According to this article from Reuters, it takes up to eight hours for a coal plant to reach maximum generation. This means that four up to eight hours after it starts up, the plant is burning coal at a lower efficiency (Btu/calorie per kWh) than what it's capable of during longer-term operation.

In the U.S. cycling of coal plants is something that happens on a daily basis in some areas. The chart below (from the Energy Information Agency) shows power sources over an average day in the western U.S.:

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For a huge area of the U.S., coal plants are reducing output for several hours each day. Presumably this involves turning off boilers or even whole plants.

This seems similar to the question on shutting off a car's engine at a stoplight. Could keeping a coal plant running, in some circumstances, result in less CO2 emissions than stopping and restarting it?

To figure that out, I'd need some data on how much coal a plant burns during shutdown and start up, and how much power is produced during this period, compared to how much coal is used to produce the same amount of power after it's been online for some time.

This question was prompted by the recent news that Britain went a week without burning coal. There's also a good discussion over on skeptics.se about a similar occurrence a few years ago: During Great Britain's coal-free day, were coal plants shut off, or active and generating power GB didn't use?

2 Answers 2


TL;DR: No -- due to the length of time it takes a coal plant to return to full output after shutting down, it will not produce more emissions during startup than a fully-loaded plant over the same time period.

Typical coal plant in the U.S.

From the monthly generator inventory I determined that in the U.S. the median utility and independent power producer (IPP) coal plant burns subbituminous coal, and has a capacity of 339 MW. Additionally, slide 11 of this EIA presentation indicates that 71% of coal plants in the U.S. are subcritical.

Length of time from shutdown to full output

The article "Review of the operational flexibility and emissions of gas- and coal-fired power plants in a future with growing renewables" provides typical shutdown and startup times for different power plant types. For a typical 300 MW subcritical coal plant, the minimum downtime (after shutdown and before startup can commence) is 120 minutes. For a "hot start" (occurring within eight hours of a shutdown), the minimum time required to return to full output is 130 minutes. This gives a minimum total time of 250 minutes between shutdown and full output.

Startup emissions

The article "Cost-Benefit Analysis of Flexibility Retrofits for Coal and Gas-Fueled Power Plants" provides data on emissions from a coal plant during start-up:

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According to the U.S. Energy Information Agency, burning coal subbituminous results in emissions of 214.3 lbs of CO2 per MMBtu (source).

This means that for a hot start, a typical plant produces 287 metric tons of CO2. Assuming a roughly linear output, power generation during the startup time would be 229 MWh, for an emissions rate of 1,250 kg CO2 per MWh.

Emissions for a non-shutdown plant

Using data from EIA, I calculated an average emissions factor in the U.S. of 2,240 lbs (1,016 kg) of CO2 per MWh of electricity generated by coal plants.

Over the same period of 250 minutes, a typical plant would produce 1,664 metric tons of CO2, and 1,638 MWh of electricity.


While the emissions rate during startup is somewhat higher (1,250 vs 1,016 kg CO2 per MWh), the length of time required for a complete shutdown and startup cycle (over four hours) means that the plant releases far less CO2 than if it were fully operation over the same period. Thus if the coal plant is being shut down to accommodate increased wind generation (or any other emissions-free source), there will always be net reduction in CO2 emissions.


or oilI disagree with your assumption that power demand ever goes to zero. The demand fluctuates and the the amount of coal burned fluctuates. It will be minimal at night and the utility does all it can to match the coal used to the demand. A large coal unit can take a day or more to start and stop ; It is not like a car. But , gas turbines are very fast to start and stop, so many utilities will use them for "peak shaving" , running them only during high demand. Then they do not need to adjust a coal fired unit as much. A large gas or oil fired electric plant is similar, ( As the reference in the question notes , UK is able to fire the coal plants with gas/oil) it takes hours to balance the burners to the steam and power generation. There are problems like cracked refractories caused by cyclic heating and cooling. Not to mention the corrosion problems of a cold coal fired heater ; Various deposits on the tubes are no problem when hot/dry but are corrosive when cold and damp. And not considering those units that sell steam on a continuous basis.

  • Rereading the question , I think there is a misunderstanding. The coal fired boilers were not shut down , they were switched to oil and gas from coal for fuel. Modern coal units blow in pulverized coal , it is not very difficult to replace the coal with oil and gas . This change has been going on in the UK for decades ,utilizing oil and gas from the North Sea offshore production. Commented May 9, 2019 at 14:48
  • Hmm I've never heard of coal plants switching to burning oil/gas -- can you provide a source? Either way, according to the map in this article, the "newest" coal plant in the UK came online in 1987, and most of them are nearly 20 years older than that, so I don't know if that counts as modern. However if you're correct that would answer my question -- any coal in the burner would probably continue contributing to generation while the switch to oil/gas was occurring.
    – LShaver
    Commented May 9, 2019 at 20:12
  • Put "coal to gas conversions" into an internet search. I see there are fewer in the US than I thought because coal is still lower cost than gas here but the UK, plentiful North Gas and higher carbon taxes on coal change the economics. The references do not emphasize enough the lower operating cost of a coal unit switching to gas; minerals in the coal ( silica, limestone,sulfur, etc) cause significant problems in the boiler which gas does not ( in my judgement). Commented May 11, 2019 at 17:29
  • According to this article under "co-firing gas and coal," the conversion is a one-time operation -- there aren't plants which can switch back and forth between fuels. My question is about a short-term shut down. According to the first link in my question, coal plants in the UK are now occasionally shut down for days at a time, with the expectation that they will again burn coal. I want to know how much coal is burnt during that transition when no power is being produced.
    – LShaver
    Commented May 11, 2019 at 18:06
  • After thinking about this I ended up editing my question quite a bit to give it a better focus and connection to sustainability.
    – LShaver
    Commented May 13, 2019 at 2:27

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