Burners and industrial furnaces: controlling combustion

In one line

Before a burner can ignite, the firebox has to be purged with air to sweep out any explosive gases, and once lit it needs a flame detector that cuts the fuel at the first sign of trouble — these are the two non-negotiable points, whatever else is or isn't on the maintenance plan.

What industrial furnaces are

There is a huge variety of industrial furnaces, but they group into two large types: burner furnaces (running on conventional fuels) and electric arc furnaces (running on electricity). Furnaces are heavy energy consumers and, in general, their energy efficiency is low — which is why their maintenance is aimed mainly at savings and efficiency, not just at preventing failure.

For burner furnaces, that maintenance concentrates on 5 operations: checking the condition of the refractory and insulation, detecting infiltrations, controlling the burners, controlling the combustion process and controlling the flue gases.

Checking the refractory and insulation

The most effective way to check the condition of the refractory and insulation is thermography — the same technique used on electrical and mechanical installations, applied here to the furnace's thermal envelope.

Burner cleaning and adjustment

Burners are kept in good condition by controlling three things: how clean they are, scale buildup, and the response of the mechanical systems. The first two are controlled with additives — dispersant-type additives prevent fouling and scale, while solvent-type ones are used to clean an already dirty burner.

Before every ignition it is essential to purge the firebox to prevent explosions: the purge consists of sweeping with air, without letting fuel in until any accumulated explosive gases have been completely cleared.

Once running, you need to check that the burner ignites instantly and keeps running without interruption — for that, a flame detector is used that responds to any anomaly by cutting the fuel supply.

The most frequent routine operations on the burner are:

• Frequent, periodic inspection of the nozzles
• Frequent, periodic inspection of the flame control system and the purge system
• Cleaning dirty parts
• Not changing the nozzle orifice size or the atomization angle
• Maintaining and checking the specifications of the mechanical systems

Combustion control

The goal of combustion control is to get maximum efficiency. The points to control are the excess-air coefficient, unburned fuel, ash and the flame detectors.

The basic defects that ruin combustion are: insufficient air, uneven distribution of fuel and/or air in the combustion zone, insufficient temperature in that zone, and insufficient turbulence to achieve a good air-fuel mix.

Controlling the flue gases

This is done periodically with a gas analyzer, which gives information on both the combustion process and any infiltrations or dilutions. The flue gas parameters to control are: gas flow, flue gas temperature, % O2, % CO2, CO (in ppm) and solid particles.

An excessively high flue gas temperature can have several causes: excess draft, an undersized firebox, a defective combustion chamber, excess combustion, a poorly adjusted draft regulator, or insufficient flue gas travel.

What this sequence of checks is for

The order matters: the purge prevents an explosion before ignition, the flame detector cuts the fuel if the flame fails during operation, and combustion and gas control optimize efficiency once the burner is already running safely. These are three different layers — safety before ignition, safety during operation, process efficiency — and none replaces the other two.

Benefits

• The purge and the flame detector are simple, low-cost safety measures against the risk they prevent (a firebox explosion)
• Gas analysis gives an objective reading of combustion efficiency, instead of judging it only by the flame's color or size
• The causes of high flue gas temperature are already classified — this speeds up diagnosis when gas analysis shows a problem

Limitations to keep in mind

• The gas analyzer and the thermographic camera are instruments that need their own calibration and maintenance — they are not a free visual inspection
• Dispersant and solvent additives treat the symptom (dirt, scale); if the underlying cause is a persistently poor combustion adjustment, the problem comes back
• The 5-operation list is for burner furnaces — electric arc furnaces follow a different maintenance logic, not covered here

In summary

In a burner, efficiency control and safety control are the same routine: purging before ignition, watching the flame during operation and analyzing the gases to adjust combustion are steps that, besides saving energy, are what keeps a furnace or boiler from turning into a hazard.