EP0152506A1 - Process for the oxidative conversion of coal - Google Patents

Abstract

The macromolecular coal structure is broken down by oxidative attack by means of peroxoacids or peroxoanions to form low-molecular products. The oxidising agent is dissolved in an aqueous electrolyte, and the reaction takes place below the boiling point. The peroxo compounds used are preferably regenerated electrochemically in the process. This can be carried out either in the reactor containing the coal or in an external reactor. Apart from the production of low-molecular compounds from coal, this process can also be used for breaking down coal underground and thus converting it into an easily extractable form.

Description

The invention relates to a method for converting coal in an aqueous electrolyte, this being carried out by oxidation using peroxo anions, which are either added to the electrolyte or generated electrochemically in the electrolyte.

The increasing scarcity and the uncertainties in the availability of mineral oil have intensified the search for alternative energy sources and chemical raw material sources. In addition to reactivating the classic coal upgrading processes, new methods for the chemical use of coal are also gaining in importance. Furthermore, in view of the large depths in which German hard coal can be found, methods of mining are of interest with which the coal can be converted underground into an easily extractable form (e.g. liquefaction). In addition to the classic processes of thermal coal conversion such as coal gasification, liquefaction and coking, there has been no lack of attempts to develop new processes which should work in particular at lower temperatures and normal pressure.

From publications and patents, efforts are known to convert coal under the mild conditions mentioned using electrochemical processes. (RW Coughlin, G. Kreysa: Chem.-Ing.-Tech. 55, 8, 1983; CS Lynch, AR Colett: Fuel 11 (1932) p. 408; RE Markby HW Sternberg, I. Wender: Nature 199 (1963 ); HW Sternberg, CL Delle Donne, RE Markby, I. Wender: Fuel 45, (1966) p. 469; RW Coughlin, M. Farooque: Nature 279, (1979) p. 301; M. Farooque, RW Coughlin: Fuel 59, (1979) p. 705; RW Coughlin, M. Farooque: Ind. Eng. Chem., Process Des. Dev. 19, (1980) p. 211; RW Coughlin, M. Farooque: J. Appl. Electrochem . 10, (1980) p. 729; VA Vaseen: US Pat. 4,226,683 (1981); G. Kreysa, G. Collin: DPA 3150729.8 (1981); G. Kreysa, W. Kochanek: J. Elec. Soc., in press; PM Dhooge: J. Electrochem. Soc. 130, 1539 (1983)).

In this work, coal suspensions are either electrochemically reduced in a non-aqueous solvent and thus partially hydrogenated, or electrochemically oxidized in an aqueous solvent. It is also known from the work published by Bockris and Kreysa that the electrochemical oxidation of the coal is not a direct electrode reaction, but rather that the oxidation takes place by means of redox mediators (eg Fe ³⁺ or Ce ⁴⁺ ) present in the electrolyte, from which the higher oxidation level at the electrodes is regenerated. In all of this work, it has so far not been possible to dissolve the coal to any appreciable extent and thus to convert it into liquid products. Only small amounts of dissolved substances, primarily carboxylic acids of the humic acid type, are formed, while well over 90% of the coal used is retained as a solid. After this electrochemical-oxidative treatment, the remaining coal shows a changed elemental composition (see the Kreysa work cited above).

The object of the present invention is to develop an effective method for converting and dissolving coal in aqueous electrolytes, in which the macromolecules of the coal are changed or reduced in such a way that a considerable proportion of them goes into solution.

The invention is based on the principle of using peroxy acids or peroxo anions instead of the oxidizing agents (such as Fe ³ ; Ce ⁴ ; Mn0 ₄ , Cr ₂ 0 ₇ 2) previously described in the literature, the effects of which on coal have not been described previously. The use of peroxyacids or peroxo anions offers, in addition to the fact that they still have to be demonstrated, that they cause extensive chemical attack on the coal, the advantage that they can be generated in situ in a simple manner from the corresponding acids or anions in the coal suspension .

The object of the invention is achieved in that the coal is reacted in a strong acid with peroxo acids or peroxo anions or hydrogen peroxide. The preferred oxidizing agents are peroxodisulfuric acid and Caro's acid, which are either produced electrochemically in the sulfuric acid or sulfate-containing electrolyte by anodic oxidation or are added to the electrolyte as peroxodisulfate. Under the strongly acidic reaction conditions, peroxodisulfuric acid or Caro's acid is then also present in the latter case. When the reaction has ended, the possibly unreacted coal is filtered off and a deep brown colored filtrate is obtained which contains the converted coal in dissolved form. The separation of the dissolved organic products can then be carried out using various work-up techniques which are known in principle and are explained in more detail in the examples.

• 1. In der Kohle vorhandene hydrochinoide OH-Gruppen, Methyl- und Methylen-Gruppen werden durch Peroxosäuren und Peroxoanionen teilweise zu Keto-Gruppen oxidiert, wie dies auch von anderen Oxidationsmitteln bekannt ist (vgl. die oben zitierte Literatur).
• 2. In der Kohle von vornherein vorhandene oder in der oxidierenden Umgebung gebildete Keto-Gruppen reagieren im Sinne der Baeyer-Villiger-Oxidation zu den entsprechenden Estern (Baeyer, Villiger; Ber. 32, 3625 (1899), C.H. Hassall; Org. React. 9, 73 (1957)).
• 3. Die durch Baeyer-villiger Oxidation gebildeten Ester, werden in dem stark sauren Elektrolyten säurekatalysiert verseift, wobei die entsprechenden Alkohole bzw. Phenole und Carbonsäuren entstehen.
• 4. Die durch Verseifung entstehenden Phenole und Alkohole unterliegen in dem stark oxidierenden Medium einer weiteren Oxidation zu Aldehyden bzw. Carbonsäuren und werden damit kontinuierlich aus dem Verseifungsgleichgewicht entfernt.

In the oxidative dissolution of coal according to the invention by means of peroxo acids or peroxo anions, the following reaction steps take place in principle, as can be seen from our own investigations and from the behavior of various organic compounds described in the literature, which can be regarded as model compounds for chemical groups present in the coal.

• 1. Hydrochinoid OH groups, methyl and methylene groups present in the coal are partially oxidized to keto groups by peroxy acids and peroxo anions, as is also known from other oxidizing agents (cf. the literature cited above).
• 2. Keto groups present in the coal from the start or formed in the oxidizing environment react in the sense of the Baeyer-Villiger oxidation to the corresponding esters (Baeyer, Villiger; Ber. 32, 3625 (1899), CH Hassall; Org. React 9, 73 (1957)).
• 3. The esters formed by Baeyer-villiger oxidation are acid-saponified in the strongly acidic electrolyte, the corresponding alcohols or phenols and carboxylic acids being formed.
• 4. The phenols and alcohols formed by saponification are subject to further oxidation to aldehydes or carboxylic acids in the strongly oxidizing medium and are thus continuously removed from the saponification equilibrium.

The decisive reaction, which, in contrast to all other oxidizing agents previously investigated in connection with coal, causes the coal to dissolve to a large extent, is obviously the Baeyer-Villiger oxidation, to which only peroxo acids, peroxo anions and hydrogen peroxide are capable.

The following examples demonstrate the feasibility of the process according to the invention and demonstrate some of the possible process variants. In the examples, finely ground (grain size <500 µm) hard coal (Westerholt coal) was always used. Peroxodisulfuric acid or sodium peroxodisulfate were used as examples of the oxidizing agent.

example

1.5 g of starting carbon are suspended in 250 ml of 85% sulfuric acid with stirring. After twice adding 7.5 g of sodium peroxodisulfate, the mixture is stirred at 35 ° C. for 12 hours. The unreacted coal is then filtered off. A deep brown colored filtrate is obtained and 0.3 g of the coal used is retained as a filter cake.

The acidic filtrate is neutralized, concentrated in vacuo and, after the addition of concentrated hydrochloric acid, evaporated to dryness. The organic compounds contained in the salt residue can be dissolved by acetone, methanol or pyridine.

Example 2:

1.5 g of starting coal are suspended in 100 ml of 85% sulfuric acid with stirring. A 1 cm ² platinum anode and a platinum cathode separated by a ceramic diaphragm were located in the reaction vessel. Electrolysis was carried out at an anodic current density of 1 A / cm ² at 30 ° C. for 24 h. After the reaction had been terminated, the remaining coal was filtered off from the solution as in Example 1, and the same deep brown colored filtrate as in Example 1 was obtained. Working up was carried out analogously to Example 1.

Example 3:

1.5 g of starting carbon were suspended in 250 ml of 85% sulfuric acid and stirred at 35 ° C. as indicated in Example 1 for 12 hours. The coal was then filtered off. The entire amount of coal used is retained and the clear sulfuric acid is obtained as the filtrate.

This experiment shows that the coal dissolves only under the action of the added or electrochemically generated in situ peroxodisulfate.

Example 4:

1.5 g of starting carbon are suspended in 100 ml of 70% perchloric acid and 7.5 g of sodium peroxodisulfate are added. The suspension is moderately stirred at 35 ° C for 24 h. After filtration, 0.8 g of the coal used is retained and a deep brown colored filtrate is obtained. The organic compounds contained in the acidic filtrate can be extracted directly with acetophenone after dilution with a little water, leaving behind a clear, very slightly yellowish aqueous phase. The acetophenone phase is colored deep brown. The organic compounds present in the acetophenone can be extracted back into the aqueous phase using strong alkalis (e.g. sodium hydroxide solution).

Example 5:

100 ml of a 70% perchloric acid are added to 22 g of sodium sulfate (10-hydrate). By electrolysis on a platinum anode with a current density of 1.A / cm ² at 35 ° C it could be shown that peroxodisulfate is formed in this solution with a current efficiency of 60%. 40% of the amount of electricity used leads to the formation of oxygen. This shows in principle that the coal dissolution can also be carried out indirectly electrochemically in perchloric acid.

Example 6:

1.5 g of starting carbon were suspended in 100 ml of 70% perchloric acid and stirred at a temperature of 35 ° C. for 12 hours. It was then filtered as in the previous examples. You win back all of the coal used and get a clear, not colored one Filtrate. This shows that even in perchloric acid the coal dissolves only under the oxidizing effect of peroxodisulfate (or peroxodisulfuric acid or Caro's acid).

Example 7

The deep brown sulfuric acid-containing filtrate from Example 1 was again mixed with 10 g of sodium peroxodisulfate and reacted with stirring at 35 ° C. for 48 hours. The deep brown color changes to a yellow-brown color by post-oxidation.

Example 8_

The perchloric acid filtrate from Example 4 was post-oxidized with peroxodisulfate as described in Example 7. The color of the initially deep brown solution changed from a deep orange to a yellow.

Example 9:

The yellow perchloric acid solution from Example 8 obtained by post-oxidation was reduced to about 50% perchloric acid with zinc after being diluted slightly with water. The initially odorless solution smells clearly of organic compounds after the reduction.

Claims (12)

1. A process for the oxidative conversion of coal, characterized in that it is reacted with peroxy acids, peroxo anions or hydrogen peroxide or a mixture of these reagents. 2. The method according to claim 1, characterized in that the oxidizing agents (peroxo acids, peroxo anions or hydrogen peroxide) are added directly to the reaction mixture. 3. The method according to claim 1, characterized in that the oxidizing agents used are only generated chemically or preferably electrochemically in the reaction mixture from suitable precursors. 4. The method according to claim 1, characterized in that the oxidizing agents used from suitable precursors are first generated outside the reactor in which the reaction with the coal takes place chemically or preferably electrochemically and then fed to the reaction mixture. 5. Process according to claims 1, 3 and 4, characterized in that the electrochemical generation of the oxidizing agent on the Unused parts of this oxidizing agent on the cathode can be reduced if these interfere with the separation or processing of the organic products, or another redox system is reduced, or the organic substances formed are reduced, or hydrogen is evolved 6. Process according to claims 1 to 4, characterized in that sulfuric acid is used as the electrolyte and peroxodisulfuric acid or Caro's acid or its salts are used as the oxidizing agent. 7. Process according to claims 1 to 4, characterized in that perchloric acid is used as the electrolyte and peroxodisulfuric acid or Caro's acid or its salts serve as the oxidizing agent. 8. The method according to claims 1 to 7, characterized in that the coal to be dissolved is previously pre-oxidized in another reaction step with another oxidizing agent (eg Fe ³⁺ or Ce ⁴⁺ . 9. The method according to claims 1 to 7, characterized in that the pre-oxidizing agent according to claim 8 and the main oxidizing agent according to claim 1 are used simultaneously. 10. The method according to claims 1 to 9, characterized in that after separation of the coal, the initially obtained aqueous solution with the organic ingredients is post-oxidized with an oxidizing agent according to claim 1 or an oxidizing agent according to claim 8. 11. The method according to claims 1 to 10, characterized in that the initially obtained or obtained by post-oxidation aqueous solution of the coal directly electrochemically or chemically a reducing treatment, preferably with zinc, under will throw. 12. Application of the method according to claims 1 to 11 for mining underground coal, which is converted into a dissolved and thus easily extractable form by the method.