Firebox oxygen

The only correct oxygen target is that firebox oxygen content which maximizes the process-side heat absorption for a given amount of fuel, or if you prefer, we could say it is that the firebox oxygen target which minimizes fuel consumption for a given process-side heat absorption. 

The answer is it will always be more than the firebox oxygen content, because there are always air leaks in the convective section and stack that, with the slightest negative pressure in the stack or convective section, will allow air to leak in. These air leaks are collectively known as tramp air. ... 

Suppose we put the air registers back as they were and pinch on the stack damper instead. So let s suppose we restrict airflow via the stack damper until the firebox oxygen goes down from our base case value of 6 percent, to 3 percent. The oxygen in the convective section will also be reduced to, say, 5 percent. In this case, we see that the A02 is also much... 

The answer, of course, is to allow more air into the firebox, and thus generate more flue gas. The firebox oxygen was increased from 3 to 6 percent, which reduced the firebox temperature to its prerevamp state. This, in turn, increased the pounds of flue gas flowing through the convective section and increased the heat absorbed in the convective section. 

The answer, of course, is to allow more air into the firebox and thus generate more flue gas. The firebox oxygen was increased from... 

If you try to operate a furnace, fired heater, or boiler with too little combustion air to starve the burners of oxygen to smother or bog down the firebox, then you will likely cause afterburn or secondary combustion in the stack, you will not be able to operate on automatic temperature control, and may even destroy the equipment altogether. 

Regardless of and in preference to oxygen analyzer results, we must consider the appearance of the firebox and flames when we assess whether more combustion air is needed ... 

To be just right, the firebox should have a very slight haze and all the flames should be compact and not searching around the firebox looking for oxygen. 

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 ... 

The oxygen measured in the stack is the sum of the unused oxygen from the firebox plus the oxygen from tramp air, drawn into the convective section and stack. 

Unfortunately, many heaters have oxygen sensors in the stack and not in the firebox. An oxygen sensor in the firebox may provide a useful guide, as we mentioned earlier, although it is not actually needed to run the heater efficiently. However, an oxygen analyzer in the stack or... 

In view of the above, it is clear that if the combustion of airflow is being adjusted on the basis of the oxygen content of the stack or convective section gases (i.e., from readings taken on an oxygen analyzer with probe located in the stack or convective section), it is very likely that there will be afterburn or secondary ignition in the stack or convective section. This scenario was discussed earlier in this chapter in the section on secondary combustion and afterburn. After all, the fire is supposed to be contained within the firebox, and not in the convec-... 

Minimize poor lateral air distribution in the firebox by adjusting air registers on individual burners. Having a low airflow in one part of the heater will lead to higher overall oxygen requirements. 

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. 

The heat of combustion is a product of the amount of fuel consumed and the net heating value of the fuel. The heater s efficiency is a function of the flue-gas stack temperature, the excess air or oxygen, and the ambient-heat losses from the firebox and the convective-section structures. 

When a heater tube fails, the process fluid spills out into the firebox. Let s assume that the process fluid is a combustible liquid. Will the leaking process fluid burn The answer is, mostly not. There is probably not enough excess oxygen in the firebox to support a substantial amount of additional combustion. 

What will happen is that flames and black smoke will pour out of the heater s stack. It looks very bad, very dangerous, but it is really not. There is not enough excess oxygen in the firebox to cause a very high temperature. The combustion gases, or flue gas, are too fuel-rich to explode. We say that the flue gas is above the explosive region. 

Mixed with fresh air flowing through convective section leaks, the flue gases may reignite. This phenomenon, called afterburn, results in damage to the convective section finned tubing. Afterburn is promoted by insufficient oxygen in the firebox, excessive draft, and leaks in the convective section exterior walls. 

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. 

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