Patent Application
20110265445
This application relates reducing CO2 emissions in combustion streams.
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), carbon monoxide (CO), and carbon dioxide (CO2), generated by the power industry. In particular, carbon dioxide has been identified as a greenhouse gas, resulting in various techniques being implemented to reduce the concentration of carbon dioxide being discharged to the atmosphere.
There are three generally recognized ways currently employed for reducing CO2 emissions from such power stations. The first method is to capture CO2 after combustion with air from the exhaust gas; wherein the CO2 produced during the combustion is removed from the exhaust gases by an absorption process, adsorption process, membranes, diaphragms, cryogenic processes or combinations thereof. This method, commonly referred to as post-combustion capture, usually focuses on reducing CO2 emissions from the atmospheric exhaust gas of a power station. A second method includes reducing the carbon content of the fuel. In this method, the fuel is first converted into H2 and CO2 prior to combustion. Thus, it becomes possible to capture the carbon content of the fuel before entry into the gas turbine and the formation of CO2 is hence avoided. A third method includes an oxy-fuel process. In this method, pure oxygen is used as the oxidant as opposed to air, thereby resulting in a flue gas consisting of carbon dioxide and water.
The main disadvantage of the post-combustion CO2 capture processes is that the CO2 partial pressure is very low in the flue gas (typically 3-4% by volume for natural gas fired power plants). Although the CO2 concentration at the stack and thus the partial pressure could be increased by partial recirculation of the flue gas to the compressor of the gas turbine (in this respect see, for example, U.S. Pat. No. 5,832,712 and WO 2009/098128), it still remains fairly low (about 6-10% by volume). And, due to somewhat lower isentropic exponent (also known as ratio of specific heat) of the flue gas compared to pure air, penalties in power and efficiency are expected for natural gas fired power plants when exhaust gas recirculation is employed. For the same reason, it is less than ideal to compress a mixture of flue gas and air in the gas turbine compressor. These factors significantly increase the cost of electricity generation. In fact, the cost of CO2 capture is generally estimated to represent three-fourths of the total cost of a carbon capture, storage, transport, and sequestration.
As a result, there is a continuing need for cost-effective CO2 removal technologies.
In one embodiment, a method for reducing CO2 emissions in an exhaust stream is provided. The method comprises generating an exhaust stream, and compressing the stream. A first flow path of the compressed exhaust stream is recirculated back to the generating step. A second flow path of the compressed stream is provided to a separator where CO2 is then separated from the compressed exhaust stream to provide a substantially CO2 free exhaust stream and a stream of liquid CO2.
An industrial plant is also provided. The plant comprises a manufacturing assembly for producing a product and an exhaust stream comprising CO2 and further comprises a compressor, recirculation line and carbon dioxide separation system. The compressor receives the exhaust stream comprising CO2 and generates a compressed exhaust gas. The compressor comprises a first conduit configured to recirculate a first flow path of the compressed exhaust gas to an upstream point in the manufacturing assembly. The compressor further comprises a second conduit configured to provide a second flow path of the compressed exhaust gas to the CO2 separation system. The CO2 separation system is configured to receive the compressed exhaust gas and generate a substantially CO2 free exhaust stream and stream of liquid CO2.
A natural gas combined cycle power plant is also provided. The plant comprises a semi-open combustion cycle and a closed steam cycle and in operation generates an exhaust stream comprising CO2. The plant further comprises at least one compressor downstream of the combustion cycle and steam cycle, as well as a CO2 separator. The compressor is coupled to a recirculation line that fluidly connects the compressor with the open combustion cycle. The compressor is also fluidly connected to the CO2 separator.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Any compositional ranges disclosed herein are inclusive and combinable (e.g., ranges of “up to about 25 wt %”, or, more specifically, “about 5 wt % to about 20 wt %”, are inclusive of the endpoints and all intermediate values of the ranges). Weight levels are provided on the basis of the weight of the entire composition, unless otherwise specified; and ratios are also provided on a weight basis. Moreover, the term “combination” is inclusive of blends, mixtures, reaction products, and the like. Furthermore, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.
The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The modifier “about” used in connection with a quantity is inclusive of the stated value, and has the meaning dictated by context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., “the stream(s)” may include one or more streams).
Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments.
Provided herein are methods and systems for reducing CO2 in emissions streams, e.g., of power plants. The present methods not only make use of exhaust gas recirculation, but also, compression of the exhaust gas. Importantly, the compression of the exhaust gas is done prior to the introduction thereof into the gas turbine compressor and/or its mixture with pure air. And so, penalties in power and efficiency that may otherwise be expected for natural gas fired power plants that employ exhaust gas recirculation due to lower ratio of specific heat of the exhaust gas as compared to pure air, can be minimized, or even eliminated.
The compression of the exhaust gas also serves to increase the pressure and thus, decrease the volume, of the exhaust gas. Recirculation of the compressed exhaust gas increases the concentration of CO2 in the exhaust gas. As a result, removal of CO2 from the exhaust gas is thus simplified, and the capital and energy expenditures required to do so reduced as compared to those associated with CO2 removal from a non-compressed exhaust gas, since less energy may be required to freeze out the CO2 from a compressed exhaust gas stream as compared to a non-compressed exhaust stream. Finally, the CO2 is cryogenically separated at pressures greater than or equal to ambient pressure, but lower than the pressure at the triple point of CO2. And so, the recovered CO2 can be pumped to its final pressure, rather than compressed.
As a result, the methods and plants disclosed herein may use at least 10% less energy, or at least 20% less, or even at least 30% less, than conventional methods and plants that provide from the removal of CO2 from an exhaust stream. These energy savings can be further maximized in those embodiments of the methods and/or plants wherein heat is recovered from the hot exhaust gas.
The present methods comprise generating an exhaust stream comprising CO2. The exhaust stream is compressed and recycled to increase the CO2 concentration therein. Generally speaking, any amount of compression that will provide even a minimal increase in pressure in the exhaust stream may be used, and the exact amount may be dictated by the initial concentration of CO2, the other components in the exhaust stream, the CO2 separation mechanism desirably employed, and the like. On the other hand, in those embodiments wherein the CO2 separation mechanism desirably comprises a cryogenic separator, the exhaust gas will desirably not be compressed to a pressure greater than the triple point of CO2, i.e., about 5 atmospheres.
A first flow path of the compressed exhaust stream is recirculated back to the generating step. The particular amount of the compressed exhaust stream recirculated in the first flow path can be selected based upon the increase in CO2 concentration in the exhaust gas desired. Generally speaking, increases in CO2 concentration in the exhaust stream may be expected to be seen when at least about 10%, or about 20%, or about 30%, or about 40%, or even up to about 50% of the compressed exhaust stream is recirculated to the generating step.
A second flow path of the compressed exhaust stream is provided to a separator where CO2 is then separated from the compressed exhaust stream to provide a substantially CO2 free exhaust stream and liquid CO2. The separator desirably comprises a cryogenic separator, also commonly referred to as a “CO2 freeze out unit”, either used alone, or in combination with other CO2 separation processes such as CO2 selective membrane technologies, sorption processes (adsorption and/or absorption), diaphragms, and the like. Such methods, as well as the parameters for their operation, are well known to those of ordinary skill in the art. Examples of suitable membrane technologies and details of their use are disclosed in US Patent Publication Nos. 2008/0104958 and 2008/0127632 to Finkenrath, which publications are hereby incorporated herein by reference to the extent that they do not conflict with the teachings herein.
In some embodiments, one or more cryogenic separators are used to remove CO2 from the exhaust stream. Cryogenic separators for the removal of CO2 are known in the art, many are commercially available, and any of these may be utilized in the methods. As is known to those of ordinary skill in the art, cryogenic separators operate by “freezing out” the CO2 as a solid from the compressed exhaust stream. The CO2 “snow” is then collected, compressed and melted. The melted CO2 is then pumped to its final pressure for storage or use.
Because of the cost and energy savings they provide, the present methods are advantageously incorporated into industrial processes and plants that generate exhaust streams comprising CO2. Further, the methods are easy to implement on all existing and future power plants, as no integration with the main power system is required. In some embodiments, such industrial plants may incorporate a heat exchanger, which may be integrated with the main power system, if desired. Such integration could lead to a reduction of the power requirement needed to drive the other components of the industrial plant, or even help to make the CO2 separation components energy self-sustainable.
Examples of industrial plants that could benefit from incorporation of the principles described include combustion processes, such as coal fired power plants, oil-fired boilers, cement or steel factories, etc. Generally speaking, such plants will comprise a manufacturing assembly for producing a product and an exhaust stream comprising CO2. Such plants will further desirably comprise a compressor, recirculation line and carbon dioxide separation system. The compressor receives the exhaust stream comprising CO2 and generates a compressed exhaust gas. The compressor comprises a first conduit configured to recirculate a first flow path of the compressed exhaust gas to an upstream point in the manufacturing assembly. The compressor further comprises a second conduit configured to provide a second flow path of the compressed exhaust gas to the CO2 separation system. The CO2 separation system is configured to receive the compressed exhaust gas and generate a substantially CO2 free exhaust stream and stream of liquid CO2.
One particular class of industrial plants that could benefit from incorporation of the methods and principles described herein includes natural gas power plants, e.g., natural gas combined cycle power plants.
Plant 100 includes a semi-open combustion cycle 101, comprising first second compressor 102, natural gas inlet 134, combustor 104 and expander 106, and a closed steam cycle 103, comprising steam turbine 108, and generator 110. Semi-open combustion cycle 101 and closed steam cycle are mounted on the same shaft, and so, as shown in
Plant 100 further comprises heat exchanger 116. Heat exchanger 116 is in flow communication with expander 106 and steam turbine 108. In operation, the relatively hot exhaust stream discharged from expander 106 is channeled through heat exchanger 116. The heat energy from the hot exhaust stream is transferred to the working fluid flowing through heat exchanger 116, e.g., in some embodiments a heat recovery steam generator, or HRSG, to generate steam that is used to produce further power in steam turbine 108. In some embodiments, heat exchanger 116 is a non-contact heat exchanger, i.e., in which water or steam from closed steam cycle 103 is provided to and passed through tubes (not shown) in heat exchanger 116 via conduit 120 and exhaust gas from semi-open combustion cycle 101 is provided to and passes around the tubes (not shown) within heat exchanger 116 via conduit 118.
A condenser 112 can be located downstream from steam turbine 108 to convert the stream discharged from steam turbine 108 to water by lowering the temperature. A pump 114 may also be employed downstream of the condenser 112 to increase the pressure of the water prior to entry into the heat exchanger 116.
Cooled exhaust gas exits heat exchanger 116 and is provided to first compressor 118. In the embodiment shown in
A second flow of compressed exhaust gas is provided to CO2 separation unit 122 from first compressor 118 via conduit 124. In some embodiments, CO2 separation unit 122 comprises a CO2 cryogenic separator, either used alone, or in combination with other CO2 separation processes such as CO2 selective membrane technologies, sorption processes (adsorption and/or absorption), diaphragms, and the like. CO2 membrane technologies are disclosed, for example, in US Patent Publication Serial No. 2008/0134660, hereby incorporated herein by reference in its entirety.
CO2 separation unit 122 produces a substantially CO2-free exhaust gas, discharged out conduit 126, and the frozen out CO2 collected, compressed, melted and delivered to pump 128 where it is pumped to supercritical pressure for transport via conduit 130.
Natural gas combined cycle plant 100 is operated as known in the art, and as such, produces an exhaust stream having a temperature of from about 600 degrees Fahrenheit (° F.) (316 degrees Celsius (° C.)) to about 1,300° F. (704° C.). The exhaust stream discharged from open combustion cycle 101 is channeled through heat exchanger 116 wherein a substantial portion of the heat energy from the exhaust stream is transferred to the closed steam cycle 103, with the working fluid channeled therethrough to generate steam that can be utilized to drive steam turbine 108 and generator 110. In other embodiments, the exhaust stream can be simply cooled without utilizing the heat rejected to useful purpose, and/or it can be linked to another process to provide heat in the form of steam or hot water.
Heat exchanger 116 facilitates reducing the operational temperature of the exhaust stream to a temperature that is between about 75° F. (24° C.) and about 248° F. (120° C.). In some embodiments, heat exchanger 116 facilitates reducing the operational temperature of the exhaust stream to a temperature that is approximately 100° F. (38° C.).
The relatively cool dry exhaust stream is then compressed in first compressor 118. If desired, prior to providing the exhaust stream to first compressor 118, the temperature thereof may be further reduced by passing the exhaust stream through a heat exchanger, wet scrubber, or the like (not shown). In some embodiments, such a heat exchanger/wet scrubber (not shown) can be used to condense the water present in the exhaust gas as well as to reduce the temperature of the exhaust stream, e.g., to about 40° C., so that the compression power required is reduced.
First compressor 118 will desirably be utilized to increase the operating pressure of the exhaust stream channeled there through to a pressure that is up to about four or five times greater than the operating pressure of the exhaust stream discharged from heat exchanger 116. Moreover, channeling the exhaust stream through first compressor 118 causes the temperature of the exhaust stream to increase. And so in some embodiments, once discharged from first compressor 118, the exhaust stream may optionally be passed through heat exchanger or wet scrubber to reduce the temperature thereof.
Such a heat exchanger may be operatively disposed relative to conduit 124 or conduit 120, as desired. When operatively disposed relative to conduit 124, such a heat exchanger or wet scrubber may facilitate reducing the operational temperature of the exhaust stream, which in turn, may be advantageous for operating CO2 separation unit 122.
The CO2 rich exhaust stream discharged from first compressor 118 enters the CO2 separation unit 122 via conduit 124. As described above, CO2 separation unit 122 comprises a CO2 freeze out unit, either used alone, or in combination with other CO2 separation processes such as CO2 selective membrane technologies, sorption processes (adsorption and/or absorption), diaphragms, and the like.
The CO2 freeze out unit comprises an advanced refrigerant process, preferably a mixed-refrigerant cycle, which is able to reduce the temperature of an exhaust stream down to −150° C. and frost CO2 at pressures greater or equal to atmospheric, but lower than the pressure at the triple point of CO2. As the CO2 freezes, it is separated from the substantially CO2 free exhaust stream. Subsequently the solid CO2 is collected and melted, using for instance the low-temperature heat from the exhaust stream. Once the CO2 is in liquid state, it is pumped to a supercritical pressure, which is required for transport, sequestration and reinjection purposes.
The introduction of compressed air at an inlet to expander 206 process can act to cool down the blades of the expander, reducing or eliminating to divert air from compressor. That is, since the pressure of the exhaust gas exiting first compressor 218 is limited by the pressure acceptable within CO2 separation unit 222 to the pressure at the triple point of CO2, or to about 5 atmospheres, the pressure of the exhaust gas recirculated and added to semi-open combustion cycle 201 after combustor 204 and prior to expander 206 must be raised to substantially equivalent to the pressure within expander 206, e.g., to about 20 to 40 atmospheres, or flow in the conduit 220 will reverse. Compressed exhaust gas can also cool down the blades of expander 206, and reduce or eliminate the need to divert air from compressor 202 for this purpose. As a result, this embodiment can provide further reductions in penalties to semi-open combustion cycle 201.
Upstream plant 540 is operated as known in the art, and as such, produces an exhaust stream having a temperature of from about 600 degrees Fahrenheit (° F.) (316 degrees Celsius (° C.)) to about 1,300° F. (704° C.). The exhaust stream discharged from semi-open combustion cycle 501 is channeled through heat exchanger 516 wherein a substantial portion of the heat energy from the exhaust stream is transferred to the closed steam cycle 503. More particularly, Heat exchanger 516 facilitates reducing the operational temperature of the exhaust stream to a temperature that is between about 75° F. (24° C.) and about 248° F. (120° C.), or to a temperature of about 100° F. (38° C.). The exhaust stream from upstream plant 540, and more particularly, heat exchanger 516, is provided to downstream plant 542, which then operates substantially as described above in connection with
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
