“Afterburning” is a term that is used to describe abnormally high temperatures that occur in the dilute phase, cyclones, or overhead line of an FCC regenerator. This phenomenon is also referred to as “post-combustion”.
Basically, afterburning is the result of combustion in the flue gas leaving the dense bed, of CO that is leaving the bed unburned, with excess O2. Ideally, in a full-burn regenerator, all the carbon in coke is burned essentially completely to CO2, with only a trace of CO remaining in the flue gas. However, many regenerators do not operate at ideal conditions, and are therefore subject to afterburning. This can also occur in a partial-burn regenerator, where there is a high CO content, but where there should not be any significant amount of O2 in flue gas.
The most severe afterburn is usually seen between the dilute phase and the cyclone outlets. The reason for this is that most of the entrained catalyst has been removed in the cyclones, and therefore a significant “heat sink” has been removed. Thus, for the same amount of CO burning, the temperature rise is much greater, because the heat capacity of the “cleaned” flue gas is much lower than that of the flue gas which contains catalyst.
Primary Causes of Afterburning
These are some reasons why afterburning occurs in a regenerator:
- Inadequate combustion kinetics
- Maldistribution between coke and combustion air
- Efficiency of mixing between catalyst and combustion air
In some units, more than one cause may be present simultaneously.
1. Combustion Kinetics
Completeness of combustion of C to CO2 depends largely on the conditions that exist in the dense bed. For example, too low a regenerator temperature can promote the occurrence of afterburning. Other factors are low residence time or poor mixing in the bed. If any or all of these are not adequate, some CO can leave the bed with excess O2 (which is normal), and that CO burns in the upper parts of the regenerator.
2. Maldistribution in the Bed
Just because a full-burn unit maintains an excess of O2 in flue gas does not mean that afterburning cannot take place. This excess O2 is measured after any afterburn has already occurred. Somewhere in the vessel, there is sufficient excess O2 to support the combustion of any CO that has left the bed. If the distribution of air in the bed were to exactly match the distribution of coke (on catalyst), combustion would be perfectly uniform, and no significant amount of CO would leave the bed. It is only when that distribution is not perfect that afterburning can occur. Thus, CO can be exiting part of the bed where there is not sufficient O2, while from another part of the bed the flue gas exits with surplus O2. These then can combine downstream in the regenerator to result in the phenomenon of “afterburning”
Although not as common, this can also occur in a partial-burn regenerator if the maldistribution is particularly bad.
3. Mixing Efficiency
This is a fairly obvious possible contributor, which can be affected by many factors, many of which are a function of the specific unit design.
Although a certain amount of afterburning is normal in most FCC regenerators, it can become critical if the temperatures approach or exceed the mechanical or metallurgical design conditions of regenerator components, such as cyclones. Some actions can often be taken to lessen the degree of an afterburning problem.
If the cause is likely to be one of inadequate combustion kinetics, it may be possible to operate at higher bed temperatures, or greater bed depth (residence time). Alternatively, the use of CO combustion promoter can be considered. Promoter “speeds up” the combustion.
If the cause can be attributed to maldistribution between coke and combustion air, it may be possible to either:
- improve the catalyst distribution (some designs are available in the industry), or
- change the air distribution to the bed to more nearly match the distribution of coke. An analysis of temperature profiles from the dense bed, through the dilute phase and cyclone outlets, can assist in determining how and where to make the changes in air distribution.