Stations share FGD facilities to avoid capital expenditures
Station Two is a generating station owned by Henderson Municipal Power and Light (HMPL) and operated by Big Rivers Electric Corp. (BREC). The station is located 20 miles southeast of Henderson, Ky., and includes two coal-fired units totaling 337 MW. HMPL began a flue gas desulfurization (FGD) retrofit project in the fall of 1992 as part of a compliance plan to meet the Phase I SO2 requirements of Title IV of the Clean Air Act Amendments of 1990.
The Station Two plant site is comprised of six units built over a 28-year period. The two-unit Green Station, one unit Reid Station and one gas turbine generating unit also share the site. These units are described in Table 1. The two units for Station Two are 167 MW each and are coal fired. The units are forced draft and the flue gas for each unit passes through an electrostatic precipitator before discharge through a common chimney.
The two units for the Green Station are also coal fired and equipped with magnesium promoted wet lime FGD systems, including barge unloading facilities for the lime, lime storage silos and paste slakers. The waste is mixed with fly ash and trucked off site for disposal. Of the units owned by BREC, three units already had FGD systems. Neither of HMPL`s two stations had any post-combustion SO2 control. As a result, the 485 MW unit owned by BREC and the 337 MW unit owned by HMPL fell under the Phase I requirements for SO2. The decision to add FGD systems to the two units at Station Two was the result of Acid Rain Compliance Studies conducted during 1990 and 1991.
The project entered active development in October 1991 with the selection of Burns & McDonnell to engineer the project. A process selection study was performed to select the wet FGD process. The magnesium-promoted wet lime process was selected primarily for economic reasons. Since the same system was already in operation at Green, low cost sources were in place for lime supply and waste disposal. Also, a considerable capital investment could be avoid by sharing existing facilities.
The FGD system contract was awarded on Feb. 6, 1993, allowing HMPL to submit a timely application to the Environmental Protection Agency and qualify for Phase I bonus and extension allowances. In April 1993, a deal was consummated to sell a portion of Station Two`s Phase I SO2 allowances, enabling the FGD systems to be installed without issuing bonds for financing. The proceeds from the sale of the allowances covered approximately 70 percent of the capital cost of the new FGD systems, with the balance of the cost paid from existing revenue funds and cash flow.
The FGD system installed at Station Two is a magnesium-promoted wet lime system provided by Wheelabrator Air Pollution Control. The basic design parameters are shown in Table 2. One absorber module was provided for each of the Station Two units and a single dual flue chimney was added to exhaust the wet flue gas from the modules. The existing lime preparation, dewatering and waste handling systems for the Green Station were expanded to handle the increased capacity for Station Two. Each absorber module is a tray tower, constructed of 317LMN stainless steel material. Two recycle pumps are provided for each tower with each pump dedicated to a spray header located above the tray. Each recycle pump and spray header are sized for 100 percent of the design slurry recirculation flow, so there is a 100 percent redundancy in the spray system. The design liquid-to-gas ratio of 30 gallons/1,000 cubic feet was selected to achieve 95 percent SO2 removal with a tray tower.
Both units of Station Two are forced draft. The flue gas of each unit passes through an ESP before being discharged through a common chimney. Dampers were added to divert the flue gas to the FGD system before it enters the existing chimney. One booster fan is provided on each unit upstream of the absorber module. The same magnesium-lime reagent is used for both the Green Station and Station Two. The barge unloading, handling and storage facilities were in place for Green Station and had capacity to serve both stations. The four existing silos provided adequate lime storage. The discharge systems of two silos were modified to supply lime to either station`s lime preparation system.
The Station Two lime preparation system adds three six-tons per hour detention slakers, two operating and one spare. The lime slurry is stored in an additive hold tank with 10 hours of storage capacity. The slurry is pumped from the lime preparation area through about 1,500 feet of pipe to the Station Two absorbers. Two 100 percent slurry pumps and piping systems are used. The dewatering and waste handling system for Green Station was expanded to process the material from the Station Two FGD systems. The primary dewatering of the waste from the Green Station FGD system was accomplished with four thickeners.
These thickeners have the capacity to handle the extra waste material. However, the magnesium oxide content of the lime has been limited to about 3 percent since the dewatering characteristics of the waste have been shown to deteriorate as the magnesium content goes up. Three existing 50 percent vacuum filters provide the secondary dewatering. The vacuum filters were not operated full time, and no additional capacity was required to handle Station Two. Additional piping was added to handle the increased flow between the thickeners and dewatering area. After mixing with fly ash and lime, the sludge is stock piled on the plant site before trucking off site for disposal. HMPL reports that the FGD system installation is complete and running smoothly, with the shared facilities saving “at least” $3 million to $4 million.
By Ann Chambers, Associate Editor