Airflow and Oxygen Trim
Forced draft fans are typically placed in automatic after the ID fan master is placed in automatic. Usually, the FD fan master is only controlling airflow; however, some boilers are designed with secondary airflow dampers that control the airflow. In this case the FD fan will control the secondary air duct pressure to the dampers (Figure 2).
2. Favorite trend. I typically monitor airflow, O2 content in the flue gas, and furnace pressure control when I tune airflow. The particular response of those variables was observed after a 20% load increase in coordinated control mode. Source: Tim Leopold
Air and, consequently, O2 control are critical to the safe and efficient operation of a boiler. The airflow signal is normally measured in terms of a percentage and is usually not available in volumetric or mass flow units. The obvious question is, "Percentage of what?" The answer is the percentage of airflow that is available from a given fan or system of fans. The actual measured pounds per hour of air does not matter, because air is free, and the final arbiter of proper airflow is the O2 content in the flue gas (gases leaving the furnace). Because of variations in coal heat content, air temperature, and combustion conditions inside a boiler, we ensure proper burning by measuring the amount of oxygen content in the flue gas, commonly referred to simply as O2.
Pulverized coal has an interesting property: Under certain conditions of heat in a low-oxygen atmosphere, coal can self-ignite or even explode. Therefore, personnel safety and equipment protection require boiler operators to maintain excess O2 in the flue gas. The amount of excess O2 is determined by the load on the plant and the type and design of boiler. Typically, the load signal used is steam flow. In any coal-fired boiler, airflow demand is a function of the boiler firing rate or boiler demand (Figure 3). Gas- and oil-fired boilers have lower O2 requirements at higher loads.
3. Extra air is a good thing. A typical O2 set point curve for a coal-fired plant is a function of boiler firing rate or boiler demand. Minimum levels of air are required so that reducing conditions in the furnace never occur. Source: Tim Leopold
The term cross-limiting refers to the function of fuel flow that limits the decrease in air demand and the function of airflow that limits the increase in fuel demand. When decreasing load, the air demand follows its lag function and the fuel demand follows the boiler demand to ensure that there is always more air than fuel going into a furnace so explosive conditions never develop inside the furnace. When increasing load, the opposite is true. This is truly an elegant piece of logic.
The output from the boiler master is the boiler demand. Cross-limited air demand is developed by choosing the highest of four calculated values: boiler demand function, the lag of the boiler demand signal, a minimum value (per the boiler manufacturer under the NFPA codes), and a function of the actual fuel flow. The cross-limited fuel demand is selected from the least of three signals: boiler demand function, a lag of boiler demand, and a function of actual airflow. When load is increased, air demand follows the function of the boiler demand and the fuel demand follows its lag of the boiler demand.
To develop the air demand for your boiler, hold your O2 trim controller in manual at 50% output. At a low, medium, and high load, place your FD fan master, or secondary airflow dampers (if the boiler is so equipped), and your fuel master in manual. Then manipulate the airflow until you find the amount that satisfies your O2 set point requirement, using stack opacity as a reality check on the O2 set point. Next, manipulate the airflow characterization curve as required to allow the air demand to equal or slightly exceed the fuel flow or boiler demand. Record the airflow required for that fuel flow and then move on to another fuel flow setting. Three points should be sufficient for a good airflow curve.
Typically, the airflow measurement is a differential pressure taken in air ductwork and requires a square root in order to make it linear. Ensure that your signal is also temperature-compensated. Each boiler should have an airflow characterization curve that should be a virtual straight line. If it isn’t, I would be concerned about unexplained "correction factors" or "magic numbers" that should not be necessary.
Next, the characterized airflow is multiplied against a function of the O2 trim controller. The O2 trim control loop uses the set point curve, discussed above, plus an operator bias to calculate an O2 set point for various loads. This set point is compared with the O2 content of the flue gas used by the control system. It is best to have several O2 measurements because of striations or variations of temperature and oxygen that are present across the stack cross-section.
Different plants use different measurement schemes, selecting the average, the median, or the lowest measurement to control. O2 trim is designed to be a steady state trim of the airflow. If you, or your tuner, are trying to control airflow with the trim controller, stop it. The O2 trim controller should be mostly integral action with very little proportional and no derivative gain. Your time is better spent reworking your air demand curves or airflow characterization than attempting to tune the airflow using the O2 controls.
The output from the O2 trim control station then goes through a function generator such that a 0% to 100% input signal equals a 0.8 to 1.2 output signal. This value is then multiplied against the characterized airflow. This means that the O2 trim controller can adjust the airflow ±20%. In some extreme cases this amount can be varied, but for most boilers ±20% is more than sufficient. The final result is a signal referred to as "O2 trimmed airflow." This value is then used by the airflow controller to modulate the ID fans or dampers.
Because O2 trim control uses a primarily integral-only controller, it does not have the dynamic capabilities of most controllers. As a result, there are times when the controller should not be allowed the full range of control. At low loads, typically less than 30% to 35%, output from the O2 trim controller should not be allowed to go below 50% but should be limited to some minimum setting so that an air-rich atmosphere is always maintained in the furnace.
Also, when the lag function in the cross-limited air demand is driving air demand, airflow will lag behind. That is, the air will remain elevated for a period of time as the load, and the fuel flow, decreases. As a result, oxygen in the flue gas will spike up. If the O2 trim controller is not limited, the controls would see the O2 go higher than the set point and start cranking, cranking, cranking down. Then, when the load gets to where the operators have set it and the fuel flow is no longer decreasing, airflow demand will catch up with the boiler demand, and the O2 will quickly begin to fall. The controller will see the O 2 falling and begin to crank up. But because there is very little, or no, proportional gain, it will take a long time to bring the air back. This can result in an unsafe or, at the least, a nerve-wracking condition.
The NFPA requires some additional logic for the airflow control loop. There should be high and low furnace pressure logic to block the airflow from increasing or decreasing, as is appropriate. Because this fan forces air into the furnace, on high furnace pressure, the fan should be blocked from increasing speed; on a low furnace pressure signal, it should be blocked from decreasing.
Also, on an MFT there are NFPA and boiler manufacturer requirements that must be considered. One important consideration is the need to hold the air in place for a time after an MFT or if the airflow should drop very low during or just after a trip. The dampers should go to a full open position shortly after the loss of all FD or ID fans (providing a natural draft air path). Moreover, in the typical boiler air control system, if the ID fan is placed in manual, then the FD fan is normally forced to manual. If the FD fan is in manual, then O2 trim is forced to manual.
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