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Semi-industrial tests on enhanced underground coal gasification at Zhong-Liang-Shan coal mine.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
Published online 7 July 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.337
Special Theme Research Article
Semi-industrial tests on enhanced underground coal
gasification at Zhong-Liang-Shan coal mine
G. X. Wang,1 * Z. T. Wang,1,2 B. Feng,1 V. Rudolph1 and J. L. Jiao3
1
School of Engineering, University of Queensland, Qld 4072, Australia
State Key Laboratory of Coal Resources and Safety Mining, School of Mining, China University of Mining and Technology, Xuzhou 221008, China
3
Chongqing Zhongliangshan Coal-electricity-gas Co. Ltd., Chongqing 400052, China
2
Received 28 October 2008; Revised 18 March 2009; Accepted 21 March 2009
ABSTRACT: A new process of enhanced underground coal gasification (EUCG) has been demonstrated recently in
successful semi-industrial tests at Zhong-Liang-Shan (ZLS) coal mine in China. The EUCG is featured using manbuilt pinnate channels and controlled moving injection points along coal-bed channel to produce syngas through
underground coal gasification (UCG). To optimize the EUCG process, this field trial was performed with various
operational manoeuvres, such as implementing controlled moving injection points, O2 -enriched operation and variation
of operational pressure. The results showed that these operational techniques of the EUCG can ensure the gas flow
comparatively controllable and hence improve UCG performance significantly, providing both a higher efficiency of
heat and a higher quality of the production syngas.  2009 Curtin University of Technology and John Wiley & Sons,
Ltd.
KEYWORDS: coal; syngas; underground coal gasification (UCG); semi-industrial test
INTRODUCTION
The underground coal gasification (UCG) technology
is a proven technology that burns coal in a controlled
manner and gasified under in-suit conditions to produce
syngas which can be converted into various types of
fuel, such as gas for electricity generation or even
oil. This technology, initially developed by the former
Soviet Union in 1950s,[1] is now receiving renewed
interest in the world due to increasing environmental
pressure associated with conventional use of coal and
expanding demand for alternative energy, particularly
in China.[2 – 5]
The rapid economic expansion in China in recent
years has been accompanied by massively increasing demand for energy. The largest indigenous energy
source is coal, which provides about 70% totally in
the national energy consumption,[6] but environmental
and supply considerations mean that the development
of unconventional natural gas from coal has become
an important part of energy policy in China.[6,7] An
example is the development of new operating UCG
systems to produce synthetic gas directly from underground coal seams, providing energy source for residential and industrial uses.[8] Since the late 1950s, China
*Correspondence to: G. X. Wang, School of Engineering, University
of Queensland, Qld 4072, Australia. E-mail: gxwang@uq.edu.au
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
has been working independently to develop UCG technology which mostly suits its particular needs. The first
successful UCG field trial ran for 10 months at Xuzhou
coal mine in 1994. Following this trial, several commercial UCG gasifiers have been constructed in Hebei,
Shandong and Shanxi. More full-scale UCG operations
are planned in China with some currently underway.[9]
The effective implementation of UCG technology is
largely dependent on a clear understanding of the process, including the reaction kinetics, heat transfer, gas
flow and thermally affected geophysics and hydrology.
The highly heterogeneous properties of coals and diversity of geological conditions increase uncertainty and
risk for UCG processes, and could lead to poor technical performance and economic outcomes. Furthermore,
environmental issues arising from UCG, such as CO2
emission and groundwater contamination are topics of
significant concern with UCG,[2] and need extensive
and ongoing evaluation. Field tests play more important
roles in optimization of UCG processes, in particular for
new designs.
This article describes a series of field tests on a
new process of enhanced UCG (EUCG) developed in
China recently. This EUCG process applies the manbuilt pinnate channel with controlled moving injection
point techniques to an abandoned coal mine for syngas
production through UCG. The results obtained from the
semi-industrial tests conducted at Zhong-Liang-Shan
772
G. X. WANG ET AL.
(ZLS) coal mine are discussed. This provides insight
towards providing the improved design and operation
of the UCG process to maximize utilization of the
abandoned coal resource.
SEMI-INDUSTRIAL TESTS OF THE EUCG
SYSTEM
Features of the EUCG process
Traditionally, an underground gasifier is constructed
by directional drilling, based on technology originally
developed in the former Soviet Union and tested also
in the US and Europe. This method is conceptually
very simple, but relatively difficult to control the
reaction and produce a consistent gas quality under a
variety of geological and coal conditions in practice.
The process involves a controlled retractable injection
procedure (CRIP), by which a series of dedicated
inseam boreholes need to be created using drilling and
completion technologies.[10] Most of the well drilling
and completion technologies are established and can be
used to construct inseam boreholes for UCG, usually
adapted from the upstream oil and gas industry.[11] The
CRIP relies on a moveable injection point between inlet
and outlet boreholes and uses oxygen, rather than air for
gasification, aiming to control the transport of the gases
from inlet to outlet boreholes. This technology will be
tested with modifications in this study.
The CRIP technology has been successfully used in
the Australian energy and mining industry to improve
the conventional UCG system using closely spaced vertical boreholes.[12] This improvement will significantly
shorten the internal pathway of gases in the coal seam,
resulting in the UCG process easily controllable. Technically, this method mostly suits production of synthetic
gas in situ, with a view to exploitation of unmineable deep coal. However, implementation of this UCG
process also implies the increased costs as it needs
increased numbers and depth of well boreholes, and
more reliable drilling and completion technologies as
well.
However, a considerable amount of the coal is
supplied by small-scale, geographically scattered coal
mines in China. Most of these coal mines only have
recovery rates below 50%.[13] Consequently, there exists
a large number of abandoned mine shafts, resulting in
the increased coal resources wasted. UCG is a potentially useful method to access these coal resources for
additional energy supply in a way that facilitates supply
of energy for domestic cooking and heating or industrial
use. However, it is often difficult to apply the conventional UCG technologies to such coal resources due
to lack of reliable operation method and undetermined
scattering of coal seams. To maximize the utilization
of these coal resources, an improved UCG method was
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
developed in China, by which abandoned mine shafts
can be used to construct the underground gasifiers. Such
gasifiers usually consist of man-built channels and pinnate horizontal wells in underground coal seams, mainly
making use of abandoned mine shafts. As a result, the
UCG process becomes more easily controllable and
the coal combustion and gasification take place more
efficiently, forming an EUCG process. In this EUCG
process, usually operated by alternating air and steam
injections, newly drilled boreholes in coal seams or
pipes constructed through abandoned mine shafts are
used to communicate with the surface. The main features of the EUCG process are its capacity to provide a
mechanism for gaining additional value from an abandoned or difficult-to-mine coal through an alternative
technology for UCG. It can take advantage of the more
controllable conditions permitted by the pre-constructed
gasification channels. Well-designed man-built channels and pinnate horizontal wells can effectively prevent abnormal surface cave-ins which sometimes happen during conventional UCG processes and causes the
unstable UCG or failure in UCG operation.[14,15]
The EUCG process is typically shown in Fig. 1.
It applies the man-built pinnate channels and controlled moving injection point technology, consisting of
five systems: air/stream injection, underground gasifier,
syngas cleaning-up, production gas transfer and process monitoring. In this process, air with/without oxygen addition and pressure water/steam can be injected
into underground gasifier through blast blowers and
a water/steam system, respectively. The blast blowers
were installed in pre-constructed underground wells.
The syngas produced by the gasifier was first introduced to the main syngas drainage pipes through a
syngas pathway (tunnel K3 or K4 in this case) and a
syngas collecting room (i.e. chamber C1). The syngas
pathway and collecting room were pre-constructed, connecting the gas chamber to the coal seam to be gasified.
The gas chamber was equipped with a water injection
system, providing initial cooling for high temperature
syngas with water injection. The raw syngas entered
into the main syngas drainage pipes which were sealed
with water and then sequentially introduced into the
cooling tower, tar extractor, dust arrester and filtrate
tank. Finally, the syngas was induced into clean gas
drainage pipes by a drawing fan on surface for further
treatment of desulfation and decarbonization, providing
the clean fuel (production syngas) for storage and industrial and/or residential uses. The key component is the
design of the EUCG gasifier which will be discussed
later.
To optimize the coal utilization and meet the increasing requirement for clean fuel, the ZLS coal mine
launched this EUCG project early in 2005 for a series
of semi-industrial tests. The ZLS coal mine, situated
near Chongqing in southwest China, has a methaneenriched coal reserve of more than 78 million tons
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SEMI-INDUSTRIAL TESTS AT ZHONG-LIANG-SHAN COAL MINE
Figure 1. An enhanced UCG system (EUCG) used in ZLS
coal mine.
(Mt), containing more than 400 billion m3 coal-bed gas.
The coalfield distributes about 10 km in length along a
south–west orientation, and consists of ten coal seams
with a total thickness of 9.42 m. Currently, there are two
underground mining shafts at the south and west ends,
respectively, which have produced approximately 25 Mt
coal since mining operations began. The mining operation also produces substantial volumes of coal mine
methane (CMM), which until recently was released to
atmosphere.
Design and construction of the EUCG gasifier
The design and construction of the EUCG gasifier,
which depends on the local geological conditions
and special requirements for the UCG process, plays
an important role in achieving stable operation and
expected performance of UCG process.
The site for the UCG field tests includes six coal
seams within an area of 130 m × 130 m at +150 m
above sea level, named with K1 to K5 and K7 as
indicated in Fig. 2a. These coal seams distribute almost
side-by-side with each other with an inclined angle
of approximately 65–70◦ toward northwest and are
separated by two fracture planes (i.e. F0 and F10)
as shown in Fig. 2a. Coal seams K3 and K4 were
chosen for gasification while coal seams K1 and K2
above +150 m had been mined and formed two empty
shafts nearly up to the cross fracture plane F0. The
latter was kept closed and has very low permeability
to the coal seams K3 and K4. These coal seams are
1.15 m and 1.45 m in thickness, and 23 m and 21 m in
width (incline), respectively. The roof and floor rocks
in contact with the coal seams are sandstone and clay
marl, respectively. All the coal seams have no direct
connection to aquifer.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
(a)
(b)
Figure 2. Configuration of the EUCG gasifiers: (a) the
profile chart; (b) the cross-section.
Figure 2b illustrates the partial cross-section of the
UCG system at +150 m above sea level, consisting
of a main channel, connecting tunnels, seal cabins for
air/steam injection, a syngas connecting chamber and
pipelines. Coal seams K3 and K4 were chosen for
gasification while coal seams K1 and K2 had been
mined and formed two empty shafts. The main channel,
originally pre-constructed for degasification and water
drainage associated with coal mine operation, was
modified by adding a gas chamber (Fig. 2b) at ends
of the coal seams K3 and K4 for syngas collection
and water drainage. A tunnel was built in coal seam
K7 horizontally, connecting the main channel with
the coal seams K3 and K4 to allow installation of
pipelines connecting to air/oxygen blast blowers and
the water/steam supply system located on surface for
air/steam injection into the coal seams for gasification
(Fig. 1). A brick wall (i.e. I3 in Fig. 2) was constructed
along the tunnel to isolate the coal seam K7 from
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
773
774
G. X. WANG ET AL.
Asia-Pacific Journal of Chemical Engineering
ones to be gasified for safety. Two seal cabins made
from concrete and refractory bricks were installed at the
entrances with short tunnels to connect with coal seams
K3 and K4, respectively, to ensure the injected air/steam
directly transfer into the coal seams for gasification. The
collecting chamber was constructed using refractory
bricks and concrete to prohibit the syngas from flowing
into the channel (Fig. 2) and connected to the gas clean
system surface through pipes installed along the main
channel. The syngas, cooled down in the collecting
chamber using water, is introduced to the syngas clean
system using gas pipe and the dust/slurry with water
can be drained through water pipe to precipitation pool
on surface for further cleaning (Fig. 1).
Before construction of the seal cabins, a resistance
portfire was pre-installed by placing under loosely stock
wood and dry coal blocks in each connecting tunnel
in coal seams K3 and K4. The portfire is operated
through electricity to ignite the coal seams for starting
up the gasification process. To prevent gas percolation,
the connecting tunnels close to the coal seams (gasifier) were constructed with refractory bricks and heatresistant packing materials, forming a protective wall
of 1 m thickness. Moreover, a number of thermocouples were pre-installed along the gasifier, in conjunction
with other computerized sampling instruments, provided monitoring and controlling for the temperature,
pressure, flow rate and composition of production gas.
Operational conditions and test procedure
Coal seams K3 and K4 (Fig. 2) to be tested for
underground gasification in this study comprises from
bright and dull coals, classified as cooking coal. The
coals have been characterized in laboratory, including
proximate and ultimate analyses, and ash fusibility and
CO2 reactivity tests. The typical properties of the coals
are listed in Table 1. The low heat value of the coals
is 25.32–28.10 MJ/kg; initial deformation temperature
of ash is <1150 ◦ C; softening temperature is about
1360 ◦ C; and flow temperature is higher 1450 ◦ C.
The UCG field tests were conducted on June 19, 2005
and operated continuously for a period of 3 months. The
test was started by igniting the pre-installed portfire in
the underground gasifier with air injection. Followed
the ignition, three cycles of operations were carried out,
that is
1. UCG test using pure steam injection;
2. UCG test using air/steam mixture injection;
3. UCG test using O2 -enrichment blast.
The air injection was used for a basic operation
and almost performed through the whole test period.
To maintain stable operations of the UCG process,
the given air/steam injection points were used based
on the design which will be described later. The O2
concentration in the injectant and the injection pressure
were controllable as required by the procedures. The
key operating conditions of these UCG tests are listed
in Table 2. Over this period, the UCG operational
conditions and their effect on gas production were
investigated.
Moreover, a pressure check on the underground
gasifier and the man-built pinnate channel including
the pipe network system must be carefully conducted
before starting the field tests to ensure the operation
safety of the UCG test system. The pressure check was
carried out by measuring the gas leak rate under various
pressures from 5 to 30 kPa for 2–4 days followed the
CH4 drainage from the gasifier. The measured gas leak
rate was reported less than 5% which has been set as
a critical safety index for the UCG test system. During
Table 2. Operational conditions of the EUCG field tests.
Air + steam
Injection media
Air
Flow rate, m3 /h
Pressure, kPa
Temperature, ◦ C
Case 1 – O2 , %
Case 2 – O2 , %
Case 3 – O2 , %
Steam
Flow rate, m3 /h
Pressure, kPa
Temperature, ◦ C
Gas temperature control
Outlet tunnel, ◦ C
Gas drainage pipe, ◦ C
Gas clearing, ◦ C
Air inlet cabin, ◦ C
Gas pressure control
Outlet tunnel, kPa
Gas clearing, kPa
Others
CO in inlet and outlet tunnel, ppm
O2 in clean gas, %
1400–1650
11–16
28–31
21 (natural air)
40
65
∼450
>18–25
>100
<80
<80
<50
<70
<10
<−5
<240
<2
Table 1. Typical properties of ZLS coal.
Component, air-dry base, wt. %
C
72.09
H
O
N
S
Volatile
Vad , wt. %
3.86
1.38
1.06
2.55
16.04
Moisture
Ash
Aad , wt. %
Mad
Mtot.
Density
ρc , kg/m3
Lower heat value, MJ/kg
18.19
1.06
5.3
1420
26.71
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
SEMI-INDUSTRIAL TESTS AT ZHONG-LIANG-SHAN COAL MINE
the UCG tests, a UCG-PMS200 computer system on the
surface was used for online control of the UCG process,
including data logging and other safety monitoring such
as temperatures, pressures and CO/CH4 concentrations
at all the designed points.
the gasifier was successfully ignited on first try within
a short time. After about 3 h following the ignition,
the system started to produce syngas in which the
composition of various components apparently changed.
As a result, the product rate and the heat value of
the syngas were rapidly increasing towards a normal
production stage with air injection, implying that the
ignition was successful. Then maintaining air injection
operation by controlling 14–16 kPa blast pressure led
to a production rate of 1500–2200 Nm3 /h and the heat
values of 1200–1600 kCal/Nm3 , as shown in Fig. 3.
After 2-1/2 days, the discharge tower on surface was
fired and a stable EUCG process was established.
Figure 4 shows the gas composition and its heat value
for the first 10 days, following the successful ignition
of the gasifier. This represents the syngas composition
at the outlet, consisting of 10–30% H2 , 15–25% CO
and 5–8% CH4 .
The steam injection operation was tested three times
on July 4, September 18–19 and 23, 2005, respectively,
in this study. The first steam injection test was conducted after about 2 weeks under the stable UCG operation with air injection. Within a short time before being
switched to steam injection operation, an increased
blast flow rate was implemented by increasing the blast
RESULTS AND DISCUSSION
Overall performance
The EUCG field tests provide clean syngas of 1500–
2200 Nm3 /h with a heat value of 1200–1600 kCal/Nm3
under basic operation using air injection. Depending
on the different operational conditions, the overall
performances of the EUCG were different and resulted
in variable product rates and heat values of the syngas.
A summary of the overall performances under various
operation conditions are shown in Figs. 3 to 5.
Successful ignition of the gasifier is a most important
stage of the EUCG process as it is essential to establish
the stable UCG. Unlike the traditional UCG process,
the EUCG one allows pre-construction of a portfire
in the underground. This makes the ignition of the
gasifier easier to control. In these EUCG field tests,
Product rate
or heat value of syngas
2500
Heat walue - kCal/m3
Product rate - m3/h
2000
1500
1000
500
Ignition stage on Jun 19-22, 2005
0
10
20
30
40
Time, hours
50
60
70
2500
H2
CO2
CO
O2
CH4
Heat value
2000
1500
1000
500
19-Jun-05
20-Jun-05
Heat Value, kCal/m3
50
45
40
35
30
25
20
15
10
5
0
1:50:00
7:20:00
9:00:00
11:45:00
11:45:00
13:01:00
13:30:00
14:13:00
15:06:00
15:37:00
15:59:00
16:00:00
17:22:00
18:13:00
19:16:00
20:25:00
19:30:00
5:45:00
6:37:00
10:50:00
10:10:00
14:00:00
19:05:00
22:10:00
6:30:00
12:00:00
10:00:00
23:40:00
7:40:00
10:00:00
11:55:00
16:00:00
4:00:00
11:35:00
16:00:00
19:45:00
4:00:00
11:37:00
15:45:00
23:35:00
7:20:00
9:50:00
11:38:00
19:40:00
3:45:00
11:30:00
15:50:00
19:55:00
2:00:00
11:35:00
19:45:00
23:37:00
6:46:00
12:00:00
19:45:00
23:52:00
4:07:00
7:10:00
15:50:00
19:30:00
Gas composition, %
Figure 3. Product rate and heat value of syngas at ignition stage of the EUCG field tests.
0
21-Jun 22-Jun 23-Jun 24-Jun 25-Jun 26-Jun 27-Jun 28-Jun 29-Jun
Figure 4. The composition of product syngas obtained from the EUCG field tests using air gasification.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
775
G. X. WANG ET AL.
Asia-Pacific Journal of Chemical Engineering
Product rate
or heat value of syngas
4000
(a) Steam Injection
on July 4, 2005
3000
Product rate - m3/h
Heat Value - kCal/m3
2000
1000
0
2
4
6
8
10
12
14
16
18
20
Time, hours
Product rate
or heat value of syngas
4000
(b) O2 Injection
on Sept 15-16, 2005
3000
2000
Product rate - m3/h
Heat Value - kCal/m3
1000
0
2
4
6
8
10
12
14
16
18
20
Time, hours
4000
Product rate
or heat value of syngas
776
(c) Air/Steam Injection
on Sept 22-23, 2005
3000
Product rate - m3/h
Heat Value - kCal/m3
Product rate - m3/h
Heat value - kCal/m3
2000
1000
0
2
4
6
8
10
12
14
16
18
20
Time, hours
Figure 5. Product rate and heat value of syngas for the EUCG field tests under various operation
conditions: (a) steam injection; (b) O2 injection; and (c) air/steam injection (unfilled symbols for
simultaneous operation and filled symbols for alternative operation).
pressure to 18–20 kPa to increase the coal combustion
and hence the gas temperature in the gasifier before
injecting the steam. The steam injection operation was
continuously maintained for approximately 14 h. During steam injection, the blast blower was switched
off, and the steam flow rate was controlled at about
450 m3 /h. The results of this steam injection test are
typically shown in Fig. 5a, and the results from other
steam injection tests are similar. As can be seen, the
product rate of the syngas under the steam injection
operation typically dropped down to about 750 m3 /h
compared with the air injection operation. However, the
heat value of the syngas from steam injection UCG was
significantly increased due to the increased supply of
oxygen, giving the heat values of 2600–3300 kCal/m3
compared with 1200–1600 kCal/Nm3 under the air
injection operation.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
The UCG test with O2 -enrichment blast was carried out on September 15–16, 2005 and the results are
shown in Fig. 5b. The oxygen used was supplied in bottles produced in a local industrial manufactory. During
this test, the blast blower was switched off. The flow
rate of oxygen was controlled at about 1.5–1.8 m3 /h.
The results showed that the O2 -enrichment blast directly
led to promotion of temperature in the gasifier and
improvement of syngas quality. Therefore, the higher
heat values of syngas under the O2 -enriched operation
were observed, giving the 2281 kCal/m3 on average
(Fig. 5b) compared with the 1305 kCal/m3 on average
for basic operation using air injection (Figs. 3 and 4).
Figure 5c shows the typical results of a UCG test
using air/steam mixture injection conducted on September 22–23, 2005. There are two types of air/steam
mixture injection operations being tested: simultaneous
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
SEMI-INDUSTRIAL TESTS AT ZHONG-LIANG-SHAN COAL MINE
60
3000
lue
50
t
ea
2500
Va
H
40
2000
H2
30
1500
CO
N2
20
CO2
10
500
CH4
Enhancement techniques
There are a number of factors which affect the UCG
process, and the following methods have been thought
to be significant in achieving stable operation and
enhanced performance for underground coal gasification, which has been demonstrated by the EUCG test in
this study.
Controlled moving injection point technique
The CRIP technology was widely used in the traditional
UCG process,[2,3,10,12] which is also approved by the
EUCG field that using a moveable injection point
is very important in control of the coal gasification
conducted in a given route underground. In this study,
two kinds of moveable injection points were tested. One
is so-called CRIP and another the controlled advancing
injection point (CAIP). The results have shown that
the water gas production with CAIP is better than
CRIP, and the CAIP operation can ensure that the
gas flow is comparatively controllable. Therefore, the
CAIP moveable injection point technique was proved
as an effective technique to enhance the UCG process.
Using EUCG technique, the UCG performance has been
improved significantly, and a higher efficiency of heat
and a higher quality of the production syngas has been
achieved.
O2 -enriched operation
The operating conditions, most important the oxygen
content of the injected stream, affect quality of the production syngas. Figure 6 gives the syngas composition
and heat values measured in the field test as a function
of oxygen in injected air. It can be seen that the gas
quality can significantly be improved with O2 -enriched
operation. This may be attributed by two major factors
as follows:
First, the O2 -enriched operation provides an increased
O2 concentration and O2 partial pressure in the injection
stream which leads to accelerated diffusion of oxygen in
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
1000
Heat Value, kCal/m3
and alternative operations. The simultaneous operation
was conducted by running two gasifiers simultaneously,
and the results were indicated by the curves with
unfilled symbols in Fig. 5c. The alternative operation ran the two gasifiers alternatively, as shown by
the curves with filled symbols in Fig. 5c. In the former, each gasifier operated under a blast pressure of
10–12 kPa and a steam flow rate of about 400 m3 /h.
For the latter, the gasifier with steam gasification used
a steam flow rate of about 150 m3 /h, and the gasifier
with air gasification was run at an air flow rate of about
800 m3 /h, while both the blast pressures were maintained at 12 kPa. Each test ran about 12–18 h, provided
the clean syngas at 950–1700 m3 /h with heat values of
1600–2700 kCal/m3 .
Gas composition, %
Asia-Pacific Journal of Chemical Engineering
O2
0
10
20
30
40
50
60
70
0
80
O2 in injected air, %
Figure 6. Influence of O2 in injected air on properties of
product syngas.
coal and hence the gasification and combustion of coal.
As shown in the field tests, the UCG can produce higher
temperature gases and the temperature measured in
front of the combustion flame was reported to approach
1250 ◦ C on average under the 65% O2 -enrichement
ratio, compared with 1180 ◦ C using natural air (21%
O2 ) and 1230 ◦ C for operation of 40% O2 . Thus, the
operation with O2 -enrichment can maintain a relatively
active oxidation zone in the gasifier with a higher
temperature as well as the larger temperature gradient in
front of the combustion flame. This will largely benefit
the coal gasification as revealed experimentally.[16,17]
Second, the O2 -enriched operation significantly
reduces N2 in the injection stream. This not only saves
valuable energy required for heating the redundant
nitrogen gas and benefits the coal gasification and combustion under a higher temperature, but also increases
all the component gas concentrations in-product syngas.
This can generally result in an improved heat value of
the syngas, as evidenced by the recorded data during the
field tests. Therefore, reduction in N2 in the injection
stream associated with the O2 -enriched operation provides significant contributions for enhancement of the
UCG process.
As a result of the improved combustion of coal
in UCG process under the O2 -enriched operation, the
increased heat release promotes the subsequent gasification reactions and hence accelerates the release of
volatiles and decomposition of higher hydrocarbons
into H2 and CH4 . The improved reaction kinetics for
combustion and gasification in O2 -enriched operation
Asia-Pac. J. Chem. Eng. 2009; 4: 771–779
DOI: 10.1002/apj
777
G. X. WANG ET AL.
Asia-Pacific Journal of Chemical Engineering
improves the quality of the production gas particularly
the valuable CO, H2 and CH4 contents. Correspondingly, the heat value of the syngas also improves with
O2 -enrichment (Fig. 6). The H2 -to-CO ratio, which may
be important if the product gas is to be used for chemical synthesis, is sensitive to O2 content below about
40%.
Variation of operational pressure
To enhance the UCG efficiency, properly adjusting
the pressure of the gasifier during UCG process while
using O2 -enriched operation may intensify the UCG and
improve the quality of product syngas. This is because
increased pressure provides a higher gas density and
hence the improved conditions for gas–solid (coal particles) contact, leading to the increase of the gasification
reaction rate.[16,18] Figure 7 shows the results from this
field tests that aimed to investigate the effect of operational pressure on the EUCG process, mainly accounting for the quality of product gas. The investigation
conducted was associated with the steam injection test
as described earlier and showed that the gas quality in
H2 and CO concentrations can remarkably be improved
by cyclically changing the operational pressure.
In the field test, cyclically changing of the pressure
was realized by controlling the flow rate with two methods. One is to keep the same blowing velocity in the
inlet and then open and close the outlet cyclically. The
other is to open and close the inlet and outlet alternatively to make the flow rate increased and decreased
intermittently. Opening and closing the outlet cyclically
will intermittently change the flow velocity of the gas
in the gasification tunnel. This makes the direction of
heat transfer cyclically changed with reference to that
of the main air stream in the gasification tunnel, and
thus the heat transfer in the gasifier can be improved
significantly. The field test shows that under the condition of cyclically changing the pressure, the heat loss
is reduced by 58%. The heat rate and gasification rate
are approximately 1.2 and 1.6 times higher than those
under the fixed pressure condition. As a result, the heat
value of syngas is increased by 26% compared with the
fixed pressure operation.
CONCLUSIONS
1. A semi-industrial test on the EUCG process with the
man-built pinnate channels has been conducted successfully and showed that its designs are reasonable
and feasible.
2. The operation techniques of the enhanced UCG
with controlled moving injection points, O2 -enriched
operation and variation of operational pressure can
ensure that the gas flow is comparatively controllable, and hence improve the UCG performance significantly. As a result, both a higher efficiency of
heat and a higher quality of the production syngas
have been achieved in the EUCG tests.
3. The semi-industrial tests on the EUCG process were
operated continuously for a period of 3 months, providing clean syngas at 2200–3600 Nm3 /h with heat
value of 1200–1600 kCal/Nm3 under basic operation using air injection. The syngas composition
at the outlet consists of 10–30% H2 , 15–25% CO
and 5–8% CH4 , which has completely satisfied the
designed targets. The field tests show feasibility
in applying the EUCG technology to ‘re-mine’ the
abandoned coal resources, particularly in China.
Acknowledgements
50
The authors acknowledge financial support from the
Australian Research Council (ARC) and from the key
Project of Chinese Ministry of Education (No. 02019).
H2
CO
CO2
45
Concentration, %
778
40
REFERENCES
35
30
25
20
0
2
4
6
8
Pressure, kPa
10
12
14
Figure 7. Influence of pressure on composition of product
syngas.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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779
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