Eaton TN08 520-1007 Operating instructions

Brand: Eaton

Model: TN08 520-1007

Type: Operating instructions

October 2016

TN08 520-1007 Rev 3

Technical note

MTL gas analysers & systems

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]

Publication No. TN08 520-1007 Rev 3 191016

October 2016

EUROPE (EMEA):

+44 (0)1582 723633

[email protected]

THE AMERICAS:

+1 800 835 7075

[email protected]

ASIA-PACIFIC:

+65 6 645 9888

[email protected]

Stoichiometric Mix

Increasing Fuel

Oxygen Concentration in Reacted Mix

Oxygen Concentration in Un-reacted Mix

Illustration for the Combustion of Methane

% Oxygen

Zones of Optimum Efficiency

0 to 2% oxygen in reacted mix

19.0 to 19.2% in unreacted mix

Figure 1 - Methane combustion

Figure 2 - Neutralisation curve

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).

CH4 + 2O2 + 7.52 Inerts = CO2 + 2H2O + 7.52 Inerts

Air

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:

2 x 100 = 19.01%

1+2+7.52

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.

Note: The MTL Z1915C and MTL Z3000 models are used in

this type of application.

Eaton TN08 520-1007 Operating instructions

Eaton TN08 520-1007 Operating instructions

Eaton TN08 520-1007 Operating instructions

Related papers

Other documents