Patent Application
20070181854
The present invention is directed generally to gas turbine systems, and more particularly to integrated gasification combined cycle gas turbine systems.
The utilization of coal in the prior art has been minimized due to undesirable emissions, such as oxides of nitrogen and sulfur, particulate emissions and greenhouse gases such as carbon dioxide. As a result, there have been efforts to reduce these emissions and improve fuel efficiency of coal plants.
One of the systems that have been developed is the Integrated Gasification Combined Cycle (IGCC) system for use in power generation. IGCC systems were devised as a way to use coal as the source of fuel in a gas turbine plant. IGCC systems are clean and generally more efficient than prior art coal plants.
IGCC is a combination of two systems. The first system is coal gasification, which uses coal to create a clean-burning synthetic gas (“syngas”). The gasification portion of the IGCC plant produces syngas, which may then be used to fuel a combustion turbine. Coal is combined with oxygen in a gasifier to produce the syngas, hydrogen and carbon monoxide. The syngas may then be cleaned by a gas cleanup process. After cleaning, the syngas may be used in the combustion turbine to produce electricity.
The second system is a combined-cycle, or power cycle, which is an efficient method of producing electricity commercially. A combined cycle includes a combustion turbine/generator, a heat recovery steam generator (HRSG), and a steam turbine/generator. The exhaust heat from the combustion turbine may be recovered in the HRSG to produce steam. This steam then passes through a steam turbine to power another generator, which produces more electricity. A combined cycle is generally more efficient than conventional power generating systems because it re-uses waste heat to produce more electricity.
IGCC systems offer several advantages of IGCC over current conventional coal-based power generation systems. One advantage is reduced emissions. Another aspect of IGCC plants is that emissions clean-up, including removal of sulfur and carbon dioxide, may be effected upstream of the combustor system in the fuel stream. Since this stream is far smaller than the entire flue gas stream, emissions removal equipment for an IGCC plant are lower than for a conventional coal plant of like output.
IGCC systems offer other advantages, such as higher efficiency, less coal used, higher turbine outputs, and/or the production of additional chemical by-products, such as hydrogen, which may be used as an alternative source of energy in other developing technologies.
Nevertheless, IGCC systems may still suffer from reduced efficiencies as compared to other systems. Since syngas has a lower heating value than other fuels, more syngas is needed to produce a selected turbine temperature. In addition, the product nitrogen stream from the Air Separation Unit (ASU) Island of an Integrated Gasification Combined Cycle (IGCC) plant is at elevated temperatures, therefore requiring equipment for reducing the heat prior to venting.
Accordingly, it would be beneficial to provide a system that utilizes coal that has increased efficiencies as compared to prior art systems. It would also be beneficial to increase the integration of the components in the IGCC to increase efficiency and/or power out put of the IGCC systems.
This present invention provides a method of increasing the efficiency and/or power produced by an integrated gasification combined cycle system by increasing the integration between the air separation unit island of the integrated gasification combined cycle system and the remainder of the system. By integrating one or more product streams from the air separation unit in the remainder of the integrated gasification combined cycle system, heat may be utilized that may have otherwise been lost or used further downstream in the system. The integration helps to increase the efficiency of the combustion reaction and/or the gasification reaction used to produce the syngas utilized in the integrated gasification combined cycle system.
In particular, in one aspect, the present invention provides a method for increasing efficiency of an integrated gasification combined cycle system including the steps of producing a nitrogen gas product stream and an oxygen gas product stream using an air separation unit, feeding the oxygen gas product stream to a gasifier, producing a syngas stream in the gasifier using the oxygen gas product stream and coal, and heating at least one of the nitrogen gas product stream, the oxygen gas product, or both using the syngas stream.
In another aspect, the present invention provides a system for increasing efficiency of an integrated gasification combined cycle system including an air separation unit for producing a nitrogen gas product stream and an oxygen gas product stream, a gasifier for producing a syngas stream in the gasifier using the oxygen gas product stream and coal, and a syngas cooler for cooling the syngas using at least a portion of at least one of the nitrogen gas product stream, the oxygen gas product, or both.
These and other embodiments are described in more detail below.
Other objects, features and advantages of the present invention will become apparent upon reading the following detailed description, while referring to the attached drawings, in which:
The present invention is more particularly described in the following description and examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an,” and “the” may include plural referents unless the context clearly dictates otherwise. Also, as used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.”
The present invention provides a method for increasing the efficiency of an integrated gasification combined cycle (IGCC) gas turbine system and an IGCC having increased efficiency. The present invention accomplishes the improved efficiency of the system by increasing the integration between the IGCC and the air separation unit (ASU) portion of the IGCC. In the present invention, heat from one area of the system is transferred and/or used in another area of the system to increase the overall efficiency of the system.
In one embodiment of the present invention, the improved integration utilizes one or more product gases from the ASU as part of a method for transferring heat from the IGCC system such that it may be used upstream in the IGCC system. For example, in one embodiment, one or more product gas streams may be used to heat the syngas entering the combustor. Since the excess heat is used and not wasted and/or the syngas enters the combustor at higher temperatures, the system operates at higher efficiencies than prior art systems. In addition, since the heat is recycled, less fuel may be used and/or more syngas may be generated, which may also increase the efficiency of the system.
In a standard IGCC system, with reference to
In one aspect of the present invention, one or more product streams are used to transfer heat to the gasifier 110, which produces a synthetic gas (“syngas”) product stream 115, which may then be used as the fuel source for the combustor 130. The ASU 100, which may be a cryogenic ASU, is used to provide pure or substantially pure oxygen to the gasification reactor and, in alternative embodiments, may include a post-compression air bleed from the gas turbine 175. The ASU produces the oxygen gas product stream 105 and the nitrogen gas product stream 120, which are generally below the temperatures of other streams in the IGCC system, such as the syngas stream 115. As a result, the present invention utilizes one or more of these ASU product streams as heat sink sources, such that heat may be transferred to one or more other areas of the IGCC wherein the increased temperatures help to increase the efficiency of overall system.
Alternatively, or in addition thereto, the one or more gas product streams from the ASU may be used to transfer heat away from syngas, such as through the use of one or more syngas coolers 145. Since syngas must be cooled prior to being cleaned, cooling the syngas using the one or more product streams from the ASU increases the efficiency of the syngas cleaning and cooling process. The syngas is then cleaned in an acid gas removal stage 165. The syngas from the reactor is generally cleaned before it is used as a gas turbine fuel. The cleanup process typically involves removing sulfur compounds, ammonia, metals, alkalytes, ash, and/or particulates to meet the gas turbine's fuel gas specifications. The syngas may then be heated in a syngas heater 170 before being used in the combustor 130.
The one or more product streams may also be used to increase the heat of the fuel mixture 140 that enters into the combustor 130. For example, in one embodiment, the nitrogen gas product stream may be used to dilute the syngas stream to achieve a selected heating value of the fuel mixture 140 entering the combustor. In a standard IGCC system, the nitrogen gas product stream 120, if used to dilute the fuel mixture 140, may first be passed through a heat exchanger 125 which is used to heat the nitrogen stream 120 since the product streams from the ASU 100 are typically at cooler temperatures since most air separation process are performed at sub-zero temperatures. In select embodiments, the nitrogen product stream 120 may be further heated, with the heat then being used in the combustor 130 to increase the efficiency of the system.
Accordingly, by using one or more product streams from the ASU as a means to utilize heat that would otherwise be lost and/or used downstream, the present invention increases the efficiency of the IGCC system by increasing the amount of syngas created per unit of coal feedstock supplied, by increasing the temperature of the fuel mixture, and/or by increasing the amount of power generated by the system. These concepts may be accomplished using a variety of embodiments.
In one embodiment, product nitrogen (N2) gas that is generated from the Air Separation Unit (ASU) is routed to a heat exchanger where it is heated by the syngas that is produced in the gasifier. After leaving this heat exchanger, the heated nitrogen is mixed with the fuel stream and the mixture enters the combustor of the gas turbine. The syngas is cooled in the heat exchanger. In this embodiment, a portion of the ASU nitrogen product gas may be routed through the heat exchanger; however, it is also contemplated that, in an alternative embodiment, the entire product nitrogen gas stream could be routed in this manner.
In an alternative embodiment, product nitrogen (N2) gas generated from the ASU is mixed with the syngas that is produced in the gasifier, such as through using a mixing valve. The nitrogen stream enters at a much colder temperature than the syngas, resulting in a cooler mixed fuel stream temperature entering the syngas cooler. In this embodiment, a portion of the ASU nitrogen product gas may be routed to the syngas stream; however, it is also contemplated that, in an alternative embodiment, the entire product nitrogen gas stream could be routed in this manner.
In yet another alternative embodiment, product nitrogen gas generated from the ASU is mixed with the syngas that is produced in the gasifier, such as through use of a mixing valve. The nitrogen stream enters at a much colder temperature than the syngas, resulting in a mixed fuel stream temperature that can be accepted by the cold gas clean-up system, thus obviating the need for a syngas cooler. In this embodiment, a portion of the ASU nitrogen product gas may be routed to the syngas stream; however, it is also contemplated that, in an alternative embodiment, the entire product nitrogen gas stream could be routed in this manner.
In still another embodiment, the product nitrogen gas stream generated from the ASU is routed to a heat exchanger where it is heated by the syngas that is produced in the gasifier. The heated nitrogen stream is then routed to a heat exchanger where it is cooled by the oxygen stream produced in the ASU. As with the previous embodiments, all or a portion of the ASU product nitrogen stream may be routed through the heat exchanger. In addition, it is possible that only a portion of the oxygen leaving the ASU will be heated prior to entering the gasifier. As shown in
In yet another embodiment, which is related to the previous embodiment, product nitrogen gas from the ASU may be routed to a heat exchanger where it may be heated by the syngas that is produced in the gasifier. The heated nitrogen stream may then be routed to a heat exchanger where it may be cooled by the oxygen stream produced in the ASU. After leaving this heat exchanger, the nitrogen stream may be fed into another heat exchanger where it is re-heated by the syngas before being mixed with the syngas. As with previous embodiments, all or a portion of the nitrogen gas stream may be routed. Also, in an alternative embodiment, only a portion of the oxygen leaving the ASU may be heated prior to entering the gasifier.
In still another embodiment, the nitrogen product gas from the ASU is not used and is mixed with the syngas prior to combustion. In this embodiment, however, product oxygen gas generated from the ASU is routed directly to a heat exchanger where it is heated by the syngas that is produced in the gasifier, effecting simultaneous cooling of the gasifier product syngas and heating of the gasifier oxygen feed stream. Again, since the syngas is cooled prior to being cleaned, the cleaning process is more efficient. And since the oxygen gas is heated, the gasification process is more efficient.
In yet another embodiment, the present invention utilizes a product stream from the ASU not as a source of heat transfer throughout the system, but as a source of additional power. In one embodiment, the product nitrogen stream is used as an additional source of power. In this embodiment, excess nitrogen that would normally be vented from the ASU is instead routed to a turbine, where it is expanded down to ambient pressure to generate torque for power generation, driving a compressor, or some other application. If the nitrogen stream is at an elevated temperature, then, in one embodiment, it may be routed through the HRSG with the flue gas. In an alternative embodiment, it may be vented to atmosphere.
The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.
