Desktop version

Home arrow Environment

  • Increase font
  • Decrease font


<<   CONTENTS   >>

Quencher Optimization

Probably, the most severe environment that air pollution control equipment must resist occurs in high-gas temperature applications such as hazardous waste incinerators. Gas inlet temperatures can exceed 2200 degrees F (1200 degrees C). The combination of corrosive gases, high gas to liquid temperature differentials, rapid water evaporation rates, and the rapid near explosive flashing of water to steam all combine to create an exceedingly aggressive environment. It is not uncommon to hear from nearby the hostile environment going on in a hot gas quencher.

The quencher must operate reliably. Maybe the word should be must. Not only does the quencher reduce hot gas temperatures in preparation for gas cleaning but also likely protects expensive downstream equipment from thermal damage and/or destruction. Most quencher designs are therefore configured to be reliable. It is not uncommon that the quencher is over- designed mechanically (incorporating conservative gas velocities, residence time, and using excessive water for example) and its associated equipment such as piping, and water source is sized and selected for reliability. It seems simple. Just cool the gases with water. But it is not simple. In a quencher, the cooling is not immediate. There is a time delay factor (much like hysteresis in magnetism) as the water, whether injected as a spray or in sheets, absorbs heat, increases in temperature, and changes phase (from liquid to vapor). This all can occur within a fraction of a second but not instantaneously in the quencher.

Possible Problems

Conservative design aside, some problems can occur with quenchers that often are revealed only after the installation. Some of those possible problems are as follows: (the following assumes a vertically mounted quencher)

  • 1. Insufficient gas cooling (higher than desired gas outlet temperature)
  • 2. Spray drying to particulate
  • 3. Poor gas distribution causing localized "hot spots" or refractory wear

FIGURE 30.1

Quencher, Bionomic Industries.

  • 4. Inability to handle peak temperature spikes
  • 5. Thermal expansion issues causing structural problems
  • 6. Local corrosion issues

Optimization Techniques

Some optimization techniques to investigate include:

  • 1. If insufficient cooling is observed and the quencher's size (usually length) cannot be changed given lack of space for a new one, the focus turns to the liquid circuit. With quenchers, the liquid injection amount and method are critical. The quenching rate, like absorption, is surface area dependent. The liquid's surface area is extended so that the water can convert to water vapor rapidly. The designer typically selects nozzles and spray patterns to provide the required liquid coverage in the quencher. A uniform gas flow pattern is usually assumed. The liquid flow rate (volume) is selected to saturate the gas stream at least adiabatically plus a safety factor. Some of the water will be sensibly heated but not participate significantly in the conversion to water vapor. That safety factor flow leaves as heated water. It is the droplets that heat and then evaporate to near completion that do the work. The nozzle droplet size is often determined to prevent the droplet from drying completely (more on that subject follows below). Reality can step in however requiring a change in the nozzle type, spray pattern and droplet distribution. Sometimes turning a spray header upward and changing to a full cone nozzle can both increase gas/liquid residence time and provide additional cooling without increasing the quencher vessel size. In addition, the quenching can often be “staged" by using multiple headers or altering the flow rate to existing headers through external piping changes. “Staging" injects the higher volume of water at the upper portion of the quencher wherein the evaporation rate is greatest. The goal is to drop the gas temperature rapidly to below about 500 degrees F (260 degree C) wherein the evaporation rate per unit volume tends to decrease. The decrease occurs because the gas surrounding a droplet already contains significant water vapor. "Staging" takes advantage of the high evaporation rate of that hottest zone. "Staging" also rapidly reduces the gas velocity so the residence time in the quencher is optimized. Nozzle suppliers such as BETE Fog Nozzle, Spraying Systems and others can help select alternate nozzles and perhaps recommend changes in the spray location whether staging is used. Experimenting with a different set of spray nozzles is often worth the effort prior to investigating more expensive and difficult options. The nozzle vendors do not, however, guarantee results since there are too many operating variables that ultimately determine success. You would need to provide operating data such as the existing nozzle size and type, location, and process conditions (gas inlet and outlet temperature, pressure, and humidity) to their applications professionals. A drawing showing the quencher header size and location would also be needed. If the ductwork allows it, sometimes a high pressure hydraulically atomized or an air-atomized spray can be added ahead of the quencher to provide additional cooling prior to the quencher. But with a caveat...
  • 2. If the droplets are too small and contain dissolved and/or suspended solids, spray drying of those solids into gas borne particulate can occur. This drying occurs where the differential in gas to liquid temperature is the greatest. At that point, the heat transfer is greatest as is the evaporation rate. Thus, if a spray is added or nozzles are changed, it is best to apply the cleanest, lowest dissolved solids water at the hottest zone of the quencher. Quite often, the optimization involves using softened water or stripped condensate to the highest temperature liquid injection point. Some facilities blend recycled water with softened make-up water at that point to reduce spray drying. Sometimes adding a filter for that portion of the quencher circuit can provide enough suspended particle reduction. It is helpful to perform a water analysis to determine if filtration alone can help. If the dissolved solids are minor and the droplet size is large enough, the spray drying will be minimized. Thus, filtering the water and shifting to larger droplets may be what is needed for optimization if the dissolved solids are low.
  • 3. Poor gas distribution at the inlet of a quencher is all too common. Often the gas inlet ductwork includes an elbow or other directionchanging elements are located ahead of the quencher. To reduce costs, these typically refractory lined pieces are made to a minimum size and are located as close to the quencher inlet as possible. In addition, there may be expansion joints or the like that can upset the gas flow pattern. If the hot gases are not distributed evenly in the quencher, the quencher liquid circuit (usually comprised of spray nozzles) is not optimized and hot spots can be created. A thermal imaging scan of the quencher can help locate those hot spots. Local passageways of hot gases can occur thus resulting in higher than desired gas outlet temperatures. Usually at least two duct diameters of straight ductwork are required ahead of the quencher gas inlet to provide acceptable gas flow patterns. Turning vanes are usually not an option given the cost of applying such vanes in such a high temperature environment. If the inlet ductwork cannot be raised to create a uniform gas inlet pattern, the options are limited and usually expensive. A CFD model can be run on the quencher spray pattern and nozzles can be relocated to place the greatest spray in the hottest gas stream. Some optimization methods have included adding a "core buster" disc in the center of the quencher at a point below the uppermost spray injection point. The "core buster" makes the gas stream divert around the disc thus acting as a gas distribution device. The residual spray from the uppermost liquid injection area falls onto the disc thus enhancing the cooling. The disc, however, must be supported from the quencher wall and must be resistant to both the corrosive environment and the temperature extremes. The disc need only reduce the quencher area by less than 10% or so. The disc just provides some resistance to gas flow that aids in stabilizing the gas flow. The pressure drop on the quencher however will increase. The quencher supplier, if they recommend a disc, will need to calculate the pressure drop increase. The capacity of the prime mover would then need to be checked to see if the addition of a disc will allow adequate process ventilation. As can be gleaned from the above, it is best to provide the 2+ duct diameters above the quencher inlet.
  • 4. Some processes cruise along in a controllable steady temperature condition while others exhibit temperature spikes. These spikes are often unpredictable and could cause damage to the quencher and related equipment. Often, if spikes are expected, the liquid circuit of the quencher is oversized (excessive water flow continually) or an additional emergency header is used. To keep the emergency header cool, a constant flow of water at a minimum is provided. If the spike occurs "on schedule" the emergency header flow is switched to full flow prior to the "event". Often two solenoid valves are used in parallel with a small one providing the constant flow and the larger one opening when a spike is expected. If the spikes are random or cannot be predicted and an oversized liquid flow is not practical, temperature sensors located at the point closest to where the temperature spike could occur a long with a temperature sensor after the quencher can be used. The sensor signals are monitored and the rate of change of both sensors is used to trigger the supplemental water addition. In other words, if the sensor closest to the hot source rises rapidly, an output signal is sent to the larger solenoid valve. The sensor after the quencher also tracks the temperature. If the postquencher temperature continues to rise an additional solenoid valve can be opened. Control valves are often avoided since their response time is slower and they are more expensive. In addition, these solenoid valves may also be used to administer gravity fed water in case of an emergency. To reduce water hammer, pilot operated solenoid valves are sometimes selected. The emergency valve is typically normally open so that under power loss water can flow for cooling purposes.
  • 5. Thermal expansion, particularly in alloy quenchers, can reduce the life of this often expensive piece of hardware. Alloy is expensive and the quencher vessel usually uses as little of the alloy (thickness) as possible. This means that the vessel is an essentially a piece of ductwork designed to support itself with no external load. It is not a structural member. Expansion joints at the gas inlet are therefore commonly used if the design calculations reveal excessive expansion is possible and the vessel is supported in structural steel. Another technique involves "hanging" the quencher from higher-level structural steel and allowing the quencher to "grow" downward. Often an expensive expansion joint at the gas inlet can be avoided. However, if the quencher is attached to downstream equipment an expansion joint may be needed at the connection of the quencher outlet and that downstream equipment. Hanging the vessel can also make header access easier thus simplifying maintenance.
  • 6. Local corrosion issues usually occur where liquid can "pool" and thereby create a corrosive galvanic cell. When doing an interior inspection, care should be taken to discover and repair any areas wherein liquid can pool. Horizontal surfaces in a vertically oriented quencher should be avoided if possible. If corrosion is noticed on the outside of the quencher, an inspection (if accessible) on the related interior surface should be made. Instead of patching from the outside, patching from the inside may produce a better result. With refractory lined quenchers, a multiple layer type construction may be used. That construction may include an elastomer lining (often rubber) between the vessel wall and the refractory. Sometimes rammed or cast refractory is used. Inspection should include finding and repairing cracks that could allow corrosive water to pass behind the refractory. Though in operation the vessel wall temperature may be within guidelines, the corrosive water could be gradually reducing the quencher's life. Keeping the quencher vessel in optimum condition helps to optimize its performance.

31__

 
<<   CONTENTS   >>

Related topics