Transcript PPT2
Gas Hydrates: Our Energy (and Climate) Future? Lecture Outline: 1)What are gas hydrates anyway? 2)Gas hydrates as an energy source – pros and cons 3)Gas hydrates and climate change: adding fuel to the flames? Hydrates - What are they? 2 Gas Hydrates are solids formed from hydrocarbon gas and liquid water They resemble wet snow and can exist at temperatures above the freezing point of water They belong to a form of complexes known as clathrates Clathrates - What are they? Clathrates are substances having a lattice-like structure or appearance in which molecules of one substance are completely enclosed within the crystal structure of another Hydrates consist of host molecules (water) forming a lattice structure acting like a cage, to entrap guest molecules (gas) CH4 (most common), CO2, H2S form hydrates 3 98% in ocean 2% on land white dot = gas samples recovered black dot = hydrate inferred from seismic imaging dotted lines = hydrate-containing permafrost http://walrus.wr.usgs.gov/globalhydrate/images/browse.jpg using seismic-reflection profiles Bottom Simulating Reflection (BSRs) http://woodshole.er.usgs.gov/project-pages/hydrates/hydrate.htm Methane Hydrate stability diagram -methane hydrates can occur at water temperatures up to 30°C, if the pressure is high enough -stable over most of ocean floor! a methane hydrate lattice redrawn after Kvenvolden (1993) “The Burning Snowball” Methane hydrate supporting its own combustion 7 Methane Hydrates as an energy source BENEFITS: - 1 cubic meter of gas hydrate (90% site occupied) = 163 m3 of gas -there is A LOT of it, and it’s everywhere -clean-burning natural gas • USA has gas hydrate reserves estimated between 112000 tcf and 676,000 trillion cubic feet (tcf) • USA has 1400 tcf of natural gas reserves • USA uses 25-30 tcf/yr of natural gas • India and Japan are leading the charge to hydrate recovery Methane Hydrates as an energy source PROBLEMS: -hydrate dissociation upon recovery; engineering challenge -expense of long pipelines across continental slope, subject to blockage with solid hydrate -methane release into atmosphere problem for climate change -fragile ecosystems surround sediment surface hydrates & seeps ice worm that lives in hydrate photo by Ian Mc Donald 1 cubic meter of gas hydrate (90% site occupied) = 163 m3 of gas + .87 m3 Undersea slides (slope failures) may be caused by methane hydrate dissociation; implications for pipeline? Large, expensive pilot programs focus on drilling in frozen permafrost areas Ex: Mallik, Canada http://energy.usgs.gov/other/gashydrates/mallik.html New ocean sediment drilling technologies invented for hydrate recovery and storage an Ocean Drilling Program core locker with lone hydrate core in pressurized chamber dissociating methane hydrate at sediment/water interface Westbrook et al., 2009 -lots of CH4 escaping from melting gas hydrates -powerful positive feedback on global warming because CH4 is a powerful greenhouse gas -most likely oxidizes to CO2 before it enters the atmosphere… but still! -see Archer et al., 2007 for detailed investigation of methane hydrate dissociation during global warming Westbrook et al., 2009 An interesting twist: - replace CH4 with CO2 in the hydrate lattice - have your energy cake and eat it too? Park et al., PNAS, 2006 Take-home point Methane hydrates represent the largest fossil fuel reservoir, but problems ranging from yet-to-be-developed technologies and climate change feedbacks remain to be resolved.