Plasma Reformers
Figure courtesy: Robert A Hefner III (Age of Energy Gases 2007)
International Energy Agency’s projection in the World Energy Outlook 2022 [1] shows that even by 2050, half of the total energy supply will still depend on fossil fuels. IEA’s projection of World Energy Outlook scenario based on prevailing policy settings shows that the global demand for each of the fossil fuels would exhibit a peak and then reduce. Total fossil fuel usage sees a clear peak for the first time. Despite this, the fraction of fossil fuels in the energy mix will remain around 60% in 2050 [2].
Fossil fuels remain so entrenched in the energy system for several reasons. First, they are accessible in one form or another with no geographical constraint. Second, they are so versatile that we have adapted them to provide energy for innumerable applications of extreme variety. As transportation fuel, they are unique because of their portability and high chemical energy density. It is thus obvious that our energy supply for the near future will surely be based on fossil-derived hydrocarbon fuels and that the spectre of smokestacks spewing out the dreaded Carbon Dioxide is going to stay with us.
Carbon dioxide produced in burning fossil fuels handles approximately 57% of all anthropogenic greenhouse gas emissions [4]. Global carbon dioxide emissions from fossil fuels and cement manufacturing have increased by 1.0% in 2022, hitting a record high of 36.6bn tons of CO2 (GtCO2) [2]. 41% of the emissions are associated with power generation. The large CO2 emissions show a positive correlation to global warming trends with profound consequences.
The villain in the energy and environment problem is the carbon content of the fuel. Fossil fuels have been steadily shedding their carbon content historically. In the 19th century, the primary energy source was wood, which releases ten CO2 molecules for every hydrogen molecule burnt. Coal has much fewer carbon atoms per H. Liquid fuels have even fewer: two Hs per C, as in kerosene or jet fuel. Methane, the typical natural gas is hydrogen rich, with 4 H for each C [3].
Reducing Carbon intensity in energy utilization is called Decarbonization. Historically, the world has been pursuing a steady path of decarbonization, driven by the preference for higher energy density and easier portability. This path ends naturally in an energy system based on hydrogen as the fuel. As an energy carrier, hydrogen has the potential to address many aspects of the energy problem, particularly for transportation. Hydrogen can be used in fuel cells to generate electricity or in combustion reactions to produce heat.
The entire world travels using petroleum-based fuels, gasoline, and diesel. No large-scale hydrogen distribution infrastructure has been built until now, and probably this situation may continue for the near future. Hence in situ reforming of fuels for vehicles is an idea being pursued by many auto manufacturers and research laboratories.
Historically, the first ever plasma reformer was built at the Plasma Science and Fusion Center (PSFC) of the Massachusetts Institute of Technology (MIT) [4]. The application was developed to enrich the fuel with syngas to improve the combustion for traditional internal combustion engines (lower consumption, reduced emissions of particulates and NOx)
In a plasma torch, chemical reactions were enhanced because of the presence of very reactive species in an extremely hot medium though paying high energy consumption. It was shown that [Bromberg 2001] that comparable H2 yields could be attained with a non-thermal plasma at significantly lower energy consumption. GEN 2 Low current plasmatron fuel converter uses a spark plug cathode. GEN 3 has concentric electrodes and allows liquid fuel injection through an axial nozzle and at three separate locations (axial, near the wall or between the two electrodes with a swirl motion). PSFC’s work has led to a technology commercialized by Arvin Meritor [Bromberg 2006].
Among emerging reforming technologies, plasma-assisted fuel reforming technology appears quite attractive for automotive applications. They score highly on response time, compactness, and absence of catalyst element [1–3]. The major advantages of non-equilibrium plasma reforming technology are that they have low energy demand, are flexible, they need no catalysts, and they are compact and have low-cost.
The Renault-Nissan collaboration is directed to on-board hydrogen generation from reforming multiple fuels. In 2003, Renault and the Center for Energy and Process in France (CEP) have started a collaborative research program on this subject. In reforming studies, many versions of plasma assisted reforming reactors based on gliding arc technology have been developed. The first one [Paulmier 2005] worked in auto-thermal or steam reforming conditions for pressures up to 3 bars and pre-heating temperatures up to 773 K.
The plasma reformer comprises a compact non-equilibrium plasma torch and a downstream reactor. Plasma torch geometry is a traditional high current DC plasma device. An electric arc is established between two concentric electrodes. The two concentric electrodes are separated by a ceramic high voltage insulation material. A high velocity gas mixture injected radially near the central electrode blows away the low current — high voltage arc discharge generated between the electrodes. The design of the plasma reactor geometry is aimed at enhancing plasma homogeneity and efficient mixing of reagents [6].
A power supply based on resonant-converter technology was developed to drive the low current — high voltage arc discharge (15 kV 660 mA). This power supply can provide continuous control of the arc current within a range of 200–600 mA. Depending on the operating parameters conditions the regime of the discharge can vary from streamer over gliding arc to continuous discharge [6].
The analysis of the experimental results will allow us to study the influence of the main processing parameters like the flow rates, pressure, inlet composition, power of the plasma, etc. shall allow us to understand their interplay. Preliminary results show a 45 % fuel reforming efficiency considering the plasma generating power. The process of generation of CO: 18 %, Hz: 17 %, CO2: 4 % and CH4: 2 % appear very promising since they have been obtained without optimization.
Laboratoire de Physique des Gaz et des Plasma (Orsay, France) in partnership with Peugeot has investigated the reforming of iso-octane, with a DBD reactor [6]. The efficiency, the specific energy requirement, and the conversion rate, appear to be good indicators for quantifying reforming systems in syngas production.
Non-thermal reforming for onboard applications makes sense: good H2 yields, compactness, reactive system, non-deactivation because of coke deposition, sulfur presence or elevated temperature. More work is needed to compare it with existing onboard catalytic systems. The lack of information concerning performances during transient regimes (cold start-up, acceleration, shutdown) and NOx production makes the evaluation of the technology for onboard applications difficult.
References
[2] Mike Scott, The Global Energy System Is Becoming More Electric, But Not Fast Enough, https://www.forbes.com/sites/mikescott/2018/07/23/energy-becomes-more-electric-but-not-fast-enough/?sh=15fa14b306c8
[2]. https://www.carbonbrief.org/analysis-global-co2-emissions-from-fossil-fuels-hit-record-high-in-2022/
[3] J. H. Ausubel, The Industrial Physicist, Dec 1999
[4] L. Bromberg, D.R. Cohn, A. Rabinovich, J.E. Surma and J. Virden, Compact plasmatron-boosted hydrogen generation technology for vehicular applications, Int. J. Hydrogen Energy, 24, 4, 341–350, 1999.
[5] T. Paulmier, L. Fulcheri, Use of non-thermal plasma for hydrocarbon reforming, Chemical engineering journal, 106, 59–71, 2005.
[6] Adeline Darmon, Jean-Damien Rollier, Emmanuelle Duval, Jose Gonzalez-Aguilar, Rudolf Metkemeijer, et al.. Plasma assisted fuel reforming for on-board hydrogen rich gas production. The 16th World Hydrogen Energy Conference, Jun 2006, Lyon, France. 6 p. hal-00526494