– when the data fit the model, and mercury emissions come for free
Mercury is a menace. It is an environmental liability and global emissions of this toxic metal continue to increase.
(Several reports on mercury have been produced by the IEA Clean Coal Centre and all are available as free, one-click downloads from our library.)
Mercury behaviour in coal-fired power plants is complex. It can be modelled and predicted, to some extent, but low concentrations and complex measurement challenges mean that data gathered in the field are often … well, different to what one might expect. Mass balances, the complete tracking of mercury from coal through to emissions and waste materials, rarely actually balance in practice. Measured concentrations will imply that either mercury has vanished into thin air, or even more weirdly, has appeared out of nowhere. This is generally accepted as something we just have to deal with – statistical variance and uncertainty.
And so there are often generalisations made about mercury behaviour to help those working with coal plants to make expert judgments on the best methods to reduce emissions. These are included within the BAT/BEP (best available technology, best environmental practice) guidance documents which support the Minamata Convention, the global legally binding instrument ratified in 2017 designed to control and reduce mercury emissions to the atmosphere from all sources. For example, it is accepted that plants firing low grade coals are likely to release more mercury than plants firing higher grade coals. Similarly, plants installed with control systems for sulphate and nitrate control (flue gas desulphurisation systems and selective catalytic reduction), are likely to report emissions up to 80 or even 90% lower than plants without these devices. Plants with electrostatic precipitators (ESPs) only particulate control systems such as baghouses/fabric filters, would be expected to control mercury emissions by around 30-70% (usually at the lower end of this range) depending on plant and coal parameters. Plants with ESPs would be expected to achieve significantly lower mercury emissions, commonly well under 40%, and would only achieve up to 60% control under very rare conditions.
And so when our hosts in Bogota reported that their ESPs were controlling 90% of the mercury emissions from TermoPaipa unit 4, we smiled politely and assumed that their data were wrong. 90% mercury control with an ESP alone? That was clearly either bad data or black magic!
But we visited the plant, looked at the coal data (produced in a separate project with Alan Kolker of the US Geological Survey) along with the data for the waste streams and the stack. The coal mercury content was low, but not uniquely so. But the stack emissions were at or below 1 microgram/m3 – a value which would be in compliance with the most challenging of global emission limits. Most plants in the USA and the EU can only dream of reaching a mercury emission value of 1 microgram/m3 without spending significant money on mercury control measures. And we couldn’t for the life of us understand why this plant was achieving such low emissions “for free”. So we sat down and literally plugged the numbers available to us – coal, boiler and other plant operating conditions, all the data we could get our hands on – into the iPOG, the mercury emission calculation tool produced by Niksa and Associates in the USA on behalf of IEA CCC and the UN Coal Partnership. The iPOG tool is an excel programme based on data from over 1,000 data sets from actual plant measurements in the USA, India, Russia, South Africa and China. It is available as a free download from the UN Coal Partnership website.
And lo and behold – the data matched the model! A Christmas modelling miracle. A plant fitted only with an ESP was indeed achieving fantastically low mercury emissions to air. And the reason why this coal/plant configuration achieved such a feat? – A combination of a very clean coal and a boiler running just a little less efficiently than it should. Mercury in Colombian coal is low. Chlorine is moderate – helping to oxidise the mercury to make it “sticky” enough to attach to any fly ash materials which have some form of sorbent characteristics … unburned carbon being the ideal surface. And so, a boiler running at a slightly lower efficiency than perhaps it should, producing 10-12% LOI (loss on ignition/unburned carbon) provides inherent sorbent, perfectly placed to capture the now oxidised mercury within the fly ash. Voila! 90% of the mercury is captured. Consistently.
This is excellent news for Colombia, a country which produces and sells a lot of high quality coal worldwide. And, although Colombia is largely powered by hydro plants, it is possible that coal use could increase in future to provide baseload to a growing population where hydro potential has peaked.
HT/Crispin in Waterloo