From the Microgrid Knowledge guide, Reciprocating Engine Generators and Microgrids: The Last Defense Against a Power Outage, this article looks at the pros and cons of the fuels used in reciprocating engines.
Electricity costs are determined largely by the price and availability of fuels used in power production. As a result, the power industry values generation technologies that offer fuel diversity and flexibility.
This is part of the reason reciprocating engines are in widespread use in the power sector, and dual-fuel units are becoming increasingly popular.
Reciprocating engines can be designed to consume a wide variety of individual fuels or—in the case of dual-fuel units—to be capable of using gaseous fuels (such as natural gas or propane) as well as liquid fuels such as diesel.
A dual-fuel reciprocating engine generally runs primarily on natural gas (or propane when pipeline gas is not available). It also needs at least a small amount of diesel fuel to ignite the mixture. A dual-fuel engine can also run entirely on diesel, and typically this is done when limited or no natural gas is available.
Generally, reciprocating engines fueled either entirely by natural gas or—in dual-fuel units—by a mix of mostly natural gas and small amounts of diesel create lower overall levels of harmful emissions than engines that are fueled entirely by liquid fuels such as diesel, gasoline or jet fuel.
Significant emission reductions
It is important to note, however, that tightening federal requirements and technological improvements in recent years have resulted in significant reductions in emissions from newer diesel and other liquid-fueled reciprocating engines.
The U.S. Environmental Protection Agency’s rules for regulating reciprocation engines exempt ‘existing emergency’ engines from compliance with emissions limits, with ‘existing’ referring to engines manufactured or ordered before 2002, or up through 2009 for certain categories.
Tightening federal requirements and technological improvements in recent years have resulted in significant reductions in emissions from newer diesel and other liquid-fueled reciprocating engines.
EPA also allowed emergency engines to operate up to 100 hours a year for non-emergency purposes while maintaining their emergency status, but a federal appellate court later overturned that part of the agency’s rules. Now, emergency engines can only operate during emergency situations or for a limited number of hours annually for testing and maintenance purposes.
Fuel availability and cost, of course, are important considerations in determining which fuel would be best for reciprocating engines used in the power sector.
Advantages of natural gas
The availability and use of natural gas for power generation in the U.S. has increased considerably in recent years due primarily to the success of gas producers in extracting vast quantities of gas from shale deposits through the use of hydraulic fracturing. Natural gas prices are considerably lower as a result—and generally less volatile than they had been, though price spikes still are possible during periods of high demand, pipeline constraints and/or gas-supply disruptions caused by hurricanes or other events.
Also, natural gas pipelines do not extend into all parts of the U.S., and in some regions such as New England existing pipeline infrastructure cannot always ensure delivery of all the gas that may be needed, especially during winter peak demand periods when gas demand for space heating is very high.
Natural gas price spikes still are possible during periods of high demand…
As a result, it is common for utilities, industrials, hospitals, government facilities and others to turn to either dual-fuel or diesel-only reciprocating engines — and for owners of dual-fuel units to stockpile diesel for use in the event that natural gas is not available when emergency back-up generation is needed. (Diesel can be stored for a year or more without any significant degradation with proper treatment.)
Because diesel is readily transportable, additional supplies of the fuel can be delivered by truck if reciprocating engines need to be used for extended periods, such as in the days after a devastating ice or wind storm.
Two more points should be made regarding fuel price
While natural gas prices are currently near historic lows and projected to rise only gradually over the next several years, the prices for diesel, gasoline and jet fuel also are lower than they have been in some time.
Since most reciprocating engines in the power sector are operated for only limited periods, the fuel costs associated with their operation generally are not as important a factor as the cost of fuels used in larger plants that operate most hours of the year.
Housley Carr contributed to this article.
What role do reciprocating engines play in the emerging microgrid market? Watch for our next installment in this series or read more in Reciprocating Engine Generators and Microgrids: The Last Defense Against a Power Outage, available for free download courtesy of Fairbanks Morse Engine.