The exhaust flue gas oxygen value
In order to improve the combustion process we need to introduce another measurement which we call the exhaust or flue gas oxygen value (3). For that we would need to have an in situ continuous oxygen measurement as close as possible to the spot after which the combustion process is expected to be finalized, which is in fact inside the combustion air port or regenerator top. It is difficult to measure in those places, which have a very harsh environment; extreme high temperature, and carry over dust along with all kinds of evaporated chemicals. However, a specially designed zirconium-oxide (ZrO2) based probe is able to do such a job. Over the last few years many of these probes have been installed, mainly situated in the crown of the regenerator with a lifetime of approximately 3 years. Such a ZrO2 sensor is normally placed inside the regenerator top, through the crown. In that environment, one side of the sensor is surrounded by the exhaust gases where we like to determine the amount of O2 and the other side of the sensor is surrounded by a small amount of normal purge air – at 20.9% O2.
According to Nernts:
If oxygen is to be measured, using a zirconium ceramic electrolyte and atmospheric air, with pO2 = 0.209, as a reference, this equation simplifies to
pO2 = 0.209 exp(-46.421 E/T)
The equation may be re-arranged, to allow calculation of the emf from a known reference concentration and a measured oxygen concentration:
E = 0.0496 T log(pO2/0.209 )
Since “T” represents the probe temperature there is a type R, S or B thermocouple inside the probe near the ZrO2 sensor.
Eurotherm T2550/2750 system has ZI – Zirconia input module
Input Types: Analogue voltage, Channel 1 – mV (TC), and Channel 2 – 2V (Zirconia probe)
Thermocouple Input Specification (Ch1 ONLY)
Input Range: -77mV to +100mV
Zirconia Input Specification (Ch2 ONLY)
Input Range: -10mV to +1800mV
Next to the measurement of the combustion air forced into the system by the combustion air fan, the precise measured amount of fuel applied to the burners now gives us a third value which represents the amount of oxygen in the flue gas. By having these three parameters we are able to calculate the amount of air which was available for the actual combustion process and the air which entered or left the system without being measured by the combustion air venturi called “parasitic air”.
Both parameters are equally important. The oxygen parameter will be used to trim the amount of combustion air to a value at which the combustion process is near stoichiometric; all reagents are consumed, there is no shortfall of reagent and no residues remain. The other parameter, which shows the amount of “parasitic air”, will be used to safeguard the control system. If this parameter shows unusual behavior it will freeze the ratio tuning algorithm until further inspection of the oxygen probe, combustion air measurement (obstructions) and regenerator and furnace refractory damages.
There are several significant advantages of controlling the excess air. First of all, next to the 20.9% oxygen which we need for combustion, the air also contains 78% nitrogen. Nitrogen is of no use to the combustion process but it has to be heated up to combustion temperature. In fact the combustion process produces, next to CO2, H2O, a lot of very hot useless nitrogen. Like nitrogen, the excess of oxygen represents an efficiency loss and has a nasty side effect as it tends to react with nitrogen at a high combustion temperature resulting in unwanted NOx emissions.
Generally: Using the exhaust flue gas oxygen values coming from the ZrO2 probe to tune the combustion air input to a level at which the air available for combustion, (thus including the parasitic air) together with the applied fuel reaches stoichiometry will increase fuel efficiency and improve NOx emissions.
