Flue Gas Analysis as a Boiler Diagnostic Tool0 pages

Application Data Sheet

Power

Flue Gas Analysis as a Boiler Diagnostic Tool
Overview, and Traditional Application
Combustion flue gas analysis has been used by Power Plant
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, plant operators can operate at the best
heat rate efficiency, 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 and air through t
a burner.

% of
Stack Gases

CO

12
11
10
9
8

4

CO

O2

7
6

CO2

5
4
3
2

Air-to-Fuel
Mixture

1

O2

Area of Maximum
Combustion Efficiency
CO Due to Poor Mixing
of Air and Fuel

2

Rich
(Deficient Air)
Stoichiometric
Point

Figure 1 - Key flue gas measurements relating to ideal combustion
stoichiometry.
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 boiler,
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”.

0

10 min

Figure 3 - DCS trend depicting the relationship of O2 and CO indications
at CO breakthrough point.