JOURNAL OF SCIENCE & TECHNOLOGY * No 88 2012 MODELLING OF CATALYTIC CONVERSION OF TOXIC GASES IN MOTORCYCLE EXHAUST AFTER TREATMENT SYSTEM MO HiNH HOA QUA TRINH XUC TAC TRUNG H 6 A KHI THAI DOC HAI TR[.]
Trang 1MODELLING OF CATALYTIC CONVERSION OF TOXIC GASES
IN MOTORCYCLE EXHAUST AFTER-TREATMENT SYSTEM
MO HiNH HOA QUA T R I N H XUC TAC TRUNG H 6 A KHI THAI D O C HAI TRONG H $ T H 6 N G X I T L Y KHI THAI XE MAY
Hoang Dinh Long HanoiUniversity of Science and Technology
Received March 8, 2012; accepted May 4, 2012
ABSTRACT
The emission contml of motomycles in Vietnam is pariiculariy imporiant due to the high and fast raising number of these means of transpori One of the most effective method sused for emission capacity motomycles here This paper pmsents the study results on heat transfer and catalytic conversion modelling of the exhaust gas in motorcycle exhaust after-tmatment system equipped with
a three-way catalytic converter for optimum design of exhaust emissions contml system The paper
of nitmgen (NO) with detailed heat transfer modelling, which includes conductive, eonveetive and radiative heat transfers The modelling results show that using exhaust after-treatment system equipped with a three-way catalytic converter for emission control of motomycles can reduce exhaust emissions of NO and CO by 99% and HC by 70% after 300s from engine start
TOM TAT
Vide kiem sodt khi thdi xe mdy & Vidt Nam cd y nghTa ddc bidt quan trong vi s6 lugng cdc phuang tidn giao thdng ndy rit l&n vd vin dang ldng nhanh Mdt trong nhOng cdng nghd kiim sodt khi ndy lai chua dugc s& dqng trdn xe mdy dung tich nhd Bdi bao ndy trinh bdy kit qua nghien ciru md
bj bd xOc tdc tmng hda khi tlidi ba chirc ndng Idm ca sd di thiet ke tdi uv hd thing xir ly khi thdi Bai bao di cap din sg biin ddi hda hoc cua d xit cdc bon (CO), hydrd cdc bon chua chay (HC) vd d xit nito (NO) va sg trao ddi nhidt cOa khi thdi v&i dng thdi vd ra mdi tru&ng gdm cdc qud trinh trao doi nhidt ddi luv ddn nhidt vd buc xp nhidt Kit qua nghidn ciru chi ra rang su dung he thing xif /)? khi thdi trang bi bg xuc tdc tmng hda khi thdi ba ch&c ndng cho xe mdy cd thi gidm phat thai NO vd CO den 99%, gidm HC din 70% sau 300s tir liic khai ddng ddng ca
1 INTRODUCTION which include the improvement of petroleum
„ , originated fuel quality, mandatory installation
'n Vietnam, following the socio- of three-way catalytic converters (TWCs),use of
economic development, the number of j„,„^,i,e f„e| vehicles such as "plug-in" motorcycles as individual transport means electric cars, LPG and CNG buses and hybrid
increases very fast By the mid of 2008, the ^^^.^|_.^ ^^^ 1,^^^ ^^^ ^^^^ f^^ ^„,^ number of these transport means in the country ^^^^^ ,^^^^ ^^^^^^ „ ^ ^ ^ j ^ ^ ^ ^ ^ ^ ^ j , ^ j ^ ^
is nearly 22 millions and the expected number treatment to achieve a significant reduction in for the 2020 will be more than 5 millions [1] ,^^.^ emissions They have been considered to
This has caused the growing concern on ^^ ^^^ ^^ ,^^ ^^^^ ^fj^^,;^^ technologies of
environmental pollution and public health, emission control in cars [4]
which has forced the government to be more
proactive in emission control of motorcycles as For small capacity motorcycles, however, well as ofolher transport vehicles [2], the design and installation of TWCs have not
been of interests The study of emission control
For motor vehicles (cars and trucks), ^^ ^^^^^ motorcycles have mainly focussed on many effective technological advancements _ ^ ^^^ ^^ of alternative fuels [5 6]
Trang 2emission standards, it is necessary to further
reduce exhaust emissions by applying advanced
sludies the conversion efficiency of exhaust gas
of the motorcycle exhaust after-treatment
system equipped with a TWO The modelling
approach is applied for the study of heat
transfer and catalytic conversion of exhaust gas
in the system for optimum design and
installation of a TWC in the system
2 MATHEMATICAL MODELS OF HEAT
TRANSFER AND CATALYTIC
CONVERSION OF EXHAUST GASES
2.1 Motorcycle
system exhaust artcr-lrcalmcnt
Figure I Motorcycle exhaust system equipped
with a TWC (applied lo Honda SuperDream)
1- Flange-joint to engine exhaust port
2- TWC: 3- Exhaust pipe: 4- Muffler
Figure I describes a motorcycle exhaust
system equipped wilh a TWC for exhaust
treatment TWC 2 is installed somewhere at the
exhaust pipe between flange I and muffler 4
The appropriate installation position of TWC in
the system is experimentally or theoretically
detennined
^^Liu.:
JoanL
^ zinnnP
r - ^nr^
Figure 2 Diagram of a l tVi'
Ishell 2-munolilh: 3- heat isolate layer:
4- Alumina layer; 5-ealalysl layer
Figure 2 shows the diagram of a TWC
used in this study It has cell density of 62 cm'
catalyst material JM of 2200g/m' TWC, catalyst area of ISmVg The size of TWC is primarily chosen and then evaluated and modified accordingly so that it works with conversion efficiency of 70-99% and pressure loss less than 10% In Ibis case the catalyst diameter of 4cm and length of 4cm are primarily determined
2.2 ilcat transfer models In exhaust pipe The heal transfer model simulates thermal response of the exhaust gas and predicts gas temperature along the exhaust pipe
It helps to predict accurate gas temperature al the inlet face of the TWC and serves as the input data to Ihe catalyst model.By assuming balance equation for Ihe exhaust gas and the exhaust pipe are written as follows:
Hi
dt '
dt
u - •
dx D'T
ct—f"
P.^pn^>
-9 rod
^r'^/
(1)
(2)
Where T \s temperature;/ - time; x - length co-ordinate; p- density; c- heat capacity; V- control volume of gas (Ki) or pipe (F;); u- gas velocity; q- transferred heat flux ffrom gas to inner wall
of pipe q^, convecdon to ambient q„„ radiation
to surroundings^„j); subscripts g for gas, p for
pipe.aforambienl
q^=h^nd,A.x{T^~T^) (3)
•/n«='' T'/:iv(7:^-7;,) (4)
<?^=£07r<.(7;;-7;;)Av (5) Where (/, rfj- inner and outer diameter of exhaust pipe;Ax- length element of
pipe;a-Siefan-Boltzmann constani;r,p Top- inner and
outer pipe wall temperatures; heat transfer coefIlcients/ij,p, /i^.^, and emissivily coefficient Eare taken from [7, 8]
2.3 Heat transfer and catalytic conversion in the catalyst
a) Chemical reaclion scheme
The main engine-out emissions
Trang 3Of the NOx emission components from petrol
engine, N O represents the main proportion
while proportion of other components is
relatively small (less than 10% of NO^) Hence,
the present model considers Ihe oxidation of
CO and HC and the reduction of NO The
oxidation of total unbumed hydrocarbon (HC)
is divided into 2 mechanisms, i.e rapidly
oxidised hydrocarbon considered as C3H6
representing 8 6 % of HC and slowly oxidised
hydrocarbon CH^, representing 14% of HC [9],
H,is also considered because it is associated
with high heal release Hence, 5 chemical
reactions are considered in this model
R,: CO -( O, - > C 0 ,
2 ' (6)
constants: k,(j =1-5)- kinetic constants AT.and
A,are determined from Chan and Hoang [7]
b) Governing equations
To simulate the thermal and emissions responses of the catalytic converter, a one-dimensional model described in figure 3 is developed The parameters of gas at the front face ofthe monolith are assumed to be uniform The basic equations of the model are energy phase ofthe converter
Q.o,
Q s
Q,ou, I
^Q^rA
R3: C H , + 2 0 3 - > C O , + 2 H , O (8)
R,: H j - t - - ! - 0 , - > H j O (9)
The kinetic rate expressions were based on the
w o r k o f V o l t z e l a l [10]
+k,C,%C^J,C°^l, IG, (11)
^ - ^ A = ^ Q c H Q o , ^ G (12)
^3 =^CH, =*3C,,.CH^C,o, IG (13)
R,-^Ryi^ =k^C,^C^aJG (14)
^ 5 = ^ « , ^ ' t j C ; t v , C w , , C ? J ^ , , / C , (15)
where
G = 7;-(l + /:,Cs(.o + l^iC^ (.,H, )" ^
G,-(i-r/c,c:;o)'
The rale expression of reaction of O; is
calculated by balancing oxygen concentration
in the reactions Thus
R^^R,^ ^-(R^-R^) + -R,+2R,+-R,{\6)
Here, C is gas concentration; subscript S is at
Figure 3 Ileal transfer in a catalyst channel Q-heatfiux, Ax- control length of catalyst
For the gas phase, the energy and mass balance are as follows:
^ T dT
Spc^.—^ = -puc,^—^- + hS{T-T){M)
* " • ^ t *^ ™ £ ? X
*?C„, (^C.,
- / i „ 5 ( C ^ ^ - C , , ) (18) Energy and mass balances for the solid phase are as follows:
5,„,ii = ^ / i „ 5 ( C ^ - C , ) (19)
M '
( l - < 5 ) A V ^ - ( l - < y ) t , ^
' dt dx
+hS{T^-TJ + S,.,„Y,(.-^j)li (20)
where C denotes gas component > (/' =1 for
CO, 2 f o r C j H ^ j forCH4 4 for H, 5 for NO
and 6 for O2); (-AHj) is the heat release from
reaction of gas J and is determined from [7];
p., p.- gas and wall densities: Cp^, Cp, - heat
capacities of gas and wall; ti- gas velocity; Saa
S-calalyst area and heat transfer area per unit of
volume of the converter; h- heat transfer
coefficient; /jD.diffusion coefficient
Boundary conditions are determined based on the physical phenomena in the
Trang 4catalytic converter That is, at the inlet face of
(flow rate, temperature and concentrations)
were known Before the engine is started (time
^ 0) Ihe wall temperature of the monolith is
equal to the ambient tcmperalure Also, because
the heat conductivity of Ihe monolith is much
higher than the eonveetive heat transfer
coefficient at its front and rear faces, the heat
flux at these two locations can be neglected
Therefore the boundary conditions are written
a s : r ^ ( 0 / ) - 7 ; ' - : C^,(0./) = c;", ; 7;,(:c.0)^7;, ;
^(o,o.^(U)=o
dx dx
The input data for the complete exhaust
system model arc the system geometry
parameters in Figure I (L|+L„+Li= 400mm
pipe diameter=25mm, pipe wall
ihickness=2.5mm) and gas parameters al the
exhaust port underidling and full load condition
which are taken from [t 1] and estimated as in
inlet face is predicted from the exhaust pipe
heat transfer model The predicted gas
temperatures serve as Ihe inputs lo the catalyst
model together with Ihe gases concentrations at
catalyst upstream so Ihat the tailpipe gas
concentrations and temperatures are predicted
The finite difference numerical melhod is
adopted with defined space grid and time
marching interval for the entire exhaust system
Table I Exhaust port gas parameters of Honda
motor.a) under full load 7000rpm:b) under
idling I450rpm: V, O O S U ' m i n It; 5K0"C
' v.iip ; T g ~ ^ n ) T n r ^ \ g o v ' " o : j f o
' '""" i ! Q ! i^'>_i_laii2LL.' " i""i 1 '."-L:
\ 0,28 710 , 9.7 650 450 0.5 ! 1 ' i 710 9.7
3 K K S l l l AND D I S C I S S I O N S
The mathematical models of heat transfer
and catalytic conversion in motorcycle c\liaust
system described by equations (1), (2) and
equations (17) to (20) together wilh initial and
boundary conditions above applied to a Honda
Super Dream motorcycle are numerically
solved using FORTRAN codes
The simulation results describing the
behaviour of the exhaust gas in the exhaust
4 describes the change of exhaust gas temperature along the exhaust pipe under full load and no load running conditions For the catalyst to work well under both idling and full load condition, it should be installed at the position of 12 cm from the exhaust port At this position, the gas temperature is not loo hot (<600''C} to avoid fast aging catalyst under full load condition and is hot enough (>3S0"C) for catalyst lighloff under no load condition
Dsiance from exhaust port (cnj
Figure 4 Gas temperature along exhaust pipe
al no load and full load condition
Figure 5 shows Ihe increase in catalyst wall temperature diuing wami up period It can
be seen that after 250s from engine start, the
entire catalyst wall reaches the active temperature for ealaiysl lighloff
600
Tirre (s)
Figure 5 i 'attily.si wall temperature al the front surjace middle (0 51) and rear surface of the monolith I- length
Figure 6 shows Ihe catalyst conversion ellTciency of NO C O and HC Il is clear that after 300s from engine start, the conversion clllciency of NO and C O reach near 100% This means that these gases are approximately totally cut down The catalytic conversion efficiency of HC reaches 70%, a bit lower than that of the two other gases The standard TWC only operates well on fuel injection engines wilh close-loop air-fuel ratio (A/F) control (for
Trang 5(Honda Super Dream motorcycle), the TWC
added with CeOi can operate efficiently by its
storage or release of Oj under the change of
A/F ratio [12]
behaviour of CO, HC and NO emissions in the exhaust system from engine start The simulated results are summarised as follows: Knowing the main engine-out exhaust gas characteristics, this model is able to predict the spatial and temporal variations of gas temperature and concentrations of gases at converter The latter determines the lightoff behaviour ofthe catalyst In addition, solid phase temperature ofthe monolith in axial direction can be also determined The modelling results can help to improve catalyst design and installation for optimum working conditions, which increase catalyst conversion efficiency as well as avoid fast aging the catalyst
Exhaust after-treatment using TWC is also
an effective method of emission control in motorcycles
0 50 100 150 200 250 300
"nme(s)
Figure 6 Catalyst conversion efficiency of NO
CO and HC under full load
4 CONCLUSION
In this study, a t-D quasi-steady heat
transfer and chemical catalytic conversion
model of Honda super-dreammotorcycie
exhaust system equipped with TWC has been
successfully developed which simulates the
heat transfer, chemical reactions and conversion
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10 Voltz, S E., Morgan, C, R., Liederman, D Kinetic Study of Carbon Monoxide and Propylene Oxidation on Platinum Catalysts Ind Eng Chem Prod, Res Dev., Vol 12, No 4, (1973)
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Author's address: Hoang Dinh Long - Tel (+84)983.658.884, Email longhd-ice@mail,hut.edu.vn
School of Transportation Engineering, Hanoi Univeraty of Scierxs 3rd Technology
No I Dai Co Viet Str., Ha Noi, Viet Nam