Determining an O2 Target Range
Targeting is the process of determining the stack O2 level to use during normal furnace operations that allows for maximum furnace efficiency. The targeting process is shown in Figure 1000-2. Targeting is accomplished in three phases:
• burner balancing
• measuring stack O2 level at CO breakthrough
• defining an appropriate O2 operating cushion
The theoretical optimum level for efficiency is 250 ppm CO and 0.5% combustion O2. However, this theoretical optimum O2 level is that which would be measured directly above the flame.
The actual optimum O2 for a real furnace will likely be higher than 0.5% for two main reasons:
1. air leaks in the furnace
2. burner imbalance or bad burners
Air Leaks. Air leaks can increase the stack O2 reading, i.e., burners can be starved for oxygen while stack readings show excess air because air is leaking into the furnace (as most furnaces operate with negative pressure).
Burner Imbalance. Different burners in the same furnace can operate at different excess air levels. This is caused by:
• differences in air register settings
• corrosion or plugging of burner tips or holes
• improper maintenance of burner tips
• differences in fuel pressure
Burners are balanced by using a portable stack gas analyzer to verify that O2 and CO levels are uniform above each burner in the furnace. The O2 level must be measured above the flames. If the levels are uneven, adjust air registers and clean or repair burners, on-line if possible.
Measuring Stack O2 Level at CO Breakthrough
The next step in the targeting phase is to slowly reduce combustion air in the furnace until CO starts to increase. This increase in CO defines the stack O2 level at which CO breakthrough starts. Once this O2 level is defined, then increase O2 back to normal operating levels.
Defining Appropriate O2 Operating Cushion
Defining the target O2 range involves looking at historical O2 trends to identify two types of changes in the O2 value.
First, it is important to identify the size of any sudden downward changes in the O2 reading. These downward changes are common and can result from a variety of operational changes. The lower limit of the O2 operating range is defined as the point of CO breakthrough, plus the size of these downward changes. This process will insure that while the furnace is operating within the O2 target range, sudden operating changes will not decrease the O2 level below CO breakthrough.
The second change to identify is the small swings and variations in the O2 reading that occur during normal steady operation. The size of the these swings will be added to the lower limit of the operating range to help define the upper limit of the O2 operating range.
Figure 1000-3 shows an actual O2 target range developed for an existing furnace.
When these tasks are complete, targeting will have provided an adjusted, appropriate stack O2 target to use during the tuning phase. Even if a furnace needs some repair, its O2 target range will be appropriately based on the current condition of the equipment.