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On Friday 21st April, the UK National Grid announced the following on twitter:

National Grid can confirm that for the past 24 hours, it has supplied GB's electricity demand without the need for #coal generation.

"no coal generation for 24 hours"

They further clarified in a following tweet:

Today's average generation mix so far has been gas 50.3%, nuclear 21.2%, wind 12.2%, imports 8.3%, biomass 6.7%, solar 3.6%

When I speak to people living here in the UK or abroad about this, they bring up that it is unclear whether:

  • coal stations were totally shut down for that 24 hour period (and other 6–8 hour periods earlier that week) and thus not burning coal or generating pollution, or
  • coal stations were running, burning coal, and generating power (which was either consumed elsewhere or entirely wasted) but Great Britain was powered without coal generation for the period.

I agree with them that it's ambiguous. A common point of doubt has been that surely they can't just shut down and start up the coal plants that fast.

Someone in the twitter thread requested some clarity around this, to which National Grid did not reply: “Were there plants burning coal, but they weren't feeding the grid? Or were they all totally cold?”

Were coal power stations running and generating power that Great Britain didn't use, or were they actually shut down?

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doppelgreener
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    Coal plants used to be exclusively base-loaded, but it's fairly common now to cycle them - either through large load swings, or daily on/off cycles. It leads to higher failure rates and maintenance costs, but has been driven by economics for quite some time because of low natural gas prices. Here's an old article about it http://www.powermag.com/make-your-plant-ready-for-cycling-operations/?pagenum=1 – Mark May 07 '17 at 13:09
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    This might be a false dichotomy. Could it be that the plants were running at "idle", such that a small amount of coal was burned but no significant amount of electrical energy was produced? – Nate Eldredge May 07 '17 at 14:42
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    And how much that that "imports 8.3%" was from coal plants outside the U.K.? – Michael May 07 '17 at 19:08
  • @Michael none, the countries we import from (Ireland, France and the Netherlands) were also coal free I believe but I can't find a source. – Tim May 08 '17 at 06:06
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    Related link: Current energy sources from the power grid dashboard: http://www.gridwatch.templar.co.uk/ – Sobrique May 08 '17 at 09:15
  • Another possibility is multifuel power stations running on biomass – Chris H May 08 '17 at 09:48
  • @Tim the Netherlands uses a lot of coal for power generation. And as domestic gas production is set to decline in the Netherlands, it'll only increase. – jwenting May 08 '17 at 10:31
  • @jwenting it could be the amount we imported could be counted as part of their renewable quota, but I do have a feeling they were all 0 coal too. – Tim May 08 '17 at 10:40
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    @Tim they'd have no way of knowing, unless they exclusively import from countries that have 0 coal generation capacity. – jwenting May 08 '17 at 10:43
  • There is [a follow up tweet explaining the countries imported from during that period](https://mobile.twitter.com/NGControlRoom/status/855707579401592832), but the drive of this question isn't about whether coal power was used, but what was going on with the coal generators in GB. – doppelgreener May 08 '17 at 10:46
  • @jwenting I'm certain there is data for each of those countries - we're all part of the EU. However I don't know if they were 0, I just have a feeling they were. My point was that the 2% of their energy which we imported (say) could have been counted as part of the 20% of their wind. – Tim May 08 '17 at 10:46
  • Given that National Grid [originally announced](http://www.bbc.co.uk/news/business-39668889) this coal-free day during the working day on Friday, it suggests that they had told the coal plants which it had dispatched/used earlier in the week that they would not be needed that day, probably due to a short-term increase in wind generation combined with the usual decline in total demand at the start of the weekend – Henry May 08 '17 at 10:48
  • Although it's been almost 35 years since my last experience with a steam plant, and it was oil-fired and sea-going rather than coal-fired and land-based, I can attest that we could go from cold plant to underway in 6 hours. – Bob Jarvis - Слава Україні May 08 '17 at 18:38
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    A couple of points worth noting : Apart from the 24 hour "day without coal", the entire preceding week had relatively little coal generation : often 300MW, some plateaus at about 600MW, and even the peak was below 1 GW. The West Burton plant alone is said in another answer te be rated at 2GW capacity. Therefore it seems likely that most, probably all but one, plant were cold, having produced no power for the entire week. However it's quite possible that one plant was kept up to temperature or spinning, with minimal coal consumption. – user_1818839 May 08 '17 at 21:26

2 Answers2

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Quick Take: There doesn't appear to be available information to determine if the coal plants were burning coal, although not contributing electricity to the grid; there is sufficient information to say that the coal plants were not exporting electricity out of Great Britain.

It looks like the event actually included all the coal-fired power plants in Great Britain going off-line:

The control room tweeted the milestone on Friday. It is the first continuous 24-hour coal-free period for Britain since use of the fossil fuel began. West Burton 1 power station, the only coal-fired plant that had been up and running, went offline on Thursday.

Source: British power generation achieves first ever coal-free day, The Guardian

This sort of thing is very possible - Wikipedia only lists 9 extant working coal power plants left in Great Britain.

It is, however, not clear what "offline" means in this context. Offline could mean that the West Burton (and other coal-fired) plants were in "Non-spinning Reserve," which would mean the turbines were not being driven by any coal burning and a considerable start-up time would have been required to start the power plant up again; or it could mean the at least some of the coal-fired power plants in Britain were in "Spinning Reserve," where they would not be actively contributing electricity to the grid, but coal was being burned and the turbines were spinning so that contributions of electricity to the grid could start quickly if demand picked up.

Without information on which of these states the power plants were in, it is not possible to say whether coal was being burned in Britain. However, the National Grid did clarify in a later tweet that coal did not contribute to the generated mix of power including exports (which is why the percentages in the tweet below add up to higher than 100%.)Tweet from National Grid Control Room Apr. 26, 2017

Imported power, on the other hand, did include some fraction of coal generation, as clarified by this tweet:2nd tweet from National Grid Control Room, Apr. 26, 2017

So even if Great Britain wasn't generating any electricity produced by coal, imports from continental Europe mean that coal is being burned somewhere. Even when all British coal plants are phased out, as is being planned for the 2020s, electricity production on the continent may provide some fraction of coal-generated electricity unless coal plants are phased out there as well.

Esteve
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    Furthermore, West Burton is a 2000 MW coal fired plant with 4 boilers and 4 turbines rated at 500 MW each (https://www.edfenergy.com/energy/power-stations/cottam-west-burton-a) If only one boiler/generator is running the max power would be 500 MW and min power probably somewhere in the 50% to 70% range. Looking at the graph of coal power in the question, it looks like the lowest coal power rate is around 250 MW. From there it drops to zero or climbs higher. Looks like only one boiler at this plant was online, and it shuts down for large parts of most days. – Mark May 07 '17 at 16:31
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    One more comment - Here's a reference indicating that this plant runs base loaded in the winter, and cycles on and off daily during the summer:https://www.hadek.com/industrial-chimney/west-burton-power-station/ – Mark May 07 '17 at 16:49
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    I think "what does 'offline' really mean?" is the point of the question. Good find though! – fectin May 07 '17 at 20:10
  • I see what you mean, fectin. The U.S. equivalents of the terms we're looking for are "Reserve Capacity - Spinning" and "Reserve Capacity - Non-Spinning." I have not seen any references that demarcate which of the two the West Burton Plant was in that day, although Mark's cites above imply the plant goes cold (no coal burning at all) - "This cycling operating schedule creates many challenges for the station. The chimney is one of them: during every start-up, the chimney will be relatively cool."... from the hadek.com cite Mark provided. – Esteve May 07 '17 at 21:28
  • Added due to length of the earlier comment - the hadek.com site does say "*relatively* cool" which is why I think it only implies that the West Burton Plant shuts down completely rather than burning some minimal amount of coal - I could be wrong in this. Perhaps someone will be able to find a better cite. – Esteve May 07 '17 at 21:33
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    This contribution does not provide enough information do answer the question. Offline can probably mean a lot of different things. – NoDataDumpNoContribution May 08 '17 at 08:09
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    "offline" means they weren't delivering to the national grid, it does not mean they weren't running idle. – jwenting May 08 '17 at 10:32
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    "Without information on which of these states the power plants were in" - this is the crux of the answer. Can we not gather this information? – Oddthinking May 08 '17 at 13:44
  • @Esteve Exactly. For example, my understanding is that most of the "backstop" generation in the US is coal. Those plants charge exorbitant prices for occasional short-term generation, because they can spin up on-demand to meet demand spikes. If a backstop generator is standing by, what does that even mean for whether it's providing power? (+1) – fectin May 09 '17 at 00:10
  • @fectin good point - one of those things where simple answers don't really fit and context becomes very important. Thanks for bringing it up. – Esteve May 09 '17 at 04:49
  • @jwenting - Any reference for that? I suspect it's true, but this is skeptics.SE after all. – AndyT May 09 '17 at 15:00
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    @AndyT just the generic terms as I remember them being used when working (on the sidelines, admittedly) in energy distribution research as a grad student. It's the same meaning as a computer being offline. It doesn't mean it's turned off, only that it has no network connection. Which may of course mean it's turned off as well, but doesn't have to. And given the cost of completely shutting down and powering back up a station (it can take days to have everything spool up and down slowly to prevent damage) it's unlikely they completely turned them off just to make a point about how green they are – jwenting May 10 '17 at 06:07
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    @AndyT This is normal practice for power stations of all kinds. You keep them running at very low power constantly unless you need to do maintenance on the turbines themselves. They call it hot (or warm) standby. Cold standby stations are mothballed, often take days if not weeks to bring back online. – jwenting May 10 '17 at 06:12
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We don't know. As you said in the question, the National Grid did not answer when asked about it and "being offline" is sufficiently undefined to not answer the question.

It's actually four twitter posts in this thread which ask this question and none of them is answered.

I searched the press coverage of the events, but none of what I could find answers this question. BBC, Guardian, NY Times, and Fortune.

Looking at public available information about the West Burton Power station (the one that needed to be taken offline) does unfortunately not answer this question. Wikipedia, EDF energy, Hadek.com.

In absence of specific information of what happened in the West Burton power station on 21st of April I guess that most likely the coal consumption was reduced but maybe not to zero.

Coal power stations are known to need some time to stop and again to start again. It all comes down to the technology and economy of coal power plants.

Please see the article about cycling of coal power stations cited by Mark in the first comment under the question.

The Technical Assessment of the Operation of Coal & Gas Fired Plants by the UK government lists some technical limitations for starting and stopping coal power plants as well as some typical cold/hot startup times in the order of hours.

From reading this brochure The Future Role of Fossil Power Generation by Siemens I gather that:

  • It is unlikely the coal-fired power plant West Burton 1 was cold (completely stopped). So some fraction of coal was probably still burned, but it may be considerably less than during normal operation.
  • It could have been down cycled, because this is needed for frequent fluctuations like day-night cycles anyway. So if the technology is available it might as well have been used. However I don't know if this was the case here. The cycle times seem to be within hours, so a down regulation during 24 hours is definitely possible.
NoDataDumpNoContribution
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    This answer basically takes the tack of "I don't know, therefore no-one knows." You haven't shown that the information being sought is unknowable. – Oddthinking May 08 '17 at 13:43
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    @Oddthinking Thanks. I agree. I have added information of where I searched and could not find the information. In general, it seems impossible to show that an information is unknowable least you ask everyone, doesn't it. I wonder what the general stance of the community is towards "negative answers"? Is showing your research enough or should one just refrain from answering? Is there a meta question dealing with this? – NoDataDumpNoContribution May 08 '17 at 14:24