Table of Contents:
The Requirement of Synergy
To achieve the highest performance at lowest cost, few air pollution control devices require more "synergy", more "balance" between the primary and ancillary components than precipitators. The "primary" components are the vessel, the "can", and the ancillary components are the devices used to make the "can" work.
Whether a dry or wet design, these components must all work together. Changing a process or moving a precipitator to a new or modified process often requires going back to the beginning. For a dry application, resistivity testing of the particulate may be required. If acidic or other corrosive gases are now part of the input contaminant mix adjustments may be required. For wet electrostatic designs, the inlet particulate loading, humidity, or settling characteristics have changed, the "problem" may have to be redefined.
For these reasons and others, to optimize the performance of a precipitator, it is suggested, by this author at least, that you seek the advice of the original device supplier or enlist the engineering expertise of consultants in that field. Investing in a thorough study of the problem and focused definition of the project goals up front, increases the likelihood of project success. There are a variety of sources, at the time of this writing, of locating experienced precipitator consultants. One combined source is the www.apcnetwork.com.
Primary Precipitator Components
Why the need for a study? Take the "can" velocity. In a dry precipitator, during the rapping of the collecting plates, the particulate is discharged and hopefully falls to the collection hopper. Given the impact energy of the rappers, plate spacing, particulate aerodynamic characteristics and can gas velocity, some of this particulate may be entrained and leave the collector. Precipitator designers carefully design the vessel based upon these characteristics and choose the vessel dimensions so that the local gas velocity is below the carrying velocity of the particulate. Change the particulate characteristics and/or the treated gas volume, and adjustments to the vessel may be needed. For a modular precipitator, perhaps a new module may be needed. For major changes, an entire new device may be the best solution.
Gas Inlet Ductwork
Say the inlet gas ductwork had to be changed and the device performance deteriorated. These devices, wet or dry, for best operation require uniform inlet gas distribution. Inherently low in flange-to-flange pressure drop, energy wasting changes of gas direction are minimized. But minimization does not mean elimination. Often perforated plates, turning vanes, or other such devices are needed to achieve acceptable inlet gas distribution. The goal is to ensure that the internal collecting surfaces, wet or dry, are functioning as designed.
Importance of Particulate Characteristics
Changes in the particulate characteristics may also force a re-definition of the problem. The particulate's characteristics may need to be modified through inlet gas conditioning. Though typically less important in the use of wet precipitators, inlet gas conditioning may be required to enhance the collection characteristics of the particulate. So called "dry fog" inlet gas conditioning may be required wherein atomized water is administered under controlled conditions wherein the fog dries to completion (minimal residual droplets). Sometimes specific chemicals are added to the spray based upon particulate characteristics testing.
After the particulate is captured, it must be removed from the device. It must be removed before it causes trouble. In a dry precipitator, the control of the dust level in the hopper is particularly important. Excessive particulate may entrain out of the device. If the particulate is hygroscopic, local buildup and plugging could occur. These hoppers are equipped with dust-level sensors such as a Bindicator shown in Figure 29.1.
The setpoint of the highest level of particulate may need to be tuned to minimize the entrainment of particulate. Often an internal inspection of the device will reveal traces of dust carry-over. The lowest particulate level may impact the performance of the particulate removal devise (often a rotary lock). To be properly "fed" some locks enjoy the use of a small inventory of particulate above the lock. Some applications, prone to build up or "bridging" require the zone above the discharge device to be kept free. Local disrupting devices may be needed in those applications.
Collecting Surface Characteristics
For wet precipitators, if the water is in whole or in part recycled as a portion of the collecting surface irrigation, the setting characteristics of the collected particulate could be significant. Often the lowest irrigation water speed is in the device itself and not in the irrigation recycle loop. Some wet electrostatic precipitators use internal distribution weirs. If use of a weir causes the water velocity to fall below the setting velocity of the irrigation water, local buildup can occur. Further separation of recycle solids through settling external to the device and/or increases in makeup water to the internal distribution network may be required.
Electrode Type and Characteristics
The internal electrode design, location, and function are of critical importance. Individual precipitator supplying firms have developed, modified, and applied electrode design configurations that they feel best suit the performance needs. Some after-market suppliers have also tried to make improvements in electrodes. Caution must be taken, however, if the precipitator is operated under a Permit that explicitly defines the device. In that case, it is suggested that you contact the original supplier. If the Permit simply requires periodic testing or continuous monitoring, you may be able to change the electrodes or other internal components if the performance is maintained or is improved. It is best to check with the local regulator agency if these changes are contemplated.
Some of the Primary Component requirements are quite simple but are overlooked or change with time.
Is the device installed so that the internal components are vertical? Sounds simple but these devices can settle over time. Dry or wet, the internal devices
“hang". In a precipitator the electrodes (even if restrained) tend to hang. In a dry device, the electrodes may be weighted but they need to be in the proper orientation to the collection plates to maximize performance. In a wet device, the tubes need to be truly vertical so that, in similar fashion, the distance between the electrode and the wetted tube is maintained within the design distance parameters. A double check, and adjustment if needed, of the vertically of the device may be required.
The ancillary components for this discussion include the rapper design and application for dry precipitators; the irrigation system for wet precipitators, and the power supply for both.
As mentioned earlier, synergy is needed for success. For dry precipitators, the original supplier and/or a qualified consultant is suggested to be retained for any investigation of the rapper design, operation, or adjustment. Though the rapper section is in the vessel, it may be helpful to view them as ancillary. Some adjustments can be made to improve performance if done under the watchful eye of the original supplier or experienced consultant.
For a wet device, the irrigation system can often be refined. If the irrigation is by city or plant water, it is likely that few adjustments are needed. Sometimes if flow monitoring is lacking, flow instrumentation and control is added. Doing so helps minimize water consumption and helps identify and solve local buildup issues in the vessel. If the irrigation water is recycled in whole or in part, solids characteristics and concentration may need to be addressed.
Often, increasing blowdown can reduce solids concentrations to acceptable levels. Settling characteristic tests can be conducted to estimate the settling rate if the recycle is held in an external tank prior to injection. The tank may simply be of improper size and may need agitation. If fresh make up water is added to the tank, it may be best, instead, to add the water directly into the recycle header (thus diluting the recycle and forcing the "clean" water to at least take a once-though path through the precipitator). In some cases where the solids have value, it may be best to invest in a higher efficiency solids reclamation device. The existing system may just use settling or a liquid cyclone so filtration may be a substitute to consider and offer both recovery and performance advantages.
The Power Supply
For both wet and dry, advancements have been made and are being made in the power supplies that provide the high voltage to the electrodes and that control that voltage. Improvements in the control electronics allow the
NWL power supply.
more modern power supplies to provide the high voltage more uniformly this improving collection efficiency.
High frequency power supplies as provided by NWL and others might be considered as a substitute and improvement over older or lower efficiency devices. The NWL design unit is shown in Figure 29.2.
These newer power supply devices apply improved voltage regulation and stability thus maintain the high voltage application within a narrower range. The devices require considerable investment thus their need requires detailed investigation and study.
After design parameters of the application have been defined and any updated testing has been completed, changes to the Primary Components (and related costs) can be obtained followed by an analysis of the need for any upgrades to the Ancillary Components. Properly applied, the precipitator should show improvements in performance through optimization of these critical components.