the dynamic temperature field of two stage ucg

7/30/2019 The Dynamic Temperature Field of Two Stage UCG

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Energy Sources, Part A: Recovery, Utilization, andEnvironmental Effects

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The Dynamic Temperature Field of Two-StageUnderground Coal Gasif ication (UCG)Lanhe Yang

a

aCollege of Mineral Resources and Geosciences, China University of Mining and Technolog

Xuzhou, Jiangsu Province, China

Version of record first published: 16 Aug 2006.

To cite this art icle: Lanhe Yang (2006): The Dynamic Temper at ure Field of Two-Stage Underground Coal Gasif icat ion (UCG)Energy Sources, Part A: Recovery, Uti l ization, and Environmental Effects, 28:7, 667-680

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Energy Sources, Part A, 28:667680, 2006

Copyright Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/009083190951438

The Dynamic Temperature Field of Two-StageUnderground Coal Gasification (UCG)

LANHE YANG

College of Mineral Resources and Geosciences

China University of Mining and Technology

Xuzhou, Jiangsu Province, China

Two-stage UCG is an effective technique to produce water gas with high heatingvalue; its gas producing process is mainly determined by temperature. On the basisof the model experiment, via the analysis of the temperature field distribution regu-larity in the gasified coal layers in the gasifier and the generalization and treatmentof the boundary conditions, two-dimension nonlinear unstable mathematical modelsof the temperature field in the two-stage UCG are established, and the method ofselecting model parameters is illustrated. Solution is made through the method ofvolume controlling. This article also analyzes the results of calculation. In the lightof the numerical computation results, the calculation value of the temperature field

for coal seams of combustion and gasification can better fit with the experimentalone under the condition of the model experiment. Except for some measuring pointsin the vicinity of the flame working face, where the relative error between computa-tion value and test value is comparatively high, those of other measuring points areall below 15%, which completely meets the accuracy requirements for the numericalsimulation on the temperature field of UCG. The consistency between the calculationvalue and the measurement value indicates that the numerical simulation of dynamictemperature field of coal media in the gasifier is correct, which provides necessarytheoretical basis for further quantitative study of the UCG process.

Keywords UCG, two-stage, nonlinear unstable, temperature field, numerical simu-lation

The process of UCG is virtually a self heat-balance process. The heat produced by coal

combustion contributes to the establishment for ideal temperature field in the underground

gasifier and also leads to the occurrence of reduction reactions and decomposition reac-

tions and, eventually, gas production. Hence, in the process of UCG, what plays a critical

role is the temperature field in the gasifier, two-stage UCG in particular. Two-stage UCG

is a technique of supplying air and steam cyclically (Yang, 1995a, 1995b; Li, 1995). In

the first stage, air is supplied to make the coal burn and store heat to produce air gas;

in the second stage, steam is supplied to produce water gas. Only if sufficient heat is

stored in the first stage can the decomposition reactions in the second stage run smoothly

This work was supported by the National Natural Science Foundation of China (RatificationNos. 59906014 and 50276066). The technical contributions of Professor Yu Li, Doctor ShuguangJiang and Mrs. Suqin Liu are gratefully acknowledged by the author.

Address correspondence to Lanhe Yang, College of Mineral Resources and Geosciences,China University of Mining and Technology, Xuzhou, Jiangsu Province, 221008, China. E-mail:[email protected]

667


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668 L. Yang

and the water gas with high heating value be ensured. Meanwhile, the degree of the

coal layer decomposition and the production volume of the gas are totally determined by

the temperature distribution in the coal layers (Guo, 1994). In the past, scholars in the

same profession overseas studied the law of change in a temperature field for the air-gas

producing (the first stage) in the process of UCG (Massaquoi et al., 1983; Guntermannet al., 1986; Thorsness and Britten, 1986; Mortazavi et al., 1986; Advani et al., 1986;

Britten and Thorsness, 1985; Thorsness and Rozsa, 1977). As to the simulation of tem-

perature field for two-stage UCG, at present, however, no articles concerning it have been

included in the literature. Based on the model experiment, the two-dimension on-linear

dynamic mathematical models of the temperature field in the coal layers to be gasified

are established in this study. Additionally, the solution of the mathematical models is

made via the volume-controlling method and satisfactory results are achieved.

Conditions of Model Experiment

The model gasifier used in the experiment is an oblong steel box (Figure 1) 7.8 m

1.36 m 0.52 m (length height width). It consists of top and base. It acceptsnatural big coal chunks in the process of coal injection so as to keep the state of the

coal as natural as possible. The interstices will be injected with small pieces of coal, and

finally, smeared with a mixture of a small amount of cement and pulverized coal. During

coal injection, set aside room for gasification channel with 65 mm diameter as planned

in advance. The strike length of the coal layer in the gasifier is 6.0 m. Slope length of

the coal layer is 0.8 m with the thickness of 0.24 m and 70 angle of inclination. So,

the coal layer is a steep one. A number of blasting caps are buried in the coal layers.

Controlled blasting with temperature can loosen the coal layers, which is conducive to

the combustion.

The gasifier is made up of a fire-resistance layer, a heat preservation layer and an

insulator. The fire-resistance layer is made from the mixture of fire-resistance bricks,

reef, and fire-resistance cement (its volume proportion is 4:1:1). Heat insulator also has

a little function of insulation. Heat preservation materials employ vermiculite powder,surrounding the heat insulator. The outmost layer is an A3 steel plate with the thickness

of 8 mm. It also functions as an insulator.

Figure 1. Cutaway view of the gasifier model.


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Underground Coal Gasification Temperature 669

The major operational parameters in the first and second stage are: air fluxes 20 m 3/h,

steam fluxes 3.1 m3/h. In addition, 22 rows of temperature-measuring points are arranged

with seven in each row (Figure 1). Thus, the temperature-measuring points in the coal

layer to be burned total 154. Temperature-measuring elements use the strictly standardized

NiCr-NiSi thermal couple, whose data are collected by the automatic data collectorregulatory and displayed on the screen automatically.

Establishment of Mathematical Models

Suppositions

In the process of combustion and gasification of the coal layer, various complicated reac-

tions occur in the gasifier. Meanwhile, energy and mass transfer take place simultaneously

between gaseous phase and solid phase (Guo, 1988; Debelle and Malmendier, 1992). So,

in order to simplify computation, the following suppositions will be made:

1. The gasifier itself is in stable working condition. Major thermodynamic parame-

ters, such as heat transfer coefficient, specific heat and heat exchange coefficient,etc. do not vary with time (Bischoff, 1969).

2. In the process of heat transfer, the diffusion effects and the impact of mass transfer

will be ignored.

3. Media of the coal layer are homogeneous and isotropic.

Differential Equation

Because the gasifier model takes the shape of cuboid and models on the steep coal

layer, the coal layers in this article will be simplified as a two-dimension model (i.e., we

consider the temperature of the coal layer along the thickness unchanged). Thus, along

the strike and slope of coal seam, the temperature will vary simultaneously with time.

The combustion zone can be held up as a moveable changing heat source. From this, we

can see the changing regularity of the media of coal layers to be gasified is virtually atwo-dimension nonlinear unstable heat transmit problem with a moveable changing heat

source. Therefore, the heat transmission equation has the following form:

x

x )(T )

T

x

+

y

y )(T )

T

y

= qv( x ,y ,t ) = Cp(T )

T

t, (1)

where x (T ) = heat transmission coefficient along x axis; y (T ) = heat transmission

coefficient; Cp(T ) = the specific heat of coal; = the coal density; qv( x ,y ,t ) = heat

emission rate of heat source; T = temperature; and t = time.

Generalization of Boundary Conditions

The physical model of coal layers to be gasified is shown in Figure 2, whose fourboundaries are: the top boundary y = H; the bottom boundary y = 0; the left boundary

x = 0; the right boundary x = L.

Because the exterior surface of the gasifier contacts the air outside, the heat on the

boundary of the coal layers will exchange heat indirectly with the air around via the heat

conduction effect of the heat insulator and heat preservation layer. Hence, the boundary

x = 0 and x = L can be classified into the third boundary condition (i.e., the temperature


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670 L. Yang

Figure 2. The physical model of the coal layer to be gasified.

of the air, Tf and comprehensive heat exchange coefficient on the boundary of the

gasifier are known).Boundary y = 0, that is, gasification channel. According to the experimental data

measured, the temperature distribution on the wall plane of the channel can be known.

Therefore, it belongs to the first boundary condition.

The top boundary y = H, can be regarded as the outward heat diffusion at a constant

heat current (Liu, 1991), therefore, belongs to the second boundary condition.

The Initial Conditions

The average temperature value, taken continually in different parts of the gasifier before

ignition, will be thought of as initial temperature.

Mathematical ModelsBased on the above analysis, the two-dimension nonlinear and dynamic mathematical

models of the temperature field in the gasified coal layers of the gasifier are as follows:

x

x (T )

T

x+

y

y (T )

T

y

qv(x,y,t) = Cp(T )

T

t,

t= 0, T (x, y, t) = T ( x ,y , 0),

S= ST, T (x, y, t) = T (x, yw,t),

S= Sa , (T Tf) =

T

n

,

S= Sq , q = T

n

,

0 x L, 0 y H, 0 t Nt

(2)

where q = the density of heat current on the boundary; = convection coefficient;

Nt = total time; n = the direction of exterior normal of surface; subscript indicates

boundary surface; other symbols have the same meaning as those above.


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The Solution of Model

The Establishment of Discretization Equations

The solution of the above-mentioned temperature field model is made via the method ofcontrol volume. We make the integral of interior control volume (Figure 3) of Eq. (1) at

the interval [t, t+ t].

Cp

T

tdtdxdy =

x

x

dxdydt+

y

T

y

dydxdt

qvdxdydt.

(3)

Assuming the temperature of the unstable terms in the equation vary with time and

space trapezoidally; diffusion terms vary with space in a subsection and linear way, and

with time in a trapezoidal way. Substitute corresponding distribution function (Guo, 1993)

into Eq. (3) and make the integral, it follows that:

The left side = Cp(Tp Tpo )xy;

The first term on the right side =

e

TE Tp

x w

Tp Tw

x

yt;

The second term on the right side =

n

TN Tp

y s

Tp Ts

y

xt; and

The third term on the right side = qvxyt.

Substitute the above terms into Eq. (3), and reorganize the equation to universally-

used discretization equation form.

apTp = aETE + aWTW + aNTN + aSTS+ b, (4)

Figure 3. The schematic diagram of control volume division.


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672 L. Yang

where the specific expressions of every coefficient are as follows:

apo =CPxy

t; b = aPo TP; aE = e

y

x; aW = w

y

x;

aN = nx

y; aS = s

x

y; aP = aE + aW + aN + aS + aPo = Kxy,

where K is the coefficient of qv .

In the above-mentioned expressions, e, w, n, s are the coefficients of heat

conduction for control volume at interface. Superscript 0 indicates point P has the nature

of previous time. Because heat transmission is a function of temperature, according to

the node temperature, only E , W, N, S, coefficients of heat conduction at nodes can

be obtained. Therefore, the coefficients of heat conduction on the boundary of control

volume can be calculated via the compromising average method (Jie, 1997).

The Establishment of Boundary Node Equations

Figure 4 is the schematic diagram of near boundary control volume and boundary controlvolume. Here, we adopt the treatment method of regarding boundary conditions as addi-

tional source terms, thus, boundary conditions can be substituted into the source items of

the discretization equation of boundary node, and the discretization equation of boundary

node B can be omitted, which lowers the dimensions of the equation group and increases

solving speed. According to Figure 4, the relational form between near boundary node

P and neighboring node can be expressed as the following

aPTP = aETE + aB TB + aNTN + aSTS+ b, (5)

where B is boundary node and satisfies

aB =2B y

x,

QB =aB (TB TP)

y.

(6)

Figure 4. Boundary control volume.


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Subtracting aB TP simultaneously from the two sides of Eq. (5), it follows that,

(aP aB )Tp = aETE + aB (TB TP) + aNTN + aSTS+ b. (7)

From heat conductivity formula, we know that

aB (TB TP) = qB y.

As for the second kind of boundary condition, qB is a given value. Substitute the

above expression into Eq. (5) and then reorganize it.

aPTP = aETE + aNTN + aSTS + b, (8)

where aP = (aP aB ) = aE + aN + aS+ aPo Kxy, and b = qB y + aPo TPo =

qB y + b.

As for the third kind of boundary condition, from heat balance, we know that QB =

(Tf TB ) = 2B (TB Tp)/x. Eliminate TB , and it follows that

QB = (Tf TP)

1

+

x

2B

. (9)

Substitute Eqs. (6) and (9) into Eq. (7) and reorganize it, and it follows that

aPTP = aE TE + aNTN + aSTS+ b. (10)

Thus, we still obtain an expression similar to commonly-used discretization equation,

except for the slight differences of coefficient included, where

aP = aP aB +y

1

+x

2B

,

b = b +Tfy

1

+

x

2B

.

Likewise, as for the top boundary and right boundary, an equation similar to Eq. (5)

can be obtained. As for the circumstances involving two boundary nodes (Figure 5), a

similar method mentioned above can be adopted to treat it and reorganize it into the form

similar to Eq. (5)

aPTP = aE TE + aSTS+ b, (11)

where

aP = aP aW aN +y

1

+

x

2W

+ x1

+

y

2N

,

b = b +Tfy

1

+

x

2W

+

Tfx1

+

y

2N

.


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674 L. Yang

Figure 5. Angle point control volume.

Likewise, as for the angle point contained by top boundary and right boundary, its form

of equation is similar to Eq. (11). Thus, on the basis of the above work, simultaneous

algebraic equation discreted can be formed so as to conduct an iterative solution.

The Selection of Model Parameters

1. In the process of UCG, the process of heat transfer in the gasifier occurs in two

waysheat conduction and radiation between incandescent coal chunks. Yang

et al., 2001 give the relation between coefficient of heat conduction and temper-

ature on the nonisothermal condition.

k(T ) = 0.0003 +AT

1000+

BT2

10002, (12)

where A, B = specific constant, select A = 0.0013, B = 0.0010 in the model

experiment. Overall heat transfer coefficient consists of heat conduct and radiation,whose calculation method is shown in Wen (1965).

2. Specific heat CP. As for the coal type with known composition and sedimentary

environment, its specific heat is linked to temperature, whose relation is illustrated

in Figure 6. CP in the inversion calculation will be determined according to

Figure 6.

Figure 6. The relation between the specific heat and temperature of coal.


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Figure 7. The boundary structure of gasifier. 1air, = 6 w/m (C). 2steel plate, h = 8 mm,

= 50 w/m (C). 3vermiculite powder, h = 60 mm, = 0.05 w/m (C). 4fire-resistance

brick, h = 180 mm, = 0.7 w/m (C). 5fire-resistance cement, h = 60 mm, = 0.3 w/m (C).

3. Density : = 1280 kg/m3.

4. Coefficient of heat transfer z of the boundary x = 0, x = L, and Y = H will be

determined on the basis of the boundary structure of the gasifier. The boundary

structure of the gasifier is shown in Figure 7, where material, thickness and heat

conduct coefficient of each layer are marked. According to the calculation method

of coefficient of heat conduction for composite structure (Yang, 1980), we can

obtain that z = 6.6 J/m2

.

C.s, hence, the heat loss coefficient on the exteriorsurface of the gasifier Kz = z, therefore, we select the heat current of heat loss

on the second kind of boundary q = KzT.

5. Heat emission rate of heat source qv is expressed with the effective heat current

of the heating coal layer. As for the heat generation terms, qv is regarded as a very

important parameter in the numerical calculation. Whether the sampling of qv is

reasonable or not is related to the dependability of the simulated results of the

temperature field. The sampling value of qv depends on the gasification process.

According to the rationales of Underground Coal Gasification (Figure 8), in the

oxidization zone, the multi-phase chemical reactions occur between the oxygen in

the gasification agent and the carbon in the coal layer, producing a large amount

of heat, which is accumulating in the coal layer:

C + O2 CO2 + 393.8 MJ/kmol, (13)

2C + O2 2CO + 231.4 MJ/kmol. (14)

Figure 8. The diagram of the rationales of UCG.


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676 L. Yang

In the reduction zone, the major reactions occur between CO 2, H2O(g) and the

incandescent coal layer. Under the effect of the high enough temperature, CO 2 is

reduced to CO, and H2O(g) is decomposed to H2 and CO:

CO2 + C 2CO 162.4 MJ/kmol, (15)

H2O(g) + C CO + H2 131.5 MJ/kmol. (16)

Additionally, under the catalyst effect of the coal ash or the metallic oxides, etc.,

a certain methanation reaction will occur:

C + 2H2 CH4 + 74.9 MJ/kmol. (17)

In the dry distillation zone, the temperature of the gas and coal layer is still very

high. The thermal effect will make the pyrolysis of the coal layer happen and the

volatile matter will be dried and separated out.

From this, we can see that, the major influencing factors of qv include: the

amount of heat discharged by the oxidization reactions, the heat absorbed by thereduction and decomposition reactions, temperature and the fluxes of the gases, the

moving state of the flame working face, and other heat losses. Nevertheless, it is

very difficult to determine the functional relation between qv and the influencing

factors. In the identification process of the mathematical models, the inversion

calculation method is adopted: first of all, give the initial value of qv based

on the method described in Yang (1980), then calculate it. If the error between

the calculation results and the experimental results are greater, the value of qvwill be continuously revised according to the thermal effects of the reactions

near the nodes of the network, temperature, the fluxes of the gases, and the

moving state of the flame working face. If the measuring points are not on the

nodes of the network, the linear interpolation method will be adopted to obtain

the value of the variables mentioned above. This process will be repeated again

and again until the calculation results are within the tolerance of the precision

requirement.

6. Initial temperature T ( x ,y , 0). T ( x ,y , 0) takes the measured mean temperature at

furnace mouth before ignition, i.e., 15C.

7. The first kind of boundary temperature T (x, y, t). On the basis of the tem-

perature data collected in the process of the experiment, the author conducted

regression for the temperature distribution along the gasification channel surface.

At the stage of air supplying and steam supplying, respectively, we obtain the

distribution function fitted for channel surface temperature of two-stage under the

unstable circumstances.

The Design and Writing of the Program for Model Solution

The computer program for model solution is written in FORTRAN77, which primar-

ily includes main-control module, data input module, net division module, pretreatment

module, equation iterative solution module, and output module. The fitting calculated

results of temperature field are shown in Figures 9, 10, and 11. The measured results of

corresponding time are shown in Figures 12, 13, and 14.


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Figure 9. The calculated value of the gasifier temperature field at the initial stage of the UCG

(36 h after the blast air).

The Analysis of Calculated Results

Comparing Figures 9, 10, and 11 with Figures 12, 13, and 14, it can be found that the

calculated results of temperature field agree virtually with measured ones. Except for the

measuring points in the combustion zone, where the comparative differentiations between

calculated results and measured ones are greater (some specific points above 20%), the

comparative differentiations of other measuring points are below 15%, most of which,

within 10%, which meets accuracy requirements of numerical simulation of temperature

field. The reasons resulting in the above differentiations are as follows: (1) when calcu-

lating the overall coefficient of heat conduction, the selection of intermediate parameters

is affected by personal factors and empirical ones, which cause certain differentiations

for the value of (T ) and by the vacillation phenomena at the nearby flame workingface; (2) not having taken the impact of temperature on the density of coal layer into

Figure 10. The calculated value of the gasifier temperature field 23 min after supply steam.


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680 L. Yang

relations cannot be determined. The simulation of a two-stage temperature field for UCG

is an attempt, and on the basis of that, according to the theory of three transfers,

the united mathematical models can be established in hope for the interrelationship and

distribution regularity among temperature, concentration, and pressure in the process of

UCG. Therefore, the study results will undoubtedly provide valuable theoretical evidencefor the further comprehensive quantitative study on the process of UCG.

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the dynamic temperature field of two stage ucg

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