Amended Silicate vs Activated Carbon

Activated carbon removes mercury from flue gas by adsorbing mercury, which means that the mercury adsorbed also desorbs.  This is true whether or not the activate carbon is brominated or not.

For this reason, activated carbon is limited to low utilization, and mercury capture from the flue gas stops when the desorption rate reaches the adsorption rate.

More importantly, when fly ash is disposed of in waste ponds or landfills, especially if not disposed of in lined and monitored landfills, the mercury will continue to leach from the carbon in the fly ash until the mercury level in the ash is as clean as the groundwater, thus releasing all of the mercury captured into local water ways.

This is a significant issue, because the only significant pathway whereby mercury can impact the environment and human health is through depositing in the sediments under shallow water ways, where it is converted by micro-organisms into an extremely toxic form of mercury, methyl mercury.

When that happens, the methyl mercury can readily bioaccumulate up the food chain into the fish we eat, particularly in predator fish like tuna.

Consequently, Activated Carbon has the Following Issues:

  • Activated carbon (even if brominated) is limited to low utilization, typically < 0.2%
  • Mercury captured by activated carbon will leach in ponds and landfills, thus
  • Making the impact of mercury on health and environment worse than not capturing it.
  • The U.S. Department of Energy has published that Hg leaches from fly ash, with or without activated carbon present.
  • Activated carbon generally prevents fly ash being used in concrete, because of the negative impact the carbon has on the concrete properties.
  • However, when ash with activated carbon and mercury has been used in concrete, studies have shown that the mercury leaches from the concrete structures, such as buildings and bridges.

Amended Silicates on the other hand, reactively capture mercury in the form of mercury sulfide, which is the most stable form of mercury in the environment.  Once Amended Silicates capture mercury in the form of mercury sulfide (HgS), it will not leach or release from the Amended Silicate particles.

Because of this difference:

  • Amended Silicates are able to obtain very high sorbent utilizations, above 95% and even near 100% product utilization in systems that allow for long contact times, such as dry scrubbers and circulating dry scrubbers.
  • Amended Silicates do not need the mercury to be oxidized in order to capture it, as activated carbon and wet scrubbers require. Therefore, halides are not needed.
  • Amended Silicates render the mercury captured in the HgS form completely stable, and the fly ash suitable for landfill, road filler, for use in concrete, or other uses, and
  • Mercury will NOT leach from the Amended Silicates in fly ash used in concrete to build buildings, bridges, schools, libraries, or other structures.