Combustion Theory
Combustion is what a fired heater runs on. Understanding it — even at a practical rather than a chemistry level — explains most of what you observe at the peephole, most of what stack analysers tell you, and most of what goes wrong.
The combustion reaction
Combustion is the rapid chemical reaction between a fuel and oxygen that releases heat, light, and combustion products. In a fired heater, the fuel is typically refinery fuel gas (a mixture of hydrogen, methane, and heavier hydrocarbons) or fuel oil. The oxygen comes from air.
For a simple hydrocarbon fuel, the complete combustion reaction looks like this — using methane (CH₄) as the example:
In reality, refinery fuel gas is a mixture of components, and combustion is never perfectly complete. When oxygen is insufficient or mixing is poor, incomplete combustion occurs, producing carbon monoxide (CO) and unburned hydrocarbons instead of CO₂:
CO in flue gas is always a signal that fuel is being wasted and combustion is unsafe. Even small amounts of CO represent unburned fuel passing to the stack — reducing efficiency and potentially accumulating in the convection section where it can reignite.
Stoichiometry and excess air
Stoichiometric combustion means supplying exactly the right amount of air to completely burn the fuel — no more, no less. In theory this is ideal. In practice it is impossible to achieve, because fuel and air never mix perfectly in the firebox.
To ensure complete combustion despite imperfect mixing, fired heaters always run with excess air — more air than stoichiometry requires. Excess air is expressed as a percentage above stoichiometric.
The excess air tradeoff
Too little excess air risks incomplete combustion — CO formation, smoking, and potentially unburned fuel in the flue gas. Too much excess air carries heat to the stack unnecessarily, reducing efficiency and increasing NOx formation at high temperatures.
Products of combustion — what they tell you
Flue gas analysis is one of the most useful diagnostic tools available to the operator. What the stack analyser reads reflects exactly what is happening inside the firebox.
| Component | Typical range | High reading means | Low reading means |
|---|---|---|---|
| O₂ (oxygen) | 2–4% (target) | Excess air too high — efficiency loss, potential NOx increase | Risk of incomplete combustion — check for CO |
| CO₂ (carbon dioxide) | 8–12% (gas fuel) | More complete combustion or richer fuel — generally good if O₂ is acceptable | Incomplete combustion or dilution with excess air |
| CO (carbon monoxide) | <200 ppm (target) | Incomplete combustion — insufficient air, poor mixing, or burner fault. Investigate immediately. | n/a — zero CO is always the goal |
| NOx (nitrogen oxides) | Varies by permit | High flame temperature or excess air — consider air reduction or low-NOx burner tuning | Good — lower NOx is the environmental target |
| SO₂ (sulphur dioxide) | Depends on fuel sulphur content | High-sulphur fuel or increased fuel rate | Low-sulphur fuel or reduced firing rate |
Flame appearance as a diagnostic tool
What you see through the peephole is a direct readout of combustion quality. Every operator should be able to interpret what they're looking at. The table below describes the key flame appearances and what they indicate.
| Appearance | Colour / Character | What it indicates | Action |
|---|---|---|---|
| Normal (gas fuel) | Clear blue / blue-orange tips | Good air-fuel mixing, complete combustion, correct excess air | None — monitor only |
| Rich / lazy flame | Long, luminous yellow-orange, flame tips touching tubes | Insufficient air or too much fuel. Incomplete combustion likely. CO may be rising. | Increase air — open register or stack damper. Check fuel pressure. |
| Smoking flame | Black or dark grey smoke from flame or stack | Significant incomplete combustion. Unburned carbon. Possible fuel oil carryover or very low air. | Increase air immediately. If no improvement, reduce firing rate. |
| Lean / tight flame | Very short, intensely blue, possibly unstable/noisy | Excess air too high, or fuel pressure low. Flame stability at risk. | Reduce air slightly. Check fuel pressure and composition. |
| Flame impingement | Flame visibly contacting tube surface — bright hot spot on tube | Flame too long, burner misaligned, or tube too close. Tube overheating risk. | Reduce that burner's firing rate. Adjust air register. Investigate burner alignment. |
| Lifting / detached flame | Flame lifted off burner tile — gap between tile and flame base | Air velocity too high relative to fuel, or fuel pressure low. Flame-out risk. | Reduce air on that burner. Check fuel pressure. Monitor closely for flame-out. |
Fuel gas composition — why it matters operationally
Refinery fuel gas is not a fixed composition. It is a blend of streams from across the refinery — hydrogen, methane, ethane, propane, and heavier components — and its composition shifts as operating modes change. This matters because different compositions have different heating values and require different air-fuel ratios.
Hydrogen-rich fuel gas has a much higher heating value per unit volume than methane-rich gas, and burns with a much hotter, shorter, more intensely blue flame. If the fuel gas suddenly becomes richer in hydrogen — after a reformer upset, for example — the heater will fire harder than expected at the same fuel gas pressure. COT will rise and O₂ may drop.
Heavier fuel gas (more propane, butane) burns with a longer, more luminous flame and requires more air per unit volume. A sudden shift to heavier fuel gas at the same fuel pressure can cause a rich flame and CO rise if air is not adjusted.
Summary — what to carry forward
Complete combustion requires the right amount of air, good mixing, and adequate temperature. In normal operation, the operator's job is to maintain the correct excess air — enough to ensure complete combustion, not so much as to waste heat up the stack.
The three indicators you use in real time are: stack O₂ (is the excess air in range?), CO in flue gas (is combustion complete?), and flame appearance (is the burner behaving normally?). Together they give you a complete picture of what is happening in the firebox without needing to go inside it.