Legislation has been enacted to limit the emission of HC hydrocarbons, NO x oxides of nitrogen, CO carbon mon-oxide and particulate matter including lead compounds.. Permitting metered
Trang 1Mobile sources, alternatively called transportation or vehicular
sources, include cars, trucks, buses, ships and various aircraft
Air pollutants emitted will vary, depending on the fuel being
combusted or reacted (in the case of fuel cells or batteries) and
the engine design of each vehicle
THE AUTOMOBILE
The automobile’s discovery appears to satisfactorily
com-bine a human desire for rapid transportation with the desire
for independence and fl exibility However, the increasing
vehicle population poses a series of threats to continued
physical and psychological well-being and to the future of
our environment
As the automotive industry expands, other auxiliary
industries such as petroleum production and concrete and
tire manufacturing also grow, with additional potential for
pollution problems Additional roads also have to be built,
with a negative impact on both ecology and landscape A
bal-ance must be made between the right of an individual to use
his own car when and where he drives and the harm brought
upon society as a whole by his doing so If it is accepted that
society needs to be protected, a number of legislative and
economic measures can be initiated to discourage
automo-tive usage
Legislation has been enacted to limit the emission of HC
(hydrocarbons), NO x (oxides of nitrogen), CO (carbon
mon-oxide) and particulate matter including lead compounds
However, other waste materials such as tires and the
auto-mobile itself (including the repugnant abandoned cars) must
be disposed of
The broad approach to automotive pollution control is to
encourage alternative means of transportation This would
include improvements in mass transit such as high-speed
trains, moving sidewalks, increased links and modernization
In the city, bicycle riding and walking are low pollution, high
exercise alternatives; in rural areas car pools might be formed
Cars have been built using different energy concepts
(ex battery, turbine engine, sterling engine) and to run on
different fuels (ex natural gas, alcohol) Special control
devices (ex catalytic reactors, afterburners) placed at the
exhaust of internal combustion engines are the primary
means of reducing emissions of various materials The total
solution to the problem will probably combine technical
and strategic methods
The US EPA and US DOE produce the Fuel Economy Guide to help car buyers choose from the best fuel gallons per mile (mpg) ratings for both city and highway traffi c modes A pdf version may be found at http://www.epa.gov/
cgibin/epaprintonly.cgi 1
In the present discussion, we shall concentrate on the major moving source of pollutants, the internal combus-tion engine In the automobile, evaporative losses of pol-lutants occur from the fuel tank and carburetor (ca 5%), fumes from the crankcase (ca 20%) and the exhaust system (ca 75%) The major offenders are unburned hydrocarbons, carbon monoxide, nitrogen oxides and HC oxidants Positive Crankcase Ventilation (PCV) on all modern cars reduces the emissions by pulling air and fumes into the engine by main-taining a vacuum at the engine Of course a highly effi cient combustion process will eliminate the partially oxidized substances One reason for incomplete combustion is that as the “fl ame front” generated from the spark moves toward the relatively cool cylinder walls, a quenching action takes place preventing further reaction, the kinetics of which are very temperature sensitive Other factors infl uencing incomplete combustion are improper dilution by poor cycle timing and less than the proper excess of oxygen admitted at the carbu-retor Standard engine cycles have been developed so that pollutant guidelines might be drawn The Federal Driving Cycle initiated in 1972 in considered the standard of vehi-cle testing A federal short cyvehi-cle Table 1 has been used for convenience in some instances Measured emission values averaged for various test sites in representative US cities are presented in Table 2 for steady state at various miles per hour (mph) and for the short cycle Carbon monoxide and NOx levels increase as the mph level increases The federal 1975 proposed standards for CO, hydrocarbons and NOx are 11, 0.5 and 0.9 grams per mile, respectively, and 0.5%, 40 ppm and 225 ppm, respectively Their extent of enforcement is increasing as of the early 1990s
The CO standard is based primarily on tests which have shown that the capacity of blood to carry oxygen is hampered
by CO absorption. 2 Brain function is retarded after an expo-sure of 10 to 15 ppm CO for several hours The HC and NOx levels are mainly based on their role in photochemical smog
using ambient levels Shy et al 3 found in their Chattanooga study that “NO 2 alone and exposure to suspended particulate matter alone appear to be the most probable explanation for the observed excess in respiratory illness rates.” The data are not completely convincing as pointed out by automobile
© 2006 by Taylor & Francis Group, LLC
Trang 2industry researchers. 4 However, the U.S government took
the initiative for the welfare of the public in not waiting for
all the details to be perfectly established Certainly there are
those among us (aged, with heart conditions, etc.) who will
be more severely affected than the average by typical
ambi-ent pollutant concambi-entrations and will need protection
At least two approaches to the removal of internal
combustion engine pollutants have come into wide
accep-tance One is the improvement of the combustion process
itself This includes reducing manifold vacuum (and hence
dilution of charge) between exhaust and intake steps;
increasing the cylinder wall temperature; and designing
for less maximum horsepower by minimizing the surface
to volume ratio
The other approach is controlling exhaust emissions
by further reaction, either after-burning or catalytic After-
burning can be accomplished slightly downstream from the
exhaust valve by additional oxygen injection converting HC
and CO to CO 2 and water vapor The after-burner has the
dis-advantage of not being able to remove NOx In fact, because
of its high temperature, still more NOx is formed The
cata-lytic approach is to fi nd a catalyst or set of catalysts and
temperature or set of temperatures which will completely
oxidize both CO and HC, but also will reduce the NOx These
two approaches will be discussed in detail below
Meteorological and vehicle persistence factors have been
developed for estimation of 1 hour carbon monoxide
concen-trations defi ned as worst-case total persistence factors. 33
IMPROVEMENT OF THE COMBUSTION PROCESS
As mentioned above the explosion of fuel accounts for
chemical pollutant formation In the Rankine cycle engines,
liquids may continually be vaporized and recondensed, and
in the Sterling cycle engines a gas is repeatedly heated and
cooled Both cycles are accomplished in a sealed container
and the heat used for the process comes from an open fl ame
external to the engine Very little pollution is generated in
such a controlled rather than explosive fl ame
Exhaust gas recirculation (EGR) is used on American Motors, Chevrolet and Chrysler automobiles This includes
a diaphragm-actuated fl ow control valve located between the exhaust and intake manifolds The valve is operated by ported vacuum directed through hoses and a coolant temper-ature override Permitting metered amounts of exhaust gases
to enter the intake manifold, which are mixed with incoming fuel mixtures, lowers the combustion temperatures within the cylinders Reducing maximum cylinder combustion temperatures minimizes the creation of Oxide of Nitrogen (NOx) EGR operation does not take place until engine oper-ating temperature has reached a preset level and engine load
is suffi cient to permit proper EGR operation
Almost all autos now contain a PVC system which directs fi ltered air into the crankcase and channels vapors but toward the manifold leading to the combustion chamber
Fuel tank vapors are also concentrated by charcoal canisters
in American Motors vehicles for recycled combustion
An H.E.W report 5 discusses engine modifi cation sys-tems, “Features shared by essentially all versions of the engine modifi cation system include calibrated carburetors that provide (a) relatively lean air–fuel mixtures for idle and cruise operation and (b) higher engine idle speeds
Refi ned control of spark timing is also used, and, in some cases, regarded spark timing at idle is employed In addition, many engines are fi tted with special air cleaners and ducting designed to supply heated air at nearly constant temperature
to the carburetor, to permit even leaner mixture settings
Most versions also incorporate high-temperature radiator thermostats to raise coolant temperatures, and thus improve mixture distribution and promote complete combustion In some cases, higher capacity cooling systems are used to handle the additional cooling load at idle that results from wider throttle openings and retarded ignition timing during this operating condition In addition, combustion chamber design attempts to avoid fl ame quenching zones where com-bustion might otherwise be incomplete, and result in high hydrocarbon emissions.”
Hydrocarbon and CO emissions are reduced by adjust-ing the carburetor to a fuel-lean mixture duradjust-ing part throttle
TABLE 1 Federal short cycle
Mode
Average acceleration rate (mph/sec)
No Type Speed range Time in mode (sec) Average speed (mph)
Trang 3and idle operation “Lean surge during cruise has been largely
overcome through improvement in manifolding (better
mix-ture distribution), better carburetor fuel-metering
character-istics, higher coolant temperatures, increased heating of the
air–fuel mixture, and, in some cases, provision for heating the
incoming air to the carburetor
Exhaust emissions of CO and HC are particularly diffi cult
to control during engine idle and closed-throttle operation
(deceleration) Considerable effort has gone into
design-ing carburetor idle systems that will provide a lean air–fuel
mixture and minimize emissions during these periods To
ensure that idle air–fuel mixture cannot be adjusted to be too
rich (which would tend to increase CO and HC emissions
appreciably), some means of limiting idle-mixture
adjust-ment is used on most carburetors Such devices allow idle
mixture to be adjusted leaner than a predetermined value,
but not richer.”
The effects of charge dilution on the exhaust emission of
nitric oxide (NO) from a single-cylinder engine were
evalu-ated over a range of engine design and operating parameters. 6
Nitric oxide emission decreased as much as 70% as charge dilution fraction was increased from 0.0065 to 0.164 due
to increased valve overlap, external exhaust recirculation, and reduced compression ratio NO emission was strongly dependent on charge dilution fraction, but was independent
of the specifi c method used to change charge dilution The combined effects of increased charge dilution and 10 degree spark retard reduced NO emission 90% However, defi nite limits of operation were observed on the single-cylinder engine with high charge dilution
The Ford Motor Company uses a system which reduces the hydrocarbon and carbon monoxide content of exhaust gases by continuing the combustion of unburned gases after they leave the combustion chamber This is achieved
by injecting fresh air into the hot exhaust stream leaving the exhaust ports At this point, the fresh air mixes with hot exhaust gases to promote further oxidation of both the hydrocarbons and carbon monoxide, thereby reducing their concentration and converting some of them to carbon dioxide and water
TABLE 2 Summary of results, steady state tests and federal short cycle vehicle exhaust emissions in grams per vehicle mile a
CO2 72.32 409.53 333.12 357.90 372.47 438.48
CO2 60.54 345.65 297.92 330.09 368.91 348.85
CO2 63.85 358.70 318.61 357.65 417.11 392.28
CO2 76.13 391.06 318.01 356.13 404.79 417.03
CO2 64.28 334.24 301.64 315.26 362.26 378.28
CO2 65.04 380.44 343.54 390.83 452.50 417.79
CO2 68.35 374.23 323.03 355.55 401.60 408.96
a Idle results in grams per minute NOx not corrected for humidity.
b FSC—Federal Short Cycle.
© 2006 by Taylor & Francis Group, LLC
Trang 4EMISSION CONTROL DEVICES
Additional Combustion
An afterburner is an additional baffl ed tubular reactor in
which the gases are reignited and burned to completion An
air pump provides the necessary oxygen-rich mixture and
the heat of reaction maintains a high temperature to speed
its completion
It is not necessary in all cases to have a separate
after-burner The aforementioned report 5 states, “An injection
sys-tems decrease exhaust CO and HC emissions by injecting air
at a controlled rate and at low pressure into each exhaust port
Here, the oxygen in the air reacts with the hot exhaust gases,
resulting in further combustion of the unburned hydrocarbons
and CO that would otherwise be exhausted to the atmosphere
Optimum reduction of emissions by this method depends on
proper air injection rates over a wide range of engine operating
conditions, carefully tailored air-fuel mixture ratios and spark
advance characteristics, and in some cases the use of heated
carburetor air Some engines also provide for retarded ignition
timing during closed-throttle operation
All air injection systems use essentially the same basic
air pump, a positive displacement rotary-vane type
To guard against excessive temperature and back
pres-sure in the exhaust system resulting from high air delivery
rates at full throttle and high speeds, a pressure-relief valve is
installed in the pump housing The valve opens to bleed off
some of the pump fl ow at a predetermined pressure setting
Output from the air pump is directed through hoses and
an air distribution manifold (or two manifolds—one for each
bank on V-8 engines) to the air injection tubes located in
each exhaust port A check valve between the air
distribu-tion manifold and the air pump prevents reverse fl ow of hot
exhaust gases in the event that pump output is interrupted
A vacuum-controlled antibackfi re valve is used to
pre-vent fl ow of air to the exhaust ports during the initial stage
of closed-throttle acceleration The high vacuum that occurs
during deceleration causes rapid evaporation of liquid fuel
from the intake manifold walls The resulting rich mixture
creates a potentially explosive vapor in the exhaust manifold
if injected air is present
As with engine modifi cation systems, most air
injec-tion systems also employ spark retard during idle or idle
and deceleration through use of ‘ported’ vacuum sources of
dual-diaphragm distributor-vacuum-advance mechanisms.”
Besides residual gas dilution and wall quenching, engine
variables have the most effect on the amount of hydrocarbons
in exhaust gases Over sixty privately owned and operated
automobiles fueled with commercial leaded gasoline have
been tested and seven main engine variables were found
which changed the hydrocarbon concentration in the exhaust
gases These results can be briefl y summarized as:
1) Air-fuel ratio: Low value for gas mixture results
in higher hydrocarbon concentration in exhaust gases
2) Ignition timing
3) Speed: Increase in speed of engine decreases the amount of hydrocarbon
4) Air-flow rate: The effect of air-flow rate on the total hydrocarbon concentration depends on the air-flow ratio, ignition timing combination
5) Compressor-ratio: Increasing the compression ratio, by decreasing head-to-piston distance, increases the total hydrocarbon concentration
6) Exhaust Back Pressure: The amount of hydrocar-bon in exhaust decreases with increasing exhaust pressure
7) Coolant Temperature: Increasing the coolant tem-perature decreases the hydrocarbon concentration
Catalytic Reactions
The engine exhaust gases may be passed through a cylindrical shaped canister packed with catalytic particles
Although this method has great potential, two problems may arise The long term stability of the catalyst (50,000 miles) is diffi cult to maintain since lead and other chemi-cals in trace amounts poison the catalyst The catalyst structural stability is diffi cult to maintain under the infl u-ence of varying gas fl ow rates and fairly high temperature
Also, removing three or four pollutants simultaneously can prove diffi cult for any single type of catalyst The removal
of NOx , for instance, requires a reduction catalyst, whereas
CO requires an oxidation catalyst For this reason dual stage catalytic reactors have been proposed The technical prob-lems for this method are greater but the potential advantages are even greater
Nitric Oxide Removal A review of some chemical
reac-tion data which might be useful in automobile pollureac-tion con-trol work has been presented by Shelef and Kummer. 7
One approach to solving the stability problems has been
to avoid leaded fuel in automobiles containing converters
The catalytic approach to conversion for American Motor cars is to use a pellet-type of catalyst with a monolithic-type warm-up feature for California and high altitude cars The warm-up converter is separately mounted ahead of the cata-lytic converter Chrysler Corporation and Chevrolet use a cat-alyst support coated with platinum, palladium and rhodium
Hydrocarbons, CO and NOx are all reduced by this three component catalyst An extensive literature exists on more economical active phases 11,13,16,22 which are not as effective converters The air to exhaust ratio in catalytic converters is computer controlled in American Motor cars
For this reason, reactions which involve combination with rather than decomposition of NO 2 are being studied very carefully The equilibrium constants in terms of par-tial pressure are given in Table 3 for NO combination with hydrogen, CO and methane at various exhaust temperatures
The thermodynamic conditions (large K p values) are gener-ally favorable for conversion (reduction) A Monel (nickel–
copper alloy) catalyst has been found reasonably successful for removing NO by combining it with residual CO in the exhaust stream The Monel dissociates the oxygen from NO and then oxides CO to the harmless dioxide
Trang 5In this system, when the engine operation is too rich (too
little oxygen) nitrogen oxide reduction is found to be
excel-lent, but carbon monoxide conversion poor The nitrogen
oxide will readily combine with Monel, forming Monel oxide
and nitrogen since there is little oxygen to compete with the
nitrogen oxide for active sites on the Monel However, since
the CO:NO ratio is so large, there will be insuffi cient Monel
oxide formed to give up its oxygen to the carbon monoxide
On the other hand, if the engine operation is too lean, NO
reduction will be poor and CO conversion excellent since the
reaction becomes signifi cant and oxygen will compete with
the nitrogen oxide for sites on the Monel
Some typical conversion data 1 at space velocities of
50,000 v/vhr indicated that conversion of NO in synthetic
gas mixture (2% CO, 1000 ppm NO) was over 95% if the
temperature was above 700⬚C, but fell off sharply at
temper-ature below 650⬚C Problems associated with this technique
are the formation of NH 3 if any residual H 2 is present and
back pressure buildups with the current catalyst structure
In addition, dusts form which are emitted as particulates It
is essential for effi cient performance of this catalyst that it
warms up to operating temperatures very rapidly Lead in the
fuel reduces the chemical activity and ultimately increased
the rate of deterioration of the catalyst
Another catalyst which showed promise for the same
reaction but at lower temperature (200 to 350⬚C) has also
been mentioned. 18 Despite the fact that conversions of better
than 90% were reported equal amounts of CO and NO were
used Automotive exhaust have about 16 times more CO than
NO Activated carbon has been used 19 successfully with H 2 gas at 600⬚C However, activated carbon lost a considerable
portion of its activity after only 7 hours service
CO and Hydrocarbon Removal Major automotive and
petroleum companies have combined efforts in the develop-ment of an inexpensive multi-thousand mile catalytic pack-age for reducing CO and HC exit concentrations Since the input of gasoline and hence the effl uent gases are rarely at steady state, any study of a catalytic reactor must consider the dynamic situation To give some idea of magnitude, the exhaust gas fl ow in standard cubic feet per minute (SCFM)
is roughly twice the miles per hour equivalent of an auto-mobile and the temperatures of the exhaust gas change from that of the ambient to well over 600⬚C Figure 1 shows typi-cal CO and temperature levels in the exhaust stream after engine startup in a Federal cycle
For balancing pollution problems with no catalytic or afterburner control an air–fuel ratio of about 16 is recom-mended Above this level the NOx level increases markedly and below it the amount of unburned HC and CO substan-tially increases With catalytic devices this ratio may no longer be optimum, since the catalyst selectivity may be greater towards removal of one of the pollutants than any of the others
Wei 20 has noted that the kinetics of CO oxidation over
an egg-shell catalyst turn out to be fi rst order for CO and 0.2 order for O 2 in the range of 1 to 9% oxygen The curva-ture in the Arrhenius plot (Figure 2) is believed to be caused
by a pore diffusional phenomena As the catalyst ages and activity falls the reaction rate becomes controlling and the Arrhenius plot becomes a straight line The catalyst of the
fi gure is best above 350⬚C, but a lower operating
tempera-ture may be preferred for longer catalyst life Wei 20 found that, “As the catalyst lost 90% of its activity, the emission rose by only 30%; but the last 10% of activity loss would result in a precipitous rise of carbon monoxide emission
A catalyst with 50% reduction in heat capacity performs much better; a reactor with 50% reduction in volume performs better when the catalyst is fresh and worse then it is aged.” His phi-losophy is, “It is our engineering goal to produce a low-cost and convenient solution However, any solution requires some inconvenience and cost from everyone Quick warm-up is no problem if we are willing to sit and wait in the car for 2 min for an auxiliary heater to warm up the catalyst bed before the car moves We have ninety million cars on the road, and a
$100 device will cost us nine billion dollars How much are
we willing to pay for 90% cleaner air? These decisions belong
to the public, not the engineers For the sake of everyone we hope to be able to present to the public an economical and convenient solution in the near future.”
Stein et al 21 have evaluated the effectiveness of possible catalysts by a microcatalytic technique based on gas chroma-tography The technique which is described in detail allows
a large variety of hydrocarbons and catalysts to be rapidly tested over a wide range of temperatures (100–600⬚C) In
general oxides of cobalt, chromium, iron, manganese, and nickel are the most effective catalysts The higher molecular weight hydrocarbons are more easily oxidized than the lower
TABLE 3 The equilibrium constants for some reduction reactions of nitric oxide 17
log Kp
Reactions 600 700 800 900 1000K
NO ⫹ 5H 2 → 2NH3⫹ 2H 2 O 49.0 48.1 32.13 26.44 21.8
NO ⫹ 2H 2 → N2⫹ 2H 2 O 51.7 43.4 37.1 32.2 28.3
NO ⫹ CO → ½N2⫹ CO 2 27.3 22.6 19.7 16.46 14.29
NO ⫹ CH 4 → ½N2⫹ 2H 2 19.59 18.1 17.6 16.2 15.5
NO ⫹ Hat → HNO 12.4 9.77 7.79 6.24 5.0
NO ⫹ ½ H 2 → HNO ⫺3.7 ⫺3.6 ⫺3.52 ⫺3.46 ⫺3.42
Temperature
600 7
CO %
0
° C
FIGURE 1
© 2006 by Taylor & Francis Group, LLC
Trang 6and hydrocarbons of a given carbon number increased in
reac-tivity according to the series: aromatic ⬍ branched paraffi n
or alicyclic ⬍ normal paraffi n ⬍ olefi nic ⬍ acetylenic The
olefi nic hydrocarbons, generally considered the most
unde-sirable, are relatively easy to remove Other results could be
summarized in the following manner:
1) With most of the catalysts tested, some
crack-ing occurs durcrack-ing the oxidation of hydrocarbons
Oxidation without CH 4 formation was possible with the oxides of Co, Cr, Cu, Mn and Ag only
Zirconium oxides are unique in that they pro-duce only cracking products, mainly methane and smaller amounts of intermediate hydrocarbons
2) Complete removal of all hydrocarbons was not
attained with some of the catalyst, even at 600⬚C
The oxidation is not a simple function of temperature
The potential of copper oxide-alumina catalyst for air
pol-lution control has been studied by Sourirajan and Accomazzo. 22
They stated that, “the simultaneous removal of hydrocarbons
and carbon monoxide present in the auto exhaust gases has
been tested making use of a six-cylinder Chevrolet engine
run on leaded gasoline fuel The hydrocarbon and carbon
monoxide concentrations encountered in these studies varied
in the range 170–17,000 ppm and 1–7%, respectively It was found that the minimum initial temperature of the catalyst bed required for the complete removal of both hydrocarbons and carbon monoxide, simultaneously, was 226⬚C under no
load condition, 342⬚C, under an engine load of 2.5hp, 400⬚C,
under an engine load of 5.1hp or higher, and 236⬚C under
deceleration conditions The catalyst showed no deterioration
in performance even after 100 hours of continuous service in conjunction with the above auto exhaust gases The extent of removal of hydrocarbons from the exhaust gases was found
to depend on the initial temperature of the catalyst bed and the engine load condition
It is realized that a successful 100 hour run does not constitute a life test on the catalyst, but it does indicate the potential applicability of the catalyst in air pollution control devices The engineering design of the suitable converter for any particular practical application of the catalyst should naturally take into account the heat liberated during oxi-dation Instantaneous catalyst temperatures of the order of
900⬚C have been encountered in this work with no
deleteri-ous effect on the subsequent effectiveness of the catalyst the heat liberated during the reaction can be advantageously used to maintain the full effectiveness of the catalyst under all conditions of engine operation encountered in normal practice.”
When contaminants are passed through a Hopcalite (unsupported coprecipitate of copper and manganese oxides) catalyst burner, the results vary from almost complete oxida-tion of some organics to very slight oxidaoxida-tion of the lower molecular weight aliphatic hydrocarbons 23 at some 300⬚C
Nitrogen compounds form N 2 O when oxidized and haloge-nated compounds indicate a strong acid reaction when the reactor effl uent is tested with detector paper
An interesting example of the use of exhaust gas recy-cle and catalysts has been presented by the Esso Research Group In Figure 3, typical hot cycle traces of CO, O 2 and
NO are presented for cases before the catalyst, after Monel catalyst and after a 2nd stage Platinum-alumina catalyst, for instance, with and without recycle The major benefi cial effect of recycle is on the NO concentration
The combustion of gasoline is more or less incomplete regardless of the quantity of excess air used About 1% of the exhaust gas is composed of harmful products chiefl y carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC) A signifi cant variable affecting each of these pollutant concentrations is the air to fuel ratio (ATFR) The stoichio-metric value, (ATFR) STO is about 14.7:1.0 on a weight basis
Using a catalytic three way converter, more than 90%
of the pollutants can be converted to harmless substances
To avoid catalyst contamination lead free gasoline must
be used In the closed loop electro-mechanical control of (ATFR) STO described by Robert Bosch, 34 1985, an oxygen sensor in the exhaust gas transmits a signal which is used
to correct ATFR deviations This control method is particu-larly effective on fuel injection engines because they do not have the additional delay times of carburetor engines For catalytic converter operation, the optimum ATFR range is
X X
X
X
X
X
100
50
k
SEC–1
INTRINSIC
120 SCFM
ACCELERATION 60
30 needed for 80%
conversion
10 IDLE
AGED FRESH
10
5
1
0.5
.25mm 1.5mm
I / T °K
FIGURE 2 Carbon monoxide activity of an eggshell catalyst.
Trang 7between 99 and 100% of (ATFR) STO Above this range NO x
levels increase markedly as ATFR is increased; below this
range CO and HC levels increase as ATFR is decreased
Electric vehicles (EV’s) operated by high energy high
powered batteries are making great strides toward
commer-cialization The near term goal is to provide over 100 miles
per recharge at accelerations capable of matching the internal
combustion engine The California Air Resources Board has
issued a technical document in December of 1995 supporting
the concept of such vehicles The key performance parameter
for EV’s is its specifi c energy (or energy density), measured
in watthours per kilogram, with a near term goal of 80–100
wh/kg, suggested by the US Advanced Battery Consortium
(USABC). 38 Another important measure of performance is the
peak specifi c power (or power density), which gives us an idea
of an EV’s acceleration It is measured in units of watts per
kg with an USABC near term goal of 150 w/kg that can be
sustained for 30 seconds during discharge down to 80% depth
of discharge A comparison of different chemical system
per-formance parameters is presented in Figure 4 All of the
bat-teries are expected to achieve the USABC’s midterm goals for
these EV parameters, which coincide with lasting about fi ve
years (-600 cycles) and costing no more than $150 per kwhr of
battery capacity Hybrid power electric-diesel engines became
available in 1995 They alternate between diesel power opera-tion at 2000–2600 RPM where its effi ciency is best to battery power at other RPM
RELATED TRANSPORTATION PROBLEMS
Diesel Exhaust Odors
Diesel engines are found in buses, trucks, off-road vehicles and power applicators and increasingly in automobiles Public reaction to diesel-engine exhaust odors provides the impetus for controlling effl uents of that type of fuel combustion. 24
A list of oily kerosene and smokyburnt odor compound iden-tifi ed by A.D Little, Inc is included in Table 4 Exhaust odor and smoke from diesel engines are more objectionable than those from spark ignition CO emissions are generally less serious but NOx is troublesome (4 to 10 g/mile) Improved fuel injection and afterburners are considered to be the most promising of the existing control methods The injection dif-fers from the internal combustion engine in that fuel does not enter the cylinder as a mixture with air, but is injected under high pressure into the chamber in exact quantities through low tolerance nozzles For NOx removal the basic approaches
Without Recycle
With Recycle
Without Recycle With Recycle
Without Recycle With Recycle
CO = 0
TIME, SECONDS
1000
4
2
4
2
60
40
20
BEFORE CATALYSTS AFTER MONEL AFTER BOTH CATALYSTS
~
FIGURE 3
© 2006 by Taylor & Francis Group, LLC
Trang 8SPECIFIC ENERGY USABC long-term goal
USABC midterm goal
Sodium-nickel chloride
Sodium-sulfur
Lithium-ion
Nickel-metal hydride
Nickel-cadmium
Lead-acid
1998 1996
1998
2001 2001
1998 1998
2004
1996
250
200
150
100
50
0
PEAK SPECIFIC POWER
USABC long-term goal
USABC midterm goal
Sodium-nickel chloride
Sodium-sulfur
Lithium-ion
Nickel-metal hydride
Nickel-cadmium
Lead-acid
500
400
300
200
100
0
1998
2004
2004
Trang 9are engine refi nement, fuel additives, catalytic conversion
and reduction of peak combustion temperature
Gas Turbines
The exhaust of gas turbines contains perhaps less pollutants
than that of any other internal combustion process The
high airport traffi c density makes the problem a real one,
however Sulfur dioxide emissions are low, but the smoke
and odor producing compound levels are high For
auto-mobile use, the total mass of gas turbine exhaust is many
times greater than that for the gasoline engine of equivalent
power Hydrocarbon and CO mass emissions are known to
be low and diffi cult to reduce A greater deal of the
pollu-tion control work underway is in the engine modifi capollu-tion
area In aircraft the turbofan engine is largely replacing the
turbojet Turbofans bypass some of the air past the engine
and rejoin it with the burner gases at the exhaust tailpipe
Modern dry NOx combustion systems can obtain emissions
of 25 ppm at 15% O 2 . 36,37
The airplane is a much more effi cient carrier
(pollu-tion wise) than the automobile, on a people × miles basis
Aircraft engine research has been concerned primarily with
smoke reduction Fuel type, fuel additives and combustion chamber design have been the primary variables studied
PARTICULATE EMISSIONS
The particulate matter emitted from automobiles has been characterized 25 as consisting of “lead salts, alkaline earth compounds, iron oxides, soot carbonaceous material, and tars This material ranges in size from large fl akes to submi-cron particles and varies in consistency from hard and brittle particulate to oil mists Some of the particulate material is generated in the engine combustion chamber and nucleated and agglomerated in the vehicle exhaust system before it passes out the tail pipe On the other hand, a large portion of the particulate material generated in the engine subsequently deposits on various surfaces of the exhaust system At some later time, this deposited material fl akes off and becomes reentrained in the exhaust gas to be emitted from the vehicle
Obviously, opportunities exist for various types of chemi-cal and physichemi-cal processes to take place and, as a result, the overall particulate emission process for a vehicle is quite complex and diffi cult to defi ne.”
CYCLE LIFE
2004
2000
2000
1998
2002 USABC long-term goal
USABC midterm goal
Lead-acid
Nickel-cadmium
Nickel-metal hydride
Lithium-ion
Sodium-sulfur
Sodium-nickel chloride
2500
2000
1500
1000
500
0
FIGURE 4 : Performance Parameter Comparison for Electric Vehicles as a Function of Advanced Battery Type (After Moore 38 ).
© 2006 by Taylor & Francis Group, LLC
Trang 10Much of the particulate is in the form of polynuclear
aro-matic (PNA) hydrocarbons and tars As for gaseous products,
standard federal cycles have been designed to test for
particu-late emissions
Lead particulate matter is emitted at a rate of about
0.16g/mile The average particle size emitted is slightly less
than a micron, but sizes are substantially distributed over
the 0.10–10 micron range In Table 5, the effect of mileage
on particle distribution may be observed As the number of
miles driven increases, the segment of particles greater than
9 microns also increases, probably because of deposition
buildup on exhaust pipes
Using 500 microns as a divider, about 30% of the lead burned over the lifetime of a car is in the form of fi nes 40%
in the form of coarse particulate
Research studies 27 showed that a relationship exists between traffi c volume, proximity to the highway engine
acceleration vs constant speed, wind direction and the
amount of lead in the air Lead values can be expressed as
a quadratic function of time and a linear function of
traf-fi c volume Ter Haar 28 and coworkers found that for Federal 7-mode cycle conditions (Z) cars vary widely in the amounts and composition of their particulate emissions; (2) cold cycle operation gives 2–8 times more particulate than hot engine operation; (3) lead compounds represent less than one-third
of total particulates, the remainder being carbon compounds along with ammonium and nitrate ions and unknown mate-rials; (4) carbon emission for stabilized cars using leaded gasoline varies widely but averages about 35% of the total;
(5) suspended particulate emissions are nearly equal with new cars whether or not lead is present; (6) exhausted lead varies with the condition of the exhaust system and ranges between 7 and 30% of the lead consumed by the engine;
(7) fuel additives affect the amount of emitted particulates;
(8) probe sampling techniques underestimate by a large factor of amount of particulates emitted by vehicles; and (9) trapping systems offer potential for greatly reducing the emission of suspended total particulates Nonleaded-fuel cars were found to exhaust 0.165g of particulate per mile while leaded-fuel cars exhausted 0.152g/mile The reason for this is that nonleaded fuel is more aromatic in nature and that the percentage emission for aromatic hydrocarbons
is considerably greater than that for paraffi nic material An increase in the volume of aromatics in the fuel from 10 to 70% doubles the amount of carbon particulates emitted It has been claimed 29 that a reduction of 80–90% of the lead emitted is possible by trapping systems
Two simple traps have been compared for particulate removal and the results presented in Table 6
More complex traps are likely to be very successful in reducing both lead and total particulate matter
A more detailed chemical analysis of particulates has been described under Federal emission test conditions. 30 This investigation found that a decrease in the total mass of partic-ulates especially particles of very small size, occurred when using nonleaded fuel The discrepancy with the aforemen-tioned work 28 may be a result of differences in exhaust depo-sition for the two systems Particles emitted in the vehicle exhaust varied in size from 0.01 to 5000 microns, the latter for such products as rust scale Typical chemical composi-tions for the particulate exhausts are presented in Figure 5 The ratio of Pb to Br is relatively constant 2.1:1 The ratio on impactor plates was somewhat lower, indicating that the chemical composition of lead salts may be related
to particle size Considering some of the differences in the
fi ndings for different particulate matter studies it is critical that such investigations be perfected and intensifi ed An interesting additional discussion of the characterization of particulate lead in vehicle exhaust may be found in the work
of Habibi 31 and Mueller. 32
TABLE 4 Partial list of odorifous compounds
General classification Compounds
Indans and tetralins Methyl indan
Tetralin Dimethyl indan Methyl tetralin Dimethyl tetralin Trimethyl tetralin Alkyl tetralin Alkyl-substituted naphthalenes Methylnaphthalenes
Dimethylnaphthalenes Indenes, acenapthenes, and Alkyl-substituted indenes
benzothiophenes Dimethylbenzothiophenes
Acenaphthene General class Carbon range
Alkenone C5 to C11
Furan C6 to C10
Diene/ C9 to C12
Furfural C6 to C7
Methoxy benzene C8 to C9
Phenol C7 to C12
Benzaldehyde C7 to C10
Benzofuran C8 to C9
Indanone C6 to C10
Indenone C9 to C12
Naphthol C10 to C14
Naphthaldehyde C11
TABLE 5 Lead particulate emissions as a function of size and mileage 25
Average
mileage
Average lead salt emissions, g/mile
>9 microns <1.0 microns <0.3 microns