Process for depolymerizing coal to co-produce pitch and naphthalene
A method of depolymerizing coal includes preparing a high temperature depolymerizing medium consisting of heavy hydrocarbon oils and mixing it with coal to form a mixture, performing an optional first distillation at a temperature below 250° C. to recover naphthalene, heating the mixture to a temperature between 350° C. and 450° C. to create a digested coal, centrifuging the digested coal to remove ash and obtain a centrate, and distillation of the centrate into separate fractions. The high temperature depolymerizing medium may be a heavy hydrocarbon with a hydrogen to carbon (H/C) ratio higher than 7.0% and may include liquids chosen from the group consisting of: coal tar distillate, decant oil, anthracene oil, and heavy aromatic oils. The high temperature depolymerizing medium may be blended with an oil, preferably with H/C ratio higher than 10.0%, such as soybean oil, other biomass derived oil, lignin, petroleum oil, pyrolysis oil such that the overall hydrogen-to-carbon mass ratio in a digestion reactor is over 7.0% for the mixture of depolymerizing medium and coal. The depolymerized coal is an aromatic liquid that can itself be, either wholly or in part, a depolymerizing medium so that the process can be repeated.
1. A method of depolymerizing coal, comprising:
preparing a high temperature depolymerizing medium consisting of heavy hydrocarbon oils and mixing it with coal to form a mixture;
performing a first distillation at a temperature below 250° C. equivalent atmospheric pressure to recover naphthalene;
heating the mixture to a temperature between 350° C. and 450° C. for a period of at least one minute to create a digested coal slurry;
optionally centrifuging the slurry to produce a centrate liquid with ash content less than 0.5% by mass; and
optionally distilling the centrate liquid to produce a pitch residue with hydrogen content between 4.0% and 5.0%, and optimally with a softening temperature of about 110° C. and ash level less than 0.5% by mass.
2. The method of claim 1 , wherein the high temperature depolymerizing medium is a heavy hydrocarbon with H/C ratio higher than 7.0%.
3. The method of claim 1 , wherein the high temperature depolymerizing medium consists of liquids chosen from the group consisting of: coal tar distillate, decant oil, anthracene oil, and heavy aromatic oils.
4. The method of claim 1 , wherein the high temperature depolymerizing medium is blended with an oil, preferably with H/C ratio higher than 10.0%, chosen from the group consisting of: soybean oil, pine tar, aromatic petroleum distillate, biomass gasification tar, biomass pyrolysis tar, and oils where the overall hydrogen-to-carbon mass ratio in a digestion reactor is over 7.0% for the mixture of high temperature depolymerizing medium and coal.
5. The method of claim 1 , wherein the coal is chosen from the group consisting of bituminous coal, sub-bituminous coal and lignite coal.
6. The method of claim 1 , wherein the coal has an H/C ratio of 7.0% or higher, volatile content of 28% or higher, and ash content less than 10.0%.
7. The method of claim 1 , wherein the coal is dried via air drying or heating to above 100° C. prior to exposing the coal to the depolymerizing medium.
8. The method of claim 7 , wherein the coal is heated in a water-insoluble liquid to above 100° C.
9. The method of claim 8 , wherein the water-insoluble liquid is the high temperature polymerizing medium.
10. The method of claim 1 , in which the centrate, either wholly or after being separated into different fractions, is used as the depolymerizing medium in subsequent executions of the method.
11. The method of claim 1 , wherein the mixture comprises a ratio of 1 part coal to a range of at least 1.5 parts to 2.5 parts of depolymerizing medium.
12. The method of claim 1 , wherein the mixture is loaded into a pressurized vessel at a temperature of not less than 380° C. and not more than 420° C. and an operating pressure between about 200 psig and 1500 psig.
13. The method of claim 1 , wherein the mixture is loaded into a pressurized vessel at a pressure of about 400 psig and temperature of about 400° C.
14. The method of claim 1 , wherein centrifuging is done at a temperature of at least 100° C., or above the softening temperature of the liquid constituents of the coal digest.
15. The method of claim 1 , wherein the coal is a low rank non-coking coal with ash below 5% by mass, wherein the low rank non-coking coal is used to produce a pitch by virtue of having exchanged molecules with the aromatic depolymerizing medium.
16. The method of claim 15 , wherein the pitch is converted to a solid carbon coke, in an oxygen-depleted environment at above 600° C., and also co-produces thermally liberated volatile molecules.
17. The method of claim 15 , wherein the coal is selected such that it has less than 6.0% ash.
18. The method of claim 1 , further comprising, distilling the centrate liquid to further separate it into different fractions according to boiling point.
19. The method of claim 16 , wherein said solid carbon coke is selected from the group consisting of: a metallurgical grade coke, a foundry grade coke, feedstock for other furnace grade carbon such as injection carbon, charge carbon or recarburizer carbon.
20. The method of claim 1 , wherein the depolymerizing medium has an H/C ratio of less than 0.70, and the coal is selected such that its H/C ratio is greater than 0.75, thus resulting in an overall digest with H/C ratio greater than 0.70 on an ash-free basis.
21. The method of claim 20 , wherein a bio-oil is not present, and hydrogenation is not used to enhance the hydrogen content of the depolymerizing medium.
22. The method of claim 1 , further comprising, filtering the slurry to produce a centrate liquid with ash content less than 0.5% by mass.
23. The method of claim 1 , wherein the ash content is less than 0.2% by mass.
24. The method of claim 1 , wherein the high temperature depolymerizing medium is blended with an oil selected from the group consisting of: a biomass-derived oil, a lignin, a petroleum oil, a pyrolysis oil, and an oil from vegetable matter.
25. The method of claim 1 , wherein when the optional centrifugation step is skipped, an ash containing medium suitable for conversion to a solid carbon containing not more than 12% ash content is produced.