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:
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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
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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