1. Field of the Invention
The present invention relates to an optimized power generation system and process. More specifically, the present invention relates to an integrated power generation system and process comprising an oxygen-fired combustor integrated with an air separation unit.
2. Brief Description of Art
The products of air separation units can be used in various power generation schemes and can enhance the performance of existing power generation systems. Such products may therefore play key roles in the high-efficiency, low or zero-emission power generation schemes of the future. For example, oxygen and oxygen-enriched air have been demonstrated to enhance combustion, increase production, and reduce emissions. Oxy-combustion also has the inherent advantage of producing a CO2-rich flue gas, which can be more easily processed than flue gas from air-blown processes. With the increasing interest in global climate change, more attention will undoubtedly be focused on technologies that facilitate the capture of CO2. The greater ease with which CO2-rich flue gas produced by oxy-combustion may be processed to capture CO2 therefore suggests that the further development of this technology would be beneficial.
Nitrogen product streams can also offer benefits to a power generation system. For instance, high-pressure nitrogen, available from the high-pressure column of a cryogenic air separation unit (ASU), when appropriately heated and expanded in an integrated power generation scheme, can further increase power output.
The integration of air separation units with power generation processes has been the topic of several patent applications and technical articles. For example, in U.S. Pat. No. 6,282,901 (Marin et al), an oxygen-enriched stream from an ASU is fed to a combustor. The combustor flue gases are used to raise steam and generate power in several distinct embodiments. A nitrogen-enriched stream from the ASU is also heated and expanded to generate additional power. High cycle efficiencies, with low or zero emissions, are possible with these highly integrated schemes. However, the specific concept described in the present invention is not discussed.
In commonly-assigned U.S. Provisional Patent Application No. [60/356,105], entitled “Integrated Air Separation Unit and Oxygen-fired Power Generation System”, an integrated air separation and oxygen-fired power generation system is disclosed. The power generation system includes one or more oxygen-fired combustors that supply drive gas to the expander section of a gas turbine, as well as to other turbines in an optimized configuration. The turbines directly drive the compressor section of the gas turbine, which functions as the main ASU air compressor. With an optimized turbine configuration, and high level of heat integration, this scheme has the potential to reduce the power costs of the ASU below levels attained with an onsite power plant. The effluent from this process is a concentrated carbon dioxide stream that can be further purified and sold as a byproduct. Unlike the present invention, the disclosed system is a self-powered, multi-product gas generator i.e., the system produces oxygen, nitrogen, argon, and carbon dioxide, with fuel and air as the only inputs.
U.S. Pat. No. 6,148,602 (Demetri) describes a power generation system in which an oxygen-fired combustor produces drive gas for a turbine. The turbine drives an air compressor and an oxygen compressor on a single shaft. The air compressor supplies an ASU with an air feed stream, while the oxygen compressor supplies high pressure oxygen to the combustor. The combustor receives gaseous fuel from a solid fuel gasifier, and water is recycled to the combustor to control the outlet temperature. Downstream of the turbine, CO2 is separated in a condenser, and delivered to a sequestration site. Additional energy is said to be possibly recovered from the ASU nitrogen stream, although no details are provided concerning how this would occur. Ideal operating parameters of the turbine are not specified.
Bolland et al (Energy Conversion & Mgmt, V. 33, No. 5-8, 1992, p. 467) proposed a scheme that consists of supplying a combustor with oxygen from an ASU, reacting the oxygen with a fuel, adding water or steam to control the combustor outlet temperature, and passing the combustor gases through a turbine to generate power. A water inlet stream is used in a heat recovery scheme to cool the discharge of the ASU main compressor. The scheme includes a power generation process that receives an oxygen inlet stream from an ASU. However, the degree of integration between the ASU and the power cycle is limited.
E. I. Yantovskii (Proceedings of World Clean Energy Conference, Geneva Switzerland, 1991, pp. 571-595) proposes a scheme that employs oxygen-fired combustion with water recycle. A high-pressure combustor receives oxygen from an ASU, hydrocarbon fuel, and recycled water and produces a steam/CO2 drive gas that enters a turbine. This is followed by two stages of reheating and expansion. The CO2 is separated in a condenser, and the condensate is recycled to the high-pressure combustor. In this scheme, the ASU is treated as a supplier of oxygen, and the ASU is not integrated with the power generation system.
In U.S. Pat. No. 5,956,937 (Beichel), a power generation system is described that utilizes an oxygen-fired gas generator and at least one oxygen-fired reheater to produce drive gas for a series of turbines. The key features of this system are depicted in FIG. 1. In this scheme, oxygen, a gaseous hydrocarbon fuel, and water/steam are supplied to a high-pressure combustor or gas generator. This device produces drive gas for a high-pressure turbine. The discharge from the high-pressure turbine is reheated in a second combustor fired with additional fuel and oxygen. The discharge enters one or more turbines to produce additional power. Since the drive gas is produced by contacting water/steam with the combustion products, it contains significant levels of carbon dioxide. In practice, the gas generator and reheater will be operated with excess oxygen to ensure complete combustion. As a result, the drive gas will also contain significant levels of residual oxygen.
Any increase in the operating pressure and temperature of the high-pressure turbine will raise the overall efficiency of this cycle. Current steam turbine temperature limitations are in the range of 1050-1100° F. (840-870K) and pressure limits are around 3500 psi (240 bar). Steam turbines, however, have been designed to operate with pure steam as the drive gas. Their performance in the presence of impurities, especially at higher pressures and temperatures, is questionable. Therefore, there is a level of risk involved in using the gas generator as a source of high-pressure drive gas.
In U.S. Pat. Nos. 6,202,442 and 6,272,171 (Brugerolle), an integrated power generation system is described in which part of the air from a gas turbine compressor is separated in a single nitrogen wash column to remove oxygen. Gaseous nitrogen produced at the top of the column is then sent back to a point upstream of the expander of the gas turbine.
In U.S. Pat. No. 6,247,315 (Marin et al), an improved combustion process for use in, e.g., a combined cycle co-generation installation is described.
Even though various processes and systems for the generation of power and the separation of air have been developed, as briefly noted above, a need continues to exist for the improvement of integrated systems for power generation and air separation.