Download Supporting Information – A review of methane mitigation

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
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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
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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:
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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.
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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)
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