GB2094340A - Preparation of solid fuel-oil dispersions - Google Patents

Abstract

Uniform dispersions of a friable solid fuel in oil are prepared by dispersing the solid fuel in a medium containing pre-oxidised fuel oil until the dispersion contains 15 to 55% by weight solid fuel, preferably 30 to 45% by weight, and the mean particle size of the solid fuel is in the range 1 to 15 micron, preferably 2 to 8 micron. Coal is the preferred solid fuel. The use of oxidised fuel oil enhances the stability of the dispersions.

Description

SPECIFICATION Preparation of solid fuel-oil dispersions This invention relates to the production of dispersions of solid fuel and oil.

Our British Patent Specification No. 1 523193 describes and claims a method for the preparation of a uniform coal oil dispersion which method comprises grinding coal in a medium consisting 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.

USP 1432178 discloses "a fuel product in which solid fuel particles are maintained in suspension in a fluid fuel medium by changing the fluid fuel medium into a medium of greater thickness or viscosity by the formation of addition or substitution products of oxygen or hydrogen or other suitable substances." We have now discovered that the stability of solid fuel-oil dispersions can be improved by the use of a pre-oxidised fuel oil. Alternatively, the grinding time required to produce a dispersion of desired stability is reduced.

Thus, according to the present invention, there is provided a method for the preparation of a uniform dispersion of a friable solid fuel in oil which method comprises dispersing the solid fuel in a medium containing pre-oxidised fuel oil until the dispersion contains 1 5 to 55% by weight solid fuel, preferably 30 to 45%, expressed as a percentage by weight of the total dispersion and the mean particle size of the solid fuel is in the range 1 to 1 5 micron, preferably 2 to 8 micron.

Preferably the medium consists essentially of the pre-oxidised fuel oil.

The solid fuel may be ground to the desired particle size either by dry grinding or by wet grinding in the presence of a light liquid grinding medium which is subsequently removed.

Dry grinding may be carried out in a jet mill.

Wet grinding may be carried out by the method disclosed in our copending application No.

8137729. The ground material is then subsequently dispersed in the pre-oxidised oil, e.g. by a high speed vortex mixer.

Alternatively, the solid fuel may be wet ground in the presence of the pre-oxidised oil. In this case, size reduction and dispersal occur simultaneously.

Suitable friable solid fuels include coals of various ranks, solvent refined coal, coal coke and petroleum coke. The preferred solid fuel is bituminous coal.

Preferably the solid fuel supplied to any grinding process is pre-ground to a particle size not greater than 250 micron.

The preferred oil is a petroleum based fuel oil fraction having a viscosity after oxidation of not more than 600 cSt, preferably not more than 380 cSt, at 50 C. The required viscosity may be achieved by "cutting back" if necessary with, for example, gas oil.

The oil may be oxidised by blowing air through it at elevated temperature.

Wet grinding can be carried out in commercially available ball mills, e.g. vibratory, agitatory or tumbling ball mills.

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

Grinding is preferably carried out until the solid fuel mean particle size is reduced to a value in the range 2 to 8 micron.

The grinding time will depend on the nature of the mill.

The dispersions will generally be prepared, used and stored at elevated temperature and under these conditions will be more stable than corresponding dispersions prepared in unoxidised fuel oil.

The solid fuel-oil dispersions are suitable for use in blast furnaces, cement kilns and in industrial, marine and utility boilers.

The stability of the solid fuel-oil-dispersion is a function of three important variables-the oxidation of the oil, the final particle size of the solid fuel and the concentration of solid in oil. If all three are chosen correctly then the dispersion is of enhanced stability at elevated and ambient temperatures.

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.

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

The invention is illustrated by the following examples. Example 1 is illustrative of the invention and Example 2 is provided for comparison only.

Example 1 The coal was a bituminous coal ex Durham coal field of Rank 501 with the following ultimate analysis (air dried basis): Carbon content % wt : 78.4 Hydrogen content % wt : 4.66 Nitrogen content % wt : 1.5 Sulphur content % wt : 1.8 Oxygen content % : 6.5 Moisture content % wt : 2.2 Ash content % wt : 4.4 Volatile matter % wt : 35.0 The oil was a mixed source fuel oil having a viscosity of 950 Redwood No. 1 seconds at 1 00CF (37.80C) and 120 cSt at 500C.

Initially it had the following properties: Pour point OC : 15 Sulphur content % wt : 2.93 Water content % wt : 0.05 Relative density 15.50C/15.50C : 0.9535 Kinematic viscosity at 600C cSt : 95.1 80 C cSt: : 41.3 100"C cSt : 21.7 The oil was oxidised by maintaining 500 g at 1 O00C for 24 hours and purging dry air through it at the rate of 500 ml/min.

The oxidised fuel oil had the following properties: Kinematic viscosity at 600C cSt : 129.6 80"C cSt : 53.3 100 C cSt : 26.6 The coal was pre-ground to 75% wt less than 75 micron.

The pre-ground coal (1.05 kg) was then added to n-heptane (3.1 5 kg) at ambient temperature.

The coal was dispersed using a high speed vortex mixer until a homogeneous mixture was obtained.

The mixture was then pumped to a stirred ball mill sold under the trade name of "Dyno Mill" Type KDL by Willy Bachofen Maschinefabrik, Basle, Switzerland, at a rate of 200 mI/min-1. The grinding chamber, a horizontally mounted cylinder of volume 600 ml, contained 1 mm steel balls (500 ml). 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 4500 rpm giving a disc peripheral speed of 1 5 m/sec. The product was collected and passed back through the mill at the same flow rate. This procedure gave a total residence time in the grinding chamber of three minutes. The coal was subsequently evaporated to dryness in air at ambient conditions. The resultant mean particle size of the coal was 4.3 Mm, as determined by optical microscopy.

The surface oxygen content of the coal particles as determined by ESCA with the intensity corrected for the atomic sensitivity factor was 7.3 on the basis of atomic concentration relative to carbon equal to 100.

This is close to the initial bulk oxygen content of 6.3 on the same basis.

83.6 g oxidised fuel was warmed to 500C and 45 g ground coal dispersed in it using a Silverson high speed vortex mixer for 10 minutes.

The resulting dispersion was a thick, black, lustrous fluid.

After standing for 24 hours at 1000C the dispersion appeared to be quite stable.

The stability 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 Mixture Combustion, Danvers, Massachusetts, U.S.A., 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 1000C 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.

The dispersion had an instability of 3.6% wt.

Example 2 To provide comparison, Example 1 was repeated using an unoxidised fuel oil the viscosity of which was greater than the viscosity of the oxidised fuel oil of Example 1.

The oil was a mixed source fuel oil having a viscosity of 3500 Redwood No. 1 seconds at 1000F (37.80C) and 350 cSt at 5O0C.

It had the following properties: Pour point OC : 15 Sulphur content % wt : 2.19 Water content %wt : 1.2 Relative density 1 5.50C/1 5.50C : 0.9675 Kinematic viscosity at 600CcSt : 219.0 800CcSt : 72.5 1000C cSt : 37.5 The dispersion had an instability of 6.65 wt.

The greater stability of the dispersion of Example 1 when compared with that of Example 2 is not simply caused by viscosity. If this were the case, one would expect that the dispersion of Example 2 would be the more stable since its oil has the greater viscosity. This is not the case and therefore some other factor present in the oxidised oil is involved.

Claims (7)

Claims

1. A method for the preparation of a uniform dispersion of a friable solid fuel in oil which method comprises dispersing the solid fuel in a medium containing pre-oxidised fuel oil until the dispersion contains 1 5 to 55% by weight solid fuel, expressed as a percentage by weight of the total dispersion and the mean particle size of the solid fuel is in the range 1 to 1 5 micron.

2. A method according to claim 1 wherein the medium consists essentially of pre-oxidised fuel oil.

3. A method according to either claim 1 or 2 wherein the friable solid fuel is coal, solvent refined coal, coal coke or petroleum coke.

4. A method according to claim 3 wherein the friable solid fuel is bituminous coal.

5. A method according to any of the preceding claims wherein the solid fuel supplied to the grinding process is pre-ground to a particle size not greater than 250 micron.

6. A method according to any of the preceding claims wherein the oil is a petroleum based fuel oil fraction having a viscosity after oxidation of not more than 600 cSt at 50"C.

7. A method according to claim 1 as hereinbefore described with reference to Example 1.