Marrying CO2 and EGS: The waste gas presents many advantages and some unknowns

Much has been said about the vast amounts of carbon dioxide humans generate daily, primarily from the creation and use of energy. What to do about it – before its accumulation makes our planet unlivable – is a topic for other publications. Here at the Digest, we have reported that the waste gas, in supercritical form, may find use in enhanced geothermal systems (EGS), as the DOE is allocating some $16 million to study the concept.

A conversation with materials scientist Miroslav Petro of Symyx Technologies suggests that there are a number of advantages to the use of CO2 over water in EGS systems. Symyx, a combinatorial chemistry firm, will take part in the DOE-funded projects, which will be led by Lawrence Berkeley National Laboratory in collaboration with Idaho National Laboratory. A number of other universities in the U.S. will take part as well. Petro will serve as the project leader at Symyx.

The idea for the use of supercritical CO2 was first proposed in 2000 by Los Alamos National Laboratory physicist Donald Brown. Supercritical CO2, a pressurized form that is part gas and part liquid, is less viscous than water and so should flow more easily through rock – advantage number one. In 2006, Karsten Pruess, a hydrogeologist at Lawrence Berkeley, performed the first detailed modeling of the technology. Pruess projected that an EGS project could produce approximately 50% more heat with carbon dioxide than with water. The latest round of DOE funding will put this to the test.

Advantage number two is in carbon sequestration. This has been discussed as well, here and elsewhere, and while Petro says there remains a great deal unknown about how and how much carbon might be captured in rock strata, the fact remains that any capture benefits the planet while simultaneously aiding in the production of a clean, sustainable energy. Any CO2 lost in the process is simply replaced in the closed system. Symyx will be studying the interaction between CO2 and the wide variety of minerals likely to be encountered at depth. How readily CO2 will be captured and by what may be one of the results of the research.

We wondered if, in fact, the loss of CO2 to capture would reduce the efficiency of an EGS system. The answer, according to Petro is yes, but this led to advantage number three. The loss of CO2 will be far less than the loss of water in water-based systems. CO2 is virtually inert whereas water behaves as a solvent in rock formations, effectively replacing some elements in rock and simply hydrating others. This leads to or enhances the briny solutions that are returned to the surface, which forces the use of binary systems to extract heat.

This leads to advantage number four. Assuming a dry hole and a non-permeable environment, EGS systems require that the rocks at depth be fractured. According to Petro, the interaction of water with these rocks inevitably lead to the briny solutions and then to minerals precipitating out, sealing the very fractures that have proven thus far to be so difficult to create. The inert nature of CO2 should eliminate this. However, a potentially disturbing problem – and the only disadvantage we found so far – is that, should CO2 encounter water, a form of harsh acidic soda water could be formed. This would dissolve more minerals than water alone.

Petro notes that there is a great deal of fundamental CO2 science and the deep crustal environment that is not as yet understood. And while the current round of research will hopefully lead to enough answers to the remaining questions to move the technology forward, it would seem that all that CO2 we have created over the last century may find a second, and far more useful life.