Systems and methods for producing substitute natural gas

1. A method for producing synthetic natural gas, comprising:

gasifying a carbonaceous feedstock to produce a raw syngas;

cooling the raw syngas to produce a cooled raw syngas;

methanating the cooled raw syngas to produce a synthetic natural gas, a first liquid, and heat;

removing a portion of any volatile components in the first liquid to produce a gaseous product and a second liquid; and

gasifying at least a portion of the gaseous product with the carbonaceous feedstock.

2. The method of claim 1 , further comprising producing steam from the heat produced by methanating the cooled raw syngas.

3. The method of claim 2 , wherein the steam is at a temperature of about 450° C. or more and a pressure of about 4,000 kPa or more.

4. The method of claim 2 , further comprising expanding at least a portion of the steam to produce power.

5. The method of claim 1 , further comprising converting at least a portion of any carbon monoxide in the cooled raw syngas to carbon dioxide prior to methanating the cooled raw syngas.

6. The method of claim 1 , further comprising contacting the cooled raw syngas with at least a portion of the second liquid to produce a saturated syngas, wherein the saturated syngas is cooled and methanated to produce the synthetic natural gas, first liquid, and heat.

7. The method of claim 1 , wherein the raw syngas comprises about 1 mol % or more methane.

8. The method of claim 1 , wherein the synthetic natural gas comprises about 85 mol % to about 100 mol % methane.

9. The method of claim 1 , wherein the gaseous product comprises ammonia and gasifying the ammonia produces nitrogen and hydrogen.

10. A method for producing synthetic natural gas, comprising:

gasifying a carbonaceous feedstock to produce a raw syngas;

cooling the raw syngas by indirectly exchanging heat from the raw syngas to a first steam product to produce a cooled raw syngas and a second steam product;

purifying the cooled raw syngas to produce a treated syngas;

converting the treated syngas to synthetic natural gas, wherein conversion of the treated syngas further produces a first liquid and heat;

producing the first steam product from the heat produced by methanating the cooled raw syngas;

flash separating the first liquid to produce a gaseous product and a second liquid;

gasifying at least a portion of the gaseous product with the carbonaceous feedstock; and

expanding at least a portion of the second steam product to produce power.

11. The method of claim 10 , wherein purifying the cooled raw syngas comprises:

hydrolytically treating the cooled raw syngas to produce a syngas lean in carbonyl sulfide; and

scrubbing the syngas lean in carbonyl sulfide to produce a waste water comprising ammonia and the treated syngas.

12. The method of claim 11 , further comprising flash separating the waste water with the first liquid to produce the gaseous product and the second liquid.

13. The method of claim 10 , wherein the raw syngas comprises at least 3 mol % methane.

14. The method of claim 10 , wherein the power is converted to electrical power, and wherein at least a portion of the electrical power is used to satisfy at least a portion of an electrical power load required to produce the synthetic natural gas.

15. The method of claim 10 , further comprising contacting the cooled raw syngas with at least a portion of the second liquid to produce a saturated syngas, wherein the saturated syngas is cooled and methanated to produce the synthetic natural gas, first liquid, and heat.

16. The method of claim 10 , wherein the second steam product is at a temperature of about 450° C. or more and a pressure of about 4,000 kPa or more.

17. A system for producing synthetic natural gas comprising:

a gasifier for gasifying a carbonaceous feedstock to produce a raw syngas;

a heat exchanger for cooling the raw syngas to produce a cooled raw syngas;

a methanator for methanating the cooled raw syngas to produce a synthetic natural gas, a first liquid, and heat;

a flash separator for removing a portion of any volatile components in the first liquid to produce a gaseous product and a second liquid; and

a conduit in fluid communication with the gasifier and the flash separator for introducing the gaseous product to the gasifier, wherein at least a portion of the gaseous product is gasified with the carbonaceous feedstock.

18. The system of claim 17 , further comprising a saturator for contacting the cooled raw syngas with at least a portion of the second liquid to produce a saturated syngas.

19. The system of claim 17 , further comprising a purification unit for treating the cooled raw syngas prior to methanating the cooled raw syngas, wherein the purification unit comprises at least one of a carbon monoxide shift converter, a carbonyl sulfide hydrolysis unit, an ammonia scrubber, a particulate control device, and a saturator.

20. The system of claim 17 , further comprising a pipeline for transporting at least a portion of the synthetic natural gas.