FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & The Patent Rules, 2003 COMPLETE SPECIFICATION (See section 10 and rule 13) TITLE OF THE INVENTION "A FUEL COMPOSITION AND METHOD OF PREPARATION THEREOF" We, BHARAT PETROLEUM CORPORATION LIMITED, of Bharat Bhawan, 4 & 6, Currimbhoy Road, Ballard Estate, Mumbai-400 001, Maharashtra, India. The follounng specification particularly describes the nature of the invention and the manner in which it is performed: A fuel composition and method of preparation thereof Field of the invention The present invention provides a refinery sludge and spent catalyst based fuel composition. The present invention relates to a method for using solid waste materials like refinery tank sludge and disposable spent catalyst to form a new fuel material which can be burned in conventional kilns, or industrial furnaces designed for burning solid or liquids. The present invention also provides a blending process comprising sludge and spent catalyst mixing, extrusion in the form of desired shape and drying to obtain a product which can be readily used as fuel which releases heat when burned and results into a non hazardous material. Background of the Invention Petroleum refineries need to dispose wastes like oil sludge's (oil-solid or oil-solid-water mixtures) produced in waste water treatment facilities and hydrocarbon storage tanks. In recent times tank bottom wastes, sometimes referred to as sludge and often shortened to just tank bottoms, have become an increasing problem for disposal since the regulation has have classified them as environmentally hazardous materials. However by using suitable methods these materials can be converted into value added useful products. The sludge material from the tank builds to large quantities because of the large amount of crude and finished product storage in many refineries and marketing locations. This material is stored in pits until some disposal method is employed. Particularly in remote locations sludge storage has continued for many years. Many of the refiners dispose the sludge by bioremediation following microbial treatment on large area of land at site. However, certain types of oily sludge such as sludges from coker hot slop tank, vacuum column bottoms contain viscous oil and high levels of refractory sulphur components; polynuclear aromatic compounds etc are not suitable for disposal through bioremediation. Removal of refractory components from oily sludge to make them suitable for microbial treating can be an attractive waste management option. However conventional de-oiling processes and services are costly and in some cases do not achieve the desired result. For example extraction based technologies using special solvents rely on recovery and recycle of the solvents and can leave asphaltene fractions of the oil on the solids. Similarly disposal of spent catalyst is also a problem due to the presence of trace metals. The present invention is directed to a method of recovering the energy of oil present in sludge and of carbon in refinery spent catalyst and a method for use of such materials as a fuel source in industries, where there is a need of burning fuel. US patent 5269234 discloses a method for utilizing land fill prohibited solid hazardous waste material as an energy source in a cement kiln. The method comprises combining the solid waste materials with a liquid fuel in a milling apparatus to form a mixture, grinding the mixture in the milling apparatus in a manner reducing the particulate size of the solid hazardous waste material to form fuel slurry, and delivering the fuel slurry directly into the flame of the cement kiln. The present invention of fuel composition differs in materials used and the procedure used for generating it. US patent 5495986 teaches a method for providing a fuel mixture by processing refinery sludge and blend stock which provides fuel. The process contains steps of grinding the waste material in a tank containing the blend stock with the grinding being at least in part being provided by the co-action between a rotating impeller and a stationery plate. This patent does not teach about the usage of spent catalyst and tank sludge in fuel mixture. US patent 5259945 discloses a process for the recovery of tank bottom wastes employing a flash tank fed with hot, pressurized tank bottom wastes and separating a vapour component and heavier bottoms then fed to a stripping reactor along with hot gas producing an overhead vapour with a solid remainder. Tank bottom sludge is employed as fuel. This patent does not teach about the usage of spent catalyst and tank sludge in fuel mixture. US patent 5288391 discloses a process for deoiling wastes. The process is integrated with a refinery and utilizes a refinery intermediary hydrocarbon stream as a solvent to recover waste oil for reprocessing in the refinery and to produce waste solids, which are either reused or disposed for land treatment. This patent does not teach about the usage of spent catalyst and tank sludge in fuel mixture. US patent 5078593 discloses method for disposal of oily refinery sludges containing a hydrocarbon component, a solid component comprising sediments such as sand, silt, rust or scale, and water in an operating rotary kiln. This patent does not teach about the usage of spent catalyst and tank sludge in fuel mixture. US patent 4786401 teaches a liquid sludge disposal process where in hydrocarbon-containing liquid waste sludge from refinery waste streams can be disposed of by blending with the feedstock being passed in a fluid catalytic cracking (FCC) unit. The sludge is premixed with a hydrocarbon, such as light oil, prior to mixing with the feed. But by mixing with FCC feed, it can change the product yield, deactivate the catalyst due to metals,, heavy nitrogen compounds and lead to environmental pollution. This patent does not teach about the usage of spent catalyst and tank sludge in fuel mixture. From the prior art it is understood that no reports were disclosed wherein two disposable refinery materials like sludge and spent catalysts have been used to prepare an environmentally friendly, disposable new fuel substance. Notwithstanding the amount of material available in the prior art, there is still need for a meaningful disposal of refinery tank sludge and spent catalyst both of which pose environmental problems. Summary of the invention The present invention provides fuel composition comprising sludge and refinery spent catalyst and a method for safe disposal of refinery tank sludge as well as refinery spent catalyst. The method also provides for recovery of energy values of the final sludge and spent catalyst fuel composition. This method also describes the process of making fuel by using refinery tank sludge and spent catalyst in a desired shape with the help of mechanical devices which are easy to store, transport and handle. The new fuel composition possesses good calorific value which is comparable with the coal used in furnace for heating. The fuel obtained in the present invention can be used as substitute or mixed with in-use material. Preferred embodiments of the invention can be more fully understood by reference to the following detailed description of the invention and to the accompanying drawings and or tables. Objects of the invention The main object of the present invention is to prepare a fuel composition with calorific value above 2000 cal/gm comprising sludge and discarded spent refinery catalyst. Another object of the present invention is to provide a fuel composition as claimed wherein the sludge and discarded spent refinery catalyst are present in a ratio of 1:1 to 4:1. In yet another embodiment of the present invention the sludge and discarded spent refinery catalyst are present preferably in a ratio of 3:1 to 4:1. In still another embodiment of the present invention the calorific value of the fuel composition is above 4000 cal/gm, preferably above 5000 cal/gm, most preferably above 8000 cal/ gm. In another embodiment of the present invention the discarded spent refinery catalyst comprises substantially of alumina and silica. In yet another embodiment of the present invention the discarded spent refinery catalyst is selected from the group comprising catalytic cracker, hydro cracker, catalytic reformer, hydro treating units or combinations thereof. In still another embodiment of the present invention the spent catalyst is preferably from catalytic cracker unit. In another important embodiment of the present invention the sludge is from crude tank bottom with any composition of oil, water and sediments, fresh free flowing refinery product tank bottom or stored semisolid sludge or solid refinery tank sludge preferably solid refinery tank sludge. In another important embodiment of the present invention the solid refinery tank sludge contains 10 to 30% wt water. In yet another embodiment the present invention provides a method for preparing fuel composition with calorific value above 2000 cal/gm comprising sludge and discarded spent refinery catalyst said method comprising: (a) mixing said sludge and discarded spent refinery catalyst at a ratio of 1:1 to 4:1; (b) loading to an extruder; and (c) drying. In still another embodiment of the method for preparing fuel composition ratio of said sludge and discarded spent refinery catalyst in step (a) is preferably 3:1 to 4:1. In another embodiment the method for preparing fuel composition optionally comprises blending the mixture of sludge and discarded spent refinery catalyst with coal or coke. Brief description of the drawings Figure 1: Schematic representation of fuel mixture preparation Figure 2: Drawing showing finished product in pellet form. Detailed description of the invention The present invention provides a new combustible product or fuel and method of preparation thereof for using refinery tank sludge and spent catalyst. Refinery sludge contains both organic and inorganic material, varying from one refinery to the other and often varies within a refinery depending upon source of crude oil and storage time. Factors likely to affect the nature of sludge include crude oil characteristics, quantity of water, storage environment, blow down etc. Refinery sludge's contain oil, gas, water and solid fraction. The oil and gas typically constitute about 15-45% by weight of the sludge and include paraffins, napthenes, asphalts or other oils. Prior to dewatering, trie solid fraction typically constitutes sediments such as sand, silt, rust, and scale and other inorganic components including various sulfur containing compounds and metals such as arsenic, barium, cadmium, chromium, lead, mercury, nickel, selenium, silver, and vanadium and compounds containing such metals. As noted previously, such sludge's typically contain about 40 to 90 percent water by weight. In accordance with the present invention, it has been found that such refinery sludge's resulting from the treatment and handling of aqueous effluents, and in particular, from tank sludge along with disposable catalyst can safely be used as fuel source in mineral processing rotary kilns. The sludge maybe charged to a rotary kiln containing in-process mineral matter and burned to recover the energy value of its organic constituents. With the high temperatures required for processing of the mineral matter in rotary kilns, fuel consumption is one of the significant factors to calculate product pricing. The typical composition of the spent catalyst and tank sludge are given in the Tables 1-2, The main inorganic constituents of spent catalyst and sludge composition are alumina and silica and other metals go through a chemical and/or physical change in the process of the present invention. Particularly in the elevated temperatures of the firing zone of the kiln, toxic metals are generally chemically combined to form non-leachable complex silicates and aluminates. These chemical components of sludge and spent catalyst mixtures have good cementation properties which can add value to the final burned material. Table: 1 Composition of spent catalyst Composition Spent Catalyst Pt (wt%) 0.01 Ni (Wt%) 0.04-0.1 V (Wt%) 0.5-0.7 Na (wt%) 0.2-0.5 Fe