Several methods for recovering carbon dioxide (CO2) from large combustion processes are being studied in the United States and abroad for possible adoption in power plant operations in the event
that limits on CO2 emissions into the atmosphere might be legislated. We discuss here, briefly, the key carbon capture cost factors for combustion technologies capable of using coal,
natural gas and biomass fuels. These technologies also have widely differing fuel combustion efficiencies as discussed on our website under the tab "Benefits".
There is a wide variance in the cost of separating CO2 from the exhaust gases of state-of-art and new combustion technologies. We divide the methods into three classifications as follows.
Class 1: Combined Cycle (CC) and Integrated Gasification Combined Cycle (IGCC)
Both CC and IGCC have CO2 separation costs equaling two or three times power plant fuel costs due to the low concentration of CO2 contained in a massive volume of hot exhaust gas comprised mostly of nitrogen and oxygen. Because all of the gas turbines used in the system require 250 to 300% excess air beyond the stoichiometric requirement for fuel combustion, the CO2 concentration is 3% or less in the exhaust. The present choice for CO2 recovery from CC and IGCC systems is between chemical absorption and compression/membrane separation technologies. Either method is energy intensive with high operating costs.
Class 2: O2/CO2 Combustion
Several concepts of this method, which excludes nitrogen from the combustion gas, have been proposed and demonstrated. They are similar to the process of the 1975 Japanese Patent No. 50-19026 and also similar to the method researched by Argonne National Laboratory/ United States Department of Energy during the 1980s. One current research project is described in the April 2007 issue of PowerEngineering, p. 10. All of the above capture CO2 without the high cost of separation required for the CC and IGCC technologies of Class 1. However this saving is offset by the cost of the air separation plant needed to supply oxygen for nitrogen-free fuel combustion. Studies in Japan have shown a reduction in plant efficiency to about 27% for oxygen combustion from about 34% for a state-of-art air-fired plant. In all of the above, little has been done so far to reduce the large waste heat losses of these plants, whether based on a steam Rankine-cycle or gas turbine Brayton-cycle, or on the combination of both systems in CC and IGCC.
To date, the one exception to the above is the high efficiency closed Rankine-cycle O2/CO2 combustion method of ourU.S. Patent No. 6,907,845, wherein by recovering most waste heat, the plant can operate at a relatively low cost while recovering all exhaust gas CO2 without any costs for separation. For electric power production operations near metropolitan areas, we are recommending use of the recovered CO2 for sewage treatment. See "Combining Municipal Services".
Class 3: Retrofitting
Falling between Class 1 and 2 in CO2 separation costs are retrofits for state-of-art Rankine-cycle plants. Some may be retrofittable to O2/CO2 combustion so that no CO2 separation costs will be required. [See "Retrofitting" on our website] Others may remain air-fired but can be adapted for exhaust gas separation by adding equipment following an existing electrostatic precipitator (ESP) or baghouse and SO2 scrubber. In the latter case, CO recovery will be less costly than either combined cycle or IGCC because of the lower exhaust gas volume: the air input for fuel combustion is only 10 to 20% excess of stoichiometric compared to 250 to 300% excess air in CC and IGCC. So for air-fired retrofits, the exhaust gas CO2 content will be in the range of 7 to 15% depending on the fuel (natural gas or coal) and depending also on the level of excess air (10 to 20%) utilized for fuel combustion.
At the present time we believe that our closed Rankine-cycle O2/CO2 combustion method with nearly complete waste heat recovery is the only technology capable of economically capturing CO2 from large combustion systems without government subsidies and large electric power rate increases.
Krebs & Sisler L.P.