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Study on the model experiment and numerical simulation
for underground coal gasification
Lanhe Yang*
College of Mineral Resources and Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221008, China
Received 22 August 2002; accepted 4 August 2003; available online 21 September 2003
Abstract
The gas production process in underground coal gasification is closely linked to the temperature distribution and seepage conditions of the
gasifier. In this paper, mathematical models on the underground coal gasification in steep coal seams are established according to their
storage conditions and features of gas production process. Additionally, the paper introduces ways to determine model parameters and the
control volume method to solve the model. On the basis of the model experiment, the calculation results are analyzed. From the distribution
of temperature field, the calculation value is slightly higher than measurement one. The relative errors of all measuring points are virtually
within 10%. According to the simulated calculation results, with the increase of the length for the gasification channel, the heating value of
the gas improves. However, behind the reduction zone, it increases with a smaller margin. The influence of the temperature field on the
heating value for the gas is noticeable. Due to the effect of temperature, in high temperature zone, the change gradient of the measured value
of the concentration field for the gas compositions is larger than that of calculated value. The simulated results of the pressure field in the
gasification panel 22 and 75 h after the ignition indicate that the relative calculation errors are 5.00–14.29 and 9.68–17.24%, respectively,
with a corresponding pressure drop rate of 9.5 and 11.97%. On the whole, the simulation value conforms with the experiment value, which
supports the numerical simulation on the temperature field, concentration field and pressure field of underground coal gasification in the steep
coal seams is correct. Thus, it lays a necessary and scientific theoretical foundation for further quantitatively studying the process of
underground coal gasification and forecasting the change patterns.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Underground coal gasification; Steep coal seam; Temperature field; Concentration field; Pressure field; Mathematical model
1. Introduction
Thanks to the basic theoretical research on the technique
of underground coal gasification and its increasing appli-
cation and promotion as well as the joint efforts made by the
workmates home and abroad, great progress has been made
in the technique of this country and it is developing into a
new discipline. Because the gas production process
completely depends on the temperature conditions and
convection conditions of gasifier, the research of tempera-
ture field, concentration field and pressure field has always
been the focus of the researchers in the profession. As of the
beginning of 1980s, a large number of experimental and
theoretical studies on the technique of underground coal
gasification had been made in many countries such as the
Former Soviet Union, Belgium, France, Germany and
the United States, etc. [1–5], resulting in great progress in
this field, which contributes a lot to the popularization and
application of the underground coal gasification. While, for
the last two decades, except China, all other countries in the
world has discontinued the research on this field. Over the
past decades, research on the gasification of different coal
seams, ranging from model experiment, field experiment to
numerical simulation [6–8], was made. Large number of
data and a lot of experiences are accumulated [9–12].
According to the data, the storage conditions of coal seams
in this country are diversified. Steep coal seams account for
8.3% [13]. In old and abandoned coalmines, high-angle coal
seams are becoming more and more [14,15]. In order to
further study the regularities of the gasification process in
the steep coal seams, the mathematical models on the
temperature field, concentration field and pressure field of
the underground gasification, are established. The numerical
solution is found. Through the model experiment,
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2003.08.011
Fuel 83 (2004) 573–584
www.fuelfirst.com
* Tel.: þ86-516-3885-762; fax: þ86-516-3888-682.
E-mail address: lhyang2053@sina.com (L. Yang).
the reasonability of the mathematical models is verified,
which provides an important theoretical basis for the
quantitative analysis and scientific guidance of the process
of underground coal gasification.
2. The features of the underground gasification
in the steep coal seams
The underground gasification in the steep coal seams
eminently features the following three aspects:
(1) The thickness of steep coal seams is much smaller
than its slant height and strike length; therefore, the
temperature conduction and retort gas movement in
the coal seams can be regarded as a matter of 2D
plane. Meanwhile, the heat absorption of aqueous
phase change and coal pyrolysis, together with the
heat losses in peripheral rocks, can be held up as
different minus source items in heat transfer
equations. Likewise, the height and width of the
gasification channel are far smaller than its
length, its field model can be thought of as 1D
matter, as shown in Fig. 1. The contribution of the
output of retort gas in the coal seams to the gas
compositions in the channel can be thought of as
source items.
(2) With the consumption of coal, due to the dead weight
and the effect of protruding pressure of retort gases,
the unburned coal falls onto the gasification panel,
which leads to a percolation-patterned channel with
comparatively big porosity [16]. Assuming the
porosity is the same everywhere.
(3) Because the coal seams are thin, the downside
and upside of the reaction zone can be thought of
as floor and roof. There is lime-ash layer under the
reaction zone. Therefore, there is material flow only
at the upper boundary.
According to the features mentioned above, the simpli-
fication of the control equation set of the gasification
process in percolation-patterned channel and the model
equation set of dry and pyrolytic model gives the
mathematical models on the gasification process of steep
coal layers, where, only seven kinds of compositions are
taken into account in gas phases, i.e. O2, CO, H2, CH4, CO2,
N2, H2O(g). Six kinds of compositions are taken into
consideration in solid phases. They are C, H, O, N, ash
matter, and water in coal. The corresponding relationship
between the compositions of gas phase, compositions
of solid phase, temperature and the order number is shown
in Table 1.
3. Mathematical models
3.1. Model of seepage gasification field
of gasification channel
Theequationof theconservationofgasphasecomposition:
f›ðCgiÞ
›t¼
›
›xfCgiUg þ CgD
›ygi
›x
� �þX
gjiRj þ bpi r
pi ði ¼ 1–7; j ¼ 9–14Þ ð1Þ
where f is the porosity; Cgi; the mole concentration of
composition i (mol/m3); Ug; the velocity of gas phase (m/s);
Cg; the overall concentration of gas phase (mol/m3); D; the
effective diffusion coefficient (m2/s); t; time (s); ygi; the mole
percent of the composition i; bpi ; r
pi ; the weighing and
measures coefficient of pyrolysis reaction and reaction rate of
the composition i (mol/m3 s); gji; the weighing and measures
coefficient of chemical reaction; and Rj is the reaction rate of
chemical reaction j (mol/m3 s).
The equation of the conservation of overall gas phases
f›Cg
›t¼ 2f
›ðCgUgÞ
›xþXX
gjiRj þX7
i¼1
bpi r
pi ð2Þ
The equation of the conservation of the compositions for
solid phase
ð12fÞ›Csi
›t¼ð12fÞ
›ðCsiUsÞ
›x
2X
gjiRjAMi ði¼ 9–14; j¼ 1–7Þ ð3ÞFig. 1. Physical model of the underground gasification in the steep coal
seam.
Table 1
The corresponding relationship between the compositions of gas phase,
compositions of solid phase, temperature and the order number
No. 1 2 3 4 5 6 7 8
The compositions of gas
phase and temperature
O2 H2O(g) CO2 CO H2 CH4 N2 Tg
9 10 11 12 13 14 15
The compositions of solid
phase and temperature
C H O N Ash H2O Ts
L. Yang / Fuel 83 (2004) 573–584574
where Csi is the concentration of the composition i for solid
phase (kg/m3) and AMi; the atomic (molecular) weight of
the composition i for solid phase (kg/mol).
The equation of the conservation of overall solid phases
ð1 2 fÞ›Cs
›t¼ 2ð1 2 fÞ
›Cs
›x2XX
gjiRjAMi ð4Þ
where Cs is the concentration of solid phase (kg/m3).
Assuming that the homogeneous reaction heat is exerted
on the gas phase, while nonhomogeneous reaction heat is
exerted upon the gas and solid phases, so the equation of
energy [17] is as follows.
Gas phase:
fX7
i¼1
CgiCpgi
›Tg
›t¼2f
X7
i¼1
Cpgi Cgi
›ðUgTgÞ
›x
� �
þCgDTg
›2ygi
›x2þfg
X4
i¼1
RiDHi
!
þR5DH52a1A1ðTg2TsÞ2Qg ð5Þ
where Tg is the temperature of gas phase (K); Cpgi; the
specific heat of the composition i for gas phase
(kJ/mol K); fg; the distribution coefficient of nonhomo-
geneous reaction heat; DHj; the chemical reaction heat
(kJ/mol); R5; the rate of homogeneous reaction (5) (the
transformational reaction of CO) (mol/m3 s); DH5;
the reaction heat of homogeneous phase reaction (5)
(the transformation reaction of CO) (kJ/mol); a1; the
overall heat exchange coefficient of solid and gas phases
(kJ/m3 s K); A1; the contact area between solid phase and
gas phase (m2/m3); and Qg is the heat losses of gas
phase (kJ/m3 s).
Solid phase:
ð12fÞX14
i¼9
CsiCps
›Ts
›t¼2 ð12fÞ
X14
i¼9
CsiCps
›ðUsTsÞ
›x
þ›
›xKs
›Ts
›x
� �þð12 fgÞ
X4
j¼1
ðRj DHjÞ
þa1A1ðTg 2TsÞ2Qs ð6Þ
where Ts is the temperature of solid phase (K); Cps; the
specific heat of solid phase (kJ/kg K); and Qs is the heat
losses of solid phase (kJ/kg s).
The equation of gas phase flow
›Pg
›t¼ as
›
›x
1
a0 þ b0Ug
›Pg
›x
!þ Wg
" #ð7Þ
where Pg is the gas pressure (Pa); as; the coefficient of
conduction pressure (m2/s); a0; b0; the coefficients related to
the permeability and porosity of media; and Wg is the
source-sink item.
The equation of gas state
Pg ¼ CgRTg ð8Þ
where R is the constant of gas (J/mol K).
The moving speed of flame working face is the amount of
coal consumed per unit area [18], i.e.
Us ¼V
A0
ð9Þ
where V is the amount of coal consumed within a period of
time (m3/s) and A0 is the cross section area of gasification
channel (m2).
Eqs. (1)–(7) can be generalized into the following:
›F
›t¼
1
b2
›
›xfj þ gjs 2 a
›h
›x
� �þ S
� �ð10Þ
The initial condition
Fiðx; 0Þ ¼ Fi;0 ð11Þ
The boundary condition
Fið0; tÞ ¼ Fi;0 ð12Þ
›FiðL; tÞ
›x¼ 0 ð13Þ
3.2. The model on the temperature field of coal seam
and the flow field of the retort gas
The model on the temperature field
rCp
›T
›t¼
›
›xl›T
›y
� �þ
›
›yl›T
›y
� �
2 ET 2X7
i¼1
bpi r
pi CpgiT Tðx; y; 0Þ ¼ T0 ð14Þ
2l›Tð0; y; tÞ
›x¼ a1½Tð0; y; tÞ2 Tin� ð15Þ
2l›TðL; y; tÞ
›x¼ a1½TðL; y; tÞ2 Tout� ð16Þ
2l›Tðx; 0; tÞ
›y¼ a2½Tðx; 0; tÞ2 Tg� ð17Þ
2l›Tðx;H; tÞ
›y¼ a3½Tðx;H; tÞ2 T0� ð18Þ
where T is the temperature of coal seam (K); Cp; the
specific heat of coal seam (kJ/kg K); r; the density of coal
seam (kg/m3); l; the coefficient of heat conductivity for
coal seam (kW/m K); E; heat losses coefficient
(kW/m2 K); T0; the initial temperature of coal seam (K);
a1; the heat convection coefficient (kW/m2 K); Tin; the
temperature of the gasification agent at the inlet (K); Tout;
the temperature of gas at the outlet (K); a2; the heat
convection coefficient, when the mass flow exists
L. Yang / Fuel 83 (2004) 573–584 575
(kW/m2 K); and a3 is the heat losses coefficient
(kW/m2 K).
The model on the flow field (quasi-stable state)
0 ¼›
›x
KPs
mRT
›Ps
›x
� �þ
›
›y
KPs
mRT
›Ps
›y
� �þX7
i¼1
bpi r
pi ð19Þ
Psð0; yÞ ¼ Pin ð20Þ
PsðL; yÞ ¼ Pout ð21Þ
Psðx; 0Þ ¼ PN ð22Þ
Psðx;HÞ ¼ P0 ð23Þ
where Ps is the pressure of coal seam (Pa); m; the dynamic
viscosity (Pa s); K; the permeability (D); Pin; Pout; PN; the
pressure at the inlet, outlet and gasification channel,
respectively (Pa); and P0 is the pressure of natural coal
under the ground pressure (Pa).
4. The discretization and solution of the control equation
In view of the nonlinearity of the above control
equation set and the strong coupling between equations, it
is difficult to solve the model in analytical method. As a
result, numerical solution is adopted. In the paper, we use
control volume method [19], which belongs to finite
difference method in the form. In methodology, it is a
kind of discretization having no difference from finite
element method. This method is designed to keep the
balance of integral within control volume and uses nodes
to represent control volume. The discretization of the
domain to be solved normally consists of even network
and uneven network. According to the features of the
problem to be addressed in this paper, the spatial
distribution of each variant is the function of time.
Thus, when the solution is made, it will be divided in the
form of even network. Along the x-axis, the network
division of channel is identical with that of coal seam, as
shown in Fig. 2.
Eqs. (14)–(19) have the same form. Selecting one of
them gives the general form. Take the discretization of the
equation of temperature field as a proxy [20].
Taking the derivative of the internal control volume P
during the interval from t to tþ Dt in Eq. (14)ðn
s
ðe
w
ðtþDt
trCp
›T
›tdt dx dy
¼ðtþDt
t
ðn
s
ðe
w
›
›xl›T
›x
� �dx dy dt
þðtþDt
t
ðe
w
ðn
s
›
›yl›T
›y
� �dy dx dt
2ðtþDt
t
ðn
s
ðe
wEþ
X7
i¼1
bpi g
pi Cpgi
!T dx dy dt ð24Þ
Assuming the variants change in a stepped way
according to time and space in the unstable items; the
variants in the diffusion items change according to space in
a piecewise linear way, to time in a stepped way; in the
linear source items, the variants change according to time
and space in a stepped way, thus, the above integral of each
equation can be expressed as the following.
The left side is:
rCpðTP 2 TP0ÞDx Dy ð25Þ
The first item on the right side is:
le
TE 2 TP
dx
2 lw
TP 2 TW
dx
� �Dy Dt ð26Þ
The second item on the right side is:
ln
TN 2 TP
dy
2 ls
TP 2 TS
dy
" #Dx Dt ð27Þ
The third item on the right side is:
ðSC þ SPTPÞDx Dy Dt ð28Þ
where
SC ¼ 2 E þX7
i¼1
bpi r
pi Cpgi
!TP0 ð29Þ
SP ¼ 2 E þX7
i¼1
bpi r
pi Cpgi
!p
TP ð30Þ
Substitute the above equations back into Eq. (24), and
simplify it into the commonly used form of discretization
equation:
aPTP ¼ aETE þ aWTW þ aNTN þ aSTS þ bT ð31Þ
where the specific expression of each coefficient is as
follows:
aP0 ¼rCP0 Dx Dy
Dt; bT ¼ SC Dx Dy þ aP0 TP0 ð32Þ
aE ¼ le Dy=dx; aW ¼ lw Dy=dx ð33Þ
aN ¼ ln Dy=dy; aS ¼ ls Dy=dy ð34ÞFig. 2. The network division of the domain to be solved.
L. Yang / Fuel 83 (2004) 573–584576
aP ¼ aE þ aW þ aN þ aS þ aP0 2 SP Dx Dy ð35Þ
In the above equations, superscript ‘0’ denoting the
feature of point P has the same feature with that of the
previous period; le; lw; ln; and ls are heat conduction
coefficients in the interfaces of control volume, which are
the function of temperature. According to the temperature of
nodes, the heat conduction coefficients lE; lW ; lN ; and lS
can be obtained. The heat conduction coefficients at the
boundary of control volume can be obtained through
compromise averaging method, thus
le ¼2lElP
lE þ lP
; lw ¼2lWlP
lW þ lP
;
ln ¼2lNlP
lN þ lP
; ls ¼2lSlP
lS þ lP
ð36Þ
The establishment of the nodal equation at the boundary.
The four boundaries of the coal seams to be gasified are: the
left boundary x ¼ 0; the right boundary x ¼ L; the left and
right boundaries exchange heat through convection with
gasification agent and gas all the time. Thus, it belongs to
the third kind of boundary condition; the lower boundary
y ¼ 0; is mainly the heat convection between the air current
with high temperature in the gasification channel and wall
plane of the channel, belonging to the third kind of boundary
condition; the upper boundary y ¼ H; will be considered
under the following three circumstances: the first, assuming
the temperature is fixed. It is initial temperature and belongs
to the first kind of boundary condition; the second, there is
heat radiation at a constant heat current, which belongs to
the second kind of boundary condition; the third, in the
model experiment, because the gasifier is exposed to the air,
it is also regarded as the third kind of boundary condition.
As for the first kind of boundary condition, because the
boundary node temperature TB is known, it is not necessary
to add the additional boundary nodal equation, but directly
substitute the boundary node temperature into the algebraic
equation of neighboring node.
As for the treatment of the second and the third kinds of
boundary conditions, the additional source item method is
adopted in this paper. Thus, we can substitute the boundary
conditions into the source items close to the boundary node
in the discretization equation. Remove the discretization
equation of the boundary node B; which can reduce the
dimension of the equation set and increase the solution
speed. According to the control body close to the boundary
(Fig. 3), the relational form between the close boundary
node P and the neighboring node can be obtained:
aPTP ¼ aETE þ aBTB þ aNTN þ aSTS þ b ð37Þ
where B is boundary node, and
aB ¼lB Dy
dx
ð38Þ
qB ¼ aBðTB 2 TfÞ ð39Þ
Subtracting aBTP simultaneously from the two sides of
Eq. (37) gives
ðaP 2 aBÞTP ¼ aETE þ aNTN þ aSTS þ aBðTB 2 TPÞ þ b
ð40Þ
From the heat conduction formula, we know
aBðTB 2 TPÞ ¼ qB Dy ð41Þ
When the second kind of boundary condition is given, qB
is a given value. Substituting Eq. (41) back into Eq. (37) and
simplifying it, we get
a0PTP ¼ aETE þ aNTN þ aSTS þ b0 ð42Þ
where
a0P ¼ ðaP 2 aBÞ ¼ aE þ aN þ aS þ aP0 2 SP Dx Dy ð43Þ
b0 ¼ qB Dy þ SC Dx Dy þ aP0 TP0 ¼ qB Dy þ b ð44Þ
When the third kind of boundary condition is given, i.e.
the heat convection coefficient a and the fluid temperature
Tf are given, from heat balance, we obtain
qB ¼ aðTf 2 TBÞ ¼lBðTB 2 TPÞ
dx
ð45Þ
Eliminating TB gives
qB ¼Tf 2 TP
1
aþ
Dx
2lB
ð46Þ
Substitute Eqs. (46) and (47) back into Eq. (43), and
arrange it
a0P ¼ aP 2aB þ
Dy
1
aþ
Dx
2lB
b0 ¼ bþTf Dy
1
aþ
Dx
2lB
ð47Þ
As for the turning node involving two boundaries, the
similar method mentioned above can be adopted to treat it.
Fig. 3. Boundary control volume.
L. Yang / Fuel 83 (2004) 573–584 577
5. The determination of major model parameters
5.1. The heat exchange coefficient a1 between gas
flow and solid particles
The reaction between gas and solid mainly occurs in the
gas holes of solid phase. The effect of nonhomogeneous
reaction heat is exerted on the solid phase. When the
oxidization reactions occur, the solid phase will transfer
heat to gas phase. While the reduction and drying occur, the
gas phase will transfer heat to solid phase.
The heat exchange between gas and solid consists of the
heat convection and radiation. Especially in the period of
high temperature, heat transfer in the form of radiation plays
a major role. Furthermore, when the diameter of the
particles is comparatively large, the heat conduction
among themselves should be taken into consideration.
According to Ref. [21], a1 can be determined through the
following formula:
a1 ¼1
1
aL þ a0S
þ1
aK þ aS
ð48Þ
where, aL; and a0S are the heat convection coefficient and
heat-exchange by radiation coefficient through the lime-ash
layer, respectively (kW/m2 K), and aK ; and aS are the heat
convection coefficient and heat-exchange coefficient by
radiation through the convert film, respectively (kW/m2 K).
5.2. The heat exchange coefficient a2 of wall plane
The percolation-patterned gasification channel, the heat
exchange coefficient between gas flow and the wall plane of
channel can be obtained through the following formula [22]:
a02
Cpgug
Pr2=3 ¼0:6Re1=2 ðRe , 40Þ
0:2Re20:2 ðRe . 40Þ
(ð49Þ
where Pr is Prandtl number and Re is Reynolds number,
Re ¼ rgdPug=m; rg is the density of gas phase (kg/m3).
Because Eq. (49) does not take into account the influence
of mass flow opposite to the direction of heat transfer at the
wall plane on the heat exchange coefficient, the existence of
mass flow (the dry distillation gas) decreases the gradient of
temperature in the proximity of wall plane, which, as a
result, reduces the heat transfer coefficient. Therefore, on
the basis of the establishment of a simple membrane model,
Eq. (49) is revised in Ref. [23], which gives the following
expression:
a02
a2
¼lnð1 þVÞ
Vð50Þ
where a02 is the heat exchange coefficient without mass flow
(W/m2 K); a2; the heat exchange coefficient with compara-
tively big mass flow (W/m2 K); and V is the ratio of the heat
in the channel brought by mass flow and the overall heat
transferred to wall plane through heat convection.
5.3. Heat conduction coefficient l
On the condition of the combustion and gasification of
coal seams, the overall heat conduction coefficient of media
consists of heat conduction and heat radiation, whose
expression is as follows:
l ¼ f1ðTÞ þ f2ðTÞ
where f1ðTÞ is the function of heat conduction coefficient of
the coal body (kW/m K) and f2ðTÞ is the function of
radiation and heat exchange coefficients between coal
chunks (kW/m K).
5.4. Permeability K
According to the seepage experiment, the permeability of
coal is the function of temperature [24] and increases with
the rise in temperature. The relationship between them can
be expressed as follows
K ¼ 0:2286 þ 0:01041T þ 0:0001786T2 ð51Þ
5.5. The specific heat Cp
The specific heat of coal is related to its compositions and
temperature [25,26]. In this paper, the specific heat formula
of the gas-fat coal adopted can be expressed as follows
Cp ¼X
½An cosðnpT =9Þ þ Bn sinðnpT=9Þ� ð52Þ
where the sampling of the coefficients An and Bn is as
follows:
A0¼1:69 A1¼20:60A2¼0:40 A3¼20:17 A4¼0:027
A5¼20:037A6¼0:046 A7¼20:05 A8¼0:04 A9¼20:012
B0¼0:00 B1¼0:333 B2¼0:017 B3¼20:131B4¼0:054
B5¼0:003 B6¼0:007 B7¼20:012B8¼0:026 B9¼0:00
The solution flow of numerical model on the computer is
shown in Fig. 4.
6. Model experiment
The model platform is 7.5 m long by 2 m high by 1 m
wide, consisting of base and lid, shown in Fig. 1. Under the
bottom of the gasifier a line of hydraulic jacks was installed,
which makes the gasifier rest in any angle. The hearth is so
spacious that it can be injected with other materials, which
are used to simulate coal seams with different thicknesses.
On the sides of the gasifier there are a number of gas inlets,
outlets, and slip casting holes used to simulate various
L. Yang / Fuel 83 (2004) 573–584578
gasifiers and study different modes of air pumping and
gasification channel with different lengths. A circular air
(steam) injection pipeline is equipped in the platform in
hope for supplying the air (steam) in a forward-inverse way
with moving pumping points. The gasifier adopts hydraulic
insulation technology. Over the roof of the gasifier is
installed a piece of pedal pushed by hydraulic push rod,
which imposes a certain pressure on the coal seam through
the pedal in the course of gasification, in order to simulate
the pressure on the coal seam.
The simulated coal seam in the gasifier is 6.8 m long by
1.1 m high by 0.25 m wide, with the angle of 68 8C. The
properties of the coal sample used in the model experiment
are shown in Table 2.
Fig. 4. The box diagram of the numerical calculation program.
L. Yang / Fuel 83 (2004) 573–584 579
The gasification panel is 6.0 m long by 1.0 m high by
0.25 m wide, and the initial equivalent diameter of
gasification channel is 70 mm.
The system of the model experiment is shown in Fig. 5.
The pipeline system is designed as armillary circuit.
Through the reversing valve, we can pump the air and
steam negatively and positively. Coal gas cleaner is mainly
to remove the tar from the gas and lower the temperature of
the resulting gas. In order to measure the temperature
distribution in the gasifier accurately, we deploy the
temperature-measuring points in an intense way. In the
gasification panel, 22 rows of temperature-measuring points
are buried, with 7 each row. The number totals 154. The
temperature-measuring elements adopt the strictly standar-
dized NiCr–NiSi thermal couples. The data are collected by
data acquisition system automatically and displayed on the
screen of the computer and recorded. The resulting gas will
be analyzed in gas phase chromatographer, which can tell
the contents of different compositions.
After the experimental system has been prepared, before
the formal ignition, the cold test should be carried out,
whose main purpose is to measure the leakage rate of the
gasifier and check whether all the system is in a good
working state or not.
Suppose the flowing capacity of the air supplied at the
inlet is Q1; the flowing capacity of air discharged at the outlet
is Q2; the quantity of wind leaked in the gasifier is
DQ ¼ Q1 2 Q2; thus the leakage rate of the gasifier is
h ¼ DQ=Q1
The experimental results of the cold test are shown
in Table 3.
From Table 3, we can see that the leakage rate generally
falls below 8.00%, with the maximum of only 10.16%. With
the increase in the air quantity blasted, leakage rate tends to
increase but still within the tolerance of losses, which is the
basic condition for the smooth run of the model experiment.
First of all, ignite the gasifier by means of an electrically
fired igniter, then, blast air with small quantity, sample the
gas from the outlet every 5 min, analyze its compositions
and heating value, half an hour later, the interval of
sampling is prolonged to 10 min, increase air blast quantity
gradually, the interval of sampling is so long as 1 h 15 min
later. When the heating value of the gas reaches 1300 kJ/m3,
the ignition process is considered successful. According to
the measurement results of the temperature, after the stable
temperature field in the gasifier is formed, the interval of
collecting temperature data at all the temperature-measuring
points is 1 min.
The model experiment lasts for 117 h, undergoing four
phases, namely, gasification preheating (24 h), stable gas
production (58 h), intermittent air pumping (18 h) and
inverse air pumping (17 h). The operating conditions are:
the gasification agent, air; the pressure at the inlet, 2400 Pa;
and the flowing capacity at the inlet, 20 m3/h. The results of
the experiment are shown in Figs. 6(a)–9(a), 10 and 11.
7. The analysis of calculation results
The calculation results of numerical simulation are
shown in Figs. 6(b)–9(b), 10 and 11.
Comparing Figs. 6 and 8, we can find that the calculation
results of temperature field virtually conform with the
measurement results. Except the measuring points in the
combustion zone, where the relative errors between
calculated value and measured value of temperature are
comparatively high (certain points, over 20%), the relative
errors of other points are no more than 14%, majority of
which are within 10%, which completely meets the precision
Table 2
Properties of the coal sample studied
Proximate analysis (wt%) Ultimate analysis (daf, wt%) Heating value (MJ kg21)
Moisture Ash Volatile matter C H O N S
4.18 7.61 23.08 72.72 4.71 8.32 1.13 1.33 28.14–29.31
Fig. 5. The system of the model experiment.
Table 3
The data of the cold test (m3 h21)
Modes of air supply Forward air supply Inverse air supply
Q1 36.4 32.1 27.3 21.5 40.0 33.8 24.7 19.5
Q2 32.7 29.5 25.5 20.6 36.1 31.6 22.9 18.8
DQ 3.7 2.6 1.8 0.9 3.9 2.2 1.8 0.7
h (%) 10.2 8.1 6.6 4.2 9.7 6.5 7.3 3.6
Average (%) 7.3 6.8
L. Yang / Fuel 83 (2004) 573–584580
requirements [27] of numerical simulation on the tempera-
ture field. From the distribution of temperature field, the
calculation value is a bit bigger than measurement one. The
reasons are as follows: first of all, heat losses coefficient E in
mathematical models is determined, on the basis of
composite structure calculation formula of heat transfer
coefficient for the fixed conditions (temperature, wind
velocity). In the process of experiment, due to the natural
ventilation or forced ventilation, the heat losses coefficient
increases, which contributes to the slight drop of the
temperature. Then, during calculation, the heat conduction
coefficient of coal seams is not the one of body coal but
the equivalent heat conduction coefficient taking the
influence of convection and heat radiation into consider-
ation. The coal seams in the model experiment are piled up
by various sizes of coal chunks. Though the interstices are
filled with coal powder, its holes are far bigger than the real
one. Therefore, heat exchange is not a single form of
heat conduction. With the effect of convection intensifying,
the surface heat conduction coefficient of the coal
seam improves, which influences the precision of calcu-
lation results.
Because the reaction rate heightens rapidly with the
increase of temperature, as a result, due to the impact of
temperature, the change gradient of the compositions
concentration for measurement value in high temperature
zone is bigger than that of calculation value (Figs. 7 and 9).
According to Figs. 7 and 9, with the expansion of
gasification channel, the heating value of the gas increases
gradually. However, behind the reduction zone, the extent
of increase decreases. The influence of the temperature field
on the heating value of the gas is remarkable. In the first
period, in spite of the comparatively long gasification
channel, due to the low temperature in the oven, the heating
value of the gas is relatively low; in the second period, the
temperature of the oven increases in the comparatively
shorter gasification channel, but the heating value of
resulting gas is higher than that in the first period. Therefore,
maintaining a temperature field with high temperature and
Fig. 6. The temperature field in the gasification panel 22 h after the ignition.
Fig. 7. The concentration field in the gasification channel 22 h after the ignition.
L. Yang / Fuel 83 (2004) 573–584 581
comparatively longer gasification channel is conducive to
the stability and increase of the heating value for the gas.
Figs. 10 and 11 demonstrate that, the simulated
calculation value of the pressure for gas flow basically
conform with the measured value. The comparative error in
the first period between calculation value and measurement
value is 5.00–14.29%, with its average drop rate for the
pressure of fluid 9.5% (þ15.2, 213.1%). From Fig. 11, we
know that, 75 h after the commencement of gasification, the
error of the simulated calculation for pressure is 9.68–
17.24%, with its average drop rate of the pressure of gas
along gasification channel 11.97% (þ21.39, 217.33%). It
can be concluded that, with the prolonging of gasification
time, the extent of the pressure drop of fluid increases and
the calculation error becomes bigger. The major reasons are
the following, at the very beginning of the experiment, the
gas moves along the free gasification channel with little
resistance on the fluid and small fluctuation of pressure.
Accordingly, the drop rate and calculation error are
comparatively low. With the development of gasification,
the top coal layer over the gasification channel, due to
the effect of high temperature, expands, inbreaks and
falls onto the gasification channel because of dead
weight, which fills the gasification channel with loose
coal chunks. Thus, the free gasification channel becomes
percolation-patterned. The resistance over the gas move-
ment increases dramatically, so does the extent of the drop
rate of pressure. The curve of the experiment takes on wild
fluctuation. Considering the changes of movement con-
ditions and the determination of major model parameters,
such as conduction pressure coefficient, permeability as well
as the involvement of human factors and empirical factors in
the calculation results in a certain error in the parameters
calculation, which causes oscillation in the value of
numerical simulation.
In short, the simulated results support that the calculation
value conforms with the measurement value well, which
shows that the establishment of mathematical models on
Fig. 8. The temperature field in the gasification panel 75 h after the ignition.
Fig. 9. The concentration field in the gasification channel 75 h after the ignition.
L. Yang / Fuel 83 (2004) 573–584582
the temperature field, concentration field and pressure field
in the gasifier, the determination of parameters, the analysis
and treatment of boundary conditions and the solution
method are correct. This provides necessary theoretical
basis and scientific guidance for the comprehensive
quantitative study and production practice of underground
coal gasification.
8. Conclusions
(1) According to the storage conditions and the features of
gasification process for the steep coal seams, on the
basis of model experiment, the mathematical models
on the gasification process of steep coal seams are
established. The simulated results demonstrate that the
calculation value can virtually conform with the
measurement one, which supports that numerical
simulation on the temperature field, concentration
field and pressure field is reasonable in the under-
ground gasification of steep coal seams on the
experimental condition.
(2) The numerical simulation shows that, in high tempera-
ture zone, the calculation value of the temperature field
is a bit bigger than measurement value; the change
gradient of the measurement value of the concentration
of various compositions for the gas is bigger than that
of the calculation value; temperature field has a great
influence on the heating value of the gas. The
heating value of the gas increases with the rise in
the temperature of gasifier. The ideal temperature
field with high temperature is conducive to the
improvement of the gas and the stability of the
gasification process.
(3) According to the calculation results, the relative error
between the calculation value and measurement one of
the fluid pressure and its drop rate increase gradually
with the gasification process. The change in the
seepage condition of gasification channel is mainly
responsible for the comparatively big calculation error.
(4) The numerical calculation results basically demon-
strate the real change patterns of the temperature field,
concentration field and pressure field in the process of
underground gasification of steep coal layers. Though a
certain error exists between the calculation value and
experiment one, however, according to the specific
combustion and gasification conditions of coal seams,
choosing appropriate parameters or optimal parameters
with the help of inverse calculation, the above
mathematical models can be fully applied to pro-
duction practice and to predict the change patterns of
‘three fields’ of the process of underground coal
gasification.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Ratification No. 59906014,
50276066). The technical contributions of professor Yu Li
and Dr Liang Jie are gratefully acknowledged by the author.
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