What Is an In-Situ Flue Gas Oxygen Analyzer?
In every combustion process, slightly more air is supplied than is theoretically needed so that the fuel can burn completely. The direct measure of that surplus is the oxygen remaining in the flue gas. Measuring excess oxygen continuously and correcting the combustion air accordingly (O2 trim) is one of the main tools for cutting energy costs, reducing emissions and ensuring safe operation of boilers and furnaces.
In an in-situ analyzer, the measuring element is not outside the stack but directly in the gas stream. A zirconia cell at the tip of a probe flanged to the stack or duct wall measures the flue gas that reaches it through a diffuser. With no sample line, pump, cooler or condensate drain, sample delay and conditioning-related errors are eliminated, and the measurement is made on the actual composition of the flue gas.
The analyzers described on this page are available in general-purpose and hazardous-area versions. In the digital architecture the probe itself generates the HART or FOUNDATION Fieldbus signal; in the traditional architecture it sends the raw sensor signal to remote analyzer electronics. For applications where unburned combustibles must be monitored in addition to oxygen, oxygen and combustibles transmitters can be considered as an alternative solution.
How a Zirconia Oxygen Probe Works
A zirconia oxygen probe converts the difference in oxygen between the two faces of a ceramic electrolyte, which conducts oxygen ions at high temperature, into an electrical voltage. The in-situ measuring chain works in the following steps:
1. Flue gas reaches the cell through the diffuser: The diffuser at the probe tip lets flue gas reach the cell by passive diffusion while limiting particulates and the direct impact of the gas flow. The measurement takes place where the gas is, without carrying it out of the stack.
2. Cell heating and temperature control: Because zirconia becomes an ion conductor only at high temperature, a heater inside the probe holds the cell at a constant operating temperature. The cell temperature is measured continuously by a thermocouple and is used both for temperature control and in the oxygen calculation.
3. Comparison with reference air: The inner face of the cell is exposed to clean, dry reference air supplied through the probe. The difference in oxygen partial pressure between the flue gas on the outer face and the reference air on the inner face drives oxygen ions through the ceramic.
4. Generation of the cell voltage: Ion migration produces a millivolt-level voltage between the platinum electrodes of the cell. As the oxygen in the flue gas decreases, the voltage rises logarithmically.
5. Signal conversion and transmission: In the digital architecture, the cell voltage and thermocouple signal are converted to percent oxygen in the probe electronics and transmitted as 4–20 mA with HART or via FOUNDATION Fieldbus; in the traditional architecture they are sent raw to the remote analyzer.
6. Calibration: At intervals, calibration gases with low and high oxygen content are applied to the probe tip to correct the cell slope and zero offset. With an integral automatic calibration housing or an external automatic calibration unit, this can be done without an operator.
The Nernst Equation: From Cell Voltage to Oxygen Concentration
The voltage produced by a zirconia cell is described by the Nernst equation: E = (R·T / 4F) · ln(P_ref / P_gas). Here R is the universal gas constant, T the absolute temperature of the cell, F the Faraday constant, and P_ref and P_gas the oxygen partial pressures in the reference air and the flue gas respectively; the 4 in the denominator reflects the fact that each oxygen molecule takes up four electrons to form two oxygen ions.
Two practical consequences follow from the equation. First, because the voltage is directly proportional to temperature, the cell temperature must be measured precisely and held constant; the thermocouple and heater control are therefore an integral part of the measurement. Second, because the relationship is logarithmic, the same absolute change in oxygen produces a larger voltage change as the oxygen level falls — which makes the zirconia cell particularly well suited to controlling boilers and furnaces that run at low excess air. Since the measurement is made without removing water vapor from the flue gas, the result is an oxygen value on a wet basis.
Digital and Traditional System Architecture
In the digital architecture, the probe itself generates the 4–20 mA signal together with the HART or FOUNDATION Fieldbus digital signal; a customer-supplied 18 AWG two-wire shielded cable is all that is needed for the connection. The probe can be configured from the remote analyzer, a handheld communicator or directly from the PLC/DCS.
In the traditional architecture, the probe transmits the raw sensor and thermocouple voltages over a 7-conductor cable to the remote analyzer, which produces the 4–20 mA and HART output. This arrangement is preferred in replacement projects where the existing wiring is to be kept. The remote analyzer can bring two digital probes or one traditional probe onto a single interface and provides a central platform for automatic calibration devices, alarm relays and diagnostic tools.
Reducing Conditions and the Stoichiometer Indicator
When combustion air is insufficient, the free oxygen in the flue gas is used up and the atmosphere becomes reducing. The stoichiometer indicator still gives a meaningful reading in this situation, showing how far the process is short of oxygen; its accuracy is ±0.1% of the reading or 0.1% O2. System response is T90 120 seconds when moving from oxidizing to reducing conditions and T90 30 seconds in the opposite direction.
System response to calibration gas is under 3 seconds for the initial response and under 8 seconds for T90. Response to changes in the process gas can differ, depending on process conditions and on how long the sensor has been in service.
Key Features
· Measurement directly in the stack: Oxygen is measured without a sample line, pump or conditioning equipment, so delay and sample-related errors are eliminated.
· Wide choice of probe lengths: Probe options of 457 mm, 0.91 m, 1.83 m, 2.74 m and 3.66 m are offered in the general-purpose version, and 457 mm, 0.91 m and 1.83 m in the hazardous-area version.
· Probe options against abrasion: In addition to the standard probe tube, there is an abrasion-resistant probe body and an abrasive shield accessory package used together with a dust-sealed diffuser.
· Diffuser selection by temperature: The snubber diffuser is designed for service to 400 °C, the ceramic diffuser to 825 °C and the high-nickel alloy metal diffuser to 705 °C; the general-purpose version also offers options with a flashback arrestor.
· Poison-resistant cell options: The standard cell is protected against sulfur and other poisoning compounds by catalytic protection beads; the acid-resistant cell, with additional protection beads, is intended for more aggressive flue gases.
· Integral automatic calibration housing: Solenoids that apply the calibration gases to the probe tip in sequence can be integrated into the probe housing; this option is also approved on the hazardous-area version.
· Advanced sensor diagnostics: A calibration-recommended alert, a diagnostic that measures response time to detect a plugged diffuser or an empty calibration bottle, and a low-oxygen diagnostic allow maintenance to be planned according to need.
· Field-repairable design: The analyzer can be repaired entirely in the field; on the hazardous-area version, a modular assembly combining the diffuser and the process flame arrestor reduces maintenance time and cost.
· Fits existing installations: A variable insertion mount and adapter plates for existing 3, 4 and 6 in. 150# and 3 and 4 in. 300# flanges suit most existing probe installations from other manufacturers.
· Remote analyzer and safety interlock option: The remote analyzer communicates with the PLC or DCS over HART/4–20 mA; with the flame safety interlock option for the heater, it can be tied into the burner safety logic.
Technical Specifications
The data in the table are typical performance and installation values for the in-situ zirconia analyzer; they are confirmed in a project-specific technical sheet once the probe length, diffuser, housing and system architecture have been selected.
Parameter | Technical data |
Measuring principle | In-situ heated zirconia oxygen cell, temperature control by thermocouple |
Oxygen range | Factory calibration 0–10% O2 · user-configurable lower range value 0–10%, upper range value 0–50% O2 |
Repeatability | ±0.75% of reading or 0.05% O2 (whichever is greater) |
Process temperature effect | 0.05% O2 over 100–700 °C |
Lowest detection limit / calibration gas repeatability | 0.02% O2 / ±0.02% O2 |
Response to calibration gas | Initial response < 3 s · T90 < 8 s |
Reducing-condition performance | Stoichiometer accuracy: ±0.1% of reading or 0.1% O2 · oxidizing to reducing T90 120 s, reverse T90 30 s |
Process temperature (by diffuser / accessory) | Snubber diffuser 0–400 °C · ceramic and high-nickel alloy diffuser 0–705 °C · abrasive shield 0–705 °C · bypass accessory 0–1,050 °C |
Process mounting zone temperature | Maximum 200 °C (190 °C for the hazardous-area version) |
Probe lengths and insertion depth | 457 mm (409 mm insertion) · 0.91 m (826 mm) · 1.83 m (1,740 mm) · 2.74 m (2,655 mm) · 3.66 m (3,569 mm); the last two lengths in the general-purpose version |
Calibration gases | Low: 0.4–2% O2 · high: 8–21% O2 (balance nitrogen) · 137.9 kPa, 2.36 L/min |
Reference air | Clean, dry instrument air · 34 kPa, 0.94 L/min |
Output and communication | Digital: 4–20 mA + HART or FOUNDATION Fieldbus (from the probe) · Traditional: raw sensor and thermocouple signal to the remote analyzer, 4–20 mA + HART from the remote analyzer · minimum loop resistance of 250 Ω for HART communication |
Power supply | Probe: 120/240 Vac, 50/60 Hz, max. 260/1020 VA · remote analyzer (digital inputs): max. 12 VA · automatic calibration unit: 100–240 Vac, 15 VA |
Ambient temperature | Probe electronics –40 to +70 °C · remote analyzer –20 to +50 °C · automatic calibration unit –40 to +65 °C · storage –40 to +70 °C |
Wetted parts and process connection | 316L or 304 stainless steel · 2 in. 150# (121 mm bolt circle) or DIN (145 mm bolt circle) · ½-14 NPT conduit entries |
Cell options | Standard cell (with catalytic protection beads) or acid-resistant cell (with additional protection beads) |
Shipping weight | Approx. 7.3–17.7 kg depending on probe length and version |
The service temperature of the ceramic diffuser is given as 825 °C in the ordering options and 705 °C in the process temperature limits; the lower value should be used for design and confirmed for the application. The remote analyzer is compatible only with probes that have a 120 V heater.
Key Advantages
· Real-time O2 trim control: Fast measurement without sample delay lets combustion air follow load changes immediately, helping to lower energy costs on boilers and furnaces.
· No sample system to maintain: Without sample line components such as pumps, filters, coolers and condensate drains, the faults and maintenance items associated with that equipment disappear.
· Calibration without human intervention: Automatic calibration triggered by a timer, a contact relay or the calibration-recommended diagnostic keeps the measurement accurate without a trip to the field.
· Predictable maintenance: Plugged-diffuser and calibration-recommended diagnostics allow maintenance to be planned by actual need rather than by calendar, reducing unplanned shutdowns.
· Suited to difficult stack conditions: The abrasion-resistant probe, abrasive shield, acid-resistant cell and diffusers for different temperature classes support measurement continuity in dusty, abrasive and high-temperature flue gases.
· Low-cost replacement: Adapter plates that fit existing flanges and the traditional architecture option make it possible to replace older oxygen analyzers without major modifications to the stack.
· Suitable for hazardous areas: The Ex d design and the integral automatic calibration housing approved for hazardous areas simplify calibration equipment in classified locations.
Application Areas
In-situ oxygen analyzers can be used on any process that produces flue gas from combustion; they are most often found on boilers, process heaters and kilns, working within oxygen trim systems.
Industrial and Large Commercial Boilers
In oxygen trim systems the analyzer signal provides feedback to combustion air control, raising plant energy efficiency and lowering fuel costs. On steam boilers, the general-purpose version with the integral automatic calibration housing is a common combination.
Process Heaters (Fired Heaters)
The Ex d version is used on process heaters located in classified hazardous areas. Where the fuel gas composition varies, heating value tracking can be supported by process gas chromatographs; the flame safety interlock option of the remote analyzer contributes to the heater safety logic.
Kilns and Dusty Flue Gases
In kiln gases with a heavy dust load, the combination of an abrasion-resistant probe body, the abrasive shield accessory and a dust-sealed diffuser extends probe life; the plugged-diffuser diagnostic sets the timing of maintenance.
Very High-Temperature Flue Gases
Above the diffuser limits, the bypass accessory allows measurement in flue gas up to 1,050 °C. A spacer that moves the electronics housing away from the hot duct can be used to keep the mounting-zone temperature from exceeding 200 °C.
Replacing Existing Oxygen Analyzers
A direct replacement probe option, adapter plates for existing flanges and a housing option with cold junction compensation for working with other manufacturers’ electronics allow older systems to be renewed while keeping stack openings and cable infrastructure.
How to Select a Flue Gas Oxygen Analyzer
With in-situ measurement, choosing the right measuring point is as decisive as the accuracy of the analyzer itself. We recommend covering the following points in the front-end engineering study:
Measuring Point and Probe Length
· A point in the duct or stack cross-section where the gas is representative; assessment of stratification and air in-leakage risk
· Required insertion depth and the corresponding probe length (457 mm to 3.66 m)
· Minimum clearance needed to withdraw the probe (686 mm to 3,926 mm with the standard housing; more with the automatic calibration housing)
· Presence and size of an existing flange on the stack wall; need for a new weld plate or an adapter plate
Flue Gas Conditions and Diffuser Selection
· Maximum gas temperature that can occur at the measuring point, to select the diffuser or bypass accessory
· Temperature in the mounting zone (maximum 200 °C; 190 °C for the hazardous-area version) and need for a spacer
· Dust and abrasive particulate load; need for an abrasion-resistant probe and abrasive shield
· Sulfur and acid-forming components; choice of standard or acid-resistant cell
· Likelihood of reducing conditions and need for the stoichiometer indicator
Calibration Approach
· Preference for manual, semi-automatic or fully automatic calibration
· Choice between an integral automatic calibration housing and an external automatic calibration unit
· Initiation of calibration by timer, contact relay or diagnostic trigger
· Routing of calibration gas cylinders and the reference air supply to the mounting point
System Architecture and Integration
· Digital (HART or FOUNDATION Fieldbus directly from the probe) or traditional (with remote analyzer) architecture
· Monitoring one or two probes with a remote analyzer; need for alarm relays and a flame safety interlock
· Cable type and distance (18 AWG two-wire shielded or 7-conductor cable from 6 to 45 m)
· Area classification of the mounting point and ATEX/IECEx or CSA approval requirements
Evaluating the oxygen measurement together with the flue gas temperature is important for combustion efficiency calculations. For this purpose, thermocouple and RTD temperature sensors and temperature transmitters can be planned close to the same measuring point.
Ex d Approvals, Communication and Connection Standards
The hazardous-area approval options of the in-situ oxygen analyzer and the communication and connection standards it supports are summarized below.
Standard / approval | Scope and description |
ATEX / IECEx (Ex d) | Flameproof protection method on the hazardous-area version, including the integral automatic calibration housing option. |
CSA Class I, Division 1 / Zone 1 | Approval option for North American hazardous-location requirements. |
HART | Digital communication on 4–20 mA — from the probe in the digital architecture and from the remote analyzer in the traditional architecture. |
FOUNDATION Fieldbus | Fully digital communication on the digital probe with integral automatic calibration housing; in this version, the stoichiometer indicator, programmable reference, extended temperature and diffuser warning functions can be enabled in the probe. |
4–20 mA | Standard analog oxygen signal to the control system. |
ANSI and DIN flanges | 2 in. 150# or DIN process connection; adapter plates for existing ANSI 150# and 300# flanges. |
The remote analyzer and the automatic calibration unit are intended for general-purpose locations; the remote analyzer interconnecting cables are not suitable for hazardous-area use and must be installed in line with local regulations. Details of the approval markings should be verified against the certificates once the order configuration is finalized.
Combustion Optimization and Oxygen Measurement Solutions from TLY Enerji
At TLY Enerji we see in-situ oxygen measurement less as an analyzer purchase and more as the starting point of a combustion optimization project. Our engineers evaluate the stack or duct geometry, gas temperature profile and dust load to define the measuring point, probe length, diffuser and cell type together; in replacement projects, we look first at solutions that allow the existing flange and cable infrastructure to be kept.
We provide technical support on the automatic calibration concept, planning of calibration gas and reference air supplies, integration of the oxygen signal into the O2 trim loop and the DCS via HART or FOUNDATION Fieldbus, commissioning and periodic maintenance planning. Scheduling cell and diffuser replacements on the basis of diagnostic data helps the analyzer deliver reliable data over the life of the boiler or furnace.
Frequently Asked Questions
What is the difference between an in-situ and an extractive oxygen analyzer?
In an in-situ analyzer, the zirconia cell sits at the probe tip directly in the flue gas, and the gas reaches the cell through a diffuser. In extractive systems, the gas is drawn out of the stack and carried to a measuring cell. Because in-situ measurement needs no sample line, pump or conditioning equipment, the associated delay and maintenance burden disappear, and the measurement is made on the actual composition of the gas.
How does a zirconia oxygen sensor work?
Zirconia is a ceramic material that conducts oxygen ions at high temperature. One face of the cell sees the flue gas and the other sees reference air. The difference in oxygen partial pressure between the two sides produces a logarithmic voltage according to the Nernst equation. Because the cell temperature is measured by a thermocouple and held constant, the oxygen concentration can be calculated directly from this voltage.
How is the probe length determined?
The probe should be long enough for the cell to reach a point in the stack cross-section where the gas is well mixed and representative. The general-purpose version offers five lengths from 457 mm to 3.66 m; the hazardous-area version offers 457 mm, 0.91 m and 1.83 m. Enough clearance must also be left outside the stack to withdraw the probe; this clearance depends on probe length and housing type.
Which diffuser type should be selected?
The choice is driven by the flue gas temperature and dust load at the measuring point. The snubber diffuser is used up to 400 °C, and the ceramic and high-nickel alloy metal diffusers up to 705 °C. At higher temperatures, measurement up to 1,050 °C is possible with the bypass accessory. For abrasive, dusty gases, a dust-sealed diffuser used together with the abrasive shield is preferred.
How does automatic calibration work and how often should it be done?
In automatic calibration, solenoids apply low- and high-oxygen calibration gases to the probe tip in sequence, and the analyzer calculates the new calibration values itself. The sequence can be started manually, by a timer, by a contact relay or automatically by the calibration-recommended diagnostic. Diagnostic-triggered calibration means the sensor is calibrated according to its actual condition instead of a fixed calendar.
What happens when the sensing cell fails?
The analyzer is designed to be repaired entirely in the field; components such as the cell and the modular diffuser assembly can be replaced on site. The calibration-recommended and plugged-diffuser diagnostics give warning of cell or diffuser problems before they turn into failures. Choosing the acid-resistant cell for flue gases containing sulfur and acids helps extend the replacement interval.
Can the analyzer replace an existing oxygen probe?
Yes. With adapter plates for existing ANSI 150# and 300# flanges and a special adapter option, the analyzer can be fitted to most existing probe installations from other manufacturers. Direct replacement probe and traditional architecture options allow existing cable infrastructure to be kept, and a housing option with cold junction compensation makes it possible to work with other manufacturers’ electronics.
What does the analyzer show under reducing conditions?
When combustion air is insufficient, the free oxygen in the flue gas is used up. In this situation the stoichiometer indicator provides a reading that shows the degree of oxygen deficiency, telling the operator how far short of air the combustion is running. In applications where unburned combustibles must also be measured, an oxygen and combustibles transmitter may be the better choice.
Can a flue gas oxygen analyzer be used in a hazardous area?
Yes. The hazardous-area version has a robust flameproof design that satisfies the approval requirements of CSA Class I, Division 1 / Zone 1 and of ATEX/IECEx Ex d. The integral automatic calibration housing is also approved on this version and simplifies calibration equipment in the hazardous area. The remote analyzer and the external automatic calibration unit, however, should be located in general-purpose areas.