GB2240164A - Combustion furnace - Google Patents

Abstract

A solid fuel combustion furnace comprises a grate on which a fuel bed can be supported, a combustion chamber above the grate, means for moving fuel on the grate from a front end in which fuel is fed to the bed and a rear end in which ash is discharged from the grate, and a heat shield over the bed and extending from the rear end towards the front end and arranged to shield partially burnt fuel char and ash from heat emissions from the combustion chamber. In this way the partially burnt fuel etc., in the shielded portion of the bed will burn at a lower temperature than would otherwise be the case. <IMAGE>

Description

TITLE: COMBUSTION FURNACE DESCRIPTION The invention relates to solid fuel combustion furnaces more particularly, but not exclusively, intended for burning not only coal, but waste derived fuels such as domestic refuse derived fuel (DRDF), scrap rubber, straw and the like. Particular problems associated with the burning of such fuels are (1) the evolution of noxious chemical compounds (2) the evolution of micro-particulate pollutants and (3) the fouling of the interior of the furnace.

The furnace of the invention may be used to produce process steam or hot water but alternatively the furnace of the invention may be a so-called mass burn municipal incinerator with or without the recovery of heat. Such mass burn incinerators are used to burn waste products which may contain poisonous chemicals such as mercury which must be burned in a controlled manner if environmental pollution is to be avoided.

It is an object of the invention to provide means whereby fuels and particularly waste derived fuels and other waste products can be burnt in a satisfactory manner.

According to the invention a solid fuel combustion furnace comprises a grate on which a fuel bed can be supported, a combustion chamber above the grate, means for moving fuel on the grate from a front end in which fuel is fed to the bed and a rear end in which ash is discharged from the grate, and a heat shield over the bed and extending from the rear end towards the front end and arranged to shield partially burnt fuel char and ash from heat emissions from the combustion chamber. In this way the partially burnt fuel etc. in the shielded portion of the bed will burn at a lower temperature than would otherwise be the case.

Preferably the heat shield will also be effective in controlling gas flow over the bed and can thus be used to prevent or minimise the entrainment of ash particles in the combustion gases and to promote more intimate mixing of combustion air and volatiles thus assisting clean and complete combustion.

Advantageously the grate is a chain -grate.

Preferably the rear end of the heat shield is sealed against the ash discharge end of the chain grate by means of a gravity actuated hinged flap, to prevent "tramp" air from entering the furnace from its rear end.

The shield may comprise a metal plate or bars covered with refractory material but preferably comprises refractory bricks. The length of the shield will vary depending on the nature of the fuel but may extend between one quarter and two-thirds of the length of the grate. The shield is preferably inclined upwards towards the front of the grate, desirably at an angle slightly greater than the expected natural angle of slope of the bed. The purpose of inclining the heat shield is to prevent combustion air supplied from below the grate from entraining ash into the combustion gases.

Steam may be injected from below the grate at a position below the heat shield or alternatively may be injected downwardly towards the grate from the heat shield.

For certain fuels, e.g. waste rubber, it may be advantageous to inject steam into the combustion chamber from above the grate from a position close to its front end. Preferably the steam jets will be directed towards the downstream end of the grate and will be angled downwards towards the grate.

The heat shield may be flat and may be supported on the side seals of the furnace or in any other convenient manner e.g. on pins.

The invention is diagrammatically illustrated by way of example in the accompanying schematic drawings, in which Fig. 1 is a side view of a furnace in accordance with the invention, Fig. 2 is a perspective view of a refractory brick and pivot for the heat shield, and Fig. 3 is a side view of a modified form of furnace and boiler in accordance with the invention.

In the drawings there is shown a combustion furnace 1 in which is disposed a generally conventional chain grate 6 which feeds fuel 5 into a combustion chamber 2 via a hopper 4. The fuel 5 forms a bed 3 on the grate 6.

Air is supplied from below the grate as indicated by arrows and combustion products leave the combustion chamber 2 as indicated by further arrows. The chain grate 6 conveys the fuel along the combustion chamber from its front end and finally discharges ash from the rear end of the conveyor and into a discharge chute 9.

A heat shield 7 is arranged in the combustion chamber and is disposed above the bed 3 to extend from the rear end of the chain grate towards the front end thereof. The heat shield terminates somewhere between one quarter and two-thirds of the distance between the rear end of the grate and its front end depending on the nature of the fuel. A fuel such as straw requires a longer heat shield and a fuel such as DRDF requires a relatively short heat shield. The heat shield slopes upwardly towards the front of the grate with the object of creating a gap between the fuel bed and the underside of the heat shield which is greater at its front end than at its rear end. A hinged flap 8 extends down from the rear end of the heat shield and seals against or is adjacent to the rear end of the chain grate 6 to prevent the entry of tramp air into the combustion chamber from the rear end.

Both steam and compressed air may be injected concurrently, horizontally over the the fuel bed, i.e., a nozzle might suitably be positioned approximately 10-12" above the grate, just behind the point where the fuel enters the furnace.

The heat shield works satisfactorily using individual (high alumina) "half bricks11 10 drilled and threaded on to stainless steel rods 11 as a means of support. These assemblies may rest upon supports which are loosely mounted on the side seals of the grate. The hinged end plate 8 may utilise half bricks with a stainless steel rod as a hinge pin. In this instance the bricks may be drilled approximately one third along their length (see Fig.2). This arrangement gives a counter balance effect and enables the hinged bricks to respond to the slight pressure exerted by ash trying to discharge off the grate.

The heat shield may be manufactured out of carefully cut sections in order to achieve a self-supporting shallow arch. This would preferably be a very shallow arch (approximately 2.5in across 2ft 6in).

Compressed air atomised water injection may be used as a method principally of controlling HCl and avoiding excessive grate temperatures. It is injected under the grate, inside the wind box of the grate, at an angle calculated to affect only the last half of the grate. It is desirable to include oxygen sensing fuel feed trim control via appropriate electrical interfaces such as variable speed drives (DC motors) or frequency inverters, 12,13, preferably programmable, for AC drive systems, or alternative devices such as variable pulse timers. This enables optimum process conditions and maximum combustion efficiencies to be achieved and reduces down time due to damage or block of tubes through bad combustion conditions.

Conventionally, in a shell and tube chain grate boiler there is a refractory brick wall towards the back of the furnace tube. The object of this is to reduce the amount of grits and fly ash blown into the back of the boiler. It has been found to be unnecessary as a result of the inclusion of the features of the present invention. Additionally, it masks a large part of the lower section of the furnace tube from radiant heat transfer. Its complete removal improves combustion efficiency significantly.

The furnace of the invention thus permits unrestricted rapid pyrolysis of the fuel followed by burn out of the resulting char in a temperature limited zone to prevent the evolution of unwanted matter in the gas flow.

The device will thus alleviate the formation of slab ash on the grate and the resultant fuming at high temperatures of low melting point ashes and which associated with deposits forming on heat transfer surfaces and high dust burdens in exhaust gases.

Particularly in the combustion of domestic refuse and manufactured pellet form refuse there is a significant amount of chlorine in the fuel. The invention permits a method of temperature control which enables the addition of activated hydrated lime to neutralise the majority of the chlorine which is associated with acid corrosion and the formation of dioxins.

The device will also increase the residence time of airborne carbon at or above its combustion temperature.

This is of particular importance when burning shredded rubber tyre scrap.

The application of a variable amount of steam can be used to control temperatures within the shield area by promoting the endothermic reaction of carbon and water to carbon monoxide and hydrogen, and can be used to adjust beneficially the air flow within the heat shield area.

Claims (12)

1. A solid fuel combustion furnace comprising a grate on which a fuel bed can be supported, a combustion chamber above the grate, means for moving fuel on the grate from a front end in which fuel is fed to the bed and a rear end in which ash is discharged from the grate, and a heat shield over the bed and extending from the rear end towards the front end and arranged to shield partially burnt fuel char and ash from heat emissions from the combustion chamber.

2. A furnace according to claim 1, in which the heat shield is adapted to control gas flow over the fuel bed.

3. A furnace according to claim 1 or claim 2 in which the grate is a chain grate.

4. A furnace according to claim 3 in which the rear end of the heat shield is sealed against the ash discharge end of the chain grate by means of a gravity actuated hinged flap which prevents air from entering the furnace from the rear end of the grate.

5. A furnace according to any preceding claim in which the heat shield comprises refractory bricks.

6. A furnace according to any of claim 1 to 4 in which the heat shield comprises a metal plate.

7. A furnace according to any of claims 1 to 4 in which the heat shield comprises bars covered with refractory material.

8. A furnace according to any preceding claim in which the heat shield extends between one quarter and two thirds of the length of the grate.

9. A furnace according to any preceding claim in which the heat shield is inclined upwards towards the front of the grate.

10. A furnace according to any preceding claim in which means are provided for injecting steam and/or compressed air from above and/or below the grate.

11. A furnace substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

12. A furnace substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.