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Supporting Information – A review of methane mitigation technologies with application to rapid release of methane from the Arctic Joshuah K. Stolaroff,∗ Subarna Bhattacharyya, Clara A. Smith, William L. Bourcier, Philip J. Cameron-Smith, and Roger D. Aines Lawrence Livermore National Laboratory, Livermore, CA, USA Assessment of scale for atmospheric methane capture Boucher and Folberth (1) propose atmospheric methane capture as a means of climate mitigation. Regardless of the technology used, a successful capture regime would have to remove methane from the atmosphere at a significant rate compared with natural sinks in order to reduce atmospheric concentrations and thus climate forcing. The current lifetime of methane in the atmosphere is about 12 years (2). It is primarily oxidized by OH· radicals in the troposphere and stratosphere, with the remaining 10% being taken up by soils. Roughly speaking, in order to reduce atmospheric methane concentrations in the atmosphere by one half, one must match the natural sink – that is, remove 1/12th of atmospheric methane each year. The lifetime of methane in the atmosphere increases somewhat with methane concentration (3), complicating the argument, but for an order-of-magnitude calculation we can ignore this effect. ∗ To whom correspondence should be addressed 1 Industrial capture Industrial process units similar to power plant cooling towers have been proposed to capture CO2 from air (4–6). These are proposed to operate with inlet air velocities up to about 2 m/s. Much above this value, the energy required to move the air becomes prohibitive. Ignoring the internals of a hypothetical methane-capture device, we can estimate the total area of inlets required to process the necessary volumes of air by: Ainlet = 1 12 Matm (1 yr) vinlet ρair where Matm is the mass of the atmosphere, 5.15 × 1018 kg or 1.8 × 1020 mol (7), vinlet is the average inlet velocity (suppose 2 m/s), and ρair is the average density of air at the inlets (suppose 44.2 mol/m3 for STP). Then we get Ainlet = 5.9 × 109 m2 = 5300 km2 , close to the size of the state of Delaware. The largest cooling towers have outlet areas of 2000–3000 m2 . More than a million such towers would be required. When compared with atmospheric CO2 capture, methane is much more difficult owing to both its lower concentration and shorter atmospheric lifetime. Catalytic particles Instead of an industrial process system, one might take advantage of the thermodynamic favorability of methane oxidation and deploy a material to catalyze the reaction in situ. Suppose that one has a room-temperature catalyst that operates with 100% efficiency, that is, every methane molecule that hits the surface is oxidized (such a catalyst is not currently known). In the best case, transport of methane to the surface will be diffusion-limited. This condition is satisfied by aerosol particles. For aerosols in the continuum regime (diameter larger than about 0.2 µ m), we can estimate the flux of a gas to the surface of a particle by (8): JCH4 = 2 DCH4 CCH4 π d p where DCH4 is the diffusivity of methane in air (2.0 × 10−5 m2 /s at STP (9)), CCH4 is the concentration of methane in air (1.8 ppm or 8.0 × 10−5 mol/m3 at STP), and d p is the particle diameter. The 2 mass of a catalytic particle with density ρ p is ρ p π d 3p /6, which gives a mass of methane destroyed per unit mass of catalyst per unit time: 12 DCH4 CCH4 mass methane = mass catalyst × time ρ p d 2p In order to oxidize 1/12th of atmospheric methane over a year (ignoring the settling of the particles over this period), we would need to satisfy: 1 12 Matm f CH4 1 yr = =⇒ Mcat = 12 DCH4 CCH4 × Mcat ρ p d 2p Matm fCH4 ρ p d 2p 144 (1 yr) DCH4 CCH4 where fCH4 is the mole fraction of methane in the atmosphere (1.8 × 10−6 ) and Mcat is the total mass of catalyst. Suppose we generate particles of surface-area-average diameter 1 µ m, small enough to have a significant but not indefinite atmospheric lifetime, and suppose the particles have density of 2000 kg/m3 (typical for a mineral). Then we get Mcat = 8.9 × 104 kg = 89 t. This is small compared to some flows to the atmosphere. For example, global sulfur emissions are about 7 × 107 t. This calculation ignores the dependence of diffusivity on temperature and the dependence of the density of air on altitude. These should be relatively small corrections for an order-ofmagnitude calculation. A larger source of uncertainty is the settling time of particles (which may be much shorter than 1 yr). The largest source of uncertainty is the catalytic efficiency of the particles, which could be orders of magnitude lower than 100%. This factor likely determines the feasibility of catalytic aerosols for methane mitigation. 3 Summary of estimates for Arctic sources Source Ocean clathrates (>300m Potential Characteristic Geographical total release release rate extent [Gt C] [Tg CH4 /yr] [106 km2 ] 400 (10) 16–80 (3) 0.3 (3) 1400 (11) 1–65 (12) 2.1(13) 4500 (12) 90 (13) 2.1 (13) depth) East Siberian Continental Shelf (ESAS) sediments (organic C) and free gas ESAS clathrates East Siberian Yedoma Non-yedoma (highland) 400–500 (14) 1 (14) > 400 (14) 22 (14) permafrost Wetlands in West Siberian 50 (15) 6–8 (15) 0.6–1 (16) 23 (17) 24 ± 11 (17) 1.1 (17) 180–455 (18) 20–45 (18) 4 (18) Lowlands Northern lakes Peatland (mire, meltpond) References (1) Boucher, O.; Folberth, G. A. New Directions: Atmospheric methane removal as a way to mitigate climate change? Atmospheric Environment 2010, 44, 3343 – 3345. (2) Solomon, S. et al. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, UK, 2007. (3) Reagan, T. M.; Moridis, J. G.; Elliot, M. S.; Maltrud, M.; Cameron-Smith, P. Basin-scale 4 assessment of gas hydrate dissociation in response to climate change. Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), 2011. (4) Mahmoudkhani, M.; Heidel, K.; Ferreira, J.; Keith, D.; Cherry, R. Low energy packed tower and caustic recovery for direct capture of CO2 from air. Energy Procedia 2009, 1, 1535. (5) Heidel, K.; Holmes, G.; Singh, A.; D. Keith, D. Process Costing of A Contactor for Air Capture. 10th International Conference on Greenhouse Gas Control Technologies, Amsterdam, 2010, Available: http://keith.seas.harvard.edu/Misc/Process% 20simulation%20of%20direct%20CO2%20capture%20from%20air.pdf. (6) Stolaroff, J. K.; Keith, D. W.; Lowry, G. V. Carbon Dioxide Capture from Atmospheric Air Using Sodium Hydroxide Spray. Environmental Science & Technology 2008, 42, 2728–2735, PMID: 18497115. (7) Trenberth, K. E.; Smith, L. The Mass of the Atmosphere: A Constraint on Global Analyses. J. Climate 2005, 18, 864–875. (8) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley and Sons: New York, 1998. (9) Massman, W. J. A review of the molecular diffusivities of H2O, CO2, CH4, CO, O3, SO2, NH3, N2O, NO, and NO2 in air, O2 and N2 near STP. Atmospheric Environment 1998, 32, 1111–1127. (10) Maslin, M.; Owen, M.; Betts, R.; Day, S.; Dunkley Jones, T.; Ridgwell, A. Gas hydrates: past and future geohazard? Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences 2010, 368, 2369–2393. (11) Shakhova, N.; Semiletov, I.; Leifer, I.; Salyuk, A.; Rekant, P.; Kosmach, D. Geochemical and geophysical evidence of methane release over the East Siberian Arctic Shelf. Journal of Geophysical Research-Oceans 2010, 115, C08007. 5 (12) Shakhova, N.; Semiletov, I.; Panteleev, G. The distribution of methane on the Siberian Arctic shelves: Implications for the marine methane cycle. Geophysical Research Letters 2005, 32, L09601. (13) Shakhova, N. E.; Sergienko, V. I.; Semiletov, I. P. The contribution of the East Siberian shelf to the modern methane cycle. Herald of the Russian Academy of Sciences 2009, 79, 237–246. (14) Zimov, S. A.; Schuur, E. A. G.; Chapin III, F. S. Permafrost and the Global Carbon Budget. Science 2006, 312, 1612–1613. (15) Anisimov, O. A. Potential feedback of thawing permafrost to the global climate system through methane emission. Environmental Research Letters 2007, 2, 045016. (16) Repo, M. E.; Huttunen, J. T.; Naumov, A. V.; Chichulin, A. V.; Lapshina, E. D.; Bleuten, W.; Martikainen, P. J. Release of CO2 and CH4 from small wetland lakes in western Siberia. Tellus Series B-Chemical and Physical Meteorology 2007, 59, 788–796. (17) Walter, K. M.; Smith, L. C.; Chapin, F. S. Methane bubbling from northern lakes: present and future contributions to the global methane budget. Philosophical Transactions of the Royal Society A 2007, 365, 1657–1676. (18) MacDonald, G. M.; Beilman, D. W.; Kremenetski, K. V.; Sheng, Y.; Smith, L. C.; Velichko, A. A. Rapid early development of circumarctic peatlands and atmospheric CH(4) and CO(2) variations. Science 2006, 314, 285–288. 6