WO2008056130A1

WO2008056130A1 - Reduction of nox emissions

Info

Publication number: WO2008056130A1
Application number: PCT/GB2007/004230
Authority: WO
Inventors: Trond Myrstad; Geir Remo Fredriksen
Original assignee: Statoilhydro Asa; Campbell, Neil
Priority date: 2006-11-08
Filing date: 2007-11-06
Publication date: 2008-05-15
Also published as: GB2443609B; GB2443609A; GB0622267D0

Abstract

A method for reducing the NOx emissions from a CO boiler wherein the gases added to the CO boiler comprise flue gas from a fluid catalytic cracker regenerato operated in partial burn mode and at least part of a NO containing exhaust gas stream from the same and/or a different CO boiler.

Description

Reduction of NOx Emissions

This invention relates to a new method for the reduction of NOx emissions during fuel refining, in particular to a method of reducing emissions from a fluid catalytic cracking process operating in partial burn mode.

In recent years there has been an increase in concern about air pollution from industrial emissions of oxides of nitrogen (NOx), sulphur (SOx) and carbon (CO₂). Growing environmental concerns mean the trend in the industry is clearly in the direction of increasingly stringent restrictions on emissions of these gases. Oil refineries in general, and Fluid Catalytic Cracking Units (FCCU) in particular, are large sources of these emissions. FCCUs process heavy hydrocarbon feeds containing sulphur and nitrogen compounds and these provide the source for the NOx and SOx emissions.

When cracked in a FCCU, parts of the nitrogen components in the feed end up in coke on the Fluid Catalytic Cracking (FCC) catalyst. During catalyst regeneration, the coke is burned off in air, and parts of the deposited nitrogen components in the coke are converted to NOx, or to other nitrogen containing species which are converted to NOx downstream of the FCCU. AU FCCUs processing nitrogen containing feed will thus have a NOx emissions problem. The term NOx includes all chemical components consisting of oxides of nitrogen.

Whilst NOx consists mainly of NO, other oxides, such as NO₂, N₂O and N₂O₂, as well as mixtures of these components can also be present.

A fluid catalytic cracking regenerator can be operated in several modes. The two most common modes are full burn and partial burn, both described below. If operated with sufficient air to convert essentially all of the coke on the

FCC catalyst into CO₂ and H₂O, the gas effluent from the regenerator will contain excess oxygen, typically in the range 0.5 to 4 % of the total flue gas. This combustion mode is called full burn. In full burn mode, the conditions in the FCC regenerator in general are oxidizing and there is enough oxygen to oxidise all reducing gas species, such as CO, NH₃ and HCN. Under these conditions, essentially all of the nitrogen deposited in the coke on the FCC catalyst is converted into molecular nitrogen or NOx, and exits the regenerator with the flue gas. On the other hand, if the amount of air added to the FCC regenerator is insufficient to fully oxidize the coke on the FCC catalyst to CO₂ and H₂O, a significant amount of the burnt coke carbon is oxidized only to CO. In this case, the gas effluent from the regenerator typically will contain between 1 and 7.5 vol% CO. This mode of operating FCC regenerators is called partial burn. When the FCC regenerator is operated in partial burn, the conditions in the regenerator, where the oxygen added with air has been depleted and CO concentration has built up, are overall reducing. Under these conditions, nitrogen deposited in the coke on the FCC catalyst is mainly converted to nitrogen species other than NOx, such as NH₃ and HCN, in addition to molecular nitrogen. Small amounts of NOx may however be present in the flue gas from a FCC regenerator operated in partial burn.

The flue gas NOx content in partial burn is low most of all due to a low (or zero) oxygen content in this mode, but also due to a lower temperature (around 700°C) in partial burn mode compared with full burn. The HCN content is high due to conversion of nitrogen components in coke in the devolatization phase. Ammonia might also be formed under these conditions. Which mode is used is broadly dependent on the nature of the feed to the FCC unit.

Since the flue gas from an FCC regenerator operated in partial burn contains high levels of CO, a toxic gas, it is necessary to convert this to carbon dioxide (CO₂) before the flue gas can be vented to the atmosphere. This is achieved by passing the flue gas from the regenerator into a carbon monoxide boiler (CO boiler).

However, when the flue gas is burnt in the presence of oxygen in the CO boiler, whilst CO is converted to CO₂, HCN is converted into NOx (mainly NO). Ammonia, if present, is also converted into NOx. Thus, however the FCCU is run (i.e. full burn or partial burn), NOx emissions represent a problem.

There are several known approaches for reducing NOx emissions from FCCUs. One approach is the utilization of a FCC catalyst additive. Several patents describe this approach, but it is only suitable for reducing the NOx emissions from an FCC regenerator operating in full burn. The additives do not, in general, reduce the emission of reduced nitrogen species, such as NH₃ and HCN and hence do not prevent the subsequent formation of NOx compounds in the CO boiler. As a result; even if such an additive was able to remove 100 % of NOx, the actual reduction of nitrogen containing species in the FCC regenerator flue gas would be very low.

Thus additives are of limited use in partial burn operation. US 6,660,883 does however, envisage reduction of both NOx and reduced nitrogen compounds in the FCC unit using a complex mixed metal oxide additive.

Other approaches for reducing the NOx emission from an FCC regenerator are post-treatment of the FCC regenerator flue gas, either catalytically, non- catalytically, or by adsorption. One example of this process is given in US 2004/0074809 where the inventors try to minimise the nitrogen species entering the CO boiler by post treatment of the flue gas from the FCC regenerator. US 4744962 offers an alternative solution where ammonia content in the flue gas of the FCCU is reduced by addition of NOx to the flue gas. NOx reacts with ammonia in the presence of oxygen to form nitrogen and water vapour which can be passed safely into the CO boiler. NOx is generated by the separate reaction of ammonia and oxygen.

US 5173278 offers a still further solution in which the flue gas from the regenerator is catalytically treated to remove HCN and NOx again minimising the presence of reduced nitrogen species which will enter the CO boiler. Zeolites or vanadium oxides are the catalyst of choice. The present invention relates to a method for reduction of NOx emissions, in particular NO emissions, from a FCCU having a regenerator being operated in partial burn mode. The inventors have surprisingly realised that significant reductions in NOx emissions from a CO boiler, and hence from an FCC process as a whole, can be achieved if part of the exhaust gas from the CO boiler is recycled back into the boiler or added to another CO boiler along with flue gas from the regenerator.

It is known that with an optimal burner in the CO boiler, the amount of thermal NOx formed by oxidizing nitrogen in the air will be very low, and since the amount of NOx entering the CO boiler is also low when the regenerator is operated in partial burn mode, the major source for NOx-formation will be HCN, i.e. HCN is oxidised to NO in the presence of oxygen in the air. - A -

The inventors have realised that HCN itself can be used to reduce the NOx content in a NOx containing gas, since HCN and NO can react and form N₂. (The reactions between HCN, NO and O₂ are very complex. A more indepth discussion of the reaction network can be found in Dagaut et. al., Comb. Sci. Tech., 139, 329 (1998) and in Glarborg et. al., Progress in Energy and Combustion Science, 29, 89 (2003).)

The inventors have further realised that by utilizing these reactions in the CO boiler, i.e. initial formation of NO from HCN combustion, and the reaction between HCN and NO to form N₂, it is possible to get a significant reduction of the overall NOx emissions, in particular NO emissions.

Moreover, the flue gas from the regenerator may .also contain minor amounts of ammonia. Like HCN, ammonia reacts to NO in the presence of oxygen, and like HCN, ammonia can react with NO forming N₂.

Thus, through contact of the flue gases from the FCCU regenerator with NOx, significant reductions in the reactive nitrogeneous species which cause eventual NOx formation can be achieved.

It is appreciated that a CO boiler operating in typical fashion will contain NOx, however under normal reaction conditions there is little or no contact between the HCN containing feed and the NOx containing flame gas meaning the chemistry above does not occur.

The present inventors have realised that contact between the reduced nitrogen compounds and NOx can be economically and easily achieved simply by partially recycling the exhaust gas from the CO boiler back into the boiler or by adding at least part of the exhaust from one CO boiler to another CO boiler which has its own regenerator flue gas input. Due to the formation of NO from HCN and ammonia combustion, the exhaust gas from the CO boiler provides a constant supply of NO and by recycling it or adding it to a further boiler, the amount of NOx emitted to the atmosphere is reduced as the NOx precursors (e.g. HCN and NH₃) are converted to non reactive nitrogen. Thus, viewed from one aspect the invention provides a method for reducing the NOx emissions from a CO boiler wherein the gases added to the CO boiler comprise flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and at least part of a NO containing exhaust gas stream from the same and/or a different CO boiler.

Viewed from another aspect the invention provides a method for reducing the NOx emissions from a CO boiler having an input gas stream and an exhaust gas stream, said input gas stream comprising flue gas from a fluid catalytic cracker regenerator operated in partial burn mode wherein at least part of the exhaust gas stream is recycled into the boiler, preferably via the input gas stream.

Viewed from another aspect, the invention provides a process for operating a CO boiler having at least one input conduit and an exhaust gas conduit comprising causing said an input conduit to comprise flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and causing an input conduit, preferably the same input conduit, to comprise at least part of a NO containing exhaust gas stream from the same and/or a different CO boiler.

Viewed from another aspect the invention further provides an apparatus comprising a CO boiler having an input conduit and an exhaust conduit, said input conduit comprising flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and recycled exhaust gas from the same CO boiler or a NO containing exhaust gas from a second CO boiler. Such an apparatus is suitable for the reduction of NOx emissions from a CO boiler. Viewed from another aspect the invention further provides an apparatus comprising a CO boiler having at least two input conduits and an exhaust conduit, a first input conduit comprising flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and a second input conduit comprising recycled exhaust gas from the same CO boiler or a NO containing exhaust gas from a second CO boiler. Such an apparatus is suitable for the reduction of NOx emissions from a CO boiler.

Viewed from another aspect the invention further provides an apparatus comprising a CO boiler having an input conduit and an exhaust conduit, said input conduit is adapted to carry flue gas from a fluid catalytic cracker regenerator operated in partial burn mode wherein a further conduit is provided to allow recycling of at least part of the exhaust gas in the exhaust conduit to the CO boiler, preferably via the input conduit. Such an apparatus is suitable for the reduction of NOx emissions from a CO boiler.

Viewed from another aspect the invention further provides an apparatus comprising a first CO boiler having a first input conduit and a first exhaust conduit and a second CO boiler having a second input conduit and a second exhaust conduit wherein said first and second input conduits are adapted to carry flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and said first exhaust conduit is adapted to transfer at least a part of the exhaust gas from the first CO boiler to the second CO boiler, preferably via the second input conduit. Such an apparatus is suitable for the reduction of NOx emissions from a CO boiler.

By reducing NOx emissions from a CO boiler is meant that the amount of NOx emitted by the boiler is reduced relative to a boiler operating in the absence of NOx recycling. The invention particularly concerns reduction of NO emissions.

By fluid catalytic cracker regenerator operated in partial burn mode is meant that the amount of air added to the FCC regenerator is insufficient to fully oxidize the coke on the FCC catalyst to CO₂ and H₂O, and a significant amount of the burnt coke carbon is oxidized only to CO (e.g. 1 to 7.5 vol% CO). Partial burn is a term of the art.

By input gas stream is meant one which contains flue gas from an FCC regenerator operated in partial burn mode. Oxygen and other fuel which may be added to the boiler to ensure CO combustion is typically added through a separate stream.

By exhaust gas stream is meant the gases which exit the CO boiler after combustion of the CO to carbon dioxide has occurred. A NO containing exhaust gas stream is one which contains NO.

By recycling part of the exhaust gas stream to the CO boiler is meant that a portion of the gas in the exhaust gas stream from the CO boiler is transported back into the boiler. Preferably, this is effected by recycling part of the exhaust gas stream via the input conduit . By conduit is meant a vessel which carries a gas stream.

By said input conduit comprises flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and NO containing exhaust gas from a different CO boiler means that as well as the flue gas from the FCC regenerator, the CO boiler has as a feed exhaust gas which exits a different CO boiler which also has as its input, flue gas from an FCC regenerator. The exhaust gas must contain NO and hence the different CO boiler is preferably not one which itself has a feed which is a recycled exhaust gas stream.

This invention relates to a method of reducing NOx emissions from a fluid catalytic cracking process operating in partial burn mode. This is achieved by adding at least a part of the exhaust gas from a CO boiler into the same boiler (i.e. a recycle) or another boiler, depending on reactor set up. The operation of a FCCU is well known in the art and is carried out throughout the World and will only be briefly discussed herein.

Catalytic cracking is an established and widely used process in the petroleum refining industry for converting heavy oils of relatively high boiling point to more valuable lower boiling products including gasoline and middle distillates such as kerosene, aviation fuel and heating oil. A preheated feed is normally brought into contact with a hot cracking catalyst that is in the form of a fine powder, typically with a particle size of 10-300 μm for the desired cracking reactions to take place.

During cracking, coke is deposited on the catalyst and this results in a loss of activity and selectivity. The coke is removed by continuously removing the deactivated catalyst from the cracking reactor and oxidatively regenerating it by contacting it with air in a regenerator. The combustion of the coke not only removes the coke but also serves to heat the catalyst to temperatures appropriate for the cracking reaction.

In the invention, the catalyst regeneration process is typically carried out in an integrated unit comprising the cracking reactor, the regenerator and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor to regenerator and back to the reactor. Zeolite catalysts are routinely employed.

The feed to the catalytic cracker is typically a gas oil (e.g. straight run gas oil, vacuum gas oil and coker gas oil, atmospheric residua, vacuum residua and residual fractions from other refining processes. Oils from synthetic sources such as Fischer-Tropsch synthesis, coal liquefaction, shale oil or other synthetic processes may also yield high boiling fractions which may be catalytically cracked either on their own or in admixture with oils of petroleum origin.

The organic compounds which make up the feed are a complex mixture of paraffins, naphthenes and aromatic compounds and these typically include a small but significant complement of sulfur, nitrogen and oxygen heteroatoms. These elements contribute to catalyst deterioration in processing, and to air pollution when the product is used as a fuel.

The amount of organic nitrogen in the feed to the FCCU may typically be in the range 0.05 to 0.5 weight percent nitrogen (500-5000 ppmw). Organic nitrogen in the feed can be divided into two component groups; basic and non-basic nitrogen. It is commonly accepted that NOx emissions are caused by basic nitrogen components. Typically, basic nitrogen typically accounts for one third of the total nitrogen in the feed to the FCCU.

In the catalytic cracker, the feed together with its organic nitrogen and other impurities is raised to a cracking temperature in the range of about 480°C to 600°C under which conditions a portion of the feed is cracked, with simultaneous formation on the catalyst of a carbonaceous deposit, i.e. coke. The coke contains an amount of organic nitrogen, typically in the range 30 to 40 percent of the nitrogen in the feed. When the coked catalyst is regenerated in partial burn mode (i.e. with insufficient oxygen to fully oxidize the coke on the catalyst to CO₂ and H₂O), a significant amount of the burnt coke carbon is oxidized only to CO (typically. 1 to 7.5 vol%). Regeneration typically occurs in air at a temperature of about 600°C to 800°C,and the flue gas from the regenerator then consists of elemental nitrogen introduced with the air, together with relatively large amounts of the expected combustion products, including water vapour, carbon monoxide and carbon dioxide. The flue also contains various nitrogen compounds such as HCN and ammonia which are the reaction products of the incomplete combustion of the nitrogen deposits on the coked catalyst. Some NOx may also be present. Of these combustion products, the water vapour is inherently benign, and the carbon dioxide, is regarded as a necessary waste product of catalytic cracking. Carbon monoxide however is toxic so is passed to a CO-boiler for combustion to CO₂. The concentration of nitrogen containing species in the flue gas from the FCCU regenerator depends on the FCC feed and on the operating conditions in the FCCU regenerator.

The flue gas from the FCCU regenerator operated in partial burn may typically comprise 200 to 1000 ppmw (parts per million by weight) of HCN, such as 400 to 800 ppmw.

The flue gas from the FCCU regenerator operated in partial burn may comprise 0 to 1000 ppmw of ammonia.

The flue gas from the FCCU regenerator operated in partial burn may comprise 0 to 50 ppmw of NOx, such as 2 to 20 ppmw.

The flue gas also typically contains around 2 to 10 wt% of CO and 10 to 15 wt% of carbon dioxide.

The flue gas from the regenerator is incinerated by mixing with a high excess air flame from a conventional burner operated typically on refinery gas fuel. The fuel gas burner supplies both heat and oxygen for oxidation of virtually all of the CO to carbon dioxide.

Simultaneously however, the nitrogen compounds are oxidised to NOx. The exhaust stream from the CO boiler is essentially free of HCN, i.e. only traces or no trace of HCN can be found. The exhaust stream from the CO boiler is also essentially free of ammonia, i.e. only traces or no trace of ammonia can be found. More significantly, the exhaust stream from the CO boiler may comprise 50 to 500 ppmw of NOx, such as 100 to 300 ppmw, mainly present as NO

According to the invention and in order therefore to take advantage of the chemistry hereinbefore described, at least part of this exhaust gas is recycled back into the same CO boiler or added to another CO boiler having an FCC regenerator flue gas stream, preferably one which derives from the same regenerator as that supplying the first CO boiler. Whilst the exhaust gas stream can be added into the boiler via its own dedicated input, it is preferred if the exhaust gas stream is added to the input stream containing flue gas from the regenerator . Where the exhaust gas is recycled to the input gas stream, it may be preferred to thoroughly mix the gas streams before the mixed stream enters the CO boiler. Where the exhaust gas is recycled into the same boiler, at least 5 wt% of the exhaust stream, preferably at least 10 wt%, such as at least 15 wt%, more preferably at least 20 wt% could be recycled. Recycling amounts may still further be at least 25 %, at least 30 wt%, at least 35 wt%, at least 40 wt%₅ at least 45 wt%, at least 50 wt%, at least 55 wt%, at least, 60 wt%, at least 65 wt%, at least 70 wt%, at least 75 wt%, at least 80 wt%. The amount recycled may vary depending on the nature of the original feed being cracked and hence the amounts of nitrogen compound present in the flue from the regenerator, as well as the conditions in the CO boiler itself. By recycling a NOx containing exhaust gas into the boiler, especially into the input gas stream to the CO boiler, NO can react with HCN and NH₃ and form N₂ as described above. If the recycled gas is added to the input gas stream (either from the same boiler or a different boiler), it is envisaged that formation of N₂ by the reactions between NO, HCN and NH₃ may occur in the input gas stream before the stream enters to CO boiler. In this way, the nitrogen compounds HCN and NH₃ which provide the source for NOx can be converted to non-reactive nitrogen gas before entering the boiler thus reducing the amount of NOx formed by the CO boiler.

It will be appreciated that during recycling using a single CO boiler NOx reduction can never reduce to zero as some NOx needs to be formed and hence recycled to allow the reactions between NO and HCN and NH₃ to take place in this embodiment. The exhaust gas stream may contain 50 to 300 ppmw NOx.

To maximise the potential for NOx emission reduction, it is preferred therefore if at least part, e.g. all, the exhaust gas from a first CO boiler is added to another CO boiler having a FCC regenerator flue gas input, i.e. at least two CO boilers are connected in series. It is preferred if at least 80 wt%, especially at least 90 wt%, e.g. all the exhaust gas from the first boiler is added to the second boiler.

In this reactor set up, both boilers have an FCC regenerator flue input and the second boiler can have a NOx emission which theoretically can approach or equal zero.

Where two CO boilers are connected in series, it is preferred if the feed to them derives from the same FCC regenerator. For example, the flue gas from such a regenerator could be split into two portions and fed to each boiler. The split may be 1:10 to 10:1 by weight preferably 1:5 to 5:1. Most preferably between 30% to 60% of the flue gas from the regenerator is added to the first boiler, the rest to the second boiler. To ensure reaction, the exhaust gas must contain NO. This means that the exhaust gas employed should preferably be one which is emitted from a CO boiler which does not itself comprise exhaust gas as a feed. The exhaust gas stream feed to the second boiler may contain 50 to 300 ppmw NOx.

The discussion which follows concerns CO boilers which contain exhaust gas as a feed. Boilers which do not have such a feed (e.g. the first boiler when two boilers are connected in series) can be operated using well known conditions.

Irrespective of whether the exhaust gas is recycled to the same boiler or added to a different CO boiler, in order to maximise conversion of the HCN into nitrogen, the oxygen content in the CO boiler should be kept low. The oxygen content must however be high enough to ensure complete combustion of CO to carbon dioxide. There is therefore a balance to be struck regarding the amount of oxygen which is added to the CO boiler. The relative amounts of fuel and air are characterised by the global fuel equivalence ratio which is defined as follows:

φ = . \^X fuel I^X air ) O 'fuel 1¹^ 'air)

\^Xfuel,stoich. / ^Xalr,stoich. ) V^ fuel, stoich. / ^ 'air .stoich. )

where x; is the mole fraction of species i

Wi is the weight fraction of species i and the index stoich denotes the stoichiometric conditions.

Reciprocally, the global air equivalence ratio λ =1/Φ and is used in this field to express the fuel and air conditions with regard to stoichiometry. Accordingly, premixed combustion can be divided into three groups:

Fuel rich combustion Φ> 1 (λ <1)

Stoichiometric combustion Φ=l (λ =1) Fuel Lean combustion Φ< l (λ>l)

In this invention Φ is preferably larger than 1, more preferably in the range 1 to 1.2. Oxygen is preferably added to the CO boiler in a separate stream from the flue gas from the regenerator. Preferably, the CO boiler is provided with additional fuel to help fire the boilers. Such a fuel is typically what is available at the refinery where the FCC unit operates but is typically high in low molecular weight hydrocarbons such as methane and hydrogen. This fuel is preferably added in a separate stream.

The CO boiler may operate at an average temperature of 600 to 1400⁰C, preferably 700 to 1100°C, especially 700 to 1050°C.

In a preferred embodiment the exhaust and input gases are allowed to contact one another for a period of time sufficient to permit mixing and reaction to occur in the input stream before entry to the CO boiler. This period of time may be less than 3 seconds, preferably less than 2 seconds, e.g. less than 1 second. .

There is an optimum ratio of NO to HCN added to a CO boiler to achieve NO reduction. Taking into account all NO and HCN fed to a CO boiler (whether via the regenerator flue gas or via the exhaust gas), the weight ratio is preferably between 10: 1 to 1 : 1 HCN to NO, more preferably 8: 1 to 3 : 1.

The temperature in the zone where ammonia/HCN are converted to nitrogen should preferably be between 800 and 1400°C, more preferably between 900 and 135O°C, e.g. between 1000 and 1300°C. This can be achieved by heating the recycling stream/transfer stream prior to admixture if necessary. Ideally therefore the exhaust gas feed enters the boiler at a point where the reactor temperature is in these ranges.

The invention has been described above in relation to a single CO boiler with a recycle or two boilers connected in series however it will be appreciated that the invention could be effected using a wide variety of set ups as long as part of the . exhaust from one CO boiler, containing NOx, is recycled to the same or another boiler. It is possible, for example, for a CO boiler to have exhaust gas feeds from a plurality of other boilers or for an exhaust gas to be split and fed to a plurality of CO boilers. It would be possible to have more than two CO boilers in series or a number of boilers in parallel which then feed a number of CO boilers in series and so on.

What is critical is the use of the exhaust gas of a CO boiler as the source of NO to cause the reactions with HCN and/or NH₃ to N₂, and hence reduce the overall NOx emissions. Thus, viewed from a still further aspect the invention provides the use of the exhaust gas stream of a CO boiler as a source of NO for addition to a CO boiler to thereby reduce NOx emissions therefrom.

This invention can allow NOx reductions of at least 20%, preferably at least

25%, especially at least 30%, e.g. at least 35%. Thus, the NOx content of the exhaust gas from a CO boiler run in accordance with the method herein may be less than 150 ppm, preferably less than 120 ppm, especially less than 100 ppm by weight.

In an industrial refinery, there may be many CO boilers connected in series or parallel and the invention can be effected in such boilers simply by recycling the exhaust from one boiler into another or the same boiler. These reactor set ups are described more fully in relation to the figures which follow.

In a highly preferred embodiment, the invention involves two CO boilers connected in series in which the flue gas from a regenerator is split and added into both boilers. The exhaust gas from the first boiler is passed into the second boiler, preferably together with the flue gas from the regenerator. By operating the invention with this CO boiler set up, it is in theory possible to reduce the NOx emissions down to zero.

Brief Description of the Figures

Figure 1 shows two CO boilers arranged in parallel. The flue gas from FCCU regenerator (1) is split and passes to CO boilers (2) and (3). Part of the exhaust stream (5) from each CO boiler is recycled to the input stream (4) of each CO boiler. In the CO boilers, HCN is converted to NO; as described above. A fraction of the NO containing exhaust gas from each CO boiler, l-,α, is recycled to its respective CO boiler, where HCN reacts with NO, forming N₂. The overall result is that the fraction of the exhaust gas emitted to the atmosphere, α, has a reduced content of NOx.

The flue gas from the FCCU regenerator may also contain NH₃, which also reacts with NO forming N₂. It will be appreciated that the principle shown here could be expanded to a plurality of CO boilers. It is also within the scope of the invention for the recycled stream to be passed to a different CO boiler. The invention therefore covers a situation where the recycled stream passes to a different CO boiler, preferably to be added to the input stream of that boiler along with the flue gas from the FCCU regenerator. This is shown in Figure 2.

In Figure 2, two CO boilers (2) and (3) are operated in series. The flue gas from the FCCU regenerator (1), containing HCN, is divided into two streams. The first stream (6), containing a fraction α of the flue gas is sent to a first CO boiler (2), where HCN is converted to NO. The second stream (7), containing a fraction 1-α of the FCCU regenerator flue gas is sent to a second CO boiler (3) together with exhaust gas (5) from the first CO boiler. In the second CO boiler HCN from the FCCU regenerator flue gas reacts with NO from the exhaust gas from the first CO boiler, forming N₂. The flue gas from the FCCU regenerator may also contain NH₃, which also reacts with NO forming N₂. Figure 3 shows a more complete set up of a FCC unit with its regenerator being operated in partial combustion mode, including CO boilers for combustion of CO to CO₂. Spent catalyst is added to regenerator (1) through conduit (10) which connects to an FCC reactor (20) with stripper (21). Oxygen is supplied by means of conduit (11) to the regenerator to provide operation of the regenerator in partial combustion mode. Regenerated catalyst is withdrawn in conduit (12) and passed by back to the FCC reactor. Flue gas is removed from regenerator (1) in conduit (13) and split in conduits (14) and (15) which pass to CO boilers (2) and (3). Oxygen, usually in the form of air, is added to the CO boilers through conduits (16). A sufficient amount of oxygen is added in to combust all CO to carbon dioxide. Fuel is added through conduits (19).

The flue gas from the CO boilers is partially recycled through conduits (17) and (18) back into conduits (14) and (15). On recycling, some the ammonia and/or HCN will react with the NOx to form nitrogen. After the CO boilers the flue gas may be further treated in cyclone dust separators, electrostatic precipitators and/or scrubbers (not shown on Figure 3) for removal of catalyst particles and other impurities. Figure 4 shows NOx reduction as a function of temperature for a reactor simulation of the invention.

Figure 5 shows NOx reduction in real and percentage terms as a function of NOx inlet concentration.

Example 1

Analysis of commercial FCC regenerator flue gas pre and post CO boiler

The flue gas from a commercial FCCU regenerator and the flue gas from the CO boilers was analysed. The regenerator in this FCCU was operated in partial burn, illustrated by the content of CO and CO₂ in the flue gas; 7.0 and 13.8 vol%, respectively. The content of basic nitrogen in the feed to the FCCU was 0.055 wt%, resulting in the following composition of nitrogen species in the regenerator flue gas:

NOx 12.2 ppm

HCN 559 ppm

NH₃ O ppm

The flue gas from the CO boiler was burnt in a CO boiler, resulting in the following composition of nitrogen species in the CO boiler flue gas:

NOx 189 ppm

HCN O ppm

NH₃ O ppm

Comparative Example 1

Simulation of use of NOx reducing additive If a commercial NOx reducing additive had been used in the FCCU described in Example 1, and assuming that this additive did not reduce the content of reduced nitrogen species from the FCCU (such as HCN or NH₃), and further assuming that the additive would remove 100 % of NOx, the reduction of nitrogen containing species in the FCCU regenerator flue gas (NOx₅ HCN and NH₃), would be (using numbers from Example 1):

₁₀₀ ^o/₀ * ≡PSH « ₂ ^o/_o

559 ppm + \2.2ppm

Example 2

Simulation showing that HCN is a reducing agent of NO.

In a reactor, simulated as a perfectly stirred reactor, two streams are added, and the reactor simulations are made at various temperatures.

Stream 1 is a gas with a typical composition of the exhaust gas from a CO boiler:

CO₂ 13.6 vol%

O₂ 3.0 vol%

H₂O 9.1 vol%

N₂ 74.3 vol%

NO 172 ppm

HCN O ppm

Stream 2 is a HCN-containing gas with a HCN concentration of 172 ppm, simulating the flue gas from the FCC regenerator.

The results are presented in Figure 4, and show that above an inlet temperature of approximately 100O⁰C, the HCN reacts with NO, causing a decrease in NO. At higher temperatures, NO increases because conversion of HCN to NO increases, and because another mechanism causing oxidation of molecular N₂ to NO becomes important. The overall NO reduction, i.e. the sum of the mechanism reducing NO via HCN and the mechanism producing NO from N₂ and HCN combustion through temperature increases, balances at approximately 1300°C. It should be noted that the chosen reactor residence time in the simulations are relatively long, which is particularly favourable to the oxidation of N₂ to NO through the thermal NO mechanism.

This example demonstrates however that there exists a temperature window associated with a proper gas composition, where HCN can reduce NO.

Example 3

Simulation showing reduction of NO emissions from a CO-boiler.

Flue gas from a FCC regenerator with 559 ppm HCN, and cooling air is burned in two CO boilers connected in series. Zero NO is assumed in the flue gas from the first boiler which passes into the second boiler. At these conditions, the concentrations of HCN and NO at the inlet of the second boiler are 275 and 4 ppm respectively (i.e. NO from the FCC flue gas only). At a temperature of 1256 K in the second CO-boiler, the resulting NO emission is 61 ppm.

In a second simulation, it is assumed that the flue gas from the first CO-boiler contains 172 ppm NO. At these conditions, the concentrations of HCN and NO at the inlet of the second boiler are 275 and 98 ppm respectively. At a temperature of 1256 K in the second CO-boilers, the simulated NO emission from the second boiler is 101 ppm.

The simulations show that an increase in the inlet concentration of NO of over 24 times does not result in a corresponding increase in the NO emissions from the second boiler, which increases by just under 2 times, meaning that conversion from HCN to NO has decreased. There exists an optimal inlet NO concentration which gives a maximum overall reduction in NO. Based on the simulations, the optimal inlet HCN/NO ratio probably is in the range 8: 1 to 3 : 1 , and a split factor for the regenerator flue gas between the first and the second CO boiler in the range 0.3 to 0.6 will be optimal.

Example 4 Simulation showing reduction of NO emissions from a CO-boiler.

In yet another simulation example, hi this simulation, flue gas from a FCC regenerator with 559 ppm HCN, and cooling air is burned in two CO boilers in series, and a split factor for the regenerator flue gas between the first and the second CO boiler has been fixed at a value of 0.5. In the simulations, the inlet concentration of NO to the second boiler has been varied from 4 to 98 ppm. The simulation results at a reaction temperature of 1256 K are shown in Figure 5, presented as the outlet concentration of NO and % NO reduction at various inlet concentrations of NO. The results show that the outlet concentration of NO increases less than the corresponding increase in the inlet concentration of NO. Therefore the NO reduction increases with increasing inlet concentrations. With the conditions used in this simulation example, a NO reduction of approximately 35 % is achieved.

Claims

1. A method for reducing the NOx emissions from a CO boiler wherein the gases added to the CO boiler comprise flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and at least part of a NO containing exhaust gas stream from the same and/or a different CO boiler.

2. A method as claimed in claim 1 wherein said CO boiler has an input gas stream and an exhaust gas stream, said input gas stream comprising flue gas from a fluid catalytic cracker regenerator operated in partial burn mode characterised in that at least part of the exhaust gas stream is recycled into the boiler, preferably via the input gas stream.

3. A method as claimed in claim 1 wherein said CO boiler is the second in a series of CO boilers and the exhaust gas stream supplied thereto derives from the first boiler.

4. A method as claimed in any preceding claim wherein the NO containing exhaust gas stream is mixed with flue gas before addition to the CO boiler.

5. A method as claimed in claim 4 wherein the flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and NO containing exhaust gas stream enter the CO boiler in the same gas stream.

6. A method as claimed in any preceding claim wherein the flue gas comprises HCN.

7. A process for operating a CO boiler having at least one input conduit and an exhaust gas conduit comprising causing said an input conduit to comprise flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and causing an input conduit, preferably the same input conduit, to comprise at least part of a NO containing exhaust gas stream from the same and/or a different CO boiler.

8. An apparatus comprising a CO boiler having an input conduit and an exhaust conduit, wherein said input conduit comprises flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and either recycled exhaust gas from the same CO boiler or a NO containing exhaust gas from a second CO boiler.

9. An apparatus comprising a CO boiler having an input conduit and an exhaust conduit, said input conduit is adapted to carry flue gas from a fluid catalytic cracker regenerator operated in partial burn mode characterised in that a further conduit is provided to allow recycling of at least part of the exhaust gas in the exhaust conduit to the CO boiler, preferably via the input conduit.

10. An apparatus comprising a first CO boiler having a first input conduit and a first exhaust conduit and a second CO boiler having a second input conduit and a second exhaust conduit wherein said first and second input conduits are adapted to carry flue gas from a fluid catalytic cracker regenerator operated in partial burn mode and said first exhaust conduit is adapted to transfer at least a part of the exhaust gas from the first CO boiler to the second CO boiler, preferably by way of the second input conduit.

11. Use of the exhaust gas stream of a CO boiler as a source of NO for addition to a CO boiler to thereby reduce NOx emissions therefrom.