O2 trim is a closed-loop control that continuously adjusts a burner’s fuel/air ratio based on measured flue gas oxygen, keeping excess air close to the optimum as firing rate, weather and fuel change. Too much excess air carries heat up the stack; too little produces carbon monoxide, soot and unsafe conditions. CO-based control goes a step further by finding the point where unburned gases begin to form and operating just on the safe side of it.
What is O2 trim?
O2 trim is a correction applied on top of a burner’s characterized fuel/air curve. The base curve, set during commissioning, positions the air damper, fan and fuel valve for each firing rate. The trim loop then measures actual flue gas oxygen and nudges air (or fuel) to hold an oxygen setpoint that also varies with firing rate.
Trim is necessary because conditions drift after commissioning. LAMTEC, which brought O2 control to market in 1982 while its team was still part of ABB, describes the variables its systems compensate for: combustion air temperature, barometric pressure, humidity and fuel quality. Mechanical wear and hysteresis in linkages add further drift.
Why does excess air matter so much?
Every pound of air beyond what combustion needs is heated and exhausted, so excess air is a direct fuel loss. The U.S. Department of Energy’s Steam Tip Sheet #4 states an often-cited rule of thumb: boiler efficiency can increase by 1% for each 15% reduction in excess air or 40 °F reduction in stack gas temperature. DOE also notes that well-designed natural gas-fired systems can attain about 10% excess air.
DOE’s natural gas table shows the effect at a 300 °F difference between flue gas and combustion air temperature:
| Flue gas O2 | Excess air | Combustion efficiency |
|---|---|---|
| 2.0% | 9.5% | 83.1% |
| 3.0% | 15.0% | 82.8% |
| 5.0% | 28.1% | 82.1% |
| 7.0% | 44.9% | 81.2% |
A worked example
These numbers are illustrative; your boiler’s data will differ. Assume a natural gas boiler burns 200,000 MMBtu per year at $4.00/MMBtu and averages 5% O2.
- O2 trim holds 3% O2: efficiency rises from 82.1% to 82.8%. Fuel savings = 1 − (82.1 ÷ 82.8) ≈ 0.85%, or about 1,690 MMBtu and $6,800 per year.
- CO/H2 control holds 2% O2: efficiency reaches 83.1%. Total savings versus the original 5% ≈ 1.2%, or about 2,400 MMBtu and $9,600 per year.
Savings scale with fuel use and fuel price, and multi-boiler plants multiply them. Lower excess air also reduces flue gas volume, which can help with fan power.
How does a zirconia O2 sensor work?
A zirconia oxygen sensor uses a ceramic cell that conducts oxygen ions when heated, typically somewhere around 700 °C. Reference air sits on one side of the cell and flue gas on the other. The voltage across the cell follows the Nernst equation, varying logarithmically with the ratio of oxygen partial pressures, so the transmitter can calculate flue gas O2.
Practical points that affect trim performance:
- In-situ, wet-basis measurement. The probe measures directly in the flue with no sample conditioning, and it reads lower than a dry-basis portable analyzer on the same gas.
- Net oxygen. Combustibles can react at the hot electrode, so a heated zirconia cell reports oxygen net of combustibles.
- Fast response, which suits closed-loop control.
- Air in-leakage. Tramp air entering the flue upstream of the probe makes O2 read falsely high, and a trim loop may then remove air the burner actually needs. Probe location matters.
LAMTEC’s LT1 (with the LS1 probe) measures O2, while the LT2 and LT3 transmitters, used with the LS2 or KS1D combination probe, add unburned gases reported as CO equivalent (COe). LAMTEC’s brochure lists an O2 range of 0–21% and a COe range of 0–10,000 ppm, with the KS1D rated for flue gas up to about 1,400 °C.
How does CO-based control go further than O2 trim?
CO/H2 control minimizes excess air by operating near the point where unburned gases begin to form, instead of holding a fixed oxygen margin. As excess air falls, CO stays low until the burner reaches a “CO edge,” then rises sharply. The location of that edge shifts with burner condition, load and fuel.
A fixed O2 setpoint must include a safety margin to stay clear of the edge under worst-case conditions, so most of the time the burner runs with more air than necessary. LAMTEC’s adaptive CO/H2 control uses the KS1D probe to detect the onset of unburned gases and continually steers toward the edge while keeping emissions in check. LAMTEC rates this function SIL 2 and offers it for its CMS, BT300 and ETAMATIC systems. For biomass combustion, LAMTEC offers a modular CO/lambda control.
| O2 trim | CO/H2 control | |
|---|---|---|
| Controlled variable | Flue gas O2 against a load-based setpoint | Onset of CO/H2 formation, with O2 as a limit |
| Sensor | Zirconia O2 probe | Combination O2 and COe probe (such as the KS1D) |
| Excess air margin | Fixed safety margin | Adapts to actual burner behavior |
| Best fit | Most gas and oil burners | Burners with a clear CO edge and meaningful fuel cost |
| Commissioning effort | Moderate | Higher; needs careful setup and limits |
How does trim relate to the burner management system?
A burner management system (BMS) is a safety system; combustion control, including O2 trim, is a process control function. The BMS handles purge, ignition, flame supervision, safety shutoff valves and interlocks. Combustion control sets firing rate and fuel/air ratio within the limits the BMS allows.
The codes that govern the BMS depend on the equipment:
- NFPA 85 covers boilers and combustion systems, including single burner boilers with fuel input of 12.5 MMBtu/hr or more.
- ASME CSD-1 covers controls and safety devices for automatically fired boilers below that size.
- NFPA 86 covers ovens and furnaces.
- API RP 556 addresses instrumentation and protective systems for gas-fired heaters in refineries and petrochemical plants.
Trim should have limited authority and must never be able to drive the burner below a safe fuel/air ratio. LAMTEC’s Combustion Management System (CMS) and compact BurnerTronic BT300 combine burner control, electronic fuel/air ratio control, flame monitoring, valve leak testing and optional trim in one architecture. LAMTEC lists SIL 3 certification and EN 298 approval for the CMS, UL 60730 and CSA approvals for the CMS, and UL 1998, UL 372, UL 353 and CSA C22.2 No. 199 for the BT300 family. FM approval could not be confirmed, so check with LAMTEC before specifying for FM-insured facilities.
Is there a regulatory benefit?
For major-source boilers and process heaters under the Boiler MACT (40 CFR Part 63, Subpart DDDDD), a unit with a continuous oxygen trim system that maintains an optimum air-to-fuel ratio may conduct tune-ups every five years instead of more frequently, provided the oxygen level is set no lower than the concentration measured during the most recent tune-up (40 CFR 63.7540(a)(12)). Confirm applicability with your environmental staff and permit.
O2 trim retrofit checklist
- Record flue gas O2, CO and stack temperature across the firing range.
- Confirm the burner has electronic ratio control or can be upgraded to it.
- Choose a probe location with representative flow and no air in-leakage upstream.
- Define trim limits, fallback behavior on sensor failure and alarm handling.
- Review the design against the BMS, applicable codes and insurer requirements.
- Commission setpoints at multiple firing rates and verify with a portable analyzer.
Gulf Coast combustion applications
Across Louisiana and the Gulf Coast, fuel-intensive combustion equipment is everywhere: package and field-erected boilers in chemical plants, fired heaters in refineries, power boilers at pulp and paper mills, and steam plants at food processors. Variable fuels such as refinery gas are exactly where DOE recommends online oxygen monitoring and trim.
Local combustion control support
Vaughn Associates represents LAMTEC across all Vaughn territories, with our Baton Rouge office supporting Gulf Coast plants. LAMTEC America, Inc., the manufacturer’s U.S. subsidiary founded in 2016, supports sales and service in the Americas alongside regional representatives. We can help review burner data, scope O2 or CO/H2 trim retrofits, and coordinate quotes and commissioning support with LAMTEC.
See combustion control and O2 trim, the LAMTEC line page, our territories map, or request application help.
