Fuel composition

Abstract

 A fuel composition is prepared from a friable solid fuel, e.g. coal, and a crude oil. The solid fuel is ground in the crude oil until the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 10 micron and the composition contains 30 to 60% by weight of the solid fuel. Air is excluded during the grinding operation. The composition is suitable for use as a liquid fuel for industrial burners.

Description

[0001] This invention relates to fuel compositions which are dispersions of solid fuel in crude oil and to a method for their preparation.

[0002] Coal oil slurries have previously been disclosed, see for example British Patent Specification 975687. Whilst these have behaved as near-Newtonian non-settling fluids in pipelines, they separate on standing. Thus such slurries are suitable for use immedi.ately after preparation or pipelining but are not suitable for transportation by tanker nor for storage.

[0003] Our British Patent Specification No. 1523193 describes and claims a method for the preparation of a uniform coal oil dispersion which method comprises grinding coal in a medium consi.sting essentially of gas oil and/or a heavier petroleum fraction until the particle size is reduced to a value below 10 micron and the dispersion contains 15 to 55% by weight coal, expressed as a percentage by weight of the total dispersion, and until a stable dispersion results on ceasing grinding.

[0004] We have now discovered that stable dispersions can be prepared from solid fuels and crude oil.

[0005] This is surprising, since although the heavy petroleum fractions are clearly present in the crude oil, lighter fractions are also present which would be expected to have a destabilising effect.

[0006] Thus, according to the present invention, there is provided a fuel composition containing 30 to 60%, preferably 40 to 50%, by weight of a friable solid fuel, uniformly di.spersed in crude oil, the mean particle size of the solid fuel being in the range 1 to 10 micron, preferably 2 to 5 micron, the percentage being expressed as a percentage by weight of the total weight of the dispersion.

[0007] Suitable friable solid fuels include coals of various ranks, solvent refined coal, coal coke and petroleum coke. A very suitable coal is bituminous coal.

[0008] The nature of the crude oil is not absolutely critical insofar as the preparation of the dispersions is concerned and their storage and use at ambient or slightly elevated temperatures. However, waxy crude oils are somewhat less suitable.

[0009] For storage and use at more elevated temperatures, e.g. at 80°C and above, medium and heavy crudes are preferred since dispersions prepared from light crudes tend to lose a significant proportion of volatile material under these conditions.

[0010] By a light crude oil we mean a crude oil having an API Gravity of 40° or higher, such as Asmix, an Algerian/Sahara blend of 44.6° API; Attaka, an Indonesian crude of 43.7° API; and Zakum, an Abu Dhabi crude of 30.1° API.

[0011] By medium crude oil we mean a crude oil having an API Gravity in the range 39° to 25°, such as Forties, a North Sea (UK) crude of 36.6° API; Burgan, a Kuwaiti crude of 31.2° API; and North Slope, an Alaskan crude of 26.8° API.

[0012] By a heavy crude oil we mean a crude oil having an API Gravity below 25°, such as Basra Heavy, an Iraqi crude of 22.7° API; Eocene, a Neutral Zone crude of 18.6° API; and Boscan, a Venezuelan crude of 10.3° API.

[0013] Dispersions prepared from light crude oils are less viscous than those prepared from medium and heavy crude oils.

[0014] The compositions are prepared by grinding the solid friable fuel in the crude oil in the absence of air.

[0015] Thus, according to another aspect of this invention, there is provided a method for the preparation of a uniform dispersion of a friable solid fuel in a crude oil, which method comprises grinding the solid fuel in the crude oil until the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 10 micron, preferably 2 to 5 micron, and the dispersi.on contains 30 to 60X solid fuel, preferably 40 to 50%, the percentage being expressed as a percentage by weight of the total dispersion, air being excluded from the grinding operation.

[0016] Preferably the solid fuel is pre-ground to a particle size not greater than 250 micron before bei.ng ground in the presence of the crude oil.

[0017] Grinding can be carried out in vibratory, agi.tatory or tumbling ball mills.

[0018] When using an agitatory or vibratory ball mill, the pre-ground solid fuel is preferably pre-mixed with the crude oil before grinding, e.g. in a high speed vortex mixer.

[0019] The grinding time will depend on the nature of the mill.

[0020] It is necessary to exclude air during the grinding operation. This can be easily achieved in the case of an agitatory ball mill by filling the mill completely. In the case of vibratory and tumbling ball mills, it is not practical to fill the mill completely and the milling should be carried out under an inert gaseous atmosphere, e.g. a blanket of nitrogen.

[0021] When using a ball mill it is, of course, desirable to use balls made of a material which does not react with the solid and which does not wear unduly either itself or the interior surface of the mill during grinding. Ball mills usually contain steel or glass balls and these are suitable for the present purpose.

[0022] The stability of the solid fuel dispersion is a function of three variables - the method of grinding, the final particle size and the final concentration of solid in crude oil. If all three are chosen correctly, then the resulting dispersion is of enhanced stability at elevated and ambient temperatures.

[0023] At ambient temperature, the dispersion is in the form of a weak, thixotropic gel in which a physical network is formed by solid particles in oil. It is a uniform structure from which the solid particles cannot settle out because they form part of it. This is unlike coal oil dispersions prior to those disclosed in 1523193 which were merely slurries in which the coal particles were suspended in the oil from which they would eventually settle out, or colloidal suspensions.

[0024] At elevated temperature, although the gel-like structure is less apparent, the same interactions occur to confer enhanced stability.

[0025] If the solid particles are not ground in the oil in the absence of air, the solid particles wi.11 become oxi.di.sed and interact unfavourably. If the solid particle size is too great, forces will be insufficient to confer stability. The concentration of the solid particles is also critical. If it is too low, the dispersion will be unstable. If it is too high, the dispersion will become too solid- like for pumping.

[0026] Such solid fuel crude oil dispersions are suitable for use in blast furnaces, cement kilns, industrial boilers, marine boilers and utility boilers.

[0027] In the case of certain heavy crude fractions, it may be necessary to heat them in order to render them sufficiently mobile to permit use as a grinding fluid.

[0028] The invention is illustrated with reference to the following examples.

[0029] In the examples, the stability of the dispersions was measured by the technique set out on page 3 of the Paper "Stable Coal/Fuel Oil Dispersions" presented at the 2nd International Symposium of Coal Oil Mixture Combustion, Danvers, Massachusetts, USA, 27th-29th November 1979, by Veal et al. In short, this involves determining the concentration of solids present at the base of a standard tube kept at a temperature of 100°C for 24 hours and subtracting from it the initial concentration of solids in the dispersion. Clearly, the greater the difference, the more the solids have separated and the more unstable is the dispersion. Similar stability tests were also carried out at 60°C.

[0030] The size of the particles i.n the final dispersions was determined by an optical microscope technique.

[0031] The results are set out in the Table following the Examples.

Example 1

[0032] The coal was a bituminous coal ex Durham coalfield of Rank 501, with the following ultimate and initial particle size analyses:

[0033] The oil was Forties crude oil typically with a kinematic viscosity of 4.4 cSt at 37.8°C, a density at 15°C of 0.842 kg/litre and an-API Gravity of 36.6°.

[0034] The coal (2.7 kg) was mixed with Forties crude oil (3.3 kg) with continuous stirring using a high-speed vortex mixer. The resulting slurry was then pumped at a rate of 300 ml/min, giving a nominal residence time of 2.3 min, to a stirred ball mill sold under the name of Dyno Mill Type KDL Pilot by Willy Bachofen Maschinenfabrik, Basle, Switzerland. The mill grinding chamber, a horizontally-mounted cylinder of volume 1.4 litres, contained 2 mm steel balls (nominally 5 kg). The balls were stirred by agitator discs mounted on a horizontal shaft which ran parallel with the axis of the cylinder. The shaft speed was set at 3350 rpm to give a disc peripheral speed of 14 m sec-¹. The produce was collected as it emerged from the mill and passed through the mill twice more giving the final product a total of three passes. The flow rate was maintained as close as possible to 300 ml/m.

[0035] The resulting coal/crude oil dispersion was stable at ambient temperatures for at least four weeks and was also effectively stable after 24 hours at 60°C or after 24 hours at 100°C.

Example 2

[0036] A coal/crude oil dispersion containing 45 % wt coal was prepared in Asmix (Algeria) crude oil (typical density at 15°C of 0.803 kg/litre and API Gravity of 44.6°). The preparative conditions were similar to those used in Example 1. The resulting dispersion was effectively stable after 24 hours at 60°C. Stability tests at 100°C were invalidated due to the high volatility (15.2 % wt recovered at 100°C in Distillation Test IP 24/VE) of this particular crude.

Example 3

[0037] A coal/crude oil dispersion containing 45 % wt coal was prepared in Bonny (Nigeria) medium crude oil (typically kinematic viscosity at 37.8°C of 12.1 cSt; density at 15°C of 0.902 kg/litre, and API Gravity of 30.4°). The preparative conditions were similar to those used in Example 1. The resulting dispersion was also effectively stable after 24 hours at 60°C or 24 hours at 100°C.

Example 4

[0038] A coal/crude oil dispersion containing 45 % wt coal was prepared in Arabian Light crude oil (typically kinematic viscosity at 50°C of 4.99 cSt; density at 15°C of 0.857 kg/litre, and API Gravity of 33.4°). The preparative conditions were. similar to those used in Example 1. The resulting dispersion showed no signs of instability after 24 hours at 60°C and was also effectively stable after 24 hours at 100°C.

Example 5

[0039] A coal/crude oil dispersion containing 33 % wt coal was prepared in Boscan (Venezuela) crude oil (typically kinematic viscosity at 37.8°C of 19,400 cSt; density at 15°C of 1.00 kg/litre and API Gravity at 10.3°). The preparative conditions were similar to those of Example 1, except that the crude oil was warmed to 50°C before blending with the coal. The resulting dispersion was effectively stable after 24 hours at 60°C or 24 hours at 100°C.

Claims

1. A fuel composition containing 30 to 60% by weight of a friable solid fuel uniformly dispersed in crude oil, the mean particle size of the solid fuel being in the range 1 to 10 micron, the percentage being expressed as a percentage by weight of the total weight of the dispersion.

2. A fuel composition according to claim 1 wherein the composition contains 40 to 50% by weight of the solid fuel.

3. A fuel composition according to either of the preceding claims wherein the mean particle size of the solid fuel is in the range 2 to 5 micron.

4. A fuel composition according to any of the preceding claims wherein the friable solid fuel is coal, solvent refined coal, coal coke or petroleum coke.

5. A fuel composition according to claim 4 wherein the friable solid fuel is bituminous coal.

6. A method for the preparation of a uniform dispersion of a friable solid fuel in crude oil which method comprises grinding the solid fuel in the crude oil until the mean particle size of the solid 'fuel particles is reduced to a value in the range 1 to 10 mi.cron and the dispersion contains 30 to 60% by weight of solid fuel, the percentage being expressed as a percentage by weight of the total dispersion, air being excluded during the grinding operation.

7. A method according to claim 6 wherein the solid fuel is pre-ground to a mean particle size not greater than 250 micron before being ground in the presence of the crude oil.

8. A method according to either of claims 6 or 7 wherein grinding is carried out in an agitatory or vibratory ball mill.

9. A method according to claim 8 wherein the solid fuel is dispersed in the crude oil prior to grinding.

10. A method according to either of claims 6 or 7 wherein grinding is carried out in a tumbling ball mill.