Diesel Engine System for Power Generator
Diesel engines have historically been the most common type of reciprocating engine for both small and large power generator applications. However, in the United States and other industrialized nations, diesel engines are increasingly restricted to emergency standby or limited duty-cycle service because of air emission concerns and also because of the high cost of fuel. Consequently, the natural gas-fueled SI engine is now the engine of choice for the higher duty cycle stationary power market (over 500 hr/yr) and is the primary focus of this report.
Compression ignition diesel engines are among the most efficient simple-cycle power generation options on the market. Efficiency levels increase with engine size and range from about 30 percent (HHV) for small high-speed diesels up to 42 to 48 percent (HHV) for the large bore, slow speed engines.
High speed diesel engines (>=1,000 rpm) are available for up to about 4 MW in size. Low speed diesel engines (60 to 275 rpm) are available as large as 80 MW. Medium speed diesel engines (400 – 1000 rpm) are available for up to approximately 17 MW.
Diesel engines typically require compression ratios of 12:1 to 17:1 to heat the cylinder air to a temperature at which the injected fuel will ignite. The quality of fuel injection significantly affects diesel engine operating characteristics, fuel efficiency, and emissions. Fine atomization and good fuel dispersion by the injectors are essential for rapid ignition, ideal combustion and emissions control.
Manufacturers are increasingly moving toward electronically controlled, high pressure injection systems that provide more precise calibration of fuel delivery and accurate injection timing.
Depending on the engine and fuel quality, diesel engines produce 5 to 20 times the NOx (on a ppmv basis) of a lean burn natural gas engine. Diesel engines on marine engines often emit over 20 lbs NOx/MWh and present on road engines emit less than 13 lbs NOx/MWh. New diesel engines will achieve rates of approximately 0.65 lb NOx/MWh. Diesel engines also produce assorted heavy hydrocarbons and particulate emissions. However, diesel engines produce significantly less CO than lean burn gas engines.
The NOx emissions from diesels burning heavy oil are typically 25 to 30 percent higher than diesels using distillate oil. Common NOx control techniques include delayed fuel injection, exhaust gas recirculation, water injection, fuel-water emulsification, inlet air cooling, intake air humidification, and compression ratio and/or turbocharger modifications. In addition, an increasing number of larger diesel engines are equipped with selective catalytic reduction and oxidation catalyst systems for post-combustion emissions reduction.
High speed diesel engines generally require high quality fuel oil with good combustion properties. No. 1 and No. 2 distillate oil comprise the standard diesel fuels. Ultra-low sulfur diesel with sulfur contents of less than 0.15 ppm is now required for the new Tier 4 diesel engines to reduce sulfur emissions. High speed diesel engines are not suited to burning oil heavier than distillate. Heavy fuel oil requires more time for combustion and the combination of high speed and contaminants in lower quality heavy oils cause excessive wear in high speed diesel engines. Many medium and low speed diesel designs burn heavier oils including low grade residual oils or Bunker C oils.
