WO1984001958A1

WO1984001958A1 - Coal-water dispersion

Info

Publication number: WO1984001958A1
Application number: PCT/SE1983/000397
Authority: WO
Inventors: Lars Lennart Stigsson
Original assignee: Fluidcarbon International Ab
Priority date: 1982-11-12
Filing date: 1983-11-14
Publication date: 1984-05-24
Also published as: SE8303364D0; AU2262283A; KR840008681A; AU9059782A; ZA84860B; ES8501793A1; SE451198B; ES529456A0; SE8303364L; ZA84859B; EP0120953A1

Abstract

Liquid fuel based on pulverized coal, water and additives, the coal content being at least 55 %. The fuel is suitable for transport in pipelines or direct combustion. A heavily increased dispersion effect as compared with prior art additives is achieved by chemical addition or connection of several aromatic rings to a hydrophilic polymer chain. The polymer chain preferably comprises an anionic or non-ionic graft block copolymerisate.

Description

COAL-WATER DISPERSION

The present invention relates to a liquid fuel comprising pulverized coal dispersed in a liquid, which has a ve ry long storage life and is suitable for transport and energy production. More particularly, the invention relates to a dispersion comprising a liquid which at least partially consists of water, pulverized coal and additives, the coal content being at least 60 % by weight.

A dispersion of coal is considerably less polluting and can be more easily handled than solid coal and also eliminates some risks involved in transport and storage. Such a dispersion is also preferred from an economical point of view.

The reason for using coal dispersions is the planned increased use of coal as a basic energy source in large as well as small plants for producing electricity and heat. The handling of solid coal in this connection is difficult for several reasons and, therefore, the transformation of the coal to liquid form generally is considered an interesting method. Chemical conversion of coal to a liquid product, so-called liquefaction, still cannot compete with oil and it is considered that this method can contribute to the global energy provision only marginally before the year 2000. Chemical conversion of the coal to a gaseous product, so-called gasification, seems to be a more prosperous method of utilizing coal. However, also this method still involves considerable technical difficulties although large resources have been spent on technical devel opment.

Physical conversion is another method of transforming the coal into liquid form, and the invention relates to this method. It is practised by dispersion in a liquid which may consist of water only or water in combination with some organic fuel such as heating oil, methanol, etc.

The present invention principally relates to a dispersion of pulverized coal in water.

One of the problems in producing a coal-water fuel is that the surface structure of the coal is modified in removing impurities, often by means of chemical additives, and that the dispersion of the coal particles in a later processing stage will be more difficult as a consequence thereof.

When coal-water dispersions which by definition are thermodynamically instable, are being stabilized the sedimentation and aggregation rates are reduced, in applying the invention, by creating a barrier which counteracts the particle attraction. This repulsive effect can be achieved by three main principles: electrostatic stabilization, sterical stabilization, and stabilization by means of hydration forces. The stabilization changes the energy of the particles and/ or creates a high barrier preventing particle attraction. On the basis of these principles it is possible to stabilize high contents of coal particles in a water solution by the addition of small amounts of organic additives.

In order to create new colloidal systems it is necessary to impart to the system such properties that the attraction forces between the particles are minimized and a repulsive barrier against flocculation and subsequent sedimentation is developed. Some form of sterical stabilization by means of hydrophilic polymers provides favourable conditions for a long-term stabilization of coal-water dispersions.

The process of manufacturing a coal-water fuel often includes removal of inorganic constituents in one way or the other.

Initially, the coal is ground in one or more mills to a polydisperse particle size distribution. All particles are smaller than about 300 microns and about 35 % of the total amount are smaller than 40 microns.

When manufacturing a coal-water fuel according to the present invention, a main process step included therein is a flotation enrichment of the coal, which by selective hydrophobization and charging separates coal and contaminants. For this purpose, different types of reagents are used which generally comprise mixtures of aliphatic alcohols together with a frothing agent. Other reagents which are suitable in the flotation of coal are mixtures of zwitterionic surfactants and oils insoluble in water together with a frothing agent and trace amounts of al kylsulpho-succinates.

After the flotation process the coal is dewatered mechanically, e.g. by means of a rotating vacuum filter. The use of surface-active substances in dewatering coal has been studied in laboratories as well as pilot scale. It has been found that there are two types of surfactants improving the degree of dewatering: 1/ Surfactants changing the surface tension of the water.

2/ Substances having hydrophobi zing properties.

Among surfactants of the first main type anionic surfactants have shown to be the most suitable. Examples of surfactants which can be used are al kylaryl-sulphonates and alkylsulpho-succinates.

In the final processing stage in manufacturing the coal-water fuel a dispersion agent and possibly a stabilizer are added. Among dispersion agents already known for pulverized coal in water or oil are e.g. anionic polynaphtalen sulphonates, polycarboxylic acids and non-ionic ethylene oxide derivatives.

It is previously known that the total amount of dispersion agent calculated as a weight percentage of the total dispersion, is completely decisive for the physical properties of the dispersion. Apart from the coal, the dispersion agent is one of the significant items of expenditure in the manufacture of the dispersion.

Surprisingly, it has been found that amphifilic non-ionic and slightly anionic graft and block copolymers added to aromatic compounds having at least two aromatic groups, have a heavily increased dispersion effect (2a). The effect increases as the aromatic content of the hydrophobic part increases to an upper limit at mole weights about 2,000 of the aromatic fraction. Ethoxylated and propoxylated naphta are particularly effective from a technical and economical point of view as dispersion agents. The mole weight of the dispersion agent can vary within a wide range (2,000 - 15,000) but technically and economically is optimal at about 5,000. To attain the same dispersing effect by means of previously known e.g. non-ionic ethylene oxide - propylene oxide derivatives, double the amount of dispersion agent for equivalent rheological properties of the dispersion is required.

According to the present invention, also slightly ionic compounds can be used, e.g. between a slightly anionic aromatic compound or another compound with high affinity to the coal surface and one or more slightly anionic or non-ionic polymer chains with high solubility in the water phase (2b). This type of compounds also has excellent dispersion properties for coal particles with zwitterionic surfactants or other hydrophobi zing substances already adsorbed on the coal surface. Particularly favourable compounds which can be used, having high affinity to the coal surface, are e.g. aromatic residues of tall-oil distillation. Particularly favourable compounds having low affinity to the coal surface and high solubility in the water phase are polyalcohols, polyamides, certain polyacrylates and polyethylene oxide derivatives. The hydrophilic chains possibly also can be sulphated to obtain a negative charge. In this way, more significant hydrophilic properties of the copolymer are obtained. The mole weight of the dispersion agent can vary between 1,500 and 20,000.

The invention will be described in more detail by means of an illustrative example. Example

1/ A non-micelle-forming zwitterionic surfactant such as lecithin or al kylbetai ne, which is thus relatively difficult to dissolve in water, is emulgated in water to form a liquid crystalline phase. The emulsion (0.03 % by weight of the coal) is supplied to the grinding step of the fuel preparation process. In a mixing step immediately before the flotation step, a preparation consisting of lecithin, hexadecane and methyl isobutyl ketpne is added in the ratio 1/5/2. The total additive amount and the composition can be varied for different coal qualities (0.01 - 0.05 % by weight of the coal). After the flotation process, a small amount of al kylsulpho-succinate (10 - 50 g/ton coal) is added to improve the dewatering degree in the filter. The filter cake contains between 15 and 22 % moisture. 2a/ Totally, 90 grammes of the moist purified coal product (22.2 % ) is homogenized and mixed with 0.25 grammes dispersion agent based on ethoxy lated and propoxylated naphta (mole weight 5,500) dissolved in water (2.5 % sol.). The resulting coal content of the dispersion is chosen to be 70 % by adjusting the concentration of the dispersion agent in the water solution. The resulting dispersion has very good rheological properties and is well suited for direct transport in pipelines. 2b/ The dewatered purified coal product is supplied to a mixer step wherein a dispersing agent dissolved in water is added. The dispersion agent comprises 0.5 % by weight of the dispersion and comprises a slightly anionic graft copolymer based on an ethoxylated residue of a tall-oil distillation. The mole weight of the dispersion agent can be varied between 1,500 and 20,000 but preferably is about 3,000. A fuel product having a low viscosity (< 1,000 cP Br) is obtained containing more than 65 % solid substance, ready for delivery to consumers. In order to achieve storage stability, stability against coalescense and sedimentation a gelation agent can be added to the dispersion. The gelation can be reversible as well as irreversible. Reversible gelation can be attained with small amounts of polysaccharide, xantan gum, guar-gum, alginate or a modified starch in combination with a metal ion, e.g. aluminium or boron. The reversible gelation process can be controlled by changing the temperature or the pH.

Irreversible gelation of the coal-water dispersion can be effected e.g. by means of different clays, bentonite, Chinese clay, etc. The fuel product obtained has now a clearly pseudopl astic rheology.

The coal-water fuel has been found to be a very good oil substitute for heat as well as steam production. The fuel is purified from a lot of inorganic contaminants and is preferred from an environmental and handling point of view to conventional coal technology.

Claims

1. Dispersion containing water, additives and pulverized coal, the surface of which has been modified by adsorption of zwitterionic or hydrophobizing substances, the coal content being at least 55 % by weight, c h a r a c t e r i z e d in that the dispersion contains as additives at least one anionic or non-ionic hydrophilic polymer chain or an anionic or non-ionic graft block copolymer with amphifilic properties, (added) chemically connected to at least one hydrophobic part segment having at least two aromatic ring structures with high affinity to the coal surface.

2. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that the aromatic hydrophobic segment has a mole weight exceeding 120.

3. Dispersion as claimed in claims 1 and 2, c h a r a c t e r i z e d in that the hydrophobic part segment of the copolymer has slightly anionic groups.

4. Dispersion as claimed in claims 1 and 2, c h a r a c t e r i z e d in that the part segment of the additive having high affinity to the coal surface, consists of amphifilic non-volatile aromatic and aliphatic residues from the distillation of oils or comprises propoxylated naphta compounds.

5. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that the hydrophilic polymer chain is slightly anionic or non-ionic.

6. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that the hydrophilic polymer chain is based on vinylalcohol, vinylpyrrolidone, ethylene oxide, or an acrylate.

7. Dispersion as claimed in claims 1 to 6, c h a r a c t e r i z e d in that the additives included in the dispersion comprise totally 0.05 2.0 % of the total weight of the dispersion.

8. Dispersion as claimed in claims 1 to 6, c h a r a c t e r i z e d in that the additives included in the dispersion comprise totally 0.1 - 0.6 % of the total weight of the dispersion.