22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Diesel oxidation catalyst for CO and unburned hydrocarbons removal from
diesel exhaust under plasma discharge conditions
A. Leray1, M. Makarov2, J.M. Cormier1 and A. Khacef1
1
GREMI-UMR 6744, CNRS-Université d'Orléans, 14 rue d’Issoudun, P.O. Box 6744, FR-45067 Orléans Cedex 02, France
2
Renault SAS FR TCR RUC T 62, 1 avenue du Golf, FR-78288 Guyancourt, France
Abstract: The use of non-thermal plasma for the improvement of diesel oxidation catalyst
performance was investigated. Instantaneous concentration and cumulative mass of
regulated gaseous emissions derived from simulated diesel engine exhaust (CO, HC, NO x )
have been studied in test bench for both steady-state conditions and during the New
European Driving Cycle (NEDC). The combination of the DOC downstream the plasma
reactor decreases the catalyst light-off temperature and the CO and HC emissions.
Keywords: non-thermal plasma, diesel oxidation catalyst, Light-off temperature, CO,
Unburned hydrocarbons
1. Introduction
Exhausts emissions of carbon monoxide (CO), volatile
organic compounds (VOCs), nitrogen oxides (NOx), and
particulate matter (PM) are regulated by EU directives, as
are evaporative emissions of VOCs. Various unregulated
gaseous pollutants are also emitted, but these have
generally been characterised in less detail.
Current catalyst technology is capable of reducing PM,
CO, and unburned hydrocarbons (HCs) at the diesel
exhaust temperature levels. While the diesel oxidation
catalyst (DOC) technology has progressed to the point
where CO and HCs emissions can be greatly reduced,
however no satisfactory solution currently exists for
reducing harmful emissions at low temperature (i.e., cold
start regime). The search for effective and durable
catalysts that work in diesel exhaust environments at low
temperature is a high-priority issue in emissions control
and a subject of intense investigations by engine and
catalyst researchers. Among the several approaches under
development to reach these requirements such as the use
of high precious group metal (PGM) loading and HCs
adsorbers [1-3], the research efforts are focusing on
finding a catalyst that can be combined with non-thermal
plasma (NTP) to reduce pollutants from lean exhausts and
especially for diesel exhaust [4-6]. In that field, corona
discharges and DBD (dielectric barrier discharge) in
combination with number of materials having catalyst
activity for NO x reduction in lean exhaust have been
extensively studied [7-8]. To be effective, this technology
should improve pollutant conversion efficiencies and
lower the catalyst light-off temperature.
One direct measurement to evaluate the catalyst
performance is to determine its light-off temperature, the
temperature at which significant oxidation reactions occur
(T 50 corresponds to temperature to attain 50%
conversion). In general, it is reported that the lower the
light-off temperature, the catalyst possess the better
performance [9]. A difference between conventional
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catalytic and plasma enhanced reactions is that in the
latter many active species, free electrons, ions, and UV
light could be generated at low temperature and thus
could be used for the catalyst activation (reactions with
the catalyst surface). Moreover, that can also be used to
prevent the catalyst poisoning.
Under oxygen-rich
conditions, a fraction of input plasma energy is dissipated
in the dissociation of O 2 that become the dominant
process in the production of radicals which is the initiator
of the hydrocarbon chemistry according to wellestablished reaction pathways [10-12].
The present study aims to investigate the oxidation
mechanisms induced by plasma with DOC using test
bench scale simulated diesel engine exhaust running in
two regimes: (i) steady-state regime (flow rate up to
260 L/mn), and (ii) New European Driving Cycle
(NEDC) regime (variable exhaust flow rate in the range
60-900 L/mn depending on the time in the cycle).
A special focus is made to quantify the instantaneous
concentrations and cumulative mass of gaseous
emissions, as well as to investigate the effectiveness of
the processes with regard to light-off temperature of DOC
for CO and THC oxidation.
2. Experimental
The experiments were conducted using Renault test
bench facility. The experimental arrangement, shown
schematically in Fig. 1, consists of a continuous flow gas
stream generation and heating systems, a DBD reactor,
and analytical system.
The gas mixture simulated diesel exhaust during the
experiments containes mixture of O 2 (10%), CO 2 (4%),
CO (8500 ppm), total hydrocarbon THC (1500 ppm C),
NO (100 ppm), H 2 O (4%), and N 2 (balance) was
prepared in a gas handling system and the gas flow was
controlled by the calibrated mass flow controllers (MFC).
THC is a mixture of propene, toluene, decane, and
methane (THC). For the European Driving Cycle
1
(NEDC)
tests,
the
CO
concentrations.
3. Results
For the steady-state regime, we focus on T 50 light-off
temperatures of the catalyst for CO and THC oxidation.
Fig. 2 shows the CO and THC conversion rate as a
function of temperature for "only DOC" and "plasmaDOC" (catalyst placed downstream the plasma reactor)
systems.
Fig. 1. Schematic diagram of the experimental set-up.
and THC concentrations were increased in order to
simulate diesel engine exhaust in HCCI mode
(Homogeneous Combustion Charge Injection). NEDC
was used for EU type approval testing of emissions and
fuel consumption from light duty vehicles.
The plasma reactor is multi-DBDs reactor in a planar
geometry provided by HK-MnS Company Ltd. (Korea)
according to our technical specifications.
Each
elementary part of the reactor consists of a pair of thin
metal electrodes covered by alumina plates separated by a
gap of about 1.5 mm. The plasma reactor was powered
by a high voltage, high frequency AC generator (11 kV 15 kHz) and the subsequent injected power can be varied
from 50 to 300 W. The plasma reactor and the power
supply device are designed to fit an integrated aftertreatment system, which can be directly installed on the
vehicle exhaust line canning.
The electrical energy efficiency of the plasma reactor
was evaluated through the specific input energy (SIE)
which is the energy deposited per unit volume of gas in
the discharge reactor (J/L) at standard conditions (25 °C
and 1 atm) as given in Eq. 1.
The electrical
measurements were made using a Tektronix high voltage
P6015A probe (75 MHz 1:1000 ratio) and Pearson 4001
current probe connected to Tektronix DPO 3054
oscilloscope (500 MHz, 2.5 Gs/s).
𝐽
𝐿
𝑆𝑆𝑆 ( ) =
𝑃 (𝐷𝐷𝐷𝐷ℎ𝑎𝑎𝑎𝑎 𝑝𝑝𝑝𝑝𝑝 (𝑊))
𝐿
𝑄 (𝐺𝐺𝐺 𝐹𝐹𝐹𝐹 𝑟𝑟𝑟𝑟 �𝑠 �)
(1)
A commercial Euro 5 diesel oxidation catalyst (DOC),
honeycomb structure monolith-supported Pt-Pd/Al 2 O 3
ratio 2/1, was used.
The sample catalyst was
hydrothermally aged for 5 h at 750 °C under N 2 -O 2 -10%
H 2 O mixture.
The exhaust line was equipped with temperature
sensors (K-type) and temperature programmed surface
reaction (TPSR, 80-400 °C) was used to determine CO
and THC light-off curves for plasma, DOC, and plasma
assisted DOC systems.
For all experiment, the outlet gas (O 2 , CO 2 , CO, HCs,
NO, and NO 2 ) compositions were measured using
Pierburg bay analyzer (AMA 2000). Accuracy of the
measurements was ± 2 ppm for NO and NO 2
concentrations, and ± 5% for CO, CO 2 , and HCs
2
Fig. 2. (a) CO, and (b) THC conversion rate for DOC and
plasma-DOC systems (SIE = 96 J/L).
T 50 for CO is about 149°C when the DOC is combined
to plasma reactor at an input power of 280W
(SIE = 95 J/L). This value has to be compared to those
obtained when the DOC is used without plasma reactor.
As shown in Fig. 2a, the plasma discharges contribute to
improve the DOC activity towards the low temperature
(improvement of the T 50 was about 48 °C). THC
conversion efficiency measured under the same
experimental conditions shows the same behaviour. THC
light-off curve indicates the presence of adsorbed
hydrocarbons on the catalyst.
For only DOC
configuration, the hydrocarbons are partially stored before
begins to be released at a temperature of about 100 °C.
Desorption process starts at 140 °C while the catalyst was
not activated yet.
The decrease of hydrocarbons
concentration at about 197 °C shows the beginning of
DOC activation and T 50 is reached at 205 °C. When the
catalyst was combined to plasma reactor, THC conversion
rate is always more than 50%. In that case, T 50 of THC
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corresponds to the temperature at which the THC
conversion starts to increase (144 °C). From these data,
T 50 for hydrocarbons is improved by about 61 °C and the
hydrocarbons released into the atmosphere is minimised.
Vehicle exhaust emissions are inherently rather
variable, and the best way to ensure that an emission test
is reproducible is to perform it under standardized
laboratory conditions. In the following section, gaseous
emissions are investigated for a driving cycle NEDC [13]
specifically used for approval of light-duty vehicle
models in the European Union. It consists of four
repeated ECE-15 urban driving cycles (UDC) and one
Extra-Urban driving cycle (EUDC). The test cycles
include the measurement of CO and THC, as well as NOx
(NO and NO 2 ) concentrations. The engine temperature
before starting the test was about 50 °C. During the
NEDC driving cycle, the exhaust flow rate varies
significantly depending on the driving type as shown in
Fig. 3.
high speed) could be attributed to the high exhaust gas
temperature leading to the continuous DOC activation.
Fig. 4. Cumulative (a) CO and (b) THC emissions during
the NEDC driving cycle (Plasma input power = 230 W).
Fig. 3. Exhaust flow rate during driving cycle NEDC.
Cumulative mass of each pollutant (CO and THC)
during the cycle was estimated by the following equation:
𝑇
𝑚 𝑇 = ∫0 𝑄𝑣 (𝑡). 𝜌. 𝜂𝑝 . 𝑑𝑑
(2)
where Q v is the flow rate of the exhaust gas in L/s, ρ the
pollutant mass density in g/L, η p the fraction of pollutant,
and T the duration of the cycle (1200 s). The total
distance for the cycle is 11.007 km.
Cumulative CO and THC mass emissions along NEDC
are presented in Figs. 4a and 4b, respectively. Each
figure depicts comparison between results related to the
four configurations studied (without plasma and catalyst,
plasma, DOC, and plasma-DOC). In the absence of both
plasma and catalyst, about 59.1 g of CO and 11.1 g of
THC have been emitted at the end of the test. The plasma
alone results show that only 25% of THC were oxidized
and 62.4 g of CO have been emitted. On the other hand,
the diesel oxidation catalyst used alone converts more
than 72% of the CO and THC released during the
complete NEDC. For CO emissions, the "plateau"
observed during the extra-urban driving cycle (t > 800 s,
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When the plasma was combined to DOC, the
cumulative mass of CO and THC was notably lower than
those obtained when the DOC was used alone. The
benefits with the use of plasma are more as the cycle
progresses, these differences being more notable after the
first urban-driving cycle (t > 200 s). It is important to
remark that, during the urban driving cycle for which the
exhaust gas temperature is low (~150 °C), the plasma
avoids the DOC deactivation limiting the CO emissions.
Due to the HC storage on DOC, the plasma effect is more
noticeable on CO emissions than on THC emissions.
Thus, the THC emission observed during the plasma
operation is a result of both HC removal from the bulk
gas by the plasma and HC desorption from the DOC due
to the increase of temperature.
The combination of plasma-DOC shows that the
cumulative mass of pollutants measured during the NEDC
decrease by about 63% for CO and 42% for THC,
compared to the values obtained without plasma. These
emissions correspond to 536 mg/km and 176 mg/km,
respectively. Results of CO have to be compared to
Euro 6 regulation for diesel engines; the limit is
500 mg/km (EU standards for THC emissions are coupled
with those of NO x ).
3
4. Conclusion
The use of non-thermal plasma assisted diesel oxidation
catalyst (Pt-Pd/Al 2 O 3 ) was investigated in test bench
simulated diesel engine exhaust for two different modes
of operation: steady-state mode (constant gas flow rate)
and NEDC mode. Focus was made on the DOC light-off
temperature behavior and the amount of CO and THC
emissions.
The main contribution of the plasma,
upstream the catalyst, is that it has improved the DOC
activity, especially during the engine cold start phase, and
significantly decreases the emission of CO and THC in
the NEDC even during the acceleration phase. It could be
suggested that, the observed plasma-DOC activity can be
enhanced by increasing the plasma-injected power.
5. Acknowledgment
This study was supported by national French CIFRE
program (agreement no. 774/2009) with a financial
support of Renault SA Company. The authors gratefully
acknowledge A. Guy and K. Lombaert for their fruitful
discussions and technical support.
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