Alexeev, N.

Bromberg, L.

Cohn, D.R.

Rabinovich, A.

Ramprasad, R.

Samokhin, A.

Tamhankar, S.

DSpace@MIT

PSFC/JA-99-17SYSTEM OPTIMIZATION ANDCOST ANALYSIS OF PLASMA CATALYTICREFORMING OF HYDROCARBONSL. Bromberg, D.R. Cohn, A. Rabinovich, N. Alexeev,A. Samokhin and R. Ramprasad* and S. Tamhankar*June 1999Revised: August 1999Plasma Science and Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139http://ww\w\.pfc.mit.edu/library/99ja/99ja0l 7_abs.htm l* BOC GasesMurray Hill NJ 07974This work was supported by Grant No DE-FG04-95AL88002 of the US Department ofEnergy Office of Concentrating Solar Power, Biopower and Hydrogen Technologies.Reproduction, translation, publication, use and disposal, in whole or in part, by or for theUnited Stated government is permitted.AbstractThermal plasma reforming offers advantages in hydrocarbon reforming especially insmall to medium scale plants and in plants with fast transients. The combination of athermal plasma reformer operating with partial oxidation with a catalyst bed will bedescribed. Reduced concentrations of CO (1-3% vol) can be achieved, with highhydrogen yields and minimal plasmatron electrical power requirements. A model for thecost of hydrogen production by this method, including hydrogen cleanup utilizing aconventional pressure-swing absorption unit, has been developed. The model usesexperimentally determined conversion yields and operational parameters. The conditionsthat result in system optimization and cost minimization have been determined.21 IntroductionManufacturing of hydrogen from natural gas, biofuels and other hydrocarbons, is neededfor a variety of applications. Plasma technology could provide important improvementsin reforming hydrocarbon fuels for the production of hydrogen-rich gas for fuel cells andother applications. The plasma conditions .(high temperatures and a high degree ofionization) can be used to accelerate thermodynamically favorable chemical reactions orprovide the energy required for endothermic reforming processes. Plasma reformers canprovide a number of advantages:" economically attractive operation in small scale hydrogen production applications* operation with a broad range of fuels, including natural gas and biofuels" decreased problems of catalyst sensitivity and deterioration" compactness and low weight (due to high power density)" fast response time (fraction of a second)* minimal cost" high conversion efficienciesHydrogen-rich gas could be efficiently produced in compact plasma reformers with avariety of hydrocarbon fuels including natural gas, biomass, and others. The technologycould be used to manufacture hydrogen for a variety of stationary applications (e.g.,distributed, low pollution electricity generation from fuel cells or hydrogen-refueling gasstations for fuel cell driven cars). It could also be used for mobile applications (e.g., on-board generation of hydrogen for fuel cell powered vehicles).In this paper, the cost issues of a plant that incorporate a plasma reformer areinvestigated. It is assumed that the plant operates on natural gas, although the plasmatronis capable of operating in a wide range of fuels. The experimental results of plasmareforming of natural gas are briefly reviewed in section 2. A model of a process plant isdeveloped, and a model for determining the hydrogen cost is described in section 3. Thesensitivity of the cost to capital cost, cost of natural gas and to manpower requirementsare then presented in section 4. The tradeoff between higher conversion and increasedelectrically energy consumption in the plasmatron is described in section 5. Finally, theconclusions and direction of additional work is described in section 6.2 Plasma reformingPlasma devices referred to as plasmatrons can generate very high temperatures (>2000 C)with a high degree of control, using electricity [1]. The heat generation is independent ofreaction chemistry, and optimal operating conditions can be maintained over a wide rangeof feed rates and gas composition. Compactness of the plasma reformer is ensured byhigh energy density associated with the plasma itself and by the reduced reaction times,resulting in short residence time. Hydrogen-rich gas can be efficiently produced inplasma reformers with a variety of hydrocarbon fuels (gasoline, diesel, oil, biomass,natural gas, jet fuel, etc.) with conversion efficiencies into hydrogen-rich gas close to100% [2,3].3The plasma conditions (high temperatures, high degree of dissociation and substantialdegree of ionization) can be used to accelerate thermodynamically favorable chemicalreactions without a catalyst or provide the energy required for endothermic reformingprocesses.The technology could be used to manufacture hydrogen for a variety of stationaryapplications e.g., distributed, low pollution electricity generation from fuel cells [4]. Itcould also be used for mobile applications (e.g., on-board generation of hydrogen for fuelcell powered vehicles) and for refueling applications (stationary sources of hydrogen forvehicles).A previous paper [5] presented results of plasma catalytic conversion of methane, usingpartial oxidation. In this section, more recent results are very briefly described. Figure 1shows the hydrogen yield as a function of the specific energy consumption (electricalpower required for a given hydrogen throughput). The best results are 95% yield at aspecific energy consumption of 13 MJ/kg H2. These results were obtained without the useof a heat exchanger. It is estimated that with a heat exchanger and with improved thermalmanagement, the specific energy consumption can be decreased to 7 MJ/kg H2 (0.18kWhr/m 3 H2) at slightly higher yield (97%). Equivalently, 20 kW of H2 gas are producedper kW electric power into the plasmatron. These numbers will be assumed in thecalculations below.1.00.80.602 0.40.20.010.0 20.0 30.0 40.0 50.0energy consumption, MJ/kg H2Figure 1. Hydrogen yield as a function of specific energy consumption for plasmacatalytic reforming of methane.A schematic diagram of a plasmatron is shown in Figure 2. The arc is generated betweenthe water-cooled cathode with a hafnium tip, and a water-cooled tubular anode. Theplasmatron gas has a large vorticity in order to spin the arc, centering the cathode arc rootin the hafnium tip, and rotating the anode spot. The natural gas was injected downstreamfrom the arc, as shown in Figure 2.4UQi IWPAir/water/Naturalgas mixtureWater inlet*- AirCathodeAnodeWater outletFigure 2. Schematic diagram of the plasmatron.Both gas phase reactions as well as catalytic reactions were studied for air and air/watervapor mixtures injection into an air plasma. In the heterogeneous experiments, nickelbased catalyst on alumina support was used. The catalyst (United Catalyst C-11) came inrings, and best results were obtained with crushed catalysts to 5 mm average size.Drinking water was the source of the steam. The gas analysis was performed using a HPM200D GC, with two columns and with two thermal conductivity detectors. Calibrationgases were provided by Matheson Gas.3 Costing modelThe cost of the hydrogen produced through a plasmatron process is investigated in thissection. The flow diagram is shown in Figure 3. It is assumed that the hydrogen rich gasis produced at pressure so that it is not necessary to pressurize the hydrogen rich gas priorto the pressure swing absorption section. Air, not oxygen, is used as the oxidizer, whichmeans that the synthesis gas contains a large amount of nitrogen. It is also assumed thatmethane is the hydrocarbon used, although other hydrocarbons can also be utilized.Further assumptions are that the system has good thermal management, and the powerconsumed by the plasmatron as well as the exothermic energy released by the partialoxidation process are used to produce the steam required for the system. Steam isrequired mainly in the water-shift reaction. There is an excess of steam for the steamshifter of 200%.The capital costs of the system, for a 571 m3/hr (20000 scthr) are shown in Table 1. Thecorresponding plasmatron power is 90 kW. The syngas is at pressure, and there is no5need for boiler to produce steam since it can be produced by the syngas itself prior to thewater shift reaction, which occurs at relatively low temperatures. If the plant is amortizedover 15 years, then the amortized capital cost is on the order of $1 50k per year.BFV shft-product _citooollWater-heater Shn Cooler HdoeShift PSAFeed HydrogenReactor0PSArAi Shift liq KOPot0 PSAWasteAi~opCompAwop HPAir U)MixedFeed 0_Plasma CondensateNGfeea Feed-MixerBoiler ynga Lq roSteam PlasmatronFigure 3. Flow diagram of plasmatron fuel converter based hydrogen generation systemTable 1. Capital costs for system with 571 m3/hrAir compressor $100,000Reforming Reactor $5,000plasmatron $50,000convertor (power supply) $18,182spares $100,000Boiler package $30,000Shift Reactor/cooler/KOD $20,000H2 PSA $200,000Feed filter/KOD $3,500Controls/Instrumentation $90,000Engg./Skidding/Packaging $65,000Cap cost total $681,682Installation (10% cap) $68,168Plant Cost (Installed) $749,850Cost per year, amortized over 15 years $149,970The operational costs depend on the number of personnel operating the plant. Since thehigh power plasmatron is a well-established technology, it is assumed that the system isautomated and needs minimal supervision. Plasmatron maintenance, consisting inreplacing the electrodes, needs to be performed every 1000 hours of operation, and lastsonly a few minutes. During plasmatron maintenance, the system can continue to operate6at reduced level. Several plasmatrons are used in the system, and electrode replacementis carried out one by one.The power requirements are shown in Table 2. The syngas is at pressure when leavingthe plasmatron reactor. Air compression requires more power than the correspondingoxygen compression. The power requirements are dominated by the plasmatron powerrequirements of 90 kW.Table 2. Power consumption in 571 m3/hr plantAir compressor power, kW 130Natural gas compressor power, kW 0Syngas compressor power, kW 0Plasmatron power, kW 91Total Power, kW 221The cost of the natural gas is assumed to be $2.5/MMBTU, and the electricity cost is$0.04/kWhr. Assuming that 0.5 full time people (all shifts) are required in the plant (at acost of $70000/person-year), then the annual costs are given in Table 3, for capacity of571 m3/hr of H2 gas.Table 3. Annualized costs for a 571 m3/hr (20000 scfh) hydrogen plantLabor $35,000Catalyst $10,000Power $73,636Natural gas $221,952Total Op. + Util. $340,588Total cost Op+util+cap $490,558With an annual hydrogen production of 60k MMBTU, then the cost of the hydrogen isabout $8.5/MMBTU. This is a very preliminary look at the cost, and more detailedcalculations will be presented at a later time.4 Sensitivity of hydrogen costThe dependence of the hydrogen costs on the plant capacity, capital cost, the methanecost and the required personnel is described in this section.The model can be used to determine the cost of the hydrogen as a function of the plantcapacity. Figure 4 shows the results from this calculations, assuming that the flowdiagram remains constant (i.e., no changes in energy recovery or in specific energyconsumption by the plasmatron fuel converter). It is assumed that the cost of thecompressor, the reformer reactor, the plasmatron and its power supply, the shift reactor,the PSA unit and the feed filter scale as the 0.7 power of the plant capacity. The controland instrumentation scales as 0.2 power of the plant capacity, and theengineering/skidding/packaging scales as the 0.3 power of the plant capacity. In addition,7it is assumed that the personnel requirement scales as the 0.3 power of the plant capacity.The cost of hydrogen decreases from about $8.5/MMBTU to about $6.8/MMBTU in therange considered. The cost of the hydrogen, in the range considered, scales inversely asthe 0.1 power of the plant capacity. Above 3500 m3/hr, the cost model needs to berevised.8.40 - - - ----8.208.007.807.60C.C7.407.20-7.006.80 --- -0 600 1000 1500 2000 2500 3000 3500 4000Hydrogen generation rate (m^3/hr)Figure 4 Cost of hydrogen as a function of the plant capacity.The sensitivity of the cost of hydrogen to the cost of natural gas in shown in Figure 5.The cost of the hydrogen is about $5.5/MMBTU more expensive than the cost of thenatural gas. For free natural gas, the cost of the hydrogen is about $4.5/MMBTU.12.00 - - - - -----10.008.006.00002.000.000.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Natural gas cost ($MMBTU)Figure 5. Hydrogen cost as a function of natural gas cost, for a 571 m3/hr (20000 scfh)plant capacity8From Table 3, it can be seen that about half the cost of the hydrogen is due to the naturalgas, with the electrical power about 1/3 of the natural gas cost. Increasing the cost of thecapital equipment by a factor of 2 increases the hydrogen cost from $8.2/MMBTU to$10.7 /MMBTU, for 571 m3/hr plant capacity. For the same plant capacity, increasingthe cost of electricity to $0.08/kWhr increases the cost of hydrogen to $9.5/MMBTU.5 Tradeoff between methane conversion and specific energy requirementAs mentioned in the sections above, there is a tradeoff between the methane conversionand the hydrogen yield. A simplified curve illustrating this tradeoff is shown in Figure 6.For a specific energy consumption less than about 0.17 kWhr/m3 there is a sharp drop inthe methane conversion. At higher specific energy consumption, the methane conversiondoes not increase much.8.70 - - -- 100%8.60 -i% j 90%-. 9508.40IsoC4 8.308.20 U9.1060%8.007.90 50%0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40Specific energy consumption (kWhrlm^3)Figure 6. Methane yield and cost of hydrogen as a function of the specific energy, for571 m3/hr plant capacityFigure 6 also shows the hydrogen cost as a function of the specific energy consumption.All the other parameters are the same as in the previous sections. There is a well-definedminimum in the cost. To the left of the minimum, the hydrogen yield is decreasingbecause of insufficient methane conversion, and to the right of the minimum the electricalcost is increasing faster than the hydrogen yield. It is important to determineexperimentally the minimum cost.6 Discussion and conclusionsPlasma reforming of natural gas has been briefly described. A process involving plasmareforming of natural gas for the production of hydrogen is then modeled to determine the9cost and the cost-sensitivity of hydrogen produced by the process. Although preliminary,the costs indicate that plasma reforming may be a means for hydrogen production atcompetitive costs, even in small capacity hydrogen generating units.Several assumptions need to be demonstrated experimentally. It is necessary todemonstrate the lower specific energy consumption assumed in the paper, by the use ofbetter thermal management and heat recovery. It is necessary to operate the plasmatron athigh pressure, to minimize the required pumping power. This could have substantialeffect in the case of liquid feedstock, since it requires little power to pressurize the liquidbut substantial power to pressurize the reformate. If electrode life or other effects limitthe plasma reformer operation at high pressure, then a reformate compressor needs to beincluded in the system. Heat recovery requires a heat exchanger that was not included inthe cost calculations.The lifetime of the catalyst is another unknown. Sulfur in the feedstock is readilytransformed into H2S which needs to be removed from the system but which does notaffect the catalyst. The costing model is continuously being improved.AcknowledgementsThis work was supported by Grant No DE-FG04-95AL88002 of the US Department ofEnergy.References[1] For a recent review of plasma technology, see: Fauchais, P. and Armelle Vardelle,Thermal Plasmas, IEEE Transactions on Plasma Science 25 6 1258-1280 (1997).[2] Kaske, G., L. Kerke, and R. Muller, Hydrogen Production in the Huls Plasma-Reforming Process, Hydrogen Energy Progress VI, 1 (1986).[3] Muller, R., The Use of Hydrogen Plasma Processes in the Petrochemical and Iron-Smelting Industries, in: Hydrogen Energy Progress IV, 3 885 (1982).[4] L. Bromberg, D.R. Cohn, A. Rabinovich and N. Alexeev, Plasma CatalyticReforming of Methane accepted for publication in Int. J. of Hydrogen Energy.[5] L. Bromberg, .R. Cohn, A. Rabinovich and N. Alexeev, presented at the 1999 AnnualPeer Review of the Hydrogen Program, Denver CO 1999 (to be published)10

System optimization and cost analysis of plasma catalytic reforming of hydrocarbons

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System optimization and cost analysis of plasma catalytic reforming of hydrocarbons

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