TECHNICAL FIELD

The present invention relates to a solid biofuel additive.

BACKGROUND OF THE INVENTION

Burning of solid biofuels is beset with a number of problems. For reasons of a handling and combustion-technical nature, solid fuels are often compressed to form pellets, briquettes or the like, particularly when the fuels are in a finely-divided state, such as wood chips, sawdust and certain types of bio-waste.

Compression of the fuel often requires the addition of a binder, so that the compressed product will remain intact after compression. The binder used may also require the presence of or the addition of water, which reduces the calorific value of the fuel, among other things. The manufacture of pellets from sawdust involves, among other things, the use of lignin compounds which require the presence of water in order to provide a satisfactory binding effect, therewith increasing the moisture content of the sawdust. Many additives, such as those proposed in accordance with U.S. Patent No. 3,240,573, have an environmentally suspect origin and produce doubtful degradation products, since they constitute rest products from mineral oil and coal-refining processes, such as tar, asphalt products and synthetic waxes, for instance. According to U.S. Patent No. 3,240,573, polymer binders, such as formaldehyde-based binders, are also used. U.S. Patent No. 4,357,145 describes a method of manufacturing briquettes from powdered coal, in which, prior to briquetting, fuel and binder are first mixed in a complicated mixing process, to form an aqueous slurry.

When firing boilers continuously on an industrial scale, a high boiler efficiency and long operating times between consecutive boiler shutdowns for cleaning purposes are of fundamental significance. When firing with oil and solid fuels, the fuel additives may be used to modify fuel ignition temperature and combustion temperature for instance, so as to reduce the occurrence of surface coatings and their negative effects and therewith improve efficiency and reduce dust emissions. These additive substances are often metal compounds that are undesirable from an environmental aspect, such as lead, tin, zinc, barium, copper or manganese.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a solid biofuel additive which is both active as a binder that has lubricating properties when compressing solid biofuels and which will improve boiler efficiency and reduce dust emissions, probably by virtue of a catalyzing effect. The additive substance will also preferably function to reduce the extent of deposits on the heat-transfer surfaces of the boiler. According to the invention, the additive substance will often have a calorific value which is as high as or higher than the calorific value of the fuel and will preferably reduce or essentially eliminate other of the aforesaid deficiencies of those additive substances used at present.

These and other objects of the invention are achieved with a substance that has the characteristic features set forth in the following Claim 1.

By solid biofuels is meant in accordance with the invention combustible, non-fossil substances, essentially substances which derive from the plant and animal kingdoms and which are not gaseous or liquid at temperatures beneath +100°C. Examples of such solid biofuels are wood in different forms, forest waste, grass, bark and other forest by-products and rest products, straw and other agricultural by-products and rest products, wastepaper, garbage, peat, etc., and also mixtures of these and other products. The biofuel may also include other substances, preferably combustible substances, which are present in larger or smaller amounts and which may derive from the mineral kingdom, for instance plastic material. On the other hand, the term biofuels does not include those which are comprised substantially of fossil fuels or are otherwise derived from the mineral kingdom, such as coal, coke, brown coal and oil-shale.

The inventive additive substance includes metal-organic compounds, such as metal carboxylates. Metals from the groups alkaline earth metals, rare earth metals and the iron group (group 8 in the periodic system) can be used.

When used as an additive in the compression of solid biofuels in accordance with the present invention, the binding and lubricating properties of these carboxylates and fatty acids are combined with the polymerizing or "drying" properties of unsaturated and/or multi-unsaturated carboxyl and fatty acid groups as well as unsatored and/or multi-unsaturated carboxyland fatty acids to hold together the compacted solid fuel, at the same time as the metals of the carboxylates improve combustion in the boiler and boiler operation. Resin acid is also used in the present invention.

The carboxylates used in the present invention are carboxylates that have 7 to 22 carbon atoms, preferably 10 to 20 carbon atoms and suitably 18 to 20 carbon atoms. These carboxylates may be saturated or single-unsaturated or multi-unsaturated. The metals of the carboxylates are one or more of the metals from the group alkaline earth metals, rare earth elements and the iron group (group 8 in the periodic system). Lanthanides are an example of particularly preferred metals (elements) from the group rare earth elements. The additive contains preferably 100-200 g/litre of the aforesaid metals. The relationships between the metals is preferably 10-30% magnesium, 40-80% calcium and 10-30% iron.

The metals are intended to enhance combustion and improve boiler operation, for instance by modifying the combustion temperature, modifying the smelt temperature of boiler coatings, and reducing dust emissions.

Although mineral oils can be used as a solvent for the carboxylates and as a means for lowering the viscosity of the additive substance, it is preferred to use esters which occur naturally or esters with naturally occurring fatty acids, such as sunflower oil, soya oil and rape oil and mixtures thereof. Rape (oil) methyl ester (RME) is particularly preferred.

The above metal carboxylates and the solvent are then admixed with fatty acids and/or resin acids. The fatty acids used are saturated and/or unsaturated fatty acids which preferably have 10 to 22 carbon atoms. The resin acids used are preferably those that have 15 to 22 carbon atoms.

In one preferred embodiment, the additive substance is comprised of carboxylates that contain magnesium, calcium and iron, and a solvent, such as rape methyl ester (RME) in a mixture of fatty acids and resin acids, such as crude tall oil. By crude tall oil is meant in this case the undistilled acid-treated and heat-treated crude soap obtained in sulphate mills. If desired, the additive substance may be supplemented with other additive substances that improve combustion in accordance with known techniques, such as oxidizing substances, for instance nitrates, nitrites, manganates, permanganates and other metal-organic cyclic and/or polycyclic compounds.

The additive substance is prepared by mixing the metal carboxylates with the solvent and then mining the resultant mixture with a solution containing essentially fatty acids and resin acids, at a temperature of between 30 and 60°C.

The additive substance is suitably added in its liquid state to the solid biofuel, by applying said substance to the solid biofuel either prior to and/or in a mixture prior to compressing the biofuel, e.g. by spraying. The additive substance is metered to the solid biofuel so that the ratio of metal carboxylates to the solid biofuel (calculated on a bone dry basis) will be between 1:500 and 1:100,000, preferably between 1:1,000 and 1:8,000, suitably between 1:1,500 and 1:4,000.

The use of the inventive additive substance has given very positive results, both with regard to compaction of the solid biofuel to pellet form and with regard to burning of the fuel. For instance, the compressor capacity has been increased, the moisture content of the solid biofuel has decreased and the density of the compressed fuel has increased, which together provides a compressed solid biofuel of higher calorific value which can be transported more easily and at lower transport costs than biofuels produced in accordance with earlier known techniques. Furthermore, the use of the inventive additive substance enables the solid biofuel to be admixed with other materials that could not earlier be admixed with the fuel while still obtaining a coherent compressed product. With regard to burning of the solid biofuel in a boiler, the solid biofuel has been found to produce less soot, less coatings on the heat-transfer surfaces of the boiler, less dust emissions and a more neutral ash at the same air/fuel ratios. Further tests have also shown an improved efficiency, calculated on the heat generated with each kg of solid biofuel delivered, and greater power in comparison with solid biofuel that has been pelletized in accordance with known techniques.

Further characteristic features and aspects of the invention and advantages afforded thereby will be apparent from the following Claims and from the following description of a conceivable embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

In one preferred embodiment of the invention, the additive substance contains the metals magnesium, calcium and iron. The relationships between the metals is 20% magnesium, 60% calcium and 20% iron.

A first component i) of the additive substance is comprised of a known fuel additive, such as Bycosin DP91 004 from BYCOSIN AB, Box 627, S-65114, Karlstad, Sweden, this component constituting 5% of the additive substance.

A second component ii) of the additive substance is comprised of rape methyl ester, this component constituting 5% of the additive substance.

A third component iii) of the additive substance is comprised of crude tall oil, this component constituting 90% of the additive substance.

In the preferred embodiment, the additive substance is prepared by mixing the metal carboxylates with rape methyl ester and thereafter mixing the resultant solution with the crude tall oil. The tall oil is heated to 50°C at normal air pressure. The mixture of metal carboxylates and rape methyl ester is then added while vigorously agitating the mix.

When pelletizing sawdust to pellets having a diameter of about 10 mm and a length of about 20 mm, 1.4% of the additive substance, calculated on the weight of the sawdust, was sprayed onto the sawdust in a mixing chamber prior to compressing the sawdust to pellet form. In comparison with earlier known techniques using lignin compounds as a binder, the pellets had a much lower moisture content, 4.5% instead of 9.5%, owing to the fact that there was no need to add water in order to "activate" the binder. The relative density of the pellets also increased from about 650 g/litre to about 690 g/litre. These property improvements alone result in a higher calorific value and improved transport economy with regard to transportation of the pellets from the pelletizing plant to the combustion plant.