GB2145731A - Improvements in or relating to fuel combustion - Google Patents

Abstract

The combustion of straw briquettes and other briquetted cellulose- based solid fuels is improved by incorporating in the briquettes during their manufacture a combustion modifier such as a sulphur oxyacid derivative and/or a phosphorus oxyacid derivative and/or a boron oxyacid derivative and/or a boron oxyacid derivative in an effective amount.

Description

SPECIFICATION Improvements in or relating to fuel combustion The present invention is concerned with improving the combustion efficiency of fuel compositions, particularly cellulose-based solid fuels, by reducing the formation of tarry deposits and residues.

In many corn-growing areas of the United Kingdom there is little market for the straw produced and it is frequently burned in the fields. Such burning has been condemned as harmful to the environment and there is a possibility that it may be banned in the future. Consequently it would be desirable to provide alternative uses for the straw.

Straw is already being converted into cattle feed by treatment with caustic soda and it has recently been proposed that it should be formed into briquettes and sold as solid fuel composition. However it is a drawback of such straw briquettes that they burn with a smoky and tarry flame and generate substantial tarry residues. It is therefore one object of the present invention to improve the combustion of cellulose-based solid fuel compositions such as straw briquettes.

According to one aspect of the invention it has been found that the combustion efficiency of straw briquettes can be improved by incorporating into the briquettes during their manufacture an effective amount of a combustion modifier.

The combustion modifiers are chemical compounds which, themselves or as derivatives produced during the combustion, have either a catalytic effect or a modifying effect on the combustion to make it more complete at the primary burning site and reduce the amount of volatile combustible material and flame generated. Because less volatiles are generated the spreading of the flame is slowed down and this also aids the completeness of burning and further suppresses the formation of tarry residues in favour of friable ash and char. The precise mechanism of action of the modifier is, however, not yet known.

A variety of chemicals can be used as combustion modifiers. Amongst them are salts, which contain combined oxygen, such as phosphates and other phosphorus oxyacid salts, sulphates and other sulphut oxyacid salts, carbonates, borates and silicates, both of inorganic and of organic bases, for example aluminium sulphate, sodium polyphosphate, sodium metabisulphate, trisodium phosphate, sodium hydrogen phosphate, calcium sulphate, borax, sodium carbonate, ammonium dihydrogen orthophosphate, melamine pyrophosphate, sodium silicate and calcium silicate; oxygen-free inorganic salts such as chlorides, for example sodium calcium oxide and zinc oxide; and weak oxyacids, for example boric acid. Mixtures of two or more chemicals can be used and may be more effective than single compounds. Preferred are phosphorous oxyacid salts alone or in admixture with other chemicals.

The combustion modifiers are incorporated into the straw briquettes during their manufacture in order that they become well distributed through the briquettes and thus affect combustion of the briquettes throughout their thickness.

The combustion modifiers are employed in amounts which are effective to catalyse/modify combustion. Amounts as small as 0.01% by weight can be effective but usually at least 0.1% by weight will be employed. The use of amounts above 10% by weight is not normally worthwhile economically and ranges of 0.2 to 5%, more narrowly 0.5 to 2%, by weight are preferred.

Two particularly preferred combustion modifiers for use in straw briquettes are as follows: A) 1% by weight of a 50:50 mixture of melamine pyrophosphate and borax, and B) 0.5% by weight of a 50:50 mixture oftrisodium phosphate and calcium sulphate.

Mixtures of such materials, as well as chemically closely related salts of the acids, are also preferred.

By use of a combustion modifier of this type it has been shown to be possible to reduce the proportion of tarry deposits in a chimney above a stove burning straw briquettes from 4.8% in the absence of the modifier to 0.4% in the presence of the modifier, a dramatic improvement.

Although the above remarks have been directed primarily to the combustion of straw briquettes, similar effects may be achieved by addition of the modifiers during manufacture of other cellulosebased solid fuels such as wood chips or sawdust formed into briquettes or the like.

1. A process for the manufacture of solid fuel briquettes orthe like from a cellulose-based material comprising densifying the material and forming it into briquettes orthe like, characterised in that an effective amount of a combustion modifier is incorporated into the briquettes or the like during their manufacture.

2. A process as claimed in claim 1, characterised in that the cellulose-based material is straw.

3. A process as claimed in claim 1 or 2, characterised in that the combustion modifier is selected from salts of phosphorus oxyacids. salts of sulphur oxyacids, salts of boric acid and mixtures thereof.

4. A process as claimed in claim 3, characterised in that 0.5 to 2% by weight of combustion modifier is used.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to fuel combustion The present invention is concerned with improving the combustion efficiency of fuel compositions, particularly cellulose-based solid fuels, by reducing the formation of tarry deposits and residues. In many corn-growing areas of the United Kingdom there is little market for the straw produced and it is frequently burned in the fields. Such burning has been condemned as harmful to the environment and there is a possibility that it may be banned in the future. Consequently it would be desirable to provide alternative uses for the straw. Straw is already being converted into cattle feed by treatment with caustic soda and it has recently been proposed that it should be formed into briquettes and sold as solid fuel composition. However it is a drawback of such straw briquettes that they burn with a smoky and tarry flame and generate substantial tarry residues. It is therefore one object of the present invention to improve the combustion of cellulose-based solid fuel compositions such as straw briquettes. According to one aspect of the invention it has been found that the combustion efficiency of straw briquettes can be improved by incorporating into the briquettes during their manufacture an effective amount of a combustion modifier. The combustion modifiers are chemical compounds which, themselves or as derivatives produced during the combustion, have either a catalytic effect or a modifying effect on the combustion to make it more complete at the primary burning site and reduce the amount of volatile combustible material and flame generated. Because less volatiles are generated the spreading of the flame is slowed down and this also aids the completeness of burning and further suppresses the formation of tarry residues in favour of friable ash and char. The precise mechanism of action of the modifier is, however, not yet known. A variety of chemicals can be used as combustion modifiers. Amongst them are salts, which contain combined oxygen, such as phosphates and other phosphorus oxyacid salts, sulphates and other sulphut oxyacid salts, carbonates, borates and silicates, both of inorganic and of organic bases, for example aluminium sulphate, sodium polyphosphate, sodium metabisulphate, trisodium phosphate, sodium hydrogen phosphate, calcium sulphate, borax, sodium carbonate, ammonium dihydrogen orthophosphate, melamine pyrophosphate, sodium silicate and calcium silicate; oxygen-free inorganic salts such as chlorides, for example sodium calcium oxide and zinc oxide; and weak oxyacids, for example boric acid. Mixtures of two or more chemicals can be used and may be more effective than single compounds. Preferred are phosphorous oxyacid salts alone or in admixture with other chemicals. The combustion modifiers are incorporated into the straw briquettes during their manufacture in order that they become well distributed through the briquettes and thus affect combustion of the briquettes throughout their thickness. The combustion modifiers are employed in amounts which are effective to catalyse/modify combustion. Amounts as small as 0.01% by weight can be effective but usually at least 0.1% by weight will be employed. The use of amounts above 10% by weight is not normally worthwhile economically and ranges of 0.2 to 5%, more narrowly 0.5 to 2%, by weight are preferred. Two particularly preferred combustion modifiers for use in straw briquettes are as follows: A) 1% by weight of a 50:50 mixture of melamine pyrophosphate and borax, and B) 0.5% by weight of a 50:50 mixture oftrisodium phosphate and calcium sulphate. Mixtures of such materials, as well as chemically closely related salts of the acids, are also preferred. By use of a combustion modifier of this type it has been shown to be possible to reduce the proportion of tarry deposits in a chimney above a stove burning straw briquettes from 4.8% in the absence of the modifier to 0.4% in the presence of the modifier, a dramatic improvement. Although the above remarks have been directed primarily to the combustion of straw briquettes, similar effects may be achieved by addition of the modifiers during manufacture of other cellulosebased solid fuels such as wood chips or sawdust formed into briquettes or the like. CLAIMS

1. A process for the manufacture of solid fuel briquettes orthe like from a cellulose-based material comprising densifying the material and forming it into briquettes orthe like, characterised in that an effective amount of a combustion modifier is incorporated into the briquettes or the like during their manufacture.

2. A process as claimed in claim 1, characterised in that the cellulose-based material is straw.

3. A process as claimed in claim 1 or 2, characterised in that the combustion modifier is selected from salts of phosphorus oxyacids. salts of sulphur oxyacids, salts of boric acid and mixtures thereof.

4. A process as claimed in claim 3, characterised in that 0.5 to 2% by weight of combustion modifier is used.