Furnaces | Crude Distillation Unit (CDU) Energy Components and Guidelines

A large potential for energy saving is available especially to oil refineries through the optimization of fired heaters and boilers. Typically, more than 75 % of energy consumed in a refinery is fuel for fired heaters and boilers/cogeneration units. The average thermal efficiency of furnaces is estimated at 75-90%. Accounting for unavoidable heat losses and dew point considerations, the theoretical maximum efficiency is around 92% (HHV).

Furnaces | Crude Distillation Unit (CDU) Energy Components and Guidelines

This suggests that on average a 10% improvement in energy efficiency can be achieved in furnace and burner design.
The simple guidelines introduced below are oriented towards fired heaters, however, most of it are also applicable to fired boilers and if applied by plant’s operators can result in fuel saving.

Fired heaters convert chemical energy of the fuel into thermal energy. Again, the efficiency of such transformation can be written as follows:
Heater Efficiency = Heat absorbed by process stream (useful heat output)/Heat Input
In other words, it is the heat input to the heater minus losses divided by the heat input to the heater.

It is normally accepted in oil and gas industry to calculate the heater efficiency using the fuel low heating value (LHV). Hence, the heat fired term/heat input is equal to the LHV of the fuel multiplied by the fuel flowrate.

If some boiler manufacturer report the efficiency based-upon high heating value of the fuel. The efficiency value will be less than the actual and to correct it additional losses must be included for the moisture and hydrogen in fuel.

Naturally, if more than one fuel is being fired, the total heat fired is the sum of each fuel’s heat fired including continuous pilot fuel. When additional heat inputs to the heater is practiced, such as for instance combustion air preheating by an external source, sensible heat in fuel stream, atomizing steam, or the sensible heat content of gas turbine exhaust used as a combustion air, it must be added to the total heat input to the heater.

Heat Input = Summation of {(Fuel rate* LHV)i }+ External heat input to the system
Having said that, any energy conservation initiative we propose should maximize heater’s efficiency. For instance, minimizing losses from heater would automatically would simply means achieving such objective.

Heat losses in fired heaters can be divided into two categories; major and minor ones.
1. Minor losses which means plus or minus 2% in the calculated heater efficiency, include for instance, radiation losses; convection section losses through the casing; incomplete combustion and the latent heat of any atomizing steam.
2. The major losses in the fired heater are concentrated in the enthalpy/ heat content of the stack gases. Such major loss normally can be minimized via reducing the quantity and the temperature of such gases. The quantity and temperature of the flue gas can be reduced via operating at minimum excess air. Temperature of the flue gas can be further reduced via heat recovery applications.

Combustion as we simply know is a rapid run away reaction between fuel and air/oxygen at ignition temperature in which the released heat is the desired product rather than the end products of this chemical reaction. The amount of heat released can be controlled by limiting the amount of fuel chosen. Supplying the fuel or air to the combustion reaction in excess of the stoichiometric amount causes a loss of efficiency because unreacted materials normally leave the heater at an elevated temperature carrying away some of the heat produced in the process.

For an efficient combustion process, fuel is burned completely to CO2 and H2O with no excess air. However, since the mixing efficiency of the reactants is not ideal, therefore, excess air is required to guarantee complete combustion in the fired heater. When excess air is used, energy is lost for two reasons. First, the extra air requires additional fuel to heat the air from ambient to stack temperature. This extra air is cooling down the heater, since it takes heat with it while leaving the heater. Second, the extra air is cooling the flame and reduce amount of heat transferred in the radiation section of the heater.

Potential fuel savings from reducing excess air can be calculated now using modern heaters commercial software (in the past using heat available curves).
Heat available is defined as the LHV minus enthalpy change of the products of combustion between the indicated/measured temperature and 60°F, expressed as Btu/Lb of fuel.

The heat available curves were plotted as a family of curves at different excess air levels. Then, these curves can be used to determine the amount of heat absorbed in a process heater if the stack temperature and excess air are known. The heat available is equal to zero at the adiabatic flame temperature and is equal to the LHV at 60°F. No credit is taken for the condensation of water vapor, in such curves, to be consistent with the definition of the fuel low heating value (LHV).

Plant operators can use heat available curves to calculate quickly the efficiency of a fired heater using the unit’s operating conditions of excess air and stack temperature. The heat available at the stack temperature is the heat absorbed in the unit/heater per pound of fuel fired. Then, the division of such heat by the fuel LHV yields the heater efficiency neglecting minor losses.

For many years it was acceptable to work at high excess air as high as 30 to even 50% of theoretical air. Rising fuel prices now has increased the incentive to control the heater excess air and it is now very common to see process heaters operating in the 10% excess air range.

Many quick graphs are available nowadays in many books where we can use it to get the percent of fuel saving upon given the original oxygen content and stack temperature. It calculates directly for us the potential fuel savings in fired heaters and boilers by reducing the oxygen in the flue gas from the current operating condition to the 2 5 which is considered a practical goal of most of the furnaces, according to the type of the fuel.

The use of such curve by plant’s operators is easy and allows the operators to determine quickly the consequences of not making furnace adjustment to keep excess air low.
A common cause of high oxygen levels in the stack is air leakage through holes and openings in the fired heater. Proper adjustment to fired heater draft will help reduce air leakage. CO analyzer is used sometimes instead of oxygen analyzer to more accurately calculate the amount of excess oxygen in the flue gas.

It is important to note here that the cost of air leaks in fired heaters are ignored, while it can cost up to $60,000 per year depending on the stack temperature and draft as well as the size of the leak (chirp door) when fuel is valued at $60 a barrel. Normally, the major sources of air leakage are flanges; cracks; casing corrosion; open peep doors and so on.

It is instructive to mention here that even though it is obvious that reducing the excess oxygen is very beneficial for fired heaters there are some constraints that sometime hinder its optimum adjustment such as smoke, combustible breakthrough and flame shape.

Each of these constraints may have a different threshold value of excess air, which varies from heater to heater. Hence, each heater or boiler must be individually evaluated to determine its minimum excess air level.

Smoking is caused by incomplete combustion of carbon particles or soot. Soot formation is an undesired byproduct of the combustion process due to the existence of poly aromatic hydrocarbons in the fuel burned. Besides the reason of insufficient air for the smoking phenomenon, smoke can also be formed due to poor atomization, poor mixing of the air with fuel and in appropriate residence time in the radiant section. In case of multiple burners, uneven distribution of combustion air can lead to smoking in one burner and non-smoking in another.

Therefore, it is important when burners are working on low excess to make sure that the both fuel and air are evenly distributed among burners.

Combustion air mal distribution can usually be corrected by adjusting fuel registers or air dampers. Fuel mal distribution can be caused by plugged fuel tips, partially closed shut-off valves or unsymmetrical burner piping. Mal distribution can be detected by traversing at the top of the radiant section with a probe connected to a portable oxygen analyzer.
It is common practice that some operators will keep the excess air at higher level than necessary to prevent smoking during load changes. Smoking can normally be handled using feed forward control strategy.

Combustible break through is normally a result of unburned fuel due to again air of fuel mal distribution.
Maintaining proper flame shape is another important task in fired heaters. Any change from the designer’s desired flame shape would result in tube over heating due to the change in heat flux profile. In the worst case tube failure can result from flame impingement. When the excess air is reduced for sake of energy efficiency, the flames tend to become longer and to lose definition. The length of the flame is of concern in both heaters and even more in boilers. The loss of the flame definition can cause impingement if the tubes are close to the burners.

Excess Air Control

The currently used oxygen analyzers generally employ either a zirconium oxide cell inserted directly in the stack or an external zirconium oxide cell with a very short sample system operating above the dew point to eliminate any potential for sample system corrosion. Such systems have high reliability, low maintenance and fast response (less than one second).

A permanent oxygen sample point should be positioned in the fired heater in order to obtain a sample as uniform as possible and to avoid flue gas dilution from air leakage. Placement of the sample point after the convection section should result in a uniform sample, but may result in high oxygen reading depending on the quantity of air leakage into convection section. Header boxes can allow excessive air leakage and thus flue gas dilution if proper sealing have not been used. Locating the sample point before the convection section avoids header box air leakage but may result in non-uniform oxygen sampling due to flue gas mal distribution. This location may tend to indicate the oxygen level associated with the nearest burner. Therefore, the location of the sample point should be customized to each heater considering the number of services in the radiant section, the flue gas mal distribution and convection section air leakage.

In many facilities now especially oil refineries, portable oxygen analyzers can be particularly useful even if the permanent oxygen analyzers are reliable. They give the plant operators a chance to make an easy check of the accuracy of the permanent analyzer. Portable oxygen analyzers are preferred to carbon dioxide analyzers because oxygen readings are more sensitive to the exact amount of excess air present. Portable analyzers can also be used to help adjust burners and find air leaks. To measure the quantity of air leakage into the convection section, the O2 should be measured at the entrance to the convection section, and in the stack. Any increase in oxygen between these two points is due to leakage.

Air to fuel ratio control systems are of two types, feedforward and feedback control strategies.

A combination of these two strategies is normally the best approach.
In the feedforward loop the air and fuel rates are monitored and the air rate is manipulated in order to maintain the fired heater near optimization. In simple application, a flow limit is applied to the fuel so that more fuel cannot be supplied than air is available to burn it. If an increase in firing is needed due to switching in fuel type or a process reason, control logic increases air flow rate first, followed by a fuel increase. When a decrease in firing is called for, fuel is reduced followed by a reduction in air. This type of system is called cross limited combustion control.

In many modern applications, duty control loops do exist if the switching in fuel type is frequent. Back to this system of cross limiting control loop, described which is known to be subjected to error due to change in air humidity, atmospheric pressure, and fuel quality as well as instrument error to know how the addition of oxygen analyzer and feedback control loop can greatly reduce the effects of these errors that can be significant and can contribute to higher than expected excess air levels or unsafe operating conditions. The advantage of this system over a straight forward oxygen feedback system is the elimination of momentary smoking during times of fast response. For variations of fuel type, the air can be ratioed to the Btu value when firing with paraffin gases and fuel oil since the amount of air required per Btu is constant.

This combined feedback, feed forward control loops approach is the best one to fired heaters control and essentially guarantees that combustible break through will not occur unless there are hardware mal functions.

Let us now move to another important opportunity in process heaters for energy conservation which is the control of draft. The difference in pressure between inside the fired heater and the outside atmosphere at the same elevation is known as draft, a positive draft is defined to mean that the pressure is less in the fired heater than the atmosphere. This is the normal operating mode for process fired heaters. In setting draft, it is recommended to avoid very high draft that could cause excessive air in leakage. A condition of negative draft should also be avoided to insure that hot flue gases do not escape through any cracks or openings causing structural damage and safety issue to personnel. Fired heater draft is usually maintained at about 0.1 inches of water column just below the convection section by adjustments to the stack and burner dampers.

In natural draft fired heaters, the stack damper can be under remote control but the burner air registers normally must be manipulated manually so it is not possible to independently control both air flow and draft. For forced draft furnaces, direct control of both box pressure and air flow is fairly common. In this situation the air flow is usually controlled by the forced draft fan. The draft pressure can then be controlled by manipulating the stack dampers or the induced draft fan, if one is used.

Furnace stack waste heat recovery is one of two major and common opportunities for efficiency enhancement in fired heaters. The second one is combustion air preheating. Both of them are recovering heat available in the stack flow by lowering the stack temperature. While air preheating results in direct fuel fired reduction, waste heat boilers are producing steam that can be used for other process application that might lead to another again fuel consumption not in the process heater but in the plant’s boilers.

Air preheating system saves fuel via transferring heat from flue gas to the combustion air. As a result, the fired heater tack gas temperature is reduced and the operating efficiency is increased. By lowering the stack temperature to about 300 to 350°F, which is typical with air preheaters’ installations, fired heater efficiency can reach more than 90%. Air preheating systems can be added to any furnace natural or forced draft. The only considerations governing each installation are economic return on investment, plot plan available for the new equipment, and fired heater and burner construction which must be compatible with the required hot air ductwork and plenums. In order to evaluate the potential application of an air preheater system to an existing fired heater, operating data must be obtained, such as process heat duty, heat fired, stack temperature and flue gas excess air levels. For instance, assume you have a furnace with a stack temperature of 850°F and the process heat duty 150 million Btu/hr. In such case about, 30.0 MMBtu/hr can be saved via reducing the stack temperature to 300°F.

Generally, there are four types of air preheaters. Rotary/regenerative; tubular; circulating fluid and heat pipe. Each type has inherent advantages and disadvantages and such information is beyond the purpose of this chapter. However, there are some system component guidelines for the design of such air preheaters. For instance, pressure drop for both air stream and the flue gas stream should be limited to approximately 3 inch water each and the minimum flue gas outlet temperature is normally in the range of 300 to 350°F (sulfur content of the fuel fired, type of the preheater and the material of construction may limit the outlet temperature to a slightly higher level). Soot blowing facilities are required with fuel oil and dirty gas firing services.

The main constraint for maximum benefit is the cold end temperature corrosion protection. The temperature of the flue gas leaving the preheater will determine the system efficiency. The flue gas temperature should be as low as possible without risking significant low temperature corrosion of the air preheater elements. Besides, consideration must be given to corrosion in downstream ducting. The minimum flue gas temperature in the downstream ducting should be above the actual dew point with reasonable safety factor.

The flue gas temperature leaving the preheater will be affected by the inlet end temperature and the firing rate of the equipment. In general, lower inlet air temperature and firing rate will result in lower flue gas exit temperature.

The following methods can be utilized to negate the effects of cold end metal temperatures due to variations in ambient air temperatures and firing rates:
– Use of a steam/air heater to preheat the cold air ahead of the main air preheater
– A cold air bypass around the preheater which limits the minimum flue gas temperature at the cold end when necessary

– A hot air bypass back to the inlet of the forced draft fan to raise the air temperature entering the preheater in much the same way as the steam/air heater Forced Draft Fan
The fan is the key element in a forced-draft system. Fans are not usually 100 % spared and failure of a fan can result in furnace shutdown in a single fan installation or in reduction of unit throughput if more than one fan is used to supply combustion air. Therefore, to provide a fan/driver combination with desired run length, capabilities critical design features must be carefully selected. For instance, for natural draft burner, a single fan should be sufficient if drop-out doors are provided in the ductwork to allow for natural draft operation in case of fan failure. For forced draft burners, two 50% fans are normally provided where one fan only enable the furnace to be operated at about 85% of its design capacity.

The fan flow rate should include burner requirements (hot air and cold air operation) plus 5% for potential ductwork leakage. Static head requirements include the pressure loss through the preheater, ducts, and burners. Typical fan heads are 6 to 8 inch water with natural draft burner and 15 to 20 inch water with forced draft burners. The recommended fan margins are 15% on air flow rate and 15% on fan head.

Induced Draft Fan

In such case, a single fan is sufficient if the fan fails since the furnace stack damper opens and the hot flue gas does not pass through the air preheater, returning back the furnace to cold air operation mode. The flow rate is based on the flue gas rate and the air preheater outlet temperature. The static head requirements include the draft required at the preheater inlet; plus the pressure drop through the preheater, cold flue gas duct, and stack; minus the stack effect. Typically, the fan head is in the range of 5 to 7 inch water. The recommended fan margins are 25% on flue gas flow rate and 50% on fan head.

Air Ducts

The recommended arrangement of the air ducts includes a main supply duct, a distribution header and feed ducts to a limited number of burners. Isolation dampers should be provided in the feed ducts to allow for burner maintenance with the preheater in operation. The designed air velocity in the supply duct is about 50 feet per second. The distribution header velocity should be limited so that the velocity head pressure is equal to less than about 5% of the pressure drop through the feed ducts and burners.

In any case, the design shall provide a bypass duct around the air preheater to allow for control of the flue gas temperature out of the air preheater and also for operation with preheater out of service. With natural draft burners, drop out doors should be included in the air ducts. These doors will allow the furnace to operate under natural draft conditions in case of forced draft fan failure as mentioned before. The hot air ducts should be insulated to minimize heat loss.

Flue Gas Ducts

The design velocity for all ducts is about 50 feet per second. Again, we have to insulate the ducts to minimize the heat loss. Normally, we protect against corrosion downstream of the air preheater through proper insulation. Generally, for operation with an air preheater, the stack must be lined for corrosion protection. A combined cost of lining the existing stack and providing cold flue gas ductwork back to original stack may be more expensive than the installation of a new ground supported stack. The flue gas exit velocity from the existing stack will be less at the lower flue gas temperature.

This may increase ground level pollution. If the addition of a reducer at the stack exit to achieve a high velocity is not sufficient to overcome the reduced buoyancy of the colder gas, a new taller stack may be required. While we are evaluating the installing of a new stack, we shall keep in mind that a reducer added to the existing one will not allow the full operation with the air preheater bypassed. Besides, less flue gas is emitted with the air preheater in operation thereby reducing pollutants.

Dampers

Furnace stack dampers on automatic control are closed for normal operation, but should be wide open in case of induced draft fan failure or air preheater failure. Forced draft fan dampers are used to control oxygen while induced draft fan dampers are used to control box pressure.

Manually operated isolation dampers include air preheater bypass damper. This damper is used to control the preheater flue gas outlet temperature during cold weather operation to avoid condensation corrosion. Isolation dampers are required in the air duct on either side of the preheater for maintenance with the forced draft fan in operation. With natural draft burners, isolation dampers are required in the air feed ducts to allow for burner maintenance with the preheater in service. With forced draft burners, isolation dampers are required in feed ducts to individual burners for maintenance during furnace operation with either preheated or ambient air.

It is instructive to note here regarding furnace’s air preheater that, if you have two furnaces having similar operating characteristics and are located in a very close proximity, you can connect them to a single air preheater system. Such single system will have an investment which is at least 20% less than that for two individual systems.
Let us now move from combustion control and air preheating in furnaces to waste heat recovery using waste heat boilers, which is another very important opportunity for furnaces’ efficiency enhancement.

Steam generation, or waste heat boiler system, recovers heat that would be normally lost by high temperature furnace stack. In some furnace design and in some special process application, it is sometimes possible to use the convection section to heat up portion of the main feed to the furnace. Only steam generation option for waste heat recovery using waste heat boiler systems is discussed here, since steam generation system can be attached to any furnace.

For most furnaces, steam generation system involves the installation of a ground level waste heat boiler and induced draft fan. These systems are generally more expensive and offer less credits than air preheaters systems. Therefore, their use is usually considered only when additional steam generation capacity is required to balance demand and avoid the installation of new boiler.

Steam generation facilities can be added to the convection section of some furnaces without major structural or stack modifications. For such furnaces, the extent of modification and the associated downtime involved must be analyzed to determine whether this approach is more attractive than a ground level waste heat system.
In order to evaluate the addition of a waste heat boiler system to a process furnace, operating data such as heat fired, stack temperature, and excess air should be obtained.

With these data and with simple computer program or even literature charts, the potential heat recovery rate and stream credits can be estimated. For example the potential heat recovery rate from a furnace having a duty of 150 million Btu/hr with a stack temperature of 850°F is about, 18 million Btu/hr.

Atypical waste heat boiler installation consists of three main components, the waste heat boiler; the induced draft fan, and the flue gas ducts. The induced draft fan is required to compensate for the additional flue gas pressure drop and the loss in stack effect due to the lower stack temperature. The waste heat boiler consists of an extended surface tube bank in an enlarged section of ductwork. Most of the units are double drum designs and operate via natural circulation. The mud drum on the bottom of the system serves as a tube header and settling area for solids in the system.

The steam drum serves as a steam/water separating device and provides the static head for water circulation.
As like as air preheater systems in furnace, there are some design considerations for the waste heat boiler system components that are briefly mentioned below.

For the waste heat boiler, the outlet temperature approach (difference between flue gas and steam saturation temperature) should not be less than 40°F, since lower approach temperatures would result in excessive surface area requirements. The flue gas pressure drop is normally within the range of 3 to 5 inch water. Again, the sulfur level in the fuel burnt will determine the minimum acceptable metal temperature that can be tolerated with carbon steel (normal tube material of construction). If sulfur corrosion will be a problem when designing to a 40°F approach temperature, then the flue gas outlet temperature should be increased, which of-course will decrease the heat recovery.

For the induced draft fan, a single fan is sufficient. If the fan fails, the damper in the furnace stack opens and the furnace reverts back to pre-waste heat recovery operation mode. The static head required includes the draft required at the waste heat boiler inlet, plus the pressure drop through the waste heat boiler and duct minus the stack effect. The flow rate is based on the flue gas rate and the waste heat boiler outlet temperature. The recommended fan margins are 25% on flue gas flow rate and 50% on fan head.

Design guidelines for flue gas ducts include; recommended design velocity of 50 feet per second; insulation of the hot flue gas ducts to minimize heat loss and protection of the cold flue gas duct downstream of the waste heat boiler against condensation and corrosion.

Dampers including the stack damper and isolation dampers have similar design guidelines as per the previous air preheating application. Furnace stack damper is closed during normal operation. However, in case of induced draft fan failure, the damper opens to bypass the waste heat boiler system. The two isolation dampers are again required here around the waste heat boiler system to allow for maintenance. One should be located upstream of the waste heat boiler and the other downstream of the induced draft fan.
Last but not least, another important opportunity for furnace/boiler efficiency enhancement is burner design.

New burner designs aim at improved mixing of fuel and air and more efficient heat transfer. Many different concepts are developed to achieve these goals, including lean-premix burners, swirl burners, pulsating burners and rotary. At the same time, furnace and burner design has to address safety and environmental concerns. The most notable is the reduction of NOx emissions. Improved NOx control will be necessary in almost all new facilities to meet new air quality standards.

Briefly, the efficiency of process heaters can be improved by improving heat transfer characteristics, enhancing flame luminosity, installing recuperators or air-preheaters, improved controls and/or using waste heat recovery boiler system.

Maintenance

Regular maintenance of burners, draft control and heat exchangers is essential to maintain safe and energy efficient operation of a process heater. An audit of an old refinery identified excess draft air in six process heaters. Regular maintenance (twice per year) can reduce the excess draft air and would result in annual savings of nearly 100,000 GJ/year.

Draft Control

Badly maintained process heaters may use excess air and this reduces the efficiency of the burners. Excess air should be limited to 2-3% oxygen to ensure complete combustion.

Furnace’s advanced control system allows running the furnace with 1% excess oxygen instead of the regular 3 to 4%. This system does not only reduce energy use by 3 to 6% but also reduce NOx emissions by 10 to 25%, and enhance the safety of the heater. An energy audit of another refinery found that reduction of excess combustion and draft air would result in annual savings of almost $1.2 million per year.

Air Preheating

Air preheating is an efficient way of improving the efficiency and capacity of process heater. The flue gases of the furnace can be used to preheat the combustion air. Every 35°F drop in the exit of the flue gas temperature increases the thermal efficiency of the furnace by about 1%. Typical savings in fired heaters due to air preheating is about 8 to 16% and is economically attractive for flue gas temperature above 600°F and heater duty above 100 MM Btu/h. However, the optimum flue gas temperature is determined by the sulfur content and dew point of the flue gases to reduce corrosion possibilities.


 

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