5.13
5.10 Calculation of total nitrogen oxides (NO
x
) concentration
For analysers fitted with the Gfx 1210 transducer option for measuring nitric oxide (NO)
the facility exists within the analyser software to calculate an estimate of the total
nitrogen oxides (NOx) level, based on the measured NO level. This estimate of total
NOx concentration can also be displayed, and output, as a separate measurement in
the normal way.
The total NOx estimate is derived from the measured NO concentration according to
the formula:
[NOx] = a * [NO] + b * [NO]
2
where a = a linear term
b = a quadratic term
The use of both linear and quadratic coefficients provides the opportunity to
accommodate a non-linear relationship between NO concentration and estimated NOx
measurement.
In most simple combustion processes it is generally recognised that the NO
concentration accounts for approximately 95% of the total NOx. In this case the total
NOx content can be estimated from the NO measurement using a simple linear factor
(a) of 100/95 ( = 1.053), and a quadratic term (b) of zero (0.00). These should be
regarded as the default coefficients when this feature is utilised.
In more complex combustion processes the user may have available, or decide to
obtain, actual data relating the NO levels to total NOx levels, using other measurement
methods (e.g. other analysers or wet chemical testing), under various operating
conditions. This data can then be used to derive the practical relationship of NO
concentration versus NOx concentration for the process being monitored, from which
ideal linear (and quadratic, if necessary) terms can be obtained for use by the 4900
using a suitable mathematical (software) method.
The procedure for entering the coefficients in the conversion equation is given in Table
5.11. After this has been entered the NOx estimate can be added to the display as a
measurand (see Section 5.6) and be given an output allocation (see Section 5.3).
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