EP0118440B1

EP0118440B1 - Coal-water dispersion and method of the manufacture thereof

Info

Publication number: EP0118440B1
Application number: EP82903239A
Authority: EP
Inventors: Lars Lennart Stigsson; Björn LINDMAN
Original assignee: Fluidcarbon International AB
Current assignee: Fluidcarbon International AB
Priority date: 1982-04-05
Filing date: 1982-11-12
Publication date: 1988-03-02
Also published as: DK558483D0; ES519418A0; FI834462A0; WO1983003617A1; BR8306789A; CA1216425A; IT8224584A0; FI834462A; AU1223183A; ES8402341A1; IT1207967B; EP0118440A1; NO834432L; DK558483A; EP0105878A1; WO1983003618A1; JPS59500520A

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

The present invention relates to the manufacture of a coal-water dispersion which is very stable in storage and is suitable for transport and direct energy production without preceding dewatering. More particularly the invention relates to the manufacture of a dispersion comprising water, pulverized coal and additives including polymers, the coal content amounting to 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, 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 although large resources have been spent on technical 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 heating oil, methanol, etc.
The main problem of water dispersion of pulverized coal is to make 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 81/01152, 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 polyphosphate. 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 mentioned above but cannot be used directly for energy production without preceding dewatering.
EP-A-0,041,337 describes dispersions containing water and 65% by weight of powdered coal and as a dispersing agent a quaternary ammonium synthetic polymer.
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 polyelectrolytes 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 without preceding dewatering by using the technique available today possibly with minor modifications.
The invention relates to a method of manufacturing a dispersion containing water and pulverized coal, the coal content ranging from 60 to 85% by weight, and further including at least one polymer containing segments of hydrophobic as well as hydrophilic character, and lecithin which is adsorbed to the surfaces of the coal particles and provides repulsion between the coal particles by hydration forces, said polymer co-operating with the lecithin, and according to the invention this method has obtained the characterizing features appearing from claim 1.
The dispersion obtained by the method 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 equipment.
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 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 the forces between surfaces mutually spaced some Angstrom. Atheoret- ical 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 be 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.
The invention will be described in more detail below with reference to an example.

- Example

1. Lecithin is first dissolved in water by the addition of an additive having the property of creating soluble mixed aggregates with lecithin, viz. an organic salt, e.g. an alkali salt of a fatty acid or an alkali sorbate, to form a surfactant preparation.
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 200 pm (preferably <150 pm) 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 in 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 hydrophilic anionic or non-ionic polymers may be 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 polyethyleneoxide, copolymers of the polyethyleneoxide-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 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 precipitator.
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.
The invention provides 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 heavy oil have shown that the dispersion is very suitable to replace oil completely or partly.

Claims

1. Method of manufacturing a dispersion containing water and pulverized coal, the coal content ranging from 60 to 85% by weight, and further including at least one polymer containing segments of hydrophobic as well as hydrophilic character, and lecithin which is adsorbed to the surfaces of the coal particles and provides repulsion between the coal particles by hydration forces, said polymer co-operating with the lecithin, characterized in that the lecithin 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 alkali sorbate.

2. Method as claimed in claim 1 wherein the solid coal is pulverized by wet grinding and is then beneficated and dewatered, characterized in that the lecithin is supplied in the process of grinding or benefication or after the process of dewatering.