While, currently, the use of activated carbon in this industry has been less than 50 million pounds per year (compared to 500 million pounds per year in water treatment), this has sharply increased in the past two years as a number of utilities have been required to meet MACT rules promulgated by individual states. When the federal MACT rule takes effect, the utility air market alone will exceed the water market, McIlvaine suggests.
The industry has anticipated this demand, with traditional supplier, Calgon Carbon and Norit expanding capacity and Albemarle acquiring Sorbent Technologies, positioning itself as a major supplier. ADA-ES has entered the market with the construction of a large activated carbon plant.
However, McIlvaine stresses that there is considerable uncertainty about the ultimate size of the market. There is attractive potential for activated carbon in related applications. Presently, selenium is being captured in power plant scrubbers. It is subsequently removed from the wastewater with an expensive biological wastewater treatment system. Plants with scrubbers would not normally consider activated carbon purchases. However, the removal of the selenium may warrant the purchase.
Another use in scrubbers is to recirculate activated carbon in the scrubber slurry. Evonik has pioneered this approach as a means of removing the mercury from the wastewater sludge. The mercury-laden carbon is separated from the wastewater using hydrocyclones. This allows disposal of the sludge as a non-hazardous material.
There is also uncertainty, as other chemicals will take market share away from activated carbon. There are some sorbents which are injected in a manner identical to activated carbon. The purpose is to adsorb the mercury by presenting a large surface area. So far none of these has proven to be more cost effective.
The bigger threat comes from chemicals injected with coal for the purpose of chemical change rather than adsorption. Oxides of mercury are much more soluble than elemental mercury. Halogen compounds including bromine and chlorine can oxidise most of the mercury in the flue gas. The result is that scrubbers can then efficiently remove the oxides. The uncertainty arises relative to how efficient this removal process may be.
There are also wide variations between fuels and boiler types and the many factors which influence efficiency of removal. There is further uncertainty as to the influence of the multi-pollutant requirement impact on mercury removal choices. The necessity to remove hydrogen chloride and toxic metals other than mercury is a factor.
As a result, McIlvaine states that the PM2.5 ambient air quality rules may have the largest impact on mercury reduction choices, potentially forcing scrubbers to achieve 98% or even 99% SO2 removal. This would dictate the use of wet rather than dry scrubbers and chemical oxidation rather than adsorption.
The impact of PM2.5 is not likely to manifest itself until 2016 or 2017 and utilities are likely to take a two-stage approach. According to McIlvaine, many companies may ultimately remove mercury first with activated carbon and later with chemical oxidation and scrubbing.
