Stacks afterburn

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. 

The products of incomplete combustion are hot and just waiting to catch fire and will ignite as soon as they find sufficient oxygen. This usually results in afterburn in the convective section or stack and can even lead to explosive detonations. 

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. 

When the combustion air was cut back this time, fire started to come out of the stack. Now Operator A was forced to admit that they would have to increase the combustion air again. The problem they then faced was that the amount of draft in the heater seemed to be less than before and they were not quite able to reestablish the same airflow. The reason for the restricted airflow was that they had caused afterburn in the... 

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-... 

Conversely, as we open the stack damper, the pressure at the convective-section inlet will decrease, so that we have more draft. If we have too much draft, we will increase the risk of afterburn as we are increasing the rate of air in-leakage in the convective section and stack. 

There is incomplete burning of the CO to CO2 in the dense phase of the catalyst bed in the regenerator. However, in the dilute phase above the bed, this reaction proceeds further. Since the CO + 5O2 — CO2 reaction is very exothermic, there is an increase in temperature between the catalyst bed and the stack gas. This is called afterburning. If the stack gas temperature gets too high, there may be thermal damage to the cyclones. 

Air emission problems of breweries are relatively minor since mass emission rates are low and discharges are largely nontoxic. Emission control measures focus on containment of process odors. Water scrubbers are used on brew-house stacks to eliminate this potential problem area [7]. If spent yeasts and grains are dried on site, the resulting odors are controlled by afterburner pyrolysis before the dryer vent stacks [18]. 

During operation, the TDU s operating parameters are measured and recorded to ensure correct operation and to prevent accidental releases of contaminants. While there are several possible parameters to monitor, the following are recommended where applicable treated soil exit temperature, vacuum in the TDU, pressure drop in the APC, waste feed rate, afterburner temperature, off-gas exit temperature from the TDU, stack gas velocity and temperature, and the flow rate and pH of acid-gas scrubber liquor. ... 

While seeking approval to by-pass the lock out of the burners and restart the afterburner, the common stack automatic continuous air monitoring system (ACAMS) alarmed at 11 26 PM. The site was immediately masked. A depot area air monitoring system (DAAMS) tube was taken for analysis at 11 38 PM and another put in its place. ACAMS readings as high as 3.63 allowable stack concentration (ASC) were obtained. The furnace was bottled up (dampers closed to slow airflow) at 11 44 PM. By 12 18 AM on May 9.2000 the ACAMS had cleared and the order to unmask given. 

A 45-stack baseline system test (fuel cell stack and reformer) was conducted to demonstrate stability and high H2 utilization (75%-80%). This was followed by an 80-stack design test at full power for over 45 minutes, resulting in high H2 utilization. Cold start of the reformer without afterburner was completed. 

One way to minimize the possibilities of afterburn is to pinch back on the stack damper and further open the burner air registers (Fig. 15-6). The resulting higher pressure (low draft) reduces the rate of air into the convective section tube banks. 

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