4 Increasing energy out

Geoengineering methods that increase outgoing energy from the Earth include carbon dioxide removal (CDR). These methods reduce the greenhouse effect by decreasing atmospheric carbon dioxide concentrations (note, not emissions). You will look at two key CDR methods.

Unlike SRM methods, CDR methods are not instantaneous in taking effect: there is a limit to how quickly a difference can be made to global CO₂ concentrations.

Once you have extracted CO₂, you need to put it somewhere. So there are two parts to CDR:

1. extraction: capturing CO₂ from the atmosphere, and
2. sequestration: storing the CO₂ or carbon so that it does not re-enter the atmosphere.

Planting trees is not enough to extract CO₂ from the atmosphere for the long term. When the trees respire, burn or decompose they return carbon to the atmosphere.

Bioenergy with carbon capture and storage (BECCS) extracts CO₂ as a natural process,but then goes one step further in that itcreates energy as a by-product of CO₂ removal.

In BECCS, biomass such as wood, sugar cane or switchgrass is used for energy, and the emitted CO₂ is captured and stored. Carbon capture and storage (CCS) itself is not unique to bioenergy power stations – it can also be used to reduce carbon emissions from fossil fuel power stations. But using it with bioenergy breaks the closed loop system and removes carbon from the system into long-term storage.

The carbon sequestration is usually proposed to be in geological reservoirs – in the porous spaces of rocks, including those left behind when oil and gas are extracted.

Listen to the discussion of CCS and BECCS in the audio, which is an extract from the BBC’s ‘Changing Climate: The Solutions’ broadcast, presented by Roger Harrabin (November 2015). Then study the diagram of a BECCS plant in Figure 10 and answer the questions that follow.

Download this audio clip.Audio player: Audio 1  Extract from the BBC’s ‘Changing Climate: The Solutions’, presented by Roger Harrabin (November 2015).

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Transcript: Audio 1  Extract from the BBC’s ‘Changing Climate: The Solutions’, presented by Roger Harrabin (November 2015).

COMMENTATOR:

Carbon capture is where you strip CO2 from power station chimneys and pump that CO2 into porous underground rocks where it remains locked up, supposedly forever. Sally Benson from Stanford University, a UN adviser, is a leading expert on Carbon Capture and Storage.

SALLY BENSON:

I started working in 1998 on Carbon Capture and Storage and when I first heard about it I thought it sounded like a really ridiculous idea, but as I got into it more deeply I actually became persuaded that one day it would play an important role in decarbonisation of the energy system.

I did see it as an option of last resort, that something that after we've tried all the things like PB and wind, and we did as much as we could of those, then that’s when CCS would come along, so that would be, you know, in the 20, 30's, 40's, 50's was my view. Of course everything turned out quite differently.

MAN:

Well a lot of governments, including the UK government, still have Carbon Capture and Storage as a main plank of their energy supply system, in the UK, for instance, after 2030. Is that realistic?

SALLY BENSON:

Well, when we talk about a main plank I, you know, I see CCS as one of those, you know, a 20% solution, you know it's not a silver bullet, it's not the whole story.

COMMENTATOR:

Carbon Capture and Storage is the technology the Intergovernmental Panel on Climate Change, the IPCC, says we'll need if, or when, we overshoot our carbon targets, which most experts think we will. When it comes to that point, they say, we're going to have to engineer global warming into reverse by using plants to suck CO2 from the atmosphere. It may sound bizarre but it is genuinely being considered.

SALLY BENSON:

The only real option for taking carbon dioxide out of the atmosphere, or so-called negative emissions today is something called BECCS, which is Bio-Energy with Carbon Capture and Storage.

COMMENTATOR:

So how would it be done? Well, first you plant crops that grow fast and absorb CO2, then you burn the plants and get electricity in the process. Then, here's the magic bit, you capture the CO2 emissions from burning the plants and bury it under the ground and, there you are, that's Bio-Energy Carbon Capture and Storage, BECCS.

End transcript: Audio 1  Extract from the BBC’s ‘Changing Climate: The Solutions’, presented by Roger Harrabin (November 2015).

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Audio 1  Extract from the BBC’s ‘Changing Climate: The Solutions’, presented by Roger Harrabin (November 2015).

Figure 10 Diagram of a BECCS plant (adapted from Canadell and Schulze, 2014).

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Figure 10 is a drawing showing an industrial plant and trees. Arrows represent flows of carbon dioxide.

One flow is from the air to trees. The trees are labelled with 'Energy Crops and Biomass residue'.

Another flow is from the trees to the industrial plant, and is labelled with 'Combustion, Fermentation, Aerobic digestion and Gasification’.

A flow outwards from the plant is towards 'energy products' and is labelled 'Heat, Biohydrogen, Synthetic biofuels and electricity'.

A flow downwards into the ground from the plant is towards 'Geological Storage' and is labelled 'Saline aquifers’ and ‘Depleted oil and gas fields'.

Figure 10 Diagram of a BECCS plant (adapted from Canadell and Schulze, 2014).

In principle, BECCS could be both achievable in the near term and effective over the long term. Lenton and Vaughan (2009) estimate that BECCS could offset two-thirds of a doubling of atmospheric CO₂ concentrations by the end of the century, while the assessment of McGlashan et al. (2012) is that the technologies ‘could be introduced relatively easily in today’s energy system’ and reduce current UK CO₂ emissions by at least 10% by 2030.