GB2055633A - Pneumatic conveyance of solid fuel to a boiler - Google Patents

Abstract

A feeding device for supplying solid fuel to a boiler has a supply tube (10) extending through the shell (11) of the boiler and terminates above the firegrate, means (12) for supplying secondary air to mix with solid fuel as it emerges from the lower end of the supply tube, a pneumatic conveyor (13) for solid fuel connected to an input (15) of a cyclone (14) which has a first output (16) connected to the supply tube (10) to supply solid fuel thereto, and a second output (17) for discharging transport air separated from the solid fuel, a venting arrangement (18) connected to said second output (17), and a pressure controller (19) arranged in said venting arrangement and operable in use to maintain a pressure differential between the cyclone and the combustion chamber in the boiler sufficient substantially to prevent hot gases from the combustion chamber passing upwardly to the cyclone via the supply tube, but not sufficient to cause any appreciable amount of transport air to pass to the boiler via the supply tube. <IMAGE>

Description

SPECIFICATION Pneumatic conveyance of solid fuel to a boiler This invention relates to the pneumatic conveyance of solid fuel to a boiler.

It is known to provide mechanical feeding devices which feed solid fuel to the fire grate of a boiler, which comprise a vertical dropping tube taken downwardly through the shell of a boiler and terminating above the fire grate. As disclosed in the specification of our application No. 7913735, a metered supply of solid fuel is fed to the upper end of the dropping tube by a reciprocating ram which, in turn, is supplied with fuel from a storage hopper. The storage hopper may be charged with fuel by a conventional screw elevator, or a pneumatic conveyance system may blow the coal to the storage hopper. In the latter case, the transport air employed is discharged to atmosphere via a filter, so as to avoid entrainment of transport air into the boiler and dilution of flue gases, which would otherwise cause a reduction in efficiency and uneven fuel bed temperature and distribution.

As is well known, a solid fuel fired boiler requires a supply of primary air which is blown upwardly through the fire grate, and a supply of secondary air which is blown over the surface of the fire grate. To achieve efficient and substantially smoke free combustion of the fuel, it is most important that the ratios of primary air and secondary air should be carefully controlled, and it is for this reason that the use of pneumatics in the direct conveyance of solid fuel to the interior of the boiler has not proved satisfactory to date, in that the admission to the boiler of "transport" air renders it difficult to maintain the required ratios between primary and secondary air.

Furthermore, although not yet widely recognised, there is a growing acceptance of the view that transport air used to deliver fuel into a boiler has an adverse effect on the combustion taking place in the boiler. Indeed, although it has been claimed for one known pneumatic system that there is the advantage that the transport air also provides a source of secondary combustion air, the pressure of the air used to operate the pneumatic delivery system is insufficient to provide a proper secondary air function, and tests have shown that this transport air is quite inoperable as secondary air and indeed has a cooling effect on the fire bed on the grate.

As indicated above, it has been proposed to provide pneumatic conveyance of fuel to a storage hopper, in conjunction with a metering ram as per our application No. 7913753, and this is satisfactory provided that care is taken to ensure that the transport air is not admitted to the interior of the boiler. However, there is a need to provide for pneumatic conveyance of solid fuel to a boiler, within a fuel supply arrangement which is less bulky and space consuming (and therefore neater in appearance) than hitherto, while still ensuring that transport air is not admitted to any substantial extent to the interior of the boiler.

According to the invention there is provided a feeding device for supplying solid fuel to a boiler and comprising: a supply tube arranged to be taken downwardly through the shell of a boiler and to terminate above the fire grate of the boiler; means for supplying secondary air to the device so as to mix with solid fuel as it emerges from the lower, delivery end of the supply tube; a pneumatic conveyor for supplying solid fuel to the supply tube; a cyclone having an input connected to the output of the pneumatic conveyor to receive therefrom a mixture of solid fuel and transport air, and having a first output connected to the upper end of the supply tube to supply solid fuel thereto, and a second output for discharging transport air separated from said mixture; a venting arrangement connected to said second output;; and a pressure controller arranged in said venting arrangement and operable in use to maintain a pressure differential between the cyclone and the combustion chamber in the boiler sufficient substantially to prevent hot gases from the combustion chamber passing upwardly to the cyclone via the supply tube, but not sufficient to cause any appreciable amount of transport air to pass to the boiler via the supply tube.

Bearing in mind that typically the pressure generated in the combustion space of a boiler is of the order of four to eight inches (water gauge), a positive pressure in the cyclone of about one inch (water gauge) should suffice.

The secondary air may be supplied in any convenient manner, to mix with the fuel as it leaves the lower end of the supply tube, e.g.

a series of jets, but preferably it is provided by an annular pipe which surrounds the supply tube.

Preferably, the cyclone has a further inlet which is connected to a grit return line which conveys pneumatically grit to the cyclone or re-cycling to the fire grate via the cyclone and the supply tube. The grit may be re-cycled in the manner described in more detaii in our specification No. 7913735.

One embodiment of feeding device according to the invention, for supplying solid fuel to a boiler, will now be described in detail, by way of example only, with reference to the accompanying diagrammatic drawing.

The solid fuel feeding device comprises a supply tube 10 which is taken downwardly through the shell 11 of a boiler so as to terminate above the fire grate (not shown) of the boiler. Means is provided for supplying secondary air so as to mix with solid fuel as it emerges from the lower, delivery end of the supply tube 10, and in the illustrated arrangement comprises a further pipe 1 2 which surrounds the supply tube 10 so as to form an annulus within which the secondary air is contained. The outlet of the pipe 1 2 is located adjacent to the lower end of the supply tube 10, and conveniently this may be as described and illustrated in Fig. 1 of the specification of our application No. 7913735.

Although not shown, a source of primary air will be provided in order to blow primary air upwardly through the fire grate, and the supply of primary air and secondary air will be controlled in conventional manner, so as to obtain efficient combustion of solid fuel in the boiler.

A pneumatic conveyor is provided for supplying solid fuel to the supply tube 10, and is illustrated diagrammatically by the pneumatic pipe 1 3. A mixture of solid fuel and transport air is conveyed along the pipe 13, and a cyclone 14 has an input 1 5 connected to the output of the pipe 13, a first output 1 6 connected to the upper end of the supply tube 10 to supply solid fuel thereto, and a second output 1 7 for discharging transport air separated from the mixture of solid fuel and transport air. The cyclone 14 will take the form of any suitable conventional apparatus, having the purpose of separating the solid fuel from the transport air.

A venting arrangement 1 8 is provided to vent the transport air to atmosphere, and is connected to the second output 17. Preferably, the venting arrangement 1 8 discharges the transport air to atmosphere via an eight inch diameter filter. Typically, the cyclone 14 may have dimensions of two foot diameter by two foot depth.

A pressure controller 1 9 is arranged in the venting arrangement 1 8 and is operable in use to maintain a pressure differential between the cyclone 14 and the combustion chamber in the boiler sufficient substantially to prevent hot gases from the combustion chamber passing upwardly to the cyclone 14 via the supply tube 10, but not sufficient to cause any appreciable amount of transport air to pass to the boiler via the supply tube.

Typically, the pressure generated in the combustion space of a boiler is of the order of four to eight inches (water gauge), and the purpose of the pressure controller 1 9 is to maintain a positive pressure in the cyclone 14 of about one inch (water gauge).

In addition to supplying a fresh charge of solid fuel, on demand, to the boiler, the fuel feeding device may also serve to re-cycle grit.

To this end, a grit return line 20 is provided which is connected to the cyclone 14 in generally similar manner to the pipe 1 3.

Thus, transport air and solid fuel is supplied tangentially via the pipe 1 3 to the top part of the cyclone 14, whereas transport air and grit from an external grit cyclone arrester is supplied also tangentially to the cyclone 14. The re-cycling of grit may be achieved by apparatus as disclosed in the specification of our application No. 7913735.

Accordingly, a solid fuel feeding device is disclosed herein which enables fuel to be delivered pneumatically direct to the boiler, while requiring less space than hitherto, and also providing a neat installation. The solid fuel is supplied directly by pneumatic conveyance to the boiler, via the cyclone, and without entraining any substantial amount of low velocity transport (delivery) air into the boiler.

The transport air is separated from the solid fuel, and is vented to atmosphere.

By providing the pressure controller 19 in an eight vent line (18) a differential pressure can be maintained in use between the cyclone 14 and the combustion space of the boiler, whereby hot gases from the combustion chamber are prevented substantially from entering the cyclone 14, whereas the transport air does not pass, to any appreciable extent, to the interior of the boiler via the cyclone 1 4 and supply tube 10.

Claims (3)

1. A feeding device for supplying solid fuel to a boiler and comprising: a supply tube arranged to be taken downwardly through the shell of a boiler and to terminate above the fire grate of the boiler; means for supplying secondary air to the device so as to mix with solid fuel as it emerges from the lower, delivery end of the supply tube; a pneumatic conveyor for supplying solid fuel to the supply tube; a cyclone having an input connected to the output of the pneumatic conveyor to receive therefrom a mixture of solid fuel and transport air, and having a first output connected to the upper end of the supply tube to supply solid fuel thereto, and a second output for discharging transport air separated from said mixture; a venting arrangement connected to said output; ; and a pressure controller arranged in said venting arrangement and operable in use to maintain a pressure differential between the cyclone and the combustion chamber in the boiler sufficient substantially to prevent hot gases from the combustion chamber passing upwardly to the cyclone via the supply tube, but not sufficient to cause any appreciable amount of transport air to pass to the boiler via the supply tube.

2. A feeding device according to claim 1, in which the secondary air is supplied, to mix with the fuel as it leaves the lower end of the supply tube, via an annular pipe which surrounds the supply tube.

3. A feeding device according to claim 1 or 2, in which the cyclone has a further inlet which is connected to a grit return line which conveys pneumatically grit to the cyclone for re-cycling to the firegrate via the cyclone and the supply tube.