14 August 2022

combustion control for boiler | different types of combustion control

combustion control for boiler

Combustion Controls :

Combustion controls assist the burner in regulation of fuel supply, air supply, (fuel to air ratio), and removal of gases of combustion to achieve optimum boiler efficiency. The amount of fuel supplied to the burner must be in proportion to the steam pressure and the quantity of steam required. The combustion controls are also necessary as safety device to ensure that the boiler operates safely.

Based on master demand signal for loading and unloading of the boiler, the signal is the set point of each controller of fuel, the signal ratio distribution among all fuel controllers is decided by the boiler operator, as per the availability of different fuels. Each controller of fuel changes the rate of fuel flow. The same master signal becomes set point of airflow controller. The trimming of excess oxygen in flue gas is brought down through Oxygen controller. The amount of Excess Air V/s Boiler Load pattern is different for each boiler manufacture. This graph is fed into excess oxygen controller. Hence based on various load conditions the excess oxygen is controlled
by oxygen control by generating its output to make set point of airflow controller.

Hence during change over of boiler load the master signal give command to (1) Fuel Controller and (2) Airflow Controller. During ‘increasing’ trend of Boiler Load, Air Flow is increased first and after actual increase of Air Flow the Fuel Flow is increased. This is done for assuring that sufficient air in furnace is available before injection of extra quantity of fuel. Similarly when the boiler load is decreasing the fuel is decreased first and Air Flow reduced later. This control system has been designed by introducing feed back/feed forward or lead-lag net after master signal.


Different types of combustion control :

1. On/Off control: The simplest control, ON/OFF control means that either the burner is firing at full rate or it is OFF. This type of control is limited to small boilers.

2. High/low/off control: Slightly more complex is HIGH/LOW/OFF system where the burner has two firing rates. The burner operates at slower firing rate and then switches to full firing as needed. Burners can also revert to the low firing position at reduced load. This control is fitted to medium sized boilers.

3. Modulating control: The modulating control operates on the principle of matching the steam pressure demand by altering the firing rate over the entire operating range of the boiler. Modulating motors use conventional mechanical linkage or electric valves to regulate the primary air, secondary air, and fuel
supplied to the burner. Full modulation means that boiler keeps firing, and fuel and air are carefully matched over the whole firing range to maximize thermal efficiency.

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