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