EATON TN08 520-1007 MTL Gas Analysers and Systems Instructions
- June 12, 2024
- EATON
Table of Contents
CROUSE-HINDS SERIES
Technical note
MTL gas analysers & systems
TN08 520-1007 MTL Gas Analysers and Systems
Measuring the Air/Fuel Ratio for Pre-Mix Burners
Used for Heating and Surface Treatment
When hydrocarbon fuels burn in air they do so according to a precise
chemical equation. An example of such an equation is shown below.
This equation illustrates the combustion of methane (CH4) in air
(approximately 21% oxygen + 79% inerts).
This equation shows amounts (volumes) of each component required to perform a
complete reaction of the methane and air. A mix of reactants where each is in
a proportion to react completely is called a ‘stoichiometric’ mixture. Similar
equations can be written for other fuels.
Typically mixtures of air and gas burnt for heating etc. contain a small
amount of excess air for optimum efficiency.
Measuring and controlling the air/fuel ratio is crucial to maintaining an
efficient combustion and to create the correct surface tension in flame
treatment processes.
One way of measuring the ratio is to measure the oxygen in the unreacted fuel
air mix. For methane the oxygen concentration in the unreacted mixture at
stoichiometry would be:
So the change in oxygen concentration from air to a stoichiometric mixture is
only 2% (21-19). Although oxygen analysers exist to measure these
concentrations, to get any appreciable accuracy (± 0.01% at least) requires a
very expensive device.
An alternative way of determining the air fuel ratio is to pre-burn a small
amount of the mixture and measure the oxygen remaining. In this way, the
difference in oxygen from air to a stoichiometric mix is 21% – i.e. more than
ten times the shift in the in-reacted mixture. The graph above illustrates the
difference.
MTL Zirconia oxygen analysers are ideal for measuring oxygen in these
combusted mixtures for two main reasons. Firstly, they operate at elevated
temperatures (600+°C/1100+°F) and so it is not necessary to cool or otherwise
condition the sample. Secondly, they can measure the low levels of oxygen
present at stoichiometry (and beyond into the reducing zone if necessary) with
high accuracy. This equates to an accuracy in the unreacted mixture of about ±
0.002% – significantly better than any alternative. They measure oxygen
concentration in a way analogous to pH electrodes measuring acidity. And in a
similar way to a pH electrode being used to monitor the neutralising of an
acid by an alkali so a MTL zirconia oxygen sensor can be used to monitor a
“neutralisation” of oxygen with the fuel. The graph in Figure 2 shows the
classic ‘neutralisation’ curve for such a process. The “neutralisation” point
is where the reactants are in exactly the correct proportions to produce no
excess of either.
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Note: The MTL Z1915C and MTL Z3000 models are used in this type of application.
The given data is only intended as a product description and should not be regarded as a legal warranty of properties or guarantee. In the interest of further technical developments, we reserve the right to make design changes.
Eaton Electric Limited,
Great Marlings, Butterfield, Luton
Beds, LU2 8DL, UK.
Tel: + 44 (0)1582 435600 Fax: + 44 (0)1582 422283
www.mtl-inst.com
E-mail: [email protected]
EUROPE (EMEA):
+44 (0)1582 723633
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THE AMERICAS:
+1 800 835 7075
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ASIA-PACIFIC:
+65 6 645 9888
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© 2016 Eaton
All Rights Reserved
Publication No. TN08 520-1007 Rev 3 191016
October 2016