Monday, 14 June 2021

How clean is 'clean' coal?

The G7 summit has agreed to to move away from making electricity from coal plants, unless they have technology to capture carbon emissions [1]. What does this mean in terms of greenhouse gas emissions? Carbon capture and storage  (CCS) is not a perfect solution, sadly. From my calculations, taking into account upstream emissions,  the emissions from a coal power station even with CCS are worse than those for UK grid average electricity now and with little hope of further improvement. In 2019 a little more than a third of UK electricity was from renewable and this continues to increase.  Of course the UK is not representative of the global average, but it shows what is possible, even starting with legacy power infrastructure. By 2050, average UK grid emissions should be down another 90%, but this may not be possible if much of our power comes from coal with CCS. Natural gas with CCS would be more sensible.

Comparing emissions from current UK coal and gas power stations, the same with CCS and average grid emissions. Coal with CCS is slightly worse than grid average, gas with CCS is considerably better. Estimates include emissions upstream of the power station. Data sources and assumptions are described below.

There are several reasons why clean coal is less than 100% clean:
  • There are upstream emissions, from mining and handling the coal before it gets to the power station.
  • Carbon capture at the power station does not catch all the carbon.
  • Carbon capture uses energy which makes the power station less efficient - so you need more coal (and hence more emissions) to make the same amount of energy.

Upstream emissions account for more than half

Some of the emissions occur before you even get to the power station. In the above graphs, upstream emissions account for more than half the total with CCS. Most of this is due to methane leakage from coal mines. Methane leaks out from the seams - this has caused many accidents in the past and is a huge danger to miners. Methane continues to leak from seams for decades even after the mines have been closed. Overall, more methane leaks from coal mines than from oil and gas production [2].

Carbon capture rates are typically 90%

There are several ways to capture CO2 from burning coal. You can burn coal in air, as we do now, and separate the CO2 from the flue gases. Or you can burn coal in pure oxygen which means that the CO2 is much more concentrated (though you have to separate the oxygen from the air first). Either way, capture rates higher than 90% are hard to achieve. Higher capture rates need more extra energy. It is possible to do better with natural gas power stations - up to 97% is claimed for oxy-fuel processes [3].

Carbon capture reduces the overall efficiency of the plant by 20% or more.

Not surprisingly, capturing the carbon takes energy and then the captured gas has to be compressed and transported to a storage location. This uses some of the energy generated by the power station and reduces the overall efficiency - typically at least 20% more coal is needed to make the same amount of energy [3].

There is a place for CCS - but not with coal

We will almost certainly need some power stations with CCS in order to meet our GHG emissions targets. It may be that some of them use biomass, although it looks increasingly difficult to produce biomass at scale without damaging loss of biodiversity. We may end up using hydrogen for some of them - preferably generated from surplus wind or solar electricity. However, even if we end up with fossil fuel power it makes no sense to use coal, rather than gas. Gas is cleaner to start with, can be fitted with more effective carbon capture technology, and does not have the problem of continuing emissions from mines even after they have been closed. Upstream emissions cannot be captured at the power station - and these account for more than half the total emissions so improving CCS has only limited impac. 

Assumptions behind the chart above

  • Emissions from coal and power stations: 920 and 380 gCO2e/kWh [4] 
  • Upstream emissions calculated 
    • plant efficiencies: (31.9% coal, 48.8% gas) [4]
    • well to tank upstream for the fuel 51  and 23 gCO2e/kWh (gross) [5]
  • Average grid electricity emissions 2019: 198gCO2e/kWh [4]
    • and I used the trend for the three years 2016-2019 to calculate the emissions in 2021, ignoring effects of Covid. 
  • Capture rate 90% [3]
  • CCS means that 20% more coal is needed to make the same amount of electricity[3]

[1] G7 to agree tough measures on burning coal to tackle climate change ((BBC) June 13 2021

[2] Coal mines emit more methane than oil-and-gas sector, study finds (Carbon Brief) March 2020

[3] 'Clean Coal' Technologies, Carbon Capture & Sequestration (World Nuclear Organisation) April 2021

[4] Digest of UK Energy Statistics 2020 (July 2020)

[5] UK Government GHG Conversion Factors for Company Reporting (2020)

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