O2 Trim in combustion PID logic

O2 Trim

The O2 sensor detects the "excess air" - the % oxygen present in the flue gas after combustion. Typically at high loads, 3-4% O2 is desirable (earth's atmosphere is 21% O2).

It is dangerous to put more fuel into a furnace than the air can fully combust. The products of incomplete combustion (synthesis gas is H2 and CO) or unburned fuel could fill a space in the furnace or ductwork, then later mix with air and explode. Insufficient air also results in environmentally undesirable emissions.

Too much excess air is a major cause of inefficiency. We want to heat water, but we must bring cold air (mostly nitrogen) into the furnace and exhaust warm air. The more air we blow through the furnace, the more fuel we burn just to heat nitrogen to blow out the stack instead of heating water. Excess air can also produce excess NOx in some burners.

So we want to control O2 at its ideal amount for the load. The burner manufacturer should provide an O2 trim SP curve - this should be plugged into a PWC with the output running through a bias station (always in auto!) to set the SP of the O2 trim PID loop. Operators should NOT have the option to set the O2 Trim SP - they can put the loop in manual, but in auto, the SP should always come from the biased SP curve. The O2 trim loop will then adjust the air flow PV, causing the air flow loop to add or subtract air to maintain it at the same % as the fuel flow % and boiler master %. Burner O2 trim is typically limited to adjust the air by only ±10% to ±20%.

This trim requires a bit of math in the CCS logic. There are three common ways to handle this:

1. The old-school method was to have an O2 trim loop with an output scaled 0-100%. 50% is "neutral" or "centered" - no adjustment to the FAR output. The output is clamped to something like 40-60%, and that output is added to 50% (or subtracted from 150%) and then multiplied by 0.01 and the air flow FAR PWC output for the flow loop PV.
2. Some modern systems have the output limited to 0.8 to 1.2, with 1.0 being centered. The output of the air flow FAR PWC can then be multiplied by the O2 trim output to set the air flow PV. This is the most simple scheme to program.
3. Other modern systems have the output limited from -20% to +20%, with 0% being centered. The output is then divided by 100%, added to 1 (or subtracted from 1), and multiplied by the air flow FAR PWC output to set the air flow PV. This is a bit more complicated to program, but is more natural for operators to understand, since zero is centered.

The direction of the O2 trim loop depends on how the internal math is configured - some are direct acting (raising their outputs in response to high or rising O2), while others are reverse acting.