Improvements to Dakota Gas' CO2 compressor station improve safety and reliability

Dakota Gasification Company continues to invest in the safety and reliability of the plant by making improvements to the plant’s carbon dioxide (CO2) compression area’s air-cooled heat exchangers (ACHE).

The ACHEs are used for interstage cooling of the three CO2 compressors. Each integral geared CO2 compressor takes feed gas from the rectisol unit composed of 95% CO2 and increases the pressure of the CO2 to around 2,300 pounds per square inch (psi) discharge pressure. To get to these high discharge pressures, the CO2 is compressed through eight different stages of each 20,000-horsepower compressor.

CO2 compressor at Dakota Gas
Carbon dioxide compressor, motor, and interstage piping.

Since their startup in the early 2000s, the exchangers have had cooling shortfall issues during the summer months. These issues have increased over the years as the exchangers became fouled and corroded, and now they are at the end of their useful life.

With the help of an outside engineering firm, Dakota Gas has been working to evaluate options to correct and improve the deficiency that occurs during seasonal weather changes. "The current projects include replacement of ACHE tube bundles and louvers for stages 1 through 5. In stage 6, ACHE will be replaced completely and relocated south of the CO2 compressor building, and the existing liquid accumulating knock-out will be located next to it. It is being reengineered so that the ACHE will be properly sized to eliminate the cooling shortfall and provide a benefit to the other ACHEs as there will be less hot air/discharge that could be recycled," said Jesse Kaelberer, manager of the utilities section at Dakota Gas.

Dakota Gas CO2 compressor station
Compressor station at Dakota Gas showing the compressor outlets and air-cooled heat exchangers.

Another upcoming project is adding side shutters to aid in cold weather operation. These additions will help aid in winter weather start-ups when the heat needs to be conserved.

"Items that are still being investigated are the addition of a high-pressure fogging system and improvements to the fan blades, fan motors, and gear boxes to mitigate long-term operational expenses," Kaelberer said.

Overall budgeted expenses for improvement projects in the CO2 compression area are approximately $20 million dollars each year for the next three years. These improvements will occur during planned outage time frames each year.