Vehicular Pollution Management: Diesel Exhaust Emission Control

Dr. Nitin K. Labhsetwar Scientist

Air Pollution Control Division, NEERI, Nagpur - 440 020

The increased use of diesel driven vehicles for all categories of

commercial automobiles and even for private cars is the major trend observed

worldwide over the last two decades in transportation field. While the energy

advantages of the diesel engines are unquestioned, lower cost of diesel fuel is

also responsible for its increasing popularity, particularly with respect to the less

developed countries. Although, cleaner than gasoline engines from the

standpoint of view of carbon monoxide (CO) and hydrocarbons (HCs), diesel

engines produce more aldehydes, sulfur oxides (because of the higher sulfur

content in diesel fuel) and nitrogen oxides. Smoke and odor emissions are also a

problem of great concern, most importantly, however, uncontrolled diesel

engines emit significant amounts of particulate. These particulate emissions are

a direct health concern as well as a serious source of overall environmental

degradation.

Diesel particulate matter consists mostly of three components: soot

formed during combustion, heavy hydrocarbons condensed or adsorbed on the

soot, and inorganic compounds, mainly sulfates. In order, diesel engines soot

was typically 40 to 80 percent of the total particulate mass, but soot

emissions from modern emission-controlled engines are much lower. Most of the

remaining particulate mass consists of heavy hydrocarbons adsorbed or

condensed on the soot. This is referred to as the soluble organic fraction (SOF)

of the particulate matter. The SOF is derived partly from the lubricating oil, partly

from unburned fuel, and partly from compounds formed during combustion. Most

of the soot formed during diesel combustion is subsequently burned during the

later stages of the expansion stroke. Typically, less than 10 percent of the soot

formed in the cylinder should be emitted into the atmosphere; in modern

emission-controlled diesel engines, even lesser amount of soot may be emitted.

Black smoke from diesel engines is due to the soot component of diesel

particulate matter. Under some conditions, diesel engines may also emit white,

blue, or gray smoke. These are due to the presence of condensed hydrocarbon

droplets in the exhaust. Unlike soot, these droplets scatter light, thus giving the

smoke a bluish or whitish cast. Blue or gray smoke is generally due to vaporized

lubricating oil and indicates an oil leak into the cylinder or exhaust system. White

smoke is common when engines are first started in cold weather and usually

goes away when the engine warms up.

Environmental and Health Impacts of Diesel Particulate

Uncontrolled diesels emit approximately 30 to 70 times more particulate

than gasoline-fueled engines equipped with catalytic converters and burning

unleaded fuel. These particles are a concern from several standpoints:

• Many areas already experience unhealthy air quality levels for total

suspended particulate (TSP) matter. TSP comes from many sources but

diesels also contribute considerably. These particles in urban air are of

concern because a strong correlation between suspended particulate and

variations in infant mortality and total mortality rates has been established.

Further, clear evidence emerges from the epidemiological literature that

implicates particles in aggravating disease among bronchitis, asthmatics,

cardiovascular patients and people with influenza. Any significant increase

in diesel particulate emissions would add to the difficulty of solving this

problem.

• Beyond the overall impact on TSP, diesel particles raise a special health

concern because they are very small (averaging about 0.2 microns in

size). Small particles, which are much more likely to be deposited in the

deepest recesses of the lung (alveolar region) and which require much

260

longer periods of time to be cleared from the respiratory tract, have a

greater potential to adversely affect human health than larger particles. In

addition, when emitted, they remain suspended in the air near the

breathing zones of people for long periods of time.

• In addition, diesel particulate has also been recognized as especially

hazardous and toxic because of its composition. The U.S. EPA has noted

that up 10,000 chemicals may be adsorbed on the surface of diesel

particles and drawn deep into the lung with them. Many of these chemical

compounds are known to be mutagenic in short term bioassays, and to be

capable of causing cancer in laboratory animals.

• While health issues have been the cause of most concern, diesel and

other particles can also become a nuisance, degrade aesthetics and

material usage through soiling and may contribute directly, or in

conjunction with other pollutants, to structural damage by means of

corrosion or erosion.

• Impairment of visibility has been widely noted as an adverse effect of

increased particulate. Diesel particles because of their composition

(primarily carbon based) and size (in the size range of 0.2 microns) are

very high light absorbers and scatterer and therefore, have the potential to

be especially harmful to visibility.

Diesel Particulate Emission Control

Some major efforts have been made towards the improvement in diesel

engine technology, which resulted in considerable reduction of both NOx and

particulate emissions from the modern engines. The cost of these engine

modifications has been broadly compensated by the improved fuel efficiency,

which in addition also resulted in environmental benefits. However, it is now

widely accepted that the future emission norms for diesel vehicles are not

achievable only through these engine modifications and some kind of emission

control technology is indispensable.

As regards to the diesel particulate emission control, following control

options have found success, at least towards partly controlling the emissions:

• Diesel Particulate Filters (DPF)

• Diesel Catalytic Converter

• Fuel Additives

Diesel Particulate Filter

A DPF system has a particulate filter media in the engine exhaust stream

and some means of burning (oxidizing) of collected particulate matter from the

filter. Manufacturing a filter capable of collecting soot and other particulate matter

from the exhaust stream is straightforward, and effective trapping media have

been developed and demonstrated. The shallow wall type wall-flow particulate

filter is today the most efficient soot collection device, attaining filtration

efficiencies of the order of 90% at nominal operation conditions. The most

successful trap-oxidizer systems use either the ceramic monolith or the ceramic -

fiber coil traps. Recently SiC based shallow wall type DPF are becoming most

popular due to their better thermal stability and chemically stable nature. The

main problem of trap-oxidizer system development is how to remove the soot

effectively and regenerate the filter. Diesel particulate matter consists of solid

carbon coated with heavy hydrocarbons. This mixture ignites at 550 to 600°C,

well above the normal range of diesel engine exhaust temperatures (150-400°C).

Special means are therefore, needed to ensure ignition. The regeneration

behavior of DPF depends on many factors including inlet gas temperature,

oxygen concentration, particulate loading, gas flow rate etc.

Many techniques for regenerating particulate trap-oxidizers have been

proposed, and much development effort has been invested. Regeneration

techniques can be divided into passive and active approaches. Passive systems

attain the conditions required for regeneration as a result of normal vehicle

operation. This requires a catalyst (as either a coating on the trap or a fuel

9R0

additive) to reduce the ignition temperature of the collected particulate matter.

Regeneration temperature of lower than 400°C has been reported with catalytic

coatings, and further lower temperatures can be achieved with fuel additives.

Active systems monitor particulate matter in the trap and trigger specific actions

to regenerate it when needed. A variety of approaches to trigger regression have

been proposed, including diesel-fuel burners, electric heaters, and catalyst

injection systems.

DPF regeneration can be broadly achieved by one of the following

means:

• Thermal regeneration by use of engine measures or by applying external

heating source

• Catalytic regeneration, which eventually reduce the regeneration

temperature to the exhaust temperature range. This may also includes

fuel additives.

• Aerodynamic cleaning using compressed air.

Applications of Catalysts

Catalyst can be applied in the following ways for the particulate

emission control :

• Catalyst fuel additives, mixed with the fuel

• Continuous oxidation of SOF through an oxidative catalytic converter

• Catalytic oxidation of soot for regeneration of DPF.

Catalyst Fuel Additives

Catalyst fuel additives have found useful applications in diesel engines.

Most of these are organo-metallic compounds of various metals and are mixed

with the fuel in a very small quantity. The additive is oxidized in the combustion

chamber and its oxides form the kernels of particulates, which are incorporated

with the soot on their way to the exhaust stream. Minor secondary affects that

may arise by the use of catalytic fuel additives include incomplete filter cleaning

and filter back-pressure increase due to the retaining of fuel additive ash. New

generation ash-less additives offer the remedy to this problem however.

Catalytic fuel additives are nevertheless, found as a useful catalytic option.

Oxidation Catalytic Converter

In cylinder diesel PM control has greatly reduced PM emission levels.

Progress has been most effective in reducing the soot portion of PM emissions,

so the soluble organic fraction (SOF) of particulate matter now accounts for a

much larger share. Depending on engine and operating conditions, the soluble

organic fraction may account for 30 to 70 percent of PM emissions. Oxidation

catalysts of a diesel catalytic converter oxidizes a large portion of the

hydrocarbons present in the soluble organic fraction of PM emissions, as well as

gaseous hydrocarbons, carbon monoxide, odor (from organic compounds such

as aldehydes), and mutagenic emissions. Oxidation catalytic converters have

been used in light-duty vehicles and demonstrated to be effective for heavy-duty

applications as well. They have little effect on nitrogen oxide emissions, but can

reduce volatile organic compound and carbon monoxide emissions by up to 80

percent. The durability of oxidation catalytic converters on heavy-duty engines

has yet to be determined, but it is likely to be acceptable. The main difficulty with

using oxidation catalytic converters on heavy-duty diesel engines is that they can

cause the formation of sulfuric acid and sulfates from sulfur dioxide in the

exhaust. If fuel sulfur levels are significant, these compounds can add

considerably to particulate mass.

DPF Regeneration Catalysts

Diesel Particulate Filter remains by far the most effective option to collect

the particulate matter of diesel vehicle exhaust. Therefore, intensive efforts are

under way to overcome the problems associated with this technical option. The

most important being efficient regeneration. Catalyst is mostly applied as a fine

264

coating on the wall-flow filters. Although a large number of catalyst compositions

have been studied, however, very effective lowering of regeneration

temperatures could not be attained. Even under the most favorable conditions,

less than 100 °C decrease in regeneration temperature is attained, this is one of

the areas, which is gaining increased attention, and development of improved

regeneration catalyst could possibly make the DPF technology an efficient and

practically feasible option for particulate emission control. Many reports suggest

the favorable participation of S02 and NOx in the soot oxidation reactions,

however, development of an active and stable soot oxidation catalyst without

involving these components would be more feasible on practical fronts.

Table-1

Estimates of Emission Control Technology Costs for Diesel-Fueled Vehicles (percent)

Technology Engine cost increase

Baseline engine, no emission control equipment

0

Injection timing retard 0

Low sac volume and valve covering nozzle Minimal

Turbocharging 3-5

Charge cooling 5-7

Improved fuel injection 13-15

High-pressure fuel injection with electronic control

14-16

Variable geometry turbocharging 1-3

Particulate trap 4-25

Particulate emission control from diesel vehicle exhaust is a major

environmental concern and deserves immediate attention. However, no

straightforward and techno-economically feasible option is so far available, and a

rather comprehensive approach will be required for diesel exhaust emission

management. Apart from the development of suitable after treatment technology,

it is equally important to improve the fuel quality, vehicle maintenance and other

related issues. The experience of developed countries will again be useful in this

regard.

References:

1. A. Faiz, C.S. Weaver and M.P. Walsh "Air Pollution from Motor ehicles;, Standards and Technologies for controlling Emissions" World Bank Pub No. -TL214 P6F35.

2. J.H. Johnson, T.M. Baines & J.C. Clerc, "Diesel Particulate Emissions; Measurement Techniques, Fuel Effects & Control Technology", Pub by SAE International, Pub No - PT 42 (1992).

3. Howitt J.S., and Montierth M. R., " Cellular Ceramic Diesel Particulate Filters", SAE Paper 810114 (1981).

4. Abthoff J., Schuster H.D., Langer H.J. and Loose G., "The regenerable trap oxidiser-an emission control technique for diesel engines" SAE 850015(1985).

5. Johnson J.H., Bagley S.T., Gratz L.G., and Leddy D.G. "A review of diesel particulate control technology and emission effects" SAE Paper 940233 (1994).

6. Pattas K.N., Samaras Z., Patsatzis N., Michalopoulou C., Zogou O., Stamatelos A., and Barkis M., SAE paper 900109(1990).

7. Koltsakis G.C., and Stametalos A.M., Prog. Energ. Comb. Sci., 23 (1997) 1.

8. Konstandopoulos A.G., Gratz L.D., Johnson J.H., Bagley S.T., and Leddy D.G. SAE Trans. J. Engg., 97 (1998) 37.

266