Røggasrensning på skibe Flue gas cleaning on ships ......Røggasrensning på skibe – Flue gas...

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Røggasrensning på skibe – Flue gas cleaning on ships Slutrapport for projektet : 2012-22, DTU Kemi, røggasrensning, Støttet af Den Maritime Fond Project introduction The overall aim of the project is to elucidate how a new filter technology for cleaning of flue gases on ships for nitrogen oxides, NO x , might be developed based on so called ionic liquids. Legislation regarding emission of NO and NO 2 has been tightened considerably in the last number of years and seems to be further tightened in the coming years. Today we have two main technolgies available for reducing the NOx emisson on ships namely EGR and SCR SCR is a well known technology from stationary sources like power plants which if implemented on ships would require transport of considerable volumes of ammonia (or urea) aboard the ship. This is not desirable regarding logistics, safety and economy. In addition the technology is rather energy demanding since the flue gas has to be reheated after the motor to reach an effective performance of the catalyst. In addition the technology is not optimal during variable motor load. EGR (recirculation of exhaust gas) reduces the NOx formation in the motor but it reduces the combustion temperature and thus also the efficiency of the motor and reduces the motor life time. EGR can only in part reduce the NOx emission and is considered non-satisfactory towards the increasingly stricter emission limits as described in Tier-III by 2016. The solution proposed in the present project utilizes an ionic liquid selective absorbent towards NOx in the flue gas. Impregnated in a high surface area porous support the ionic liquid appears solid and can be extruded to a filter with low pressure drop regarding the passing flue gas. Also the negligible vapor pressure of the ionic liquid and its thermal stability up to around 160 C makes the filter stable in a temperature swing process where NOx is absorbed from the flue gas at end-of-pipe (after the SO2 wet-scrubber) conditions i.e. at around 60-80 C and desorbed as HNO3 by e.g. rotating the filter into a channel parallel to the chimney in a small stream of hot air at 130 C, the boiling temperature of nitric acid. The desorbed acid is then condensed in a sea water cooled heat exchanger and collected as commercial grade nitric acid to be unloaded ashore. This technology is therefore much more attractive than the available ones described above since no chemicals have to be added, the energy consumption is low and a waste is transformed to a commercial product, indeed a green solution to an environmental problem. . This work was sparked by the experiments performed in Centre for Catalysis and Sustainable Chemistry (CSC) at DTU Chemistry, addressing absorption of unwanted off-gasses like NO x , SO x and CO x by ionic liquids as alternative to cumbersome and costly catalytic or wet-scrubber technologies. Initial research including the apparently solid form of the absorber as Supported Ionic Liquid Phase (SILP) materials has proven promising for further technological development which in the present project is performed regarding selective absorption of NOx in flue gases on ships.

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  • Røggasrensning på skibe – Flue gas cleaning on ships

    Slutrapport for projektet : 2012-22, DTU Kemi, røggasrensning, Støttet af Den Maritime Fond

    Project introduction

    The overall aim of the project is to elucidate how a new filter technology for cleaning of flue gases on ships

    for nitrogen oxides, NOx, might be developed based on so called ionic liquids.

    Legislation regarding emission of NO and NO2 has been tightened considerably in the last number of years

    and seems to be further tightened in the coming years. Today we have two main technolgies available for

    reducing the NOx emisson on ships namely EGR and SCR SCR is a well known technology from stationary

    sources like power plants which if implemented on ships would require transport of considerable volumes

    of ammonia (or urea) aboard the ship. This is not desirable regarding logistics, safety and economy. In

    addition the technology is rather energy demanding since the flue gas has to be reheated after the motor

    to reach an effective performance of the catalyst. In addition the technology is not optimal during variable

    motor load. EGR (recirculation of exhaust gas) reduces the NOx formation in the motor but it reduces the

    combustion temperature and thus also the efficiency of the motor and reduces the motor life time. EGR

    can only in part reduce the NOx emission and is considered non-satisfactory towards the increasingly

    stricter emission limits as described in Tier-III by 2016. The solution proposed in the present project utilizes

    an ionic liquid selective absorbent towards NOx in the flue gas. Impregnated in a high surface area porous

    support the ionic liquid appears solid and can be extruded to a filter with low pressure drop regarding the

    passing flue gas. Also the negligible vapor pressure of the ionic liquid and its thermal stability up to around

    160 C makes the filter stable in a temperature swing process where NOx is absorbed from the flue gas at

    end-of-pipe (after the SO2 wet-scrubber) conditions i.e. at around 60-80 C and desorbed as HNO3 by e.g.

    rotating the filter into a channel parallel to the chimney in a small stream of hot air at 130 C, the boiling

    temperature of nitric acid. The desorbed acid is then condensed in a sea water cooled heat exchanger and

    collected as commercial grade nitric acid to be unloaded ashore. This technology is therefore much more

    attractive than the available ones described above since no chemicals have to be added, the energy

    consumption is low and a waste is transformed to a commercial product, indeed a green solution to an

    environmental problem.

    .

    This work was sparked by the experiments performed in Centre for Catalysis and Sustainable Chemistry

    (CSC) at DTU Chemistry, addressing absorption of unwanted off-gasses like NOx, SOx and COx by ionic liquids

    as alternative to cumbersome and costly catalytic or wet-scrubber technologies. Initial research including

    the apparently solid form of the absorber as Supported Ionic Liquid Phase (SILP) materials has proven

    promising for further technological development which in the present project is performed regarding

    selective absorption of NOx in flue gases on ships.

  • The report consists of 3 semi-annual reports over the periods:

    Period 1: 1. June 2012 - 31. November 2012

    Period 2: 1. December 2012 – 31. May 2013

    Period 3: 1. June 2013 – 31. November 2013

    Figure 1 illustrates the overall time plan of the project as visualized at the project start 1. June 2012.

    Figure 1: GANNT diagram and time plan for the project. The project is divided into 3 periods of six months

    each.

  • Period 1:

    Primary work force: Postdoc Anders Theilgaard Madsen

    Participation from Ph. d. student Andreas Jonas Kunov-Kruse and Master student Peter Langelund

    Thomassen

    Supervision: Assoc. Professors Susanne Mossin and Anders Riisager

    Project leader: Professor Rasmus Fehrmann

    Focus points

    Investigation of SILP materials for NOx absorption

    Literature study of deNOx- and absorption systems

    Acquisition of instrumentation for efficient data collection

    Evaluation of diffusion limitations and absorption kinetics

    Description of absorption test rig

    Temperature measurement

    Furnace

    Absorber bed

    Desorption bubbleflask (H2O)

    Reactant bubbleflask (H2O)

    Mass flow controllers

    1% NO/He

    Air or O2

    Air or O2(for humid)

    UV-vis spec.Gas outlet

    Gasoutlet

  • Figure 2: Schematic of the absorption test rig with gas inlet controls, heated absorber bed, UV-vis

    spectrophotometer and bubble flasks for gases

    Investigation of SILP materials for NOx absorption

    Til nu har vi i projektet udført absorptionsforsøg på nogle mesoporøse bærematerialer, der alle har været imprægneret med den ioniske væske [BMim+][NO3

    -]. Forsøgene er udført med 2000 ppm NO, 2000 ppm H2O samt ca. 11% He i atmosfærisk luft. Forsøgene er udført på et Jasco UV-vis spektrofotometer. Først og fremmest er tre forskellige bærematerialer – SiO2, TiO2 og Carbon – undersøgt med [BMim

    +][NO3-]

    som aktiv fase; af Figur 3 ses, at [BMim+][NO3-]/TiO2 og [BMim

    +][NO3-]/SiO2 har størst umiddelbar kapacitet

    og absorberer al NO i et antal minutter ved 40°C mens den C-bårne ioniske væske ikke på noget tidspunkt absorberer al NO i gassen. Det ses også, at den umiddelbare absorption – dette kan enten være tegn på en overfladereaktion uden absorption (oxidation af NO til N2O3, NO2 eller N2O5), eller at absorptionen efter den umiddelbare absorption er meget langsom (opblanding af NOx i det ioniske væskelag begrænser reaktionshastigheden). Den umiddelbare absorption svarer kun til maksimalt 2 % (for TiO2-bærer) af den beregnede kapacitet for den ioniske væske. Det formodes, at den umiddelbare absorption kun sker med den ioniske væskes overflade, mens selve absorptionen (opløsning af HNO3 i væsken) tager længere tid. Det er uvist hvorfor, men forskellen forhindrer en effektiv anvendelse af den ioniske væskes absorption.

    Figur 3: Absorption af flow af NO på [BMim+][NO3

    -] supporteret på tre forskellige porøse bærematerialer v. 40°C

  • Figur 4: Absorption af NO i gasflow på [BMim+][NO3

    -]/SiO2 ved 25, 40, 50 og 60°C. Det ses af Figur 4 at absorption ved lavere temperatur medfører en marginal kapacitetsforøgelse, men reaktionshastigheden falder samtidig. Desorptionen af NOx ved 90-120°C ses i Figur 5, og det ses, at der stadig ved 120°C kan bindes en lille smule NO i absorberen. Ligeledes ses, at den fuldt mættede absorber ikke frigiver NO-molekyler men kun andre NOx-specier (primært HNO3 og N2O5) – også selvom NO anvendes som reaktantgas.

    Figur 5: Koncentrationsprofiler af NO gasflow under desorption af NOx fra [BMIM][NO3]/TiO2 absorber.

    Literature study of deNOx- and absorption systems

    Et litteraturstudium er blevet gennemført mhp. at undersøge absorptionsteknologier for NOx-specier samt eventuelle alternative teknologier til DeNOx røggasrensning. I litteraturen er foreslået og undersøgt en række vandige systemer til absorption og/eller metal-kompleksering af NO eller NO2, men disse må ofte kombineres med oxidation af NO, som har en lav opløselighed i vand – oxidationsmidlerne der foreslås er f.eks. O3, H2O2, MnO4

    - eller CrO42-.

    Oxidationsmidlernes pris og de lave stofovergangstal for NO gør disse systemer ret uanvendelige på nuværende tidspunkt.

  • Den selektive katalytiske reduktion (SCR) af NOx er det mest åbenlyse alternativ til absorption. SCR med ammoniak er vidt udbredt industrielt og anvendes f.eks. på kraftværkerne til DeNOx. Det opereres typisk ved 300-450°C. Varianter af denne teknologi er på forskellige udviklingsstadier, f.eks. lavtemperatur DeNOx med NH3, reduktion med kulbrinter eller sågar enzymatisk reduktion. Endvidere udvikles elektrokemiske metoder med elektrolyseceller til selektiv NOx-reduktion. En udvikling af kulbrinte-SCR er NOx-Storage-Reduction (også kaldt Lean-NOx-Trap, LNT), udviklet mest indenfor bilindustrien til udstødning fra diesel-biler med forbrænding i luftoverskud, hvor NOx-specier absorberes i et fast stof, for derefter kortvarigt at blive reduceret til N2 i et reducerende gasmiljø. Endelig kendes den selektive ikke-katalytiske reduktion (SNCR), som dog kræver længere opholdstid ved høj temperatur (750-1100°C) og en NH3-kilde som reduktionsmiddel.

    Acquisition of instrumentation for efficient data collection

    En del af den første halvårsperiode i projektet har været dedikeret hjemskaffelse af forbedret analyseudstyr til at måle reaktanter og røggaskomponenter - konkret et Evolution 220 UV-Vis-spektrofotometer fra Thermo-Fisher Scientific. Det nye UV-Vis-spektrofotometer måtte anskaffes for fremover bedre at kunne måle flere af NOx-komponenterne i røggassen (primært NO2) samt måle hurtigere, hvilket er nødvendigt for at få et retvisende billede af absorber-materialernes funktion og effektivitet. Spektrofotometret samt dets reservedele er nu, medio januar 2013, hjemkommet og blevet installeret, hvorved vi nu kan genoptage og videreføre målingerne på SILP-materialer samt kontrollere absorptionsmålingerne fra vores gamle UV-vis-spektrofotometer.

    Evaluation of diffusion limitations and absorption kinetics

    Industri-deltagerne spurgte ved projektmødet ultimo september 2012 til eventuelle begrænsninger af absorptionen i en SILP-monolit, hvilket er blevet søgt vurderet. Generelt træder tre typer kinetiske begrænsninger frem i heterogene absorbere af SILP-typen (såvel som i heterogene katalysatorer): Selve absorptionen af NOx i det ioniske væskelag er den primære begrænsende faktor, men også diffusion igennem det porøse absorber-bæremateriale samt endda diffusion gennem filmlaget mellem SILP'ens overflade og midten af gasstrømmen kan være en kinetisk barriere. Absorptionshastigheden i SILP-materialerne er næppe begrænset af filmlags-diffusion ved de beskedne temperaturer som anvendes her, men vil sandsynligvis med nogle materialer være begrænset af diffusion i absorberens porer og med andre af både porediffusion og absorption på SILP-overfladen. De kinetiske begrænsninger ændrer sig afhængigt af temperatur, røggassens sammensætning og SILP-materialernes både fysiske og kemiske egenskaber som porøsitet, fordeling af ionisk væske i absorberens poresystem og den molekylære absorptionshastighed. Nylige undersøgelser i vores laboratorium tyder på at absorptionen på overfladen af en ionisk væske følger en 2. ordens afhængighed af NOx-koncentrationen. Dette betyder, at særligt absorption af lave NOx-koncentrationer begrænses af langsomme hastigheder.

  • Period 2:

    Primary work force: Postdoc Anders Theilgaard Madsen (1/2) and postdoc Søren Birk Rasmussen (1/2)

    Participation from Master student Peter Langelund Thomassen

    Supervision: Assoc. Professors Susanne Mossin and Anders Riisager

    Project leader: Professor Rasmus Fehrmann

    Focus points

    Optimization of temperature and ionic liquid loading

    Surface characterization

    Assess if absorption rate and capacity is compatible with a reasonable reactor design

    Development of test rig to separate oxidation and absorption steps

    Optimization of temperature and ionic liquid loading

    A series of experiments have been made to elucidate the influence on absorption of temperature and of

    the percentage of active ionic liquid absorbent loading in the pore system of the carrier material. A number

    of supported ionic liquid-phase (SILP) absorber were prepared by impregnating a well-defined porous silica

    (SiO2 Gel 60: average pore diameter 60 Å) with various pore fillings of butylmethyl imidazolium nitrate

    ([BMIM]+[NO3]-). The absorption capacities of these prepared materials were tested at 50, 60, 70, 80, 90

    and 100°C by exposition of the materials to a gas mixture consisting of 0.20% NO, 0.20% H2O, 19.6% He

    (spectator gas) per volume and balance atmospheric air.

    It was expected that a reduction of the liquid loading would lead to a better overall absorption capacity

    during the initial NO silent period with 100% NOx absorption. However, we have not reached a

    breakthrough in terms of absorption capacity with ionic liquid loading under normal flue gas

    concentrations. As is seen from Figure 6, the maximal silent period of the NO signal is 4.75 min for the SILP

    with 20% pore filling, corresponding to an absorption of mol NO/mol IL. This contrasts the fast and

    equimolar absorption observed using high-concentration NO gas on a pure ionic liquid droplet. We

    speculate that the limiting factor might be a higher order rate of oxidation of NO to higher NOx compounds

    on the ionic liquid surface.

    At high temperatures and high IL loadings (100°C and SILP with 20% pore filling of IL) we observed a

    considerable oxidation of NO to NO2 over the absorber bed. The continuous removal of NO under these

    conditions is so significant that it corresponds to a marked effect of catalytic oxidation.

  • Figure 6: Initial time periods of the NO signals after the IL bed for various loadings of [BMIM]+[NO3]-/SiO2 at

    low temperatures (left-hand side) and comparison of high and low temperature absorption for 20% SILP and

    carrier material (right-hand side) .

    Surface characterization

    The optimal amount of IL depends on the interior surface properties of the porous carrier. This is currently

    being elucidated primarily using the BET method with the aim of optimizing the liquid loading and choosing

    the optimal carrier material for the SILP absorber.

    Assess if absorption rate and capacity is compatible with a reasonable reactor design

    The initial experiments on IL films have been conducted with high concentration of NO (6.7% of the gas). In

    the SILP reactor we observe a of low initial silent periods of the NO signal during initial NOx-absorption

    when using simulated flue gas with a much lower concentration of NO in the gas.

    To elucidate the effect of O2 and NO concentration and to stress the absorber, new experiments were

    conducted. The prepared SILP with 20% ionic liquid pore filling of SiO2 Gel 60 was exposed to a gas

    consisting of 6.7% NO gas, 60% He (spectator), respectively 6.7% or 33% O2 and balance N2, at respectively

    25 and 60°C. This can be seen in Figure 7. Surprisingly, the absorption capacity during the silent period was

    0.43 mol NO/mol IL at 25°C and in 33% O2, corresponding to a factor of at least 20 compared to the highest

    measured SILP capacity at 50°C and 0.2% NO. Such a high NOx capacity would allow the design an absorber

    at around 0.3 m3/min absorption/MW engine power, see tabulation of the assumptions and calculation in

    Appendix. The high amount of both O2 and NO in the gas are not realistic in ship flue gases.

    0

    0,02

    0,04

    0,06

    0,08

    0,1

    0,12

    0,14

    0 2 4 6 8 10

    NO+NO2

    Time/minutes

    2000 ppm NO50-60°C

    60°C - 2.5%

    60°C - 5%

    50°C - 10%

    50°C - 20%

    1min 3.25min 4.75min0min

    0

    0,02

    0,04

    0,06

    0,08

    0,1

    0,12

    0,14

    0 20 40 60

    NO

    sig

    nal

    time (min)

    2000 ppm NO 20 % SILP Loading

    100°C 0%

    60°C - 20%

    100°C - 20%

  • Figure 7. Absorption of NO and NO2 with 20% pore filling of [BMIM]+[NO3]

    -/SiO2 in 6.7% NO gas and

    6.7%oxygen (air mix) or 33% oxygen(O2 mix). The ordinate units are arbitrary proportional with NO

    concentration, thus denoting the missing NOx due to absorption.

    At such high NO and O2 gas concentrations a substantial degree of oxidation of NO to NO2 in the gas piping

    around the reactor was observed, which was fairly constant throughout the experimentation. The oxidation

    is not dependent on the exposure of the system to light from the surroundings, but it is well-known from

    atmospheric NOx-chemistry as well.

    Interestingly the results in Figure 7 indicate a much higher preference for absorbing the formed NO2

    compared to pure NO when we quantified the NO and NO2 separately. This selectivity for NO2 absorption –

    which is not hindered by kinetics – could allow for a more efficient absorption if one could obtain the more

    oxidised NOx species.

    Development of test rig to separate oxidation and absorption steps

    Following up upon the data acquired in the first two periods it was suggested to separate the oxidation and

    the absorption. Previous experimentation with pure [BMIM]+[NO3]– ionic liquid in situ in a FTIR

    spectrometer has suggested that the oxidation-absorption of NO has a higher reaction order with respect

    to NO, and that the absorbed NOx species affect the composition of the bulk ionic liquid immediately. The

    rate constant is temperature dependent and the process will be more efficient at higher temperatures. The

    absorption, however, depends on the NOx gas and water to be present in the IL and the solubility of gasses,

    water and of the resulting HNO3 in the IL are all lower at higher temperatures. The setup was therefore

    altered to investigate this strategy.

    Change of strategy for the last period

    In conclusion the non-positive results on extending the observed silent period by only optimizing IL and

    support materials led us to the following:

    The exposure of the SILP to flue gas onboard a ship and the construction of a pilot plant scheduled to take

    place in the 3. period was not initiated. Instead the last period shifted focus to understanding the details of

    the mechanism and the kinetics of the chemical reactions as well as concentrating on optimizing for

    oxidation and absorption separately.

    60°C air mix

    0.05 mol

    NOx/mol IL

    1.35 mg NO/g

    adsorber

    25°C air mix

    0.15 mol Nox/mol IL

    4.29 mg NO/g adsorber

    60°C O2 mix

    0.13 mol NOx/mol IL

    3,68 mg NO/g adsorber

    25°C O2 mix

    0.43 mol Nox/mol IL

    12.3 mg NO/g adsorber

  • Figur 8 Proposed dual reactor set up

  • Period 3:

    Primary work force: Postdoc Anders Theilgaard Madsen

    Participation from Master student Peter Langelund Thomassen

    Supervision: Assoc. Professors Susanne Mossin and Anders Riisager

    Project leader: Professor Rasmus Fehrmann

    As outlined at the end of Period 2, the absorption rate in a single SILP setup under realistic flue gas

    conditions is assessed to be too low, it has been determined that the efforts should be put into elucidating

    the reaction mechanism and to pursue the dual-bed reactor setup.

    Focus points

    Elucidation of kinetic and mechanistic models for oxidation of NO in ionic liquids

    Synthesizing and testing alternative SILP formulations

    Assessment of absorption efficiency of pre-oxidized gas mixtures

    Testing of dual-bed reactor system

    Patenting of the dual reactor setup for removal of NOx from flue gasses

    Elucidation of kinetic and mechanistic models for oxidation of NO in ionic liquids

    In order to map the reaction mechanism, ATR-FTIR experiments were run at different NO concentrations

    Under the assumption that O2 is present in large excess compared to NO, the rate of reaction can be

    expressed as:

    r = k [NO]2

    k is a rate constant, which is dependent on the temperature and the concentration of O2

  • Figure 9: The reaction rate was recorded for different concentrations of NO and the results are plotted as a

    double logarithmic plot. The plot is close to linear with a slope of 2.01 indicating that the rate has a second

    order dependence on [NO].

    The spectral data obtained was also analyzed thoroughly in an attempt to identify intermediate species in

    the reaction. Doing this has led us to propose a reaction scheme for the steps involved in total oxidation of

    NO to HNO3:

    Reaction step Reaction coefficient

    (1) NO + NO3– N2O4

    – 6

    (2) N2O4– NO2 + NO2

    – 6

    (3) 2 NO2 N2O4 4

    (4) N2O4 + H2O HNO2 + H+ + NO3

    – 4

    (5) H+ + NO2– HNO2 6

    (6) 2 HNO2 [HNO2]2 5

    (7) [HNO2]2 + O2 2 HNO3 3

    (8) HNO3 H+ + NO3

    – 2

    (9) [HNO2]2 H2O + NO + NO2 2

    Total reaction 4 NO + 3 O2 + 2 H2O 4 HNO3

    Table 1: Proposed intermediate reactions involved in oxidation of NO to HNO3. The total reaction is the sum

    of the reaction steps (1)-(9) multiplied with the reaction coefficient.

    The sequence of reactions is complicated in order to obtain the stoichiometry of the total reaction. The

    most important reactions are number (1), where the inert NO molecule is activated by the nitrate in the IL

    and number (7) where the oxidation with O2 takes place. The overall reaction scheme is in corroboration

    with the reaction order of 2 with respect to NO if the reaction (1)-(6) are relatively fast and (7) is the rate-

    determining step.

  • The dimer of nitrous acid, HNO2 is believed to be the activated species and it shows us why IL media is

    relevant for this reaction and is performing so well at high concentrations. The IL provides activation of the

    inert NO molecule by providing a good initial oxidant, NO3–. Once NO is caught, the IL it provides a

    condensed media where two HNO2 molecules can cooperate on activating the also inert O2 molecule. The

    product is retained in the IL since HNO3 and NO3– can make especially strong symmetric hydrogen bonds.

    Once the resulting solution is heated above 130 C, the hydrogen bonds are broken and HNO3 desorb from

    the IL.

    In order to assess the dependence on the combined oxidation-absorption reaction on the temperature the

    IR experiment was performed at different temperatures.

    Figure 10: Rate of maximal linear rate of HNO3 generation in the IL as a function of the temperature.

    It was found that the combined oxidation and absorption does not follow the Arrhenius expression. On the

    contrary, there seems to be a maximum temperature which is around 30C. Chemical reactions run faster

    at higher temperatures but the capacity of the IL with respect to all reacting gasses and the products

    decrease with temperature.

    The reactor design of the dual bed should therefore be based on optimizing the oxidation and absorption

    reactions separately and trying to absorb above 60 C does not seem to be relevant.

    Synthesizing and testing alternative SILP formulations

    A series of SILP absorbers based on alternative support materials have been synthesized but testing was

    interrupted by the employment of the post doc at an external industrial company just after the expire of

    the present project.

  • Assessment of absorption efficiency of pre-oxidized gas mixtures

    In order to assess the absorption dynamics and efficiency of a pre-oxidized gas mixture, FTIR experiments

    were performed with NO2 in the feed gas instead of NO. This corresponds to entering at reaction (3) in

    Table 1. The results show that absorption of NO2 in the IL is significantly faster and that HNO3 builds up in

    the IL much faster than in the same reaction with NO at low concentrations. The results are corroborated

    by the UV-Vis monitoring of the SILP measurements. The first species that appear after breakthrough is not

    NO2 but NO, which was not in the feed gas. This corresponds to reaction (9) from Table 1 being active.

    In conclusion the IL is excellent in absorbing NO2 and the generation of HNO3 is much faster than in the

    combined oxidation and absorption of NO of similar concentration. The absorption rate can become faster

    than the rate of oxidation and the excess HNO2 dimer can decompose to NO and NO2. The different

    reactions in Table 1 are probably scrambling all nitrogen species to attain their equilibrium values. Some of

    the decomposition products will be caught further down the stream in a flow system.

    The absorption is optimal at lower temperatures (25-40 C) and demands the presence of water.

    Testing of dual-bed reactor system

    A schematic overview of the proposed dual-reactor setup can be found in Figure 8 (Period 2). NO is

    observed to be oxidized over the SILP bed even if no water is present. Both NO2 and N2O5 can be observed

    by UV-vis after the SILP bed. This effect has been found to increase significantly at higher temperatures (80-

    100 C), where a significantly portion of the NO in the simulated flue gas is converted into higher NOx

    species. The effect is also present in the bypass stream, but the SILP is more efficient. If the temperature

    approaches 130C, the catalytic effect of the SILP bed decreases again and the difference between the

    oxidizing bed and the bypass is less significant.

    Another possible configuration is also to pre-oxidize the NO using another method, and then absorb and

    perform the final oxidation of the NOx species in a temperature-swing absorber containing SILP. A technical

    scheme of the suggested oxidation – temperature-swing absorber is outlined in Figure 11.

    The first experiments with this setup show promising results. The difficult step is still the oxidation where

    we unfortunately were focusing on too high temperatures in the first experiments. Therefore the first

    results regarding the assessment of different IL/support combinations did not result in selecting a better

    combination than the first suggested combination of [BMIM]+[NO3]– and silica or anatase.

    The water necessary for the absorption step is luckily not found to be interfering with the oxidation step.

  • Figure 11: Scheme of the proposed temperature-swing absorber.

    Patenting of the dual reactor setup for removal of NOx from flue gasses

    In order to protect IP in connection with this project the dual reactor approach is in the process of being

    patented.

  • Final conclusions and outlook

    The knowledge that has been gained on the nature of the reaction can be used to improve the SILP formulations expected to be the final formulation of an absorber to be used on an industrial scale. The typical concentration of NO in a flue gas is below 4000 ppm. Therefore, the finding that this reaction is second order in NO is not optimal. However, if it was possible to increase NO concentration by pre concentration, this would greatly improve the reaction rate. Also, a pre oxidation to NO2 would help, as it has previously been proven that NO2 is more readily oxidized to nitrate. Another possible way to optimise the absorber would be to optimize the formulation of the IL. It is highly unlikely that the anion can be changed since this has been found to be exchanged with NO–3 during absorption. The cation however can be changed to improve the solubility of oxygen and possibly also NO. If this is possible, the rate of the initiation (1) and oxidation (7) reactions (Table 1) would be increased. Also, a different cation formulation could help improve dispersion of IL on the support material, giving a higher contact area. A higher contact area for the gas can also be obtained by optimising the support material. Given that the initiating reaction runs through the anion, the optimal support should have a high surface area and promote the anion. The pore structure of the support is also important, so optimising the pore size and distribution will likely improve the absorption capacity significantly. Based on the findings in the temperature measurements, it seems that the reaction rate depends largely on having the optimal temperature. These findings are inconclusive for now but should be investigated further in the near future. If there is as much to gain as indicated, by optimising the temperature, finding a good way to implement the absorber at this temperature should be first objective.

    As described in the introduction the development of a rotating ionic liquid absorber filter solution to be

    used onboard ships is obviously more attractive than the methods , SCR and EGR, available today . The

    present project have discovered challenges that were not to be foreseen at the launching of the project, i.e.

    regarding the kinetics of the absorption process, see above. However, during the project it was discovered

    that a tandem set-up consisting of an ionic liquid catalytic converter of NO to NO2 upstream at a higher

    temperature (e.g. 100-140 C) with an ionic liquid NO2 absorber downstream at a lower temperature (e.g.

    60-80 C) might be a sustainable solution.

    At the end of this project an application for further development of the technology by a ½ year post doc project was approved by the so called GAP-funding (CleanTek cluster funding). Here proof of concept for the efficiency of the dual reactor (tandem) set-up followed by extrusion of the absorber SILP material in the form of monoliths with channels for gas penetration will be attempted. If laboratory tests of these monoliths are promising additional tests under more realistic conditions will be performed in collaboration with our industrial partners.

  • Appendix: Calculations on absorber dimensions

    Assumptions:

    • 10 MW engine, 50% energy efficiency, fuel ~CH1,8, λ = 3,5. Absorb all NOx produced

    • NOx: 1100 ppm @ 13.2 Nm3/s (= 0.606 mol/s)

    • Flue gas temperature: ~60oC

    • SILP: [BMIm][NO3] / TiO2 (10 wt% IL)

    • Stoichiometry & saturation, IL:NOx = 1:0.43

    • Monolith absorber: Quadratic, each canal 4 · 4 mm2

    • (500 x 500 canals in monolith)

    • 10 wt% of monolith is active IL material)

    Absorber saturation time

    Base: Measured capacity

    • Absorber volume: 3m·2,5m·2,5 m=18,75m3

    • SILP capacity demand: 36,4 mol NOx/min

    • (=> 170 kg SILP/min)

    • Saturation time: 423 sec (7 min 3 sec)

    • Space time: τ = 0,82 s

    • Flow profile: Re = 720 (laminar)

    • Pressure drop: Δp = 4,8 mbar