CO2 Key to
Geothermal Plan


July 2011

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A novel geothermal energy project with the potential to enhance geothermal’s range and appeal is moving forward in New Mexico. The pilot project will use carbon dioxide rather than water to produce steam to power turbines and produce 2 MW of renewable energy and cost between $2 million and $4 million. If successful, experts believe it has the potential to capture large amounts of CO2 now emitted to the atmosphere from fossil-fueled power plants, considered by many to be a large contributor to climate change.

Greenfire Energy, a startup independent power developer based in Salt Lake City, UT, is now seeking permits to drill wells for the renewable energy project in northwest New Mexico, near the Arizona state line and atop a geologic formation called St. Johns Dome. The formation contains CO2 produced from past volcanic activity, about one-half mile below the surface. Greenfire’s scheme, which has landed a $2-million grant from the U.S. Deparatment of Energy, calls for the gas to be tapped and then compressed to a supercritical state, reinjected to a deeper geologic formation about 10,000 ft below the surface to extract heat, and brought to the surface to generate steam and power turbine-generators. The closed-loop system would then separate the CO2 and reinject it to begin the process again.

Developers say the pilot is a precursor to what eventually may become 50-MW modular plants that could use carbon-dioxide feedstock supplied by nearby powerplants. Two coal-fired plants, the Coronado and Springerville stations, are close to the geothermal site, near Springerville, AZ. Together they emit some 11,000 tons of COannually.

Image courtesy of Greenfire Energy

Conventional geothermal systems use water—and lots of it—as a medium, generally tapped in aquifers at tectonic hot spots, where it is heated naturally by rock formations. Then it is brought back to the surface and pumped into a heat exchanger, where it is turned to steam to power turbines and produce electricity. On the surface, the water is then either captured or added for reinjection. Enhanced geothermal systems pump water to subsurface dry hot rocks, where it is heated and then pumped back to the surface.

Substitute

Greenfire’s renewable-energy process substitutes geothermally heated CO2 for the water, a medium that some engineers believe may offer better efficiencies. Greenfire officials say it should produce the lowest-cost baseload energy of any renewable-energy source that is commercially scalable.

“Carbon dioxide has a very low viscosity,” says Alan Eastman, one of four Greenfire partners. “When you go to supercritical … it has a density of one-seventh that of water. It’s much easier to pump around.”

He and Greenfire's partners, all of whom are veterans of the engineering and oil business, are betting on the Thermosiphon Effect to reduce capital and operating costs. "At 2,000 psi, a cool carbon-dioxide column weighs more than water," says Eastman. That should reduce pumping requirements, reducing capital and operating costs. Although thermosiphoning has been used in industrial applications, it has never been used for power generation.

Image courtesy of Greenfire Energy

Before drilling can begin, developers must first perform seismic monitoring. Eastman says the site is in a very low seismic zone but points to instances where geothermal drilling triggered earthquakes, one a magnitude 4 quake in Switzerland.

Induced seismic events are more apt to occur in areas of active faults where tectonic plates meet. The Springerville site falls outside of those zones and offers “hot rocks” at great depths, about 6 km beneath the surface. Tapping the resource with surface water is not practical in the arid West; carbon dioxide could prove to be the medium to open up the resource, which is widespread throughout the region, says Eastman.

Unknowns

Still, developers aren’t really sure of the geologic specifics below the sedimentary layer, at about 3,000 ft. Greenfire’s joint-venture partner, Ridgeway Arizona Oil Co., St. Johns Dome, AZ, owns the rights to St. Johns Dome, but has only explored to that level. But Eastman and others believe granitic rock below that level is “fairly fragmented.” To test the theory, they will use the DOE grant to hydraulically fracture the rock, drilling a well to about 6,000 ft, and then sending CO2 down the hole.

“That should make new fissures,” Eastman says. New and larger cracks in the rock will provide a greater area for the gas to settle and heat to temperatures reaching 150 degrees C. The gas will be drawn back to the surface when pressure on the well is released.

Greenfire is searching for another $5 million to $6 million to take the project to the next level, building the pilot powerplant.

Carbon capture—or carbon sequestration—is an added bonus to the plan, says Eastman. Although the project is designed to use CO2 naturally occurring in the ground, the technology can use CO2 now emitted to the atmosphere, and store it within St. Johns Dome.

Greenfire officials estimate the amount of gas contained in St. Johns Dome is enough to generate 800 MW of electricity. Although carbon-capture technology has not yet been demonstrated, developers are confident the technology is viable. The ultimate driver, however, is climate-change legislation that would cap CO2 emissions. Congress has yet to act on such a law.

If the pilot is successful, Eastman says it is simple to scale the plant for commercial production. The prevalence of dry hot rocks throughout the western U.S. also provides opportunity to expand the technology and bring power sources closer to users. Eastman points to sizable transmission losses of electricity in power lines connecting powerplants to cities. If this geothermal concept proves feasible, similar plants could be sited closer to cities and users, greatly reducing transmission costs and losses, he says.

The proposed system has the potential to capture carbon dioxide now emitted to the atmosphere from fossil-fueled power plants for use in geothermal power generation.

 
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by John Kosowatz, ASME.org