SOx and NOx emission abatement in the FCC
The complexities of sulphur and nitrogen species formation in the FCC call for tailor made catalyst additive chemistry to limit formation of polluting emissions.
KATE HOVEY and RICK FISHER
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Some people’s offices smell like potpourri, some people’s smell like disinfectant and cleaning products, and some just smell like ‘new furniture’. For refinery workers, on the other hand, their place of work generally smells like rotten eggs. Delightful, right? Yes, we all know that the smell is coming from hydrogen sulphide (H2S) and is something that goes part and parcel with the oil refining industry, albeit not the nicest of smells. This article focuses on SOx and NOx abatement from the FCC/RFCC, which is not the most glamorous of topics, but an important one nonetheless. It certainly should not be overlooked, and goodness knows how anyone could avoid thinking about it while having the occasional whiff of rotten eggs – a constant reminder that our refineries are generating unwanted emissions.
The facts are simple: emissions cause environmental problems, damaging to both our ecosystems as well as human health. NOx emissions essentially cause smog and undergo a photon reaction to form ozone. SOx emissions react with moisture in the air to form acid rain. It is estimated that 80 million tonnes of SOx was emitted last year, with the majority coming from the combustion of fossil fuels in power stations and other industries. It is also estimated that a whopping 107 million tonnes of NOx emissions were released last year, with over 50% of this being generated from the transport industry alone.1
Oil refining and the FCC sit in the ‘fuel supply’ category of emission sources, which in the grand scheme of things looks to be having only a small impact on global SOx and NOx emissions (see Figure 1). However, we are talking about millions of tonnes, so the small segments that come from the ‘fuel supply’ category are in fact highly significant.
Fortunately, there are environmental regulations around the world that enforce strict limitations on emissions from the ‘fuel supply’ sector, and it is the refineries’ responsibility to take the necessary action to comply with these regulations. One particular set of regulations in Europe is outlined in the BAT Reference Document (BREF) produced by the European IPPC Bureau which are set to tighten their limits in the coming year. Fortunately for refiners who have strict environmental limitations to abide by, solutions for emissions control have been developed and successfully commercialised. This article will focus solely on SOx and NOx emission abatement from the FCC.
Emissions from the FCC – where do they come from?
For a chemical engineer, the FCC is an incredible unit to work on. It is full of interesting chemistry, mechanical complexity and, no matter how many years’ experience you have with the technology, there always seems to be unknowns. One thing that is known for certain, however, is that part of the feed forms coke on the surface of the catalyst as cracking reactions occur in the riser. This coke could be deemed a byproduct of the reactions but it is in fact an essential part of the process, required to fulfill the overall heat balance. Without coke, there would be no energy source to satisfy the endothermic cracking reactions in the riser. Typically, 5-6 wt% of the feed ends up as coke which is transferred to the regenerator on the ‘spent catalyst’. In full burn regenerators, the coke primarily burns to carbon dioxide (CO2) and in partial burn regenerators, some of the carbon is left in the partially oxidised form of carbon monoxide (CO). However, coke is not made up of carbon alone. The FCC feed is a complex mix of heavy hydrocarbon streams that typically contain heavy metals and contaminants, which will ultimately form part of the coke composition. What is important for this article is that 10-20% of feed sulphur and 40% of feed nitrogen end up in coke. When these are combusted in the regenerator, SOx and NOx are liberated.
Focus on SOx emissions from the FCC
As was mentioned, 10-20% of FCC feed sulphur ends up in coke. When the coke is combusted, SO2, SO3, COS and H2S are released. In full burn regenerators, there is about 90% sulphur dioxide (SO2) and 10% sulphur trioxide (SO3) with ppm levels of H2S. When excess oxygen is limited in partial burn regenerators, the reduced form carbonyl sulphide (COS) is also present in the flue gas; levels are dictated by how deep the partial burn is.
With environmental regulations dictating that SOx emissions need to be controlled below specific limits, refiners are legally obliged to utilise available solutions. An option for some is to process low sulphur feedstocks or hydrotreat the feed. However, hydrotreaters require high capital expenditure and additionally have a negative impact on gasoline octane. Another solution is to capture SOx emissions from the flue gas using a wet gas scrubber. Again, this requires high capital expenditure and also incurs additional operational costs through caustic usage and disposal. A popular solution for many is to use SOx reduction additives, which will pick up SOx in the regenerator and release it in the reactor as H2S. This released H2S goes along with the FCC dry gas which passes through amine contactors to remove the H2S. It can then be recovered as elemental sulphur in a sulphur recovery unit (SRU).
SOx additive chemistry is not as straightforward as one might think and it is definitely not the case that ‘one size fits all’ when it comes to SOx reduction additives. There are multiple vital components that make up a SOx reduction additive and these can, and should, be altered depending on the unit configuration and operation.
Understanding SOx additive functionalities
SO3 sorption capacity
One of the major components of a SOx additive is the functionality of adsorbing SO3 out of the combustion gases in the regenerator. The sorption capacity is directly proportional to the amount of magnesium oxide (MgO) that is contained within the additive. More MgO means more SO3 ‘pick up’ and therefore less additive consumption. The most effective SOx reduction additives are based on a hydrotalcite structure, while some still use a spinel type substrate. The difference between these two is that hydrotalcites allow for a much higher (as much as 50% more) MgO content than the spinel alternative. This is of most importance in full burn regenerators as the sorption capacity is often the limiting factor, and these account for ~80% of the SOx additives used by refiners today.
Oxidation package for SO2 to SO3 conversion
Another important function of the SOx additive is its ability to oxidise SO2 to SO3. Under normal full burn conditions, only about 10% of the sulphur is in the form of SO3; the remainder is mainly SO2. This means that without any assistance in the oxidation of SO2 to SO3, SOx additives could only be effective at reducing SOx emissions by about 10%. This is not really a problem in full burn regenerators because the additives have an inbuilt oxidation package that aids the oxidation of SO2 to SO3. There is also plentiful O2 available for forward oxidation to occur. However in partial burn applications, which account for ~17% of SOx additive users, the story is quite different. The limited oxygen availability can hinder SO2 to SO3 conversion. A reduced sulphur combustion product, COS, is also present; this is very difficult to oxidise in the regenerator once it is formed. COS mainly gets a free ride through the regenerator and leaves with the flue gas to the CO boiler where it is combusted to produce SOx. So it goes without saying that the deeper into partial burn the regenerator is operating, the less SOx reduction is possible. Figure 2 shows how the ‘maximum SOx reduction level’ is related to the amount of CO present in the flue gas and, hence, how deep the partial burn is. The figures include the theoretical relationship based on the following equation, as well as a few commercial cases where the SOx reduction is plotted against flue gas CO level, further validating this correlation:
Maximum SOx reduction level
= 100% - (%CO*10) (1)
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