Over the past three years, mercury emissions from the cement industry have seen greater scrutiny by local and federal regulators throughout North America. Currently, most plants in the US fall below existing guidelines for mercury emissions. However, process changes such as the substitution of alternative fuels or possible new government regulations may result in more plants seeking solutions for mercury mitigation.
The easiest way to reduce mercury emissions is to avoid introducing mercury into the cement kiln. This usually means the use of fuels and raw materials with limited mercury content. However, this solution is not always economically feasible. Traditionally, producers could choose from three different technologies: the wasting of main filter dust, the use of wet scrubbers, or the injection and collection of activated carbon (ACI). All of these methods have drawbacks. Wasting of filter dust and the use of wet scrubbers can provide for very low mercury removal rates. Wet scrubbers and activated carbon systems can be very expensive to install and operate.
Targeted investments in R&D
Recognising the market’s lack of attractive options for mercury reduction, FLSmidth focused its development efforts. The goal of these efforts was to create a system that not only reduces mercury emissions at similar or better rates than existing methods, but also minimises installation and operating costs. These efforts were successful; FLSmidth is proud to announce a new patent-pending process, the Mercury Roaster.
How it works
In the raw mill system, dust captured in the main baghouse acts as a natural adsorbent for mercury. This mercury enriched dust that is captured in the main baghouse is taken to the new mercury roaster process for cleaning before the dust is returned back to the system. The figure illustrates an example of a mercury roaster installation. The baghouse dust is fed to a roasting system which uses a heat source (for example kiln bypass gas, cooler vent gas, or hot gas generator) to heat the dust above the boiling point of mercury compounds. While the mercury is still in the gas phase, the gas stream enters a hot electrostatic precipitator which removes most of the cleaned dust. This dust is taken back to the blending silo to be part of the kiln feed. After the electrostatic precipitator, the gas stream is cooled below the mercury boiling point so that the mercury can condense on the dust particles that were not captured in the electrostatic precipitator. An adsorbent may also be added to the gas stream here, as needed, to help capture the mercury. The cleaned gas after the baghouse is vented to the atmosphere and the mercury enriched dust/adsorbent collected in the baghouse can be transported to the finish mill area to be added to the cement. In some cases, the baghouse dust may need to be removed as a waste stream.
Proven performance, minimised total cost of ownership
Through extensive laboratory testing, the Mercury Roaster has successfully demonstrated its ability to reduce mercury emissions. In addition, because the air and sorbent flows are significantly smaller than would be seen with a full ACI system, the operating costs are significantly reduced. Finally, the Mercury Roaster requires a much smaller installation footprint than competing technologies and can be applied even in applications that cannot accommodate a large baghouse.
