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Title:
FLUIDISED BED REACTION METHODS
Document Type and Number:
WIPO Patent Application WO/2018/130544
Kind Code:
A1
Abstract:
The present invention relates to fluidised bed reaction methods.

Inventors:
PRUSISZ BARTLOMIEJ (NL)
MAST ADRIANA PIETERNELLA (NL)
Application Number:
EP2018/050516
Publication Date:
July 19, 2018
Filing Date:
January 10, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SIBELCO NEDERLAND N V (NL)
International Classes:
F23C10/01; F23G5/30; F23G7/10
Foreign References:
US20020134019A12002-09-26
JP2003160789A2003-06-06
US20130123564A12013-05-16
US20100326888A12010-12-30
Attorney, Agent or Firm:
BOND, Christopher William (GB)
Download PDF:
Claims:
Claims

1 . A method of oxidising fuel in a fluidised bed, the method comprising the steps of:

adding bed materials to a fluidised bed reactor, wherein the bed materials are capable of forming a fluidised bed;

adding fuel to the fluidised bed reactor;

adding both a clay mineral and a catalyst to the fluidised bed reactor; forming a fluidised bed by applying gas to the mixture of bed materials, clay mineral, catalyst and fuel; and,

applying heat to oxidise the fuel.

2. The method of claim 1 , wherein the method of oxidising fuel in a fluidised bed is a method of fluidised bed combustion.

3. The method of claim 1 , wherein the method of oxidising fuel in a fluidised bed is a method of fluidised bed gasification.

4. The method of any one of claims 1 to 3, wherein the step of adding both a clay mineral and a catalyst to the fluidised bed reactor occurs before, during or after the step of adding bed materials.

5. The method of any one of claims 1 to 4, wherein the step of adding both a clay mineral and a catalyst to the fluidised bed reactor occurs before, during or after the step of adding fuel.

6. The method of any one of claims 1 to 5, wherein during the step of adding both a clay mineral and a catalyst: the clay mineral and the catalyst are added at the same time; or, the clay mineral is added first; or, the catalyst is added first.

7. The method of any one of claims 1 to 6, wherein the steps of the method occur in the following number order:

(i) adding bed materials to a fluidised bed reactor, wherein the bed materials are capable of forming a fluidised bed;

(ii) forming a fluidised bed by applying gas to the bed materials;

(iii) adding fuel to the fluidised bed reactor;

(iv) adding both a clay mineral and a catalyst to the fluidised bed reactor;

(v) forming a fluidised bed by applying gas to the mixture of bed materials, clay mineral, catalyst and fuel; and,

(vi) applying heat to oxidise the fuel.

8. The method of any one of claims 1 to 7, wherein when both the clay mineral and the catalyst are added, they are added separately or as a mixture; optionally, the clay mineral and the catalyst are in the form of separate powders, mixed powders, separate pellets, mixed pellets, separate granules, mixed granules or slurry.

9. The method of any one of claims 1 to 8, wherein when both the clay mineral and the catalyst are added, the rate of addition of either one or both of the clay mineral and the catalyst is from 0.05 to 2% by weight of the fuel mass.

10. The method of any one of claims 1 to 9, wherein when both the clay mineral and the catalyst are added, either one or both of the clay mineral and the catalyst are added in one shot or over time.

1 1 . The method of any one of claims 1 to 10, wherein the clay mineral is formed of clay mineral particles with a particle size of 250 μιτι or less; optionally, a particle size of from 0.1 to 50 μιτι.

12. The method of any one of claims 1 to 1 1 , wherein the catalyst is formed of catalyst particles with a particle size of 250 μιτι or less; optionally, a particle size of from 0.1 to 50 μιτι. 13. The method of any one of claims 1 to 12, wherein the relative amount of catalyst in the clay mineral and catalyst added to the fluidised bed reactor ranges from: 50% catalyst to 1 % catalyst (by weight); or 30% catalyst to 1 % catalyst (by weight); or 20% catalyst to 1 % catalyst (by weight); or 10% catalyst to 1 % catalyst (by weight)..

14. The method of any one of claims 1 to 13, wherein the clay mineral contains at least 5%, or at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% kaolinite or halloysite by weight; optionally, additionally containing minerals from the illite and/or smectite groups of minerals.

15. The method of any one of claims 1 to 14, wherein the catalyst is any one or more of a metal, a metal oxide and/or a metal carbonate. 16. The method of any one of claims 1 to 15, wherein the catalyst is any one or more of: nickel-chromium (optionally forms of olivine which include both nickel and chromium or compounds of the formula Ni2-xCrxAl3 where x = 0.07 or 0.1 1 ), iron (metallic Fe), iron oxides (optionally FeO, Fe3O4, Fe4O5, Fe5O6, Fe5O7 or Fe2O3), FeTiO3, NiO, CaO, CaCO3, Na2CO3, K2CO3, nickel enriched olivine, olivine and/or CuO.

17. The method of any one of claims 1 to 16, wherein the bed materials comprise any one or more of: ash, sand (for example quartz sand), limestone, feldspar, crushed ceramics and/or calcined clays.

18. The method of any one of claims 1 to 17, wherein the fuel is any one or more of: fossil fuels (for example coal, peat or natural gas); wood (for example untreated wood, treated wood, recycled wood or wood pellets); char; torrefied biomass; energy crops (for example Miscanthus, Switchgrass, Giant Reed, Reed Canary Grass, Cardoon, Willow, Poplar or Eucalyptus); animal manure (for example cow, horse, pig or poultry manure); organic residues/products (for example agricultural waste, horticultural waste, bagasse, black liquor, food industry products, food industry waste, grain, meal, organic domestic waste, paper, paper pulp, slaughterhouse residue, textile waste or organic residue); municipal solid waste; refuse-derived fuel; plastic; sludge (for example drainage culvert, food industry sludge, paper sludge or sewage); straw (for example stalk, cob or ear straw); a mixture of virgin wood (90% by weight) and grass (10% by weight); or any combination of any one, two, three, four, five, six, seven, eight, nine, ten or more of these fuels.

19. The method of any one of claims 1 to 18, wherein the gas is an oxidising gas.

20. The method of any one of claims 1 to 19, wherein the gas comprises oxygen. 21 . The method of any one of claims 1 to 20, wherein the gas is any one of air, steam or a mixture of air and steam.

22. The method of any one of claims 1 to 21 , wherein the gas is at atmospheric pressure (101 ,325 Pa) or at a pressure higher than atmospheric pressure.

23. The method of any one of claims 1 to 22, wherein the heat of the method of oxidising fuel is: from 100 to 1 ,000°C; or, from 200 to 900°C; or, from 500 to 850°C; or, from 750 to 850°C; or, from 650 to 750°C.

24. The method of any one of claims 1 to 23, wherein the clay mineral comprises or consists of kaolin.

25. The method of any one of claims 1 to 24, wherein the catalyst comprises or consists of: olivine; ilmenite; or, olivine and ilmenite.

26. The method of any one of claims 1 to 25, wherein the fuel is a mixture of virgin wood (90% by weight) and grass (10% by weight).

27. The method of any one of claims 1 to 26, wherein the bed material comprises or consists of quartz sand.

28. A method as hereinbefore described, with reference to Figure 1 .

29. Any novel feature or combination of features disclosed herein.

Description:
Title: Fluidised bed reaction methods

Description of Invention The present invention relates to fluidised bed reaction methods.

Combustion is the high-temperature exothermic reaction between a fuel and an oxidant. Gasification is a method that converts organic (for example biomass) or fossil fuel based carbonaceous materials (fuels) into carbon monoxide, hydrogen and/or carbon dioxide. Combustion and gasification are both fluidised bed reaction methods. Combustion and gasification can be considered oxidation methods in the sense that the fuel is oxidised in both cases. Fluidised bed technology is used in the combustion and gasification of fuels.

In fluidised bed combustion, particles of fuel are suspended in a bed of particulate materials (for example ash, sand and/or limestone). Jets of oxygenated gas (for example air or oxygen depleted air) are blown through the bed to provide the oxygen required for the combustion of the fuel. In some examples, fluidised bed combustion occurs at a temperature of combustion of from 750 to 850°C. Fluidised bed combustion can occur at different temperatures, depending on the boiler design and the fuel. Similarly, in fluidised bed gasification, particles of fuel are suspended in a bed of particulate materials (for example ash, sand and/or limestone). Jets of oxygenated gas (for example air) and/or steam are blown through the bed to effect gasification of the fuel. One difference between fluidised bed combustion and fluidised bed gasification is the amount of oxygen in the gas. Generally, less oxygen is required for fluidised bed gasification because the amount of oxidation of the fuel in gasification is less than in fluidised bed combustion. Another difference between fluidised bed combustion and fluidised bed gasification is the temperature of the fluidised bed; combustion utilises higher temperatures than gasification, for any particular fuel. In some examples, fluidised bed gasification occurs at temperature of gasification of from 650 to 749°C. Fluidised bed gasification can occur at different temperatures, depending on the boiler design and the fuel.

Fluidised bed combustion is an increasingly common source of energy because fluidised bed combustion can be used to burn fuels which prove difficult to burn using other technologies. Furthermore, SO x emissions can be precipitated out using limestone in a fluidised bed. Fluidised bed combustion releases lower levels of NO x emissions than other combustion methods because the temperatures of combustion are relatively low.

A fluidised bed, used in either fluidised bed combustion or fluidised bed gasification, can be formed by introducing pressurised gas (pressurised higher than atmospheric pressure) through a bed material. The bed material is formed of a plurality of solid particles. The solid particles are typically formed of ash, sand and/or limestone. A fluidised bed consists of a gas-solid mixture (the gas being the pressurised gas; the solid being the solid particles) that exhibits fluid-like properties. A fluidised bed can be considered a heterogeneous mixture of gas and solid.

A fluidised bed can be used to promote contact between gases and solids and enhance reactions between gases and solids.

Agglomeration of bed materials and fuel ash may cause problems during fluidised bed combustion or gasification. In other words, during fluidised bed combustion or gasification, the ash (inorganic fine particles produced by combustion or gasification of fuel) may agglomerate with one or more components of the bed materials. Ohman et al. (in "The Role of Kaolin in Prevention of Bed Agglomeration during Fluidized Bed Combustion of Biomass Fuels", Energy & Fuels, 2000, 14, 618-624) taught that adding kaolin to a fluidised bed prevented the agglomeration of the fuel ash and bed materials. Prevention of agglomeration led to increased efficiency of fluidised bed combustion of biomass. Furthermore, prevention of agglomeration broadened the range of possible fuels and eliminated operational problems.

There is a need to further increase the efficiency of fluidised bed combustion and fluidised bed gasification.

The present invention relates to a method of oxidising fuel in a fluidised bed, the method comprising the steps of:

adding bed materials to a fluidised bed reactor, wherein the bed materials are capable of forming a fluidised bed;

adding fuel to the fluidised bed reactor;

adding both a clay mineral and a catalyst to the fluidised bed reactor; forming a fluidised bed by applying gas to the mixture of bed materials, clay mineral, catalyst and fuel; and,

applying heat to oxidise the fuel.

Preferably, wherein the method of oxidising fuel in a fluidised bed is a method of fluidised bed combustion.

Further preferably, wherein the method of oxidising fuel in a fluidised bed is a method of fluidised bed gasification. Advantageously, wherein the step of adding both a clay mineral and a catalyst to the fluidised bed reactor occurs before, during or after the step of adding bed materials.

Preferably, wherein the step of adding both a clay mineral and a catalyst to the fluidised bed reactor occurs before, during or after the step of adding fuel. Further preferably, wherein during the step of adding both a clay mineral and a catalyst: the clay mineral and the catalyst are added at the same time; or, the clay mineral is added first; or, the catalyst is added first. Advantageously, wherein the steps of the method occur in the following number order:

(i) adding bed materials to a fluidised bed reactor, wherein the bed materials are capable of forming a fluidised bed;

(ii) forming a fluidised bed by applying gas to the bed materials;

(iii) adding fuel to the fluidised bed reactor;

(iv) adding both a clay mineral and a catalyst to the fluidised bed reactor;

(v) forming a fluidised bed by applying gas to the mixture of bed materials, clay mineral, catalyst and fuel; and,

(vi) applying heat to oxidise the fuel.

Preferably, wherein when both the clay mineral and the catalyst are added, they are added separately or as a mixture; optionally, the clay mineral and the catalyst are in the form of separate powders, mixed powders, separate pellets, mixed pellets, separate granules, mixed granules or slurry.

Further preferably, wherein when both the clay mineral and the catalyst are added, the rate of addition of either one or both of the clay mineral and the catalyst is from 0.05 to 2% by weight of the fuel mass.

Advantageously, wherein when both the clay mineral and the catalyst are added, either one or both of the clay mineral and the catalyst are added in one shot or over time. Preferably, wherein the clay mineral is formed of clay mineral particles with a particle size of 250 μιτι or less; optionally, a particle size of from 0.1 to 50 μιτι. Further preferably, wherein the catalyst is formed of catalyst particles with a particle size of 250 μιτι or less; optionally, a particle size of from 0.1 to 50 μιτι.

Advantageously, wherein the relative amount of catalyst in the clay mineral and catalyst added to the fluidised bed reactor ranges from: 50% catalyst to 1 % catalyst (by weight); or 30% catalyst to 1 % catalyst (by weight); or 20% catalyst to 1 % catalyst (by weight); or 10% catalyst to 1 % catalyst (by weight)..

Preferably, wherein the clay mineral contains at least 5%, or at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% kaolinite or halloysite by weight; optionally, additionally containing minerals from the illite and/or smectite groups of minerals. Further preferably, wherein the catalyst is any one or more of a metal, a metal oxide and/or a metal carbonate.

Advantageously, wherein the catalyst is any one or more of: nickel-chromium (optionally forms of olivine which include both nickel and chromium or compounds of the formula Ni 2- xCr x Al3 where x = 0.07 or 0.1 1 ), iron (metallic Fe), iron oxides (optionally FeO, Fe3O 4 , Fe 4 O 5 , Fe 5 O6, Fe 5 O 7 or Fe2O 3 ), FeTiO3, NiO, CaO, CaCO3, Na2CO3, K2CO3, nickel enriched olivine, olivine and/or CuO. Preferably, wherein the bed materials comprise any one or more of: ash, sand (for example quartz sand), limestone, feldspar, crushed ceramics and/or calcined clays.

Further preferably, wherein the fuel is any one or more of: fossil fuels (for example coal, peat or natural gas); wood (for example untreated wood, treated wood, recycled wood or wood pellets); char; torrefied biomass; energy crops (for example Miscanthus, Switchgrass, Giant Reed, Reed Canary Grass, Cardoon, Willow, Poplar or Eucalyptus); animal manure (for example cow, horse, pig or poultry manure); organic residues/products (for example agricultural waste, horticultural waste, bagasse, black liquor, food industry products, food industry waste, grain, meal, organic domestic waste, paper, paper pulp, slaughterhouse residue, textile waste or organic residue); municipal solid waste; refuse-derived fuel; plastic; sludge (for example drainage culvert, food industry sludge, paper sludge or sewage); straw (for example stalk, cob or ear straw); a mixture of virgin wood (90% by weight) and grass (10% by weight); or any combination of any one, two, three, four, five, six, seven, eight, nine, ten or more of these fuels.

Advantageously, wherein the gas is an oxidising gas.

Preferably, wherein the gas comprises oxygen.

Further preferably, wherein the gas is any one of air, steam or a mixture of air and steam.

Advantageously, wherein the gas is at atmospheric pressure (101 ,325 Pa) or at a pressure higher than atmospheric pressure.

Preferably, wherein the heat of the method of oxidising fuel is: from 100 to 1 ,000°C; or, from 200 to 900°C; or, from 500 to 850°C; or, from 750 to 850°C; or, from 650 to 750°C.

Further preferably, wherein the clay mineral comprises or consists of kaolin.

Advantageously, wherein the catalyst comprises or consists of: olivine; ilmenite; or, olivine and ilmenite.

Preferably, wherein the fuel is a mixture of virgin wood (90% by weight) and grass (10% by weight). Further preferably, wherein the bed material comprises or consists of quartz sand. Embodiments of the invention are described below with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a fluidised bed reactor, which can be used in either fluidised bed combustion or fluidised bed gasification.

Some of the terms used to describe the present invention are set out below:

"Fluidised bed combustion" refers to the combustion of fuel, typically solid fuel, in a hot, bubbling bed, or circulating bed, of bed materials.

"Fluidised bed gasification" refers to the gasification of fuel, typically solid fuel, (forming carbon monoxide, hydrogen and carbon dioxide) in a hot, bubbling bed, or circulating bed, of bed materials. "Fluidised bed reactor" refers to a reactor that can be used in fluidised bed combustion and/or fluidised bed gasification methods. Typically, fluidised bed reactors are lined with a ceramic material. Examples of suitable ceramic materials include C71 refractories according to the ASTM standard ("non- metallic materials having those chemical and physical properties to make them applicable for structures, or as components of systems, that are exposed to environments above 538°C": see "Circulating Fluidized Bed Boilers: Design, Operation and Maintenance", Prabir Basu, Springer, 2015, the contents of which are hereby incorporated by reference). "Fuel" refers to a material that can react with other materials to release chemical energy as heat through oxidation of the fuel. Non-limiting examples of fuel include: fossil fuels (for example coal, peat or natural gas); wood (for example untreated wood, treated wood, recycled wood or wood pellets); char; torrefied biomass; energy crops (for example Miscanthus, Switchgrass, Giant Reed, Reed Canary Grass, Cardoon, Willow, Poplar or Eucalyptus); animal manure (for example cow, horse, pig or poultry manure); organic residues/products (for example agricultural waste, horticultural waste, bagasse, black liquor, food industry products, food industry waste, grain, meal, organic domestic waste, paper, paper pulp, slaughterhouse residue, textile waste or organic residue); municipal solid waste; refuse-derived fuel; plastic; sludge (for example drainage culvert, food industry sludge, paper sludge or sewage); straw (for example stalk, cob or ear straw); a mixture of virgin wood (90% by weight) and grass (10% by weight); or any combination of any one, two, three, four, five, six, seven, eight, nine, ten or more of these examples of fuel. "Bed materials" refers to the materials making up the bed in a fluidised bed reactor. Bed materials are typically solid particulate materials. Examples of bed materials include ash, sand (for example quartz sand), limestone, feldspar, crushed ceramics and calcined clays. Bed materials are typically inert but can agglomerate with fuel ash under high temperatures and/or pressures.

"Kaolin" refers to minerals that are rich in kaolinite. The amount of kaolinite present in kaolin is at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%.

"Kaolinite" refers to a clay mineral with the chemical composition AI 2 Si2O 5 (OH) . It is a layered silicate material, with one tetrahedral sheet of silica (SiO 4 ) linked through oxygen atoms to one octahedral sheet of alumina (AIO 6 ) octahedral.

"Halloysite" refers to a clay mineral with the empirical formula AI 2 Si2O 5 (OH) . The structure and chemical composition of halloysite is similar to that of kaolinite but the unit layers in halloysite are separated by a monolayer of water molecules. As a result, hydrated halloysite has a basal spacing of 10 A which is approximately 3 A larger than that of kaolinite. "Clay mineral" refers to minerals which contain at least 5%, or at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50% or at least 60%, or at least 70%, or at least 80%, or at least 90% kaolinite or halloysite by weight; optionally, additionally containing minerals from the illite and smectite groups of minerals.

"Catalyst" refers to a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Examples of catalysts used to catalyse oxidation of fuel in fluidised bed combustion or fluidised bed gasification include metals, metal oxides and metal carbonates. Particular examples of catalysts used to catalyse oxidation in fluidised bed combustion or fluidised bed gasification include: nickel-chromium (for example particular forms of olivine which include both nickel and chromium or compounds of the formula Ni 2- xCr x Al3 where x = 0.07 or 0.1 1 ), iron (metallic Fe), iron oxides (for example FeO, Fe3O 4 , Fe 4 O 5 , Fe 5 O6, Fe 5 O 7 or Fe2O 3 ), FeTiO 3 (ilmenite), NiO, CaO, CaCO 3 , Na 2 CO 3 , K 2 CO 3 , nickel enriched olivine, olivine and CuO.

Figure 1 is a schematic diagram of a fluidised bed reactor 1 , which can be used in either fluidised bed combustion or fluidised bed gasification. In the fluidised bed reactor 1 of Figure 1 , there is an input 2 for solid materials 7 and an input 3 for gas materials 6. The solid materials 7 sit on a porous plate 4, sometimes referred to as a distributor. The gas materials 6 enter the reactor 1 though the input 3 and then through the porous plate 4, in an upwards direction as shown schematically by arrow 5.

The solid materials 7 are illustrated schematically by the circles 7; this is a simplification, there are in fact many millions of particles of solid material in a charged fluidised bed reactor. The gas materials 6 are illustrated schematically by the diamonds 6; this is a simplification, there are in fact many millions of particles and/or bubbles of gas material (formed of gas molecules) in an active fluidised bed reactor.

The gas materials 6 are forced into the input 3 and then through the porous plate 4. At lower gas velocities, the solid materials 7 remain in place as the gas materials 6 pass through the voids between the solid materials 7 (the fluidised bed reactor at this stage is sometimes called a packed bed reactor). As the gas velocity is increased, the reactor 1 reaches a stage where the force of the gas materials 6 on the solid materials 7 balances the weight of the solid materials 7 (this stage is sometimes called incipient fluidisation). As the gas velocity is increased further, the solid materials 7 act like a fluid, for example like water in a boiling receptacle of water (the bed in the fluidised bed reactor at this stage is now called a fluidised bed).

The above description relates to a bubbling bed reactor. Alternatively, with a circulating fluidised bed, as the gas velocity is increased, the solid materials 7 travel with the gas materials 6 to the top of reactor 1 . The solid materials 7 reach a stage where their weight is greater than the force of the gas materials 6 and the solid materials 7 fall back down into the bed of solid materials 7.

With reference to Figure 1 , solid materials 7 are introduced through input 2. Typical examples of solid materials 7 added to a fluidised bed reactor for fluidised bed combustion or fluidised bed gasification include bed materials; for example ash, sand, limestone, feldspar, crushed ceramics and calcined clays. Fuel is also introduced through input 2, either in one shot or in a number of shots over time as the fuel is depleted. Gas materials 6 are introduced through input 3. Typical examples of gas materials 6 include air (in the case of fluidised bed combustion) and steam optionally mixed with air (in the case of fluidised bed gasification). Waste gas materials are taken out of the reactor 1 through output 8. Some components of the waste gas materials can be used in refining processes, particularly in the case of waste gas materials from fluidised bed gasification.

Waste solid materials are taken out of the reactor 1 through output 9, or through other outputs (not shown).

According to the present invention, both a clay mineral and a catalyst are added to a fluidised bed reactor before or during fluidised bed combustion or fluidised bed gasification in the fluidised bed reactor. In Figure 1 , both the clay mineral and the catalyst would be added through input 2 or another input (not shown). Non-limiting examples of catalysts, which can be added to a fluidised bed reactor along with a clay mineral, and some reactions they catalyse in fluidised bed reactors include:

1 . Gaseous hydrocarbons and superheated steam over a nickel- chromium catalyst (for example particular forms of olivine which include both nickel and chromium or compounds of the formula Ni 2-x Cr x Al3 where x = 0.07 or 0.1 1 ) react to produce carbon monoxide and hydrogen according to reaction 1 : C n H m + nH 2 O -> nCO + (2n+m)/2 H 2 (Reaction 1 )

Without catalyst some of the hydrocarbons could not react or will only partially react in a fluidised bed reactor. Here, the nickel-chromium catalyst increases the reaction rate and prevents emission of unburned hydrocarbons. 2. The gaseous mixture containing carbon monoxide, obtained from biomass containing an excess of steam, over an iron catalyst (for example iron metal or an iron oxide), reacts according to reaction 2: CO + H 2 O -> CO 2 + H 2 (Reaction 2)

Here the iron catalyst shifts the CO/H 2 equilibrium in the favor of hydrogen gas. Hydrogen gas is a preferable product because it can be used as a fuel in other methods.

Reactions 1 and 2, above, do occur in combustion and gasification. In combustion, the gases will combust in the fluidised bed reactor. In

gasification, the produced gases can be removed from the reactor and used in further methods.

Another catalyst, and a reaction that it catalyses in a fluidised bed reactor, is:

3. Ilmenite (FeTiO 3 ). These reactions are shown in Reactions 3(1 ), (2), (3) and (4) below:

(Reactions 3(1 ), (2), (3) and (4)) In reactions 3(1 ), (2), (3) and (4), the substrate of the first reaction is FeTiO3. After undergoing reactions 3(3) and 3(4), one product is FeTiO 3 ; therefore FeTiO3 is acting as a catalyst because it increases the rate of the overall chemical reaction (depleting carbon monoxide and methane and producing carbon dioxide and water) without itself undergoing any permanent chemical change. (This set of reactions was proposed in, "Using an oxygen-carrier as bed material for combustion of biomass in a 12-MW th circulating fluidized-bed boiler", Henrik Thunman, Fredrik Lind, Claes Breitholtz, Nicolas Berguerand, Martin Seemann, Fuel, 1 13 (2013), 300-309; the disclosure of which is hereby incorporated by reference).

Other examples of catalysts, and some methods they catalyse in fluidised bed reactors, include:

4. Ilmenite (FeTiO 3 ) as a catalyst for fluidised bed combustion of agricultural waste or sludge and their mixes with wood.

5. Nickel oxide (NiO), and minerals containing nickel oxide (for example olivine), as a catalyst in fluidised bed gasification of plant based fuels.

6. Nickel oxide (NiO), and minerals containing nickel oxide (for example olivine), as a catalyst in fluidised bed combustion of agricultural waste, municipal solid waste and refuse-derived fuel.

7. Calcium oxide (CaO) as a catalyst in fluidised bed gasification of rice husk, wood or straw.

8. Calcium oxide (CaO), calcium carbonate (CaCO 3 ), sodium carbonate (Na2CO 3 ) and/or potassium carbonate (K2CO3) as catalysts in catalytic fluidised bed hydrogasification of coal char.

9. Nickel enriched olivine as a catalyst in catalytic tar reduction in fluidised bed biomass steam gasification.

10. Copper oxide (CuO) as a catalyst in fluidised bed combustion of coal. 11. Calcium oxide (CaO) as a catalyst in cracking tar formed by rice husk fluidised bed gasification.

12. Olivine ((Mg, Fe) 2 SiO 4 ) as a catalyst in fluidised bed gasification of biomass.

13. Nickel-chromium compounds of the formula: Ni 2-x Cr x Al3 where x =

0.07 or 0.1 1 as a catalyst for fluidised bed gasification or fluidised bed combustion.

In some examples, the size of the catalyst particles added to a fluidised bed reactor are 250 μιτι or lower. Optionally, the size of the catalyst particles added to a fluidised bed reactor are from 0.1 to 50 μιτι. The particle sizes are measured by whether the particles fit through a suitably sized filter, for example catalyst particles of 250 μιτι or lower fit through a filter with a mesh size of 250 μιτι. Alternatively or additionally, the particle sizes can be measured by laser diffraction (for example using a Malvern™ Mastersizer 3000).

A non-limiting example of a clay mineral which can be added to a fluidised bed reactor, along with a catalyst, is kaolinite. In some examples, the size of the kaolinite particles added to a fluidised bed reactor are 250 μιτι or lower. Optionally, the size of the kaolinite particles added to a fluidised bed reactor are from 0.1 to 50 μιτι. The particle sizes are measured by whether the particles fit through a suitably sized filter, for example kaolinite particles of 250 μιτι or lower fit through a filter with a mesh size of 250 μιτι. Alternatively, the particle sizes can be measured by laser diffraction (for example using a Malvern™ Mastersizer 3000).

In some examples, the mixtures of clay mineral and catalyst are added to a fluidised bed reactor in the form of separate powders, mixed powders, separate pellets, mixed pellets, separate granules, mixed granules or slurry. By adding both a clay mineral and a catalyst (for example a clay mineral and catalyst mixture) to a fluidised bed reactor, before or during fluidised bed combustion or fluidised bed gasification in the fluidised bed reactor, the clay mineral acts as a support to the catalyst and can form layers of clay mineral/catalyst mixtures on the walls of the fluidised bed reactor and/or on the surfaces of other solid materials (for example bed materials; for example ash, sand, limestone, feldspar, crushed ceramics and calcined clays) in the fluidised bed. By acting as a support, the catalyst can adsorb on, or form bonds to, the surface of the clay mineral. In other words, by adding both a clay mineral and a catalyst, there is a synergistic effect. Without wishing to be bound by theory, this synergistic effect occurs at least because: the clay mineral acts to prevent agglomeration of bed materials, fuel ash and/or catalyst; the clay mineral acts as a support for the catalyst which increases the contact surface area between the catalyst, other solid materials (i.e. bed materials and fuel) and gas materials in the fluidised bed reactor; and the clay mineral can act to mitigate poisoning of the catalyst by fuel ash or other reaction by-products. Since the layer of catalyst adsorbed on to, or bonded to, the surface of the clay mineral, is constantly formed, any poisoned catalyst is replaced by fresh catalyst. Furthermore, by adding both a clay mineral and a catalyst, there is a large surface area of the catalyst in the boiler - effectively all walls of the boiler and surfaces of the bed materials become a catalytic surface; hence increasing the efficiency of the catalytic reactions. By way of these benefits, adding both a clay mineral and a catalyst (for example a clay mineral and catalyst mixture) to a fluidised bed reactor increases the efficiency of the oxidation of the fuel (combustion or gasification) in the fluidised bed reactor.

With respect to the layers of clay mineral/catalyst mixtures on the walls of the fluidised bed reactor, over time these layers can detach from the walls and fall into the fluidised bed. This is a beneficial process because the layer formed does not become particularly thick (for example the layer formed has a thickness of up to 3cm, or up to 5cm) and plays an active part in increasing the efficiency of the fluidised bed combustion or fluidised bed gasification.

In some examples, the clay mineral and catalyst are added to a fluidised bed reactor continuously during fluidised bed combustion or fluidised bed gasification. This continuous addition is beneficial because it permits the formation of layers of clay mineral/catalyst mixtures on the walls of the fluidised bed reactor and/or on the surfaces of other materials in the fluidised bed, the subsequent depletion/reaction of those layers and the formation of new layers. In some examples, the rate of addition of the clay mineral and catalyst is from 0.05 to 2% by weight of the fuel mass. Typically, for 1 ,000 kg of fuel, from 0.5 to 20 kg of clay mineral and catalyst is added, regardless of the rate of fuel consumption. Typical fuel consumption is around 20,000 kg of fuel per hour. Fuel consumption can reach 100,000 kg per hour in large fluidised bed reactors.

The amount of catalyst in a clay mineral/catalyst mixture added to fluidised bed reactors ranges from: 50% catalyst to 1 % catalyst (by weight); or 30% catalyst to 1 % catalyst (by weight); or 20% catalyst to 1 % catalyst (by weight); or 10% catalyst to 1 % catalyst (by weight).

Examples

Kaolin was mixed with olivine to form a mixture of clay mineral (kaolin) and catalyst (olivine). Two mixtures were formed as shown in Table 1 :

Table 1 : example clay mineral-catalyst mixtures In table 1 , the kaolin contained 82% kaolinite by weight the balance being unavoidable impurities, the olivine was generally pure olivine (as produced and sold by Sibelco™). Examples 1 and 2 were added to a fuel at 0.5 % by mass of the fuel. The fuel consisted of virgin wood (90%) and grass (10%). The bed material was quartz sand.

In further examples, kaolin was mixed with ilmenite to form a mixture of clay mineral (kaolin) and catalyst (ilmenite). Two mixtures were formed as shown in Table 2:

Table 2: example clay mineral-catalyst mixtures In table 2, the kaolin contained 82% kaolinite by weight the balance being unavoidable impurities, the ilmenite was generally pure ilmenite (as produced and sold by Sibelco™).

Examples 3 and 4 were added to a fuel at 0.5 % by mass of the fuel. The fuel consisted of virgin wood (90%) and grass (10%). The bed material was quartz sand.

The mixtures of examples 1 , 2, 3 and 4, in combination with the fuel and bed material described above, were subjected to fluidised bed combustion in a boiler at 850°C and an oxygen gas concentration of 6% by volume of the gas bubbled through the bed materials. The results were compared with data obtained for experiments with pure quartz sand as bed material without the addition of clay mineral or catalyst to the fuel. Data were also obtained by adding pure kaolin (containing 82% kaolinite by weight the balance being unavoidable impurities) without any addition of catalyst.

In these comparative experiments, the addition of catalyst (olivine or ilmenite, as in examples 1 , 2, 3 and 4) to the kaolin led to a reduction in CO emissions of at least fiftyfold. This was achieved without changing operational conditions and with constant NOx emissions (plus or minus 2%). The reduction in CO emissions shows more efficient combustion of the fuel.

The results are shown in Table 3 below:

Table 3: NOx and CO emissions from fluidised bed combustion

There was a small difference observed for the data obtained for 5% and 10% catalyst addition. This indicates that the 5% by weight addition of catalyst was sufficient to reach a beneficial performance of the mixture.

The present inventors additionally found that addition of catalyst (absent any clay mineral) increases the efficiency of the fluidised bed combustion compared with no catalyst. However, the CO emissions from the addition of catalyst (olivine or ilmenite absent any clay mineral) are approximately 50% of the value from adding pure kaolin (as shown in Table 3). In other words, the addition of clay mineral and catalyst provides a reduction in CO emissions (compared to no mineral additive) which is much greater than the sum of the reductions in CO emissions from adding either pure clay mineral (absent any catalyst) or catalyst (absent any clay mineral).

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.