The startup characteristics of plasmatron reformers with gasoline fuel for the generation of hydrogen rich gas is discussed. A brief set of experiments has been carried out at a single fuel and air flow rate. The air/fuel mixture preparation, especially the implications of the gasoline vaporization, has been modeled using a CFD code. I

Bromberg, L.

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PSFC/JA-05-49Startup characteristics ofplasmatron gasoline reformersL. BrombergDecember 15, 2005MIT Plasma Science and Fusion CenterWork supported by the US Department of Energy, Office of FreedomCar and VehicleTechnologiesAbstractThe startup characteristics of plasmatron reformers with gasoline fuel for the generationof hydrogen rich gas is discussed.  A brief set of experiments has been carried out at asingle fuel and air flow rate. The air/fuel mixture preparation, especially the implicationsof the gasoline vaporization, has been modeled using a CFD code.I. IntroductionWe have previously reported in a series of papers [1-6] the performance of a plasmatronreformer using methane and propane. That work provides increased understanding of thebehavior of the plasmatron reformer operating with heavier hydrocarbons. In a previouspaper the homogeneous reformation of refined and unrefined vegetable oils and ethanolhave been reported [7].  In this paper the results of plasmatron gasoline reformeroperation during startup are discussed..The performance of liquid fuels and gaseous fuels is very different due to the latent heatof vaporization of the liquid fuel.  In reference 7 we reported exclusively in the steadystate characterization of the plasmatron.  There are three main differences between liquidand gaseous fuels.  First, there is a finite rate of vaporization of the fuel, and thus eithercombustion occurs as a diffusion flame (as is the case in a diesel engine).  The second hasto do with the heat of vaporization of the fuel, which cools the air/fuel mixture. Finally,because of the need to prevent wall coating, the liquid fuel is exclusively injected throughthe axial port, while the gaseous fuels could be injected through any set of ports, andindeed had best performance when injected through the swirl gas.Section II  discusses the setup and the characteristics of the plasmatron gasolinereformers, in particular the effect of the fuel atomization and the cooling of the air/fuelmixture due to the latent heat of vaporization of the gasoline.  Section III describes asimple CFD model that is useful to understand the air/fuel mixing, as well as thevaporization of the gasoline droplets.  Section IV describes the experimental results forgasoline transients.  Section V summarizes the work.II.  Experimental setupThe experimental setup has been described in [1] and will only be briefly described here.The plasmatron used is the same used in [1-7] and is shown in Figure 1.The experiments reported in [1-6] were carried out with gaseous fuels, and there was noneed for an additional pump for the liquid fuel or for liquid fuel atomization. For liquidfuels, the approach used in the plasmatron fuel reformer work has been to form a finespray from the liquid fuel, followed by air assist atomization.  We expect to have dropletsize on the order of < 20 µm. The liquid fuels in the present tests have been introducedinto the plasmatron through a nozzle that forms a fine spray of droplets.  The nozzle usedin these experiments is a B-37 nozzle. The characteristics of this nozzle are given inReference [7].The atomization air flow rate was monitored by a TSI air flow sensor.  The TSI sensorwas also used to confirm that the two other mass-flow controllers (for the wall air and forthe plasma air) were within certification.The fuel pump used to pressurize and monitor the flow rate was a variable displacementpump attached to a variable speed drive from Fluid Metering. The pump was calibratedby capturing and then weighing the fluid for a given amount of time. The adjustedparameter was the speed of the drive. The pump provides constant flow rate for pressureslower than about 200 psi, and we have stayed below this limit, operating close to 50 psi.Figure 1.  Photograph of the plasmatron used in these experiments, as well as those inreference [1-7].The transient gas analysis was determined using an electrostatic quadrupole massspectrometer, from Pfeiffer. The mass spectrometer setup was described in [2].  TheSoot/raw fuel droplets were measured using a Wagner 2000 opacity meter.Conventional 87 octane gasoline was used in the experiments.III.  CFD Modeling in the absence of chemistryThe calculations were performed using FLUENT 6.0 CFD package. The solution is for asteady state solution, axisymmetric, with air as the fluid. A Reynolds-Average Navier-Stokes model has been chose, with a k-ε formulation for the turbulence.  The modelincludes swirl and compressibility of the air.  The energy equation is solved. The solutionis shown on half of the cross section of the plasmatron.  A small section (1 in) long of thereaction extension cylinder is also included in the model.No chemistry is assumed.  Wall temperatures are at the same temperature as the gas(300K). The outlet condition is atmospheric pressure.  Inlet conditions in all three ports(plasma air, wall air, atomization air) are as mass flow rate. Under-relaxation has beenused for the pressure (0.3) and momentum (0.7).  The scheme for solution is 2nd orderupwind.Table 1 shows the parameters used in the calculations. The gasoline flow rate assumedwas 1 g/s, twice that of the experiments.  The fuel was injected using “surface” injectionat the port for the atomization air (uniformly distributed). Fuel droplets were injectedexclusively in the axial direction, with no radial or tangential component, that is, with azero spray angle.  It was assumed that the injection velocity of the droplets was 50 m/s.The droplets composition was n-octane and uniform in diameter, with the droplet size inthe first set of calculations is assumed to be 50 µm.  This is a relatively large drop size.Evaporation of the droplets was included, and the concentrations of the n-octane vaporshave been calculated. Smaller drop sizes will be considered later in this paper.   Finally,the effects of spray distribution are discussed in the last section.A. 50 µm diameter dropletsIn this section, the upper range of the particulate size is investigated.Table 1Plasmatron gen 3APlasma air 0.0018 g/sVelocities: swirl 50./s                   Radial -12 m/sWall air 0.0028 g/sAtomization air 0.0013 g/sAtomization air temp 300 KFuel injectionDroplet size 50 µmInitial droplet speed -50 m/sComposition of droplet n-octane liquidSpray angle 0Figure 2. Axial velocity of the gas (a) with fuel injection (1 g/s) (b) no fuel injectionFigure 2 shows the axial velocity of the gas in the plasmatron for the conditions of Table1, with and without fuel. Note that the axial velocity of the atomization air is lower in thecase of the fuel injection than without, reflecting the fact that momentum is beingtransferred from the gas to the droplets, which are injected with lower speed (50 m/s).Figure 3.  Droplet average path (a) droplet axial velocity (b) droplet residence timeFigure 3 shows results of calculations of n-octane droplets.  The calculations areperformed using stochastic air and droplet flow fields, with multiple droplets launched atthe surface of the nozzle. Figure 3a shows the calculated droplet velocity on the dropletpath, while Figure 3b shows the residence time.  The droplets quickly achieve thevelocity of the gas (air), by the time that they reach the interlectrode spacing they are wellentrained in the atomization air flow.Figure 4. Mass fraction of Gaseous C8H18 (relative to nitrogen)The mass concentration of gaseous n-octane is shown in Figure 4 for the case of 50 mmdroplets.  Only a small fraction of the fuel has vaporized, and the conditions forstoichiometric conditions (about 6% by mass for an air/fuel ratio of 15) are not reachedwith the fuel vapor.B.  10 µm diameter dropletsIn order to examine the other extreme of the droplet size distribution, the calculationswere performed with 10 mm diameter droplets. The smaller droplets size results inincreased evaporation of the droplets, due to increased surface area, as well as in reducedparticulate inertia, with increased impact on the droplet paths due to the turbulence of thegas.Figure 5. Stochastic particle path vs (a) residence time (b) particle diameterThe random paths of some droplets, launched at different locations on the nozzle surface,are shown in Figure 5. Different sets of droplets have been used in Figures 5 (a) and (b),as noticed by the outermost droplets. Figure 5a shows the residence times of theparticulates, while Figure 5b shows the droplet diameter. Some evaporation of theparticle is seen, more than in the case with 50 mm particles.  However, even at the inter-electrode location (where the plasma is expected), the droplet diameter has decreased byonly about 1%, corresponding to about 3% of the droplet. The correspondingconcentration of n-octane is shown in Figure 6.:Figure 6.  Concentration (by mass) of gaseous n-octaneFigure 7.  Axial velocity (a) of droplets along their path (b) of continuous mediaAs expected, with the smaller particle size there is stronger interaction between thecontinuous media and the droplets.  Thus, in the case with smaller droplet size theatomization air slows down quicker and the droplets gain speed faster than in the casewith larger droplet size. Also, the path of the smaller droplets (stochastically calculated inevery plot) is influenced more by the local turbulence of the flow.C. Finite spray angleThe surface injection has no capability of spray angle distribution, but it can have a non-zero radial speed (with no spread, therefore resulting in a cone-shape). Since the effect ismore pronounced with the smaller droplets, the calculations of the effect of finite sprayangle  has been performed for 10 µm droplets.The initial radial speed of the droplets was varied from 0 through 30 m/s.  The initialaxial speed of the droplets was kept constant at 50 m/s.  Therefore, relatively wide sprayangles were modeled.Figure 8. Stochastic paths for initial radial velocities of (a) 0 m/s, and (b) 10 m/sFigure 9. Stochastic paths for initial radial velocities of (a) 20 m/s, and (b) 30 m/sThe radius of the droplet “cloud” increases slightly when the initial radial speed of thedroplets increases from 0 to 10 m/s.  However, the location of the paths in the inter-electrode region do not change substantially when the initial radial velocity of thedroplets increases further from 10 through 30 m/sThe complex vaporization process made modeling of the system very difficult, and it hasbeen analyzed in a simplified CFD model. The simple models of the plasmatron, withoutchemistry, show that indeed very small fraction of the gasoline is vaporized.D. Cooling of the gas by evaporating fuelThe gasoline droplets need some heat in order to vaporize.  The vaporization rate of thedroplets follows the ruledD2/dt = -KeWhere Ke is the evaporation constant and is a function of temperature.  For stoichiometriccombustion, the change in the temperature of the air due to the vaporization of thegasoline is about 24 K, assuming constant pressure process.  However, for partialoxidation, where the air/fuel mass ratio is about 1/3 that of stoichiometric combustion,the temperature of the air is reduced by as much as 70 K (assuming constantthermodynamic properties of the air).  These large drop in temperature means that only asmall fraction of the gasoline will be vaporized prior to entering the reaction zone, whereexothermic reactions raise the temperature of the gas.V. Transient experiments with gasolineThe plasmatron setup described in section II was used to determine the startupcharacteristics of the plasmatron. Only one set of experiments was carried out, at an O/Cratio of the experiments is 1.06, near ideal partial oxidation, and at a power of 300 W.The measured hydrogen concentration is shown in Figure 10. The experiments were carriedout by turning on the air flow, the plasma, and then the fuel.Figure 10.  Measured concentration of H2 as a function of time, for 3 startups.After the early startup, the hydrogen concentration drops to ~ 4 %.  It is likely that hisprocess occurs because of changes in the air/fuel mixture characteristics.  After a fewseconds, the hydrogen concentration starts to increase rapidly.The soot concentration, as measured with the opacity meter, was below measurable values.VI. SummaryExperiments and modeling of the vaporization with the time response of a plasmatrongasoline reformer were carried out.We have determined that the response time of the mass spectrometer is on the order of 1 s.Thus, it is likely that the hydrogen concentration reaches about 4% 1 s after turn-on. Bycomparison, the response time with gaseous fuels was on the order of 2 s, at substantiallyhigher O-to-C ratios..0123456789100 1 2 3 4 5time (s)Hydrogen concentration (% mol)AcknowledgementWe want to thank the continued support of Dr. Sidney Diamond.  His encouragement, depthand width of knowledge, enthusiasm and `Joye de Vivre’ will serve as standards for us toemulate. Dr. Kamal Hadid helped to operate the mass spectrometer during the experiments.References[1] L. Bromberg, K. Hadidi and D.R. Cohn, Experimental Investigation of Plasma AssistedReforming of Methane I: Steady State Operation, Plasma Science and Fusion Center ReportJA-05-10[2] L. Bromberg, K. Hadidi and D.R. Cohn, Experimental Investigation of Plasma AssistedReforming of Methane II: Start-up, Plasma Science and Fusion Center Report JA-05-11[3] L. Bromberg and N. Alexeev, Plasma Assisted Reforming of Methane: Two StagePerfectly Stirred Reactor (PSR) Simulation, Plasma Science and Fusion Center Report JA-05-12[4] L. Bromberg, Modeling of Plasma Assisted Reforming of Methane II: Partially StirredReactor (PASR) Simulation, Plasma Science and Fusion Center Report JA-05-13[5] L. Bromberg, CFD modeling of Plasmatron Methane Reformers, Plasma Science andFusion Center Report JA-05-14[6] L. Bromberg, K. Hadidi and D.R. Cohn, Experimental Investigation of Plasma AssistedReforming of Propane, Plasma Science and Fusion Center Report JA-05-15[7] L. Bromberg, K. Hadidi, D.R. Cohn, Plasmatron Reformation of Renewable Fuels,Plasma Science and Fusion Center Report JA-05-3

Startup characteristics of plasmatron gasoline reformers

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Startup characteristics of plasmatron gasoline reformers

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