Allam power cycle
The Allam power cycle or Allam cycle is a process for converting fossil fuels into mechanical power, while capturing the generated carbon dioxide and water. The main inventor behind the process is English engineer Rodney John Allam.
A conventional combined cycle power station burns natural gas or gasified coal with air to drive a gas turbine and then uses the hot exhaust gas to heat water and drive a steam turbine. The working fluid for the gas turbine is carbon dioxide and water vapor from the combustion, along with nitrogen from the air (partially converted into nitrogen oxide). With this system, it is possible to achieve an LHV efficiency of around 55 to 59 percent. The gases driving the gas turbine will also constitute the principal emissions from the station, unless captured by an external system, at expense of a lower total efficiency. In addition, the station requires a cooling tower or access to large amounts of water for cooling.
In contrast, the Allam cycle uses a single turbine, driven by a working fluid consisting of only water and carbon dioxide, the mass of the latter being around 97 percent of the total. This is achieved by using pure oxygen instead of air to burn the fuel. Similarly to combined-cycle plants, the intended fuel is natural gas or gasified coal. The exhaust gas is cooled in a heat exchanger, and the steam is condensed and separated from the flow, becoming a potential source of fresh water. The carbon dioxide is compressed mechanically, and a small amount, matching the amount continuously added through combustion, is captured at high pressure, ready for pipeline transmission. The rest of the carbon dioxide is reheated in the heat exchanger and recycled into the combustion unit, where it continues to form the vast majority of the working fluid.
|Stage of the cycle||Oxygen||Natural
|Intake to combustion||4.75%||1.25%||–||94% (hot, compressed, recycled)|
|Intake to turbine||–||–||2.75% (very hot)||97.25% (very hot)|
|Intake to cooling side of heat exchanger||–||–||2.75% (hot)||97.25% (hot)|
|Output from cooling side of heat exchanger||–||–||2.75% (condensed and
extracted as clean water)
|97.25% (sent to compressor)|
|Output from compressor||–||–||94% (sent to heat exchanger)||3.25% (extracted
at high pressure)
|Intake to heating side of heat exchanger||–||–||94% (compressed)|
|Output from heating side of heat exchanger||–||–||94% (hot, compressed, to be recycled)|
Even though only one turbine is used, the Allam cycle can potentially achieve efficiencies up to about 59 percent (LHV) for natural gas and 51–52 percent (LHV) for gasified coal. This is due to the use of carbon dioxide as the main constituent of the working fluid. In its supercritical state, which it reaches in the combustion unit, carbon dioxide is very efficient for driving a turbine. Also, energy losses from phase transitions of water are avoided, which allows Allam cycle plants to recover more energy in their heat exchangers than combined cycle plants can do. At this efficiency, carbon dioxide capture is already a part of the Allam cycle, whereas if desired in conventional plants, the capture has to be added as an external function that may decrease the overall efficiency by around 10 percent.
Unlike the combined cycle, the Allam cycle requires an air separation unit, to supply the cycle with oxygen. On the other hand, there is no need for large-scale external cooling, there is only one turbine, and small components can be used, as the system operates at high pressures and thus high power densities. Complete Allam cycle power stations can therefore be smaller than combined cycle stations, even before adding the considerable size of carbon dioxide capture equipment. This potentially makes the capital costs for the Allam cycle lower than for the combined cycle at equivalent capacities. Other advantages include the complete absence of nitrogen oxide in the waste, and for stations located in dry environments, the integrated generation of water.
A 25 MWe/50 MWt demonstration plant for electricity generation using the Allam cycle is being built in La Porte, Texas. The project is a collaboration between, among others, Exelon, Toshiba, CB&I, and 8 Rivers Capital's portfolio company, NET Power. Construction started in March 2016 and was completed in 2017. As of June 2018 it is undergoing testing prior to commencement of service.
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