Researchers at ETH Zurich are using heated oxygen, ethanol and water pumped into their reactor burner through various pipelines and valves and mix them under temperature and pressure conditions, which correspond to the supercritical state of water (see illustration below) in an effort to get to energy rich deep geothermal rock.  The researchers observe the mixture’s auto-ignition through small sapphire-glass windows using a camera. A newly developed sensor plate measures the heat flux from the flame to the plate of simulated rock and records the temperature distribution on the surface for different distances between the burner outlet and the plate.

ETH Zurich Drilling Test Chamber.  Click image for the largest view.

ETH Zurich Drilling Test Chamber. Click image for the largest view.

Inside the chamber shown in light blue is the super critical water.  Above a temperature of 374.12°C and a pressure of 221.2 bars, water vapor and liquid water can no longer be distinguished from each other in terms of their density. In this supercritical state, water is less polar, has no phase boundaries and is a good solvent for non-polar gases like oxygen. Under these conditions fuel and oxygen can be mixed without any bubble formation and in the case of ethanol for the fuel, auto-ignition occurs at approximately 450°C.

Based on the experimental results, conclusions can be drawn concerning the heat transfer from the flame to the rock. Philipp Rudolf von Rohr, professor at the Institute of Process Engineering of the ETH Zurich and supervisor of the three PhD students explains, “The heat flux is the crucial parameter for the characterization of this alternative drilling method.”

Rapid heating of the upper rock layer induces a steep temperature gradient in it.  Doctoral student Tobias Rothenfluh explains, “The heat from the flame causes the rock to crack due to the induced temperature difference and the resulting linear thermal expansion.”  The experimental flame reaches a maximum temperature of about 2000°C.  The expansion of the upper rock layer causes natural flaws that already existing in the rock to act as origin points for cracks.  Then disc like rock fragments in the millimeter scale are formed in the spallation zone. Those particles are transported upwards with the ascending fluid stream of the surrounding medium.  “One of the main challenges of the spallation process is to prevent the rock from melting, whilst it’s being rapidly heated.  The larger the temperature gradient in the rock, the faster you can drill,” says Rothenfluh.

Rothenfluh’s job is to climb up a small ladder into the three-story pilot plant where pipelines lead through metering and safety valves into the reactor, which is affectionately known as “Betsy”. Inch thick plates, made of heat-resistant steel, prevent the reactor from bursting, even at pressures up to 300 bars.  “In our experimental reactor we are able to ignite a flame underwater at a pressure of around 250 bars and 450 degrees Celsius” says Rothenfluh. “Thus we are able to experimentally simulate the temperature and pressure conditions prevailing in a borehole, about three kilometers below the earth’s surface.”

Rothenfluh and his colleagues Martin Schuler and Panagiotis Stathopoulos constructed the first burner prototype over a few months

At increasing depths geothermal energy offers an almost inexhaustible potential for renewable energy. But drilling costs rise exponentially with depth in the case of conventional rotary drilling. A thermal drilling method, which could allow for reaching greater drilling depths in a more efficient and more cost-effective way should answer the need for less costly boreholes to such depths and temperatures.

The ETH Zurich method is particularly suitable for the hard dry rock sought for geothermal heat sources normally encountered at depths deeper than three kilometers, which is a little less than 10,00 feet. In such rock at such depths conventional drilling bits wear out much faster and their frequent replacement renders the conventional drilling techniques uneconomic. For example a 10-kilometer (almost 33,000 feet – nearly 6 miles) borehole costs around $60 million. Using the new Zurich method coined “hydrothermal spallation drilling” the burner’s bit wear is considerably less, because there is no mechanical contact with the rock. “It is expected that the drilling costs will rise linearly with depth in the case of spallation drilling, instead of exponentially, which is the case of the conventional methods”, says Professor von Rohr who oversees the three PhD students.

In order to test the flame’s behavior under different conditions, doctoral student Martin Schuler is developing a tool for the numerical simulation of the reaction and transport processes in cooperation with the master’s student Karl Goossens. “The simulation enables us to change and optimize parameters like fuel mass flow rates, temperature and pressure, as well as the geometry of the burner”, says Schuler.

Stathopoulos is designing a pilot plant using the experimental results from the current test set-up.  The 1.2 million Swiss franc (about $1.16 million) plant should demonstrate that it is actually possible to drill through rock by means of hydrothermal flames. The project is already funded by the Swiss Federal Office of Energy, the industrial organization Swisselectric Research, ETH Zurich and the Swiss National Science Foundation.  No surprise, as the Swiss are always drilling some mountain or another for tunnels and other projects.  If anyone needs to efficiently get through hard rock fast and cheap – it’s the Swiss.

von Rohr offers the interest of the Swiss Federation and industry confirms the high potential of “hydrothermal spallation drilling”. Some time will pass until the method is industrially applicable, but the feasibility is without doubt so far. “It is for sure possible to speed up the project towards the industrial application, but we still want to focus on basic research at a university like the ETH Zurich.”

Perhaps that means prying the technology out for use could be a little difficult or time consuming.  The team currently is the only group worldwide investigating the heat transfer characteristics of a flame in supercritical water. “We literally want to research in both depth and breadth”, says Rothenfluh. In the future, the knowledge acquired might be useful not only for geothermal energy, but also for other applications.


10 Comments so far

  1. Matt on September 17, 2009 7:32 AM

    I wonder about the potential use in hard rock mining?

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