Because my Earth’s too bootylicious for you, babe xo

Ok, so we know how to capture CO₂, we can even store CO₂; but how much junk can the Earth actually fit in the trunk? Formally, what is the expected global storage capacity for fixed Carbon Dioxide? Whilst we shall discuss methods of Carbon Storage (there are many), the one important thing to note now: Carbon is not stored in a gaseous state, but a denser super – critical state which means we can store a lot more than you’d think!

The IPPC Climate Change Synthesis Report estimates anthropogenic 2010 emissions at ~49±4.5 GtO₂-eq/yr^-1. To give a little perspective, anthropogenic emissions in 1970 were just 27 Gigatons (Gt) – a ~100% increase in 40 years, well done society. The figures for global capacity carry great amounts of uncertainty and range from320 Gt to 10,000 Gt. The main argument for geo – engineering and CCS is to prevent tipping points in the climate system. Even if we take a more reserved storage estimate of 3,000 Gt CO₂, and we store 25% of emissions per annum we are working on a timescale exceeding 75 years! The Manhattan Project took 4 years. Look what the world’s leading economies can do if they really put the effort in…

Figure 1. Density of Carbon Dioxide as a function of depth plotted against geo-thermal gradients, extracted from E.Lindeberg, 1996.

Taking off the mask.

So far we have discussed methods of geo – engineering as if wearing a mask, now we look towards an opposition; a concept by which carbon is extracted from the climate system and stored, Carbon Capture and Storage (CCS). The idea of CCS is often considered as a mitigation strategy to reduce emissions at a large scale, primarily targeting electricity generating power plants.

There are two leading ideas to tackle this problem:

• Capturing CO₂ from a source gas prior to its release into the atmosphere
• Directly extracting CO₂ from the air in the climate system 

To discuss carbon capture leaps into the world of advanced chemistry and chemical engineering, as so, let’s start with a picture. The figure below, courtesy of Terry Wall, provides a concise and simplified summary allowing us to grasp the three operations which are currently being implemented at select power stations.

Kanniche, Mohamed et al. 'Pre-Combustion, Post-Combustion And Oxy-Combustion In Thermal Power Plant For CO2 Capture'. Applied Thermal Engineering 30.1 (2010): 53-62.

Pre – combustion or the Integrated Gasification Combined Cycle (IGCC), if you want to make things difficult, describes an operation which separates the Hydrocarbons into Hydrogen and Carbon (shock) and burns them at increased pressures with pure oxygen. The result is a gas of Carbon Monoxide (CO) and Hydrogen (H₂). Clearly we can’t leave things like this, to capture CO₂ we actually need some CO₂, this is done by reducing the temperature and reacting the CO with steam (H₂O). Finally, the gas needs to be separated, this is done using physical and chemical solvents and the left over Hydrogen is utilised for further energy production.

Post – combustion, as you might have guessed, captures the CO₂ after the electricity has been generated. Things start warming up and the gas is scrubbed (with a solvent of aqueous amine solution) at about 50°C. It then starts to get really hot (~100 – 120°C) and it is stripped down, molecule by molecule to regenerate the solvent with water vapour. The condensed vapour leaves a gas of pure CO_2.

The M&S advert is over now. Oxy – fuel combustion takes the natural air and separates out the Oxygen. The extracted oxygen (~95% pure) is burned with a recyclable gas to maintain volume and thus pressure in the system (why? Only ~21% of natural air is oxygen!) and produces a gas of majority CO₂ and water.

Members of the academic community including David Keith and Klaus Lackner have progressed the idea of air capture directly from natural air by building artificial trees… and I’m not talking about the plastic bonsai on your bedside table. The idea is to create a structure of blinds embedded with plates of Sodium Hydroxide which absorb CO₂ from passing air with a collection of efficiency of ~30%. The idea remains rogue in the scientific community, whilst yes, you could capture CO₂ from anywhere, the issue still remains of transporting and storing the CO₂, Although the ability of such technology exists, Keith quotes the realism acts as an, ‘unlikely means to achieve air capture,’ in the foreseeable future.

On a final note, considering such technology actually exists, is feasible and has been proved successful; you may ask, why is it not implemented at every power station? It boils down to money. The UK Government, just days before the Paris climate summit, retracted the £1bn effort to implement CCS in conjunction with Shell and Drax. Realistically, the decision showed a complete lack of appreciation for the cost of doing nothing with expected savings of ~£32bn by 2050.