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Removal of NOx from FCCU regenerator vent gases

Design of FCCUs has traditionally been aimed at optimising process yields and heat recovery, but legislation on emissions from refineries requires changed investment criteria

Peter Brook, Bernard Hagger, John Wood, Foster Wheeler Energy
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Article Summary
Refinery upgrades are increasingly becoming directly or indirectly driven by environmental legislation. The Scanraff refinery at Lysekil, Sweden, is no exception. Sweden is currently recognised as one of the world leaders in setting environmental legislation standards for emission and control of pollutants. The process modifications at Scanraff, recently completed by Foster Wheeler Energy Limited (FWEL), were a reflection of this governmental focus on the environment.

Various studies were carried out to identify how the Scanraff refinery could best comply with current environmental legislation for emissions of NOx and particulates.

The initial work by Scanraff identified that the flue gas from the catalyst regenerator, part of the fluidised catalytic cracking unit (FCCU), was the largest single source of NOx emissions from the refinery. To comply with current and anticipated legislation, it would be necessary for the NOx in the flue gas emissions to be reduced by 90 per cent, from about 400ppm to approximately 40ppm. In addition, the particulate emission levels from the FCCU were required to be reduced from 125mg/Nm3 to a limit of 75mg/Nm3 (with a possible target level of 50mg/Nm3).

This initial work, completed by Scanraff, was followed by a more detailed study, carried out in July 1993 by FWEL. This study identified the process modifications required to bring the emissions down to the levels required. The conclusions led to the revamp of the FCCU regenerator flue gas system, which included the addition of a selective catalytic reduction unit (SCR) and a ceramic hot-gas filter. These technologies were selected to give a cost-effective and reliable solution.

SCR technology using ammonia is well established as a method of reducing NOx in combustion-related applications such as power generation and boilers. However, there are few installations in the world where this technology is used in FCCU plants. There are three in Japan and one in Taiwan. The application of SCR technology at Scanraff is the first such use in a European refinery. This meant a considerable degree of  innovation was required on the part of FWEL and its subcontractor, Austrian Energy & Environment, in the design and installation.

Applying the technology successfully required a number of creative solutions to unusual problems:
- Typically, power plants operate for a maximum of 8000 hours a year, with an annual shutdown. But an FCCU operates continuously for 20000 to 35000 hours between shutdowns, so reliability of the plant items is paramount
- Compliance with stringent emission levels of NOx, particulates and ammonia slip (from SCR process) is required
- There was a space availability problem at Scanraff which necessitated the use of novel technological solutions
- The project involved the application of ceramic-filter technology (Schumacher hot-gas filter), which has been used in Europe, but not, as yet, become universal
- The characteristics of the particles are very different from those from combustion sources. For example, power plant dust is soft and sooty, containing various quantities of ash, soot, SiO2 or Fe2O3. On the other hand, FCCU regenerator dust is hard and contains pulverised zeolite catalyst material, contaminated with traces of heavy metals.

The SCR and hot-gas filter units were started up in October 1994. Scanraff has advised that the SCR is performing perfectly, reducing NOx emissions by 340 tonnes in 1995. Currently, emissions of particulates were 74mg/Nm3. This is just below the design limit of 75mg/Nm3, and only marginally above the target level of 50mg/Nm3.
The work at Scanraff is seen as a positive and unique step forward in the application of novel techniques to emission control in Europe. The reduction of pollutants from the refinery will have a beneficial effect for both the local environment and to Europe as a whole.

The FCCU plant
The FCCU at Scanraff was built in the 1980s to Kellogg’s design, with a capacity of 30000 barrels per stream day (bpsd). An FCCU is a refinery unit used for the conversion of gas oils and heavy stocks recovered from other operations into lighter products.

The cracking reactions are carried out in a vertical reactor (riser) in which vaporised oils rise with fluidised catalyst particles. The reactions are very rapid and petroleum coke, produced during the reaction, is deposited on the catalyst, making it ineffective.

The converted oils are separated from the spent catalyst, which then passes to the regenerator where the coke is burned off the catalyst in a fluidised bed, using air and producing a flue gas. The regenerated catalyst (less particulate losses to the flue gas) is conveyed down to the bottom of the riser, where the cycle begins again.

Before the Scanraff revamp, the flue gas passed through a series of cyclone separators for particulate removal before entering the waste heat boiler (WHB) for heat recovery. The flue gas was then vented to atmosphere through a stack. A manual bypass was provided around the WHB for use during start-up and shutdown.

The flue gas composition direct from the FCCU catalyst regeneration system was:
Particulates    105mg/Nm3average
(125mg/Nm3 max)
NOx     400ppm
SO2     300ppm (700ppm max)
SO3     20ppm
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