Add a New SCR to Further Reduce NOx
In 2006, Hitachi Power Systems America Ltd. (Hitachi) was awarded an engineering, procurement, and construction contract by SECI for a selective catalytic reduction (SCR) system retrofit on both SGS units (Figure 3).
3. Latest air emissions reduction upgrade. Hitachi designed and installed selective catalytic reduction (SCR) systems on both units at SGS. Post-installation testing showed the SCRs exceeded the 90% contract guaranteed NOx removal rate. Courtesy: Hitachi Power Systems America Ltd.
Hitachi’s contract included the design, supply, and erection of the SCR systems to achieve 90% NOx reduction. This goal was achieved through the integration of Hitachi’s process system design with Hitachi’s low SO2 -oxidation, plate-type catalyst. The SCR system also includes:
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An ammonia injection grid/static mixing system to promote thorough mixing of ammonia and NOx prior to entering the SCR.
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Hitachi’s patented reactor hood design.
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A complete air preheater rebuild and induced-draft (ID) fan replacement, including foundation rework.
Ammonia required for the SCR systems is produced on site by a urea-to-ammonia system in which a urea solution is converted to gaseous ammonia. This system safely avoids the onsite storage and handling of ammonia. SECI believed that investing about $25 million in this system would return a "good neighbor" dividend in the form of increased safety.
Integration of the new SCR system with the existing boiler was handled in two phases for each of the two SCR projects. Phase 1, scheduled for a 30-day outage, included installation of the new economizer outlet flues, SCR diverter dampers, and partial replacement of the air heater inlet flues. Temporary blanking plates were installed at the SCR inlet and outlet connections, allowing the unit to safely operate at the conclusion of this phase and making installation of the entire SCR possible prior to Phase 2. Phase 2 included upgrading the air heater rotors, installing the underground utilities, modification of the ID fan concrete foundations, replacement of four ID fans, and commissioning.
The four ID fans, each rated at about 33% of full-load capacity, were replaced because of the pressure drop that SCR added to the gas path. The choreography associated with this upgrade was well planned months in advance.
The upfront fan foundation work started shortly after completion of the first phase and was completed prior to start of the second phase. This initial work included installation of the dowels and rebar in and around the existing motor and fan pedestals. As this work was performed with all fans in service, the bearing vibration levels had to be monitored closely by the operators. Twelve days prior to beginning the second outage, the first fan was taken out of service and the primary fan work started. The existing fan was demolished, modification of the foundation was completed, and the new fan was installed.
Two additional fans were removed from service at the start of the outage and the same process was repeated. The fourth fan remained in service for the initial two days of the second outage and was used to cool down the unit. Afterward, it was taken out of service and replaced using the same sequence as for the first three fans. By end of the second outage, which lasted 30 days, all four fans were ready for operation.
When the dust settled, Phase 1 was completed in 31 days for Unit 1 and 30 days for Unit 2; Phase 2 was completed in 35 and 34 days, meeting or improving upon the original schedule milestones. By utilizing temporary isolation plates and expansion joints placed at strategic locations, the entire SCR, including the inlet and outlet flues, was erected between the end of the first outage and the start of the second outage.
The 2 x 60% load forced-draft fans were also upgraded as part of a post-SCR project because they would have run short of capacity during the summer.
Operation of the gas diversion system is relatively straightforward (Figure 4). Diverter dampers and SCR outlet dampers are installed in the flue gas path and operated together to divert the flue gas through the SCR (SCR mode) or to bypass the flue gas around the SCR (SCR bypass mode). SCR mode is achieved with diverter dampers open toward the SCR and the SCR outlet dampers open. SCR bypass mode is obtained with diverter damper closed toward the SCR and the SCR outlet louver damper closed. NOx monitoring is performed before and after the SCR.
4. Pick your path. The flue gas path of each SCR system is illustrated. During the Phase 1 outage the existing economizer outlet flue was demolished, with the exception of the last 20 feet of the air heater inlet flue, and new diverter dampers and economizer outlet/air heater inlet flues were installed. Source: Hitachi Power Systems America Ltd.
Final Test Results
Preliminary performance testing results indicate that the SCR system has significantly outperformed its NOx removal guarantee of 90.3% reduction of NOx in the flue gas, from 0.413 lb/million Btu to 0.04 lb/million Btu. The ammonia slip, which was guaranteed at 2 ppm, was measured at 0.2 ppm. The SO2 oxidation was guaranteed at 0.5% and measured less than 0.1% across the entire SCR. The system pressure drop was measured at less than 5 inches of H 2 O — well under the guaranteed value. The precipitator particulate removal efficiency is 99.7%.
What about mercury? Prior to the pollution control enquipment upgrades, testing indicated that approximately 82% of the mercury was being removed. With the scruber upgrade and the addition of the SCRs, the removal rate increased to 90%.
After a decade of hard work and an investment of about $300 million, SECI can now say that the Seminole Generating Station has reduced its air emissions well below what is required by the plant’s air permit and has minimized its solid waste disposal problems with a robust recycling program. In fact, should the supercritical Unit 3 be built, the total air emissions of all three units will be less than those of just Units 1 and 2 a decade earlier. Few coal-fired plants can make these claims.
—Dr. Robert Peltier, PE is POWER's editor-in-chief.
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