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, writes Lauri Myllyvirta of Greenpeace East Asia. Courtesy Reneweconomy.com
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).
Moral of the story: Emissions regulation matters a lot, whether a plant is ultra-supercritical matters little
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.
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.
I hope you’re not too shocked that coal advocates are not mainly motivated by health concerns
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.
Coal and gas plants
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:
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.
Lauri Myllyvirta works for Greenpeace East Asia. This article was first published on Reneweconomy.com and is republished here with permission.
Nigel West says
Australian coal is low sulphur content so flue gas desulphurisation might not be justified.
UK may need to build new coal plants. UK’s huge offshore Rough gas storage facility is being closed. UK is becoming over dependent on gas for power generation and heating. Coal stored at a power station would improve UK energy security considerably.
New nuclear stations are taking too long to build too and costs are borderline affordable for the UK. New coal stations would be a good substitute as plants can be built in under 5 years and cost effective without CCS.
Karel Beckman says
Ever heard of UK climate policy? Or you just don’t give a hoot?
Nigel West says
I acknowledge, even new ultra critical coal with FGD would produce more carbon/MWh than a gas fired plant. However UK energy policy requires clean, secure and affordable supplies. Security of energy supplies ultimately is more important than climate policy.
Problem is UK heating is predominantly gas and around half the electricity produced in the winter is generated burning gas. Rough storage has gone, old coal stations are closing and a replacement for Rough is not likely. Without storage the UK will have to buy more LNG during the winter when prices are likely to be higher so not good for affordability. Drax is looking at converting 2GW of coal capacity to gas firing. Will be expensive and low efficiency. They should maintain coal firing too.
Perhaps the UK could land LNG in the summer and pipe it to Germany for storage where they have vast capacity. Would help meet winter demand in the UK.
Jan Veselý says
British houses are notoriously leaky. They should stop to heat up the air outdoors first. The insulation is cutting the gas consumption by several percent per year in my home country.
And of course, heat pumps and solar thermal should become the preferred form of heating. When combined with CCGT plants it makes at least 150% of heat energy than present in natural gas itself.
Bob Wallace says
Exactly. Cut useage first.
” Without storage the UK will have to buy more LNG during the winter ”
Build more storage. Why put the UK in the position of having to buy LNG or make climate (and air pollution) problems worse by burning more coal.
With a fossil fuel plant you have to keep feeding it fuel and that is an unnecessary expense these days.
There’s no need to look first to fossil fuels for your answer. The UK has massive amounts of wind resources and no shortage of places to install storage. Clean, cheap and safe.
Nigel West says
“no shortage of places to install storage.”
That statement is not factually correct if you are not referring to storing gas. Even if >3TWh of PHS or battery storage were feasible it would be too expensive.
Bob Wallace says
PuHS + wind/solar is cheaper than nuclear.
The UK has plenty of places to install PuHS.
Nigel West says
That has been looked at by a former UK Government Chief Scientific Advisor – the UK does not have plenty of places.
Storage and wind/solar (plus epic transmission reinforcement) would not be cheaper than nuclear. Overbuilding wind would halve the CF so doubling the cost of offshore wind – onshore not being acceptable for a small island to accommodate.
Bob Wallace says
McKay made a point of proving renewable energy could not work.
He belongs in the same basket as Tom Murphy and Euan Means. And all the other people who stretch and create facts in order to boost nuclear energy.
The UK has thousands of abandoned mines and rock quarries. Many of those can be fine places to build PuHS.
Closed loop PuHS needs only a change in altitude over a short area. We call those hills.
“Overbuilding wind would halve the CF so doubling the cost of offshore wind – onshore not being acceptable for a small island to accommodate”
Overbuilding would mean that some potential electricity would not be used. But overbuilding in many cases will be cheaper than building storage or nuclear.
I’ve been working with a model which looks at powering ERCOT with nothing but onshore wind from the Panhandle region. (Meaning that all turbines are operating in roughly the same wind conditions at the same time.)
Overbuilding 300% greatly lowers the amount of storage needed to make a 100% Panhandle wind 24/365 grid while increasing the cost of wind-electricity from $0.02/kWh to only $0.06/kWh. We have no way to store electricity for four cents.
CO2 stays in the atmosphere during hundreds and thousands of year so a coal powerplant and a natural gas powerplant have strictly the same effect on climate, it’s just that one is eating the carbon budget a little bit slower than the other. An old coal powerplant sets to retire in 10 years is more climate friendly than a brand new CCGT that will stay 40 years. Only fossil fuel with CCS are considered as low carbon technology.
Also in terms of emission a coal power plant connected to a district heating is emitting less than a CCGT with gas boilers.
Nigel West says
Fine, but security of UK power supplies comes first. Growing dependency on gas undermines security.