Flarestack Combustion Method

Abstract

A burner for disposing of low pressure fuel gases with a steam inlet adapted to direct steam (originally at high pressure) to flow over a director surface so that entrainment of surrounding air occurs (Coanda effect) and towards a fuel gas inlet supplying low pressure fuel gas, the feed line for the fuel gas being an internal passage of the director body.

Description

This invention relates to a burner for disposing of waste combustible gas, and in particular it relates to the disposal of refinery waste gases.

Refinery and chemical plant operation often requires that a vessel is vented through pressure relief valves into a vent system running at near atmospheric pressure. Gas from this low pressure vent system is then disposed of by flaring from an elevated stack so as to aid the dispersion of any oxides of sulphur that may be formed during combustion.

Since the low pressure of the gas precludes the use of air entrainment devices, and the possible sulphur content makes ground level flaring in a natural draught flare impracticable, often the only way to improve combustion and reduce the amount of smoke formed during such emergency flaring operations is to add steam to the flared gas, which then reacts with any carbon particles by undergoing a water gas reaction, thus preventing smoke formation.

It is an object of this invention to improve the method of injecting steam.

According to the invention a burner for disposing of low pressure fuel gas comprises a fuel gas inlet adapted to supply fuel gas (originally at low pressure) to a combustion zone, a director body at one end of which the fuel inlet is situated, the director body having an external director surface and an internal passage for conveying fuel gas to the fuel gas inlet, the burner also comprising a steam inlet adapted to direct a stream of steam (originally at high pressure) over the director surface towards the fuel gas inlet the director surface being curved so as to initiate the flow of steam and air towards the fuel gas inlet.

Suitably the steam is supplied at a pressure in the range 70 kN/m.sup.2 to 300 kN/m.sup.2.

It is known that a stream of gas will "stick" to a suitably shaped surface and a curved surface can deflect a stream of gas and thereby produce a low pressure zone. In the burner according to the invention the initial portion of the director surface is shaped so as to deflect the flow of steam and to produce a low pressure zone into which atmospheric air flows.

Preferably the burner comprises a director tube the outer surface of which is the director surface and the bore of which is a fuel gas channel which terminates at the fuel inlet.

Preferably the initial portion of the director surface is the surface of revolution formed by the rotation of a quadrant of a circle about the longitudinal axis of the director body, the curved section of the quadrant being tangential to the steam inlet.

Flame stabilisation can be improved by fitting a wind deflector. A suitable deflector is disclosed in British Pat. No. 795,664.

According to another aspect of the present invention there is provided a method for disposing of low pressure fuel gases which method comprises inducing steam (originally at high pressure) to flow over a director surface which is shaped so as to cause the steam flow to initiate flow of steam and air towards a fuel inlet which supplies fuel gas (originally at low pressure) into the flow of steam and air.

The invention will now be described by way of example with reference to the accompany drawing which shows a burner according to the invention half in side view and half in cross section.

The burner shown comprises a director surface 10 which forms the outer surface of a director tube which has a steam inlet 11 for stream at its lower end and a secondary inlet 12 for fuel gas at its upper end. During use the steam flows over the director surface 10 and this flow initiates flow of steam and air towards the secondary inlet 12.

The director tube has a flat base 13 and the steam inlet 11 takes the form of a circular slot formed between the wall of the steam line 14 and the flat base 13 so that the steam leaves the steam inlet 11 as a thin horizontal sheet.

The director surface 10 comprises two portions, namely a deflector portion 15 which turns the direction of flow of the steam from the horizontal to vertical, and the continuation of the portion 16 which maintains the flow of steam and air between the deflector portion 15 and the fuel gas inlet 12. The purpose of this curved continuation 16 is to allow a suitable separation between the steam inlet 11 and the secondary inlet 12, while maintaining the skin effect up to the secondary inlet 12.

The shape of the deflector portion 15 is most conveniently specified as the surface of revolution formed by the rotation of a quadrant of a circle about the longitudinal axis of the director body, the curved section of the quadrant being tangential to the steam inlet; as shown in the drawing the distance between the axis of rotation and the centre of the quadrant is several times the radius of the quadrant. The continuation portion 16 is again a surface of revolution, but of an arc of larger radius than that of the quadrant, thus giving rise to a tapered portion.

As the steam flows around the deflector portion 15 its direction of flow is changed from (initially) horizontal to vertical. This induces a low pressure zone in the surrounding air and hence it induces movement of air as well as steam towards the secondary inlet 12.

The fuel is conveyed to the secondary inlet by the fuel gas line 17 (which forms an annular configuration with the steam manifold 14) and fuel which issues from the fuel line 17 meets the converging steam of steam and air moving over surface 16.

Claims

What we claim is:

1. A method for disposing of low pressure fuel gases, comprising the steps of:

causing said low pressure fuel gases to flow in a stream;

passing a stream of high pressure steam over a deflector surface which comprises a surface of revolution formed by the rotation of a quadrant of a circle and an inwardly curved continuation about the axis of the direction of flow of said low pressure fuel gases,

whereby said stream of high pressure steam is directed in a first direction generally perpendicular to the direction of flow of said stream of low pressure fuel gases and is thereafter caused to flow in the same general direction as the direction of flow of said stream of low pressure fuel gases but angularly thereto so that said stream of high pressure steam and said stream of low pressure fuel gases converge,

the change in the direction of flow of said stream of high pressure steam inducing a low pressure zone in the air surrounding said stream of high pressure steam to thereby entrain said surrounding air into said stream of high pressure steam, and

said steam, entrained air and fuel gases being mixed upon the respective streams of steam and fuel gases converging together; and

igniting and burning said mixture of steam, air and fuel gases.

2. A method as claimed in claim 1, wherein said steam is supplied at a pressure of between 70 and 300 kN/m.sup.2.