WO1983000500A1 - Coal-water dispersion - Google Patents

Abstract

A dispersion comprising water, pulverized carbon and additives, stable in storage, can be pumped and burned by a conventional technique. The carbon content of the dispersion is between 60 and 80% by weight and the dispersion contains at least one amphiphilic substance adsorbed on the surfaces of the carbon particles, which causes repulsion between the carbon particles by hydration forces, as well as polymers acting in concert with this substance, at least one of these polymers containing segments of both hydrophobic and hydrophilic character.

Description

COAL-WATER DISPERSION

The present invention relates to a coal-water dispersion which is ve ry stable in storage and is suitable for transport and energy production. More particularly the invention relates to a dispersion comprising water, pulverized coal and additives, the coal content ranging from 60 to 80 % 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 development.

Physical conversion is another method of transforming 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 some organic fuel such as heating oil, methanol, etc, or of water. The problem of water dispersions of pulverized coal mainly is to make the dispersion stable at low viscosity. The coal particles must not sediment during storage or transport of the coal dispersion. Moreover, the coal 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.

Among previous proposals of stabilizing coal-water dispersions the proposal could be mentioned, which is described in the Russian publication Khim. Pereab. Topi. 1975 30 (2) 19-29, which is summarized in Chemical Abstracts 87 : 55532b. In that case salts of polycarboxyl acid and polyphosphates are used among others as stabilizers. The viscosity of the optimal mixture is about 5 P at a coal content of 57 to 63 %, and it follows thereof that this dispersion does not satisfy the demands. Moreover, the stability of sedimentation is deficient. In the laid-open Swedish specification 7805632-2, a coal-water dispersTon is disclosed wherein a stabilizing action against sedimentation is obtained by conventional polyel ectrolytes, among them polyphosphate, according to the same principles as those described in the Russian publication mentioned above. Also in this case the stability against sedimentation is not satisfactory.

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 p rovi des an improvement in relation to the dispersions mentioned above.

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 surface-active substances and polyel ectrolytes and sterical stabilization by means of. polymers there is a further central effect so-called hydration forces. 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 by using the technique available possibly with minor modifications, According to the invention, this dispersion has obtained the characteristics of claim 1.

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,

When coal-water dispersions which by definition are thermodynamically instable, are heing 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 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.

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 declination over 2 - 3 Ångstrom. 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 (l i q ui d 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 the forces between surfaces mutually spaced some Angstrom. 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 zw i tte ri on i c groups should be common in micro- and macro-heterogeneous systems. They are utilized according to this invention in order to impart desired properties to a suspension of a low dielectric substance. 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.

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

Examples

1. A non-mieelle-forming zwitterionic surfactant such as lecithin or 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.

2. The surfactant preparation is added to a dispersion of pulverized coal and water. Suitable fractions of the coal powder range from 1 to 200 μm (preferably

150 μm) with a broad size distribution (polydispersedcoal powder). The particle size and the size distribution can be chosen according to the desired stability. 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 75 % by weight are of particular interest because these dispersions have 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.

After the adsorption .process two or more of hydrophilic anionic and non-ionic polymers are added to the dispersion in order 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 polyethyleneoxi de, copolymers of the polyethyl eneoxide-polypropyleneoxide type, carboxymethyl cell ulose, and xanthan gum. 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 about 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. 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 percipi tator.

In order to prevent the water from evaporating from the coal-water dispersion an agent can be added to the dispersion which forms a monomol ecu! ar layer in the interface. Such an agent is cetyl alcohol or hexadecanol .

The invention provides substantial advantages over the prior art technique of stabilizing coal particles in a water solution. 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 heavy oil have shown that the dispersion is very suitable to replace oil completely or partly.

Claims

1. Dispersion comprising water, pulverized coal and additives, the coal content ranging from 60 to 80 % by weight, c h a r a c t e r i z e d in that the dispersion contains at least one amphiphile substance which is adsorbed to the surfaces of the coal particles and 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.

2. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that the amphiphile substance is lecithin or another zwitterionic surface-active substance.

3. Dispersion as claimed in claim 1, 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 polyethyleneoxide with a dominating amount of polyethyleneoxide.

4. Dispersion as claimed in claim 3, c h a r a c t e r i z e d in that the molecular weight of the copolymer ranges from 5,000 to 50,000.

5. Dispersion as claimed in claim 4, 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.

6. Dispersion as claimed in claim 1, c h a r a c t e r i z e d in that one of the polymers is a polysaccharide.

7. Dispersion as claimed in claim 6, c h a r a c t e r i z e d in that the polysaccharide is xanthan gum or guar gum.

8. 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.