A Mathematical Model of Biomass Briquette Fuel Combustion

doi:10.4236/epe.2013.54B001

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Energy and Power E ngineering, 2013, 5, 1-5

doi:10.4236/epe.2013.54B001 Published Online July 2013 (http://www.scirp .o rg/journal/epe)

A Mathematical Model of Biomass Briquette Fuel

Combustion

Jingxia Sui1, Xiang Xu1,2, Bo Zhang1*, Changjiang Huang2, Jinsheng Lv1

1School of Energy and Power, Dalian university of technology, Dalian, China

2Wen Zhou Speci al Equipment Inspection Center, Wenzhou, China

Email: *[email protected]

Received March, 2013

ABSTRACT

The numerical simulation model is proposed according to the characteristics of the biomass briquette fuels, which in-

volves two main areas o f i nterest: t he solid combustion mod e l in the bed and the out-of-bed gas c o mb ust ion mod el . The

contents and characteristics of 3 kinds of biomass and coals were experimentally tested. The biomass fuels compared

with the coal fuel have the following characteristics: 1) Higher volatile content, lower fixe d c ar bo n co nte nt and calorific

value; 2) Lower carbon content, higher oxygen content; 3) Lower ignition temperature, faster burning velocity. The

discredited equations were established by the finite element analysis method, which analyzed the fuel endothermic

process on the grate, while the out-of-bed gas combustion process was simulated by CFD. These two processes are

strongly coupled . The results of the numerical simula tion contain the stead y state temper ature distribution, oxyge n dis-

tribution, carb on dioxide distr ibution and so on, which are used to judge bur ning e ffec t and p rovide the correct furnace

transformat i on me th o d.

Keywords: Biomass; CFD; Simulation; Boiler; Combustion

1. Introduction

At present, the outbreak of the global energy crisis is

making energy situation more serious in the world. M.

Fatih Demirbas [1] pointed out that the biomass fuel is a

renewable energy source and its importance would in-

crease along with the national energy policy, focusing

more on renewable sources. China is rich in bio mass but

its utilization is still lacking, causing a large amount of

energy wasted. If these fuels could be effectively and

efficientl y used, China and even the world will benefit on

the energy supplyin g and environmental prote c tio n.

Using the coal-fired boilers to burn the biomass fuels

is an ineffective method of utilization, according to pre-

viou s studie s, whic h invol ved the follo wing phe no menon,

such as the combustion instability, heat congregation

before furnace, the flame into fuel damper to ignite the

fuels in the hopper, black smoke causing from bad air

distribution, low thermal efficiency and poor environ-

mental benefits. Therefore, careful study on biomass

furnace design based upon the coal-fired’s becomes im-

portant. The topic is to improve the biomass fuels com-

bustion efficiency. Guo Y [2] pointed out that the coal-

fired boilers have to be improved to adapt to biomass

fuel, because so many differences exist between the bio-

mass and the coal such as the fuel properties, the com-

bustion phenomenon, the coking properties and the most

suitable furnace structure.

Currently, many domestic and oversea experts are

working on improving the biomass briquette combustion

efficiency. Liang YD [3] pointed out that reducing the

grate length, changing the front arch and the rear arch

structures and setting the dust barrier wall are necessary

way s t o transform the existing coal-fired bo iler . Li n P [4]

studied the combustion characteristics of different bio-

mass fuels, analyzed the main influence factors on com-

bustion (fuel characteristics, shapes and sizes, and air

distribution, etc.). Wan g L [5] inve sti gated o n the coking

problem in bio mass c ombustio n process, and discussed a

variety of measures to reduce the ash. The main research

methods on biomass combustion are experiments and

simulations. Experimental studies can accurately and

directly determine the burning effect, but the investment

is large. Simulations have the advantages of s imple, effi-

cient and s mall inves tment, b ut it is idealized, so there is

a big gap compared to the actual res ults. Therefore, more

accurate numerical simulation methods become particu-

larly importan t.

In this paper, the discrete equations were established

by t he fi nite e le ment ana l ysis metho d whic h anal yzed the

fuel endothermic process on the grate, and the results as

*Corresponding author.

J. X. SUI ET AL.

the boundary condition of the out-of-bed gas combustion

models, and finally the results of the gas combustion are

displayed by CFD.

2. Experiments

2.1. Analysis Experiments

One kind of coal fuel and two kinds of woody biomass

fuels are selected in this paper, and their physical proper-

ties were shown through industry analysis, elemental

analysis and heat measurements. S1 is the coal fuel, S2

and S3 are the biomass fuels, and results are shown in

Table 1 and Table 2.

The volatile content in the table refers to the dry ash-

free basis volatile content. Fixed carbon content, mois-

ture content and ash content refer to the content in the air

dried basis. Comparing the biomass fuel with the coal

fuel, it can be seen that:

• The biomass fuels contains a large proportion of

volatile, a small amount of fixed carbon, so the combus-

tion is more close to the gas combustion, which is clea rl y

different from the fixed-carbon coal combustion.

• The b iomass fuel s co ntai n mo re oxyge n co ntent and

less carbon, so they need less theoretical air quantity.

• The b iomass fue ls have l ow calorie values, so when

the same heat load required, the biomass fuel consump-

tion i s highe r tha n the coal.

2.2. The TGA Experimental

Wu HX [7] made research on the pyrolysis performance,

as well as their mixture, pointing out the different bio-

mass at initial volatile releasing temperature and the first

maximum peak temperature of co-pyrolysis increasing

with more lignite in blend by t he r mo gr avi me tr ic analysis.

It can be found that the differences between the biomass

and the coal in thermal decomposition and combustion

Table 1. Industrial analysi s.

Fuel

name

The fuel main components

Volat ile

(%) Fixed carbon

(%) Moisture

(%) Ash

(%)

S1 6.89 83.14 7.75 3.21

S2 80.13 17.67 9.87 1.32

S3 83.61 14.64 10.14 0.58

Table 2. Element analysis and the calorie value.

Fuel

name C

(%) H

(%) O

(%) N

(%) S

(%) LCV

MJ/kg

S1 84.81 1.97 8.93 0.94 0.13 30.49

S2 48.56 0.47 49.28 0.24 0.12 18.5

S3 48.78 1.15 49.3 0.09 0.11 18.47

by ma king t he T GA exp er ime nts i n thi s pap er . The expe-

rimental conditio ns wer e that the temperature rise rate of

20 degrees per minute in an air atmosphere. The experi-

mental results are shown in Figure 1.

On the basis of the TGA experimental results, it co uld

be seen that:

• Comparing to the coal fuel, the extrapola ted onset

and decomposition temperatures are 200℃ lower, and

the termination temperature is only a half, which means

that the biomass fuel is easier to be decomposed and its

ignition temper a ture is lower.

• All the fuels contain higher moisture, so at the be-

ginning of the experiment, there is precipitation of the

moisture and the fuel quality reduced, and then their

mass close to the same stage in a period of time.

• As far as bio mass fuels, the extrapolated onset tem-

perature is around 250℃, the dec omposition temper ature

is of 50%, around 350℃ and ep ita xial ter minate temper-

ature around 500℃.

3. The Numerical Simulation of the Biomass

Particles

Many numerical researches related to the combustion

simulations have been carried out. For example, Chaouki

Ghenai [6] directly simulated pulverized biomass mixed

with pulverized coal combustion, providing select ions of

models and drawing the conclusion that increasing the

residence time and enhancing vortex could improve the

combustion efficiency. The shape of the fuel has an im-

portant effect on the combustion, so the simulation met ho d

of the pulverized fuels combustion is not suitable for the

biomass briquette fuels. T here are three ways to solve the

above problem.

Firstly, a simple approach is to use inlet conditions for

the top of the fuel bed based on the experimental mea-

surements [8].The inlet conditions contain the gas tem-

perature, speed and species. Secondly, a more complex

method is to develop a separate sub-mode l that ca lcula tes

the temperature, velocity and species at the top of the

Figure 1. Three fuel TGA experiments.

J. X. SUI ET AL.

fuel bed. The CFD code can then be coupled with the bed

sub-model, and the radioactive flux emi tted by the flame

and furnace walls to the top fuel layer, fed back for the

next interaction of the bed model [9].The third type of

approach, which is not so commonly adopted, is to define

a user defined sub-ro utine s (U DF) withi n the F luent cod e.

This code contains the essential details to characterize

the solid and gas phase interactions [10, 15].

In this paper, the second app roach is combined with

the third. The mathematical model consists of two sub-

models: one model for the burning bed of biomass bri-

quette, and the other model for the gas co mbustion in the

furnace above the bed. The fue l is continuously lost mass

during moving in the bed ( moisture evaporation, devola-

tilization and char combustion). The conditions for the

top of the bed are calculated from overall heat and mass

balances of the fuel components and the primary air ve-

locity. To simulate the gas ph ase reactio ns within the full

geometry of a biomass furnace in CFD, a model for tur-

bulence flow and radiation transfer is need. The inlet

boundary condition based on the conditions for the top of

the bed, is achieved by a user defined sub-routines (UD F)

within the CFD code. The modeling schematic view of

the model is shown in Figure 2:

3.1. The Combustion Model on the Grate

In order to model the heat and mass transfer process in

the bed, the fuel in the bed along with the grate move-

ment is discredited, and the mass conservation equations

for each element are based on moisture evaporation, vo-

latile release and char combustion. Heat conservation

equations for each element are based on radioactive heat

transfer, convective heat transfer and combustion heat

production. The interaction between the gas and solid

phase occurs through the relevant source terms in the

conservation equation. The mass and energy conversion

relationship for an element is shown in Figure 3.

CFD

Inlet boundary

Grate

Primary air

Bed model

Bed-gas interface

Radiative transfer

Height

Out boundary

(Gas combustion model)

Figure 2. Numerical simulation model.

Inlet Outlet

Radiactive heat transfer

Convective heat transfer

Combustion heat production

Primary air

Mixture gas

Figure 3. The computing element.

It is assumed that the fuels in the bed are continuous

porous medium, the bulk density of the fuel and the bed

voidage remain unchanged; the heat transfer in the fuel

height direction is i gnored and the volatiles in t he release

process have no combustion reaction. The primary air is

evenly distributed. According to the formulas about

moisture evaporation rate and volatile release rate [11,

12], and combi ned wit h the combustion characteristics in

this paper, the followin g d iscrete equations can b e got.

1) Char without oxidatio n:

a) Mass conservation,

ρudy / (ydx)=RevpR−−

(1)

b) Energy conservatio n,

ρu2dT / (dx)=Sh(TT) /1000

a1 sg

Sεσ(T T

2/10 0

−−

+−

(2)

2) Char oxidatio n:

a) Mass conservation,

V C(s)

ρudy / (ydx)=RevpRR− −−

(3)

b) Energy conservatio n,

ρu2dT / (dx)=Sh(TT) /1000

a1 sg

Sεσ() /10T0TW

a2 lchar

−−

++−

(4)

3) The rate of moisture evaporation can be express as:

a) when T<100 ℃,

w,s w,g

Revp Sahs CC= −（）

(5)

b) when T ≥100℃,

cr evp

RevpQ/H=

(6)

( )

' 44

cra1 sga2l

QSh TTSεσ(TT )= −+ −

(7)

4) The rate of volatile devolati satio n can be express as,

VV v

RρY Avexp/(E )(RT )= −

(8)

J. X. SUI ET AL.

5) The rate of char oxidation can be express as,

C(s) o2rd

RC/(1/k1/k)= +

(9)

The moisture cont ent and volatile content,

w,s w,sw,s

CCSahsC dt= −

(10)

V VVVV

YYρY Aexp/(( ERT))dt

=−−

(11)

where ρ is the bulk density of the fuel, u is the speed of

the grate, Sa is the particle surface area, hs is the co nnec-

tive mass transfer coefficient bet ween solid and gas, C(w,s)

is the moisture concentration at solid surface, C(w,g) is the

moisture concentration in the gas phase, YV is the mass

fraction of volati le matter remaining in biomass briquette,

Av is the pre-exponential factor, Ev is the activation

energy, R is the gas constant, Sa1 is the particle convec-

tive heat transfer surface area, Sa2 is the particle radioac-

tive heat transfer surface area, Tg is the primary air tem-

perature T1 is the furnace temperature, T is the fuel tem-

perature.

Several results can be got according to the above cal-

culat io n, such as the f ue l hei g ht, the moi s tur e a nd volat il e

content, the fuel temperature, the mass fraction in the

mixture, the temperature and the velocity about the mix-

ture, the mixed gas temperature and velocity. All the re-

sults as the inlet boundary conditions to the Fluent to

simulate the gas phas e.

3.2. Out of Bed Gas Combustion Model

Volatile combustion occurs in the gas phase, and Fluent

was selected to simulate the gas combustion. To simulate

the gas phase reactions within the full geometry of a

biomass furnace, models for turbulence and radiation

need to be selected. To model the turbulent interactions

within the boiler, the standard Realizable k-epsilon Mod-

el was adopted. Radiation heat transfer in biomass fur-

nace was modeled with the P1 radiation model [12].

The kinetics of gas phase reaction and the mixing rate

of oxidizer and fuel are the dominant factors to influence

the gas combustion rate. The gas phase reactions are ki-

netically fast above the fuel bed, where the temperature

is high and the reactions quickly, leading to the combus-

tion is only controlled by the mixing rate The rate of a

reaction in particular cell is determined by the minimum

limiting value between the kinetics and the mixing

rate[13,14].

4. Results Analysis

The simulation model is used to calculate the biomass

briquette combustion in the grate. Several results can be

got according to the calculation on the solid combustion

in the bed, which are shown in Figure 4.

The figure shows t he mixture fraction distributio n and

temperature distribution above the grate. These results

wo u ld be as the boundary condition for the furnace si-

mulation.

The simulation on biomass combustion in the two dif-

ferent furnaces under the condition that keep the fuel

characteristic, the grate speed, the air temperature, flow

rate and velocity constant. The simulation results about

the bi omass b urning in the coal -fired b oiler are shown in

Figure 5, contain the temperature distribution and the

mass fraction of O2.

Figure 4. The solid combus ti o n re sul ts.

Figure 5. The combus t ion results of coal-fired boiler.

J. X. SUI ET AL.

Figure 6. The combus t ion results of transformation boi ler.

The simulatio n resul ts sho ws t hat the b iomass b urnin g

in the coal-fired boiler have some problems such as the

high temperature.

The simulation results about the transformation boiler

are shown in Figure 6, contain the temperature distribu-

tion and the mass fraction of O2. Compared with the

coal-fired boiler, the arch angle increase and the front

arch covered area decrease.

Through the simulation results, the conclusion can be

drawn:

1) The o xyge n i s l i kely lack t o b ur n t he vo la ti le s in th e

middle of furnace.

2) The combustion of the biomass is closer to the gas

combustion because the fuel is about around 80% vola-

tile, which is have a huge different from the coal.

3) The flame positio n in the furnace clearly down after

the transformation, which improved the convective heat

transfer of the flue gas and the efficienc y of the boil er.

4) A large space to ensure the stable and complete

combustion of volatile gases after the transform the fur-

nace structures are needed, but the transformation wea-

kens t he disturbance of the mixture.

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