Teledyne PEM-9004 - Portable emissions analyzer - Manual (Page 20)

Operational Theory PEM-9004
Teledyne Analytical Instruments 8
The calculation is done on a dry basis. This means that the term
representing the energy necessary to heat the water vapor from ambient
to exhaust temperature is omitted.
To obtain the precise value for combustion efficiency one must
account for many variables. The most important are:
• Radiative heat losses
• Variation in fuel composition
• Air that leaks into the flue gas mixture raising oxygen
reading
• Heat loss in ash
• Incomplete combustion
• Boiler blowdown.
The PEM-9004 necessarily makes simplifying assumptions in its
determination of efficiency. Therefore it must be considered a trend
indicator that will correctly show an increase or decrease in efficiency,
whereas the absolute value of the number applies to the ideal case only.
2.4.4 Calculating Excess Air Value (Lambda)
The minimum amount of air that supplies sufficient oxygen for the
complete combustion of a fuel is referred to as the theoretical or
stoichiometric
amount of air. Most combustion processes involve an
amount of air different than stoichiometry requires. The differential is
called excess air and can be a positive or negative amount. The
stoichiometric requirement for air depends on the type of fuel used and
to a lesser extent, the temperature and pressure of the reaction. Nitrogen
is normally considered unaltered in lower temperature reactions and
simply adds to the heat balance of the combustion process. As the
combustion temperature increases however, increasing amounts of
nitrogen oxides are formed which must be considered in the overall
chemical balance.
In the PEM-9004, the excess air (Lambda) value is calculated from
the ratio of CO
2max
to CO
2observed
as follows:
2 2
2MAX
O 9 . 20
9 . 20
CO
CO
Lambda
−
= =