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


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


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


Autothermal gasification (1)

Product gas

Air / O2 –Steam Biomass

GASIFIER

Product gas

Biomass

GASIFIER No external heat is needed


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


Char conversion in autothermal “stand-alone” FBG

• Not enough conversion time:– Slow reactivity (C+H2O)– Entrainment (high gas velocity)

FLY-ASH

BOTTOM ASH

FUEL


Need for tar removal?

ASH

AIR

BIOMASS

GASIFIER

FUELFEEDING

LARGEBIOLER

TO STACK

COALOIL

GASCOOLER

PRODUCT GASFILTER

ASH


ASH

AIR

BIOMASS

GASIFIER

FUELFEEDING

Need for tar removal?

STACK

~POWER

GAS ENGINE(S)

HEAT RECOVERY

HEAT

GASFILTER

TARCRACKER

GASCOOLING

GASCOOLING


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


Concluding remarks

• Increasing char conversion improves efficiency• Increasing tar conversion improves gas

utilization• Optimization of char and tar conversion in FBG

is necessary


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


Solid mixing and devolatilizationDadev = rate of devolatilisation/rate of solid mixing


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


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


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)


Low temperature

tarsAromatic

structures Soot

Time-Temperature

Gas H2

Lower mass hydrocarbons

Gas

Tar generation and conversion


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,


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


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


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


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 (…)


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


Secondary measures 1: scubbing

keys:• Solvent loss• Gas cooler

plugging• Heat integration• Capture

efficiency

New concept in development:

• Tar scrubbing by tar


Example: 3 MWth BFBG in Seville (Spain)


Catalytic cracking/reforming of tars


Performance of tar reforming


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


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


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 ?


FLETGAS at BEGUS: Flexible Three-Stage Gasifier

Biomass

1. BFB devolatilizer

3. Chemical quench

2. Non catalytic reformer


Fluid-dynamics: Cold model


• 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


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


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

mailto:[email protected]

http://www.grupobioenergia.com/

improving char and tar conversion in fluidized bed...

Documents