Abstract
Large-scale atmospheric removal of greenhouse gases (GHGs) including methane, nitrous oxide and ozone-depleting halocarbons could reduce global warming more quickly than atmospheric removal of CO2. Photocatalysis of methane oxidizes it to CO2, effectively reducing its global warming potential (GWP) by at least 90%. Nitrous oxide can be reduced to nitrogen and oxygen by photocatalysis; meanwhile halocarbons can be mineralized by red-ox photocatalytic reactions to acid halides and CO2. Photocatalysis avoids the need for capture and sequestration of these atmospheric components. Here review an unusual hybrid device combining photocatalysis with carbon-free electricity with no-intermittency based on the solar updraft chimney. Then we review experimental evidence regarding photocatalytic transformations of non-CO2 GHGs. We propose to combine TiO2-photocatalysis with solar chimney power plants (SCPPs) to cleanse the atmosphere of non-CO2 GHGs. Worldwide installation of 50,000 SCPPs, each of capacity 200MW, would generate a cumulative 34PWh of renewable electricity by 2050, taking into account construction time. These SCPPs equipped with photocatalyst would process 1atmospheric volume each 14–16 years, reducing or stopping the atmospheric growth rate of the non-CO2 GHGs and progressively reducing their atmospheric concentrations. Removal of methane, as compared to other GHGs, has enhanced efficacy in reducing radiative forcing because it liberates more °OH radicals to accelerate the cleaning of the troposphere. The overall reduction in non-CO2 GHG concentration would help to limit global temperature rise. By physically linking greenhouse gas removal to renewable electricity generation, the hybrid concept would avoid the moral hazard associated with most other climate engineering proposals.
Highlights
IntroductionTagedPEven if humans stop combusting fossil fuels and discharging CO2 into the atmosphere, the average global temperature of the Earth will continue to increase for the rest of the century Àfor several reasons
TagedPThis review shows that a large scale method of removal of greenhouse gases (GHGs) other than CO2 is possible, as these GHGs can be mineralized by photocatalysis, using simple metal oxides like MgO or ZnO, cheap TiO2 derivatives and zeolites
TagedPA giant Photocatalytic Reactors (PCRs) supported by the infrastructure of a solar chimney power plants (SCPPs) is proposed as a device able to: TagedP transform CH4 into CO2 which has global warming potential (GWP) >10 times lower per mole on a 100 years’ basis; TagedP transform several CFCs and HFCs into CO2 C HCl and HF and neutralize the halo-acids by-products; TagedP reduce the atmospheric concentrations of CH4, N2O and halocarbons, achieving GHG removal; TagedP require minimum additional investment for the PCR
Summary
TagedPEven if humans stop combusting fossil fuels and discharging CO2 into the atmosphere, the average global temperature of the Earth will continue to increase for the rest of the century Àfor several reasons. The long lifetime of CO2 (estimated in the 100À300 year range [1]) means that the excess atmospheric stocks (515 Gt Carbon) would continue to drive radiative forcing and global warming for many decades [2]. Even if atmospheric concentrations were to decrease, CO2 would outgas from the oceans and offset this decrease, because of the dynamic equilibrium between the CO2 in the atmosphere and the (bi)carbonates HCO3¡/CO32¡ dissolved in the oceans [3]. Even if all excess anthropogenic atmospheric CO2 were removed, radiative forcing would only be reduced by half [5]
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