Sorbent supported dust shuttling
In cases where dust shuttling is technically restricted, the guideline mentions that it may be supported by the injection of sorbents into the gas stream upstream of a particulate matter control system. Their high surface area or specific chemical properties may increase the rate of mercury bound to particles. Activated carbon, for example, is commonly injected in power plants or waste incineration plants. However, in the cement industry, the technology is applied in very few plants and so far only in order to limit peak emissions in mill-off operation. As shuttled dust is often used as an additive in the cement mill, it has to be ensured that the cement quality is not harmed by the sorbent.
Oxidised mercury is adsorbed on dust and sorbents to a higher extent than elemental mercury, thus oxidising agents such as bromine or sulfur can even further enhance the sorbents’ adsorption capacity. However, if shuttled dust is used in the cement mill, again a possible impact on the product quality needs to be considered. Sorbent injection in the clean gas would require an additional polishing filter, making this technology unacceptable costly.
Based on published results from lab-scale tests, alternative concepts to remove mercury from the process have been suggested recently by technology suppliers using a separate thermal treatment of precipitated dust in a separate installation. Mercury is re-volatilised and subsequently separated again by the use of sorbents. The scale of this installation would be smaller than a separate tail-end activated carbon injection system with a separate polishing filter for the entire exhaust gas stream. These techniques are not yet applied on an industrial level today. Therefore, because of the lack of experience these technologies are far from being seen as BAT/BEP.
All measures suggested have to be seen in the light of the efforts to reduce mercury emissions on a global scale. Existing measures show that the cement industry already has proven means at hand to cope with the challenges faced. Beyond this it is constantly developing and investigating techniques to achieve sustainable mercury abatement.
This is part three 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 one here and part two here.
Read the article online at: https://www.worldcement.com/europe-cis/13042015/challenging-mercury-emission-limits-and-measures-673/