Short of replacement, what are your options when your original electrostatic precipitator fails to meet your current emissions and opacity requirements? The management of Big Stone Plant chose the unconventional, yet economic approach of building a pulse jet fabric filter inside the casing of the old electrostatic precipitator. The upgrades restored plant availability and prepare the plant to meet the next regulated reductions in particulate matter emissions.
Otter Tail Power Co.’s Big Stone Plant Unit 1, located in Milbank, South Dakota, is a 475-MW plant that burns Powder River Basin (PRB) coal (Figure 1). This unit was originally outfitted with an electrostatic precipitator (ESP) when it was constructed in 1975. During the mid-1970s the number of ESPs installed on coal-fired boilers across the U.S. spiked in response to the Clean Air Act of 1970. The long-established technology of the ESP was at the time considered the best choice to meet the particulate emission requirements of the Clean Air Act.
1. ESP upgrade needed. Big Stone Unit 1 elected to use the casings of its electrostatic precipitator (ESP) to house a pulse jet fabric filter. The upgrade not only ended unit derates for opacity excursions and induced draft fan pressure drop limitations, but the new equipment also performs well enough to meet anticipated future fine particulate matter legislation. Courtesy: Buell APC
Many of those original ESPs are now candidates for an upgrade or replacement to meet the latest emissions and opacity requirements. The precipitator’s collection efficiency can be maintained within the normal variations in boiler operation, but its performance is sensitive to the electrical characteristics of the fly ash as defined by the type and source of the coal burned. Many of these "first-generation" ESPs have been rebuilt with new and improved plate and emitting electrode systems, power supplies, and control systems to extend their life. Others cannot meet current outlet emissions and stack opacity rules, so plant owners face eventually replacing their aged ESPs with a pulse jet fabric filter (PJFF).
At Big Stone Unit 1, a different and more cost-effective alternative was selected: Buell APC was retained to replace the existing ESP with a PJFF configured to fit within the ESP casing. After overcoming the inevitable challenges that occur when putting a square peg in a round hole, the conversion was completed in late December 2007. These challenges were plant-specific, but many, as you will see, also apply to other utilities that might be considering similar projects.
Many Conversion Considerations
There are many reasons, beyond a poorly performing ESP, that would push a utility to consider an ESP replacement. Some plants have completed a fuel switch that has adversely affected ESP performance. Others may have added a flue gas desulfurization (FGD) system or scrubber upstream or downstream of the ESP for SO 2 reduction, thereby reducing the effectiveness of the ESP. Others are looking ahead to the advantages of a modern fabric filter system to help meet mercury emission limits or future fine particulate control (PM2.5) legislation. When considering future emission requirements, replacing an ESP with a PJFF is probably on many plants’ short list of large-dollar projects.
Another issue: If the ESP is not consistently meeting outlet emission or opacity requirements due to aged internals and close electrical clearances between electrodes, then a rebuild may solve these problems. However, performance problems may be compounded if the ESP is treating a higher-than-original design gas volume that often results from a fuel switch.
Switching to a low-sulfur coal, especially a subbituminous PRB, will also result in higher-resistivity ash and degradation in ESP performance. Derating of the entire unit has been required in many cases to maintain emission and/or opacity requirements. Depending on the severity of the performance deterioration, upgrade requirements may include the addition of sections to the ESP, gas conditioning, or replacement/conversion to a PJFF system.
Some plants have selected a spray dryer FGD system for SO2 reduction. In such cases, a pulse jet baghouse downstream is a logical addition given the additional adsorption of SO 2 in the baghouse filter cake that reduces lime consumption. If a wet limestone forced oxidation FGD system producing commercial grade gypsum is selected, the amount of ash exiting the ESP and its chemical composition can cause potential problems with the quality of the byproduct and the chemistry of limestone dissolution that affects overall SO2 removal efficiency. The role of the existing ESP, when adding a wet FGD system, thus extends beyond achieving stack opacity requirements. The mechanical condition of the ESP and its ability to consistently meet the required fly ash loading limitations to the scrubber have pushed many to add a PJFF.
Mercury removal efficiency with sorbent injection is also highly dependent on coal type, loss on ignition, flue gas temperature, chlorine content in the coal, and SO3 content of the flue gas. A number of test programs with injection upstream of an ESP of various sorbents and enhanced sorbents, conducted by a number of sorbent suppliers with funding from the Department of Energy, show that mercury removal rates of 30% to 90% are achievable. For many coal-fired plants, the best option that gives consistent, high mercury removal efficiencies (frequently >90%) is sorbent injection followed by a PJFF. The PJFF is an integral part of this mercury removal system.
In the future, standards will likely be enacted for stationary sources, limiting their release of particulate matter that is 2.5 micron diameter and less (PM2.5). These solid particulates will likely be limited to between 0.01 lb/million Btu and 0.015 lb/million Btu of fuel burned. Utilities may also need to control air toxics, with particulate matter becoming the surrogate for a group of hazardous air pollutants in the form of heavy metals, a large percentage of which are emitted from coal-fired boilers as fine particulates. For many older, relatively smaller specific collection area ESPs (collecting plate area/1,000 acfm of flue gas treated), performance upgrades will be required to achieve PM2.5 emission limits.
In sum, there are many site-specific requirements that must be carefully studied when considering a PJFF upgrade or conversion. Some utilities will conclude, after weighing their options, that removing the ESP and replacing it with a PJFF is the right decision. Others, such as Otter Tail, will conclude that the conversion of an existing ESP casing to a PJFF is the most flexible and economic path forward.
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