GB2204877A - Coal pyrolysis pretreatment - Google Patents

Abstract

A pretreatment for flash pyrolysis of coal, giving increased tar yields, comprises contacting the coal at elevated temperature with a liquid which enters the coal pores but does not form char at pyrolysis temperatures. The liquid is preferably a polynuclear aromatic.

Description

COAL PYROLYSIS PRETREATMENT This invention concerns coal pyrolysis. Nore especially it concerns a coal pyrolysis pretreatment process which increases eventual tar yields.

Coal carbonisation has been the conventional source for coal tar, from which significant quantities of chemical feedstocks such as benzene are obtained. The production of coal tar from carbonisation has, however, been considerably reduced because of the worldwide fall in demand for metallurgical coke. Alternative methods for producing coal tar are therefore of potential commercial interest. Research carried out by a number of organisations has shown that entrained flow or fluidised bed pyrolysis of coal, in which rapid devolatisation of coal occurs, is capable of increasing significantly the yield of tar, compared to coal carbonisation in which yields are typically less than io: of dry ash free coal.The char from such processes could find a ready market as an energy source for power generation and generally contain less sulphur and chlorine than the initial coal.

We have suggested, in GB 2,061,998B, that quenching tar vapours from high yield pyrolysis processes, using a hydrogen donor liquid, prevents combination reactions occurring that would result in a hevier (higher molecular weight) tar. There remains, however, a need for a coal pyrolysis process which offers increased overall conversion and tar yields without the expense of hydropyrolysis.

The present invention provides a coal pyrolysis pretreatment process comprising treating particulate coal with a liquid capable of penetrating coal pores but which does not form significant amounts of char under subsequent pyrolysis of the coal, at an elevated temperature at which the liquid penetrates the coal pores and removing excess liquid so that a proportion of the liquid is retained within the coal structure. The invention also provides coal so pretreated, and further provides a coal pyrolysis process comprising the pretreatment as a first step and a flash pyrolysis as a second step.

It is preferred to use as the liquid substituted or unsubstituted polynuclear aromatic compounds, which may include heteroatoms in the ring structure. Routine testing can establish whether a particular liquid provides a desirable increase in tar yields compared to the same pyrolysis without pretreatment and does not cause char formation from cracking. For example, hydrogen donor solvents (or hydroaromatics), commonly used in coal liquefaction processes, crack at relatively low temperatures (below 500it), below the pyrolysis temperature where maximum tar yields ar obtained (about 600C), and are undesirable. The liquid may be a solid at room temperature.It need not be a single compound, but may be a mixture of compounds, e.g. a cut of a petroleum or coal- derived oil or product of processing such materials or a chemical process by-product, provided that it exhibit the correct characteristics. For example1 good results have been obtained in experimental work with l-methylnaphthalene and quinoline.

It is considered that the following theory applies to this invention, although we do not wish to be bound by it. Under the pretreatment conditions, the coal pore structure expands and the liquid penetrates the pores. Experimental work indicated that if the pretreated coal was cooled, drained and washed, about 3% or less, by weight of the dry, ash free coal, of solvent was imbibed, or trapped, in the inner pores. During flash pyrolysis, the trapped liquid is thought to play an important role in preventing char-forming reactions occurring.

It is preferred to carry out the pretreatment at a temperature in the range 180 to 300it, more especially from 200 to 250it. The pretreatment may conveniently be carried out at atmospheric pressure, although elevated pressures may be found useful. It is especially convenient to carry out the pretreatment at reflux temperatures for the liquid selected, for an hour or more. At the end of the pretreatment, excess liquid is desirably drained from the coal particles, which will retain liquid on the surface and in surface pores as well as in internal pores. The liquid may be recovered, for example, by collection of the correct fraction in the liquid products from pyrolysis.

The coal starting material may te y hlh volatile bituminous coal, including coking coals. The coal is finely divided, and suitable particle sizes are below about 250 microns.

It is preferred that the coal particles do not include any significant proportion of fines because of handling problems and their tendency to be elutriated from fluidised bed pyrolysers.

The pretreatment is desirably followed by a flash pyrolysis which will be understood to mean a pyrolysis process in which the coal is rapidly devolatilised, especially in which the coal is heated to 500 to 7000C and has a residence time of at least a few seconds. Fluidised bed or entrained flow pyrolysis processes are especially suitable. The gas used to fluidise or transport the coal, and to sweep the volatiles out of the pyrolysis reactor, may be any gas which does not adversely affect the pyrolysis process, and specifically may be mentioned nitrogen, carbon dioxide and carbon monoxide, and mixtures thereof.

The present invention will now be illustrated with reference to the Example below.

EXAMPLE Samples of coal from Linby Colliery, England (a high volatile, very weakly caking coal), of particle size between 75 and 150 microns, were refluxed for 1 hour in the desired pretreatment liquid. For experimental purposes, to eliminate the effect of free liquid on the mass balance during pyrolysis, the coal-liquid slurry was cooled and the liquid was drained off. Surface liquid, and liquid in surface pores, was removed by washing the coal particles vith n-pentane or methanol. Thermal gravimetric analysis was used to establish that the concentrations of trapped solvent in the internal pores were less than 3t by wt of daf coal.

5g samples of the treated coal were pyrolysed in an atmospheric pressure fluidised bed, by feeding the sample onto a fluidised bed of sand at the desired pyrolysis temperature, over a period of 15 minutes. The bed was fluidised with nitrogen at a velocity well. in excess of the minimum fluidising velocity. The gas exhaust from the bed was passed through two liquid nitrogen cooled traps for liquor, and the yield of char produced was determined from the increase in reactor weight. A steel wool plug was used to prevent loss of char and sand particles from the reactor. Tar soluble in dichloromethane was recovered from the liquid traps for analysis, phase separating paper being used to separate water from the liquor collected. The results obtained at a pyrolysis temperature of 6000C are summarised in the table below.

TABLE

Pretreatment None (Control) 1-methylnaphthalene Quinoline Washing solvent in pre-treatment None n-pentane methanol n-pentane methanol methanol Char 60 64 69 51 52.5 55 % daf C1 - C4 2.7 2.7 3.2 3.5 5.1 4.5 Coal hydrocarbons Tar & liquorA 35 32 26.5 43.5 41 39 % of tar soluble in 85 84 67 65 66 75 dichlormethane % Aromatic H of total H in tar 23 29 27 20 22 31 Key A = by difference, 100 - % ( char + gas), not all the tar could be trapped consistently It should be observed that the n-pentane and methanol washed controls show a reduction in tar yield because the washing solvents removed bitumen-like material which contributes to tar formation.

Significant improvements in tar yields were observed by pretreating the coal with l-methylnaphthalene and quinoline. Additional tests carried out with tetralin and 9,10-dihydro-phenanthrene, hydrogendonor materials known as active coal solvents, did not produce tar yield increases, but did increase the char formed. The tars obtained from the l-methylnaphthalene and quinoline treated coal samples appear slightly heavier than that normally obtained (measured by slightly higher concentrations of dichloromethane insoluble material present). However, the treated coal tars are not much more aromatic than normal by considering the aromatic hydrogen figures, confirming that only low concentrations of imbibed liquid are present in the tar (both liquids are highly aromatic and, if released during pyrolysis, would significantly increase the aromatic H values).

A series of tests at different pyrolysis temperatures for the untreated Linby coal and pyrolysed in nitrogen in similar nanner to the above, were carried out and the major yields are represented in the accompanying Figure. These tests confirm that the maximum tar yield is obtained close to 6000C.

Claims (10)

1. A coal pyrolysis pretreatment process comprising treating particulate coal with a liquid capable of penetrating coal pores but which does not form significant amounts of char under subsequent pyrolysis of the coal, at an elevated temperature at which the liquid penetrates the coal pores and removing excess liquid so that a proportion of the liquid is retained within the coal structure.

2. A process according to claim 1, wherein the liquid is selected from substituted or unsubstituted polynuclear aromatic ccmpounds, optionally including one or more heteroatoms in the ring structure.

3. A process according to claim 1 or 2, wherein the coal is treated by refluxing it with the liquid.

4. A process according to claim 3, wherein the pretreatment temperature is from 200 to 2500C.

5. A process according to any one of the preceding claims, wherein the coal is sized at below about 250 microns.

6. A process according to claim 1, substantially as hereinbefore described.

7. A coal pyrolysis process comprising a first step of a pretreatment according to any one of the preceding claims and a second step of a flash pyrolysis.

8. A pyrolysis process according to claim 7, wherein the second step comprises a fluidised bed or entrained flow pyrolysis.

9. A pyrolysis process according to claim 7 or 8, wherein the second step comprises heating the coal to 500 to 7000C for at least a few seconds.

10. A pyrolysis process according to claim 7, substantially as hereinbefore described.