Thursday, June 7, 2012

Methane sequestration in hydrates

Could natural gas should be regarded as "clean energy", or as a "bridge fuel" on the road to a clean energy society? This is questioned by a Cornell University study that concludes that emissions caused by natural gas can be even worse than emissions by coal and diesel oil, especially when looked at over a relatively short period (image below).

Robert Howarth et al. - Methane and the greenhouse-gas footprint of naturalgas from shale formations
Not surprisingly, many people call for a ban on drilling in the Arctic, where factors such as remoteness, low temperatures of the water, presence of sea ice, shallowness of seas, long sea currents and lack of bacteria and hydroxyl combine to further increase environmental concerns about spills, leakage and fugitive gases.

Such factors should also make drilling in the Arctic more expensive. At first glance, one would therefore think that over time, in a world shifting to genuinely clean energy such as produced by solar panels and wind turbines, such more expensive "unconventional" sources of fuel will never become economic anyway. Many were therefore caught by surprise when the Energy Department announced the completion of a "successful field trial of methane hydrate production technologies". The announcement adds that a mixture of carbon dioxide and nitrogen was injected to promote the production of natural gas, and that ongoing analyses will be needed to determine the efficiency of simultaneous carbon dioxide storage in the reservoirs.

Indeed, there are concerns about the stability of the sequestered carbon dioxide (i.e. about possible leakage of carbon dioxide over the years), while there are also concerns about emissions caused in the process of producing this carbon dioxide in the first place. A concern voiced by Holly Moeller in a recent post is that any carbon dioxide sequestered as part of the methane extraction process will quickly be replaced through burning of the extracted methane. One should consider that methane in hydrates is highly compressed -- when taken out of the hydrate, it expands some 170 times in volume. And of course, there are also concerns about fugitive releases during capture and leakage during transport and distribution of the methane.

Ironically, environmental concerns can lead both to calls for bans on drilling and to calls for capture of methane in the Arctic. Large amounts of methane are present in undersea sediments in the Arctic. There are indications that much methane is on the verge of abrupt release any time now, due to rising temperatures worsened by the risk of hydrate destabilization due to seismic activity. Some therefore argue that drilling could risk destabilizing the hydrates. Others, on the other hand, argue that to reduce the risks of large methane releases, preemptive action is needed to remove methane from such locations.

In the ANGELS proposal methane is extracted, stored and sold as LNG, for distribution as fuel. There are a number of alternative proposals, each with their advantages and disadvantages. One alternative is to store captured methane in hydrates. Methane hydrates only remain stable within a limited range of  temperatures and pressures, i.e. between 290 and 5,076 psi (2-35 MPa). A group at the University of California - Irvine, led by Prof. Kenneth Yanda, does important research on hydrates. The group proposes to produce hydrates stabilized partly by other gases such as propane, to makes it possible for the hydrates to remain stable at a relatively low pressure of 25 psi (0.172 MPa). Hydrates can be produced that contain larger cages for other gases, as well as smaller cages for occupancy by methane. The group produced propane-methane hydrates that can be stable at temperatures of up to 288 K (14.85 degrees Celsius) and can fill up to 50% of the cages. In other words, such hydrates can store a combination of propane and methane at near ambient temperature and pressure conditions.

Further research is needed, such as into the possibility of converting methane into propane and other gases using UV light. The eventual goal could be long-term storage of such gases in the form of hydrates. In conclusion, rather than using the methane captured in the Arctic as fuel, it could be relocated to places where it can be expected to remain stored long-term, in the form of hydrates, e.g. in the deeper waters north of Alaska.

This may also lead to smart ways of sequestration of carbon removed from the atmosphere. Indeed, when considering places to sequester excess carbon, why not look at where nature stores most carbon, i.e. in hydrates? The amount of carbon stored naturally in hydrates is huge -- the 1992 image below illustrates this well, even though it's dated and estimates have changed a bit since.

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