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Nonthermal plasma torch reforming of exhaust gas for NOx trap regeneration

Researchers from the Center for Energy and Processes (CEP), MINES ParisTec, and Renault are exploring the non-catalytic reforming of diesel fuel with diesel engine exhaust gas (i.e. a mixture of air, CO2, and H2O mixture) using a nonthermal plasma torch for use in a NOx trap regeneration application. They report in a paper in the ACS journal Energy & Fuels that, even if the plasma torch technology is far from mature, the it is an interesting option for onboard NOx trap regeneration to meet coming more stringent NOx emissions requirements.

NOx trap catalysts are one of the solutions under industry-wide development to meet the further emission regulations. NOx trap technology (NOx storage and reduction, NSR) operates with cycles composed of successive storage and regeneration modes. The classical way to regenerate NOx trap catalysts consists of operating the engine under rich combustion conditions for a short while in order to produce reducing species in the exhaust gas, which will convert NOx into N2.

This method is not totally satisfying, the authors note, because of the problem of diesel fuel mixing together with engine oil (the oil dilution problem); this requires increasing the oil changing frequency.

Catalytic diesel fuel reforming has been explored for a number of years as a pathway to produce NOx reducing species (such as H2 and CO to avoid the oil dilution problem. In such a scenario, when the NOx trap is full, a small fraction (typically 3.5%) of the diesel engine exhaust gas is bypassed toward the reformer and is mixed with a small amount of diesel fuel, which provides the necessary species to regenerate the NOx trap catalyst.

For several years, intensive studies have been dedicated at CEP on reforming processes for fuel cell powering using a nonthermal plasma torch. An alternative to the catalytic exhaust gas reforming of diesel fuel method is presented in this paper and consists of using an NTP torch. Using a plasma torch as a reformer has been previously explored by several research teams.

Contrary to catalysts, plasma processes are nonsensitive to sulfur, light and compact device, and have a short transient time. The amount of oxygen in the plasma gas is a key point for this application because it directly affects the performance of the system and the electric power needed for the reforming reactions. Indeed, at high load engine, the oxygen fraction in the exhaust gas becomes low (typically in the range of 5-15%).

...Some studies have been dedicated to the plasma-assisted diesel fuel reforming for NOx trap regeneration application by Bromberg et al. in association with ArvinMeritor and, more recently, by Park et al., but all these techniques have used the partial oxidation of diesel fuel with an additional air pump...To avoid the cost of an additional pump and injector, the plasma-assisted exhaust gas fuel reforming of diesel fuel, with a high content of CO2 and H2O, can be directly realized and is presented in this paper.

—Lebouvier et al.

The plasma technology they developed is based on a high voltage/low current nonthermal plasma torch. In the first part of the paper, they report the experimental results on synthesis gas production from exhaust gas fuel reforming of diesel fuel. In the second part of the paper, these experimental results are compared with a 1D multistage model using n-heptane as a surrogate molecule for diesel fuel.

In their experimental setup, the plasma reactor comprises two consecutive zones: a plasma zone and a post-discharge zone. The plasma zone is the part where the arc plasma really takes place. The post-discharge zone is a passive zone, located downstream of the plasma zone where most of the reforming reactions ignited in the plasma zone continue to take place depending on their kinetic speed. The power supply is a resonant converter controlled in current. Three mass flow controllers supply a mixture of air, N2 and CO2.

They studied two compositions of synthetic diesel engine exhaust gas, corresponding to high and low engine loads. Among the findings of their study:

  • Low O2 availability in the plasma gas made the plasma-assisted diesel fuel reforming harder than partial oxidation. The oxygen from CO2 and H2O hardly ever intervenes in the exhaust gas diesel fuel reforming. On contrary, they absorb a part of the calories and lower the temperature. This implies lower temperatures, lower kinetic reaction speed, and lower energy efficiency compared with the POx reaction. To raise the temperature, more oxygen is needed, but local combustion can happen and promote H2O and CO2 production.

    As a consequence, they found, a compromise has to be made between diesel fuel consumption, electric consumption, and methane production. At high engine load, the most suitable condition is reached for O/C = 1. At low engine load, they achieved an energy efficiency of 40% and a conversion rate of 95%, corresponding to a 25% of syngas dry molar fraction.

  • Their 1D model showed good consistency with experimental and thermodynamics trends but with a significant shift deriving above all from the strong model hypotheses (adiabaticity and perfectly and instantaneously gas mix). In a further step, to obtain better correlations between modeling and experimental results, they will have to take into account thermal losses and a non-perfect mix together with a 2D fluid model.

  • Assuming (i) a 100 kW car engine thermal power (i.e., 40 kW mechanical power), (ii) that the plasma will treat only a small fraction of the exhaust gas (typically 3.5%), (iii) that the plasma will operate under a cycling operating mode, and (iv) an 80% efficiency for the onboard production of electricity from the car engine, they estimated that the electric power needed to run the plasma will be around 2.2% of the engine power only during 12 s every 11 km (6.8 miles), that is, 12 s every 6 min assuming a 110 km/h (68 mph) average car velocity.

In the first case, corresponding to the least oxidant environment, the plasma solution will hardly compete with catalytic reforming because it would necessitate a too long regeneration time. For this particular case, one solution could be the use of a hybrid plasma catalysis system where the plasma could favorably allow one to reduce the catalyst volume and, consequently, to decrease the amount of precious metal and catalyst price. The plasma could also give interesting benefits by quickly heating the catalyst during the startup phase. Plasma catalysis technology could compensate the energy cost of heating it up by another way, and it allows preactivating the reforming reactions. As a conclusion, even if the technology is far to be mature, the plasma torch technology is, therefore, an interesting option for onboard NOx trap regeneration.

—Lebouvier et al.

Resources

  • Alexandre Lebouvier, François Fresnet, Frédéric Fabry, Valérie Boch, Vandad Rohani, François Cauneau, Laurent Fulcheri (2011) Exhaust Gas Fuel Reforming of Diesel Fuel by Nonthermal Arc Discharge for NOx Trap Regeneration Application, Energy & Fuels doi: /10.1021/ef101674r

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