EP0068622A2

EP0068622A2 - Dispersions of solid fuel in oil

Info

Publication number: EP0068622A2
Application number: EP82302578A
Authority: EP
Inventors: Alan Grint; Christopher John Veal
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1981-05-21
Filing date: 1982-05-20
Publication date: 1983-01-05
Also published as: EP0068622A3; JPS57198795A

Abstract

Dispersions of a friable solid fuel in oil are prepared by grinding coal together with a minor proportion of green petroleum coke in a fuel oil medium. Air is excluded from the grinding operation and the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 15 micron. The dispersion contains 15 to 55% by weight of solid fuel.

The presence of the coke enhances the stability of the dispersions which are suitable for use as industrial fuel oils.

Description

This invention relates to dispersions of coal in oil and to a method for their preparation.
Coal oil slurries have previously been disclosed, see for example, British Patent Specification No. 975687. Whilst these have behaved as near-Newtonian non-settling fluids in pipelines, they separate on standing. Thus such slurries are suitable for use immediately after preparation or pipelining but are not suitable for transportation by tanker nor for storage.
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 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.
We have now discovered that the stability of dispersions of coal in oil can be increased by the addition of green petroleum coke.
Alternatively, the grinding time required to produce a dispersion of desired stability may be reduced.
Thus according to the present invention there is provided a method for the preparation of a dispersion of friable solid fuel in oil which method comprises grinding coal together with a minor proportion of green petroleum coke in a fuel oil medium, air being excluded from the grinding operation, until the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 15 micron and the dispersion contains from 15 to 55X by weight solid fuel, expressed as a percentage by weight of the total weight of the dispersion.
Preferably the combined quantities of coal and coke amount to 30 to 45% by weight of the total weight of the dispersion.
Suitably the coke is present in amount 0.1 to 40%, preferably 2 to 20% by weight, expressed as a percentage by weight of the total solid fuel, i.e. coal plus coke.
Preferably grinding is continued until the average particle size of the ground solid particles is reduced to a value in the range 2 to 8 micron.
The fuel oil may be a petroleum based fuel oil 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 heavier 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 an agitatory or vibratory 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 filing the mill completely. It is not practical to fill a vibratory ball mill completely and milling should be carried out under an inert gaseous atmosphere such as nitrogen.
In the case of 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.
According to another aspect of the present invention there is provided a solid fuel-oil dispersion comprising (a) coal particles ground in oil in the absence of air, (b) a minor proportion relative to (a) of green petroleum coke particles ground in oil in the absence of air, and (c) a fuel oil, the solid fuel being present in amount from 15 to 55% by weight of the dispersion and having a mean particle size in the range 1 to 15 micron.
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 coal in oil - enhanced by a fourth - the coke.
At ambient temperature the disperson 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.
If the solid particles are not ground in the oil in the absence of air, the solid particles will become oxidised and interact unfavourably. If the solid particle size is too great, forces will be insufficient to confer stability. The concentration of 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.
Such solid fuel oil dispersions are suitable for use in blast furnaces, cement kilns, industrial boilers, marine boilers and utility boilers.
The invention is illustrated with reference to the following example.

EXAMPLE

The coal was a bituminous coal ex National Coal Board (UK) of Rank 902 with the following ultimate analysis (air dried basis):
Size Analysis
The coke was a green petroleum coke with the following ultimate analysis (air dried basis):
Size Analysis
The oil was a mixed source heavy fuel oil having a nominal viscosity of 380 cSt at 50°C.
Initially it had the following properties:
The fuel oil (227.5 g) was warmed to 60°C and the pulverised coal (122.5 g) was added slowly with continuous stirring with a high speed vortex mixer. The resulting slurry containing 35X wt coal in fuel oil was added to a batch Megapact vibratory ball mill made by Pilamec Limited, Lydney, Gloucester. The mill grinding chamber, a horizontally mounted cylinder with total internal volume of 635 ml, was filled with 1750 g of 10 mm steel balls as grinding media occupying 235 ml. This left a free space of 400 ml. On milling each chamber contained approximately 360 ml of dispersion leaving a free space which was occupied by nitrogen.
The grinding chamber was vibrated on the horizontal cradle of the mill for two hours.
The resultant dispersion was a thick black lustrous fluid. After standing for twenty four hours at 100°C the dispersion showed considerable signs of instability.
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 Oil Mixture Combustion, Danvers, Massachusetts, USA, 27-29th 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 twenty four hours and subtracting from it the initial concentration of solid 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 8.3% wt. The mean particle size as determined by an optical microscope technique was found to be 5.3pm.
A second dispersion was prepared in a similar manner. A further sample of the same fuel oil (227.5 g) was warmed to 60°C and a mixture of pulverised coal (90% wt 110.25 g) and green petroleum coke (10% wt 12.25 g) was added slowly with continuous stirring with a high speed vortex mixer. The resultant slurry was milled under identical conditions again using the Megapact vibratory ball mill.
The resultant dispersion was a thick black lustrous fluid. After standing for twenty four hours at 100°C the dispersion appeared to be quite stable.
The dispersion had an instability of 2.0% wt and a mean particle size of 4.9 pm.
Thus, although the mean particle sizes of the two dispersions are not significantly different, the difference, indeed, being within the limits of experimental error, the stabilities are significantly different. This shows that the presence of a relatively small proportion of coke results in a considerable improvement in the stability of a coal oil dispersion.
A third dispersion was prepared in a similar manner using coke alone as the solid fuel component. This dispersion had an instability of 1.4% and a mean particle size in the range 2 to 5 micron.

Example 2

The dispersions of Example 1 were ground for further periods and the instabilities again determined. Results are given in the following Table together with those from Example 1 for ease of comparison.
These results clearly show that the stabilities of the mixtures are disproportionately enhanced by the presence of the coke.

Claims

1. A method for the preparation of a dispersion of a friable solid fuel in oil which method comprises grinding coal together with a minor proportion of green petroleum coke in a fuel oil medium, air being excluded from the grinding operation, until the mean particle size of the solid fuel particles is reduced to a value in the range 1 to 15 microns and the dispersion contains from 15 to 55X by weight solid fuel, expressed as a percentage by weight of the total weight of the dispersion.

2. A method according to claim 1 wherein the combined quantities of coal and coke amount to 30 to 45X by weight of the total weight of the dispersion.

3. A method according to either of the preceding claims wherein the coke is present in amount 0.1 to 40% by weight of the total weight of solid fuel.

4. A method according to claim 3 wherein the coke is present in amount 2 to 20% by weight of the total weight of solid fuel.

5. A method according to any of the preceding claims wherein grinding is continued until the average particle size of the ground solid particles is reduced to a value in the range 2 to 8 micron.

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

7. A method according to claim 6 wherein the fuel oil has a viscosity of not more than 380 cSt at 50°C.

8. 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.

9. A method according to any of the preceding claims wherein grinding is carried out in a ball mill.

10. A solid fuel-oil dispersion comprising (a) coal particles ground in oil in the absence of air, (b) a minor proportion, relative to (a) of green petroleum coke particles ground in oil in the absence of air and (c) a fuel oil, the solid fuel being present in amount from 15 to 55X by weight of the dispersion and having a mean particle size in the range 1 to 15 micron.