United States Industrial Environmental Research EPA-600/7-79 232b
Environmental Protection Laboratory October 1979
Agency Research Triangle Park NC 27711
Assessment of Diesel
Participate Controj:
Direct and Catalytic
Oxidation
Interagency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
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mental issues.
EPA REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-232b
October 1979
Assessment of Diesel Particulate Control:
Direct and Catalytic Oxidation
by
M.J. Murphy, LJ. Hillenbrand, and D.A. Trayser
Battelle Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
Contract No. 68-02-2629
Program Element No. EHE623
EPA Project Officer: John H. Wasser
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
The technology and potential for disposal of diesel particulate
by oxidation are discussed. Relevant properties of typical diesel particu-
late are given; particular note is taken of the small size (on the order of
0.1 pm diameter) and the presence of a portion extractable with an organic
solvent. Available reaction rate data is used to derive particle lifetimes
at various temperatures; these exceed likely exhaust system residence times.
The use of catalysts to increase oxidation rates and lower ignition tempera-
tures is discussed. Small amounts of many metals are effective in increas-
ing the rate of oxidation by 2 to 5 orders of magnitude. Chemical reactor
theory is used to derive ignition and operational characteristics of trap/
oxidizers. Special note is taken of the tendency of these devices to go
rapidly from a cold unignited state to an ignited state close to the adia-
batic flame temperature of the fuel. Design techniques to ameliorate
undesirable temperature excursions are presented.
ii
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TABLE OF CONTENTS
Page
Abstract ii
Figures iv
Tables iv
1. Introduction.
2. Diesel Particulate 2
Properties of Diesel Particulate 2
Particulate Emission Rate Characteristics
of Diesel Passenger Cars 2
3. In-Stream Oxidation of Particulate 6
Thermal In-Stream Oxidation 6
Catalytic In-Stream Oxidation 9
4. Oxidation of Filter-Trapped Particulate 10
Trap/Oxidizer Strategies 10
The Ignition Concepts 11
Strategy 1-Thermal Ignition of Trapped
Particulate 15
Strategy 2-Use Ignition Devices 15
Strategy 3-Catalytic Ignition of Particulate .... 16
Strategy 4-Combined Use of Catalytic Hydro-
carbon Oxidizer and Particulate
Trap/Oxidizer 20
Condensation Effects on Particulate 21
Approaches to Trap/Oxidizers Design 21
Combustion of Collected Material 23
5. Conclusion 23
References 25
iii
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LIST OF FIGURES
Figure 1. Number, Surface Area, and Volume Distributions
for Mode 3 of the EPA 13 Mode Cycle 3
Figure 2. Temperature Relationships for Exothermic
Reactors 13
LIST OF TABLES
Table 1. Soluble Organic Fraction and Sulfate Emissions
from Heavy-Duty Diesel 4
Table 2. Fractional Breakdown of Soluble Organic
Emissions from Heavy-Duty Diesel 4
Table 3. Light-Duty Vehicle Particulate Emission
Rates as a Function of Vehicle Type
and Weight 5
Table 4. Light-Duty Vehicle Particulate Emissions Rates
and Concentrations for Cyclic and Steady
Speed Operation g
iv
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SECTION 1
INTRODUCTION
Diesel engines are becoming available in increasing numbers in
passenger car service because of their good fuel economy in comparison to
conventional gasoline engines. About 160,000 diesel-powered light-duty
vehicles were sold in the 1978 model year in the United States. This
figure is expected to double in 1979, and sales of light-duty diesels are
expected to continue to increase substantially over the next 10 years.
In 1990 15 to 20 percent of all light-duty vehicles sold are predicted to
be diesel powered.
Though the diesel exhaust is relatively clean with respect to
unburned hydrocarbons and carbon monoxide, it contains particulate emissions
that are 30 to 50 times greater than those produced by the catalyst-equipped
gasoline engine. These diesel particulate emissions will contribute to
already high levels of total suspended particulate (TSP) in urban areas.
Furthermore, certain components of the particulates have been identified
as carcinogenic, thereby creating a potentially greater health hazard.
The U. S. Environmental Protection Agency (EPA) has issued an
NPRM (Notice of Proposed Rule Making) which will impose limits on the amount
of particulate that may be emitted by each light-duty diesel vehicle. The
proposed standards, based on the presently used FTP with a particulate measure-
ment procedure added, are 0.6 grams per mile (0.37 g/km) for 1981 model year
vehicles and 0.2 grains per mile (0.12 g/km) for 1983 model year vehicles.
These standards are based on the need to reduce (or prevent an increase in)
the TSP levels in urban areas as diesels become more numerous. It is quite
possible that even more stringent particulate emission standards will have to
be set in the future to control the toxicity problem.
There are several approaches to the control of diesel emissions
that are being pursued by the automotive industry, EPA, and others. These
include operating mode modifications, engine design and component modifica-
tions, fuel modifications, and exhaust devices.
In this report we discuss the technology of and potential devices
for: thermal and catalytic oxidation of particulate while suspended in the
exhaust stream, thermal oxidation of filter-trapped particulate, catalytic
oxidation of particulate, oxidation of particulate, oxidation of particulate
-------
trapped by other techniques, and catalytic oxidation of the organic vapors
before deposition on the particulate.
The following discussions will center primarily on the applica-
tion of "add-on" devices in the engine's exhaust systems. However the
successful application of a thermal or catalytic oxidation system for
particulate or organic vapor control may also depend on operating mode
engine design, or fuel modifications for maximum effectiveness. These*
aspects will also be covered in the discussions where appropriate.
SECTION 2
DIESEL PARTICULATE
The particulate matter emitted by the engine consists of solid
liquid, and adsorb _d vapor. The particles can be considered either as single
entities or as aggregates of subunits. Because of this complexity, exact
specification is difficult; one must measure properties that seem to be
meaningful in the design of control strategies. Moreover, each engine fuel
and set of operating conditions causes the properties of the particulate to '
vary. In the following section we have tried to give representative values
for some of the properties that are important in the design of control
strategies.
PROPERTIES OF DIESEL PARTICULATE
Physical Form: Small, solid, irregularly shaped particles
agglomerates of roughly spherical subunits!
High molecular weight HC adsorbed on surface.
Also may have liquid coating.
Size: Number median diameter 0.3 ym; subunits about 10 nm
Size Distribution: See Figure l^1) for sample size distribution.
Composition: Mostly carbonaceous material 10-50 percent
extractable with organic solvent.
Bulk Density: "bulk * 0.075
on "
Ash: One value of 16 percent
Extractable Hydrocarbons: See Table
Chemical Composition: See Table
PARTICULATE EMISSIONS RATE CHARACTERISTICS
OF DIESEL PASSENGER CARS
Measurements of light-duty diesel particulate emissions under
various conditions of operation have been reported in the literature These
measurements provide some insight into the factors that influence the emission
r ci c •
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MODE 3
(25°/eload. 1800 rprr )
Volume Dilution Ratio (D)-55
£S/Log Dp
Surface Area
Distribution
EV/Log Dp
Volume
Distribution
i i i i i 11 il
j i i i i 11 il
i i i i i 111
.001 .01 O.I
Dp- Particle Diameter (micrometers)
1.0
FIGURE 1. NUMBER, SURFACE AREA AND VOLUME DISTRIBUTIONS
FOR MODE 3 OF THE EPA 13 MODE CYCLE
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TABLE 1. SOLUBLE ORGANIC FRACTION AND
SULFATE EMISSIONS FROM HEAVY-
DUTY DIESEL(a)
Soluble Organic
Fraction
Sulfate
Emissions
of Total Participate
Light Diesel Fuel
Average Diesel Fuel
Heavy Diesel Fuel
17
16
32
9
9
12
(a) Caterpillar naturally-aspirated, direct-injec-
tion V-8 engine, EPA mode 5, 8:1 dilution
ratio.
TABLE 2. FRACTIONAL BREAKDOWN OF SOLUBLE ORGANIC
EMISSIONS FROM HEAVY-DUTY DIESEL
Paraffin
Oxygenated
Aromatic
Transitional
Acid
Base
Ether and Hexane
Insoluble
Light
43
14
3
6
10
2
22
Fuel Type
Average
43
11
6
3
15
1
21
Heavy
56
8
8
3
10
1
15
(a) Caterpillar naturally-aspirated, direct-injec-
tion V-8 engine, EPA mode 5, 8:1 dilution ratio.
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Particulate emissions from passenger-car diesels range from
0.2 to 0.7 gm/km on the 1975 FTP cycle, and from 0.1 to 4.0 gm/km on steady
speed/load operation. Expressed in terms of concentration in the exhaust
gases, the participates range from about 20 to 130 mg/rn^.
In general, the particulate emission rate is a function of
vehicle weight, the heavier vehicles produce more particulates in the same
manner in which they produce more gaseous emissions. EPA certification-
vehicle emissions data for the 1979 model year are presented in Table 3.
TABLE 3. LIGHT-DUTY VEHICLE PARTICULATE EMISSION RATES
AS A FUNCTION OF VEHICLE TYPE AND WEIGHT*
Vehicle
(Diesel)
VW Rabbit
Peugeot 504
Mercedes 300 D
Oldsmobile 4.3 1
Oldsmobile 5.7 1
Approximate
Curb Weight,
Ibs
2000
3000
3500
3500
3800
FTP Particulate
Emissions Rate,
gm/km
0.14
0.18
0.52
0.45-0.63
0.53
Particulate emissions also appear to increase with increasing
aromaticity of the fuel, with lower speed and load operations, with higher
intake air temperature, with higher fuel temperature, and with the use of
EGR (exhaust gas recirculation). The relationship between particulate
emissions and EGR is an especially troublesome problem for the industry
because EGR is currently the best hope for meeting the more stringent NOX
emission standard scheduled for the 1981 model year. EPA has proposed a
particulate emission standard of 0.373 gm/km also to take effect in the
1981 model year; the industry claims that it cannot meet both NOX and
particulate standards in 1981 with current technology.
Data in the literature relating particulate emission rates to
vehicle speed and load under steady state conditions is sparse. The
following data may or may not be representative, but is presented in Table
4 to illustrate what is available.O)
Federal Register, Vol 44, No. 23, February 1. 1979
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TABLE 4. LIGHT-DUTY VEHICLE PARTICIPATE EMISSION RATES
AND CONCENTRATIONS FOR CYCLIC AND STEADY SPEED
OPERATION
Particulate Emissions, gm/km (mg/m^)
Vehicle
Mercedes 240 D
VW Rabbit
1975 FTP
0.31
(97)
0.28
(100)
Low
Speed
2.8
(83)
1.8
(46)
50 kph
0.13
(70)
0.12
(78)
85 kph
0.17
(95)
0.14
(87)
The values in Table 4 are averages of a number of tests, with
individual test results sometimes varying quite widely. In general it appears
that the low speed condition is the worst particulate emission mode in terms
of gm/km although the particulate concentration is relatively low. Also,
it is evident that steady speed/load conditions result in lower particulate
emission rates than cyclic conditions.
SECTION 3
IN-STREAM OXIDATION OF PARTICULATE
One of the simplest control techniques would be an exhaust system
that completes the combustion of the carbon particles. Some aspects of
this method are explored below.
THERMAL IN-STREAM OXIDATION
One strategy for the control of diesel particulates is the use of
in-stream oxidation. Since the exhaust always contains some oxygen, the
diesel particulate can be eliminated by oxidation if the temperature is high
enough and the time for burning long enough. However, raising the particle
temperature by heating the bulk of the exhaust gas requires a large amount
of energy. Since such very small particles will reach temperature equilibrium
with the surrounding fluid very rapidly, the bulk of the gas must be kept
at the temperature desired for the particles. Unless almost all of the energy
used to heat the bulk gas could be recovered, this method of control would
be quite impractical. Design of a compact heat exchanger for the temperatures
and flow rates involved would be difficult.
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The analysis below explores some aspects of in-stream oxidation
of particulate. The following exhaust conditions are assumed:
exhaust temperature
exhaust flow rate
particle size
particle concentration
200 C
0.05 kg/s
1 ym, 0.1 ym or 0.0287 ym*
100 mg/m3
* Particles of this size are estimated to have a surface
area of 112 m2/g. Lee, Thring and Beer^) report that
soot from a hydrocarbon flame has a BET surface area of
112 m^/g. Surface areas for various diesel particulates
need to be measured in order to refine these calcula-
tions .
These conditions might represent a 5.7 1 diesel engine at medium speed and
high load.
The particles must be kept at a high temperature long enough to
allow oxidation to take place. In order to get an idea of how much time
is available, exhaust velocity of 21 m/s was calculated based on a 50-mm
ID exhaust pipe. If the path at the high temperature is 1-m long then
the time available for oxidation will be 50 ms. This does not take into
account the additional residence time in the muffler or in a control device,
but it does illustrate magnitude of the time available.
The residence time at high temperature could be increased by
increasing the volume of the afterburner device. However, since the
possibility exists that the engine may supply a flammable mixture to the
exhaust system, a large confined volume would present a safety hazard.
For particles in this size range the rate of oxidation should be
kinetically controlled at all reasonable temperatures(5). Thus, by examining
the chemical kinetics, the temperature necessary for oxidation in this time
interval can be determined. Good data for the oxidation of diesel particulate
is not available, so this calculation was carried out using data for graphite.
It was also assumed that the particulate consisted of small spheres. The
kinetic expression of Nagle and Strickland-Constable'^) is used for these
calculations. This expression gives the mass of carbon oxidized per unit
surface area per unit time at a given temperature and partial pressure of
oxygen. For this type of rate expression
~ = RS or dm = RS dt
dt
where m = mass of carbon burned
t = time
R = reaction rate
S = surface area
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Since for a sphere S = irD2 where D = sphere diameter
dm = R IT D2 dt
To find the change in sphere volume we note that:
dV = jirD2
dD 2
By considering the change in surface area with diameter, the burning times
of the particles can be found. The following expression results:
t =
(D
where p = density of carbon
D = particle diameter
R = reaction rate
Using equation (I) and the kinetic expression of Nagle and Strickland-
Constable the time required to oxidize completely a particle of a given
size can be found. Some results of this calculation are shown below:
Particle Size Temperature Burning time, approx.
1 ym 1000 K (727 C) 20 s
1 ym 1500 K (1227 C) Is
1 ym 2000 K (1727 C) 50 ms
0.1 ym 1000 K (727 C) 2s
0.1 ym 1500 K (1227 C) 100 ms
0.1 ym 2000 K (1727 C) 5 ms
0.0287 ym 1000 K (727 C) 575 ms
0.0287 ym 1500 (1227 C) 30 ms
0.0287 ym 2000 (1727 C) 1 ms
It can be seen from the above results that the particle temperature
must be quite high no complete oxidation in a reasonable time. It is of
interest to estimate that heat required to bring the exhaust to this tempera-
ture.
To heat a kg of exhaust from 200 C to 1200 C requires about 1120
kJ of heat. At an exhaust flow of 0.08 kg/s, 90 kJ/s of heat must be supplied,
At this speed and load the engine uses about 13 kg/hr of fuel or
0.0036 kg/s. If the heat content of the fuel is taken as 45,000 kJ/kg, then
the engine is using 162 kJ/s of energy. It can be seen that the energy re-
quired to heat the exhaust to the particulate burning temperature is over
half of that used by the engine. This is obviously unacceptable.
-------
It might be argued that heat will be released when the particu-
late burns. The heat released will be given by
Q = rop H
where Q = heat released
mass flow of particulate
/ m3ex \ /0.08 kg ex\ n, ,
0.737 kg ex r—' = U mg/S
y 8 / \ /
H = heat of combustion for particulate
Assuming that H is 45,000 kJ/kg, we find 0.5 kJ/s released by the burning
particulate. It can be seen that this is negligible compared with the 90
kJ/s required to heat the exhaust.
If it were possible to recover most of the heat in the hot exhaust,
this afterburning concept might be feasible. Some kind of heat exchanger
would be required for this purpose. This heat exchanger would have to operate
at temperatures of 1200 C and durability may be a real problem.
CATALYTIC IN-STREAM OXIDATION
The burning rate of the diesel particulate appears to be not rapid
enough at typical exhaust temperatures to eliminate a significant amount of
particulate. One way to speed up a reaction rate would be to use a suitable
catalyst.
Such a catalyst would, of course, have to be incorporated in the
particle as it is formed. This suggests that the catalyst be an additive
to the diesel fuel. Although the exact mechanism that would cause incorpora-
tion is not known, several could be suggested involving either chemical
reaction of the soot with the additive, agglomeration of soot and additive
particles, additive particles acting as nuclei for soot formation, or
adsorbtion of the gaseous additive by the soot. If such incorporation can
be achieved and a suitable catalyst found, the burning rate at exhaust
temperatures could increase dramatically.
There are a number of metals that show catalytic activity for the
oxidation of graphite. Some likely candidates are discussed in detail in
connection with the oxidation of material collected on traps. As the following
tabulation^ ' shows, the reaction rate can increase dramatically. And we
can expect that the presence of the catalyst will be most noticeable at lower
temperatures.
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Rate of Reaction Relative
Metal to Pure Graphite
Ba 100
Na 230
Au 240
V 340
Cu 500
Ag 1,340
Cs 64,000
Mn 86,000
Pb 470,000
There would be toxicity problems with some of these metals, but others may
be acceptable. Even lead may be acceptable since the amount required may
be very small—in tests with graphite a lead concentration of 1.3 ppm was
enough to increase the rate of oxidation at 430 C by a factor of 120.
Exactly how large an increase in the rate of oxidation will occur
is uncertain. One reason is that the impurities and trace elements in the
diesel fuel may already be acting to catalyze the oxidation reaction. It
has been suggested that this is the case for most coal combustion where
the measured energy of activation is about 170 kJ/mol although the energy
of activation of pure graphite is reported to be 250 kJ/mol. Certainly
diesel fuel does not have as many metallic impurities as coal and probably
lacks the optimum amount of metallic catalyst.
Such a catalytic approach could make in-stream oxidation of the
diesel particulate feasible. This would have the advantage of not requiring
any add-on devices. Or, if a trap were found necessary, the particulate
loading would be reduced, and the light-off characteristics would be improved
SECTION 4
OXIDATION OF FILTER-TRAPPED PARTICULATE
A basic problem with the particulate in the diesel exhaust is that
it is too dilute to burn. This can be remedied by collecting the material
on a filter. Although a filter is usually thought of as a way to remove
particles from the flowstream, it can also be thought of as a device for
concentrating the particulate material. In this concentrated form the average
heat loss is reduced and conditions for combustion are more favorable. In
fact they may be so favorable that damaging temperature excursions may occur.
TRAP/OXIDIZER STRATEGIES
There are a number of design concepts for trapping and oxidizing
the particulate emissions from diesel engine operation. Although the require-
ments for successful application of each of these concepts can be described
qualitatively, neither the character of the particulate, the service require-
10
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merits, nor the limitations imposed by automobile application are well enough
described to permit firm choice among these concepts of the complete rejection
of any. Some concepts, such as the catalytic oxidation of a solid carbon
particle that has been casually deposited on a solid catalytic surface,
immediately imply such severe limitations as to appear improbable. In this
section of the report it is assumed that the particulate has been trapped
in a fashion suitable to each concept.
Four broadly based strategies must be considered:
Strategy 1 - The trapped particulate is brought to ignition
by heat derived from the engine operation,
usually from the hot exhaust gases themselves.
Strategy 2 - The trapped particulate is brought to ignition
by an auxiliary heating device used to trigger
ignition at programmed times in the engine
operation cycle.
Strategy 3 - The particulate is brought to ignition at
substantially lower temperatures than
either of the above cases by use of catalytic
materials that promote particulate oxidation.
Strategy 4 - The condensible portion of the material forming
the ambient particulate is first oxidized
(catalytically), then the remaining particulate
is burned in a trap-oxidizer unit according to
any of the above three strategies.
These four strategies are used as a vehicle to discuss the problems
and critical features associated with the burning of trapped particulate
emitted by automotive diesel engines. It is probable that any of these
strategies might be applied successfully in the laboratory; the primary
source of the difficulties that arise is the automobile application with its
adjunct limits on space, cost, and energy consumption.
In any of these choices the trapped particulate must be ignited and
burned, and the implications of this are basic to all strategies. In the next
section a kinetic model for ignition is discussed.
The Ignition Concepts
The trap/oxidizer type of particulate control device is basically
a chemical reactor in which oxidation occurs. The rate of oxidation will
vary with the temperature—as the temperature increases, the rate increases
rapidly due to the exponential nature of the kinetic rate expression.
Eventually, the rate of the reaction is limited by mass transfer (diffusion
of oxygen to the reacting surface and reaction products away from the surface),
and no longer increases with temperature. As the temperature rises the heat
loss from the device also rises. Thus, the operating temperature will be that
temperature for which the heat released by the reaction is equal to the heat
loss. Because the reaction rate curve flattens at high temperatures, the
11
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heat loss curve intersects in two places and the device will tend to be in
one of two conditions—unignited or ignited. Once ignition occurs the
temperature very rapidly reaches a value near the combustion temperature of
the fuel. This is because the exhaust flow through the filter bed serves
to increase mass transfer to the carbon on the filter. Thus, the mass
transfer limitation is not severe. Also, the heat loss occurs at the
surface of the device and conditions at the center will be near adiabatic
So the two things that tend to limit the temperature are not very effective.
The ignited condition therefore may destroy the control device.
Application of chemical reactor theory allows a more detailed
picture to be drawn.
Salient features of any exothermic reaction (e.g., combustion or
oxidation) in a flow-through reactor are the transient nature of both the
temperature of any volume element with time, and the large gradients of
temperature throughout the reactor. Afterburners, wh? aer catalytic or
not, exhibit ignition characteristics similar to flame processes and the
performance of the afterburner is directly related to the achieving and
stabilizing of a suitable "ignited" state.
The situation for some volume element of the reactor is described
with the help of Figure 2 using theory similar to that employed by Wagner(?)
and Frank-Kanenetskii^8). The rate of heat generation (Qj) in some small
volume element is described by Curve 1 which exhibits the anticipated
exponential rise in reaction rate with temperature at the lower end and th
levelling-out effect at the upper end due to transport limitations. An
increase in reaction kinetics would be expected to move the lower end of
the curve upward, as illustrated by Curve A, and an increase in transport
flow or an increase in combustible concentration would raise the upper
end, as illustrated by Curve 3.
For the given volume element, temperature stability is achieved
when the rate of heat_generation, Qj, equals the rate of heat loss from
that volume element, QH> due to the sum of the processes of convection
conduction, and radiation. In Figure 2, Curves 5 and 6 represent situations
created by different levels of the heat loss processes, QJJ. The heat loss
curves are much stiaighter than those for Qj since conduction and convection
tend to be more linearly related to the AT generated. Significant contri-
butions by radiative processes at high temperatures would make the QTT curves
concave upward. In the example of Figure 2 the heat loss relationships are
shown as straight lines.
Figure 2 can be used to demonstrate how the stable operating
temperature of the exothermic reactor, here assumed to be catalytic, will
vary with the initial temperature conditions. In each case the intersection
of the assumed Qj and QJJ curves represents the stable operating temperature
for the volume element under consideration.
12
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Or
OK
Q*
T
FIGURE 2. TEMPERATURE RELATIONSHIPS FOR
EXOTHERMIC REACTORS
13
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For a cold start with catalytic surface temperature less than TA,
and assuming Curves 1 and 5 are valid, an inlet gas with temperature less
than TA but no further, since above TA the rate of heat loss exceeds the rate
of heat generation.
Incremental decreases in the rate of heat loss would raise the
operating temperature, TA, by moving Curve 5 toward Curve 6. An equivalent
effect on operating temperature would be achieved by an increase in heat
release, e.g., by raising Curve 1. When the circumstance represented by
TC is reached as a result of such changes, the system temperature is no
longer stable since any further temperature increase causes the operating
temperature to rise continuously until Tg is reached. Temperature T^ is
the ignition temperature for the system. It is usually the case that TA
is not enough to give a "practical" rate of oxidation so that the ignited
state, represented by T£ is the one that is sought.
Later, decreased catalytic activity or a lower combustible
concentration can cause Curve 1 to bend over more sharply (e.g., to Curve
2) until a tangential situation analogous to that of Curves 2 and 6 is
generated. The new temperature represented by the tangential contact of
Ql and Qxi» ID in Figure 1, is the "flameout" temperature. Any further
decrease in reactor temperature causes the steady state temperature to
fall abruptly to the lower intersection at TB, and again all intermediate
temperatures are unstable. This is the ignition characteristic commonly
observed for exothermic reactors, whether catalytic or not, and the
stable upper operating temperature, the "ignited" condition, in many cases
tends to approach the adiabatic temperature that can be calculated for
the fuel/air mixture represented. This is especially true for local catalytic
sites where the heat of reaction is momentarily shared by a relatively few
atoms of the adsorbed gas and the catalytic site. Thus, frequently it is
a good assumption that if the catalytic reactor, here the afterburner, cannot
tolerate the adiabatic combustion temperature for the fuel/air mixture
represented it probably will deteriorate seriously in activity during use.
For oxidation processes such as this, catalysts serve to lower
the ignition temperature of the combustion system, e.g., by raising Curve 1
in Figure 2, but the thermal (noncatalytic) processes tend to predominate
kinecically at Tg. For this reason the catalyst is best viewed in most of
such cases as an igniter which must be stable against the thermal combustion
temperatures of the system represented if it is to be reused.
In applying this model for ignition processes to the diesel
particulate oxidation problem we must consider several cases and keep in
mind that the catalyst employed may or may not be a preformed solid body
contained in the afterburner. Consideration of these several cases helps
to outline the possibilities available.
14
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Strategy 1 - Thermal Ignition of Trapped Particulate
Experimental trials in the BCL Laboratory indicate that accumula-
tions of diesel particulate ignite at temperature of about 600 C. Below this
temperature oxidation rates appear very slow and the results seem to confirm
the expectation that only the ignited condition will serve this purpose.
Such temperatures are unrealistically high for any reliable mechanism whereby
the particulate is regularly ignited by waste heat from the engine without
some assistance. No further consideration is given to this strategy but the
temperature cycle for the exhaust gases can serve as the basis for more
frequent and/or scheduled ignition in the options discussed for the other
strategies.
Strategy 2 - Use Ignition Devices
Since the heat from the engine alone is insufficient to ignite
the deposit of particulate material on the trap, some type of ignition
device must be used. One possible device is a glow plug-type of electric
resistance heater. The basic problem with this strategy is that the
automotive electrical system is not capable of delivering an amount of
energy that is significant compared with the energy that might be lost
to the relatively cool exhaust flowing through the device. For example,
the electrical system is hardly able to supply more than 1 kW or 1 kJ/s.
Something like 50 kJ/s would be required to heat the exhaust stream to
the deposit ignition temperature. Obviously this is not the purpose of
the ignition device. The point is that if one tried to heat the entire
deposit on the trap to the ignition temperature, the heat losses to the
exhaust would be too large. The only alternative is to provide a few hot
spots in the deposit and let the combustion propagate through the remainder.
This could present a problem, since the deposit must be sufficiently evenly
distributed for this propagation to occur. If it is not, fuel may accumu-
late in part of the trap, only to ignite in an unscheduled or uncontrolled
fashion.
Alternatively some diesel fuel could be burned to raise the
trap/oxidizer device to its ignition temperature. The diesel fuel would,
in turn, be ignited by a spark. This approach has a number of advantages.
The amount of heat that can be put into the system is large. The fuel is
already at hand. A spark ignition system should pose no problems. Since
the additional fuel needs to be supplied for only a few seconds, the effect
on overall fuel economy will be small.
There will be design problems, however. The fuel spray must
ignite smoothly—a controlled addition of heat is required, not a backfire.
The fuel would probably be added with a small spray nozzle; this nozzle
must not foul when not in use. The heat from the burning ignition spray
must not harm the device—the additional heat may make burn-off temperatures
even higher. The presence of another fuel and ignition system adds cost
and complexity.
15
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Either system would need some kind of control to insure that
ignition occurred at necessary intervals. This could be done on the basis
of engine hours, pressure drop across the device, miles traveled, or fuel
used. Because of the wide variation in ambient conditions, it is anticipated
that the control system will have to be set for worst case conditions with
extra cycles at other times.
It seems then, that there is no barrier to making an auxiliary
ignition device for the trap/oxidizer. Of course it would be more desirable
that the device stand alone. If the ignition temperatures were lower,
additional ignition would not be necessary. It is possible that a catalytic
approach may help here.
Strategy 3 - Catalytic Ignition of Particulate
For the present application, the catalytic effect must be obtained
at the surface of carbon particulate at temperatures less than that at which
the unassisted thermal oxidation is detectable. The heat release rate due
to this catalytic oxidation must be sufficient to achieve the ignition
temperature described earlier. Trials(10) of the uncatalyzed ignition of
furnace soot placed the ignition temperature (point C in Figure 1) at 615 C,
and it was observed that insignificant oxidation rates were obtained below
480 C. This helps to define the position of the Qj curve in Figure 1.
The reduction of inlet oxygen concentration from normal air to 8 percent
had little effect on the ignition temperature for untreated samples. Samples
treated with certain inorganic salts showed substantially lowered ignition
temperature, but the reduced 02 content caused these ignition temperatures
to rise somewhat above those found in normal air. These catalysts are
discussed in the following paragraphs.
Both the design of the catalytic chamber and the activity of the
catalyst influence the ability of the chamber to achieve this ignition.
Once the most favorable form of catalysis is identified a study of chamber
design must be undertaken to maximize the repeatability of the catalytic
ignition at acceptable performance levels.
The oxidation of a solid carbon particle casually implanted on a
solid catalytic surface is most difficult to catalyze since the contact
between the particle and the catalyst is uncontrolled and its area is
anticipated to be small. The presence of hydrocarbon condensate in that
carbon particle (as in diesel particulate) may alleviate this difficulty
somewhat since that condensate may transfer to the catalytic surface and
the higher H/C ratio, expected for the condensate as compared to the solid
carbon portion, augments both the rate and heat release of the catalytic
oxidation. This anticipated influence of the level of condensate accumula-
tion on either the thermal or catalytic ignition of diesel particulate
does not appear to have been examined systematically.
16
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In some unspecified temperature range the condensate may be
anticipated to accumulate rapidly on the particulate as the temperature
falls, but this same fall in temperature increases the level of catalysis
that will be needed for ignition. In the diesel exhaust situation the
temperatures that can be relied on are already lower than desired for reliable
ignition and so catalytic trap-oxidizers have not been easy to develop.
Catalytic oxidizers for hydrocarbon vapors located as close to the exhaust
manifold as possible have been reported effective for oxidation of the
condensate portion of the particulate without materially oxidizing the solid
particulate itself(H). Such experience implies that the condensation of
hydrocarbons onto the particulate was not yet complete at this stage of the
exhaust system. Catalytic oxidizers of this sort function by adsorption and
oxidation of hydrocarbon vapors and are not necessarily designed as particulate
traps. These units in themselves are not sufficient for the particulate
problem under consideration but will be considered further in Strategy 4.
The only form of catalysis that has been described in any detail
for oxidation of carbon particulate is that developed years ago for removal
of soot accumulation from flues and furnaces by the use of inorganic salts.
The U. S. Bureau of Mines Bulletin 360^^) and the more recent work of
and coworkers provide an understanding of the level of development of this
form of carbon particulate oxidation catalysis.
It is of interest to note that English patents for inorganic
soot removers began to appear in 1856 covering the use of ordinary alkalis
or salts such as Nad, quicklime plus soda ash, magnesia, copper salts, and
others. American patents started in 1892 covering similar compounds and
especially chlorides of a number of elements such as zinc. From the earliest
patents the use of chlorides has been favored for such purposes. Bulletin
360 of the U. S. Bureau of Mines lists 59 compositions tried by them and
exhibiting decreases in ignition temperatures of as much as 287 C. By this
means the ignition temperature was lowered from 613 C to a minimum of 326 C
in normal air. Simple salts such as ammonium chloride and CaCl lowered the
ignition temperature 137 C and 92 C, respectively.
In all of these tests the volatility of the salt used was an
important characteristic. Initially, the salt was applied by vaporizing
it and allowing it to deposit onto soot that had been accumulated previously
on a test screen. The action by which the salt catalyzed burning was not
specified, but some relatively contemporary publications suggest mechanisms
that do not seem to have been further investigated.
Thus, Cassel^ ^ noted that soot deposited on the etched or
ground surfaces of Jena glass ignited much more easily and burned more
rapidly than soot on adjacent smooth surfaces of the glass. From evidence
for oriented crystal growth of soot particles on surfaces he suggested that
the ground surfaces interrupted crystal growth during soot deposition because
of surface irregularity, and also helped to prevent secondary crystal growth
during oxidation (burning). He reasoned that the salt deposits condensed
on soot similarly promote ignition and burning by maintaining the highly
dispersed soot structure prior to and after ignition.
17
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Alternatively, Taylor and Neville^-*) ancj later Day, et al.
noted large increases in soot burning rate following deposition of soot onto
surfaces previously coated with salts by evaporation. Both of these investi-
gations concluded that the salts probably functioned by hastening the
decomposition of carbon-oxygen surface complexes. With this barrier removed
the carbon surface was believed to be more readily attacked by oxidizing
gases. A brief review of the succeeding years publications has failed to
disclose any further discussion of these or other hypotheses for the action
of the salts. It is of interest to note that the promotion effect is claimed
either for salts vaporized onto the soot or for soot condensed (deposited)
onto the previously deposited salt. In the more recent work of Duval and co-
workers^) inorganic cations were impregnated into graphite carbon from water
solution and then the rates of oxidation were noted. The results showed that
the introduction of cations at 120 ppm concentration produced rates of oxida-
tion up to 470,000 times that of the untreated graphite. In this study the
effective cations were identified as representing those elements that have
variable oxidation states, and can exist in defect states of oxidation. By
this means they serve as oxygen carries to the graphite interface.
Table 5 shows an interesting contrast in the relative effectiveness
of the various metal salts employed in the studies of impregnated graphite
and of vapor deposited compounds on soot particulate. The differences seem
to underline differences in mechanism of action that have occurred, either
because of the difference in method of application or more probably because of
the differences in reactivity and structural stability of soot and graphite.
The variable valence postulate for the catalytic action on graphite is the
same as that widely used to explain the catalytic action of high-surface-
area oxide catalysts in catalytic hydrocarbon oxidizers. Two additional
observations are of interest here:
1. Some of the most active catalytic materials for
solid catalysts in catalytic oxidizers, i.e.,
the platinum metals and cobalt oxides, were not
tried.
2. Low melting oxides, such as lead oxide, have low
activity in solid-state catalytic oxidizers in
spite of their usefulness as oxygen carriers
because on melting they destroy the high area
dispersion needed for the solid-vapor contact
between catalyst and reactant. In the present
application the support for the catalyst is the
reactant (carbon) itself and the ease of melting
may imply good dispersion and mobility for the
catalyst on the oxidizing graphitic interface.
The large increases in oxidation rate illustrated by these catalytic applica-
tions indicate that considerable catalytic assistance is available for the
burning of diesel-exhaust particulate.
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TABLE 5. RELATIVE ACTIVITY ESTIMATES FOR CATALYTIC
INORGANIC COMPOUNDS ADDED TO CARBON
Na
Ca
Zn
Mn
Fe
K
NH4
Sn+2
Ni
Pb
Cu
Ba
Au
V
Ag
Cs
Lowering of
Ignition Temperature
•c(D
92
124
130
130
131
137
137
153
162
180
284
—
—
—
—
—
Relative
Oxidation Rate'2'
230
4
—
86,000(3)
—
—
—
—
32
470,000(3)
500
100
240
340
1,340(3)
64,000(3)
(1) Compound added to soot particulate by
vaporization of the corresponding chloride.
(2) Compound added to graphite powder by
impregnation of a soluble salt. Oxidation
rate compared to untreated graphite.(6)
(3) For most of these elements the normal oxide
structure melts below 500 C; in the case of
Mn a series of defect oxides are known with
uncertain melting points. All of these
elements exhibit thermal decomposition of
normal oxide structure at or below 500 C.
19
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Preformed catalyst surfaces, even if effective, may be anticipated
to accumulate ash residues from lubricating oil and/or fuel components of
the particulate and the active life of such surfaces may be materially
shortened by such accumulations. On the other hand, catalysts incorporated
in the particulate by inorganic residues from fuel or oil additives, by spray
application or by vaporization onto the accumulating particulate are self-
renewing and should occupy very little residual volume once burning is
complete. The effectiveness of such inorganic catalyst applications for
burning of diesel exhaust particulate should be easy to demonstrate. The
means for application of such catalytic methods to automobile service remains
to be examined.
Strategy 4 - Combined Use of Catalytic Hydrocarbon
Oxidizer and Particulate Trap/Oxidizer
The condensate portion of diesel particulate is a potential problem
in operation of the trap/oxidizer because of its volatility. Balanced
against the usefulness of the condensate for promotion of ignition is the
possibility that partially oxidized portions of this condensate may be re-
evaporated from the advancing combustion front and emitted with the exhaust
gases. Some recent reports indicate that such re-emitted material may be
highly toxic. Elemental carbon can be expected to burn largely to CO, C02,
and H20 and we anticipate that the troublesome re-emissions derive from the
condensate portion of the trapped particulate.
Separate oxidation of the condensibles would reduce the mass load
on the trap oxidizer by 5 to 40 percent depending on the engine operation
conditions. The question arises then of the degree to which separate oxida-
tion is feasible and the effect of that oxidation on the particulate trap.
The published experience^1) with catalytic hydrocarbon oxidizers for
stationary diesel service seems to imply that the condensation of hydro-
carbons onto the particulate has been far from complete when the oxidizer
is located as close to the exhaust manifold as possible. These units,
built for minimum back pressure, do not function as particulate traps so
that the hydrocarbon portion apparently is being oxidized at a catalyst
surface without any effective retention of the solid particulate. Thus,
Serconibe'11' has described a platinum catalyst system that is claimed to
provide overall control of hydrocarbon, nitrogen oxide, and CO emissions
from diesel engines but which apparently has no effect on the particulate.
Similarly, another hydrocarbon oxidizer system was described^!*) as able
to remove 70 percent of the hydrocarbon emissions from diesel engine that,
without the oxidizer, produced particulate with 35 percent condensibles.
Catalytic hydrocarbon oxidizers of this sort perform well only if "ignition"
is attained and maintained, and failure to achieve this may have contributed
to the comparatively poor performance of the oxidizer tried by Seizinger,
et. al.,(l°) in their trials of two derated diesel engines operating at low
emission levels.
20
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Condensation Effects on Particulate
As the temperature falls the least volatile portions of the con-
densibles may be anticipated to collect first in liquid form in the micro-
pores of the soot particulate. Micropores in the particulate might be
expected to cause condensation of a vapor, whose liquid form is capable of
wetting the surface of the particulate, at partial pressures only a few
percent of the saturation vapor pressure of that vapor. As the exhaust
gases cool, this condition would be reached first for the least volatile
components and would cause condensation of that vapor at temperatures well
above the'dewpoint of that vapor.
On the other hand, as condensation proceeds with falling tempera-
ture, each particle of solid carbonaceous material can serve as a condensa-
tion nucleus for the condensable vapor, even though the pores of the particle
may be already filled with condensate. Because of the small size of the
elementary particles of diesel particulate, e.g., 200 A, this condensation
would be occurring on convex surfaces of small radii and would proceed only
at temperatures somewhat below the vapor dewpoint. Here the concept of
critical size for condensation nuclei applies: vapor pressures on convex
surfaces of small radii are greater than those over surfaces of larger radii
so that condensation will start on the biggest particles and, for some given
condition, may proceed only on particles larger than some critical size. Thus,
if there is time for these fractional condensation processes to occur, we can
anticipate differences in amount and in composition of condensate in particu-
late fractions of different size and structure. The steepness of the
temperature gradient for the exhaust stream would be the determining factor
here.
Assuming that the exhaust condition can be obtained where con-
densibles have not yet been incorporated into the particulate, a catalytic
hydrocarbon oxidizer can be expected to be able to oxidize this organic
portion of the exhaust without being effective as a particulate trap. The
longevity of such units may be a problem if the ash content of the exhaust
accumulates on that catalyst as part of the organic phase being oxidized.
On the other hand, if the ash is not collected but remains part of the
particulate that passes through, the poisoning action of the ash may not
be serious. These features must be investigated. The remaining particu-
late with condensibles largely removed could be trapped and oxidized,
probably by Strategies 2 and/or 3. The relative difficulty of ignition
of such particulate remains to be demonstrated; in any event a combination
of Strategies 2 and 3 seems best for the particulate trap-oxidizer.
APPROACHES TO TRAP/OXIDIZER DESIGN
The trap/oxidizer is a chemical reactor in which oxidation occurs.
As outline above such a reactor tends to be in either an ignited condition
or an unignited condition. When the device is in the ignited condition, the
temperature very rapidly reaches the combustion temperature. The ignited
condition therefore may destroy the control device.
21
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Application of chemical reactor theory, along with data on the
heat transfer, mass transfer and chemical kinetics of the device would allow
a quantitative analysis of the above phenomena.
But even in the absence of such an analysis, it is clear from the
experience that is available for trap/oxidizers that the qualitative picture
presented above is correct and that ignition of the particulate often
destroys the device.
In view of the above, a number of approaches to trap/oxidizer
design present themselves.
• If the flow through the device were stopped when ignition
occurred, the mass transfer limitation would occur at a
lower temperature. This could be accomplished by tem-
porarily bypassing the device, or by switching to an
alternate. A possible disadvantage of this approach is
that the combustion reaction will be oxygen-starved and
may produce noxious products.
• The heat transfer can be increased. The critical area
is in the center of the reactor where conditions are
adiabatic rather than at the edge. The solution is to
make the reactor all "edge", that is, to increase the
surface to volume ratio. This could be done by making
a flat pancake-shaped device or by including cooling
passages in the internal structure. It would be de-
sirable that each filter fiber would be able to "see"
a relatively cool wall, and thus be cooled by radiation.
• The chemical kinetics could be changed through use of an
inhibitor. The rate of reaction at a given temperature
would be less and there would be more time for the heat
to dissipate, thus making the device temperature lower.
• The situation can be accepted as unchangeable and
the filter designed to withstand the temperatures
expected during burn-off. It may well be that
ceramic fibers would have the necessary ability to
keep their integrity when very hot.
• The device will proceed to the adiabatic flame
temperature only if a sufficient supply of fuel is
available. If by the time the temperature had
reached Tp on Figure 2 all the carbon had been
oxidized, the high temperature of point TE would
never be reached. Also the reactor does not consist
entirely of fuel, but contains the filter as well.
The filter has a small thermal mass that requires
heat energy to be raised in temperature. Thus,
this approach would be to insure that the trap/
22
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oxidizer always runs out of fuel before reaching
a temperature that would be too high for the filter.
The oxidation would never reach steady-state. This
might be accomplished in a number of ways.
• If ignition occurs frequently enough, the
amount of fuel accumulated will necessarily
be small.
• Frequent ignition would be encouraged by
the presence of a catalyst.
• Fuel could be added to the exhaust and
ignition supplied regularly.
• The thermal mass can be deliberately
increased. Ceramic "marbles" might be
"mixed" with the filter to insure that
the fuel was gone before the ceramic
(and the filter) were overheated.
A successful device might well use several of these approaches.
However, each should receive a detailed evaluation to determine their effectiv-
ness.
COMBUSTION OF COLLECTED MATERIAL
A general discussion about the combustion of collected material
must be prefaced with the warning that the physical configuration of the
device will be a major factor in its operation. If particulate material is
removed (say, by rapping) from a filter, it will be in some kind of container,
and it will be in a thick layer (many particles thick). Thus, when this
material burns the situation will be similar to the combustion of a piece of
solid fuel. Just as a thin piece of wood will not support a flame if the
heat loss is too great, a layer of particulate will not burn if too much heat
is lost to the container. It is expected that it will be diffcult to strike
a balance between maintaining combustion and preventing excessive temperatures.
SECTION 5
CONCLUSION
This report lists a number of conceptually different ways to
attack the diesel particulate problem. Each of these concepts could
become the basis for different ingenious applications of creative design.
The acquisition of engineering data relevant to the various concepts will
make that design effort easier and will suggest the concepts that have the
greatest potential for success. The specific design will insure that
success; it is difficult to consider this factor in a general evaluation.
23
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With this caveat in mind, it appears that the catalytic oxidation
of the particulate on some type of filter-trap holds the greatese promise
for the removal and destruction of the diesel particulate. The key to
the successful operation of such a device would appear to be frequent, if
not continuous, ignition combined with filter fibers that maintain their
integrity at high temperatures.
The work in progress, at Battelle and elsewhere, will help fill
the need for a store of experience with these devices. This experience is
needed both to identify the concepts worthy of future development and to
identify the specific materials and dimensions of the final choice.
24
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REFERENCES
1. Khatri, N. J., and J. H. Johnson. SAE 780788.
2. Frish, L. H., J. H. Johnson, and D. G. Leddy. SAE 790417.
3. Hare, C. T., and T. M. Baines, SAE 490424.
4. Lee, K. B., N. W. Thring, and J. M. Beer. Combustion and Flame.
(6):137, 1962.
5. Mulcahy, M.F.R., and I. W. Smith. Reviews of Pure and Applied Chemistry.
(19):81, 1969.
6. Nagle, J., and R. F. Strickland-Constable. Proceedings of the Fifth
Conference on Carbon. (1):154, 1962.
7. Heuchamps, C., and X. Duval. Carbon. (4):243-253, 1966.
Amoriglio, H., and X. Duval. Carbon. (4):323-332, 1966.
8. Wagner, Carl. Chem. Tech. Leip2ig(18):28-34, 1945.
9. Frank-Kamenetskii, D. A. Diffusion and Heat Transfer in Chemical
Kinetics. Plenum Press, New York, 1969.
10. Nicholls, P., and C. W. Staples. U. S. Dept. of Mines Bulletin 360,
1932.
11. Sercombe, E. J. Platinum Metals Review (Johnson Matthey). 19(1):2-11,
1975.
12. Cassel, H. M. J. Am. Chem. Soc. (58):1309-10, 1936.
13. Taylor, H. S., and H. A. Neville. J. Am. Chem. Soc. (43):2055, 1921.
14. Day, J. E., R. F. Robey, and H. J. Dauben. J. Am. Chem. Soc. (57):
2725-6, 1935.
15. Mooney, John. Englehard Industries, Personal Communication.
16. Seizinger, D. E., B. H. Eccleston, and R. W. Hurn. SAE 790420.
Detroit, 1979.
25
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing]
1. REPORT NO.
EPA-600/7-79-232b
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE ANDSUBTITLE
Assessment of Diesel Particulate Control:
Catalytic Oxidation
Direct and
5. REPORT DATE
October 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
M.J. Murphy, L.J. Hillenbrand, and D.A. Trayser
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
10. PROGRAM ELEMENT NO.
F.HF.fi?^
11. CONTRACT/GRANT NO.
68-02-2629
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; 3/79 - 7/79
14. SPONSORING AGENCY CODE
EPA/600/13
Wasser, Mail Drop 65, 919/541-^
15. SUPPLEMENTARY NOTES IERL-RTP pro j ect officer is John H.
2476.
16. ABSTRACT
report discusses the technology and potential for disposal of diesel
particulate by oxidation. Relevant properties of typical diesel particulate are
given; note is taken of the small size (on the order of 0.1 micrometer diameter)
and the presence of a portion extractable with an organic solvent. Available reaction
rate data is used to derive particle lifetimes at various temperatures; these exceed
likely exhaust system residence times. The use of catalysts to increase oxidation
rates and lower ignition temperatures is discussed. Small amounts of many metals
are effective in increasing the rate of oxidation by 2 to 5 orders of magnitude.
Chemical reactor theory is used to derive ignition and operational characteristics
of trap/oxidizers. Note is also taken of the tendency of these devices to go
rapidly from a cold unignited state to an ignited state close to the adiabatic flame
temperature of the fuel. Design techniques to ameliorate undesirable temperature
excursions are presented.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Diesel Fuels
Diesel Engines
Dust
Aerosols
Oxidation
Catalysis
Assessments
Pollution Control
Stationary Sources
Particulate
13B
21D
21G
11G
07D
07B, 07C
14B
3. DISTRIBUTION STATEMENT
Release to Public
19 SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
30
20. SECURITY CLASS (This page>
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
26
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