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Combined Cycle Plants

The combined-cycle unit combines the Rankine (steam turbine) andBrayton (gas turbine) thermodynamic cycles by using heat recoveryboilers to capture the energy in the gas turbine exhaust gases for

steam production to supply a steam turbine as shown in the figure"Combined-Cycle Cogeneration Unit". Process steam can be alsoprovided for industrial purposes.

Fossil fuel-fired (central) power plants use either steam or combustionturbines to provide the mechanical power to electrical generators.Pressurized high temperature steam or gas expands through variousstages of a turbine, transferring energy to the rotating turbine blades.The turbine is mechanically coupled to a generator, which produceselectricity.

Steam Turbine Power Plants:

Steam turbine power plants operate on a Rankine cycle. Thesteam is created by a boiler, where pure water passes through aseries of tubes to capture heat from the firebox and then boilsunder high pressure to become superheated steam. The heat in thefirebox is normally provided by burning fossil fuel (e.g. coal, fuel oilor natural gas). However, the heat can also be provided bybiomass, solar energy or nuclear fuel. The superheated steamleaving the boiler then enters the steam turbine throttle, where it

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powers the turbine and connected generator to make electricity.After the steam expands through the turbine, it exits the back endof the turbine, where it is cooled and condensed back to water inthe surface condenser. This condensate is then returned to theboiler through high-pressure feedpumps for reuse. Heat from the

condensing steam is normally rejected from the condenser to abody of water, such as a river or cooling tower.

Steam turbine plants generally have a history of achievingup to 95% availability and can operate for more than a yearbetween shutdowns for maintenance and inspections. Theirunplanned or forced outage rates are typically less than 2% or lessthan one week per year.

Modern large steam turbine plants (over 500 MW) haveefficiencies approaching 40-45%. These plants have installed

costs between $800 and$2000/kW, depending on environmentalpermitting requirements.

Combustion (Gas) Turbines:

Combustion turbine plants operate on the Brayton cycle.They use a compressor to compress the inlet air upstream of acombustion chamber. Then the fuel is introduced and ignited toproduce a high temperature, high-pressure gas that enters andexpands through the turbine section. The turbine section powersboth the generator and compressor. Combustion turbines are alsoable to burn a wide range of liquid and gaseous fuels from crude oilto natural gas.

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The combustion turbines energy conversion typically rangesbetween 25% to 35% efficiency as a simple cycle. The simple cycleefficiency can be increased by installing a recuperator or wasteheat boiler onto the turbines exhaust. A recuperator captureswaste heat in the turbine exhaust stream to preheat the

compressor discharge air before it enters the combustion chamber.A waste heat boiler generates steam by capturing heat form theturbine exhaust. These boilers are known as heat recovery steamgenerators (HRSG). They can provide steam for heating orindustrial processes, which is called cogeneration. High-pressuresteam from these boilers can also generate power with steamturbines, which is called a combined cycle (steam and combustionturbine operation). Recuperators and HRSGs can increase thecombustion turbines overall energy cycle efficiency up to 80%.

Combustion (natural gas) turbine development increased inthe 1930s as a means of jet aircraft propulsion. In the early 1980s,the efficiency and reliability of gas turbines had progressedsufficiently to be widely adopted for stationary power applications.Gas turbines range in size from 30 kW (micro-turbines) to 250 MW

(industrial frames). Industrial gas turbines have efficienciesapproaching 40% and 60% for simple and combined cyclesrespectively.

The gas turbine share of the world power generation markethas climbed from 20 % to 40 % of capacity additions over the past20 years with this technology seeing increased use for base loadpower generation. Much of this growth can be accredited to large

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(>500 MW) combined cycle power plants that exhibit low capitalcost (less than $550/kW) and high thermal efficiency.

The capital cost of a gas turbine power plant can varybetween $35000-$950/kW with the lower end applying to large

industrial frame turbines in combined cycle configurations.Availability of natural gas-fired plants can exceed 95%. In Canada,there are 28 natural gas-fired combined cycle and cogenerationplants with an average efficiency of 48 %. The average poweroutput for each plant was 236 MW with an installed cost of around$ 500/kW.

Simple Cycle Power Plants (Open Cycle)

The modern power gas turbine is a high-technology package that iscomprised of a compressor, combustor, power turbine, and generator,

as shown in the figure "Simple-Cycle Gas Turbine".

In a gas turbine, large volumes of air are compressed to high pressurein a multistage compressor for distribution to one or more combustiongases from the combustion chambers power an axial turbine thatdrives the compressor and the generator before exhausting toatmosphere. In this way, the combustion gases in a gas turbine powerthe turbine directly, rather than requiring heat transfer to a

water/steam cycle to power a steam turbine, as in the steam plant.The latest gas turbine designs use turbine inlet temperatures of1,500C (2,730F) and compression ratios as high as 30:1 (foraeroderivatives) giving thermal efficiencies of 35 percent or more fora simple-cycle gas turbine.

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