Flue Gas Analysis as a Diagnostic Tool for Fired Process Heater Furnaces0 pages

Application Data Sheet

Refining

Flue Gas Analysis as a Diagnostic Tool for
Fired Process Heater Furnaces
Overview, and Traditional Application
Combustion flue gas analysis has been used by process heater
operators for decades as a method of optimizing fuel/air ratio.
By measuring the amount of excess oxygen and/or CO in the
flue gases resulting from combustion, furnace operators can
operate at optimum efficiency and lowest NOx, and also generate
the least amount of greenhouse gas.The theoretical ideal, or the
stoichiometric point is where all fuel is reacted with available oxygen
in the combustion air and no fuel or O2 is left over.

Figure 2 - CFD Depiction of the Turbulent Mixing of Fuel tt
t
and Air Through a Burner

Figure 1 - Key Flue Gas Measurements Relating to Ideal tt
t
Combustion Stoichiometry
% of
Stack Gases

CO

The "combustion process" is the burner.

12
11
10
9
8

Figure 3 - DCS Trend Depicting the Relationship of O2
t
and CO Indications at CO Breakthrough Point

7
6

CO2

5
4

CO

4

3
2

Air-to-Fuel
Mixture

1

O2

Rich
(Deficient Air)
Stoichiometric
Point

Operating furnaces never attain this ideal, however, and the best
operating point usually will result in 1–3 % excess air and 0–200 PPM
of CO. This optimum operating point is different for every furnace
and also varies for differing loads, or firing rates. A higher firing rate
induces greater turbulence through the burner(s), providing better
mixing of fuel and air, and enabling operation with a lower excess
O2 before unburned fuel (represented by CO) appears, or “breaks
through”. For most furnaces, this point of CO breakthrough is fairly
repeatable, and reduces the need for a separate CO or combustibles
sensor.

2

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10 min