Sorbents for the oxidation and removal of mercury

Abstract

A promoted activated carbon sorbent is described that is highly effective for the removal of mercury from flue gas streams. The sorbent comprises a new modified carbon form containing reactive forms of halogen and halides. Optional components may be added to increase reactivity and mercury capacity. These may be added directly with the sorbent, or to the flue gas to enhance sorbent performance and/or mercury capture. Mercury removal efficiencies obtained exceed conventional methods. The sorbent can be regenerated and reused. Sorbent treatment and preparation methods are also described. New methods for in-flight preparation, introduction, and control of the active sorbent into the mercury contaminated gas stream are described.

1 . A promoted carbon sorbent comprising a base activated carbon that has reacted with a promoter selected from the group consisting of halides, halogens, and combinations thereof, wherein the sorbent is effective for the removal of mercury from a gas stream located downstream of a boiler, said removal of mercury yielding a cleaned gas stream, and wherein the composition of the promoted carbon sorbent is controlled by the mercury content of the cleaned gas stream. 
   
   
       2 . The promoted carbon sorbent of  claim 1  wherein the base activated carbon is selected from the group consisting of powdered activated carbon, granular activated carbon, carbon black, carbon fiber, aerogel carbon, pyrolysis char, activated carbon with an average particle size greater than 40 micrometers, and combinations thereof, and the promoter is selected from the group consisting of molecular halogens, Group V halides, Group VI halides, hydrohalides, and combinations thereof. 
   
   
       3 . The promoted carbon sorbent of  claim 2  wherein the sorbent comprises an activated carbon matrix in a fixed or a moving bed structure. 
   
   
       4 . The promoted carbon sorbent of  claim 3  wherein the structure is selected from the group consisting of moving fiber screens and monoliths. 
   
   
       5 . The promoted carbon sorbent of  claim 2  further comprising an optional alkaline component selected from the group consisting of alkali elements, alkaline earth elements, alkali salts, alkaline earth salts, and combinations thereof. 
   
   
       6 . The promoted carbon sorbent of  claim 5  wherein the ratio of base activated carbon, the promoter, and the optional alkaline component is chosen for optimum economic removal of mercury from the gas stream. 
   
   
       7 . The promoted carbon sorbent of  claim 2  further comprising a mercury-stabilizing reagent selected from the group consisting of S, Se, H 2 S, SO 2 , H 2 Se, SeO 2 , CS 2 , P 2 S 5 , and combinations thereof. 
   
   
       8 . The promoted carbon sorbent of  claim 7  wherein the promoter is selected from the group consisting of Br 2 , HBr, and combinations thereof. 
   
   
       9 . The promoted carbon sorbent of  claim 7  further comprising an optional alkaline component comprising an alkaline or alkaline earth element wherein the promoter is selected from the group consisting of Br 2 , HBr, and combinations thereof. 
   
   
       10 . The promoted carbon sorbent of  claim 2  further comprising an optional secondary component comprising a halogen or a hydrohalide such that the reactivity and mercury capacity of the sorbent are enhanced. 
   
   
       11 . The promoted carbon sorbent of  claim 10  wherein the concentration of the optional secondary component on the finished sorbent is within the range of from about 1 wt-% to about 15 wt-% of the concentration of the promoter on the finished sorbent. 
   
   
       12 . The promoted carbon sorbent of  claim 10  wherein the optional secondary component is selected from the group consisting of Group V halides, Group VI halides, HI, HBr, HCl, and combinations thereof. 
   
   
       13 . The promoted carbon sorbent of  claim 12  wherein the optional secondary component is selected from the group consisting of I 2 , HI, and combinations thereof. 
   
   
       14 . The promoted carbon sorbent of  claim 13  wherein the promoter is substantially in vapor form when combined with the base activated carbon. 
   
   
       15 . The promoted carbon sorbent of  claim 13  wherein the promoter is combined with an organic solvent prior to reaction with the base activated carbon. 
   
   
       16 . The promoted carbon sorbent of  claim 13  wherein the promoter and optional secondary component are combined with the base activated carbon substantially simultaneously. 
   
   
       17 . A method of preparing a promoted carbon sorbent comprising providing an activated carbon; and forming a reaction product by reacting the activated carbon with a promoter selected from the group consisting of halogens, halides, and combinations thereof, such that the reaction product comprises a promoted carbon sorbent effective for removal of mercury from a gas stream downstream of a boiler, thus yielding a cleaned gas stream, and wherein the composition of the sorbent is adjusted based upon the mercury content of the cleaned gas stream. 
   
   
       18 . The method of  claim 17  wherein the activated carbon has an average particle size greater than that of the components of the gas stream such that the promoted carbon sorbent produced therefrom can be substantially separated by physical means from the cleaned gas stream. 
   
   
       19 . The method of  claim 18  wherein the reaction product has an average particle size distribution greater than about 40 micrometers. 
   
   
       20 . The method of  claim 17  further comprising preparing the sorbent in-flight by reacting the activated carbon, the promoter, and an optional secondary component to enhance the reactivity and mercury capacity of the sorbent. 
   
   
       21 . The method of  claim 17  further comprising continuously monitoring the mercury content of the cleaned gas stream. 
   
   
       22 . The promoted carbon sorbent prepared by the method of  claim 21 . 
   
   
       23 . The method of  claim 17  wherein the sorbent comprises an activated carbon matrix in a fixed or a moving bed structure. 
   
   
       24 . The method of  claim 23  wherein the structure is selected from the group consisting of moving fiber screens and monoliths. 
   
   
       25 . The method of  claim 17  wherein the ratio of activated carbon to promoter is adjusted for optimization of economic removal of mercury from the gas stream. 
   
   
       26 . The method of  claim 17  wherein the reaction product comprises from about 1 to about 30 grams promoter per 100 grams activated carbon. 
   
   
       27 . The method of  claim 26  wherein the promoter is in an organic solvent when contacting the activated carbon. 
   
   
       28 . The method of  claim 26  further comprising providing an optional alkaline component to improve the effectiveness of mercury removal. 
   
   
       29 . The method of  claim 26  wherein the promoter is in the gas phase when contacting the activated carbon. 
   
   
       30 . The method of  claim 29  wherein the promoter is selected from the group consisting of Br 2 , a Group V bromide, a Group VI bromide, and combinations thereof. 
   
   
       31 . The method of  claim 29  further comprising reacting the activated carbon with an optional secondary component comprising a halogen or a hydrohalide such that the reactivity and mercury capacity of the sorbent are enhanced. 
   
   
       32 . The method of  claim 31  further comprising providing an optional alkaline component to improve the effectiveness of mercury removal. 
   
   
       33 . The method of  claim 31  wherein the promoter and optional secondary component are contacted substantially simultaneously with the activated carbon.