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Application Note

THERMOX®

THE OTHER SIDE OF ZIRCONIUM OXIDETHE OTHER SIDE OF ZIRCONIUM OXIDETHE OTHER SIDE OF ZIRCONIUM OXIDETHE OTHER SIDE OF ZIRCONIUM OXIDETHE OTHER SIDE OF ZIRCONIUM OXIDEThermox Analyzers forThermox Analyzers forThermox Analyzers forThermox Analyzers forThermox Analyzers for

Excess Fuel, Equivalent Combustibles and PremixExcess Fuel, Equivalent Combustibles and PremixExcess Fuel, Equivalent Combustibles and PremixExcess Fuel, Equivalent Combustibles and PremixExcess Fuel, Equivalent Combustibles and Premix

Regular Fuel Efficiency ApplicationsRegular Fuel Efficiency ApplicationsRegular Fuel Efficiency ApplicationsRegular Fuel Efficiency ApplicationsRegular Fuel Efficiency ApplicationsSeparate Oxygen and Combustibles SensorsSeparate Oxygen and Combustibles SensorsSeparate Oxygen and Combustibles SensorsSeparate Oxygen and Combustibles SensorsSeparate Oxygen and Combustibles Sensors

In standard flue gas applications for Thermox analyzers,the customer generally tries to extract the maximumenergy from the fuel and transfers as much of that energyas possible to the process. These applications require theprocess to be operated as efficiently as possible bykeeping both oxygen (excess air) and combustibles(incompletely burned fuel) to a minimum. The best wayto do this is to measure both oxygen and ppm combus-tibles simultaneously, using two separate detectors inone analyzer. Thermox WDG-IVC and WDG-HPIICanalyzers have both oxygen and combustibles detectors.Here the term combustibles describes the backgroundamounts of CO and H2 which occur during normalcombustion, typically 50-200 ppm.

What applications maintain excess airWhat applications maintain excess airWhat applications maintain excess airWhat applications maintain excess airWhat applications maintain excess airconditions?conditions?conditions?conditions?conditions?

Most standard heating processes including boilersoperate with excess air (excess oxygen)—for example:

Steam or power boilerProcess heaterSludge incineratorReformer furnaceRecovery boiler, etc.

Excess Fuel or High Millivolt ApplicationsExcess Fuel or High Millivolt ApplicationsExcess Fuel or High Millivolt ApplicationsExcess Fuel or High Millivolt ApplicationsExcess Fuel or High Millivolt ApplicationsOxygen Oxygen Oxygen Oxygen Oxygen ororororor Combustibles from ZrO Combustibles from ZrO Combustibles from ZrO Combustibles from ZrO Combustibles from ZrO22222

Some applications use the flue gas not only togenerate heat, but also to alter the properties of theproduct exposed to it. Here fuel efficiency is not asimportant as the oxidizing or reducing properties ofthe flue gas. This is called heat treatment and is usedon many metals, ceramics, glasses and other materials.By adjusting the air/fuel ratio to the burner, the fluegas is controlled to be as oxidizing or reducing asneeded. The exact air-to-fuel ratio which givestheoretically perfect combustion is called stoichio-metric (approximately 9.5 parts air to 1 part fuel formethane). Using more air than the stoichiometricamount creates an oxidizing effect; less air thanstoichiometric creates a reducing effect.

Representation of a furnace showing oxidizing and reducing zones

FUELAIR

PRODUCT

FUELAIR

Oxidizing Zone(excess air)

Reducing Zone(excess fuel)

This latter condition is known as sub-stoichiometric (other terms include excessfuel, reducing or Lambda <1) and can beused to alter the physical properties ofproducts or to prevent the formation ofNOx. For example, reducing conditions

are necessary to make colored glass. In some applications,such as bright annealing, a highly reducing atmosphere isrequired, but the product would be damaged by carbon-containing gases so a flue gas from fossil fuels cannot beused. In such cases a mixture of nitrogen and hydrogen (orcracked ammonia) is commonly used. It is easier and lessexpensive to use a flue gas as the furnace atmosphere when-ever possible. The zirconium oxide cell can also be used tofollow sub-stoichiometric as well as oxidizing atmospheres.

What applications run underWhat applications run underWhat applications run underWhat applications run underWhat applications run underexcess fuel conditions?excess fuel conditions?excess fuel conditions?excess fuel conditions?excess fuel conditions?

In the following applications it may sometimes bedesirable to run sub-stoichiometrically and to have someway of knowing at all times how oxidizing or reducingthe process is:

Heat treatment of metalsWire manufacturingHeat treatment of ceramics and porcelainsPrimary stage of a thermal oxidizer (incinerator)Manufacture of specialty glass, e.g. colored glassPrimary stage of a low NOx burner

When there is excess oxygen, the output of the cell followsthe normal Nernst equation from 0 mV to about 130 mV. Inexcess fuel (or sub-stoichiometric) conditions, the same cellproduces a high output signal from about 550 mV to wellover 1000 mV.

More fuel in the flue gas reduces the oxides-to-fuelsratio (or increases the fuels-to-oxides ratio) so the cellmillivolt output rises. Thus the oxygen cell gives us away of following how oxidizing or reducing theprocess is. The oxides-to-fuels ratio depends on thehydrogen-to-carbon ratio of the fuel being burned andso the millivolt signal can be calibrated to read excessfuel, combustibles, oxides/fuels ratio or other units if thefuel is known. In many cases the fuel is natural gas,which has a hydrogen-to-carbon (H/C) ratio of 3.7(methane is 4.0, coal is 0.68). Common fuel types arelisted and selectable using the Thermox Series 2000control unit. If a fuel is not listed, the customer canenter the actual hydrogen-to-carbon ratio. If the fuelcomposition varies, such as in a waste incinerator, thenthe measurement will be less accurate and the worstcase limits can be calculated.

In the past we had to make these measurements withcomplicated options on the analog analyzers, such aslean/rich modules, function generators and hand-drawn scales. Now the excess fuel option allows anyWDG-IV or WDG-HPII to display and output in sub-

Oxygen or Combustibles from an Oxygen-Only Analyzer?Oxygen or Combustibles from an Oxygen-Only Analyzer?Oxygen or Combustibles from an Oxygen-Only Analyzer?Oxygen or Combustibles from an Oxygen-Only Analyzer?Oxygen or Combustibles from an Oxygen-Only Analyzer?How does it work?How does it work?How does it work?How does it work?How does it work?

From this output reading it is possible, using a singlesensor, to determine whether the process is oxidizing orreducing. How can the cell measure anything in reducingatmospheres since there is no longer any oxygen? Thezirconium oxide cell is still responding to oxygen—thetiny amount of oxygen produced by dissociation of watervapor and carbon dioxide at the high cell operatingtemperature.

The cell is actually measuring the water-to-hydrogen ratioand also the carbon dioxide-to-carbon monoxide ratio.Since both are present in flue gas, the cell is really respond-ing to the ratio of total oxides (carbon dioxide and water) tototal fuels (hydrogen and carbon monoxide).

This is the oxides-to-fuels ratio.H2O + CO2H2 + COO2 ∝

CO2 CO + ½ O2 or O2 ∝

H2O H2 + ½ O2 or O2 ∝H2OH2

CO2CO

Zirconia cell output for Excess Oxygen and Excess Fuel

0

1500

10 % Oxygen 0 % Combustibles 10

Cel

lMill

ivol

ts

Oxidizing

Net Excess Oxygen

Lambda > 1

Reducing

Net Excess Fuel

Sub-stoichiometric

Lambda < 1

600

650

700

750

800

850

900

0.10 1.00 10.00

% Excess Fuel (lower lines) or % Combustibles (upper lines)

Cel

l Mill

ivol

ts

Natural GasOilCoal

Zirconia cell output (sub-stoichiometric) for various fuels

Equivalent CombustiblesEquivalent CombustiblesEquivalent CombustiblesEquivalent CombustiblesEquivalent CombustiblesReturning to Separate Oxygen and Combustibles DetectorsReturning to Separate Oxygen and Combustibles DetectorsReturning to Separate Oxygen and Combustibles DetectorsReturning to Separate Oxygen and Combustibles DetectorsReturning to Separate Oxygen and Combustibles Detectors

stoichiometric conditions. This unique combinationprovides analog outputs and alarms for the followingmeasuring units:

% combustibles % excess fuel to % excess oxygen % excess fuel % combustibles to % excess oxygen % excess oxygen % excess oxygen to % excess fuel oxides/fuels ratio % excess oxygen to % combustibles fuels/oxides ratio cell millivolts

For example, the output can be set so that 4 mArepresents 5% Oxygen and 20 mA represents 5% ExcessFuel. The position of the zero point can be adjusted tosuit.

Note that here the terms “Excess Fuel” and “Combus-tibles” both refer to sub-stoichiometric conditions. Thedifference is that “Excess Fuel” is calculated on the air/fuel mixture coming to the burner, whereas “Combus-tibles” is calculated in the flue gas and is a highernumber than the excess fuel value for the samereducing conditions.

“Combustibles” in this context (which is really excess combus-tibles) should not be confused with the common use of theword to describe the presence of CO and H2 together withexcess oxygen in normal (oxidizing) combustion.

On the standard WDG-IVC and WDG-HPIIC, as long asthe oxygen reads 0.01% or higher, there is sufficient oxygento burn any combustibles that may be present and thecombustibles detector is reading correctly. Don’t forgetthat zirconium oxide gives the net oxygen—that is, theoxygen left over after any combustibles are burned. If theprocess goes sub-stoichiometric, there is no oxygen to burnany combustibles on the detector, and you might expect itto read zero. The detector does not read zero however,because the signal from the zirconia cell is now higherthan 130 millivolts and this is used as a trigger to send thecombustibles detector to full scale. For example, if thecombustibles output range is set at 4-20 mA correspondingto 0 to 10,000 ppm, then the output would jump to 20 mAand the display would read 10,000 ppm.

For normal combustion applications you never want to beoperating under sub-stoichiometric conditions becausethis is very costly in fuel, bad for emissions and perhapsdangerous. When you reach zero oxygen, and thereforesub-stoichiometric or excess fuel conditions, the very factthat the combustibles detector is at full scale and theoxygen reads zero should be telling you that the processneeds more air. So normally you will want to get back toexcess air conditions as soon as possible and it is enough toknow that your process has gone reducing. Some applica-tions may occasionally need to be sub-stoichiometric.

Then you will want to know not only that your process isreducing, but by how much. The equivalent combustiblesoption for the WDG-IVC and WDG-HPIIC uses the highmillivolt signal from the oxygen cell to calculate the amount ofexcess fuel or equivalent combustibles, as discussed above. Itadds this to any signal from the combustibles detector, so thatthe combustibles reading is provided under all conditions.

What applications would useWhat applications would useWhat applications would useWhat applications would useWhat applications would useequivalent combustibles?equivalent combustibles?equivalent combustibles?equivalent combustibles?equivalent combustibles?

The Equivalent Combustibles option is used where there areoccasional sub-stoichiometric conditions such as the followingexamples:

Decarburizing furnacesCement kilnsPulverized coal dryers and injectorsOre roasting ovens

IMPORTANT NOTE: If the process is alwaysreducing, the Excess Fuel option on a WDG-IV orWDG-HPII should be used instead.

PreMix 2000PreMix 2000PreMix 2000PreMix 2000PreMix 2000

The Thermox PreMix 2000 is the replacement for the CMA. Itaccurately and continuously measures the proportions ofoxygen and fuel in pre-mix gases. The analyzer receives asmall sample of the air/fuel mixture, burns it and thenmeasures the results.

CMFCMFCMFCMFCMFA-P 2000A-P 2000A-P 2000A-P 2000A-P 2000

The CMFA-P 2000 is a portable combination premix and fluegas analyzer. The operating mode is selected by moving avalve to either the premix or flue gas position. In the premixmode the premix gas is applied via a flowmeter to the premixinlet port. In the flue gas mode, a suitable probe is screwedinto the flue gas inlet on the back of the sensor and the sensorcan be mounted directly at the measuring point on thefurnace.

The CMFA-P 2000 can also operate under either excessoxygen or excess fuel conditions.

A small portion of the samplegas stream enters the analyzerthrough the sample flowmeterand flashback arrestor to thecombustion chamber, wherethe mixture is burned. The

Premix AnalyzersPremix AnalyzersPremix AnalyzersPremix AnalyzersPremix AnalyzersThe PrThe PrThe PrThe PrThe PreMix 2000 and CMFeMix 2000 and CMFeMix 2000 and CMFeMix 2000 and CMFeMix 2000 and CMFA-P2000A-P2000A-P2000A-P2000A-P2000

What applications can use the PreMix 2000 orWhat applications can use the PreMix 2000 orWhat applications can use the PreMix 2000 orWhat applications can use the PreMix 2000 orWhat applications can use the PreMix 2000 orCMFCMFCMFCMFCMFA-P 2000?A-P 2000?A-P 2000?A-P 2000?A-P 2000?

The Premix analyzers measure the air/fuel ratio in open flameapplications where flue gas measurements are not practical.The value measured by the analyzer accurately predicts theconditions in the open flame area.

Flame treatingGlass forehearthTempering furnacesAir/fuel mixturesGas generatorsGlass fiber manufacturingMetals and metal formingOpen flame brazing and soldering

Most of the sample gasentering the analyzer passesthrough a bypass flowmeter toensure a fast response andkeep the sample inlet purgedof dead volume.

resulting products of combustion, either oxygen or combus-tibles (excess fuel), are measured with a zirconium oxide cell.The PreMix 2000 can operate under either excess oxygen orexcess fuel conditions in the same way as described earlier forthe zirconium oxide cell.

PreMix 2000

CMFA-P 2000 Portable

Premix flow diagram

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