Combustion Control Strategy


Thread Starter


What is the best control system strategy for combustion control in a oil fired boiler? Oil flow, air flow, online oxygen analyser signals are available. At present we are using a master steam pressure controller with a multiloop programmable controller for combustion control. Oxygen trim control is being used for correction in the air flow damper.But results are not very good.


Assuming there are three boilers generate steam individually and send to a main header. Main header Pressure decides how much fuel is needed from each boiler (either shared equally or partly depending on the healthiness of individual boilers). This is called load sharing. This is done with individual load sharing ratio relays. Master Pressure controller's output is the input of all these ratio relays. Their output have become automatically the setpoints of fuel oil controllers, which in turn drive the fuel valves. These ratios can be varied
while in line, to alter load sharing ratio.

A sudden closure and immediate recovery of fuel oil controller output, may put off the flame and continue to flow fuel inside furnace. This is explosive and dangerous. High signal Selectors are used to choose between fuel controller output and a biased signal for minimum fuel valve opening.
Dear friends,

We are not able to go down on the oil/steam ratio. hence seeking a better control strategy from those who have experience in combustion control system with oxy trim control.

Fabian Monino

Given your instrumentation, the best control strategy is full-metered cross-limited control with O2 trim. Drum pressure controls the firing rate.

A lot of energy is lost up the stack. An economizer recovers energy from the flue gases and provides a significant improvement in the boiler efficiency (oil/steam ratio).

O2 trim is an optimization of the combustion controls. It reduces energy lost by minimizing the amount of combustion air used. You should see a few percentage points improvement.

At time of commissioning, the air/fuel ratio curve is established at several load conditions. The O2 trim adjusts the ratio by 10-15%. This prevents creating an unsafe condition. The excess oxygen required depends on the burner. You need just enough excess oxygen to reduce carbon monoxide formation. The amount required may also vary with load. Some O2 trim loops adjust the O2 set point based on steam demand or firing rate.
You do not say if the oil is a heavy fuel oil or bunker oil. If it is, then it will be pre-heated prior to injection. Check if the heater is controlled using a temperature sensor or a viscometer.

If a temperature sensor then you may need to combine strategies of routinely testing the fuel to determine the optimum temperature it needs heating to as this can vary significantly and often due to the quality variation of the fuel.

Even so you may also need to run excess oxygen and periodically do a flame inspection.

If you are using a viscometer it will automatically compensate for fuel quality variation by responding to viscosity changes by controlling the fuel heater to maintain the optimum viscosity.

Viscosity is important to droplet size, dispersion, mixing with the air flow and burning in the optimum part of the boiler.