Emissions from diesel engines are a significant air-quality issue, accounting for about one-third of the nation’s NOx emissions and one-quarter of the PM emissions from mobile sources. The U.S. EPA has responded with new, tougher emissions standards, the first tier of which took effect in 2004 (October of 2002 for engine builders affected by the consent decree). Even more stringent rules will take effect in 2007 and 2010, along with new diesel fuel standards requiring lower sulfur levels.
With these changes, a review of technology options available to OEMs, fleet owners and operators is in order.
First, let’s take a brief look at the standards themselves. The EPA’s standards introduce a lower NOx limit of 0.2 g/bhp-hr that must be fully phased in by 2010. Between 2007 and 2009 at least 50% of this NOx emissions limit must be achieved. Manufacturers can satisfy this requirement by bubbling the emissions and certifying that the NOx limit for all their 2007-2009 model engines is approximately 1.1 g/bhp-hr. In other words, the phase-in will be on a percent-of-sales basis - i.e., a 50% reduction from 2007 to 2009 and 100% by 2010. The EPA has also introduced a new emission standard of 0.01 g/bhp-hr for PM (down from the current 0.1 g/bhp-hr) that must be met by 2007. These standards thus include flexibility provisions to facilitate the transition to the new standards, and encourage early introduction of clean technologies for both new and existing sources.
|2004 EPA Heavy-Duty Engine Standards||Recommended Strategies for Meeting Standards|
|2007-10 EPA Heavy-Duty Engine Standards||Recommended Strategies for Meeting Standards|
Major manufacturers such as Caterpillar, Cummins, Detroit Diesel and International Truck and Engine have adopted diesel particulate filters as the preferred strategy/technology for PM reduction, but there is no consensus on NOx control technologies. The two most practical and cost-effective approaches to lower NOx emissions from diesel trucks are in-cylinder techniques such as a high-rate of EGR and exhaust system technologies such as the urea-SCR, which is being adopted in the European Union starting in 2005 (Euro 4).
These new standards will also indirectly affect existing diesel trucks and fleet operators in several non-attainment regions of the nation. There are approximately 11.3 million diesel trucks operating in the United States, and an estimated four million trucks operate in various non-attainments areas. Such fleet owners will be subject to a voluntary retrofit program accompanied by financial incentives to encourage compliance.
By June 1, 2006, fuel refiners will be required to start producing diesel fuel for use in on-highway vehicles with sulfur levels no more than 15 parts per million (ppm). The program offers some flexibility to refiners such as an averaging, banking and trading component, beginning in June 2006 and lasting through 2009, with credit given for early compliance before June 2006.
Several options are available to manufacturers and fleet owners who are considering improving the emissions performance of their diesel engines. These include converting engines to alternative fuels like compressed natural gas and retrofitting older engines with modern emission control technologies.
Retiring older engines and converting to natural gas can be very expensive to a diesel truck operator, especially given the current high price of natural gas. Hence engine retrofits such as EGR and exhaust control technologies emerge as the most practical option for cost-effective emission compliance for new and existing trucks.
An accompanying table provides a comparative analysis of emission reduction technologies to meet the new on-road diesel vehicle emission standards, as well as for use in retrofit programs for engines already on the road. Most manufacturers and fleet owners expect to implement one or some combination of these technologies. The NOx and PM control devices cited are at various levels of development and commercialization. Some devices are verified by the EPA and/or the California Air Resources Board (CARB) to achieve various levels of NOx and PM emission reduction efficiency in retrofit applications.
|Emission Control Device||Description||Expected NOx Efficiency||Expected PM Efficiency||Status|
|Exhaust Gas Recirculation (EGR)||Recycles the exhaust gas back to the engine intake system||50% - 60%||n/a||In commercial use; still concerns about condensation, packaging and engine integration constraints such as fuel and air management system upgrades.|
|NOx Adsorber||Adsorbes NO and oxygen during lean operation, uses CO and HC from periodic rich operation to convert to nitrogen||>80%||n/a||In development; may become available by 2010.|
|SCR||Converts NOx to nitrogen and oxygen in the presence of urea||70% - 90%||20% - 30%||Used in marine and stationary engines; first commercial application in heavy duty engines underway.|
|Non-thermal Plasma||High energy electrons convert exhaust pollutants to inert species||50% - 60%||30%||Under development.|
|Oxidation Catalyst||Oxidizes HC and CO in catalyst||n/a||30%||Established commercial technology.|
|Diesel Particulate Filter||Collects particles in diesel exhaust||n/a||80% - 90%||Commercially used in light duty; in development for heavy-duty applications.|
Many methods of engine-based emission control have been investigated since the advent of emission standards for diesel engines. These methods range from minor hardware improvements to a redesign of major engine components such as the combustion chamber, to introduction of electronic control of the fuel delivery system. Another common engine based technique for low-cost compliance is exhaust gas recirculation (EGR). While cooling of the exhaust gases and advanced fuel-injection systems used in an EGR provides some benefits such as higher injection pressures and flexible fuel injection timing, concerns exist about condensation, packaging and engine integration constraints such as the fuel and air management system upgrades needed to counteract increased PM from EGR.
Moreover, there are questions as to whether these and other engine design improvements can comply with the 2007 and 2010 standards. It seems unlikely that the earlier standard can be met through EGR- based approaches alone and there is unanimous agreement that aftertreatment or exhaust control treatment such as SCR and NOx adsorbers will be required to meet the 2010 numbers.
While SCR’s ability to achieve high levels of reduction is well documented, NOx adsorber technology is not capable of achieving the 90% reduction level at the present time. NOx adsorbers require engine integration and a means for supplemental fuel injection. NOx adsorbers reduce nitrogen oxides in two steps. First, the catalyst chemically traps and stores the nitrogen oxides. Eventually the catalyst’s active sites “fill up,” setting off the second step-regeneration, in which diesel fuel or other hydrocarbons are injected directly into the exhaust gas. This artificial hydrocarbon-rich (reducing) environment triggers the release of oxygen and the conversion of nitrogen oxides to nitrogen and water.
On the downside, regeneration exacts a penalty in fuel economy, projected at over 5% as a result of the injection of fuel into the exhaust gas. Adsorbers are also quickly poisoned by sulfur, and their efficiency drops substantially even when exposed to very low levels. Although this can be reversed through desulfurization, it can be expensive and uneconomical depending on sulfur levels in the fuel.
Urea-SCR is a technology where a NOx reducing agent – an aqueous solution of urea – is injected into the exhaust gas upstream of the SCR catalyst. The urea solution must be carried onboard in a tank.
Compared to other technologies, the use of urea-SCR could potentially provide better fuel economy and lower operating costs. These systems have been shown to reduce NOx by 65% to 99% over a range of diesel operating conditions. SCR technology has been employed on stationary sources for over 15 years. However, mobile SCR technology must achieve smaller packaging, be durable and function effectively in a diverse range of a truck engines. Moreover, systems must be designed to prevent ammonia slip in which unreacted ammonia escapes out the tailpipe.
Recent developments in SCR technology such as the ELIM-NOx system designed and developed by Monroe, Conn.-based Combustion Component Associates (CCA) has addressed this problem by tailoring the amount of urea injected into the catalyst as a function of engine control parameters such as RPM and load while providing on-board diagnostics for the various functions.
A recent survey conducted by Manufacturers of Emission Control Association (MECA) shows that the cost for an SCR system for engines in the 300 hp to 500 hp range can vary from $11,000 to $50,000 per system depending on the volume of the order. As the market for diesel exhaust emission control technology for new and retrofit application grows, it is anticipated that the costs of the SCR system will continue to drop over time. Moreover, the cost for the manufacturer or the diesel truck operator depends on the volume of SCR systems ordered and the amount of NOx removal desired. The unit cost of a single system will be significantly higher than a volume order of 500 systems.
An accompanying table analyzes the annual compliance cost of an SCR system over a five-year life on engines with a variable NOx emission rating based on the engine model year. Assuming a NOx reduction requirement of 70%, the cost per ton of NOx removed for the SCR system analyzed ranges from $1,400 to $2,600 per ton depending on the NOx emission rating of the truck. Greater emission reduction is possible on an older truck; consequently the fixed system cost is allocated among more tons resulting in a lower cost per ton.
To help achieve air quality goals and encourage diesel truck manufacturers and owners to comply, EPA, through its Voluntary Diesel Retrofit Program, is developing policy options for using emissions credits generated by retrofitting diesel engines as credits to be traded and as stationary source offsets. New trucks that install emission controls prior to 2007 can be eligible for Early Compliance Credits. Some other incentive programs in development are averaging, banking and trading (ABT), emissions trading, market-based allowance trading, mobile source emission reduction credits (MERCs) and open market emissions trading.
While engine manufacturers and OEMs can apply early compliance credits to other non-compliant diesel engines, operators of existing diesel trucks who voluntarily reduce emissions can be eligible to a State Implementation Plan (SIP) credit, an economic incentive to recover the costs of the pollution control device within the NOx SIP region.
Most of the economic incentives for diesel truck operators are likely to be centered in the ozone non-attainment regions. These regions are most likely to make voluntary compliance as economically attractive for fleet owners as possible, given the severe ozone non-attainment levels in the region. A lowering of NOx emissions levels from diesel trucks in these severe non-attainment regions will contribute significantly to the reduction of acid-rain, ground level ozone and reduced visibility.
|Engine Model Year||2002||1998||1991||1988||Units|
|NOx Reduction||70%||70%||70%||70%||Percent Reduction|
|Engine Load Factor||70||70||70||70||Load Factor|
|Annual Cost||$ 3,120||$ 3,980||$ 4,320||$ 5,360||Capital & Operating Cost/Yr|
|5-Year Cost||$ 15,600||$ 19,900||$ 21,600||$ 26,800||Estimated Life Cycle Cost|
|Tons Eliminated (5 Years)||6||9.5||12||19||Tons for 5 Years|
|Cost Per Ton||$ 2,600||$ 2,095||$ 1,800||$ 1,411||Cost/Ton of NOx Removed|
About the Authors:
T.J. Tarabulski is product manager, Combustion Components Associates (CCA), global provider of air-pollution control technologies for boilers, stationary and mobile IC engines. His phone number is (203) 268 3139; email firstname.lastname@example.org. For more information on CCA’s ELIM-NOx SCR system, visit www.cca-inc.net
Ravi Krishnan is principal and managing director of Krishnan & Associates Inc., a Norwalk, Conn.-based provider of sales & marketing, market analysis and strategic investment planning services for the power industry, with a focus on energy and environmental industries. He can be reached at (203) 854 6700, e-mail email@example.com, web site www.krishnaninc.com