Method of using a structural member of anti-sulfur-attack cr-ni-al-si alloy steel for coal gasification system

A structural member to be subjected to a hot gas atmosphere produced through reaction between coal and a gasifier such as oxygen, air, steam or hydrogen, in a gasification furnace for example. The structural member is made of an anti-sulfur attack Cr-Ni-Al-Si alloy steel which has a composition essentially consisting of, by weight, 0.03 to 0.3% of C, 1 to 10% of Si, not greater than 2.0% of Mn, 8 to 14% of Ni, 16 to 20% of Cr, 0.5 to 10% of Al and the balance not less than 50% of Fe.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel metallic material suitable for use 
as material of constituent members of coal gasification processes and 
other processes in which the constituent members are used in an atmosphere 
of hot gas containing sulfides. More particularly, the invention is 
concerned with a sulfidation resisting Cr-Ni-Al-Si alloy which is capable 
of suppressing high temperature corrosion caused by combustion gases and 
other product gases. 
The oil crisis triggered by the Arab-Israeli war of 1973 has given rise to 
a demand for developing alternative fuels as substitutes for petroleum. 
Among these substitute fuels, coal is considered most significant as the 
basic fuel of the future because of an abundancy of deposits as compared 
with petroleum. However, since coal is a solid fuel, it is difficult to 
store and transport as compared with liquid fuel. This in turn has 
promoted development of techniques for converting coal into a fluid fuel 
which is easy to store and transport, and also for obtaining clean energy 
sources through removal of ash, SOx, etc. Typical examples of such 
techniques are liquefaction and gasification of coal. The gasification of 
coal is a process in which coal is caused to react with a gasifier such as 
oxygen, air or steam, thereby obtaining a product gas consisting mainly of 
hydrogen, carbon monoxide, methane and so forth. Three types of coal 
gasification processes have been proposed: namely, the fixed bed type, 
fluidized bed type and entrained bed type. The process type, i.e., the 
furnace type, and the reaction temperature are selected in accordance with 
the use of the product gas. 
A typical example of a furnace used for the fixed bed type process is a 
furnace called a "Lurgi furnace." A large scale commercial plant of this 
type is operating in Sasol in the Union of South Africa. In this process, 
lumps of coal of sizes ranging between several tens of millimeters and 
several millimeters are fed from the top of a furnace and are gasified 
while the coal is held in the form of a bed which is kept stationary. The 
gasification is effected by the heat which is produced as a result of 
partial burning of coal with the aid of a gasifier which is supplied from 
the bottom of the furnace. This process is advantageous in that a high 
thermal efficiency is obtained by the counter-flow contact between the 
coal moving downwardly and the gasifier flowing upwardly, but suffers from 
various disadvantages such as generation of tar in the low temperature 
region due to a large temperature gradient in the furnace. In addition, 
this process cannot be applied to the processing of powdered coal and 
caking coal, and the processing rate is impractically small. 
The fluidized bed type process and the entrained bed type process do not 
suffer from the disadvantage of the fixed bed type process, and are also 
capable of treating the remnant of crude oil which has to be utilized. For 
these reasons, intense study and development of these types of coal 
gasification process are being vigorously undertaken, particularly in U. 
S. A. and West Germany. In the fluidized bed type process, powdered coal 
of particle sizes falling within a predetermined range of between several 
millimeters and several hundreds of microns are charged into a 
gasification furnace. The powdered coals are fluidized and gasified by a 
gasifier which is also blown into the furnace. By virtue of the use of 
powdered coals, this process exhibits a superior heat conduction through 
convection, so that the reaction takes place uniformly, thus reducing the 
tendency for tar to be generated as a byproduct. The disadvantage of this 
type of coal gasification process is that the coal ued in this process has 
to have such a particle size that adequate fluidity of the coal is 
maintained. 
The entrained bed type process is a process in which pulverized coal of 
particle sizes ranging between several tens of microns and several 
hundreds of microns is blown into the furnace from the bottom and is 
gasified at a high temperature. This process can gasify any type of coal 
without requiring mechanical stirring or pre-treatment, and is able to 
gasify the coal almost completely without generation of tar. This process, 
however, requires pulverization of the coal, and difficulty is experienced 
in controlling the residence period of the coal in the furnace, as well as 
in connection with certain problems concerning the system such as 
facilities for discharge of slag and utilization of sensible heat. 
The metallic materials used in coal gasification furnaces are inevitably 
subjected to high temperature as a result of burning of the coal, unlike 
the material used in coal liquefaction systems. This imposes a problem of 
corrosion of the metallic materials by hot gases such as CO, CO.sub.2, 
H.sub.2, H.sub.2 S and CH.sub.4 which are generated as a result of burning 
of the coal. In particular, H.sub.2 S at high temperature causes heavy 
corrosion which is usually referred to as sulfur attack. 
In order to put the developed process into practical use on a greater 
scale, it is necessary to construct a highly reliable plant through 
development of economical materials or working techniques which enable the 
constituent elements of the furnace to withstand severe conditions in the 
gasification process. Thus, the constituent metallic materials used in 
coal gasification plants are required to withstand the hot corrosive coal 
gases to which they will be exposed, particularly H.sub.2 S which causes 
serious sulfur attack. 
Among various austenitic steels proposed hitherto, AISI 304 (18Cr-8Ni 
steel), AISI 316 (18Cr-8NiMo steel), AISI 321 (18Cr-8Ni-Ti steel) and AISI 
347 (18Cr-8Ni-Nb steel) are used broadly as the constituent materials for 
various plants by virtue of their high-temperature strength and 
workability, as well as low cost and the ease with which they can be 
manufactured. The use of these austenitic steels is spreading also to the 
field of piping in nuclear plants and boilers, as a result of improvements 
in anti-stress corrosion cracking sensitivity through reduction of C 
content and improvements in anti-steam corrosion properties by refining of 
the crystal grains. Using these materials for which the ease of production 
and other properties are known is advantageous from the viewpoint of 
design, cost and reliability. 
These austenitic stainless steels, however, exhibit serious corrosion 
degradation due to corrosion by gases at high temperatures, particularly 
grain boundary attack by sulfides. 
It has been proposed that the anti-corrosion properties at high temperature 
may be improved by increasing the Cr content. Examples of materials having 
increased Cr content are: AISI 309S (21Cr-13Ni steel), AISI 310S 
(25Cr-20Ni steel), Incoloy 800 (21Cr-32Ni-Ti, Al steel), Inconel 671 
(50Cr-50Ni steel) and so forth. These materials have been proposed in view 
of the fact that inclusion of at least 20 to 25% of Cr is necessary for 
attaining high corrosion resistance of materials in long use. Attention 
has been given to these materials because of their ease of manufacture and 
good workability, but the improvement in their resistance to corrosion by 
sulfides such as H.sub.2 S is still unsatisfactory due to the fact that 
the Ni content is necessarily increased in correspondence with the 
increase in the Cr content in order to maintain the workability and 
austenitic structure. 
Under these circumstances, there is an increasing demand for development of 
an inexpensive material easy to produce and having high workability, as 
well as high corrosion resistance equivalent to that of AISI 309S, AISI 
310S and Incoloy 800. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide an alloy steel which 
exhibits high anti-sulfur attack property in an atmosphere of hot coal 
gas. 
Through intense study and experiments on the materials which can be used 
for structures to be employed in coal gasification processes, the present 
inventors have found that the grain boundary corrosion caused by sulfides 
can be suppressed in ordinarily used austenitic steel materials such as 
AISI 304, AISI 316, AISI 321 and AISI 347 if a suitable amount of Si is 
added besides Al to these austenitic steels. Such austenitic steels 
containing Si and Al in combination have proved to exhibit superior 
corrosion resistance in a coal gas atmosphere, i.e., high resistance to 
grain boundary corrosion by sulfides at high temperatures, and are 
therefore better suited for use as the structural materials employed in 
coal gasification plants than known austenitic steels such as AISI 309S, 
AISI 310S and Incoloy 800. 
According to the invention, there is provided a structural member to be 
subjected to a hot gas atmosphere produced through reaction between coal 
and a gasifier such as oxygen, air, steam or hydrogen, the structural 
member being made of an anti-sulfur attack Cr-Ni-Al-Si alloy steel which 
is prepared as follows. 
Namely, the alloy steel in accordance with the invention can be obtained 
from one of the following four types of steels: namely, a steel 
containing, by weight, not greater than 2% of Mn, 8 to 11% of Ni and 18 to 
20% of Cr; a steel containing not greater than 2.0% of Mn, 10 to 14% of 
Ni, 16 to 18% of Cr and 2 to 3% of Mo; a steel containing not greater than 
2.0% of Mn, 9 to 13% of Ni, 17 to 20% of Cr and not greater than 0.6% of 
Ti; and a steel containing not greater than 2.0% of Mn, 9 to 13% of Ni, 17 
to 20% of Cr and not greater than 1% of Nb+Ta. In one of these steels, the 
C content is increased to 0.03 to 0.3%, and 0.5 to 10% of Al and 1 to 10% 
of Si are added in combination, such that the balance is constituted by 
not less than 50% of Fe. 
The alloy steel in accordance with the invention exhibits a remarkable gas 
corrosion resistance at high temperatures, when used in a coal gas 
atmosphere generated in a coal gasification process in which a product gas 
consisting mainly of hydrogen, carbon monoxide and methane is produced 
through reaction between coal and a gasifier such as oxygen, air, steam 
and so forth. 
In case of the 18% Cr-8% Ni austenitic stainless steel, the gas corrosion 
resistance at high temperatures was significantly improved by the addition 
of a suitable amount of Al followed by addition of Si, even when the C 
content was increased to range between 0.03 and 0.3%. Steels having large 
Al and Si contents are rather inferior in workability. Therefore, the 
constituent member which requires a high workability of the material is 
formed by forging or rolling from an alloy steel which has comparatively 
small Al and Si contents, whereas the constituent member in which 
preference is given first to the corrosion resistance rather than 
workability is made by casting from an alloy steel having large Al and Si 
contents. It has thus been proved that various constituent members for use 
in coal gasification process can be obtained by suitably selecting the Al 
and Si contents of the alloy steel, without impairing the functions 
required for such members. 
The reasons for limitation of content in respect of each alloy element in 
the alloy steel of the invention will be described hereinunder. 
C: C is an important element because it serves as an austenite former and 
because it ensures a high strength at high temperatures. In order to 
permit the addition of Al and Si and to stabilize the structure as much as 
possible without impairing the corrosion resistance, the C content 
preferably ranges between 0.03 and 0.3%, more preferably between 0.07 and 
0.15%. 
Si: Si is a significant element for attaining the properties required for 
the alloy steel of the invention. For obtaining a high corrosion 
resistance, the Si content should be not smaller than 1%. Addition of Si 
in excess of 10%, however, causes saturation in the effect of improvement 
in corrosion resistance, and undesirably impairs the workability and the 
castability. The effect produced by the addition of Al varies according to 
the amount of addition of Si. When Si is added alone, no substantial 
improvement in the corrosion resistance is achieved when the Si content is 
not greater than 1.0%, and Si content should not be smaller than 2.0% if 
an appreciable effect is to be obtained. A greater effect is produced when 
Si is added in combination with Al than when it is added alone. The Si 
content preferably ranges between 3 and 5%. 
Mn: Mn serves as an austenite former but the Mn content is preferably 
relatively small because Mn tends to impair the oxidation resistance. For 
this reason, the Mn content is selected to be not smaller than 2%, 
preferably between 1 and 2%. 
Ni: Ni is one of the basic constituent elements of austenitic stainless 
steel. The Ni content should be not smaller than 8%, in order to maintain 
an austenite structure in spite of the addition of Al and Si which are 
ferrite formers. Addition of Ni in excess of 14% impairs the resistance to 
sulfur attack in a coal gas atmosphere. 
Cr: This element is the most fundamental element for improving the gas 
corrosion resistance at high temperatures. The Cr content should be 16% or 
greater but is limited to be not greater than 20% in view of the Ni 
content. 
Ti, Mo, Nb: Ti, Mo and Nb are elements which are effective in improving the 
high temperature strength through formation of safe carbides and nitrides. 
In order to obtain an appreciable effect, Ti, Mo and Nb content should be 
so selected as to be not greater than 0.6%, 2 to 3% and not greater than 
1%, respectively. Preferably, Ti content and Nb content should range 
between 0.2 and 0.5%, respectively. 
Al: Al is an important element which provides, in cooperation with Si, a 
superior anti-sulfur attack property. The Al content ranges between 0.5 
and 10%, preferably between 2 and 5%. This element improves the gas 
corrosion resistance at high temperatures even when it is added alone to 
the austenite stainless steel. The effect is further increased, however, 
when Al is added together with Si. In order to attain an appreciable 
effect, the Al content should not be less than 0.5%. Addition of Al in 
excess of 10% causes saturation in the improving effect and, instead, 
causes problems in workability and castability. For this reason, the Al 
content is limited to be not greater than 10%. When the need for 
workability is not so strict, an Al content ranging between 2 and 5% 
provides satisfactory corrosion resistance. When the material is to be 
forged, the Al content can be increased up to 10%. 
The alloy steel in accordance with the invention can contain other elements 
which are inevitably included in the course of production, besides the 
elements mentioned hereinabove. 
In some cases, the knowledge concerning the contents of elements as 
specified above may prove insufficient for the practical production of an 
alloy steel in accordance with the invention. There is also a risk that 
the alloy steel of the invention will become cracked during subsequent 
working. In order to obtain a material having practical utility, 
therefore, it is necessary to suitably adjust the contents of the alloy 
elements in relation to each other. 
More specifically, when Al is added alone, the material can be formed into 
sheets, bars and pipes, regardless of whether the work is done in a hot or 
cold state, provided that the Al content is not greater than 5%. However, 
an Al content exceeding 5% causes a risk of cracking during working. 
Addition of Si by an amount equal to the amount of Al causes a more 
serious effect on workability than in the case where Al is added alone. 
When Si is added together with Al, therefore, it is necessary to effect an 
adjustment of the alloy elements by increasing the C content, while 
suppressing the Ni and Cr contents, or to use the material in its as-cast 
state. Such an adjustment of the properties of alloy elements facilitates 
the application of the material of the invention to equipment and members 
which are subjected to gas atmosphere containing sulfides produced in coal 
gasification systems, e.g., the water-cooled wall tube of a coal 
gasification furnace, the constituent members of a heat exchanger, valves, 
nozzles and so forth. 
Briefly, the material in accordance with the invention is an anti-sulfur 
attack Cr-Ni-Al-Si alloy which has an improved resistance to grain 
boundary corrosion which is caused by hot gas produced as a result of 
reaction between coal and a gasifier such as oxygen, air and steam, 
particularly resistance to grain boundary sulfur attack by sulfides in 
such hot gas. The composition of the material in accordance with the 
invention contains: 0.03 to 0.3% of C; not greater than 2% of Mn; Ni, Cr, 
Mo, Ti and Nb in amounts falling within the ranges of austenitic stainless 
steels AISI 304, AISI 316, AISI 321 and AISI 347; 0.5 to 10% of Al in 
combination with 1 to 10% of Si; and the balance substantially Fe and 
impurities inevitably included during production. 
The invention will be described hereinafter with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Table 1 shows the chemical compositions of examples of alloy steels in 
accordance with the invention, together with comparison steels. The 
contents of elements in this Table are shown in terms of weight percent. 
In each steel, the balance is substantially Fe and inevitable impurities 
such as P, S, etc. The sample Nos. 1 to 18 are alloy steels in accordance 
with the invention, while sample Nos. 26 to 30 are comparison steels. 
These samples were prepared by vacuum-melting and casting the materials, 
followed by 1-hour water cooling at 1100.degree. C. Test pieces of 6 
mm.times.20 mm.times.25 mm were prepared from them. Sample Nos. 19 to 25 
are forged materials. The test pieces of the sample Nos. 19 to 25 were 
prepared after being heated for 30 minutes at 1100.degree. C. and 
subsequent water cooling, while the test pieces of sample Nos. 24 and 25 
were formed after 30 minutes of water cooling at 1150.degree. C. and 
subsequent water cooling. 
TABLE 1 
__________________________________________________________________________ 
No. C Si Mn Ni Cr Al Mo Ti Nb Cu 
__________________________________________________________________________ 
Alloy Steel 
of invention 
1 0.08 
2.15 
1.75 
8.81 
18.75 
0.51 
2 0.07 
2.31 
1.69 
8.77 
18.69 
1.09 
3 0.07 
2.25 
1.81 
8.68 
18.81 
2.11 
4 0.08 
2.18 
1.79 
8.15 
18.62 
5.09 
5 0.09 
2.19 
1.72 
8.68 
18.65 
9.88 
6 0.28 
2.15 
1.74 
8.29 
18.71 
9.53 
7 0.08 
1.08 
1.71 
8.61 
18.75 
2.15 
8 0.08 
3.15 
1.81 
8.72 
18.79 
2.21 
9 0.07 
5.18 
1.74 
8.69 
18.68 
2.18 
10 0.08 
9.88 
1.76 
8.80 
18.69 
2.23 
11 0.29 
5.16 
1.75 
9.62 
18.18 
5.11 
12 0.28 
9.11 
1.89 
10.59 
18.11 
9.51 
13 0.07 
2.15 
1.79 
12.25 
17.93 
2.23 
2.05 
14 0.08 
5.13 
1.82 
13.65 
16.23 
5.05 
2.01 
15 0.08 
2.21 
1.91 
12.61 
17.15 
2.18 0.45 
16 0.07 
5.51 
1.93 
12.75 
17.19 
4.98 0.39 
17 0.07 
2.18 
1.82 
12.63 
17.18 
2.01 0.38 
18 0.07 
5.01 
1.85 
12.75 
17.25 
4.99 0.39 
Comparison 
Steel 
19 
AISI 304 
0.05 
0.46 
0.83 
8.79 
18.21 
20 
AISI 316 
0.06 
0.63 
0.83 
11.79 
17.52 2.23 
21 
AISI 321 
0.03 
0.87 
0.92 
9.13 
17.43 0.29 
22 
AISI 347 
0.03 
0.63 
1.02 
9.43 
17.76 0.43 
23 
AISI 631 
0.08 
0.78 
0.69 
7.35 
17.81 
1.08 
24 
AISI 310S 
0.05 
0.75 
0.99 
19.38 
25.28 
25 
Incoloy 800 
0.08 
0.37 
0.90 
33.69 
19.94 
0.41 0.43 0.48 
26 0.07 
0.73 
1.75 
8.15 
18.81 
0.49 
27 0.06 
0.71 
1.72 
8.23 
18.92 
2.12 
28 0.06 
0.75 
1.85 
8.92 
18.63 
4.48 
29 0.07 
0.68 
1.79 
9.01 
18.78 
6.25 
30 0.06 
0.59 
1.76 
8.85 
18.77 
9.49 
__________________________________________________________________________ 
These test pieces were subjected to a corrosion test in which they were 
held for 100 hours within an atmosphere simulating a coal gas, containing 
24% of H.sub.2, 18% of CO, 12% of CO.sub.2, 6% of CH.sub.4, 0.5% of 
H.sub.2 S and the balance H.sub.2 O. The test temperature was 850.degree. 
C., while the pressure was 30 atm. The corrosion loss is expressed in 
terms of the sum of reduction in thickness and depth of corrosion (grain 
boundary corrosion). The results of the test are shown in Table 2. 
TABLE 2 
______________________________________ 
Thickness Internal Corrosion 
No. reduction corrosion 
loss 
______________________________________ 
Alloy steel of invention 
1 0.306 0.089 0.395 
2 0.214 0.075 0.289 
3 0.050 0.051 0.101 
4 0.025 0.025 0.050 
5 0.024 0.027 0.051 
6 0.024 0.025 0.049 
7 0.090 0.062 0.152 
8 0.055 0.051 0.106 
9 0.055 0.032 0.087 
10 0.054 0.026 0.080 
11 0.054 0.027 0.081 
12 0.046 0.027 0.073 
13 0.084 0.052 0.136 
14 0.044 0.028 0.072 
15 0.087 0.048 0.135 
16 0.046 0.025 0.071 
17 0.086 0.058 0.142 
18 0.047 0.028 0.075 
Comparison Steel 
19 AISI 304 0.371 0.150 0.521 
20 AISI 316 0.382 0.182 0.564 
21 AISI 321 0.342 0.215 0.557 
22 AISI 347 0.543 0.148 0.691 
23 AISI 631 0.293 0.075 0.368 
24 AISI 310S 0.195 0.152 0.347 
25 Incoloy 800 
0.324 0.149 0.463 
26 0.313 0.102 0.415 
27 0.046 0.088 0.134 
28 0.029 0.052 0.081 
29 0.028 0.051 0.079 
30 0.024 0.054 0.078 
______________________________________ 
As will be understood from Table 2, the alloy steel of the invention 
exhibits a remarkably improved resistance to corrosion by gas at high 
temperature as compared with comparison steel Nos. 19 (AISI 304), 20 (AISI 
316), 21 (AISI 321) and 22 (AISI 347). In particular, the alloy steel 
sample Nos. 7 to 10 of the invention, to which Si is added together with 
2% of Al, exhibit superior corrosion resistance even over the comparison 
steel sample Nos. 24 (AISI 310 S) and 25 (Incoloy 800) which have large Cr 
contents and, hence, exhibit high corrosion resistance. It will be 
understood also that, when the Al content is the same, higher corrosion 
resistance can be obtained by addition of not less than 1.5% of Si, as in 
the case of alloy steel sample Nos. 1 to 5 and 12 in comparison with 
comparison steel No. 23 (AISI 631) and comparison steel Nos. 26 to 30. 
FIG. 2 shows the relationship between the Al content and the corrosion loss 
in the alloy steel of the invention to which 2% of Si is added together 
with Al, in comparison with that of the comparison steels to which Al is 
added alone. It will be seen that the corrosion resistance is improved by 
the combined addition of Al and Si as compared with the case where Al is 
added alone. 
FIG. 3 shows a relationship between the Si content and the corrosion loss, 
in alloy steels to which Si has been added in various amounts in addition 
to 2% Al. It will be understood from this Figure that the resistance to 
corrosion by hot gas is increased by increase in the Si content, as 
compared with the case where Al is added alone, and also that the Si 
content should be not smaller than 1.5% in order to obtain an appreciable 
effect. 
As explained before, the alloy steel in accordance with the invention is 
effectively used as materials of devices and members which are subjected 
to an atmosphere containing sulfides produced in a coal gasification 
system, e.g., a water-cooled tube wall of a gasification furnace, members 
of a heat exchanger, valves, nozzles and so forth. A coal-gasification 
combined cycle power plant, which employs such a coal gasification system, 
will be explained hereinafter by way of example. 
FIG. 4 is a block diagram of a coal-gasification combined cycle power plant 
which has various parts made from the material in accordance with the 
invention. FIG. 5 is a schematic vertical sectional view of an entrained 
bed type coal gasification furnace, while FIG. 6 is a sectional view taken 
along the line VI--VI of FIG. 5, showing an upper water-cooled structure 
of the gasification furnace. 
As shown in these Figures, coal 1 is introduced by means of a burner 3 into 
a gasification furnace 4 to which also introduced is oxygen as a gasifier 
2. The coal 1 thus introduced is gasified in a gasification zone 5. The 
gasification zone 5 is defined by a refractory structure 6 because a high 
temperature exceeding 1600.degree. C. is established in the gasification 
zone. The coal gas of high temperature is delivered to a heat collecting 
zone 8 which is constituted by a water-cooled structure 17 made from an 
alloy steel of the invention and is cooled down below 900.degree. C. 
before it reaches the outlet of the gasification furnace 4. The gas as a 
crude gas 10 coming from the outlet of the gasification furnace 4 is sent 
to a steam generator 11 which is made of an alloy steel in accordance with 
the invention, so that the crude gas 10 is cooled through a heat exchange 
in the steam generator 11. Thus, the sensible heat posessed by the crude 
gas 10 is collected as the energy of steam 12. The crude gas 13 coming out 
of the steam generator 11 is sent to a gas-gas heat exchanger 14 which is 
made of an alloy steel in accordance with the invention, where heat is 
exchanged between the crude gas 13 and the refined gas 15, so that the 
crude gas is cooled down to the temperature suitable for the refining 
before it is sent to a gas refining section 16. The gas 15 refined in the 
gas refining section 16 makes a heat exchange with the crude gas in the 
gas-gas heat exchanger 14 such as to be heated by the crude gas, and is 
supplied as a fuel gas 18 to a gas turbine combustor 19. The hot 
combustion gas expands through a gas turbine to drive a generator, thereby 
generating electric energy. 
This composite plant has a heat recovery system which will be explained 
hereinafter. The exhaust gas 20 exhausted from the gas turbine is 
introduced into a heat recovery boiler 22 such as to produce a sensible 
heat of steam. On the other hand, the crude gas 10 available at the outlet 
of the gasification furnace 4 delivers sensible heat to water in the steam 
generator 11. The steam generated in the heat recovery boiler 22 and the 
steam generated in the steam generator 11 merge in each other and the thus 
mixed steam is superheated in a superheater such as to become superheated 
steam which is sent to a steam turbine 23. The superheated steam expands 
through the steam turbine 23 which in turn drives a generator thereby 
generating electric energy. The steam discharged from the steam turbine 23 
is condensed in a condenser 24 to become condensate which in turn is fed 
as feedwater to the heat recovery boiler by a feedwater pump.