Patent Application
20130269346
The present invention pertains to a combined cycle power plant for the generation of electrical power comprising a gas turbine, a steam turbine, a heat recovery steam generator, and a plant integrated in the power plant for the capture and compression of carbon dioxide in the flue gases of the gas turbine. It furthermore pertains to a method of operating the power plant.
Combined cycle power plants with carbon dioxide (CO2) capture are well known in the literature and have been realized in first pilot power plants, for example in the USA and Sweden.
Known concepts for such power plant with carbon capture plants include a combined cycle power plant with a gas and steam turbine and a heat recovery steam generator (HRSG) designed to generate steam using the heat from the gas turbine flue gases. The flue gases exhausted by such HRSGs typically contain CO2 only up to about 4% of volume flow. CO2 capture plants have been proposed that capture the CO2 by means of an absorption process, for example using an absorption solution such as monoethanolamine. Following the CO2 absorption, the absorber solution is reheated in order to release the CO2 in pure gaseous form. The released CO2 gas is then compressed and cooled to allow storage.
Most importantly, the operation of a CO2 capture facility in connection with a fossil fuel fired power plant results in a considerable drop of the overall efficiency of the plant due to the heat consumption, typically in the form of extracted steam, and auxiliary power required to operate the CO2 capture plant. Efforts directed to lower this efficiency drawback are discussed For example in O. Bolland and S. Saether, “New concepts for natural gas fired power plants which simplify the recovery of carbon dioxide”, Energy Conversion Management, Vol. 33, No. 5-8, pp. 467-475, 1992. Based on the direct relation between the efficiency of the CO2 absorption process and the concentration of the CO2 in the flue gases, various methods of increasing the CO2 concentration in the flue gases are proposed. For example, a recirculation of the flue gas back to the inlet of the gas turbine compressor can result in a doubling of the concentration of the flue gas led to the CO2 capture plant and a consequential increase of the CO2 capture plant efficiency.
It is also known that the concentration of oxygen in the flue gas affects the operation of the CO2 capture plant as oxygen may cause degradation of the absorbing solution, which can result in an increase in CO2 capture plant operation and maintenance costs.
A typical combined cycle power plant 1 according to the prior art as illustrated in the schematic of
The CO2 capture plant 15 is supplied with steam extracted from the steam turbine 6 via line 17 in order to regenerate the CO2-rich absorption solution. Resulting condensate is directed via line 18 back to the water steam cycle of the steam power plant, for example to the HRSG 5 via the feedwater tank. Part of the flue gases from the exhaust from the HRSG 5 can be recirculated via line 19 and blower 20 back to the inlet of the gas turbine compressor, whereby the recirculated flue gases are mixed with fresh ambient air in line 22. A power plant with such flue gas recirculation as shown in
It is an object of the present invention to propose a combined cycle power plant for the generation of electrical power with a CO2 capture plant having an increased CO2 capture efficiency compared to power plants of this type of the prior art. It is a further object of the present invention to propose a method to operate such power plant allowing decreased operating costs.
A combined cycle power plant for the generation of electrical power comprises at least one gas turbine, a first or main heat recovery steam generator HRSG for generating steam by means of gases exhausted by the gas turbine and directed to the HRSG via an exhaust gas line, and furthermore at least one steam turbine driven by steam generated in the HRSG. The power plant is furthermore operationally connected with a CO2 capture plant arranged and configured for the absorption of CO2 contained in the flue gas by means of a lean absorption solution, where after absorbing CO2 gas the CO2-enriched absorption solution may be regenerated by reheating in order to release CO2 in pure gaseous form.
According to the invention, the power plant comprises a second heat recovery steam generator (HRSG) or a boiler and a line for gas turbine exhaust gas leading from the combined cycle power plant to the second HRSG or the boiler. The second HRSG or boiler is configured and arranged to receive the exhaust gas from the gas turbine and to transfer the heat of the exhaust gas to feedwater and/or steam and to generate steam for the operation of a steam turbine or the CO2 capture plant or both. The combined cycle power plant comprises in particular a damper or flow divider in the exhaust gas line, which is arranged to divide the exhaust gas flow into a first and second partial exhaust gas flow in a first and second gas line respectively, where the first gas line for the first partial flow leads to a stack and the second gas line for the second partial flow leads to the second HRSG or the boiler.
The power plant according to the invention with a damper in the exhaust gas flow allows the direction of the first partial exhaust gas flow to the stack in two different ways, either directly to the stack or indirectly via the first HRSG, depending on the placements of the damper in the exhaust line, namely after or prior to the first HRSG.
In a first embodiment of the invention, the damper or flow divider is arranged in the gas turbine exhaust that is in the line leading exhaust gas from the gas turbine to the first HRSG. It directs the second partial exhaust gas flow to the second HRSG and the first partial exhaust gas flow to the first HRSG. In this arrangement exhaust gas of high temperatures (above 500° C., for example ca. 600° C.) may be led to the second HRSG.
In a second embodiment of the invention, the damper or flow divider is arranged in the HRSG exhaust, that is in the exhaust gas line leading away from the first HRSG. This directs a second partial exhaust gas to the second HRSG and a first partial exhaust gas flow to the stack. In this arrangement exhaust gas of lower temperatures (for example from 80 to 100° C.) may be led into the second HRSG.
In an exemplary embodiment of the invention, the power plant further comprises a line directing the exhaust gas from the second HRSG or boiler to the CO2 capture plant.
The power plant according to the invention allows a full capture of the CO2 contained in the exhaust gases and resulting from the combustion of a fossil fuel for the gas turbine. In particular, the invention allows also a partial capture of the CO2 in the exhaust gases. Additionally, it allows an increase in the CO2 concentration of the exhaust gases intended for CO2 capture by means of the supplementary firing within the second HRSG.
In a further exemplary embodiment, the second HRSG or boiler comprises in particular a supplementary firing assembly, which augments the heat available in the HRSG and reduces the oxygen content of the exhaust gas passing through it.
The supplementary firing also effects a reduction of the oxygen concentration of the flue gases, which in turn slows down the rate of degradation of the CO2 absorption solution in the CO2 capture plant. The operating lifetime of the solution is thereby increased, and maintenance costs of the CO2 capture plant can be lowered.
Furthermore, it allows the use of the heat in the exhaust gases by transfer of this heat to flow media, for example the rich absorption solution of the CO2 capture plant, thereby supporting the regeneration of the solution. Extractions of steam from the steam turbine of the combined cycle plant to support the CO2 capture plant are thereby avoided or can at least be reduced such that valuable steam can further be used to drive the turbine, and steam turbine efficiency can be upheld.
A partial CO2 capture is advantageous when it sufficiently reduces the power plant's CO2 emission within the limitations as required by authorities and regulations. It can enable improved performance and profitability compared to power plants with full CO2 capture, where the flue gases are treated for separation of the CO2 at a rate of up to 90% capture.
The second HRSG or boiler with supplementary firing allows on one hand a reduction of the residue oxygen content in the flue gases passing through it and thereby a CO2 absorption process with a lower O2 content such that the CO2 absorption solution is loaded with less oxygen. This results in a reduced rate of degradation of the absorption fluid due to O2 and thereby in a longer operational lifetime of the absorption solution. Furthermore, the combustion due to the supplementary firing produces exhaust gases from the second HRSG or boiler having an increased CO2 volume concentration of about 6%. This allows an increased efficiency of the CO2 capture plant in that the requirement of steam and energy consumption per flue gas mass flow is reduced.
The operation of the second HRSG with the gas turbine exhaust gases allows a more flexible operation of the CO2 capture plant and of the steam turbine in that heat can be provided by two different HRSGs for the reboiling of the CO2 rich absorption solution and/or the steam extraction, for example low-pressure steam from a cross-over pipe from an intermediate-pressure turbine to a low-pressure turbine, or cold reheat steam, or live steam from a steam turbine.
A further advantage of the power plant according to the invention is that the flexibility in part load operation of the power plant can be greatly increased. At power plant part load operation, the exhaust gas flow diverted to the second HRSG or boiler can be kept constant such that the CO2 capture plant treating this exhaust gas is also run at a constant load. The flow to the second HRSG could be also reduced at part load if less CO2 needs to be captured.
In an exemplary embodiment of the invention, the damper or flow divider in the exhaust gas line is arranged for division of the flows such that the second flow to the second HRSG or boiler to the exhaust gas flow is in the range of 10% up to 60% of the total exhaust gas flow exiting from the gas turbine. This may be the case at full-load operation as well as part-load operation of the power plant.
In place of the second HRSG, the power plant may comprise a boiler, where this boiler can be a gas-fired boiler, a coal fired boiler, or an oil-fired boiler.
In the embodiment of the power plant comprising a boiler instead of a second HRSG, a nearly stoechiometric combustion can be achieved in the boiler, such that the resulting CO2 concentration in the exhaust gases from this boiler reaches a level of 6-8%. This facilitates greater CO2 capture plant efficiency due to a reduction of the steam needed to operate it.
In both cases, a power plant according to the invention with a second HRSG or a boiler enables an increased part load operation flexibility as well as increased part load power plant performance while the requirements on CO2 capture are still fulfilled.
In a further exemplary embodiment of the invention, the power plant comprises additionally a backpressure steam turbine configured and arranged to receive steam from the second HRSG or a boiler, where the steam turbine drives a generator. A further low-pressure steam turbine may be arranged on the same shaft as the backpressure steam turbine and the generator by means of an automatic clutch, for example a synchro-self-shifting (SSS) clutch. Steam expanded in the back pressure steam turbine can be led to both the low-pressure steam turbine for the generation of electricity and/or to the CO2 capture plant reboiler for rich absorption solution regeneration or as regulation steam combined with HRSG tail-end rich absorption solution heat exchanger.
A further embodiment of the invention comprises a CO2 compressor arranged on a single shaft together with a steam turbine driving the compressor. The steam turbine is operated by live steam generated in the second HRSG.
In all the disclosed embodiments, the power plants may include means to introduce augmenting fresh ambient air to the second HRSG or boiler as needed in order to stabilize the supplementary firing within the HRSG or the combustion within the gas, oil, or coal fired boiler.
Further specific embodiments of the power plant are disclosed in connection with the figures.
A method according to the invention to operate a combined cycle power plant comprising a gas turbine generating a exhaust gas flow, a first HRSG, a steam turbine, and a CO2 capture plant comprises:
A particular method according to the invention comprises—diverting the second portion of the exhaust gas flow immediately following its exit from the gas turbine and directing it to the second HRSG or boiler.
A further method according to the invention comprises—diverting the second portion of the exhaust gas flow after its passing through the first HRSG and its exiting from the first HRSG and directing it to the second HRSG.
In a further particular method of the invention, the residue oxygen content in the exhaust gas is reduced by means of supplementary firing in the second HRSG. As mentioned, a reduced oxygen content in the exhaust gas increases the operating lifetime of a CO2 absorption solution.
A particular use of the steam generated in the second HRSG or boiler include the reheating a CO2 rich absorption solution in the CO2 capture plant. Depending on the operation load of the power plant and/or CO2 capture plant, this can allow for greater flexibility and performance increase due to a decrease use of extraction steam from the main combined cycle steam turbine.
For yet greater flexibility in the partial CO2 capture, the exhaust gas flow exiting from the second HRSG or boiler is directed to the CO2 capture plant as well, however can also by means of damper in the exhaust line from the second HRSG be diverted to a stack.
A further particular method according to the invention further comprises one or more of the following:
The supplementary firing allows the combustion of residual oxygen contained in the flue gases to maintain a constant CO2 concentration in the flue gas leaving the second HRSG or boiler. A minimum oxygen concentration can remain in the flue gases to allow complete combustion, typically with flue gases, which contain practically no residual unburned hydrocarbons and practically no CO. The constant CO2 concentration in the flue gas leaving the second HRSG or boiler assure good operating conditions for the subsequent CO2 capture plant, thus enhancing the overall plant efficiency. Due to the subsequent firing the flue gases directed to the second HRSG or boiler lead to a higher steam and power production than the flue gases directed to the first HRSG or boiler. As a consequence by controlling the ratio of flue gas mass flow directed to first HRSG or boiler to the flue gas mass flow directed to the second HRSG or boiler the overall power output of the plant can be controlled.
Same reference numerals in different figures refer to same elements in same configuration.
A power plant 30 for the generation of electricity according to the invention as shown in
Steam generated in the first HRSG is led via a live steam line 40 to a steam turbine 39, which in turn drives a further generator. The steam exhausted by steam turbine 39 is led into a condenser 39′ and resulting condensate is directed into the HRSG 36 via line 39a for the generation of steam, thereby completing the water steam cycle.
Unit 30b of the power plant 30 mainly comprises a second HRSG 43, which is operated with hot exhaust gases from unit 30a branched off from exhaust gas line 34 by means of a damper or flow divider 34′. The exhaust gases are used for heat transfer to various fluids in various pressure and temperature ranges and finally directed first to an exhaust gas cooler 47 via line 46 and then to the CO2 capture plant 48. A further damper in line 46 allows a further division of the exhaust gas flow, a portion of which is directed via line 46′ to a stack 38′ or to line 37 and the stack 38.
The first damper 34′ in line 34 is arranged to divert from 10 to 60% of the exhaust gas flow exiting from the gas turbine flowing in line 34 to the second partial flow line 42 and the second HRSG 43. A supplementary firing assembly 43′ supplied with fuel via line 43″ is operated within the HRSG 43 in order to reduce as much as possible the oxygen content in the flue gases and to be treated in the CO2 capture plant by means of the CO2 absorption solution. In order to stabilize the combustion within the second HRSG and to augment the power, the supplementary firing assembly can be supplied by additional air via a line 44 directed into the supplementary firing fuel line 43″.
The two HRSGs in this power plant are used for generating steam from condensate from the steam turbine condenser, as well as for heating or reheating steam from various sources. The first and second HRSGs 36, 43 are provided with condensate from the steam turbine condenser 39′ via lines 39a and 39b respectively leading to their low-pressure, low temperature portions. Further condensate from the CO2 capture plant resulting from steam condensing in the CO2 absorption solution reboiler within the CO2 capture plant and having a temperature higher than that of the condenser 39 is directed via lines 50 and 50′ to the first and second HRSGs 43 and 36, respectively.
Both first and second HRSGs 36 and 43 generate live steam that is directed to steam turbine 39 via lines 52 and 40, respectively. They are also configured to generate reheat steam that is directed to same steam turbine 39 via lines 54 and 41, respectively. They furthermore are configured to generate steam for direct or indirect use in the CO2 capture plant such as the regeneration of the CO2 absorption solution.
The second HRSG 43 is also configured for the preheating of a CO2-rich absorption solution prior to a regenerator reboiler of the CO2 capture plant so that the required steam in the reboiler can be minimized to a level sufficient for regulating the required temperature. This configuration will both increase the thermodynamic efficiency of the regeneration process and reduce the investment cost.
The CO2 capture plant is operated by and provided with steam used for the regeneration of the CO2 absorption solution, where this steam is extracted from the combined cycle steam turbine 39 via line 49′ together with steam from the second HRSG 43 provided via line 49. This configuration allows the steam extraction from the steam turbine 49′ to be reduced compared to a power plant with a CO2 capture of the prior art thereby improving the steam turbine performance.
Optionally, an additional steam line leading from away the second HRSG and designated with R can provide steam to the CO2 capture plant reboiler, to further support its operation.
It can further more be used for power augmentation or for grid frequency response.
The second HRSG 43 and single-shaft arrangement thereby further supports the CO2 capture process, specifically the compression of the captured CO2 for transport and/or storage.
Steam generated in the second HRSG 43, which contains the heat of the gas turbine exhaust gas flow in line 37″ is provided to a high- or intermediate-pressure steam turbine 60 and a low-pressure steam turbine 61, which drive a generator 76. Again, as is the case in the embodiment of
| Number | Date | Country | Kind |
|---|---|---|---|
| 10186603.6 | Oct 2010 | EP | regional |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2011/067179 | Sep 2011 | US |
| Child | 13852485 | US |
