METHOD AND SYSTEM FOR CONTROLLING MERCURY EMISSIONS FROM COAL-FIRED THERMAL PROCESSES

1 . A method, comprising:

selecting a mercury-containing feed material natively including mercury and halogen, the halogen comprising one or more of bromine and iodine;

contacting the mercury-containing feed material with a halogen-containing additive to form a treated feed material, the halogen-containing additive comprising one or more of bromine and iodine, wherein the one or more of bromine and iodine concentration of the treated feed material relative to a weight of the feed material is about 30 ppmw or less;

generating, from the treated feed material, a mercury-containing gas stream comprising vapor-phase mercury and vapor-phase halogen derived from the one or more of bromine and iodine; and

removing 50% or more of the mercury from the gas stream.

2 . The method of claim 1 , wherein the one or more of bromine and iodine comprises bromine and wherein the mercury-containing feed material has from about 0.003 to about 100 ppmw mercury.

3 . The method of claim 1 , wherein the one or more of bromine and iodine comprises iodine, wherein the halogen-containing additive facilitates the vapor-phase mercury to be removed from the mercury-containing gas stream, and wherein the mercury-containing feed material natively comprises no more than about 5 ppmw halogen.

4 . The method of claim 1 , wherein the mercury-containing feed material natively has one of:

(i) no more than about 5 ppmw iodine;

(ii) no more than about 4 ppmw iodine;

(iii) no more than about 3 ppmw iodine

(iv) no more than about 2 ppmw iodine; or

(v) no more than about 1 ppmw iodine.

5 . The method of claim 1 , wherein the mercury-containing feed material comprises a high alkali, high iron, and/or high sulfur coal.

6 . The method of claim 1 , wherein the halogen-containing additive comprises iodine and/or bromine.

7 . The method of claim 1 , wherein the halogen-containing additive comprises at least one of an iodine containing coal, an alkali halide, an alkaline earth halide, and diatomic halide.

8 . The method of claim 1 , wherein at least one of the following is true:

(i) the mercury-containing gas stream comprises about 3.5 ppmw or less of the vapor-phase halogen derived from the treated feed material;

(ii) in the mercury-containing gas stream, a molar ratio of the vapor-phase halogen derived from the treated feed material to the vapor-phase mercury derived from the treated feed material is no more than about 600; and

(iii) at an air preheater outlet, a concentration of the vapor-phase halogen derived from the treated feed material ranges from about 0.1 to about 10 ppmw.

9 . The method of claim 1 , wherein the mercury-containing feed material is combusted in a thermal unit to produce the mercury-containing gas stream.

10 . The method of claim 1 , further comprising passing the mercury-containing gas stream through a particulate removal device to form a treated gas stream free of particulates and mercury.

11 . A method, comprising:

contacting a mercury-containing feed material comprising a feed material iodine with a halogen-containing additive to from a treated feed material having an iodine content of about 30 ppmw or less, wherein the halogen-containing additive comprises one or more of bromine and iodine and wherein no more than about 2 ppmw of the treated feed material iodine comprises the feed material iodine;

generating, from the treated feed material, a gas stream comprising vapor-phase mercury and vapor-phase iodine derived from the treated feed material; and

removing 50% or more of the vapor-phase mercury from the gas stream.

12 . The method of claim 11 , wherein the one or more of bromine and iodine comprises iodine and wherein the mercury-containing feed material has from about 0.003 to about 100 ppmw mercury.

13 . The method of claim 11 , wherein the one or more of bromine and iodine comprises bromine, wherein the mercury containing feed material natively comprises bromine, and wherein the bromine concentration is:

(i) no more than about 10 ppmw of bromine; or

(ii) no more than about 5 ppmw of bromine.

14 . The method of claim 11 , wherein the halogen-containing additive comprises at least one of an iodine containing coal, an alkali iodide, an alkaline earth iodide, and diatomic iodine.

15 . The method of claim 11 , wherein the halogen-containing additive comprises iodine and:

(i) no more than about 25% of halogens other than iodine;

(ii) no more than about 10% of halogens other than iodine; or

(iii) no more than about 5% of halogens other than iodine.

16 . The method of claim 11 , wherein the halogen-containing additive comprises iodine and bromine.

17 . The method of claim 11 , wherein at least one of the following is true:

(iv) the mercury-containing gas stream comprises about 3.5 ppmw or less of the vapor-phase halogen derived from the treated feed material;

(v) in the mercury-containing gas stream, a molar ratio of the vapor-phase halogen derived from the treated feed material to the vapor-phase mercury derived from the treated feed material is no more than about 600; and

(vi) at an air preheater outlet, a concentration of the vapor-phase halogen derived from the treated feed material ranges from about 0.1 to about 10 ppmw.

18 . A method, comprising:

selecting a first mercury-containing feed material that natively includes mercury and less than about 2 ppmw iodine;

selecting a second mercury-containing feed material that natively includes mercury and greater than about 2 ppmw iodine;

mixing, in no particular order, the first mercury containing feed material, the second mercury containing feed material, and a halogen-containing additive to form a treated feed material, wherein the halogen-containing additive comprising one or more of iodine and bromine, and wherein the treated feed material has a halogen concentration relative to a weight of the treated feed material of about 30 ppmw or less;

generating, from the treated feed material, a mercury-containing gas stream comprising vapor-phase mercury and vapor-phase halogen derived from the treated feed material; and

removing 50% or more of the mercury from the gas stream.

19 . The method of claim 18 , wherein the second mercury-containing feed material natively has one of:

(i) greater than about 3 ppmw iodine;

(ii) greater than about 4 ppmw iodine; or

(iii) greater than about 5 ppmw iodine.

20 . The method of claim 18 , wherein the halogen-containing additive comprises iodine and the treated feed material has a total iodine concentration relative to a weight of the treated feed material of about 30 ppmw or less.

21 . The method of claim 18 , wherein the halogen-containing additive comprises bromine.