GB2110714A - Fuel/oil dispersions - Google Patents

Abstract

A dispersion of a friable solid fuel, eg, coal, in fuel oil is prepared by grinding 15 to 55% by weight of solid fuel particles in the fuel oil, air being excluded during the grinding operation. A surfactant and water are added before or during grinding. This enhances the stability of the dispersion. The dispersion is suitable for use as a liquid fuel for industrial burners.

Description

SPECIFICATION Dispersions of solid fuel in oil This invention relates to a method for the preparation of dispersions of solid fuel in oil.

Coal-oil slurries have previously been disclosed, see for example, British Patent Specification No. 975687. Whilst these have behaved as near-Newtonion non-setting fluids in pipelines, they separate on standing. Thus such slurries are suitable for use immediately after preparation or pipelining but are not suitable for transporation by tanker nor for storage.

Our British Patent Specification No. 1 5231 93 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 1 5 to 55% by weight of coal expressed as a percentage by weight of the total dispersion and until a stable dispersion results on ceasing grinding.

United States Patent Specification 2668757 describes the preparation of coke oil mixtures stabilised by the formation of soap in-situ.

The stability of solid fuel oil dispersions of the type described in 1 523193 is believed to be promoted by the formation of oleophilic surfaces on the coal by the method of grinding.

On the other hand, the stability of solid fuel oil mixtures stabilised by a surfactant and water is believed to be promoted by water adsorbing on the coal surface leading to the formation of water bridges between particles. The tendency for water to adsorb will be enhanced with increasing hydrophilicity of coal surface which in turn will be promoted by adsorption of surfactant or wetting agent.

We have now discovered that grinding a solid fuel in oil in the absence of air but in the presence of a surfactant and water results in a product of enchanced stability. Alternatively, the grinding time required to produce a dispersion of desired stability may be reduced.

This is indeed surprising since one might reasonably have expected that the two distinct methods of stabilisation acknowledged as prior art would interfere with one another, in view of the fact that one depends on increasing the oleophilicity of the solid fuel particles and one on their hydrophilicity, and not reinforce one another as we have found.

Thus according to the present invention there is provided a method for the preparation of a uniform dispersion of a friable solid fuel in fuel oil which method comprises grinding 1 5 to 55% by weight, preferably 30 to 45% by weight, of solid fuel particles in the fuel oil, air being excluded during the grinding operation, and, before or during grinding, adding a surfactant and water, the percentage being expressed as a percentage by weight of the total dispersion.

Preferably grinding is continued until the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 1 5 micron, preferably 2 to 8 micron.

The preferred surfactants are of the anionic type, most preferably alkyl aryl sulphonates.

The concentration of the surfactant is suitably in amount 0.01 to 5% by weight, most preferably 0.05 to 0.5% by weight, of the total weight of the dispersion.

Suitable friable solid fuels include coals of various ranks, solvent refined coal, coal coke and petroleum coke.

The preferred fuel oil is a petroleum based fuel oil fraction having a viscosity of not more than 600 cSt at 50-C, 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.

In the case of certain heavy fuel oil fractions it may be necessary to heat them in order to render them sufficiently mobile to permit dispersal of the solid fuel particles therein.

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

Grinding can be carried out in vibratory, agitatory or tumbling ball mills.

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

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

It is necessary to exclude air during the grinding operation. This can easily be achieved in the case of an agitatory ball mill by filling the mill completely.

It is not practical to fill a vibratory mill completely and the milling should be carried out under an inert gaseous atmosphere, e.g. a blanket of nitrogen.

In a tumbling ball mill, the contents will take up a kidney shaped configuration as the mill rotates. This cascades smoothly with little splashing or surging. Grinding takes place in the interior of the kidney shaped mass and few, if any freshly ground surfaces are exposed to the gaseous atmosphere in the mill. The gaseous atmosphere is thus excluded from the grinding operation.

If no special precautions are taken, this atmosphere will normally be air. However, in order to reduce the risk of free oxygen contamination still further, in view of the adverse effect which this has on product stability, it is preferred to operate the tumbling ball mill with the contents under an inert blanket, e.g. of nitrogen. This has the further advantage of eliminating the risk of forming hazardous vapour mixtures. Inert gas blanketing is particularly advantageous when grinding at relatively high temperatures, e.g. close to or above the flash point of the liquid.

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 the grinding. Ball mills usually contain steel or glass balls and these are suitable for the present purpose.

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

The invention is illustrated with reference to the following examples.

Examples 2 and 3 are provided for the purpose of comparison only and not in accordance with the invention.

Example 1 A fluid coke had the following ultimate analysis (air dried basis): Moisture content % wt 1.9 Ash content % wt 1.2 Carbon content % wt 85.0 Hydrogen content % wt 2.22 Nitrogen content % wt 0.8 Oxygen content % wt 2.9 Sulphur content % wt 5.9 Volatile matter % wt 6.6 and initial particle size: + 212,us %wt % wt 1.9 212-180ym % wt 7.8 180- 125 /lm %wt 22.0 125-75 ym % wt 23.2 75-53sim %wt 14.1 53 ym %wt 31.0 The fuel oil was a mixed source fuel oil with the following properties: Sulphur content % wt : 2.26 Water content % wt : 0.05 Relative density at 15.6'C/15.6'C : .9740 Kinematic viscosity at: 60 C cSt: 444.0 80"C cSt : 144.5 100 CcSt: 61.5 Asphaltenes % wt : 6.1 Asphaltum % wt: 9.8 Vanadium content ppm : 55 The additive was an alkylbenzene sulphonate alkanolamine salt sold under the Trade Name Lutensit A-LBA by BASF, AG. It was used as a 16% by wt solution in water.

25 g of the additive solution was added to 350 g of the fluid coke which was then added to 625 9 of the fuel oil warmed to 60"C and dispersed in a high speed vortex mixer.

The resulting slurry was then ground in a vibratory ball mill sold under the Trade Name Megapact by Pilamec Limited, Lydney, Gloucestershire. This comprises two grinding chambers, each a horizontally mounted cylinder of capacity 635 ml. One cylinder was charged with 245 ml 10 mm steel balls and 350 ml slurry leaving a free space of 40 ml which was filled with nitrogen.

The cylinders are mounted on a cradle which was vibrated in the horizontal plane for one hour.

At the end of this period the product was a thick, black, lustrous fluid material.

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 Combination, Danvers, Massachusetts, USA, 27th-29th November, 1 979 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.

The dispersion had the very slight instability of 1.7% by wt.

Example 2 By way of comparison, a similar dispersion was prepared in a similar manner but without the additive solution.

The resulting dispersion had an instability of 10.1%.

Example 3 For further comparison, a mixture of fluid coke (350 g), additive solution t25 g) and fuel oil (625 g) was prepared by hand stirring.

The resulting dispersion had an instability of 23.9%.

Examples 2 and 3 illustrate that grinding alone and the presence of the additive alone are not sufficient to confer stability.

Claims (9)

1. A method for the preparation of a uniform dispersion of a friable solid fuel in fuel oil which method comprises grinding 1 5 to 55% by weight of solid fuel particles in the fuel oil, air being excluded during the grinding operation, and before or during grinding, adding a surfactant and water, the percentage being expressed as a percentage by weight of the total dispersion.

2. A method according to claim 1 wherein grinding is continued until the means particle size of the solid fuel particles is reduced to a value in the range 1 to 1 5 micron.

3. A method according to either of the preceding claims wherein the surfactant is an anionic surfactant.

4. A method according to claim 3 wherein the anionic surfactant is an alkyl aryl sulphonate.

5. A method according to any of the preceding claims wherein the surfactant is present in amount 0.05 to 0.5% by weight of the total weight of the dispersion.

6. A method according to any of the preceding claims wherein the solid fuel is coal or petroleum coke.

7. A method according to any of the preceding claims wherein the solid fuel particles are preground to a particle size not greater than 250 micron.

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

9. A method as hereinbefore described with reference to the example.