Method for avoiding expensive sour water stripper metallurgy in a gasification plant

A method of producing synthesis gas (syngas) comprising:

• (a) converting a carbon containing fuel to raw syngas in a gasification unit, wherein the carbon containing fuel comprises nitrogen in an amount of equal to or greater than about 0.5 wt. %, and wherein the raw syngas comprises carbon monoxide, hydrogen, cyanide, and solid particulates;
• (b) introducing at least a portion of the raw syngas and water to a syngas scrubber to remove at least a portion of the solid particulates from the raw syngas to produce unshifted syngas, and wherein a molar ratio of hydrogen to carbon monoxide in the raw syngas is about the same as a molar ratio of hydrogen to carbon monoxide in the unshifted syngas;
• (c) introducing a first portion of the unshifted syngas to a first cooling unit to produce a cooled unshifted syngas and a first aqueous condensate, wherein the first aqueous condensate comprises cyanide in an amount of from bout 20 ppmw to about 100 ppmw;
• (d) recycling at least a portion of the first aqueous condensate to the syngas scrubber;
• (e) introducing a second portion of the unshifted syngas to a water gas shift unit to produce a shifted syngas, wherein a molar ratio of hydrogen to carbon monoxide in the shifted syngas is greater than the molar ratio of hydrogen to carbon monoxide in the unshifted syngas, and wherein at least a portion of the cyanide in the second portion of the unshifted syngas is converted to ammonia in the water gas shift unit;
• (f) introducing at least a portion of the shifted syngas to a second cooling unit to produce a cooled shifted syngas and a second aqueous condensate, wherein the second aqueous condensate comprises cyanide in an amount of less than about 1 ppmw;
• (g) contacting at least a portion of the cooled shifted syngas with at least a portion of the cooled unshifted syngas to produce modified syngas, wherein a molar ratio of hydrogen to carbon monoxide in the modified syngas is lower than the molar ratio of hydrogen to carbon monoxide in the shifted syngas, wherein the unshifted syngas is characterized by a molar ratio of hydrogen to carbon monoxide of from about 0.5:1 to about 1.25:1, and wherein the modified syngas is characterized by a molar ratio of hydrogen to carbon monoxide of from about 1.7:1 to about 3.2:1;
• (h) introducing at least a portion of the second aqueous condensate to a sour water stripper to produce an acid gas and stripped water, wherein the acid gas comprises hydrogen sulfide, carbon dioxide, and ammonia, and wherein the sour water stripper and associated piping and equipment are fabricated from materials comprising carbon steel;
• (i) communicating a pump-around stream from the sour water stripper to a third cooling unit to produce a cooled pump-around stream, wherein at least a portion of the cooled pump-around stream is recycled to the sour water stripper, wherein the cooled pump-around stream comprises cyanide in an amount of less than about 20 ppmw, wherein the cooled pump-around stream is characterized by a pH of from about 7.5 to about 11, and wherein the third cooling unit and associated piping and equipment are fabricated from materials comprising carbon steel;
• and
• (j) introducing at least a portion of the acid gas to a sulfur recovery unit to produce sulfur.