Plasmatron reformers can provide attractive means for conversion of diesel fuel into hydrogen rich gas. The hydrogen rich gas can be used for improved NOx trap technology and other aftertreatment applications

Bromberg, L.

Cohn, D.R.

Heywood, J.

Rabinovich, A.

DSpace@MIT

       PSFC/JA-02-30  Onboard Plasmatron Generation of Hydrogen Rich Gas for Diesel Engine Exhaust Aftertreatment and Other Applications  L. Bromberg1, D.R. Cohn1, J. Heywood2, A. Rabinovich1   December 11, 2002    1 MIT Plasma Science and Fusion Center 2MIT Dept of Mechanical Department and Sloan Automotive Lab.            Submitted for publication to International Journal of Hydrogen Energy    Work supported by US Department of Energy Office of Transportation Technologies.   ABSTRACT  Diesel plasmatron reformer technology can provide substantial throughputs without the need of using a catalyst The use of the special plasma facilitates robust non-catalytic conversion of diesel fuel into hydrogen rich gas with hydrogen yields that could be sufficient for NOx trap regeneration applications.  Higher yields can be obtained with the use of a catalyst.  Plasmatron reformers could also be used for manufacturing of ethene, which could be useful as a selective reducing agent.  In addition, plasmatron reformer technology can be used for converting a range of biofuels into hydrogen rich gas.  These fuels include various oils.  This capability may improve the prospects for use of renewable fuels in a variety of vehicular applications. This paper discussed the status of plasmatron diesel fuel reformers and possible applications to diesel engine exhaust aftertreatment.   Onboard Plasmatron Generation of Hydrogen rich Gas for Diesel Engine ExhaustAftertreatment and Other ApplicationsL. Bromberg*, D.R. Cohn*, J. Heywood**, A. Rabinovich** Plasma Science and Fusion Center**Dept. of Mechanical Engineering and Sloan Automotive Laboratory.Massachusetts Institute of TechnologyCambridge MA 02139 I. IntroductionPlasmatron reformers can provide attractive means for conversion of diesel fuel intohydrogen rich gas.  The hydrogen rich gas can be used for improved NOx trap technologyand other aftertreatment applications [1].Plasmatron reformers developed at MIT use a special low power, low current electricaldischarge to boost partial oxidation conversion of hydrocarbon fuels into hydrogen andCO.  This reformer technology provides the advantages of• Rapid startup and response to transient conditions• Efficient conversion• Compact size• Relaxation or elimination of reformer catalyst requirements• Capability to process difficult to reform fuelsDiesel plasmatron reformer technology can provide substantial throughputs without theneed of using a catalyst The use of the special plasma facilitates robust non-catalyticconversion of diesel fuel into hydrogen rich gas with hydrogen yields that could besufficient for NOx trap regeneration applications.  Higher yields can be obtained with theuse of a catalyst.  Plasmatron reformers could also be used for manufacturing of ethene,which could be useful as a selective reducing agent.  In addition, plasmatron reformertechnology can be used for converting a range of biofuels into hydrogen rich gas.  Thesefuels include various oils.  This capability may improve the prospects for use ofrenewable fuels in a variety of vehicular applications. This paper discussed the status ofplasmatron diesel fuel reformers and possible applications to diesel engine exhaustaftertreatment. II. ReformationPlasmatron fuel converters operate through the generation of a continuous dischargewhere fuel enters the reformer.  This discharge provides large volume reaction initiationof the reforming reaction.  The discharge generates numerous localized regions of intenseheating (even though the resulting spatially averaged temperature increase can be quitemodest), as well as large number of radicals that promote the reaction. The discharge andthe subsequent exothermic reactions generate enough energy for the gasification of theremaining liquid fuel.The plasma induced turbulence also improves the mixing of the air/fuel mixture, as wellas mixing of those regions were partial oxidation has started with those regions whereconversion has yet to begin.  Thus, reactions occur over essentially the entire reformervolume facilitating high conversion efficiency, in a device with small volume.  Rapidheating of the fuel and good air/fuel mixing are also helpful in the minimization of sootproduction.In some circumstances, the plasma energy can be used to provide additional enthalpyincrease of the air/fuel mixture.  This capability can be utilized to meet demandingrequirements, such as rapid startup and quick transient performance. (a) (b)Figure 1 Schematic diagrams of plasmatron fuel converters developed at MIT.  (a)Thermal plasmatron (High current, high power, water cooled electrodes, circa1999); (b) Low current plasmatron (Low current, low power, long electrode lifetimeCirca 2001)Present plasmatron development at MIT involves low current plasmatron reformers.Typical parameters of operation for a low current plasmatron reformer are given in Table1.  The electrical power can be varied, and the device has been operated at powers from50 W to > 800 W.  We have used this type of device to generate a reformate with anequivalent heating value power of 10-40 kW, from diesel and other heavy hydrocarbons.The power supply is current controlled, and the plasma current entry in Table 1 refers tothe average value after breakdown.Table 1.Low current plasmatron operating parameters.Power W 300-600Plasma Current A .1-.4H2 flow rate slpm 40Length cm 40Volume cm^3 2000Weight kg 3 III. Diesel fuel reformingCharacteristics of thermal and low current diesel fuel plasmatron reformers are shown inTable 2.  The thermal plasmatron operates at ~100V, 15A DC using an arc plasma, whilethe low current plasmatron reformer operates at 500-1000V, 0.2-0.8 A AC.Table  2Diesel Plasmatron Reformer ParametersThe reformate composition for homogeneous (noncatalytic) and catalytic reforming(where the fuel first passes through the discharge which precedes a catalyst) for the lowcurrent plasmatron is shown in shown in Table 3.  The ratio of oxygen to carbon atoms inthe air/fuel mixture is 1.2-1.4. The thermal plasmatron was operated without a catalyst.Table 3Reformate parameters for low current plasmatron IV. Biofuels reformingAs is the case with diesel fuel, bio-oils are difficult to properly reform because of theirpropensity to generate soot in the partial oxidation process.Plasmatron reformer conversion of a number of biofuels has been investigated. Corn oil,canola oil and soy oils have been studied. Ethanol has also been reformed.Thermal plasmatron 1999Low current plasmatron 2001Electrical power W >1500 100-800O/C ratio ~1 1.2-1.4Fuel flow rate g/s 0.6 0.4Hydrogen yield 0.9 .5-.9Power conversion efficiency 0.9 0.85Reformate temperature K 1200 1000-1300Homogeneous CatalyticConcentration (vol %)H2 8% 18%CO 15% 22%C2's 3% 0.6%ConversionH2 in fuel to Hw in reformate 50% 80%C in fuel to C2's 25% 5%Energy conversion efficiency 75% 70%Reformate temperature (K) 1000 850Table 4 shows results of tests using a thermal plasmatron fuel converter.  These tests arebeing extended to include use of a low current plasmatron reformer.Table 4Plasmatron fuel converter reforming of biofuelsThermal plasmatron V. Diesel aftertreatment exhaust applicationsPotential diesel exhaust aftertreatment applications using plasmatron generation ofhydrogen rich gas include:• NOx absorber catalyst regenerationo NOx regenerationo Sulfur regeneration• Hydrocarbon regeneration of SCR catalyst• Onboard ammonia manufacturingThese potential applications are briefly described below. The SCR regeneration byhydrocarbons, and the ammonia production applications are speculative at this time, butmerit further study.a) NOx Absorber Catalyst Regeneration using Diesel plasmatron reformerThe regeneration process of the NOx absorber trap uses a reductant.  Hydrogen rich gas isa significantly stronger reducing agent than diesel, with reduced fuel penalty comparedwith diesel fuel.  Although this is important in itself, a potentially more importantadvantage of using hydrogen as a reductant, instead of diesel fuel, is due to the fact thathydrogen can regenerate traps at lower catalyst temperature than possible with straightdiesel fuel.  This is important for trap regeneration under conditions where catalyst doesnot warm up sufficiently, such as urban drive cycles.As a consequence of the use of a stronger, more efficient reductant (higher concentrationof hydrogen), it should be possible to decrease the regeneration time and the fuel penalty.This could be particularly important for the case of a single leg NOx trap configuration(only one trap, regenerated on-line). In this case decreasing the regeneration time couldhave an especially significant beneficial effect on the fuel penalty. In either the case ofsingle or tandem leg configuration, plasmatron diesel fuel reformers can operate with theCANOLA CORN OILFuel flow rate (g/s) 0.3 0.47Hydrogen concentration (% vol) 25.6 23Methane concentration (% vol) 1.7 7.6Carbon Monoxide concentration (% vol) 26 18.6Carbon Dioxide concentration (% vol) 0.3 2.1Hydrogen yield 92% 84%engine exhaust as the source of the oxidizer.  This minimizes the amount of fuel requiredfor generating the hydrogen rich gas or the high temperatures required for regeneration.In addition, using hot hydrogen rich gas, Parks [2] has obtained good desulfurization atlower temperatures in a net reducing atmosphere.  A comparison of desulfurizationperformance using reformate and diesel fuel has yet to be performed.b) Light Hydrocarbon catalyst regenerationThe regeneration of certain NOx catalysts requires light hydrocarbons. The dieselplasmatron reformer, when operated in the homogeneous mode, can produce largeamounts of ethylene.  Unfortunately, we have not been able to create slightly heavierhydrocarbons, such as propane, which seem to be more effective in catalyst regeneration,such as in the case of nonthermal plasma aftertreatment.Table 5 shows the results of operation a low current plasmatron reformer (with improvedreactor) for optimal production of C2’s.  In this case, it is possible to have carbonconversion to C2’s of about 30%. The measured energy conversion efficiency (heatingvalue of the reformate divided by the heating value of the fuel) is higher than in the caseof high hydrogen yields.Table 5Homogeneous plasmatron conversion for production of light hydrocarbonsImproved reactor configurationc) Onboard ammonia manufacturingDiesel fuel plasmatron reformers have potential for onboard ammonia generation forSCR catalytic regeneration.  Two options for manufacturing of ammonia are:1. Ammonia production from hydrogen rich gas (N2, H2 --> NH3)2. Ammonia from onboard generated NO (NO, H2 --> NH3)Electrical power (W) 500Reformate compositionH2 9.5CO 15.2CO2 4.1N2 63.5CH4 3.4C2H4+C2H6 3.1C2H2 0.1YieldsCmHn to H2 44%CmHn to CO 58%CmHn to C2's 28%Reformate thermal efficiency 88%Reformate power (HHV, kW) 36In both approaches, a plasmatron fuel converter might provide the hydrogen required forthe ammonia manufacturing. The practicality of using these processes is uncertain.  Proofof principle experiments for these options have not been performed.A system scheme based on approach 2 is shown in Figure 2. NO, from engine exhaust,with additional NO generation from a thermal arc discharge, is converted to ammonia byreacting it with hydrogen over a catalyst. One source of system inefficiency comes fromthe arc requirement for increasing the NO concentration in the exhaust gas (to minimizethe free oxygen).  A second  inefficiency comes from the hydrogen/CO required toremove the free oxygen prior to converting the NO into ammonia.  However, even withthese parasitic inefficiencies, system fuel penalties on the order of a few percent might beobtained under appropriate assumptions.Figure 2. Onboard manufacturing of ammonia from NO and hydrogen. VI. ConclusionsPlasmatron reformers can provide substantial advantages in converting diesel fuel andother difficult to reform fuels (e.g. bio-oils) into hydrogen rich gas. Use of the hydrogen rich gas could significantly improve prospects for practicalutilization of NOx traps, both for NOx removal from the trap, and for periodicdesulphurization of the trap.                                          .Diesel fuel plasmatron reformers might be also used to generate gases with relativelyhigh concentrations of ethylene.  This gas could be used for aftertreatment systems thatutilize hydrocarbon driven SCR.  Another possible application is onboard generation ofammonia. The ammonia could be used in conjunction with SCR catalysts.  However,these concepts are speculative at the present time.NO to NH3 reactorPlasmatronfuel converterairfuelH2 rich gasUntreatedexhaustSCRcatalystDieselengineexhaust gasArcdischargeNO/airProgress is continuing in improving the operating characteristics of low current dieselfuel plasmatron reformers.Acknowledgements   This work has been supported by DoE Office of Transportation TechnologiesReferences[1] Emissions reductions using hydrogen from plasmatron fuel converters, Bromberg L,Cohn DR, Rabinovich A, Heywood J, Int. Hydrogen Energy 26, 1115-1121 (2001);presented at 2000 DEER meeting.[2] Parks, James, Emerachem, private communication (2002)

Onboard Plasmatron Generation of Hydrogen Rich Gas for Diesel Engine Exhaust Aftertreatment and Other Applications

Heywood,J.

UNT Digital Library

Onboard Plasmatron Generation of Hydrogen rich Gas for Diesel Engine ExhaustAftertreatment and Other ApplicationsL. Bromberg*, D.R. Cohn*, J. Heywood**, A. Rabinovich** Plasma Science and Fusion Center**Dept. of Mechanical Engineering and Sloan Automotive Laboratory.Massachusetts Institute of TechnologyCambridge MA 02139 I. IntroductionPlasmatron reformers can provide attractive means for conversion of diesel fuel intohydrogen rich gas.  The hydrogen rich gas can be used for improved NOx trap technologyand other aftertreatment applications [1].Plasmatron reformers developed at MIT use a special low power, low current electricaldischarge to boost partial oxidation conversion of hydrocarbon fuels into hydrogen andCO.  This reformer technology provides the advantages of• Rapid startup and response to transient conditions• Efficient conversion• Compact size• Relaxation or elimination of reformer catalyst requirements• Capability to process difficult to reform fuelsDiesel plasmatron reformer technology can provide substantial throughputs without theneed of using a catalyst The use of the special plasma facilitates robust non-catalyticconversion of diesel fuel into hydrogen rich gas with hydrogen yields that could besufficient for NOx trap regeneration applications.  Higher yields can be obtained with theuse of a catalyst.  Plasmatron reformers could also be used for manufacturing of ethene,which could be useful as a selective reducing agent.  In addition, plasmatron reformertechnology can be used for converting a range of biofuels into hydrogen rich gas.  Thesefuels include various oils.  This capability may improve the prospects for use ofrenewable fuels in a variety of vehicular applications. This paper discussed the status ofplasmatron diesel fuel reformers and possible applications to diesel engine exhaustaftertreatment. II. ReformationPlasmatron fuel converters operate through the generation of a continuous dischargewhere fuel energy the reformer.  This discharge provides large volume reaction initiationof the reforming reaction.  The discharge generates numerous localized regions of intenseheating (even though the resulting spatially averaged temperature increase can be quitemodest), as well as large number of radicals that promote the reaction. The discharge andthe subsequent exothermic reactions generate enough energy for the gasification of theremaining liquid fuel.The plasma induced turbulence also improves the mixing of the air/fuel mixture, as wellas mixing of those regions were partial oxidation has started with those regions whereconversion has yet to begin.  Thus, reactions occur over essentially the entire reformervolume facilitating high conversion efficiency, in a device with small volume.  Rapidheating of the fuel and good air/fuel mixing are also helpful in the minimization of sootproduction.In some circumstances, the plasma energy can be used to provide additional enthalpyincrease of the air/fuel mixture.  This capability can be utilized to meet demandingrequirements, such as rapid startup and quick transient performance. (a) (b)Figure 1 Schematic diagrams of plasmatron fuel converters developed at MIT.  (a)Thermal plasmatron (High current, high power, water cooled electrodes, circa1999); (b) Low current plasmatron (Low current, low power, long electrode lifetimeCirca 2001)Present plasmatron development at MIT involves low current plasmatron reformers.Typical parameters of operation for a low current plasmatron reformer are given in Table1.  The electrical power can be varied, and the device has been operated at powers from50 W to > 800 W.  We have used this type of device to generate a reformate with anequivalent heating value power of 10-40 kW, from diesel and other heavy hydrocarbons.The power supply is current controlled, and the plasma current entry in Table 1 refers tothe average value after breakdown.Table 1.Low current plasmatron operating parameters.Power W 300-600Plasma Current A .1-.4H2 flow rate slpm 40Length cm 40Volume cm^3 2000Weight kg 3 III. Diesel fuel reformingCharacteristics of thermal and low current diesel fuel plasmatron reformers are shown inTable 2.  The thermal plasmatron operates at ~100V, 15A DC, while the low currentplasmatron reformer operates at 500-1000V, 0.2-0.8 A AC.Table  2Diesel Plasmatron Reformer ParametersThe reformate composition for homogeneous (noncatalytic) and catalytic reforming(where the fuel first passes through the discharge which precedes a catalyst) for the lowcurrent plasmatron is shown in shown in Table 3.  The ratio of oxygen to carbon atoms inthe air/fuel mixture is 1.2-1.4. The thermal plasmatron was operated without a catalyst.Table 3Reformate parameters for low current plasmatron IV. Biofuels reformingAs is the case with diesel fuel, bio-oils are difficult to properly reform because of theirpropensity to generate soot in the partial oxidation process.Plasmatron reformer conversion of a number of biofuels has been investigated. Corn oil,canola oil and soy oils have been studied. Ethanol has also been reformed.Thermal plasmatron 1999Low current plasmatron 2001Electrical power W >1500 100-800O/C ratio ~1 1.2-1.4Fuel flow rate g/s 0.6 0.4Hydrogen yield 0.9 .5-.9Power conversion efficiency 0.9 0.85Reformate temperature K 1200 1000-1300Homogeneous CatalyticConcentration (vol %)H2 8% 18%CO 15% 22%C2's 3% 0.6%ConversionH2 in fuel to Hw in reformate 50% 80%C in fuel to C2's 25% 5%Energy conversion efficiency 75% 70%Reformate temperature (K) 1000 850Table 4 shows results of tests using a thermal plasmatron fuel converter.  These tests arebeing extended to include use of a low current plasmatron reformer.Table 4Plasmatron fuel converter reforming of biofuelsThermal plasmatron V. Diesel aftertreatment exhaust applicationsPotential diesel exhaust aftertreatment applications using plasmatron generation ofhydrogen rich gas include:• NOx absorber catalyst regenerationo NOx regenerationo Sulfur regeneration• Hydrocarbon regeneration of SCR catalyst• Onboard ammonia manufacturingThese potential applications are briefly described below. The SCR regeneration byhydrocarbons, and the ammonia production applications are speculative at this time, butmerit further study.a) NOx Absorber Catalyst Regeneration using Diesel plasmatron reformerThe regeneration process of the NOx absorber trap uses a reductant.  Hydrogen rich gas isa significantly stronger reducing agent than diesel, with reduced fuel penalty comparedwith diesel fuel.  Although this is important in itself, a potentially more importantadvantage of using hydrogen as a reductant, instead of diesel fuel, is due to the fact thathydrogen can regenerate traps at lower catalyst temperature than possible with straightdiesel fuel.  This is important for trap regeneration under conditions where catalyst doesnot warm up sufficiently, such as urban drive cycles.As a consequence of the use of a stronger, more efficient reductant (higher concentrationof hydrogen), it should be possible to decrease the regeneration time and the fuel penalty.This could be particularly important for the case of a single leg NOx trap configuration(only one trap, regenerated on-line). In this case decreasing the regeneration time couldhave an especially significant beneficial effect on the fuel penalty. In either the case ofsingle or tandem leg configuration, plasmatron diesel fuel reformers can operate with theCANOLA CORN OILFuel flow rate (g/s) 0.3 0.47Hydrogen concentration (% vol) 25.6 23Methane concentration (% vol) 1.7 7.6Carbon Monoxide concentration (% vol) 26 18.6Carbon Dioxide concentration (% vol) 0.3 2.1Hydrogen yield 92% 84%engine exhaust as the source of the oxidizer.  This minimizes the amount of fuel requiredfor generating the hydrogen rich gas or the high temperatures required for regeneration.In addition, using hot hydrogen rich gas, Parks [2] has obtained good desulfurization atlower temperatures in a net reducing atmosphere.  A comparison of desulfurizationperformance using reformates and diesel fuel has yet to be performed.b) Light Hydrocarbon catalyst regenerationThe regeneration of certain NOx catalyst requires light hydrocarbons. The dieselplasmatron reformer, when operated in the homogeneous mode, can produce largeamounts of ethylene.  Unfortunately, we have not been able to create slightly heavierhydrocarbons, such as propane, which seem to be more effective in catalyst regeneration,such as in the case of nonthermal plasma aftertreatment.Table 5 shows the results of operation the plasmatron reformer (with improved reactor)for optimal production of C2’s.  In this case, it is possible to have carbon conversion toC2’s of about 30%. The measured energy conversion efficiency (heating value of thereformate divided by the heating value of the fuel) is higher than in the case of highhydrogen yields.Table 5Homogeneous plasmatron conversion for production of light hydrocarbonsImproved reactor configurationc) Onboard ammonia manufacturingDiesel fuel plasmatron reformers have potential for onboard ammonia generation forSCR catalytic regeneration.  Two options for manufacturing of ammonia are:1. Ammonia production from hydrogen rich gas (N2, H2 --> NH3)2. Ammonia from onboard generated NO (NO, H2 --> NH3)Electrical power (W) 500Reformate compositionH2 9.5CO 15.2CO2 4.1N2 63.5CH4 3.4C2H4+C2H6 3.1C2H2 0.1YieldsCmHn to H2 44%CmHn to CO 58%CmHn to C2's 28%Reformate thermal efficiency 88%Reformate power (HHV, kW) 36In both approaches, a plasmatron fuel converter might provide the hydrogen required forthe ammonia manufacturing. The practicality of using these processes is uncertain.  Proofof principle experiments for these options have not been performed.A system scheme based on approach 2 is shown in Figure 2. NO, from engine exhaust,with additional NO generation from a thermal arc discharge, is converted to ammonia byreacting it with hydrogen over a catalyst. One source of system inefficiency comes fromthe arc requirement for increasing the NO concentration in the exhaust gas (to minimizethe free oxygen).  A second  inefficiency comes from the hydrogen/CO required toremove the free oxygen prior to converting the NO into ammonia.  However, even withthese parasitic inefficiencies, system fuel penalties on the order of a few percent might beobtained under appropriate assumptions.Figure 2. Onboard manufacturing of ammonia from NO and hydrogen. VI. ConclusionsPlasmatron reformers can provide substantial advantages in converting diesel fuel andother difficult to reform fuels (e.g. bio-oils) into hydrogen rich gas. Use of the hydrogen rich gas could significantly improve prospects for practicalutilization of NOx traps, both for NOx removal from the trap, and for periodicdesulphurization of the trap.                                          .Diesel fuel plasmatron reformers might be also used to generate gases with relativelyhigh concentrations of ethylene.  This gas could be used for aftertreatment systems thatutilize hydrocarbon driven SCR.  Another possible application is onboard generation ofammonia. The ammonia could be used in conjunction with SCR catalysts.  However,these concepts are speculative at the present time.NO to NH3 reactorPlasmatronfuel converterairfuelH2 rich gasUntreatedexhaustSCRcatalystDieselengineexhaust gasArcdischargeNO/airProgress is continuing in improving the operating characteristics of low current dieselfuel plasmatron reformers.Acknowledgements   This work has been supported by DoE Office of Transportation TechnologiesReferences[1] Emissions reductions using hydrogen from plasmatron fuel converters, Bromberg L,Cohn DR, Rabinovich A, Heywood J, Int. Hydrogen Energy 26, 1115-1121 (2001);presented at 2000 DEER meeting.[2] Parks, James, Emerachem, private communication (2002)

Onboard Plasmatron Generation of Hydrogen rich Gas for Diesel Engine Exhaust Aftertreatment and Other Applications

http://dspace.mit.edu/bitstream/1721.1/94030/1/02ja030_full.pdf

Onboard Plasmatron Generation of Hydrogen Rich Gas for Diesel Engine Exhaust Aftertreatment and Other Applications

Abstract

Similar works

Full text

Available Versions

DSpace@MIT

UNT Digital Library