2012

101 11 25~27 Paper number: X00-002

1

Pressurized Fluidized Bed Combustion Concept Design and the Cold Model

Numerical Simulation Zu-Zen Chu C.P. Chen Chien-Hui Shen Heng Lin

2nd

Division Chung Shun Institute of Science and Technology

E-mail:zuzenchu@gmail.com

NSC Project No: NSC 101-3113-P-008-002

Abstract

This paper presents a 10 atmospheric pressure and

1.0MW fluidized bed combustion boiler and proceeds to

the cold model numerical simulation to verify the

conceptual design is suitable. The results will provide a

basic blueprint for the future demonstration PFBC

boiler. Key word: PFBC,Two phase Flow

1. Introduction

The pressurized fluidized bed combustion (PFBC)

comes from traditional fluidized bed technology. The

major concept of PFBC is to increase the boilers

pressure from ambient pressure to 0.6Mpa~1.6Mpa.

Because the pressurized chamber was adapted, the

efficiency of combustion was enhanced and the size of

the boiler can be largely reduced. The most important

achievement is that it can reduce the emission products

significantly. Carbon dioxide becomes the main exhaust

gas. The concentration of carbon dioxide can reach

around 95%. If we can store it in underground,

environment will suffer with no pollution impact.

Beside this, it can increase the power generation

efficiency from 3% to 4%. This is a worthy technology

for generating electricity of the future.

2. The Concept Design of PFBC 2.1. The Design Requirements

The bed material for the PFBC includes bituminous

coal, water, limestone and round sand. While in

operation, the bed temperature needs to keep under

900 by using the immersed tube to extract the heat

flux .The sieve diameter of the coal particle is chosen as

two mm. Contains of coal are listed in below:

Car Har Oar Sar Nar Mar

68.8% 4.5% 9.1% 0.56% 1.63% 2.6%

The feeding system of the bed material contains

coal silo and screw feeder. Before enter the furnace and become coal water slurry, limestone and water need to

be added to the coal. The required coal for the

combustion is 155 kg per hour. The required water for

the coal water slurry is 31 kg per hour. The required

limestone for the combustion is 10.84 kg per hour which

was used as desulfurization.

When operating in 10 atmospheric pressure and

273oK condition, theoretical calculation of air volume

required for burning one kilogram coal v0 is

ARAR

ARar

OS

HCV

00333.000333.0

0265.000889.00 (1)

Substitute CarHarSarOar into equation (1), we

can get air required V0 0.735 m

3/kg or 110.25 m

3 per

hour. Transfer to the oxygen requirement, burning one

kilogram coal needs 23.11m3/kg or 332.8 kilogram.

Due to 10% oxygen-enriched combustion is used,

practical oxygen required needs to be modified to

366.087 kilogram per hour. The theoretical flue gas Vy can be derived as

22

0

22 ObOHNROy VVVVVV

ararRO SCV 007.001866.02

arN NV 008.02

079.0 VVb

0

2 021.0 VVO

OHararOH MHV 22 /2.00124.0111.0 (2)

In equation (2), VRO2 is volume of tri-molecular

gas. Vb is the recirculation flue gas volume which will

take place the role of nitrogen in combustion. VO2 is the

residual of oxygen when the burn is finished. When

burn one kilogram coal, the final volume for flue gas are

VRO2 VN2 Vb VH2O VO2

0.1288 0.0013 0.5512 0.0813 0.0149

The total flue gas volume Vy is around 0.7597 m3/kg.

The recirculation flue gas ratio is defined asVy divide by Vb. From this definition, the recirculation flue

gas ratio is 72.56%The final emission of the waste gas

contains as

VCO2 VN2 VH2O VO2

92.095% 0.181% 5.650% 2.073%

2.2. Critical Velocity Calculation

In operating the PFBC, the gas velocity usually has to reach a certain value such that bed material can

switch from fixed bed to fluidized bed .This velocity is

called critical velocity umf. The calculation of critical

velocity usually depends on approximate solution or

through experimental method. The formulation is

derived as:

pmf

g

gpp

mf

gdu 3

75.1

)(

(3)

Critical Reynolds number Remf is defined as

2012

101 11 25~27 Paper number: X00-002

2

g

gmfp

mf

udRe (4)

Archimedes number Ar is defined as

2

3 )(

g

gpgp

r

gdA (5)

The input parameters include sphericity p 0.581,

the apparent density p 2594 kg/m3, tapped b

1350 kg/m3 and the critical porosity mf 0.48. The

average diameter dp of ground sand is set to 1 mm.

Under ten atmospheric pressure and 900 furnace

temperature, the characteristic of the furnace are

Remf Ar umf

42.808 50044.304 0.467 m/s

To make sure the furnace get a fully fluidized

situation, a fluidization velocity u0 was used which

usually assumes 2 or 3 times of the critical velocity. In

our design, the fluidization velocity u0 was set to 1m/s.

It can guarantee us that a fully developed condition

in our furnace. For the reason to understand the fluidized

phenomenon, the Technical Institute of Swiss Union in

1971, have proposed a fluidization regime map, shown

in fig.4. From the Reynolds number Re and drag

coefficient 1/Ctwhich is calculated approximately

equals to 0.126, we can figure out our PFBC furnace

probably falls into a bubbling bed zone.

2.3. Furnace Configuration Design Due to the fluidized bed is located in the bubbling

beds. It can be divided by two clear types of zone. The

lower one is called dense phase and the upper one is

called the dilute phase. In the dense phase zone, a

rectangular shape was designed with conical opening in

axial direction which can increase the fluidized velocity

and reduce the sintering. In the dilute phase zone, a

straight rectangular shape was adapted. In the last

section of the furnace, a divergent section is used to

speed up the flue gas. Hoping that can offer a sufficient

velocity for the particles while entering the cyclone

separator.

The sizing of the furnace is decided by(1)the mass

flow rate under the air distributor 0.502314 kg/m3

(2)The total mass flow rate in the furnace 0.55699

kg/m3 (3)coal particle burning time 1.5 second

(4)furnace cross-section heat release rate 1.2996 MW.

From above values, we can calculate the width of lower

dense phase zone is 26.7 cm. In actual design, a value of

30 cm was taken. The width of the upper dilute zone is

54.8 cm. In actual design, a value of 57 cm was taken. If

we assume the furnace cross-section heat release rate Qs

equals to 4 MW/m2 and the lower heating value (LHV)

Qnet,ar is 27017.65 KJ/kg. The height of dense zone can

be evaluated as 1.2573 m. The volume of bed material

required is 0.3 m3. Consider the capacity of immersed

tube and the future expanding ability, the height of

dense zone assumes 1.8 m. The height of dilute zone

usually depends on the value of coal particle burning

time. For the time being, the height of dilute zone is

temporarily set 1.5 m. We will modify this to a proper

value when finishing the hot model CFD simulation. In

order to increase the flue gas velocity from 0.43 m/s to

25 m/s before entering the cyclone separator, the last

part of the furnace is using a converge section. The bent

rectangular tube which connects furnace and cyclone

separator is designed with 99cm rectangular

cross-section.

Outer housing of the furnace adopts two different

layers design. The outer shell uses a round shape

stainless steel to resist the high pressure. The inner shell

use rectangular shape refractory bricks to resist the high

temperature. The outer diameter is set to 2 meter and the

thick of refractory brick is set to 32 cm. The detailed

furnace design is shown in fig.1.

The bed pressure drop is another key parameter

which can be calculated as

gHp gpbb )()1(0 (6)

In equation (6), H0 is the height of moving packed bed.

b is the moving bed porosity which is set to 0.66667. g

is the gas density which equals to 4.266038427 kg/m3.

Finally we get the bed pressure drop 10643.77 pa.

2.4. Gas Distributor Design The Basic concept design of the gas distributor

requires:(1)uniform flow avoids choked area(2)strong

dynamic energy in the exit of nozzle in order to mix up

the bed material(3)proper pressure drop dont waste

too much pressure head(4)prevents the deformation of

gas distributor due to the heating effect(5)prevents the

nozzle from plugging and easy to clean the fouling.

From volume rate below the air distributor and

gas temperature at 117,the total area for the orificef

can be calculated.

273

273

3600

0

0t

u

VBf

or

j (7)

Where Bj is the coal consumption per hour. The

orifice velocity uor is usually assigned around 35m/s at

ambient pressure condition. We take the value of 38 m/s

then modifies to ten atmospheric pressure criteria.

P

uu oror

(8)

From equation (8), uor equals to 12.02m/s. The

total area for the orifice f from equation (8) is

0.003717081 m2. The number of nozzles needed for the

gas distributor can be calculated by following equation:

2

4

ordn

fm

(9)

In equation (9), dor is the diameter of orifice, set to

5 mm. n is the number of orifice for each nozzle, set to

8. From equation (9), the required nozzles are twenty

four. The open area ratio is a key parameter for the

nozzle design which was defined as the gas distributor

area Ab divided by total area of the orifice f.

2012

101 11 25~27 Paper number: X00-002

3

%1004

%1002

b

or

b A

dnm

A

f (10)

Where the gas distributor area Ab is 0.09 m2. The open

area ratiofrom equation (10) is 4.2% which satisfies

the design required criteria from 4% to 8% .

Summarizing the above results, 24 cap type

nozzles were decided to use for the 3030 cm

bowl-shaped gas distributor. Each nozzle contains

eight orifices. In the center of the gas distributor, a 50

mm diameter ash hopper was used to get rid of the ash.

The final furnace sizing is shown in figure2 and

figure3.

The distributor pressure drop is another key

parameter. For a proper design, the ratio of distributor

pressure drop over the bed pressure drop, called

which is required 0.4 to 0.5. The way to calculate the

distributor pressure drop is as following:

2

2

org

bf

up (11)

Where is the drag coefficient equals to 5.0. The

parameter g is the gas density 12.6377 kg/m3. The

parameter uor is the orifice velocity 12.02m/s. The

pressure calculated in equation (11) is 4562.227pa. The

ratio of distributor pressure drop over the bed pressure

drop is calculated as 0.43 which can satisfy the design

requirement.

3. Cold Flow Numerical Simulation 3.1. Numerical Modeling The most commonly used numerical scheme for

the two phase flow is Eulerian-Eulerian method, which

treats the gas and particles as continue mediums. They

both exist and penetrate to each other and the gas and

particles are coupled through the drag force. A

conservation form can be applied to describe this

phenomenon. This method is regarded as the most

advance model in today.

3.2. Governing Equations The governing equation for Two- phase flow is as

following

ijiii

ii mvt

)()( (12)

1sg (13 )

Where i represents the porosity of a small

volume. The index i and j represent different phase.

The sub-index g means gas phase and the s means

solid phase. mi,j represents the chemical reaction and

mass interchange. The conservation form for the gas

phase and particle phase are as following:

The stress-strain tensor is shown below:

)3

2()( iiii

T

iiiii vvv (16)

In equation(16), i i are the shear stress and bulk

viscosity respectively for i phase. iFis the external

forces which include lift force iliftF , ,virtual force ivmF , ,

and pressure force.jiv , is the relative velocity which is

defined as:

If mi,j>0(mass from phase i to phase j) jiv , = iv

If mi,j

2012

101 11 25~27 Paper number: X00-002

4

Where srv , is defined as:

))2(Re12.0)Re06.0(

Re06.0(5.0

22

,

AAB

Av

ss

ssr (27)

The coefficients of A and B are defined as: 28.114.4 8.0, gg BA for 85.0g

65.214.4 , gg BA for 85.0g

g

gssg

s

vvdRe

In our numerical analysis, we adopt the

commerical software Fluent to simulate the cold model

of two phase flow under the Eulaerian coordinate. A real

three-dimensional fluidized bed, which includes the

complicated gas distributor, was used to simulate the

fluidized particle moving behavior, velocity distribution

and pressure drop of the bed material under ten

atmospheric pressure.

3.3. 3D Cold Model Flow Simulation

In this chapter a real three-dimensional, unsteady,

two phase flow simulation was conducted. The real

fluidized bed boiler includes gas distributor and 24 hat

type nozzles. Each nozzle is opened with 8 orifices

around it. The grid point of the numerical analysis is

about 1,000,000. The pressure is set to 10 atmospheric

and the temperature is set to 600oK inside the boiler.

The velocity at the entrance of the gas distributor is set

to 0.8 and 1.3m/s which equals to the mass flow rate

0.36 and 0.59 kg/s respectively. The results of three-dimensional simulation are

shown in fig.5 to fig.9. The velocity distribution near

the orifice for the case of bed height 50cm and mass

flow rate 0.59kg/s at time 6.8 second is shown in Fig.5.

The result shows that the average velocity is around 10

to 19 m/s which are slightly higher than the design value

12.02 m/s. We take a further observation into the flow

field where a uniform flow without any choking

condition can be observed. The solid volume fraction

distribution varied with time for the case of bed height

50cm and mass flow rate 0.59kg/s is shown in Fig.6.

The red zone shows a higher concentration of solid

volume. In here, a rising bubble formation can be

watched at time 0.8 second. The bubble keeps on

growing and rising up until 1.2 second finally breakup

at the surface. When time came to 4 seconds, the solid

volume distribution becomes very uniform. The red

zone begins to shrink into a very small area near the gas

distributor. The solid volume fraction distribution varied

with time for the case of bed height 50cm and mass

flow rate 0.36kg/s, corresponding to a fluidized velocity

0.8m/s, is shown in Fig.7. Because the lower fluidized

velocity, the flow field is not uniform and the red zone

of solid volume distribution is larger than the previous

case. The solid volume fraction distribution varied with

time for the case of bed height 75cm and mass flow rate

0.59kg/s, corresponding to a fluidized velocity 1.3m/s,

is shown in Fig.8. A rising bubble formation can be

watched at time 0.8 second. The bubble breaks up at

time 1.6 second. When time came to 4 seconds, large

concentrated solid volume fraction can be observed. The

solid volume fraction distribution varied with time for

the case of bed height 75cm and mass flow rate 0.36kg/s

(fluidized velocity 0.8m/s) is shown in Fig.9. Because

of the higher bed material and lower fluidized velocity

the solid volume fraction becomes very non-uniform.

The red zone was enlarged.

Rearrange the numerical results and make a list

for the pressure drop and expansion ration.

Height

of

bed(cm)

m

(kg/s)

Pressure

Drop

(pascal)

Expansion

Ratio

Case1 50 0.59 9920 2.29

Caae2 50 0.36 8450 1.72

Case3 75 0.59 12640 2.04

Case4 75 0.36 11500 1.43

4. Conclusion Accomplish the 1MW10 atmospheric pressure

pressurized fluidized bed combustion concept design

and the cold model numerical simulation. The numerical

results show the bed material can be in a fully fluidized

condition and match with the design requirements. In

next year a cold model experiment will be conducted in

CSIST in order to further verify the proper design for

the furnace.

5. Acknowledgements NSC Project No.SC101-3113-P-008-002. Thanks

for the support from the vice director of 2nd

division

CSIST and my colleagues in the aerodynamics group.

6. References 1. Dong-Fang Li The Numerical simulation of two

phase flow in CFBC China Petroleum University

thesis S0604521, 2009.

2. Chun-Mei Lu, Shi-Qing Cheng, Yong-Zheng

Wang, Kui-Hua Han, Jian-Li Zhao The

Construction and Operation of CFBC China

Electric Power Press.

3. Hsien-pin Sun, Chung Huang The Apply of

Large Scale CFBC Technology in Industry China

Electric Power Press.

4. Chien-Sung Tsien The Technology of FBC Kau

Li Press, Taiwan.

5. Fluent Inc. ANSYSFLUENT12.1 Users Guide .

Fluent Inc, 2011.

6. Taghipour, F. Experiment and computational

study of gas-solid fluidized bed hydrodynamics,

chemical engineer science 60, 2005.

2012

101 11 25~27 Paper number: X00-002

5

Fig. 1 1MW10 ATM fluidized bed concept design

Fig. 5 The velocity distribution near the nozzle

Fig. 2 Configuration of gas distributor

Fig. 3 Configuration of gas nozzle

Fig. 4fluidization regime map

Fig. 6 The solid volume fraction varies with time(Bed

height=50 cmm=0.59 kg/sV= 1.3 m/s)

1.6 2.0 2.4 3.2 sec

0 0.4 0.8 1.2 sec

4.0 4.8 5.6 6.4 sec

2012

101 11 25~27 Paper number: X00-002

6

Fig. 9 The solid volume fraction varies with

time(Bed height=75 cmm=0.36 kg/sV= 0.8 m/s)

0 0.4 0.8 1.2 sec

4.0 4.8 5.6 6.4 sec

1.6 2.0 2.4 3.2 sec

1.6 2.0 2.4 3.2 sec

Fig. 8 The solid volume fraction varies with time(Bed

height=75 cmm=0.59 kg/sV= 1.3 m/s)

0 0.4 0.8 1.2 sec

4.0 4.8 5.6 6.4 sec

Fig. 7 The solid volume fraction varies with

time(Bed height=50 cmm=0.36 kg/sV= 0.8 m/s)

0 0.4 0.8 1.2 sec

1.6 2.0 2.4 3.2 sec

4.0 4.8 5.6 6.4 sec

5.6 6.4

1

1

Email:zuzenchu@gmail.com

NSC101-3113-P-008-002

1.0MW