Incineration [PDF]

Zitiervorschau

COMMON INCINERATION PROBLEMS

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COMMON INCINERATION PROBLEMS This document lists common problems encountered with industrial incinerators handling vapor and liquid wastes. The list is not complete, of course, since the waste streams are quite variable and people are inventive. New problems surface all the time, but many involve these issues: 1. A particular operating condition or waste characteristic was overlooked in the specification phase.

2. The equipment supplier didn't design the system correctly or there were fabrication errors. 3. The installation crew omitted some important part of the system. 4. The incinerator safeties were poorly maintained or preventative maintenance was not performed.

5. Plant production changed, the waste changed, but incinerator was left "as is". Ultimately all users would like their incinerator system to run like a Swiss watch and need no maintenance, ever! But routine checks, cleaning and refurbishment will be necessary to avoid nuisance shutdowns and even safety problems. Spare parts is big business for hardware suppliers and for good reason. Click on the topics below - maybe one fits your situation. If not, or if you want to contribute, contact us! 1. 2. 3. 4. 5. 6. 7. 8. 9.

Definitions Burner Related Problems Combustion Pulsation and Noise Waste Handling Problems Corrosion / Erosion Thermal Damage Thermal Expansion Problems Scrubber Problems Boiler Problems

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DEFINITIONS

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DEFINITIONS: Waste: Usually a byproduct of hydrocarbon processing or chemical manufacture. If a liquid, it may be mostly water or can range up to waste solvents with fuel qualities. If a vapor, it can range from lightly contaminated air to near fuel gas. Some vapors are an inert such as nitrogen or CO2 mixed with oxygen and/or hydrocarbons. Vapor waste compositions often vary according to process plant operation. Both liquid and vapor wastes can contain acid components (Cl, F, S) and "ash" such as Na, Fe, etc. Incinerator: Refractory lined chamber with a fuel burner, air supply, quench medium (for high BTU wastes), waste injector(s) and optional heat recovery and flue gas scrubbing. An instrumented safety and control system is used to assure proper addition of fuel, air, wastes and quench plus operation of the optional hardware. Chemistry: Waste is heated above its ignition temperature in the presence of fuel and air to provide for complete combustion. Time is allowed for the combustion reactions to proceed far enough to satisfy legal emission requirements. Safety: Avoiding dangerous conditions which may lead to injury to personnel or damage to equipment. Performance: Ultimately each incinerator system has to meet the requirements of the air permit under which it operates. Specified levels of destruction for each waste component must be achieved. Specified limits on NOx, CO, acid gases, particulates, etc. must not be exceeded. Failures in this area are mostly caused by mis-specified waste streams or errors in sizing the equipment - fixes may require major and costly modifications. Scrubber: Any device intended to remove particulates, acidic gases or odorous compounds from a gas stream. Usually scrubbers absorb the target material into a liquid stream for later separation or treatment. Occasionally a dry scrubber is used, where adsorbent particles are used to remove components (usually acidic gases) from a fouled gas stream. Boiler: A heat recovery device where hot combustion products transfer heat to water, forming steam for use elsewhere. Boilers may include Economizers and Superheaters for more complete heat recovery. Boilers may be "firetube" where the hot gas passes through tubes (water outside) or "watertube" where the evaporating water is contained within tubes and the hot gases flow outside. Return to Home or Problems Page. Send us a comment about this topic?

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BURNER STABILITY:

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BURNER STABILITY: Stable burner operation is critical to good incinerator operation. A few incinerators can operate with zero fuel, but most benefit from at least a small burner. Where the waste contains almost all of the heating value needed for operation, the entire waste entry area should be considered part of the "burner"! At best, an unstable burner will cause variations in waste destruction efficiency or irritating combustion noise. At worst, burner instability can be responsible for repeated nuisance shutdowns of the incinerator system and damage to expansion joints and other components. Air Availability: Most often the total air supply is OK, but air and fuel / waste positioning within the burner or furnace is the problem. Errors in mixing the air and fuel components create localized instability although the stack gas may contain precisely the amount of excess O2 desired. Combustion noise (pulsation) is generated as the flame enters and then leaves zones where the air / fuel mixture is in the combustible range. Too much air: Quench combustion reactions => flame unstable or generates products of incomplete combustion (PIC) like carbon monoxide and even soot. Excessive fuel usage - if you heat a little air that you don't have to, you are wasting fuel. Too little air: Combustion reactions cannot go to completion => flame unstable or generates PIC. Stack smokes. Combustion noise may be present as the flame pulsates. Source of problem: Air flow control (control logic problem), air blower too small, or bad air placement relative to fuel placement (burner layout problem)

A well designed burner injects high BTU (high air demand) fluids well inside the main air flow path. Inerts (nitrogen, aqueous wastes) are added away from the main flame area in order to avoid quenching the flame before it fully develops. Switching the locations of the fuel and inert streams here would make the burner unworkable. Waste Flow / Composition Variations: Waste is an undesired byproduct plant production. The flow and composition might swing as the plant’s control loops focus on keeping the main product on spec. Waste flow varies too rapidly: air flow cannot track and air availability is a problem or auxiliary fuel / quench medium

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BURNER STABILITY:

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cannot track and furnace temperature varies out of range (possible system shutdown). Waste composition varies too rapidly: can cause the same problems as rapid waste flow changes. Source of problem: the control system can't anticipate the shifting combustion requirements so adjustments are often out of phase.

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Combustion Pulsation and Noise

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Combustion Pulsation and Noise All commercial burner systems make some amount of noise, starting with the white noise generated by a gas fired home heater. Systems firing millions of BTUs produce correspondingly louder noise. Improper design can generate lower frequency noise, sometimes at very high decibel levels. Such noise is usually accompanied by visible pressure fluctuations in the furnace or boiler. If a furnace is involved, and is designed (accidentally, of course) to resonate at a critical frequency, then much higher frequency noise is possible. For more detail, see the fact sheet. Back to Common Incineration Problems page

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Fact Sheet

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Fact Sheet: Combustion Pulsation & Noise All flames make noise and larger ones make more. You can’t hear the noise from a pocket lighter, but you know when your gas water heater or gas fired central heat burner starts. And any industrial size furnace makes enough noise that you can tell if it is on without looking. Usually combustion noise can be ignored – it seldom causes problems. In rare cases it’s loud enough to draw complaints from the neighbors and even vibrate equipment to destruction. Here we will consider cases like that. A Little Theory: Any noise has a frequency and an amplitude. You can hear noise over a frequency range of about 20 to 20,000 Hz or cycles per second. Noise at a frequency less than 20 Hz might be felt but not heard – you might notice the floor or windows rattling even though you can’t hear the sound. If you have good hearing, the quietest sound you can hear is slightly louder than zero decibels (db). The loudest (I’m told) is 120 db, at which point your ears are quickly damaged. (http://en.wikipedia.org/wiki/Psychoacoustics) In the combustion world, there seem to be two categories of noise: z z

Noise formed by the resonant characteristics of furnaces, ducting, stacks, etc. Noise formed by a flame only.

Actually noise in the first category is seldom a problem unless a flame is also involved. Noise inherent in the system appears to be amplified by the flame energy. A few field reports illustrate this: Field Report No. 1 "Claus Plant Incinerator" A new Claus tail gas incinerator was built at a refinery in the Houston area. It included a relatively long tail gas duct running from the last sulfur condenser down to the incinerator burner. The burner was natural gas fired and included a centrifugal air blower to overcome the pressure drop across a water tube waste heat boiler positioned at the incinerator furnace outlet and exhausting to a carbon steel stack. This system was designed for future plant expansion, so throughput was only a fraction of design. Immediately upon startup, the operators heard a very loud, relatively high frequency noise. The noise disappeared when the burner was fired at its design rate, and of course it went away when the burner was off. Attempts to modify the burner fuel gas injection geometry had little effect on the noise, even though this step often eliminates or at least reduces similar problems. An acoustics expert was called in. Microphones were installed at several locations throughout the system but the problem source remained elusive until, with the burner turned off, low level noise at the problem frequency was detected. This meant that the noise was being triggered somewhere in the system and the burner was simply amplifying the signal into a container accidentally sized for resonance.

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Fact Sheet

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Ideas to alter the resonant frequency of the equipment were considered. For instance, shortening or lengthening the stack would have changed the resonant point and cut the noise amplitude. The fix finally selected consisted of installing a perforated stainless steel plate across the boiler outlet flange. The perforations were sized to create as much flue gas pressure drop as the blower and Claus plant could handle. That fixed the noise problem by interrupting the resonance. Field Report No. 2 "Chemical Plant Thermal Oxidizer" At a chemical plant, also near Houston, a new thermal oxidizer system was installed to dispose of offgas from an acrylonitrile plant. The system was to have a waste heat boiler installed later, so the horizontal furnace was connected via a long duct to the exhaust stack. A forced draft natural gas burner was used to bring the furnace up to temperature. Once the refractory was cured out, the operator switched the vent gas stream away from atmospheric vent and into the burner. Immediately a very loud, high pitched noise was heard. Apparently the furnace system was sized just right for resonance and the burner provided the amplification, while the noise was started at the inlet valve. Fortunately, a simple change to the inlet valve position changed the frequency enough to reduce the noise to acceptable levels. In this case the change was easy to accomplish and worked "like magic". Field Report No. 3 "Elevated Flare" A refinery in California installed a large, steam assist, elevated flare. Steam injection was necessary for smokeless burning of the waste gases. Upon startup, a very loud, low frequency (2 Hz) noise was produced. It resulted in complaints of vibrating floors and walls from workers well away from the flare site and steps were immediately taken to find the cause and eliminate it. With trial and error and a few lucky guesses, the cause was determined to be the steam jet position in relation to the waste gas injection passage. By raising the steam jet elevation about 2 inches, the problem noise was eliminated and smokeless operation maintained. Field Report No. 4 "North Dakota Hydrogen Vent Incinerator" A petrochemical plant in North Dakota produced a waste gas rich in hydrogen. It was to be used to fire a process heater. The heater was equipped with a number of floor mounted burners, each with a central fuel gas gun mounted inside a refractory throat. Upon startup, a very loud noise was produced. In this case there were operator complaints, and the amplitude was so great that it was loosening the nuts holding the furnace platforms and other hardware in place. With nuts falling, the furnace could not operated while personnel were below. This problem was solved by adding a central gas passage through the center of each burner gas gun. This changed the flame geometry enough to "detune" the burners and reduce the noise to acceptable levels. Field Report No. 5 "Houston Chemical Plant Waste Heat Recovery Plant" A petrochemical plant near Houston operated a large horizontally fired incinerator mated to a water tube heat recovery boiler and economizer. The flue gas was ducted to atmosphere through a 100 ft. stack close coupled to the economizer. The waste gas had very low heating value and was injected, along with the combustion air, through a series of stainless pipes installed through the furnace refractory lining. A natural gas burner started the combustion and two waste liquids were sprayed in between the burner and the waste gas injectors. With increased plant production, a "noise" began to appear. In this case, the frequency was only 1 or 2 cycles per second, and was detected by rhythmic swelling of the fabric expansion joints connecting the economizer to the stack. Expansion joint life was reduced to about 6 months, and the system had to be taken down for fabric replacement (total shutdown about 3 days each time). Previously, the plant could be run about 2 years between shutdowns. In this case, modifying the waste distribution across the pipe injectors resulted in enough amplitude reduction to solve the problem. The low frequency noise was still present, but flexing of

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Fact Sheet

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the expansion joint fabric was eliminated. More Theory: How can a fuel gas burner can act as an amplifier for noise? It appears that the pressure changes which cause the noise are created when the fuel and air streams are combined improperly. If the two streams are combined incorrectly, pockets of rich or lean mixtures are created which are not "flammable". As additional fuel or air enters the pocket, the mixture becomes flammable and burning continues, increasing the volume of the pocket, which displaces the air and fuel streams, forming a nonflammable pocket again. As this cycle continues, the on/off nature of the combustion is seen as pressure fluctuations. With many industrial size furnaces, the pulsation frequency is in the range of 1 or 2 per second, but with hydrogen rich fuel gas or fuel gas under high pressure, the frequency can be greater. Premixing the fuel and air before ignition eliminates the problem. This "premix" burner method is found in the small pilot burners used to ignite an adjacent larger burner. This is also the type of burner used in kitchen ranges, propane grills and gas fired central heat units. Larger burners are typically "diffusion type" burners, meaning that the fuel (gas or oil or pulverized coal) is injected adjacent to the combustion air stream. As the fuel diffuses into the air stream (and vice versa) the burner flame develops and grows until all of the fuel is oxidized. So how would a fuel burner "amplify" an existing noise, as apparently happened in Report Nos. 1, 2, 3 and 5 above? In Report No. 1, the inert waste gas was injected through an annular gap surrounding the flame zone. Apparently the pressure fluctuations in the waste gas distorted the fuel gas / air mixing process at the existing frequency, and the energy already available from the flammable mixture pockets was redirected from standard combustion noise to the new frequency, which happened to be the same as the resonant frequency of the furnace system. Bad luck. The other cases involved a similar effect, although the circumstances look quite different. Conclusion The negative effects of combustion pulsation include; z z

z z z

Rhythmic flame body displacement (flame detection device may lose sight of the flame), Brief loss of combustion air flow as furnace pressure peaks, briefly reducing air flow (low flow switch may trip), "Breathing" of the furnace shell and expansion joints leading to material failure through fatigue. Structural damage to the furnace if the amplitude is high enough. Extremely irritated operators and neighbors.

Eliminating combustion pulsation and excessive noise is possible through changes in burner operation (reduce or increase air flow, change steam flow, etc.), but often some sort of hardware change is required. One approach involves changing the location or velocity of fuel injection. Another involves changing the location direction of combustion air flow, steam flow or waste flow. And in some cases, steps must be taken to change the resonant frequency of the furnace/stack assembly. These changes are typically accomplished in an experimental manner - change only one thing at a time, record results and move on till all options have been tried. Sometimes observation of the flame body will give a clue as to the best approach. There is some hope from acoustic analysis of the resonating chamber(s) as in Report No. 1, but this is presently very difficult and typically too expensive to use prior to a problem showing up.

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Fact Sheet

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Combustion noise is inevitable, but excessive noise can always be controlled without resorting to magic. Dan Banks, P.E. Banks Engineering Inc. Tulsa, OK www.banksengineering.com Phone 877-747-2354

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WASTE HANDLING

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WASTE HANDLING: Liquid wastes can normally be run thru tankage to even out flow and composition swings. Some liquid wastes must be hot in order to flow, so heat tracing and insulation design are critical. Solids in liquid wastes can deposit on injection equipment, causing plugging or poor distribution inside the burner. Vapor wastes must flow directly to the incinerator, so variations have to be accommodated quickly by the burner and instrument system.

Waste Stream Isolation: Many incinerators have to handle multiple waste streams. Mixing several vapor streams or several liquid streams for movement from the source to the incinerator can save money, but if mixing causes reactions and deposits, the cost savings are soon forgotten. Prematurely mixing a gaseous waste high in oxygen with one high in hydrocarbons can create a highly combustible mixture, and should be avoided if possible! Keeping these streams isolated until injected into the incinerator is always preferable. Where these problems are not present, it may still make sense to avoid premature mixing, since differing stream characteristics often dictate specific injection points in the burner or furnace - loss of one of the streams changes the combined waste stream characteristics and may cause burner stability problems. Source of problem: Premature mixing of waste streams.

Flame Arrestor Plugging: Waste gases with the potential to be in the combustible range are normally run though a flame arrestor (or detonation arrestor) just prior to incinerator entry. Some waste gases can be saturated with a heavy compound such as tar. Cooling in the waste gas duct can create droplets of tar, plugging most types of flame arrestor. Some wastes contain ash or other particulate matter, creating similar plugging problems. Usually duct heat tracing, ash filtering or selection of a packed bed or other type of arrestor more resistant to fouling will solve the problem. Source of problem: Contaminants within a vapor waste and improper flame arrestor selection.

Flash-back Protection: Sometimes a waste gas stream will pass through the combustible range in the course of plant operations. The incinerator is an ignition source. Once started, a flame proceeds at specific velocities through a combustible mixture until it is cooled below ignition temperature or passes into a noncombustible zone. Flash-back from the incinerator to the source is a common safety hazard. Methods to prevent flash-back are (1) dilution with air or inert gas, (2) enrichment with fuel gas, (3) maintenance of gas velocity above the flame velocity, (4) cooling the traveling flame using a commercial flame arrestor or (5) breaking the continuity of the combustible mixture (typically by bubbling it through a water bath). Flashback can still occur if these measures are applied improperly. Source of problem: Waste gas enters the combustible range and proper safety hardware and logic is not present.

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About Flame Arrestors and Detonation Arrestors

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About Flame Arrestors and Detonation Arrestors Flammability and Flashback Prevention (a work in progress) Dan Banks, P.E.

Flammability: Overview – Flammability refers to the ability of a mixture of fuel and air to sustain combustion when ignited. Hydrocarbon molecules will react with oxygen (burn) if heated sufficiently, for instance by a spark or similar ignition source. The required temperature is different for different compounds and is called the "Ignition Temperature". The heat released by burning the hydrocarbons in the vicinity of the spark is absorbed by the hydrocarbon/air mixture nearby. If the nearby mixture picks up enough heat, it will also burn, releasing heat into the adjacent gas and resulting in burning of all of the surrounding mixture. If the mixture contains too few hydrocarbon molecules, the released heat will be too little and burning will not progress. If the mixture contains too few oxygen molecules, only part of the hydrocarbon molecules will be burned and again the heat released will be insufficient for burning to continue beyond the ignition source. In laboratory testing, various pure hydrocarbons are mixed with air to form mixtures with different hydrocarbon/air ratios. As the hydrocarbon fraction is increased, the first point at which sustained burning is observed is noted as the Lower Explosive Limit (LEL). As the hydrocarbon fraction is increased, eventually the fraction of oxygen is reduced enough that sustained burning is no longer achieved. This point is the Upper Explosive Limit (UEL). Any hydrocarbon/air mixture between the LEL and the UEL will burn, while any mixture outside of this range will not burn. Testing with methane, for instance, shows that 5% (by volume) of methane in air is the LEL. Methane’s UEL is 15%. Laboratory values for a few of the tested hydrocarbons is listed below. Note that several of the compounds actually require no oxygen at all for combustion (UEL = 100), indicating that a tank of the pure hydrocarbon will burn completely once ignited. The lowest LEL in this group is 1.05% for n-Heptane; if you mix 1.05% of n-Heptane with 98.95% air, it will burn. This mixture of n-Hexane, however, would not burn: Hydrocarbon

Formula

LEL in air (%)

UEL in air (%)

Ignition Temperature, oF

Methane

CH4

5.0

15.0

1202

Ethane

C2H6

3.0

12.4

959

Propane

C3H8

2.1

9.5

871

n-Butane

C4H10

1.8

8.4

896

n-Pentane

C5H12

1.4

7.8

878

n-Hexane

C6H14

1.2

7.4

527

n-Heptane

C7H16

1.05

6.7

491

Dimethyl ether

C2H6O

3.4

27

662

Hydrogen

H2

4.0

75

1062

Ethylene oxide

C2H4O

3.6

100

804

Acetylene

C2H2

2.5

100

581

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Effect of temperature – Almost all published flammability values are measured using hydrocarbon/air mixtures at room temperature. If the mixture temperature is higher, the LEL is reduced. Reference #3 reports that increasing the temperature 100oC = 180oF decreases the LEL value about 8%. The equation to use is Lt = 1.02 x L x (1-7.75 x10-4 x T), where L is the laboratory value of LEL, T is the elevated temperature and Lt is the LEL at T. The UEL value increases with increase in mixture temperature, also by about 8% with 180oF increase in T. Effect of pressure – Increasing the mixture pressure above atmospheric affects the LEL very little. The UEL, however, increases greatly. One source reports that for several saturated hydrocarbons, the UEL increases in proportion to the logarithm of the pressure. Another source reports an opposite effect for some hydrocarbons and warns that pressure effects vary according to the hydrocarbon considered. Effect of inerts – Inert gases play no part in combustion reactions, but absorb heat when present in a hydrocarbon/air mixture. For that reason, adding inerts to a mixture tends to reduce the spread between LEL and UEL until finally the mixture is no longer flammable. Reference 2 provides the graph below showing specific effects for hydrogen, carbon monoxide and methane when inerted with nitrogen and carbon monoxide. For example, methane in air with no inerts (Ratio = 0) has LEL = 5 and UEL = 15. But if 3.5 mols of CO2 are added to a mixture containing 1 mol of CH4 (in air), the mixture is no longer flammable.

Minimum Oxygen for combustion – Another way to look at the presence of inerts is to calculate the Minimum Oxygen for Combustion (MOC). The table below from Reference 4 provides values for some compounds. The data came from Reference 5.

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Flammability of hydrocarbon mixtures The flammability limits of a mixture of various hydrocarbons can be calculated using Le ChatelierÂ’s law, which states that a mixture at the lower limit of flammability mixed with other mixtures which are also at the lower limits of flammability will yield a resulting mixture at the lower limit of flammability. For example, to calculate the LEL of a hydrocarbon mixture which is 70% CH4, 20% C2H6 and 10% C3H8, LEL = 100% / (70/5.0 + 20/3.0 + 10/2.1) = 3.9% This mixture has a LEL of 3.9% hydrocarbons in air. The mixture UEL is calculated the same way.

Ignition Sources: Hot refractory – If the refractory lining a burner or furnace is hot enough to bring the hydrocarbon/air mixture to the autoignition temperature, rapid combustion will start. Heat transfer from the hot surface depends on gas velocity and turbulence, explaining why "swirl" type burners often seem more stable than more linear types. Flame – Flames from pilot burners are the typical means of initiating combustion of a hydrocarbon/air mixture. Nozzle mix burners (where the fuel mixes with the combustion air within the furnace) have zones that are too lean or too rich for combustion, so the pilot flame must be positioned to heat a volume of well mixed gas. Large pilot flames can overcome poor positioning of the pilot tip. Sparks – Sparks are used to ignite pilot burners and also main burners in some cases. Occasionally small sparks (static electricity) are capable of initiating combustion, but the extra energy in a large spark helps insure lightoff. Undesired sparks, such as those resulting from debris moving through steel ducting or fans, can initiate combustion and require careful design to avoid. Flashback Prevention Methods: Enrichment – By adding natural gas or other hydrocarbon, the mixture can be brought above the UEL, preventing combustion.

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Dilution – By adding inerts such as N2 or CO2, the mixture can be brought to a nonflammable state. Velocity – Flames progress at a defined rate through a flammable mixture. Considerable lab testing with non-turbulent mixtures has been done – data for a few gases is listed below from Reference 7. For instance, the "maximum flame velocity" of a methane/air mixture is 1.48 ft/sec under lab conditions. If this mixture flows through a pipe at 1.5 ft/sec, any flame will be unable to propagate against the flow. This fact is used in designing flare tips, burner nozzle and some flame arrestors – by designing for gas velocity above the flame velocity, the flame can be prevented from moving upstream from the point of ignition. NOTE: as a flame front moves through a vessel or pipe, the flame velocity increases. With long enough piping the velocity can increase to detonation levels, which are supersonic.

Flame Arrestor Tips – Mechanical flame arrestors stop flame propagation into or through a pipe (more information below). By placing a flame arrestor at the end of the flammable mixture pipe feeding a flare or burner, flame can be prevented from moving into the pipe regardless of the mixture velocity. End-of-pipe flame arrestors – Mechanical flame arrestors may be attached to vent pipes on hydrocarbon storage tanks to allow passage of potentially flammable hydrocarbon/air mixtures but preventing passage of flame into the tank from outside. This protects storage tanks from explosions triggered by lightning ignition of vented gases outside the tank. Cooling – For a flashback to progress into equipment, combustion heat must be transferred into the combustible mixture. By passing a potentially flammable mixture through a water spray chamber or some sort of heat sink, a flashback can be stopped. Mechanical inline flame arrestors and detonation arrestors are common heat sinks (see below).

Flashback Interruption Methods:

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About Flame Arrestors and Detonation Arrestors

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Many methods to stop flashbacks have been devised. "Active" methods require maintenance of certain parameters, such as liquid level or gas velocity. "Passive" methods require only routine inspection and typically have no moving parts or instrument requirements. Venturi type flame Arrestors (active) – Venturi flame arrestors simply create a restriction in the hydrocarbon/air mixture delivery pipe so that the gas velocity is faster than the flame speed, preventing progression of a flashback upstream. Flashback in the direction of flow can still happen. Even a partly closed valve can create a high velocity for flashback prevention, but a venturi shape creates much lower pressure drop. If gas flow stops, the venturi is no longer effective, so methods to measure flow and add makeup gas (nitrogen, for instance) are often included. Note: initially flame velocity is limited to the values in the literature, but extended pipe runs and fittings act to increase the velocity, eventually reaching detonation velocities. A venturi arrestor must be located close to the point of ignition to avoid problems. Inline flame arrestors (passive) – Mechanical flame arrestors are filled with metal or ceramic, which absorbs heat from a flashback, quenching it to a temperature below what is needed for ignition. This stops the flame. With a low enough hydrocarbon/air mixture flow rate, if a flame travels to the face of the arrestor, it can become stable at that point. Heating of the arrestor body and internals results. Once the arrestor temperatures increase enough, ignition temperature can be reached on the upstream side of the arrestor and the flashback can proceed. For this reason, a temperature switch is often installed on the flame side of each arrestor (adding an "active" element). If an elevated temperature is detected, an alarm sounds and steps can be taken to stop flow completely. An Enardo flame arrestor is shown below (www.enardo.com).

Flame Arrestor with removable element from Enardo

How a Flame Arrestor works

Inline detonation arrestors (passive) – Detonation arrestors are stronger, more effective versions of standard flame arrestors. They are certified after extensive testing per U.S. Coast Guard standards, which specify piping arrangements certain to accelerate a normal flash back to detonation speeds. The certified detonation

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arrestor must stop the flash back without damage to the arrestor itself, so it can be used repeatedly if necessary. Detonation arrestors can be certified for various hydrocarbons, which have been divided into groups according to how difficult flash backs with them are to stop. A list from Protectoseal (www.protectoseal.com) derived from National Electric Code (NEC) Article 500 is shown below:

Detonation Arrestor Certification Classes Group A

- acetylene

Group B - butadiene, ethylene oxide, hydrogen, manufactured gases containing more than 30% hydrogen by volume and propylene oxide Group C - acetaldehyde, cyclopropane, diethyl ether, ethylene, and unsymmetrical dimethyl hydrazine Group D – acetone, acrylonitrile, ammonia, benzene, butane, butyl alcohol, secondary butyl alcohol, n-butyl acetate, isobutyl acetate, ethane, ethyl alcohol, ethyl acetate, ethylene dichloride, gasoline, heptanes, hexanes, isoprene, methane (natural gas), methanol, isobutyl alcohol, methyl isobutyl ketone, isobutyl alcohol, tertiary butyl alcohol, petroleum naphtha, octanes, pentanes, amyl alcohol, propane, propyl alcohol, isopropyl alcohol, propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylenes.

Note that a detonation arrestor certified for Group B hydrocarbons is also suitable for Group C and D hydrocarbons. There is some evidence that methanol belongs in Group C or B, but recertification is not complete. Currently the European Union is defining separate standards for testing and certification of detonation arrestors acceptable in that jurisdiction.

Detonation Arrestors from Protectoseal

Liquid seal flame Arrestors (active) – This type of flame arrestor works by bubbling the hydrocarbon/air mixture upwards through a liquid bath (usually water), forming discrete bubbles. The gas exits above the liquid to the ignition source. A flash back is stopped when flame is unable to move from bubble to bubble in order to reach the upstream pipe. Some certification work has been done in Europe on this type arrestor, but so far there are no certified models on the market. A common liquid seal flame arrestor design is shown below. Note the water level must be maintained safely above the level of the sparger at all times to insure bubbles.

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About Flame Arrestors and Detonation Arrestors

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Example of a Liquid Seal Flame Arrestor

Quick acting valves (active) – By combining a very quick closing valve with very quick flame detection, Fike (www.fike.com) and probably others have built systems which reliably stop flashbacks. The Fike valves are actuated with explosive charges similar to those used in car air bags. Each time the valve actuates, replacement of the charge (and inspection of the valve) is required. The flame detector can sense the radiation from a flame or can detect the quick pressure rise associated with a flashback, and Fike supplies a special control panel to integrate the system. Below is a Fike valve drawing from their web site:

Quick Acting Flashback Prevention Valve

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About Flame Arrestors and Detonation Arrestors

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Inerts injection (active) – When a flash back is detected, one way of stopping it is to temporarily modify the hydrocarbon/air mixture so that it is no longer in the explosive range. Often this is done by injecting an inert gas such as nitrogen. Using this method, the mixture is diluted so that the final oxygen concentration is below the Minimum Oxygen for Combustion (defined above). This approach is sometimes used when a standing flame at a flame arrestor is detected – inert gas is added upstream, snuffing the flame. The system then returns to normal operation. It can also be used for routine operation, in order to prevent the possibility of any flashbacks.

References: 1. PerryÂ’s Chemical EngineersÂ’ Handbook, 4th Edition 2. Bureau of Mines Bulletin 503 "Limits of Flammability of Gases and Vapors" 3. Flammability Properties of Hydrocarbon Fuels, Wilbur A Affens, Journal of Chemical and Engineering Data, Vol. 11, No. 2, April 1966. 4. Industrial Explosion Prevention and Protection 5. National Fire Protection Association, Standard on Explosion Prevention Systems, NFPA 69, Boston, 1973. 6. Flammability Calculations for Gas Mixtures, W.M. Heffington and Gaines, W.R., Oil & Gas Journal Nov. 16, 1981. 7. North American Combustion Handbook, Second Edition.

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CORROSION

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CORROSION / EROSION: This problem is common when wastes being combusted contain acidic components or when burners / furnaces are designed with swirling flow (for high intensity operation.) Acid Dew Point: Waste containing acidic components form combustion products which are corrosive when combined with water. Methyl Chloride contains chlorine, which forms HCl. When cooled to 120 - 150oF, the water vapor / HCl mixture in the flue gas condenses. Corrosion of refractory or vessel shell material results. Sulfur compounds burn to form SO2 and SO3 (usually 3 to 10% ends up as SO3). The SO3 combines with water vapor when flue gas cools to the dew point temperature - often between 200 and 400o F, condensing to form H2SO4. Dewpoint problems often occur at the furnace shell when the refractory / rainshield system allow a low shell temperature.

Refractory linings are relatively porous. Flue gas components penetrate to the steel shell. If the shell temperature is below the acid dewpoint of the gas, condensation will occur.

Other problem areas are any heat recovery system (air preheater) or particle removal system (baghouse) where large surface areas and reduced flue gas temperature are required. Adding heat tracing and/or external insulation may prevent the condensation. Source of problem: Acidic flue gas is inadvertently cooled below dewpoint temperature or the materials of construction were not properly selected for the resulting acid contact.

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CORROSION

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Thermal Shock: Rapid temperature changes can cause localized stresses in refractory materials as the areas more easily heated or cooled expand or contract faster than the rest of the material. The resulting stresses overcome the molecular bonds holding the material together and pieces "spall" off. Commonly happens when a furnace is started or shutdown too quickly (see refractory guidelines in this website). Thermal shock can also be caused by spraying liquid water directly on hot refractory usually caused by poor atomization of aqueous wastes or water quench streams. Corrosion or heat damage to water spray gun tips is a common cause. Source of problem: Heating or cooling refractory too fast relative to its strength. Particulate "Scrubbing": Any material can be eroded by a harder material. If ash or refractory particles strike refractory or vessel shell material, some "sanding" can take place. Excessive contact can remove enough of the base material to cause equipment failure. This is most common where waste gas with entrained particulate enters a furnace or where burner / furnace swirl is maintained using an exit choke ring. Source of problem: Construction of the equipment of materials which are not sufficiently erosion resistant, given the waste composition. Alternately, equipment design which traps particles and "recycles" them across a refractory surface.

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THERMAL DAMAGE:

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THERMAL DAMAGE: The environment near the incinerator burner flame can easily reach 3000o F or higher. Loss of temperature control or selection of materials with poor high temperature properties account for most thermal damage. Metal Selection: Since incinerators operate at high temperatures, carbon steel can't be used in certain areas. Burner parts are normally constructed of 304, 316 or 310 stainless steel in order to handle normal flame temperatures. Operation with corrosive wastes may require use of Inconel, Hastelloy or other specialty metal for both temperature and chemical resistance. The degree of air cooling and radiation affect metal selection. Incinerating acid wastes usually requires flue gas scrubbing. A scrubber circulating 15% HCl at 200o F may require Zirconium or nonmetallic construction - even Hastelloy may prove inadequate at an acid temperature above 150o F. Trial-and-error or past experience with similar waste may be required for a long lasting design. Source of problem: Wrong metal selected for the waste or combustion products being handled.

Refractory Selection: An operating temperature above the service limit for a particular refractory can weaken or melt the refractory. Operation under reducing conditions (starved air) in the furnace reduces the service limit of most refractories by several hundred degrees (use 400oF as a rule of thumb). Reducing operation can cause severe shrinkage in phosphate bonded refractories (causing large cracks in castable refractory or collapse of large sections of brick linings). Waste containing certain ashes, particularly sodium compounds, can create corrosive deposits which dissolve certain refractory constituents, thinning the lining in a matter of months. Source of problem: Improper refractory selection or placement.

Radiation / Convection: Waste gases must be transported to the incinerator and then injected into the burner or furnace at the proper location and velocity for stable operation. Intermittent flow may allow hot flue gases to circulate back into the waste delivery hardware, overheating carbon steel or nonmetallic materials. Maintaining minimum flow or using a different material of construction can solve the problem. Hot refractory radiates heat, and a straight view line into an exterior waste gas plenum or duct can allow localized overheating some distance from the heat source. A change of sight line or use of high temperature materials can be a solution. Source of problem: Improper configuration, flow control or materials of construction.

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THERMAL DAMAGE:

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THERMAL EXPANSION

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THERMAL EXPANSION: Of necessity parts of any incinerator system are hot while other parts may be near ambient temperature. This temperature differential causes many problems.

Refractory Lining - Expansion Joints: A refractory lined metal vessel is constructed of several layers - steel plus one or more layers of brick or hard castable refractory. During operation the layer closest to the combustion area is hotter than the outer layer. Expansion differences require that soft joints be built into the refractory layer to avoid pressure stresses, crushing and loss of refractory material. Refractory linings constructed of ceramic fiber blanket or board don't usually have these problems, but might have others! Source of problem: Poorly designed or installed refractory lining.

Steel Shell - Expansion Joints: Metal vessel shells expand when hot. Incinerator shells are often designed for operation as high as 400o F to guard against dewpoint problems. A furnace or duct anchored at both ends can exert tremendous stresses when heated, causing deformation unless a bellows or other expansion joint is included. A cold waste gas plenum or relatively cool boiler shell expands less than the hot furnace shell it may be welded to, causing deformation or cracked welds. A hot flue gas tubesheet may expand more than the associated tubes or shell in a waste gas preheater, causing cracking at the welds. Source of problem: Inadequate provision for unequal expansion.

Tie Down Points: Horizontal vessels are normally supported by saddles. Even if an expansion joint between the vessel and associated stack or boiler is installed, if the saddle at each end of the vessel is bolted securely to the concrete foundation, then the vessel is not free to expand and deformation will result. One saddle should be equipped with a slide plate to allow movement. Source of problem: Improper saddle design.

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THERMAL EXPANSION

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Stacks: Incinerator exhaust stacks expand when heated. Stack length can increase substantially, depending on the shell temperature, which may range above 400o F. A stack supported by guy wires can exert enough force to deform or break the wires unless provision is made when the guys are tensioned during installation. A freestanding stack subject to rain or winter weather will experience uneven cooling - result: "bowing" of the stack as one side becomes cooler than the other. External insulation or a standoff "rainshield" with air gap will protect from uneven cooling. Source of problem: Improper stack design.

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About Refractory

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About Refractory: Refractory Property Definitions from Harbison Walker A 1953 Refractory Philosophy from A.P. Green Refractory Selection Comments from A.P. Green Color vs. Refractory Temperature Chart (General Refractories)

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A.P. Green on Refractories (from a 1953 paper): Over-insulation in high temperature furnaces has, to some degree, the same effect as exposing the lining to heat on two sides. If the temperature drop through the lining is so gradual that an appreciable thickness of the lining has a mean temperature within the softening range of the brick, the refractories may deform under their own weight. Particular caution should be taken in insulating sprung arches in furnaces operating at high temperature for long periods of time. The weight of the arch plus the tremendous expansion thrust developed during heating may cause the soft portion of the brick to compress and the arch to settle. It is for tile above reasons that, at the of thermal efficiency, many furnaces are left uninsulated in the hot sections, while the cooler portions are well insulated.

TYPE OF REFRACTORY By "type of refractory" we mean the major classifications of refractories - such as silica, fire-clay, high-alumina, magnesite, chrome, etc. Chemical characteristics of the furnace process usually determine the type of refractory required, although this is not universally true. We have the familiar and greatly overworked example of silica brick, the most acid of refractories, being successfully used in the roofs of basic open hearth steel furnaces. In this case the physical properties of the brick, particularly load bearing strength, are more important than the chemical characteristics. On the other hand, susceptibility to spalling prevents the successful use of silica brick in the standpoint of slag resistance. Numerous examples could be given illustrating the fact that for successful refractory practice the physical and chemical properties of the refractory must be compatible with the furnace operation and nature of the process. In order to judge the type of refractory material most likely to give satisfactory service in a certain furnace, one must have a knowledge of the limitations as well as the strong points of the principal types of refractories. Very often it is desirable to use different types of refractories in various parts of the same furnace to obtain uniformly good or balanced refractory service. General statements, such as the above, are of little help to the person trying to explain why a refractory lining failed to give satisfactory service. The following brief discussions of the outstanding characteristics of the various types of refractories is an attempt to be more specific.

(a) Fire-Clay Brick Fire-clay brick comprise about 75% of the production of refractories on a volume basis. They form the backbone of the industry - the general all purpose refractory - applicable to every service except where the conditions prescribe some one property or group of properties as being of far greater importance than others. There is no such thing as a perfect refractory. It is, therefore, necessary to balance service conditions against refractory properties to obtain , the most economical combination. Price and local availability are important factors in the use of fire-clay brick. As a type they are the least costly of all refractory brick, and there are few communities in which fire-clay brick cannot be purchased from local stocks. Fire-clay brick are also the most versatile of refractories. The fact that they have no one outstanding property is more than offset by the lack of a specific weakness. This accounts for their successful use in practically every type of industrial and domestic furnace - from the largest iron blast furnace, requiring more than a million 9" equivalent down to domestic heating furnaces or fireplaces, in which less than a hundred brick are used. All fire-clay brick are not alike and the total ranges of their properties are quite broad. A.S.T.M. subdivides fire-clay brick into four major classifications depending primarily upon fusion temperature (P.C.E.) which, within limits, is a function of the alumina-silica ratio. Other differences in fire-clay brick are the result of differences in raw materials, manufacturing process (dry press. stiff mud, wood mold, et cetera), and the proficiency of the manufacturer. From the standpoint of fusion temperature, super duty fire-clay brick have a P.C.E. of approximately Cone 33, which corresponds to 3175oF. This does not mean that a brick with a P.C.E. of Cone 33 can be used. in furnaces operating at 3175oF. On the contrary, it means that at this temperature in a clean, slag free, neutral to slightly oxidizing atmosphere, this brick would be unable to support its own weight. It is impractical to attempt to designate a safe operating temperature for any fire-clay brick because furnace conditions other than temperature affect its service life materially. Characteristically, fire-clay brick begin to soften far below their fusion temperature (P.C.E.), and under load actual deformation takes place. The amount of deformation depends upon the load, and, once started, this deformation is a slow but continuous process unless either the load or the temperature is reduced. It Is for this reason that fire-clay brick are not well adapted for use in wide sprung arches in furnaces operating continuously at high temperatures. The best load bearing fire-clay brick are the so-called high fired super duty brick. The load bearing ability of these brick is greatly enhanced by the development of a stabilized mineral structure, consisting mostly of needlelike mullite crystals, during the high temperature firing process.. The spilling resistance of fire-clay brick as a type Is good. There are variations in spalling resistance between brands depending largely upon the manufacturing process (stiff mud, dry press, et cetera), raw material characteristics, and the technique of manufacture; but fire-clay brick are most generally used in intermittently operated furnaces and wherever rapid changes in temperature would promote spalling. -This is true even when, from the standpoint of chemical composition, some other refractory material might be preferable. A.S.T.M. defines slagging as "the destructive chemical reaction between refractories and external agencies at high temperatures, resulting in the formation of a liquid". Although slagging is thus easily and simply defined, no satisfactory laboratory test for measuring it has as yet been developed and the trial-and-error method in actual service is the only reliable guide. Fire-clay brick withstand the action of both siliceous and basic stags surprisingly well. For example, note the contrast in service conditions between fire-clay glass tank blocks in contact with siliceous slag in the form of molten glass, and a fire-clay blast furnace lining which , must resist the high lime slags used as a flux in the production of pig iron.

A.P. Green on Refractory – 1953

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In addition to the properties of refractoriness and resistance to spalling, slagging, and load, mentioned above, there are other properties of fire-clay brick which vary widely between different brands but which are often the controlling factor in determining their selection for a particular application. Some of these properties are:

1. 2. 3. 4.

5.

Resistance to abrasion and erosion Permeability to gases and liquids Permanency of volume (lack of shrinkage at high temperatures) Resistance to destructive effect of certain gases (carbon monoxide, for example) Thermal conductivity

(b) High Alumina Brick High alumina brick carry the all purpose characteristics of fire-clay brick into higher temperature ranges. There are classes of high alumina brick ranging from 50% Al2O3 to 99% A1203 and with fusion temperatures from Cone 35 (3245°F) to Cone 42 (3720oF). They are generally good in spalling resistance and the ability to withstand loads. Aside from temperature alone, their most valuable property is resistance to chemical attack. That is why high alumina brick are being used in cement and lime kilns and in lead drossing furnaces. Recent test installations indicate that they will be economical far the lower sections of soaking pits in the steel industry, primarily because of their resistance to iron oxide slags. Manufacturing cost and, therefore, price of these brick increase more rapidly than the alumina content, so it is essential to determine experimentally or by test installations the most economical alumina content for each service.

(c) Silica Brick From a volume standpoint silica brick are second only to fire-clay brick. They have one outstanding advantage and one equally outstanding weakness. Their excellent mechanical strength at temperatures approaching their actual fusion is their most important property and accounts for their successful use in the wide sprung arches of open hearth steel furnaces, glass tanks, and reverberatory furnaces for the melting and refining of copper and many other metals. Their weakness lies in their susceptibility to spalling at temperatures below 1200oF. Temperature fluctuations above 1200oF do not affect silica brick adversely and in this range it is classed as a good spalling resistant brick. Many furnaces, however, must be cooled to nearly room temperature at frequent intervals and for such processes silica brick are not practical. As would be expected silica brick give good service In contact with high siliceous stags but they are surprisingly resistant td many basic slags, particularly those high in lime or iron oxide. This fact is explained by a property known to geologists and mineralogists as "Immiscibility", but merely noting the fact Is sufficient for this discussion.

(d) Basic Brick In this group we include the magnesite, chrome-magnesite, magnesite-chrome, forsterite, and periclase brick. The physical properties of this class of brick are generally poor, and their great value is primarily in their resistance to basic slags. . A brick with the physical strength and ruggedness of a silica brick would find immediate and wide use in the metallurgical industry. Many combinations of materials have been tried with varying degrees of success to approach this goal as closely is possible. Recognizing the advantage to be derived from magnesite brick In open hearth roofs and ends, an extensive program is under way to see if the lack of physical strength can be overcome by providing support by external means. Success or failure will, of course be determined on a dollars and cents basis. The cost of refractories per ton of steel produced will be the deciding factor. In addition to metallurgical furnaces, basic brick are now being successfully used in glass tank checkers and In lime and cement kilns Research and development by the refractories' companies, in cooperation with the consumers, have resulted in a great improvement in the properties of basic brick during the past 26 years:

(e) Special Refractories Besides the general classes of refractories already mentioned, there are quite a few kinds of brick produced with unusual properties which fit them to specific application. In this group are included silicon carbide, zircon, stabilized zirconia, carbon, mullite, and the fusion cast refractories. In general they are expensive and their application limited to those parts of a furnace in which their unusual properties will extend the over-all life of the furnace.

OPERATING PRACTICE "Our operating practice has not been changed in years." "Your brick are just not as good as they used to be.", These are statements frequently heard during the investigation of cases of premature failure of refractories, That they are entirely honest and sincere expressions of opinion, there can be no doubt. The point that is sometimes overlooked, however, is that there can be unintentional changes in furnace conditions long enough in duration to destroy a refractory lining but not long enough to be considered as a change in practice. We refer to such things as a burner getting out of adjustment so that the furnace atmosphere becomes highly reducing for a short period of time; possibly the operating temperature of the furnace may become abnormally high and damage the gridwork before it is corrected; or perhaps the chemical composition of raw material charge or the fuel may have changed although purchased from the same sources. Such variations in practice are usually extremely difficult to discover because they leave no record except for their effect on the lining. Another complicating factor is that the failure of the refractory may not become apparent until weeks or even months after the actual damage was done.

A.P. Green on Refractory – 1953

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Therefore, in investigating cases of refractory failure, it is advisable not to jump to apparently obvious conclusions. It is much better to obtain all the information possible and then, by a process of elimination strike out all the factors that are reasonably certain to have had thorough examination before the field can be narrowed still further. In this way it is almost always possible to arrive at a conclusion as to which factor is the one most likely to have caused the trouble. If you feel there are too many "weasel words" in that statement, the investigation of a few cases of refractory failure will go far toward changing your mind

QUALITY OF REFRACTORY All too frequently, from the refractory manufacturer's standpoint that is, it happens that by our process of elimination we find the finger of guilt pointing directly at the refractory material itself. When this happens perhaps we should be relieved (although we rarely are) because it is the easiest to correct of all causes of failure. It does not involve alterations in furnace design, modifications in the process to take care of raw material variations, better instrumentation for closer burner control, or the selection of a different type of refractory. All that is required is the replacement of the lining with the same brand of brick. It is almost certain that this replacement lining will be up to standard, normal quality and no further difficulty will be encountered In any mass produced material, such as refractories, there are unavoidable variations in properties. This Is particularly true when the finished product is made directly from raw materials without intermediate refining or beneficiation. It is the constant endeavor of the refractories industry to reduce the range of these variations. The testing of raw materials, even before mining; the selection and blending of clays; and the inspection testing at all stages of the manufacturing process contribute to this end, Finally, the inspection of the finished product before shipment is tremendously important to insure the consumer of uniform and consistent quality.

WORKMANSHIP It is easy to blame the brickmasons for every refractory failure because by the time failure occurs the evidence is pretty weII destroyed. Good brickwork is extremely Important but we have tried to make the poi that there are other factors Involved, probably of more frequent occurrence than either poor brickmasonry or brick that are not up to standard quality. Perhaps we should make it clear that we do not include the proper size and location of expansion joints, the rise of an arch, or the thickness of a wall as properly coming under the head of masonry workmanship. These should be taken care of in the furnace design. In practice we realize that many of these design details are left to the brickmason doing the work, and in most cases this is the best. way to handle it. There are instances, however, of brickmasons being called on to do furnace work that have had no experience except in the building trade. This is not likely to happen in large companies employing their own masons, but smaller companies will be well advised to keep this point in mind and assure themselves that the person in charge of the masons understands high temperature furnace work. At the beginning of this discussion we listed the five most important factors governing refractory service:

1. 2. 3. 4. 5.

Furnace Design Type of Refractory Operating Practice Quality of Refractory Workmanship

.

Arrange them in the order to suit the conditions existing in any particular furnace, and we believe they will help you analyze any refractory troubles.

A.P. Green on Refractory – 1953

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SCRUBBER PROBLEMS:

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SCRUBBER PROBLEMS: Most scrubbers for incinerators handling vapor or liquid wastes are of packed bed design. Variations include spray towers, trayed towers and venturi scrubbers. Acid gases are usually the target of scrubber operation. Water chemistry is usually simple, but there are always surprises.

Scale Deposition: Definite solubility limits exist for all minerals in water. The limits are composition and temperature dependent. Scrubbers remove acidic components from incinerator exhaust gas using absorption in water, and a neutralizing chemical such as NaOH or calcium carbonate are used to convert the acid to its salt (NaCl, Na2SO4, etc.). These salts, combined with whatever minerals (hardness) came with the fresh water feed stream, affect the scaling potential of the circulating solution. Once the solubility limit is exceeded, the salts begin coming out of solution on any available surfaces, such as the packing or other scrubber internals, eventually blocking flow. Deposition can be avoided by keeping a low level of dissolved salts in the scrubber liquid (increase liquid blowdown rate, use only caustic for neutralization or replace hard scrubber makeup water with deionized water). Source of problem: Operating with excessive dissolved solids.

Hypochlorite: Sodium hypochlorite (NaOCl) is a product of neutralizing Cl2 with NaOH. The reaction is 2NaOH + Cl2 ==> NaCl + NaOCl + H2O. NaOCl attacks glass, so improperly designed FRP shell construction or FRP circulating pipes can be ruined as the glass fibers are consumed. In addition, if the scrubber blowdown liquid is later acidified (as in a waste water treatment plant) the NaOCl breaks down, releasing chlorine gas! Source of problem: Poor FRP fabrication; improper waste water treatment.

Thermal Damage: Most incinerator flue gas scrubbers are constructed of the least expensive acid proof materials available. These are usually FRP, polypropylene or similar and are sensitive to high temperatures. Prior to scrubbing, the hot flue gas must be cooled. Safety items include high temperature switches in the quenched gas duct, quench liquid low-flow switches, etc. Should these safeties be bypassed, a quench system failure (like a damaged pump or water spray tip) can result in burned or melted scrubber internals. Source of problem: Inadequate maintenance of flue gas quench system.

Salt Particles / SO3 aerosols: Scrubbers designed to remove acid gases typically are not very good at removing small particles. Organic liquid wastes containing salts are fed directly to the flame zone of an incinerator burner in order to achieve stable combustion. The high temperature in this area can vaporize the salt compounds. When the salt vapor cools to furnace temperature it

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SCRUBBER PROBLEMS:

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condenses to droplets, which further cool in the gas quench section. Result: very small salt particles. Similar particle size can be seen when burning any waste containing sulfur. The SO3 from the

furnace forms tiny droplets of H2SO4 when quenched with water prior to scrubbing. This "aerosol" acts like a fine particulate and passes through the acid gas scrubber. A high energy scrubber, electrostatic precipitator, or filter must be used in these cases. Source of problem: Waste combustion forms small particles; additional equipment must be specified.

Pump Cavitation: Most flue gas scrubbers recirculate the scrubbing liquid in order to reduce the rate of blowdown to water treatment. Circulating pump cavitation can sharply reduce the rate of scrubbing liquid available and damage the pump. Source of problem: Incorrect pump specification, faulty pump suction piping design, poor scrubber sump design or foreign matter clogging the pump suction.

Channeling and flooding: Incinerator flue gas scrubbers often incorporate beds of random packing. If the scrubbing liquid is not well distributed across the bed, the incoming gas can flow preferentially up one side of the scrubber, reducing acid gas removal efficiency. If gas flow is too high the downward flow of liquid can be slowed, flooding parts of the bed, increasing gas pressure drop and reducing scrubbing efficiency. Source of problem: Improper scrubber design or possible fouling of the scrubber packing.

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BOILER PROBLEMS:

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Boiler Problems: Since incinerators are usually installed only to satisfy emission requirements, heat recovery boilers sometimes offer the only positive payback for the project. Designed correctly, they are one of the most trouble free components in any system. But: Steam Blanketing: Firetube boilers are most often used for incinerator heat recovery for economic reasons. Carbon steel boiler tubes are commonly used, and the tube metal temperatures are kept sufficiently low by the very rapid rate of heat removal as water flashes to steam on the outer surface of the metal. As steam bubbles form they float upward away from the tube surface, allowing fresh water to reach the tube. Excessive steam production in one area can "blanket" the area with steam, impeding water entry and allowing tube metal to approach flue gas temperature, damaging the tube. A ceramic "ferrule" sleeve at the entrance to each tube will prevent steam blanketing in this area of high gas velocity. Installation of ferrules to eliminate steam blanketing is possible, as long as gas side pressure drop does not become excessive. Source of problem: Improper boiler tube layout or construction.

Solids Fouling: Some ash components in waste streams can have relatively low melting points. Should a molten salt droplet contact a cool boiler tube, the salt will solidify. The effect is similar to painting the tube, and the accumulation of salt cuts boiler efficiency. To avoid this, the flue gas should be cooled enough to solidify the salt droplets prior to boiler entry and soot blowers should be installed for removal of the solid salt particles. Watertube boilers with unfinned (bare) tubes in the entry zone are preferred in this service, even though the first cost of a firetube boiler might be less. Source of problem: Improper system design. For more about Boilers, click here.

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