WO1983003618A1 - Coal-water dispersion and method of the manufacture thereof - Google Patents

Abstract

Dispersion which comprises water, pulverized coal and additive and is stable in storage, can be pumped and can be burnt by conventional technique. The coal content of the dispersion ranges from 60 to 85 % by weight and the dispersion contains at least one substance adsorbed to the surfaces of the coal particles, which provides repulsion between the coal particles by hydration forces.

Description

COAL-WATER DISPERSION AND METHOD OF THE MANUFACTURE

THEREOF

The present invention relates to a coal-water dispersion which is ve ry stable in storage and is suitable for transport and direct energy production without preceding dewatering. More particularly the invention relates to a dispersion comprising water, pulverized coal and additive, the coal content amounting to at least 60 % by weight. A dispersion of coal is consi derably 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, steam 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 a l th ough l a rge res o u rces h ave be en s pen t on te ch n i ca l development.

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

The main problem of water dispersions of pulverized coal is to ma.ke the dispersion stable at low viscosity. The coal particles must not sediment during storage or transport of the coal-water dispersion. Moreover, the coal-water dispersion shall have a high pumpability and shall have rheological properties so as to be suitable for burning by conventional technique probably modified to a minor degree. According to the published international application WO 31/01152, anionic surfactants and at least one electrolyte among other additives are used to prevent sedimentation and improve rheology. Tests have shown that the sedimentation stability as well as the rheological properties do not satisfy the desired demands.

In the laid open Swedish specification 7805632-2, a coal-water dispersion is disclosed wherein a stabilizing action against sedimentation is obtained by conventional polyelectrolytes, among them poly phosphate. Also in this case the stability against sedimentation is not satisfactory. The necessary atomization of the fuel when being burnt moreover causes problems e.g. due to formation of agglomerates. U.S. patent specification 4,242,098 describes a coal-water dispersion wherein the stabilization is obtained by the addition of a number of water soluble polymers (polyethyleneoxide, polyacrylamides, etc.). This dispersion provides an improvement in relation to the dispersions mentioneα. above but cannot be used directly for energy production without preceding dewatering.

The theory of the stability of dispersions has recently been considerably refined. The theory has been developed to include more concentrated systems but above all a new type of interaction has been proved. In addition to previously known effects such as electrostatic stabilization by means of s u rface-acti ve substances and polyelectrolytes and sterical stabilization by means of polymers there is a further central effect so-called hydration forcesv These forces have recently been proved and have also been explained theoretically. The hydration forces, possibly together with other types of interaction, form the primary basis of the present invention providing a long-term stable coal-water dispersion of the type referred to above, which is well suited for direct burning without preceding dewatering by using the technique available today possibly with minor modifications.

The disperston according to the invention is characterized in that the dispersion contains at least one substance adsorbed to the surfaces of the coal particles, which provides repulsion between the coal particles by hydration forces.

Preferably, said substance comprises a zwitter ionic amphiphile, or a derivative thereof with amphoteric behaviour.

Suitably said substance is present in an amount of about 0.01 to about 0,15 per cent by weight of the total dispersion, preferably about 0.1 per cent by weight of the total dispersion.

The invention also provides a method of manufacturing the dispersion, wherein said suhstance providing the repulsion between the coal particles by hydration forces is supplied to the pulverized coal either dispersed in wa,ter or dissolved in an organic solvent.

The dispersion has rheological properties which allow the dispersion to be pumped and transported through pipelines at a greatly reduced friction, and above all has a considerably improved stability against flocculation as well as sedimentation. The dispersion moreover has such properties that it can be atomized without forming agglomerates in a suitable burner equi pment.

When coal-water dispersions which by definition are thermodynamically instable, are being stabilized the sedimentation and aggregation rates are reduced 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 medium by the addition of small amounts of organic additives.

In order to create stable 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.

From studies of lamellar liquid crystalline phases in systems of ionic surface-active substances it has been shown that these phases can swell and incorporate large amounts of water. This is explained by a repulsion over the water layer between adjacent layers of surface--active substance and can be related to electrostatic double-layer forces. For zwitterionic substances such as the phospholipid lecithin the swelling is less pronounced but nevertheless very clear. This shows that also in the absence of a net charge there is a considerable repulsion. This repulsion force, the so- -called hydration force, is approximately exponentially varying with the distance with a characteristic length of 0.2 - 0.3 nm. The force is of a general character and does not vary with the length of the alkyl chain or with the physical condition of the chains (liquid or solid) and is present also when a quantity charged surface-active substance is included into the system. Hydration forces have been proved also by directly measuring th e fo rces betwee n s u rfaces mu tua l ly s paced s ome An gström. A theoretical model for hydration forces has recently been developed, and then it has been possible to relate these forces to the presence of so- -called mirror charges over interfaces where the effective dielectric constant is being changed. Such mirror charges with zwitterionic groups should he common in micro- and macro-heterogeneous systems. They are utilized according to the present invention in order to impart desired properties to a suspension of a low dielectric substance in a high dielectric medium. Then, a zwitterionic surface-active substance such as lecithin can be adsorbed onto the surfaces of the solid particles. Strong repulsion forces between the particles then exist at short distances. The principle therefore will be particularly useful for concentrated dispersions.

In order to obtain a pseudopl astic, thixotropic rheology, water-soluble polymers are also added to the dispersion. Suitable polymers which co-operate well with the hydration effect according to the invention are among copolymers of polyacrylates, polysaccharides, polyoxyethylenes, and polysulphonates with a balanced hydrophilic-lipophtlic content.

The invention will be described in more detail below with reference to an example. Example

1. A non-micelle-forming zwitterionic surfactant such as lecithin or an alkylbetain, which is thus relatively difficult to dissolve in water, is first dissolved in an organic solvent. The solvent can consist of for example octanol, hexadecane or methanol and can be recovered in a suitable manner or can comprise an insignificant portion by weight of the dispersion. Alternatively, the surfactant can be dispersed directly in water probably in the form of a lamellar liquid crystalline phase, or can be dissolved in water by the addition of an additive having the property of creating soluble mixed aggregates with lecithin.

2. The surfactant preparation is added to a dispersion of pulverized coal or in connection with the pulverization step. For example, the pulverized coal can be produced by wet grinding solid coal, the pulverized coal thus obtained being beneficated and dewatered. In that case the surfactant preparation can be added to the pulverized coal in the process of grinding as a grinding aid or in the process of benefication as a flotation reagent making the coal surface more hydrophobic or after the process of dewatering. Suitable fractions of the coal powder in the final product range from 1 to 200μm (preferably < 150 μrn) with a broad size distribution (polydispersed coal powder). Optimum packing is obtained according to Fuller. The smaller particle size, the greater stability but it is expensive to grind coal down to submicron particle sizes. The particle concentration can be varied within a broad range. Considering economical and technical aspects the particle concentration should be optimized from one case to the other. Coal--water dispersions with a dry substance content between 65 and 80 % by weight are of particular interest because these dispersions have a high energy content and good rheological properties for-example for transport in pipelines. In order to obtain the highest coal contents (70 - 80 % by weight) the size distribution must be particularly taken into account. In the normal case this can be done on the basis of simple geometrical considerations as to the minimization of the free volume when packing particles of different sizes. It is also possible to add larger coal particles in the range from 0.5 to 5 mm to a dispersion in order to obtain in this manner a more energy-rich coal-water dispersion, particularly tn view of transporting.

Another possibility according to the invention includes addition of oil to the slurry (0 - 10 % ) either in the pulverization step or later to improve flame stability when the slurry is being burnt, especially in small scale applications.

After the adsorption process one or more of hydro philic anionic or non-ionic polymers may be added to the dispersion in order to achieve the desired rheology and to provide some form of a steric barrier and in order to reduce the friction between the particles. For example one can choose between polyethers, polysaccharides, polyalcohols and polyacrylates. Particularly suitable according to the invention are polyethyleneoxide, copolymers of the polyethyleneoxi de-polypropyleneoxide type, carboxymethylcellulose, xanthan gum, and polymers of the type block or graft copolymers with balanced hydrophilic-lipophilic content. The concentration of polymers in percent by weight based on the total weight can be varied between 0.1 and 5 % but is economically optimal at ahout 0.5 % .

For the purpose of making the dispersion more attractive as a non-polluting substitute for oil some alkali salts or salts of the alkaline earths can be added to the dispersion suitably in amounts correspond ing to the stoichiometric content of sulphur in the fuel in order to obtain the desired reduction in oxides produced when the fuel is utilized. Preferably, the salt is calcium hydroxide or dolomite powder. The salt neutralizes acid gas components generated at the oxidation of the fuel and can be recovered in a particle percipitator.

In order to prevent the water from evaporating from the coal-water dispersion an agent can be added to the dispersion which forms a monomolecular layer in the interface. Such an agent is cetyl alcohol or hexadecanol and suitably is added in amounts of 1 to 10 ppm by volume.

The invention proyides substantial advantages over the prior art technique of stabilizing coal particles in water. Due to an excellent sedimentation stability combined with favourable rheological properties for pumping, the dispersion obtained is well suited for transporting coal in an appropriate manner in conduits or pipelines for use for example in the chemical industry or for direct energy production.

Combustion tests in boilers designed for heayy oil have shown that the dispersion is very suitable to replace oil completely or partly.

Claims

1. Dispersion containing water, pulverized coal and additive, the coal content ranging from 60 to 85 % by weight, c h a r a c t e r i z e d in that the dispersion contains at least one substance which is adsorbed to the surfaces of the coal particles and which provides repulsion between the coal particles by hydration forces.

2. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that said substance comprises a zwitterionic amphiphile or a derivative thereof with amphoteric behaviour.

3. Dispersion as claimed in claim 2, c h a r a ct e r i z e d in that the zwitterionic amphiphile is a zwitterionic phospholipid for example lecithin, an alkyl betain or polymerized derivatives thereof.

4. Dispersion as claimed in claim 3, c h a r a c t e r i z e d in that an increased repulsion between the coal particles is obtained by the phospholipid being partly zwitterionic and partly anionic.

5. Dispersion as claimed in any of claims 1 to 4, c h a r a c t e r i z e d in that the dispersion also contains polymers cooperating with said substance, at least one of said polymers containing segments of hydrophobic as well as hydrophilic character.

6. Dispersion as claimed in claim 5, c h a r a ct e r i z e d in that one of the polymers is of the type hlock or graft copolymer.

7. Dispersion as clatmed in claim 5, c h a r a ct e r i z e d in that one of the polymers is a copolymer having a hydrophohic base with a large attraction to the coal surface and that the hydrophilic tail is a polymer based on a polysaccharide, polyethylene oxide, polycarboxylήte or polyamide.

8. Dispersion as claimed in claim 5, c h a r a c t e r i z e d in that one of the polymers is a copolymer of polypropyleneoxi de and polyethylene oxide with a dominating amount of polyethyl eneoxide.

9. Dispersion as claimed in claim 8, c h a r a ct e r i z e d in that the molecular weight of the copolymer ranges from 5,000 to 50,000.

10. Dispersion as claimed in claim 9, c h a r a c t e r i z e d in that the copolymer contains at least 70 % by weight of polyethyleneoxide and that the molecular weight ranges from 8,000 to 15,000.

11. Dispersion as claimed in claim 5, c h a r a c t e r i z e d in that one of the polymers is a polysaccharide.

12. Dispersion as claimed in claim 11, c h a r a c t e r i z e d in that the polysaccharide is xanthan gum or guar gum or an alginate.

13. Dispersion as claimed in claim 1 , c h a r a c t e r i z e d in that the total amount of additives is below 2 % by weight.

14. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that it contains 0 to 10 % by weight of oil.

15. Method of manufacturing the dispersion as claimed in any of claims 1 to 14, c h a r a c t e ri z e d in that said substance providing the repulsion between the coal particles by hydration forces is supplied to the pulverized coal either dispersed in water or dissolved in an organic solvent.

16. Method as claimed in any of claims 1 to 14, c h a r a c t e r i z e d in that said suh stance is supplied to the pulverized coal dissolved in water by means of an organic salt for example an alkali salt of a fatty acid or an alleali sorbat.

17. Method as claimed in claim 15 or 16 wherein the solid coal is pulverized by wet grinding and is then beneficated and dewatered, c h a r a c t e r i z e d in that said substance is supplied in the process of grinding or benefication or after the process of dewatering.