Oxygen and Combustibles Transmitters

What Is an Oxygen and Combustibles Transmitter?

The most direct indicator of combustion efficiency is the oxygen left in the flue gas. When more air is supplied than necessary, that air is heated and discharged up the stack and fuel is wasted; reducing the air raises efficiency, but beyond a certain point the fuel can no longer burn completely and unburned combustibles — carbon monoxide above all — appear in the flue gas. These combustibles mean both an energy loss and a safety problem, because they can accumulate in the furnace and ducts.

By monitoring both variables together, an oxygen and combustibles transmitter lets operators find the optimum excess air point with confidence. Oxygen is measured with a zirconium oxide (zirconia) cell and combustibles with a catalytic bead sensor, the result being reported as CO equivalent (COe). An analyzer that reads oxygen only cannot show on its own why the combustion air has become insufficient or whether incomplete combustion has started at the burner; the combustibles signal fills that gap.

The transmitter described on this page consists of a probe that seats on a flange in the stack or duct wall, a heated sensor housing and an electronics unit that can be mounted integrally or remotely. Versions are available for general-purpose locations and, with explosion-proof housings, for hazardous areas. For applications where oxygen measurement alone is sufficient and direct measurement at the probe tip is preferred, in-situ oxygen analyzers can be considered as a separate solution.

How O2 and Combustibles Measurement Works

The transmitter draws a continuous sample flow from the flue gas and passes it through two different sensors in succession. The measuring chain consists of the following steps:

1.      Drawing the sample through the probe: An eductor driven by clean, dry instrument air pulls a continuous flue gas sample through the probe and sample tube that extend into the stack. An optional in-situ filter at the probe tip retains particulates; after measurement the gas returns to the stack through an exhaust tube.

2.      Heated sensor housing: The sample reaches the sensor housing located outside the stack, which heaters hold at a constant operating temperature. Heating provides the condition needed for ion conduction in the zirconia cell and also prevents moisture in the sample from condensing.

3.      Oxygen measurement in the zirconia cell: When the oxygen on the sample side is lower than on the reference air side, a potential difference develops between the electrodes of the zirconia cell. This voltage varies with the logarithm of the oxygen ratio and is converted to percent oxygen in the electronics unit.

4.      Combustibles measurement on the catalytic bead: In the combustibles sensor, CO and other combustibles are oxidized on the surface of an active catalytic bead; the heat of reaction creates a temperature difference between the active bead and a reference element. This difference is proportional to the amount of combustibles and is reported as CO equivalent (COe). Dilution air added to the sample provides enough oxygen for oxidation even in reducing conditions, when the flue gas contains no oxygen.

5.      Signal processing and outputs: The electronics unit processes the signals from both sensors and generates two separate isolated 4–20 mA outputs, one for oxygen and one for combustibles. The oxygen output carries the HART digital signal; FOUNDATION Fieldbus communication can be selected as an alternative. A dry-contact relay and an optional alarm module report low oxygen and high combustibles conditions to the control system.

6.      Calibration: Low and high oxygen calibration gases and a CO-containing combustibles calibration gas are delivered to the sensors through the calibration line. In versions with automatic calibration solenoids, this is carried out without operator intervention.

Reading Net Oxygen and Combustibles Together

The hot platinum electrodes of the zirconia cell act as a catalyst; if combustibles are present in the sample, part of them reacts with oxygen at the cell surface. The value read by the zirconia cell is therefore the net oxygen remaining after the oxygen consumed by the combustibles has been subtracted. The combustibles sensor, in turn, shows the unburned fuel in the same sample separately.

Read together, the two signals make the state of combustion clear: if oxygen is adequate and COe is low, combustion is in the safe zone; if COe rises sharply as oxygen falls, the burner is approaching the limit of incomplete combustion. The aim of combustion control is to run at the lowest excess air level just before COe begins to rise.

Protecting the Sample Path: In-Situ Filter and Blowback

In dusty flue gases, a stainless steel, high-surface-area stainless steel or high-temperature alloy filter can be fitted at the probe tip. To delay filter plugging, an optional blowback arrangement periodically sends pressurized instrument air in the reverse direction and cleans the sample path.

In this sampling-based design, the measuring sensors are not inside the stack but in the heated housing outside it. This arrangement allows two different sensors to measure the same sample at a controlled temperature, and it lets the probe material be chosen independently of the sensors in high-temperature stacks.

Key Features

·         Two measurements, one transmitter: The zirconia oxygen cell and the catalytic combustibles sensor share the same heated sensor housing, so separate analyzers, sample lines and mounting points are not needed.

·         Probe options for high temperatures: The 316 stainless steel probe is designed for flue gas temperatures up to 704 °C, the Alloy 600 probe up to 1,000 °C and the ceramic probe up to 1,427 °C.

·         Poison-resistant zirconia cell: Catalytic platinized beads extend cell life in flue gases containing sulfur and other poisoning compounds. A high-sulfur oxygen cell option is available for applications with a heavy sulfur load.

·         Reliable COe measurement in reducing conditions: A combustibles sensor with a large active surface, together with dilution air, delivers correct combustibles readings even under reducing conditions in which the oxygen has been used up.

·         Field-selectable ranges: The oxygen range can be set in the field between 0–1% and 0–40%, and the combustibles range between 0–1000 ppm and 0–5%, via HART or the local operator interface.

·         Fast system response: System response to test gas is T90 10 seconds for oxygen and T90 25 seconds for combustibles, strengthening the feedback to combustion control during load changes.

·         Automatic calibration option: Versions with autocalibration solenoids apply the calibration gases in sequence and complete calibration without an operator; semi-automatic calibration is also supported.

·         Integral or remote electronics: The electronics unit can be integrated into the sensor housing or mounted remotely, with cable options from 6 to 46 m. The remote-mounting cable is rated up to 200 °C.

·         Local operator interface: An optional local operator interface gives on-site access to commissioning, maintenance and diagnostic information with no extra tools; the display can be rotated in 90° steps.

General-purpose and explosion-proof versions: The compact housings, which mount directly on the process, are offered for general-purpose locations and in an explosion-proof design for hazardous areas.

Technical Specifications

The table summarizes the measuring, process and electrical data of the two-sensor transmitter. Static performance values apply with operating variables held constant; final values are confirmed for the project once the probe, cell and electronics configuration has been defined.

The upper temperature limit of the ceramic probe is stated as either 1,426 °C or 1,427 °C; staying on the safe side is recommended in design. Probe temperature limits are set by the probe material, not by the process conditions.

Key Advantages

·         Efficient combustion at the safe limit: Using the oxygen and COe signals together helps reduce excess air to the lowest level reached before combustibles form, and so lowers fuel consumption.

·         Early detection of incomplete combustion: A high-combustibles alarm makes conditions such as a burner fault, an air damper problem or a disturbed fuel/air ratio visible in the flue gas at an early stage.

·         One mounting point, simple installation: With both sensors in a single housing, the stack needs only one flange, one set of wiring and one calibration infrastructure.

·         Long sensor life with difficult fuels: Catalytic protection beads and the high-sulfur cell option help reduce how often cells must be replaced in plants that fire sulfur-bearing fuels.

·         Lower maintenance workload: Automatic calibration, blowback and the local operator interface allow routine maintenance to be done without extra equipment or lengthy site visits.

·         Diagnostics from the control room: HART and FOUNDATION Fieldbus options let diagnostic and operating data be monitored from the control room, so site visits can be planned.

·         Stable signal on noisy sites: The design meets industrial-environment requirements for electromagnetic and radio-frequency interference, and output values are unaffected when shielded twisted-pair cable is used.

Application Areas

Oxygen and combustibles transmitters can be used in processes where fuel is burned with air and the flue gas leaves through a stack or duct. The deciding factors in application selection are flue gas temperature, the sulfur content of the fuel and the area classification of the mounting point.

Industrial Boilers and Steam Generation

Monitoring oxygen and COe together in the boiler stack allows combustion air to be matched to demand. Where the composition of the fuel gas supplied to the burner varies, this data can be supplemented by process gas chromatographs.

Process Furnaces and Heaters

In furnaces running at high heat duty, operating at low excess air saves a significant amount of fuel; combustibles measurement helps hold that limit safely.

High-Temperature Stacks and Ducts

Because the Alloy 600 probe can sample flue gas up to 1,000 °C and the ceramic probe up to 1,427 °C, measurements can be taken close to the firebox or in hot duct sections upstream of heat recovery.

Plants Firing Sulfur-Bearing Fuels

In plants that burn sulfur-containing liquid or gaseous fuels, the high-sulfur oxygen cell and catalytic protection beads help preserve sensor life.

Combustion Units in Classified Hazardous Areas

The explosion-proof version is used on combustion units in areas with a potentially explosive atmosphere. Ambient gas leak detection around the unit, however, should be handled independently of the process measurement, with infrared gas detectors.

How to Select an Oxygen and Combustibles Transmitter

The right configuration is a chain of linked decisions, from probe material to cell type and instrument air infrastructure. Answering the following questions before the quotation stage helps get the configuration right the first time:

Flue Gas and Process Conditions

·         Maximum and normal flue gas temperature, which determines the probe material

·         Fuel type and sulfur content; need for a standard or high-sulfur cell

·         Dust and particulate load in the flue gas; need for an in-situ filter and blowback

·         Expected oxygen and combustibles ranges; likelihood of reducing conditions

·         Stack or duct pressure (the maximum process pressure is 8 in. water column)

Mounting and Mechanical Layout

·         Stack wall type: metal or masonry wall, and a matching mounting plate

·         Required insertion depth and probe length; clearance needed for probe removal (from 864 mm for a 457 mm probe up to 3,150 mm for a 2.7 m probe)

·         Preference for ANSI or DIN flanges; whether existing mounting plates will be reused

·         About 228 mm of free space for cover removal and service access

·         Ambient temperature at the mounting point; preheating of instrument air and housing insulation in cold climates

Instrument Air and Calibration Infrastructure

·         Clean, dry instrument air regulated to 2.4 barg for reference, eductor and dilution air

·         An air supply of at least 4.1 barg if blowback is used

·         Supply of low/high O2 and CO calibration gases

·         Loose or panel-mounted calibration and reference gas flow meters and regulator set

Electronics, Communication and Alarms

·         Integral or remote electronics; cable length for remote mounting

·         Preference for HART or FOUNDATION Fieldbus communication

·         Need for a local operator interface and automatic calibration solenoids

·         How low-O2 and high-combustibles alarms will be passed to the control system

·         Hazardous-area classification of the mounting point

When choosing the probe material, measuring the flue gas temperature with industrial temperature sensors and passing the signal to the control system through temperature transmitters helps establish the true maximum temperature across different load conditions.

Communication Protocols and Protection Standards

The communication protocols, protection ratings and connection standards supported by the transmitter are summarized below.

An explosion-proof version is available for hazardous areas. The protection method and scope of certification are assessed per project together with the classification of the installation area.

Combustion Analysis Solutions from TLY Enerji

At TLY Enerji we approach oxygen and combustibles measurement where combustion efficiency and combustion safety meet. Our engineers evaluate the fuel type, burner design, flue gas temperature profile and accessibility of the mounting point to define the probe material, cell type and the need for a filter and blowback.

We provide technical support throughout the project: planning the instrument air and calibration gas infrastructure, wiring alarm and output signals to the combustion control system, configuring digital communication on the PLC or DCS side, commissioning and setting up a periodic maintenance program. Under difficult conditions — sulfur-bearing fuel, high temperature or a hazardous area — choosing the right configuration is the foundation for a measuring point that delivers reliable data for many years. Re-evaluating an existing measuring point after process changes such as a fuel switch or burner revamp is also part of this approach.

Frequently Asked Questions

What does an oxygen and combustibles transmitter do?

This transmitter measures the residual oxygen and the unburned combustibles in flue gas at the same time. The oxygen value shows how much excess combustion air is being supplied, while the combustibles value shows whether combustion is complete. Used together, the two readings allow boilers and furnaces to run without wasting air and fuel and without drifting into incomplete combustion.

Why measure combustibles as well as oxygen in flue gas?

Oxygen on its own tells you how much excess air the combustion is using, but when the mixture at the burner is disturbed, unburned fuel can form even while oxygen is still present. Combustibles measurement shows this directly. Excess air can then be reduced safely, and an alarm can be raised when combustibles rise, keeping both energy losses and the safety risk from unburned gas accumulation under control.

What does COe (CO equivalent) mean?

The catalytic combustibles sensor measures the heat released when carbon monoxide, hydrogen and other combustible components are oxidized on a catalytic surface; it does not distinguish which combustible produced the signal. Because the sensor is calibrated with a CO-containing calibration gas, the result is reported as the CO-based value of the total combustible effect — the CO equivalent (COe).

What does it mean that a zirconia sensor measures net oxygen?

The hot platinum electrodes of the zirconia cell have a catalytic effect. If combustibles are present in the sample, part of them reacts with oxygen at the cell surface, and that oxygen is not measured. The value read is therefore the net oxygen remaining after the oxygen consumed by the combustibles has been subtracted. Because the combustibles sensor measures the same sample separately, this effect is easier to interpret.

Which probe material should be selected?

The selection is driven by the maximum flue gas temperature at the mounting point. A 316 stainless steel probe can be used up to 704 °C, an Alloy 600 probe up to 1,000 °C and a ceramic probe up to 1,427 °C. Metal probes are offered in lengths from 457 mm to 2.7 m, ceramic probes in 457 mm and 0.91 m. Probe length should be chosen for the insertion depth that reaches a point where the gas is representative.

How is sensor life protected with sulfur-bearing fuels?

Catalytic platinized beads on the zirconia cell reduce the effect of sulfur and other poisoning compounds before they reach the cell, extending cell life. In plants that fire fuels with a high sulfur content, a high-sulfur oxygen cell is recommended instead of the standard cell. The right cell choice makes maintenance intervals and spare-part costs predictable.

How is the transmitter calibrated?

Calibration is semi-automatic or automatic. Two nitrogen-balanced gases containing 0.4% and 8% O2 are used for oxygen, and an air-balanced gas containing 1000 ppm CO for combustibles; each gas is applied at a regulated flow rate. In versions with automatic calibration solenoids, the gases are applied in sequence without operator intervention.

Which instrument air supplies are required?

The transmitter runs on clean, dry instrument air regulated to 2.4 barg. Reference air flows at about 1 L/min, the eductor air that draws the sample at 2.5 L/min and the dilution air for the combustibles sensor at 2.8 L/h. If the blowback option is used, air at a pressure of at least 4.1 barg is required. Preheating the supply air is recommended in cold climates.

Can the transmitter be used in hazardous areas?

Yes. Alongside the general-purpose version, there is a version with an explosion-proof housing for hazardous areas. In this version the electronics are offered integrally mounted or remote-mounted, with the remote option supplied without interconnecting cable. The classification, gas group and temperature class requirements of the installation area should be defined at project stage to confirm the appropriate configuration and certification scope.