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SO3 poisoning of PAC can be countered by duct sorbent injection of hydrated lime [Ca(OH)2] upstream of the ESP or first fabric filter. Lime sorbent removes SO3 to improve mercury removal and lower PAC feed requirements. Sorbent suppliers also are developing SO3-tolerant materials. Some of these chemically prevent SO3 from preferentially adsorbing on the carbon, leaving the pores active for mercury capture.
PAC feed locations, feed rates and collection
Excluding the influence of poisons, the two factors that have the most impact on mercury removal efficiency are residence time and temperature. Very little mercury removal occurs above 450°F with non-brominated PACs. The adsorption reaction efficiency increases as temperature decreases, with adequate reaction rates from about 350°F and below. This is the temperature between the air heater outlet and the particulate collection device inlet. Thus, if PAC is fed at this point, the residence time might be too short. It might be necessary to feed carbon ahead
The injection point might be significantly influenced by the particulate collection device. If it is a baghouse, the activated carbon will lodge on the bags and continue to remove mercury as the flue gas passes by. For an ESP, the reaction must essentially be complete by the time the flue gas reaches the precipitator. The criteria by which activated carbon feed rates are evaluated and reported is typically as pounds of sorbent per million acfm (actual cubic feet per minute) of flue gas; a measurement that is usually taken at the air heater outlet regardless of the actual injection location. Optimizing reagent utilization is important to minimize costs, but also has implications for utilities that sell fly ash. Feed rates as low as 1-2 lbs per acfm are possible with baghouse collection, while the rates might be 3x-4x higher with an ESP at the backend.
What about activated carbon effects on flyash?
Many utilities sell at least part of their flyash to the cement industry. Excessive carbon in the ash can ruin its use for this application. PAC interferes with concrete quality by decreasing the air bubbles in concrete that are required for proper freeze/thaw properties. The activated carbon adsorbs the organic air entrainment admixtures, e.g., surfactants, used in concrete production.
More than one possibility exists for solving this problem, albeit sometimes with additional equipment. One potential method is shown in figure 2 on page 23, and goes by the name of TOXECON.
As is evident, the primary particulate removal device removes flyash ahead of activated carbon injection. So, the ash byproduct remains pristine with regard to its application for cement production. A polishing fabric filter device is utilized for carbon capture and disposal.
Some concerns exist with this approach. First and most obvious is the additional capital and operating cost of a second particulate control device. The extra pressure drop of the polishing fabric filter must be overcome by the existing, or possibly new, induced-draft fan. Polishing filter cleaning might be problematic, since the collected particulates are very fine and can be difficult to dislodge during cleanings, although the variety of particle sizes common to a single fabric filter is easier to clean from the bags.
Proper PAC distribution techniques
Good PAC distribution in the flue gas is essential for high mercury removal at the lowest injection rate. Computational Fluid Dynamics (CFD) modeling of critical sections of the flue gas system is necessary to assure proper distribution. CFD modeling is a computer simulation of flue gas and sorbent particle flow and interaction in the ductwork. The model can be used to recommend designs and locations for sorbent injection and mixing patterns of the flue gas both upstream and downstream of the injection point. The model requires operating data for the existing flue gas and duct system, including velocities and temperatures.
What equipment is needed for PAC injection?
The equipment for PAC injection is proven and can usually fit in a small place at the plant site. Truck access for PAC delivery is critical. The following equipment is typically part of a PAC system.
- Storage silo (5-30 days of capacity) with bin vent filter and jib crane
- Air compressor and air fluidization system for the silo
- Air blower for pneumatic conveying of the PAC
- Volumetric or gravimetric feeder from the silo to the eductor
- PAC conveyor piping, manifolds and splitters
- PAC injection lances