By Gerd A. Folberth. 19th July 2010.
Air capture of carbon dioxide (CO2) has been proposed as a solution to climate change but the grand challenge is to scale up prototypes to provide cost-effective solutions. Surprisingly air capture of methane (CH4) has not received the same attention as air capture of CO2. In a recently published article (Boucher and Folberth, 2010) we discussed the advantages and disadvantages of artificial methane removal from the atmosphere. Here, we present a summary of the principal arguments.
Methane is the second most important anthropogenic greenhouse gas and its atmospheric abundance has come close to tripling since the beginning of the industrial revolution as a consequence of human activity. Thus, a reduction to pre-industrial levels would alleviate a radiative forcing of 0.48 Wm-2 amounting to roughly one third of the radiative forcing by CO2 (Forster et al., 2007).
Atmospheric methane concentrations are currently about 0.5 percent that of CO2 and the methane atmospheric lifetime (~10 years) is one order of magnitude smaller than the CO2 lifetime (~100 years). However, the methane radiative efficiency is one order of magnitude larger (3.7 10-4 versus 1.548 10-5 Wm-2ppbv-1 – Ramaswamy, V. et al., 2001). The relatively short methane lifetime can be seen as an advantage (fast response to emission reductions or increased sinks) or a disadvantage (response diminishes quickly). The attached figure shows the temperature change expected from removing 1 kg of methane with (black curve) and without (red curve) concurrent capture of CO2 produced by methane oxidation (for details cf. Boucher et al., 2009). Most of the climate benefit is realised within 60 years of methane capture, but a net climate benefit persists for as long as 500 years even without capturing the CO2 stemming from methane oxidation.
From first principle considerations it can be concluded that:
1) the minimum energy required to separate methane from air is given by the opposite of the change in Gibbs free energy when the two gases are mixed. It can be approximated by
Q = (R T / M) (1 + ln (1/x) )
where R is the universal gas constant, T is the temperature, M is the molecular mass of methane and x is its volume mixing ratio (x<<1). Therefore, the minimum thermodynamic energy to separate 1 kg of methane from ambient air is 2.2 MJ or 35.2 kJ/mol; in practice the energy required to separate methane is likely to be much larger.
2) Methane carries significant amounts of chemical energy. The standard enthalpy ΔH0 for methane combustion is -55.6 MJ/kg or -890.4 kJ/mol. In contrast, carbon dioxide, since it is fully oxidized, essentially carries no exploitable energy.
At this most simple level the energy budget would seem to favour air capture of methane quite substantially. However, using a detailed energy balance for CO2 air capture (Zeman, 2007 and references therein) to derive a more realistic assessment of air capturing energy costs for methane shows that “air contacting”, i.e., pumping of air through the removal apparatus, becomes the crucial factor for engineering solutions.
Essentially, due to the lower atmospheric concentration of methane air contacting becomes substantially more expensive energetically. At current concentration methane air capture could require between approximately 300 and 2100 kJ/mol (where we account for the mitigation benefit of removing a much more potent greenhouse gas through normalising the energy costs by the methane 100-year GWP on a molar basis; for details see Boucher and Folberth, 2010) as compared to capturing CO2 which would require between roughly 450 and 675 kJ/mol according to Zeman (2007). The challenge, hence, is to minimise the air contacting costs in the methane removal process.
In essence, the main goal is be to increase the methane sink to balance the increased sources or ultimately surpass them. Secondary benefits are 1) reduced tropospheric ozone production, 2) decreased stratospheric forcing through methane-derived water vapour, 3) a possible further reduction in atmospheric CO2 through CO2 capture and storage in the methane oxidation process and 4) energy recycling by exploiting the methane chemical energy. Potentially, a few hundred kJ/mol of “portable” chemical energy can be recycled by either using methane directly or making accessible the energy via abstracting the hydrogen from the methane molecule or its oxidation products.
The above arguments demonstrate that there are good physical reasons to consider methane air capture in addition to greenhouse gas emission reductions and carbon dioxide air capture as part of a portfolio approach to mitigate climate change. There are however inherent difficulties associated with the very small concentration of methane in the atmosphere and its low chemical reactivity at ambient conditions but it is possible that these can be overcome. However, the substantial challenges of engineering methane air capturing facilities do seem a feasible endeavour in light of all arguments.
Boucher, O. et al., Indirect GWP and GTP due to methane oxidation, Environmental Research Letters, 4, 044007, doi: 10.1088/1748-9326/4/4/044007, 2009
Boucher, O. and G.A. Folberth, New Directions: Atmospheric methane removal as a way to mitigate climate change? Atmospheric Environment, 44, pp. 3343–3345, doi:10.1016/j.atmosenv.2010.04.032, 2010
Forster, P. M. de F. et al., Climate Change 2007: The Physical Science Basis, pp. 129-234, Cambridge University Press, 2007
Ramaswamy V. et al., Climate Change 2001: The Scientific Basis, pp. 349-416, Cambridge University Press, 2001
Zeman, Energy and material balance of CO2 capture from air, Environ. Sci. Technol., 41, pp. 7558-7563, 2007