GB2107728A - Diesel fuel oils from coal - Google Patents

Abstract

Diesel fuel J having a mid boiling point of about 255 DEG C and having 90% W/W boiling in the range 225-280 DEG C is produced from coal- derived liquid by deep hydrogenation at 4 (e.g. at 350-450 DEG C, 50-750 bar hydrogen pressure and a LHSV of 0.1 to 1 using a Gp VI B plus Gp VIII B metal sulphide catalyst. The coal- derived liquid is obtained from coal by extraction at 1, under hydrogenation or non-hydrogenation conditions, with a liquid or gaseous solvent followed by removal of solids or is obtained from coke oven by-products and can be treated by hydrocracking prior to the deep hydrogenation. <IMAGE>

Description

SPECIFICATION Diesel fuel oils from coal This invention relates to coal-derived Diesel fuels, a process for their manufacture, and a method of fuelling a diesel engine using a coalderived fuel.

Diesel fuels are currently manufactured from petroleum oils, and are known as gas-oil.

Petroleum and its refined products are intermittently subject to shortages for political reasons and it is envisaged that pressure on supplies will further increase in the medium to long term because of exhaustion of reserves. It is an aim of the present invention to produce Diesel fuels from coal, of which there are substantial reserves in the United Kingdom and certain other countries.

In the period around World War II, there was interest in converting coal to fuel oils. Various processes were proposed, and in British Patent Specification No. 484,127 it was suggested that a middle oil could be produced which could be used as Diesel oil. The starting material described therein is a distillable carbonaceous material which contains asphaltic substances and the primary starting material is tar from the destructive distillation of bituminous or brown coal. Nevertheless, it is suggested, although not illustrated in the Examples, that other starting materials could be obtained by "pressure extraction and/or destructive hydrogenation" of coals.This prior patent specification specifies that the starting material can be freed of asphaltic substances by a mild liquid phase hydrogenation, in which the asphaltic substances are reduced at least 90% and less than 20% of the material is converted to materials boiling below 3500C. It is suggested that the middle oil fraction of the product may be used as a source of Diesel fuel. Ail or part of the product may, it is suggested, be treated by gas phase hydrogenation over a catalyst to produce mainly high octane benzine plus a middle oil which is said to be a very good Diesel oil. In the Examples in which Diesel is said to be produced, tar, of which about 50% boils above 35000 its hydrogenated to produce a gas oil which is mixed with a gas oil produced by hydrogenating a paraffin wax from the initial hydrogenation.The Diesel (gas) oils produced have cetene numbers of 57 and 68 (equivalent to cetane numbers of 50 and 60); other product details are specific gravity of 0.855 and solidifying point of-i 60C. We interpret this data to mean that the product Diesel fuel contains a significant proportion of straight or branch chain paraffins, for reasons which will be discussed below, and this is the major reason for a high cetane number.

As has been stated, it is believed that the above described process starting from tar leads to gas oils containing significant amounts of paraffins, i.e.

straight and branched chain paraffins. One indicator is the presure of paraffin waxes, which are not detected in the case of a starting material from the liquid extraction of coal. Furthermore, high cetane numbers and solidification points of -20 to OOC also indicate high proportions of paraffins. Thus it is relatively easy to obtain a Diesel fuel having a high cetane number when the oil contains a high proportion of paraffins.This interpretation is supported by an article entitled "Chemistry and Technology of Synthetic Liquid Fuels" by I. B Rapaport, translated from Russian and published for the National Science Foundation and the Department of the Interior, Washington, 1 962. Details are given of gas oils produced by the hydrogenation of brown coal (cetane number = 47.8, solidification temp. 15C, density 0.855 [containing 54% alkanes]; after further hydrogenation cetane number = 52.4, density = 0.837 and solidification temperature = --16.20C), and the hydrogenation of middle oil from carbonisation of bituminous coal (aniline point 50540C, 54% alkanes).Tars from carbonisation can be considered as a high quality material, i.e. hydrogen-rich, which are obtained only in low yields, up to 10% by weight of the coal but more usually up to about 5% in coke oven technology. This can be contrasted with coal extracts obtained by the solution of coal in liquid aromatic solvents, in which yields of up to 85%, and in some cases 95% (daf), can be obtained, the product being of low quality and having a high proportion of polynuclear ammarics. Coal extracts can thus be considered as an unpromising source of starting material for Diesel fuels.

Another prior proposal to make Diesel fuel from coal was made in British Patent Specification No.

730,030, which is primarily concerned with the manufacture of gasoline. It is proposed to hydrogenate coal in the liquid phase to gasoline and heavy oil, with only a minor proportion of middle oil. The heavy oil and solid residue is coked to yield more gasoline and middle oil and then a mixture of gasoline and middle oil is hydrogenated in the vapour phase to give an improved gasoline.

Although gasoline is the main product, it is said that the process can be operated to produce an excess of middle oil which can be sold for Diesel fuel. The specification does not describe the production of Diesel fuel further. We believe that any middle oil produced in such a process, although boiling in the gas oil range, would be a very poor Diesel fuel indeed, and if useful at all, would be of the lowest grade suitable only for low speed engines of the marine type.

In our published British Patent application No.

2 060 681 A we describe a process for the manufacture of Diesel fuel suitable for high speed engines from coal-derived materials, comprising hydrogenating, over a hydrogenation catalyst, a middle oil which is a fraction of a partially hydrogenated coal oil, which oil contains at least 90% of polycyclic hydrocarbons, contains a major proportion of naphthenes and does not contain any significant amount of mineral matter or straight or branch chain paraffinic material, and fractionating and hydrogenated oil to yield a gas oil. Preferably the middle oil derived from coal is a fraction boiling in the range 170 to 3500C and is conveniently the product of hydrocracking coal extract.

Using this process it has proved possibie to produce a Diesel fuel having a cetaine number of at most 46. The fuel has a lower cut point of about 1 800C and an upper cut point of about 3000 C.

There are different grades of Diesel fuel for example as specified in British Standard 2869: 1970 Amended 1977 -- "Petroleum Fuels for Oil Engines and Burners". In general, grades Al and A2. having minimum cetane numbers of 50 and 45 respectively, are suitable for high speed engines (e.g. capable of running at 6,000 r.p.m. or more). It is believed that the invention described in our published application provides for the first time a grade A2 Diesel fuel derived from coal.

However, it is desirable to be able to produce grade Al Diesel fuel from coal, According to the present invention, a Diesel fuel comprises a fraction of a deeply hydrogenated coal-derived liquid having a mid boiling point of about 255us and at least 90% W/W of which boils within the range 225-2800C.

Preferably, at least 95% W/W of the fraction boils within the specified range, and advantageously the specified range is 235-2700C.

The invention also includes a method of producing such a fuel.

Th" coal derived liquid may be produced by the extraction of coal using a liquid oil or gaseous solvent under hydrogenative or non-hydrogenative conditions, followed by separation of mineral matter (ash) and undissolved coal. The techniques of extraction by liquid or gaseous solvents are known in the art. The separation of ash and undissolved coal may be carried out in a number of ways, but it is believed that filtration and centrifugation are the most practical mefliods. The coal may be bituminous or a brown coal or lignite.

The coal derived liquid may also be a fraction of a coal extract and may be a mildly hydrogenated fraction. The coal derived liquid may also be a byproduct of other coal conversion processes, for instance by-products from coke ovens.

Preferably, the coal derived liquid is firstly hydrocracked to reduce the average molecular size of .he material and to effect mild hydrogenation.

The catalytic hydrocracking of coal derived liquids has been proposed in the art. Suitable catalysts are those of the type Co or Ni and Mo or W sulphides, or a combination thereof, on a catalyst support which may be y-alumina, clay, active carbon, zinc oxide, magnesium oxide, aluminosiiicates, silica, chromia, etc. A number of hydrocracking catalysts of this type are commercially available. The conditions are preferably selected to yield an oil boiling between 50 and 4500 C, with less than 1 5% by weight boiling above 4500 C. It is desirable to fractionate ihe hydrocracked oil to select a middle oil fraction suitable for further processing.The cut points are preferably within the range 1 70 to 3500C and are suitably 1 80 to 300"C.

The coal derived liquid may itself be deeply hydrogenated, although it is preferable to hydrogenate deeply a middle oil fraction from the catalytic hydrocracking of the coal derived liquid.

The hydrogenation catalyst for deeply hydrogenating the coal-derived liquids may be a metal sulphide from Group VI B or Group VIII B of the Periodic Table, and may be identical to or different from the hydrocracking catalyst mentioned above.

Alternatively, the hydrogenating catalyst may be a supported precious metal catalyst (e.g. Pt, Pd, Rh, Ru) or a supported precious metal sulphide catalyst. Hydrogenation conditions are selected according to the catalyst used, but would generally be within a temperature range of 350 to 4500C and a hydrogen pressure range of 50 to 750 bar, preferably 100 to 230 bar. Hydrogen concentrations are suitably in the range of 40 to 87%, preferably 85 to 95%, this being dependent upon the source of hydrogen. Liquid hourly space velocities are suitably in the range 0.1 to 1.0 h-', preferably about 0.5 h-1 in order to effect deep hydrogenation.

The deeply hydrogenated oil is then carefully fractionated to remove material boiling below 2250C, preferably removing material boiling below 2350C, and material boiling about 2800 C, preferably about2700C.The resulting gas oil is useful inter alia as a grade Al fuel oil for Diesel engines.

The fuel oil produced according to the present invention may, if desired, be blended with petroleum gas oils. Such blending can give a Diesel fuel oil improved in such characteristics as cloud point compared with conventional petroleum gas oil.

Fuel oil manufactured according to the invention has been used to fuel a single cylinder research Diesel engine. The middle oil which may be used as a starting material for the present invention was also tried in the research engine but gave very poor results. The middle oil had poor ignition qualities and had to be blended with petroleum gas oil to run the engine. The fuel oil according to the invention on the other hand gave satisfactory performance of the engine generally competitive with that of petroleum gas oil, and appears to offer the possibility, especially after optimisation of engine design, of lower pollution.

The invention, therefore, also provides a method of fuelling a Diesel engine comprising the use of a Diesel fuel according to the invention.

The invention also comprises a method of producing a coal-derived Diesel fuel comprising deeply hydrogenating a coal-derived liquid and fractionating the hydrogenated liquid to give a fraction having a mid boiling point of 2550C and at least 90% W/W of which boils in the range from 225 to 2800C.

It is thought that the fraction of deeply hydrogenated coal derived liquid will contain mainly highly hydrogenated bicyclo alkanes which are highly substituted with straight and branch chain C2 to C4 alkyl groups. It will contain very few mono or tri cyclic alkanes and have a very low percentage of aromatic or straight or branched chain hydrocarbons. It is thought that this fraction therefore has a composition which balances the average boiling point of the components, their specific gravities and their hydrogen content in a manner which allows the achievement of a cetane number of about 50. The careful selection of mid boiling point, boiling range and degree of hydrogenation appear to be essential in producing a grade Al Diesel fuel from coal-derived liquids.

The Applicants do not wish to be limited by this explanation and therefore the scope of the invention should be determined by the claims attached hereto.

The invention may be more fully appreciated by reference to the accompanying schematic flow diagram, illustrating a process according to the invention and including the production of a coal derived liquid by liquid extraction of coal using as a solvent a recycle oil produced in the process. As the individual unit processes present no difficulty to the skilled man, these are not described in detail but are in accordance with the foregoing description.

Raw coal is fed to the process as stream A and admixed with solvent oil in an approximately 1:3 ratio; the solvent oil being stream G which is the +3000C fraction from distillation column/separator 3. The oil-coal mixture is digested by heating in a digester generally indicated by 1. C1-C4 gases formed during digestion are taken off as stream B, and residues containing ash and undissolved coal are filtered off and removed as stream C. The filtrate coal derived liquid, stream D, is passed to a catalytic hydrocracker 2, which is supplied with make-up hydrogen E in addition to recycled process hydrogen. The product from the hydrocracker is fed to distillation column/separator 3.From separator 3, light gases, especially C,--C, gases, are taken off as stream F, -1 800C liquids, largely containing mononuclear aromatics, are removed as stream I, the desired feedstock which is the 1 8O-300"C cut is taken off as stream H, and the +3000C fraction is recycled, as has been stated, as stream G.

Stream H is passed into a catalytic hydrotreater vessel 4 together with hydrogen (E). The hydrogenated product passes to a distillation column/separator 5 in which it is separated into a light fraction (-2350C) K which is useful as a jet fuel or a steam cracker feedstock, and a 235--2700C fraction J, which is the desired coalderived gas oil.

The invention will now be described by way of the following non-limiting example.

EXAMPLE A coal-derived liquid from stream D of the plant described above, having a boiling range from initial boiling point (IBP) to 3000C was fractionated to provide a naphtha stream (IBP - 1 700C) and a mid-oil stream (170--3000C). The mid-oil stream was hydrogenated over a sulphided commercial cobalt-molybdenum on alumina catalyst at 4350C, a hydrogen pressure from 1 50-210 bar and a liquid hourly space velocity of 0.5 h-'. The hydrogenated fraction was again fractionated to give a gas oil fraction having a boiling range from 235-2700C (about 95% W/W of the fraction boiled within this range).

The gas oil fraction was found to have a midboiling point of 2550C.

A sample of the gas oil fraction was used by an independent firm of consulting engineers to fuel a single cylinder research compression ignition engine of 1.93 1 swept volume, having a classical toroidal bowi combustion system in combination with a helical swirl-producing intake port. The compression ratio was 1 6.0:1.

After thoroughly warming the engine by running for 1 hour using conventional petroleum gas oil Diesel fuel, the fuel system was drained and replenished with the coal-derived gas oil fraction to be tested. The cetane number of the fraction was established to be approximately 50.

The engine ran well and performance levels were extremely competitive with that achieve with petroleum gas oil. A small fuel economy penalty of approximately 2% occurred at mid to high load conditions, but no other significant brake performance differences were noted.

The fuel had a similar energy value per gallon to petroleum gas oils, and was considered to be competitive with current Diesel fuels on a miles per gallon basis.

The coal derived gas oil was found to have far less "Heavy ends" than petroleum gas oil, leading to faster rates of burning during the latter stages of combustion, enabling a less advanced start of combustion timing for optimum performance, particularly at higher speeds. It was considered likely that the performance of the current generation Diesel engine could be improved by the matching of injection equipment and combustion systems to run on a fuel similar to coal derived gas oil. The improvements would be directly attributable to the presence of lower quantities of heavy ends and could manifest themselves in terms of lower smoke and particulate levels or possibly lower NOx emissions.

The product according to the invention had a density 4% higher than that of the petroleum gas oil used as a standard in the tests. Proton NMR spectroscopy indicated that the coal derived fraction comprised mainly highly hydrogenated, highly substituted bicyclo paraffins, having mainly C2C4 alkyl substituents. This is thought to contribute to the optimisation of the density of the coal derived fuel oil.

Claims (16)

1. A diesel fuel comprising a fraction of a deeply hydrogenated coal-derived liquid having a mid boiling point of about 2550C and at least 90% W/W of which boils within the range 225--2800C.

2. A diesel fuel according to Claim 1 , wherein at least 95% W/W of the fraction boils within the range 225--2800C.

3. A diesel fuel according to either one of Claims 1 and 2, wherein the boiling range is from 235--270"C.

4. A diesel fuel substantially as hereinbefore described with reference to the Example.

5. A method of producing a diesel fuel according to any one of the preceding claims comprising deeply hydrogenating a coal-derived liquid and fractionating the liquid to give a fraction as defined in any one of the preceding claims.

6. A method according to Claim 5, wherein the coal-derived liquid is produced by the gaseous or liquid extraction of coal.

7. A method according to Claim 6, wherein the extraction is carried out under hydrogenative conditions.

8. A method according to either one of Claims 6 or 7, and including a separation step in which the coal-derived liquid is separated from ash and undissolved coal.

9. A method according to any one of Claims 6 to 8, and including a hydrocracking step in which it is hydrocracked to reduce its average molecular weight and to effect mild hydrogenation prior to the deep hydrogenation.

10. A method according to any one of Claims 6 to 9, and including a fractionation step to produce a middle oil fraction for deep hydrogenation.

11. A method according to any one of Claims 5 to 10, wherein the deep hydrogenation is carried out using a supported Group VI B or Group VIII B metal sulphide catalyst.

12. A method according to any one of Claims 5 to 10, wherein the deep hydrogenation is carried out using a supported precious metal catalyst.

13. A method according to any one of Claims 5 to 12, wherein the deep hydrogenation is carried out at a temperature in the range 350 to 450 C, a hydrogen pressure in the range 50 to 750 bar and a liquid hourly space velocity in the range 0.1 to 1.0 h-'.

14. A method of producing a Diesel fuel, substantially as hereinbefore described with reference to the drawing.

1 5. A Diesel fuel made according to the method of any one of Claims 5 to 14.

16. A method of fuelling a Diesel engine comprising the use of a Diesel fuel according to any one of Claims 1 to 4 and 1 5.