In July 2005, the U.S. Environmental Protection Agency (EPA) proposed New Source Performance Standards (NSPS) for stationary diesel engines. The proposal included an interim phase-in period, followed by full certification requirements for most stationary diesel engines built on or after January 1, 2007. The proposed NSPS will essentially harmonize the stationary requirements with the existing EPA mobile off-highway (EPA non-road) requirements that became effective beginning in 1996. It is important that all who specify, install, or operate on-site diesel power systems understand how these proposed standards will affect the products they install over the next several years. Diesel generator set manufacturers are meeting these new requirements in various ways, but all are striving to create cleaner diesel generator sets that meet or exceed environmental regulations while maintaining performance and reliability.

Diesel Technology

Diesel-powered generator sets remain the preferred choice for standby and emergency power systems around the world. With the growth of distributed generation applications in recent years, more diesel generator sets are being used for utility peaking and commercial load-shedding because of the proven reliability, low lifecycle cost, high efficiency, ready availability, ease of installation, operational flexibility, and high-quality electrical performance of these products.

Diesel generator sets are used at industrial sites all around the world to supply temporary or replacement power.

As a mature engine technology, diesels offer these advantages over other prime movers, in addition to having a well-established service and fuel supply infrastructure. Exhaust emissions from diesel-powered generators include oxides of nitrogen (NOx), hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM) that would include any visible smoke and soot. Current standards enacted by the EPA and various state agencies mandate significant reductions in all these substances—particularly NOx—in an effort to reduce impact on the environment and public health. The most stringent of these regulations will ultimately reduce diesel NOx emissions to be on par with prime movers burning natural gas equipped with the best available control technology (BACT).

Currently, the major manufacturers offer products that meet applicable Tier 1 control levels established by the pre-existing EPA non-road regulations. Compared to older engines, NOx emissions from stationary and mobile EPA non-road diesel engines have been reduced 54%, and PM emissions have been reduced 70% to meet EPA Tier 1 standards.

Regulatory Landscape

Prior to the new standards, there were no EPA emissions regulations for stationary diesel engines. in sharp contrast to regulations for mobile off-highway (EPA non-road) diesel engines. Emissions regulations for stationary engines were usually governed by state and local permitting authorities and varied by the annual operating hours for the application. The exact number of permissible operating hours varied by state and locale, but as an example, base-load generators running in excess of 2,000 hours per year in areas with the worst air quality faced the most stringent local emissions standards.

Standards for emergency standby generators operating only 200 hours per year had the most attainable permitting requirements. These general standards were established by local authorities in response to an EPA requirement for clean air standards. EPA’s new NSPS stationary emissions standards are intended to help local authorities meet EPA’s clean air standards in regions that are non-compliant.

Stationary vs. Mobile Off-highway

Many US utilities used diesel generator sets to supply seasonal peaking power at critical load centers.

The mobile off-highway (EPA non-road) category of engines was established to describe engines used in mobile equipment such as farm equipment, construction equipment, trailerized generator sets, and other portable industrial engines used in temporary off-road applications. The off-highway category was created by the EPA, and standards were established because local permitting authorities only had jurisdiction over stationary sources. EPA regulations already in effect for mobile off-highway engines will continue to be applied according to the evolving mobile off-highway schedule.

Stationary engines are defined as any off-highway engine that is permanently installed and used as a power source. This category includes standby generator sets, on-site prime, and distributed energy power systems and a wide variety of industrial engines mounted on permanent bases or foundations. Under EPA regulations, stationary generators may be identified as “emergency” or “non-emergency.” There is no limit on the operation of emergency generators during normal utility outages, but they are limited to 30 hours per year for normal maintenance and testing. Non-emergency generators are for any other intended power application and could face curtailed operating hours due to stricter local regulations. Operational restrictions depend on total installed generation and total site emission levels among other criteria. Trailerized engines and generator sets are considered stationary when they are installed at a single location for more than 12 months. After January 1, 2007, both the mobile off-highway regulations and the stationary engine regulations will be essentially identical and based on the existing mobile-off highway regulations.

Interim Period

The relatively rapid enactment of the proposed emissions regulations for stationary generator sets has made it difficult for some manufacturers to adjust their factory build schedules. As a result, EPA established an interim period that addresses the manufacturers’ needs while introducing lower-emitting products to the field as soon as possible.During this interim period, the NSPS for stationary diesel engines will apply to each diesel engine for stationary use that is both ordered after July 11, 2005, and built after April 1, 2006. All generators using engines ordered and built after these respective dates have to comply with EPA non-road Tier 1 emissions requirements. Beginning January 1, 2007, all diesel engines used in generator sets will be required to be built to the prevailing regulations in the mobile off-highway/EPA non-road emissions regulations that are in effect when an engine is built. The regulation levels (Tier levels) depend on the horsepower of the engine (see the figure). There will be two significant exceptions:

• Stationary engines over 3000 hp will remain at Tier 1 until their Tier 4 requirements become effective in 2011
• The intent is that requirements for emergency standby generators will not advance beyond the last tier that requires no exhaust after treatment

State and Local Regs

This environmental award-winning diesel peaking installation uses selective catalytic reduction (SCR) to beat even the most stringent air quality regulations.

For the first time, the NSPS establishes uniform Federal standards for emissions from stationary generator sets. However, these regulations do not prevent state and local authorities from imposing even more restrictive standards based on prevailing local air quality conditions. In the U.S., certain state and local emissions standards for diesel-powered generators sets require an additional level of control, primarily for NOx and PM. While EPA-designated ‘nonattainment areas’ represent a fraction of the land area of the country, they are typically heavily populated areas—where diesel standby and peaking generator sets are most likely to be deployed. In effect, these nonattainment areas have served to drive development of NOx and PM control strategies and to economically bring them to market in advance of the EPA standards alone.

The figure summarizes the EPA emissions regulations for both mobile off-highway (EPA non-road) and proposed stationary diesel engine generators for Tier 1, Tier 2, Tier 3, and Tier 4 out to 2017. Due to the interim regulations established by EPA, engines used in all stationary generator sets that are both ordered after July 11, 2005, and built after April 1, 2006, will have to comply with Tier 1 regulations. Beginning with engines built on January 1, 2007, all stationary generators with engine ratings of 49-99 hp will have to comply with Tier 2 regulations; all generators with engines in the range of 100-751 hp will have to comply with Tier 3 regulations; and all generators with engines over 751 hp will continue at Tier 2 until 2011 as previously noted. Stationary engines over 3000 hp will remain at Tier 1 until their Tier 4 requirements become effective in 2011.

Emissions Control

The real challenge in designing today’s diesel engines involves a trade-off between NOx and PM emissions. Most engine modifications that decrease NOx have a tendency to increase PM. Conversely, techniques to reduce PM tend to increase the production of NOx. Both are linked by combustion temperatures: as in-cylinder temperatures increase, PM goes down but NOx goes up; as temperatures decrease, NOx goes down but PM goes up. The following control strategies are primarily concerned with optimizing the control of these two constituents, either during combustion or after. All engine modification strategies have been aimed at optimizing the combustion process while producing the least amounts of both NOx and PM per unit of power output.

Eight major strategies have enabled diesel manufacturers to achieve Tier 2 and Tier 3-level emissions reductions. These include:

• Electronic engine controls. The addition of electronic sensors and microprocessor-based controls has greatly improved fuel efficiency and power output while decreasing the production of both NOx and PM. By controlling fuel quantity, injection timing, turbocharger boost pressure and other factors, electronic engine controls maintain optimum combustion efficiencies by compensating for load, temperature, fuel energy content, barometric pressure, and even engine wear.
• Injection systems. Injection timing, injection pressure, nozzle design, and electronic injection systems have all proved significant in controlling both NOx and PM.

Retardation of injection timing along with increased injection pressure has been shown to reduce NOx without significantly increasing the production of hydrocarbons (HC) or PM. Higher injection pressures improve fuel atomization and combustion chamber penetration that simultaneously improve fuel economy while reducing PM.

• Combustion chamber geometry. Combustion chamber design goals include achieving the optimum compression ratio and thorough mixing of fuel and air prior to combustion. Designs that optimize the air swirl and turbulence provide the best mixing and therefore the lowest emissions consistent with high power output.
• Turbocharging systems. All medium and large diesel generator sets employ turbocharging to boost power, improve combustion efficiency, and reduce emissions. The most sophisticated systems use aftercoolers, which are water- or air-cooled heat exchangers that increase the density of the charge air and therefore increase specific power output.
• Exhaust Gas Recirculation (EGR). EGR is a well-proven method of reducing NOx in internal combustion engines, but also reduces power output and fuel efficiency. By recycling a portion of the inert gases of the exhaust gas stream with incoming engine air, combustion temperatures are reduced and, therefore, so is NOx formation. While not employed widely in stationary diesel engines at the present time, EGR may be used on selected engines to achieve compliance with EPA regulations.

Enlarge this picture Figure shows how new stationary diesel environmental regulations will be phased in based on various horsepower ranges.

Aftertreatment strategies

For larger engine size categories, achievement of Tier 3 stationary and mobile off-highway levels of NOx and PM are about the maximum limit for diesel engine in-cylinder control strategies. To reach lower levels of NOx and PM, aftertreatment of exhaust gas will likely be necessary. The following strategies have already achieved a practical level of commercialization in a variety of applications. Most aftertreatment strategies require the use of low-sulfur diesel fuel to prevent catalyst contamination.

Selective Catalytic Reduction (SCR) systems. Already in commercial application on stationary diesel engines, SCR systems incorporating aqueous urea injection into the exhaust stream passing over a suitable catalyst reduces NOx up to 90%. Systems consist of a SCR catalyst, urea injection system, urea tank, pump, and a control system.

PM traps. Diesel particulate matter (PM) traps are designed to physically capture PM from the exhaust stream. They can either be simple mechanical filters requiring frequent replacement, or they can be catalytic filters that provide periodic or continuous oxidation (regeneration) of the trapped particulates into CO2. PM traps with continuous regeneration have already reached a high level of commercialization and are being employed on stationary diesel engines in areas with strict PM emissions regulations. Ultra-low sulfur diesel fuel is needed to prevent contamination of the conversion catalysts. However, filtration efficiencies up to 90% have been demonstrated.

In remote areas not served by utilities, diesel generators offer a low-cost solution for clean, reliable electric power.

Conclusions

According to the interim requirements being established by the EPA, all stationary diesel gen sets ordered after July 11, 2005, with engines built after April 1, 2006, will have to comply with Tier 1 regulations at a minimum. Beginning with engines built on January 1, 2007, all generators with engine ratings of 49-99 hp will have to comply with Tier 2 regulations; all generators with engines in the range of 100-751 hp will have to comply with Tier 3 regulations; all generators with engines greater than 751 hp will continue at Tier 2 until 2011. Stationary engines over 3000 hp will remain at Tier 1 until their Tier 4 requirements become effective in 2011. Strategies are already in place to accomplish these and future emissions reduction goals mandated by the EPA through in-cylinder combustion improvements, new engine controls, and after treatment of the exhaust. Thanks to these technological refinements taking place today, the electric power industry will continue to enjoy the performance advantages that diesel generators offer well into the foreseeable future.