Reducing NOx emissions from an FCC unit (TIA)
Refinery processes account for a considerable share of the overall emissions of air pollutants in Europe. The Industrial Emissions Directive (IED) is the main EU instrument regulating pollutant emissions from industrial installations.
Dimitrios Gkanis, Vasileios Tatsis, Charitos Damianou and Constantinos Plellis-Tsaltakis
W R Grace & Co
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The IED entered into force in 2011 and had to be transposed by Member States by 2013. As a result, many refiners in the EU are facing more stringent SOx and NOx emissions, with additional emission reduction targets coming into effect in 2018. One such refinery is Hellenic Petroleum’s Aspropyrgos refinery in Greece which has successfully managed to reduce NOx emissions using Grace FCC additive technology.
Options for NOx reduction
One route to lower NOx is through choice of feedstock, though this is not a particularly flexible but costly approach. Regenerator conditions also play a big role in NOx formation and can present an easier route for NOx reduction. For example, lower oxygen levels and regen temperatures typically help to reduce NOx. In addition, optimising air distribution and regen bed levels can be beneficial.
The use of platinum based combustion promoters can lead to excessive NOx formation, with many refiners switching from platinum to palladium based promoters, such as Grace’s CP P combustion promoter, in recent years. After combustion promoter usage has been optimised, specific NOx reduction additives such as Denox can provide additional levels of NOx reduction.
Decreasing NOx emissions at Aspropyrgos refinery
The FCC unit at Aspropyrgos refinery is an Exxon Flexicracker design operating in full burn with close coupled cyclones and modern feed nozzles. The unit processes a combination of hydrocracker unconverted oil and atmospheric residue. Hellenic and Grace initiated a NOx reduction project at the start of 2017 involving commercial trials of FCC additive technology. The objective was to determine the lowest achievable emissions of NOx to avoid the future need for high capex modifications such as retrofitting furnaces with low NOx burners while remaining within compliance of future BREF NOx emission legislation.
Up until 2017, Aspropyrgos refinery had been using Grace’s traditional Pt based combustion promoter technology for afterburn control. The first step of the NOx reduction project was to transition to CP P, a non-Pt based low NOx combustion promoter. A key consideration in addition to NOx reduction was not to compromise the level of afterburn in the regenerator. NOx emissions from the FCC unit are measured using a portable flue gas analyser, and based on a five-year period of operational data the baseline NOx emission levels could be determined.
During the switch from a Pt to a Pd based combustion promoter, elemental analysis of the equilibrium catalyst (E-cat) enabled the technology change-out to be monitored. Once complete change-out was achieved, a NOx emission reduction of 45% was consistently noted using the CP P combustion promoter. Importantly, the catalyst remained well promoted using CP P with no afterburn issues observed.
The next step of the project was to examine the additional benefits of combining the low NOx combustion promoter with Grace’s Denox technology, a standalone additive for NOx reduction. The target was to reduce the NOx emissions to as low as possible. The Denox additive was preblended with the fresh catalyst to ensure consistent additive injection. Elemental analysis of the E-cat was again used to monitor the level of additive technology in the circulating inventory. The target level of Denox additive was 0.6 wt%, and this was achieved over a four-month period without the use of a baseloading period. During this time, the frequency of NOx measurement was increased to ensure the performance of the additive was determined in the most accurate way possible. Using a combination of CP P and Denox technologies enabled NOx emissions to be reduced by 65%, fully meeting the ambitious expectations of the refinery.
The 45% reduction of NOx using CP P, followed by the additional NOx reduction through the use of Denox additive, is shown in Figure 1.
The use of the NOx reduction technologies described had no negative impact on unit performance or the FCC products yield structure.
By establishing the levels of NOx emissions achievable using Grace’s CP P and Denox technologies, Aspropyrgos refinery is well prepared for the upcoming impact from BREF limitations and expect to be able to mitigate any potential capex impact.
This short case study originally appeared in PTQ's Technology In Action feature - Q3 2018 issue.
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