This invention relates to the production of synthetic r solid fuel.

It is known to provide a "wood substitute" for fires by mulching and compacting newspapers to form blocks which are subsequently dried. ^" The fuel blocks which are thus formed have slow burning characteristics.

It has been found that a suitable slow-burning solid fuel may be formed by the compaction and drying of paper which has been mixed with or impregnated with animal manure or sewage sludge. The resulting dried product can be produced so as to be virtually free of odour, and the manure/sludge impregnant adding to the calorific value of the fuel and making it superior to fuel blocks made from pulped paper alone. -

According therefore to the present invention there is provided a method Of making synthetic fuel in which paper fragments are mixed with animal manure and /or sewage sludge and subsequently compacted and dried to form solid blocks or pellets of combustible material.

By using as the raw material at least some paper fragments which have been used as animal bedding material and which are impregnated with animal manure it is possible to dispense with the separate step of mulching the paper to form a pulp prior to compaction..

It may be found that sufficient drying heat can be .generated by compacting the impregnated paper fragments in a die or press , thereby heating the material frictionally The fuel blocks or pellets can therefore be produced in. a dry semi-tlry ^* state, avoiding ^* the need for a separate drying stage . Thus the material may be compacted in an extrusion - press , the feed to which mixes up the material , so that the mixing, compacting and drying of the material takes place in a single passage of the material through the press .

If the material is produced in an extrusion press it may be broken up into convenient lengths for use in a furnace hearth. For example, the material may be produced in the form of cylindrical "logs" which may be stacked conveniently. Such logs are easy to handle and enable a boiler hearth to be stoked effectively in a single operation, without the need for constant attention, as will be necessary if particulate fuel were used.

For some purposes it may be desirable to produce the solid fuel blocks or pellets in a specially shaped form, for example in the form of artificial logs. For this purpose the material may be compacted in moulds or presses, rather than extruded.

Where sewage sludge is used as a raw material, the mixed paper fragments and sewage sludge are preferably subjected to an anaerobic decomposition before being compacted. ^* Preferably the anaerobic decomposition is followed by an aerobic composting- stage, to exhaust bacterial and chemical activity before the compacting stage.

Prior to the compaction of the mixed raw materials various additives may be introduced, according to the intended use. For example, charcoal, colouring agents, deodorising agents or odoriferous agents may be .added, particularly if th resulting fuel blocks are to be used on open domestic fires.

The invention also comprehends a synthetic fuel in block or pellet form comprising compressed and dried paper fragements or other cellulose fibres impregnated with animal manure or composted or unσαmposted sewage and bound together.

In one method according to the invention for the production of fuel -in block or pellet form used animal bedding material in the form of diced or shredded news- papers is collected from farms and other -places of use after impregnation with animal manure. The manure may be horsemanure, cattle manure or m.anure from pig or poultry farms. In the course of each such bedding material becomes partially broken down into cellulose fibres, and the collection of such material in batch or bulk

form will have the effect of mixing the material thoroughly. The collected material is passed through a compacting plant such as an extrusion press having individual or gang moulds in pelleting dies. The ~ frictional heat generated as the material is compressed through the extrusion dies generates enough heat to dry the material while the resulting pressure and heat binds the cellulose fibres together to form solid blocks or pellets of combustible material. The synthetic fuel thus produced is clean to handle and substantially odour-free. The degree of compaction applied to the material in its production has the effect of at least partially re-constituting the cellulose fibres to form a monolithic structure which has slow-burning characteristics, making the material ideal for use in industrial boiler furnaces and in domestic open hearths. Such slow-burning fuel is particularly desirable in a boiler furnace where only occasional fuel replenishment is desirable.

Although the method described above envisages the use of soiled animal bedding material, it is also within the scope of the invention ot use clean shredded or diced paper, preferably highly absorbent paper such as newsprint, and to admix this with animal manure, and/or with liquid sewage sludge. If sewage sludge is used, the mixture may be subjected to a bacterial and/or chemical decomposition stage or stages to produce a composted mixture which is then dried and moulded, or extruded as previously

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described. The decomposition may, for example, take place in a first anaerobic stage, accompanied by the extraction of methane as a useful by-product, and a second aerobic composting stage.

5 It is envisaged that for some applications uncomposted sewage may be mixed with shredded or diced paper, possibly with the addition of deodorising and/or odoriferous agents.

Control of the moisture content of the resulting solid fuel, which is important in determining the burning 10 characteristics of the fuel, is easily exercised, for example by regulating the extrusion or compacting rate.