NOx control additives continue to reduce emissions (TIA)
Johnson Matthey’s (JM) NoNOx technology, in conjunction with a non-platinum CO promoter like JM’s COP-NP, can reduce NOx emissions below increasingly strict permit requirements without costly capital investments.
Michael Talmadge, Ron Butterfield and Martin Evans
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The following commercial examples from full-burn FCC units demonstrate how refineries continue to utilise NoNOx and COP-NP products to control both NOx and CO emissions.
Nitrogen compounds in FCC regenerators are largely derived from basic nitrogen compounds in FCC feed that adsorb on the acid sites of the catalyst. The nitrogen remains bound to the catalyst sites through the stripper and enters the regenerator where it is combusted with coke. In full burn FCC units, where coke is combusted in the presence of excess oxygen, the main nitrogen species formed in the regenerator are oxidised compounds like N2 and NOx. However, the chemical reactions to these oxidised compounds are not direct and follow the pathways shown in Figure 1.
During coke combustion, much of the nitrogen is first converted to HCN, which is thermally unstable at FCC regenerator operating conditions and will convert to other nitrogen species. In full burn units, most of the regenerator nitrogen (approximately 75%) is converted to inert nitrogen (N2). The remainder is converted to NOx (NO, NO2, N2O) through further reactions with O2, NH3 through hydrolysis by steam, or remain HCN due to kinetic limitations, such as non-homogeneity or imperfect mixing, in the regenerator. Reduced nitrogen species (HCN, NH3) are partially converted to NOx due to oxygen-rich environments in the regenerator and the relatively slow pathway from NOx to N2.
More detailed explanations of regenerator nitrogen chemistry have been published by JM in conference presentations and journal articles.1
NOx reduction strategy
JM recommends the following series of steps for refiners pursuing reduced NOx emissions from their FCC units:
• Optimise excess O2 and other process variables: oxygen is a key factor impacting reaction pathways of FCC regenerator nitrogen. Refiners should test full burn units by varying excess flue gas oxygen to understand its impact on NOx and optimise operations to other constraints like regenerator CO or afterburn. Other variables known to impact regenerator NOx are antimony (Sb) and regenerator bed levels.
• Minimise platinum based promoters: platinum combustion promoters effectively reduce afterburn and increase HCN conversion. However, they also contribute to increased regenerator NOx. Avoiding pre-blending platinum promoters is an important step in reducing NOx.
• Replace combustion promoter with non-platinum: laboratory testing and many commercial trials of non-platinum CO promoters, like COP-NP, have demonstrated significant reductions in NOx emissions relative to platinum promoters. Non-platinum alternatives enable control of regenerator afterburn and CO emissions, while substantially reducing NOx.
• NOx reduction additive: if NOx emissions are still above the required level, then a NOx reduction additive like JM’s NOxGetter or NoNOx product lines will provide further reductions. We recommend a product trial to assess the performance on each specific FCC unit.
• Mechanical modifications: further improvements to regenerator NOx performance are achieved through hardware technology enhancements to maximise the homogeneity of the regenerator.
Impact of NOx reduction additive
The plots showing regenerator NOx as a function of excess O2, presented in Figure 2, represent the results of two commercial trials with JM NOx reduction additives. Both trials were performed on full burn FCC units in the US where the refineries had completed NOx reduction strategy steps through replacement of platinum promoter with COP-NP non-platinum promoter.
In commercial Example 1, the refinery tested NoNOx additive at 1 wt% and 2 wt% concentrations in inventory. With 1 wt% and 2 wt% NoNOx in inventory, the NOx emissions in the FCC regenerator flue gas reduced by 35% and 39%, respectively, at constant levels of excess oxygen.
In Example 2, three products were tested separately – Additive X, NOxGetter and NoNOx. NoNOx is currently the most advanced NOx reduction technology in the JM product line. The results of the trials showed Additive X reduced NOx by 30%, NOxGetter by 37% and NoNOx by 49% at constant levels of excess oxygen.
In both commercial examples, the refineries have a adopted a long term additive control strategy and continue to utilise NoNOx and COP-NP products to control NOx and CO below permitted levels. Other commercial successes of NOx emissions reductions with JM NOx additives have been published in the public domain2 and the company will continue to publish the results of future technology enhancements demonstrated in laboratory tests and commercial trials.
The commercial examples prove that NOx reduction additive technologies are effective and continue to improve. The technology enhancements enable refiners to further improve environmental performance with increasingly strict regulations and avoid costly capital investments. In addition, there are opportunities for refiners to optimise operating expenses with non-platinum CO promoters and NOx reduction additives. For example, refiners may explore cost reductions from ozone consumption in low temperature oxidation processes.
There are opportunities to reduce NOx emissions by up to 75% relative to base emissions with platinum CO promoters and up to 50% relative to base emissions with non-platinum CO promoters as demonstrated by commercial case examples.
NONOX, NOXGETTER, and COP-NP are marks of the Johnson Matthey group of companies.
This short case study originally appeared in PTQ's Technology In Action feature - Q3 2019 issue.
For more information: Michael.Talmadge@matthey.com
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