F. Scala - processEng

C.N.R.
Istituto di Ricerche sulla Combustione
ATTRITION
ATTRITION OF
OF LIGNITE
LIGNITE CHAR
CHAR
DURING
DURING FLUIDIZED
FLUIDIZED BED
BED
GASIFICATION:
GASIFICATION: EXPERIMENTAL
EXPERIMENTAL
AND
AND MODELING
MODELING STUDIES
STUDIES
Paola Ammendola and Fabrizio Scala
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Introduction
Introduction
 The
Theurgent
urgentneed
needtotocapture
captureand
andsequester
sequesterCO
CO22ininparticular
particularemitted
emittedfrom
from
the
the use
use of
of coal
coal has
has triggered
triggered the
the development
development of
of new
new processes
processes like
like
chemical
chemicallooping
loopingcombustion
combustionand
andsorption
sorptionenhanced
enhancedgasification.
gasification.
 Both
Both these
these processes
processes require
require that
that fuel
fuel gasification
gasification isis carried
carried out
out atat aa
relatively
relativelylow
lowtemperature,
temperature,limiting
limitingthe
thetypes
typesof
offuel
fuelthat
thatcould
couldbe
beused
usedtoto
those
thosewith
withhighly
highlyreactive
reactivechars,
chars,such
suchas
aslow-rank
low-rankcoals
coalsand
andbiomass.
biomass.
 Lignite
Lignite isis an
an abundant
abundant fuel
fuel and
and makes
makes up
up approximately
approximately 40%
40% of
of coal
coal
reserves
reserves inin the
the world.
world. Its
Its commercial
commercial use
use isis forecast
forecast toto grow
grow
substantially.
substantially.
 However,
However,lignite
ligniteusually
usuallyhas
hashigh
highwater
waterand
andoxygen
oxygencontent,
content,low
lowenergy
energy
density,
density, and
and itit isis expensive
expensive toto transport.
transport. All
All these
these characteristics
characteristics have
have
limited
limitedits
itsutilization.
utilization.
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Introduction
Introduction
 Fluidized
Fluidized bed
bed (FB)
(FB) gasification
gasification isis acknowledged
acknowledged toto have
have great
great flexibility
flexibility inin
conversion
conversionofofseveral
severalsolid
solidfuels,
fuels,including
includinglow
lowrank
rankcoals,
coals,into
intosynthesis
synthesisgas.
gas.
 Fuel
Fuel attrition
attrition and
and fragmentation
fragmentation phenomena
phenomena are
are well
well known
known toto affect
affect the
the
reliability
reliabilityand
andefficiency
efficiencyofofFB
FBcombustion
combustionand
andgasification
gasificationprocesses.
processes.
 Attrition/fragmentation
Attrition/fragmentation cause
cause the
the elutriation
elutriation ofof fine
fine material
material from
from the
the bed
bed that
that
results
resultsininthe
theloss
lossofofunconverted
unconvertedcarbon.
carbon.
 Attrition/fragmentation
Attrition/fragmentation may
may also
also significantly
significantly change
change the
the particle
particle size
size
distribution
distributionofofthe
thematerials
materialsininthe
thebed
bedwhich
whichinfluences
influencesthe
thebed
bedfluid-dynamics,
fluid-dynamics,
heat
heatand
andmass
masstransfer
transfercoefficients
coefficientsand
andreaction
reactionrates.
rates.
 Several
Severalattrition
attritionstudies
studiesare
arereported
reportedunder
underFB
FBcombustion
combustionconditions,
conditions,but
butonly
only
limited
limitedactivity
activityisisreported
reportedunder
undergasification
gasificationconditions,
conditions,and
andno
nostudy
studycould
couldbe
be
found
foundininthe
theliterature
literatureon
onattrition
attritionofoflignite
ligniteduring
duringgasification.
gasification.
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Introduction
Introduction
The
The authors
authors started
started aa study
study on
on attrition
attrition ofof lignite
lignite char,
char, pointing
pointing out
out that
that
fragmentation
and
attrition
of
the
char
particles
during
gasification
were
significant,
fragmentation and attrition of the char particles during gasification were significant,
and
andsuggested
suggestedaagasification-assisted
gasification-assistedattrition
attritionenhancement
enhancementeffect.
effect.
This
This mechanism,
mechanism, associated
associated toto the
the low
low reactivity
reactivity ofof the
the generated
generated fines,
fines, made
made the
the
loss
lossofofcarbon
carbonby
byfines
fineselutriation
elutriationduring
duringchar
chargasification
gasificationmore
moresignificant
significantthan
thanthat
that
typically
typicallyfound
foundunder
undercombustion
combustionconditions.
conditions.
t = gasification time, XC = carbon conversion degree
T= 850 °C, U= 0.4 m/s, Gas = 100% CO2
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Scope
Scope of
of the
the work
work
 In
In this
this work
work the
the attrition
attrition behaviour
behaviour of
of char
char from
from an
an Italian
Italian lignite
lignite (Sulcis)
(Sulcis) was
was
studied
gasificationconditions
conditionsininaalab-scale
lab-scaleFB
FBapparatus.
apparatus.
studiedunder
underCO
CO22gasification
 In
In particular,
particular, attrition
attrition was
was characterized
characterized as
as aa function
function of
of the
the char
char conversion
conversion
degree.
degree.
 The
Theinfluence
influenceof
ofbed
bedtemperature
temperatureand
andCO
CO22concentration
concentrationon
onattrition
attritionand
andcarbon
carbon
conversion
conversionwas
wasinvestigated.
investigated.
 On
On the
the basis
basis of
of the
the experimental
experimental results,
results, aa kinetic
kinetic model
model of
of lignite
lignite char
char under
under
CO
gasificationconditions
conditionswas
wasdeveloped
developedininorder
ordertotopredict
predictthe
thetime
timeresolved
resolved
CO22gasification
profiles
profilesof
ofcarbon
carbonconversion.
conversion.
 The
The mechanical
mechanical resistance
resistance of
of the
the char
char particles
particles was
was also
also characterized
characterized atat
different
differentstages
stagesof
ofchar
charconversion
conversionby
byspecific
specificattrition
attritionexperiments.
experiments.
 AA semi-empirical
semi-empirical model
model was
was finally
finally developed
developed toto simulate
simulate the
the experimental
experimental
profiles
profilesof
ofcarbon
carbonelutriation
elutriationrate
rateas
asaafunction
functionof
oftime.
time.
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Two-exit head FB
Experimental
Experimental
Sulcis lignite properties
ID = 40 mm
T= 800-900°C
U= 0.4 m/s
CO2 = 20-100%
dfuel = 6.3-8.0 mm
Wsand = 180 g
dsand = 0.3-0.4 mm
1) gas preheating section; 2) electrical furnaces; 3)
ceramic insulator; 4) gas distributor; 5)
thermocouple; 6) fluidization column; 7) head with
three-way valve; 8) sintered brass filters; 9) hopper;
10) SO2 scrubber; 11) stack; 12) cellulose filter; 13)
membrane pump; 14) gas analyzers; 15) personal
computer; 16) manometer; 17) digital mass
flowmeters ; 18) air dehumidifier (silica gel).
Proximate analysis, % (as
received)
Moisture 6.3
Ash 16.9
Volatile Matter 49.7
Fixed Carbon 27.1
Ultimate analysis, % (dry
and ash free basis)
LHV, kcal/kg
Carbon 71.7
Hydrogen 5.7
Nitrogen 1.8
Oxygen 13.1
Sulphur 7.7
5137
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Char
Char attrition
attrition tests
tests results
results –– temperature
temperature effect
effect
Elutriation rate
Carbon conversion
0.0010
1.0
A
B
0.8
900°C
850°C
800°C
0.0006
U= 0.4 m/s
Wbatch = 1 g
Gas = 100% CO2
0.6
Xc, -
EC/WC0, min-1
0.0008
0.0004
0.4
900°C
850°C
800°C
0.0002
0.2
0.0000
0
100
200
300
400
500
0.0
0
Time, min
100
200
300
400
500
Time, min
Operating Temperature
Gasification time
Gasified carbon
Elutriated carbon
Unconverted
(min)
(%)
(%)
carbon (%)
T = 800°C
480
86.4
8.9
4.7
T = 850°C
240
91.4
4.8
3.8
T = 900°C
160
96.2
3.4
0.4
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Char
Char attrition
attrition tests
tests results
results –– CO
CO22 effect
effect
Elutriation rate
Carbon conversion
0.0010
1.0
A
B
0.0008
0.8
0.0006
T= 900°C
U= 0.4 m/s
Wbatch = 1 g
Gas = 20-100%
CO2 in N2
0.6
XC, -
EC/WC0, min
-1
20%
50%
100%
0.0004
0.4
20%
50%
100%
0.0002
0.2
0.0000
0
50
100
150
200
250
300
0.0
0
Time, min
50
100
150
200
250
300
Time, min
CO2 inlet concentration (in N2)
Gasification time
Gasified carbon
Elutriated carbon
Unconverted carbon
(min)
(%)
(%)
(%)
20%
300
85.4
10.0
4.6
50%
200
91.8
5.2
3.0
100%
160
96.2
3.4
0.4
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Kinetic
Kinetic model
model
 Char gasification in the present operating conditions is controlled by intrinsic kinetics.
 The concentration of produced CO is negligible (< 1% of CO2 concentration).
dX C
 A  X C   k 0  exp   E a / RT   CCO2

dt
C
CO 2

 

A  XC   a  b  X C  c  XC2
a (-)
b (-)
c (-)
k0 (s-1)
Ea/R (K)
 (-)
0.964
0.460
1.42
1.53·105
22300
0.625
U= 0.4 m/s, Gas = 100% CO2
0,0006
T= 900 °C, U= 0.4 m/s
0,0006
900°
850°
800°
0,0005
0,0004
dXC/dt, s
-1
-1
0,0004
dXC/dt, s
100%
50%
20%
0,0005
0,0003
0,0003
0,0002
0,0002
0,0001
0,0001
0,0000
0,0000
0,0
0,2
0,4
0,6
XC, -
0,8
1,0
0,0
0,2
0,4
0,6
0,8
1,0
XC, -
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Kinetic
Kinetic model
model -- results
results
0,0006
U= 0.4 m/s, Gas = 100% CO2
T= 900 °C, U= 0.4 m/s
0,0006
900°
850°
800°
900° modello
850° modello
800° modello
0,0005
0,0004
dXC/dt, s
dXC/dt, s
-1
-1
0,0004
100%
50%
20%
100% modello
50% modello
20% modello
0,0005
0,0003
0,0003
0,0002
0,0002
0,0001
0,0001
0,0000
0,0000
0,0
0,2
0,4
0,6
XC, -
0,8
1,0
0,0
0,2
0,4
0,6
0,8
1,0
XC, -
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Elutriation
Elutriation model
model -- 11
E C  k   U  U mf    WC d 
Attrition rate
WC  WC0  1  X C 
Carbon mass
In order to understand the relationship between the char attrition constant (k) and carbon
conversion (XC), attrition experiments were carried out under inert conditions (100% N2)
using char samples pre-gasified for different times (ranging from 10 to 50 min).
1.6e-5
Experimental data
Fitting curve
1.4e-5
1.2e-5
1.0e-5
k(xC), -
Initial
values
k  1.5 106  exp  2.31 X C 
8.0e-6
6.0e-6
4.0e-6
2.0e-6
b
0.0
0.0
0.2
0.4
0.6
0.8
1.0
X C, -
Pre-conversion of the char significantly enhances the mechanical attrition of the particles.
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Char particle size
EC/WC0, min-1
 Only limited secondary fragmentation occurs at low
carbon conversions, while significant shattering is
evident at conversions larger that 50%.
0.0012
a
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0
20
40
60
80
100
120
140
160
180
0.0012
EC/WC0, min-1
Elutriation
Elutriation model
model -- 22
b
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0
d  XC   d 0  3 1 n  XC 
50
100
150
200
250
EC/WC0, min-1
0.0012
Number of char particles
0.0008
0.0006
0.0004
0.0002
0.0000
*
C
XC* = 0.5
nmax = fitting parameter
Comparison between experimental and model time
resolved carbon elutriation rates obtained during CO2
gasification experiments at different operating conditions
EC/WC0, min-1
 1  X 
0
100
200
300
400
500
600
0.0012
d
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0
50
100
150
200
250
0.0012
EC/WC0, min-1

n  X C   1   n max  1   X C  X*C
c
0.0010
e
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0
50
100
150
200
250
300
350
Time, min
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Conclusions
Conclusions -- 11
 AA gasification-assisted
gasification-assisted attrition
attrition mechanism
mechanism isis proposed.
proposed. Internal
Internal reaction
reaction progressively
progressively
weakens
the
structure
inside
the
particle,
enhancing
particle
attrition.
weakens the structure inside the particle, enhancing particle attrition.
 The
Thelow
lowreactivity
reactivityofofthe
thegenerated
generatedfines
finesunder
undergasification
gasificationconditions
conditionsmakes
makesthe
theloss
lossofof
carbon
carbonby
byfines
fineselutriation
elutriationsignificant.
significant.
 The
Thetotal
totalelutriated
elutriatedcarbon
carbonincreased
increasedwhen
whenthe
thebed
bedtemperature
temperatureisisdecreased.
decreased.Two
Tworeasons
reasons
are
are likely
likely toto determine
determine this
this behaviour:
behaviour: i)i) atat lower
lower temperatures
temperatures the
the residence
residence time
time for
for
complete
complete gasification
gasification increases;
increases; ii)
ii) the
the extent
extent ofof in-bed
in-bed conversion
conversion ofof the
the attrited
attrited fines
fines
increases
increaseswith
withthe
thebed
bedtemperature.
temperature.
 The
Thetotal
totalelutriated
elutriatedcarbon
carbonincreased
increasedwhen
whenthe
theCO
CO2 2concentration
concentrationwas
wasdecreased.
decreased.At
Atlower
lower
concentrations
the
residence
time
of
particles
in
the
bed
increases
and
the
extent
CO
CO2 2 concentrations the residence time of particles in the bed increases and the extentofof
in-bed
in-bedconversion
conversionofofthe
theattrited
attritedfines
finesdecreases.
decreases.
 On
Onthe
thewhole,
whole,the
thecarbon
carbonloss
lossby
byelutriation
elutriationisiscertainly
certainlyone
oneofofthe
thecritical
criticalfactors
factorsduring
during
the
gasification
process,
especially
at
low
gasification
rates
(i.e.
at
low
CO
concentration
the gasification process, especially at low gasification rates (i.e. at low CO2 2 concentration
and
andbed
bedtemperature).
temperature).
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012
C.N.R.
Istituto di Ricerche sulla Combustione
Conclusions
Conclusions -- 22
 AAchar
chargasification
gasificationkinetic
kineticmodel
modelwas
wasproposed
proposedwhich
whichwas
wasable
abletotocorrectly
correctlypredict
predict
the
thecarbon
carbonconversion
conversionasasaafunction
functionofoftime
timefound
foundininthe
theexperiments,
experiments,including
includingthe
the
effect
effectofoftemperature
temperatureand
andCO
CO22concentration
concentrationon
oncarbon
carbonconversion.
conversion.
 AA semi-empirical
semi-empirical carbon
carbon elutriation
elutriation model
model was
was also
also presented,
presented, where
where the
the char
char
attrition
attritionconstant
constantwas
wasexpressed
expressedasasaafunction
functionofofthe
thecarbon
carbonconversion
conversiondegree.
degree.
 The
The change
change ofof the
the average
average char
char particle
particle size
size due
due toto secondary
secondary fragmentation
fragmentation was
was
taken
takenininaccount.
account.
 Model
Model results
results were
were inin good
good agreement
agreement with
with the
the curves
curves obtained
obtained from
from the
the
experimental
experimentaltests.
tests.
 Particle
Particlefragmentation
fragmentationappears
appearstotobe
beaakey
keyphenomenon
phenomenoninindetermining
determiningthe
theattrition
attrition
rate
rateofofthe
thechar
charininthe
thebed.
bed.
 Nevertheless,
Nevertheless, the
the presence
presence ofof aa fitting
fitting parameter,
parameter, i.e.,
i.e., the
the maximum
maximum number
number ofof
generated
generatedfragments,
fragments,implies
impliesthat
thatthe
themodel
modelisisnot
notyet
yetpredictive
predictiveatatthis
thisstage.
stage.
Attrition of lignite char during fluidized bed gasification: experimental and modeling studies
Paola Ammendola, Fabrizio Scala
th
th
65 IEA-FBC Meeting – 18 Symposium on Fluidization and Particle Processing – Osaka – Japan – 8-9.11.2012