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  5. New CO2-fueled gas turbines can revolutionize power plant technology

New CO2-fueled gas turbines can revolutionize power plant technology

Author: Prof. Dr. Jochen Fricke, Energy Technology Cluster (as of July 2017)

In the U.S., a 25 MW demonstration power plant will go on line this year near Houston that burns natural gas not in air but in oxygen to produce CO2 and H2O. The hot CO2 will be used in a newly developed gas turbine to generate electricity.

Operation

The combustion of natural gas in oxygen is not new; it is known as the oxyfuel process. For this purpose, the air is separated into oxygen and nitrogen in a cryogenic process  (Fig.1).

Innovative is the use of a newly developed gas turbine to utilize the CO2 gas produced at high temperature (about 1150°C) and high pressure (300 bar) in a combustor. The new turbine and combustion chamber were developed and built by the Japanese company Toshiba. The natural gas is burned in a natural gas-oxygen-CO2 mixture, because without added CO2 the temperature in the combustion chamber would be excessively high.

Gas turbine 1
Fig. 1: Adam cycle, a modified Brayton cycle (Image: J. Fricke)

The "expanded" gas mixture flowing out of the turbine has a significantly lower pressure and temperature than at the turbine inlet. In a regenerative heat exchanger, it still gives off residual heat to a high-temperature heat exchanger or regenerator (see also TS diagram, Fig. 2). Downstream of the heat exchanger and any downstream cooler, the condensed water vapor is extracted and can be used for irrigation purposes, for example.

Fig.2. Temperature entropy (TS) diagram of a Brayton process running above the critical point (KP) of CO2. The 2-phase or wet steam region is not traversed. This is a significant difference from the Rankine process using water. The Adam process described above uses even much higher temperatures, maximum 1150°C. Image: J. Fricke, after Y.Ahn et al.

Oxyfuel process crucial for efficiency

The cooled CO2 is now brought back to high pressure in a compressor. Its temperature rises again as a result of the compression. In the heat exchanger, the CO2 again absorbs heat and is then injected into the combustion chamber to limit the combustion temperature. Between the compressor and the heat exchanger, a partial CO2 stream is extracted at high pressure and is  -  in the case of the pilot power plant in Houston - used to optimize oil recovery. This is known as "enhanced oil recovery," or EOR for short. Another option is to inject the CO2 underground, which is known as carbon capture and storage (CCS).

Image 2
Fig. 2: Temperature entropy (TS) diagram of a Brayton process.

The small amount of CO2 extracted just corresponds to the amount created by oxidation in the combustion chamber, so that the amount of CO2 pumped around in the cycle remains constant.

The efficiency of the new process is decisively determined by the oxyfuel process. In the conventional combustion process with air as oxidant, the CO2 (to be extracted for CCS) accounts for only about 14% of the flue gas, after all, a large amount of nitrogen is entrained. In the oxyfuel process, the flue gas consists only of CO2 and condensable water vapor.

CO2 turbines are more effective

The idea for the new process for generating electricity came from Rodney Adam, a retired chemical engineer. He was bothered by the bulky steam turbines and huge amounts of flue gas in conventional power plants.  CO2 turbines are much smaller and less expensive than steam turbines for the same output. Since CO2  s critical parameters are only 31°C and 73 bar, the CO2 in the combustor and turbine is supercritical (Fig. 2). It resembles a liquid more in its flow properties than a gas.

Because CO2 has a molar mass 2.5 times higher than water, as a supercritical fluid it has a density significantly higher than water vapor. CO2 turbines are therefore up to a factor of 10 smaller than steam turbines for the same power output.  Adam calculates the overall efficiency of the CO2 pilot power plant to be 56%. It is also noteworthy that the CO2 extracted from the cycle is already at high pressure and can be injected directly into the subsurface for EOR, making money.

The new pilot power plant, called the "NET Power Demo Plant," is being financed and also operated by two major industrial companies, Exelon and Chicago Bridge Iron, at a cost of $140 million. Such innovative power plants will certainly not be able to operate without further F&E without problems. This is especially true of the turbines and the combustors. The U.S. DOE is also promoting the development of efficient CO2 compressors at General Electric and at Hanwha. In expert circles, the CCS process in the power plant sector is now regarded as indispensable for efficient reduction of CO2 input into the atmosphere. Here, the pilot power plant in the USA points the right way. But then the storage of CO2 underground (without EOR) must also be given a chance, also in Europe. 

Sources: L.Irvin, Y.L.Moullec, Science356,805-806(2017) R.F.Service, Fossil Power, Guilt Free, Science356,796-799(2017) Y.Ahn et al. "Review of Supercritical CO2 Power Cycle Technology," Nucl. Eng. Technol 47,647-661(2015).