How do burner combustion control systems work?
Gas and oil burners are everywhere. They power package boilers, start up larger furnaces with fluidized beds and grates, and heat many other processes. To understand how to tune gas and/or oil burners, it helps to first understand how and why they work the way they do.
Gas and oil burners are everywhere. They power package boilers, start up larger furnaces with fluidized beds and grates, and heat many other processes. Larger burners have combustion control systems (CCSs) that should be tuned periodically.
To understand how to tune gas and/or oil (usually #2 diesel oil) burners, it helps to first understand how and why they work the way they do.
What is a combustion control system?
Larger burners are controlled with a combination of a CCS and a burner management system (BMS). The BMS determines if there will be a fire or not, and is primarily responsible for shutting down the system if conditions become unsafe, as well as enforcing purge requirements on restart.
The CCS determines how much fuel, air, and water to put into the boiler, and should prevent unsafe conditions from happening in the first place. This post does not cover the BMS; it focuses primarily on the fuel and air aspects of the CCS (rather than water and other auxiliary components).
This post also does not apply to jackshaft burners, which have a single actuator driving both the combustion air damper and the fuel valve. The fuel/air ratio (FAR) for jackshaft burners is set by mechanically adjusting set-screws on the fuel valve-the CCS sends a single output to the actuator to drive both fuel and air together.
Bumpless transfer with setpoint and output tracking
A boiler CCS's proportional-integral-derivative (PID) loops and auto/manual bias (A/M/bias) stations should generally be configured to track as much as possible. Setpoints (SPs) should track process variables (PVs) when not in auto. Cascade master outputs should track slave SPs if the slave is not in cascade (auto output and remote SP). This is important for bumpless transfer, so that loops can be switched between manual, auto, and cascade without bumping the boiler.
I recommend only two exceptions to forcing everything to track:
- The SP for drum level-please see my earlier blog post on Optimizing drum level measurement for details on SP ramping.
- The O2 trim output should generally just be forced to the center when the air loop is in manual. This will cause a "bump" in reported airflow percentage on the transition from auto to manual, but that does not cause a real bump; the mass-flow of air is unchanged, and there is no bump when returning to auto.
Plant master and boiler masters
The plant master is a pressure control PID loop with the main steam header pressure as PV, and the output setting the firing rate of all boilers running in auto. Each steam header should have no more than one active plant master; this means most facilities have just one plant master. Each boiler has a boiler master, which is an A/M/bias station that handles the firing rate for that boiler. In AUTO, the boiler master receives the plant master output, biases it, and sends that percentage to the fuel and airflow controllers. In MANUAL, the boiler master output is adjusted by the operator. But if the fuel is not in cascade, the boiler master output tracks the fuel flow SP. If the boiler master is in manual or tracking, its bias tracks the difference between it and the plant master output.
If there is only one boiler, the output of the plant master is the boiler master and no bias is needed.
A piecewise characterization (PWC) is a function block in the CCS that approximates a smooth curve by stringing together many short lines. Typically, the PWC block defines the line segments with from 10 and 21 pairs of (X,Y) points. The block figures out which line segment the input is on and produces the output based on the two Y values surrounding that line. The FAR, O2 trim setpoint, and output linearization curves are contained in PWC blocks.
What were they smoking?
If you are experienced with process controls, but this is the first time you've seen the standard scheme for combustion air for a gas/oil burner, you probably think my logic drawing above is wrong. You might think, "Why would any sane engineer mess with the PV so extensively? Why not set the FAR by adjusting the airflow cascade setpoint?"
That was my initial reaction too, but the drawing is correct and there is a good reason to do it that way: cross-limiting (more on that below).
The idea is this: The system needs to control fuel and air as "the amount of fuel and air needed for this percentage load." So we convert fuel flow to percent by multiplying it by a constant. We convert the airflow to percent by running mass flow through a PWC to get raw percentage, then trimming that percentage from the output of the O2 trim loop. The resulting percent fuel and airflows will be controlled to match the boiler master output percentage.
You should think of an airflow 5% below SP as "the airflow appropriate for when the boiler master is 5% lower" rather than "5% less air than I want." That is a subtle distinction, but the resulting numbers may differ unless the FAR curve is perfectly straight.
In the absence of a proper air mass-flow meter, the windbox pressure is sometimes used as an indication of volumetric airflow, and is fed into the FAR PWC instead of flow. Otherwise the logic is the same.