Emission Control Technology
There have been tremendous developments in the design and application of emission control technologies in the last decade to substantially reduce levels of particulate matter (PM), carbon monoxide (CO), nitrogen oxide (NOx), and hydrocarbon (HC) pollutants. The two most common technologies – diesel particulate filters (DPF) and oxidation catalysts (DOC) – effectively control the levels of pollutants in the exhaust on their own or when used together. For example, a diesel oxidation catalyst can lessen the formation of particulate matter prior to the exhaust passing through a particulate filter, thereby increasing the performance and longevity of the filter. Additional technologies are designed to control specific pollutants, such as NOx.
While some of these technologies are affected less by the sulfur content of diesel fuel, all perform better at reducing emissions when used with ultra-low sulfur diesel fuel (ULSD), which has a sulfur content of less than 15 ppm. For example, diesel oxidation catalysts and some DPFs can reduce CO, HC, and PM emissions with fuels that contain sulfur levels greater than 15 ppm while catalyst-based DPFs are more sensitive and are more effective with ULSD. (See the insert on “Alternative Fuels” for more information.)
Costs for individual technologies vary. This insert cites costs from an independent cost survey conducted in November 2000 by the Manufacturers of Emission Controls Association (MECA). Generally, the larger the engine being retrofitted, the more expensive the device. However, higher sales volumes will begin to lower the costs of these technologies. Given the recent market penetration, costs should begin to decrease. Prices cited in association with specific technologies and their pollution reduction potential are provided by the U.S. Environmental Protection Agency (EPA). The reader is encouraged to contact individual manufacturers for exact costs.
Diesel Particulate Filters (DPF)
| NOx | PM | HC | CO | Price | Base Metal Oxidizing PM Filter | -- | 80% | 50% | 50% | $6.5-10K |
| Highly Oxidizing Precious Metal PM Filter | 0-5% | >90% | 90% | 90% | $6.5-10K |
| NOx | PM | HC | CO | Price | |
| Base Metal Oxidation Catalyst | -- | 10-30% | 50% | 50% | $1-2K |
| Precious Metal Oxidation Catalyst | -- | >20-40% | 90% | 90% | $1-3K |
Diesel particulate filters (DPFs) are one class of emission control technologies that lower PM emissions. By trapping the particulates as the exhaust gas passes through the filter, DPFs are able to achieve PM reductions of 80 – 90 percent. Numerous studies have documented the effectiveness of DPFs in both on- and off-road applications. The systems are relatively easy to maintain, but do require users to monitor their condition and occasionally remove the filter, blowing out the ash and replacing it.
Fuel sulfur content plays a key role in the performance of DPFs since it has a direct impact on the level of particulate matter in the exhaust. Numerous studies have found that DPFs, regardless of their manufacturer, achieve higher PM emission reductions with the use of ultra-low sulfur diesel fuel.
Two DPF products – Engelhard’s DPX Catalyzed DPF and the Johnson Matthey Continuously Regenerating Technology (CRT) Particulate Filter – reduce PM, CO, and HC by 60 percent as verified – but are capable of reducing emissions by 80 – 90 percent. Both technologies are verified by EPA’s National Voluntary Diesel Retrofit Program – which tests and validates technologies for fleet managers and operators – for their performance. These products are verified with ULSD. Today’s technology could be utilized in many off-road applications but requires active regeneration technology being developed for on-road use to make it applicable to all off-road applications. DPF retrofit programs for trucks and buses are underway in California and New York City, where the city plans to retrofit its 3,500 buses with DPFs by the end of 2003.
Diesel Oxidation Catalysts (DOC)
Diesel oxidation catalysts (DOCs) are a section of the exhaust system coated with metals that trigger chemical reactions which breakdown pollutants (CO, HC, PM) into harmless gases, when engine exhaust passes through it. Since 1995, more than 500,000 trucks and buses have been retrofitted with DOC systems.
On- and off-road applications of DOCs are virtually maintenance free, requiring only periodic inspections. DOCs also work to improve the effectiveness and performance of DPFs, by attracting excess soot from the exhaust before it passes through the filter. The cost of diesel oxidation catalyst devices range from several hundred to several thousand dollars per device depending on engine size, sales volume, and whether the installation is a muffler replacement or an in-line installation. MECA’s 2000 survey reported that average diesel oxidation catalyst costs ranged from $465 to $1,750 per vehicle. The majority of devices are designed to replace the muffler and installations typically take less than two hours.
Like DPFs, DOCs are also affected by sulfur. The sulfur content of diesel fuel is critical to applying catalyst technology, as the reaction caused by the catalysts rely on the sulfur content and the temperature of the exhaust gases.
NOx Reduction Technologies
The first verified system to reduce NOx and PM is a NOx reduction catalyst. This system combines a NOx catalyst with a particulate filter or oxidation catalyst to provide additional PM reductions. The Longview system from Cleaire (and offered by Fleetguard Emission Solutions) is verified to reduce NOx by 25 percent and PM by 85 percent.
In addition to the exhaust gas recirculation (EGR) technology to lessen NOx during the combustion process (see the insert on “Advances in Diesel Engine Technology” for more information), post-combustion emission controls for NOx include selective catalytic reduction (SCR) and NOx adsorber technologies.
SCR devices have been used for years to control NOx from stationary sources and are now being applied to mobile sources to cut the pollutant by over 70 percent. Unlike DOCs, the SCR system requires the addition of a reductant (typically urea or ammonia) to convert NOx pollutants to nitrogen and oxygen. Based on the oxidizing metals used in the SCR, additional pollutant reductions can be achieved. (See the insert on “Off-Road Heavy-Duty Diesel Vehicles” for more information.)
NOx adsorber catalyst technology is also undergoing extensive research and development in anticipation of the 2007 on-road, heavy-duty diesel engine regulations. Researchers have demonstrated the ability of NOx adsorbers to control up to 90 percent or more of NOx emissions over a broad temperature range.
NOx adsorbers act to store NOx emissions during lean engine operation and release the stored NOx by periodically creating a rich exhaust environment by either engine operation or the injection of a reductant in the exhaust stream. While EPA estimates that the technology can cut NOx (as well as HC and CO) by more than 90 percent, it is still largely in the research and development phase for on-road applications.
Crankcase Emission Control
In the majority of turbo-charged diesel engines, the crankcase breather is vented to the atmosphere often using a downward directed draft tube, therefore allowing a substantial amount of PM to be released into the atmosphere. One solution to this emissions problem is the use of a multi-stage filter designed to collect and return the emitted lube oil to the engine’s sump or a CCV system (available from Fleetguard). These systems allow filtered gases to return to the intake system, balancing the differential pressures involved and allowing the systems to eliminate crankcase emissions. EPA has verified one manufacturer’s crankcase filtration system. In addition to the Donaldson closed crankcase filtration system’s ability to lower crankcase emissions, it also reduces PM emissions by 25 – 32 percent and CO by 14 – 18 percent, according to EPA.
Additional Technology Potential
The California Air Resources Board recently verified the use of a diesel engine retrofit technology that simultaneously achieves reductions of at least 85 percent in PM and 25 percent in NOx emissions. The system produced by Cleaire Advanced Emission Controls is actually a combination of a lean NOx catalyst and a diesel particulate filter. The system has been verified for use on specific on-road diesel engines operating on ultra-low sulfur diesel fuel. In addition to DOC technology used to treat exhaust gases, EPA estimates that catalysts included in diesel fuel for commercial use will cut NOx up to 10 percent, PM up to 33 percent, and HC and CO up to 50 percent during the combustion process.
The Lubrizol Corporation has developed a water-in-diesel fuel emulsion product that produces a low-emission, emulsified diesel fuel. PuriNOx reduces NOx emissions up to 30 percent and PM up to 65 percent when compared to conventional No. 2 diesel fuel. Average emission reductions, considering data from numerous tests, indicate a NOx reduction of approximately 20 percent and a PM reduction of approximately 54 percent. The application areas for fuel powered by PuriNOx are centrally-fueled fleets, such as pick-up and delivery vehicles, urban and school buses, waste management fleets, and agricultural, mining, and construction equipment.
Sources
DieselNet – www.dieselnet.com
Manufacturers of Emission Controls Association – www.meca.org
U.S. Environmental Protection Agency, Voluntary Diesel Retrofit Program – www.epa.gov/otaq/retrofit
