Transient behaviour of a fixed bed countercurrent...
Transcript of Transient behaviour of a fixed bed countercurrent...
Transient Transient behaviourbehaviourof a fixed bed countercurrent of a fixed bed countercurrent gasifiergasifier::
one dimensional one dimensional modellingmodelling
M. BrunduM. Brundu, G. Mura, G. Mura
UniversityUniversity ofof CagliariCagliariDepartmentDepartment ofof ChemicalChemical EngineeringEngineering
and and MaterialsMaterials
A brief introduction
COCO2
H2
CH4
O2
N2 H2
O
Gasification is the termochemical conversion of a solidfuel in a gas mixture
mainly composed byCO CO2
CH4
H2
Gasification reactors
Fluidized bedEntrained bedFixed (moving bed)
1. Updraft (countercurrent)2. Downdraft (cocurrent)3. Cross current
0
CO
CO2
CH4
TAR
H2
O
H2
O2
N2\
H2
O
The mathematical model
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Mass balance
Energy balance
Kinetic description
I
Bound and interstitial water is released whenever a wet fuel isexposed to high temperature
Drying
H2O
Istantaneous release of water at a fixed solid temperature isconsidered;A complete model of the water diffusion inside the particle isdeveloped;Drying is treated as a 1st order reaction depending on moisturecontent.
Moistrure
loss calculation
A kinetic approach is used to describe every reaction inserted in the model .
Heterogeneousness
of
the system is
partially
described
in the model
and most
part of
gas solid
reactions
have
a pseudo‐homogeneous
kinetic
descrption
Kinetic description
II
TAR
CO
CO2
CH4
H2
CHAR
CO
CO2
CH4
H2
CHAR
If
exposed
to
high temperature a solid
fuel
decomposes
to
a gas mixture
(CO, CO2, H2, CH4), a solid (CHAR) and a volatile product
(TAR).TAR, once formed, if
exposed to high temperature decomposesto yeld more gas and char
Pyrolysis
Global kineticmodels
Network models
FLASHCHAINFGDVCCPD
DAEMCRMSFORM
Global kineticmodels
Network models
FLASHCHAINFGDVCCPD
DAEMCRMSFORM
Kinetic description
III
Gas phase
chemistry
Char gasification
and combustion
Shrinking
core
reaction
model
External diffusion
Kinetic resistance
α C+O2 2(α-1) CO + (2-α) CO2
C + 2H2 CH4
C + CO2 2 CO
C + H2 O H2 + CO
H2O
CHAR
CO
CO2
CH4
H2
O2
CO+ 0.5O2 CO2
CH4 + 1.5O2 CO + 2H2 O
Secondary reactions of TAR
CO + H2 O H2 + CO2
Complex
homogeneous
kinetic
models
Other assumptions
Cylinder shape
Plug flow with variable axial velocities for gas and solid phases
Variation of solid and gas phase physical properties with temperature
Variation of void fraction of the bed during pyrolysis
Wall temperature of the gasifier is considered to be constant and equal to 100°C
Constant pressure along the bed (1 Atm)
Coal is considered free of sulfur compounds
Numerical method
The system of
24 differential
equations
is
solved
through
an
explicit
finite difference
method. To
avoid
stability
problems
and keep
a reasonableintegration
step
a fictious
diffusivity
and a fictious
diffusivity
of
the gas phase
is
introduced.
INPUT
Gas and solid
feed
properties
and
operative conditions
as
well
as
initial
conditions
inside the reactor
OUTPUT
Concentration
and temperature profiles
along
the reactor
for
both, the gas and
the solid
phase
for
different
time
steps
Input data
Weight0.3Steam‐solid feed ratio RVCWeight2.5Air‐solid feed ratio RAC
° C100Wall temperature° C287Steam feed temperature° C287Air feed temperature° C25Solid feed temperature‐0.3Initial void fraction
Kg/m31260Initial bed densitycm1.27Particle’s diameterKg/s0.005Coal feedAtm1Bed pressurem2Bed heightm0.5Inside diameter
14.7O1.S1.2N5.6H77.5C
Elemental analysis wt%35.1Volatile matter16.8Moisture42.4Fixed carbon5.7Ash
Proximate analysis wt%Initial
condition
Coal
properties
and operational
conditions
At
the
initial
state
the
reactor
is
filled
up
with
charcoal
and nitrogen. A linear temperature profile ranging between 900°C and 25°C is also considered along the bed. A step variation is suddenly introduced to gas and solid feed. Agas
flow
constituted
by
air
and
steam
and
a
coal
feed
is
fed respectively to the bottom and to the top of the reactor
1. Main case
2. Parametric analysis
Results
Main case
I
Main case
II
Parametric analysis
I
Gas flowrates
influence
Parametric analysis
II
Solid
feed
influence
Parametric analysis
Initial
bed
height
influence
Conclusion
A mathematical
model of an
updraft
(countercurrent) fixed
bed
gasifier
basedon microscopic
mass and heat
balances
has
been
developed
in this
work.
INPUT
Gas and solid
feed
properties
and
operative conditions
as
well
as
initial
conditions
inside the reactor
OUTPUT
Concentration
and temperature profiles
along
the reactor
for
both, the gas and
the solid
phase
for
different
time
steps
I
1.
An air to fuel ratio equal to 1.8, a steam to fuel ratio of 0.7, a coal feed
of
0.05 Kg/s
and an
initial
bed
height
of
2m is
taken
as
the main
case under study.
2.
For
these
values
of
the operating
parameters
after
80000s the solid
compositionalong
the bed
hasn’t still
reached
a steady state condition
while
gas phasecomposition
reaches
the steady state condition
faster.
Conclusion
3.
The
reversible
CO shift
reaction
is
found
to
govern
the composition
of
the gas phase
.
4.
Because
of
a slow heating
by
the gas phase, solid
temperature in the upper part of
the gasifier
is
low and the reactor
is
driven
to
a pyrolysis
and combustion
system.
5.
The assumption
made
for
void
fraction
was
also
found
to
be
in accordancewith
experimental
data found
in literature.
6. A cooling down of reactor was observed for some parameter combination.
7.
The parametric
analysis
doesn’t show any
relevant
difference
on the transient
behaviour
of
reactor
for
a change
in steam
to
fuel
ratio, coal
flow rate and initial
bed
height
in the range
studied
II
Transient Transient behaviourbehaviourof a fixed bed countercurrent of a fixed bed countercurrent gasifiergasifier::
one dimensional one dimensional modellingmodelling
ThankThank youyou
M. BrunduM. Brundu, G. Mura, G. Mura