February 1, 2012

Colstrip’s Cure for Mercury

By John J. Iovino and Richard LaFlesh, Alstom Power

In January 2012 a new mercury control system at the Colstrip power plant in Montana reached its first major milestone: two years of operation with mercury emissions below the state regulatory limit. The plant uses Alstom’s unique Mer-Cure technology to capture up to 90% of the mercury leaving the stack.

Like many power plants in the U.S., the Colstrip power station in Montana—owned by PPL Montana LLC (a subsidiary of PPL Generation LLC), Puget Sound Energy Inc., Portland General Electric Co., Avista Corp., PacifiCorp, and NorthWestern Energy LLC—has had to comply with new legislation that sets limits for mercury (Hg) emissions. When the state adopted the Montana Mercury Rule in 2006, annual mercury emissions were limited to no more than 0.9 pounds of mercury per trillion Btu (lb/TBtu) of heat input for units firing subbituminous coal. Faced with having to capture up to 90% of mercury in the coal, the plant owners contracted Alstom Power to test and install its Mer-Cure technology on all four of the station’s units.

As of January 2012, the new system achieved a significant milestone: two years of operation with rolling 12-month average mercury emissions below the state regulatory limit.

Varying Amounts of Mercury

The Colstrip power plant, located east of Billings, is the second-largest coal-fired project west of the Mississippi. It comprises four coal-fired generating units capable of producing close to 2,100 MW. Units 1 and 2 began commercial operation in 1975 and 1976; Units 3 and 4 started in 1984 and 1986. Units 1 and 2 both have about 310 MW generating capacity. Units 3 and 4 have about 740 MWe net of generating capacity each (Figure 1).

1. High plains plant. The Colstrip plant is the second-largest coal-fired facility west of the Mississippi. It burns one rail car’s worth of coal every five minutes. Courtesy: PPL Corp.

The plant sits next to the Western Energy Co. Rosebud mine, a 25,000-acre surface mine complex in the northern Powder River Basin (PRB) near Colstrip, Mont., and the Northern Cheyenne Indian Reservation. Rosebud is a large operation with three active pits; it supplies almost all of its current production to Colstrip.

Although coal chemical composition is generally consistent from this expansive mine, coal mercury content varies substantially. Units 3 and 4 can go from 5 to 10 lb/TBtu Hg.