"You complete me."

(Famous line from character 'Jerry Maguire' in the movie "Jerry Maguire")

PTOA Readers and Students have already learned that oxygen deficiency in the combustion reaction causes:

• Soot formation.
• Flame impingement and Flameout.
• Globally warming carbon monoxide.

So, why not solve these problems by opening up air registers on the burners to insure that there is more than enough oxygen for combustion?

Fired heaters were operated just like that ... with way too much excess oxygen ... for decades.

The "problem" is that the combustion reaction is very specific on how much oxygen is needed for complete combustion.

Too little oxygen may cause the above problems but too much excess oxygen provided for the combustion reaction contributes to smog, acid rain and ground level ozone.

Furthermore, too much oxygen makes heater operations inefficient.

Since creating radiant heat accounts for 66% of a fuel budget, Plant Owners expect Process Operators to operate all fired equipment optimally with just enough excess oxygen.

This PTOA Segment clarifies why and how Process Operators must be vigilant to provide just the right amount of combustion oxygen.

The segment concludes by listing all the ways that air (hence, oxygen) can enter the firebox.

 

JUST RIGHT O₂ COMPLETELY COMBUSTS THE FUEL SOURCE

The top graphic reiterates that a hydrocarbon fuel mixed with a specified amount of oxygen (from air) is ignited to generate the desired combustion product heat and byproducts that are called  "combustion products and flue gases."

What are the "combustion products and flue gases?"

PTOA Readers and Students learned in PTOA Segment 69 that when the fuel is natural gas (CH₄) the completed combustion reaction makes one carbon dioxide molecule (CO₂) and two water molecules (H₂0).

Completing the combustion reaction means that just the right amount of oxygen (from air) has been supplied for the reaction.

By the way, the big word for "just the right amount" is called the "stoichiometric amount of oxygen."

 

TOO LITTLE O₂ MAKES CO AND ADDS TO GLOBAL WARMING

Carbon dioxide (CO₂) and water (H₂O) are the gaseous products formed via complete combustion.

PTOA Readers and Students already know that carbon monoxide (CO) is a flue gas generated when the combustion reaction does not provide sufficient oxygen for the hydrocarbon fuel.

Carbon monoxide contributes to global warming.

PTOA Readers and students already know that soot buildup, flame impingement and the dangerous situation of flameout are caused by  insufficient oxygen to support complete combustion.

And yet, opening up on the air registers in the burners is not a good solution.

 

TOO MUCH O₂ INCREASES ACID GAS, ACID RAIN, BAD GROUND LEVEL OZONE, AND SMOG

All fuels for fired heaters have some trace amounts of sulfur in them. When the sulfur reacts with the oxygen in air at a high temperature, the flue gas sulfur dioxide (SO₂) is generated.

Sulfur dioxide is a lung irritant; it also dissolves in water and creates acid rain.

The below graphic shows that 69% of sulfur dioxide (SO₂)emissions are due to industrial combustion.

Oxides of nitrogen (NOx) are flue gases that are generated when the nitrogen gas (N₂) and oxygen gas (O₂) in air are present in a hot atmosphere ... exactly the conditions that exist in a fired heater.

The "x" in NOx means the generated gas could be NO (nitric oxide)  or NO₂ (nitrogen dioxide).

Nitrogen oxides create smog, acid rain, and are a precursor to ground level ozone (the bad kind of ozone).

The graphic above shows that  32% of NOx are generated by industrial combustion.

The formation of NOx is related to how hot the flame is as well as the amount of excess oxygen.  For this reason, EPA has required the installation of Low NOx burners because they promote efficient combustion while using less excess air.

Low NOx burners are yet another example of technology we can all live with!

 

TOO MUCH O₂ WASTES FUEL AND DISRUPTS HEAT TRANSFER 

In addition to creating acid gas, heater operations are inefficient when air registers are opened up to provide more air than the stoichiometric amount required for the fuel to completely combust.

Since air is 78% nitrogen, heating up a large volume of excess oxygen/air requires much more fuel to raise the temperature of all that unneeded hot oxygen and nitrogen.

All that hot, unneeded air whooshing through the air registers on the burners does not have enough time to get hot in the radiant section at the bottom of the fired heater.

The end result is that the temperatures in the upper areas of the firebox and the convection section become hotter than the lower parts of the radiant section.

In summary, inefficient operations of a fired heater can be noticed by the alert Process Operator and Board Operator as a "flip-flopped" temperature profile between the radiant and convection sections of the fired heater:

Too much excess air/oxygen transfers the heat duty of the fired heater upward and simultaneously used too much fuel because the lower tubes in the radiant section are being under-utilized.

 

TECHNOLOGY HELPS OPTIMIZE EXCESS O₂

Fortunately for modern Process Operators, technology is available to help control the optimum air flow for combustion through each fired heater.

The fact that the combustion reaction requires a stoichiometric amount of oxygen to combust a specific fuel makes it possible to optimize the air-to-fuel ratio needed at the burners.

Each fired heater is permitted a maximum amount of  "Excess O₂" under the EPA's Clean Air Act.  The "Excess O₂" will be detected and measured by an Analytical Device installed in the stack

PTOA Readers and Students must not stress about fully understanding the graphic to the right.

The graphic shows the control scheme for a Yokagawa air flow controller that determines how much to adjust air flow (the outgoing arrow extending from the left side of the graphic) from 3 inputs: the current air flow, the current firing rate at the burner, and the excess oxygen measured at the stack.

PTOA Readers and Students should notice that analytically measured process variables have ISA tag names that start with "A." An "AT" is an "Analytical Transmitter."

 

AIR APPARENT

To help maintain the proper air-to-fuel ratio used for the combustion reaction, Process Operators must understand the possible ways that air can enter a fired heater/furnace/boiler.

Recall that the combustion structure is built to enhance creation of draft; any air that can get into the structure will get into the structure!

Leaks in the fired heater/furnace structure would entrain air as would any leaks in the gasket materials wedged in the  fabricated openings that accommodate process fluid piping or instrumentation.

Thermal imaging as shown in the above right picture can detect hot areas that reveal refractory troubles and air leaks.  The white areas in the above photo would be worth checking out when the heater is offline.

On a daily basis, Process Operators must be aware how the following actions impact the air-to-fuel ratio:

• Adjustments to primary combustion air flow on the controller.
• Adjustments to secondary air flow on the burner register, and  (in the case of Low NOx Burners) adjustments to the tertiary air register.
• Opening peepholes during rounds to check heater operations.

 

 

TAKE HOME MESSAGES:  The products of compete combustion are heat, carbon dioxide, and water.

Flue gases include carbon monoxide, SOx, and NOx, unreacted nitrogen, unreacted excess oxygen, unreacted entrained fuel.

The air used in the combustion reaction must be optimized:

• Too little air creates carbon monoxide which is global warming gas, flame impingement, and soot formation.
• Too much air creates NOx and SOx which create smog, acid rain and bad, ground-level ozone.
• Too much air disrupts the intended heat transfer profile for the fired heater and wastes fuel.

Each fired heater will have a permitted maximum excess oxygen level that Process Operators must not exceed. 

In line analytical devices help detect and measure concentrations that are used in automatic control schemes. The ISA tagname for such devices will start with an A.

Board Operators will use the analytically measured concentrations of Excess O₂ to help keep permitted emissions within compliance maximums. 

Board Operators will impact air-to-fuel ratios upon adjusting primary air flow via controllers.

Outside Operators will impact air-to-fuel ratios upon adjusting secondary, tertiary air registers on the burners and when opening the peephole when visually verifying firebox operations.

Outside Process Operators and Board Operators are in the driver's seat with regard to minimizing local hazardous air emissions and global warming associated with fired heaters.

 

©2015 PTOA Segment 00073
Process Industry Equipment Operations
Process Industry Regulation Compliance

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