A Guide to Assist in Evaluating Liquid Fuel Flames
Frank Beale (P.E.), Coen Company, Inc.
Liquid fuel firing is by far a more challenging feat than firing a gaseous fuel. The liquid fuel first, must be atomized to a very fine misty vapor, somewhat like a heavy fog, before the liquid will burn.
Atomization of the oil must provide a large oil surface area for contact with the combustion air. The more surface area exposed, hence, the smaller the oil particles, the more rapid the combustion process takes place. There are several ways to atomize fuel oil but the two most popular methods are using an atomizing medium, such as steam, air or gas or by straight mechanical means, either directly or by a return flow technique.
The least used of the two methods is straight mechanical for two main reasons. First, the atomization process is a function of the oil pressure and is accomplished strictly by utilizing the energy in the liquid stream, namely the energy stored as burner pressure. Secondly, very high oil pressures are required to obtain decent turndown. This method is by far the simplest from piping and a control stand point. For industrial boilers, liquid fuel burner pressures in the range of 300 to 400 PSIG are required to obtain a turndown of 3:1. In applications for power boilers, oil pressures are typically operated at 1000 PSIG at maximum flow rate with only a turndown capability of 5:1.
The second and most used technique is using either steam or compressed air for atomization. Normally, compressed air is used on light oils, such as No. 2 fuel oil, and steam is used on heavy oils, such as No. 6 fuel oil and Bunker “C”. There again are two types of oil gun designs. The first design requires a constant steam pressure at the oil gun over the entire operating range of firing. The second design, requires a steam pressure over oil pressure differential anywhere between 15 PSIG and 30 PSIG typically over the entire operating range. In both of the types above, the required oil pressure at the burner is 150 to 200 PSIG with 6:1 turndown.
Steam is widely used to atomize heavy residual fuels or tars which require heat to obtain the desired viscosity. However, steam can be a deterrent when atomizing light fuels because the steam temperature can cause carbonization of distillates within the burner. Steam used for atomization should be dry so that full expansion of the steam will be available to shear the oil. The use of steam per pound of oil will vary normally from 0.1 lb. of steam/lb. oil to 1.0 lb. steam/lb. oil with the norm being about 0.25 lb/lb. On a pound for pound basis, less steam is required per pound of oil fired at higher loads than at lower loads, due to the utilization of the higher pressure oil at higher loads.
There are two types of gun designs with respect to atomization; internal and external. Most oil gun designs which use an atomizing medium are called internal port mix guns. The steam and the oil are mixed together before they leave the oil tip. A few of the configurations used in port mixed guns are normally referred to as the “Y” jet, the “Skew” jet, or the “F” jet. Concentric steam and oil tubes or parallel steam and oil tubes can be used with either configuration.
External atomization, however, is used on very dirty waste stream, where the liquid and the steam are kept separated until they exit the tip. The atomizing medium is aimed to intercept the waste liquid just outside the tip in a perpendicular direction to “blast” the waste stream into droplets. Externally, atomization is far less economical in point of steam usage. Typically, with external atomization, the atomizing steam rate will range from 0.5 lb. stm/lb. oil up to 1.0 lb. stm/lb of oil. Normally, the waste oil is directed down the center with fairly large ports to prevent plugging.
Viscosity is one of the key parameters when we atomize heavy liquids. Viscosity is that property of any fluid which tends to resist a shearing force. Viscosity for most liquid fuels is available and normally plotted on a graph using temperature on the “Y” axis and viscosity on the “X” axis. Viscosity is normally, in our world, given the units of SSU (Seconds Saybolt Universal) or in Centistokes. For a typical example, a specific No. 6 fuel oil at 160°F would have a viscosity of 175 SSU which is equivalent to 37.5 Centistokes. A specific No. 2 fuel oil at 70°F, would have a viscosity of 36 SSU equivalent to 3.0 Centistokes. Viscosity is temperature dependent, the higher the temperature of the liquid, the less the viscosity. As a general rule, No. 6 fuel oil must have a viscosity of less than 10,000 SSU in order to pump the liquid. For comparison, an average grade of blackstrap molasses at 130°F would have a viscosity of 40,000 SSU or 8,800 Centistokes.
Vaporization of the liquid fuel droplets is required before the liquid will burn and is strongly dependent on the surface area of the drops. By increasing the droplet surface area, in other words reduce the droplet size, the rate of fuel vaporization and hence, the combustion process is shorten. It has been demonstrated that the lifetime of a fuel droplet is directly proportional to the square of the droplet diameter. Therefore, liquid fuel combustion is optimized by minimizing the average droplet size of the fuel spray.
The average diameter most commonly utilized to characterize liquid fuel sprays is the “Sauter Mean Diameter”. If we define a quantity called the specific surface of a droplet as its surface area divided by the volume then we may define the Sauter Mean Diameter (SMD) as follows: The “SMD” is defined as the diameter of a droplet whose specific surface is the same as that for the total number of droplets in the spray. Ideally, we want the liquid fuel introduced into the furnace at a minimum SMD.
There are many factors which affect the performance of firing a liquid fuel. They are the viscosity, heating value, specific gravity, constituents in the fuel, and the boiling points of each constituent to name a few.
Fuel oil constituents such as olefins, sulfur, nitrogen, vanadium, sodium, and asphaltenes cause increased pollutant formation, burner carbonization, poor atomizer performance and boiler corrosion.
Heavy fuel oils require substantial preheating in order to be atomized properly. For example, No. 6 fuel oil normally has to be heated to the 225°F to 260°F range in order to obtain a viscosity of less than 200 SSU. Oils containing olefin hydrocarbons such as ethylene will have a tendency to polymerize and form gummy substances which will detrimentally effect atomization. If excess preheat is applied the tar formed will harden in the fuel system causing maintenance problems. Care must be taken to maintain the proper preheat for a specific oil to prevent burner carbonization due to fuel polymerization.
The presence of sulfur, sodium and vanadium in heavy fuels can cause severe corrosion problems as well as pollutant emissions known as acid smuts. During the combustion process, sulfur is oxidized to sulfur dioxide (SO2). When high excess oxygen is present in the flame zone some SO2 will be converted to SO3 which leads to the formation of sulfuric acid (H2SO4). The dew point of sulfuric acid is around 270°F for most atmospheric conditions. Therefore, as a good rule, the stack exit temperature when firing oil should always be above 300°F to keep sulfuric acid attacks to a minimum.
Vanadium and sodium can cause metal attacks on lower temperature metal surfaces. Vanadium attacks are greatly enhanced in the presence of sodium (salts) and can occur at temperatures as low as 900°F. Vanadium, as low as 50 PPM, also can cause great harm to refractory.
The presence of asphaltenes in fuel oil causes increased particulate emissions and carbon carryover. Asphaltenes, which are heavy solid combustible substances (often containing some organic-metalic species) tend to be non-volatile, and therefore are hard to vaporize and burn. The presence of large amounts of asphaltenes will limit the minimum excess air levels at which burners can operate.
Enough said about the burner and the chemistry of the fuels. The intent of this paper is to provide some insight for operators when looking at a liquid fuel flame and to determine the condition of the burner. So lets begin with the visual aspects of evaluating a flame.
The appearance of a flame provides a good indication of combustion conditions. It is difficult however, to generalize the characteristics of a “good” flame since there is a certain amount of operator preference and variations due to different burner designs. From years past, before emissions were a factor, an ideal oil flame was short, bright, crisp, and highly turbulent with no flame impingement seen on any water-cooled surface. However, operations today mostly require low NOx burners which provide a different flame appearance than conventional burners of years past, but a good sighting of the flame can still be evaluated to troubleshoot a combustion problem. In order to minimize NOx for example when firing heavy No. 6, the best techniques are to use staged air in combination with special oil tip port drilling patterns and with a minimum of excess O2, the lower the better.
For example, firing with low excess O2 will normally produce a flame with the following characteristics:
- Flames that actually grow in volume and tend to completely “fill” the furnace at less than full load.
- Flames exhibiting a long, lazy “rolling” appearance instead of a short intense, highly turbulent flame.
- The overall color of the flame will change as O2 is lowered. Oil flames become darker yellow or orange and the furnace will become hazy, not clear and clean. Natural gas flames become more visible or luminous with yellow and the furnace will become hazy, not clear and clean.
- When firing No. 2 oil, CO must be watched as the O2 is reduced. (The same is true for natural gas firing.) Normally when firing No. 6 fuel oil, smoke will be your first indication when the O2 is too low.
In general, poor oil firing will exhibit six (6) problems, either singly or in combination. They are:
- Smoky flame
- Fire flies or sparklers in the furnace
- Long, lazy or uncontrollable flame pattern
- Instability (motor boating or lift-off)
- Coke formation
- Oil spills (dripping off the tip on to the floor beneath)
Some of the most likely causes for these problems are discussed herein as a trouble shooting aid. In order to trouble shoot an oil burner problem, a good view of the oil flame at the root of the flame as well as the flame envelope is invaluable.
Having said that however, it is very difficult to describe what an oil flame should look like due to the many variables which can effect the flame. I have made an attempt to provide a trouble shooting guide to assist in tuning a liquid fuel burner.
Occurs when an insufficient amount of air to burn the fuel passes through any burner. Heavy black smoke in the furnace and emitting from the stack indicates a large air deficiency.
Any burner within a group of burners may be responsible for smoke. When any burner is operating with a considerable air deficiency, or an excess of fuel, heavy black smoke will issue from the stack. As the air flow is increased, the smoke lightens in color until a light brown haze is produced, which is normally referred to as the “economy haze”. This is usually the most efficient operating condition but in today’s world, a clear stack is normally required. A further increase in air quantity results in a clear stack; a greater excess of air results in dense white smoke.
If a O2 indicator or other combustion guide is fitted in the stack, the operator should use this as a guide in adjusting the air supply.
Smoke may result from any one of the following causes:
- A tip in any burner has a higher capacity than the others, all other things being equal.
This type of problem makes it practically impossible to adjust the common air supply to prevent smoke. All tips should be marked; only tips in good condition and of the same size should always be installed in the same furnace.
- Oil spray impinging on the diffuser or tile throat directly will form carbon and produce smoke.
This condition is usually caused by a dirty atomizer, improper atomizer position, a damaged diffuser, or worn tips.
- Improper oil tip/stabilizer position in the burner throat proper
Too far in, causes incomplete mixture and fluttering. Too far out, causes carbon formation.
- Improper air mass flow
Too little air gives a long smoky flame that tends to fill the entire furnace and continues far into the heat absorbing surface. Too much air results in a ragged flame (tends to rip the flame apart) without definite shape and a shower of sparks (tends to cool the oil droplets and results in fire flies). However, in the case of fuels containing a considerable amount of ash, it may be impossible to eliminate incandescent particles from the flame or furnace.
- Fluctuating air supply caused by hunting fan controls.
Air flow must match fuel and follow the combustion curve.
- Fluctuating fuel pressure which corresponds directly to fluctuating flow rate.
Fuel pressure must be steady in order to maintain steady flow rate. Again, fuel flow and air flow must follow the combustion curve.
- Viscosity too high.
Oil is too cold or the fuel grade has changed from the design.
- Either the air register, the burner throat, or oil gun and swirler assembly, are not centered.
Eccentricity as small as 1/8″ can cause problems.
- Inconsistent fuel.
Fuel that varies in constituents, such as water slugs, heavy ends, light ends, all can cause smoke, everything else being equal. In other words, a consistent mass fuel flow may not provide a constant fuel heat input and with a metered system, we can’t anticipate this inconsistency. An O2 trim system can be installed to compensate and automatically correct for small variations (±5%) in the heating value of the fuel.
- Uneven air flow.
Faulty windbox air distribution on a multiple burner installation will cause some burners to fire fuel lean and some burners to fire fuel rich. Poor air flow distribution can be a problem even with a single burner boiler installation. If the flame is not symmetrical or favors one side, and everything else is within specifications, then poor air flow distribution could be the gremlin. Remember, air flow is 90% of the mass flow passing through the burner with only 10% being fuel flow. A clean oil tip positioned correctly will provide even oil flow distribution.
A tinted glass either blue, green, or black in different levels of tint, is of great value to the operator in viewing the flames especially when firing No. 6 fuel oil which can be blinding bright. By observing the flame through the furnace view ports, poor combustion can be detected to the trained eye.
FLAME CHARACTERISTICS AND FURNACE APPEARANCE:
The oil flame should be clearly attached to the oil tip (the base of the flame should resemble a tornado – rotating tight conical or funnel shaped expanding flame front) and maintain a consistent profile. The furnace should be clean and clear without haze. Haze indicates poor combustion and normally results when unburned fuel is present. The flame color at the root of the flame should be brilliant white with the majority of the flame being a clean yellow with some orange tails on the ends. (Sometimes short fingers of oil can be seen as it exits the port.)
A dark orange flame at the root of the flame may indicate that viscosity and oil temperature are not properly adjusted or that degradation of the sprayer plate has occurred.
The oil tip ports should be drilled on a port circle large enough to allow air to channel and penetrate into the root of the flame with an included angle to match the tile exit. A well designed oil tip will provide flames which are tangent to the tile exit diameter without direct impingement. When the flame fills the burner exit, air is not permitted to bypass the flame but is forced through the flame envelope to provide a clean burn.
In order to optimize the oil flame pattern to the boiler configuration several different oil tips with different port drilling patterns will be required. (Trial and error so to speak.) This is due to the many variables involved, namely, the port circle diameter, the number of ports, the included angle, the port drill pattern such as little hole-big hole, two or more port circles each with different size ports, and the actual geometry of the tip itself (round nose, flat beveled nose). However, many of these options can be eliminated because we know the results from past experience. For one example, a very narrow included angle may be perfect to prevent flame impingement on the side walls but as we increase the burn rate, a too narrow included angle will cause the flame to collapse with heavy black smoke filling the furnace, and ultimately results in a flame out.
A bright, whitish flame (not just the root but most of the main flame) may indicate that the oil is too hot or that too much excess air is being used.
A sputtering, crackling flame is usually indication of water contamination of the oil.
Smoky flames can result from such conditions as worn, plugged, or dirty sprayer plates, insufficient oil pressure and/or temperature, reduced steam atomizing pressure, or operation with too little excess air.
For good combustion, the flame should be bright, clean, and uniform in color, not smoky and not streaked. The flame should maintain a pronounced outward thrust with a “jet” type discharge from the burner throat and be without any noticeable recirculation or cyclic rotation in the flame profile.
- “Fireflies”, as normally called in the arena, appear as sparklers similar to what come off of a grinding wheel when grinding steel, can be caused by any one of the following:
- Wet atomization steam (moisture in the steam)
- Poor atomization. Large drops of oil, or poor atomization, will generate fire flies. If the droplets are too large, there is not enough time for complete vaporization before the droplet escapes from the flame and the burning continues to cause either sparks or fire flies to appear. This problem also deposits coke on the furnace water wall tubes.
- Very high excess air which cools the oil droplets and quench the oil droplet before they vaporize and burn.
- Solid particles in the oil, such as “catalyst fines” or ash, which will not burn, will glow and fall back to the furnace floor.
- Cold fuel oil.
PULSATION OR PANTING:
Pulsation or panting normally occurs from operating with a considerable deficiency of air through the burners. This condition is usually accompanied by smoke.
Increasing air supply through the burners or decreasing fuel supply usually stops this condition. Decreasing fuel supply slowly is recommended over increasing the air supply in order to prevent a possible puff.
Pulsations may also be caused by too high an oil temperature with vaporization in the tip or gun, fluttering of a loose damper, or hunting of a fan.
Always maintain the steam at the required pressure, and ensure that the steam is dry or dry saturated. Very wet steam will cause poor atomization, and when combined with incorrect pressure it will also effect the through-put of the burner. Symptoms of the above will appear in the form of streaks in the flame with probable fall out of burning oil droplets extending beyond the main flame configuration, and will show up as “fire flies” or “sparklers” consisting of particles of unburnt fuel falling back to the furnace floor, or being deposited on the “cool” water walls.
If the atomizing steam is too hot, and a concentric tube oil gun design is used, the high steam temperature can crack the oil and deposit carbon residue in the tip causing plugging of the tip. Normally, an oil gun design consisting of dual parallel tubes will solve the problem.
LOW ATOMIZING PRESSURE:
Each oil gun capacity curve should indicate both the type of oil used for design sizing and the suggested atomization pressure for that oil.
Generally, the lower viscosity oils require lower atomizing pressures than the higher viscosity oils. If a change in viscosity has been experienced either through a change in oil from design or a change in operating temperature, it is possible that a revision in atomization pressure may be required.
An oil gun which utilizes an internal mixing chamber is subject to variations in oil flow at any single oil pressure. A small variation in atomizing medium supply pressure can significantly change the internal pressure of the mixing chamber. Therefore, low atomizing supply pressure will increase the net oil flow, increasing BTU input, thereby causing that burner to be “over fired” at a pressure which it normally would have sufficient air. As a result, smoky flames, fire flies, a lazy flame, and coke build-up may occur.
LACK OF ATOMIZING FLOW:
Pluggage of atomizer steam ports with pipe scale, dirt, and/or oil borne particulates will cause a reduction in atomizing medium flow, effectively reducing the mixing chamber back pressure, resulting in the same effect as low atomizing pressure.
Specifically, on those burners designed to use steam as the atomizing medium, it should always be clean, dry steam. A suggestion of 10 – 20°F superheat should ensure this situation, and as a rule of thumb, the steam temperature should be normally not be greater than 500°F. However, all atomizing steam systems should be trapped. Dry steam at the burner requires excellent insulation of the steam line plus adequate trapping of condensate. If there is a pressure variation in the supply system due to sudden use of large steam quantities at points in the system, the atomizing steam pressure should be regulated to avoid firing irregularities.
Since water is lower in energy than steam, atomization with wet steam is not as effective as with dry steam. This lower energy atomization will result in a larger droplet size and some oil coated water droplets being dispersed into the combustion zone. These larger droplets and water droplets are slower burning and can often be seen floating on the internal firebox currents. These “fireflies” are a major source for ash and soot build-up on radiant and convective tubes.
A second effect from wet steam atomization results in slower burning with possible accumulation (or sludging) of slugs of water which is incombustible, severely reducing burner stability.
COLD OIL (HEAVY OIL):
On those burners operating on heavy or viscous oils, the oil should be heated to a temperature sufficient to achieve 150 – 200 SSU. This suggested viscosity range is sufficient for proper atomization of most heavy oils and any reduction in the oil temperature will increase the oil’s viscosity.
Viscosity is a measure of the oil’s resistance to break up, and as the viscosity increases the quality of atomization and combustion will decrease.
Cold heavy oil can cause smoky flames, fire flies, delayed combustion resulting with a lazy long flame, and coke deposits. Sometimes even flame-out will result.
Most oil gun designs are dependent on a number of machined orifices, channels and seals. These pieces are subject to high velocity abrasive flows and corrosive action dependent on the type of oil fired.
Clearly these orifices, channels and seals are subject to some “normal” wear, making them a “maintenance item”. Additionally, this condition is aggravated by the common, and some not so common, contaminants found in many oils. Coke or carbon particles, catalyst fines, and silica particles have a highly erosive action on metal parts when subjected to high pressure, high velocity metering, while sulfur, chloride compounds and, in some cases, anhydrous acids will severely attack, through corrosion, the materials of the atomizer and dispersion nozzle.
The use of case hardened tip and atomizer materials for erosive oils and 300 series stainless steel or higher for corrosive oils is suggested. Sometimes tungsten is required.
Some typical effects of tip and/or atomizer deterioration are as follows:
- Enlargement of oil orifice – high oil flow, low atomizing medium ratio, poor atomization, burner overfiring.
- Enlargement of atomizing orifices – high atomizing medium flow, low oil flow, reduction in oil gun capacity, reduction in low fire stability.
- Enlargement of atomizer exit – lowered mixing chamber pressure, reduced atomization quality, burner overfiring.
- Deterioration of atomizer seal – steam bypassing of atomization chamber, poor atomization, instability, unsymetrical flame patterns, fire flies.
- Deterioration of dispersion chamber – reduction of exit port L/D, deterioration of dispersion pattern, coking, oil spills.
- Enlargement of exit ports – reduction of exit port L/D, an increase in oil flow for the same pressure.
THROAT TILE/DIFFUSER FAILURE:
Commonly a failure in either the refractory of the throat tile or the metallurgy of a diffuser cone is the result of some other oil firing problem. However, it should be noted that these parts are integral and necessary to the proper function of the burner. Failure of these parts should be acted upon with replacement immediately. Smoky flames and coke build up will result.
As a secondary consideration, non-concentricity of these parts with respect to tile throat and oil tip will cause poor air distribution, non-uniform flame patterns, coking and oil spills. All parts should be centered within ±1/8″ for proper performance. Ideally, ±1/16″ should be the target.
LOW OIL FLOW/LOW OIL PRESSURE:
Extreme reduction in oil firing rate and/or plugging of the oil orifice from pipe scale or oil borne contaminants can cause severe burner stability problems, while the lowered exit port velocities can cause dripping or internal oil spills. If for any reason the burner capacity requirements are reduced by any appreciable percentage, new reduced capacity oil guns are suggested.
HIGH ATOMIZING PRESSURE:
High atomizing medium pressure will increase the mixing chamber back pressure thereby, reducing the oil flow. In many cases, this raised atomizing medium/oil ratio can cause severe stability problems. Excessively high atomizing steam pressure will tend to uncouple the flame from the tip and cause lift-off of the flame which can lead to severe instability.
LEAKING STEAM CROSSOVER:
An oil gun operated with the atomizing medium at a higher pressure than the oil, leaking steam into the oil path can cause severe disruption in oil flow to the oil gun and be detrimental to the atomization of the oil supplied.
The bypassing of atomizing medium into the oil supply, is typically characterized by what is commonly called “Motor Boating”. This continuous disruption of oil flow is clearly audible, thus deriving its name from the similar sounds.
HIGH ATOMIZING STEAM TEMPERATURE:
High temperatures on the atomizing medium cause problems in two separate ways, but these problems can be directly tied to the medium temperature.
Light oils often can be adjusted to very clear, yellow fires, however, this same fuel will often exhibit instability, pulsation in flow, clear blue/bright yellow flame envelope and haze at the flame boundary. These are all indicators of fuel oil vaporization within the oil gun. This two-phase or vapor flow through orifices originally designed for liquid flow will severely reduce the oil gun capacity and stability.
Heavy oils which contain residual or added light oil will exhibit these same problems, as the lights flash, with the added problems of heavy oil slug flow and resulting smoke and poor atomization of the heavy ends.
OIL TIP MISPOSITION:
By far the most common oil firing problem and the most detrimental condition to oil firing, is the mispositioning of the oil tip with respect to its air supply/stabilization source. Due to variations in oil, atomizing medium, oil temperature, atomizing medium quality, burner air side DP, operating oil/atomizing medium pressures, and furnace requirements for flame pattern, all oil guns should be supplied with oil tip insertion adjustability, which should be optimized at start-up. Normally, the business end of the oil tip is fixed in relationship to it’s flame stabilizer. Only the center guide tube with the flame stabilizer mounted in a fixed position can be axially moved in or out.
While the tip position is located on the burner assembly drawing supplied with every job, the final position requires a burner field service adjustment for optimum operation.
A mispositioned oil tip can cause any one or all six (6) oil firing related problems.
FIRING WASTE LIQUIDS:
Typical waste liquids might include “ASO”/”HF Polymer Waste Oil”, “Waste Olefins”, “Waste Alcohol”/”Fusal Oil”. The acid soluble oil (ASO) is a mixture of hydro-fluoric acid and hydrocarbons. It is particularly corrosive and requires the use of monel tips, atomizers and oil tubes.
It may be preferable to use gas atomization in an internally atomized gun when firing these waste liquids.
It is preferred that the burners be horizontally fired. In this way, any oil drippage will run into the furnace instead of down into the burner or plenum.
Alcohols, ethers, naptha, butane, and other light liquids should be burned in a duel tube gun rather than the concentric design. This will avoid the premature flashing that sometimes occurs with steam atomization. Premature flashing will lead to unstable combustion (“motor boating”) and eventual flame out. These fuels can usually be burned in any type of burner designed to fire fuel oil. “Motor boating” also occurs when firing oil contaminated with water caused either from water in the oil or a leaking steam bypass in the oil tip.
Heavy fuel oils, asphalt, asphaltene, phrolysis fuel oil, etc., can be successfully burned, but they require careful attention to detail and more maintenance in the field. These fuels must be heated to a high enough temperature to achieve a viscosity of 200 SSU or less but not so high that some of the light ends are flashed off causing unstable firing. This viscosity must be available at the burner. The steam must be dry, either superheated, or the steam lines must be well trapped. In some cases, it may be advisable to provide knockout pots and steam traps on the oil body itself.
Wear-resistant alloys and case hardened tips can be used to handle oils containing abrasive particles (catalyst fines), but these measures are only “stop-gap”. These alloys will wear over a period of time and the enlargement of the oil tip ports will lead to flame deterioration, coking and drippage. Only regularly scheduled, cleaning and tip replacement will minimize the above problems. Higher quality burner tiles and refractory must be used when the oil contains high levels of alkali salts or other compounds that attack refractory. This will prolong the life of the refractory.
Depending on the flow rate and fuel, external atomization may be the solution.
Turbulence is demanded both in the zone where the fuel and air are mixed and in the zone where the fuel is burning. A state of turbulence as the fuel is discharged and mixed with air prior to burning produces noise. This is understandable because in gas burning, gas pressures are normally in the range of 10 to 30 PSIG, which establishes the gas fuel discharge at sonic velocity. Higher pressures are required for oil burning, normally in the 150 to 300 PSIG range. Energy is proportional to the square of the velocity (MV2/2). Therefore, when firing a liquid fuel, more combustion noise will be generated than when gas firing, everything else being equal. However, in most installations, rotating machinery, such as the forced draft blower, produce the majority of the noise pollution and is considered the greatest significance. Attenuation devices are normally fitted on the fan’s suction to silence the noise. Combustion noise, however, can not be confined to the internal areas of the furnace because the internal noise “telegraphs” through the boiler’s structure.
A fuel oil behaves and is identified by various grades of oil; mainly No.1, No.2, No.4, No.5 and No.6. No.1 and No.2 are classified as distillates, and No.4, No.5 and No.6 residual oils. No.4 can be a distillate or a mixture of refinery products. All oils are classified according to their physical characteristics by the specification set forth in ASTM standard D396.
No. 6 fuel oil, sometimes called residual, Bunker C, vacuum bottoms, or reduced crude is produced by many methods, but basically, it is the residue left after most of the light volatile products have been distilled from the crude. It is a very heavy oil, with a viscosity ranging from 900 to 9000 Saybolt Universal Secondary (SUS) at 100°F. Thus it can be used only in installations with heated storage tanks and with a recirculating piping return back to the tank in order to circulate hot oil at the burner front for correct atomization. No. 6 oil essentially is a refinery by-product.
Due to the demand of low sulfur content and low fuel bound nitrogen fuel oil to meet the stringent limits dictated by the environmental protection agency regulations, light distillates with characteristics as having low sulfur content and low fuel bound nitrogen are blended with high sulfur and high fuel bound nitrogen residue to produce an improved fuel. However, blending light distillate oils with heavy #6 influence other properties of oil. For example, API gravity, heating value per gallon of oil; viscosity, and the ash content as well as the emissions.
Firing so-called “blended fuel” can present major combustion problems. This “blending” technique is used to produce a product called a Low NOx fuel oil to reduce the fuel bound nitrogen content of a heavy fuel. A lighter oil is blended with the heavy oil to obtain say a 0.3% by weight nitrogen fuel bound fuel from a 0.5% by weight fuel bound nitrogen fuel. If the blend is not consistent, many problems will appear. A blend of distillate and residue can produce an unstable fuel due to stratification or separation of the two blending components in the storage tank. If this is suspected, pull oil samples for analysis every 15 minutes over an 8 hour run. Also, another trick is to put the combustion control system in manual and observe the stack or flame under steady state firing.
Blended fuel oil comes in two classes, depending on the percentage of the light distillate used which range from 20 to 85 percent. The two classes are light (or cold) with a viscosity range of 150 to 300 SUS at 100°F and heavy (hot) with a viscosity range of 350 to 750 SUS at 100°F. The light oil blend is capable of atomization without preheating; but the heavy oil requires some preheat.
Blended residual oils do not have predictable physical characteristics. Some will follow the normal viscosity/temperature curve once they are heated above the pour point; others become highly fluid only a few degrees above the pour point. Viscosity must be maintained at a constant value for efficient atomization. Controlling the viscosity by controlling the oil’s preheat temperature can be difficult when firing blended fuels. If the oil’s preheat temperature gets too hot, the distillates will gasify and produce varying viscosities. This property creates the need for greater attention to the fuel’s preheat temperature, to prevent viscosity from dropping too low at the burner tip for proper atomization. Raising and lowering the storage tank temperature can create the poor characteristic.
Another potential problem resulting from using blended oil is sludge. Sludge found in some heavy fuels often is classified incorrectly as sediment. Actually, it is a mixture of organic compounds that have precipitated after different oils have been blended. The most notable is the asphaltene group, consisting of heavy hydrocarbons. After precipitation, they disperse in the oil and contribute to particulates. Another heavy oil contaminant is wax.
Unfortunately, asphaltenes and wax are not detected with normal test methods because the solvents used, benzol and toluene, dissolve them. The presence of these compounds usually is not detected until they cause problems. Heat can eliminate wax, but asphaltenes require a solvent for dissolution, and this generally is impractical in a fuel oil system.
Blended oils also produce special combustion characteristics depending on how the distillate was produced. Distillate oils can be divided into two classes; straight-run and cracked. A straight run oil is refined directly from crude oil by heating it and then condensing the vapors at various temperatures at atmospheric pressure. Cracking processes depend on higher temperature and pressures, or a catalyst, to produce distillate from heavier fractions. The difference between the two types of oil is that cracked distillate contains a substantial amount of olefinic and aromatic hydrocarbons, which are more difficult to burn that the paraffinic and naphthenic hydrocarbons produced in the straight-run process.
Unfortunately, the straight run grades of No. 1 and No. 2 are exclusively used for home heating and as a light grade diesel fuel. No. 2 straight-run oil is frequently called “gas oil”.
The industrial No. 2, cracked distillate, is used mainly in fuel burning installations such as smelling furnaces, ceramic kilns, small package boilers, and blended with No. 6 to lower the sulfur and fuel bound nitrogen content.
One of the most prevalent concerns with firing blended oil is flashing of the light ends. When an oil contains both high and low boiling components, the volatile portions volatilize and burn more rapidly leaving the heavy ends. If there is not sufficient time for complete combustion of the heavy ends, carbon particles will be discharged from the stack producing opacity problems.
Sometimes in order to dispose of waste crank case oils, spent lube oils, and spent motor oils, these waste oils are mixed with a heavy oil to produce a “blended fuel”. This blended fuel generally produces an unacceptable fuel for boilers due to metal shavings in the oil, sludge, and unpredictable properties due to the spent lube oil. In addition, spent lube oil and crank case oil are very difficult to burn clean in a boiler. An incinerator should be used.
BLENDED FUELS – ATOMIZING:
Atomizing a blended fuel can also create combustion problems.
Atomization is the process whereby a volume of liquid is converted into a multiplicity of small drops. Its principal aim is to produce a high ratio of surface area in the liquid phase, resulting in very high evaporation rates. Heavy oil fired burners use both pressure and steam atomization in combination. The function of the atomizer is to attenuate the fuel exiting the fuel nozzles into fine jets from which ligaments and ultimately drops will be produced, and the distribution of the resulting drops provide a controlled spray pattern with direction.
The spray characteristics of importance to combustion performance include means drop size, drop-size distribution, patternization, cone angle, and penetration. Special importance is attached to mean drop size, drop-size distribution, and patternization because they are solely dependent on the atomizer design, whereas cone angle and penetration are governed partly by the atomizer design and partly by the aerodynamics influences to which the spray is subjected after atomization is complete.
Having said that, firing blended fuel with both light and heavy ends, uniform droplet sizes are not possible and disintegration of the fuel jets are highly irregular; consequently drop sizes are much more varied due to the volatile portions (light end) flashing leaving the heavy portions as irregular drop which are difficult to burn in this situation.
BLENDED FUELS – PARTICULATE EMISSIONS:
The burning of fuel oil can produce sulfur oxides, inorganic ash, oxidizer of nitrogen, carbon and unburned and partially oxidized hydrocarbons. Most of these contaminate, notably sulfur oxides and inorganic ash, are attributable directly to the fuel and are independent of the burner or operation. Sulfur in equals sulfur out. If the proper combination of burner and fuel has been selected, and if the burner is operated property, no visible emissions should be caused by oxidizable air contaminates and the concentration of items such as aldehydes and carbon monoxide should be negligible. Never the less, smoke and oxidizable material are often found in boiler stack products.
The quantity of inorganic solid particulates in flue gases is entirely dependent upon the characteristics of the fuel. There is no measurable inorganic ash in the flue gas from the combustion of natural gas or other clean gaseous hydrocarbons, except for that small quantity attributable to the dust usually present to some degree in all air used for combustion. Distillate fuel oils do not contain appreciable amount of ash. Typical analyses show variations from a “trace” to about 0.03% by weight. In residual oil, however, inorganic ash-forming materials are found in quantities up to 0.1 percent by weight. Most of this material is held in long-chain organo-metallic compounds. The strong oxidation conditions present in most boiler furnaces convert these materials to metallic oxides, sulfates, and chlorides. As would be expected, these compounds show up as finely divided particulates in the flue gases.
For instance, the efficient burning of a residual oil of 0.1 percent by weight ash results in a stack gas concentration of 0.03 grains per SCF at 12% carbon dioxide. A clear or nearly clear stack when firing oil must contain less than or equal to 0.003 grain per ACF at a stack exit gas temperature between 300 and 400°F. When unburned particulate concentrations of ash exceeds this limit, opacity measured in percentage or Ringelmann number will exhibit visible contaminates in varying degree, dependent upon the density of the inorganic particulates.
BLENDED FUELS – OPACITY:
Zero opacity is smokeless combustion. For smokeless combustion not only must the air supply be efficient, but it must be mingled as intimately as possible with the escaping fuel spray, and there must be no local cooling. In order to maintain smokeless combustion, the atomizer must provide uniform and homogeneous fine jets of fuel and steam mixed to promote disintegration into fine droplets of fuel. In order for the atomizer to provide consistent spray, the fuel supplied to the atomizer must all have the same fuel properties. Whenever the fuel type is changed, the system must be changed in order to provide acceptable results because fuel preheat, fuel pressure, and fuel properties will affect the flame and emissions produced. Blended fuel, with both light and heavy portions, are difficult to burn clean because as flashing at the tip takes place leaving heavy irregular droplets of aromatics and olefins which are difficult to burn even with fine regular droplets.
Also note that the presence of a glycol or fatty alcohol in the fuel blends in combination with water found in the fuel, to form an emulsion which will cause a thickness in the mixture in spite of the higher percentage of light hydrocarbons. Actually, the light hydrocarbons will flash inlieu of forming the preferred or desired droplets, leaving the remaining heavy ends to irregular droplet sizes, hence prone to opacity or smoking.
It can be noted that during opacity excursions with visible smoke, the CO may remain well below the 100 ppm limit. This is typical of a fuel with heavy fractions and irregular droplet sizes. The large droplets form straight carbon particles and cause smoke without generating CO which is an indication of incomplete combustion. This is an indication that atomization quality is off and also indicates that the volatile light ends are flashing causing irregular diameter droplets of heavy ends.