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sCO2 and CO2 as working
Too much carbon dioxide in the atmosphere is clearly a problem. But using the thermodynamic properties of this versatile molecule in modern power systems can be an opportunity
Stephen B. Harrison sbh4 consulting, Germany The Allam–Fetvedt cycle enables thermal power
generation from fossil fuels with zero CO2 emissions to air. It uses supercritical CO2
(sCO2
from oxyfuel combustion as the main working fluid in a semi-closed Brayton cycle. sCO2
with concentrated solar power, or on modern nuclear reactors. Gaseous CO2
as a working fluid in reversible power storage cycles to complement non-programmable renewable power generation.
) is also
at the heart of the indirect-fired supercritical CO2 recompression Brayton cycle that can be used
can also be used sCO2
as a working fluid in the Allam–Fetvedt cycle
The Allam–Fetvedt cycle can enable the use of gasified coal or natural gas to contribute to a net-zero future. It offers high-efficiency power generation from traditional fossil fuels in an innovative cycle that avoids greenhouse gas emissions. The process relies on oxy-fuel combustion, which also ensures that pollutant emissions are avoided and enables post- combustion CO2
and low cost.
As in conventional air-fed, gas-fired turbines, combustion gases spin the main turbine in the Allam-Fetvedt cycle. However, in the Allam–Fetvedt case, they consist of a mixture of approximately 97.3% CO2 Supercritical CO2
and 2.7% water. is recirculated within the to be captured at high pressure
Allam–Fetvedt cycle as the main component of the working fluid for power generation. Transfer of heat from the hot turbine exhaust gases to the burner inlet gases is essential to achieve efficient operation of the cycle. Printed circuit heat exchangers have been used for this application to combine high efficiency and process intensity. The main turbine for the original Allam–Fetvedt cycle demonstration facility at La Porte, Texas, was built by Toshiba. Recently, Baker Hughes has taken on the role of turbine development and production for future deployments of the Allam- Fetvedt technology.
The temperature and pressure mean that the CO2
is supercritical at the turbine inlet. And the presence of moisture means that carbonic acid formation is possible. Materials selection and coating technologies are the key to successful turbine operation in this environment. Post-combustion CO2
capture generally
takes place at the end of a power generation combustion process where the gas stream is very low pressure, and the CO2
is often diluted with nitrogen from combustion air.
One of the challenges of this mode of post- combustion CO2
gas stream requires a very large CO2 compressor
capture is the low-pressure flue absorber
tower to handle the high volume of low-pressure flue gas. Additionally, a large CO2
is required to blow the captured CO2
suitable sequestration location. Pre-combustion CO2
away to a capture (for example
in steam methane reformers that are used for hydrogen production) can take place at high pressure and high CO2
concentrations,
capture. Uniquely, the Allam–Fetvedt cycle enables low-cost, high-pressure, post- combustion CO2
capture because CO2 compression. Furthermore, pure CO2 Above: Net Power La Porte demo site | January/February 2024|
www.modernpowersystems.com
thereby reducing the capital and operating costs of CO2
is released
from the cycle at about 80 bar. This avoids the need for CO2
is released directly from
capture equipment to separate the CO2 nitrogen, oxygen, and other typical flue gases.
the Allam–Fetvedt cycle avoiding the need for CO2
from
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