Fuel Oil

Fuel oil is classified as a hydrocarbon fuel. It is called that because it is composed principally of carbon and hydrogen.

Combustion of fuel oil, according to our previous definition, would be the rapid combining of carbon and hydrogen with oxygen.

This is a chemical reaction, which produces an entirely new product, called heat.

As we stated above, oil is composed of carbon and hydrogen the breakdown of typical No. 2 oil shows approximately 86% C and 14% H2 by weight, therefore, the air which is supplied to burn oil gives up its oxygen to form not only CO2 but also a compound with the hydrogen.

If exactly the right amount of air were supplied for complete combustion of the carbon and hydrogen in the fuel oil, the products of combustion would be heat and a mixture of other gasses and water.

With practical equipment, however, it is generally not possible to get a perfect mixture in which all of the combustible elements are supplied with the exactly correct quantity of oxygen, and therefore it is necessary to supply what is known as excess air.

This excess air is simply air over and above the theoretical requirement for the burning of the fuel.

It must be supplied because the equipment, which mixes the air with the fuel, is not capable of bringing oxygen into intimate enough contact with the molecules of fuel to produce complete combustion without some excess air being present.

The efficiency of the particular burner is indicated by the amount of excess air required to produce clean combustion. When more excess air is required to do the job, the efficiency of the unit is lower.

This excess air takes no part in the combustion process as there is Nitrogen as well as Oxygen this Nitrogen plus other inert gasses enters the furnace at ambient temperature and is discharged at the temperature of the exit gases and carries away that much heat.

So the above tells us what happens when we have a mixture of fuel and air in the correct proportions and what it produces.

Now we must decide which fuel to use and why

All fuels are rated on their heat content, that is, the amount of heat generated when a given quantity is 100% consumed by the chemical process called "burning". This heat is measured by the amount of energy needed to raise one pound of water one degree, called British Thermal Unit or (BTU). Heating oil contains 140,000 BTU's per gallon, whereas propane contains only 91,300 BTU's per gallon. Therefore, it takes 1.53 gallons of propane to provide the same amount of heat as one gallon of heating oil, whilst this statement is true, it fails to take into account the fact that gas requires considerably less air to be 100% burnt and the resulting heat is not wasted in the exhaust gasses but is put to good use.

By contrast, Fuel oil will not burn if a match is dropped onto liquid fuel, gas on the other hand is difficult to find naturally in a liquid state to perform this test but if you did then you would need new eyebrows,

This means that the fuel oil needs to be vaproised first in order to provide a suitable gas like vapour to burn cleanly

Fuels and Ignition Temperatures

Ignition temperature:  The minimum temperature required to ignite gas or oil vapour in air without a spark or flame being present. 

Fuel	oC
Acetylene	305
Benzene	415
Bituminous coal	300
Butane	420
Carbon	700
Carbon monoxide	300
Coal-tar oil	580
Coke	700
Ethane	515
Heavy hydrocarbons	750
Hydrogen	500
Light gas	600
Light hydrocarbons	650
Methane	580
Naphtha	550
Natural gas	600
Peat	227
Petroleum	400
Producer gas	750
Propane	480
Semi anthracite coal	400
Wood	300

Calorific value of fuels

The major problem with fuel oil is that it requires much more preparation to be combustible before it will ignite.

This means that the oil either needs to be heated to the point where it vaporises or forced into a mist that closely approximates gas vapour, This requires either the fuel line to be coiled and be in direct contact with a heat source to force the fuel to vaporise when exiting the nozzle which also requires that it be kept clean.

Also a cold furnace would be difficult and time consuming to start, if a mist spray approach were to be used then extra equipment would need to be installed, with the required necessary servicing and maintenance.

To mix the fuel oil with air into correct proportions there are several tried and tested means for doing this.

• Drip fuel into a heated pot where it will vaporise, ignite, and thus heat the pot, blown air is also added at this point this will improve the combustion and provide heat output. This method is used but it is very difficult to regulate the heat output it will be all or nothing and the flame will be orange smoky and sooty.

• Pressurise the fuel oil and pass through a coil of heated pipe to a jet, this method has been used for a long time with kerosene lamps and blowlamps and we have all heard of the odd explosion with this type of apparatus. It also has the disadvantage that it is difficult to heat the oil sufficiently to produce vapour, the heating coil will by nature of the device be close to the heat of the furnace, and the hotter the furnace gets the less heat is required to vaporise the fuel, this method is also difficult to regulate as there is a severe risk that should the pressure drop then the oil will rise above the ignition temperature in the heating area with severe risk of explosion and fracture, this would result in  a fuel leak close to a source of heat. however This method will produce a reasonable heat output without too much smoke and soot,  with careful tinkering this method should produce a usable heat output but it is difficult to control.

• The most popular tried and tested method to burn fuel oil, is to spray the oil in to a mist and ignite directly inside the furnace, this method is in use in many central heating boilers around the world and is the easiest to control, unfortunately extra equipment is required to perform this task

As it can be seen from the tables above that the oil will produce more Btu’s than gas, for a given quantity of fuel, but a greater quantity of air is required to do this and as already been stated this air will remove heat from the furnace

After a lot and I mean a lot of web trawling I have come up with the answer to the eternal question of life the universe and every thing....oh sorry wrong story,