0.25 MW to 500 MW
Gas turbines are currently the favoured prime mover in larger-scale cogeneration wherever natural gas is available at costs less than 3 to 4 times the equivalent energy cost of solid fuels. For operation, intake air passes through a compressor before being heated by the combustion of the fuel. The expanding air is then used to drive a turbine before exiting through the exhaust and heat processes (see figure below). Compressors require a large amount of energy, making the choice of compressor crucial to the overall efficiency of the turbine.
|Gas Turbine Schematic|
| Source: WADE, 2003|
Due to the high oxygen content in the exhaust gas, the combustion of further fuel can be supported without the addition of extra air to raise the quality of heat. This process is known as supplementary firing, and can efficiently raise the exhaust gas temperature from around 500ºC to 1000ºC or more, raising the overall heat:power ratio of the cycle. This can be useful for industrial processes, which require high-temperature steam, such as some chemical processes.
Performance and Efficiency
The electrical efficiencies of modern gas turbines range from 28% to 42% simple cycle efficiency, with typical efficiencies of 32%. For systems larger than 3MW, the gas turbine exhaust, typically around 540ºC, can be used to produce high-pressure steam, which then powers a second generator. Such combined-cycle gas turbines (CCGT) have electric efficiencies of 35%-55%. The pass-out steam from the steam turbine can be used to meet on-site heat requirements increasing overall efficiencies to 75% to 90%. This lowers electricity production, but improves overall economics. To improve electrical generating efficiency and reduce NOx it is possible to inject steam into the combustion chamber. Current production gas turbines have NOx emissions from 2 to 25 ppm, before external controls. Additional NOx reduction methods have been successfully developed for gas turbines, so where very low emission levels are specified, it is possible to attach end of pipe solutions such as Selective Catalytic Reduction (SCR).
Since the combusted fuel passes through the turbine, clean gases must be used to avoid blade erosion. Natural gas is the main fuel source, but other fuels can be used. Distillate oils and gas oils are often used in combination with cheaper interruptible gas supplies. Waste fuels such as biogas, coke-oven gas and landfill gas can be used provided that their composition is consistent and their calorific values relatively constant.
- Simple cycle gas turbines are generally used to provide peaking power or back up power without any provision of heat.
- Recuperated cycle gas turbines utilise the exhaust gas to preheat the compressed air before it enters the combustion chamber.
- Cogeneration cycle gas turbines are suitable for industrial and commercial applications. In industrial applications the exhaust gases can be used to produce process steam or chilling, or directly for drying processes if direct contact with exhaust gases is permissible.
- CCGT systems are best suited to public utility companies (without heat recovery) and industrial plants where there is an abundant supply of natural gas or other gaseous fuel.
Advantages and Disadvantages
| Cost Range for Open Cycle Gas Turbines and CCGT/CHP|
|Installed Capital Cost ($/kW)||800 - 1,800 ||800 – 1,300|
|Operating and Maintenance ($c/kWh)||0.3 - 1.0||0.3 – 1.0|
|Levelized Cost ($c/kWh)|
|8000hrs/year||4.0 - 5.5||4.0 – 4.5|
|4000hrs/year||5.5 - 8.5||5.5 – 6.5|
|Source: WADE, 2006|
Gas turbines are a mature and economically efficient technology with broad acceptance in the electricity market place. The installed capital cost of a gas turbine cogeneration system varies between $800-$1,800/kWh. This is due to large variations in turbine size from a few kW to many hundred of MW. The O&M costs range from $0.3c-$1.0 c/kWh.
| Installed Costs for Gas Turbines|
| Source: Delta Energy and Environment, 2006|
Inspections and blade washing must be carried out, around every 4,000 hours or so, to ensure the turbine is free of excessive vibration due to worn bearings, rotors and damaged blade tips. Entire hot section replacement is often required at roughly five-year intervals, and usually involves a complete inspection and rebuild of components. Therefore, O&M costs vary significantly depending on the quality of regular servicing