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Combined cycle is a term used when a power producing engine or plant employs more than one thermodynamic cycles. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 30%). The remaining heat from combustion is generally wasted. Combining two or more "cycles" such as the Brayton cycle and Rankine cycle results in improved overall efficiency.
Although traditionally combining cycles has been reserved for large power plants BMW has a proposal to use exhaust heat to drive a steam turbine."BMW Turbosteamer gets hot and goes" by John Neff, AutoBlog, December 9, 2005
There are also possibilities being considered to use the pistons in a reciprocating engine for both combustion and steam expansion. "Inside Bruce Crower’s Six-Stroke Engine" By Pete Lyons, AutoWeek, Febuary 23, 2006
In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat from the gas turbine is used to make steam to generate additional electricity via a steam turbine, this last step enhances the efficiency of electricity generation. Most new gas power plants are of this type. In a thermal power plant, high-temperature heat as input to the power plant, usually from burning of fuel, is converted to electricity as one of the outputs and low-temperature heat as another output. As a rule, in order to achieve high efficiency, the temperature of the input heat should be as high as possible and the temperature of the output heat as low as possible (see Carnot efficiency). This is achieved by combining the Rankine (steam) and Brayton (gas) thermodynamic cycles. Such an arrangement used for marine propulsion is called COmbined Gas (turbine) And Steam (turbine) (COGAS).
Design principle
In a steam power plant water is the working medium. In this case high pressure has to be employed which leads to bulky components. High cost of special alloys that endure high temperature limit practical steam temperature to 655°C. For compact gas turbines this limitation does not apply and gas cycle firing temperature in excess of 1,200°C is practicable. In the combined cycle plant the thermodynamic working cycle is operated between the high firing temperature and the ambient temperature at which low temperature waste heat can be disposed.
In a gas turbine set, composed primarily of a compressor, burner and the gas turbine proper, the input temperature to the gas turbine is relatively high (some 900°C to 1,350°C) but the output temperature of the flue gas is also relatively high (some 450°C to 650°C). Flue gas temperature is sufficient for production of steam in the second, steam cycle (Rankine cycle), with live steam temperature in the range of 420°C to 580°C. The lowest temperature of the steam cycle depends on the ambient temperature and the method of waste heat disposal, either by direct cooling by lake, river or sea water, or using cooling towers. Therefore, by combining both processes, high input temperatures and low output temperatures can be achieved and the power plant efficiency can be increased.
The output heat of the gas turbine flue gas is utilized to generate steam by passing it through a heat recovery steam generator (HRSG) and therefore is used as input heat to the steam turbine power plant.
Efficiency of CCGT plants
The thermal efficiency of a combined cycle power plant is normally in terms of the net power output of the plant as a percentage of the lower heating value (LHV) or net calorific value (NCV) of the fuel. In the case of generating only electricity, power plant efficiencies of up to 59% can be achieved. In the case of combined heat and power generation, the efficiency can increase to about 85%.Supplementary firing
The HRSG can be designed with supplementary firing of fuel after the gas turbine in order to increase the quantity or temperature of the steam generated. Without supplementary firing, the efficiency of the combined cycle power plant is higher, however supplementary firing allows the plant to respond to fluctuations of electrical load. Supplementary burners are also called duct burners.Supplementary firing is possible because turbine exhaust gas (flue gas) contains considerable fraction of unused oxygen. Due to temperature limitation at the gas turbine inlet, excess air, above the optimal stochiometric ratio is used. Often in gas turbine designs part of the compressed air flow bypasses the burner and is used for cooling of the turbine blades.
Fuel for combined cycle power plants
Typical combined cycle plants are powered by natural gas, although other sources of fuel can be used such as fuel oil or synthetic gas. Supplementary fuel may be natural gas, fuel oil or coal.
Integrated Gasification Combined Cycle (IGCC)
Integrated Gasification Combined Cycle, or IGCC, is a power plant using synthetic gas (syngas) as a source of clean fuel. Syngas is produced in a gasification unit built for Combined Cycle purposes, hence name Integrated. Steam generated by waste heat boilers of the gasification process is utilized to help power steam turbines. Heavy petroleum residues and coal with high sulfur content and even biomass are possible feeds (raw material) for gasification process. There are only a handful of IGCC plants in operation to date for power generation. These plants are by far the best environmental use of coal. The main inhibiting factor for IGCC is high capital cost, upwards of $1400/kW.
The DOE Clean Coal Demonstration Project facilitated the construction of 3 IGCC plants. One in Indiana, another in Tampa, Florida (online 1996), and another in Reno, Nevada. In the Reno demonstration project, researchers found an inherent problem with the design of IGCC technology such that it would not work above 300 feet from sea level elevations. Joe Lucas, Executive Director of Balanced Energy Choices, as interviewed on NPR's Science Friday, Friday May 12, 2006
The power generation industry also is yet to be convinced about the reliability of IGCC technology as the five demonstration facilities that have gone into operation have not consistently demonstrated availabilities comparable to conventional CCGTs or coal-fired power plants.
References
External links
- Hunstown: Ireland's most efficient power plant @ Siemens Power Generation website
- Natural Gas Combined-cycle Gas Turbine Power Plants Northwest Power Planning Council, New Resource Characterization for the Fifth Power Plan, August 2002
"Combined cycle"