Optimization of the Fired Heaters in the
Oil and Gas industry
INTRODUCTION
Fired heaters, which are sometimes called process heaters or direct fired heaters, are
designed to heat petroleum products as well as chemicals and different liquids and gases.
The heating process is done to increase the temperature of a fluid for a further downstream
process in the petrochemical industry (Baukal and Charles 2001,page 11) In this research
project, the parts and working principle of a fired heater will be described. Then different
modifications that can be done in order to increase reliability, efficiency and
environmentally suitability of the fired heater and as well as the safety of the equipment will
be described.
Parts and Working Principle
The heating process is conducted by increasing the temperature of the fluid for next
processes where the heat is delivered. The working principle of the device based on the
theory of heat transfer. Heat is transferred in three ways: Radiation-where heat is
transferred by electromagnetic radiation like the Sun heats the Earth, Conduction-heat is
transferred between two materials by movement of atom and molecules, convection-heat is
transferred by mixing or carrying a part of a medium to another part (Alfa Laval 2004). All
three ways are applied in fired heaters to gain maximum heat production. As a heater fuel,
low molecule weight hydrocarbons, reformers and waste gases are used by many refineries.
Usually natural gas is mixed with waste and other off-gases as the primary fuel for the
heating process. Alongside these, some heavy weight molecule hydrocarbons can also be
used such as tar, pitch and Bunker C (heavy oil).
Samir Nabiyev
55449
Team 9
A conventional fired heater (figure 1) consists of 5 sections: Radiant, Shield, Convection,
Breeching sections and Stack.
Firstly, the radiant section is the most important part of the fired heater where 85% of the
heat is gained. Depending on the type of the equipment, it consists of vertical or horizontal
tubes which are located along the walls. Provided they are near the flame, the tubes get
heat directly from the burners. (AMETEK 2014) Secondly, the convection section reduces the
remaining temperature of the exhaust gas by removing heat from it and preheating the
contents of the tube. An excess amount of heat gained in convection section indicates the
existence of high draft. The temperature of the tubes is measured in both radiant and
convection sections. (AMETEK 2014) Following this the shield section containing sets of
tubes in row between the radiant and convection section prevents the convection section
from being heated directly by radiant heat. Important measurements are usually made
below the shield section. The temperature of bridgewall (or breakwall) is considered the
temperature of flue gas after the radiant heat is removed by the radiant tubes and before it
reaches the convection section. (AMETEK 2014) Finally, the breeching section is the
transition from the convection section to the stack from where the exhaust gas leaves the
furnace (AMETEK 2014)
Turning to how it works, the basic process starts from combustion, where the rapid
combination of fuel and Oxygen results in an exothermic reaction and for a completed
combustion, the fuel and Oxygen should be mixed thoroughly (Grag, 2010). While in theory
the burning of the fuel completely is achievable, in actual operation the calculated amount
of fuel and air will not burn sufficiently due to the need for excess air (Grag, 2010). The
combustion process is started and maintained in burners which also deliver fuel and air in
the correct proportion, mix them and ignite and stabilize the flame. (Grag, 2010) As the
combustion starts, negative pressure is created inside the heater due to lightness of the hot
flue gas inside and this pressure is called draft pressure or just draft that needs to be
maintained to be slightly higher than atmospheric air pressure to prevent by-products from
escaping into the surrounding area and to carry flue gas to stack (SIEMENS 2012). Providing
the heater is under negative pressure, it makes the operation safe and prevent hot flue
gases from leaving the furnace without giving off heat (Grag, 2010). Depending on the fired
heater and where it is installed, draft or negative pressure can be created in three ways:
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natural draft, forced draft and induced draft (NYB, no date). Natural draft uses the stack
effect which is created when flue gas are hotter and less dense than around the stack
opening. When flue gas rises a natural convection current is formed. To get sufficient draft,
stack should be high (SIEMENS, 2012). In order to get high air velocity and better air/fuel
mixing with smaller burner, forced draft is used. In this case a centrifugal fan is used to
supply air continuously (Grag, no date). Induced draft is created by installing a fan to draw
the glue gas out of the heater. Control of the draft is regulated by changing the fan speed.
The induced draft makes high stack unnecessary (SIEMENS, 2012). When preheating
installations are needed both of the forced and induced draft fans are this is known as
balanced draft (Garg, no date).
Requirement for Optimization
Fired heaters were usually considered to be less efficient than boilers due to the following
reasons: firstly, high stack temperature and excess air level is usually observed, secondly,
they are a part of a large process unit and that’s why they have shifts operations during the
course of a year, thirdly, the demand for response to process is higher than demand for
efficiency. (James, 1983). But nowadays by means of applying automated controls, safety
has improved greatly. The reason why today many companies and governments invest in
automation control in each plant is higher reliability and lower lifecycle costs in the
automated process (YOKOGAWA, no date). As the majority of safety issues have been
solved, the need for efficiency became a priority. “Even a 1% improvement in thermal
efficiency translates into energy savings of $600,000 per year”. (Grag, no date) Having
looked at optimization it needs to be remembered that fired heaters can be a source of
environmental pollutions. The main issue with the use of fired heaters is Nitrogen Oxides
which is released into the atmosphere during operation. This is why the improvement on
fired heaters to keep emission level below the federal and local mandates is important
(Optimized Process Furnaces, 2007-2015). The main cost of a fired heater is its fuel and
maintenance when fuel costs cannot be reduced, maintenance cost can be decreased by
implementing an optimal maintenance inspection strategy (Vicente, no date)
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Emission and Efficiency Optimization
The usual problems with fired heaters that results excess costs are issues like excessive air,
fouled convection sections, high temperature of stack, bad flames and overfiring (Garg, no
date). The main optimization is required in the combustion part. There are two main
aspects that must be considered at first stage: Nitrogen and fuel-air quantity. These two
factors have common trends in term of air amount. For every part of oxygen four parts of
nitrogen enter the burner. As the nitrogen is not burning, it absorbs the heat and carries it
to the stack. As a result, the generated heat is lost and more nitrogen oxide is emitted into
the atmosphere (Grag, no date). While trying to stop excessive air from entering the
combustion, the amount of air mixed with fuel can be less than required. It can lead to
incomplete combustion where the fuel leaves the stack without being ignited and it means a
loss of energy as well as a lack of oxygen in combustion process which creates CO and NOx
emissions (Cooper, 2008). One of the concepts to maintain proper air/fuel mixture in a
forced draft using fired heater is given in (figure 2). This is an automated system where flow
rate is adjusted to maintain the temperature of the heat transfer fluid exiting a furnace. The
temperature controller maintain amount of fuel which enters into combustion, and the flow
sensor F, attached to sense flow rate of fuel, sends the flow rate data to flow fraction
controller FFC. According to the present flow rate, FFC calculates the required amount of air
and changes it accordingly. (Cooper, 2008). By using this system, full combustion can be
achieved and it can increase efficiency and decrease emissions (Cooper, 2008). Further
optimizations based on this concept can be done by attaching sensors/transmitters on the
stack to monitor amount of O2 and CO (indicates all lost fuel) and can provide flow fraction
controller with more accurate information in a short time (Cooper, 2008). Another point is
that, despite using a blower, exhaust gas stream is delivered to the stack at low pressure
due to passing through the tubes and twists and turns of irregular ductwork. The difference
in the pressure is a good alternative to measure flow rate more accurately and easily than
measuring in the air inlet pipe. Sensors/transmitters are attached to indicate differential
pressure between two across point of stack (figure 3). Instead of using a valve as final
control element, damper louvers are also used by which the FFC controls differential
pressure by opening and closing louvers (Cooper, 2008). Taking all things into consideration
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the system adjusts both draft and air-mixing as a result efficiency and fewer emissions are
achieved.
There are some other methods which are applied to decrease the amount of NOX
released into atmosphere. One of them is installing Low-NOx burners which controls flame
temperature and reduces oxygen concentration and it is possible to cut emissions by 60%
(Optimized Process Furnaces 2007-2015). Another method is flue gas recirculation (FGR) in
which external flue gas is blown into combustion air and it lowers the flame temperature
and cuts formation of NOx(Optimized Process Furnaces, 2007-2015). Alongside these,
chemical reactions also used to separate NOx into harmless nitrogen and water molecules
(Optimized Process Furnaces, 2007-2015).
Revamping
One of the ways to increase efficiency and productivity is revamping where by additional
rows of tubes are added which increase surface area. The increasing of capacity of a fired
heater by firing it harder, results in problems such as over-firing, high tube metal
temperatures, positive pressure inside fired hater. This fact makes the revamping process
inevitable by which the heater capacity and efficiency, NOx emission is decreased. The
smaller differential temperature is achieved which increases tube life as well. In Figure 4
two additional rows of tubes are installed without making a major change. If space for extra
tubes is less, the convection section can be extended into the breeching section to get extra
space (Garg, no date). Replacing tubes with small diameter ones in order to increase surface
area and space inside heater, can lead to heat transfer problems because of
spacing.(Muhlenforth, 2014)
Safety
As the operation is conducted at high temperature with flammables, risk of explosion, fire
and other accidents still exist. Modern control systems are applied successfully to increase
safety. They include combustion control, interlock mechanism and a safety shut-off
mechanism to prevent explosion. To achieve this a burner management system (BMS)
monitors pressure, temperature, CO in flue gas and flame and according to this information
the controller makes decision. A typical BMS is shown in figure 5 which has capability to
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shut-off system which detects CO concentration as well. The BMS can also determine the
level of methane levels providing an extra level of safety during start up where a flame out
or a failed burner ignition can cause an explosion (Yokogawa, 2014).
Conclusion
In conclusion, I would emphasize the importance of the application of automated control in
the process heaters, which gives multiple benefits in terms of efficiency, lower emissions,
increased safety and equipment durability and reliability. The proper mixture of air and fuel
is the key for optimization where excessive air results in a loss of energy and NOx emissions
whereas lack of air results in uncompleted combustion and CO emissions. These problems
are easily mitigated after introducing automated control systems.
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Bibliography
Alfa Naval Company. (2004). The theory behind the heat transfer. [online]. Available at:
http://local.alfalaval.com/en-gb/aboutus/news/Documents/The%20Theory%20behind%20heat%20transfer.pdf [Accessed 16 Jan.
2015].
Ashutosh, G (no date) Optimise fired heater to save money. Sugar Land Texas. [Online].
Available from:
http://www.researchgate.net/profile/Ashutosh_Garg2/publication/236543849_Optimize_fi
red_heater_operations_to_save_money/links/00b7d53178b4462cbd000000 [Accessed on
16 January 2015]
Baukal, Charles E., Schwartz, Robert E., Baukal, Charles E. Jr., The John Zink Combustion
Handbook, CRC Press, Boca Raton, Fl., March 27, 2001.
Controlguru.com, (2015). Ratio control and metered-air combustion processes - practical
process control by control guru. [online] Available at:
http://www.controlguru.com/2008/012708.html [Accessed 16 Jan. 2015].
Equipnet. (2015). Heaters from Kinetic Technology International Listing. [online]
Equipnet.com. Available at: http://www.equipnet.com/kinetic-technology-internationalfired-htf-heater-93-mmbtu-hr_listid_298618/ [Accessed 16 Jan. 2015].
Greg, A (2010) New approach to optimizing fired heater [online]. Sugar Land Texas,
available: http://repository.tamu.edu/bitstream/handle/1969.1/94037/ESL-IE-10-05-01.pdf
[accessed 16 January 2015].
Grag, A. (1998). Revamp fired heater to increase capacity. 1st ed. [ebook] Sugar Land, TX:
Yogitadoda, pp.1-4. Available at: http://www.scribd.com/doc/94566782/Revamp-FiredHeaters-to-Increase-Capacity [Accessed 16 Jan. 2015].
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Muhlenforth, C. (2015). Revamping a Process Fired Heater | Carmagen Engineering. [online]
Available at: http://www.carmagen.com/news/engineering_articles/news27.htm [Accessed
16 Jan. 2015].
Optimized Process Furnaces, INC, (2015). OPF Emissions Control | Optimized Process
Furnaces, INC | Chanute, KS. [online] Available at: http://www.firedheater.com/emissionscontrol_id72.html [Accessed 16 Jan. 2015].
Siemens Application Data, (2012). Procidia™ Control Solutions Combustion Management
Solutions Furnace (Draft) Pressure Control. [online]. Available at:
http://cache.automation.siemens.com/dnl/TE/TEyOTc2MQAA_51436060_HB/AD353106r3.pdf [Accessed 16 Jan. 2015].
Shriver, J. (2015). Improving fired heater efficiency. [ebook] Foxboro, Massachusetts: The
Foxboro Company, p.602. Available at:
http://repository.tamu.edu/bitstream/handle/1969.1/94621/ESL-IE-83-0496.pdf?sequence=1 [Accessed 16 Jan. 2015].
The New York Blower Company, (n.d.). Fans and blowers for combustion process. [online]
Available at: http://www.nyb.com/Catalog/Letters/EL-10.pdf [Accessed 16 Jan. 2015].
Yokogawa.com, (2015). Fired heater safety & optimization - combustionONE | Yokogawa
America. [online] Available at: http://www.yokogawa.com/us/products/processperformance-safety-solutions/process-safety-management/fired-heater-safetyoptimization.htm [Accessed 16 Jan. 2015].
Yokogawa.com, (2015). Optimizing Combustion Control with the TDLS200 | Yokogawa
America. [online] Available at: http://www.yokogawa.com/us/technical-library/applicationnotes/optimizing-combustion-control-with-the-tdls200.htm [Accessed 16 Jan. 2015].
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Appendix
Figure 1. Typical Fired Heater
AMETEK 2014 (left), and http://www.equipnet.com/kinetic-technology-international-firedhtf-heater-93-mmbtu-hr_listid_298618/ (right)
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Figure 2. Ratio Control Architecture
http://www.controlguru.com/2008/012708.html
Figure 3. Ratio Control Using Air flow From Differential Pressure
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Figure 4. Revamping
http://www.scribd.com/doc/94566782/Revamp-Fired-Heaters-to-Increase-Capacity
Figure 5. Safety Control
http://www.yokogawa.com/us/technical-library/application-notes/optimizing-combustioncontrol-with-the-tdls200.htm
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