GB2259712A

GB2259712A - Preparing fuel oil - water emulsion

Info

Publication number: GB2259712A
Application number: GB9218253A
Authority: GB
Inventors: Ievan Hopkin John
Original assignee: British Steel PLC
Current assignee: British Steel PLC
Priority date: 1991-09-18
Filing date: 1992-08-27
Publication date: 1993-03-24
Also published as: GB9218253D0

Abstract

The atomisation of a heated and pressurised heavy fuel oil in a burner is improved by emulsifying the oil with 5% or less of water by steam injection. Steam (6) and fuel oil are emulsified in an emulsifier (5) before passing to a burner 3. <IMAGE>

Description

IMPROVEMENT IN FUELS This invention relates to fuels for burners and in particular to improving the burning of fuel in burners designed for use with heavy fuel oil under pressure.

Combustion of heavy fuel oil is not an easy process to conduct efficiently and in an environmentally benign mode, owing to the fact that the heavy fuel oil usually used is the residual mixture left over when the more valuable and volatile components such as petrol and kerosene have been extracted from the crude oil at the refinery. Various techniques are employed to enable the heavy fuel oil to be burnt efficiently and generally the fuel must be preheated to a temperature of the order of llO"C and atomised. In some circumstances, where the oil is used to fire a boiler, a rotary spinning cup can be used to atomise the fuel oil, although these are associated usually with high capital and maintenance costs.Spinning cups are not generally suitable in process furnaces and often here auxiliary steam jets are used to further atomise the fuel oil after it has been ejected from the burner jet. Considerable quantities of steam can sometimes be employed in these circumstances, and figures of 0.3 to 0.4 kg of steam per kilogram of oil have been found to be necessary with high grade burners, and for burners of a lower quality equal amounts of steam and oil can be required to achieve sufficient atomisation.

It is therefore an object of the present invention to provide a means of treating heavy fuel oil so as to prove its atomisation in a burner.

According to the present invention there is provide a method of preparing a fuel oil for use in a burner to be supplied with pressurised preheated fuel by emulsifying the fuel oil with a substance which will cross its liquid vapour phase line in passing through a burner jet. The substance may be water, and the quantity of water in the emulsion may be 5% or less. The emulsion may be formed by the injection of steam into the fuel supply and the latent heat of the steam in condensing may be used to preheat the fuel supply. The emulsion may be generated and fed to a plurality of burners either associated within the same heating unit or in different heating units. The fuel oil may be a heavy fuel oil.

The invention will now be described with reference to the accompanying drawings of which Figure 1 is a schematic diagram of a fuel system feeding a burner in a process furnace, and Figure 2 is a phase diagram for water.

Turning to Figure 1, heavy fuel oil is stored in a tank 1 and is fed in the conventional manner, having been filtered by a filter 7, through a pump 2 to a burner 3. The purpose of the burner is to heat a furnace 4 which may for example contain steel slabs which are being reheated in preparation for rolling. These have not been shown. The burner 3 has been shown diagrammatically, since there are many designs of burner but in all cases to which the invention applies the fuel oil fed to the burner is under pressure. The fuel oil is released through jets in the burner into the furnace which is at substantially atmospheric pressure. The temperature in the furnace will in general be high, but the temperature of the fuel immediately after is has passed through the burner jet will not have risen substantially.Therefore the fuel oil is subjected to a relatively rapid decompression at constant temperature as it passed through the burner jet. The fuel oil has been preheated to about llO"C in the preheater 8.

So far this does not differ from normal technology. In Figure 1, however an emulsifier 5 has been inserted into the fuel supply line 9 and is fed with a supply of steam 6. The pressure of the steam is regulated to be about one bar above the pressure of 6 or 7 bars in the fuel supply line, and steam is injected into the fuel oil in a conventional manner so that it both heats the fuel oil and creates a liquid/liquid emulsion which is adjusted to be about 5% of water and 95% oil. The 5% emulsion at a pressure of six to seven bars and approximately 110-150"C is fed to the burner 3, which may be a standard burner such as can burn heavy fuel oil.

The action of the emulsion as it passes from the inside to the outside of the burner can be followed by looking at Figure 2. Figure 2 shows pressures - above atmospheric - against temperatures and line 10 divides the paper into areas 20 where water is a vapour and 21 where it is a liquid. Inspection of Figure 2 will show that the emulsion is in the range of temperatures and pressures inside rectangle A,B,C,D as fed to the burner. As it passes through the burner jets the pressure falls to substantially atmospheric. For example, for an emulsion that was at 1600C immediately before it passed through kthe burner jet the end point would be point L in figure 2 in Figure 2 and the temperature remains substantially constant as previously explained at least in the immediate facility of the burner. An instantaneous change of state of water takes place.The effect of this change of state causes the water to flash vaporise and in doing so it had been found that a very effective atomisation of the heavy fuel oil is achieved. The fuel oil has also been heated in part by the incoming steam that has created the emulsion and will burn very satisfactorily. Among the benefits that have been noticed for burners fired with such an emulsion are that the ignition lag has been reduced to almost zero, oil burn out rates have been increased and flames are shortened; the residence time of combustion products in the furnace chamber has been increased, thus in the case of a heating furnace improving the efficiency and the reducing temperature in the waste gas stack.

The reduction in the consumption of steam will be obvious from the figures already given. 5% addition of steam is used to create the emulsion; up to 100% steam was as stated previously sometimes needed to achieve adequate atomisation with a low grade burner. The emulsifier 5 is a conical injection type and it has been found possible to arrange such an emulsifier to provide a satisfactory emulsion of rates from 1800 to 3600kg of fuel oil per hour and only when the rate drops below 900kg per hour does the atomisation degrade to an unacceptable level.

Although the invention has been described by reference to a single burner fed from a single tank, one of the advantages of this method of treating fuel oil is that the supply may be emulsified at any point1 and therefore the supply to many burners from a single tank can be emulsified at a single point and the benefits of improved atomisation achieved by one modification to an existing system; the existing burners and pipe work remain intact and by the emulsifying the oil atomisation is achieved intrinsically when passing through a standard burner. It is of course possible to heat fuel as well by conventional means as by steam, and this may be necessary when it is necessary to keep the emulsion within the boundaries of the appropriate point on the phase diagram.

Although water has been used as the emulsifying liquid, and in most cases it is the clear choice, any other liquid that emulsified with oil, passed through the furnace without deleterious effects and had a high enthalpy change across the burner tip would be a candidate for use in such a system.

Claims

1. A method of preparing a fuel oil for use in a burner to be supplied with pressurised preheated fuel in which the fuel oil is emulsified with a substance with will cross its liquid-vapour phase line in passing through the burner jet.

2. A method as claimed in claim 1 in which the substance is water.

3. A method as claimed in claims 1 or 2 in which the quantity of water in the emulsion is less than 5%.

4. A method as claimed in claims 2 or 3 in which the emulsion is formed by the injection of steam into the fuel supply.

5. A method as claimed in claim 4 in which the latent heat of the steam is used to preheat the fuel supply.

6. A method as claimed in any preceding claim in which an emulsion formed at a single emulsion generating point is fed to a plurality of burners.

7. A method as claimed in any preceding claim in which the fuel oil is a heavy fuel oil.

8. A method substantially as herein before described and with reference to the accompanying drawings.