Power plants – Combustion products used as motive fluid – Multiple fluid-operated motors
Reexamination Certificate
2001-05-21
2004-06-15
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Multiple fluid-operated motors
Reexamination Certificate
active
06748733
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to an apparatus and method for utilizing waste heat in a combined cycle power plant. More particularly, hot combustion gases are selectively combined with the waste exhaust gases through a diverter system to permit the temperature, pressure and quality of the steam generated in the combined cycle plant to be selectively controlled.
BACKGROUND OF THE INVENTION
The simplicity and flexibility of gas turbine power plants have made them increasingly attractive to electrical utilities as a means for generating electrical power. The low capital cost, ease of operation and variety of fuels that can be accommodated in gas turbine plants are viewed as significant advantages generally not available in conventional boiler-fired power plants. In particular, the brief start up period associated with gas turbine power plants have made them particularly attractive as a supplemental electrical generating means during short periods of peak electrical loading.
When a gas turbine power plant is used on a sustained basis for the generation of electricity, however, differences inherent in the thermodynamic processes governing a steam power cycle and a gas turbine power cycle generally favor the use of a steam power cycle for sustained operation. For example, although the pressure of the exhaust gas leaving a gas turbine plant is approximately atmospheric, the temperature of the exhaust gases is still relatively high. Since no further expansion of the exhaust gas is possible, the heat in the exhaust gas is generally wasted, resulting in relatively low energy conversion efficiencies for the gas turbine plant. In contrast, a conventional steam power plant, when operated with a condenser, permits heat to be rejected at a temperature much closer to the surrounding environmental temperature, which results in a greater energy conversion efficiency.
The elevated exhaust gas temperatures generally associated with gas turbine power plants suggests that a gas turbine power plant may be combined with the advantageous features of a conventional steam power plant to achieve cogeneration in a combined cycle power plant, where the source of thermal energy for the steam cycle is provided at least in part by the hot exhaust gases from the gas turbine plant. As a result, considerable effort has been expended in developing methods to recover the available energy in the gas turbine exhaust flows through the use of combined cycle power plants, particularly where the gas turbine power plant operates on a sustained basis.
Referring now to
FIG. 1
, a combined cycle power plant
20
according to the prior art is shown. The combined cycle power plant
20
generally consists of a gas turbine section
18
that is coupled to a steam cycle section
19
. The power plant
20
as shown in
FIG. 1
is structured to utilize solid fuels, as will be discussed in further detail below. The gas turbine section
18
generally includes a compressor
2
that receives and compresses atmospheric air
1
and delivers the compressed air to a combustion chamber
3
. The combustion chamber
3
also receives pulverized fuel material from a fuel pulverizer and conveyor
21
that, in turn, receives material from a fuel source
6
. The pulverized fuel is directed to the combustion chamber
3
where it is mixed with the compressed air and burned with the pulverized fuel. The hot gases resulting from the combustion are routed to a cyclone separator
22
to separate fly ash from the combustion gases. The combustion gases are routed to a turbine
4
, where they are partially expanded to recover sufficient mechanical energy to drive the compressor
2
through a power transmission shaft
5
. The combustion gases are further expanded through a power turbine
24
that is coupled to an electrical generator
17
through a power transmission shaft
13
. Electrical energy produced by the generator
17
may be supplied to an external electrical supply grid
23
. Subsequent to the expansion of the exhaust gas in the power turbine
24
, the gases
7
are routed from the turbine
24
to the steam cycle section
19
, which generally includes a heat recovery steam generator (HRSG)
9
that receives the exhaust gas
7
. Steam
14
is generated in the HRSG
9
when the latent heat of evaporation is transferred from the exhaust gas
7
to the feed water
15
within the HRSG
9
. The exhaust gas
7
is then released to the atmosphere through a stack
8
. The steam
14
thus generated is routed to a steam turbine
10
and expanded to recover mechanical energy. The steam turbine
10
is coupled to an electrical generator
16
by a transmission shaft
11
. Electrical energy produced by the generator
16
may also be supplied to the grid
23
. The steam
14
exhausted from the turbine
10
is routed to a condenser
12
, and then returned to the HRSG
9
for reheating.
Still referring to
FIG. 1
, the steam
14
generated in the HRSG
9
preferably attains sufficient pressure and temperature to obtain acceptable efficiencies from the steam cycle section
19
, and to minimize the moisture content in the steam
14
. However, a particular shortcoming associated with the foregoing combined cycle plant
20
is that the temperature of the exhaust gas
7
frequently limits the temperature of the steam generated in the HRSG
9
. Moreover, the exhaust gas temperature similarly limits the maximum pressure of the steam since the saturation temperature of the steam increases with its pressure and only the portion of the heat in the exhaust gas
7
, which is above the saturation temperature of the feed water
15
in the HRSG
9
, is available for the generation of steam. Unless the temperature of the exhaust gas
7
entering the HRSG
9
can be augmented, a lower thermal efficiency from the steam power section
19
is often encountered.
Reduced exhaust gas temperatures are particularly problematic in combined cycle plants
20
that employ biomass fuels, since the heating values of these fuels is significantly lower than heating values associated with hydrocarbon gases, coal or petroleum distillate fuels. Biomass fuels are defined as a solid fuel material of plant origin, consisting, for example, of wood chips or scrap residues, tree barks, or bagasse from sugar cane processing. As a result, the low temperature of the exhaust gas
7
prevents biomass combustion plants from using the exhaust gas energy for cogeneration. For example, U.S. Pat. No. 5,720,165 to Rizzie, et al., discloses a biomass combustion system that generates steam for injection into the power turbine of the gas turbine plant. The Rizzie patent does not disclose a system may be used in a combined cycle plant, as described above.
Other prior art systems have addressed the problem of insufficient exhaust gas temperatures in combined cycle plants by relying on sophisticated feed water management systems, and therefore do not propose augmenting the energy in the exhaust gases exhausted from the gas turbine plant. For example, U.S. Pat. No. 5,799,481 to Fetescu discloses that the performance of the steam power cycle portion of a combined cycle plant may be enhanced through a sophisticated feed water control system that uses an HRSG of complicated design. Similarly, U.S. Pat. No. 4,976,100 to Lee discloses that the exhaust gas energy of the gas turbine portion of the combined cycle plant may be more effectively recovered by allowing the exhaust gases, rather than steam exhausted from the steam turbine, to heat the feed water prior to entry into an HRSG. Accordingly, the prior art systems as disclosed in the Fetescu and Lee references are directed only towards more careful management of feed water heating, and cannot overcome the basic limitation of insufficient exhaust gas temperature inherent in combined cycle plants.
Thus, those concerned with the design and operation of combined cycle plants are highly aware of the need for a system that will permit the augmentation of the energy available in the gas turbine exhaust in a simple and convenient manner, thereby permitting the gene
Arterberry Steven H.
Casaregola Louis J.
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