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How much do ultra-supercritical coal plants really reduce air pollution?

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The coal industry and some power utilities have been talking up the supposed benefits of ultra-supercritical power plants – leading some decision-makers and commentators to mistakenly believe that the choice of ultra-supercritical technology for a power plant will substantially mitigate air pollution and carbon dioxide emissions.

To understand the difference between subcritical, supercritical and ultra-supercritical power generation technology on the air pollutant emissions from a coal-fired power plant, the most important thing to know is this: which type of steam cycle is used has no impact on the emissions per tonne of coal burned.

Taking the example of sulphur dioxide (SO2) emissions, emissions per tonne of coal depend solely on the amount of sulphur contained in the coal, essentially all of which is oxidized into SO2 during combustion, ending up in the raw flue gas.

For example, for typical “low-sulphur” coal containing 0.5 per cent of sulphur when fed into the boiler, every tonne of coal will contain 5 kilograms of sulphur. When burnt, this sulphur turns into 10 kilograms (kg) of SO2. (Every sulphur atom joins with two oxygen atoms to produce one SO2 molecule which is twice as heavy as a sulphur atom.)

The only difference between different steam cycles in terms of emissions is how much power they can generate from one tonne of coal.

A typical new subcritical plant will have a thermal efficiency of 38 per cent, meaning that 38 per cent of the thermal energy contained in the fuel is converted into electrical energy fed into the grid.

A supercritical plant will have an efficiency of maybe 42 per cent and a typical ultra-supercritical plant will achieve around 44 per cent (designs going up to 47 per cent are being developed).

This means that a 1000 megawatt (MW) coal-fired plant using subcritical technology will need to burn coal at a thermal input rate of 1000 MW / 38 per cent = 2630 MW-thermal to generate its full output. This corresponds to 410 tonnes of coal per hour, assuming a typical calorific value of 5500kcal/kg, and 4100kg/h of SO2 in raw flue gas.

If the plant uses ultra-supercritical technology, it needs thermal input of 1000 MW / 44 per cent = 2270 MW-thermal. As a result, it burns 350 tonnes of coal per hour, or 14 per cent less than the subcritical plant and generates 14 per cent less SO2.

If the plant is not equipped with SO2 emission control technology, that’s the end of the story.

However, if the environmental regulators require the plant to meet SO2 emission limits that cannot be met without installing SO2 control devices, the plant will have to make additional investments.

Cleaning the air pollution

In essentially all countries except the US, SO2 emission limits are set in terms of SO2 concentrations in flue gas. The project developer will have to design a control device that removes enough of the SO2 from the flue gas to get below the limits.

Some of the toughest limits for SO2 emissions are found in China, where flue gases from coal-fired power plants are not allowed to contain more than 35 milligrams of SO2 for every cubic meter of dry flue gas.

The untreated flue gas from the example plants above will contain about 1200mg/m3 of SO2. Therefore, the plants will have to install SO2 control devices that remove about 97.5 per cent of the SO2 contained in untreated flue gas.

The difference between subcritical and ultra-supercritical technology is that the total amount of flue gas emitted from the ultra-supercritical plant is about 14 per cent smaller, and hence the capacity of the SO2 control device can be about 14 per cent lower, resulting in savings in investment and operating costs. Resulting SO2 emissions associated with a given emission standard will also be about 14 per cent lower.

The same logic applies to the emissions coal plant (NOx), particulate matter (PM), mercury and other heavy metals. The air quality and health impacts are directly proportional to emissions.

Moral of the story: Emissions regulation matters a lot, whether a plant is ultra-supercritical matters little.

So why are the coal industry and its advocates always going on about ultra-supercritical coal plants and not about emissions regulation?

Simple: ultra-supercritical plants are usually more profitable than subcritical plants, since they have lower fuel and other operating costs.

Stringent emission regulation, in contrast, increases both investment and operating costs. I hope you’re not too shocked that coal advocates are not mainly motivated by health concerns.

It is worth noting that Australia, the main peddler of “High Efficiency Low Emissions” (HELE) coal plants along with Japan, hasn’t even required flue gas desulphurisation equipment on its own coal plants, making them some of the dirtiest in the world.

Below is a simple graph illustrating the effect of emissions regulation versus type of steam cycle on SO2 emissions:

coal fired plant emssions graph

The chart below shows a comparison between coal and gas plants following the same Chinese emission standard.

SO2 and particle emissions from gas are a tiny fraction of those from coal, while NOx emissions are similar. It would be technically easy for the gas plant to go a lot lower but this is what current standards require.

air pollutant emissions

 

Lauri Myllyvirta works for Greenpeace East Asia.  

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  • David leitch

    Excellent article Lauri. Write us some more. What’s happening with the constraints on China wind these days? I note that electricity demand is well up in China this year, might push the coal plants to over 50% cap rate this year?

    • Peter F

      David
      Renewable capacity is up 50GW +, coal is up gas is up and I think nuclear is up. Unless there have been extremely rapid decommissioning of old coal plants, coal plant utilisation has probably fallen further

  • Tim Buckley

    Likewise, Lauri – great article. Could you give Josh Frydenberg a quick 101 in being Energy Minister, please? He seems to have read the QRC songbook and forgotten to ask any experts. Either that, or he is playing politics ahead of actually doing his highly paid job serving the people of Australia.

    • Alastair Leith

      “I certainly hope the coal and mining industry reward a good [servant]”

  • https://uploads.disquscdn.com/images/0fb828c0d7337a4ecc70197121b5967cd72b5783ce90a2b8acb80ff6c537d864.png The coal lobby is quite calculating in trying to foist this particular coal technology on unsuspecting governments. Chinese and Japanese corporations are doing deals to build these plants in Vietnam, Pakistan and Kenya – and probably elsewhere.

    Pakistan realised too late that this technology is unable to use low grade coal – of which Pakistan has substantial reserves – and obliged them to import coal, for example, from Chinese-owned coal mines in Australia.

    This obligation could persist for 40 years or more – the operating life of the new coal-fired power station.

    The coal lobby is deliberately very quiet on more advance power station technology, such as that used by South Korea and Japan itself – which can convert even low-grade coal into gas that is then used to fuel gas power stations.

    There are now at least 4 suppliers of technology to convert waste into gas at moderate temperatures, so in future waste-to-landfill to sewage treatment works will be fuel suppliers to gas power stations. Power-to-gas technology is able to convert excess solar and wind power to gas that can also be used to fuel gas power stations.

    The existing energy market segmentation of fossil fuels (coal/oil/gas), renewables (solar/wind/bioenergy/hydroelectric,etc) and energy storage (batteries/pumped storage/compressed air, etc) is dissolving.

    The coal lobby’s goal in flogging out-dated coal-fired power technology is to create a segregated market share that cannot be easily replaced with lower cost alternate fuel sources.

    The Minerals Council of Australia put out a media release in May on this obsolete coal technology claiming:
    “High efficiency, low emission coal-fired electricity generation technology will play a critical role in South-East Asia’s future economic prosperity and climate policies.”

    A report Brendan Pearson, the MCA chief executive, referenced in the above media release [Sylvie Cornot-Gandolfe, ‘The role of coal in Southeast Asia’s power sector’, Oxford Institute of Energy Studies, December 2016] states gas power stations are cheaper, more flexible and have much lower CO2 emissions.

  • brucelee

    Is there a way citizens can push for SO2 /N02 standards to be introduced?

  • raaj

    Please indicate Life cycle cost of all the dealt technologies.. plant life is not factored when half truths or post truths are told

    • Peter F

      Fuel price volatility, water, waste disposal and site rehabilitation are usually not fully costed either

  • john

    An article on this in The Conversation.
    https://theconversation.com/is-clean-coal-power-the-answer-to-australias-emissions-targets-71785?sa=google&sq=ultra+critical+coal+generation&sr=1

    Quote from the last paragraph.
    The only way shifting to ultra-supercritical coal power could meet Australia’s 26-28% climate target is if carbon capture and storage (CCS) were applied.

    Ultra-supercritical coal plants are expected to generate electricity at A$80 per megawatt-hour, according to the Australian Power Generation Technology Report. This is 45% more expensive than the average wholesale cost of electricity for 2015-16. If CCS is added, then the projected cost swells to A$155 per megawatt-hour, nearly three times last year’s wholesale cost of electricity.

    And perhaps this following quote shows the folly of the use of this technology.

    If we then replace the entire 25GW, both black and brown, with ultra-supercritical generation, according to the assumptions included in the Australian Power Generation Technology Report, emissions would total 139 million tonnes. This would represent a 16% reduction in coal emissions, but a mere 5% reduction in Australia’s total emissions in 2013-4.

    So it would appear that the technology is just not going to cut it economically and the much hyped reductions in CO2 emissions are not as fantastic as thought.

    • Peter F

      It is actually worse than you say.
      ISO standard plant efficiency is rated at full load at 15C.
      1. At part load which accounts for 50-60% of running time, coal consumption per delivered MWhr can be 10-20% higher than the sticker number
      2. Due to fundamental Carnot efficiency, At 40C, plants are typically 8-10% below rated efficiency
      3. Daily demand swings (the duck curve) are larger and faster everywhere in the world. In most cases this lowers the CF of coal plants.

      All of these factors lead to higher coal consumption and therefore annual emissions per MWhr so by the time the new USC plant is built, the savings in emissions will be only half to two thirds of the numbers projected above

  • Alan S

    I presume RenewEconomy has no control over it but amongst the ‘We Recommend’ adverts below this article is one from Australian Mining titled ‘Why do we need renewables and coal?’ It predicts a rosy future for coal and promotes Australian HELE coal. BTW that’s a risky acronym as it’s easy to maliciously swap the words Emissions and Efficiency.

    • Alastair Leith

      hella risky 🙂

  • Peter F

    A couple of questions and a statement.
    1. USC plants run higher steam temperatures and therefore probably have higher combustion temperatures and therefore higher NOx emissions. Am I wrong in those assumptions
    2. Particulates come from soot (incomplete combustion of carbon) as well as ash and minerals in the coal stream. I would have thought incomplete combustion is much less likely in gas because it is much easier to burn short chain molecules CH4 than some of the very long chain hydro-carbons, furans and dioxins found in coal so one would assume
    a) there is less soot from gas.
    b) there is no minerals and ash in the gas stream
    Therefore particulates from gas should be much lower than coal, what have I missed

    Statement. Get Australian population to demand that coal plants meet current Chinese standards within 7 years, forget about GHG concerns coal will be forced out of the market

  • Adam Smith

    Great article, shame about the gap in analysis of emissions in obtaining the gas in the first place. How about another analysis that also takes into consideration the emissions for the whole chain of production?

  • Alastair Leith

    The other thing about ultra-supercrtical is that as the efficiency increases, the ability to ramp without damaging the boilers and pipes decreases. As we all know, the Emu-curve is on the horizon and approaching fast. Coal is already in trouble, slower to ramp is even more vulnerable to emu-curve. There’s just no way one of these plants will be in operation long enough to pay investors a return unless govt gives them some kind of operation licence to operate at desired CF and override the merit order bidding system (their beloved market economy for energy that is the dogs breakfast we needed to Finkel with).