This invention relates to a method and apparatus for burning solid fuel.

By the term 'solid fuel' as used in this specification there is meant fuels such as wood, coal, straw, lignite, peat and the like, and mixtures of such solid fue Is .

Presently ava lable apparatus for burning solid fuels, particularly for domestic heating systems, suffer from problems ith regard to incomplete and inefficient combustion, smoke emission, and consequential inefficient uti li sation of fue I .

These particular problems do not arise in the same way and to the same extent with non-solid fuels such as gas or oi l. For gas and oi l, the provision of conditions permitting complete combustion is a relatively atrai ghtforward matter. However, in the case of solid fuels, the problem arises that during the first portion of the co busiton process the fuel gives off material such as volatile fuel constituents, combustion products, and the like which have been incompletely burned, or not burned at all. As a result, during this first portion of the combustion process the gases rising from the combustion zone contain significant amounts of combustible material which is unused, represents a loss of potential heat, andwhich tends to be deposited on the surfaces over which it passes. This problem is particularly acute in the case of coal since these volatile constituents, hydrocarbons and the like are given off at temperatures below their spontaneous ignition temperature, and thus

stand little chance of undergoing ignition dur ng the early stage of establishing the fire bed.

Conventional domestic heating appliances offer no real possibility of overcoming the above-identified problem. The metallic construction of these appliances in the zone above the fire bed ensures that the combustion products, volatiles and hydrocarbons are cooled as they rise, particularly so if heat exchangers are provided, and it is only in the later stages of the burn, when the fire is well established and the wood or coals are glowing like charcoal, that this emission prob lem subs ides .

There is disclosed in GB 2,091,41 A a solid fuel combustion appliance in which a down draught fire bed - i s ^" connected through a relatively narrow opening to a refractory secondary combustion zone behind the fire bed, in which volatile components of the combustion gases are burned.

However, this prior apparatus is not well suited to the mass market since it does not work well in the conventional domestic situation. - Proper control of the fire requires expert attention. This is due, at least in part, to the varying resistance of the fire bed as combustion proceeds. Moreover, it is often the case, that the secondary combustion zone does not really get hot enough for the purpose, and it certainly does not work at all at first. In addition, the refractory material ava lable hitherto has not worked well and, in short, the pr or apparatus is not suited to domestic use.

An object of the present invention is to provide apparatus for burning solid fuel, and a method thereof

offering improvements in relation to one or more of the matters discussed herein. Certain aspects of the present invention are applicable not only to domestic heating apparatus, but also more widely. The embodiment described below provides for improved secondary combustion during the first phase of combustion of a solid fuel, and enables the secondary combustion to be commenced at a particularly early stage, moreover means is provided for adapting existing domestic and other stoves and furnaces for these purposes .

According to the nvention there is provided a method and apparatus for burning solid fuel as defined in the accompanying claims.

The invention also pro ides a method and apparatus for burning solid fuel not limited by each feature of any claim herein, but comprising any novel feature or ^' novel combination of features disclosed in this app Ii cati on .

An embodiment of the invent.ioπ will now be described by way of example with reference to the accompan ing drawing which shows a vertical cross-section through apparatus for burning solid fuel according to the invention.

As shown in the drawing apparatus 10 for burning solid fuel comprises a lower portion 12 providing a primary combustion zone 14, and an upper portion 16 providing a secondary combustion zone 18. Solid fuel 20 is burnt in lower portion 12 of the apparatus. Combustion products including volatile fuel constituents, hydrocarbons and the like pass upwards through

combustion zone 18 where secondary combustion takes place. Heat is extracted by heat exchangers 22, 24,

26 and 28. Flue gases pass via a flue outlet 30. . The exten of zone 18 is shown approximately by dotted lines in the drawing .

The above main components of apparatus 10 will now be considered in more detail.

Lower portion 12 of the apparatus comprises a body portion 32 supporting a fire grate 34 including riddling means for agitating the fire bed 20, an ash pan 36, and a thermostatically controlled primary air inlet 38. Doors 40 are hinge-mounted and provided with glass windows 42 and a, anually ad ustable secondary - plate ' ^{J 7} air inlet 44. A finned 46 supports the front

/ side of fire bed 20. ATV entering via inlet 44 passes upwardly over the heated plate and thence past the glass 42 of the doors 40 and into secondary combustion zone 18.

The water-filled heat exchangers 22 and 24 extend along three sides of the fire bed 20 and are hydraulically interconnected with each other and with the water jackets 26, 28, which are themselves interconnected by a connection duct 48.

Upper portion 16 of apparatus 10 comprises lower and upper refractory members 50 and 52 respectively forming part of secondary combustion zone 18.

The refractory members are of considerable importance in relation to providing effective secondary combustion in zone 18, and will now be discussed in detai l.

As shown in the drawing, the refractory members 50 and 52 are generally of plate or baffle-like form and located directly above primary combustion zone ^" 14. They are mounted in vertically spaced positions one above the other and extend across the full width of the primary and secondary combustion zones. The arrangement is such that the combustion products from the primary combustion zone pass slowly through the secondary combustion zone, over the extensive surface areas of the refractory members, and these cause the volatile and combustible constituents of the primary combustion to undergo secondary combustion in zone 18, afte which heat is extracted by the heat exchangers 26, 28 and 48.

Lower refractory member 50 is in the form of a baffle formed from ceramic fibre board. T is material is currently available in the UK under the name Morganite ceramic fibre board. Morganite is a trade mark. Baffle 50 is mounted on supports 54 and 56, so as to be removable for servicing purposes. It has a protec lower steel plate 57 and/or a-n open-work jacket of stainless steel mesh material to -render it more durable. Baffle 52 is likewise formed of ceramic fibre board and is of greater thickness than board 50 and formed with vertical openings 58 whereby the gases passing therethrough are caused to swirl and mi .

Although ceramic fibre board is presently the preferred material for the refractory members 50 and 52, other materials may be used if desired, and better materials may emerge in the future. The preferred properties of the refractory members include having a low thermal capacity and a low thermal conductivity.

Alternative possibly suitable materials include fibreglass (but this may soften and distort), silica and asbestos. Certain fire clay materials may also be suitable. The need for both a low thermal capacity and from conductivity arises ^, the need for the refractory member to achieve a high surface temperture from a limited quantity of heat in a short time. Thus, for example, conventional refractory ceramic brick material has a relatively high specific heat and is therefore not a preferred material although it maybe usable in situations where a plentiful supply of hea.t is available. Once the secondary combustion has commenced, the requirement for low thermal capacity is of less significance. A simple test for suitability of a material for the purpose comprises heating the surface of a sample of the material with a small hand held gas torch. It is found that in the case of ceramic fibre board, such a torch produces a red hot glow in 5 seconds or less, and such a material is to be preferred. Periods of up to 10 and 20 seconds to produce such a glow may be acceptable for certain types of combustion appratus. This test serves also to test for low thermal conductivity. Ceramic fibre board of thickness approximately one centimetre can glow red hot on one side while being merely 'hand hot' on the other. The conductivity of the refractory material need not necessarily be as low as this, but must certainly be significantly less than that of a metal. Materials equivalent to ceramic fibre board may be identified thus. - The important function of the refractory members 50 and 52 is that they achieve, during the first phase of combustion, a temperature ithin zone 18 suff cient to produce spontaneous combustion of the combustion products, volatiles, hydrocarbons etc from the primary

combustion zone. The temperatures needed for this purpose are as low as 420 C to 460 C for acetylene, and 650 C to 750 C for methane and 644 C to 658 C for carbon monoxide and approxi ately 550 C for the remainder of the combustion products. A tteemmppeerraattuurree ooff 550000 C or above will usually be sufficient for most purposes.

Heat exchangers 26 and 28 are of generally plate¬ like form being disposed generally horizontally, the former being mounted as a forward extension of refractory member 52. Gases passing through holes 58 in the latter are intercepted by heat exchanger 28 and pass around the side and forward edges of the latter. A series of fins 60 are mounted on the upper surface of heat exchanger 28 and lie in planes extending from front to rear of apparatus 10. Mounted above ^' the fins is a top plate 62 having a central opening 64 through which the cooled gases pass to flue 30. A butterfly-type restrictor 66 operable by a handle 68 is provided to regulate the updraught applied to apparatus 10 by the chimney. Heat exchangers 22, 24, 26 and 28 are connected through an inlet 70 and an outlet 72 to a hot water consuming system such as a domestic central heating system.

Operation of apparatus 10 will now be described.

When fire bed 20 is first is ignited, combustion products including unburnt volati les and gases pass upwards through primary combustion zone 14 to refractory baffle 50. The latter causes the combustion products to circulate within combustion zone 14 thereby promoting a ra id rise in te perature and intimate mixing of the gases. As the temperature rises initially, the surface

temperature of the refractory members 50 and 52 rises much more rapidly than those of the surrounding structures and soon reach the level of 420 C to 460 C at which any acetylene present will ignite. At 500° C to 550 C a number of the other combustion products will ignite and then the surface temperatures of the baffles will soar, causing more of the combustible components entering the secondary combustion zone 18 to igni te.

It should be noted that the secondary combustion air entering through inlet 44 reaches combustion zone 18 in a relatively hot condition and is mixed with the combustion products there. Its relatively high temperature ensures that the gas mixture is not chilled, which in prior art apparatus has tended to damp down and sometimes prevent secondary combustion.

The relatively tortuous path which the gases adopt as they pass around the refractory members ensures inti mate mixing and intimate surface contact with the refractory members whereby combustion is promoted.

The presence of heat exchangers 26, 28 and 48 adjacent -° the secondary combustion zone produces efficient heat extraction therefrom, whereby a relatively low flue gas temperature is achieved despite the occurence of combustion at this relatively high level in the apparatus.

Control of the output of the apparatus 10 is by means of the thermostatic control of primary air inlets 38. The control may be based upon water temperature in the heat exchangers. It is particularly

to be noted that the secondary air supply is not throttled when the primary air supply is. In this way, although the fire bed combustion may be damped down by reduced air supply, the combustion in the secondary combustion zone is permitted to continue without hindrance, whereby more complete combustion is ac ieved than with conventional arrangements in which the secondary air supply is also throttled .

In a second embodiment which is not illustrated as such but which is otherwise constructed in a simi lar manner to that described above, means is provided to preheat the secondary combustion zone independently of fire bed 20. Otherwise, this embodiment is as described above.

The heating means for the secondary combustion zone may comprise surface heating means such as an electrical heating element mounted on the refractory materials themselves. However, it is preferred to provide external heating means to effect surface heating of the refractory materials, for example by means of a blast of hot air or a flame. External electrical heating means could be provided to produce a hot air supply at, for example, 750° C to 1250° C. Such a hot air blast could be introduced into the apparatus in secondary combustion zone 18 so as to impinge on the surfaces of the refractory members, thereby raising them to the above -mentioned temperatures for producing spontaneous combustion of the primary combustion products.

A supply of preheated air in the above manner may replace the secondary air supply for normal operation of the apparatus. A temperature probe may

be provided to monitor the surface temperature of the refractory materials and a control system may then control the heat supply to the air blast or current .

In use, on initial firing of fire bed 20, the maximum heat supply to the refractory members would be adopted until secondary combustion had been established, whereupon the heat supply could be reduced to a level commensurate with not inhibiting secondary combustion.

Adoption of an external heat source for preheating the refractory materials by means of an air or gas current has the advantage that this heat source can be located outside the harsh env ronment of the secondary combustion zone, and thereby its working life extended. The advantages of the present embodi ent are not limited to the general configuration shown in the accompanying drawing and the concept of preheating the secondary combustion zone is applicable to any form of solid fuel combustion appa ratus .

Amongst other modifications which could be made in the embodiment described above with reference to the accompanying drawing are the following:

1 The secondary combustion zone need not be located directly above the pri ary combustion zone. It could for example be offset to one side or otherwise displaced, but connected to the primary combustion zone to receive combustion products rising therefrom.

The invention is appl cable to any form of

solid fuel combustion appliance, and certain aspects are applicable to non-domestic combustion appliances,

3 The refractory members of the above embodiment may be retro-fitted to existing heating appliances in some cases so as to establ sh the possibility of secondary combustion.

4 Other types of heat exchange apparatus may be provided, such as ducted air systems.

5 Considerable modification of the arrangements for preheating the secondary combustion air may be made. Such preheating serves also to substantially prevent deposition of combustion products on the inner surface of the transparent/translucent inspection windows and/or remove such deposits.