The invention relates to combined cycle turbomachinery and, more particularly, to improving combined cycle efficiency by heating fuel in a gas turbine fuel line into stages.
Combined cycle turbomachines utilize gas turbines (GT(s)) as prime movers to generate power. These GT engines operate on the Brayton Cycle thermodynamic principle and typically have high exhaust flows and relatively high exhaust temperatures. These exhaust gases, when directed into a heat recovery boiler (typically referred to as a heat recovery steam generator (HRSG)), produce steam that can be used to generate more power. The produced steam can be directed to a steam turbine (ST) to produce additional power. In this manner, a GT produces work via the Brayton Cycle, and a ST produces power via the Rankine Cycle. Thus, the name “combined cycle” is derived.
Fuel gas heating in combined cycle turbomachinery is typically performed to increase the thermal efficiency. In one previous approach, with reference to
Although higher fuel gas heating temperature improves the thermal efficiency of a turbomachine, a higher operating pressure of the IP evaporator has a detrimental effect on the steam cycle power output and the thermal efficiency of the machine. Therefore, the IP evaporator is typically operated at an optimum pressure in a combined cycle turbomachine, thus limiting the fuel gas heating temperature and the efficiency of the machine.
In order to increase the temperature of the water available for fuel gas heating, water from high pressure (HP) economizers upstream of the IP evaporator may be used. However, using high pressure water considerably increases the cost of fuel gas heating while presenting a reliability concern in the event of a failure. In one known design, the available IP water temperature has limited fuel gas heating to 365° F. Thus, there is a need to improve the thermal cycle efficiency of combined cycle turbomachinery to overcome the problems faced by prior systems.
In an exemplary embodiment, a method of improving efficiency of a combined cycle turbomachine includes the steps of (a) providing a fuel line for inputting fuel into the gas turbine; (b) heating the fuel in a first fuel heater using water input from an intermediate pressure economizer of the HRSG to an inlet of the first fuel heater; (c) heating the fuel in a second fuel heater downstream from the first fuel heater using water input from a high pressure economizer of the HRSG; and (d) directing water output from the second fuel heater to one of an intermediate pressure section of the HRSG or the inlet of the first fuel heater.
In another exemplary embodiment, a method of improving efficiency of a combined cycle turbomachine includes the steps of (a) providing a fuel line for inputting fuel into the gas turbine; and (b) heating the fuel in two stages using (i) hot water from an HP economizer of a heat recovery steam generator (HRSG) in a second stage and (ii) hot water from an IP economizer of the HRSG and water output flow from the second stage in a first stage.
In yet another exemplary embodiment, a fuel heating circuit is utilized in a combine cycle turbomachine. The turbomachine includes a gas turbine, a heat recovery steam generator (HRSG) receiving gas turbine exhaust and generating steam, and a steam turbine receiving the steam from the HRSG. The fuel heating circuit includes a fuel line that supplies fuel to the gas turbine, a first heater on the fuel line that receives hot water output from an intermediate pressure economizer of the HRSG via a heater inlet, and a second heater on the fuel line downstream from the first heater that receives hot water output from a high pressure economizer of the HRSG. An output line from the second heater delivers water output from the second heater to one of an intermediate pressure section of the HRSG or the heater inlet of the first fuel heater.
With continued reference to
In order to increase the fuel temperature, the fuel is further heated in a second fuel heater or HP fuel heater 46 on the fuel line 42 downstream from the first heater 44. The second fuel heater 46 utilizes flow from the HP economizer 32, which discharge flow is typically about 650° F. Discharge water from the HP fuel heater 46 at a temperature of about 430° F. may then be mixed with the incoming flow entering the IP fuel heater 44. Alternatively, as shown in
Using the HP economizer flow will result in an increase in efficiency, but at a cost of combined cycle output. In an exemplary embodiment, the output can be replaced and additional efficiency realized using LP water injection and fuel preheating.
With reference to
The fuel temperature after water injection can reach up to 300° F. using additional LP feed water, and in some embodiments, the water may be injected again 57. Each successive water injection brings the fuel moisture closer to about 10% water by volume. The increase in fuel moisture, however, is smaller with each successive heating/water injection cycle. In a preferred construction, three cycles of water injection can be used, but a cost performance trade can be calculated to determine a number of justified cycles. After the preheating/water injection process, the fuel is directed to the IP fuel heater 44.
After the fuel is heated in the IP fuel heater, a section of pipe spray 56, with water from the IP feed pump discharge (or HP feed pump discharge if higher pressure is required to fully atomize the water spray), water is injected/mixed with the fuel in the IP injector 58. The amount of water injection is regulated so as to reach moisture saturation of the fuel.
The saturated fuel is then superheated in the HP fuel heater, giving adequate safety of downstream valves and equipment from damage from fuel borne water droplets.
The system serves to improve combined cycle efficiency. In at least one combined cycle turbomachine, efficiency is increased by 0.2% with increased output of 5-8 MW.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.