Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-09-27
2003-09-09
Koczo, Michael (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S039120, C060S039182
Reexamination Certificate
active
06615589
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the field of power generation and, in particular, to the generation of power from stream using a steam turbine system.
BACKGROUND OF THE INVENTION
Many processes for the generation of power from steam operate using two stages, the first stage involving the production of a gaseous fuel and the second stage involving the use of the fuel to generate steam which is expanded to produce power.
GB-A-1525490 (Klein et al; published on Sep. 20, 1978) discloses a power generation process in which a fuel is partially combusted in the presence of compressed air. A proportion of the heat liberated is used to produce steam from pre-heated water. The combustion gases are then cleaned, freed of H
2
S, mixed with compressed air and then combusted completely. The resultant combustion gases drive a turbine. The gases leaving the turbine are passed to an off-gas boiler in which steam generated upstream is further heated. The further heated steam is used to drive a steam turbine.
It is known in the art to use hydrocarbon or carbonaceous feedstock to provide fuel for a power generation plant. For example, it is known to convert natural gas to “synthesis gas” (a mixture of hydrogen and carbon monoxide). The gaseous fuel is then fed to a power generation plant comprising a gas turbine system, a heat recovery and steam generation system (“HRSG”) and a steam turbine system. The fuel gas is combusted in the presence of a compressed oxidant gas such as air or oxygen to form a mass of hot gaseous combustion products. At least some of the heat generated in the combustion may be recovered in the HRSG by generating steam which is then expanded in the steam turbine system to provide power and expanded steam.
The two stages of these processes are usually independent of each other, the first stage simply supplying the fuel for the second stage.
Conventional steam turbine systems use three pressure levels of steam generation with the expanded steam from the highest pressure turbine being reheated before it is introduced to the medium pressure turbine. A typical steam turbine system is shown as part of a typical two-stage power generation process depicted in FIG.
1
.
Referring to
FIG. 1
, a stream
24
of feed air is compressed C-
102
and then fed as a stream
27
to a combustion chamber R-
108
. A stream
23
of pressurized fuel gas comprising predominantly hydrogen is fed to the combustion chamber R-
108
where the air and the fuel are combined and burned and a stream
28
of pressurized gaseous combustion products is removed. This product stream
28
is expanded in a gas turbine T-
101
to produce power and a stream
29
of lower pressure gaseous combustion products. Optionally, a stream of nitrogen
76
is added to the combustion chamber R-
108
thereby increases the power produced by the expander T-
101
.
The exhaust
29
from the gas turbine T-
101
is typically at about 600° C. and is cooled to approximately 100° C. in the HRSG X-
106
. A stream
33
of 20° C. water at about atmospheric pressure is fed to the HRSG X-
106
in which it is heated to 99° C. The warmed water stream
77
is then removed from the HRSG and de-aerated in de-aerator
78
. The de-aerated water
79
is then divided into three streams
80
,
87
,
93
. The stream
80
is pumped in pump
81
to about 4 atm. (0.4 MPa) to produce a low pressure stream
82
which is vaporized in the HRSG X-
106
to produce a stream
83
of saturated steam at a temperature of 144° C. that is then fed to a low-pressure stage T-
104
of the three-stage steam turbine. The low-pressure turbine T-
104
expands the steam and the resultant exhaust stream
84
has a pressure of about 0.04 atm. (4 KPa) and a temperature of about 29° C. The exhaust stream
84
is then condensed X-
107
to form stream
85
that is then pressurized in pump P-
102
to about 1 atm. (0.1 MPa) to form stream
86
. Stream
86
is recycled by addition to the HRSG feed water stream
33
.
Stream
87
is pumped in pump
88
to about 35 atm. (3.4 MPa) to form a medium pressure stream
89
which is vaporized in the HRSG X-
106
to produce a stream
90
,
91
of saturated steam at a temperature of about 243° C. The stream
91
of medium pressure steam is fed to the medium pressure stage T-
103
of the steam turbine where it is expanded to a pressure of about 4 atm. (0.4 MPa). The exhaust stream
92
is then fed to the low-pressure stage T-
104
of the steam turbine.
Stream
93
is pumped in pump
94
to about 150 atm. (15 MPa) to form a high pressure stream
95
which is vaporized in the HRSG X-
106
to produce a stream
96
of superheated steam at a temperature of about 585° C. The superheated steam
96
is then expanded in a high-pressure stage T-
102
of the steam turbine to produce a medium pressure stream
97
at about 35 atm. (3.5 MPa). In the prior art process, the medium pressure exhaust stream
97
is then returned to the HRSG X-
106
and reheated to about 550° C. The reheated medium pressure stream
98
provides a portion of the feed stream
91
for the medium pressure stage T-
103
of the steam turbine.
The graph in
FIG. 2
depicts a typical cooling curve for a HRSG in combination with a conventional three level steam turbine system in a process according to the flow sheet in FIG.
1
. The ideal rate of cooling, represented by the upper line, would be constant thereby maximizing the efficiency of the process. Use of more pressure levels of steam generation would improve the efficiency of the power generation process as the actual cooling curves in the HRSG would match more closely the ideal cooling curve. However, increasing the number of pressure levels in this way would significantly increases the capital, running and maintenance costs of the process. It is the primary objective of this invention, therefore, for provide a modified process that strikes a balance between performance and cost.
SUMMARY OF THE INVENTION
It has been found that the primary objective of the invention can be achieved by using the heat generated in an exothermic fuel gas generation process to produce the steam for expansion in the steam turbine system. This significantly improves the efficiency of the overall power generation process. The inventors are not aware of any system in which the high pressure steam vaporisation duty is carried out outside the HRSG.
In particular, power is produced from hydrocarbon fuel gas by a process comprising generating exothermically a first fuel gas. An oxidant gas is compressed to produce compressed oxidant gas. A second fuel gas is combusted in the presence of at least a portion of the compressed oxidant gas to produce combustion product gas, at least a portion of which is expanded to produce expanded combustion product gas. Pre-heated water is at least partially vaporized by heat exchange against at least a portion of the first fuel gas to produce an at least partially vaporized water stream. This water stream is heated by heat exchange against expanded combustion product gas to produce a heated first steam stream at a pressure of from 100 atm. (10 MPa) to 200 atm. (20 MPa). The heated first steam stream is expanded in the highest pressure stage of a steam turbine system having more than one pressure stage to generate power and an expanded steam stream.
The latent heat duty for at least partially vaporising the pre-heated water is provided by the first fuel gas rather than by the expanded combustion product gas. Thermal integration of the process in this way improves significantly the overall thermal efficiency of the power generation process.
REFERENCES:
patent: 4999992 (1991-03-01), Nurse
patent: 5440871 (1995-08-01), Dietz et al.
patent: 6130259 (2000-10-01), Waycuilis
patent: 6145295 (2000-11-01), Donovan et al.
patent: 6167691 (2001-01-01), Yoshikawa et al.
patent: 6223519 (2001-05-01), Basu et al.
patent: 1448652 (1976-09-01), None
patent: 1525490 (1978-09-01), None
Allam Rodney John
Cotton Rebecca
Air Products and Chemicals Inc.
Jones II Willard
Koczo Michael
LandOfFree
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