DK152374B - DISPERSION OF COALS IN WATER - Google Patents

Abstract

Dispersion which comprises water, pulverized coal and additives and is stable in storage, can be pumped and can be burned by conventional technique. The coal content of the dispersion ranges from 60 to 80% by weight and the dispersion contains at least one amphiphile substance adsorbed to the surfaces of the coal particles, which provides repulsion between the coal particles by hydration forces, and also polymers cooperating with said substance, at least one of said polymers containing segments of hydrophobic as well as hydrophilic character.

Description

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DK 152374 B

The present invention relates to a dispersion of coal in water which is very stable during storage and is suitable for transport and energy production. More particularly, the invention relates to a dispersion comprising water, powdered coal and additives, the carbon content being from 60 to 80% by weight.

A dispersion of coal is significantly less polluting and can be more easily handled than solid coal, and it also removes certain risks posed by transport and storage. Such dispersion is also preferred from an economic point of view.

The reason for using coal dispersions is the planned increase in the use of coal as a basic energy source in large as well as small plants for the production of electricity and heat. The handling of solid coal in this connection is difficult for several reasons, and the conversion of the coal into liquid form is therefore generally regarded as an interesting method. Chemical conversion of coal into a liquid product, so-called hydrogenation, 20 still cannot compete with oil, and it is assumed that this method can only marginally contribute to global energy production before the year 2000. Chemical conversion of coal into a gaseous product, so-called gasification. , seems to be a more promising method of coal utilization. However, this method is also subject to considerable technical difficulties, although considerable resources have been devoted to technical development.

Physical conversion is another method of converting coal into liquid form and the invention relates to this method.

It is carried out by dispersing coal in a liquid which may consist of certain organic fuels such as fuel oil, methanol, etc. or of water.

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DK 152374B

The problem of aqueous dispersed powdered coal mainly consists in making the dispersion stable at low viscosity. The coal particles must not settle during storage or transport of the coal dispersion. Furthermore, the coal dispersion must be easy to pump and have rheological properties so that it is suitable for combustion by conventional techniques, possibly modified to a lesser extent.

Previous proposals for stabilizing the dispersions of coal in water include the proposal described in the Russian publication Khim. Pereab. Topi.

1975 30 (2) 19-29 and summarized in Chemical Abstracts 87: 55532b. In this case, salts of polycarboxylic acid and polyphosphates are used, among others, as stabilizers. The viscosity of the optimum mixture is approx.

15 5 P at a carbon content of 57 to 63%, and it follows that this dispersion does not meet the requirements. Furthermore, the sedimentation stability is deficient.

I. Swedish Patent No. 425.'917 discloses a dispersion of coal in water, whereby a stabilizing effect against sedimentation is obtained by means of conventional polyelectrolytes, including polyphosphate, in accordance with the same principles as those of the above-mentioned Russian publication described. Neither in this case is the stability against sedimentation satisfactory.

U.S. Patent No. 4,242,098 discloses a dispersion of coal in water, the stabilization being achieved by the addition of a number of water-soluble polymers (polyethylene oxide, polyacrylamides, etc.). This dispersion represents an improvement over the above-mentioned dispersions.

DK 152374 B

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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 demonstrated. Apart from previously known effects such as electrostatic stabilization by surfactants and polyelectrolytes as well as steric stabilization by polymers, there is a further central effect, the so-called hydration forces. These powers have recently been demonstrated and have also been explained theoretically. The hydration forces, optionally together with other types of interaction, form the primary basis of the present invention by providing a long-lasting stable hydrocarbon dispersion of the above-mentioned type suitable for direct combustion using the available technique, optionally with minor modifications. According to the invention, this dispersion has the characteristics stated in claim 1.

The dispersion has rheological properties which allow the dispersion to be pumped and transported through pipelines with a significantly reduced friction, and above all, the dispersion has a significantly improved stability against both flocculation and sedimentation.

When coal-water dispersions, which by definition are thermodynamically unstable, are stabilized, the sedimentation and aggregation rates are reduced by the formation of a barrier that discourages particle attraction. This repulsive effect can be achieved by three main principles: electrostatic stabilization, steric stabilization, and stabilization by hydration forces. The stabilization changes the energy of the particles and / or forms a high barrier ref. which impedes particle attraction. On the basis of these principles, it is possible to stabilize high carbon content.

DK 152374B

4 particles in an aqueous solution by the addition of small amounts of organic additives.

In order to form new colloidal systems, it is necessary to impart such properties to the system, minimize the forces of attraction between the particles, and develop a repulsive barrier to flocculation and subsequent sedimentation. Some form of steric stabilization by hydrophilic polymers provides favorable conditions for long-term stabilization of hydrocarbon dispersions.

From studies of lamellar liquid crystalline phases in systems of ionic surfactants, it has been found that these phases can swell and incorporate large amounts of water. This is explained by repulsion via the water layer between adjacent layers of surfactant and can be associated with electrostatic bilayer forces. In the case of zwitterionic substances, such as the phospholipid lecithin, the swelling is less pronounced, but nonetheless very evident. This shows that even in the absence of a net charge there is a considerable repulsion. This repulsive force, the so-called hydration force, is approximately exponentially varying with the distance with a decrease over 2-3 Angstroms. The force is of a general nature and does not vary with the length of the alkyl chain or with the physical state of the chains (liquid or solid), and it is also present when an amount of charged surfactant is included in the system. Hydration forces have also been demonstrated by directly measuring the forces between surfaces with a mutual distance of some Angstroms. A theoretical model of hydration forces has recently been developed, and it has then become possible to relate these forces to the presence of so-called mirror charges across interfaces, where the effective dielectric constant

DK 152374B

5 will be changed. Such mirror charges with zwitterionic groups should be common to micro- and macro-heterogeneous systems. They are utilized according to the invention in order to impart desired properties to a suspension of low dielectric. Then, a zwitterionic surfactant, such as lecithin, can be adsorbed on the surface of the solid particles. Strong repulsive forces between the particles then exist at short distances. The principle therefore will be particularly applicable to concentrated dispersions.

The invention is explained in more detail below.

A non-micelle forming zwitterionic surfactant, such as lecithin or alkyl betaine, which is thus relatively difficult to dissolve in water, is first dissolved in an organic solvent. The solvent may e.g. consist of octanol, hexadecane or methanol and it may be recovered appropriately or may constitute an insignificant equal weight portion of the dispersion.

The surfactant produced is added to a dispersion of powdered coal and water. Suitable fractions of the carbon powder are in the range of 1 to 200 µm (preferably 150 µm) with a wide size distribution (polydispersed carbon powder). The particle size and size distribution can be selected according to the desired stability. The smaller the particle size, the greater the stability, but it is expensive to grind coal down to particle sizes below 1 micron. The particle concentration can be varied within a wide range. Taking into account economic and technical aspects 35 6

DK 152374 B

the particle concentration is optimized from one case to another. Hydrocarbon dispersions having a solids content of between 65 and 75% by weight are of particular interest because these dispersions have good rheological properties, e.g.

5 for transport in pipelines. In order to achieve the highest carbon content (70-80% by weight), the size distribution must be taken into account in particular. In the normal case, this can be done on the basis of simple geometric considerations with respect to the minimization of the free volume when particles of different sizes are packed.

Following the adsorption process, two or more hydrophilic anionic and nonionic polymers are added to the dispersion to obtain some form of steric barrier and to reduce friction between the particles. Eg. one can choose from polyethers, polysaccharides, polyalcohols and polyacrylates. Particularly suitable according to the invention are polyethylene oxide, copolymers of the polyethylene oxide polypropylene oxide type, carboxymethyl cellulose and xanthan gum or guar gum. When copolymers are used, claims 3, 4 and 5 are particularly preferred. The concentration of polymers in wt%, based on the total weight, can be varied between 0.1 and 5%, but is economically optimal at approx. 0.5%. It is preferred that the total amount of additives be below 2%.

In order to make the dispersion more attractive as a non-polluting oil substitute, certain alkali salts or alkaline earth salts or hydroxides may be added to the dispersion. Preferably, calcium hydroxide or dolomite powder is used.

This neutralizes acidic gas components formed by the oxidation of the fuel and can be recovered in a particle trap.

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. One such agent is cetyl alcohol or hexadecanol.

Claims (7)

A dispersion comprising water, powdered coal and additives, the carbon content being from 60 to 80% by weight, characterized in that the dispersion contains at least one amphiphilic substance which is adsorbed on the surfaces of the coal particles and which produces repulsion between the coal particles by means of hydration forces, as well as polymers which cooperate with said substance, at least one of the polymers containing both hydrophobic and hydrophilic segments. Dispersion according to claim 1, characterized in that the amphiphilic substance is lecithin or another zwitterionic surfactant. Dispersion according to claim 1, characterized in that one of the polymers is a copolymer of polypropylene oxide and polyethylene oxide with a dominant amount of polyethylene oxide. DK 152374 B Dispersion according to claim 4, characterized in that the copolymer contains at least 70% by weight polyethylene = oxide and that the molecular weight is from 8,000 to 15,000. Dispersion according to claim 1, characterized in that one of the polymers is a polysaccharide. Dispersion according to claim 6, characterized in that the polysaccharide is xanthan gum or guar gum. Dispersion according to claim 1, characterized in that the total amount of additives is less than 2% by weight.