(173e) Modeling of Fast Cycling NOx Storage and Reduction – Effect of Reductants, Thermal Effect, and HC-Intermediate Mechanism

Modeling
of Fast Cycling NOx Storage and Reduction – Effect of Reductants, Thermal
Effect, and HC-intermediate mechanism

Allen Wei-Lun Ting, Michael P. Harold
and Vemuri Balakotaiah

Department of Chemical & Biomolecular Engineering,
University of Houston, Houston, TX 77204, USA

Abstract

A modeling study of fast cycling NOx
storage and reduction (NSR) with C₃H₆ and H₂
for emission control of lean burn gasoline and diesel vehicles is carried out
using 1-D two phase catalytic monolith reactor model including the washcoat
diffusion. Our recent experimental study shows that when cycling time is
reduced from 70 s to 7 s the thermal effect and impact of reductants, including
HC-intermediate pathway, may be minor factors in enhancement of NOx conversion comparing
to the better utilization of sites. This work proposed a global kinetics for modeling
previous mentioned experimental results and providing mechanistic insights
through calculated spatio-temporal site coverages, species concentrations and
temperature profiles.

At temperature higher than 350 ^oC,
negligible difference in NO_X conversion between H₂ and C₃H₆
is observed under slow cycling, and under fast cycling the C₃H₆
outperforms H₂ by 7 % (Fig. 1a) at temperature higher than 450 ^oC
due to higher molecular diffusivity of H₂, which results in larger
NOx puff (Fig. 1b.)

This work proposed a simplified
mechanism for HC-intermediate (selected as C₂H₃NCO)
NOx
reduction pathway:

2 C₃H₆ + Ba(NO₃)_2,fà
(C₂H₃NCO)₂-BaO_f + 3 H₂O                                                                  (R1)

(C₂H₃NCO)₂-BaO_f
+ 15/2O₂à 6CO₂ + 3 H₂O + N₂ +
2 BaO_f,                                                 (R2)

This HC-intermediate pathway can
predict the formation of double peaks of N₂ (Fig. 2), while it show
little enhancement in NOx conversion. This pathway requires ~2.5 times C₃H₆
(compared to traditional pathway) to reduce same amount of stored NOx to N₂.
Due to this stoichiometry in NOx reduction, HC-intermediate pathway may decrease
the ignition temperature by 30 ^oC but reduce NOx conversion by ~5%
at temperature higher than 450 ^oC (Fig. 3), due to larger NOx puff
resulting from insufficient reductant (Fig. 1b). This estimates the maximum
effect of HC-intermediate pathway on NOx conversion because simpler
intermediates (like NCO) from C₃H₆ may not perform beyond
this prediction, as long as no other reactions dominate.

Modeling results are also used to distinguish
NOx reduction pathways when using C₃H₆. From predicted
spatio-temporal analysis, actual regeneration of stored NOx occurs at
temperature 30 – 45 ^oC lower than cyclic averaged mid monolith
temperature. At temperature higher than 350 ^oC, most reductants are
completely oxidized by oxygen stored on ceria sites during/where most stored
NOx is reduced, while most of CO and H₂ measured at later time are actually
spectator that don’t react with stored NOx. This makes it difficult to
distinguish what pathway NOx is reduced through, but selectivity of the spectator
H₂ and CO may help estimate.

Fig.
1 (a) Comparison of the experimental (marker) predicted (line) cyclic
averaged NOx conversion between slow cycling (solid line and circle marker) and
fast cycling (dashed line and triangle marker) with anaerobic rich feed and with H₂
(black) and C₃H₆(red). (b)
comparison of the experimental (solid line) predicted (dashed line) NOx
effluent concentration under fast cycling with anaerobic rich feed and with H₂
(black), C₃H₆ (red), C₃H₆ w/o
HC-intermediate mechanism (blue) at feed temperature of 488 ^oC.

Fig.
2 Comparison of the model predicted double peak N₂ effluent
concentration (line) and experimental m/e = 28 signal (circle) under slow (a)
and fast (b) cycling at feed temperature of 326 ^oC with H₂
(black) and C₃H₆ (red).

Fig.
3 Comparison of model predicted NOx (a) and C₃H₆ (b) conversion
between with (solid line) and without (dashed line) proposed HC-intermediate mechanism
during slow (black) and fast cycling (red).