So don't play with me, cause you're playing with fire.

("Play With Fire," The Rolling Stones, 1965)
 

FLAME ON!

PTOA Readers and Students already know that an industrial process that must be absolutely certain a target temperature will be attained will incorporate a combustion reaction.

Heat Transfer by Radiation, the Mother of all Heat Transfer, is the desired outcome and purpose for creating a safe place for the combustion reaction to occur.

Hey! That must mean the combustion reaction is exothermic!

Yes, indeedo!

And the combustion reaction is the same whether radiating heat to roast marshmallows over a campfire or creating radiant heat destined to be conducted through firebox heater tubes found in reaction furnaces, package boilers, and fired heaters.

Whatever the end use of the radiation, its creation begins with mixing oxygen from air with fuel and exposing the blend to an ignition source that is at a temperature that supports combustion.

Once the combustion starts and all three elements remain available, a chain reaction continues and the combustion products of carbon dioxide (CO₂), water (H₂O), and of course Radiated Heat are generated.

 

COMBUSTION REACTION FUEL SOURCES

Everything on earth ... including PTOA Readers and Students ...  has an incineration temperature, thus everything on earth is a potential fuel.

However, the combustion fuel sources used in the process industries are limited to hydrocarbons, logically named molecules made of hydrogen and carbon.

Hydrocarbons are also called "organic molecules"; any chemical structure with the element Carbon is in the chemical classification of "organic molecules."

When complete combustion occurs, the hydrocarbon fuel chemically changes into carbon dioxide (CO₂), water (H₂O) and radiated heat.

Hydrocarbons range in size from the very smallest methane (CH₄, aka "natural gas") and grow larger with more and more carbon atoms.  Methane, ethane, and propane are gases that easily combust.

 

 

 

 

Butane and larger hydrocarbons up through the diesel range can also be used as fuel in the combustion reaction; however the fuel delivery systems and burner designs will be different.

The bigger the hydrocarbon used for fuel, the more oxygen is needed to completely combust the fuel and a greater amount of carbon dioxide, water, and heat will be generated.

When the fuel is methane, two molecules of oxygen (2O₂) are needed to completely combust one methane molecule which yields one molecule of carbon dioxide (CO₂) and two molecules of water (2H₂O).

When the fuel is propane instead of methane, five molecules of oxygen are needed to completely combust one molecule of propane which makes three molecules of carbon dioxide, four molecules of water, and much more heat and light will be generated via the combustion reaction.

In summary:

• Completely combusting propane requires two and a half times more air flow than completely combusting methane.
• The more carbon atoms in the fuel, the more air is needed for combustion and the more heat, water, and carbon dioxide will be generated.

 

BURN BABY, BURN!

The hardware component that performs the function of 'burner on a stove top' is also called a burner in the fired heater.

The job of the burner is to provide an efficient combination of fuel and oxygen in the presence of an igniting fluid (pilot gas) to initiate the burn.

Modern fired heaters will have a Programmable Logic Controller (PLC) to help with a safe ignition sequence.

On the complete other side of the spectrum there are still facilities that skate by the Process Safety Management regulations and use a light-a-fuel-soaked-rag-torch-and-hope-for-the-best ignition strategies.

PTOA Readers and Students should not ever work for Plant Owners that do not invest in protecting workers and equipment; their plants will not be around long anyway.

Start up procedures are specific to the design of the heater and whichever site-specific fuels are most cost effective.

Outside Operator Trainees must pay strict attention to the written procedures that detail step by step how to start up fired heaters, boilers and reaction furnaces.

In all cases, the goal is to ignite the burners, establish a controlled combustion chain reaction, and thence establish draft up the chimney by adjusting the dampers (a topic featured in the next PTOA Segment).

Schematic of a fired heater burner.

A schematic of an industrial burner for a fired heater is to the right.

Designs differ, and in the USA retrofit low-NOx burners will look more elongated.

However, all burners will have piping for fuel gas, pilot gas,purge gas, and air registers (primary and secondary) to provide the oxygen needed for the combustion reaction.

PTOA Readers and Students must recognize that the burners for fuel gas and/or liquid fuel are providing the fuel reactant needed for the combustion reaction. 

Likewise, the air registers provide the combustion air reactant needed for the combustion reaction to progress.

Underneath a fired heater, the piping to the burner assembly will not be as clearly marked as in the graphics above and will look something like the photo to the right.

 

WHAT COULD POSSIBLY GO WRONG?

By definition, Radiation Heat Transfer requires a fireball to be created and controlled.

Flameout

A flameout condition is present when the flame on a burner goes out while fuel is still being charged into the firebox.

Flameout is a dangerous situation. When not noticed and left unattended, flameout will result in the explosion of the fired heater.

 

Flame Impingement

Long flames that actually reach the heater tubes are caused by too little oxygen to support combustion.

If left uncorrected and sustained, the flames can cause the tubes and refractory to overheat and fatigue.

 

CLIFF-HANGER ... TO BE CONTINUED

The next PTOA Segment will feature the causes of Flameout and Flame Impingement.  These two potential operating problems are uniquely associated with the need for combustion to generate radiant heat.

Process Operators and Control Board Operators that understand how the combustion reaction generates radiant heat will be more aware to recognize and proactively resolve Flameout and Flame Impingement problems.

 

TAKE HOME MESSAGES:  Radiant heat transfer requires combustion and takes place in fired heaters, boilers, and reaction furnaces.

The duty of the Process Operator is to successfully start the combustion reaction and, once the chain reaction has been established, establish draft via positioning the dampers.

The combustion reaction requires specified amounts of fuel and oxygen in the presence of an ignition source and will generate water, carbon dioxide and ... most desired ... radiant heat.

Removing fuel, air, or the ignition source will stop the combustion reaction. Using a fire retarding material can chemically break up the the established combustion chain reaction. 

The heavier the hydrocarbon used as fuel, the more oxygen is needed to complete combustion and the more water, carbon dioxide, and heat will be generated.

Two potential operating problems related to Radiant Heat Transfer and the need to create combustion are Flameout and Flame Impingement.

Alert Process Operators and Board Operators who understand the combustion reaction will be better prepared to recognize and resolve Flameout and Flame Impingement problems.

 

©2015 PTOA Segment 00069
Process Industry Temperature-Increasing Equipment

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