GB2079780A - Agglomeration of coal - Google Patents

Abstract

In a method of forming coal fines into agglomerates which can be gasified with steam and oxygen in known gasifiers the coal fines are crushed to a powder, mixed with a calcareous hydraulic binding agent such as Portland cement, moistened with water, shaped into agglomerates and hardened. Coal fines which would otherwise be unsuitable can thus be charged to a gasifier and materials which improve the performance of the gasifier may be introduced into the charge.

Description

SPECIFICATION Improvements in or relating to agglomeration of coal This invention relates to coal treatment processes and in particular to a method for the preparation from coal fines of coal agglomerates which may be gasified with steam and oxygen.

Gasification of coal with steam and oxygen is a rapidly developing technology likely to find wide application in exploiting the world's coal reserves. Very large plants are envisaged using the whole output of dedicated mines, which would have to include the fine coal of less than about 5mm in size. Depending on the nature of the coal and the mining techniques in use, the proportion of fine coal produced could be more than half the output of the mine. Consequently ensuring that the fines can somehow be gasified is an important aspect of gasification technology.

A fixed bed gasifier, for example a Lurgi gasifier or a slagging gasifier such as that disclosed in British Patent Specification 977 1 22 and in Gas Council Research Communications GC 50 and GC112, is normally charged with coal in the size range 5-50 mm and up to about 10% of the total fuel supplied may be coal fines injected at the tuyeres. Before the whole output of a mine could be gasified with such plant, a large part of the fine coal would need to be converted into a form which would allow it to be incorporated into the main charge.

A similar need is encountered when fine coal is to be used for metallurgical purposes or upgraded to domestic fuel, for which it is known to form the coal fines into agglomerates by briquetting or rolling in pans or drums using pitch, water soluble organic materials such as starch and sugars, and certain industrial waste liquors such as spent sulphite solutions as binders. An alternative is to bind the coal particles by heating the coal to its softening point.

It is an object of this invention to form coal fines into agglomerates in a manner which is particularly advantageous when the coal is to be gasified with steam and oxygen.

According to the invention there is provided a method for forming coal fines into agglomerates suitable for gasification with steam and oxygen, which method comprises: (a) crushing the coal fines to powder, (b) mixing the crushed coal fines with a calcareous hydraulic binding agent in powder form.

(c) moistening the mixture with water, (d) shaping the moistened mixture into discrete pieces and (e) drying and curing the shaped pieces so that they harden.

In preparing a suitable powder, it is preferred to crush the coal fines such that the whole of the powder will pass through a sieve with an aperture size of 500 microns.

Of the wide range of calcareous hydraulic binding agents, including lime, which may be used and which are available either as naturally occurring substances or as manufactured products, the preferred material is Portland cement.

The proportion of binding agent to be mixed with the coal fines powder, as is explained later, will not always be governed solely by the strength of the agglomerates produced. In general, it is preferred that the mixture of crushed coal fines and calcareous hydraulic binding agent should contain not more than 20% wt. of the binding agent. More preferably, when the nature of the coal ash is such that only the strength of the agglomerates need be considered. the mixture of crushed coal fines and calcareous hydraulic binding agent should contain from 5 to 10% wt of binding agent.

The weight of water to be used to moisten the mixture of coal powder and binding agent preferably ranges from a third to a half the weight of the dry mixture.

Apparatus well known in other applications may be used to produce agglomerates of suitable shape. Conveniently, the mixture of crushed coal fines and cement may be fed to an inclined rotating pan or drum into which water is sprayed. As the pan rotates, round agglomerates form and grow to the required size by accretion of freshly added mixture. Agglomerates so formed may be allowed to dry and harden without further treatment or may be dried by heating. Strong agglomerates in the size range 5-50 mm which maintain their strength when pyrolysed in air, have been made in this manner.

In comparison with pitch, the binder commonly used in agglomerating coal fines, Portland cement is a considerably cheaper and more readily available material. But these are not its only advantages. Apart from the mechanical requirement to charge coal in a particular size range into a gasifier, there is the advantage with a slagging gasifier that the major component of Portland cement (CaO, approx 65%) improves the flow characteristics of the slag, making it easier to remove.The viscosity of the slag is greatly influenced by the 'silica ratio' of the coal ash:

SiO2 'silica ratio' = x 100 SiO2 + Fe203 + CaO + MgO (percentages in ash) High silica ratios adversely affect operation of the gasifier in that higher temperatures are required to maintain adequate mobility of the slag. Thus, when gasifying coals of high silica ratio, blast furnace slag and dolomite are commonly mixed into the coal charge as fluxes, with the penalty that the gas manufacturing cost is increased.

In comparison with blast furnace slag, Portland cement contains much more CaO and much less SiO2 and is more effective in reducing silica ratio. Dolomite, though it does not contain silica, introduces carbon dioxide into the product gas, thereby imposing a greater load on the carbon dioxide removal plant associated with the gasifier; Portland cement does not have this drawback. With a charge composed partly of agglomerated fines, the cement will behave as a flux for the slag from the whole charge.

A further advantage is that agglomerates made with Portland cement are porous, enabling catalysts for the gasification reaction to be readily introduced and distributed uniformly by subsequent impregnation with a solution of catalytic substances. Alternatively, catalytic substances may be added as powder with the cement or dissolved in the water used during manufacture of the agglomerates.

Another advantage arising from the use of Portland cement as a binder is that the caking and swelling characteristics of the coal, which are important in determining its suitability for gasification with steam and oxygen, are changed for the better. With coals which otherwise cake and swell it has been found that agglomerates prepared by the method of the invention suffer little or no change when heated to about 600"C in a nitrogen atmosphere. The proportion of cement required to prevent swelling and caking depends on the nature of the coal but 20% wt has eliminated such behaviour with a coal (Pittsburgh No. 8 seam) which is known to be singularly difficult to gasify because of its marked swellings and caking properties.The significance of this finding is that it can be exploited to make certain coals more amenable to gasification and even enable coals at present wholly unsuitable to be used in a gasifier.

The invention is illustrated by the following examples.

Example 1 For this example a quantity of NCB Rossington coal fines was dried and crushed to provide a powder having the following sieve analysis: 300 micron sieve retained 1.75% wt 212 13.25 150 34.05 106 12.8 75 17.5 53 18.0 remainder 2.6 5 Portland cement powder was thoroughly mixed into portions of the crushed coal to give cement proportions of 2.5, 5, 7.5, 10 and 20%. wt. Agglomerates were then prepared using a simple pelletiser, revolving at a constant speed of about 30 revolutions per minute, by adding some water to a batch of coal-cement mix before loading into the drum and spraying more water into the drum as the agglomerates formed. The agglomerats so formed were allowed to dry at room temperature for 72 hours.

The strength of the agglomerates produced has been measured in two ways. Firstly, about 25 pieces from each batch have been individually dropped from a height of 9 metres onto a bed of agglomerates from the same batch. The proportion surviving impact, for each proportion of added cement, is shown in Table 1. The majority of breakages gave only two fragments.

Table 1.

Cement content, % wt Survival rate, % 20 100 10 48 7.5 31 5 13 2.5 23 The second test made use of a proprietary machine (Heberlein Tablet Hardness Tester: Model 2E) which measures the load required to crush an agglomerate. The results obtained are shown in Fig. 1.

Agglomerates containing 2.5, and 10% wt of cement have also been subjected to a qualitative test of their behaviour when heated under pressure. In this test, a few agglomerates are placed one above the other in a silica tube which is then enclosed within a pressure vessel and heated at a controlled rate of 40"C per minute to about 600"C in flowing nitrogen at a pressure of 350 Ib/in2.

In each case the agglomerates remained whole and showed no signs of caking or swelling.

Example 2 For this example, further batches of agglomerates were prepared and tested as in Example 1 using Pittsburgh No. 8 coal powder having the following sieve analysis: 300 micron sieve retained 0.35% wt 212 3.2 150 8.5 106 11.6 75 15.4 53 50.6 remainder 10.35 The proportions of cement used were 2.5, 5, 10 and 20% wt. The results of the strength tests are shown in Table 2 and Fig. 2.

Table 2.

Cement content, % wt Survival rate, % 20 92 10 47 5 27 2.5 7 In the test of behaviour when heated under pressure, the results shown in Table 3 were obtained.

Table 3.

2.5% cement Swelling of agglomerates comparable with that of coal pieces.

5% cement Swelling of aggomerates approx.

half that of coal pieces.

10% cement Some swelling of agglomerates but only enough to distort their shape.

20% cement Negligible swelling of agglomerates.

Claims (8)

1. A method for forming coal fines into agglomerates suitable for gasification with steam and oxygen, which method comprises: (a) crushing the coal fines to a powder, {b) mixing the crushed coal fines with a calcareous hydraulic binding agent in powder form; (c) moistening the mixture with water {d) shaping the moistened mixture into discrete pieces and (e) drying and curing the shaped pieces so that they harden.

2. A method as claimed in Claim 1 in which the coal fines are crushed such that the whole of the powder will pass through a sieve with an aperture size of 500 microns.

3. A method as claimed in Claim 1 or Claim 2 in which the mixture of crushed coal fines and calcareous hydraulic binding agent contains up to 20% wt of the binding agent.

4. A method as claimed in Claim 1 or Claim 2 in which the mixture of crushed coal fines and calcareous hydraulic binding agent contains from 5 to 1 0% wt of the binding agent.

5. A method as claimed in Claims 1 to 4 in which the calcareous hydraulic binding agent is Portland cement.

6. A method as claim in Claims 1 to 4 in which the calcareous hydraulic binding agent is lime.

7. A method as claimed in any of the preceding claims in which the weight of water used to moisten the mixture of coal powder and binding agent ranges from a third to a half the weight of the dry mixture.

8. A method for forming coal fines into agglomerates according to Claim 1 and substantially as hereinbefore described.