Improve Particulate Control – Cheap
By Ralph Altman, Ramsay Chang and George Offen, Electric Power Research Institute
The High-resistivity Ash Produced When Burning Most Low-sulfur Coals Generally Reduces the
particulate collection efficiency of electrostatic precipitators (ESPs). Consequently, utilities have renewed their interest in finding low-cost techniques for overcoming this deficiency, and thereby for increasing fuel flexibility. Several technologies developed, or under evaluation, by the Electric Power Research Institute (EPRI) can help plant owners cost-effectively upgrade ESP performance, increase fuel flexibility, and prepare for regulatory changes.
Given near-term uncertainties about regulation of airborne trace substances (air toxics), many utilities planning ESP upgrades are considering improvements that could also facilitate compliance with stricter emission limits at moderate additional cost. In addition, to cope with uncertainties about the future ownership of power plants in the competitive environment, utility managers are placing a premium on low-cost options for extending life or enhancing ESP performance.
This article highlights three products–performance monitoring software (ESPert), the Compact Hybrid Particulate Collector (COHPAC) and a separator technology called ElectroCore–that illustrate a range of particulate control technologies (Figure 1). These products are designed to provide:
-enhanced ESP performance at low capital cost,
lower plant O&M costs,
increased fuel flexibility,
-ability to comply with more-stringent emission standards and
integrated multipollutant control.
New digital controls for ESPs can help restore or increase ESP effectiveness and thereby achieve quick fixes at units where a marginally performing precipitator must be kept in compliance with emission requirements. This becomes especially important where continuous compliance is required. Under license from EPRI, most major ESP control manufacturers can now incorporate ESPert, EPRI`s new ESP performance monitoring and troubleshooting software, into their control systems. ESPert monitors ESP operation on-line, evaluates performance, and recommends corrective actions when ESP performance problems arise. Now in use by three utilities, ESPert can help power plant personnel improve ESP performance, reduce ESP-related O&M costs, troubleshoot problems even when experienced ESP specialists are not available, continue to meet applicable emissions and opacity standards reliably, and avoid costly derates. Figure 2 shows a typical ESPert output screen.
ESPert can also help plant owners decide how to upgrade existing ESPs. With this software, plant personnel can readily model ESP performance over a range of operating conditions and fly ash properties for an individual unit, can use the model to make refined site-specific predictions concerning how ESP upgrades would affect ESP performance and can then select the most attractive option. Further, because ESPert can help define the range of coal and fly ash properties existing ESPs can handle effectively, utilities can use ESPert to screen low-sulfur coals, reducing a large number of candidate coal sources to a manageable number, without having to perform costly and time-consuming full-scale test burns on many coals.
Particulate control options providing high collection efficiencies include large ESPs and reverse-gas (RG) or pulse-jet (PJ) baghouses, as well as cheaper, smaller, and easier-to-retrofit technologies that are in early stages of development. In the past ten years, domestic utilities have equipped 20,000 MWe of capacity with RG baghouses. EPRI`s survey of recent user experience and measurements at various pilot- and full-scale plant sites show that baghouses readily keep outlet emissions below the New Source Performance Standard of 0.03 lb./MBtu. Well-maintained baghouses generally achieve good bag life (average lifetime of over four years, with many lasting over eight years in RG applications). However, PJ baghouses, although used widely abroad, have seen only limited utility-scale applications domestically.
A novel and lower-cost method of obtaining the very-low emissions levels achieved with baghouses is EPRI`s patented COHPAC. The basic concept of this process is simple: install a filtering system–typically, a PJ baghouse operated at a higher air-to-cloth ratio than in conventional PJ baghouses–downstream from an existing ESP to remove
any uncollected particles. (For a full
article on COHPAC, see Power
Engineering, July 1996.) A versatile technology, COHPAC enables utilities to upgrade ESP performance, and to meet present and potentially more-stringent future regulatory requirements at significantly lower capital costs than other options. An extension of the basic concept involves retrofitting a baghouse into the last field of an ESP, forming an even more compact, high-efficiency particulate collector. TU Electric uses 1,100 MW of COHPAC downstream of small cold-side ESPs to improve performance at its Big Brown station, and Alabama Power has installed a 275-MW COHPAC unit to improve hot-side ESP performance at its Gaston station.
EPRI has developed several tools to help utilities assess potential baghouse applications and is engaged in continuing efforts to optimize baghouse design and operation. As part of its effort to extend the service life of bags in PJ applications, EPRI is investigating ways to prevent bag deterioration caused by acid condensation in COHPAC baghouses at units using sulfur trioxide (SO3) conditioning for the existing ESP, and more generally in bags exposed to gas streams containing relatively high levels of SO3 or otherwise subject to chemical attack.
EPRI is currently evaluating the potential for injecting sorbents between the ESP and a COHPAC baghouse to capture sulfur oxides, mercury and/or other gas/vapor phase contaminants. In this configuration, the fly ash and sorbents are collected separately, thus allowing for separate disposal, sale, or recovery of fly ash and sorbents. A current project studies the impact of additional sorbent loading and sorbent size distribution on COHPAC baghouse performance.
For several years, EPRI has supported development of advanced filters for use in limited-space retrofits and multipollutant control systems. Most of the devices are barrier filters that can provide very-high collection efficiencies; all of them are suited for stand-alone applications and are being considered for use in COHPAC applications, especially when space is at a premium. The devices include high-surface-area pleated bags made from commercial polymers and membrane-coated and layered ceramic filters (Figure 3). The ceramic filters are capable of withstanding temperatures up to 1,600 F and, in some cases, could be coated with NOx-reduction catalysts. To assess their potential for long-term performance, EPRI is now conducting pilot-scale testing at utility sites (Figure 4).
EPRI`s patented ElectroCore combines electrical and mechanical forces to separate flue gas into two streams, one containing a high concentration of particles and the other containing very few particles. Flue gas enters the compact, rugged core separator through a slot that is tangential to the side of a cylinder (Figure 5). The tangential flow creates a circular motion that forces the larger particles in the gas towards the outside of the cylinder. A “bleed flow” with a high concentration of particulate matter is withdrawn from the core separator through a slot in the cylinder wall opposite the entering flow. Relatively clean gas is withdrawn through short cylinders, called “vortex finders,” located in the center of the core separator. The aerodynamic design of this system avoids the secondary vortices that form in conventional cyclone separators, entrain particles from the outer walls and, therefore, increase particle penetration (reduce collection efficiency). The ElectroCore mechanical separation mechanism is augmented electrically by placing a high charge on the particulate matter before it enters the core separator. The charged particles are further prevented from leaving the core separator through the vortex finders by forces induced by a large electric field created between a high-voltage electrode in the center of the core separator and the outer wall.
No particulate collection takes place in the core separator; it only separates the flue gas into clean and dirty streams. A collection device must be supplied externally. In a utility application, collection takes place in an existing ESP, with a bank of core separators inserted between the ESP and the existing induced draft fan; the dirty stream from the core separator is returned to the flue gas ahead of the ESP. Pilot-scale laboratory tests have shown that the device can upgrade removal efficiencies from a level of 95-97 percent to over 99 percent. To field test the concept, EPRI will soon participate in the installation of a large ElectroCore pilot at a utility plant.
Other Advanced Options
Several other technologies developed, or under evaluation or development, by EPRI are also designed to cost-effectively reduce particulate emissions and outlet opacity and/or increase fuel flexibility. These additional particulate control upgrade options, which will give plant owners a greater selection in meeting site-specific space and technical requirements, include:
-EPRICON–an easy-to-install alternative to conventional SO3 conditioning systems for small units; it requires no on-site storage of potentially dangerous chemicals and does not increase overall emission of sulfur oxides. It has been applied recently at New England Power`s Brayton Point station.
-The use of lower-cost novel additives that can reduce fly ash resistivity and increase fly ash cohesivity over a wide range of temperatures, thereby increasing ESP collection efficiency. At least one such additive may be suitable for use with ESPs operating on the hot side of the air heater, where SO3 conditioning is ineffective and sodium conditioning can cause troublesome deposits in the boiler.
-Two “non-chemical” solutions to the high-resistivity problem: precharging, which separates the particle-charging and collecting functions, allowing effective particle capture without SO3 conditioning; and pulsed energization–the superimposition of microsecond-duration, high-voltage spikes on the dc side of the power supply for an ESP–an option that has been known to greatly improve ESP performance on high-resistivity dusts but requires the development of new, solid-state switching circuits to be cost-effective.
-Waveform-shaping techniques that counter the sodium-depletion problem often encountered at plants with hot-side ESPs.
-Converting the last field of an existing ESP from dry to wet operation in a way that allows operation above-saturation conditions to minimize water usage/treatment requirements and downstream corrosion. The wet section prevents reentrainment, thereby providing very high levels of particulate capture, including fine particulate. It will also remove 60 to 70 percent of the SO3 and a small amount of SO2, and could be adapted to capture some volatile trace metals.
Utilities having to make substantial ESP upgrades will often need to take the control of other pollutants into account, and may want to integrate SO2- and NOx-removal technologies with particulate control technologies. Moreover, if utilities are required to remove vapor-phase mercury from flue gas, that would require integration of mercury control as well. Integrating SO2, NOx and mercury removal with particulate control technologies may promise benefits over the conventional use of independent control processes. EPRI is beginning to re-examine these integrated control options, including the use of multiple pollutant sorbents injected directly into the flue gas stream, catalytic filter systems, fluidized or fixed bed reactors, combined wet-dry scrubbing, and ionization processes. p
Figure 4. COHPAC installation (lower right) at TU Electric`s Big Brown plant.
Figure 3. Prototype compact ceramic filter (Courtesy Specific Surface Corp.)