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Exhaust filtration:

Fibre based solutions in diesel exhaust filtration

shish Mathur and Santosh Chavan provide us with an overview of commonly used filtration materials employed in diesel after treatment systems, and present recent progress at Ahlstrom related to the diesel filtration industry.

Diesel engines are considered the most efficient internal combustion engines and are therefore widely used throughout the world in heavy duty vehicles. However, the growing concerns regarding the harmful effects of emissions from the diesel exhaust on air quality are driving both car and engine manufacturers to continuously develop efficient solutions to manage these emissions.

Due to the various engine sizes and driving conditions, several technologies are currently being practiced in the industry to deal with both the diesel particulate matter (DPM) as well as the noxious gases present in the exhaust.

Introduction

Trends in diesel engines

While diesel engines have been widely used in heavy-duty trucks and off-road vehicles for a long time, their use in passenger cars and light trucks has risen sharply in the recent years both in Europe and the US market. Some of this growth in demand is being fuelled by the rising oil prices thus giving more impetus for the more efficient diesel engines.

However, coupled with this growth are the growing concerns of air quality from the emissions, where pollutant gases such as carbon and nitrogen oxides (NOx) and particulate matter (PM) in the diesel exhaust are considered to be very critical pollutants concerning health.

Trends in emission regulation

Clean air regulatory authorities in the EU and in the US have adopted diesel emission

standards which require the use of particulate filters to meet the PM limits. Under the new regulations in Europe, limits on both PM and NOx emissions will be reduced by more than a third of current levels.

Trends on emission regulations in Europe are shown in Figure 1. These increased complexity of emission controls demands filtration with higher porosity, lower back pressures & higher efficiencies.

Current technologies for diesel exhaust after treatment systems

Diesel emissions essentially consists of gaseous emissions such as carbon monoxide (CO), carbon dioxide (CO2), nitrogen monoxide (NO), nitrogen dioxide (NO2) and particulate emissions such as diesel particulate matter (DPM), elemental carbon and hydrocarbons.

Hydrocarbons and CO removal

Diesel oxidation catalyst convertors (DOC) are used to oxidise the pollutants in the exhaust stream and can be packaged with mufflers.

In many diesel engines Closed Crankcase Ventilation (CCV) devices direct NOx,HC and toxics to the intake system for re-combustion instead of polluting the environment.

NO and NOx removal

Selective Catalyst Reduction (SCR) systems reduce NOx by 60-90% by injecting urea or some form of ammonia into the exhaust stream, which is then catalytically reacted with NOx. However, due to storage requirements, use has been limited to large engines only.

A

Figure 1: Exhaust Gas Emissions (L) and trend of CO2 (R) regulations in Europe (Source: www.isuzu.co.jp).

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A lean NOx Catalyst (LNC) is similar to a SCR system except the LNC injects diesel fuel into the exhaust stream instead of urea.

Exhaust Gas Recirculation (EGR) devices re-circulate a portion of engine exhaust back into the engine to cool peak combustion temperatures and thus reduce NOx. EGRs have been developed for dockside and construction equipment, and highway engines. Typically they can reduce 40-50% NOx.

Diesel particulate matter (DPM)

DPM removal is achieved basically by two filter systems, Diesel Particulate Filters (DPF) Systems and Disposable Filter elements. The DPF system typically utilises honeycomb or mesh devices placed within the exhaust stream that physically trap and oxidise PM. Collected particulates are removed from the filter, continuously or periodically through thermal regeneration.

The other growing category for DPM removal is filtration devices which utilise Disposable Filter elements. The disposable approach offers a much lower cost effective solution in comparison to the above as engine manufacturers are increasing focuses on lowering overall system costs through efficient engine management to reduce the burden on the after treatment technologies.

Overview of current filter materials used for dpf

A number of filter materials are used in DPF filter systems, includes ceramic monoliths, ceramic yarns, ceramic foams, ceramic paper, synthetic fibre, metal mesh and sintered metal fibre materials. These filtering materials should

possess high filtration efficiencies, lower pressure drops, high soot holding capacity, high operating temperatures, resistance to thermal stress or shrinkage, chemical resistance and moreover compatibility to regeneration methods. Depending on the regeneration method, diesel filter materials may be exposed to very high temperatures up to or more than 1000°C and extensive thermal shocks. Such a phenomenon is responsible for most instances of filter failure, such as melting or cracking3.

DPF filter systems utilising these filter materials typically have removal efficiencies ranging from 60 to 95 percent. The two most commonly used systems utilise ceramic filters (monolith and deep bed) and disposable paper filters, The ceramic filters are typically used in heavy duty and on-road applications whereas the disposable paper filters are primarily used in stationary and off-road applications, such as underground mines, fork lifts etc. All of these filter materials have certain advantages and drawbacks associated with them are therefore used selectively.

Ceramic filter systems

Ceramic DPF systems can be either wall flow filter or wound filter cartridges. The most common types (70% of market share) of wall flow are extruded monolith substrates and are typically made from cordierite or silicon carbide. With alternately plugged channels at each end, the diesel exhaust gas is forced through the porous walls and is filtered. Wall-flow monoliths are characterised by good mechanical strength, temperature resistance and good filtration efficiencies. These substrates can be coated with catalysts to reduce the temperature necessary for regeneration.

Its drawback is a higher thermal expansion coefficient, relatively high pressure drop with the soot loading that may also lead to complete clogging. They are brittle materials and therefore susceptible to damage under thermal or mechanical shock.

Ceramic fibre based wound filter cartridges can be made by winding continuous fibres

around perforated steel support tube, wherein exhaust gas is forced to flow inside the tubes and through the yarn layer, where diesel particulates are deposited on the fibres. Regeneration is done through electrical heating.

Some cartridge designs utilise thin ceramic fibre sheets, which are made in a process similar to paper making, followed by sintering / firing of the shaped filter. Composite SiC-coated alumina fibre media and a wall-flow filter element design have been developed by Fleetguard.3, 4

Disposable/extended life/reusable paper filters

Disposable diesel exhaust filters are normally pleated filter cartridges resembling common engine intake-air cleaners with efficiencies close to 100%. As the maximum operating temperature of the filter media is below 120°C, such filters require on-vehicle exhaust coolers like water scrubbers or heat exchangers. These filters are designed to be used up to 3 shifts.

The disposable/extended life/reusable types have a higher operating temperature window up to 200°C and are typically made from synthetic fibres or with blends with ceramic fibres for even higher temperature applications. Such materials, used in fork lifts etc, can be reused several times before disposal.

Recent developments in DPF media at Ahlstrom

There is a continual demand for cost-effective and reliable materials and systems forced by increasing legal emission standards. Ahlstrom has been working on several different approaches for Diesel Particulate Filtration including the Disposable, Reusable (Extended Life) and Regenerative Technologies.

Disposable DPF filter media

Ahlstrom successfully developed a suitable DPF media in an Italian government

Figure 2; Cellular wall-flow filter substrate made of ceramic fibre media (Source: Dieselnet.com).

Figure 4: Filter system installed on heavy duty bus.

Figure 3: Cartridge with pleated filter media.

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sponsored program called “Projetto Blue” to retrofit public transport buses. The filter was deemed to be useful for a mileage of 2,500 KM or up to 10 days, considering 250 KM/day (Figures 3, 4, and 5).

The cellulose based filter media developed is suited for low temperature applications up to 120oC (250oF) with controllable pressure drop & efficiency characteristics as per the requirement.

Extended life/ high temperature resistant /regenerative fibre DPF filter media

With the efforts of past few years, Ahlstrom has now developed DPF filter media with up to 100% ceramic fibre content and thus capable of withstanding temperatures up to 8000C and more, which can be used as base material for the regenerative filtration.

Comparative filtration efficiencies of these types of media are presented in Figures 6 and 7. In the figure, CER 160 DPF, CER 140 DPF and CER 120 DPF represents media with various inorganic fibre compositions for different efficiency, pressure drop and high temperature requirements that can be tailored further depending on the needs for different engine designs.

As can be seen from the graphs, efficiencies close to 100% is possible through the inorganic fibre media with possibility of lower pressure drops in comparison to the ceramic monolithic wall flow filters.

The cartridges made with these media can help reduce the cost of current DPF systems by more than 50% of the current commercial prices.

Innovative technology solutions at Ahlstrom for DPF filter media

Ahlstrom has some unique technology platforms that can be readily exploited to develop engineered solutions for the DPF market.

Disruptor™ Technology

Disruptor™ media is Ahlstrom’s nano alumina fibre technology where alumina ceramic fibres about 2 nanometres in diameter and 100 nanometres long are grafted on micro-glass fibres. Disruptor™ fibres have the highest aspect ratio (ratio of radius to length) and surface area (up to 600 m2/gm) than any other available fibres.

A specific advantage of this technology for DPF media is that it allows the attachment of extremely fine powders such as catalysts to the nanoalumina fibre.

Trinitex® media

Trinitex® is a multilayered media (comprising a three layer web) offering the design

Figure 5: Particulate accumulation on upstream side of the filter.

Figure 6: Pressure Drop comparisons with contaminant - PTI fine (100 mg/m3), Air flow: 10.8 m3/h, Air face velocity: 30 cm/sec for 100 cm2 test area.

Figure 7: Initial efficiency results with PTI fine dust, Air flow: 10.8 m3/h, Air face velocity: 30 cm/sec for 100 cm2

test area.

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flexibility to work with different fibers and powders such as catalysts, odour control agents, etc.

Conclusion Diesel engines are highly efficient internal combustion devices and are gaining increasingly popularity worldwide. Rises in fuel prices and exhaust emission regulations

in Europe in recent years have captured public attention. As a result, the demand for high efficiency, lower pressure drop and low cost diesel exhaust filters are increasing. Ahlstrom has been actively developing a platform of disposable, extended life and regenerative fibre based DPF media for use in diesel exhaust after treatment systems and has a broad technology portfolio that could

be exploited to develop unique innovative solutions for this market. •Contact:Ashish Mathur, Vice President Research & Technology Ahlstrom Filtration Division Turin, ItalyEmail: ashish.mathur@ahlstrom.com; Tel: +39 011 9260 437

Santosh Chavan R&D Scientist, Ahlstrom Innovation Services, Pont-Eveque, France Email: santosh.chavan@ahlstrom.comTel: +33 474 57 29 02

References

1. Isuzu’s official website> www.isuzu.co.jp >Isuzu R&D Room

2. Green Power: Low-Emission TDI Engines for the USA January 7, 2007; at www.audiworld.com

3. DieselNet technical reports.

4. Sakaguchi, T., et al., 1999. «Development of High Durability Diesel Particulate Filter by Using SiC Fibre», SAE 1999-01-0463

5. Technology Achievements, Freedom car & Vehicle Technologies Program DECEMBER 2006, EERE Information Center at www.eere.energy.gov

An SEM which shows how fibrous media works in this application.

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