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kinetic parameters for this reaction have been taken from [31].In order to integrate the kinetic rate equations the Cantera open source software [32, 33] is used. The used mechanism include a thermodynamical description of all involved species expressed源'自^751;文,论`文'网]www.751com.cn
through 7 coefficient NASA polynomials [34]. In order to simulate the oxidation of fuel bound sulfur a multi-zone model is invoked and the details of the
model will summarised in following.Up to the time of fuel ignition the fresh charge of the
cylinder is considered as a single zone and the working gas is compressed isentropically. By the time of ignition and until the combustion has ended,for each crank angle degree a new zone is created by mixing the differential amount of fuel burned during this particular time step (one crank angle degree) stoichiometrically with fresh air and subsequently equilibrating [35] at the corresponding measured cylinder pressure. We will refer to such a
unit as a parcel. After its creation and subsequent equilibration no more fuel is added to this particular parcel. Only fresh air is allowed to be added at a predefined mixing rate. The multi-zone approach taken here is in close analogy with that in [36]. In
each parcel the rate equations describing O/H chemistry and sulfur oxidation are integrated up to the time of the exhaust valve opening.Some further assumptions of the model are:Sulfur contained in the fuel is modeled as elemental S in the fuel feed.The fuel is mimicked by a mixture of noctane,benzene and ethanol. The exact composition is tuned in order to match the heating value of the real fuel as well as the elemental composition.
The concentration of all gas phase species is isotropic i.e. mixing is fast and complete.
For each time step it is assumed that temperature and pressure is constant at the
starting value for that particular time step.In order for the flame temperatures calculated not being unrealistically high a temperature drop due to radiation losses is included [36]
in which Tad is the adiabatic flame temperature calculated by equilibrating fuel and air at λ=1, cp is the constant pressure heat capacity of the combustion products and Crad is a heat radiation constant with a value chosen to give a temperaturedrop of approx 100 K for the parcels created early.Throughout this paper we will refer to a so-called into SO3 and H2SO4 which is denoted ε. The definition of ε is where Σ [SOx] is the sum of the concentrations of all sulfur containing species i.e. (SO2, SO3, H2SO4 ,...).
RESULTS AND DISCUSSION
Examination of the equilibrium An overall, yet simplified, description of the oxidation of fuel bound sulfur is outlined in the following reactions.
S + ½O2 = SO (3)
SO + ½O2 = SO2 (4)
SO2 + ½O2 = SO3 (5)
SO3 + H2O = H2SO4 (6)
The enthalpy of formation, ΔH298o, of the inpidual species are 6 kJ/mol (gas), -297 kJ/mol (gas), -396 kJ/mol (gas), and -814 kJ/mol (liquid) for SO, SO2,To which degree the sulfur is oxidised or converted into H2 SO4 depends on both thermodynamics and reaction kinetics. The above reactions are listed in the order of increasing thermodynamic stability of the product. According to the principle of Le Chatelier this implies that the higher the temperature the less degree of oxidation i.e. SO is favored vice versa at lower temperatures SO3/H2SO4 is favored. Also pressure is important; the higher the pressure, the higher degree of oxidation. To get a comprehension of the quantitative influence of temperature and pressure on the distribution of sulfur containing species the O2/SO2/SO3/H2SO4 equilibrium composition is calculated. The results of the equilibrium
calculations are shown in Figure 3. As seen from the figure the main transition from SO3 to SO2 takes place at around 1100 K at 10 bar and at approx. 1200 K at a pressure of 100 bar. Thus, in the early phase of the combustion all sulfur will be in the form of SO2. No significant amounts of SO were found unless above 2200 K. At the lowest temperatures