In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat efficiently from and to use the extracted heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such steam generators have been incorporated into combined cycle electric generating plants including both gas and steam turbines wherein the exhaust gases of the gas turbine are used to heat water into steam then to be transferred to the steam turbine. Typically, steam generators include a water heating section or economizer tube, a high pressure evaporator tube and finally a superheater tube, whereby water is gradually heated while increasing levels of pressure are applied thereto to provide from the superheater tube, superheated steam to supply the steam turbine. A condenser is associated with the steam turbine to receive the spent steam therefrom and for converting it into water condensate to be fed back to the steam generator.
In a combined cycle electric power plant, the steam turbine is combined with a gas turbine whereby the heated exhaust gases of the gas turbine, otherwise lost to the atmosphere, are used to heat the circulated water and to convert it into steam to drive the steam turbine. In this manner, a significant reduction in the fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized.
Although the efficiency of the process is improved by transferring the heat from the exhaust to water to produce steam, steam as a heat source is very inefficient in itself. Using steam as a heat transfer medium for supplying heat to an oil stream in a refinery is inefficient and such a system has significant energy losses. As heat is transferred out of the steam, it is condensed, let down in pressure and reintroduced into the boiler feed water to the steam generator for re-evaporation. Although this sounds efficient, in fact there are usually significant water losses in the steam system due to steam traps and heat loss. Often as much as 50% of the condensed steam is not returned to the steam generator. Therefore, significant make-up water is required, and that water must be obtained and treated to be used to make steam. Thus, it is fairly costly to operate a steam heating system.
Many prior art processes are directed at improving the efficiency of the steam generation system. In U.S. Pat. No. 4,031,404, an improved superheat temperature control for heat recovery steam generators, particularly adapted for use in combined cycle electric power plants, is disclosed. In U.S. Pat. No. 4,501,233, an improved steam generator high and low pressure boiler drums and high and low pressure evaporators is disclosed. U.S. Pat. Nos. 5,247,991 and 5,311,844 provides improved tube arrangements in a heat recovery steam generator (HRSG). In U.S. Pat. No. 5,924,389 an improved water flow circuit for overall plant efficiency is disclosed. Finally, in U.S. Pat. No. 5,946,901 and improved flow distribution of the exhaust gas stream in a heat recovery steam generator is disclosed.
As shown above, a lot of effort has been put into improving the steam generation process. Unfortunately, each of the above mentioned prior art process does not address a major inefficiency: the steam system itself. Thus, it would be desired to develop a system that not only improves the efficiency of a heat recovery steam generator, but also provides a more efficient heat transfer means of transferring heat from a turbine exhaust to, for example, a process stream in a refinery.
The present invention has been developed in order to maximize the heat recovery from the exhaust from a combustion turbine. This is normally done in an HRSG, where the HRSG is used to preheat water, boil the water, and then superheat the steam. The present invention is directed toward integrating a heat transfer medium with the HRSG to absorb some of the heat from the exhaust.
The present invention is directed to a process to recover heat from a hot gas stream that involves exchanging heat between the combustion engine turbine exhaust and both a heat transfer medium and water. In a preferred embodiment, the process first calls for exchanging heat between the combustion engine turbine exhaust and a heat transfer medium, giving a heated heat transfer medium and a heated heat transfer medium cooled exhaust. Second, heat should be exchanged between the heated heat transfer medium cooled exhaust and steam, giving superheated steam and a superheated steam cooled exhaust. Third, exchanging heat between the superheated steam cooled exhaust and preheated boiler feed water, giving the steam and a steam generation cooled exhaust. Fourth, the steam generation cooled exhaust should exchange heat with the boiler feed water, giving the preheated boiler feed water and preheated boiler feed water cooled exhaust. It is within the contemplation of the present invention, though, that the heat transfer medium can be heated at any point within the HRSG.
It is a further object of the process to include an additional heat exchanging step. A final step of the present invention involves exchanging heat between the preheated boiler feed water cooled exhaust and the heat transfer medium fluid. This can serve as a means for either preheating or cooling the heat transfer medium fluid before the heat transfer medium is exchanged with the exhaust.
A secondary object the present invention is to provide an apparatus to recover heat from a combustion engine turbine exhaust. This apparatus provides a first heat exchanger to exchange heat between the combustion engine turbine exhaust and a heat transfer medium, giving a heated heat transfer medium and a heated heat transfer medium cooled exhaust; a second heat exchanger to exchange heat between the heated heat transfer medium cooled exhaust and steam, giving superheated steam and a superheated steam cooled exhaust; a third heat exchanger to exchange heat between the superheated steam cooled exhaust and preheated boiler feed water, giving the steam and a steam generation cooled exhaust; and a fourth heat exchanger to exchange heat between the steam generation cooled exhaust and boiler feed water, giving the preheated boiler feed water and preheated boiler feed water cooled exhaust.
It is a further aspect of the apparatus to provide a fifth heat exchanger to exchange heat between the preheated boiler feed water cooled exhaust and the heat transfer medium fluid. This can serve as a means for either preheating or cooling the heat transfer medium fluid before the heat transfer medium is exchanged with the exhaust in the first heat exchanger. Preferably, the heat exchangers of the present apparatus are integrated into one process unit for the heat recovery from a combustion engine turbine exhaust, such as an HSRG with an integrated heat transfer medium fluid loop.