Both suppliers and purchasers are dealing with a number of remaining uncertainties not only in the U.S. but in Europe and Asia where control activity is beginning.
The main regulatory driver in the U.S., The Clean Air Mercury Rule (CAMR), is very likely to be struck down by the Courts. It probably will be replaced with a tougher standard, but a slow regulatory process could mean delays for the market. On the other hand, nearly 50 percent of the power plants will be regulated by tougher State standards.
European power plants emit twice the amount of mercury as U.S. plants. Additional controls will probably be mandated, but the timing is unpredictable. Chinese power plants emit four times as much mercury as those in the U.S. The timeline for control in China is still a mystery.
The race horse out of the gate first and running hard is activated carbon injection (ACI) technology. One system is in operation at We Energies and more than 40 more systems are in some form of design or construction. If this technology were to become the leading selection by power plants, the demand for carbon would substantially exceed the current U.S. production (for all uses).
Continuous accurate measurement of mercury emissions is essential to control. Initial field results with a relatively small number of monitors are promising, but the base of experience is small. Methods for determining the relative accuracy of the monitors are still being finalized.
Remaining uncertainties include many site specific factors. But they also include the broader questions of efficiency requirements, byproduct acceptability, and technology alternatives.
Relative to site specific factors, the first is coal type. Low sulfur and high sulfur coal present two distinctly different challenges. Dry scrubbers are generally used with low sulfur coal generators. Activated carbon can be easily added ahead of these scrubbers.
High sulfur coal generators generally incorporate wet scrubbers. Many of these same units also control NOx with SCR. For these plants, all that may be needed is a halogen additive ahead of the SCR.
One big uncertainty has to do with greenhouse gases. Europe has made huge strides in greenhouse gas reduction by replacing 10-20 percent of the coal at each generator with biomass. If stringent reduction requirements were implemented in the U.S., then biomass co-firing would be likely. This could substantially change the strategy used for mercury reduction.
Multi-pollutant considerations could also alter the mercury reduction strategy. There has been pressure to require new plants to reduce SO2 by 98 percent. If this evolved, then many new coal-fired boilers (even those burning low sulfur coal) would be forced to select wet rather than dry scrubbers. This would then dictate halogen additives at the SCR rather than activated carbon.
The opposite result could occur if high efficiency SO3 reduction is required. Dry scrubbers do a better job than wet scrubbers in capturing SO3.
There are many potentially attractive removal technologies. In addition to the activated carbon and halogen compounds, there are other additives. Another alternative is a catalytic reactor to oxidize the mercury. Still another process uses gold amalgamation and regeneration. Municipal incinerators in Europe use chloride pre-scrubbers. So this is also an alternative.
Despite all the uncertainties the zig zag progress is forward. However, decision makers need to keep up with each new zig and zag. Power Plant Air Quality Decisions will help them do this.