Regenerator inventory and tail combustion and oxygen content
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Today, our plant removed some catalyst from the regenerator; as a result, the amount of catalyst stored there dropped from 120 T to 95 T. Realizing that the amount of catalyst in the regenerator was low and that the oxygen content was still around 3.5, I decided to reduce the main air flow rate, thereby increasing the density of the catalyst in that area. This also prolonged the burning time, helped to lower the oxygen content, and reduced the current consumption of the main fan. However, after making these adjustments, tail burning occurred. In order to resolve this issue, I had to increase the main air flow rate even further than before, and I also raised the regeneration pressure; only then did the oxygen content drop to 4.3, and things improved. How is this done?
Your circulation volume remains unchanged, and the amount of carbon deposit entering the regenerator stays the same; by reducing the main air flow, the oxygen content decreases, which leads to afterburning.
After removing a portion of the catalyst, the initial coking temperature drops, resulting in poor coking; furthermore, secondary combustion is likely to occur when there is an excess of oxygen. The OP’s actions can’t be called wrong. When we operate, the main air flow rate is also reduced. At the same time, if the opening degree of the double-acting slide valve is sufficient, the pressure increases. Sometimes, increasing the amount of slag added slightly can also help reduce the temperature difference
When the reserve in the regenerator decreases and the main air flow rate drops, the regeneration efficiency of the dense-phase bed deteriorates, resulting in high levels of carbon monoxide and excess oxygen in the flue gases; this can lead to secondary combustion in the dilute phase of the regenerator or tail combustion in the flue. After you increased the main air flow and the regenerator pressure, the regeneration effect improved significantly; as can be seen from the increase in oxygen content, the carbon monoxide level decreased, thereby eliminating the conditions for secondary combustion or afterburning.
There is a concentration range within which the combustion of combustible gases can continue. The decrease in the main wind speed may have led to an increase in concentration, bringing it within its specified range
Can it be explained in this way: with other operating conditions remaining unchanged, the amount of carbon per unit of catalyst increases after large-scale catalyst discharge. Coupled with a decrease in the main air flow, the burning contact area between the catalyst particles and oxygen is less sufficient compared to before the catalyst discharge. Additionally, as the reserve amount decreases, the onset temperature for burning also drops, resulting in an increase in CO levels in the dense phase, which leads to secondary combustion in the dilute phase. After adjusting the wind velocity, the coking condition of the regenerator improved, the carbon content per unit of catalyst decreased, the contact during coking became more adequate, and the phenomenon of secondary combustion was eliminated
With a decrease in reserve volume and unchanged main air flow, the line speed increases, and the coking intensity also increases accordingly. If the main air flow is reduced, the coking intensity decreases. At the same time, as the amount of catalyst available decreases, its heat capacity drops, and CO cannot burn completely in the dense phase; it ends up in the dilute phase or the flue, where secondary combustion or afterburning can occur.
If the amount of coking does not decrease after the reserve volume falls, then the coking intensity must increase; otherwise, reduced residence time of the catalyst in the regenerator will have an impact on coking. Another type of single-stage regeneration unit with a raw catalyst distributor is prone to secondary combustion when the catalyst inventory in the regenerator decreases, as there is less catalyst above the raw catalyst outlet.
The amount of reserve determines the residence time of the catalyst in the regenerator, which in turn affects the carbon content of the regenerated catalyst. Large capacity, long residence time, low carbon content in the catalyst, low oxygen content ； On the other hand, for a certain main exhaust pressure, the volume capacity is high, the gas velocity in the regenerator is low, and tail burning is less likely to occur.
I think it’s correct to reduce the reserve volume and lower the main air flow rate; then why does afterburning still occur?
The question is by how much to reduce the wind speed; it needs to be done in moderation. After the reserve amount decreases, the wind speed can be reduced, but it should not be forgotten that the amount of material that burns becomes unchanged. \"The reserve amount in the dense phase also increases, and the burning time lengthens.\" At the same time, both the oxygen content and the velocity of the main air stream decrease, which in turn reduces the intensity of burning; as a result, CO is present in the flue gases, leading to incomplete combustion. For example, if the reserve amount decreases from 100 tons to 1 ton while the feed rate remains unchanged and fluidization is not taken into account, can the main air flow be reduced to 1/100 of its original value? Of course not; it’s not an absolute proportional relationship. The balance between the oxygen required for combustion and the amount of oxygen supplied is one of the key factors for a smooth regeneration process. In practice, it is necessary to make moderate adjustments to the operations that have been analyzed; the principles governing these adjustments should be tweaked frequently, through trial and error, rather than making all adjustments at once, to avoid excessive adjustments that could cause fluctuations.