Pulverized-coal burner

Abstract

 A burner adapted for use with pulverised coal has an annular swirling coal and primary air supply (2) located inside a swirling annular secondary air supply (3) with a hollow rotationally summetrical flame stabil­ising body (7) located centrally of the swirling coal/air streams. The flame stabilising body can include a ring of nozzles (24) on a domed downstream end (8) for egress of start-up fuel therefrom.

Description

Technical Field

[0001] This invention relates to an improved form of coal burner which, inter alia, can be used to replace an oil or gas burner in a stove, furnace, boiler or the like heating plant.

Discussion of Prior Art

[0002] Coal combustion in its pulverised form has been performed for many years using complex arrangements in the burner to mix the ground coal and combustion air and thus produce a stable flame. The known burners gener­ally require extensive refractory surfaces, multiple air ports and special arrangements for light up and support fuel supply.

[0003] Most known burners have been designed for large industrial or power plant installations and are difficult to scale down to operate successfully below a fuel supply rate of a few tonnes per hour of coal. The known burners are generally also somewhat inflexible having an operating range only down to approximately 40% of design capacity.

[0004] Internal designs of some burners to induce either swirl or high velocities in the coal/air feed also result in blockages and coal holdups in the feed pipes which can result in fires. An example of a design including a turbine rotor contacted by pulverised coal can be found in GB-A-268417.

[0005] Pulverised coal is increasingly being looked at as a replacement fuel for oil and gas and being fluent, should be able to be automatically fed to the flame zone of a burner in the same way as oil and gas. In practice, however, there are problems posed by the use of pulverised coal which militate against its adoption as a substitute fuel for oil and gas. Among these problems may be men­tioned the following.

1. Storage of coal pulverised to the fineness re­quired for use in a burner (i.e. at least 80% w/w less than 75 microns), is much more dangerous than the storage of oil or gas, unless due care is taken to reduce the risk of spontaneous combustion arising in the coal.

2. The coal, although capable of flowing freely, has a tendency to form blockages in the ducts provided for its conveyance. This loss of free-flowing properties is particularly noticeable if the moisture content of the coal is above 1½ - 2% w/w.

3. The pulverised coal is less easy to ignite than oil or gas and it is less easy to maintain a steady flame zone as the fuel supply drops below that required to maintain the designed thermal output rating.

[0006] Unless the above-noted problems can be solved, the ability to replace oil and gas burners with coal burners will continue to be restricted.

[0007] One aim of this invention is to provide a burner suitable for design coal throughputs of 500 - 3000 Kg/hour (but not limited to these figures) and overcoming most of the problems mentioned above.

[0008] This invention relates to a coal burner which seeks to provide solutions to problems 2 and 3 above and which can be used in a coal fired system to provide an answer to problem 3.

[0009] GB-A-253684 discloses a coal burner comprising three concentric ducts, the inner of which serves as a supply duct for a start-up fuel, the middle of which serves as a duct for pulverised coal and the outer of which serves as a duct for combustion air. The downstream end of the middle duct is partially obstructed by an outwardly tapering body which defines with the middle duct an annular outlet through which annular outlet the coal flows into the path of the combustion air flowing out of the downstream end of the outer duct, the outer duct including swirl-generating means to cause the combus­tion air to swirl around the downstream end of the middle duct.

Summary of the Invention

[0010] According to one aspect of the invention, a coal burner of the kind just described is characterized in that the middle duct includes means to cause the coal to swirl as it enters the swirling combustion air.

[0011] Conveniently the flame stabilising body is a hollow member in the form of a body of revolution having a frusto-­conical portion (e.g. with semi-angle 30°) partially obstructing the middle duct and a downstream domed end provided with a ring of nozzles or spray device for egress of the start-up fuel therefrom.

[0012] Preferably the swirl-generating means associated with the outer duct is a ring of vanes, each of which vanes is angled relative to a respective axial plane of the burner and interposed between a windbox for the combustion air and an inwardly tapering section of the outer duct.

[0013] Suitably the swirling movement of the coal in the middle duct is created by providing the latter, at its upstream end, with an enlarged inlet region which tapers down to a circular cylindrical tube defining the down­stream end of the middle duct, the coal supply to such an enlarged inlet region of the middle duct being tangen­tially directed so that a turning movement is imparted to the coal as it enters the middle duct.

[0014] By mounting the inner duct so that its position can be adjusted axially with respect to the middle duct, the flame stabilising body can serve as a coal valve controlling the size of the annular outlet through which the coal flows outwardly into the flow of combustion air.

[0015] An electrical igniter for the start-up fuel can pass centrally through the inner tube and the stabilising body mounted at the downstream end thereof.

[0016] When a burner in accordance with the invention is mounted in an opening in a side wall of a boiler, furnace, oven or other plant to be heated by the burner (e.g. as a replacement for a gas- or oil-fired burner), the opening in which it is located will be lined with refrac­tory material. Usually this refractory lining will define a frusto-conical recess tapering away from the burner. The refractory lining plays an important role in stabilis­ing the coal flame, by maintaining a high temperature in the flame zone and controlling the spread of the pro­ducts of combustion away from the burner. With prior art coal burners it has frequently been necessary to increase the thickness of the layer of refractory material that surrounds the burner when a coal burner is used as a replacement for an oil- or gas-fired burner. Increas­ing the thickness of the refractory layer can be an expen­sive modification discouraging the change-over to coal firing. With preferred forms of burner in accordance with this invention we have found it is not necessary to modify the refractory lining in this way and this is an important further advantage which results from the invention.

Brief Description of the Drawings

[0017] The invention will now be fruther described, by way of example, with reference to the accompanying draw­ings, in which:

Figure 1 is a sectional side elevation of a burner in accordance with the invention, and

Figure 2 is a section on the line II-II of Figure 1.

Description of the Preferred Embodiment

[0018] The burner illustrated comprises an inner tube 1, a middle tube 2 and an outer tube 3. The burner is bolted onto a side wall 4 of a boiler having a refractory lining 5 defining a frusto-conical recess 6 into which the burner projects.

[0019] The inner tube 1 terminates in a bluff body stabil­iser 7 having a domed end 8, a frusto-conical region 9 and a sleeve 10 by which it is secured to the inner tube 1. The semi-angle of the region 9 is 30° in the burner illustrated, although this angle can lie anywhere in the range 15° to 45°. The semi-angle of the recess 6 is in the range 0-60° and although it is convenient to have the conical faces 9 and 7 parallel, it is not essential for this to be so.

[0020] The middle tube 2 has an enlarged inlet portion 11, fed by a tangentially directed inlet pipe 12, which tapers down , via an intermediate section 13, to a circu­lar cylindrical section 14. Because of the partial obstruction of the downstream end of the section 14 by the stabiliser 7, an annular outlet opening 15 is formed.

[0021] The outer tube 3 is located within a windbox 16, fed with combustion air via an inlet 17, which communi­cates with the interior of the tube 3 via a swirl ring 18 comprising a plurality of vanes 19. The angle each vane 19 makes with an axial plane of the tube 3 is adjust­able so that the rate of swirl of the air in the tube 3 can be varied. From the swirl ring 18, the tube 3 tapers down via a frusto-conical section 20 to a circular cylindrical outer portion 21 which terminates in a flange 22 by which the burner is bolted to the wall 4.

[0022] The inner tube 1 is fed with natural gas (although other hydrocarbon gases or even oil could be used) via an inlet 23 and the gas leaves the tube 1 via a ring of openings 24 in the stabiliser 7. The gas leaving the openings 24 is ignited via a spark generating or hot-wire igniter 25 which passes centrally through the tube 1 and is powered, when required, via leads 26.

[0023] The tube 1, with its attached stabiliser 7 and con­tained igniter 25, can be moved axially within the tube 2 in the directions of the arrows A and can be locked in its adjusted position by a clamp ring 27.

[0024] Pulverised coal and primary air are fed into the middle tube 2 via the inlet pipe 12 and flow down around the inner tube 1 to issue in a swirling diverging spray of particles from the annular opening 15, directly into the swirling secondary air flowing out from the downstream end of the outer tube 3. Because of the smoothly varying diameter of the section 13 and the region 9 and the chosen ratio of coal and primary air fed to the pipe 12, coal blockages in the middle tube 2 can be avoided even if there is a substantial moisture content in the coal. In practice moisture contents up to 16% w/w of the coal can be accepted without problems arising.

[0025] The particle size of the coal does not appear to be critical and the burner illustrated has operated successfully with normally pulverised coal"Standard PF" with a grain size such that 80% passes through a 80 micron screen) and with ultrafine or "Microground PF" (80% pass­ing through a 40 micron screen) and it is expected all degrees of pulverisation between these grades can also be used without problems arising.

[0026] To use the burner, the start-up gas would be ignited and when the flame zone (in the recess 6) has acquired a temperature of some 300-600°C, coal feed can be comm­enced with supply of secondary air and the burner tempera­ture built up to the designed level e.g. in the range up to 1300°C. When coal firing is operating in a stabilised manner (which can be viewed via a peephole 27) the gas supply can be turned off. A flame viewing head and swivel assembly 28 can be used in conventional manner to provide automatic control of the coal supply, preventing the latter until a required temperature is attained in the flame zone and shutting off coal supply if combustion ceases.

[0027] The large flow sections and smooth flow paths in the burner eliminate any coal deposition problems. This has been found to be true even when conveying and burning ground coal with a moisture content of up to 16% w/w.

[0028] The design of the bluff body stabiliser 7 produces a combined gas and oil pilot and light-up device which is also capable of providing the full thermal designed output from the burner should coal not be available, by feeding oil or gas continuously through the tube 1.

[0029] The ignition system operates through the centre of the bluff body stabiliser 7 where four activities are combined in one entrance to the burner namely, ignition, pilot, pilot/alternative fuel supply and stabiliser.

[0030] Due to the simplicity of operation, the high swirl generated in both coal and air and subsequent close con­trol of the flame, the quantity of refractory material 5 required to stabilise the flame is reduced to a minimum (less than half the amount of refractory material found to be necessary with competitive known burners).

[0031] One drawback with many known pulverised coal burners is their inablity to operate at rates much below the designed thermal output. The illustrated burner can be operated down to 20% of its design capacity - under certain conditions down to 10% of design capacity - with­out any need to preheat the combustion air.

[0032] The secondary combustion air can swirl in the same or the opposite direction to that chosen for the coal and primary air and desirably the vanes 19 allow such a range of position adjustment that a reversal of direc­tion of rotation of the swirling secondary air is possible. In addition to changing the direction of rotation of the swirling secondary air, the speed of swirl rotation can be altered, an adjustment facility which permits a burner operation to be optimised to fuel quality, degree of fineness and power output of the burner.

Claims

1. A coal burner comprising three concentric ducts, the inner of which serves as a supply duct for a start-­up fuel, the middle of which serves as a duct for pulver­ised coal and the outer of which serves as a duct for combustion air, the middle duct being partially obstructed at its downstream end by an outwardly tapering body mounted on the inner duct and defining with the middle duct an anular outlet through which annular outlet the coal flows into the path of the combustion air flowing out of the downstream end of the outer duct, the outer duct including swirl-generating means to cause the com­bustion air to swirl around the downstream end of the middle duct, characterised in that the middle duct in­cludes means to cause the coal to swirl as it enters the swirling combustion air.

2. A burner as claimed in claim 1, in which the tapering body is a hollow member in the form of a body of revolution having a frusto-conical portion partially obstructing the middle duct and a downstream domed end provided with a ring of nozzles or spray device for egress of the start-up fuel therefrom, the hollow body forming a flame stabilising body.

3. A burner as claimed in claim 2, in which the semi-angle of the frusto-conical portion of the flame stabilising body is 30°.

4. A burner as claimed in any preceding claim, in which the swirl-generating means is a ring of vanes, each of which vanes is angled relative to a respective axial plane of the burner and interposed between a windbox for the combustion air and an inwardly tapering section of the outer duct.

5. A burner as claimed in any preceding claim, in which the middle duct has an enlarged inlet region at its upstream end which tapers down to a circular cylin­drical tube defining the downstream end of the middle duct.

6. A burner as claimed in claim 5, in which the swirl in the coal fed to the combustion air is generated by supplying the coal to a tangentially directed enlarged inlet region of the middle duct so that a turning movement is imparted to the coal as it enters the middle duct.

7. A burner as claimed in any preceding claim, in which the inner duct is mounted so that its position can be adjusted axially with respect to the middle duct, the stabilising body then serving as a coal valve con­trolling the size of the annular outlet through which the coal flows outwardly into the flow of combustion air.

8. A burner as claimed in any preceding claim, in which an electrical igniter for the start-up fuel passes centrally through the inner tube and the stabilis­ing body mounted at the downstream end thereof.

Drawing