Education: Diesel

Introduction to Conventional Filtration Technologies

Although variations of diesel emission filtration technologies are numerous, there are essentially two categories into which all conventional variations fall:

1. Catalyzed Diesel Particulate Filters (“DPF’s”): DPF’s are referred to by many different names. Some of the most commonly used—and misused—are: “Catalytic Converters”, “Catalytic Reactors”, “Catalytic Purifiers”, “Exhaust Purifiers”, “Trap Filters”, “Diesel Traps”, “Exhaust Scrubbers”, “Catalyst Filters”, “Catalyzed Wall-flow Filters”, “Wall-flow Filters”, “Catalytic Mufflers”, etc (13).

2. Diesel Oxidation Catalysts (“DOC’s”): DOC’s are also commonly referred to as “Oxidation Catalysts”, “Flow-through Catalysts”, “Flow-through Devices”, etc.

Both Catalyzed Diesel Particulate Filters and Diesel Oxidation Catalysts employ the same basic method to achieve PM emissions reduction; they utilize heat to “oxidize” [burn] the particulate material. In most cases, the heat from the engine’s exhaust system is used to achieve oxidation. The reoccurring process of oxidation is also often referred to “regeneration” because the process of oxidation not only reduces PM emissions, it also “regenerates” (re-news) the catalytic device’s filtration capacity.

In order for the process of regenerative oxidation to occur, high temperatures—normally between 250° and 350° C must be attained, and preferably sustained during operation (14). In many operating conditions, attaining sufficiently high temperatures can prove difficult or unattainable. Catalytic devices (DPF’s and DOC’s) employ precious metals such as platinum, palladium and rhodium as catalysts to lower the minimum temperatures necessary to achieve “light off”, the point at which oxidation of the particulate material is initiated. Manufactures use these highly conductive [and very expensive] metals to coat or impregnate the substrate surfaces of their catalytic devices.

Passive vs. Active Catalytic Technologies

Catalytic technologies which rely on heat from an engine’s exhaust system in order to achieve oxidation are frequently referred to as “passive” catalytic devises. Other systems may incorporate fuel burners, electric heating elements, and fuel-borne additives which aid in attaining the temperatures at which oxidation occurs. Technologies which employ these types of components are often referred to as “active” catalytic devices.

For purposes of eliminating potential confusion, it should be noted that some manufactures define DPF’s which only contain precious metal catalysts as “active” devices, even though these devices rely solely upon the heat contained in an engine’s exhaust to achieve oxidation. This classification usually occurs when the manufacturer also produces a diesel particulate filter which contains no catalyst, i.e. a device which is in all other ways similar to a catalyzed diesel particulate filter, however; the device relies solely upon the heating of its component base metal to achieve temperatures sufficient to initiate oxidation. Because exhaust temperatures are commonly required to exceed 500° C for these non-catalyzed devices to affect oxidation, their widespread use is significantly restricted (15).

Primary Differences between Conventional Catalytic Technologies

The primary difference between DPF and DOC technologies is that DPF technologies physically trap and store particulate material—usually by using catalyzed ceramic, cordierite or silicon carbide wall flow monoliths, or ceramic fiber or ceramic cartridge filters (16). Once the particulate material becomes trapped, it is incinerated [oxidized] and PM emissions reduction is achieved.

Conversely, DOC technologies do not trap PM emissions. Rather, particulate materials “pass-through” the internal structures of these devices. When exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles are oxidized, thereby reducing TPM emissions (17).

Other differences between DPF and DOC technologies include: costs, filtration capacities, filtrations efficiencies, maintenance requirements, operating condition requirements, their production of chemical bi-products, etc.