Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2018-11-15.
Over recent decades mercury has gone from a widely used commodity — in thermometers and herbicides among many other products — to a substance whose name alone suggests environmental and health risk, with many uses phased out and the remainder tightly regulated. That includes emissions to air from energy production, especially coal-fired power plants, and from hazardous waste incineration, where plant-specific limits usually require flue gas monitoring. Elemental mercury and mercury compounds are toxic, with both chronic and acute risks, and exposure has to be minimised. Volcanoes are a major natural source, but man-made emissions can at least be limited: coal-fired plants are the largest potential source because the fuel contains mercury compounds, with gold production, smelters and hazardous waste incineration also significant. Elemental mercury has very favourable optical properties, so Hg⁰ can be measured to very low levels — but a substantial share of the mercury leaving a combustion process is bound in organic or inorganic compounds such as mercuric chloride, mercury sulphide or methylmercury, and an Hg⁰ monitor cannot see those. OPSIS therefore draws flue gas through a converter that releases all bound mercury as Hg⁰; a DOAS analyser then measures it, and that reading is the total mercury concentration. The same system can be configured for NO, NO₂, SO₂, CO, CO₂, NH₃, H₂O, HF and HCl.
In China mercury and its compounds are a priority for heavy-metal control, with emission limits for coal-fired boilers, cement kilns, waste incineration and non-ferrous smelting, and continuous mercury monitoring required where the rules apply — check the current standard for the specific duty, because both the limit and the monitoring requirement differ by industry. The measurement chain deserves as much attention as the analyser. Sampling must be heated and inert enough to avoid mercury loss to the walls; the converter must reduce bound mercury reliably and be checked as part of maintenance; and calibration needs a traceable mercury source rather than a surrogate. Because the limit refers to total mercury, a system reporting only Hg⁰ will read low whenever oxidised mercury is present — which is precisely the situation in plants with high chlorine, and in cement kilns the mercury recirculation makes it worse. Where the same analyser also reports NO, SO₂, CO, NH₃, HCl and water vapour, the mercury channel comes at a modest marginal cost.
Continuous Emissions Monitoring of Total Mercury — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2018-11-15. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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