GB2062824A - Method of igniting solid fuel and apparatus for use in the method - Google Patents

Abstract

Apparatus for burning solid fuel such as coal has an ignition device comprising a fan 18 connected by a duct 19 with nozzles 20 for directing a stream of hot air onto the coal in a retort 18. The air may be heated by electrical elements within a duct 23. <IMAGE>

Description

SPECIFICATION Method of igniting solid fuel and apparatus for use in the method.

The invention relates to a method of igniting a sold fuel and to apparatus for carrying out the method.

The invention has been developed primarily for use in water-heating boilers comprising means for feeding coal of other solid fuel to a combustion space defined by a structure herein referred to as a retort and for feeding air also to the retort.

It is known to feed the fuel and air automatically to the retort of a boiler of the kind referred to. A common fuel-feeding device comprises of a screw which is rotatably mounted in a tube to feed coal along the tube into the retort. A fan is generally provided to feed air through a separate duct to the retort.

Boilers of the kind referred to are commonly used in heating installations which are required to supply heat to a building at certain times, for example during the working day, and are not required to supply heat to the building at other times, for example during the night and during the weekend. However, it is usual to maintain the contents of the retort at a temperature above the minimum combustion temperature of the fuel at all times, in order to avoid the necessity for heating up and igniting the contents of the retort.

igniting of the contents of the retort from cold is an inconvenient and time-consuming operation, requires some skill on the part of the operator and a willingness of the operator to work in dirty conditions.

During a period when a boiler of the kind referred to is not required to supply heat, air and fuel are generally fed intermittently to the retort to release heat therein and thereby maintain the temperature of the contents of the retort above a minimum value. Typically, the aggregate of such intermittent operation is equal to one-quarter of the period during which the boiler is not required to supply heat.

According to a first aspect of the invention, there is provided a method of igniting solid fuel in a combustion zone in which the fuel is required to burn, wherein a stream of hot gaseous fluid is directed onto the fuel in the combustion zone to raise the temperature of the fuel until combustion of the fuel occurs.

It will be understood that, in a case where the fuel is coal, raising the temperature of the coal will cause combustible gases to be emitted by the coal and these gases will burn before the solid residue is ignited. However, once the emission and combustion of the gases is properly established, the direction of the hot gaseous fluid onto the fuel can be discontinued and combustion of the fuel will proceed provided there is an adequate supply of air.

The gaseous fluid preferably includes an oxidising agent and may be air.

The gaseous fluid may be heated electrically.

This method of heating does not reduce the proportion of oxidising agent present in the gaseous fluid.

After combustion of the fuel has begun, combustion air, which may be cold, may be fed to the fuel in the combustion zone separately from the hot gaseous fluid. We prefer that cold combustion air should not be supplied to the combustion zone during heating of the fuel to the temperature at which combustion begins.

The feeding of the stream of hot gaseous fluid onto the fuel may be controlled automatically, for example by a timing device. This enables a boiler in which the contents of the retort are cold to be brought into operation automatically. Intermittent operation of the fuel and air feed means merely for the purpose of maintaining the temperature of the contents of the retort above a minimum value can be avoided. This saves fuel, increases the working life of the feed means and boiler generally and reduces the amount of maintenance work and other attention required.

According to a second aspect of the invention there is provided apparatus for burning solid fuel and comprising a retort, fuel feed means for feeding fuel into the retort, air feed means for feeding air into the retort and an ignition device comprising means for directing a stream of gaseous fluid onto fuel in the retort and heating means for heating the gaseous fluid before it enters the retort.

The retort may include a wall having apertures for admitting air to fuel in the retort and the ignition device may be arranged to direct the stream of gaseous fluid through one or more of said apertures onto the fuel.

The heating means preferably comprises an electrically energisable heating element.

The electrically energisable heating element may be supported on a refractory tube through which the gaseous fluid is caused to flow.

The apparatus may further include control means for bringing the ignition device into operation at predetermined times, for initiating operation of the fuel feed means and air feed means when combustion is established in the retort and for discontinuing operation of the fuel feed means and air feed means if combustion in the retort is extinguished.

The control means may be arranged for causing operation of the fuel feed means for a predetermined period prior to operation of the ignition device.

One example of an embodiment of the invention will now be described with reference to the accompanying drawing wherein: Figure 1 shows diagrammatically a boiler, partly in cross section, and Figure 2 shows on an enlarged scale part of a heating means of the boiler.

The boiler illustrated in the accompanying drawing is intended for supplying hot water to a building for space heating purposes and includes a heat exchanger 10 for transferring heat to the water from burning coal and the hot products of combustion thereof. Spaced somewhat below the heat exchanger there is an air box 11 to which combustion air is fed by air feed means comprising a fan 12 connected by an air feed duct 13 with the air box.

In the air box 1 there is mounted a retort 14 defining a combustion zone in which coal is to be burnt. For feeding coal to the retort, there is provided fuel feed means including a screw 1 5 rotatably mounted in a tube 1 6 leading from a bulk supply of coal to the bottom of the retort. For rotating the screw 1 5 there is provided an electric motor (not shown).

In walls of the retort 14 at a level above the screw 15, there are provided apertures 1 7 referred to in the art as tuyeres. The tuyeres provide communication between the interior of the air box 11 and the interior of the retort 14 to admit combustion air to the retort.

The structure thus far described is well known and may be operated continuously or intermittently. Coal is fed by the screw 1 5 into the retort where it burns in the combustion air supplied by the fan 12. If the supply of air and fuel is discontinued for a period not exceeding one hour, the temperature of the contents of the retort will remain sufficiently high for vigorous combustion to be re-established if the feeding of air to the air box is re-commenced.

The boiler further comprises an ignition device for igniting coal in the retort 14 after the feeding of air and fuel to the retort has been discontinued for a period sufficiently long to permit the temperature of the contents of the retort to fall below the minimum combustion temperature so that combustion cannot be re-established merely by re-commencing the feeding of air and fuel to the retort.

The ignition device comprises a secondary fan 1 8 connected by a duct 19 with nozzles 20 for directing a stream of air onto coal in the retort 14.

The heating device further comprises means for heating air flowing through the duct 1 9. This means is in the form of an electrically energisable heating element 24 mounted on a tubular refractory body 21 disposed in the duct 19. The arrangement is such that air flowing through the duct 1 9 is caused to flow through the interior of the refractory body.

The duct 19 extends into the air box 11 and the nozzles 20 are arranged adjacent to the wall of the retort 14 but are spaced slightly therefrom.

The nozzles are aligned with corresponding openings 1 7 so that a stream of air issuing from a nozzle is directed through the corresponding opening 1 7. Any dust which falls into the opening 1 7 whilst the boiler is out of operation can fall to the bottom of the air box through the gap between the nozzles 20 and the wall of the retort. The nozzles 20 are inclined somewhat downwardly to reduce the risk of dust entering the nozzles whilst there is no stream of air flowing through the duct 19.

The refractory body 21 carrying the heating element is preferably disposed in the duct 1 9 immediately adjacent to the nozzles 20. The heating means may be of a known kind used in tools for softening and welding thermoplastics.

There is also provided in the boiler a detector 22 for detecting vigorous combustion in the retort 14. The detector may be adapted to respond to infra-red radiation above a predetermined intensity or to respond in some other way to the attainment of a predetermined temperature within the retort. The detector is connected electrically with a control device (not shown) for controlling operation of the main fan 12, the motor which drives the screw 15, the electrical heating element 20 and the secondary fan 1 8. The control device may further include a timer which can be adjusted to initiate an ignition sequence at a predetermined time.

A typical ignition sequence is as follows. If the detector 22 indicates the absence of vigorous combustion in the retort 14, the screw 1 5 is driven for a relatively short, predetermined period sufficient to carry fresh coal into the retort.

Rotation of the screw is then discontinued. The main fan 12 remains inoperative until combustion is established in the retort.

The secondary fan 1 8 is energised to feed a stream of air through the duct 1 9 into the retort.

The heating element 20 is energised to heat the stream of air which, in turn, heats coal within the retort. The heat output of the heating element 24 is typically 3.5 kw. Typically, the secondary fan 18 feeds air at the rate of 25 cubic feet per minute and the heated air entering the retort is at a temperature within the range 4000C to 6000C.

As the coal is heated, it emits combustible gases and within a period of a few minutes there is achieved a temperature which is sufficiently high for these gases to be ignited. The temperature in the retort then rises rapidly and combustion is usually well established within a period of less than ten minutes. When combustion is well established, the detector 22 provides a signal to the control device and, upon receipt of this signal, the control device energises the main fan 1 2 and energises the motor driving the screw 1 5. Energisation of the heating element 20 is discontinued and, subsequently, energisation of the secondary fan is discontinued.

In the event of combustion in the retort being extinguished, the detector 22 provides an appropriate signal to the control device'which prevents further energisation of the main fan 1 2 and motor driving the screw 1 5 unless combustion is re-established in the retort by means of the ignition device.

In the event that combustion is not established in the retort by means of the ignition device within a predetermined period, for example ten minutes, the ignition sequence is terminated. The ignition sequence may be repeated one or more times but, if vigorous combustion of the coal in the retort is not established by a predetermined number, for example four, ignition sequences, then subsequent operation of the main fan 12, motor associated with the screw 1 5, secondary fan 18 and heating element 20 is prevented until the operator has attended the boiler. The control device may also be arranged for checking the electrical continuity of the heating element 20 and for checking the air pressure at the outlet of the main fan 12.In the event of electrical continuity or the required air pressure during operation of the fan is not maintained, further operation of the boiler may be prevented until it has been attended by the operator. The detector 22 may be operative at all times when the main fan 12 and screw 1 5 are operating to discontinue operation of both of these if proper combustion in the retort ceases.

It will be noted from Figure 1 that the duct 1 9 comprises a plurality of portions which can be disconnected from each other. A portion 23 which contains the heating element 20 extends from the retort 1 4 to the outside of the boiler wall and can be withdrawn to the outside of the boiler wall along a rectilinear path. In certain boilers, it may not be convenient to arrange for withdrawal of the duct portion containing the heating element along a rectilinear path from the openings 1 7 which provide communication between the air box 11 and the retort 14.In such cases, the nozzles 20 may be disposed adjacent to a lower part of the retort, in which case it will be necessary to provide in the retort wall apertures which do nat provide communication between the retort and the air box but permit air to be directed from the duct 1 9 into the retort. Such apertures may be provided in a plug secured in an opening formed in the wall of the retort at a level below that at which combustion occurs during normal operation of the boiler. The plug may be provided with several apertures of circular cross section and there may be fitted in the free end of the duct portion 23 a similar plug having a corresponding arrangement of apertures. These plugs are aligned with each other but are spaced somewhat apart to permit dust which escapes from the retort through the apertures of the plug to fall to the bottom of the boiler.

Whilst we prefer to heat electrically the stream of hot air which is used to ignite the coal in the retort, heating may alternatively be carried out by burning a fuel, for example natural gas or oil. The hot products of combustion, containing an excess of air, may be fed into the retort. Alternatively, heat may be transferred from the products of combustion to a separate stream of air which is directed into the retort.

Claims (12)

1. A method of igniting solid fuel in a combustion zone in which the fuel is required to burn, wherein a stream of hot gaseous fluid is directed on to the fuel in the combustion zone to raise the temperature of the fuel until combustion of the fuel occurs.

2. A method according to claim 1 wherein the gaseous fluid includes an oxidising agent.

3. A method according to claim 2 wherein the .

gaseous fluid is air.

4. A method according to any preceding claim wherein the gaseous fluid is heated electrically.

5. A method according to any preceding claim wherein cold combustion air is not supplied to the combustion zone during heating of the fuel to the temperature at which combustion begins.

6. A method according to any preceding claim wherein the feeding of the stream of hot gaseous fluid onto the fuel is controlled automatically.

7. Apparatus for burning solid fuel and comprising a retort, fuel feed means for feeding fuel into the retort, air feed means for feeding air into the retort and an ignition device comprising means for directing a stream of gaseous fluid onto the fuel in the retort and heating means for heating the gaseous fluid before it enters the retort.

8. Apparatus according to claim 7 wherein the retort includes a wall having apertures for admitting air to the fuel in the retort.

9. Apparatus according to claim 8 wherein the ignition device is arranged to direct a stream of gaseous fluid through one or more of said apertures onto the fuel.

10. Apparatus according to claim 7, 8 or 9 wherein the heating means comprises an electrically energisable heating element.

11. Apparatus according to claim 10 wherein the electrically energisable heating element is supported on a refractory tube through which the gaseous fluid is caused to flow.

12. Apparatus according to any of claims 7 to 11 which includes control means for bringing the ignition device into operation at predetermined times, for initiating operation of the fuel feed means and air feed means when combustion is established in the retort and for discontinuing operation of the fuel feed means and air feed means if combustion in the retort is extinguished.

1 3. Apparatus according to claim 12 wherein the control means is arranged for causing operation of the fuel feed means for a predetermined period prior to operation of the ignition device.

1 5. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.