Patent Application
20100005966
This invention relates generally to the use of solar energy for the recovery of carbon dioxide from gas streams. The invention has particular application to CO2 recovery from flue gases generated by coal- and gas-fired power plants or from process gases in a wide variety of industrial processes including steel plants, smelters, cement kilns and calciners. The term “process gases” refer to gas streams fed to or from a process, and embraces, e.g. syngas feed to an industrial furnace, and blast furnace gas in a steel plant.
There is rapidly growing pressure for stationary sources of CO2 emissions such as power stations, to make step reductions in greenhouse gas (GHG) emissions through 1) capturing the CO2 formed from the process, and 2) storing the CO2 by various geological means. Most involve injecting CO2 in a supercritical or “liquefied” state into deep aquifers, coal seams and adjacent strata, or at depth in the ocean, or converting the CO2 into a solid mineral form.
In the case of power stations, as an example, there are at present three main approaches to CO2 separation from new or existing power plants: 1) post combustion capture, 2) precombustion capture, and, 3) oxygen combustion with flue gas liquefaction. In this context, the present invention is primarily applicable to post combustion capture, though the invention could also be used for precombustion capture where heat is required for solvent regeneration.
In post combustion capture, the CO2 in flue gases is preferentially separated from nitrogen and residual oxygen using a liquid solvent in an absorber. The CO2 is then removed from the solvent in a process called desorption (or regeneration, and sometimes termed “stripping”), thus allowing the solvent to be reused. The desorbed CO2 is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. The main disadvantage of this process is that the CO2 partial pressure is relatively low (compared to the two alternative approaches mentioned above), which necessitates the use of CO2 selective solvents. The regeneration of these solvents releases an essentially pure CO2 stream, but this step is relatively energy intensive. Overall, this reduces the electrical power output by around 20%, due to the need to provide low temperature heat (approximately 65% of the total energy required) and work to drive the CO2 liquefaction plant and other auxiliary equipment. Both heat and work are also required for dehydration of the liquefied product CO2. The net effect is to reduce the thermal efficiency of the plant by around 9 percentage points.
Post combustion capture in this form is applicable to other stationary CO2 sources, such as steel plants, cement kilns, calciners and smelters.
It has been previously noted by the present inventor that renewable energy sources can be used to provide input to the electricity grid by either direct or indirect integration, and that this synergy can reduce the emissions intensity of fossil fuels and support the uptake of renewables. An example of direct integration is the use of solar thermal energy to provide steam or hot fluids to a host power plant for the purpose of heating the working fluid (usually water), for raising steam, or for superheating steam. An Australian example is the Solar Heat and Power plant located at Liddell, NSW. This arrangement provides hot water to the power plant for feed water heating to displace extraction steam from the low pressure turbine stages.
The basis for indirect integration is that solar thermal energy can be applied anywhere to the power grid, thereby displacing the power required from fossil fuelled power stations. The avoided CO2 emissions may be allocated to a range of emitters, including non-grid sources. The grid thereby facilitates the use of solar thermal energy by providing backup and reserve power.
The use of solar thermal energy for post combustion capture has been considered in a number of studies by the present inventor and co-workers at CSIRO, with the solar heat being used for directly providing heat for the desorber of the capture plant. This direct integration is similar to that used for augmenting power station output by providing feed water heating, and involves transferring the solar energy as a hot working fluid (mostly hot water under pressure, though low pressure steam has also been considered) to the desorber of the post combustion capture process, at temperatures of up to 150° C., via thermally insulated pipes.
An object of the invention, at least in one or more aspects, is to more effectively employ solar energy to address the aforedescribed problem of the reduction of thermal efficiency incurred by post combustion capture of CO2.
It is a further object of the invention, at least in one or more aspects, to effectively employ solar energy as a source of additional energy in a power generation system or industrial process.
In the first aspect, the invention provides a method of recovering CO2 from a gas stream, including:
In the first aspect of the invention, there is further provided apparatus for recovering CO2 from a gas stream, including:
In a second aspect, the invention provides a method of injecting solar energy into a power generation or other industrial system, including:
The invention still further provides, in its second aspect, apparatus for injecting solar energy into a power generation or other industrial system, including:
Advantageously, in the methods of the first and second aspects of the invention the CO2-enriched medium and/or the regenerated solvent stream are selectively accumulated so as to respectively optimise the timing and rate of said absorption and desorption of CO2, and/or to provide a storage of solar energy.
Advantageously, the apparatus of the first or second aspect of the invention further includes a plurality of storage vessels whereby the CO2-enriched medium and/or the regenerated solvent stream are selectively accumulated in the vessels so as to respectively optimise the timing and rate of said absorption and desorption of CO2, and/or to provide a storage of solar energy.
In a third aspect, the invention provides a method of recovering CO2 from a gas stream including:
The invention still further provides, in its third aspect, apparatus for recovering CO2 from a gas stream, including:
Most conveniently, the absorber station is located adjacent a source of the gas stream, e.g. a boiler or furnace to which said gas stream is being fed or from which said gas stream is emitted as a flue gas stream. The boiler or furnace may be part of an electrical power generation plant, e.g. a coal-fired electrical power station.
Preferably, said working fluid, heated in said solar field by insolation, is also employed as an energy source in compression and liquefaction of the CO2 in said separate CO2 stream.
The arrangement of the first and second aspects of the invention allows the production of lower temperature heat from the solar energy field, thereby increasing sun-to-heat efficiency, and also substantially reduces energy losses otherwise incurred from transferring hot working fluids over long distances between the solar energy field and the desorber station, which is conventionally located adjacent to the absorber station. The separation of the desorber station from the absorber station means that the relatively cooler solvent solutions are transferred over the greater distances.
In the third aspect of the invention, storage of solvent in one or more vessels permits provision of CO2-lean solvent to the absorber during periods of low insolation, and additional flow of CO2-rich solvent to the desorber station to utilize peaks in solar energy production. This storage in effect disconnects the absorber and thereby enables solar energy to be used to capture a larger proportion of the total CO2. It also allows process optimization by varying desorption conditions to match variations in insolation, thereby improving the efficiency of solar energy production and overall solar-to-CO2 capture efficiency.
More generally, storage of the CO2-lean solvent can be a low cost manner of storing solar energy, e.g. for periods of low or no insolation.
The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:
The CO2-lean flue gas from absorber station 34 is passed to a flue stack 39 for release into the atmosphere while the desorbed CO2 from desorber/regeneration station 36 is compressed, cooled, dehumidified and liquefied, in station 38, for subsequent transport and storage. The CO2-lean regenerated solvent is recycled to absorber station 34, exchanging its heat with the incoming CO2-rich solvent at heat exchanger 35.
It will be appreciated that, in particular plants, there may be more than one CO2-absorber at station 34 and/or more than one CO2-desorber at station 36. Moreover, within an individual desorber, there may well be multiple stages within the column.
In conventional practice of post combustion capture of CO2, the heat energy for regeneration/desorber station 36 is obtained from steam turbine 16, with the effects previously discussed. In accordance with a preferred practice of the first aspect of the invention, desorber station 36 and liquefaction stage 38 are sited remote from the power plant 10 and absorption station 34, and located immediately adjacent a solar energy field 40. Field 40 may typically be an open array of solar collectors mounted above ground and sufficiently spaced to allow use of the land below for other purposes, e.g. grazing of sheep or cattle. The array may, for example, be 2×2 km in extent. The solar collectors heat a working fluid, typically water, which is then employed to provide the required energy to heat the CO2-rich solvent stream to effect desorption of the CO2 and regeneration of the solvent. The working fluid circulates to heat exchange at 37 via pipe network 41. Respective pipes 42, 44 convey the CO2-rich solvent from absorber station 34 to desorber station 36, and the CO2-lean solvent in the reverse direction.
In a modified embodiment, depicted highly schematically in
Storage of solvent in vessels 50 allows provision of CO2-lean solvent to the absorber during periods of low insolation, and additional flow of CO2-rich solvent to the desorber to utilize peaks in solar energy production. This storage disconnects the absorber, thereby enabling solar energy to be used to capture a larger proportion of the total CO2. It also allows process optimization by varying desorption conditions to match variations in insolation, thereby improving the efficiency of solar energy production and overall solar-to-CO2 capture efficiency.
CO2-lean solvent can be stored indefinitely, enabling the absorber to operate when solar energy is not available. It will be appreciated that storage of the CO2-lean solvent is an especially effective means of low-cost storage of solar energy for periods of low or zero insolation. In solar shoulder periods, the thermal efficiency of the solar energy field will be higher if a lower temperature is provided to the working fluid. Under these conditions it may be advantageous to partially strip only the richest solvent.
While it is usually preferred that the storage vessels 50 be located adjacent the desorber station, it may be more convenient in particular cases to locate them adjacent the power plant.
The solar energy field(s) 40 can be configured to provide energy for a number of other purposes which may integrate with the desorber and/or provide work or electrical energy. For example, steam or other vapours could be supplied from the solar field(s) to drive a turbine to generate electricity, and/or to provide CO2 compression work at liquefaction stage 38. The exhaust from the turbine can then provide a proportion of the heat energy for the desorption and dehydration processes, and heat from CO2 compression can be used to further augment solar heat sources. This combined heat and power option will have significantly higher efficiency than for power generation alone. This configuration is depicted schematically in
An additional embodiment is to use stored solar heat for desorption during periods of low or no insolation. There are a wide range of options for achieving this, but one suitable configuration is shown schematically in
The invention has a number of advantages over the direct use of solar heat energy for CO2 capture processes:
1. Heat transfer fluids from the solar field(s) need only be transferred to the desorber, which can be advantageously located closer to the solar field(s) 40. This reduces the energy losses and need for thermal insulation of these pipes. It also avoids the problem of heat losses associated with the transfer of hot working fluids between the solar field and a conventional absorber-desorber combination.
Overall, the concept enables the solar field(s) to be advantageously located further from the absorber without efficiency penalties caused by heat loss from transferring hot fluids over long distances.
2. The solvents rather than the solar-heated working fluid are transferred over the longer distance between the absorber and the desorber, which has a number of advantages:
Heat losses from the CO2-lean regenerated solvent returning to the absorber from the desorber storage tanks will be advantageous for the absorption stage.
3. The solvent streams are beneficially stored in a number of locations, preferably adjacent to the solar field, to enable flexible operation of the desorber to maximise use of the solar field, and to minimize the solvent pipe size 44 back to the absorber. Storage of CO2-rich solvent enables more solvent to be desorbed during periods of high insolation; the lean solvent can be stored indefinitely, enabling the absorber to operate when solar energy is not available.
4. The use of a number of solvent storage tanks with varying levels of CO2 loading will enable the desorption stage to be optimized with the available solar energy.
Moreover, storage of the CO2-lean solvent is an especially effective means of low-cost storage of solar energy for periods of low or zero insolation.
In solar shoulder periods, the thermal efficiency of the solar field will be higher if a lower temperature is provided to the heat transfer fluid. Under these conditions it may be advantageous to partially strip only the richest solvent.
The desorber can be operated at temperatures and pressures which allow optimization of the use of the solar energy for the CO2 desorption and liquefaction stages.
This combined heat and power option will have significantly higher efficiency than for power generation alone.
5. The solar field(s) can be configured to provide energy for a number of other purposes which may integrate with the desorber and/or provide work or electrical energy. For example, steam or other vapours could be supplied from the solar field(s) to drive a turbine to provide CO2 compression work, and the exhaust from the turbine can then provide a proportion of the heat energy for the desorption and dehydration processes, and heat from CO2 compression can be used to augment solar heat source(s).
6. For power generation, the invention provides a low cost method of avoiding a reduction in electricity generation due to post combustion capture. Overall the invention in effect uses solar energy to at least partially offset electricity generation and electricity storage. This approach is the lowest cost for both, and offers a low risk route for large scale use of solar thermal energy.
7. In summary, for electricity generation, the invention provides emissions-free energy for the CO2 capture process and avoids the loss of electrical output from the host power plant (20-25%), thereby giving a saving in both capital and operating costs, and a 20-25% reduction in the amount of CO2 to be captured for a given electrical output. The invention is also applicable to other large scale stationary sources of CO2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006903840 | Jul 2006 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU07/00994 | 7/17/2007 | WO | 00 | 4/8/2009 |
