|Fly ash has been collected by particulate removal technologies since the 1920s
Credit: Pittsburgh Mineral & Environmental Technology
Since the 1920s, steam generator owners have used particulate removal technologies such as cyclones, fabric filters, or dry or wet electrostatic precipitators (ESPs) to collect flue gas particles such as fly ash.
Today utilities expect ESPs to efficiently remove very fine particulates (e.g., PM10) and often rely on a wet flue gas desulphurization (FGD) system to remove smaller and harder-to-capture sub-micron particulates (e.g., Pm2.5, mercury, and other hazardous emissions) as a co-benefit. Experience has shown that problems often occur when a FGD system is added without a concurrent ESP performance upgrade.
ESP operation, in principle, is very straightforward. Particles to be removed from the gas stream are charged by a series of high-voltage electric discharge electrodes to produce negatively charged ions (corona discharge) that charge the particles in the flue gas, providing the driving force for moving particles to the collecting plates.
Next, plates stationed parallel and on each side of the gas stream are grounded in order to attract and accumulate the negatively charged particles in cake-like layers on the plate surface. Finally, the particulate matter is removed from the plates by mechanical rappers causing the material to fall into collection hoppers from which it is disposed or recycled. The ESP removal efficiency is highly dependent on the voltage differential between the discharge electrode (Eo) and the collection plate (Ep). The typical ESP operates at a voltage in the range of 30 kV and 100 kV.
The typical ESP will have multiple discharge electrodes between each set of collecting plates and multiple sets of collecting plates in a single ‘field’ in the direction of gas flow. Each field acts as an independent precipitator. Multiple fields may be added in series to improve particle removal efficiency with most plants using three or more fields.
Some plants use up to 12 fields in order to achieve collection efficiencies greater than 99 per cent. In most utility applications, each field is also electrically divided into separate compartments or ‘lanes’ to further optimize particulate removal efficiency, primarily due to temperature and flow rate gradients across the flue gas entering the ESP. The ESP has been chosen as the PM removal device in over 90 per cent of utility applications due to its low pressure drop (usually 0.5-1.0 inch water column), which translates into less auxiliary fan power.
The particle removal effectiveness of an ESP is largely based upon the resistivity of the particles (the ability of a particle to hold a charge), the gas flow properties, and the quality and strength of the electric field produced between the electrodes and collection plates. Also, a fuel change often changes the resistivity of the particles in the flue gas.
The recent US Mercury and Air Toxics Standards (MATS) also place limits on mercury and acid gas limits on boilers that may require adding activated carbon injection for mercury removal and/or dry sorbent injection to remove acid gases. Adding activated carbon or dry sorbent to the gas stream may change the particle resistivity and therefore the efficiency of ESP operation. Other changes in the flue gas, such as volumetric flow rate, moisture content, chemical composition, and temperature can also adversely impact the particle collection efficiency of an ESP.
There are a number of upgrade alternatives available to those needing to increase the particulate removal capacity or those experiencing an underperforming ESP. Options available to the end user are increase collecting surface area, improving the flow distribution entering the ESP, and upgrading or replacing the collection plates or discharge electrodes. Each of these options requires extensive physical changes to each stage of the ESP and all require an extended unit outage and significant cost.
The conventional power supply system for an ESP consists of the transformer-rectifier (TR) set current-limiting reactor (CLR), and silicone controlled rectifier (SCR) that produce the high voltage power source for the discharge electrodes. The TR set is a high voltage transformer and rectifier that converts single phase AC power to single-phase DC power with approximately 30 per cent ripple in the output voltage waveform due to 50/60 Hz operating frequency.
The CLR provides current limiting during transient overload (sparking) conditions. The SCR regulates the voltage into the TR set to adjust the output voltage and current to the ESP. Separate conventional power systems are used on each ESP stage (sometimes each lane in a field) in order to optimize individual field performance.
Anatomy of the switch mode power supply
Often a quicker, less intrusive and more cost-effective solution to improve ESP performance is replacing the conventional power supply system with a high frequency switch mode power supply (SMPS) that converts 50/60-Hz power to low ripple DC with output waveform ripple below 3 per cent.
The reduced ripple in the output voltage allows the SMPS to produce a higher average output voltage (Eo), which in turn produces higher collection efficiency. The typical voltage output from the SMPS – usually determined by ESP plate spacing, discharge electrode type and particle resistivity – ranges from 50 kV to 120 kV with improved spark handling. The SMPS also has a faster spark and arc response time – microseconds instead of milliseconds – that reduces power dissipated in sparks and arcs and reduces wear on power feed components and ESP internals.
The SMPS upgrade option has two principal benefits that are dependent on how the unit is operated. The first benefit is to improve ESP collection efficiency when less particulate emissions are desired. The SMPS puts more power (increased Eo) into the ESP resulting in higher collection efficiency.
The second benefit for plant owners is for a unit where less ESP auxiliary power use is desired in order to either improve unit heat rate or increase sellable power to the grid. With this option, the SMPS replaces the inefficient conventional power systems used on an ESP operating with acceptable collection efficiency by limiting the power output to that of the TR and CLR it replaces. The SMPS operates at >90 per cent power efficiency compared to the TR, which generally operates at <60 per cent. For the same power input into the ESP (and the same ESP collection efficiency), SMPS can reduce auxiliary power use by 30 per cent.
ModuPower SMPS systems have also been used in a variety of unique applications, as the following three case studies will illustrate.
For example, at AES Gener’s 135 MW coal-fired Norgener Power Station, located in Tocopilla, Chile, its 1990s vintage ESP met air emissions standards when built. New regulations that went into effect this year reduced the level of permitted particulates from 211 mg/Nm3 (98 per cent removal rate) to 50 mg/Nm3 (99.95 per cent removal rate). The reduced emissions limits would require a unit derate in order to continue operating.
Norgener replaced three of the four conventional power supply systems on each unit with Stock Equipment Company’s ModuPower SMPS, which allowed Norgener to increase generation to full load on each unit while maintaining particulate emissions below the new particulate discharge limits.
A 640 MW supercritical utility boiler located in Ohio was reconfigured with a wet FGD downstream of its existing four-lane/12-field ESP. It was assumed during design that the new wet FGD would capture particulates not removed by the ESP, so no enhancements to the ESP were made as part of the FGD retrofit project (see Figure 1).
|Figure 1: Ohio plant ESP roof with ModuPower ground switches installed on the insulator compartment
Credit: Stock Equipment Company
However, shortly after the FGD was commissioned, the plant determined that excessive particulates were entering the FGD and fouling pumps, pump linings, and piping. Also, the ash contamination reduced the quality of the gypsum byproduct from the FGD, negating its market value. Instead of a positive cash flow from the sale of the gypsum, the utility must pay the cost of landfilling the waste product.
During the first year, authorities allowed the plant to ‘bypass’ the FGD system when it required monthly maintenance to repair damage caused by excess particulate emissions from the ESP.
However, after the first year of operation a monthly unit outage was required to perform FGD maintenance, an untenable situation for a baseload unit. The site was limited in size so adding fields to the ESP wasn’t an option.
Stock Equipment Company was asked to evaluate options for repowering the ESP in order to reduce particulate emissions entering the wet FGD.
The solution was to add four ModuPower SMPS systems of 60 kW (83 kV/1080 mA) for each lane in the second and third fields and four 90 kW (83 kV/ 1080 mA) systems for each lane in the sixth field. This approach allowed the original inlet field TR sets to perform the relatively easy chore of removing large, friendly particles in the front of the ESP, and to use ModuPower SMPS systems to do more difficult removal work in the center of the ESP.
The ESP represented a special challenge in that access to its roof was difficult, and there was little space available in which to locate the ModuPower SMPS equipment. Also, another SMPS brand previously installed had failed to operate reliably in the high temperature and dirty environment within the ESP weather enclosure.
With this in mind, Stock Equipment worked with the customer to remote-mount the ModuPower units in a pre-wired modular control room that was located on the ground at the base of the ESP. The entire system was delivered as two structures, each containing six of the 12 ModuPower units provided. The plant was required to only connect power to the switchgear provided in each control room and run high voltage cable to the roof of the ESP.
All the installation work was completed with the unit in operation, with only a short outage required to make the final electrical tie-in to the ESP. This unique remote SMPS installation provided by Stock Equipment enhanced the project’s ROI significantly.
The ModuPower SMPS installation produced an over 100% increase in power sent to the ESP, from 464 kW to 1052 kW. With the exception of the inlet fields that are exposed to the highest dust burden, all fields are operating at the nameplate current limit rating of both the ModuPower units and the original TR sets.
More importantly, the repowered ESP has eliminated outages caused by excessive particulates entering the FGD and restored the quality of the gypsum by-product. The plant has since installed an additional 12 ModuPower SMPS systems on fields 1, 4, and 6 of the same unit.
A 670 MW supercritical utility boiler located in West Virginia, US is configured with a wet FGD after its ESP (see Figure 2). The ESP is configured as four fields deep, eight lanes wide, with each field using weighted wires on a nine-inch gas passage spacing. Though the ESP as designed did not perform effectively, it did not experience excess particulate emissions because of the downstream wet FGD. However, the two induced draft (ID) fans that are positioned between the ESP outlet and the FGD inlet were suffering significant mechanical erosion and producing increased vibration as a result of excess fly ash leaving the ESP. Quarterly fan cleaning, repair, and rebalancing, plus the associated cost of a four- to five-day outage cost millions, and easily exceeded the cost of an ESP upgrade.
|Figure 2: A 90 kW ModuPower was installed at a West Virginia coal-fired plant
Credit: Stock Equipment Company
Plant management considered a number of permanent repair options including expanding the ESP, but the cost was prohibitive and the site was space-constrained. The best option was to upgrade the ESPs to reduce the amount of particulate emissions entering the ID fans.
The plant first rebuilt all the ESP boxes to 11.5-inch spacing and retrofitted the ESP with rigid discharge electrodes in an effort to improve the particle removal efficiency. However, the existing 45 kV TR sets ran out of secondary voltage and fan wear worsened. The plant next tried to replace the existing TR sets with a higher-power design but without changing the kVA rating of the units. The new TR set produced higher voltage (kV) and lower current (mA) but produced the same primary current rating. This approach saved the cost of changing out the CLRs, power cables and other equipment inside the automatic voltage control cabinet. However, the new TR equipment hit its secondary current limit before reaching significantly higher voltage levels (Eo) and fan erosion continued.
The plant eventually decided to upgrade its ESP with an SMPS, and ModuPower units from Stock Equipment were selected. Eight 90 kW (83 kV/1080 mA) units replaced the newer 36 kW TR systems (65 kV/550 mA) on the inlet field of the ESP where the majority of the particulate removal takes place.
|AES Norgener ESP with ModuPower ground switches installed on top of its ESP
Credit: AES Norgenere
The site arrangement required the ModuPower equipment to be installed away from the roof of the ESP and required the use of a high voltage cable to bring the DC power to the ESP. In addition, the ModuPower units were installed and commissioned one at a time while the boiler and ESP remained online, thereby avoiding one or more costly plant outages. The original TR sets were disconnected but left in place as backup systems should the need arise.
ModuPower installation raised the power (kW) not only in the inlet field but also across the entire ESP. The improved collection efficiency at the inlet of the ESP allowed the downstream conventional TR sets to perform at a higher efficiency as well. Power provided to the ESP increased 204 kW and the current increased by 3114 mA, equivalent to the power input produced by five TR systems.
A year after the installation was completed the unit has not experienced a single FD fan erosion- or vibration-related outage. The utility saved over three times the cost of the ModuPower units in the first year of operation. The utility has since purchased ModuPower units for the remaining three fields on this unit, and all four fields at the other two 670 MW units located at this plant.
Upgrading the particle removal efficiency of existing ESPs is inevitable as new air quality rules take effect. However, a ModuPower upgrade can also be a good business decision because it can minimize ID fan erosion and improve the performance of a downstream wet FGD, thereby reducing the ash content in the gypsum byproduct so it can be recycled rather than landfilled. For ESPs already meeting air quality regulations, an SMPS upgrade can reduce auxiliary power requirements and pay for itself in short order. Customers and ESP OEMs around the world are standardizing on SMPS technology for ESP upgrades and new precipitator installations in place of the conventional power supply systems.
Jason Horn is director, environmental controls for Stock Equipment Company (www.stockequipment.com)
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