|
Molecular_Weight
22.415
|
x |
volume |
= |
mass |
(22.415 m3 / mole is the gas constant
at STP (0°C, 1 atmosphere)
The analyser measured
NO ppm by volume, since NO makes up 90% of the NOx volume,
(NO PPM / 0.9) = NOx PPM by volume.
|
46.007
22.415
|
x
|
NO(PPM)
0.9
|
=
|
NOx(expressed as NO2) mg/m3 |
By calculating the products
of combustion, the NOx mg/m3 calculation can be referenced
to 12% CO2 dry.
Theoretical combustion
of fuel can be shown by:
| Cm Hn |
+ (
|
4m + n
4
|
)O2 |
® |
mCO2 |
+ ( |
n
2
|
)H2O |
The furnace was fired
on naphtha, the main component of Naphtha is C6 H14, but
to calculate more accurately a laboratory analysis showed
the carbon / hydrogen ratio to be 5.2
|
2(C 0.433_H)
|
+
|
2(0.433+1/4)O2
|
®
|
2(0.433)CO2
|
+
|
H2O
|
|
2(C 0.433_H)
|
+
|
1.366O2
|
®
|
0.866CO2
|
+
|
H2O
|
To obtain smokeless combustion
the fuel is burnt using excess air. Air not oxygen is
used in the combustion process, therefore nitrogen will
be present in the products of combustion. Each mole of
O2 in air is accompanied by 3.76 moles of
N2. To obtain 12% CO2 dry in
the products of combustion the excess air was 18.68%
| Products of Combustion |
Mole
|
% Wet
|
% Dry
|
|
CO2
H2O
O2 (0.1868x1.366)
N2 (1.1868x1.366.3.76)
|
0.866
1.000
0.255
6.096
|
10.54
12.17
3.10
74.19
|
12.00
----
3.53
84.47
|
|
Total
|
8.217
|
100.00
|
100.00
|
From the above table
to correct the calculation to 12% CO2 dry,
the existing O2 % wet analyser could be used,
by referencing the calculation to the O2
wet value that would be present in the flue when the CO2
value was 12% dry. From the table this can be seen to
be 3.10% wet.
NOx (actual) x reference
conditions = NOx (reference conditions)
| NOx(actual) |
x
|
20.9-(O2_reference)
20.9-O2_(actual)
|
= |
NOx(referenced to O2)
|
Assigning instrumentation
and process values from the products of combustion table
| NOx(analyser) |
x
|
20.9-3.10
20.9-O2_(analyser)
|
= |
NOx_volume
(referenced to 12%CO2)
|
The above formulas are
combined to form the final equation:
NOx mg/m3 (expressed
as NO2) corrected to STP, 12% CO2
dry
=
|
NO(analyser)
0.9
|
x
|
46.007
22.415
|
x |
20.9-3.10
20.9-O2_(analyser)
|
This equation is implemented
in a Regulatory PV Calculation data point as:
PV = (P1/0.9)x(C1/C2)x((C3-3.1)/(C3-P2))
Where:
| P1 |
=
|
NO_analyser |
| P2 |
=
|
O2_analyser |
| C1 |
=
|
46.007 |
| C2 |
=
|
22.415 |
| C3 |
=
|
20.9 |
| C4 |
=
|
0 |
Similar methods can be used to calculate
references to O2 dry Standard Metric Conditions
etc.
The regulatory PV data point could
be used as an indication, and indication with alarms
or as an input to a controller or more sophisticated
sequence programme.
This is an example
of how G&L calls upon it experience
to provide a securely engineered system.
For examples
of the model related to the above calculations click
on one of the pages below
These calculations
are entirely original, developed by G&L
Engineering.
© 2000 G&L Engineering Ltd.
