Marine Boilers Development

Fuel Oil Atomizating

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Fuels derived from crude oil are graded as either distillates or residual. Distillates are produced by the distillation of the crude oil in a fractionating column under atmospheric or sub-atmospheric pressure. Residual fuels are produced from the residue left in the column after the lighter fractions have been distilled off. This residue is reduced in viscosity by dilution with lighter distillates, which allows the material to be pumped and stored with ease. The amount of dilution or viscosity is the basis upon which the fuel is graded. For many years, the fuel oil industry has marketed residual fuel oils in grades according to the viscosity of the fuel as measured by the Redwood Viscometer.

The burning of liquid fuels has developed into a precise science over the recent years. Research into methods of reducing atmospheric pollution has resulted in great advances being made in the study of oil flames, with design improvements to combustion apparatus following perhaps at a slower rate. Nevertheless, it is possible to purchase equipment for firing the heaviest grades of fuel under fully automatic control with low excess air requirements of 10% to 15% and with unburnt Carbon down to as low as 100 mg/m³ of flue gas. (approx. 0.1% by weight of the fuel burnt).

Several types of burner are available and are usually categorised as follows:
Pressure Jet Burner. The fuel is Atomized by the use of a high pressure pump forcing the preheated heavy oil through an Atomizer. The pressure energy alone disrupts the oil into fine particles. This type of Atomizer has a poor turn down ratio. A small drop in Atomizing pressure will result in a dramatic reduction in combustion performance. Two nozzles are sometimes fitted to achieve turn down, one in use on low fire with two for the full rate, the oil pressure being maintained constant. A variation of the pressure jet Atomizer is the spill return or recirculating burner. The problems with this type of burner are that the Atomizer has an increasing cone angle of the issuing spray as the burner is turned down with impingement on the furnace walls, being made possible and the additional problem of how to dispose of the returned hot oil if the burner has been running for prolonged periods on low fire. Cavitation or gasification takes place in the oil at the oil pump suction if the oil is so returned. Equally, one can return it to the tank, but this does not offer a sound solution as the tank can become overheated, especially if the stock level is low.

Twin fluid Atomizers. In this class one finds all the burners which require the use of a second fluid to achieve the Atomization of the oil into fine micron sized particles. The most common second fluid is air. Air Atomizing burners are classified by the pressure delicate construction. High pressure air burners are seldom used other than for start up duties. The prohibitive cost of compressed air tends to result in high running cost for this type of burner.

The use of steam as the second fluid for Atomization has been steadily gaining in popularity over the past decade. The advent of more viscous grades of fuel has thrown up the fact that Atomization from certain types of burner, in particular the pressure jet unit, does not perform well unless the fuel is maintained at a firmly set viscosity. Should the viscosity vary then the Atomization and throughput from the pressure jet type of burner changes to the detriment of the combustion as a whole.
Steam Atomized Burners were more tolerant to these changes. A variation occurred, but not so marked as with e pressure jet Atomizer. In addition to this advantage, the steam Atomized burner had better turn down, did not require high fuel oil pump pressures, and was frugal in the use of steam. The development work to reduce the specific steam consumption of the burner had been so successful that the marine industry took up the use of steam Atomized burners whereas in the past, the use of valuable fresh water at sea had meant only pressure jet burners were used.

One particular type of burner which is of the twin fluid type, yet has a novel method of presenting the oil to the Atomizing air is the so called Rotary Cup Burner. As the name implies the burner comprises a horizontal shaft running at about 5,000 rpm and having at one end of the shaft an Atomizer which is a hollow cup into which is fed the fuel to be Atomized. The oil travels down the cup under the combined forces of gravity and centrifugal force until it comes to the cup lip. The Atomizing air is so guided as to impinge upon the edge of the cup and Atomize the oil sheet which is leaving the cup. In this fashion very fine Atomization is achieved. The rotary burner finds considerable use on packaged shell type boilers. It has a good turn-down ratio of 4 to 1 and is simple to automate.