This is a continuation-in-part of application Ser. No. 07/382,654 filed as PCT/GB00058, Feb. 1, 1988 and publishedas WO 88/05807, Aug. 11, 1988, now abandoned. This invention relates to the liquefaction of cellulose. U.K. Patent Specification No.2089831 describes a process for liquefaction of cellulose which comprises hydrogenation of the cellulose in the presence of a polycyclic hydrogen donor substance such as tetralin at elevated temperature and increasedpressure. The product comprises a mixture of solid, liquid and gaseous hydrocarbons. The polycyclic hydrogen donor substance is recovered and used in the treatment of further cellulose. It has now been discovered that the liquefaction of cellulose can be effected by replacing the polycyclic hydrogen donor substance with cellulose derived oil. According to the invention there is provided a process for converting cellulose to hydrocarbon comprising subjecting the cellulose to elevated pressure and temperature in the presence of a liquefaction solvent characterised in that during atleast part of the process the liquefaction solvent is wholly a cellulose derived oil. Further, the process of converting cellulose is carried out without the addition of water or the use of additional reducing species, such as a specific hydrogen donor solvent or a reducing gas. In this regard, it has even been found that inactual practice, the addition of a reducing gas can have a deleterious effect on the process by causing improper mixing and irregular flow through process vessels due to slugging and surging. By the invention, therefore, the requirement for addition of a specific polycyclic hydrogen donor solvent is rendered unnecessary and hence the separation of polycyclic hydrogen donor solvent from the product is no longer required. In apreferred embodiment of the invention a part of the liquid aromatic hydrocarbon product oil from the process is used for the treatment of further cellulose. Thus the invention can very easily be operated continuously in which a predetermined quantity ofproduct oil is recycled for the treatment of fresh cellulose. It will be understood, however, that, even in a continuous process, the cellulose derived oil used in the invention does not have to be recycled from the product of the process actually beingoperated. The cellulose derived oil can, if desired, be taken from another suitable source. For example the oil can be obtained from another like process according to the present invention or a process as described in U.K. Patent SpecificationNo.2089831. In order to reach a state of affairs where it is possible to carry out the process of the present invention, should cellulose derived oil not be available, it may be necessary to carry out the process as described in U.K. Patent SpecificationNo.2089831 and utilise a polycyclic hydrogen donor substance until sufficient product oil has been obtained. In this embodiment of the invention product oil together with polycyclic hydrogen donor substance can be used. The proportions of polycyclichydrogen donor substance to cellulose derived oil do not appear to be critical. Thus starting from a process using a polycyclic hydrogen donor substance the proportion of cellulose derived oil in the recycle for treatment of fresh cellulose can beprogressively increased with corresponding reduction of the proportion of polycyclic hydrogen donor substance until the recycle consists entirely of product oil. It is preferred that the cellulose derived oil used in the invention should comprise a light fraction of the liquid oil product obtained from cellulose by distillation, in particular a cut from 200.degree. to 300.degree. at atmosphericpressure. Although this is preferred, it is not necessary that the cellulose derived oil should consist entirely of a fraction in this range. It is not even essential that the composition of the cellulose derived oil used as starting material shouldremain uniform during the process. For example when the process is operated using recycled product oil it may happen that the proportion of light oil in the recycled oil gradually falls. When the performance of the recycled oil reaches an unacceptablelevel, for example if the oil becomes too viscous to handle, the proportion of light oil in the recycled oil can be increased by any suitable means. The cellulose material for use in the invention can be derived from any source. Examples include cellulosic material from municipal refuse and waste biomass such as straw and sugar cane. The process of the invention is preferably carried out in the presence of a catalyst- The preferred catalysts are heterogeneous catalysts such as nickel. As stated previously the process of the invention is carried out at elevated temperature and increased pressure. The preferred temperature range is from 320.degree. to 380.degree. C. particularly preferred 350.degree. C. and pressure would be40.53 to 151.99 Bar (40 to 150 atmospheres). The following Examples further illustrates the invention. EXAMPLE I 100 grams of cellulosic material from municipal refuse were charged to a pressure vessel together with 400 grams of cellulose derived oil and 2 grams of catalyst. Air was exhausted from the vessel which was then heated to 350.degree. C. over aperiod of three hours and that temperature maintained for a further two hours. A pressure of 151.99 Bar (150 atmospheres) developed in the vessel. The vessel was then cooled. The reaction products were as follows: ______________________________________ Solids (char) 2 g Oil 433 g Gas (including steam) 65 g ______________________________________ For continuous operation, therefore, 400 g of oil can be taken from the oil product and recycled for treating a further 100 g of cellulosic material. If the 400 g recycle contains too high a proportion of heavy oils the product oil can bedistilled and 400 grams of a light cut recycled. EXAMPLE II 100 grams of cellulosic material from sugar cane bagasse were charged to a pressure vessel together with 390 grams of cellulose derived oil and 2 grams of catalyst. Air was exhausted from the vessel which was then heated to 380.degree. C. overa period of three and a half hours and that temperature maintained for a further two hours. A pressure of 177.32 Bar (175 atmospheres) developed in the vessel. The vessel was then cooled. The reaction products were as follows: ______________________________________ Solids (char) 7 g Oil 415 g Gas (including steam) 68 g ______________________________________ EXAMPLE III 90 grams of cellulosic material from straw were charged to a pressure vessel together with 390 grams of cellulose derived oil and 1 gram of catalyst. Air was exhausted from the vessel which was then heated to 375.degree. C. over a period ofthree hours and that temperature maintained for a further one hour. A pressure of 7.18 Bar (165 atmospheres) developed in the vessel. The vessel was then cooled. The reaction products were as follows: ______________________________________ Solids (char) 7 g Oil 415 g Gas (including steam) 59 g ______________________________________ This Example also illustrates a process which can readily be run continuously by recycling 390 g of product oil for treating a further 100 g of straw derived cellulosic material. * * * * * | 
