An improved process for the pyrolysis of carbonous materials

Description

[0001] The present invention relates to an ⁱnproved process for the pyrolysis of carbonous materials, such as coal, to produce liquid and gaseous products, by the use of iron and/or zinc soluble salts prior to pyrolysis.

[0002] Coal, once the leading source of energy in the United States, is beginning to play a more important role in the nation's energy future. The primary reason for the growing importance of coal is the rapid depletion of known petroleum and natural gas reserves. These known reserves are being depleted at a rate considerably faster than the rate of discovering new reserves. As the era of petroleum growth draws to a close, the world's commercial energy mix will have to change. Transition energy sources will be needed as a bridge between petroleum and the potentially unlimited energy sources of the future; such sources being, for example, solar power and nuclear fusion. Owing to their great abundance, coal and oil shale are perceived as the keystones of such a bridge. Consequently, much work is presently in progress to provide economical ways of converting these resources to valuable liquid and gaseous products. Coal liquefaction and pyrolysis processes in which coal, with or without a diluent, is subjected to elevated temperatures and pressures to convert solid coal to normally liquid hydrocarbonaceous products, are well known.

[0003] Pyrolysis of coal to yield liquids and char is an area of technology which has the potential of leading the way to a successful national synfuels program. The major limitations of present coal pyrolysis technology are the low quality of simple pyrolysis liquids combined with high sulfur content. Even when a coal in relatively low sulfur content is employed to reduce the need for hydrotreating, the product is generally low in yield and has stability problems.

[0004] By the practice of the present invention, pyrolysis in the presence of hydrogen is advantageously combined, under certain conditions, to give increased yields and more stable liquid products than those by use of pyrolysis processes conventionally known.

[0005] In accordance with the present invention, there is provided a process for converting carbonous materials . selected from subbituminous coal, oil-shale, lignite and peat, to liquid and gaseous product.s, said process comprising: (a) impregnating the carbonous material with one or more water soluble salts of iron and/or zinc; (b) treating the impregnated material in such a way as to precipitate, into the structure of the material, the metal of the soluble salt as its oxide or in a form which can be readily converted to its oxide by decomposition; and (c) pyrolyzing the treated impregnated material in the presence of hydrogen, at a temperature from about 400°C to about 700°C, and at gas residence times of less than about 30 seconds and solids residence times of from about 5 to 100 minutes.

[0006] In preferred embodiments of the present invention subbituminous coal or oil-shale is impregnated with iron sulfate, treated with sodium carbonate, and pyrolyzed at a temperature of about 550°C.

[0007] Any type of coal or oil shale may be treated according to the present invention, although for coal it is preferred to treat only the lower rank coals such as, subbituminous coal, lignite, and peat; of course, other similar solid carbonous materials may also be employed. Such coals usually have the following character: carbon content ranging from about 55 to 88 wt. %, hydrogen content ranging from about 3.8 to 6.2 wt. %, oxygen content ranging from about 2.6 to 33 wt. % (MAF basis), and a H/C ratio from about 0.3 to 1.1. In addition, oil shales, particularly those with high aromatic content, can be treated in accordance with the present invention.

[0008] It is preferred that the carbonous material have as high a surface area as possible; although, it is not economically justifiable to pulverize it to a very fine powder. Consequently, it is desirable to expose as much of the material's surface area as possible without losing material as dust or fines or as the economics of grinding or process equipment may dictate. Generally, for purposes of this invention, the material will be ground to a finely divided state and will contain a majority of,particles less than about 4 mesh, U.S. Sieve Size. The coal may be dried by conventional drying techniques, for example, heating to a temperature of about 100°C to 110°C.

[0009] The carbonous material, after grinding, is impregnated with a water soluble salt of a metal selected from iron and zinc by forming a slurry of coal and salt solution. Non-limiting examples of water soluble salts suitable for use herein include iron chloride, iron sulfate, iron nitrate, zinc chloride and zinc sulfate. More preferred is iron sulfate.

[0010] The salt solution is prepared by dissolving enough of the salt in water to result in at least a O.Ol_M solution. Preferably a 0.lM solution is employed. It is desirable to dissolve the salt in enough polar solvent, preferably water so as to have about the same weight of water as the weight of carbonous material. The salt solution and carbonous material are slurried at a temperature from about room temperature (20°C) to about the boiling point of the solution. Generally. about 1 part of salt solution by weight is employed for each part.of carbonous material by weight.

[0011] After the carbonous material has been slurried with the salt solution for an effective amount of time, it is treated in such a way as to precipitate, into the structure of the carbonous material, the metal of the soluble salt employed, in a well dispersed form as its oxide or in a form which can be readily converted to the oxide by decomposition. An effective amount of time, as used in this context, means at least that amount of time needed to ensure substantially'total impregnation of the salt solution into the structure of the carbonous material. This amount of time is primarily dependent on the moisture content of the carbonous material. For example, impregnation of dry coal is achieved in a matter of minutes whereas impregnation of wet coal may require about 24 hours owing to transfer problems of the salt into the wet coal structure.

[0012] After the carbonous material is impregnated with the water soluble salt, it is treated in such a way as to cause precipitation, into the structure of the carbonous material, of the metal of the soluble salt used for impregnation. The precipitated metal will nest likely be in a well dispersed fonn, preferably as its oxide, or in a form which can be readily converted to its oxide by decomposition. Non-limiting examples of such methods which can be employed herein, to cause precipitation, include: (a) raising the pH of the slurry to an effective pH to cause precipitation of the metal as a hydroxide or hydrated oxide; (b) introducing an anion into the slurry in form of another soluble salt which will produce the oxide of the metal or which will produce a form which can readily be converted to its oxide by decomposition, for example at the pyrolysis conditions employed herein.

[0013] Non-limiting examples of cations of which this other anion producing soluble salt is comprised include sodium, ammonium and potassium. Considerations which should be kept in mind when choosing such other salt as well as any other aqent used to cause precipitation: (a) not adversely affect the pyrolysis products or their evolution and recovery, (b) be economically feasible, and (c) be non-toxic.

[0014] If the method used for precipitating the metal of the impregnating salt employed herein is to raise the pH, an appropriate base is introduced into the slurry. Non-limiting examples of bases suitable for being employed herein include ammonia, a caustic solution, and an organic base. The pH to which the impregnated carbonous material slurry would have to be raised to affect precipitation would vary depending upon the solubility product of the salts employed. Generally a pH in excess of about 11 will precipitate most salts.

[0015] If the method used to precipitate the metal of the impregnating salt employed is to introduce an anion into the slurry, the anion chosen is one which will produce an insoluble form of the metal of the soluble salt employed herein for impregnating. Such an insoluble form should be one which is its oxide already or one which can be readily decomposed, under the pyrolysis conditions employed herein, to its oxide. Non-limiting examples of anions suitable for such purposes include carbonates, bicarbonates, hydroxides, etc.

[0016] After treating, so as to precipitate the metal in a well-dispersed form, it is preferred to dry the treated carbonous material. Any conventional drying means may be employed.

[0017] The treated and preferably dried carbonous material is then subjected to pyrolysis at temperatures from about 400°C to about 700°C, preferably from about 450°C to 600°C. The pyrolysis is performed in the presence of hydrogen, generally at hydrogen pressures from about 500 to 2500 psi. Any excess hydrogen, of course, will be recycled. Furthermore, the pyrolysis is performed at relatively short gas and long solids residence times. That is, the gas residence time should be less than about 30 seconds, preferably less than 10 seconds and the solids residence times should be from about 5 to 100 minutes, preferably from about 10 to 30 minutes. By choosing the proper residence time, substantially maximum conversion of carbonous material to liquids and gases are achieved and undesirable secondary reactions are minimized. Such undesirable secondary reactions include irreversible condensation reactions of molecules which will lead to char.

[0018] Non-limiting examples of reactors suitable for use herein include fixed and fluid bed. The primary constraint on the reactor employed is to minimize contact with the solid phase with hydrogen at reaction temperatures.

[0019] The following examples more fully des,- cribe the manner of practising the above-described invention, as well as to set forth the best nodes contemplated for carrying out various aspects of the invention.

EXAMPLE 1

[0020] 200g of Rawhide subbituminous coal-was washed with 1.0% _HC1 solutions until no calcium appeared in the wash. This also improved the wetability of the coal. The coal was then washed with distilled water until the wash was neutral. The washed coal was then dried in a vacuum oven at 105°C for three hours after which it was treated with an equal weight of a 10% by weight solution of zinc chloride. An equal weight, based on the weight of dry coal, of a 10% by weight solution of sodium carbonate was added. The mixture was allowed to stand overnight at room temperature after which it was dried in a vacuum oven at 105°C for three hours.

[0021] 150g of this dried and treated coal was pyrolyzed in a fixed bed type reactor, by heating it at a rate of 6°C/min. to 600°C-and holding it at about that temperature for 20 minutes. The fixed bed was an annular configuration having the dimensions of 4" ID x 4 1/2" O_D x 6" deep, and had a hydrogen flow rate of 2 standard cu. ft/min.

[0022] The total liquid product collected was 39.3 wt. % of the dry coal.

Comparative Example A

[0023] The procedure of Example 1 was followed except the coal was not treated with salt solutions..After pyrolysis the untreated coal produced a liquid product of 30.2 wt. % based on the weight of the dry coal.

[0024] These examples illustrate the advantages of employing the present invention to obtain maximum amounts of liquid product from coal by pyrolysis.

EXAMPLE 2

[0025] The procedure of Example 1 above was followed except ferrous ammonium sulfate was substituted for zinc chloride and 1% by weight solution of sodium carbonate was employed to precipitate ferrous hydroxide into the coal structure. The iron content of the impregnating solution was 0.14 wt. %.

[0026] Pyrolysis was carried out by heating the sample, under 400 psi hydrogen pressure, to 300°C rapidly then utilizing a heating rate of 100°C/30 min., to 500°C.

[0027] The total conversion of dry coal to volatile (licuid and gas) products was 52.8 wt. %.

Comparative Example B

[0028] The procedure of Example 2 above was followed except the coal was not treated with the salt solutions before pyrolysis. After pyrolysis, it was found that 43.0 wt. % based on the weight of the dry coal, had been converted to volatile products.

Claims

1. A method for converting a carbonous material selected from subbituminous coal, oil-shale, lignite, and peat, to liquid and gaseous products, which comprises:

(a) impregnating the carbonous material with one or more water soluble salts of iron and/or zinc;

(b) treating the impregnated carbonous material in such a way as to precipitate, into the structure of the carbonous material, said metal as its oxide or in a form which can be readily converted to its oxide under the pyrolysis conditions of step (c) below; and

(c) pyrolyzing the treated carbonous material in the presence of hydrogen, at a temperature from 400°C to 700°C, at a gas residence time of less than 30 seconds and a solids residence time of from 5 to 100 minutes.

2. A method as claimed in claim 1, wherein the water soluble salt is iron chloride, iron sulfate, iron nitrate, zinc chloride or zinc sulfate.

3. A method as claimed in claim 1 or claim 2, wherein the carbonous material is first dried before impregnation.

4. A method as claimed in any one of claims 1-3, wherein the pH of the carbonous material is raised to cause the said precipitation step (b).

5. A method as claimed in any one of claims 1-4, wherein the precipitation step (b) comprises treatment with one or more other salts selected from carbonates, bicarbonates, and hydroxides, preferably of sodium, ammonium or potassium.

6. A method as claimed in any preceding claim , wherein the pyrolysis step is conducted at a temperature of 450°C to 600°C..

7. A method as claimed in any preceding claim, wherein the pyrolysis step is conducted at a gas residence time of less than 10 seconds and a solids residence time of 10 to 30 minutes.

8. A method as claimed in any preceding claim, wherein the pyrolysis step is conducted at a hydrogen pressure of 500 to 2500 psi.

9. A method as claimed in any preceding claim, wherein the carbonous material is subbituminous coal.

10. Liquid and gaseous products wherever produced accoxding to any of the preceding claims.