Methods and apparatus for recycling tail gas in syngas fermentation to ethanol

1. A method of converting a carbonaceous feedstock to a syngas-fermentation product, said method comprising:

(a) introducing a carbonaceous feedstock and an oxidant to a gasifier, under suitable gasification conditions to produce a raw syngas stream comprising CO, H 2 , and CO 2 ;

(b) optionally feeding at least a portion of said raw syngas stream to a syngas-cleanup unit, to produce an intermediate syngas stream;

(c) feeding at least a portion of said raw syngas stream and/or at least a portion of said intermediate syngas stream, if present, to an acid-gas removal unit, to remove at least some of said CO 2 and produce a conditioned syngas stream;

(d) feeding at least a portion of said conditioned syngas stream to a fermentor, under suitable fermentation conditions and in the presence of suitable microorganisms and nutrients to biologically convert one or more of CO, H 2 , or CO 2 to a syngas-fermentation product;

(e) capturing a tail gas from an exit of said fermentor, wherein said tail gas comprises at least CO 2 and unconverted CO or H 2 ;

(f) recycling a first amount of said tail gas to said fermentor in an amount described by R 1 , the volumetric ratio of said first amount to said tail gas, wherein R 1 is selected from 0 to 1; and

(g) recycling a second amount of said tail gas to said acid-gas removal unit in an amount described by R 2 , the volumetric ratio of said second amount to said tail gas, wherein R 2 is selected from 0 to 1,

wherein R 1 +R 2 is greater than 0; and

wherein R 1 +R 2 is not greater than 1.

2. The method of claim 1 , wherein said carbonaceous feedstock comprises biomass.

3. The method of claim 1 , wherein said oxidant comprises one or more of air, oxygen, and steam.

4. The method of claim 1 , wherein said gasifier is a fluidized-bed gasifier.

5. The method of claim 1 , comprising feeding at least a portion of said raw syngas stream to a syngas-cleanup unit, to produce an intermediate syngas stream.

6. The method of claim 1 , wherein said acid-gas removal unit additionally removes at least some H 2 S, if present.

7. The method of claim 1 , wherein said first amount of said tail gas is compressed before being recycled to said fermentor.

8. The method of claim 1 , wherein said second amount of said tail gas is compressed before being recycled to said acid-gas removal unit.

9. The method of claim 1 , wherein said first amount and said second amount of said tail gas are separately compressed before being recycled to said fermentor and said acid-gas removal unit, respectively.

10. The method of claim 1 , further comprising a reformer disposed between said gasifier and said acid-gas removal step.

11. The method of claim 1 , further comprising recycling cells from said fermentor back to said gasifier.

12. The method of claim 1 , wherein the total conversion of CO and H 2 is at least five percentage points higher than the total conversion of CO and H 2 that is attained in a comparable method with R 1 and R 2 both equal to 0.

13. An apparatus for converting a carbonaceous feedstock to a syngas-fermentation product, said apparatus comprising:

(a) a gasifier for gasifying a carbonaceous feedstock with an oxidant, for producing a raw syngas stream comprising CO, H 2 , and CO 2 ;

(b) an optional syngas-cleanup unit in communication with said gasifier, for producing an intermediate syngas stream from at least a portion of said raw syngas stream;

(c) an acid-gas removal unit in communication with said syngas-cleanup unit, if present; or in communication with said gasifier, if no syngas-cleanup unit is present; for removing at least some of said CO 2 and producing a conditioned syngas stream;

(d) a fermentor in communication with said acid-gas removal unit, for biologically converting one or more of CO, H 2 , or CO 2 to a syngas-fermentation product;

(e) a tail gas conduit in communication with said fermentor; and

(f) a recycle conduit in communication with said tail gas conduit for recycling tail gas to said fermentor.

14. An apparatus for converting a carbonaceous feedstock to a syngas-fermentation product, said apparatus comprising:

(a) a gasifier for gasifying a carbonaceous feedstock with an oxidant, for producing a raw syngas stream comprising CO, H 2 , and CO 2 ;

(b) an optional syngas-cleanup unit in communication with said gasifier, for producing an intermediate syngas stream from at least a portion of said raw syngas stream;

(c) an acid-gas removal unit in communication with said syngas-cleanup unit, if present; or in communication with said gasifier, if no syngas-cleanup unit is present; for removing at least some of said CO 2 and producing a conditioned syngas stream;

(d) a fermentor in communication with said acid-gas removal unit, for biologically converting one or more of CO, H 2 , or CO 2 to a syngas-fermentation product;

(e) a tail gas conduit in communication with said fermentor; and

(f) a recycle conduit in communication with said tail gas conduit for recycling tail gas to said acid gas removal unit.