3-28-08

March 27, 2008

SO₃Capture is Proven and Economical

We have gone from the “confusion and desperation stage” with acidified towns to the “multiple solution option” stage. The availability of a number of commercial systems was clearly evident in the March 27 Hot Topic Hour on SO₃. Over 70 phones and many more people were connected to experts from the supply and A/E sectors, as well as a number of utilities.

John Boyle of Fuel Tech described SO₃ reduction systems applied at two 600 MW units at Santee Cooper. This system uses magnesium hydroxide with patented features for chemical distribution, particle size and feed rate. Not only was the SO₃and opacity reduced but there was a 20 percent reduction in the total toxic release. The Mg(OH)₂ also decreased ash fusion problems. The result was an increase in boiler efficiency and a heat rate improvement of 120 Btu/kWh. There was also a reduction in popcorn ash fouling of the SCR.

Jonathan Norman of O’Brien & Gere with co-presenter Ben D’Alessio of Nol-Tec provided a detailed description of the dry sorbent injection system and the critical components which ensure that the material is delivered, stored and injected to maximize efficiency. Specific types of valves and feeders are necessary to achieve metering accuracy and reliability. Fifteen systems are installed or being installed. Twelve utilize lime while three use Trona.

Sterling Gray of URS discussed the merits of SBS which injects clear sodium liquor into the network either before or after the airheater. In either case high efficiency is achieved. If you inject it after the air heater, there can also be an ESP efficiency improvement. If it is injected before the airheater, it can improve heat recovery by allowing a lower gas outlet temperature without fouling. By capturing the SO₃, activated carbon can capture more of the mercury. The latest installation slated for June 2008 startup at DP&L Stuart 1-4 will utilize injection at the economizer outlet. The system is being considered for new plants in competition with wet precipitators due to its high efficiency (greater than 95 percent) and low outlet guarantees (0.005 lbs H₂SO₄/MMBtu).

Mike Schantz of Chemical Lime made it clear that there is a big variation in performance of various hydrated limes. Particle size, surface area, and porosity are all important. Injection at two points (after the airheater and after the precipitator) has performance advantages. Injection after the precipitator eliminates any problems with increasing ash resistivity and allows excess lime to be used in the scrubber for SO₂ control. The caveat is the potential for increasing particulate emissions (2-3 micron sized lime particles). A number of systems using Chemical Lime hydrate are operating in the U.S. The parent company L’Hoist has done considerable research on improving hydrated product and supplies it to the European market as well.

Eric Van Rens of Mississippi Lime admitted that demand for hydrated lime is high and supplies are limited but that with expansions in Ste. Genevieve, MO and Weirton, WA there is new supply available. The company has experience with injection both ahead of and after the ESP. Flyash sales are not impacted with the lime. When injected after the ESP in full scale installations, the particulate performance of the scrubber was within the design limits. However this has been a function of scrubber design. Lime is readily available and is less costly than sodium sorbents. Mercury removal improvements are seen with and without activated carbon.

Buzz Reynolds of Siemens cited the multi-pollutant control advantage of the wet precipitator. Not only will it capture 90 percent or more of the acid mist, it will also reduce PM_2.5, mercury and other toxic metals. The Siemens WESP is being installed at Elm Rd (Oak Creek), Dallman Unit 4, Trimble County Unit 2, Thoroughbred (on hold) and Prairie State. In all of the above installations the WESP is the last piece of control equipment. Another option is to replace the last section of the dry ESP with a WESP. This would be a lower cost alternative to a new fabric filter.

A new presentation by Marsulex is now included in the FGD Decision Tree but was not covered in the recorded discussion. CleanStack relies on the injection of micronized particles upstream of the air preheater. Limestone was the choice because of its low cost and other properties. The Dominion Energy Chesterfield 6 unit with a fabric filter will be in service by June 2008. So by next summer or fall there should be results. The requirement is to reduce SO₃ concentrations to less than 7 ppm to avoid back-end corrosion, damage to fabric filters, and visible stack emissions. The fine particle size will be achieved with an attritor in the injection system. With interest in the delivery of finely ground limestone on a regional basis, it would seem that there is the potential to reduce capital cost by buying limestone already ground to size.

The power point presentations can be directly accessed in the FGD Decision Tree.

John Boyle – Fuel Tech

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