Excess Air Effects
Burners are one source of excess air in heaters. All plants watch excess air because too much means wasted fuel. The wasted fuel is that required to heat the excess air from ambient temperature to flue-gas temperature leaving the stack.
However, excess air isn’t necessarily a problem. It depends on the flue gas temperature in the stack. If the flue gas were cooled to ambient temperature, then the thermal efficiency would be 100% regardless of the amount of excess air.
As long as the stack temperature is low, most of the energy is extracted from the flue gas no matter how much excess air there is. For example, at a stack temperature of about 400°F, an increase in excess air from 10% to 25% produces less than 1% decrease in the efficiency of fuel use in the heater. Even at a stack temperature of 700°F, a 10% increase in excess air (2% excess O2) produces only about 1% reduction in fuel efficiency. (See Figure 200-10.)
But the difference between 400°F and 700°F accounts for about an 8% reduction in the efficiency of fuel use in the heater. The hot flue gas is the real problem, not excess air. However, the needed waste heat recovery equipment costs a lot of money.
So we do what we can with excess air. But if you are running into operating difficulties, don’t compromise heater operation by obsessive dedication to reducing excess air.
Nevertheless, “low” excess air burners are available today. So if you have old burners and you know they are the source of excess air, a change to new burners, together with a furnace uprate, is worth considering.
How much excess air is “too much?” A practical maximum for good combustion is about 10% for most fired heaters. With the burners well trimmed, too much excess air at the arch may be symptomatic of wasted fuel.
In some cases, however, low excess air may not provide enough cushion against abrupt changes in fuel gas heating values, and low excess air is difficult to maintain when a furnace has many burners to keep balanced. Again, if you are running into operating difficulties, it is unwise to compromise heater operation by excessive dedication to “low excess air.”
Energy economy and low excess air often go together. But with older burners, when you cut back on excess air the flames may become too large, go unstable or even leave fuel unburned. In such cases, a burner change might be the economical choice.
First, though, look for other sources of excess air. Especially in the older heaters in which those older burners are normally found, firebox leakage is significant. Refineries commonly use smoke tests to highlight leaky boxes. A burner change won’t correct fuel waste caused by air leaks in the firebox.