Start saving with bi-fuel diesel engines
To avoid confusion, let’s define what we’re going to discuss: bi-fuel engines — sometimes called dual-fuel engines — use diesel fuel and natural gas simultaneously for stationary on-site electric generation. The terms “bi-fuel” and “dual-fuel” can have various meanings. They are both used at times to describe engines that can burn multiple fuels, but individually, or engines that use combinations other than diesel fuel and natural gas. But this discussion focuses on diesel and natural gas as combination fuels for reciprocating engines.
Owners of both standby and continuous-duty engine generators operating on diesel fuel alone face several challenges. It is expensive and space-consuming to have enough diesel fuel on site for continuous use or to supply standby generation that may need to operate for several days or longer. Diesel fuel is likely to continue to rise in price. Keeping liquid fuel in good condition also adds cost. Further, emissions from diesel engines operating long hours on diesel fuel only may exceed federal, state or local air quality standards.
A practical solution is to equip an existing engine or purchase a new engine to replace 70% to 80% of the diesel fuel with natural gas supplied via pipeline. Diesel fuel is still needed for pilot compression ignition but the necessary stored fuel quantities are much smaller. Natural gas on a $/MMBtu basis is far less costly in most markets, and bi-fuel operation dramatically reduces emissions of sulfur oxides, particulates and carbon dioxide.
Convert an Existing Engine
Several companies supply systems for conversion of recip engines to bi-fuel operation. These require no or minimal engine internal changes, and can be installed in a matter of hours. Because of lower fuel costs with natural gas, some owners of peaking generation systems may choose to operate these over longer hours or even continuously, thus reducing electric demand and energy charges. With longer operating hours comes the increased practicality of heat-recovery, making these installations true combined heat and power (CHP) systems.
Even pure standby diesel generation units can benefit from installation of bi-fuel capability, because the necessary onsite fuel storage and fuel holding costs are dramatically reduced, and system reliability improves with two potential fuel sources.
Package Conversion System
One system for converting existing diesel engines to bi-fuel operation is the GTI Bi-Fuel system from Altronic LLC. With this addition to the engine, natural gas is introduced downstream of the engine air cleaner and upstream of the turbocharger. The gas is supplied at approximately atmospheric pressure using a proprietary air-fuel mixer that provides a high level of gas mixing. The air-gas mixture is compressed in the turbocharger and distributed to each cylinder by the engine air-intake manifold.
The lean air-gas mixture is compressed during the compression stroke of the piston and ignited by the diesel injector. Flow of gas to the engine varies with the engine load. The system varies gas flow according to changes in engine vacuum level, thus allowing it to respond to engine fuel requirements while maintaining the integrity of the OEM governing system.
Short Paybacks for Prime Power
According to Steven Roix, Altronic GTI Sales Manager, the bi-fuel approach offers major savings to owners. “They are seeing rapid payback and huge savings on prime power and peak shaving applications. The lower fuel cost allows extending runtimes by 300% or more, depending on the type of installation.” For standby applications, the bi-fuel approach dramatically reduces the on-site fuel storage requirement.
Roix also points out that because of the cleaner burning characteristics of natural gas, typical installations reduce air emissions significantly. He notes that although the engine burns cleaner in the bi-fuel mode, the recommended service intervals do not change. “We always encourage customers to follow engine OEM maintenance instructions.”
The Altronic system monitors engine and fuel flow characteristics, and can quickly revert to all-diesel operation if there is an interruption or change in the flow of natural gas. Roix says, “It’s a simple add-on system that can be taken on and off easily.”
Bi-Fuel Capability from the Factory
Some generator package manufacturers offer bi-fuel capability with new generator sets. Generac offers bi-fuel as an option on its offering of standby diesel generation systems. Although these units are designed for standby service only, the bi-fuel capability still reduces fuel cost and makes a major reduction in the amount of standby fuel needed on site.
According to Michael Kirchner, Technical Support Manager for Generac, bi-fuel makes sense for many buyers of standby engines. “Because they run on both diesel and natural gas, they capitalize on both the availability of on-site diesel fuel (required by many authorities) and the long running times that come from utility-supplied natural gas. Because they typically run on a mix of about 75% natural gas, their running time is extended by a factor of four without a fuel delivery.”
Where Reliable Power is Essential
According to Kirchner, Generac bi-fuel units are the only ones on the market that have this capability from the factory: Others need to be modified in the field. He notes that common applications for this product are health care facilities, data centers, and other facilities such as wastewater treatment plants and 911 call centers, where continuous access to electric power is essential.
Kirchner says that emission reduction by adoption of the bi-fuel option is variable, but the company has seen evidence of reduction of emissions of NOx and particulates in excess of 25%.
Might be an Opportunity
If your facility is currently using diesel fuel only for engine generation, it is worth considering the benefits of changing to a bi-fuel system. If you are looking at adding standby generation, it certainly is an important option. Ask your consulting engineer to look into potential savings and reduction in the need for on-site fuel storage.
– This story appeared in the Summer 2013 Gas & Technology supplement. See additional stories below.