NOx reduction RACT
compliance requires careful technology selection
By Gavin B. Heckler,
PECO Energy Co.
After the Clean Air Act Amendments passed in 1990, Title I (Attainment and Maintenance of Ambient Air Quality
Standards) and Title IV
(Acid Deposition Control) of the
Act required power plants to submit and implement compliance plans for NOx and volatile organic compounds (VOC) emissions, among other pollutants. This legislation affected PECO Energy Co.`s Eddystone Generating Station, requiring the utility to comply with the Act under reasonably available control technology (RACT) rules established by the state of Pennsylvania. The State Implementation Plan required installation of the required technologies by May 31, 1995.
In mid-1993, PECO commissioned a consultant to review the legislation and develop a compliance plan. As the state progressed with its legislative rulemaking, it became clear that Eddystone Units 1 and 2 and the station`s combustion turbines would be required to comply with presumptive RACT while the other station sources would be evaluated on a case-by-case RACT basis.
During the evaluation, PECO reviewed several combustion technologies, including low-NOx burners (LNB), overfire air (OFA), burners out of service and flue-gas recirculation. PECO also reviewed post-combustion technologies such as selective and non-selective catalytic reduction, but quickly removed these from consideration as economically unfeasible.
RACT compliance plan
After carefully considering alternatives aligned with the RACT rules for Pennsylvania, PECO adopted a compliance strategy and submitted it to the Pennsylvania Department of Environmental Protection (PaDEP) for review and approval.
Under the case-by-case RACT proposals, the proposed NOx reduction technology for Units 3 and 4 was to rehabilitate existing OFA ports which had been bricked over. Each of the four corners of these units was originally constructed with an OFA port located in the boiler side walls. Also under the case-by-case RACT proposals, the proposed NOx reduction technology for the A, B and C auxiliary boilers was to install low-NOx burners.
Under presumptive RACT proposals, PECO proposed low-NOx burners with close-coupled OFA (CCOFA) and separated OFA (SOFA) as the proposed NOx-reduction technology for Units 1 and 2. For the combustion turbines PECO proposed to reduce NOx by limiting the annual capacity factor to 5 percent or less on a 12-month rolling basis. After considering technological and economic feasibility, the utility proposed no VOC reductions because none of the available VOC reduction technologies fell within RACT guidelines.
In late 1994, PaDEP issued an operating permit to PECO, regulating Eddystone Station for “Facility
NOx & VOC RACT” compliance. The permit approved the proposals as listed above. The permit also outlined NOx and VOC emission limitations (Table 1) for compliance after installation of the proposed technology.
RACT NOx reduction
Units 1 and 2 involved the most complex, schedule-driven implementation of any of the NOx controls installed at Eddystone Station. The cost of the NOx-reduction technology for these units represented more than 95 percent of the total expenditures for NOx reduction at the station. These units are a twin-furnace design, so PECO duplicated all modifications for each furnace on each of the units.
PECO chose a low-NOx concentric firing system with CCOFA and SOFA for NOx reduction. The system, designed and manufactured by International Combustion Ltd. (ICL), reduces NOx formation by staging combustion with two levels of OFA and lengthening the flame path to reduce peak flame temperatures. Offset air nozzles minimize tube corrosion/erosion and slagging tendencies.
PECO replaced the existing eight coal burners in each corner (two per mill elevation) with four larger flame attached nozzle (FAN)-type coal nozzles (one per mill elevation). This reduced the total number of coal nozzles from 64 to 32 on each unit. To accommodate the new nozzles, PECO completely demolished the burner corners down to the burner box casing, including windbox dampers, tilt linkages, air nozzles and compartment division plates. Restacking the burner corner compartments to lower the four coal elevations provided room at the burner top for two CCOFA compartments. Between each coal nozzle is an auxiliary air compartment consisting of a triple set of nozzles, a center straight air nozzle, and an upper and lower offset auxiliary air nozzle. A single straight auxiliary air nozzle is located at the burner bottom and at the burner top just below the CCOFA nozzles.
The utility fitted new windbox secondary air dampers and support bearings for each compartment and installed new tilt support channels, tilt drive bearings, linkages and shear pins. They reused existing electrically operated secondary air damper drives and tilt drives. The new tilt system operates over a range of plus 20 to minus 20 degrees.
PECO also installed new 12-inch diameter coal piping from the burner backplate down to the operating floor elevation and added a toggle joint assembly in the coal piping horizontal run to account for boiler thermal growth. New constant load spring hangers support the coal pipe system.
SOFA is provided directly above each burner corner centered approximately 7 feet above the top of the burner box. New furnace tube panels provided the SOFA openings. On Unit 1, the opening was located in a vertical run of waterwall tubing; however, on Unit 2, the opening was located in a horizontal run of waterwall tubing, requiring a complex tube panel which repositioned 30 tubes. The SOFA box, attached to the tube panel, supports a single split-air nozzle. Tilt range is the same as the burner box but operated with a new electric drive. Secondary air is brought through new ductwork to the SOFA box and is admitted to two air compartments. Two elevations of secondary air dampers per corner control SOFA air injection to the furnace. This overall OFA arrangement provides two levels of CCOFA and two levels of SOFA per corner.
The utility installed new programmable logic controllers (PLC) for each furnace to coordinate the operation and monitoring of the LNB system. The PLCs control the windbox dampers to regulate windbox to furnace pressure differential pressure, position the fuel air dampers, monitor coal pipe temperatures, initiate logic for boiler purge, light-off and normal firing operations, and control the SOFA dampers and tilt drives. An interface to the existing distributed control system (DCS) provides operator access for control and feedback.
On Units 3 and 4, it was much simpler to implement the proposed NOx-reduction technology. After removing the fire bricks from the SOFA openings, PECO installed new SOFA duct expansion joints and repaired the casing leaks. Besides refurbishing the secondary air dampers and installing new pneumatic drives, the utility also installed new air nozzles. It added operational control of the SOFA system to the new DCS.
The NOx-reduction technology for the auxiliary boilers involved installing a LNB system with OFA. The modifications for each of the three boilers were identical. The burner system selected was a Faber low-NOx register burner. The system reduces NOx emissions by staging combustion air. The burner is a dual-fuel system firing natural gas through five gas spuds and No. 2 oil through a center-mounted oil gun. Full steaming capacity is obtained while firing either fuel, although co-fire capability is not provided. Eight ports, four above and four below the burner assembly, supply OFA.
The LNB was designed for installation into the existing boiler front wall opening with minimum modification. PECO reused all of the existing peripheral equipment (i.e., the combustion controls, windbox casing, windbox dampers, forced-draft fan, and the fuel and atomizing control valves and pipe trains) with only minor changes. By minimizing the interfaces, the utility realized considerable cost savings and reduced operator training requirements.
The auxiliary boilers were not required to install continuous emission monitors, but are required to conduct annual emission tests to verify LNB performance. PECO constructed a testing platform and stack test ports to access the boiler stacks for performing the annual emission tests.
All units have operated within the compliance limitations as outlined in the operating permit. Overall performance of the installed NOx-reduction technologies has been excellent for Units 3 and 4 and the auxiliary boilers, but the pulverized-coal-firing performance of Units 1 and 2 has been less than optimum.
As base-loaded coal-fired generation, Units 1 and 2 have high expectations for capacity factor and efficiency. Hence, installation of the presumptive RACT NOx-reduction technology can represent considerable risk in the area of performance. A wide range of performance parameters can be affected including the ability to reach maximum continuous rating (MCR) and steam temperatures, flame stability, slagging tendencies, unburned carbon losses and boiler tube erosion.
Since returning from its NOx outage in May 1995, Unit 1 has had limited operating time, due in large part to problems experienced in other plant areas. Unit 2 has operated nearly continuously since its NOx outage in July 1995. The utility has accumulated sufficient operating time on these two units in this short period to identify good results in some areas and poor results in other areas of performance.
On the positive side, both units have been able to reach MCR and steam temperatures. In fact, the units have run similarly to how they performed prior to the LNB installation with respect to maintaining steam temperatures, furnace heat absorption, and heat input and steaming capacity. Operation of the windbox dampers to maintain proper control of windbox to furnace differential pressure and the burner tilt system has improved greatly. Previously, the burner tilt system was in poor or inoperable condition.
The firing system of the LNB FAN-type coal nozzles has performed below expectations. The FAN design intends to maintain the flame front at the nozzle exit face in order to reduce NOx formation. In fact, the flame has actually propagated into the FAN nozzle causing excessive slagging in the FAN sections. The nozzles are then subjected to high temperatures causing distortion and cracking. PECO first identified the problem two weeks after the initial operation of Unit 1 with the new LNB system when the unit was removed from service for a reheater tube leak. An internal inspection of the burners identified severe enough damage to five of the nozzles to warrant replacement. Nearly all FAN nozzles in Unit 1 exhibited some thermal damage. Fortunately, Unit 2 nozzles were on site awaiting installation and could be installed immediately in Unit 1 to return the unit to service in a timely manner.
ICL began an evaluation to identify the failure mode. It developed and implemented modifications to improve the coal delivery velocity and maintain the flame front off the nozzle. The modifications also included operating restrictions to provide a minimum-fuel secondary-air flow at the coal nozzles as well as feeder speed limitations. ICL also instituted the modifications on the Unit 2 nozzles prior to installation.
FAN section slagging has continued to be a problem; however, the modifications have improved firing stability to a marginal level. Unit 2 has operated for more than five months in the restricted mode with only minor but progressing thermal distortion and cracking. ICL is continuing to review the problem and has developed a solution which will meet the guaranteed, stable-firing performance criteria. Based on its evaluation, ICL will replace the FAN nozzle assemblies with conventional low-NOx coal nozzle assemblies in mid-1996 on both Units 1 and 2.
General slagging of the furnace walls has been minimal, with most slag build-up occurring around the burner coal and air nozzles. Waterwall tube erosion has been a problem with these units in the past. While no tube wastage has been found to date, PECO will continue to monitor this parameter in the future.
The utility has deferred optimization of the LNB system due to the firing stability concerns. Even so, the LNB system has operated well below the NOx limitations of the operating permit.
The Eddystone units have demonstrated compliance with the limitations set forth in the operating
permit, but the actual operating
data represent a relatively short operating period. Table 2 summarizes the measured NOx emissions. Units 1 and 2 have undergone a very limited optimization to determine the possible NOx-reduction capabilities. PECO has attained NOx values as low as 0.27 lb/MBtu at MCR on Unit 1. Values of NOx throughout the load range have not been plotted due to both a limited number of start-ups/shutdowns and the hold on the optimization program. With operating restrictions in place to assist firing stability, these units run in the 0.33 to 0.35 lb/MBtu range at MCR, well below the permit limitations.
The auxiliary boilers have performed very well. Each of the three boilers is identical and has met identical NOx emissions of 0.11 lb/MBtu while firing natural gas. Performance has also been excellent through the load range with NOx emissions below 0.08 lb/MBtu at 25-percent load. The NOx values of 0.10 lb/MBtu were surprisingly low while firing No. 2 oil. Marginal atomizing steam (slightly wet steam) conditions and staging the combustion air through OFA ports contribute to lower the NOx value on No. 2 oil.
Units 3 and 4 have reached compliance in all cases except for No. 6 oil firing. While firing natural gas or in the co-firing mode (approximately 70/30 gas/No. 6 oil mix) the NOx emissions have been under the operating permit limitations. While not tabulated here, the NOx levels have also met limitations throughout the load range. PECO will perform additional testing while firing No. 6 oil to optimize the NOx reduction using OFA.
Units 3 and 4 discharge the boiler flue gas into a common stack. This makes NOx data gathering difficult on an individual basis since the units are usually dispatched together. Therefore, the information listed in Table 2 is representative of both units in operation at the specified conditions.
Since no RACT technology was implemented for VOC emissions, the station cannot regulate the emission rates even though a limit has been set forth in the operating permit. Units 1, 2, 3 and 4 are required to perform a VOC stack emission test to determine actual VOC concentrations after the RACT technologies are optimized. z
Stone & Webster Engineering Corp., “Title I System Planning Study, Phase I–NOx Reduction Technology Screening Analysis,” August 1993.
Stone & Webster Engineering Corp., “Title I System Planning Study, Phase II–NOx Reduction Technology Preliminary Engineering,” October 1993.
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This separated OFA unit was part of the NOx control strategy for Eddystone`s Units 1 and 2.
The low-NOx burner retrofit for the auxiliary boilers required little boiler modification. This retrofit was coupled with OFA.
Typical slagging condition of Unit 1 burner corner after several months
Eddystone Generating Station is located on the Delaware River approximately 15 miles southeast of Philadelphia, Pa. With a nominal gross capacity of 1,500 MW, the station is the largest fossil plant on the PECO Energy Co. system. The plant has a mix of coal-fired, oil-/gas-fired and combustion turbine units to supply both base load and cycling generation. In addition, three auxiliary boilers provide start-up steam and house heat.
Eddystone Units 1 and 2 are Combustion Engineering twin-furnace, balanced-draft, tangentially fired, radiant-type boilers firing western Pennsylvania bituminous coal. The Units are rated at 315 MW and 325 MW, respectively. Eddystone Units 3 and 4 are Combustion Engineering single-furnace, balanced-draft, tangentially fired, radiant-type boilers firing natural gas and/or No. 6 fuel oil and are rated at 395 MW. The three identical auxiliary boilers are Combustion Engineering `D`-tube, single-pass package boilers with superheaters. Four Pratt & Whitney combustion turbines rated at 15 to 18 MW each are also located on site.