1.2 Use of combination fuels

1.2.1 Dual-Fuel (DF)

In this diesel process, a very small percentage of between 1 and 10% liquid heavy fuel or diesel oil is injected.

During the intake stroke, gas is admitted just before the inlet valves. The liquid fuel ignites the air/gas mixture.

The advantage is that the exhaust gases are cleaner than when heavy-fuel oil or diesel oil are used. The CO₂ production is reduced, since a gaseous fuel contains less carbon than a liquid fuel and the emission of soot particles and sulphur is significantly less. Therefore, the contamination of the engine decreases.

Some LNG-tankers are presently equipped with a propulsion engine that mainly runs on the gas from the cargo. In order to keep the cargo (the liquid gas) at a sufficiently low temperature, a very small part of the cargo evaporates. This evaporation requires heat, drawn out of the cargo. This way the liquid gas is kept at the desired low temperature of -180 °C and gaseous fuel becomes available for propulsion. Previously, this gas cargo was also used as fuel for the steam boiler of the steam-turbine propulsion installation. The efficiency of such propulsion installations is much lower than that of the dual-fuel engine.

Dependent on the distance of the voyage, propulsion uses approximately 10% of the cargo gas. When a tanker is not carrying cargo, the main engine can run on 100% heavy-fuel oil via the main injector. The separate ‘pilot’ injector (nozzle) is then switched off.

1.2.2 Gas-diesel (GD)

This is similar to the Dual-Fuel principle: the supply of very little liquid fuel, often less than 1% of the fuel consumption at full load and a substantial amount of gaseous fuel.

During the intake stroke, only air is drawn in and compressed. Just before TDC position, a small amount of liquid fuel is injected, immediately followed by the injection of the gaseous fuel.

If no gaseous fuel is present, the engine can run on liquid fuel. A disadvantage of this gas/diesel system in relation to the dual-fuel system is that the gas must be compressed to a very high pressure of approximately 350 bar, so it can be injected at the end of the compression stroke. This requires a considerable amount of power for gas compression.

1.2.3 Biofuels, such as olive oil, palm oil, rapeseed oil and other vegetable oils

In approximately 1900, Rudolf Diesel demonstrated a diesel engine that could run on peanut oil. In 2010, the interest in biofuels is huge. This is the result of the following two points.

1    Due to the increasingly stringent requirements with regard to the emission of noxious substances in the exhaust gases, amongst which the toxic greenhouse gas CO₂, carbon dioxide.

Biofuels absorb CO₂ from the atmosphere during their growth process; the CO₂ is converted to vegetable matter. This is known as photosynthesis. Photosynthesis is the conversion of, amongst others, CO₂, extracted from the atmosphere as a base material and light as an energy source to (energy containing) carbohydrates and O₂, oxygen, which is released into the atmosphere. This is known as ‘CO₂ neutral’. Therefore, when biofuels are burnt, no extra CO₂ is emitted, as is the case with fossil fuels.

2    Finitude of fossil fuels

Due to the explosive growth in the use of energy, the demand for mineral fuels has increased exponentially. Countries such as China, India, Brazil and Indonesia require an ever-increasing amount of energy for their industries. Due to this, the worldwide demand is increasing, as is the fuel price. It is becoming increasingly clear how much large industrial countries are dependent on oil-producing countries. The economic and political need for the development of alternative fuel sources is increasing and in the search for alternatives, subjects such as ‘less dependent on fossil fuels’ are ever more important.

Some biofuel properties

The most important biofuels are olive oil, palm stearine and refined palm oil. Rapeseed oil is at present being investigated in Western Europe. Some properties:

–    The pour point is high.

–    A large variation in ash content.

–    A large variation in acid number.

–    A low calorific value.

–    Very low sulphur content.

The following measures are necessary for the use of these fuels. 

–    Fuel systems: the storage tanks must be warmed and the fuel pre-heated before injection.

–    Fuels with high ash and acid content, which cause contamination and corrosion, must not be used.

–    The fuel-supply and high-pressure systems have larger dimensions as they must transport more fuel, due to the lower calorific value of biofuels in comparison with diesel and heavy-fuel oils.

–    Biofuels can vary in composition. This is dependent on the refining process and the source of the vegetable matter supplied for the production of biofuels. It is therefore advisable to obtain the biological matter from the food industry, as they are required to investigate the quality and composition of their raw materials supplied.

Waste oils: Furthermore, used oils, such as, deep-frying fat, will play a part as alternative fuel sources for diesel engines in the near future.

Source: Diesel Engines from Kees Kuiken, Target Global Energy Training.

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