Burner Combustion Control for Boilers

Excess air regulation

In actual practice, gas, oil, coal burning and other systems do not do a perfect job of mixing the fuel and air even under the best achievable conditions.

Additionally, complete mixing may be a lengthy process. Figure 1 shows that in order to ensure complete combustion and reduce heat loss, excess air has to be kept within a suitable range.

The regulation of excess air provides:

• A better boiler heat transfer rate
• An ‘advance warning’ of flue gas problems (excess air coming out of the zone of maximum efficiency)
• Substantial savings on fuel

Burner modulation

Modulating control is a basic improvement in controlling combustion. A continuous control signal is generated by a controller monitoring the steam or hot water line.

Reductions in steam pressure or hot water temperature lead to an increase in firing rate. The advantages of introducing burner modulation in combustion control include:

• Fuel and air requirements are continuously matched to the combustion demand
• Steam pressure or hot water temperature is maintained within closer tolerances
• Greater boiler efficiency
• Weighted average flue gas temperature is lower

Air/fuel cross-limiting

A cross-limiting combustion control strategy ensures that there can never be a dangerous ratio of air and fuel within a combustion process. This is implemented by always raising the air flow before allowing the fuel flow to increase, as shown in Figure 2, or by lowering the fuel flow before allowing the air flow to drop.

Figure 3 depicts a simplified control block diagram of the cross-limiting combustion circuit. Combination firing of multiple fuels simultaneously can also be easily accommodated within the scheme.

Cross-limiting combustion control is highly effective and can easily provide the following:

• Optimization of fuel consumption
• Safer operating conditions by reducing risk of explosion
• Fast adaptation to variations in fuel and air supplies
• Satisfaction of the plant demand for steam

Total heat control

In situations where combustion is not the principal heat source and when several factors contribute to the total heat to be generated by a boiler, a control loop can be introduced in order to monitor and manage the generated heat. This is particularly true for CHP plants, where gas turbines and supplementary firing are used. This type of implementation is shown in Figure 4: