Part 1: Fuel Oil Blends and Applications
Welcome to the first in a three-part presentation on current trends in fuel oils and fuel oil contamination. EDT hopes you will come away from this series with better knowledge of oil as a fuel, what can contaminate fuel oil, the damages caused by contaminants, and means to prevent or identify the fuel contamination. This part of the series will cover what fuel oil is and where it is used.
When powering machinery, engineers have a limited number of viable options for the source of energy – electricity, hydraulics, air (pneumatics), or combustion (burning) of fuel. Fuel combustion is often chosen for mobile applications. While battery technology has improved to the point where the energy density can sustain motion for an acceptable period of time, combustion is still ‘king of the road.’ It’s the potential energy-to-weight (energy density) advantage – especially for liquid fuels – which will keep fuel combustion as a major source of mechanical energy for some time to come.
Fuels used for combustion have been around for a long time, and in many forms. Wood, coal, oil, gasoline, compressed hydrocarbon gases, and alcohol have all found favor at different times and in different applications. When the application needs a fuel with the highest energy density and the safest delivery, fuel oils are the ‘go-to’ combustion fuel.
The Fuel Oil Advantage
Most of us are familiar with fueling our cars. We go to the ‘gas’ station, pay the attendant (or his electronic substitute), pump the fuel, and fill our car’s tank with gasoline. Technically, gasoline is a fuel oil – it is a fuel derived from the fractional distilling, or ‘cracking,’ of raw petroleum oil. As a fuel, it has a high energy density and can be easily ignited at a lower temperature. This low flashpoint – low temperature of vaporization – can be hazardous to transport as the vapors can be ignited at room temperature if spilled.
Some of you may pump a different type of fuel oil into your cars – diesel fuel. Actually, the fuel is not ‘diesel’ fuel – your engine is running the Diesel cycle and the fuel oil you’re supplying the engine can ignite during the compression cycle of the engine. The fuel oil used is actually a blend of various oils, targeted for a specific flash point which allows combustion at a high compression in a lean air-fuel mixture. These fuel oils are safer to transport – they do not vaporize easily – and less fuel is needed for the same power, making them excellent fuels for mobile applications.
Oil as a Fuel
Fuel oil is not a single composition of petroleum. In the United States (U.S.), there are six grades of fuel oil. The grading system tries to distinguish the relative flash point of each level of fuel oil. Number 1 fuel oil would ‘flash’ off the raw petroleum soonest and Number 6 is the stuff left over (see chart). Over time, trade names have been assigned:
Low Sulphur, or Diesel. This is the fuel oil used in most passenger cars and highway tractor trucks. It is roughly equivalent to the Number 2 fuel oil on the U.S. grading scale. Diesel fuel differs from number 2 fuel oil in the amount of cetane in the fuel.
Gas Oil. Gas oil is a pure distillate of petroleum at the Number 2 grade. Previously used for home heating, it is also known as Bunker A or Marine Gas Oil (MGO). It also has the lowest flash point of any commonly used fuel oil (43° Celsius).
Distillate. Distillate, or Navy Distillate, is close to Number 3 fuel oil. It is one of the last fuel oils to distill from the petroleum. It has a reasonably high flash point and usually has very low Sulphur or other contaminants. It is the fuel oil of choice for the U.S. Navy; it can be used as both the fuel for the ship propulsion turbines of smaller warships and for aircraft – saving room by allowing the same fuel tanks to be used for multiple purposes!
Marine Diesel Oil. Marine diesel oil, or MDO, is a blend of distillate and residual fuel oils. It has a flash point similar to the residual fuel oils, but can be pumped at ambient temperature – well, above 6° Celsius – and thus does not need heating prior to combustion. It falls close to Number 4 fuel oil on the U.S. grading scale.
Intermediate Fuel Oil. As can be imagined, intermediate fuel oil (IFO) is a grade of fuel oil between the distillates and heavy fuel oil. Intermediate fuel oils actually fall into a range of blends, which are differentiated by the viscosity of the fuel at 50° Celsius – 10° Celsius below the flash point. The two most common are IFO 180 at 180 centistokes (Cst) and IFO 380 at 380 centistokes (Number 4 and 5 fuel oils, respectively).
Heavy Fuel Oil. Heavy fuel oil is, literally, the bottom of the barrel. Known as Bunker C or just Bunker oil, it actually ends up being one of the fuel oils in greatest use. This highly viscous fuel oil, when heated, can be used for combustion in the large diesel engines found in ships, power plants, and as a fuel for power boilers. Heavy fuel oil is Number 6 fuel oil.
Bio-Fuels. Commonly referred to as bio-diesel – again (and for the last time), Diesel is a cycle, not a fuel – it is a marketing misnomer in most forms. While some engines can burn nearly any liquid fuel, bio-fuels are usually blends of 75-85 percent low sulphur fuel oil and the balance some vegetable oil or rendered fat. So, reality is these are bio-blends, just like adding 15 percent ethanol to your gasoline.
Fuel Oil Applications
Fuel oils are used for fuel where their high flash point can be used to an advantage. This will be either engines, boilers, or other combustion powered cycles.
Otto Cycle. The Otto cycle is the common combustion cycle for fuel oil which is not ignited through compression. This includes gasoline, alcohol, and compressed hydrocarbon gases – Liquefied Natural Gas and Liquefied Petroleum Gas.
Diesel Cycle. The Diesel cycle can produce enough energy in compression to ignite high flash point fuels. Diesel cycle engines appear similar in arrangement to Otto cycle engines – the typical gasoline fuel cycle. Diesel cycle engines have heavier cylinder walls, longer piston strokes, and often larger piston skirts to support the higher compression.
Boilers. Boilers are used for heating other media – usually water – for energy transfer. Power boilers are an industrial application creating steam for other processes by introducing the fuel oil into an existing flame to heat and vaporize water into steam.
Gas Turbine Engines. Gas turbine engines also sustain combustion by introducing fuel oil into an existing flame. The output energy density of the air is increased by heating the air with a continuous flame and quickly expanding release of energy which, in this case, turns turbine fans in the exhaust. Many land-based turbines operate on compressed natural gas, with most mobile installations using fuel oil.
Sterling Cycle. The Sterling cycle is an interesting spin on continuous combustion – the working medium (an inert gas or liquid) cycles between the area heated by combustion, expanding the medium, which then cools at the end of the stoke, to be compressed and reheated. The Stirling cycle is, in effect, an external combustion engine.