80
Time (min)
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Improved coal char combustion, effect of
catalytic inorganic metals in biomass fuel
Raw Sawdust/coal 1000⁰C HTF nitogen char, air combustion
100
HCl extracted sawdust/coal HTF nitrogen chars and air combustion
100
KK char 1000⁰C
kkchar 1000⁰C
sawkk blend 50:50
80
50: 50wt% blend
Carbon burnout (%)
Carbon burnout (wt%)
Calculated blend
sawdust char 700⁰C
60
40
20
Predicted blend
80
Dem sawdust char 700⁰C
60
40
20
0
0
0
20
40
60
80
0
20
Review
60
80
Time (min)
Time (min)
Introduction
40
Objectives
Experimental
Results
Conclusion
Future work
conclusions
 High reactivity observed for CO₂ at high temperature due to gasification
reaction.
 No particle size effect, can use bigger particle size for pulverised biomass fuel
combustion systems
 Biomass improved coal combustion. There is chemical interaction between the
two fuels during co-combustion
 Inorganic minerals in biomass catalysed coal char combustion
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Completing PhD, what is left to
be done

Devolatilisation of sawdust in DTF at different temperatures and different
residence times
 DTF char burnout
 Co-firing at different temperatures and residence times at the two
atmospheres
 DTF burnout of blend chars
 TGA burnout analysis of DTF chars (sawdust and blend chars) in air and oxy-
fuel conditions
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work
Thanks
for
YOUR
Attention
Introduction
Review
Objectives
Experimental
Results
Conclusion
Future work