Fuel/Air Cross-Limiting

What is the Application?

Steam supply and heating loads in a boiler plant vary throughout the course of the boiler firing cycle. In response to this, the boiler firing rate output % is continuously adjusting to maintain steam or temperature setpoint. This means that the boiler controller’s outputs (typically a fuel valve positioner, air damper positioner, and variable fan speed drive) are in constant motion to stay on the commissioned “firing curve” or “Fuel/Air Ratio”.

Why is This Needed?

When a boiler control increases or decreases the firing rate command, the fuel/air ratio controller performs a function called “cross-limiting”. The purpose of this is to ensure that the burner never experiences a fuel-rich condition, which could potentially become unsafe. In a fuel-rich situation, the burner is receiving less than the required amount of air for complete combustion. This can lead to unburnt fuel collecting in the boiler furnace which has the potential to ignite and cause a boiler explosion. Additionally, the incomplete combustion will result in higher carbon monoxide emissions from the burner.

Traditional Fuel/Air Control

Traditional fuel/air cross-limiting accounts for this by commanding the air control output to lead the fuel control output when the boiler firing rate is increasing, meaning that the air damper opens first to give the burner the required air for the anticipated fuel, then the fuel valve follows bringing both outputs on to the commissioned curve. Conversely, when the boiler firing rate is decreasing, the air control output “lags” the fuel control output, allowing the fuel flow to decrease first before adjusting the air supply down to meet the required fuel/air ratio.

The drawback to this method is that the boiler controls are never truly on the commissioned firing curve that the technician set up for optimal fuel efficiency and lowest emissions. Ensuring that the burner has more than the required amount of air can reduce the boilers fuel efficiency, increasing the fuel usage for a given heat output. There must be a better way…

The Smarter Fuel/Air Control

Preferred’s BurnerMate Universal boiler control system utilizes servo “position-pacing” logic. Each time the burner goes through a start sequence, the control servos are stroked fully open and closed, and the BMU checks the open position, closed position, and travel speed data for each servo. This allows the BMU to move all servos simultaneously. When the firing rate changes, the BMU calculates the amount of rotation that each servo will need to move and determines which servo will take the longest time to get to its target position. The BMU then adjusts the movement speed of the other servos to match. This gets re-calculated every half second, to ensure that all outputs are always “on-curve” improving combustion stability, efficiency, and repeatability.

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