PROCESS FOR PRODUCING SOLID BIOMASS FUEL FIELD OF THE INVENTION The present invention relates to a process for producing a solid biomass fuel, as well as a solid biomass fuel produced by said process. Additionally, the present invention relates to a combustion process comprising combusting said solid biomass fuel so as to produce energy. BACKGROUND OF THE INVENTION Coal-fired power generation is used in power plants and industrial processes around the world. Coal and other fossil fuels are non-renewable energy resources. Over the last few decades, there have been calls to reduce the consumption of coal in coal-fired power stations and instead to use renewable resources for energy. Fuels derived from biomass are an example of a renewable energy source that can be used to replace or at least partially replace coal. Biomass derived fuels can be burned in the presence of oxygen in power plants in combustion processes to produce energy. Biomass derived fuels can be combusted in traditional power plants originally designed for coal combustion, or biomass derived fuels can be combusted in power plants built specifically for biomass combustion. Certain forms of biomass can be mixed with coal and combusted in the same combustion process within a power plant. Such a process is known as coal co-firing of biomass. To be suitable for co-firing with coal, biomass derived fuel must typically have certain properties such as a certain level of quality and homogeneity with regard to properties. For example, biomass fuel comprised of particles of a homogenous size, density, moisture content etc. are particularly desirable in co-firing processes. It is also desirable that the biomass fuel contains a low level of ash. Levels of ash in biomass derived fuels are typically higher than those found in coal. Various processes for producing solid biomass fuels from biomass sources are known. WO2014/087949 discloses a process for producing a solid biomass fuel in which a source of biomass is steam exploded before being molded into biomass blocks which are then heated so as to form the biomass fuel. The aim of the process is to produce biomass fuel with sufficient handleability during storage and with reduced chemical oxygen demand (COD) in discharged water during storage. The biomass source used in the process is palm kernel shell. WO2016/056608 builds upon the teaching of WO2014/087949, and discloses a process for manufacturing solid biomass fuel in which the steam explosion step is not required to produce the fuel. The process comprises a molding step in which a biomass source is crushed before being compressed and molded into biomass blocks, before the biomass blocks are heated. The biomass source taught for use in said process is trees such as douglas fir, hemlock, cedar, cypress, European red pine, almond old tree, almond shell, acacia xylem part, acacia bark, walnut shell, sago palm, empty fruit bunches, meranti and rubber. WO2017/175733 discloses a similar process comprising a molding step in which a biomass source is crushed before being compressed and molded into biomass blocks, before the biomass blocks are heated. The process of WO2017/175733 is directed to providing biomass fuel which exhibits low disintegration and achieves reduced COD in discharged water when exposed to rain water. The source of the biomass to be used in the process is selected from the rubber tree, acacia, meranti, eucalyptus, teak and a mixture of larch, spruce and birch. WO2019/069849 aims to provide a biomass fuel that is easy to transport and store and that is resistant to spontaneous combustion during storage. The biomass fuel is made by a process comprising a molding step in which a biomass source is crushed before being compressed and molded into biomass blocks, before the biomass blocks are heated. The biomass source for producing the fuel is selected from rubber trees, acacia trees, radiata pine, a mixture of larch, spruce and birch; and spruce, pine and fir. WO2019/069860 discloses an apparatus for producing biomass solid fuel. The apparatus comprises a carbonisation furnace for carbonising a molded biomass product to obtain a biomass solid fuel. The apparatus further comprises a yield calculation unit, a temperature measurement unit and a control unit. The control unit controls the heat applied to the carbonisation furnace based upon the spontaneous combustion properties of the biomass fuel. The molded biomass product is formed by pulverising a biomass source into pellets, before molding said pellets into a molded biomass product. The biomass source is selected from the rubber tree, acacia, dipterocarp, radia pine, a mixture of larch, spruce and birch or a mixture of spruce, pine and firs. WO2018/181919 discloses a different process to those discussed above for producing a solid biomass fuel. The process involves a step of hydrothermal carbonisation of biomass in which a biomass source is pressurised in hot water so as to carbonise the biomass. The process is reported to provide a biomass fuel with high grindability in high yield and with reduced manufacturing costs. The source of the biomass is selected from husks, palm kernel shell, coconut palm, bamboo, empty fruit bunches, apricots and aubergines. WO2017/175737 discloses a cooling apparatus for cooling carbonised biomass. The apparatus improves the cooling efficiency of semi-carbonised molded biomass. The apparatus cools the biomass by spraying water thereon. The cooler comprises a vibration flat plate and a spraying section for spraying water on the flat plate. The biomass fuel is produced by the same processes as discussed above. The source of biomass for producing the biomass fuel is douglas fir, hemlock, cedar, cypress, European red pine, almond old tree, almond shell, acacia xylem part, acacia bark, walnut shell, sago palm, empty fruit bunches, meranti and the rubber tree. Finally, WO2014/050964 discloses a process for improving the grindability of biomass such that it can be ground with coal. The process involves increasing the moisture content of ground wood biomass to between 10 to 50%; densifying the biomass to have a density of 0.55 g/cm ³ or higher, before subjecting the biomass to torrefaction. The source of biomass includes wood chips, bark, wood shavings, and sawdust. However, it is known by those of skill in the art that the solid biomass fuels and processes for their production discussed in the above documents have various problems associated with them. For example, the biomass sources described in the above documents are all plants and trees that typically only occur naturally, and are not easy to cultivate and harvest on a commercial scale. Additionally, the known sources of biomass described above are generally all comprised of wood and similar materials, which when subjected to conventional pulverising techniques, form particles with a low degree of homogeneity. Furthermore, pulverising the biomass sources is expensive due to the nature of the wood and wood-like material. Additionally, solid biomass fuels prepared from biomass sources and using known prior art processes do not have sufficient water proof characteristics. Water proof characteristics are important for solid biomass fuels since they need to be dry (or at least sufficiently dry) when used in a combustion process (either on their own or when co-fired with coal). It has also been found that the biomass fuel production processes described above do not provide fuels with sufficient quality and uniformity. In particular, the processes discussed above do not provide sufficient control of the density of the biomass during the molding step. In light of the above problems, there is a need in the art for a process for producing high performance solid biomass fuels from alternative sources of biomass. Particular solutions to these problems are disclosed in WO2020/229824, WO2021/014151, WO2021/024001 and WO2021/156628 and seek to ameliorate the problems discussed above. However, none of these documents disclose the use of wheat husk as a biomass source which can be processed into a solid biomass fuel. Wheat husk (also known as chaff) is the dry, scaly, protective casing of the seeds of wheat grains. After wheat is harvested, wheat husk is typically removed from the wheat grains since wheat husk is indigestible by humans. Wheat husk is typically separated from grains by processes known in the art such as threshing and winnowing. Wheat husk is generally considered to be an agricultural waste product, although it is known for a variety of uses such as a component of animal feed, fertilizer, and various composite materials. Pyrolysed wheat husk has also been suggested for use as a component of electrodes for electrochemical devices. However, due to its nature, it is not known as a suitable bulk material for the formation of solid biomass fuels. This is not entirely surprising given that the present inventors have found that the processes taught in WO2020/229824, WO2021/014151, WO2021/024001 and WO2021/156628 are unsuitable for the production of solid fuels derived from wheat husk as the primary biomass source for a variety of reasons. Firstly, wheat husk contains high quantities of minerals such as potassium, sodium, chlorine, calcium and phosphorus containing salts. The presence of such minerals, for example as salts, are known to cause serious problems when the biomass is combusted such as the formation of low melting point ashes which result in slagging, fouling and agglomeration of ashes and bed materials upon combustion. The salts are also known to cause corrosion. The presence of salts in the fuels are further known to negatively impact the torrefaction steps known in the art. In addition to this, wheat husk contains large quantities of dust and tiny particulates. This is due to wheat husk being a dry, scaly and brittle material. Wheat husk is typically separated from wheat grain by a threshing process that involves the application of strong mechanical forces. These high forces combined with the brittle nature of the material result in the generation of tiny particulates and dust in the wheat husk product. These high quantities of dust and small particulates have been found to impede the efficiency of pulverisation processes and also lead to a less desirable larger particle size distribution once the biomass has been pulverised. As will be discussed in further detail below, a larger particle size distribution is less desirable since it leads to less homogeneity and uniformity of the solid biomass fuel once produced. Dust is also undesirable since it pollutes the air and local environment during processing of the biomass into solid fuel. Such serious issues explain why wheat husk is only included as a minor component, alongside other fuels such as coal and material derived from other sources of biomass such as rice husk, wheat straw, corn straw and similar materials in biomass fuels. To date, the art is principally focused on the use of other (non-wheat husk) sources of biomass that do not have the problems discussed above. However, there remains a need in the art for high performance solid biomass fuels derived from other biomass waste materials such as wheat husk and suitable manufacturing processes for producing said fuels that do not have the abovementioned problems associated therewith. As will be appreciated, wheat husk has the potential to be a particularly useful as a source of biomass for fuel manufacture since it is produced as a by-product on a commercial scale since wheat is one of the most abundantly grown crops on earth. A fixed and constant source of biomass for the production of fuels can thus be provided in growth cycles. Additionally, growing wheat husk on a commercial scale enables control of the quality and uniformity of the biomass source, for example, by cultivation and breeding techniques. However, this is only feasible if the above mentioned problems can be successfully overcome. SUMMARY OF THE INVENTION The present inventors have surprisingly found a process for manufacturing solid biomass fuels from wheat husk that solves the abovementioned problems and that can be used to provide high performance solid biomass fuels from wheat husk via an economically viable production process. Indeed, the present inventors have surprisingly found that it is possible to obtain a biomass fuel having suitable water-proof characteristics, yield, quality and uniformity, along with certain other physical characteristics that are highly preferable when the fuel is used in a combustion process, and that make the fuel easier to transport and store. According to a first aspect of the invention, there is provided a process for producing a solid biomass fuel from a biomass material comprising wheat husk, wherein the process comprises the following sequential steps: (i) providing a biomass composition comprising at least 50% by weight of wheat husk; (ii) removing dust from the biomass composition; (iii) pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm; optionally, (iv) compressing the pulverised biomass composition powder to provide a compressed biomass composition powder;    (v) washing the compressed biomass composition powder or the pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder; (vi) mechanically dewatering the washed biomass composition powder to provide a dewatered biomass composition powder and an aqueous effluent; (vii) drying the dewatered biomass composition powder to provide a dried biomass composition powder; (viii) molding the dried biomass composition powder so as to provide a molded biomass composition product; and (ix) heating the molded biomass composition product to a temperature of from 160°C to 420°C for a time period of from 0.25 to 5 hours so as to provide a solid biomass composition fuel. Typically, step (ii) of removing dust from the biomass composition comprises removing dust particles from the biomass composition with a screen. Typically, the screen has a pore size of from 2 mm to 8 mm, preferably wherein the screen has a pore size of from 2 mm to 5 mm, and more preferably wherein the screen has a pore size of from 3 mm to 5 mm. Typically, a drum sieve is used as a screening device to remove the dust particles from the biomass composition. Preferably, the drum sieve comprises a rotating drum sieve. Typically, step (iii) of pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm comprises crushing the biomass composition in a negative pressure pneumatic conveyancing apparatus where the moisture content of the biomass composition is 30% by weight or less. Alternatively, step (iii) of pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm may comprise pulverising the biomass composition in a low discharge type shredder where the moisture content of the biomass composition is 30% by weight or more; preferably wherein the pulverising step (iii) comprises stirring and then spraying the biomass composition within the low discharge type shredder to provide the pulverised biomass composition powder.     The process may comprise step (iv) of compressing the pulverized biomass composition powder to provide a compressed biomass composition powder. Typically, step (iv) comprises compressing the pulverised biomass composition powder to provide a compressed biomass composition powder and an aqueous effluent. Typically, the pulverised biomass composition powder is squeezed to provide a compressed biomass composition powder with a moisture content of less than 30% by weight, preferably less than 25% by weight, and more preferably less than 20% by weight. Alternatively, the process does not comprise step (iv) of compressing the pulverised biomass composition powder to provide a compressed biomass composition powder. Typically, step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder comprises washing the compressed biomass composition powder or pulverised biomass composition powder with the aqueous wash liquid via a counter current mechanism. Preferably, step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder comprises washing the compressed biomass composition powder or pulverised biomass composition powder more than once, preferably, wherein the compressed biomass composition powder or pulverised biomass composition powder is washed in from two to ten successive washing stages, more preferably, wherein the compressed biomass composition powder or pulverised biomass composition powder is washed in from two to five successive wash stages. In some instances, each successive washing stage comprises spraying the compressed biomass composition powder or pulverised biomass composition powder with the aqueous wash liquid and/or immersing the compressed biomass composition powder or pulverised biomass composition powder in the aqueous wash liquid.     Preferably, step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder comprises washing the compressed biomass composition powder or pulverised biomass composition powder in an extrusion type apparatus comprising a screw. More preferably, the screw is adapted to transport the pulverised biomass composition powder or compressed biomass composition powder in a first direction through the extrusion type apparatus; and wherein the apparatus is adapted to receive and transport the aqueous wash liquid in a second direction through the apparatus such that that the aqueous wash liquid contacts the pulverised biomass composition powder or compressed biomass composition powder within the apparatus. In certain instances, the second direction may be opposite to or perpendicular to the first direction.     Preferably, the second direction is opposite to the first direction. Where step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder comprises washing the compressed biomass composition powder or pulverised biomass composition powder more than once such as in from two to ten successive washing stages as discussed above, preferably, the extrusion type apparatus comprises a separate screw for each successive washing stage. Typically, where step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder comprises washing the compressed biomass composition powder or pulverised biomass composition powder with the aqueous wash liquid via a counter current mechanism; and where step (v) comprises washing the compressed biomass composition powder or pulverised biomass composition powder more than once in successive washing stages; preferably, the successive washing stages are performed in a counter current manner whereby aqueous wash effluent from a later washing stage is used as the aqueous washing liquid for an earlier washing stage, preferably, wherein fresh aqueous washing liquid is used to wash the compressed biomass composition powder or pulverised biomass composition powder in a final washing stage, and wherein for each of the other washing stages, the aqueous wash liquid used comprises the aqueous wash effluent from the washing stage that immediately follows. Typically, the aqueous wash liquid is at a temperature of from 5°C to 150°C during step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder, preferably, wherein the aqueous wash liquid is at a temperature of from 5°C to 150°C during each successive wash stage of step (v) if step (v) comprises multiple washing stages. Preferably, the aqueous wash liquid is at a temperature of from 5°C to 35°C during step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder, preferably, wherein the aqueous wash liquid is at a temperature of from 5°C to 35°C during each successive wash stage of step (v) if step (v) comprises multiple washing stages. Typically, step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder is carried out at a pressure of from 1.1 bar to 15 bar, preferably, wherein step (v) is carried out at atmospheric pressure, more preferably, wherein step (v) is carried out at atmospheric pressure and a temperature of from 5°C to 35°C. More preferably, the aqueous wash liquid is at a temperature of from 5°C to 30°C during step (v) of washing the compressed biomass composition powder or pulverised biomass composition powder with an aqueous wash liquid to provide a washed biomass composition powder; and preferably, wherein the aqueous wash liquid is at a temperature of from 5°C to 30°C during each successive wash stage of step (v) if step (v) comprises multiple washing stages. Typically, step (vi) of mechanically dewatering the washed biomass composition powder to provide a dewatered biomass composition powder and an aqueous effluent comprises compressing the washed biomass composition powder. Preferably, the compressing comprises compressing the dewatered biomass composition powder in an extrusion type apparatus comprising a screw; more preferably wherein the screen gap of the extrusion apparatus is from 0.5 mm to 1 mm in size.  Alternatively, step (vi) of mechanically dewatering the washed biomass composition powder may comprise squeezing the washed biomass composition powder with a roller-type apparatus. Typically, step (vii) of drying the dewatered biomass composition powder to provide a dried biomass composition powder comprises drying the dewatered biomass composition powder to provide a dried biomass composition powder with a moisture content of from 10% to 20% by weight; preferably from 12% to 18% by weight. Typically, drying step (vii) is carried out within a multi-layer belt dryer. Preferably, the process comprises recycling heat generated in heating step (ix) and using said heat in drying step (vii) to heat and dry the dewatered biomass composition powder to provide the dried biomass composition powder.   Typically, between drying step (vii) and molding step (viii), the process may comprise a step of mixing the dried biomass composition powder with other biomass powders. Typically, step (viii) of molding the dried biomass composition powder so as to provide a molded biomass composition product comprises adapting the molding step such that the density of the molded biomass composition product is controlled. Preferably, adapting the molding step such that the density of the molded biomass composition product is controlled comprises controlling the compression ratio of a mold used in said molding step. More preferably, the compression ratio is controlled to be within the range of from 5.0 to 8.0; and most preferably from 6.0 to 8.0. Preferably, the molding step is controlled such that the density of the molded biomass composition product is from 1.0 kg/L to 1.35 kg/L. Step (ix) of heating the molded biomass composition product is typically carried out for a time period of from 0.25 to 5 hours. Typically, step (ix) of heating the molded biomass composition product comprises heating the molded biomass composition product to a temperature of from 160°C to 420°C, and preferably from 210°C to 280°C. Preferably, step (ix) of heating the molded biomass composition product is carried out for a time period of from 0.25 to 5 hours, and wherein the step of heating the molded biomass composition product comprises heating the molded biomass composition product to a temperature of from 160°C to 420°C, preferably 210°C to 280°C. Typically, step (ix) of heating the molded biomass composition product comprises heating the molded biomass composition product under conditions so as to induce torrefaction of the molded biomass composition product. Typically, step (ix) of heating the molded biomass composition product is adapted so as to control the uniformity of the solid biomass composition fuel. Preferably, adapting step (ix) so as to control the uniformity of the solid biomass composition fuel comprises conducting step (ix) in an apparatus in which the molded biomass composition product is rotated whilst being heated. More preferably, adapting step (ix) so as to control the uniformity of the solid biomass composition fuel comprises controlling the speed or direction of rotation of the molded biomass composition product. In some instances, the molded biomass composition product is rotated in a clockwise direction. In other instances, the molded biomass composition product is rotated in an anticlockwise direction. Preferably, the molded biomass composition product is rotated in the apparatus in both an anticlockwise and clockwise direction. Typically, the process further comprises a step of cooling the solid biomass composition fuel after heating step (ix). After heating step (ix), the process may further comprise a (a) removing dust particles from the solid biomass composition fuel; and/or (b) washing the solid biomass composition fuel with an aqueous wash liquid. Typically, the bulk density of the solid biomass composition fuel as determined according to DIN EN 15103 is from 1.0 to 1.4 g/cm ³ , preferably from 1.1 to 1.3 g/cm ³ , and more preferably from 1.2 to 1.3 g/cm ³ . Typically, the solid biomass composition fuel is waterproof for up to 20 days, preferably up to 30 days, and more preferably up to 40 days. Typically, the PM1.0 emissions of the solid biomass composition fuel upon combustion is less than 175 mg/kg, preferably less than 150 mg/kg. Typically, the bulk density of the molded biomass composition product is A, and the bulk density of the biomass composition solid fuel is B, and wherein B/A is 0.55 to 1, wherein the bulk density is determined in accordance with DIN EN 15103.       Typically, material derived from biomass is present in the solid biomass composition fuel in an amount of at least 95% by weight of the total fuel content of the solid biomass composition fuel. Typically, the solid biomass composition fuel has one or more of the following properties: (i) a fixed carbon content of from 20% to 40%, preferably from 25% to 35%, and more preferably from 27% to 33%, wherein the fixed carbon content is determined according to DIN EN 51734; (ii) a volatile matter content of from 45% to 65%; preferably from 50% to 60%; and more preferably from 54% to 58%, wherein the volatile matter content is determined according to DIN EN 15148; (iii) an ash content of less than 20%, preferably less than 15%, and more preferably less than 12%, wherein the ash content is determined according to EN 14775 at 550°C; (iv) an internal moisture content of less than 5%, preferably less than 3%, wherein the moisture content is determined according to DIN EN 14774; (v) a coke residue characteristic (CRC) of from 2 to 6, preferably from 3 to 5, and most preferably 4, wherein the coke residue characteristic is determined according to… (vi) a total dry sulphur content of the biomass solid fuel of 0.15 wt% or less, preferably 0.12 wt% or less, wherein the total dry sulphur content is determined according to DIN EN 15289; (vii) a calorific value of from 4300 kcal/kg to 6500 kcal/kg, preferably from 4300 to kcal/kg to 4800 kcal/kg, wherein the calorific value is determined in accordance with DIN EN 14918; (viii) a base moisture content of less than 5%, wherein the base moisture content is determined according to GB/T211-2017. The solid biomass composition fuel may have any combination of the above properties. Preferably, the solid biomass composition fuel has all of the above properties. Preferably, the solid biomass composition fuel has one or more of the following properties: (i) a fixed carbon content of from 25% to 35%, and more preferably from 27% to 33%, wherein the fixed carbon content is determined according to DIN EN 51734; (ii) a volatile matter content of from 50% to 60%; and more preferably from 54% to 58%, wherein the volatile matter content is determined according to DIN EN 15148; (iii) an ash content of less than 15%, and more preferably less than 12%, wherein the ash content is determined according to EN 14775 at 550°C; (iv) an internal moisture content of less than 3%, wherein the moisture content is determined according to DIN EN 14774; (v) a coke residue characteristic (CRC) of from 3 to 5, and most preferably 4, wherein the coke residue characteristic is determined according to… (vi) a total dry sulphur content of the biomass solid fuel of 0.15 wt% or less, preferably 0.12 wt% or less, wherein the total dry sulphur content is determined according to DIN EN 15289; (vii) a calorific value of from 4300 to kcal/kg to 4800 kcal/kg, wherein the calorific value is determined in accordance with DIN EN 14918; (viii) a base moisture content of less than 5%, wherein the base moisture content is determined according to GB/T211-2017. The solid biomass composition fuel may have any combination of the above properties. Preferably, the solid biomass composition fuel has all of the above properties. More preferably, the solid biomass composition fuel has one or more of the following properties: (i) a fixed carbon content of from 27% to 33%, wherein the fixed carbon content is determined according to DIN EN 51734; (ii) a volatile matter content of from 54% to 58%, wherein the volatile matter content is determined according to DIN EN 15148; (iii) an ash content of less than 12%, wherein the ash content is determined according to EN 14775 at 550°C; (iv) an internal moisture content of less than 5%, preferably less than 3%, wherein the moisture content is determined according to DIN EN 14774; (v) a coke residue characteristic (CRC) of 4, wherein the coke residue characteristic is determined according to… (vi) a total dry sulphur content of the biomass solid fuel of 0.12 wt% or less, wherein the total dry sulphur content is determined according to DIN EN 15289; (vii) a calorific value of from 4300 to kcal/kg to 4800 kcal/kg, wherein the calorific value is determined in accordance with DIN EN 14918; (viii) a base moisture content of less than 5%, wherein the base moisture content is determined according to GB/T211-2017. The solid biomass composition fuel may have any combination of the above properties. Preferably, the solid biomass composition fuel has all of the above properties. Typically, the biomass composition comprises wheat husk in an amount of from 75% to 100% by weight. Preferably, the biomass composition consists essentially of or consists of wheat husk. Typically, the solid biomass composition fuel comprises material derived from biomass in an amount of at least 75% by weight; preferably at least 80% by weight and more preferably at least 90% by weight. Typically, the solid biomass composition fuel comprises material derived from wheat husk in an amount of at least 75% by weight; preferably at least 80% by weight and more preferably at least 90% by weight. Preferably, the solid biomass composition fuel consists essentially or consists of material derived from wheat husk. According to a second aspect of the invention, there is provided a solid biomass composition fuel obtainable by or obtained by a process according to the first aspect of the invention. According to a third aspect of the invention, there is provided a solid biomass composition fuel derived from a biomass composition, wherein the biomass composition comprises at least 50% by weight of wheat husk; and wherein the solid biomass fuel composition comprises at least 50% by weight of material derived from wheat husk. Preferably, the biomass composition consists essentially of, or consists of wheat husk. Preferably, the solid biomass composition fuel consists essentially of, or consists of material derived from wheat husk. Preferably, the biomass composition or solid biomass composition fuel are as further defined above in accordance with the first aspect of the invention. According to a fourth aspect of the invention, there is provided a combustion process comprising the step of combusting a solid biomass composition fuel in accordance with the second or third aspects of the invention so as to produce energy. Typically, the solid biomass composition fuel is co-fired and combusted alongside a fossil fuel such as coal. Typically, the PM1.0 emissions of the process are less than 175 mg/kg, and preferably less than 150 mg/kg. According to a fifth aspect of the invention, there is provided the use of a solid biomass composition fuel according to the second or third aspects of the invention as a fuel in a combustion process, optionally wherein the use comprises using the solid biomass composition fuel in a process according to the fourth aspect of the invention, optionally wherein the combustion process comprises co-firing the solid biomass composition fuel alongside a fossil fuel, such as coal. Typically, the PM1.0 emissions of the process are less than 175 mg/kg, and preferably less than 150 mg/kg. Typically, the biomass composition comprises, consists of, or consists essentially of wheat husk. Typically, the use comprises using the biomass composition in a process according to the first aspect of the invention, and/or wherein the solid biomass composition fuel is as defined in the second or third aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The present inventions will now be described by way of example and with reference to the accompanying Figures in which: Figure 1 depicts an apparatus that may be used for rolling, rotation and vibration of biomass composition solid fuel particles in the process of the invention. Figure 2 is a photograph of a compression apparatus that may be used in the process of the invention. Figure 3 is a photograph of a compression apparatus that may be used in the process of the invention. Figure 4 a diagram of an apparatus that may be used for washing step (iv) of the process of the invention. Figure 5 a diagram of an apparatus that may be used for washing step (iv) of the process of the invention. Figure 6 is a diagram of an apparatus that may be used for washing step (iv) of the process of the invention. Figure 7 is a diagram of an apparatus that may be used in mechanically dewatering washed biomass composition powder in the process of the invention. Figure 8 is a photograph of an apparatus that may be used for washing step (iv) of the process of the invention. Figure 9 is a diagram of a drying apparatus that may be used in the process of the invention. Figure 10 is a diagram of a compression mold that may be used in accordance with the process of the invention. DETAILED DESCRIPTION OF THE INVENTION Providing a biomass composition The biomass composition comprises wheat husk. Preferably, the biomass composition consists essentially of wheat husk or consists of wheat husk. The term “comprising” as used herein is used to mean that any further undefined component can be present. The term “consisting” as used herein is used to mean that no further components can be present, other than those specifically listed. The term “consisting essentially of” as used herein is used to mean that further undefined components may be present, but that those components do not materially affect the essential characteristics of the composition. Typically, the biomass composition comprises wheat husk in an amount of 75% by weight or more, preferably 80% by weight or more, more preferably 90% by weight or more and most preferably 95% by weight or more. In instances where the biomass composition comprises sources of biomass in addition to wheat husk, the additional sources of biomass may comprise any other suitable source of biomass. For example, the biomass composition may comprise agricultural waste. The term “agricultural waste” as used herein typically refers to plant-based waste products that are produced as a by- product of agricultural operations. For example, agricultural waste may comprise left over plant-based products that are harvested, or unwanted components of harvested plant-based products. Alternatively, the additional sources of biomass may be grown specifically for the purpose of being a feedstock for the preparation of biomass composition solid fuels. Examples of further sources of biomass that may be used include calliandra calothyrsus, straw, coconut shell, acacia mangium, albizia chinensis, hevea brasiliensis, rice husk, grasses such as those of the Pennisetum genus such as Pennisetum sinese Roxb, yam, corn cob, bagasse, sunflower stalks, wheat stalks, corn stalks, sorghum stalks, soybean stalk, peanut stalks, cotton stalks, rape stalks, coconut husks, palm kernel shell (PKS), oil palm tree such as oil palm tree trunk, seaweed, peanut hulls, or any combination thereof. Preferably, the biomass composition does not comprise wood and/or the solid biomass composition fuel does not comprise material derived from wood. Each of the biomass sources discussed above can be obtained or harvested by conventional methods known in the art. As discussed above, it has been found that the wheat husk can be grown and harvested on a commercial scale, providing increased control of the quality and specific characteristics of the biomass source compared to certain materials used in the prior art such as wood. Use of wheat husk also avoids the environmental damage associated with using trees such as necessary deforestation.   Use of wheat husk has also surprisingly been found to be easier to grind than materials such as wood. This reduces the costs of the grinding process. Use of wheat husk, when pulverised, also provides a more homogenous mix of particle sizes than said prior used materials such as wood. Without being limited by theory, this is believed to impart advantageous properties to the final solid fuel product, such as greater uniformity and continuousness of the wheat husk fuel products. This is desirable in combustion processes for a number of reasons. The wheat husk may be provided in any suitable size. Typically, the wheat husks are harvested by use of a conventional combine before being threshed or winnowed so as to separate wheat husk from the grains of the wheat. Removing dust from the biomass composition comprising wheat husk The process of the invention comprises step (ii) of removing dust from the biomass composition. It has been found important to remove dust from the wheat husk prior to further pulverisation and processing into the solid biomass composition fuel. Wheat husk typically contains a large amount of dust and fine particulates due to its production process from harvested wheat. The typical threshing and winnowing process carried out to wheat husk to remove the husk from the grain and other components of the wheat causes the produced wheat husk to contain large amounts of dust. It has been found important to remove the dust prior to the subsequent pulverisation step since the presence of large quantities of dust in the wheat husk starting material has been found to impede the efficiency of the pulverisation process and can also lead to a less desirable larger particle size distribution once the wheat husk has been pulverised. As will be discussed in further detail below, this is less desirable since it may lead to less homogeneity and uniformity of the solid biomass composition fuel once produced which is undesirable for a variety of reasons. Dust and fine particulates may also pollute the environment and atmosphere during production process steps such as pulverisation, and in some instances, pose a health risk to workers operating the production process machinery. Step (ii) of removing dust from the biomass composition comprising wheat husk may comprise inducing friction between the particles of the wheat husk. For example, step (ii) of removing dust from the wheat husk may comprise subjecting the particles to vibration, rotation, rolling, or any combination thereof. Suitable apparatus for conducting rolling, rotation, and vibration of the biomass compositions comprising wheat husk are known to the person skilled in the art, and are shown in Figure 1. An example of an apparatus that may be used to remove dust from the particles is a rotating drum sieve. Step (ii) of removing dust particles from the wheat husk may comprise removing dust particles from the wheat husk with a screen. Typically, the screen has a pore size of from 2 mm to 10 mm, preferably 2 mm to 8mm, more preferably from 2 mm to 5 mm, and most preferably from 3 mm to 5 mm. Dust particles that are admixed with the wheat husk may be separated from the wheat husk particles by passing through the screen. The larger wheat husk particles do not pass through the screen and are thus separated from the dust particles. Suitable apparatus and methods for performing the screening step are known to those skilled in the art, and any of said suitable apparatus may be used. For example, an apparatus that employs screening, rolling and rotating the wheat husk particles may be used to remove dust particles from the biomass composition comprising wheat husk. In the use of such a device, the wheat husk may be laid upon a screen, and the screen may be driven to roll and rotate upon its axis by operation of a motor. During rolling/tilting and rotation of the screen, material on the sieve surface of the screen is turned over. Some material passes through the screen and is separated from material that does not pass through the screen. The rolling and rotation of the screen causes material stuck in the pores of the screen to fall through and thus clogging of the pores of the screen is prevented. Alternatively, an apparatus that vibrates and screens the wheat husk particles may be used. In this case, a motor can be used to vibrate the screen which may cause material to be thrown up on the screen surface. This process may cause small particles adhered to larger ones to come loose and then pass through the pores in the screen. An example of an apparatus that employs a screen and vibration to separate larger particles from smaller particles, where the smaller particles may or may not be adhered to the larger particles is a device as taught in CN201324717. Accordingly, methods of the invention may comprise subjecting the wheat husk particles to one or more of rolling, rotation and vibration so as to induce friction between the wheat husk particles which causes dust particles adhered to said particles to be removed from said particles. The methods then preferably comprise subjecting the mixture of wheat husk particles and dust particles to a screening step as discussed above to remove said dust particles from said wheat husk particles. Pulverisation of biomass Step (ii) comprises pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm. The biomass composition may be pulverised into a biomass composition powder by standard techniques known in the art. The biomass composition may be pulverised such that the biomass composition powder has an average particle diameter (D50) of from 1000 µm to 15,000 µm, such as from 1000 µm to 10,000 µm. In some instances, the biomass composition is pulverised to have an average particle diameter of from 1000 µm to 8,000 µm such as from 1,000 to 5000 µm. As discussed above, pulverising the biomass composition for use in the present invention has been found to provide a biomass composition powder with an advantageous smaller particle size distribution than provided by grinding prior known biomass sources such as wood. It has further been found that the smaller the particle size distribution of pulverised biomass composition powder, the greater the quality and performance characteristics of the solid biomass composition fuel product. Without being limited by theory, this is believed to be due to greater uniformity and homogeneity of the final solid biomass composition fuel product. Smaller powder particle size distribution and greater uniformity and homogeneity of the final fuel product is believed to be linked to improved performance characteristics of the fuel upon combustion, and also to improved water proof characteristics of the solid fuel product. Different pulverisation processes are preferred depending on the moisture content of the biomass composition. Where the biomass composition has a moisture content of 20% by weight or less, preferably, the step of pulverising the biomass composition involves the use of a negative pressure pneumatic conveyancing apparatus. Such negative pressure pneumatic conveyancing apparatus are known in the art. Where the biomass composition has a moisture content of greater than 20% by weight or more such as 30% by weight or more, typically, step (iii) of pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm comprises pulverising the biomass composition in a low discharge type shredder. Suitable types of low discharge type shredders are known in the art. Since wheat husk typically has a moisture content of greater than 30% by weight, preferably, in processes of the invention, the wheat husk is pulverised in a low discharge type shredder as discussed above. Where the process comprises the use of a low discharge type shredder apparatus, the process typically comprises stirring and then spraying the biomass composition within the low discharge type shredder to provide the pulverised biomass composition powder.   The stirring step beforehand has been found by the inventors to be beneficial in achieving a more uniform particle size distribution of the pulverized biomass composition powder, which is beneficial for at least the reasons discussed above. The subsequent spraying of the biomass within the low discharge type shredder is the step by which the biomass is actually pulverized within the shredder. Where step (iii) comprises pulverizing in the low discharge type shredder, it is preferred that the biomass composition has a uniform moisture content when it is pulverized in the shredder, such as in the spraying step discussed above. Accordingly, the process may further comprise a step of increasing the uniformity of the moisture content of the biomass composition prior to pulverization of the biomass composition. Such a step may be accomplished, for example, by the stirring step which is carried out in the shredder prior to the spraying step which leads to pulverization of the biomass composition. A more uniform moisture content has been found by the inventors to be preferable since it ensures that the biomass composition is evenly fed in the pulverization step (such as evenly fed to a spraying component of the low discharge type shredder) which also provides pulverized biomass composition powder of greater uniformity and with a smaller particle size distribution. Step (iii) may also comprise a step of increasing the moisture content of the biomass composition prior to pulverization in the low discharge type shredder. For example, the moisture content of the biomass composition may be increased to 10% or more by weight of the biomass composition; preferably 20% or more by weight of the biomass composition; and more preferably 30% or more by weight of the biomass composition. It has been found that a higher moisture content during pulverization is desirable since it reduces the dust present in the pulverized biomass composition powder. Accordingly, it is preferred that the moisture content of the biomass composition prior to pulverization is 10% by weight or more, preferably 20% by weight or more, and more preferably 30% by weight or more such as 40% by weight or more. Dust present in the pulverized biomass composition powder is less desirable since it may pollute the air during processing of the wheat husk biomass. The dust may also pollute the local environment. Furthermore, if left in the open air, dust particles may form mildew. It has surprisingly been found that by pulverising the biomass composition to provide a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm, certain advantages are provided to the subsequently carried out washing step (v). Without being limited by theory, it is believed that the reduced particle size aids in the efficiency of the washing step. Reducing the particle size by pulverisation increases the surface area of the biomass composition. When the pulverised biomass composition is subsequently washed, the larger surface area means that a more efficient and effective wash is carried out than when particles of a smaller surface area are washed. The greater efficiency and effectiveness of the washing process means that less harsh conditions can be used in the washing step, such as a lower temperature of the aqueous washing liquid used in the washing step. A lower temperature aqueous washing liquid means that a lower pressure may also be used, since higher pressures are not required to keep the water as a liquid since the water is at a lower temperature. Lower temperature and pressure for the process mean that the cost of the process can be reduced in comparison to prior art processes that typically use higher temperature and pressure washing steps to provide a more effective wash of biomass particles with a larger surface area. Safety is increased too by using lower temperatures and pressures. Hence, efficiencies are introduced by using a pulverisation step prior to washing. Due to the pulverisation step, it may also be possible to use less successive washing stages than prior art processes that typically use a great many successive washing stages in order to effectively wash the biomass raw materials. However, it has been found by the inventors of the present invention that increasing the efficiency and effectiveness of the washing process is not as simple as just pulverising the wheat husk biomass to any smaller biomass particle size with increased surface area. If the size of the wheat husk biomass is reduced too much (below 1000 µm) then the effectiveness of the washing process is negatively affected. This is, inter alia, due to greater difficulty in separating the water from the washed wheat husk biomass after washing (for example in a subsequent compression step) as the mixture of water and pulverised biomass composition may form a slurry where the solid particles have less structural integrity, and the fact that the wash water may carry away the pulverised biomass composition during the washing step (for example if a mesh or screen is used to drain wash water from the biomass), reducing the yield of the process, unless complex equipment is used to prevent this happening. Additionally, as discussed in further detail below, where the washing step comprises the use of an extrusion type apparatus comprising a screw (which is preferable in the process of the invention), it has been found that a particle size of from 1000 µm to 20,000 µm is optimum for flowability and transferability of the pulverised biomass composition powder through the extrusion type apparatus and whilst also achieving effective washing efficiency. Accordingly, it has thus been found that a pulverised biomass composition powder with an average particle diameter (D50) of from 1000 µm to 20,000 µm is optimum for enhancing the efficiency of the subsequent washing step. Compressing the pulverised biomass powder The process of the invention may comprise step (iv) of compressing the pulverised biomass composition powder to provide a compressed biomass composition powder. Alternatively, the process may not comprise such a step. Compression of the wheat husk biomass after pulverisation may typically be desired if the biomass composition powder has a moisture content of greater than 30% by weight, although compression may also be carried out on biomass composition powder with lower moisture contents. Typically, pulverised biomass composition powder that has been pulverised in a low discharge type shredder will have a higher moisture content due to either the original moisture content of the wheat husk source material, or any moisture that has been added in the pulverisation step. Accordingly, in some instances, the process comprises step (iv) of compressing the pulverised biomass composition powder to provide a compressed biomass composition powder if the pulverised biomass composition powder has been pulverised in a low discharge type shredder apparatus. In general, compression of the biomass powder prior to washing has been found advantageous irrespective of the moisture content of the pulverised biomass composition. The compression step may involve compressing the biomass composition powder using suitable apparatus known in the art. An example of such an apparatus is shown in Figure 2. Such an apparatus operates by compressing the biomass composition powder with a hydraulic compression device. Material can be inserted into the meshed container shown in Figure 2. The material may then be subjected to hydraulic compression with a hydraulic compression device causing water to exit the meshed container through the holes of the mesh. Another apparatus that may be used to compress biomass composition powder is that shown in Figure 3, which is a screw water squeezing machine. The material to be compressed is introduced into the spiral extrusion vessel. Moisture from the material is squeezed through the screen mesh by rotation of the motor-driven spiral screw. It has been found that carrying out a compression step after the biomass has been pulverised as discussed above provides a compressed biomass composition powder with even lower water content. Typically, the moisture content of the compressed biomass composition powder is less than 30 % by weight, such as less than 25% by weight or less than 20% by weight. Surprisingly, it has been found by the inventors of the present invention that the step of compressing the pulverised biomass composition powder to provide compressed biomass composition powder imparts various advantages to the subsequently carried out washing step. Compressing the pulverised biomass composition powder results in a reduced water content of the biomass. When the wheat husk biomass is then subsequently washed with water (for example, by spraying or immersing the wheat husk biomass in water as described in further detail below), the washing water is able to more effectively wash the compressed biomass composition powder due to its reduced water content. Without being limited by theory, this is believed to be because the reduced moisture content of the wheat husk biomass allows the wash water to better penetrate the biomass during washing due to a greater diffusion gradient. The improved efficiency and effectiveness of the washing step means that lower temperatures and pressures can be used in the washing step, providing the advantages discussed above. As also discussed above, it has been found that an average particle diameter (D50) of from 1000 µm to 20,000 µm for the pulverised biomass composition powder is particularly advantageous for the compression step, since it is easier to remove moisture from particles of such a size when compressing the wheat husk biomass. Washing of the pulverised biomass composition powder The process comprises step (v) of washing the pulverised biomass composition powder or compressed biomass composition powder. The washing step is carried out with an aqueous wash liquid to provide a washed biomass composition powder. The washing step has been found by the inventors to be advantageous in that it removes minerals such as potassium, sodium, chlorine, calcium and phosphorus containing salts from the wheat husk biomass. As discussed above, it has been found by the inventors of the present invention that washing to remove mineral salts is particularly important with wheat husk biomass. As discussed above, preferably, the washing process involves washing the compressed biomass composition powder or pulverised biomass composition powder more than once, in successive washing stages. An advantage of the present invention is that less successive washing stages may be used than washing processes known in the art, whilst achieving the same effectiveness of washing and mineral salt removal. Without being limited by theory, this is believed to be due to the adaptation of the pulverisation and compression steps prior to washing discussed in detail above, and the use of certain washing apparatus as described below. Any suitable aqueous washing liquid may be used in washing step (v). Examples of aqueous washing liquids include water. Preferably, the water is pure and does not comprise salts or other compounds dissolved or dispersed therein. However, the aqueous washing liquid may contain other components dissolved or dispersed therein if desired. In some instances, each successive washing stage may comprise spraying the compressed biomass composition powder or pulverised biomass composition powder with the aqueous wash liquid and/or immersing the compressed biomass composition powder or pulverised biomass composition powder in the aqueous wash liquid. The wash liquid may be removed from each successive washing stage by, for example, a mesh or screen that drains the wash liquid from the biomass. The mesh or screen may have a mesh size smaller than the size of the compressed or pulverised biomass composition powder such that the compressed or pulverised biomass composition powder sits upon the screen, with the aqueous wash liquid draining from the screen so as to be removed from each successive wash stage. A suitable apparatus for washing the pulverised biomass composition powder according to these instances is shown in Figure 4. The compressed or pulverised biomass composition powder enters on the left hand side of the apparatus shown in Figure 4. Water enters on the right hand side of the apparatus shown in Figure 4. There are four successive washing stages shown in the apparatus in Figure 4. The compressed or pulverised biomass composition powder is washed at each stage. Fresh aqueous washing liquid such as water is used to wash the biomass in the final successive washing stage on the far right of the diagram. The wash water from the final washing stage is then drained and collected after washing before being used as the wash water for the third washing stage. The wash water from the third washing stage is then collected and used as the wash water for the second washing stage, and so on for the second and first washing stages. In this respect, the biomass composition powder is washed in a counter current manner. Washing in such a manner has been found to advantageously provide more effective washing and mineral salt removal. As discussed above, in some instances, the compressed or pulverised biomass composition powder can be sprayed with or immersed in the aqueous wash liquid. In instances where the compressed or pulverised biomass composition powder is sprayed with the aqueous wash liquid, the washing step of the process of the invention may also comprise brushing the pulverised or compressed biomass composition powder during or after the step of spraying the pulverised or compressed biomass composition powder with the washing liquid. The brushing step may aid in cleaning the biomass by removing dirt and other impurities from the compressed or pulverised biomass composition powder. Accordingly, in certain instances, at least one washing stage of washing step (v) comprises brushing the compressed or pulverised biomass composition powder during or after washing. For example, each successive washing stage of washing step (v) may comprise brushing the compressed or pulverised biomass composition powder during or after washing. Apparatus that may be used in accordance with the washing step discussed above, such as the apparatus shown in Figure 4, may comprise a washing drum within which the pulverised or compressed biomass composition powder is placed. The pulverised biomass composition powder may then be sprayed by a suitable spraying component (typically a spraying tube) before being brushed by a suitable brushing component (typically a brush plate). Alternatively, the brushing component can brush the pulverised biomass composition powder at the same time as the pulverised biomass is sprayed. Apparatus that may be used in accordance with the washing step discussed above, such as the apparatus shown in Figure 4, may comprise water tanks, such as water tanks situated beneath a washing drum. In some instances, each successive washing stage of washing step (v) comprises a washing drum and two washing tanks. One washing tank may be used to supply fresh water to a washing tube or other suitable washing liquid providing component for spraying or immersing the compressed or pulverised biomass composition powder. The other washing tank may collect the washing liquid that has been used to spray the pulverised or compressed biomass composition powder. Once the washing liquid is collected in this water tank, it may be filtered in the water tank by a suitable filtering component so as to produce clean water that can be recycled and used again in another washing stage, for example, a preceding washing stage, such as in a counter current process described above. In some instances, the process comprises a washing step in which the pulverised or compressed biomass powder is placed in a washing drum that is rotated. For example, the pulverised biomass composition powder is placed in the rotating washing drum and sprayed with a washing liquid so as to wash the pulverised biomass composition powder, before the washed powder is brushed. Other features of washing step (iv) may be as described in WO2013162355. However, preferably, washing step (v) is not as described above and instead comprises washing the compressed biomass composition powder or pulverised biomass composition powder in an extrusion type apparatus comprising a screw. Preferably, the screw is adapted to transport the pulverised biomass composition powder or compressed biomass composition powder in a first direction through the extrusion type apparatus; and wherein the apparatus is adapted to receive and transport the aqueous wash liquid in a second direction through the apparatus such that that the aqueous wash liquid contacts the pulverised biomass composition powder or compressed biomass composition powder within the apparatus. Typically, the second direction is opposite to or perpendicular to the first direction, and preferably opposite to the first direction. Preferably, the extrusion type apparatus comprises a separate screw for each successive washing stage. Such extrusion type apparatus comprising a screw are known such as sand washers. It has surprisingly been found by the inventors of the invention that the use of such washers provides an improved washing effect of the pulverised or compressed biomass composition powder over other known washing techniques such as the use of rotary washing drums discussed above and the techniques discussed above where the pulverised or compressed biomass is immersed or sprayed with the wash liquid. Without being limited by theory, it is believed that the higher pressure forces that the pulverised or compressed biomass composition powders are subjected to in the extrusion type apparatus leads to an improved washing effect of the biomass. As discussed above, it has been found that a particle size of from 1000 µm to 20,000 µm for the pulverised/compressed biomass composition powder is optimum for flowability and transferability of the pulverised biomass composition powder through the extrusion type apparatus whilst also achieving optimum washing efficiency. The extrusion type apparatus comprising a screw preferably comprises from 1 to 5 washing stages, and more preferably from 2 to 5 washing stages, with each separate washing stage comprising a separate screw. Figure 5 depicts an example of an extrusion type apparatus, such as those discussed above, that can be used to carry out washing step (v) of the process of the invention. The apparatus comprises four successive washing stages, with each stage comprising an extrusion screw. The biomass powder to be washed enters each washing stage at one end of the screw and is transferred by rotation of the screw towards the other end of the screw whilst being washed with the aqueous wash liquid. The aqueous wash liquid enters each washing stage at the opposite end of the screw to the biomass powder. After passing through a first washing stage, the biomass powder is transferred to the next washing stage. The washing is performed in a counter current manner whereby aqueous wash effluent from a later washing stage is used as the aqueous washing liquid for an earlier washing stage. Figure 6 shows the wheat husk biomass material passing through the extrusion device in an opposite direction to the aqueous wash liquid. Accordingly, in some embodiments, washing step (v) comprises at least one washing stage that comprises washing the pulverised biomass composition powder or compressed biomass composition powder with an aqueous wash liquid, whereby the at least one washing stage is carried out in an apparatus section comprising a screw adapted to transport the pulverised biomass composition powder or compressed biomass composition powder in a first direction through the apparatus section; and wherein the apparatus section is adapted to receive and transport the aqueous wash liquid in a second direction through the apparatus section such that that the aqueous wash liquid contacts the pulverised biomass composition powder or compressed biomass composition powder within the apparatus section. Preferably, the first direction is opposite to the second direction. Preferably, washing step (v) comprises from 1 to 5 of such washing stages, and more preferably 2 to 5 of such washing stages. The term counter-current wash mechanism is used herein in the context of the normal meaning of the term in the art. For example, a typical counter-current washing mechanism would involve contacting the portion of the biomass material that has had the most exposure to aqueous wash liquid with the freshest portion of aqueous wash liquid. After washing the portion of biomass material that has had the most exposure to aqueous wash liquid, the wash liquid then goes on to contact portions of biomass material that have had successively less exposure to the aqueous wash liquid. In this manner, a dissolved salt concentration gradient may be maintained between the aqueous wash liquid and the biomass material being washed, such that salts are continuously washed and removed from the biomass material. The apparatus shown in Figures 4, 5 and 6 thus involve a counter-current washing mechanism. As described above, washing step (iv) is preferably carried out at temperatures of from 5°C to 160°C, and more preferably at ambient temperatures of from 5°C to 35°C. In some instances, washing step (iv) is carried out temperatures of from 5°C to 30°C, 5°C to 25°C, 5°C to 20°C, 5°C to 15°C, or 5°C to 10°C. The temperature referred to is the temperature of the aqueous wash liquid applied to the pulverised or compressed biomass composition powder. Preferably, no heating of the aqueous wash liquid or the pulverised or compressed biomass composition powder is carried out during the washing step of the process. The aqueous wash liquid may increase in temperature during use (for example as a result of passing through the washing apparatus). However, preferably, no additional intentional heating of the aqueous wash liquid or the pulverised or compressed biomass composition powder is carried out. The washing step (iv) is also preferably carried out at pressures of 1.1 to 15 bar, and most preferably atmospheric pressure. A key advantage of the washing process of the present invention is that it may be carried out at atmospheric pressure and ambient temperature whilst still achieving a highly effective mineral salt removal. This results in the process being more commercially viable and also safer. It is believed that this higher effectiveness at more ambient conditions is due at least in part to the adaptations of the pulverising and compression steps discussed above, and in certain instances, the use of the extrusion type apparatus comprising the screw. The adaptations mean that the process of the invention can achieve similar levels of mineral salt removal using ambient temperature and pressures that are achieved by techniques known in the art (such as those disclosed in WO2013162355) where higher temperatures are generally used to achieve effective mineral salt removal. Mechanically dewatering the washed biomass powder Step (vi) comprises mechanically dewatering the washed biomass composition powder. Preferably, the washed biomass composition powder is mechanically dewatered so as to provide a dewatered biomass composition powder and an aqueous effluent. Preferably, step (vi) comprises squeezing or compressing the washed biomass composition powder. Optionally, the compression apparatus and method used can be the same as that used for compression step (iv) of the process. In alternative instances, step (vi) can be as described in the mechanical dewatering step disclosed in WO2013162355. The purpose of compression step (vi) is to remove water from the washed biomass composition powder to improve the ease and efficiency of subsequent drying step (vii). Preferably, step (vi) of mechanically dewatering the washed biomass composition powder comprises passing the washed biomass composition powder through an extrusion type apparatus comprising a screw. Such an apparatus is shown in Figures 7 and 8. As can be seen in these figures, the washed biomass composition powder enters the extrusion type apparatus via the dosing port. The material is then pushed forward by the spiral rotating screw. The changing diameter of the screw along its length and the screw’s rotation mean that the washed biomass is subjected to a large squeezing pressure within the apparatus causing water to be squeezed from the biomass powder. The water exits the apparatus through a screen at the base of the screw. The screen gap is typically from 0.5 mm to 1 mm in size which allows water to pass through but not the biomass particles which have a larger average particle size. The dewatered biomass then exits the apparatus at the discharge port shown in the figures. Drying the pulverised biomass composition powder The biomass is dried in step (vii) of the process. Step (vii) of drying the dewatered biomass composition powder so as to provide a dried biomass composition powder typically comprises drying the dewatered biomass composition powder such that the dried biomass composition powder has a moisture content of from 10 % by weight to 20 % by weight, preferably from 12 % by weight to 15% by weight. However, it will be appreciated that it is not essential that the dried biomass composition powder has a moisture content within this range. The dewatered biomass composition powder may be dried using any suitable method, such as using standard drying cylinders known in the art. For example, in some instances, the drying step is carried out in a drying apparatus that comprises a rotating drying drum. The rotation of the rotating drying drum can also be used to mix the dewatered biomass composition powder with one or more additional sources of biomass, or other components, if desired. Typically, the rotating drying drum comprises a lifting plate. The lifting plate continuously raises material while the drying cylinder rotates. The use of a rotating drying cylinder with a lifting plate results in improved mixing of the one or more biomass powders where the one or more biomass powders are being dried with additional materials, or where two or more biomass powders are being mixed. Where the dewatered biomass composition powder is dried in one or more drying cylinders, where the dewatered biomass composition powder has a moisture content of less than 20 wt%, typically, the dewatered biomass composition powder is dried in a single drying cylinder. Where the dewatered biomass composition powder has a moisture content of greater than 20 wt%, the dewatered biomass composition powder is typically dried in multiple drying cylinders. For example, the process may comprise drying the dewatered biomass composition powder in two or more, three or more, four or more, or five or more drying cylinders. In preferable instances, drying step (vii) is carried out within a multi-layer belt dryer. Such dryers are known in the art. An example of a multi-layer belt dryer that may be used is shown in Figure 9. In preferable instances, the process of the invention comprises recycling heat generated in heating step (ix) and using said heat in drying step (vii) to heat and dry the dewatered biomass composition powder to provide the dried biomass composition powder.  Advantageously, the inclusion of such a step in the process means that the process is more economically viable and less costly since costs associated with providing heat for drying step (vii) may be reduced. The step of drying the biomass composition powder may also comprise mixing the biomass powder during drying with other sources of biomass or additional components. For example, where biomass composition comprises at least two sources of biomass, the one or more sources of biomass may be mixed during the drying step of the process of the invention, for example, in a drying cylinder, as discussed above. Alternatively, and preferably, the process of the invention comprises a step of mixing the dried biomass composition powder with other biomass powders between drying step (vii) and molding step (viii) of the process. The biomass powder may be mixed with an additional source of biomass that has also been prepared by steps (i) to (vii) of the present invention. Alternatively, the one or more additional sources of biomass are not processed as described herein. For example, the biomass powder prepared as described herein may be mixed with one or more additional sources of biomass that are prepared in different ways. Molding the dried biomass powder The dried biomass composition powder is molded so as to provide a molded biomass composition product. The molding step may be carried out in any molding apparatus known in the art and in accordance with biomass molding techniques known in the art, and may include extrusion systems. Preferably, the molding step is carried out in a compression mold. Preferably, the compression mold comprises a mold product exit hole. The molding step may be carried out using an apparatus as described in CN105435708. Preferably, the molding step comprises molding the dried biomass composition powder into pellets. Accordingly, preferably, the molded biomass composition product and solid biomass composition fuel product comprises biomass pellets. Adapting the molding step such that the density of the molded biomass composition product produced from said step is controlled so as to be within a certain range imparts certain advantageous properties to the final solid biomass fuel product. Specifically, controlling the molding step such that the density of the molded biomass composition product is within the range of from 1.0 to 1.35 kg/L has been found to impart advantageous properties to the final biomass fuel product. Preferably, the molding step is controlled such that the density of the molded biomass composition product is from 1.0 kg/L to 1.35 kg/L. Typically, the above mentioned densities are determined according to NY/T 1881.7-2010. Accordingly, in some instances, the molding step is controlled such that the density of the molded biomass composition product is from 1.0 kg/L to 1.35 kg/L, wherein the density is determined according to NY/T 1881.7-2010. The molding step may be controlled in a variety of ways. Where the molding process comprises the use of a compression mold, the density is typically controlled by using a compression ratio of less than 9, and more preferably from 6 to 8. Typically, the smaller the compression ratio, the lower the density of the molded biomass composition product. However, the higher the compression ratio, the lower the yield of the molded biomass composition product. There is thus a balance to be met between having a high enough compression ratio to achieve the target densities for the molded biomass composition product, whilst having a low enough ratio to ensure that an adequate and economically viable product yield is provided in the molding step. The optimum compression ratio will be dependent upon the nature of the biomass source. For wheat husk, it has been found by the inventors that a compression ratio of from 6.0 to 8.0 is optimum in order to achieve the target densities discussed above for the molded biomass product in order to impart advantageous properties to the final solid biomass fuel product, whilst also ensuring that the yield of the molded biomass product is high enough for the process to be economically viable. The compression ratio for a compression mold with a mold product exit hole may be defined as the ratio of the length to the diameter of the mold product exit hole. Figure 10 shows an example of a compression mold that may be used in accordance with the present invention. The dried biomass composition powder is inserted into the interior of the mold before being squeezed from inside the mold by pressure such that it exits the mold product exit hole in the Figure. The compression ratio is shown in the Figure as the ratio of the length of the product out hole to its diameter. In the process of the invention, preferably, the step (viii) of molding the dried biomass composition powder comprises adapting the molding step such that the density of the molded biomass composition product is controlled to be within the range of from 1.1 kg/L to 1.35 kg/L, typically wherein the density is determined according to NY/T 1881.7-2010. Preferably, the density is controlled by using a compression mold and controlling the compression ratio of the compression mold. More preferably, the compression ratio is from 6.0 to 8.0 Controlling the density of the molded biomass composition product during the molding step has been found, surprisingly, to provide a final biomass fuel product with increased water proof capacity. Preferably, the solid biomass fuel product produced from a molded biomass product with a density within the range of from 1.1 kg/L to 1.35 kg/L is sufficiently water proof for up to 20 days, and preferably up to 30 days. Preferably, an additive is added to the dried biomass composition powder prior to step (viii) of molding the dried biomass composition powder. Said additive is believed to improve the molding process and increase the yield of the molded biomass composition product produced from the molding step. Suitable additives are known in the art and include, but are not limited to starch, or starch derivatives. Typically, other than additives such as those discussed above, no other fuel source is added to the dried pulverised biomass composition powder during the molding step. Accordingly, the molded biomass composition product of the molding step typically comprises only material derived from biomass as the fuel source in the solid biomass fuel. For example, when the dried pulverised biomass composition powder is molded into pellets, typically, no other fuel source is added to the dried pulverised biomass composition products prior to molding such that the solid biomass fuel pellets produced at the end of the process only contain a fuel source derived from biomass. In preferable instances, the solid biomass fuel thus comprises at least 50% by weight of the total fuel content of the fuel, such as at least 60% by weight, at least 70% by weight, at least 80% by weight, at least 90% by weight and preferably at least 95% by weight of material derived from biomass. Where the term total fuel content of the solid fuel is used herein, this is intended to refer to the component of the solid fuel that is combustible material such as biomass derived material and coal. The term fuel content in relation to solid fuel is not intended to encompass additives that may be present in the solid fuel pellets that do not themselves combust to produce energy. The molding step has also been found to enhance the waterproof properties of the final biomass composition solid fuel product. The increase in density that occurs during the molding step means that it is harder for water to penetrate the denser molded biomass product particles. Furthermore, with a denser product, more biomass is concentrated in the interior of the molded product, and so is not in direct contact with water. Heating the molded biomass composition product The molded biomass composition product is heated so as to produce a solid biomass composition fuel. The heating is carried out at a temperature of from 160°C to 420°C for a time period of from 0.25 to 5 hours. Preferably, the step of heating the molded biomass composition product is carried out for a time period of from 0.4 to 2 hours. Preferably, the step of heating the molded biomass composition product comprises heating the molded biomass composition product to a temperature of from 180°C to 350°C, and more preferably to a temperature of from 210°C to 280°C. Preferably, the step (ix) of heating the molded biomass composition product comprises heating the molded biomass composition product under conditions so as to induce torrefaction of the molded biomass composition product. Torrefaction is a process of mild pyrolysis in which the heating is carried out in a low oxygen atmosphere such as an atmosphere of less than 10% oxygen content. Suitable conditions and processes of torrefaction are known in the art. Accordingly, preferably step (ix) of heating the molded biomass composition product comprises torrefaction. It is highly preferred that step (ix) of heating the molded biomass composition product comprises a step of dry torrefaction. The term dry torrefaction as used herein refers to thermal treatment of biomass in an inert environment at the temperatures discussed above so as to induce torrefaction. Dry torrefaction is carried out in the absence of water, steam and hydrothermal media. It is highly preferred that step (ix) of heating the molded biomass composition product does not comprise a step of wet torrefaction. The term wet torrefaction as used herein refers to the treatment of biomass under the conditions discussed above in hydrothermal media or hot compressed water. The heating step may be carried out in any suitable apparatus known in the art for heating the molded biomass composition product. For example, the heating step may be carried out in apparatus and using process conditions as disclosed in EP3287509A1. Preferably, step (ix) of heating the molded biomass composition product is adapted so as to control the uniformity of the solid biomass fuel, optionally wherein adapting step (ix) so as to control the uniformity of the solid biomass composition fuel comprises conducting step (ix) in an apparatus in which the molded biomass composition product is rotated whilst being heated, optionally, wherein adapting step (ix) so as to control the uniformity of the solid biomass composition fuel comprises controlling the speed or direction of rotation of the molded biomass composition product, optionally wherein the molded biomass composition product is rotated in the apparatus in both an anticlockwise and clockwise direction. The uniformity of the solid biomass composition fuel is also optimised by the heating temperatures and time periods discussed above. Typically, the process of the invention may comprise a step of cooling the solid biomass composition fuel after heating. Where the process of the invention comprises a cooling step after the step of heating the biomass, the cooling step may comprise rotating the solid biomass composition fuel. The biomass may be rotated in a suitable apparatus such as those disclosed in EP3287509A1. Preferably, both heating step (ix) and the step of cooling the biomass comprise rotating the biomass. Where the biomass is rotated in either the cooling step or the heating step, the biomass may be rotated in different directions, such as both clockwise and anti-clockwise in successive cycles. The term ‘uniformity’ of the solid biomass product is used to refer to the solid biomass composition fuel or molded biomass composition product having constant or similar properties across each particle of solid biomass composition fuel or molded biomass composition product and across the plurality of particles within a bulk sample of the solid biomass composition fuel product or molded biomass composition product. For example, but not limited to, the densities of the particles, the ease of combustion of the particles, the chemical composition of the particles, and the water resistant properties of the particles. Uniformity is a highly desirable property for biomass fuels for use in combustion processes. It has also been found by the inventors that controlling the heating step in the manner discussed above additionally aids in providing a solid biomass composition fuel product with enhanced water proof properties compared to the biomass fuels of the prior art. During the heating step, hydrophilic compounds present in the biomass powders that absorb water are degraded. Furthermore, the heating step causes oils present in the biomass powders to migrate to the exterior of the biomass powder particles, increasing the hydrophobicity of said particles. Once the solid biomass composition fuel has been produced by heating step (ix), the solid biomass composition fuel may optionally be subjected to various further treatment steps. For example, the process may further comprise (a) removing dust particles from the solid biomass composition fuel; and/or (b) washing the solid biomass composition fuel with an aqueous wash liquid. The step of removing dust from the solid biomass composition fuel particles may be the same as step (ii) discussed above, or may comprise any other appropriate steps of dust removal known in the art. The step of washing the solid biomass composition fuel may comprise washing the solid biomass composition fuel with an aqueous wash liquid. The aqueous liquid preferably comprises pure water. However, water with certain substances dissolved therein such as certain salts may also be used to wash the solid biomass composition fuels of the present invention. The solid biomass composition fuel may be immersed in or sprayed with the aqueous wash liquid. The step of removing dust is useful since dust is problematic because it may pollute the air during transport and packaging of the solid biomass composition fuel. The dust may also pollute the local environment. Furthermore, when stored in the open air, dust particles form mildew and affect the performance and quality of the solid biomass composition fuel. The washing step may also remove other impurities that may be present in the fuel. Such impurities include the salts and hydrophilic organic substances discussed above that may not have been effectively removed by a pre-manufacture washing step, and also other impurities that may have been introduced to the solid fuel during manufacture such as during molding or torrefaction steps. Surprisingly, and in contrast to solid biomass composition fuels known in the art, it has been found possible to wash the solid biomass fuel particles of the present invention with an aqueous wash liquid after manufacture because of the good water proof properties of the solid biomass composition fuel particles (as discussed above). It has been found that many prior known solid biomass fuels have not been able to be satisfactorily washed due to not having sufficient waterproof properties to withstand a washing step. With particles that are not sufficiently waterproof, application of a washing liquid may cause the biomass composition solid fuel particles to absorb too much water or degrade. In such circumstances, said wet particles may absorb water, degrade and fall apart due to reduced structural integrity. Accordingly, it has been found not possible to wash certain known biomass fuel particles with a washing liquid. In contrast, with biomass solid fuels of the present invention, the excellent water proof properties of the fuel that are believed to be imparted by the optimised pulverising, molding and heating steps discussed above, mean that the solid biomass composition fuel particles can be effectively washed without being damaged or having their advantageous fuel properties compromised. The solid biomass composition fuel product The solid biomass composition fuel product may have any of the physical properties discussed above. As discussed above, the biomass composition solid fuel of the invention preferably comprises pellets. The pellets may be any suitable size. Preferably, the pellets have a diameter of from 3 mm to 100 mm, and more preferably, 5 mm to 8mm. Preferably, the pellets have a length of from 20 mm to 60 mm, and more preferably from 30 mm to 50 mm. As discussed above, surprisingly, it has been found that the solid biomass composition fuel product of the invention has enhanced waterproof characteristics compared to solid biomass fuel products made by prior art processes. This is believed to be due to controlling the pulverising, molding and/or heating step as discussed above. Biomass fuels of the prior art have been found by the inventors to be sufficiently water proof for only up to 10 days. In contrast, it has been found the solid biomass fuel products of the invention are sufficiently water proof up to 20 days, preferably 30 days and more preferably 40 days. The water proof properties of the solid biomass fuels may be determined according to standard tests of the Energy Research Centre of the Netherlands (ECN). The moisture content of the biomass composition solid fuel of the invention may also be determined by standard ECN test methods. The internal moisture content of the solid biomass composition fuel of the invention is typically less than 8 wt %, preferably less than 6 wt%, and more preferably less than 5 wt%, wherein the internal moisture content is determined according to DIN EN 14774. The biomass composition solid fuel has a base moisture content of typically less than 10 wt%, preferably less than 8 wt%, and most preferably less than 5 wt%, wherein the base moisture content is determined according to GB/T211-2017. The solid biomass composition fuel of the invention has also been found to have unexpectedly high mechanical durability. The mechanical durability is typically higher than 90%, and preferably higher than 95%. This is advantageous since biomass pellets of 95% mechanical durability or greater have been found to be able to stored outside without damage to for periods as long as two months. In contrast, biomass pellets with less than 90% mechanical durability typically are damaged by rainfall and are not able to be stored outside. Accordingly, high mechanical durability is an additional advantage of biomass pellets of the invention. An additional advantage associated with high durability of the solid biomass composition fuel particles is that if the pellets are somehow broken by force, they fall apart in larger pieces than pellets with low mechanical durability. This minimises any dust explosion risks. As discussed above, in preferable instances, typically, other than additives such as those discussed above, no other fuel source is added to the heated biomass composition product during the molding step. Accordingly, the solid biomass composition fuel typically comprises only material derived from biomass as the fuel source in the solid biomass composition fuel. For example, when the heated biomass composition product is molded into pellets, typically, no other fuel source is added to the heated biomass composition products prior to molding such that the solid biomass composition fuel pellets produced by the molding step only contain a fuel source derived from biomass. Preferably, the solid biomass composition fuel thus comprises at least 50% by weight of the total fuel content of the fuel, such as at least 60% by weight, at least 70% by weight, at least 80% by weight, at least 90% by weight and preferably at least 95% by weight of material derived from biomass. The solid biomass composition fuel preferably comprises material derived from biomass in an amount of at least 75% by weight; preferably at least 80% by weight and more preferably at least 90% by weight. More preferably, the solid biomass fuel comprises material derived from wheat husk in an amount of at least 75% by weight; preferably at least 80% by weight and more preferably at least 90% by weight. Most preferably, the solid biomass composition fuel consists essentially of or consists of material derived from wheat husk. Example A solid biomass composition fuel was produced using the process of the invention. The biomass source used comprised only wheat husk. The solid biomass composition fuel consisted essentially of material derived from wheat husk. The properties of the solid biomass composition fuel produced are indicated in the tables below and contrasted with those of wheat husk raw material. Table 1 Table 2