This invention relates to a process for the partial combustion of solid fuel in particulate form and to a burner for carrying out such a process.

The efficient combustion of particulate fuels presents rather different problems from those associated with liquid fuels. For example, apart from the pure handling difficulties, the fact that the particle size is fixed and that the heat input to a solid fuel has to be much higher to sustain combustion has meant that there is no really effective solid fuel burner available which will operate with a short, stable flame.

An object of the present invention is to provide a process for the efficient partial combustion of a solid fuel in particulate form and a burner for carrying out such a process.

In accordance with the invention a process for the combustion of solid fuel in particulate form comprises injecting the fuel centrally in a stream into a pre-mix zone in which it encounters a plurality of streams of a primary supply of oxygen or oxygen-containing gas which impinge on it at an angle of between 30 and 60° relative to the axis of the flow of the fuel and at a velocity in excess of that of the fuel so that they penetrate the fuel stream, a secondary supply of oxygen or oxygen-containing gas being introduced into the pre-mix zone in the vicinity of the primary supply and at a velocity in excess of that of the fuel so that it forms a shroud of gas around the fuel, as the mixture of fuel and oxygen or oxygen-containing gas leaves the pre-mix zone through a converging-diverging nozzle in order to enter the combustion zone.

In operation no combustion takes place in the pre-mix zone, even in the case of the gas for combustion being oxygen under pressure. This is due to the very short residence time in the pre-mix zone, which is not long enough for sufficient heat to be transferred to the fuel to enable the more volatile components, which are necessary for combustion to commence, to he released. The velocity and distribution of the particles must therefore be such as to prevent any premature combustion in the pre-mix chamber. The converging-diverging nozzle is also designed to provide an effective screen against radiation in order to supplement that provided by the dense cloud of particles leaving the nozzle.

On leaving the nozzle the outer shroud of gas comes into contact with hot combustion products which also contain some unburned matter or gases. The latter burn with the gas shroud which as a result tends to turn inwardly into the cloud of particles. The velocity of the gas shroud being greater than that of the particles, it causes the latter to heat up very rapidly. The resulting volatile components which are thus given off then enable combustion of the solid fuel to begin. Once started, the combustion is rapid and self propagating due to the ready availability of oxygen or oxygen-containing gas at the centre of the particle stream. The flame is thus short and the combustion efficient and stable.

In the case of partial combustion of coal for gasification, on leaving the burner the combined stream of coal and oxygen or oxygen containing gas enters directly into a partial oxydation reactor. Once in the reactor the shroud of oxygen or oxygen containing gas comes into contact with hot reactor gases which start to burn. The resulting burning gases are deflected radially inwardly into contact with the fuel particles. This provokes rapid heat transfer resulting in stable combustion of the fuel particles and producing a short, hot flame. The rapid combustion is useful in that it reduces the required reactor volume necessary for gasification to take place. It also makes better use of the available oxygen by reducing the proportion of the oxygen which is lost due to complete combustion of the solid fuel or with the reactor gas.

Due to slip between the fuel particles and the gas for combustion it is not necessary that a high degree of swirl be imparted to the gas or to the fuel. ("Swirl" in this specification is defined as the non-dimensional quotient of the axial flux of the tangential momentum and to the axial flux of the axial momentum times the radius at the exit of the burner, taken at the exit of the burner.) In the process according to the invention the swirl is preferably between 0 and 1.1.

The invention extends to a burner for the combustion of fuel in particulate form comprising a pre-mix chamber having primary and secondary combustion gas inlets situated around a fuel inlet port which is disposed in the same axis as an outlet in the form of a converging-diverging nozzle, the primary gas inlets being directed radially inwardly at an angle of between 30° and 60⁰ to the axis and the secondary inlet or inlets being arranged so that in operation they provide a shroud of gas around fuel leaving the nozzle.

The secondary inlet or inlets is/are preferably situated outside the primary inlets and are at an angle of between 0 and 30 to the axis.

Whilst from a practical point of view it is simplest to form the inlets by drilling holes of the desired dimensions, in an alternative, and very effective form of the burner, the secondary inlet comprises an annular slit, or series of slits forming an annulus, in the wall of the pre-mix chamber. The disposition of the secondary inlet(s) may equally be arranged to impart a rotation of the secondary supply of gas, for example by forming them at a skew to the axis in the case of individual ports, or by fitting swirl vanes in the annular slit or slits, according to the construction of the burner.

In order to facilitate the siting of the gas inlets the wall of the pre-mix chamber diverges outwardly from the fuel inlet, and the gas inlets are formed in it. The wall may conveniently be at an angle of from 30 to 60° with respect to the axis (though in the opposite sense to that of the inclination of the primary inletsl. In its most convenient form the said wall is conical, but it may also he in the form of any concave or convex surface of revolution, or polygon, either continuous or stepped, according to normal design considerations for flame stabilisation.

The diverging section of the nozzle will normally form the mouth of the burner, which may be between 30 and 60° to the axis and from 0.5 to 2D in length, where D is the diameter of the throat of the nozzle.

The mouth may also be formed in such a way as to induce a higher swirl. One particularly suitable form is in the shape of a tulip with a sharp angle of between the throat and the beginning of the mouth and a smooth transition to a substantially conical exit. The transition may have a radius of from 0.25D to 0.6D and may be between 70° and 120°.

In order to avoid the risk of pre-combustion taking place inside the pre-mix chamber of the burner the length of the chamber measured from the fuel inlet to the start of the mouth should not be more than 3D. Its minimum length is governed by the physical constraint in providing the space for good fuel distribution in the pre-mix chamber and in practice it will not be less than about 1D.

For satisfactory operation of the burner in accordance with the invention the various inlet velocities and pressure should be controlled so that the swirl is between 0 and 1.1. This will generally imply an optimum average stream velocity at this point of 70 m/s though the necessary conditions may well be met at velocities over the range 35 to 100 m/s in a typical burner.

In most cases the fuel will be delivered to the burner using a transport gas which is inert to the fuel particles. This may be either recycled reactor gas, C0₂ nitrogen or steam, or a mixture of two or three of the said gases.

The invention will now be further described by way of example with. reference to the accompanying drawing which is a sectional side elevation of a burner in accordance with the invention for the partial combustion of fuel in particulate form. Whilst the burner is symmetrical, for convenience here two different forms of quarl have been illustrated respectively above and below the axis.

The burner JO comprises a pre-mix chamber 12 having primary 14 and secondary 16 combustion gas inlets situated around a fuel inlet port J8.

An outlet 20 to the pre-mix chamber is provided on the opposite side of the pre-mix chamber from the fuel inlet port and is disposed co-axially with it. The outlet is in the form of a converging-diverging nozzle having a converging section 22 and a diverging section 24 separated by a throat 26 of diameter D.

The diverging section 24 of the nozzle which is the mouth of the burner has the function of controlling the expansion of the gases and solids as they leave the burner and enter the reaction chamber (not shown in detail, but situated at 281. Its half-angle should be between 30 and 60° to the axis 30 of the burner depending upon the exit velocity and scale of the burner. The mouth shown in the upper part of the drawing has an angle a of 45°.

The mouth 24^J shown in the lower part of the drawing is tulip-shaped and makes an angle φ with the throat of the burner. It then has a smooth transition of radius R to a conical portion of half-angle α^1. In the burner draian φ is 95⁰and R is 0.5D; a¹ is 45° as in the straight mouth 24.

The length of the mouth is also important in preventing premature mixing with hot reactor gases and promoting turbulence in the gas-fuel mixture. Its maximum length L will be approximately 3D. A minimum length L of at least ½D is necessary in order to obtain the necessary turbulence near the exit of the burner and to protect the premix chamber from excessive heat transfer from the flame and reactor gases.

The nose 36 of the burner, which contains the mouth 24 is subjected to a considerable heat flux and needs to be cooled. The coolant flow is indicated by arrows 32, 34.

An important aspect of the burner resides in the deposition of the combustion gas inlets J4, 16. The inlets are connected with a gas supply, preferably of oxygen or an oxygen/steam mixture, via an annular duct 38.

The primary gas inlets are inclined at 45° to the axis 30 as is indicated by the angle β. The purpose of these inlets is to break up the stream of fuel particles emerging from the fuel port 18. The velocity of the gas must be such as to penetrate the stream but not to reemerge on the opposite side of it. It is important that it remains within the particle stream, though still moving at a higher velocity. In the burner shown, there are 4 primary inlets 14 which are situated adjacent to the fuel inlet port 18. The value of 45° has been found to be the optimum for the angle βin the embodiment shown.

The secondary gas inlets 16 are inclined at approximately 17 to the axis 30 (the angle is indicated by δ in the drawing). The angle δ and the deposition of the inlets 16, of which there are 8 is important. Here they are situated further from the fuel port 18 than the primary inlets 34 and are arranged so that in operation they substantially provide a shroud of gas around the fuel particles in the nozzle throat 26. As explained above the shroud not only performs the initiation of the combustion of the particles but also reduces the mechanical abrasion on the nozzle throat 26. As shown the secondary inlets are aligned with the inner side of the throat 26 and converge on the axis 30, i.e. they are not askew to it.

The premix chamber 12 which is considered to extend from the fuel inlet port 38 to the end of the throat 26, indicated by reference 40. Its length, indicated by M, should be between 1 and 3D in order to provide sufficient mixing time whilst not being so long that the fuel particles can be accelerated by the faster moving gas to such a point that the all important slip between the two phases is lost, nor the fuel from becoming so hot that the volatile components begin to be released, which could result in precombustion. In the burner shown M is approximately 1.4D.

As shown, the burner is designed for ground coal whose dimensions are consistant with normal power station milling, e.g. Sauter mean diameter of approximately 50 to 75 micron.

The coal particles. will normally be injected in combination with a small quantity of transport gas which may be steam, C0₂, nitrogen or reactor gas for the producticn of hydrogen or CO/H₂ mixtures by partial oxidation. The latter solution has the advantage that it avoids dilution of the reactor products with an inert transport gas.

The burner is designed for a mean outlet velocity of 70 m/s at full load. This permits the burner to operate at a turndown ratio of 2 at 35 m/s. Slight overload may be obtained by increasing the velocity up to 100 m/s. As shown the burner is designed to operate at a reactor pressure typically of 10 to 60 bar.