Cold plasma assisted reforming of hydrocarbons: focus on automotive applications

Transcription

1 Cold plasma assisted reforming of hydrocarbons: focus on automotive applications Laurent BEDEL ALPHEA HYDROGENE European Network and Center of Expertise on Hydrogen and its Applications Parc d'activités FORBACH Ouest - Rue Jacques CALLOT FORBACH, FRANCE Phone: +33 (0) Fax: +33 (0) laurent.bedel@alphea.com ABSTRACT: Refueling hydrogen powered car is nowadays a challenge. Developing a widespread refueling infrastructure would cost too much to be done without a strong distributed hydrogen demand itself driven by hydrogen powered car needs. On board reforming is a way to break the famous chicken and egg dilemma, bridging nowadays hydrocarbon fuels filling station with tomorrow s hydrogen cars. Conventional on-board reforming is not only made of ease. Its main drawbacks are a long start-up time, a poor compactness, a low flexibility and a difficult integration with temperature sensitive fuel cells. Cold plasma reformers could be of use to overcome conventional reforming hurdles. KEYWORDS: cold plasma, reforming. 1. Introduction Hydrogen powered vehicles are considered as the most promising transportation way in terms of greenhouse gases emissions and petroleum independence. These vehicles are actually under development. If in a long term vision, these vehicles will be directly fed with hydrogen dispensed in dedicated fuelling stations. Currently, the necessary widespread hydrogen distribution network remains conceptual, and building up such a network will require huge investments over a long period. Furthermore, actual hydrogen storage techniques have to be improved to confer these vehicles an extended autonomy that should not suffer from the comparison with present gasoline powered vehicles. On-board hydrocarbon (gasoline or diesel) reforming is of some car manufacturers interest because of solving both infrastructure (use of the existing fuelling stations) and driving range problems (energetic density of hydrocarbons is much larger than the hydrogen one). On-board reforming could then be used during the transition period to an extended hydrogen based economy. Nevertheless, on-board reforming of hydrocarbons is not without any technical hurdles: as the reforming is performed at high temperature (~700 C), the of the reformer must be heated up and requires an activation period to reach the steady state performances leading to a system start-up time of several minutes. Moreover, the s are sulphur sensitive and the coke formation on their surface leads to their deactivation. Plasmas (ionised gas) are very reactive mediums that could be used to assist the hydrocarbon reforming reaction. Thermal plasmas have proven their activity to produce hydrogen from hydrocarboned feeds but present poor energetic yields because of heating at several s K the whole reaction chamber, even inactive species. In cold plasma, or also called non equilibrium plasma, only the lightweight electrons are heated, saving so some energy. Collisions with the high energy electrons initiate radicalising reactions ending up to the production of hydrogen and carbon oxide. The Centre of Expertise of ALPHEA Hydrogène, the European network on hydrogen and its applications, performed a study on hydrogen production by cold plasma assisted reforming of hydrocarbons focused on automotive applications. In this paper, we present the results of this study. First the plasma assisted reforming systems are described (arc plasma, dielectric barrier discharge, coronal discharge and microwave discharge), then their performances in steam reforming, partial oxidation and autothermal reforming are compared. 1/5

2 2. Description of cold plasma assisted reformers However the study of cold plasma assisted reformers performed by ALPHEA Hydrogène considers all kind of cold plasma technologies suitable for hydrocarbon reforming for automotive end use; we will only focus in this paper on arc plasma. Full results are available at ALPHEA s Center of Expertise. Plasma is caused by an electrical discharge (arcing) between two electrodes plunged into a gaseous medium. Electrical field applied between the electrodes accelerates electrons of the gaseous medium and leads to its ionization. Ionized medium becomes electrically conductive and an electrical arc appears between the electrodes. Electrical energy can be supplied by two different ways: either by applying a high intensity and low voltage current, or oppositely a low intensity and high voltage current. First solution is generally used for producing permanent thermal plasma. Cold, or non-thermal, plasmas are usually obtained by using alternative low intensity current, producing periodical arcs. High difference of potential between the electrodes accelerates gas s electrons up to get enough kinetic energy to leave core attraction and cause ionization. Protons and neutrons being much heavier than electrons, they are not affected by the electrical discharge if short enough. The difference of temperature between electrons (hot) and other heavier species (cold) confers cold plasmas special behavior. In order to avoid localized erosion of electrodes submitted to arcing in oxidative atmosphere (air, O 2, CO 2 ), arcs can be motioned pushed by gas steam or induced by the action of an external magnetic field. Compared to thermal plasma generator, electrodes for cold plasma do not need any water cooling. Several architectures have been being developed. MIT Plasma Science and Fusion Center is one the leading team on the topic. A sketch of MIT plasmatron is presented on Figure 1. Arcs are obtained between electrodes 20 and 24. Fuel and air are injected in the reformer by means of injector 21. Small amount of fuelair mixture is laterally injected through orifice 19 in order to give a curved shape to the arcs. Highly reactive species (ions, radicals, excited molecules ) formed in the uniform plasma region 26 initiate reforming reaction that completes inside the reaction chamber 30. As presented on Figure 1, reaction chamber may be equipped with a catalytic zone mounted at its bottom. Noble metal s aim to enhance the hydrocarbon conversion by completing the reforming reaction in a more classical way, using the waste heat of the plasma. Applied voltage ranges from 300 V up to 60 kv, while intensity is comprised between 10 ma and 2 A. Arcs are produced at a 1-10 khz frequency range. The reformer energetic requirements are only of about % of reformate. For diesel reforming, plasma power optimum is around W while processing a kw fuel stream. MIT technology is developed and commercialized by ARVIN MERITOR i-iv. Plasma reformer commercialized by ARVIN MERITOR does not aim to feed any hydrogen powered car but rather reform small amount of diesel for producing hydrogen to be co-injected in internal combustion engine. Hydrogen co-injection is efficient for reducing pollutant emissions. 2/5

3 Figure 1 : MIT plasma reformer v 3. Catalytic performances MIT plasmatron (Figure 1) catalytic performances for diesel reforming (partial oxidation) are summarized in Table 1. 3/5

4 Electrical power (W) O/C H 2 O/C Fuel flow Reformate composition (vol. %) (kw) H 2 CO CO 2 N 2 CH 4 C 2 H 4 No Ceramic water Table 1 : MIT plasmatron catalytic performances for diesel partial oxidation vi We can notice that even with water addition to the fuel feed, the CO content of the reformate is always high. Plasma assisted reforming only performs the first step of the reforming (C x H y + x/2 O 2 x CO + y/2 H 2 ), the water gas shift reaction is not favored in these plasma conditions. Electrical power (W) Fuel flow (kw) [g/s] Hydrogen yield (%) Energy consumption (MJ/kg H 2 ) Energetical yield (%) No [0.26] Ceramic + water [0.48] [0.34] [0.31] Table 2 : Energy consumption of MIT plasmatron for diesel partial oxidation vi The energy needs for diesel reforming using MIT plasmatron are presented in Table 2. Using a noble metal based to enhance reforming reaction leads to a better energetic yield. 4. Conclusion Low electric consumption of plasma as well as quick start-up, compactness and lightweight are cold plasma main advantages. Cold plasma assisted reforming may be a promising way to produce hydrogen from liquid hydrocarbons. Feedback on this special technology is quite poor. Further research and development works have to be carried out to determine both best plasma technology and reactor design for hydrogen production. It has also to be pointed out that for feeding a fuel cell, large amount of CO must be removed. Shift reactors will then be required. Plasma reactor can also be used to assist conventional catalytic reformer when cold starting or transient operating. i R.M. Smaling, M.J. Daniel and S.D. Bauer, Apparatus and method for reducing power consumption of a plasma reformer Patent WO (2004). ii M.J. Daniel, R.M. Smaling, K.D. Zwanzig, M.M. Lee and S.D. Bauer, Plasmatron having an air jacket Patent WO (2003). iii 4/5

5 iv R.M. Smaling, Plasma fuel reformer having a shaped catalytic substrate positioned in the reaction chamber thereof and method for operating the same Patent WO (2004). v A. Rabinovich, A. Nikolai, L. Bromberg, D.R. Cohn and S. Andrei, «low current plasmatron fuel converter having enlarged volume discharges» Patent US (2003) vi L. Bromberg, D.R. Cohn, A. Rabinovich and J. Heywood, Int. J. Hydrogen Energy, 26 (2001) /5