Understanding gas engine emissions in standby power applications

Air quality regulations have a major impact on the choice of engine generator sets for standby electric power and gas engines can be a more advantageous choice than traditional diesel technology

By Diane Clifford, Caterpillar Energy Solutions, Lafayette, Indiana November 7, 2019

Learning Objectives  

  • Understand what’s required to choose a gas engine for standby duty that meets operating objectives and site-specific emission standards.  
  • Learn how to determine a best-value proposition based on the level of standby support required and potential cost of lost power, along with system, operation and maintenance costs. 

Strict air emissions rules have brought natural gas engines to the forefront for standby power applications. The latest gas engines designed for standby duty, in addition to offering lower emission profiles, offer rapid startup and load acceptance capability similar to diesel engines. They also have competitive installed cost per kilowatt and lower long-term costs for fuel and maintenance.  

Choosing gas engines for standby duty is more complex than selecting and installing a diesel generator set sized for the expected standby loads. It requires understanding of the gas quality and composition along with applicable treatment services. The optimum choice for gas equipment requires an understanding of emissions control technologies and of federal, state, regional and local emission standards. 

Engineers and facility managers need to carefully consider their options in light of operating objectives and site-specific emission standards before selecting a gas-fueled standby power system. 

Knowing the rules  

To a large extent, the expected use of gas standby equipment determines the emission standards at the site. For pure standby/emergency use only  engines expected to run only for brief spells during routine testing and emergencies  emission standards are often not a major issue and the engine is allowed to run during the duration of the outage.  

This changes for engines installed for other purposes that also offer standby capability. For example, in load management (demand response) applications, gas engine-generators deliver a financial benefit by lowering electric utility rates and have the added benefit of availability for standby duty. The same can be true of engine-generators in combined heat and power installations. In these cases, the expected hours of operation are much longer than for pure standby and emission standards are therefore more restrictive.  

Apart from those considerations, emission standards can vary from one region to another. Emission standards are a primary driver of gaseous-fueled engines as alternatives to diesels for standby power. In general, federal regulations allow Environmental Protection Agency Stationary Emergency rated engine-generator sets to operate for only 100 hours per year or less during nonemergency situations such as testing and maintenance. Applications other than providing power during a utility outage are classified as nonemergency and for diesel engines require EPA Tier 4 emission standards (see Table 1). 

Stricter federal air quality standard rules apply in areas classified by the Environmental Protection Agency as nonattainment, and threshold values for sites to become a major source vary based on nonattainment severity. For example, 50 tons/year in Chicago and New York City is required while its only 10 tons/year in Los Angeles. Major sources are subject to rigorous and expensive emission standards. As a result, major facilities with large standby diesel capacity requirements may exceed the site annual tonnage limits and become a major source when running only during federallyallowed testing and maintenance.  

The federal stationary emission standards must be met but state and local government body requirements have the authority to make emission standards stricter. For example, the California Air Resources Board sets its state’s emission standards for a range of pollution sources and 35 local air pollution control districts may further regulate emissions from various businesses and facilities based upon their local situation. All this favors natural gas fueled standby power equipment. 

Gas engine emissions  

Like diesels, gas engines have their emission ratings certified at the manufacturing factory (for example, 1-gram NOx per brake horsepower-hour for prime power, nonemergency units as seen in Figure 1). However, unlike diesels, they can be fitted with field-deployed exhaust aftertreatment devices for further emission control. In some cases, they’re subject to on-site emissions performance testing.  

Today’s lean-burn standby gas engines have substantial advantages over older rich-burn technology in fuel economy, power output, service life, reliability and emissions profile. Lean-burn engines, operating on a lean air-fuel mixture, inherently operate with low cylinder temperatures that limit formation of NOx and other emissions. They can typically meet any site NOx limit. Particulates, unburnt hydrocarbons (THC) and carbon monoxide are also extremely low and exhaust aftertreatment typically is not needed.  

However, there are cases, particularly at higher altitudes, where these emissions may be out of local compliance. For example, a 1 MW gas generator set rated at 2 g/bhp-hr NOx would produce about 6.3% more THC and 2.8% more carbon monoxide at 8,000 feet of elevation than the same unit at 500 feet. In those cases, an inexpensive oxidation catalyst can be added to control the specific emission constituents that aren’t in compliance (see Figure 1). To meet extremely low NOx limits, selective catalytic reduction also can be deployed. For nonemergency engines, an oxidation catalyst may be required to meet National Emission Standards for Hazardous Air Pollutants. 

Achieving best value  

There is no one right answer to the question of which generating technology is best for standby power. A best-value proposition should become clear based on a fair evaluation of the level of standby support the application requires, the potential cost of lost power, the all-inclusive installed system cost and operation and maintenance costs.  

It is helpful to consult with a dealer or engine-generator manufacturer representative with a track record in the standby market. These professionals have access to engine-generators in a broad range of power ratings and configurations. They can help select the most appropriate standby system at the optimum installation, operating and maintenance cost depending on whether it’s a commercial, industrial or institutional facility. They also are familiar with standby power code requirements and the preferences of local authorities having jurisdiction.  

Dealerships can manage whole-project engineering, procurement and construction and supply all engines, generators, transformers, switchgear and other required ancillaries for projects. They also have access to the manufacturer’s financing programs to help complete the project in a budget- and cash-flow friendly manner. A methodical approach can enable a sound selection of standby generator sets to meet operating goals and site-specific requirements. 

Author Bio: Diane Clifford is a senior engineer specializing in technical marketing at Caterpillar with 13 years of experience in electric power. She focuses on new product and technology introduction.