The projected global diesel engine emission standards for the 2002-2010 time frame will require revolutionary changes in exhaust system technology.
In October 1997, the EPA adopted new emission standards for heavy-duty diesel truck and bus engines beginning with model year 2004. These standards reflect the provisions of the Statement of Principles (SOP) signed in 1995 by the EPA, The California Air Resources Board (CARB), and the manufacturers of heavy-duty diesel engines. The goal was to reduce nitrogen oxide (NOx) emission levels from highway heavy-duty engines to approximately 2 grams per brake horsepower per hour (g/bhp-hr) beginning in 2004.
On May 17, 2000 the EPA announced proposed emission standards for model year 2007 and later heavy-duty highway engines. The proposed rule includes two components: (1) diesel fuel regulation and (2) emission standards.
The proposed fuel regulation limits the sulfur content in on-highway diesel fuel to 15 ppm down from the previous 500 ppm. The fuel provisions would go into effect in June 2006. The second part of the proposal introduces new, stringent emission standards. The EPA has proposed a particulate matter (pm) emission standard of 0.01 g/bhp-hr, to take full effect in the 2007 heavy-duty engine model year. The proposed standards for Nox and non-methane hydrocarbons (NMHC) are 0.20 g/bhp-hr and 0.14 g/bhp-hr, respectively. These NOx and NMHC standards would be phased in for diesel engines between 2007 and 2010. The phase-in would be on a percent-of-sales basis: 25% in 2007, 50% in 2008, 75% in 2009, and 100% in 2010. The EPA has also proposed a formaldehyde emission standard of 0.016 g/bhp-hr.
Ultra-low-sulfur diesel fuel has been proposed as a "technology enabler" to pave the way for advanced, sulfur-intolerant exhaust emission control technologies, such as diesel particulate filters and De-NOx catalysts, which will be necessary to meet the 2007 emission standards.
Global OEMs will have several aftertreatment approaches at their disposal to meet 2004/2007 emission limits. The following table lists most common technologies and describes reactions used to control emissions.
|Diesel Oxidation Catalyst (DOC)
||CO, HC (incl. PAH), PM (SOF), odor|
|Selective Catalytic Reduction (SCR)
||Selective catalytic reduction by ammonia/urea
|(Lean NOx Catalyst, LNC)
||Selective catalytic reduction by hydrocarbons
||NOx, CO, HC, PM (SOF)|
|NOx Adsorbers (Lean NOx Trap, LNT)
||Adsorption (trapping) of NOx from lean exhaust followed by release and catalytic reduction under rich conditions
||NOx, CO, HC|
Summary of After treatment Methods
The following sections provide more detailed descriptions of the various emission control technologies.
Lean NOx Catalysts (LNC) are designed to reduce nitrogen oxides from diesel or spark-ignited engine exhaust gases under net oxidizing conditions, i.e., in the presence of abundant amounts of oxygen. They employ a ceramic honeycomb monolithic substrate (support), featuring many small parallel channels running in the axial direction. An alternative design utilizes a metallic substrate, made of thin metal foil, with noble metal catalysts deposited on them, such as platinum (Pt), palladium (Pd), or mixtures of the same.
Lean NOx Trap (LNT) is a method of NOx adsorber development, the latest advance in removing NOx in lean exhaust. With this emerging technology, NO is catalytically oxidized to NO 2 and stored in an adjacent trapping site as a nitrate. The stored NOx is removed in a two-step reduction process by temporarily inducing a rich exhaust condition using a pulsed charge in fueling. NOx adsorbers employ precious metal catalyst sites to carry out the NO to NO2 conversion step. The NO2 is then chemically stored in alkaline-earth oxide as a nitrate. To operate effectively, the NOx adsorber must remain stable for extended periods, during which time the exhaust environment modulates between rich and lean conditions.
Sulfur dioxide (SO 2 ) derived from the sulfur in the fuel can react catalytically with oxygen and then with the NOx storage components, such as BaCO 3 , forming stable sulfates (BaSO 4 ) and rendering the adsorbing capabilities of the system ineffective. In addition, SO 2 can be catalytically converted to sulfate in the exhaust stream, resulting in higher particulate emissions.
Selective Catalytic Reduction (SCR) is a term frequently used as a synonym for catalytic reduction of NOx in diesel exhaust or flue gases by nitrogen-containing compounds, such as ammonia or urea. Such systems are commercially available for stationary applications.
Diesel Oxidation Catalyst (DOC) is a catalyst promoting oxidation processes in diesel exhaust. It is usually designed to reduce emissions of the organic fraction of diesel particulates, gas-phase hydrocarbons, and carbon monoxide.
Diesel Particulate Filter (DPF) is a device that physically captures diesel particulates and prevents their discharge from the exhaust pipe. Collected particulates need to be removed from the filter, usually by continuous or periodic oxidation, in a process called "regeneration."