Selection Criteria For Diesel Particulate Trap Systems: VERT Experience

Andreas Mayer
TTM
VERT Project Manager

Introduction

Hot gas filters for diesel particulates have been the focus of intense activity during the last 20 years. Numerous filter media and regeneration methods were developed. Only a few of these reached maturity and were tested. Even the mature systems were not widely deployed because there was no legislation and the engine industry did not voluntarily implement this promising development.

Meanwhile a new situation has evolved. Legislation is now effective in Germany (TRGS 554 and the German TA Luft), Austria (Occupational Health) and Switzerland (the new clean air act LRV 98).

Accordingly, VERT [1, 2, 3] initiated comprehensive investigations (1993 - 1998). These confirmed the particulate trap as the only efficient solution for sustained curtailment of the diesel soot. The investigations particularly proved that such filters can intercept at least 99% of the sub-micron particulates in the range of heightened pulmonary intrusion. Thus, the imperative to minimize carcinogenic substances in occupational health, and ensure air quality, are better fulfilled than with all other known measures. These results have been repeatedly published and discussed among specialists. The facts are now established.

Particulate filters are now imperative for many applications. They are a prerequisite for implementation of the VERT conclusions in occupational health (Suva, AUVA, TBG) and compliance with the Swiss clean air act 1998. The deployment of these traps is proved to be technically, operationally and economically feasible. The traps are state of the technology. The technology, in particular the availability of proven traps systems, was thoroughly investigated in the VERT project in collaboration with several industrial partners. Compliance with the resulting VERT specification must be individually ascertained through the VERT suitability test.

This report summarizes trap selection criteria developed by the VERT program and lists all trap systems and control techniques that gave positive results in field tests. The list includes the traps investigated during the VERT project and completed the VERT suitability tests. This report does not cover the many filter media and regeneration systems that are yet undergoing development or are being researched.

A working group "particulate trap systems" was formed on 30 September 1998 by trap manufacturers, additive suppliers and suppliers of control systems. Its task is to technically accompany the wide scale deployment of particulate trap systems. "Instructions for using particulate traps" are being prepared for this purpose. The working group will evaluate experience and periodically inform both users and the authorities. The working group has its contact address at TTM (TTM, Fohrhoelzlistr. 14b, 5443 Niederrohrdorf, AG, Switzerland, Tel.: +41.56.496.6414, Fax: +41.56.496.6415).

Objectives and criteria

Since, according to the law, the carcinogenic potential must be restricted to the technically possible minimum, the primary target is to minimize the pollutant emissions from diesel engines. The carcinogens are mainly the particulate combustion aerosols, the so-called diesel soot. These particulates have a solid core mainly consisting of elementary carbon. They also have a very surface-rich morphology. This adsorbs many other toxic substances, that are transported with the particulates, and can penetrate deep into the lungs. Besides these combustion aerosols, the exhaust gas also contains further solid aerosols, e.g. ash particulates, metallic abrasion, sulfates and silicates. The vast majority of these particulates are in the invisible sub-micron range of 100nm. Trap systems must therefore efficiently diminish the particulate count as well as the particulate mass.

Trap systems for this duty must have a very high filtration rate in the entire range of pulmonary intruding particulate sizes. Modern systems can attain values above 99%. The value of these trap systems is enhanced when they also eliminate further pollutants, in particular the carcinogenic PAH, through the binding on the particulate surface. The deployment of filtering exhaust gas treatment systems must not generate any additional pollutants (secondary emissions). This is not a forgone conclusion, as experience with metallic fuel additives has shown.

Trap operation is characterized through long periods of soot collection alternating with short burn-off sequences. These should not result in emission peaks, i.e. caused by rapid regeneration with phases of incomplete combustion or trap rupture tendencies. An additional criterion is that these traps only cause a negligible back-pressure. Otherwise the engine process is impacted and, in turn, increases fuel consumption, increases pollutant emissions or shortens durability. There are many other practical criteria, e.g. trap size, weight, thermal inertia, muffling, surface temperature. Of course, the investment and operating costs as well as the maintenance effort are decisive criteria in the success of such exhaust gas after-treatment technology.

Compliance with the outlined criteria list cannot be left to chance. The risk of set-backs, in the introduction of this technology must be minimized. Hence, the VERT principals Suva, AUVA, TBG and BUWAL decided to develop pertinent mechanisms together with the industry. These include:

• Specifications for trap systems: Besides the demands on the filtration characteristic of the filter media, the specifications also define requirements on the regeneration procedure and the self-monitoring.
• Suitability test: It is absolutely necessary to perform such a test of the trap system at engine operating conditions. During the suitability test, the regeneration response and the aging are also verified during operation.
• Field verification and field measurement: Periodic control of the trap system during operation is imperative. The measurements must be suitable to dependably verify the essential performance criteria under field conditions.

Only those trap systems can be recommended for deployment that comply with the specifications and passed the suitability test.

Available filter systems

A filter system consists of the following components:

• Filter medium
• Regeneration arrangement and procedure
• Control and monitoring

All three components must be integrated, maintained, and ecologically disposed of at the end of their life-cycle. Obviously, the operators prefer to single-source the entire system and thus ensure reliability.

Fully automatic systems are desirable, i.e. neither the driver nor the workshop need intervene during the life-cycle of the trap system. Such systems are commercially available. Systems that are not fully automatic, e.g. systems with stand-still regeneration and exchangeable filters, must be justified in a cost/benefit analysis.

A special case is the use of fuel additives to lower the soot ignition temperature. To ensure effectiveness, an on-board dosage and sufficient additive storage, is desirable. Fuel supply from pre-mixed tanks is risky, because other vehicles could be fueled from such tanks and emit additive ash particulates when not equipped with traps.

Some trap systems already comply with all these specifications and thus represent the state of the technology. Surprisingly, many filter media fulfilled the minimum criteria of the specification. The VERT field tests have, however, revealed that some traps require further refinement to dependably sustain the quality criteria during a long operational period.

There is a multitude of applications for diesel engines. Experience is not yet accumulated for all applications. In certain instances, it may therefore be necessary to specifically evaluate several systems.

Some properties of the trap systems are not addressed in these lists. Nevertheless, they are very significant for the comprehensive evaluation and recommendation for practical deployment.

These include:

• Avoiding ash: The traps retain the ash components due to the high filtration efficiency. The ash particulates are not combustible. Hence, they progressively clog the pores of the filter medium and thus increase the back pressure. The filter elements must therefore be periodically replaced or cleaned. The cleaning interval mainly depends on the fuel, lubricants, operations, filter properties and filter design. These aspects can only be satisfactorily resolved when the users collaborate with the filter manufacturers. It can be advantageous to use premium fuels (low sulfur), or better lubricants (lower TBN) or even more efficient inlet-air filters.
• Easy maintenance: The specification indeed defines the maximum maintenance interval. The maintenance effort can vary widely depending on the filter system.
• Ecological disposal: The filtration efficiency results in the trap retaining several toxic substances during deployment. Hence, simple disposal is not permissible. Many traps also contain precious metals that should be recovered. Filter disposal should therefore be contractually decided with the suppliers.

The following tables categorize the filter systems into two groups.

List A: Filter systems that satisfactorily completed the first part of the VERT suitability test

List B: Filter systems having generally acknowledge positive field performance (VERT suitability test not performed or not yet completed).

Control measures

The intention is to prevent deployment of unsuitable trap systems. Hence, the following control measures were developed, during the VERT project, in collaboration with trap manufacturers, engine operators, engine manufacturers and the authorities:

Filter specification

This specification does not reflect the best values obtained within the scope of the VERT program. Several traps demonstrated more than 99% filtration of the ultra-fine particulates. This represents the capability of the technology. The specifications only demand 95%.

The appended list distinguishes between obligatory and desirable criteria, thus emphasizing the priorities. The specification does not claim technical completeness. It must be further refined in discussions between deployer and manufacturer.

Filter suitability test

It is often impossible to verify the filtration characteristics in the field after retro-fitting. This is because the necessary test conditions cannot be represented in the field, or because the required measurement techniques are unsuitable for the field. Hence, it was decided to perform a suitability test of the trap system on an engine test bench. This suitability test is staged on a representative diesel engine. The filtration characteristic is determined under steady-state and transient (free acceleration) conditions. The mass criteria and the particulate counts for nano-particulates are measured. The regeneration is verified.

The suitability test must be repeated after an appropriate operational deployment (minimum 30% of the life-cycle). This permits a statement on the aging tendency of the entire system. A filter system can only be recommended after both the initial and the subsequent measurements are satisfactory.

Filter self monitoring

It is essential that the trap system itself monitors the back-pressure. The measurements must be stored and alarms promptly annunciated to prevent damage to the engine and the trap system.

Field measurements of emissions

Extensive field measurements of the emissions were performed during the VERT project. The pertinent proposals were formulated in a sub-project and verified in field tests.

Fleet operators in the United States are obliged to equip engines with appropriate instruments for systematic emission measurements. This is a good example for operators who deploy several diesel engines at construction sites, in enclosed spaces, or similar situations over a prolonged period. These measurements also assist diagnosis and are indicative of the engine state. Thus, preemptive maintenance can be done and damage prevented.

Particulate trap manufacturers, whose products succeeded in the VERT suitability tests, joined a working group to exchange experience. The group shall become a specialist institution that develops the VERT specifications into a product standard based on field tests.

VERT specifications for particulate trap systems on construction engines

(will be soon mandatory for occupational health in Switzerland, Austria and Germany)

Filtration efficiency (on the reference engine Liebherr 914 T)

	new	after 2000 h
Total particulates, gravimetric (ISO 8178 C1, 4 test points)	>80%	>75%
Elementary carbon EC, Coulometric (mass)	>90%	>85%
Soot puff during free acceleration: opacity measurement (turbidity)	<10%	<10%
Particulate penetration in the size range 10-500 nm (conc. count)	<5%	<10%

The above values must be maintained both for the clean filter and also for the filter clogged with soot and/or ash. Compliance is required during the entire life cycle and the limits shall not be exceeded even during regeneration processes.

Additional constraints for emissions

There shall be no clearly detectable and relevant increase of the following emissions compared to the initial engine conditions. Such increases are not permissible even during regeneration:

• Sulfuric acid and / or sulfate formation
• Secondary emissions due to fuel additives
• Secondary emissions due to dioxin formation
• Increase of basic emissions CO, HC, NO, NO₂ (cycle total)
• Mineral fiber emissions

Emission verification in the field

The following emission verification limits are control figures for checking under field conditions whether traps are operating properly. These numbers do not replace the trap filtration efficiency requirements listed above.

Vehicles: Verification is according to the method of free acceleration. Maximum opacity of construction diesel engine with soot filter: (corresponds to K = 0.24 1/m, or approx. 1 Bosch or approx. 0.03 g/m3)	< 10%
Stationary engines: Blackening measurement at full load Maximum according to MIRA correlation:	< 5 mg/m3

Pressure loss at rated RPM / full load

• Fresh filter < 50 mbar
• Limit till regeneration < 150 mbar
• Maximum burden (warning) < 200 mbar

Regeneration

• Preferably in situ and automatically during operation
• Additive dosage on board fully automatic with fill-gauge warning. The additive should not be pre-mixed in containers, if there is a risk of other vehicles being fueled from such tanks.

Filter self testing

• Pressure loss controlled electronically with 2 set warning levels
• 2 warning levels: upper level for filter deposition state / lower level for filter damage
• Data storage
• Self monitoring

Trap size

Approximately the size of the muffler replaced < 0.6 l/kW
Sight obstruction must comply with applicable regulations

Muffling

At least equivalent to the muffler replaced > 25 dBA

Costs of trap system

Rating > 200 kW	< CHF 75/kW
Rating 100 - 200 kW	< CHF 100/kW
Rating 50 - 100 kW	< CHF 125/kW
Operating costs	< CHF 0.02/kW
Annual maintenance costs	< 10% of filter cost

Durability and maintenance

	Vehicle	Stationary engine
Life expectancy	> 5'000 op.h.	> 20'000 op.h.
Usable hours till cleaning and disposal of the residues through the trap manufacturer	> 2'000 op.h.	> 2'000 op.h.
Maintenance interval	> 500 op.h.	> 1'000 op.h.

Suitability for deployment underground

• Availability in 3 shift operation
• Maintenance-free during operation
• On-site maintenance
• No additional risk of fire and ignition
• No toxicity risks
• Invulnerable to dusty air and extreme ambient temperatures
• Invulnerable to high air humidity and water spray

The quality-of-service requirements in the specification are verified according to the VERT suitability test on the reference engine Liebherr D 914 T. The measurements shall be performed in the new state and after field deployment corresponding to 30% of the life expectancy.

Filter suitability test

Objective is to measure:

• Filtration characteristics according to particulate mass and particulate count.
• Dependence of filtration response on the soot load on the filter
• Back pressure response in operation
• Efficiency of the regeneration system
• Behavior under field conditions
• Filtration characteristic after field deployment

• Test bed: LIEBHERR 914 T/105 kW construction site engine or customer's choice
• Test scope: 4 operating point according to ISO 8178 plus transient tests "free acceleration"
• Emission measurements: as per Swiss standard FAV 2 including PM gravimetric and soot-puff opacimetric
• Particulate size distribution: using the SMPS procedure

The VERT reports contain a detailed description of the measurement methods.

Fuel: Swiss standard diesel fuel as per SN EN 590 KO (sulfur content < 500 ppm)

Lubricants: preferably synthetic lubricant with TBN < 5 (ISO 3771)

Suitability test part 1: New condition

• Conditioning the particulate trap
• Filtration characteristic in deposited state
• Filtration characteristic after regeneration
• Regeneration response including emissions during regeneration

Suitability test part 2: Field deployment

• Field deployment in a typical application that the trap manufacturer selects and monitors.
• 3 filters during 30% of the life cycle, at least 1'500 operating hours per filter.
• The filter systems should be sealed to detect tampering.
• Employ data-loggers that record the temperature and pressure during the tests.
• Record all problems, break-downs and repairs, fuel and oil consumption.
• The system may only be modified after consultation with the certifying institution.
• At least two of the three systems must survive field deployment without damage.

One of the two systems must then be the object for the third part of the suitability test.

Suitability test part 3: Verification after field deployment

• Repeat the pertinent part 1
• Criteria are slightly relaxed according to the specifications (see list)

Filter self monitoring

Modern electronic control systems must be used together with filters. Two warning levels should be set. The controls ensure that the prescribed trap back-pressure is not exceeded. The controls also detect trap failure, e.g. due to rupture of the filter structure.

Desirable supplementary functions:

• Data storage
• Remote indication (driver cabin)
• Warning signals (driver cabin)
• Self monitoring
• Access to the diagnosis

Unfortunately, many trap systems are delivered without such controls. Often there is merely a recommendation to use a control manometer.

Modern automatic trap systems are supplied with the above controls integrated.

Electronic systems are commercially available that can be combined with any filter. These are recommended as autonomous control systems.

Field measurements of emissions

Within the scope of the VERT working group, the Swiss Federal Office of Metrology took the lead in preparing recommendations for emission measurement under field conditions (can be obtained from TTM).

• Smoke puff during free acceleration (Opacimetry), Switzerland VRV 59A
  This is a legislated method for the periodic inspection of diesel engines in many countries. The method is not without problems and the results can be manipulated. It is important to accurately maintain the RPM range, acceleration time and the number of measurements. Large errors can occur for careless measurements. However, the method is very simple and can be used for all engines without exception. Particularly for super-charged engines, it is usually a very sensitive method to detect operational deficiencies.
  Engines that are very clean under steady-state conditions, often have a noticeable smoke puff. Particulate traps that function well under steady-state conditions, can also very efficiently filter this smoke puff, i.e. an opacimetric signal is not visible when such a trap is deployed. This method is therefore very suitable to clearly detect filter damage.
• Blackening measurements at full load
  If operationally feasible, the full load measurement is preferable to measurements under free acceleration. Full load measurements can be difficult because many engines under field conditions are not at stand-still and cannot be operated at full load for prolonged periods. This is however usually possible for construction site engines: Torque converter engines can, for a short period, be operated at stall. Hydraulic systems, too, can be operated at full load for a short time. In both cases, the operating temperature must be carefully monitored. Usually, 2 - 3 minutes is possible and sufficient for such measurement.
  Results are generally reproducible. They give valuable information about the emission state of the engine, and enhance the opacimetry. The measurement parameter is basically the black smoke as per the filter method. It is essentially interpreted as elementary carbon and can also be calculated in g/m³ soot using the globally accepted so-called MIRA-correlation. This conversion is even built into many instruments. Increased sensitivity can be obtained with instruments having prolonged filtration times, e.g. AVL 407.
• Measuring gaseous emissions under steady-state conditions
  Measurements of the legislated pollutants NOx, HC and CO and in addition NO/NO₂, CO₂ and O₂ is possible. This is accomplished using computer-controlled instruments on the basis of electro-chemical reaction or infra-red absorption sensors. The samples are extracted using modern sampling techniques adapted to diesel engine conditions.
• Monitoring the filter back pressure, exhaust gas temperature and oil temperature.

Market information

Herewith some deployment statistics of particulate trap systems as evidence that this technology has attained certain maturity.

• Over 3000 CRT systems deployed in England, Scandinavia and Germany.
• Deutz has deployed 400 systems with full flow burners rated 50 - 600 kW
• The company Oberland-Mangold has approx. 1000 systems field deployed during 8,400 op.h. in forklifts, 10,600 op.h. in construction site engines, 19,200 op.h in stationary engines, 187,000 km in automobiles and 225,000 km in trucks.
• The company Unikat has, in Switzerland alone, over 250 filters deployed since 1989; some of these have operated more than 20,000 h without problems.
• The company Linde + Still annually installs approx. 1500 particulate traps in forklifts.
• Since 1990, there are 150 city busses running in Zurich equipped with the so-called Daimler-Benz system (wound filter, copper coated with acetyl-acetonate regeneration).
• There are 400 city buses operational in Munich/Germany since 1990.

Further, the manufacturers DAIMLER-BENZ and MAN (buses), LIEBHERR (construction engines) and DEUTZ (for construction site engines) are accepting orders for soot filter systems as original equipment. All three vendors state the maximum back pressure as 200 mbar.

Abbreviations

References

1. VERT - Available particulate trap systems for diesel engines, Report TTM W04/4/98, 22.10.98
2. VERT-Bulletins 1-4 (available Suva/Lucerne, Switzerland)
3. SAE 980539: VERT: Diesel Nano-Particulate Emissions: Properties and Reduction Strategies

###