GB2259694A - Method and apparatus for powdered solids pressurization - Google Patents

Abstract

A powdered coal pressurization method and system for coal gasification. With a sufficient weight of material, powdered material such as coal can be removed from the bottom of a column (1) under a higher gas pressure than the solids (2) which are being fed at the top of the column. To this end, and to avoid a long column height, magnetic particles such as iron spheres (3) are mixed into the coal particles to increase the mixture density. The metal is separated (4a) at the bottom and recycled (5) using liquid e.g. water. The coal is then passed to the reactor using pressurised gas. <IMAGE>

Description

METHOD AND APPARATUS FOR POWDERED SOLIDS PRESSURIZATION The present invention relates to a method and apparatus for powdered solids pressurization.

In particular, the invention relates to a method and apparatus for powdered coal pressurization for a coal feeding system to a coal gasifier.

Finely divided solid carbon-containing fuels are applied in processes for the preparation of synthesis gas by the partial combustion of a finely divided solid carbon-containing fuel with an oxygen-containing gas in a reactor wherein liquid slag formed during the partial combustion process is removed through an outlet in the bottom of the reactor and passed by gravity through a slag discharge means into a water bath or slag quenching vessel where it is solidified by quenching.

The partial combustion of finely divided solid carbon-containing fuel with substantially pure oxygen as oxygen-containing gas yields synthesis gas mainly consisting of carbon monoxide and hydrogen. When the oxygen-containing gas is air or oxygen-enriched air, the synthesis gas formed of course also contains a substantial quantity of nitrogen. By finely divided solid carbon-containing fuel is generally meant coal or another solid fuel, such as brown coal, peat, wood, coke, soot etc., but mixtures of liquid or gas and particulate solid fuels, are also possible.

Advantageously, a moderator is also introduced into the reactor. The object of the moderator is to exercise a moderating effect on the temperature in the reactor. This is ensured by endothermic reaction between the moderator and the reactants.

Suitable moderators are steam and carbon dioxide.

The gasification is advantageously carried out at a temperature in the range from 1200 to 1700 "C and at a pressure in the range from 1 to 200 bar.

The reactor in which the preparation of synthesis gas takes place may have any suitable shape.

The supply of finely divided solid carbon-containing fuel and oxygen-containing gas to the reactor can take place in any manner suitable for the purpose and will not be described in detail.

Liquid slag formed in the partial combustion reaction drops down and is drained through the outlet located in the reactor bottom.

In partial oxidation processes of finely divided solid carbon-containing fuels, such as for example coal gasification, the fuel is fed from a supply device to a gasifier by means of a suitable carrier fluid.

The hot product gas usually contains sticky particles which lose their stickiness upon cooling.

The sticky particles in the hot product gas will cause problems in the plant where the product gas is further processed, since undesirable deposits of the particles on, for example, walls, valves or outlets will adversely affect the process. Moreover, such deposits are very hard to remove. The sticky particles may be partly or completely in the molten state; they may comprise metals, salts or ashes, and, in general, these particles lose their stickiness at a temperature below about 800 "C.

Therefore, the hot product gas is quenched in a quench section which is located above the product outlet on top of the reactor. In the quench section a suitable quench medium such as, for example, water or a gas is introduced into the product gas in order to cool the product gas.

As already indicated in the above the gasification is carried out under pressure in the gasifier.

Therefore, it is required to transport (atmospheric) solids against pressure to the reactor.

The current methods of (dry) powdered solids pressurization such as e.g. the lock hopper system are both expensive and require large numbers of potentially troublesome valves. In such a system solids are fed batchwise into a vessel which is isolated and pressurized with an inert gas, e.g. nitrogen. The pressurized vessel is then discharged into a pressurized handling system.

The failing of the system is the large number and large sizes of valves which are subject to erosion/abrasion.

Therefore, it is an object of the invention to provide an apparatus and a method for powdered solids pressurization in the coal gasification field which are continuous (not batch) and have a minimum number of pieces of equipment and valves.

Further, it is another object of the invention to provide such an apparatus and such a method having potential to both simplify the current practice as well as reduce the capital and operating cost.

The invention therefore provides a method for pressurizing finely divided solid carbonaceous fuel to be fed from a vessel to a gasifier comprising the steps of supplying pulverized fuel from a vessel to a vertical fuel pressurization column, adding metal particles to the said column, said metal particles having a much higher density than the pulverized fuel and mixing said particles with said pulverized fuel to increase the mixture density; creating gravitational particle flow; transporting the mixture as a moving bed; separating the metal particles from the pulverized fuel after leaving the said column; recycling the metal particles to the top of the column, and feeding the pressurized pulverized fuel to a coal feed vessel.

The invention also provides an apparatus for carrying out the said method comprising means for supplying pulverized fuel from a vessel to a vertical fuel pressurization column, means for adding metal particles to the said column, said metal particles having a much higher density than the pulverized fuel and means for mixing said particles with said pulverized fuel to increase the mixture density; means for creating gravitational particle flow; means for transporting the mixture as a moving bed; means for separating the metal particles from the pulverized fuel after leaving the said column; means for recycling the metal particles to the top of the column, and means for feeding the pressurized pulverized fuel to a coal feed vessel.

The invention is based upon the concept of increasing the density of a first powdered material by mixing with a second much heavier material that can be easily separated from the first material by means of physical properties (e.g. magnetic properties of iron) or size. This reduces the required height to a single pressurization column, so thac it is both economically feasible and practically realistic to apply the concept.

In particular, the invention represents a concept for the pressurization of powdered solids using gravity (i.e. via hydrostatic pressure). The basis for the concept is that with a sufficient weight of material, in this case a tall column of the powder, the powdered material can be removed from the bottom under a higher gas pressure than the solids which are being fed at the top. In order to avoid the high gas pressure of escaping through the column of solids, the velocity at which the solids move down through the column must exceed or be equal to the velocity at which gas can percolate upwards through the powdered material under the driving force of the applied pressure gradient.

However, for low density materials (e.g. pulverized coal), the column required to achieve desired pressure levels is for many applications too tall and would require construction as a series of shorter columns. This presents significant solids handling problems which increase the cost and decrease the utility of the concept.

The idea of the invention is that of mixing two dissimilar materials, one with a much higher density than the other, thereby increasing the density of the mixture to the point where the required column height is achievable in a single column.

The invention will now be described by way of example in more detail by reference to the accompanying drawings, in which: Fig. 1 represents schematically the principles of operation of the invention; and Fig. 2 represents and advantageous embodiment of the invention.

Referring now to fig. 1 a coal pressurization column 1 has been shown, having a height h. At its top atmospheric pulverized coal 2 and iron powder 3 are supplied by any means suitable for the purpose. At its outlet the iron/coal separation 4 takes place by any means suitable for the purpose and iron is recycled (5) to the top of the column, whereas the pressurized pulverized coal 6 is conveyed in any suitable manner by means of a carrier gas to a coal feed vessel 7 for the gasifier (not shown for reasons of clarity).

In fig. 2 the same reference numbers as used in fig. 1 are applied.

A tall bed la of pulverized coal and iron balls has been shown. Further, revolving magnetic drums 4a with demagnetized segment have been represented as a suitable means for iron/coal separation.

After separation the iron balls are transported through any suitable medium, e.g. water to the top of the column 1. Reference numeral 8 represents a circulation pump for transport of water and iron balls.

The overall effect of mixing a very high density material (e.g. iron) with the coal is to achieve significantly increased densities of the mixture at reasonable mixture ratios (roughly 50/50 mixture by volume of iron and coal). Furthermore, by appropriate selection of the size of the iron, easy separation of the iron from the coal is possible by means of aerodynamics (e.g.

fluidized bed and/or aerodynamic pickup of coal), physical sieving/sifting, and/or use of magnetic fields.

It will be appreciated that the metal is present as separate particles, thus non-impregnated, and is essentially a pure metal and, in particular, non-alloyed iron.

The invention will now be described by reference to the following example: Example The solid to be pressurized is coal powder (90% < 90 micron) 3 with a bulk density of 800 kg/m . This material is mixed with an equal (bulk) volume of iron balls with an average diameter of 3 mm 3 and a bulk density of 4700 kg/m . The 50/50 v/v blend has a density 3 of 2800 kg/m The coal has to be transported against a pressure of 2 MPa gauge. The height of the pressurization bunker must therefore have a minimum height of 73 m. A practical value is 80 m. Upon leaving the bunker the contents have to flow between two revolving drums of which the surface is magnetic and the iron balls will adhere to the drums and are thus separated from the coal. The coal is further transported by e.g. pneumatic means to the high pressure system.

The iron balls which are attached to the drums are taken over the rim of the coal bunker and released from the drum in a section where no magnetic fields are present. The balls then fall in a bath with water.

The whole system has a temperature of about 95 "C because the coal comes from the heated coal milling and drying section. The water leaving the water circulation pump is heated to a temperature of about 120 "C. The iron balls are transported to a point above the top of the pressurization bunker entrained as a 20% volume suspension in water. The slurry then passes over a sieve whereafter the balls are mixed with the coal by proper introduction of both solids at the top of the pressurization bunker. The transportation of the iron balls in water of above 100 "C makes that the water adhered to the iron balls after passing the sieve flashes off and the balls are dry before being blended with the coal. The water from the sieve is collected and runs back through a pipe to the section of the circulation pump.

Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims (9)

1. A method for pressurizing finely divided solid carbonaceous fuel to be fed from a vessel to a gasifier comprising the steps of supplying pulverized fuel from a vessel to a vertical fuel pressurization column, adding metal particles to the said column, said metal particles having a much higher density than the pulverized fuel and mixing said particles with said pulverized fuel to increase the mixture density; creating gravitational particle flow; transporting the mixture as a moving bed; separating the metal particles from the pulverized fuel after leaving the said column; recycling the metal particles to the top of the column, and feeding the pressurized pulverized fuel to a coal feed vessel.

2. The method as claimed in claim 1 wherein the metal particles consist of pure metal.

3. The method as claimed in claim 2 wherein the metal particles consist of non-alloyed iron.

4. The method as claimed in claim 2 or 3 wherein the metal particles consist of iron balls.

5. The method as claimed in any one of claims 1-4 wherein the metal particles consist of non-impregnated particles.

6. An apparatus for carrying out the method as claimed in any one of claims 1-5 comprising means for supplying pulverized fuel from a vessel to a vertical fuel pressurization column, means for adding metal particles to the said column, said metal particles having a much higher density than the pulverized fuel and means for mixing said particles with said pulverized fuel to increase the mixture density; means for creating gravitational particle flow; means for transporting the mixture as a moving bed; means for separating the metal particles from the pulverized fuel after leaving the said column; means for recycling the metal particles to the top of the column, and means for feeding the pressurized pulverized fuel to a coal feed vessel.

7. The apparatus as claimed in claim 6 comprising a magnetic means for separating the metal particles from the fuel.

8. Method for pressurizing finely divided solid carbonaceous fuel substantially as described in the description by reference to the example.

9. Apparatus for pressurizing finely divided solid carbonaceous fuel substantially as described in the description by reference to figs. 1 and 2 of the accompanying drawings.