When considering any type of aftertreatment for retrofit, first make sure that the system is on either the EPA's Verified Technologies List or on a similar list from the California Air Resources Board (CARB).
Remember, too, that before installing a retrofit device, a qualified supplier might have to instrument and monitor your machine's operation for several days in order to competently recommend (or not recommend) a particular system. For example, does the machine's duty cycle allow exhaust temperatures to remain high enough for sufficient periods to passively regenerate a catalyzed DPF?
According to Schmidt, three basic retrofit technologies ("good, better and best," he says) typically are available for particulate control on older diesels: the DOC, the partial filter, and the DPF.
When the diesel oxidation catalyst is used alone (not in conjunction with a DPF) as a means for reducing particulates, it uses a different catalyst than when its function is to clean the DPF. The catalyst used in the stand-alone DOC basically strips off the "soluble organic fraction" (SOF) portion of PM. The SOF consists essentially of unburned portions of diesel fuel and lubricating oil that condense on the sponge-like carbon particles.
Compared to the DPF, the stand-alone DOC is perhaps 20 to 30 percent effective at reducing total PM, but does little to reduce the volume of solid carbon particles. Reducing the SOF is a plus, however, and the stand-alone DOC does not require ultra-low-sulfur fuel. The price for a retrofit DOC might be in the neighborhood of $2,000 to $4,000, maybe more if a dual system is required or if installation is difficult.
Partial filters are 40 to 70 percent effective at capturing soot and may use filtering material such as metallic fleece that is laminated between layers of corrugated steel. The filter traps a portion of the carbon particles present in the exhaust stream, but usually does not trap ash. Partial filters are passively regenerated, either by using a catalyst on the filter or by employing a diesel oxidation catalyst, thus requiring ultra-low-sulfur fuel to protect the catalyst from sulfur poisoning. The partial filter might cost between $5,000 and $6,000.
(According to some aftertreatment specialists, particulate filters that use passive catalyzed regeneration systems are limited to 1994 and newer non-EGR [exhaust-gas-recirculation] engines. Pre-1994 engines might overwhelm a passively regenerated soot-filtering system, and [to add a twist of irony here] cooled-EGR engines may not produce enough NOx for a passive system to convert NO to sufficient quantities of NO2 for combusting the soot.)
Older diesels also can be fitted with a full DPF, usually at a cost (depending on engine horsepower) between $7,000 and $10,000. But costs can vary dramatically. Some large engines, for instance, may require dual systems to handle high exhaust flows, driving the cost to perhaps $20,000 or more.
Retrofit DPFs typically require higher exhaust temperatures to passively regenerate, compared with temperatures required for partial filters. If temperatures are insufficient, then an active system is required, such as an integral electrical heater plugged in overnight. In theory, non-catalyzed, actively regenerated particulate filters could be retrofitted to any diesel.
"Low temperature" DPFs, which employ more catalyst to produce more NO2, can passively regenerate at temperatures below those required for a standard DPF. But the possibility of this process resulting in excess NO2 at the stack has the EPA and CARB concerned. (The sidebar, "DPF Passive Regeneration and NO2 Concerns," addresses this issue.)
For older trucks and machines that might require both particulate and NOx control, the "lean NOx catalyst" combined with a catalyzed DPF may be a retrofit possibility. Cleaire Advanced Emission Controls, for example, manufactures a retrofit system (the Longview) that combines these two technologies.
According to Tom Swenson, director of sales and verification for Cleaire, the lean NOx catalyst uses continual injection of a small volume of diesel fuel into the exhaust stream to reduce NOx over a proprietary platinum catalyst, producing benign nitrogen gas and oxygen. Unlike the NOx adsorber, the lean-NOx catalyst works well in an oxygen-rich environment, but it is not nearly as effective at reducing NOx as the adsorber, and thus is not considered for use on new engines.
With ultra-low-sulfur fuel, the Longview has a stated NOx-reduction capability of at least 25 percent, but, says Swenson, that number might be closer to 35 to 40 percent in off-road applications, where exhaust temperatures generally are higher. Minimum particulate reduction is rated at 85 percent. The proprietary catalyst that coats the Longview's particulate filter promotes oxidation of collected soot and also converts CO and hydrocarbons into benign gases and water.