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Mar-2009

Reducing NOx emissions from FCC regenerators

Case study demonstrates the performance of new technology for reducing NOx emissions in regenerator flue gas. The new developments reduce NOx emissions whether the FCCU is operating in full- or partial-burn mode

Ye-Mon Chen and David Brosten
Shell Global Solutions (US)
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Article Summary
The FCC regenerator is a major NOx emission source in refineries. Several existing technologies are available to reduce NOx emissions from a FCC regenerator, which include De-NOx catalyst additives, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Technology recently developed by Shell Global Solutions can be used as a standalone strategy, to reduce NOx emissions to as low as 40 ppm or less, or in conjunction with one of the previously noted existing technologies. 

NOx formation
Most NOx emissions from FCC flue gas come from nitrogen in the feed. The contribution of direct oxidation of N2 to NOx is negligible, particularly for full-combustion FCC regenerators. For partial-combustion regenerators, the contribution of direct oxidation of N2 to NOx is relatively small if low-NOx burner technology is applied in the CO boiler/incinerator.

A recent study shows1 that about 50% of nitrogen in the feed exits the FCCU on the reactor side, and the remaining 50% exits as coke on spent catalyst sent to the regenerator. Of the 50% feed nitrogen exiting from the reactor, about 10% ends up as ammonia, which is collected in sour water, and the other 40% ends up in various streams of the reactor liquid products.

This article focuses on the remaining 50% feed nitrogen, which enters the regenerator in the form of coke on spent catalyst. As the spent catalyst is regenerated and coke is burned off in the regenerator, the nitrogen species on coke are released into the flue gas. Recent studies1,2 further show that less than 5% of feed nitrogen on coke is released in the form of NOx emissions in the flue gas. More than 45% of feed nitrogen on coke is initially released in the form of NOx or other intermediates, but is converted in situ to N2 in the regenerator.

A recent study2 of batch regeneration of spent FCC catalyst with oxygen and helium reveals a close interaction between the combustion of carbon and the release of nitrogen in the coke. Figure 1 shows the concentrations of CO, CO2 and O2 in the flue gas as a function of time as coke on catalyst is burned off in the batch regeneration experiment. The amount of coke on catalyst was not directly measured, but Figure 1 implies that coke on catalyst was removed continuously, converted to CO/CO2 and became negligible after 26 minutes, as both CO and CO2 concentrations fell to negligible levels. For the first nine minutes, the O2 concentration remained low and the CO concentration was higher than CO2, indicating a reduction environment in this period of the batch regeneration. As O2 broke through the unit at ten minutes and its concentration continued to rise, coinciding with a sharp drop in CO concentration and a rise in CO2, the batch regeneration shifted gradually from a reduction environment to an oxidation environment.

Figure 2 shows the concentrations of NO, HCN and N2 in the flue gas as a function of time as coke nitrogen is released in the same batch regeneration experiment. Most of the coke nitrogen was released as N2, which peaked at 13.5 minutes at 200 ppm, under a reduction or a slightly oxidation environment. A fraction of coke nitrogen was released as HCN, which peaked at 10.5 minutes at 35 ppm under the same environment. The NO concentration was below 20 ppm for the first 14 minutes under a reduction or slightly oxidation environment when both coke on catalyst and CO were present. NOx levels increased sharply afterwards, and peaked at 18 minutes at 190 ppm, when the CO concentration fell to negligible levels and the O2 concentration increased beyond 1%, as shown in Figure 1.

The proposed reaction kinetics2 for the release of coke nitrogen in the CC catalyst regeneration process involves the initial volatilisation of coke nitrogen as HCN, which could be hydrolysed to another intermediate, NH3. Both intermediates, HCN and NH3, can be oxidised to NO, which can be reduced to N2 by the presence of CO or/and coke on catalyst.

Low-NOx regenerator
The FCC regenerator design has a direct impact on the effectiveness of in situ reduction of NOx to N2, and hence the reduction of the final NOx emissions in the flue gas. The Shell low-NOx regenerator technology shown in Figure 3 enables the unit to operate in both full- and partial-combustion modes with low NOx emissions.

As shown in Figure 3, the regenerator system (1) includes a single regenerator vessel (10) with an upper end (12) and lower end (14). The regenerator vessel (10) includes a dilute-phase catalyst zone (16) above and a dense-phase catalyst zone (18) below, with a transition surface (20) between the two. The dense-phase catalyst zone further includes a high-velocity central region (22), located in the central portion (26) of the dense-phase catalyst zone, and a low-velocity annular region (24), located in the annular portion (28) of the dense-phase catalyst zone. It is a significant aspect of the new regenerator technology that the high-velocity central region and the low-velocity annular region are formed within the dense-phase catalyst zone without the use of a structural element such as a vertical baffle or a partition. The two fluidisation regions are instead formed within the dense-phase catalyst zone by the introduction into the dense-phase catalyst zone of more than one fluidisation gas stream, each of which is directed and controlled in such a manner as to cause the formation of multiple fluidisation regions. Thus, introduced into the central portion of the dense-phase catalyst zone is a high superficial velocity fluidisation gas stream that passes by way of conduit (30) to fluidisation gas distribution ring or rings (32) near the bottom of the regenerator vessel. Introduced into the annular portion of the dense-phase catalyst zone is a low superficial velocity fluidisation gas stream that passes by way of conduit (36) to fluidisation gas distribution ring or rings (38) located within the annular portion near the bottom of the regenerator vessel.

The controlled introduction of the various fluidisation gas streams at the different fluidisation gas flow rates along with the directed introduction of the fluidisation gas streams to desired locations induces a desired circulation of the FCC catalyst within the dense-phase catalyst zone, as depicted in Figure 3 by the bold arrows (40) that show the general direction and circulation of the FCC catalyst within the dense-phase catalyst zone. As shown by the bold arrows, catalyst particles in the high-velocity central region move in a generally upward direction, and catalyst particles in the low-velocity annular region move in a generally downward direction. Catalyst from the bottom end (42) of the low-velocity annular region flows into the high-velocity central region, and most of the catalyst from the top end (44) of the high-velocity central region flows into the low-velocity annular region, thereby forming the catalyst circulation within the dense-phase catalyst zone.
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