By Jeffrey Shellenberger
The Lon D. Wright Power Plant in Freemont, Nebraska. Photo courtesy: Amec Foster Wheeler
Federal air emission regulations continue to pose challenges for coal-fired electric generating units in the United States. The City of Fremont, Nebraska recently upgraded its air quality control system (AQCS) at Lon D. Wright Unit 8 to enable compliance with the EPA’s Mercury and Air Toxics Standards (MATS) rule. The upgrade included a new pulse jet fabric filter, along with a spray dryer absorber and an activated carbon injection system.
MATS Compliance Strategies
Power plant owners and operators, such as the City of Fremont, can achieve the required MATS compliance by combining different air pollution control products and technologies with their existing equipment. The power plant and equipment designers must conduct a thorough assessment of the flue gas pollutant mix in order to select the optimum combination of technologies and achieve the lowest possible life cycle costs. Technology selection varies from unit to unit based on differences in fuels, capacity factors, existing equipment configuration, and operating practices.
Particulate Matter (PM) control is central to any multi-component AQCS. Both Electrostatic Precipitators (ESPs) and Fabric Filters (FF) provide effective PM removal and must be integrated into the overall emissions compliance strategy. The PM control technology selection will affect the approach to mercury and other acid gases. ESPs and FFs are relied upon to provide additional co-benefits, such as mercury capture in particulate form, mercury absorbed and collected with powdered activated carbon, and acid gas control by collecting Dry Flue Gas Desulfurization (FGD) sorbents.
The primary technologies for acid gas (SOx and HCl) removal are Spray Dryer Absorbers (SDA), Circulating Fluidized Bed Scrubbers (CFBS), Dry Sorbent Injection (DSI), and Wet Flue Gas Desulfurization (WFGD). Powdered Activated Carbon (PAC) injection systems provides a low capital cost technology for controlling mercury emissions, and ESPs or FF baghouses are widely applied for PM control. As shown in Figure 1, combining these technologies has marked impacts on project costs and schedule, depending on the specific complexity adopted by each plant.
ESP Design Considerations
ESPs can operate at high temperatures and can handle sticky fly ash, which can be problematic for fabric filters. ESPs are relatively low maintenance and exhibit low pressure drops. Major ESP disadvantages include a dependence on fly ash resistivity, which is affected by the addition of DSI sorbents, and the overall ESP power consumption.
Major design considerations when assessing existing ESPs include specific collection area (SCA), flue gas velocity, physical geometry, current and future coal selection, and site-specific construction restraints to implement upgrades or replacement. The two main types of ESPs: tumbling hammer designs and top-rapped / magnetic-impulse designs are often selected based on customer preference. Amec Foster Wheeler offers both types of ESPs.
There are a number of minor and major upgrade options for improving ESP PM collecting efficiency and overall performance. Amec Foster Wheeler has an extensive portfolio of rebuilds on many different types and models of ESPs. As shown in Figure 2, some ESP upgrades can be completed during minor outages while more extensive, higher cost upgrades require a major outage. Many operating ESPs in the U.S. coal fleet have been in service for over 20 years. A thorough condition assessment of a plant’s ESP equipment will help to determine if an upgrade is feasible and economical.
In cases where upgrading an existing ESP cannot sufficiently improve PM removal efficiency, or where space and economic constraints do not allow for full replacement of the ESP, plant owners and operators can install a small polishing FF downstream of the existing ESP. This configuration, which has gained a large U.S. experience base and which Amec Foster Wheeler helped pioneer, provides improved PM emission reductions and lower DSI and ACI sorbent costs. If the sorbents are added at a point between the ESP and FF, ash recovered from the ESP hoppers is not contaminated with the sorbents, thus maintaining its marketability. However, high air-to-cloth ratio pulse-jet fabric filters are more sensitive to high dust and sorbent loadings, and must be designed with additional considerations for reliable performance.
|Unit 8 AQCS Equipment (showing SDA Penthouse and PJFF Housing). Photo courtesy: Amec Foster Wheeler|
Another option is direct conversion of the existing ESP to a Pulse Jet Fabric Filter (PJFF), assuming the ESP casing and hoppers are structurally sound. For the ESP-to-PJFF conversion option, key design considerations include ESP casing size and geometry, induced draft fan capacity, ductwork layout and design, and PJFF compartmentalization requirements.
Fabric Filter Design Considerations
A large number of U.S. coal-fired power plants are adding or upgrading fabric filters to address the MATS PM emissions requirements. FFs provide a greater PM reduction than ESPs and are less dependent on consistent operating parameters. FFs can handle a wide fuel mix and emissions stay relatively constant. Additionally, a FF with upstream DSI or ACI injection improves sorbent utilization due to the increased residence time on the filter bags. The major disadvantage of a FF lies with operating costs attributable to consumable filter bags and the comparatively high pressure drop.
The air-to-cloth ratio (acfm/ft2, typically expressed as ft/min) of the FF determines the overall size and performance of the unit. In general, lowering the air-to-cloth ratio increases FF costs and improves performance, while increasing the air-to-cloth ratio lowers costs and worsens performance. Experienced FF suppliers with knowledge of similar applications should be relied upon to provide the optimal air-to-cloth ratio.
The filter bag material is the heart of any FF design and key material selection parameters include: flue gas temperature and chemistry, emissions performance, filter material longevity, and costs. In order to comply with the most stringent regulatory requirements, filter media with an expanded polytetrafluoroethylene (ePTFE) membrane typically provides the lowest PM emissions (typically <5 mg/Nm3).
|Unit 8 AQCS Equipment (left to right showing Pebble Lime Storage Silo, PJFF Housing, PAC storage Silo, Spray Dryer Absorber, and associated SDA inlet ductwork). The AQCS equipment was delivered within 18 months from the date of award, and installation was completed during a September 2015 tie-in outage. Photo courtesy: Amec Foster Wheeler|
FF cleaning cycle frequency is another key design factor that affects overall emissions performance. Most utility FFs operate in an on-line cleaning mode, which means that the filter bags remain on-line processing flue gas while they are simultaneously cleaned. There is bleed-through of PM during pulsing, and excessive pulsing leads to high emissions. For this reason, a robust and efficient pulse cleaning system coupled with a low “can velocity” gas flow distribution design, such as the Amec Foster Wheeler Jet VIP, will result in the lowest PM emissions possible. In 2012, the Electric Power Research Institute (EPRI) published a survey of pulse cleaning information for 23 fabric filter installations, of which 11 are Jet VIP designs supplied by Amec Foster Wheeler. The Amec Foster Wheeler design cleans less than half as often as the average of all baghouse suppliers included in the study.
Retrofits and upgrades of existing FFs are typically designed to lower the air-to-cloth ratio, improve the bag material selection, and/or reduce the pulse cleaning frequency.
Nebraska Case Study
The City of Fremont, Nebraska’s Lon D. Wright Unit 8 (Figure 3) is a 90 MW (gross) coal unit (1800 psig, 1005°F) fired by Powder River Basin coal. Considering the only component of the unit’s AQCS was a hot-side ESP, emissions from the unit exceeded the mercury, PM, and acid gas MATS limits. Based on an analysis by HDR Engineering, the city elected to install a PAC system for mercury control, a SDA for acid gas control, and a PJFF to remove PM. In 2013, the city awarded a contract to Fagen Inc. for execution of the project. Fagen subcontracted the complete AQCS equipment scope to Siemens Environmental (now Amec Foster Wheeler).
The Unit 8 flue gas pollutant mix, as shown in Table 2, required additional AQCS equipment to meet the MATS emission limits.
MATS Limits for Existing Electric Generating Units Burning Coal >8,300 Btu/lb
The PAC system scope provided by Amec Foster Wheeler consisted of a storage silo and redundant blower/feeder systems. For acid gas control, Amec Foster Wheeler provided one (1) 100% SDA and a lime preparation system. Two fluid nozzle SDA technology was selected for its superior reliability and operational flexibility. The two fluid nozzles can be maintained with the SDA on-line without decreasing boiler load or affecting emissions performance. This Dry FGD technology is very beneficial to the City of Fremont since it allows for the greatest flexibility in maintenance. Alternative DFGD technologies, such as rotary atomizers, require time-consuming and costly maintenance of the rotary wheel or disc. The SDA is also capable of high unit turndown, down to 25% of rated flue gas flow without recirculation of the flue gases while maintaining emission requirements. At full boiler load, the pressure drop through a two fluid nozzle SDA vessel is typically 2.5 in. H2O less than the pressure drop through a comparable rotary atomizer.
For controlling PM emissions, it was determined that the existing hot-side ESP would remain in service to capture the majority of the fly ash, and a new PJFF would be installed to collect the SDA byproducts and any remaining fly ash from the existing ESP. Amec Foster Wheeler designed and supplied a modular PJFF in order to minimize the field labor and construction. The PJFF is capable of operating and meeting emissions requirements with one (1) module off-line for maintenance. Each module is fully shop-assembled including the walls, hopper, roof, and tubesheet.
Amec Foster Wheeler selected a intermediate pressure intermediate volume (IPIV) Jet VIP pulse cleaning system. The Jet VIP pulse cleaning system utilizes 3 inch double diaphragm pulse valves, large capacity pulse air headers, and variable-hole manifold pipes for equal and efficient cleaning, which is critical for optimal performance. Industry “standard” pulse cleaning systems with smaller valves, smaller headers, and un-tuned manifold pipes are not as effective, and result in more overall cleaning and therefore less filter bag life. Amec Foster Wheeler has installed the Jet VIP system on over 50 utility and large industrial applications over the past 15 years with superior results.
The Jet VIP PJFF (shown in Figure 3) also utilizes a split-gas vertical baffle in each module to lower the overall can velocity at the bottom of the filter bags. The vertical baffle also increases large particle drop-out prior to the filter bags. Alternative “up-flow” style collectors suffer from high can velocity which affects the overall performance of the FF. The low can velocity Jet VIP design allows the PJFF to clean on-line, reduces re-entrainment, reduces pulse cleaning, and extends bag life.
The PJFF filter bag material is an ePTFE membrane supported on polyphenylene sulfide (PPS) felt to provide superior PM emissions performance. The filter cages are a two-piece design for ease of installation. Amec Foster Wheeler personnel conducted extensive trial fittings and quality assurance checks to ensure the critical bag and cage components will function properly in the field. Many FF OEM’s rely solely on the bag supplier for quality assurance, however Amec Foster Wheeler has a thorough QA program which is essential for a successful large scale PJFF installation such as the City of Fremont.
The AQCS equipment was delivered within 18 months from the date of award, and installation was completed during a September 2015 tie-in outage.
Initial performance of the SDA and PJFF system has been according to Amec Foster Wheeler predictions with no major issues to report.
Depending on operating conditions of the upstream ESP, the PJFF cleaning system has cleaned at a rate of approximately 7 – 24 pulses / bag / day, which is less than most typical Dry FGD applications. The low cleaning rate of the Jet VIP system should help extend the filter bag life for the City of Fremont.
The City of Fremont commissioned and tested the AQCS in November 2015, and as shown in Table 3, the system is already meeting the MATS rule emission limits.
Jeffrey Shellenberger is AMEC Foster Wheeler’s Product Manager for Fabric Filters. He has been with Amec Foster Wheeler for over 11 years