7/28/2019 05987373

1/4

Combustion of Industrial Gas in Porous Media

Burner

Hui Liu, Wenzhong Chen

School of Materials & Metallurgy, NortheasternUniversity

Shenyang, China

sy_liuhui@163.com

Benwen Li

Electromagnetic Processing of Materials (Ministry ofEducation)

Shenyang, China

AbstractThe stable combustion of industrial gas in porous

media burner was studied in this paper. A two-dimensional

mathematic model was set up, and the distributions of

temperature and concentrations of gas components in stablecombustion are computed by FLUENT. Results show that the

emissions in porous combustion of industrial gas are extremely

limited. Industrial gas and industrial emissions should be used

effectively, thus energy can be conserved.

Keywords- industrial gas; porous medium; combustion;numerical simulation

I. INTRODUCTION

For a long time, problem in energy and environmentbecome increasingly prominent in our country, and has becomean issue that must be properly resolved. However, on the onehand, problems in utilize of energy is low efficiency,

environmental pollution and serious waste problem, on theother hand, a large number of low-grade energy cannot bedeveloped and used, in particular, the emissions generated byindustrial production. This kind of the gas has thin combustiblecomponent, difficult to be used in conventional combustiontechnology. The industrial gas is directly discharged into theatmosphere, not only result in energy waste, the toxicsubstances will also pollute the environment. Therefore, greatefforts should be developed to using the low-grade or lowcalorific value of energy, taking environmental protection intoaccount, and to achieve low-emission of nitrogen oxides,carbon monoxide and other pollutants. The combustion of

premixed gas fuel in porous media is a new, clean, active andeffective technology that stable combustion can be achieved

with low calorific value gas. This technology has significantadvantages of stable combustion, highly combustion rate, wideflammable limits and low emissions, and has become aresearch hot spot of domestic and foreign, known as the epoch-making technology in recent years

[1.2].

Compared with the traditional free space combustion, thepremixed combustion in porous medium has an excellentcharacteristic, not only low calorific value of energy can beused, but also low emissions of pollutants can realized. Thecombustion in porous medium has the good economicefficiency and the social efficiency, which have broadapplication prospect. [3, 4]

II. PHYSICAL MODEL AND NUMERICAL METHOD

A. Physical modelThe two-section porous burner studied in this paper is

shown in Figure 1. The computational region, which is 6.05 cmlong, starts from the small-pore ceramic which is 3.5 cm longto the large-pore ceramic which is 2.55 cm long. The premixedmethane/air is preheated in the upstream section and reacts inthe downstream section. A two-dimensional physical model is

presented here.

B. Assumptions

To simplify the model, the assumptions used in the model

are as follows [5]:

x The porous ceramic acts as a gray homogeneous media.x

The boundary condition of the wall is no slip, adiabatic,and the inner wall is gray media.

x Gas radiation is not considered.x Potential catalytic effects of the solid under high

temperature are negligible.

x The Dufour effect, bulk viscosity, and body forcesare negligible.

x The reactants and products are treated asincompressible ideal gas.

Figure 1. Diagram of the physical model

C. Governing equations

The two-dimensional approach which includes the effectsof solid- and gas-phase conduction, solid radiation, solid-to-gasheat transfer, species diffusion, and chemistry [12-19] is written as:

1) Continuity equation

Fundamental Research Program of China (2006CB601203)

OutletInlet

Downstreamsection

Upstreamsection

Premixed gas

Axis

Products

0 3.5 6.05

2041

978-1-4244-9439-2/11/$26.00 2011 IEEE

7/28/2019 05987373

2/4

g g 0

t

HUHU

w

wu

(1)

wheregU is gas density; u is gas velocity vector; H is

porosity of the porous medium.

2) Momentum equation

g g p R

t

HUHU H H

w

w

uuu

(2)

where 2 g

2

CR

UP

D

u u represents viscosity resistance

and inertial resistance produced by the viscosity effect and theshape effect of the porous medium when the fluid passes

through it; D is the viscosity resistance factor;2

C is the

inertial resistance factor; is the viscosity stress tensor.

3) Species conservation equation

g g g

i

i i i i i

YY Y W

t

HUHU HU HZ

w

wu V

(3)

where iY is mass fraction of the ith species; i iV = u - u is

diffuse velocity of the ith species; iu is the velocity which

compared to the stationary coordinate system of the ith species;

iZ is the reaction rate of the ith production;

iW is the molar

mass of the ith species.

4) Gas phase energy equation

g g g

g g g v g

g g

s

i i i

i

C TC T p h T T

t

k T h Y Q

HUH U

H H U H H

w

w

u

u u u

(4)

where p is pressure;Q is the heat release rate of chemical

reaction, andi i i

i

Q hWZ ; gC is specific heat of the gas

mixture;g

k is the thermal conductivity of the gas

mixture; vh is the volumetric heat transfer coefficient between

the porous media and the gas; ih is the molar enthalpy of the ith

species; gT is the gas temperature.

5) Solid phase energy equation

s s s se s v g s

1 C Tk T h T T

t

H Uw w (5)

wheres c r

e e ek k k is the efficiency heat transfer

coefficient of the porous media;c

ek is the efficiency thermal

conductivity of the porous media;r

ek is the radiation heat

transfer coefficient. The radiation heat transfer can beapproximated by the effective heat transfer:

3(16 / 3 ) /r s s sq T dT dxV V , where V =5.6710-8 W/m2gK4

is Stefan-Boltzmann constant; sV is the extinction coefficient

of the porous media; sU is density of solid; sC is specific heat

of the solid; Ts is temperature of solid.

6) State equation

g

g

W p

RTU

(6)

where Wis the mean molecular weight;R is the universalgas constant.

D. Chemical reaction kinetics

The components of industrial gas are complex, here wechoose the gas provided by a steel plant as the research object,and the main components are as follows:

TABLE I. VOLUME FRACTION OF EACH COMPONENT IN INDUSTRIAL GAS

component C2H6 C3H8 C3H6 C4H10 CO N2

volume

fraction%

1.67 14.34 3.50 7.83 1.91 6.23

The simplified chemical reaction mechanism of combustionin industrial gas is present here:

2 6 2 2 2 2 2C H 3.5(O 3.76N ) 2CO 3H O 13.16N (11)

3 6 2 2 2 2 2C H 4.5(O 3.76N ) 3CO 3H O 16.92N (12)

3 8 2 2 2 2 2C H 5(O 3.76N ) 3CO 4H O 18.8N (13)

4 10 2 2 2 2 2C H 6.5(O 3.76N ) 4CO 5H O 24.44N (14)

2 2 2 2CO 0.5(O 3.76N ) CO 1.88N (15)

Considering the ith elementary reversible (or irreversible)reaction withKchemical species, and it can be represented bythe following general form:

1 1

K K

ki k ki k

k k

v x v x

c ccl (16)

> @ > @1 1 1

K KIv v

k ki ki fi k ri k

i k k

v v x xZ N Nc cc

cc c

(17)

The forward reaction rate coefficientfik follows Arrhenius

dependence:

2042

7/28/2019 05987373

3/4

expi ifi i

c

Ek AT

R T

E

(18)

fi

cri

k

Kk

(19)

E. Property data of the porous media

The small-pore porous material in the upstream section isPSZ (Partially Stabilized Zirconia), and the large-pore materialin the downstream section is aluminum oxide. The property

parameters of the porous media used in computations aresummarized in Table 1.

TABLE II. PROPERTY PARAMETERS OF POROUS MEDIA

Upstream Downstream

Porous media type 25.6PPC 3.9PPC

Pore diameterd 0.029cm 0.152cm

Porosity 0.835 0.87

Thermal conductivityKec 0.2W/mK 0.1W/mK

Extinction coefficient s 1707m-1 257m-1

C 0.638 0.146

m 0.42 0.96

F. Boundary and initial conditions

At the inlet, for the gas phase

0 n in300 K 0i i,iT , Y Y , u u , v (20)

for the solid phase

4 4se s,in 0( )c Tk T T

x

w

w(21)

and at the exit, for the gas phase

g g g0i

u v T Y

x x x x

w w w w

w w w w(22)

for the solid phase

4 4se s,out 0

c Tk (T T )x

w

w

(23)

Initially, the fluid velocity of the computational region isspecified according to the velocity of the inlet gas mixture, anda temperature profile in the solid is specified with a peaktemperature of 1500K in order to initiate the reaction

[7].

G. Numerical method

Using the commercial software FLUENT, stable solutionsare obtained by solving the transient governing equations. Inthe software FLUENT, only the single temperature modelcould be employed to solving the energy equations in porousmedia, i.e. assuming the solid and the gas are at thermal

equilibrium, and the temperature of solid are equal to the gastemperature. The application of the single temperature model isnot correspondent with practice. Therefore, the UDF (UserDefined Function) is necessary to modify the singletemperature model into two-temperature model, one for gas,

and the other for solid. The UDF is also used to define theproperty data of the porous media, which change with thetemperature but cannot be defined by the original property

parameter model of FLUENT.

The SIMPLE algorithm has been employed to solve thepressure-velocity coupling momentum equation. Theconvective terms are approximated by the first-order upwindschemes, while the diffusion terms are approximated by centraldifference schemes. The under-relaxation iteration is used tosolve the stiff problem in chemical reactions.

III. RESULTS AND DISCUSSIONS

The purpose of this paper is to examine the temperature

distribution and pollutant emissions of industrial gascombustion in porous media by numerical simulation.

Figure 2 shows the gas and solid temperature profiles at

equivalence ratio I=0.65, inlet velocity inu =0.6 in premixed

combustion of industrial gas and air in porous media burner.From the figure one can see that, in upstream region, the solidtemperature is higher than the gas temperature, unburned gas is

preheated by porous body; in downstream region, gastemperature is about 150 K higher than the solid temperature.As the flammable gas mixture flows through the porous media,the reaction products heat the ceramic matrix by convectionand radiation. Because of the high emissivity of the porousmedia in comparison with the gas, radiation from the high

temperature post-flame zone serves to heat the pre-flame zoneof the porous media, which, in turn, radiatively andconvectively heats the incoming reactants, so the methane/airmixture is preheated effectively.

0.01 0.02 0.03 0.04 0.05

200

400

600

800

1000

1200

1400

1600

1800

2000

Te

mperature,

K

X, m

Tg

Ts

,QWHUIDFH

Figure 2. Temperature distribution of combustion

Figures 3 and 4 show the concentrations distribution ofspecies in industrial gas and combustion product at equivalence

2043

7/28/2019 05987373

4/4

ratio I=0.65, inlet velocity inu =0.6. In upstream region, the

concentration of fuel and air maintained the value at theentrance, the molar concentration of combustion productsremain at around zero, meaning that there is no combustionreaction in upstream region. The combustion begins when the

premixed gas reaches the interface of the porous medium. Therapid reaction of fuel consumes large amounts of oxidant, andthe combustion products CO2 and H2O are produced. At this

point, the fuel is almost completely consumed, the rest gas aremost of combustion products CO2, H2O, some are O2 which didnot participate in the oxidant reaction, and a large number of N2.

Figure 3. Concentration distribution of species in industry gas

0.01 0.02 0.03 0.04 0.05

0

2

4

6

8

10

12

14

16

18

20

22

masspercent,%

X, m

CO2

H2O

O2

Interface

Figure 4. Concentration distribution of species in product

IV. SOLUSTION

In this thesis, FLUENT software is used to numerical studythe combustion properties and emission characteristics featuresof industrial gas combusting in porous media, and examinedthe effects of different equivalence ratios, extinction coefficient

and thermal conductivity of porous media on the combustionfeatures and emissions characteristics. and concluded asfollows:

1. Using the data and conditions in reference paper, thepremix combustion of industrial gas and air in porous medialayer was simulated, results show that: flame is stable at theinterface of porous media, and is near the downstream area.

2. When the premixed industrial gas and air combust in thetwo-section porous media, the emission of NOx is only 2.5ppmat =0.65, uin=0.6 m/s. The NOx emission obviously increaseswith increase of the equivalence ratio, but it is still much lowerthan the standard of the lowest NOx emission standard country.The low emission property of the porous burner is proved.

ACKNOWLEDGMENT

This work was supported by the Fundamental ResearchProgram of China (2006CB601203). The corresponding authorgratefully acknowledges Prof. Li Benwen for his great effort toimprove the quality of this paper.

REFERENCES

[1] F. W. Li, J. F. Wu, X. H. Xu. Disposal of Environmental Pollution byPorous Ceramic Filtration Media China Safety Science Journal, vol. 16,

pp. 112-117, July 2006.

[2] Y. B. Deng. Experimental and Numerical Study of PremixedSuperadiabatic Combustion in Porous Media with Reciprocating Flow.

Dalian: Dalian University of Technology, 2004[3] Der Technischen Fakultt der, Application of the porous medium gas

combustion technique to household heating systems with additionalenergy sources, ErlangenUniversity of Erlangen-Nuremberg, 2004.

[4] J. R. Howell, M. J. Hall, J L Ellzey. Combustion of hydrocarbon fuelswithin porous inert media, Prog. Energy Combust. Sci., vol. 22, pp. 121-145, 1996

[5] P. F. Hsu, Analytical and experimental study of combustion in porousinsert media. The University of Texas at Austin, 1991.

[6] A.R. Martin, C. Saltiel, J. Chai, W. Shyy, Convective and radiativeinternal heat transfer augmentation with fiber arrays [J], Int. J. HeatMass Transfer, vol. 41, pp. 3431-3440, 1998

[7] V.V. Martynenko, Mathematical model of self-sustaining combustion ininert porous medium with phase change under complex heat transfer [J],Int. J. Heat Mass Transfer, vol. 41, pp. 117-126 ,January 1998

[8] M. Kaplan, M.J. Hall, The Combustion of Liquid Fuels within a PorousMedia Radiant Burner, Experimental Thermal and Fluid Science, vol. 11,pp. 13-20, 1995.

[9] A. J. Barra, J.L. Ellzey, Heat recirculation and heat transfer in porousburners, Combustion and Flame, vol. 137, pp. 230241 , 2004.

[10] R. Mital, J.P. Gore, R. Viskanta, A study of the structure of submergedreaction zone in porous ceramic radiant burners, Combustion and Flame,vol. 111, pp. 175-184, 1997.

0.01 0.02 0.03 0.04 0.05

-2

0

2

4

6

8

10

12

14

masspercen

t,%

X, m

C3H

8

C4H

10

CO

C2H

6

C3H

6

2044