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. 

Allowing a fired heater, boiler, or furnace to operate with insufficient air is hazardous because 

Consider a forced-draft boiler producing 600-psig steam as shown in Fig. 20.2. The fuel rate on this boiler is fixed and we are going to optimize the oxygen (02) content of the flue gas by adjusting the speed of the forced-draft fan. Do we simply adjust the forced-draft (FD) fan to give 2 percent 02 in the stack because someone once said that 2 percent 02 in the stack was a good number  

The term absolute combustion is not the same as complete combustion. Complete combustion is a theoretical term, implying a theoretical goal that we might aim toward but will never quite reach on any real process heater, furnace, or boiler, whereas the point of absolute combustion represents the best achievable efficiency point of any such piece of equipment on any day of the week, at any hour or minute. 

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 

No We are going to adjust the speed of the FD fan to produce the maximum amount of 600-psig steam. In other words, we are going to 

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, 

---

It is not possible to operate on automatic temperature control on the wrong side of the point of absolute combustion. 

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

Did the oxygen analyzer really help us find the point of absolute combustion for the forced-draft boiler No, it did not. However, once you have found the point of absolute combustion and then noted the corresponding flue-gas percent 02, as long as all operating conditions remain constant for the heater (and in reality the longest time you could hope that all conditions would truly be constant would be /2 h), then you could use the percent 02 in the flue gas as a rather secondary... 

This type of operation can also be described as operating on the wrong side of the point of absolute combustion. Take another look at Figs. 20.3 and 20.4 we are talking here about the portion of the curves marked as bad operation. What would be the color of the flue gas as it emerged from the stack during this type of operation Would it be black ... 

If we do not ensure that we operate some extra air to put us on the good side of the point of absolute combustion, then we run the risk of getting into oxygen starvation on the wrong side or bad side of the point of absolute combustion. 

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... 

Note that combustion air is relatively inexpensive compared to fuel it is irrational to cut back on combustion air and waste fuel by inefficient burning. Also note that to operate fired equipment efficiently, we need to heat-prove it, or test to find the point of absolute combustion, on a regular basis, preferably at least once a day. 

Suppose we have a natural-draft heater operating very efficiently on the good side of the point of absolute combustion. The oxygen content of the firebox gases (just below the shock tubes) is 2.5 percent oxygen as shown in Fig. 20.5. What do you think the oxygen content of the flue gases in the stack will be ... 

May reduce combustion air so that it falls below that required for absolute combustion, resulting in destructive afterburn. (Note air preheaters themselves are often destroyed by afterburn.)... 

The increased combustion airflow was needed not for combustion, but to transfer heat from the radiant section (firebox) to the convective section. This is what we call heat balancing. In this situation, oxygen requirements to reach absolute combustion become irrelevant as we are now operating with a very plentiful supply of oxygen. 

Operating a process heater simply to achieve a minimum excess oxygen target in the flue gas can waste a great deal of energy. The proper way to adjust O2 to a heater is to target for the point of absolute combustion, as shown in Figure 15-5. The point of absolute combustion is defined as that air rate that maximizes heat recovery to the process. That is, either a decrease or an increase in the combustion air supply will reduce heat absorbed in the heater. 

The primary objectives, in the operation of a fired heater are to avoid excessive heat density in the firebox, maintain a small negative pressure below the bottom row of convective tubes, and obtain complete combustion of the fuel in the firebox with minimum oxygen in the flue gas. Regardless of the flue-gas oxygen content, however, the point of absolute combustion is the combustion air rate that maximizes process-side heat absorption for a given amount of fuel. 

The oxygen target required to achieve the point of absolute combustion may be 2%, or it may be 6%, depending on the operating characteristics of the heater. Often, energy is wasted because operators and engineers fail to understand ... 

For inslance, in excess of 10% of the heating value of methane can be lost in this manner, while the stack gases remain clear. However, if heavy fuel oil is burned, the flue gas will quickly turn black as the combustion air rate falls below the absolute combustion point. For an extreme contrast, the combustion of hydrogen will never turn a stack black, regardless of combustion air insufficiency. 

One of the more common factors causing high oxygen levels in flue gas at the absolute combustion point is poor air-fuel mixing. To promote air-fuel mixing, the following parameters should act as a guide ... 

FIGURE lS-5 Absolute combustion point (courtesy Oil Gas Journal)... 

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. 

When the air register is closed to suppress air flow below that required for absolute combustion, black smoke will not immediately issue from the stack. Depending on the hydrogen-to-carbon ratio of the fuel, varying amounts of the potential heating value of the fuel may go up the stack as products of partial combustion before the flue gas turns black. 

The conclusion from the preceding example is that excess air is often used to prevent overheating of the combustion zone, and the oxygen requirements to reach the absolute combustion point are, in this case, irrelevant. 

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. 

---