Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust

Stephen F. Benjamin, Carol A. Roberts

    Research output: Chapter in Book/Report/Conference proceedingChapter

    39 Downloads (Pure)

    Abstract

    Application of SCR catalyst technology in a light-duty Diesel exhaust system requires injection of a reductant. Aqueous urea is injected with a spray unit directly into the exhaust upstream of an SCR catalyst. Ideally, the aqueous urea droplets must first evaporate and break down to ammonia and HCNO before reaching the catalyst. Multiple chemical reactions then occur on the catalyst reducing NOx. Droplet size is thus critical in this process. Laser PDPA measurements have been made in the laboratory of the diameter of water droplets from a commercially available urea spray unit in hot air flow to characterize the spray. These measurements have shown significant numbers of droplets with diameters greater than 120 microns. Measurements have also been made with a typical mixer unit downstream of the spray unit and the effect of this on the droplet size distribution is reported. A simple model of the warm up and evaporation of an aqueous urea droplet convecting in a Diesel exhaust has been developed. This can be run as a single droplet simulation programme but could be incorporated into a full CFD model. The latter is required for designing SCR injection systems without recourse to testing. The model is valid for the early part of the evaporation process where it is assumed that water leaves the aqueous urea droplet and also for the later stage where the urea leaves the droplet prior to breaking down by thermolysis. The model has been tested against data available in the literature. The effect of initial droplet size, droplet injection temperature and velocity, and of the exhaust temperature, vapour pressure and flow rate under typical operating conditions are demonstrated. The implications of the model are that the urea droplets must be very small, less than 50 microns, if they are to evaporate in the short distance between the injector and the catalyst in a typical light duty Diesel exhaust system.
    Original languageEnglish
    Title of host publicationFuel Systems for IC Engines
    EditorsInstitution Of Mechanical Engineers
    Place of PublicationCambridge
    PublisherWoodhead Publishing
    ISBN (Print)9780857092106
    Publication statusPublished - Mar 2012

    Fingerprint

    Selective catalytic reduction
    Urea
    Catalysts
    Thyristors
    Evaporation
    Thermolysis
    Vapor pressure
    Water
    Chemical reactions
    Ammonia
    Computational fluid dynamics
    Flow rate

    Bibliographical note

    Paper presented at the IMechE Conference C1342: Fuel Systems for IC Engines. Held 14-15 March, 2012, Institution of Mechanical Engineers, London.

    Project ID: EP/F036175/1

    Keywords

    • selective catalytic reduction
    • diesel exhaust
    • aqueous urea
    • droplet size

    Cite this

    Benjamin, S. F., & Roberts, C. A. (2012). Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust. In I. O. M. Engineers (Ed.), Fuel Systems for IC Engines Cambridge: Woodhead Publishing.

    Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust. / Benjamin, Stephen F.; Roberts, Carol A.

    Fuel Systems for IC Engines. ed. / Institution Of Mechanical Engineers. Cambridge : Woodhead Publishing, 2012.

    Research output: Chapter in Book/Report/Conference proceedingChapter

    Benjamin, SF & Roberts, CA 2012, Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust. in IOM Engineers (ed.), Fuel Systems for IC Engines. Woodhead Publishing, Cambridge.
    Benjamin SF, Roberts CA. Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust. In Engineers IOM, editor, Fuel Systems for IC Engines. Cambridge: Woodhead Publishing. 2012
    Benjamin, Stephen F. ; Roberts, Carol A. / Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust. Fuel Systems for IC Engines. editor / Institution Of Mechanical Engineers. Cambridge : Woodhead Publishing, 2012.
    @inbook{679a6453993049f689d5ff02f0b179ba,
    title = "Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust",
    abstract = "Application of SCR catalyst technology in a light-duty Diesel exhaust system requires injection of a reductant. Aqueous urea is injected with a spray unit directly into the exhaust upstream of an SCR catalyst. Ideally, the aqueous urea droplets must first evaporate and break down to ammonia and HCNO before reaching the catalyst. Multiple chemical reactions then occur on the catalyst reducing NOx. Droplet size is thus critical in this process. Laser PDPA measurements have been made in the laboratory of the diameter of water droplets from a commercially available urea spray unit in hot air flow to characterize the spray. These measurements have shown significant numbers of droplets with diameters greater than 120 microns. Measurements have also been made with a typical mixer unit downstream of the spray unit and the effect of this on the droplet size distribution is reported. A simple model of the warm up and evaporation of an aqueous urea droplet convecting in a Diesel exhaust has been developed. This can be run as a single droplet simulation programme but could be incorporated into a full CFD model. The latter is required for designing SCR injection systems without recourse to testing. The model is valid for the early part of the evaporation process where it is assumed that water leaves the aqueous urea droplet and also for the later stage where the urea leaves the droplet prior to breaking down by thermolysis. The model has been tested against data available in the literature. The effect of initial droplet size, droplet injection temperature and velocity, and of the exhaust temperature, vapour pressure and flow rate under typical operating conditions are demonstrated. The implications of the model are that the urea droplets must be very small, less than 50 microns, if they are to evaporate in the short distance between the injector and the catalyst in a typical light duty Diesel exhaust system.",
    keywords = "selective catalytic reduction, diesel exhaust, aqueous urea, droplet size",
    author = "Benjamin, {Stephen F.} and Roberts, {Carol A.}",
    note = "Paper presented at the IMechE Conference C1342: Fuel Systems for IC Engines. Held 14-15 March, 2012, Institution of Mechanical Engineers, London. Project ID: EP/F036175/1",
    year = "2012",
    month = "3",
    language = "English",
    isbn = "9780857092106",
    editor = "Engineers, {Institution Of Mechanical}",
    booktitle = "Fuel Systems for IC Engines",
    publisher = "Woodhead Publishing",

    }

    TY - CHAP

    T1 - Significance of droplet size when injecting aqueous urea into a selective catalytic reduction after-treatment system in a light-duty diesel exhaust

    AU - Benjamin, Stephen F.

    AU - Roberts, Carol A.

    N1 - Paper presented at the IMechE Conference C1342: Fuel Systems for IC Engines. Held 14-15 March, 2012, Institution of Mechanical Engineers, London. Project ID: EP/F036175/1

    PY - 2012/3

    Y1 - 2012/3

    N2 - Application of SCR catalyst technology in a light-duty Diesel exhaust system requires injection of a reductant. Aqueous urea is injected with a spray unit directly into the exhaust upstream of an SCR catalyst. Ideally, the aqueous urea droplets must first evaporate and break down to ammonia and HCNO before reaching the catalyst. Multiple chemical reactions then occur on the catalyst reducing NOx. Droplet size is thus critical in this process. Laser PDPA measurements have been made in the laboratory of the diameter of water droplets from a commercially available urea spray unit in hot air flow to characterize the spray. These measurements have shown significant numbers of droplets with diameters greater than 120 microns. Measurements have also been made with a typical mixer unit downstream of the spray unit and the effect of this on the droplet size distribution is reported. A simple model of the warm up and evaporation of an aqueous urea droplet convecting in a Diesel exhaust has been developed. This can be run as a single droplet simulation programme but could be incorporated into a full CFD model. The latter is required for designing SCR injection systems without recourse to testing. The model is valid for the early part of the evaporation process where it is assumed that water leaves the aqueous urea droplet and also for the later stage where the urea leaves the droplet prior to breaking down by thermolysis. The model has been tested against data available in the literature. The effect of initial droplet size, droplet injection temperature and velocity, and of the exhaust temperature, vapour pressure and flow rate under typical operating conditions are demonstrated. The implications of the model are that the urea droplets must be very small, less than 50 microns, if they are to evaporate in the short distance between the injector and the catalyst in a typical light duty Diesel exhaust system.

    AB - Application of SCR catalyst technology in a light-duty Diesel exhaust system requires injection of a reductant. Aqueous urea is injected with a spray unit directly into the exhaust upstream of an SCR catalyst. Ideally, the aqueous urea droplets must first evaporate and break down to ammonia and HCNO before reaching the catalyst. Multiple chemical reactions then occur on the catalyst reducing NOx. Droplet size is thus critical in this process. Laser PDPA measurements have been made in the laboratory of the diameter of water droplets from a commercially available urea spray unit in hot air flow to characterize the spray. These measurements have shown significant numbers of droplets with diameters greater than 120 microns. Measurements have also been made with a typical mixer unit downstream of the spray unit and the effect of this on the droplet size distribution is reported. A simple model of the warm up and evaporation of an aqueous urea droplet convecting in a Diesel exhaust has been developed. This can be run as a single droplet simulation programme but could be incorporated into a full CFD model. The latter is required for designing SCR injection systems without recourse to testing. The model is valid for the early part of the evaporation process where it is assumed that water leaves the aqueous urea droplet and also for the later stage where the urea leaves the droplet prior to breaking down by thermolysis. The model has been tested against data available in the literature. The effect of initial droplet size, droplet injection temperature and velocity, and of the exhaust temperature, vapour pressure and flow rate under typical operating conditions are demonstrated. The implications of the model are that the urea droplets must be very small, less than 50 microns, if they are to evaporate in the short distance between the injector and the catalyst in a typical light duty Diesel exhaust system.

    KW - selective catalytic reduction

    KW - diesel exhaust

    KW - aqueous urea

    KW - droplet size

    M3 - Chapter

    SN - 9780857092106

    BT - Fuel Systems for IC Engines

    A2 - Engineers, Institution Of Mechanical

    PB - Woodhead Publishing

    CY - Cambridge

    ER -