Field of the invention

The invention relates to a method of combusting a solid carbonaceous fuel. The invention also relates to uses of particles of a metal oxide in the combustion of a solid carbonaceous fuel. It also concerns a fuel mixture comprising a solid carbonaceous fuel and particles of the metal oxide, and a method for its manufacture.

Background to the invention

The energy demands of many industrialised countries are increasing as the size of their population increases and the size of their industry increases. As energy demand increases, there is usually an associated increase in the cost of fuel. In many countries, the combustion of solid fuels, such as coal, provides one source of energy. However, supplies of non-renewable natural resources that are used for fuel, like coal, are diminishing. It is therefore desirable to provide methods that increase the energy output from solid fuels.

The combustion of solid fuels invariably produces a number of undesirable byproducts, such as soot or smoke, that are environmental pollutants. Soot can also be harmful to human health when inhaled. When solid fuels are combusted on an industrial scale to produce energy, then many countries require that the amount of soot and smoke in the emissions or exhaust gases from the combustion process is reduced to a negligible level. There are costs associated with the installation of apparatus to remove such undesirable by-products and its maintenance.

Summary of the invention

Combustion of a solid carbonaceous fuel together with a metal oxide as described herein may provide more complete combustion of the solid fuel, improve thermal output and reduce the amount, or inhibit the production of undesirable particulate matter (e.g. soot) produced as a by-product of combustion. In particular, reducing the amount or inhibiting the production of undesirable particulate matter produced by combustion of the solid fuel may improve thermal efficiency of an apparatus, such as by reducing energy losses at a surface of a heat exchanger in a boiler or the like. Exhaust gases may also require less cleaning or scrubbing.

The present invention therefore provides a process of combusting a solid carbonaceous fuel comprising the step of combusting said fuel in the presence of particles of a metal oxide, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

It also provides a process of combusting a solid carbonaceous fuel comprising the step of combusting said fuel in the presence of particles of a transition metal oxide or a rare earth metal oxide.

A further aspect of the invention relates to the use of particles of a metal oxide in the combustion of a solid carbonaceous fuel to improve the energy output obtained from combustion of the fuel compared to the combustion of the same solid carbonaceous fuel under the same conditions except that said particles of the metal oxide are not present, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

The invention also relates to the use of particles of a transition metal oxide or a rare earth metal oxide in the combustion of a solid carbonaceous fuel to improve the energy output obtained from combustion of the fuel compared to the combustion of the same solid carbonaceous fuel under the same conditions except that said particles of the transition metal oxide or the rare earth metal oxide are not present.

A further aspect of the invention relates to the use of particles of a metal oxide as a combustion improver in the combustion of a solid carbonaceous fuel, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

The invention further relates to the use of particles of a transition metal oxide or a rare earth metal oxide as a combustion improver in the combustion of a solid carbonaceous fuel.

Another aspect of the invention relates to the use of particles of a metal oxide in the combustion of a solid carbonaceous fuel to inhibit the production of soot, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

It also relates to the use of particles of a transition metal oxide or a rare earth metal oxide in the combustion of a solid carbonaceous fuel to inhibit the production of soot. The invention also relates to the use of particles of a metal oxide in the combustion of a solid carbonaceous fuel to reduce the amount of smoke and/or noxious smells produced by combusting the fuel compared to the combustion of the same solid carbonaceous fuel under the same conditions except that the particles of the metal oxide are not present, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

A further aspect of the invention relates to the use of particles of a transition metal oxide or a rare earth metal oxide in the combustion of a solid carbonaceous fuel to reduce the amount of smoke and/or noxious smells produced by combusting the fuel compared to the combustion of the same solid carbonaceous fuel under the same conditions except that said particles of the transition metal oxide or the rare earth metal oxide are not present.

Further, the invention provides a fuel mixture comprising a solid carbonaceous fuel mixed with particles of a metal oxide, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

It also provides a fuel mixture comprising a solid carbonaceous fuel mixed with particles of a transition metal oxide or a rare earth metal oxide.

Another aspect of the invention is a method of manufacturing a fuel mixture for combustion, comprising admixing a particle of a metal oxide to a solid carbonaceous fuel, wherein a metal of said metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

The invention also provides a method of manufacturing a fuel mixture for combustion, comprising admixing a particle of a transition metal oxide or a rare earth metal oxide with a solid carbonaceous fuel.

Detailed description

The reactions that are believed to take place when a solid carbonaceous fuel is combusted in the presence of metal oxide particles described herein are set out below. The reactions shown use cerium oxide as an example, but other metal oxides may also be able to undergo or catalyse one or more of the reactions shown in an analogous manner. After ignition and during the combustion process, cerium oxide oxidises the carbon of, or present in, the hot solid carbonaceous fuel.

CeO ₂ + 2C → Ce + 2CO 2CeO ₂ + C → Ce ₂ O ₃ + CO.

If any water is present in the solid carbonaceous fuel, then some of it will evaporate during ignition or combustion of the fuel. Some water may remain because it has had no time to evaporate due to a high combustion rate or because it is initially trapped within the body of the solid fuel and is liberated during the combustion process. Water may also be produced during combustion of the solid carbonaceous fuel.

Water that is present during combustion of the solid carbonaceous fuel may oxidise a cerium species to produce hydrogen gas.

2H ₂ O + Ce → 2H ₂ + CeO ₂ H ₂ O + Ce ₂ O ₃ → H ₂ + 2CeO ₂ .

The reaction regenerates cerium (IV) oxide, which may then undergo further reaction with the carbon present in the fuel and thereby repeat the cycle shown above. The hydrogen produced burns off in the air and generates additional heat according to the following equation:

2H ₂ +O ₂ → 2H ₂ O.

The reaction of hydrogen with oxygen proceeds at a high rate and increases the rate of combustion. Oxygen used to react with the hydrogen may also be generated by dissociation of cerium oxide particles as follows:

CeO ₂ =^ Ce + O ₂ 4CeO ₂ ^==^ 2Ce ₂ O ₃ + O ₂

Cerium (IV) oxide can become non-stoichiometric in its oxygen content (i.e. it can give up oxygen without decomposing) depending on the ambient partial pressure of oxygen.

As used herein, the term "combustion", in the context of the solid carbonaceous fuel, refers to the reaction of the solid carbonaceous fuel with oxygen to generate heat. Combustion of the solid carbonaceous fuel in the invention is typically performed in air.

It is preferred that combustion of the solid carbonaceous fuel is performed until substantially all of the solid carbonaceous fuel is consumed (i.e. the fuel is spent). In one embodiment of the invention, the solid carbonaceous fuel is continually loaded or fed into a combustion chamber to maintain combustion. For example, the fuel is replenished in a power station usually by continually feeding coal into the power station's boiler to maintain the generation of electricity. Under such conditions, it is preferable that combustion is performed until substantially each portion or quantity of solid carbonaceous fuel that is loaded or fed into a combustion chamber is consumed during a continual combustion process.

Without wishing to be bound by theory, in the combustion of many solid fuels, such as coal or coke, there are usually three distinct phases: (i) the preheating phase where the fuel is heated up to its fire point (the temperature at which it will continue to burn after ignition for at least 5 seconds) and flammable gases start to evolve from the fuel, (ii) the distillation or gaseous phase where the evolved flammable gases mix with oxygen and are ignited to produce heat and light, and (iii) the solid phase where evolution of flammable gases is too low for the persistent presence of a flame and the resulting charred fuel or ash does not burn rapidly anymore, but just glows and then smoulders. It is preferred that combustion of the solid carbonaceous fuel is performed until at least 90%, more preferably at least 95% and even more preferred is at least 99%, of the total weight of the fuel is in the solid phase of combustion (e.g. it has been converted to ash), as described above.

Typically, the solid carbonaceous fuel is coal, coke, wood, biomass, charcoal, peat, animal dung or a mixture of two or more thereof. Coal that may be used in the invention includes lignite, bituminous coal, anthracite or a mixture of two or more thereof. In principle any type of wood may be used provided that it is suitably dry to permit combustion. Wood in the form of wood pellets, wood chips or a mixture of two or more thereof may be used. Biomass is an energy source obtained from living or recently dead material. Typically, the biomass is a renewable energy source. Examples of biomass that may be used in the invention include corn, wheat, rye, rice husks, bagasse, groundnut shells or a mixture of two or more thereof. In principle, any type of animal dung may be used in the invention, provided that it has a moisture content that is suitable for combustion of the dung. Animal dung that is commonly used as a solid carbonaceous fuel is cow dung or camel dung.

It is preferred that the solid carbonaceous fuel is coal, coke or wood. More preferably, the fuel is coal or coke, and even more preferably the fuel is coal. It is to be understood that the reference to combusting a solid carbonaceous fuel in the presence of particles of a metal oxide refers to combusting a solid carbonaceous fuel that is in contact with the particles of the metal oxide. Typically, the invention involves the combustion of a mixture of solid carbonaceous fuel and particles of the metal oxide. It is preferred that the mixture is a homogeneous mixture.

A mixture comprising a solid carbonaceous fuel and the particles of the metal oxide, such as the fuel mixture of the invention, may be in the form of a pellet or a briquette. Thus, a use or process of the invention may include the step of adding a mixture comprising a solid carbonaceous fuel and particles of a metal oxide as described herein to a combustion chamber before the step of combusting the fuel. A method of manufacturing a fuel mixture of the invention may include the step of pelletising, or compressing to form a briquette, a mixture comprising a solid carbonaceous fuel and the particles of a metal oxide.

Typically, a metal oxide for use in the invention is an oxide of a transition metal or a rare earth metal. An oxide of a rare earth metal is preferred. A transition metal as described herein is a member of group 3 (also referred to as group IHB) to group 12 (also referred to as group HB) of the Periodic Table. Typically, the transition metal oxide or rare earth metal oxide in the invention is an oxide of zirconium, chromium, vanadium, manganese, iron, lanthanum, cerium, samarium, praseodymium or gadolinium. Preferably, the metal oxide is selected from zirconium, chromium, vanadium, manganese and cerium. Zirconium oxide is a preferred transition metal oxide. Cerium oxide is a preferred rare earth metal oxide. Most preferred is cerium oxide (preferably cerium (IV) oxide). The metal oxide may be a compound where the same metal is present in more than one oxidation state.

Although it will be more usual to employ a simple metal oxide in the invention, it is also possible to use mixed metal oxides. The term mixed metal oxide as used herein refers to a mixture of two or more metal oxides, which will generally be of two or more transition metals and/or rare earth metals.

In one embodiment, the metal oxide is a mixed metal oxide comprising a mixture of at least one transition metal oxide and one rare earth metal oxide.

In another embodiment, the metal oxide is a mixed metal oxide comprising a mixture of at least two different transition metal oxides. In a further embodiment, the metal oxide is a mixed metal oxide comprising a mixture of at least two different rare earth metal oxides.

When the metal oxide is a mixed metal oxide, at least one metal of the transition metal oxide or rare earth metal oxide has at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

Preferably, at least two different metals of the mixed metal oxide each independently have at least one oxidation state capable of oxidising the fuel during combustion and/or at least one oxidation state capable of being oxidised during combustion.

Typically, each transition metal oxide of the mixed metal oxide is independently selected from an oxide of zirconium, chromium, vanadium, manganese and iron.

Each rare earth metal oxide of the mixed metal oxide is, typically, independently selected from an oxide of lanthanum, cerium, samarium, praseodymium and gadolinium.

In another embodiment, the metal oxide is a mixed cerium zirconium oxide.

In some instances, the activity or efficiency (e.g. as a catalyst) of the metal oxide (especially in the case of cerium oxide) can be enhanced by addition of further components in the material.

In an embodiment of the invention, the metal oxide is doped with one or more components that result in additional oxygen vacancies being formed. Thus doping will generally be substitution doping as opposed to interstitial doping. This will clearly enhance the oxygen storage capacity (OSC) of the material, and hence its activity (e.g. its catalytic properties).

Preferably, the host lattice of the doped metal oxide is a transition metal oxide or a rare earth metal oxide. Typically, the host lattice is an oxide of zirconium, chromium, vanadium, manganese, iron, lanthanum, cerium, samarium, praseodymium or gadolinium.

Doping involves the incorporation of a dopant ion component into at least a part of a host lattice of a metal oxide. The metal of the host lattice metal oxide is generally a different element to the dopant ion. Dopant ions can be incorporated singly or in combination of two or three or more.

Such dopant ions are preferably di- or tri-valent in order to provide oxygen vacancies. They must also be of a size that allows incorporation of the ion within the surface region of the metal oxide particles. Accordingly metals with a large ionic radius should not be used. For example transition metals in the first and second row of transition metals are generally preferred over those listed in the third.

Typically, the metal oxide (i.e. the host lattice of the metal oxide) is doped with a divalent or trivalent metal, which metal is a rare earth metal, a transition metal, including a noble metal, or a metal of group 2 (also referred to as group IIA), group 3 (also referred to as group IIIB), group 5 (also referred to as group VB) or group 6 (also referred to as group VIB) of the Periodic Table.

In particular, the metal oxide (i.e. the host lattice of the metal oxide) is preferably doped with Zr, Rh, Ni, Cu, Ag, Au, Nb, Ta, Pd, Pt, Fe, Mg, Mn, Cr, Mo, Be, Co, V, Ca, Sr, Ba, Ga, Sn, Si, Al, Pr, Sm, Gd, La or Ce. Preferably, the dopant ion is selected from Mn ²⁺ , Mn ³⁺ , V ³⁺ , V ⁵⁺ and Cr ³⁺ .When the dopant ion is a rare earth metal, then the host lattice must be of a size that allows the dopant ion to be incorporated. Thus, it is preferred that the host lattice of the metal oxide is a rare earth metal oxide or is a transition metal oxide, where the transition metal is from the third row of transition metals in the periodic table, when the dopant ion is a rare earth metal.

Typically, the total amount of dopant ion in the host lattice is 0.05 mole% to 10 mole%, preferably 0.1 mole% to 5 mole%, and more preferably 0.5 mole% to 2 mole%.

In an embodiment where the metal oxide is cerium oxide, the cerium oxide serves as an oxygen activation and exchange medium during a redox reaction. When the metal oxide is cerium (FV) oxide, then it may be doped with a divalent or trivalent metal which metal is a rare earth metal, a transition metal, including a noble metal, or a metal of group 2 (also referred to as group IIA), group 3 (also referred to as group IIIB), group 5 (also referred to as group VB), or group 6 (also referred to as group VIB) of the Periodic Table. In one embodiment, the oxides will have the formula Ce _1-X M' _X O ₂ where M' is a said metal, in particular Zr, Rh, Ni, Cu, Ag, Au, Nb, Ta, Pd, Pt, Fe, Mg, Mn, Cr, Mo, Be, Co, V, Ca, Sr, Ba, Ga, Sn, Si, Al, Pr, Sm, Gd or La and x has a value up to 0.3, typically 0.01 or 0.1 to 0.2. In another embodiment, the metal oxide has the formula [(CeO ₂ ) _J-0 (REOy) _n ] _I- IcM ⁺ _I c where M* is a said metal other than a rare earth metal, RE is a rare earth metal, y is 1 or 1.5 and each of n and k, which may be the same or different, has a value up to 0.5, preferably up to 0.3, typically 0.01 or 0.1 to 0.2. If too much dopant is used, there will be an increasing tendency for it to form an oxyanion thus negating the benefits of introducing it. Dopants may be incorporated into a metal oxide, such as cerium oxide, using a method as described in WO 03/040270 or WO 2004/065529.

The particles of the metal oxide used in the present invention can be obtained in a conventional manner. Thus, the particles may be prepared by controlled precipitation, combustion synthesis, mechanochemical processing or flame pyrolysis, as well as by other methods described in the literature for the production of particles.

Typically, the particles of the metal oxide have an average particle size from 1 nm to 10 μm. It is preferred that the average particle size of the particles of the metal oxide is from 1 nm to 1 μm, more preferably the average particle size is from 1 nm to 500 nm, even more preferred the average particle size is from 1 nm to 200 nm. In particular, the particles of the metal oxide may have an average particle size from 1 nm to 100 nm, more preferably the average particle size is from 2 nm to 50 nm, and especially from 3 nm to 20 nm. X-ray diffraction (XRD) has been used to determine the size of the particles.

Normally, a distribution of particles having various sizes is obtained. Any reference to particle size as used herein refers to the size (i.e. the mean particle size) of the particles in a size distribution of the particles. If a particle of the metal oxide is coated, such as with a coating agent, then the particle size refers to the size of the metal oxide at the core of the coated particle (i.e. it does not include the thickness of the coating).

The particles of the metal oxide are preferably nanocrystalline in nature. If the metal oxide is a mixed metal oxide, then it is preferable that the nanoparticles of at least one of the metal oxides present in the mixed metal oxide is nanocrystalline. More preferably, the nanoparticles of each metal oxide of the mixed metal oxide is nanocrystalline. More preferably, the particles of the metal oxide are nanocrystalline in nature and have an average particle size that does not exceed 1 micron.

The particles of the metal oxide may have a regular or an irregular shape. Thus, any reference to particle size as used herein refers to a volume based particle size. The particle size is equal to the size of a sphere having the same volume as that particle. For spherical particles, the diameter of the particle is equal to the diameter of the sphere.

It is believed that the activity of the particles of the metal oxide in the invention is surface area dependent. A small particle size, particularly when the metal oxide is nanocrystalline, renders the material more effective as a reactant or a catalyst. In general, the particles of the metal oxide in the invention should have as large a surface area as possible, such as a surface area of at least 10 m ² /g and preferably a surface area of at least 50 or 75 m ² /g, for example 80 to 150 m ² /g, or 100 to 300m ² /g.

Typically, the particles of the metal oxide, in the invention, are present in a total amount of 0.0001% to 0.5% by weight of the solid carbonaceous fuel. It is preferred that the total amount of the particles of the metal oxide present is 0.001% to 0.05% by weight of the solid carbonaceous fuel, more preferably 0.005% to 0.035% by weight of the solid carbonaceous fuel, particularly 0.0075% to 0.025% and more preferably 0.01% to 0.02% by weight of the solid carbonaceous fuel. The quantity of particles of a metal oxide used in the invention may vary depending on the metal oxide used.

In one embodiment of the invention, the particles of the metal oxide are coated with an organic coating. The organic coating may assist dispersion (e.g. to prevent agglomeration of the particles) or admixture of the particles with the solid carbonaceous fuel, especially if the metal oxide particles are susceptible to oxidation in air at room temperature. The organic coating is burnt off during the initial stages of combustion.

The particles are coated with a coating agent that is suitably an organic acid, anhydride or ester or a Lewis base. The coating agent is preferably an organic carboxylic acid or an anhydride, typically one possessing at least 8 carbon atoms, for example 10 to 25 carbon atoms, especially 12 to 18 carbon atoms, such as stearic acid. It will be appreciated that the carbon chain can be saturated or unsaturated, for example ethylenically unsaturated as in oleic acid. Similar comments apply to the anhydrides that can be used. They are preferably dicarboxylic acid anhydrides, especially alkenyl succinic anhydrides, particularly dodecenylsuccinic anhydride, octadecenylsuccinic anhydride and polyisobutenyl succinic anhydride. A preferred anhydride is dodecenylsuccinic anhydride. Other organic acids, anhydrides and esters that can be used in the process of the present invention include those derived from phosphoric acid and sulphonic acid. The esters are typically aliphatic esters, for example alkyl esters derived from an acid and an alcohol, each of which may contain 4 to 18 carbon atoms.

Other coating agents which can be used, although are less preferred, include Lewis bases which possess an aliphatic chain of at least 8 carbon atoms including mercapto compounds, phosphines, phosphine oxides and amines as well as long chain ethers, diols and aldehydes, as well as mixtures of two or more of the coating agents mentioned above. The long chain ethers, diols and aldehydes may possess an aliphatic chain of at least 6 carbon atoms, preferably 8 to 18 carbon atoms. The oxygen atom of the long chain ether is bonded to two aliphatic groups, which may be the same (e.g. di-hexyl ether, di-heptyl ether, di-octyl ether) or different (e.g. methyl-octyl ether, ethyl-octyl ether, propyl-octyl ether, iso-propyl-octyl ether). At least one of the aliphatic groups in the long chain ethers has an aliphatic chain may contain at least 6 carbon atoms, preferably 8 to 18 carbon atoms. The other aliphatic chain may have a long chain as defined above, or may contain 1 to 5 carbon atoms.

The coating process can be carried out in an organic solvent. Preferably, the solvent is non-polar and is also preferably non-hydrophilic. It can be an aliphatic or an aromatic solvent. Typical examples include toluene, xylene, petrol, diesel fuel as well as heavier fuel oils. Naturally, the organic solvent used should be selected so that it is compatible with the intended end use of the coated particles.

The coating process generally involves comminuting the particles so as to prevent any agglomerates from forming. Techniques that can be used for this purpose include high-speed stirring or tumbling and the use of a colloid mill, ultrasonics or ball milling. Further details of methods for preparing such coatings can be found in WO 02/092703.

Typically, the particles of the metal oxide are present as an aqueous dispersion or as a dispersion in an organic solvent or dispersant. Suitable organic dispersants include an organic acid, anhydride or ester or a Lewis base, such as the materials that are described as coating agents above. This provides a convenient way of adding the particles of the metal oxide to the solid carbonaceous fuel. Thus, the invention may include the step of adding a dispersion of the particles of the metal oxide in an organic liquid or an aqueous dispersion of the particles of the metal oxide to the solid carbonaceous fuel before the step of combusting the fuel. It is preferred that the dispersion of particles of the metal oxide is sprayed onto the solid carbonaceous fuel before combustion of the fuel. More preferably, the dispersion of the particles is an aqueous dispersion.

When the particles of the metal oxide are present as a dispersion, then the dispersion of the particles may comprise a dispersion aid. The dispersion aid helps to stabilise the dispersion of the metal oxide particles in the organic liquid or water. Suitable dispersion aids, particularly for an aqueous dispersion of the particles, include glycols, such as ethylene glycol or polyethylene glycol having an average molecular weight of less than 1500, preferably a polyethylene glycol having an average molecular weight of less than 650 (e.g. PEG 600, PEG 400 or PEG 200); polyacrylic acid or polymethacrylic acid, or a salt of polyacrylic acid or polymethacrylic acid (e.g. the sodium or potassium salt). Typically, the weight ratio of the metal oxide to water or the organic liquid in a dispersion of particles of the metal oxide is from 1:10 to 1:10000. Preferably, the weight ratio is from 1:100 to 1:2500, more preferably from 1:200 to 1:1000, and even more preferred is a weight ratio from 1 :250 to 1 :750.

Typically, the combustion of the solid carbonaceous fuel in the presence of particles of the metal oxide is performed in a combustion chamber, such as a boiler, a kiln, an oven, a furnace or an incinerator. The boiler may, for example, be a power plant boiler, a hog-fuel boiler, a ship boiler or a central steam heat boiler. The kiln may, for example, be a pottery kiln or a cement kiln. The furnace may be a blast furnace, such as a blast furnace typically found in a metal works and is used for smelting a metal ore. The incinerator may, for example, be a refuse incinerator. It is preferred that the combustion chamber is a boiler.

Some solid carbonaceous fuels, such as coal, are usually obtained in raw form as sizeable lumps and it may be necessary to process the fuel before the combusting it. Typically, the invention may include the step of comminuting a solid carbonaceous fuel to obtain pieces of the solid carbonaceous fuel having an average diameter of less than 5 cm. For example, the step of comminuting the solid carbonaceous fuel may be performed in a milling machine or may involve grinding or crushing of the solid. For power stations, the step of comminuting coal is often referred to as pulverising the coal and is usually performed in a machine known as a coal pulveriser. In the invention, it is preferred that pieces of solid carbonaceous fuel, particularly coal, coke or charcoal, have an average diameter of less than 5 cm.

In one embodiment of the invention, the metal oxide is admixed with the solid carbonaceous fuel before the step of comminuting the solid carbonaceous fuel. The metal oxide and the solid carbonaceous fuel may then be comminuted together. Particles of the metal oxide may be admixed with the solid carbonaceous fuel as a powder or as a dispersion, such as an aqueous dispersion or a dispersion in an organic liquid as described above.

In another embodiment of the invention, particles of the metal oxide are admixed with pieces of the solid carbonaceous fuel having an average diameter of less than 5 cm (i.e. after the step of comminuting the solid carbonaceous fuel). The particles of the metal oxide may be admixed with the solid carbonaceous fuel as a powder or as a dispersion, such as an aqueous dispersion or a dispersion in an organic liquid as described above. Typically, the invention relates to a process for improving the energy output obtained from the combustion of a solid carbonaceous fuel, which process comprises the step of combusting a solid carbonaceous fuel in the presence of particles of a metal oxide as described herein.

Typically, the energy output obtained from the combustion of the solid carbonaceous fuel may depend on a number of factors, such as the type of fuel used, the amount of water present and the conditions used for its combustion. It is to be appreciated that the energy output referred to herein (usually given in units of MJ kg ^'1 ) is an energy output produced in the general combustion of the fuel in a combustion chamber, such as the energy output obtained from a boiler in a power station. It is to be understood that the improvement in energy output does not refer to an improvement in the intrinsic amount of energy that may be obtained from a fuel, such as that measured in a laboratory under adiabatic or isothermal conditions.

The invention may, typically, increase the energy output obtained from the combustion of the solid carbonaceous fuel in a combustion chamber by at least 0.1%, particularly by at least 1% and even more preferably by at least 5% (compared to the combustion of the same solid carbonaceous fuel under the same conditions, except that the particles of the metal oxide are not present). It is preferred that the energy output is a thermal output (i.e. heat output).

In the invention, the improvement in energy output obtained from combustion of the solid carbonaceous fuel may also be an improvement in the rate of thermal output (often given in units of BTU/hr per kilogram of fuel). Typically, the invention may provide an increase in the rate of thermal output (compared to the combustion of the same solid carbonaceous fuel under the same conditions, except that the particles of the metal oxide are not present) of at least 1%, particularly by at least 2.5% and even more preferably by at least 5%.

In general, the process of the invention of combusting a solid carbonaceous fuel is a process of improving the combustion properties of a solid carbonaceous fuel, which comprises the step of combusting said fuel in the presence of particles of a metal oxide as described herein.

As a combustion improver, the particles of the metal oxide in the invention may reduce the ignition temperature of the solid carbonaceous fuel. Typically, the particles of the metal oxide in the invention are used as a combustion improver to increase the amount of complete combustion of the solid carbonaceous fuel compared to combustion of the solid carbonaceous fuel under the same conditions except that the particles of the metal oxide are not present. In one embodiment of the invention, the particles of the metal oxide are used as a combustion improver in the combustion of a solid carbonaceous fuel.

By increasing the amount of complete combustion of a solid carbonaceous fuel, the particles of the invention may be used to reduce the amount of carbon monoxide present in the gaseous by-products produced by combustion of the solid carbonaceous fuel. This may be measured in comparison to the combustion of the same fuel under the same conditions except that the particles of the metal oxide are not present. Typically, a reduction in the amount of carbon monoxide of, for example, at least 0.1% is obtained. In particular, the reduction in the amount of carbon monoxide may be at least 1%, more preferably at least 2%. In applications where the exhaust gases from combustion of solid fuels are scrubbed, an advantage of the invention is that it may increase the lifetime of the scrubbers that are used to remove carbon monoxide.

Typically, the particles of a metal oxide in the invention may be used in the combustion of a solid carbonaceous fuel to inhibit the production of soot. Soot may coat the inner surfaces of boilers, which reduces heat transfer and hence reduces the thermal efficiency of the boiler. Thus, the process of the invention may also be a process for improving the thermal efficiency of a boiler.

The term "soot" as used herein refers to particulate matter, usually black carbon particles, which is a by-product that results from the incomplete combustion of a solid carbonaceous fuel. Soot often contains polycyclic aromatic hydrocarbons, which are known mutagens and are probable carcinogens.

Typically, the particles of the metal oxide used in the invention in the combustion of a solid carbonaceous fuel reduce the quantity of soot produced by combustion of the fuel by at least 0.1% compared to combustion of the same solid carbonaceous fuel under the same conditions, except that the particles of the metal oxide are not present. More typically, the amount of soot is reduced by at least 1%, more preferably by at least 5% or is even reduced by at least 10%.

Normally, the particles of the metal oxide used in the invention in the combustion of a solid carbonaceous fuel reduce the quantity of soot produced by combustion of the fuel that accumulates on in the flue for exhaust gases produced by combustion and/or a surface in a combustion chamber, preferably a surface for heat transfer in a combustion chamber, such as in a boiler.

Generally, the particles of the metal oxide may be used in the invention in the combustion of a solid carbonaceous fuel to reduce the amount of smoke produced by combustion of the fuel. Typically, the amount of smoke is reduced from level 3 to level 1 on the Ringelmann Smoke Chart.