May 2009

Oxygen in stack gas reflects process efficiency

The history of gas measurement has seen great changes since the first sensors allowing a continuous measurement of gas concentration came out.

These changes made it possible to combine modern electronics with gas measurement, which was not possible with the old chemical methods. Development has accelerated because of the interest in environmental matters and the realization that more care must go into the treatment of our planet.

A flue gas analyzer is a collection of different sensors and a central unit that collates all the results and provides a useful and understandable result. To carry out this task, a battery of different sensors is necessary. Most of these sensors measure gas or gas concentration.

The components of flue gases are (in order of their predominance) nitrogen (N2), carbon dioxide (CO2), oxygen (O2), carbon monoxide (CO), nitrogen oxides (NOX), sulphur dioxide (SO2), hydrocarbons (CXHX), and soot (smoke).

We measure some of these elements and compounds because they indicate the level of efficiency with which the burner is operating. We want to burn all the fuel to avoid waste and maximize profit.

Others we measure so we can comply with environmental restrictions, safety rules, and reduce the impact on the health of surrounding populations.

Oxy1

If it were possible to have perfect combustion, CO2 would be maximized, and O2 would be at, or close to, zero in the flue gas stream.

Since perfect combustion is not practically possible due, in part, to incomplete mixing of the fuel and air, most combustion equipment is set up to have a small percentage of excess oxygen present.

The lower the temperature for a given O2 or CO2 value, the higher the combustion efficiency. This is because less heat goes up the stack by the combustion gases.

Oxygen sensors

There are many different types of oxygen sensor available, depending on the application, interfering gases, and a few other factors. These range from the expensive paramagnetic sensors to the standard electrochemical sensors with a limited lifetime.

Oxygen is not optically active, unlike other effluent, so we cannot measure it with infrared technology.

In the electrochemical sensor, oxygen diffuses through a membrane and the gas contacts the sensing electrode and the base solution and reacts at the wet surface of the electrode. This reaction consumes the counter electrode. The chemical change in the counter electrode allows a circuit in the instrument to measure a potential (voltage) between the electrodes.

In reality, the oxygen sensor acts as a current source, so the voltage measurement takes place over a load resistor. This should not be large. Otherwise, the balance of the oxygen circuit will be upset.

Finally, electrochemical oxygen sensors are current generators, and the current is proportional to the rate of oxygen consumption (Faraday's Law). The device measures the current by connecting a resistor across the output terminals to produce a voltage signal.

This signal is a measure of oxygen concentration.

Nicholas Sheble writes and edits Automation Basics.