Absolute combustion heater outlet temperature

One definition of the point of absolute combustion is the point of maximum heater outlet temperature for a given amount of fuel, for any given furnace or heater (as illustrated in Fig. 20.3). Following this we... 

Figure 20.3 Point of absolute combustion in terms of heater outlet temperature.

Automatic operation linked to process outlet temperature while on the bad side of absolute combustion is potentially hazardous because the heater outlet temperature will drop as a result of the reduced heating efficiency of the fuel. The automatic control will then call for more... 

The heater outlet temperature declines as air flow is diminished past the absolute combustion point. That is, products of the partial combustion of hydrocarbons (aldehydes, ketones, CO, etc.) are formed, and the heating value of the fuel gas is effectively reduced. 

The point of absolute combustion in terms of either maximum heater outlet temperature or minimum fuel fired for a given outlet temperature would correspond to the same flue-gas oxygen content for a given furnace at the same moment in time. However, we must also note that this flue-gas oxygen content that corresponded to the point of absolute combustion on, say, heater 3304-A on Tuesday, January 2,... 

If we have the furnace on automatic temperature control while we are not using enough combustion air, and if the control valve on the fuel gas then opens to allow more fuel to the burners in order to increase the furnace outlet temperature, the extra fuel will not burn efficiently. In fact, the extra fuel is likely to reduce the heater outlet temperature rather than increase it, as there is already a shortage of air and it cannot burn properly and tends to cool the firebox. The automatic temperature controller then senses the reduction in heater outlet temperature and increases the fuel rate. Thus the furnace will spiral into an increasingly dangerous condition as the outlet temperature continues to fall and the furnace is left on automatic control. The way out of this situation is to put the furnace on manual control and manually reduce the fuel gas back to the point of absolute combustion. 

To determine if you are running above the point of absolute combustion (i.e., the safe area), observe the heater outlet temperature as you manually reduce the fuel-gas rate. If this temperature increases, then you are still on the wrong side of the point of absolute combustion. If this temperature decreases, then you are on the safe side of the point of absolute combustion. Do not open the burner air registers until you are on this safe side. Increasing air flow when operating in a fuel-rich environment may make the firebox explode. 

Automatic operation linked to process outlet temperature while on the bad side of absolute combustion is potentially hazardous because the heater outlet temperature will drop as a result of the reduced heating efficiency of the fuel. The automatic control will then call for more fuel, which, in turn, produces more oxygen starvation. The only way out of this situation is to put the furnace back to manual operation and manually increase the combustion air or reduce the fuel to get back to the good side of the point of absolute combustion. 

The heater process outlet temperature declines as airflow is reduced past the point of absolute combustion. In this situation we have products of incomplete or partial combustion such as aldehydes, ketones, and carbon monoxide going up the stack. This sets the heater up for afterburn in the stack, and the heating value of the fuel is also effectively reduced. 

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