Flue Gas Analysis for Furnace Combustion Efficiency: Methods and Optimization

Introduction to Combustion Analysis

Every gas-fired or oil-fired furnace converts chemical energy in the fuel to thermal energy through combustion. The efficiency of this conversion directly affects operating costs and emissions. Flue gas analysis is the primary diagnostic tool for assessing and optimising combustion performance. A portable flue gas analyser costing a few thousand pounds can identify tens of thousands of pounds in annual energy waste.

This guide covers the fundamentals of combustion chemistry, flue gas composition and what each component tells you, analyser operation and calibration, interpreting results, optimising combustion settings, emissions regulations, common problems, and data recording practices.

1. Combustion Fundamentals

Stoichiometric Combustion

Stoichiometric (or theoretical) combustion is the chemically perfect reaction where all fuel reacts with exactly the right amount of oxygen, leaving no excess fuel or oxygen. For natural gas (methane, CH&sub4;):

CH&sub4; + 2O&sub2; → CO&sub2; + 2H&sub2;O

Since atmospheric air contains approximately 21% oxygen and 79% nitrogen, the stoichiometric air-to-fuel ratio for natural gas is approximately 10:1 by volume (or 17.2:1 by mass). At stoichiometric conditions, the flue gas contains the maximum possible CO&sub2; concentration (approximately 11.7% for natural gas) and zero excess O&sub2;.

Excess Air

In practice, furnaces operate with excess air — more air than theoretically required — to ensure complete combustion and prevent the formation of carbon monoxide (CO). However, excess air carries a penalty: every cubic metre of excess air entering the furnace must be heated from ambient temperature to flue gas temperature, carrying that heat out of the stack as waste.

The optimal operating point for most industrial furnaces is 10–20% excess air, corresponding to 2–4% O&sub2; in the flue gas. Below 10% excess air, the risk of incomplete combustion (CO formation) increases sharply. Above 50% excess air, efficiency losses become significant.

Air-Fuel Ratio

The air-fuel ratio can be expressed as a ratio (e.g., 10:1), as a percentage of excess air, or as an air factor (λ, where λ = 1.0 is stoichiometric). Most combustion controllers work in terms of air factor or O&sub2; percentage. The relationship is:

Excess air (%) = (λ − 1) × 100

Excess air (%) ≈ O&sub2;% × 100 ÷ (21 − O&sub2;%) (simplified, for dry flue gas)

2. Flue Gas Composition: What Each Component Tells You

Oxygen (O&sub2;)

The O&sub2; reading is the most direct indicator of excess air. Lower O&sub2; means less excess air and higher efficiency — but also higher risk of incomplete combustion. Target range for natural gas: 2.0–4.0% O&sub2;.

• O&sub2; < 1%: Dangerously lean. High CO likely. Risk of flame instability and incomplete combustion. Increase air immediately.
• O&sub2; 2–4%: Optimal range. Good efficiency with safe combustion margin.
• O&sub2; 5–8%: Excessive air. Moderate efficiency loss. Reduce air supply.
• O&sub2; > 8%: Grossly excessive air. Significant energy waste. Check for air leaks in the furnace or flue system.

Carbon Dioxide (CO&sub2;)

CO&sub2; is inversely related to excess air. Higher CO&sub2; indicates less dilution by excess air. Maximum CO&sub2; for natural gas is 11.7%; for propane, 13.7%; for oil, 15.4%. CO&sub2; is a useful cross-check on the O&sub2; reading but is less commonly used as the primary control parameter in modern systems.

Carbon Monoxide (CO)

CO in the flue gas is the signature of incomplete combustion. Even small concentrations indicate wasted fuel and a potential safety hazard (CO is toxic). Target: <100 ppm CO for a well-tuned burner. Acceptable: <400 ppm. Action required: >400 ppm. Dangerous: >1000 ppm (shut down and investigate).

CO spikes can indicate:

• Insufficient air (low O&sub2;)
• Poor fuel-air mixing (burner maintenance required)
• Flame impingement on cold surfaces
• Cracked or fouled heat exchanger (indirect-fired furnaces)

Oxides of Nitrogen (NOx)

NOx (primarily NO and NO&sub2;) is formed by the reaction of atmospheric nitrogen with oxygen at high flame temperatures (above approximately 1300°C). NOx is regulated as an air pollutant. Measurement is expressed in ppm or mg/Nm³ (normalised to 3% O&sub2; reference for most standards).

NOx reduction strategies:

• Low-NOx burners: Staged combustion, flue gas recirculation (FGR), or lean premix designs reduce peak flame temperatures
• Reduce excess air: Less O&sub2; available for NOx formation (but must balance against CO risk)
• Reduce combustion air preheat temperature: Lower air temperature reduces flame temperature (at the cost of efficiency)

3. Portable Analyser Operation

Electrochemical Cell Technology

Most portable flue gas analysers use electrochemical cells to measure O&sub2;, CO, and NOx. Each cell generates an electrical signal proportional to the gas concentration. Key operational points:

• Cells have a finite life (typically 2–3 years for O&sub2;, 3–5 years for CO) and must be replaced when they fail calibration checks
• CO cells can be poisoned by exposure to high SO&sub2; concentrations (relevant for oil-fired furnaces) — use an SO&sub2; filter if testing oil-fired equipment
• High CO concentrations (>4000 ppm) can permanently damage or saturate the CO cell. If high CO is expected, use the dilution probe accessory
• Allow the analyser to complete its fresh-air zero calibration before inserting the probe into the flue. This typically takes 30–60 seconds

Sampling Probe Placement

Correct probe placement is critical for accurate readings. The probe must sample representative flue gas, not a localised pocket of air or products.

• Insert the probe through a test port in the flue or chimney, positioned at approximately one-third of the flue diameter from the wall
• The sampling point should be at least 2 flue diameters downstream of any bend, damper, or junction to allow gas mixing
• Avoid sampling immediately above the burner (gas is not yet fully mixed) or after an air dilution device
• Ensure the probe tip is in the gas stream, not touching the flue wall (which may give falsely low temperature readings)

Calibration

Portable analysers should be calibrated at least annually by an accredited service provider, with the calibration certificate retained. Most analysers also require a daily fresh-air zero check (automatic on modern instruments) and periodic span checks using certified calibration gas.

4. Interpreting Results and Calculating Efficiency

Combustion Efficiency Calculation

Combustion efficiency is calculated using the Siegert formula (or its variants), which estimates the percentage of fuel energy lost in the hot flue gases:

Flue gas loss (%) = K × (T_flue − T_air) ÷ CO&sub2;%

Where K is a fuel-specific constant (0.38 for natural gas, 0.35 for propane, 0.56 for light oil), T_flue is the flue gas temperature (°C), and T_air is the combustion air temperature (°C).

Combustion efficiency (%) = 100 − Flue gas loss (%)

For example, a natural gas furnace with flue gas at 280°C, ambient air at 20°C, and CO&sub2; at 9.5%:

Flue gas loss = 0.38 × (280 − 20) ÷ 9.5 = 10.4%

Combustion efficiency = 100 − 10.4 = 89.6%

Most modern analysers calculate efficiency automatically. Use our Furnace Design Calculator to model the impact of combustion efficiency changes on annual fuel costs.

Heat Loss in Flue Gas

The two primary components of flue gas heat loss are:

1. Sensible heat loss: The energy required to heat the flue gases (including excess air) from ambient to flue gas temperature. This is the dominant loss and is directly controlled by reducing excess air and flue gas temperature.
2. Latent heat loss: The energy contained in the water vapour produced by hydrogen combustion. This heat is only recoverable if the flue gas is cooled below the dew point (~55°C for natural gas), which requires a condensing economiser.

5. Optimising Combustion

Air Damper Adjustment

The most basic combustion optimisation: adjust the air damper (or air register on the burner) to achieve the target O&sub2; reading in the flue gas while keeping CO below the acceptable limit. Procedure:

1. With the furnace at normal operating temperature and load, take baseline flue gas readings
2. Gradually close the air damper, monitoring O&sub2; and CO continuously on the analyser
3. Stop when O&sub2; reaches the target range (2–4%) and CO remains below 100 ppm
4. If CO rises above 100 ppm before O&sub2; reaches 4%, the burner requires maintenance (poor mixing, worn diffuser, damaged electrodes)
5. Lock the damper in position and record the settings and flue gas readings

Burner Tuning

Beyond simple air damper adjustment, burner tuning involves optimising the fuel-air mixing at the burner head. This may include:

• Adjusting the gas pressure at the burner nozzle (affects flame length and mixing)
• Setting the diffuser or swirl vane position (affects flame shape and stability)
• Checking and cleaning the ignition electrode gap and positioning
• Verifying the combustion air fan speed or inlet vane position at each firing rate
• For modulating burners: checking the fuel-air ratio at low fire, mid fire, and high fire, adjusting the cam or linkage to maintain consistent O&sub2; across the range

Fuel-Air Ratio Controllers

Modern cross-connected ratio control systems (e.g., Siemens, Honeywell, Kromschroder) maintain a consistent fuel-air ratio across the full modulation range by mechanically or electronically linking the gas valve position to the air damper position. These systems should be checked at commissioning and annually to ensure the linkage has not drifted. Consult our Gas Systems Reference for manufacturer-specific setup procedures.

6. Emissions Regulations

Environmental Permitting (England and Wales)

Industrial combustion installations require environmental permits from the Environment Agency or local authority depending on their thermal input:

MCPD Emission Limits (New Plant, Natural Gas)

Existing plant may have transitional limits. Check your specific permit conditions. Continuous emissions monitoring (CEMS) may be required for installations above 20 MW_th.

7. Common Combustion Problems

8. Heat Recovery from Flue Gases

Waste heat in flue gases represents the single largest energy loss in most gas-fired furnaces. Recovering even a fraction of this energy can significantly improve overall thermal efficiency.

Recuperators

A recuperator is a gas-to-gas heat exchanger that preheats the combustion air using the hot flue gases. The simplest design is a concentric tube (radiation recuperator) mounted on the flue outlet, where flue gases pass through the inner tube and combustion air flows counter-current in the annular space. More efficient designs use multiple-pass shell-and-tube or plate-type configurations. A recuperator preheating combustion air from 20°C to 350°C can improve furnace efficiency by 15–20%. However, recuperators are unsuitable for furnaces with dirty or corrosive flue gases (e.g., salt bath furnaces) without upstream gas cleaning.

Regenerative Burners

Regenerative burner systems use a pair of burners with ceramic heat storage media (typically alumina balls or honeycomb). One burner fires while its partner's media absorbs heat from the flue gases. The system then switches: the hot media preheats the combustion air for the previously idle burner while the other absorbs heat. Regenerative burners can preheat combustion air to within 100–200°C of the furnace temperature, achieving thermal efficiencies exceeding 85%. They are cost-effective on furnaces operating above 900°C with annual gas consumption exceeding £50,000.

Economisers (Water Heating)

Where combustion air preheating is impractical, an economiser can extract heat from flue gases to heat water for process use, space heating, or boiler feed water preheating. Condensing economisers that cool the flue gas below the water dew point (~55°C for natural gas) recover both sensible and latent heat, achieving additional savings of 8–12% beyond what a conventional recuperator delivers.

9. Natural Gas vs LPG: Key Differences

Key practical implication: LPG is heavier than air and accumulates in low-lying areas. Gas detection and ventilation requirements are more stringent for LPG installations. When converting a furnace from natural gas to LPG (or vice versa), burner nozzles, gas valves, and the combustion control system must all be re-specified and re-commissioned.

10. Recording and Trending Combustion Data

A single set of flue gas readings is a snapshot. Trending data over time reveals degradation patterns that enable predictive maintenance:

• Rising O&sub2; trend: Air leaks developing in the furnace or flue system, or combustion air fan delivering more air (belt slipping on a VSD system, damper linkage loosening)
• Rising CO trend: Burner degradation (worn nozzle, fouled diffuser, electrode gap opening)
• Rising flue gas temperature: Heat transfer surface fouling (radiant tubes, recuperator), reduced furnace insulation effectiveness
• Rising NOx trend: Increased flame temperature, possibly from higher air preheat or loss of FGR flow

Record readings at least quarterly (monthly for high-consumption furnaces) with the furnace at steady-state operating conditions. Include: date, furnace load, firing rate (%), O&sub2;, CO, CO&sub2;, NOx, flue gas temperature, combustion air temperature, and the analyser serial number and calibration date.

Use our Flue Gas Analysis Tool to log readings, calculate efficiency, and generate trend charts. For detailed combustion system reference material, see our Gas Systems Reference.