IMPROVING CHAR AND TAR CONVERSION IN FLUIDIZED BED...
Transcript of IMPROVING CHAR AND TAR CONVERSION IN FLUIDIZED BED...
AGB, 2ndISGA, Fukuoka Dec10
IMPROVING CHAR AND TAR CONVERSION IN FLUIDIZED BED GASIFICATION
A. Gómez-BareaChemical and Environmental Engineering Department
Bioenergy Group, University of Seville (Spain)[email protected]
iSGA 2010 Second International Symposium on Gasification and Its Application
Fukuoka, Japan, December 5-8, 2010
AGB, 2ndISGA, Fukuoka Dec10
Contents
• Introduction• Fundamentals of fuel conversion in FBG
– Qualitative description of FBG – Generation of tar and char (devolatilization)– Conversion of tar– Conversion of char
• Optimization of char and tar conversion in FBG– Optimization of existing gasifiers– Existing measures for secondary tar removal– Staged gasification
AGB, 2ndISGA, Fukuoka Dec10
The problem
• FBG for biomass and waste has advantages compared to fixed/moving or entrained-flow bed designs
• Two main problem limits autothermal FBG:– Low carbon (char) conversion Low efficiency– High tar content in the gas limits application
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Autothermal gasification (1)
Product gas
Air / O2 –Steam Biomass
GASIFIER
Product gas
Biomass
GASIFIER No external heat is needed
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Autothermal gasification (2)
HEAT
Product gas
SteamBiomass
GASIFIER
Product gas
Biomass
GASIFIERCOMBUSTION
CHAMBERGASIFIER
Flue gas Product gas
SteamBiomassAir
Bed material + char
Bed material + heat
COMBUSTIONCHAMBER
GASIFIER
Flue gas Product gas
SteamBiomassAir
Bed material + char
Bed material + heat
Gas recirculation
COMBUSTIONCHAMBER
AirGas recirculation
COMBUSTIONCHAMBER
Air
Flue gas
Char recirculation
COMBUSTIONCHAMBER
AIR
COMBUSTIONCHAMBER
AIR
Flue gas
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Char conversion in autothermal “stand-alone” FBG
• Not enough conversion time:– Slow reactivity (C+H2O)– Entrainment (high gas velocity)
FLY-ASH
BOTTOM ASH
FUEL
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Need for tar removal?
ASH
AIR
BIOMASS
GASIFIER
FUELFEEDING
LARGEBIOLER
TO STACK
COALOIL
GASCOOLER
PRODUCT GASFILTER
ASH
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ASH
AIR
BIOMASS
GASIFIER
FUELFEEDING
Need for tar removal?
STACK
~POWER
GAS ENGINE(S)
HEAT RECOVERY
HEAT
GASFILTER
TARCRACKER
GASCOOLING
GASCOOLING
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Limitation
Lower Tar conversion Higher
Lower Char conversion Higher
Decreasing risk Sintering Increasing risk
700 °C 800 °C 900 °C 1000 °C
Agrofuels
Refusedderived
fuel
Woodybiomass Coal
Sintering limitis the thermal level of the gasifier
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Concluding remarks
• Increasing char conversion improves efficiency• Increasing tar conversion improves gas
utilization• Optimization of char and tar conversion in FBG
is necessary
AGB, 2ndISGA, Fukuoka Dec10
• Introduction• Fundamentals of fuel conversion in FBG
– Qualitative description of FBG – Generation of tar and char (devolatilization)– Conversion of tar– Conversion of char
• Optimization of char and tar conversion in FBG– Optimization of existing gasifiers– Existing measures for secondary tar removal– Staged gasification
AGB, 2ndISGA, Fukuoka Dec10
Description of fuel gasification
• Reforming• Craking• Oxidation• Polimerization• Char conversion •Secondary fragmentation•Attrition
• Thermal and chemical conversion (*)• Shrinkage• Primary fragmentation
Heat
SECONDARYCONVERSION
Tar 2Light gas 2Char 2
PRIMARY CONVERSION(GENERATION)
Fuel
Char 1
Tar 1
Gasificantionagent
Light gas 1
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Solid mixing and devolatilizationDadev = rate of devolatilisation/rate of solid mixing
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Generation of tar and char: devolatilization
• Fuel thermal transformation: shrinkage, fragmentation, gas releasing and reaction,…
• It is difficult to descript or model • Existing advanced kinetics do not predict the
yield of gas and tar specie• Yields are experimentally determined for:
– Each biomass– Temperature– Heating rate
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Yields of devolatilization
0 200 400 600 800 10000.0
0.2
0.4
0.6
0.8
1.0
Temperature (ºC)
Cha
r yie
ld
Temperature (ºC)
0 200 400 600 800 10000.0
0.2
0.4
0.6
0.8
1.0
Ligh
t yie
ld
0 200 400 600 800 10000.0
0.2
0.4
0.6
0.8
1.0
Temperature (ºC)
Tar y
ield
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Factors determining char conversion
– Intrinsic reactivity (mainly with H2O/CO2)
– Reactivity of a single particle (diffusion effects)
– Fragmentation and attrition patterns
– Char conversion in the bed (spatial distribution of particles with different conversion)
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Low temperature
tarsAromatic
structures Soot
Time-Temperature
Gas H2
Lower mass hydrocarbons
Gas
Tar generation and conversion
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Conversion of some tars under different conditions
2
55 61
100
60.373.7
94.4 99.6
0
20
40
60
80
100
Nap
htha
lene
co
nver
sion
(%)
T=900ºC, τ=0.3 s,
6
34.542.7
81.6 87.1 90 91
0
20
40
60
80
100
Phen
ol
conv
ersi
on (%
)
T=700ºC, τ=0.3 s,
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• Introduction• Fundamentals of fuel conversion in FBG
– Qualitative description of FBG – Generation of tar and char (devolatilization)– Conversion of tar– Conversion of char
• Optimization of char and tar conversion in FBG– Optimization of existing gasifiers– Existing measures for secondary tar removal– Staged gasification
AGB, 2ndISGA, Fukuoka Dec10
Optimization of the gasifier
Operational optimization:– O2/fuel and Steam/fuel ratios (steam gasification) – Optimization of air (O2)/ fuel ratio (air gasification)– Flow rate of catalyst /additive If revamping or re-design is applied:– Pre-heating of the air (heat integration)– Distribution along the gasifier (adjustment of flow rates)
Design Optimization: – Staged gasification– Other (see the paper)
* Temperature is a dependent parameter which is given by the above set of adjustable parameters
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Optimization of oxygen and steam in an autothermal pilot FBG
55%
60%
65%
70%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7SBR
Col
d G
asifi
catio
n Ef
ficie
ncy
Steam / fuel ratio
O2/ fuel ratio
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Use of in-bed catalysts: tests from autothermal pilot FBG
ER=fed O2 / stoichiometric O2
170
175
180
185
0.2 0.25 0.3 0.35 0.4
Gas d
ew p
oint
(ºC)
Alumina
Lime
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Rules of thumb: conclusions from experiments
Tar conversion: • Tar dew point (and not tar amount) is the key point• “Fresh” tar is easier to be converted downstream• In-bed catalyst are not efficient enough for fine applications• The presence of steam and char in the bed are positive
Char conversion can be improved by increasing char reaction time
• Recirculation• Pretreatment of the fuels• Adjustment of fluidization velocity• Re-design of gasifier for specific biomass (…)
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Main conclusions operational optimization
• Char conversion can be improved significantly by optimization. It is difficult to reach more than 97% carbon conversion
• Tar conversion is improved by operational optimization but it is not possible to reache the requirement for electricity in engines or finer application
• Further (design) optimization or secondary measures are needed
AGB, 2ndISGA, Fukuoka Dec10
• Introduction• Fundamentals of fuel conversion in FBG
– Qualitative description of FBG – Generation of tar and char (devolatilization)– Conversion of tar– Conversion of char
• Optimization of char and tar conversion in FBG– Optimization of existing gasifiers– Existing measures for secondary tar removal– Staged gasification
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Secondary measures 1: scubbing
keys:• Solvent loss• Gas cooler
plugging• Heat integration• Capture
efficiency
New concept in development:
• Tar scrubbing by tar
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Example: 3 MWth BFBG in Seville (Spain)
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Catalytic cracking/reforming of tars
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Performance of tar reforming
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Main conclusions of secondary measures
• Tar scrubbing with organic solvent might work but is complex for small or medium plants (OLGA, …)
• In catalytic reforming Catalyst deactivation • Demonstration is necessary
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• Introduction• Fundamentals of fuel conversion in FBG
– Qualitative description of FBG – Generation of tar and char (devolatilization)– Conversion of tar– Conversion of char
• Optimization of char and tar conversion in FBG– Optimization of existing gasifiers– Existing measures for secondary tar removal– Staged gasification
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Staged gasification
Low tar content gas
PyrolysisUnit
Charcoalbed
Steam+Air
Biomass
Gas
ifier
Gas
ifica
tion
unit
Pyrolysis gas+
CharcoalPyrolysis zone(1st stage)
Gasification zone(2nd stage)
Pyrolysis gas,Tar
Secondary air
Primary air
Biomass
Gas
ifier
Low tar content gas
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Example of two-staged gasification:Viking gasifier at DTU
- Tar levels in gas drop from 25 g/Nm3 to "no tar“ (<5 mg/Nm3)- State of Commercialization ?
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FLETGAS at BEGUS: Flexible Three-Stage Gasifier
Biomass
1. BFB devolatilizer
3. Chemical quench
2. Non catalytic reformer
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Fluid-dynamics: Cold model
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• Devolatilization tests with:– different biomass, T and size of fuel– high T steam-enriched air mixtures
• Tests for characterization of: – Generation of individual tar compounds– Char properties (reactivity and catalytic properties)– Tar conversion experiments of individual tar compounds at different:
• proportion of O2/steam • Temperature• Though in-situ generated char bed
Fuel conversion investigation:lab-scale FB fuel conversion tests
Pilot plant desing and construction
Fluid-dynamics
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1. Char and tar need to be converted in FBG2. Optimization of FBG gasifiers is not enough for
application other than thermal (because of tar)3. Secondary measures for tar removal are complex
or expensive and need to be demonstrated (primary measures are preferred)
4. Staged-gasification – seems to be an adequate concept for small to medium
electricity plants– exists in different forms (technologies)– needs commercial demonstration
Conclusions
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Contact /further information
Thank you for your attentionMore details in:• Paper in the proceeding and Fuel (to be
submitted)• PECS: Gómez-Barea and Leckner (2010)• Talking/writing to authors
[email protected]://www.grupobioenergia.com