Systems and methods for producing substitute natural gas

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

gasifying a carbonaceous feedstock in the presence of an oxidant within a gasifier to provide a raw syngas;

cooling the raw syngas within a cooler having distinct first second, and third zones for heat exchange, wherein the raw syngas is cooled within the first zone to provide a first cooled syngas;

exchanging heat within the second zone from the first cooled syngas to a first steam and a second heat transfer medium to provide a second cooled syngas and a second steam comprising superheated high pressure steam;

exchanging heat within the third zone from the second cooled syngas to a boiler feed water to provide a third cooled syngas and a feed water condensate;

processing the third cooled syngas within a purification system to provide a treated syngas, wherein the purification system comprises a saturator adapted to increase a moisture content of the third cooled syngas;

introducing the treated syngas and a first heat transfer medium to a methanator to provide a synthetic gas, the second heat transfer medium, and a methanation condensate; and

recycling at least a portion of the methanation condensate from the methanator to the saturator.

2. The method of claim 1 , wherein the superheated high pressure steam has a temperature of about 450° C. to about 750° C. and a pressure of about 4,000 kPa to about 22,100 kPa.

3. The method of claim 1 , wherein gasifying the carbonaceous feedstock in the presence of the oxidant occurs in a fluidized reaction zone operated at a temperature of about 816° C. to about 1,066° C.

4. The method of claim 1 , wherein the methanation condensate comprises about 0.01 mol % to about 1 mol % carbon monoxide, about 0.001 mol % to about 0.5 mol % hydrogen, about 0.01 mol % to about 1 mol % carbon dioxide, about 0.001 mol % to about 0.5 mol % methane, and about 0.001 mol % to about 0.2 mol % nitrogen, and wherein the raw syngas comprises about 40 mol % to about 80 mol % hydrogen, up to about 10 mol % nitrogen, up to about 3 mol % argon, about 15 mol % to about 25 mol % carbon monoxide, and up to about 40 mol % carbon dioxide.

5. The method of claim 1 , wherein the saturator provides a saturated syngas, and further comprising introducing at least a portion of the saturated syngas to a hydrolysis device to provide a hydrogen sulfide syngas.

6. A method for producing a synthetic gas, comprising:

gasifying a feedstock in the presence of an oxidant within a gasifier to provide a raw syngas;

cooling the raw syngas within a cooler having distinct first, second, and third zones for heat exchange, wherein the raw syngas is cooled within the first zone to provide a first cooled syngas;

exchanging heat within the second zone from the first cooled syngas to a first steam and a second heat transfer medium to provide a second cooled syngas and a second steam comprising superheated high pressure steam at a temperature of about 450° C. to about 750° C. and a pressure of about 4,000 kPa to about 22,100 kPa;

exchanging heat within the third zone from the second cooled syngas to a boiler feed water to provide a third cooled syngas and a feed water condensate;

processing the third cooled syngas within a purification system to provide a treated syngas, wherein the purification system comprises a saturator adapted to increase a moisture content of the cooled syngas;

introducing the treated syngas and a first heat transfer medium to a methanator to provide a synthetic gas, a second heat transfer medium, and a methanation condensate; and

recycling at least a portion of the methanation condensate from the methanator to the saturator.

7. The method of claim 6 , wherein the methanation condensate comprises about 0.01 mol % to about 1 mol % carbon monoxide, about 0.001 mol % to about 0.5 mol % hydrogen, about 0.01 mol % to about 1 mol % carbon dioxide, about 0.001 mol % to about 0.5 mol % methane, and about 0.001 mol % to about 0.2 mol % nitrogen.

8. The method of claim 6 , further comprising removing particulates from the third cooled syngas with a particulate control device within the purification system to provide a filtered syngas.

9. The method of claim 8 , wherein the saturator adapted to increase a moisture content of the third cooled syngas provides a saturated syngas.

10. The method of claim 9 , further comprising recycling at least a portion of the methanation condensate from the methanator to the saturator, wherein the superheated high pressure steam has a temperature of about 650° C. to about 750° C. and a pressure of about 11,000 kPa to about 22,100 kPa.

11. The method of claim 9 , further comprising introducing at least a portion of the saturated syngas to a gas shift device to provide a shifted syngas.

12. The method of claim 11 , further comprising:

introducing at least a portion of the saturated syngas to a hydrolysis device to provide a hydrogen sulfide syngas; and

removing ammonia from the hydrogen sulfide syngas with an ammonia scrubber to provide a scrubbed syngas.

13. The method of claim 12 , further comprising introducing the shifted syngas and at least a portion of the scrubbed syngas to an acid gas removal device to provide the treated syngas.

14. The method of claim 12 , further comprising recycling at least a portion of the scrubbed syngas to the gasifier.

15. A method for producing a synthetic gas, comprising:

gasifying a feedstock in the presence of an oxidant within a gasifier to provide a raw syngas;

cooling the raw syngas within a cooler having distinct first, second, and third zones for heat exchange, wherein the raw syngas is cooled within the first zone to provide a first cooled syngas;

exchanging heat within the second zone from the first cooled syngas to a first steam and a second heat transfer medium to provide a second cooled syngas and a second steam comprising superheated high pressure steam, wherein the superheated high pressure steam has a temperature of about 450° C. to about 750° C. and a pressure of about 4,000 kPa to about 22,100 kPa;

exchanging heat within the third zone from the second cooled syngas to a boiler feed water to provide a third cooled syngas and a feed water condensate;

processing the third cooled syngas within a purification system to provide a treated syngas, wherein the purification system comprises a saturator adapted to increase a moisture content of the third cooled syngas to provide a saturated syngas;

introducing the treated syngas and a first heat transfer medium to a methanator to provide a synthetic gas, the second heat transfer medium, and a methanation condensate;

introducing at least a portion of the saturated syngas to a hydrolysis device to provide a hydrogen sulfide syngas;

recycling at least a portion of the methanation condensate from the methanator to the saturator;

introducing at least a portion of the saturated syngas to a gas shift device to provide a shifted syngas;

removing ammonia from the hydrogen sulfide syngas with an ammonia scrubber to provide a scrubbed syngas; and

introducing the shifted syngas and at least a portion of the scrubbed syngas to an acid gas removal device to provide the treated syngas.

16. The method of claim 15 , further comprising recycling at least a portion of the scrubbed syngas to the gasifier.

17. The method of claim 15 , further comprising removing particulates from the third cooled syngas with a particulate control device within the purification system to provide a filtered syngas.

18. The method of claim 15 , further comprising recycling at least a portion of the methanation condensate from the methanator to the saturator, wherein the superheated high pressure steam has a temperature of about 650° C. to about 750° C. and a pressure of about 11,000 kPa to about 22,100 kPa.

19. The method of claim 18 , wherein the methanation condensate comprises about 0.01 mol % to about 1 mol % carbon monoxide, about 0.001 mol % to about 0.5 mol % hydrogen, about 0.01 mol % to about 1 mol % carbon dioxide, about 0.001 mol % to about 0.5 mol % methane, and about 0.001 mol % to about 0.2 mol % nitrogen.