July 1, 2010

ReACT Reduces Emissions and Water Use

By H. James Peters, Hamon Research-Cottrell Inc.

Carbon Use and Disposal

In ReACT, granular activated coke is repeatedly recycled between the adsorption tower and the regeneration tower. The size of activated coke is gradually reduced over time from mechanical wear. Fines are separated prior to returning the activated coke to the adsorber. In addition, reactions in the regenerator use some carbon, and a make-up stream of activated coke replaces these losses. Operating results show that the activated coke supply rate is less than 1.5% of the circulating rate of activated coke. Because fines removal and other carbon losses are offset by make-up, the size of the activated carbon in the system reaches a stable long-term distribution.

Unlike other catalysts—which tend to deteriorate over time with activity loss due to aging, fouling, and contamination—the performance of the activated coke increases over time. Activity is improved through regeneration, which exposes fresh surface and micropores through carbon reactions and increases the number of functional groups on the catalyst surface from residual sulfur, oxygen, and nitrogen species. Regeneration of sorbent greatly reduces site logistics for reagent make-up processing and waste handling compared with other FGD processes.

Compare Performance to ACI Systems

A comparison of the carbon make-up and disposal requirements for a typical 250-MW plant using ReACT compared to wFGD limestone and to typical activated carbon injection (ACI) for Hg control illustrates the differences in incoming reagent quantities and mercury-laden material disposal. Consider the following comparison, which uses these assumptions:

• Basis: 250-MW PC boiler burning low-sulfur Powder River Basin coal, 75% capacity factor
• Firing rate: 2,800 million Btu/hr (at 11,000 Btu/kWh)
• Flue gas flow: ~1,000,000 acfm @ 300F
• Fly ash load: 2 to 3 gr/acf (55,000 tons/year @ 2 gr/acf)
• Hg load: 5 to 10 lb/1,012 TBtu

Limestone wFGD, which addresses primarily SO₂ with some Hg co-benefits, for this typical plant would require about 15,000 tons/year incoming limestone and create approximately 25,000 of dewatered gypsum as a disposal stream.

For the ReACT process, fresh make-up activated coke replaces losses due to fines separation after regeneration and carbon consumed in regenerator reactions. Make-up for a 250-MW plant is approximately 1,250 tons/year (4 to 6 tons/year per MW), which addresses SO_x, NO_x and Hg control. Fines separation of the same 1,250 tons/year is in the form of mercury-free carbon, which can be burned for fuel value or may have beneficial reuse as a carbon sorbent at other sites.

For typical ACI systems that address Hg only, 90% mercury control requires injection of powdered activated carbon (PAC) into the flue gas at rates between 2 lb/million acfm and 10 lb/million acfm. The higher rates are generally associated with injection upstream of an electrostatic precipitator, and lower rates are associated with injection upstream of a fabric filter. Injection rates have some sensitivity to fuel type, boiler type, and the effects of increased SO₃from an upstream SCR.

When PAC is injected upstream of the primary particulate control, whether it be an ESP or a FF, the fly ash is contaminated with mercury-laden carbon, which may render otherwise saleable fly ash for cement or road material into landfill waste or pose future questions as to waste disposal.

If activated carbon is injected into a secondary FF placed downstream of an ESP (which may be reduced to control the PAC/ash mixture at the FF to about 10% carbon), activated carbon usage can be minimized and fly ash beneficial use maintained, though with additional equipment and energy requirements. In this case, the disposal stream would be in the range of 6,000 tons/year for a mercury-laden carbon/fly ash mixture.

For a ReACT system serving a typical 250-MW coal-fired boiler, the activated coke from the mercury capture zone in the regenerator weighs about 30 tons and has a disposal rate less than 15 tons/year.