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Challenging mercury: emission limits and measures – part two

World Cement,

Read part one here. 

Mercury in the clinker burning process

Extensive investigations have led to a profound understanding of the behaviour of mercury in the cement production process, which is mainly determined by the thermal conditions between the preheater, raw mill and dust precipitator. Mercury and its compounds enter the process through raw materials and fuels, evaporate and partly react with other gas constituents, and due to their high volatility leave the preheater with the hot gas stream (Figure 1).

Figure 1. Behaviour of mercury and its compounds in the clinker burning process.

In raw mill-on operation a significant share of mercury compounds condense on the raw meal. To a smaller extent, elemental mercury is adsorbed on the meal’s surface. Low temperatures and a high dust load favour the adsorption. Adsorbed mercury then evaporates once more when raw meal and precipitated dust enter the preheater again, thus forming a mercury cycle between preheater, dust precipitator and raw mill where the kiln meal silo acts as a buffer (Figure 2).

Figure 2. Mercury cycle balance in a clinker kiln.

Measures for mercury control

Within the process of implementing the Minamata Convention on a UN level, technical guidelines regarding BAT/BEP (Best Environmental Procedures) in relevant industry sectors are currently being developed. These guidelines aim at supporting governments, especially of developing/emerging countries, to later implement the Convention into national legislation. The draft guideline for the cement industry describes all known abatement techniques as well as monitoring/measurement techniques. The cement industry – represented by the Cement Sustainability Initiative (CSI) – is closely following the process. A major concern is that the assessment of the technologies does not sufficiently consider technical availability, industrial experiences and economic aspects. According to the guideline, the first measure is a careful selection of all input materials to the cement production process: while the composition of natural raw materials can hardly be influenced, a proper control of mercury contents in the alternative raw materials and fuels used is recommended.

Dust shuttling

If, in spite of a careful input control, mercury emissions are still an issue, a proven method is to limit the build-up of mercury cycles by the selective shuttling of precipitated dust. This is systematically applied in many cement plants in order to separate mercury from the process.

The efficiency of mercury removal is highest the lower the temperature in the gas stream and the higher the proportion of oxidised mercury. Therefore, in most cases the water or air quenching of the exhaust gas has to be optimised and the temperature has to be reduced to below 140°C. This leads to a significantly higher adsorption of mercury on the dust particles. Accordingly, with a given quantity of dust, more mercury can be removed from the process. However, depending on the gas atmosphere, measures against corrosion have to be considered.

This is part two of a three-part article originally published in Newsletter 1/2015 of the European Cement Research Academy and is reproduced by kind permission of ECRA. Read part three here.

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