Reducing FCC unit NOx emissions
Mechanisms of FCC unit NOx formation and logical steps to minimise these emissions are discussed. Performance examples at each stage of operation using additives are provided
Intercat (Johnson Matthey)
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NOx emissions are becoming a worldwide issue. Reductions in NOx emissions from FCC units have been a requirement at many US refineries as a part of the negotiations between the US Environmental Protection Agency (EPA) and the operators for consent decree settlements. A consent decree is effectively a negotiated voluntary agreement by the refiner to reduce emissions levels. Many US regional pollution-control authorities have also now started to impose significantly lower NOx emissions limits for FCC units.
European refiners are also now facing increasingly stringent controls, as laid down under several EU-wide emissions limits and targets for the reduction of SO2 and NOx. They include the National Emissions Ceiling Directive (NECD; 2001/81/EC) and the United Nations Economic Commission for Europe Convention on Long-Range Trans-boundary Air Pollution Gothenburg Protocol (UNECE 1999). These have culminated in the European Commis-sion’s Clean Air For Europe programme (CAFE). This thematic strategy on air quality was released in September 2005.
Reductions in NOx emissions from refineries are also becoming an issue of major public concern in other countries as air pollution levels in expanding major cities increase.
The environmental issues underlying the European legislation relate to acidification.1 Acidification is caused by emissions of sulphur dioxide, nitrogen dioxide and ammonia into the atmosphere, and their subsequent chemical reactions and deposition onto ecosystems and man-made structures. Deposition of acidifying substances causes damage to ecosystems, and corrosion damage to buildings and materials. The adverse effects associated with each individual pollutant depend on its potential to acidify, and the individual properties of the ecosystems and materials. The deposition of acidifying substances still often exceeds the critical loads of ecosystems across Europe. (Critical load is the ability of ecosystems to bear an environmental load — ie, acidifying depositions —without significant damage.) Efforts to reduce the effects of acidification are therefore focused on reducing the emissions of acidifying substances.
NOx and SO2 can also have direct and indirect impacts on human health, and can transform into small-diameter particulate matter (see EN07 Energy-related particulate matter emissions published by the European Environment Agency), which, when inhaled, can cause respiratory problems. NOx is also a tropospheric ozone precursor that reacts in the atmosphere in the presence of sunlight to form ozone, which, in high concentrations, can lead to significant health impacts and damage to crops and other vegetation. Furthermore, an excessive input of nutrients from atmospheric deposition leads to eutrophication.
The National Emissions Ceiling Directive includes emissions-reduction targets that are slightly stricter than the targets set in the Gothenburg Protocol, and requires the introduction of national emissions ceilings for emissions of SO2, NOx and NH3 in each Member State, as well as setting interim environmental objectives for reducing the exposure of ecosystems and human populations to damaging levels of acid pollutants.
In terms of the energy sector, which includes refining, the most relevant NEC Directive targets for the EU-25 as a whole are:
- SO2: emissions reduction of 74% by 2010 from 1990 levels
- NOx: emissions reduction of 53% by 2010 from 1990 levels.
Other key policies that have contributed to the reduction of acidifying emissions across Europe include the Integrated Pollution Prevention and Control (IPPC) Directive (96/61/EC). This came into force in 1999 with the aim of preventing or minimising the pollution of water, air and soil by industrial effluent and other waste from industrial installations. It defines the basic obligations for operating licences or permits, and introduces targets or benchmarks for energy efficiency. It will also require the application of Best Available Techniques (BAT) in new installations from now on (and for existing plants over ten years, according to national legislation).
The Large Combustion Plant Directive (2001/80/EC) is important in reducing emissions of SO2, NOx and dust from combustion plants with a thermal capacity of greater than 50 MW. The directive sets emissions limits for the licensing of new plants and requires Member States to establish programmes for reducing total emissions. Emissions limits for all plants were revised in 2007 under the IPPC Directive. Adding to the pressure on industry is the fact that the US refining industry has already gone down this route, driven heavily by the Consent Decree settlements with the EPA.
Refinery NOx emissions reduction
A significant portion of refinery NOx normally originates from furnaces. For this reason, low-NOx burners are usually required in furnaces to meet BAT requirements. In addition, FCC NOx emissions reductions are likely to be required to meet BAT. Reducing FCC NOx emissions can actually lead to cost reductions through reduced promoter usage. This could also lead to further savings by providing increased furnace fuel flexibility within a bubble constraint. However, it should be noted that for partial-burn FCCs, the vast majority of the NOx is produced in the CO boiler. These emissions cannot currently be controlled using catalyst additives and so the issues addressed in this discussion are not relevant to these units.
FCC NOx formation chemistry
The published and generally accepted routes to NOx formation are illustrated in Figure 1.
The UOP view on factors impacting NOx formation, as stated at the NPRA 2006 FCC conference, is summarised in Table 1.2
The complexity of the interaction between these many factors is a major issue, which makes multiple-unit NOx comparisons very difficult.
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