Olavo C. Leite, THERMICA Technologies

This article describes the design and operating conditions of liquid and gas waste fuels. Lower fuel prices can be achieved by switching from light oil to heavy ones, or to fuel gases or combining with unconventional fuels. Unconventional fuels such as low energetic off gases, high energetic off gases and organic liquid waste require especially designed combustion equipment and may also require additional equipment to comply with safety and environmental requirements. Combustible liquid waste, generally with HHV > 8000 BTU/LB, can sustain combustion without support fuel in a standard commercial burner. Common liquid waste fuels are Chlorinated compounds, Nitrogen compounds and Sulfur compounds with typical calorific values varying from less than 2160 BTU/Lb for aqueous wastes to 18000 BTU/Lb for some organic compounds. The burner type should be selected by the grade of the fuels, operation requirements and atomizing media available. Hydrocarbons with halogens, bound nitrogen, salts and/or metals are difficult to dispose. Combustion of heavy fuels and liquid wastes is not straightforward. Clean combustion without soot or residue may be achieved by the use of Combustors, Fig.1, like High Intensity Vortex Burners, used to handle combustible liquid and partially combustible liquid organic wastes. The heart of waste fuels combustion system is the burner with proper designed and operated injectors, Fig.2 and Fig.3. The burner functions are as follows:

• Deliver and mix the fuel/waste and air to the combustion zone at the required ratio and rates;
• Atomize and vaporize liquid fuel;
• Provide for continuous ignition of the fuel and air mixture;
• Give direction and shape to the flame.

Combustion Basics. Combustion may be defined as the rapid chemical reaction of oxygen with combustible elements of a fuel, resulting in heat release. Hydrocarbons are chemical compounds of carbon and hydrogen and their combustion result in carbon dioxide and water vapor. The oxygen comes from the air, which is 21% oxygen and 78% nitrogen and other inerts, by volume, or 23% and 76% respectively, by weight. Complete combustion is the combination of fuel with oxygen without fuel left over requiring time, turbulence and temperature high enough to ignite all the combustible elements.

Stoichiometric air or theoretical air is the exact amount of air required to provide the right amount of oxygen for complete combustion. If enough oxygen is supplied, the mixture is lean and the flame is oxidizing, resulting in a clear and short flame. If excess fuel occurs, the mixture is rich and the flame is reducing, resulting in a long and smoky flame, a consequent of incomplete combustion. If there is a shortage of oxygen, the final product may contain carbon monoxide, hydrogen, hydrocarbons and free carbon. Nitrogen in the air absorbs some of the heat, resulting in lower flame temperatures.

The combustion excess air can be determined from the fluegas dry composition by the Orsat method or by determining the oxygen volumetric percent content, Y, on a dry basis, using the following expressions,

% X_S air = 100 *(O₂ -CO /2) /(0.264 N₂ -O₂ +CO /2)

or % X_S air = K*Y /(21-Y)

where K=90 for natural gas

K= 95 for fuel oils

Burner Excess Air, %X, versus stack O2, %Y

%X = K*Y /(21-Y) ====> Y% =21*X /(K+X)

The heating or calorific value of a fuel can be determined experimentally in a calorimeter or from its chemical analysis. The high or gross heating value (HHV) is the total heat released of a perfect mixture of fuel and air originally at 60°F and then cooled to the same temperature. The low or net heating value (LHV) is equal to the high heat value minus the heat released by condensation of the water vapor in the flue gas, i.e., it assumes all products to remain gaseous.

The Low Heating Value, LHV, can be calculated subtracting the formation heat, Q_F, from the High or Gross Heating Value.

LHV = HHV - Q_F, BTU/Lb.

where Q_F = 1040 w

w = condensed H₂O/fuel weight ratio

Also, for hydrocarbon gas mixtures, both the High and the Low or Net Heating Values, commonly in BTU/SCF units, can be given by the following fitting equations,

HHV = 54.4 M_W + 127, BTU/SCF

LHV = 51 M_W + 94, BTU/SCF

The high heating value can be estimated by the following DuLong expression, where the symbols represent the weight fraction of the fuel elements,

HHV = 14,544*C +62,028 (H₂ -O₂ /8) +4050*S, BTU/Lb.

Where the free hydrogen is given by the following expression,

H₂ (Free) = H₂ -O₂ /8

The approximated HHV of the chlorinated organic waste is given by the DuLong modified equation:

HHV = 14,544*C +62,028*(H₂ -O₂ /8) +4,050*S -760*Cl, BTU/LB

Also, the following equation is widely used to calculate the waste gross calorific value,

HHV = 15410 +323.5(% H₂) -200.1(% O₂) -120.5(% N₂) -115(%S) -162(%Cl) -190 (%F), BTU/LB

Fuel oils are hydrocarbons containing 84-85% carbon, 12-14% hydrogen, and small amounts of oxygen, nitrogen and sulfur, as well as traces of moisture and ashes. They vary from light volatile kerosene type to heavy viscous Banker C type. Oils with higher specific gravity have lower calorific values on a weight basis, BTU/LB, but on a volumetric basis, BTU/Gal, their calorific value is higher.

The HHV of fuel oils can be determined by the use of the Bureau of Standards formula,

HHV = 22,320 –3780(SG @ 60°F /60°F), BTU/LB

Or by the Sherman-Kropf formula

HHV = 18,250 +40(°API –10), BTU/LB

Where the Specific Gravity and deg API are related by

SG = 141.5 /(°API +131.5)

The flame temperature of a mixture of hydrocarbons, can be expressed as a function of the excess air (decimal X_S), by the following fitting equation, within +100 on the high end and –100 on the low end, as follows:

T_F = 6305 /(1.85 + X_S), °F

The flame temperature of a mixture of combustibles and inerts can be expressed as a function of the excess air (decimal X_S) and the Low (Net) Heating Value (LHV, BTU/LB), as follows:

T_F = 60 + LHV /{0.3 *[1 +7.5 *10^-4 *(1 +X_S) *LHV]}, °F

Ignition temperature is reached when more heat is generated by the chemical reaction than is lost to surroundings, and combustion becomes self-sustaining. The temperature of a flame depends on the type of fuel, the starvation or excess air and the initial temperatures of both fuel and air. Maximum temperature is reached when a fuel is burning near stoichiometric conditions.

Hydrocarbon gases are important fuels, typically expressed in SCFH or sometimes in therms, that represents the volume of gas equivalent to 100,000 BTU. With gaseous fuels, instead of breaking down the hydrocarbons into their elements it is easier to use the stoichiometric air required for the different compounds. The stoichiometric air/fuel volumetric ratio will be given by

Usually, the theoretical air required for combustion varies slightly for each particular fuel. As an average, each mole of stoichiometric Oxygen demand will produce 184,000 BTU on the HHV basis. Typically Natural gas is assumed to be equivalent to CH₄, No.2 fuel oil to CH₂ and No.6 fuel oil to C₄H₅.

The amount of air required for stoichiometric combustion is a fairly constant air/gas weight ratio, with an approximate value of 16. This will result in the fairly constant value of each 105 BTU of Gross heat release requiring 1 SCF of Air (see Table I). The following expression is a widely used rule of thumb that is approximately correct for a mixture of hydrocarbons, when compared with the chemical analysis calculated value, with the total heat released based on the High Heating Value (HHV):

SAR = Q /105 SAR = air flow, SCFH

Q = Gross Heat release, BTU/Hr

The same approach can be used on the LHV basis, resulting also in a fairly constant value of each 96 BTU of Net heat release requiring 1 SCF of air (see Table I). We can simplify the calculation of the stoichiometric air required (SAR) of a mixture with several components using the following expression:

SAR = Q /96 SAR = air flow, SCFH

Q = Net Heat release, BTU/Hr

Water vapor content in the air should be taken into account resulting in a reduction of oxygen. A humidity factor as well as a temperature factor should be applied to correct air requirements. Saturated air at 100°F contains 6.45% water vapor, reducing the oxygen volume percentage from 21% to 19.7%. Also, absolute pressure correction should be used at significant elevations above sea level (500 feet or more).

In practice, the Wobbe index, W_H,L, is the heat flux of the burner, used to evaluate interchangeability of two different fuel gases, without changing the settings of burner geometry, valves and controls. A gas burner will deliver the same heat output if operating with two gases with similar Wobbe index at similar gas pressures.

W_H = HHV /Sg^0.5 and W_L = LHV /Sg^0.5

• Proper burner turndown to meet all operating conditions
• Good flame stability during all operating range
• Ability to adjust shape and dimensions of flame
• Available combustion space to meet process requirements
• Proper safeguards and automatic control to meet the process needs

Burner Flame Turndown is defined as the relation between the maximum and minimum flows of fuel, using proper controls, free of combustion pulsation, good flame stability and acceptable emission of unburned products. It can be measured also by using the square root of the fuel flow across the burner, based on sub-critical flow conditions. There are a few factors that affect burner turndown, like type of burner, fuel, piping size and layout, etc.

Burner Flame Stability is the ability of a burner to maintain the flame within the limits of control, at different operating conditions of pressure and temperature from which the burner was designed to operate.

The Flame Shape and Length are based on the burner size and type. Operating conditions like good fuel to air mixing will generate a short and intensive flame. Also larger excess air will decrease the flame length, although a short flame with reduced excess air can be achieved by high turbulence and partial recirculation of the combustion products. Increasing the combustion air pressure across the burner typically reduces the flame length due to the increase in velocity and turbulence. The flame temperature along the flame envelope is the practical parameter to control flame length.

The space required for the combustion is related with the shape of the flame and the furnace should be designed around the flame. The furnace Thermal Load is defined, as the burner heat output per unit of furnace volume and it is larger for smaller furnace volumes.

The basic formula for the Burner Thermal Input is given by

Eq.1 Q = K_p w V D²

There are several methods used for designing and scaling burners. The two most applied criteria for scaling industrial burners are the constant velocity and the constant flame residence time.

• The constant Velocity scaling is proportional to the burner throat area, typically known as the burner characteristic, based on similar burner geometry, resulting in D = f (Q^0.5).
• The constant Residence Time is based on a constant ratio between the burner throat diameter and the combustion air velocity, D / V = constant. It result in D = f (Q^0.333), if the burner thermal input is applied to replace V.

Using the Constant Residence Time scaling principle, the mixing time scale is kept constant. This method is not commonly used because it results in low windbox pressures for smaller sizes and excessive pressures for larger burner sizes. Both constant velocity and constant residence time scaling criterias have limitations, in particular when scaling the interaction between gaseous and liquid phases.

There are other factors that influence the burner design and operation.

• Burner /Furnace geometry like a proper furnace to burner diameter ratio should be maintained.
• Imparting a constant inlet swirl is required to ensure recirculation of the combustion products.
• A proper Fuel /Air momentum ratio must be maintained.
• Heavy oil momentum to combustion air momentum ratio is more difficult to reach due to atomized oil droplet size distribution be too wide, generating a difficult adjustment of the fuel momentum.

Heat Transfer in both the flame zone and the post flame zone affect the burner emissions. In the flame zone, both the flame length and radiation reduce the adiabatic flame temperature. In the post flame zone, the combustion products temperature is affected by the radiation and convection losses. Radiation losses reduce the NO_X emissions. Heat loss increase with decreasing the burner /flame size. Also, heat removed from the post flame zone and re-circulated back to the burner root reduces the flame temperature affecting the NO_X emissions. This has been the new burner technology used to achieve ultra low NO_X emissions with fuel gas.

Also, there is limited knowledge of scaling low NO_X designs, due to a lack of understanding of flame to flame interaction with respect to Heat Transfer, Fluid Mechanics and Pollution emissions. Burner design affects the shape of the flame and the system performance. The combustion times for small flames are shorter. The larger the burner, the longer the products of combustion stays at high temperature. Minimizing the peak temperature, the O₂ content at peak temperature or the time at the peak temperature can reduce the NO_X formation. Larger combustion systems probably will use in the future a well stirred reactor design with discrete injection point of fuel gas and combustion air, entraining the recirculation flue gas, resulting in an efficient system with low NO_X emissions.

The turbulent mixing of the fuel and air is a function of the burner design, which should provide efficient combustion at low excess air rates. The Swirl Number is a dimensionless number viewed as the major indicator to quantify the fuel /air turbulent mixing by the burner. The higher swirl number provides a better mixing. The swirl number is given by

Eq.2 S = G_o / R_e G_x

The swirl also contributes to another phenomenon taking place in a furnace chamber and that is the dynamic effect. Some of the burner functions are to efficiently facilitate the reaction of the air /fuel mixture and to impart the combustion products with flow patterns within the furnace chamber. The dimensionless number that characterizes the turbulent interrelationship of a flame in a furnace chamber is called the Craya-Curtet number, C_t , that is the square root of the ratio of the initial velocity of the flame in plug flow to the intensity of non-uniformity of the initial flame.

Eq.3 C_t = 1/ m^0.5

and

Eq.4 m =(U²_d –0.5U²_k)/U²_k

The value m describes how well the flame will stir up the residence chamber. A long, lazy flame will have a larger C_t number than a short well-stirred flame. The combination of high swirl number and low C_t number in a burner /residence chamber combination, results in a more efficient mixing with a more uniform temperature profile in the chamber and achieves faster a more uniform heat flux, thus improving the effective residence time, Fig.4. The design of Mixing is very dependent on past experience. Design factors and features are used to compensate for the unknown. Modeling can be a very useful approach when using math models together with experimental data to improve the models.

There is no set straightforward procedure to design a combustor, but the following approach should cover the following steps:

• Characterization of the fuel/waste
• Establish performance objectives
• Develop Heat and Mass balances
• Evaluate combustor configuration and burner dynamics

Combustors for heavy fuels and liquid halogenated wastes are high intensity vortex burners with refractory lined combustion chambers, operating at low excess air with a very compact and bushy flame. These combustors can provide the ability to burn efficiently some special fuels like heavy fuels with a high carbon/hydrogen ratio without excessive soot formation as well as fuels with high percent of inert components that generate low theoretical flame temperatures. Although operating with low excess air, these combustors can produce high NO_X levels but they may be the proper choice for some applications. The low NO_X version of these combustors operates with the combustion air staged between the burner throat and the downstream of the burner combustion chamber, Fig.5.

The combustion objective is to convert organic compounds to carbon dioxide and water. Pumpable liquid wastes are converted to gas through atomization prior to combustion and introduced through the burner, producing flue gas. This flue gas has a composition containing low concentrations of CO and total unburned H/C (THC) in addition to CO₂, H₂O, O₂ and N₂ besides the Hydrochloric acid, free Chlorine, if containing Chlorinated Organic, and Particulate. If the organic waste /fuel contains Sulfur compounds, the flue gas will have, in addition, Sulfur dioxide. Wastes can change their characteristics at anytime. In practice, combustion is never complete, resulting in combustion by-products like carbon monoxide.

Eq.5 % Combustion Eff. = 100 *CO₂ /(CO₂ +CO +THC)

Excess air is required for good combustion and it varies with the burner design, fuel and oxygen source. The burner should be designed to facilitate the reaction of the fuel and air efficiently with minimum excess air, since any excess air beyond that required to complete combustion will waste fuel by putting more sensible heat into the atmosphere. Burners with greater swirl, like the high intensity vortex combustors, operate with lower excess air, minimizing the SO₂ to SO₃ conversion, if operating below 5% excess air, due to the reduction of partial pressure of the Oxygen, significantly reducing acid formation and equipment corrosion. Normally heavy liquid fuels have more particulate than light ones. High swirl reduces the formation of particulate in the products of combustion. Particulate is also influenced by the degree of atomization.

Burner over-firing can lead to erosion and lack of recirculation. The injectors should be designed to operate properly at reduced flow, to achieve the required burner turndown. Flame length increases when firing concurrently gas and liquid, due to the reduction in recirculation. Also, low energetic wastes burn with a longer and cooler flame.

Flame stability is achieved based on the following:

• Good fuel/air mixing is the key for stable combustion, lowering the required excess air, reducing ignition temperature and increasing the speed of burning,
• Higher pressure drop across the burner throat increases the mixing due to the additional turbulence; excessive velocity reduces the mixing.
• Flame temperature is to be above the auto-ignition temperature of the fuel/air mixture; the burner block/chamber provides heat back to the mixture to keep the temperature above the auto-ignition point
• Flame instability varies with burner design, but deficient O₂, below 15% by volume, requires combustion support to maintain flame stability.

The following expressions provide the most common information to select the proper burner size and operating settings:

• Burner Excess Air, %X, versus stack O₂, %Y

Eq.6 %X = K_f *Y /(21-Y) ====> Y% =21*X /(K_f +X)

The corrected burner Pressure Drop, based on the same burner excess air, with Pressure and Temperature corrections, is given by the following equation

Eq.7 dPc = dP*(14.7/P) *(T/530) = 0.028*dP*T /P

If T=70 °F =530°R and P =14.7 PSIA====> dPc =dP

Atomized Liquid Injectors. Liquid fuels must be converted to a gas form, facilitated by increasing the exposed surface area through atomization, before combustion occurs. Atomization is the physical breaking up of a liquid into, using the oil own pressure, or the steam or air media pressure, into a fine spray. Good atomization will minimize the non-evaporated droplets, improving combustion efficiency and destruction of the liquid wastes. Droplet size distribution is a very important parameter in the evaporation of the liquid. The evaporation rate varies with the droplet surface area as well as with the physical characteristics, pressure and temperature. Smaller droplets are easier to vaporize, providing better mixing. Ideal droplet size is in the 40-100 microns range. Atomized liquid nozzles, Fig.2 and Fig.3, are used to inject liquid waste through the main burner or into the furnace. The maximum efficiency occurs when the liquid waste passes through the burner.

Steam and Compressed Air are the most utilized atomizing media for burner injectors used with different grades of fuel oil /organic wastes. The atomizing media sonic flow provokes a low-pressure zone, just downstream of the injector, providing recirculation and mixing of the atomizing fluid with the waste liquid. Partially combustible liquid requires support fuel to sustain combustion.

Atomizing steam should be dry and slightly superheated at the burner injector inlet, with a required pressure of 70 to 100psig, and as high as 400psig when atomizing extremely heavy residuals. Any released gas should be trapped and vented to avoid unstable fuel feed and burning. If air is used for atomizing, the air should be heated or the oil must be further preheated since this atomizing media will cool the oil. Mechanical burner injectors require a higher preheating temperature to reduce further the viscosity. Oil flow rates should be measured on a mass basis or a temperature correction needs to be applied, since the volume of the oil varies with the temperature. Also, the specific heat of the oil varies slightly with the temperature, and the rule of thumb for the specific heat of oil is about 0.5 BTU/LB.

Viscosity should be reduced to the proper point by elevating the temperature if required. The fluid needs to be thinned or heated to lower the viscosity to obtain a fine spray, otherwise poor ignition and combustion will result. The desired viscosity depends on the burner type, method of atomizing, ranging between 100 and 200 SSU, with a most common value at or below 150SS. The normal temperature limit to reduce viscosity is as high as 400-500°F, typically about 200-270°F. It is common practice to overheat the oil by 30-50°F to compensate for unaccounted piping heat losses. Also, the preheated temperature should be verified to avoid polymerization, nitration or oxidation. If preheating represents a problem, lower viscosity oil can be added to reduce the overall viscosity. Insufficient preheating will not lower the viscosity for good atomization and poor burner ignition, unburned fuel, smoke and carbonization will occur. Excessive preheating of the oil can provoke cracking and coking the oil in the preheater, blocking the burner injectors and loosing output heat, as well as provoking vaporization and pulsation of the pumps and burners. Also a high degree of steam superheat can vaporize the oil in the injector gun, causing vapor lock at the injector tip. Sometimes when the oil is preheated to just below a low flash point, it can be insufficient to reduce the viscosity to the desired level, but there is danger of preignition causing the burner to puff and pulsate due to the formation of light vapors. In this case, if possible, the preheating temperature should be somewhat lowered.

Low NO_X Burners. Nitrated organic, when combusted at high temperatures, will form nitrogen oxides (fuel NO_X), NO and NO₂, that need to be reduced to acceptable levels. Modifying combustion is the most widely used technology for reducing the formation of the nitrogen oxides. Combustion air temperature, excess air and theoretical flame temperature have also a heavy increasing effect on the nitrogen oxide (thermal NO_X) formation in the combustion zone. Lower nitrogen fuels and combustion modifications, like the staged fuel and/or combustion air and the flue gas recirculation, can reduce the formation of these oxides. In staged air combustion, the burner operates under sub-stoichiometric conditions, and part of the required combustion air is introduced downstream of the burner where the combustion is then completed, resulting in lower overall formation of the thermal NO_X. In Flue gas recirculation, a portion of the downstream flue gas is introduced in the burner, lowering the theoretical flame temperature and reducing the amount of oxygen concentration to react with the nitrogen, resulting in reduction of thermal NO_X.

NO_X levels are increased due to high temperature flames. Combustion air preheat increases the flame temperature and combustion efficiency, although increases the NO_X emissions too. The NO_X levels can be reduced if Staged Air burners are utilized and Staged Fuel burners will provide even larger NO_X reduction. Addition of external flue gas recirculation will achieve further reduction of NO_X levels of the Staged Fuel burners. The NO_X formation increases faster with N₂ rich fuel/wastes and with higher operating and flame temperatures. Also it increases to lesser degree with higher excess air oxidation. Single stage combustion can generate NO_X from the fuel bound nitrogen, up to 85%, dependent on the nitrogen concentration in the waste. The NO_X formation can be minimized, using a system operating at lower temperatures, lower excess air, recycle flue gas and also with part of the combustion air introduced downstream of the burner flame.

• Staged Air Burners, Fig.5, operate in two different combustion zones with a fuel rich (reducing) zone and the secondary air is introduced at a location downstream of the primary combustion zone, an oxidizing zone. The NO_X is reduced as the primary air is reduced, with an optimized point of 70% of stoichiometric, achieving reductions of 50% using natural gas and 38% using fuel oil. The lower flame temperature in both zones due to the heat losses to the surroundings reduces the thermal NO_X. The flame is cooler and longer than those of the standard burners are.
• Staged Fuel Burners operate in two different combustion zones with a fuel poor (oxidizing) zone, reducing the thermal NO_X due to the heat losses to the surroundings, followed by the introduction of the balance fuel that combines with the leftover oxygen, generating a zone with a cooler and longer flame.
• Water Injection applied as steam or atomized liquid lowers the flame temperature. Also, it reduces the formation of particulate and allows a reduction in excess air. The combined effect can be up to 65% NO_X reduction, although this method decreases thermal efficiency and increases the corrosion effects.
• Internal Flue Gas Recirculation Burners utilize the burner design for the combustion air or fuel streams to generate a low-pressure zone inducing the downstream flue gas into the combustion zone, reducing the flame temperature. Both flame stability and flashback can be of concern with flue gas recirculation burners.
• External Flue Gas Recirculation Burners utilize the cooled inert fluegas ducted back to the burner to produce the same effect as internal recirculation. Both the flame temperature and the resultant oxygen concentration are decreased, resulting in an overall NO_X reduction. Both flame stability and flashback can be a concern with external flue gas recirculation.

Burner Noise. Burner Noise consists of pressure pulses and results from the unsteady nature of the combustion process, depending on the rate of change of the rate of heat release. The overall combustion noise is a resultant of the following noise mechanisms:

• Combustion roar and super-turbulent combustion noise
• Furnace/Chamber resonance
• Burner throat resonance
• Flow noise amplification and combustion driven oscillations

The sound power level generated is affected by the burner/injector geometry and aerodynamics design. Also it increases with the following operating conditions:

• Burner size, firing rate and air /fuel ratio
• Combustion intensity
• Higher speed of fuel/air mixing.
• Fuels with higher flame speed (hydrogen content).
• Higher pressure drop and higher fluid density generate higher flow noise.
• Smaller liquid droplet sizes
• Longer flames

Burner troubles and causes. Oil firing generates a bright yellow flame; a dull orange flame denotes insufficient excess air and a white flame denotes too much excess air. When the burner injectors get eroded or partially clogged, they will not provide good pattern, droplets become coarse and the combustion efficiency drops, increasing the CO emissions. Also, the liquid can impinge on the refractory, causing future failure. High pressure and low flow conditions is an indication of a clogged tip. Low pressure and high flow conditions indicates the tip is lost or mechanically deteriorated. Liquid injector guns should be able to be removed on the fly, for easy cleaning or replacement, avoiding costly shutdowns. The most common burner troubles and causes when using liquid fuel (or organic waste) are as follows:

• Difficult start, due to wrong liquid fuel/air ratio, incorrect atomization, excessive atomizing steam, wet steam, injector partially blocked/plugged, high flash point, wrong oil flow, dirty oil, etc
• Combustion inefficiency, due to deficient combustion air, incorrect fuel/air ratio, incorrect atomization, low flash point, low calorific value of organic waste, dirty oil, etc
• Lack of atomization, due to wrong atomizing temperature, pressure or flow, incorrect atomizing media flow, wet steam, viscosity too high, incorrect oil flow settings, excessive dirt, injector eroded, plugged or carbonized, unstable oily waste, injector dripping, etc
• Smoke, due to lack of atomization, wrong fuel/air ratio, condensate in the steam atomizing media, flame impingement, cold furnace, pre-ignition, wrong injector position, etc
• Soot, due to high oil ashes or carbon particles, incorrect injector position, etc
• Unstable/pulsating flame, due to oil or atomizing media pressure fluctuations, lack of atomization, wrong injector position, damaged burner throat, low flash point, excessive preheat, low injector turndown, air in the oil line, excessive free water, etc
• Burner pre-ignition, due to low flash point, excessive preheat, atomizing media temperature, etc.
• Flame flashback, due to low oil flash point, atomizing pressure fluctuations, high oil pressure, water content, dirty oil, etc
• Carbonization of burner injector, due to deficient combustion air flow, wrong injector position, excessive viscosity, improper oil blend, deficient atomization, etc
• Burner Injector erosion due to corrosive oil, excessive free water, high content of dirt or salts in the oil/waste, etc
• Burner Refractory erosion due to excessive dirt, salts and ash content in the oil, wet atomizing steam, flame impingement, undersized combustion chamber, etc

The heavier the oil in viscosity the higher in carbon residue and heavier in weight, the rate of combustion decreases and the flame length increases. Lower is the specific gravity lower is the carbon content and higher the hydrogen content, greater is the heat of combustion on a weight basis. The proper preheat temperature should set not only to make the oil more pumpable but also to obtain proper and efficient combustion. Lack of efficient combustion will result in low heat release, fuel waste and formation of smoke and carbon. Problems due to viscosity of an organic or oily waste are both the excessive high and low viscosity.

Too high viscosity

• Difficulty in pumping, resulting in loss of pump suction or lack of flow at burner
• Poor atomization resulting in incomplete combustion and liquid dripping at the burner injector.

Too low viscosity

• Excessive flow to burner injector causing incomplete combustion
• Losses of heat release due to low energetic value and low gravity

Ashes containing high levels of Vanadium and Sodium will attack the combustion chamber refractory and also provokes undesirable metallurgical attack. The V₂O₅ acts as a catalyst for the SO₂ to SO₃ conversion, adding for the sulfuric acid corrosion. Concentrations of Vanadium up to 20 PPM provide safer operation of the furnaces. The attack is dependent on the degree of Vanadium and stronger when Sodium is also present in the fuel /waste oil.

Fuel oil switching is necessary if gas is not available and the fuel pump and heater sets need repair or cleaning. By using the same atomizer, a spare atomizer is unnecessary and the distillate oil can be substituted for the heavy oil online with no interruption to the operation. The burners will work with atomizing air as well as atomizing steam for either type of oil. A system can be designed to automatically switch from atomizing steam to atomizing air if steam pressure is lost and also switch from heavy oil to distillate oil if oil temperature or pressure, leaving the pump and heater, drops below set values. Also, the ability to switch between No. 6 and No. 2 oil without changing atomizers means that this operation can be done at any load without a shutdown. Operator involvement only is needed to return to steam atomization or to heavy oil after firing distillate.

Burner Control. Systems for Flame Safeguard and Automatic Control provide proper conditions of safety and automatic burner control output. The burners can accommodate several scanners, typically selected are self-checking ultraviolet with quick-disconnect couplings on the scanner heads. With this arrangement and a proper burner management system, the scanners can be removed and replaced without tools and without shutting down the burner. Good scanning is very important for burner control, but without a flame temperature above 2200°F, will result in a lack of flame stability regardless the other operating parameters or proper scanning. In order to operate stable and safely, we can use several operating options:

• The combustion temperature above 2200°F for gases and 2500°F for liquids, requiring extra consumption of fuel;
• Adding a continuous igniter burner, to provide continuous ignition;
• Introduce the non-combustibles downstream of the flame into the furnace residence chamber to avoid flame quenching.

The burner design easily allows the use of metered controls for fuel/air ratio, with the advantage that fuel can be changed at any load. The fuel/waste to air ratio control is designed to maintain the proper ratio while meeting the system demand on firing rate. This ratio is commonly set or maintained using Tandem Valve, Flow Control or Pressure Balance Control, as follows:

• Tandem Valve systems operate based on the combustion air and fuel valves being linked mechanically or electronically, in function of the valve flow characteristics;
• Flow Control systems operate based on the rate of one controlled flow, air or fuel, being transmitted to a ratio controller causing the other flow to stay in ratio;
• Pressure Balance systems utilize the combustion air control valve impulse pressure to set the outlet fuel pressure and flow, as the combustion air is varying, the ratio control regulates the fuel flow to follow in proportion. Combustion chamber pressure should be backloaded at the fuel regulator to compensate for pressure variations.

Typically, the term’s pilot and igniter are interchangeable. The most common pilots are the Continuous Igniter, Intermittent Igniter and the Interrupted Igniter. Direct electric igniters are generally not recommended. The different types of igniters are define as follows:

• The Continuous Igniter, Fig.6, on a system provides continuous pilot without turndown through the entire time, whether or not the Main Burner is firing. The Continuous Igniter Burner is sized to support ignition energy, under any light off or operating condition, typically at least 10% of the Main Burner capacity. A major application of this arrangement is with highly variable calorific value waste fuels.
• The Intermittent Igniter, Fig.6, is used to ignite the Main Burner, under prescribed light-off conditions, and stay on simultaneously with the main burner firing, shutting off with the main burner. This igniter burner is also used to support ignition under low load and certain adverse operating conditions. Supervision is required at both burners; the Igniter Burner capacity is generally within a 4-10% range of full load burner fuel input and is not designed to ignite main burner under uncontrolled conditions.
• Interruptible Igniters, Fig.1, are small igniters applied to gas and oil burners to ignite the fuel input to the burner under prescribed light off conditions. The igniter shall be cut off at the end of the time trial for ignition of the main burner. Interrupted type igniters are typically sized to a minimum of 3% of the main burner at its light off rating, typically at 25% burner capacity. The use of Interrupted Igniters to support ignition or to extend the burner control range should be prohibited.

The use of Continuous and Intermittent Igniters provides a very stable flame, improving flame stability and reliability on a burner system. Also, furnace stable conditions exist if the operating temperature is at or above 1400°F, consequently flame safety and process interlocks can be by-passed. Also, it is commonly used a value of at least 400°F above the fuel /waste auto-ignition temperature, if known.

The design considerations presented in this article for combustion equipment are based on proven technology, and they are commercially available to meet the required combustion and destruction efficiencies. Burner design used for a given application may not be compatible with the all the process requirements, but should match the most possible requirements, including emissions. These combustion systems must be composed of properly designed and operated components to handle the upset operating conditions and also to prevent hazardous situations due to operating error.

The use of lower grade fuels and wastes at presently increasing costs call for the application of sophisticated, cleaner and still reliable burners. Changing emissions control, efficiencies and performance requirements has been the driving force for new techniques to enable combustion of these lower grade fuels and wastes. Based on engineering and economic evaluations, it was found that retrofitting thermal systems with proper designed and operated burners could offer an alternative to new units in order to achieve the required efficiencies. When burning waste fuels, the combustion product temperatures can be very high to achieve the required destruction efficiency. The utilization of waste heat recovery equipment like boilers, to recover the waste heat is a viable concept, although the heat recovery systems from chlorinated hydrocarbons and aqueous wastes are not straight forward, presenting critical design problems. Waste energy can be environmental friendly and profitable to recover the heat of the heavy fuels and wastes, as well as the auxiliary fuel, especially in the large volumes of high temperature fluegas.

Improvements in combustion system design and control can result in capital cost increases but can reduce the operating costs. The design of safety and process control systems has increased and can provide the safety, performance and reliability expected by the public and the industry. Control system configuration, flammability monitoring operation and operating procedures must be properly established to add personnel, equipment and environmental safety. Combustion systems should offer reliable, continuous and automatic operation with minimum supervision. Control systems are not fool proof but proper operation and maintenance can improve the overall the system.

Hopefully, the considerations and factors discussed in this article will be of assistance in understanding and improving the design and operation of efficient heavy fuels and waste combustion systems.

C_t = Craya-Curtet number

D= burner throat diameter

dP= burner Pressure Drop

dPc = burner corrected Pressure Drop, dPc,

G_o = flux angular momentum

G_x = flux axial momentum

K_f = fuel coefficient, 90 for fuel gas and 95 for fuel oil

K_p = proportionality constant

m =(U²_d –0.5U²_k)/U²_k

P = absolute pressure, PSIA

Q = burner thermal input

R_e = exit throat radius

S = Swirl number

T= absolute temperature, °R

U_d = dynamic mean entry velocity

U_k = total flow uniform velocity

V = combustion air velocity

w = typically combustion air density

X = burner excess air, %

Y = stack O₂, %

1. Monroe, E.S. "Saving Fuel with Furnaces", MIT Press, 1982
2. Leite, O.C., "Equipment for Incineration of Liquid Hazardous Wastes", Environmental Technology, May/June 1996
3. Leite, O.C., "Burn Wastes Cost-Effectively", Hydrocarbon Processing, May 1998
4. Leite, O.C., "Operating Thermal Incinerators Safely", Chemical Engineering, June 1998
5. Monnot, G., "Principles of Turbulent Fired Heat", Gulf Publishing, 1985
6. Reed, R. J., "Combustion Handbook", North America Mfg. Co., 1978/1995
7. Schmidt, P.F. "Fuel Oil Manual", Industrial Press, 1985
8. Weber, R. "Scaling Characteristics of Aerodynamics, Heat Transfer and Pollutant Emissions in Industrial Flames", 20^th Symposium on Combustion /The Combustion Institute, 1996

Fig.1 High Intensity Vortex Combustor

Fig.2 External Mix Atomizing Injector

Fig.3 Internal Mix Atomizing Injector

Fig.4 Effective Residence Time

Fig.5 Low NO_X Air-Staged Vortex Burner

Fig.6 Waste Combustor with Igniter burner

Author

Olavo C Leite is principal consultant for Thermica Technologies, USA,(email THERMICATECH@yahoo.com ). He holds a five-year degree in mechanical engineering from the Technical University of Lisbon and has written in various publications on process combustion, flaring and incineration. E-mail: THERMICATECH@yahoo.com