Layered double hydroxide sorbents for the removal of SO.sub.x from flue gas and other gas streams
AssigneeBoard of Trustees operating Michigan State University
InventorsThomas J. Pinnavaia, Jayantha Amarasekera, Christine A. Polansky
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Abstract
Processes for removing noxious sulfur oxides from gas streams, particularly from flue gases of coal-burning power plants, using heated layered double hydroxide (LDH) sorbents are described. The sorbent compositions contain metal components, incorporated into the sorbents either by isomorphous replacement of all or part of M.sup.II and/or M.sup.III ions in layers of LDH structures or by impregnation as a metal salt, to promote the oxidation of sulfur dioxide.
Claims24
A sorbent composition useful for removing sulfur oxides from flue gas and other gas streams which comprises contacting said gas streams with a heated sorbent composition wherein the said sorbent before being heated has a layered double hydroxide structure of the formula:
- [M.sub.1-x.sup.II M.sub.x.sup.III (OH).sub.2 ](A.sup.n-).sub.x/n.pH.sub.2 O
- wherein M.sup.II is a divalent metal cation and M.sup.III is a trivalent metal cation, A is an interlayer anion of charge n- which forms a volatile gas at elevated temperatures, x is between 0.8 and 0.12, and contains an additional transition metal cation which is impregnated as a salt in admixture with the said sorbent composition and p is a number representing moles of water, wherein said additional metal cation which is impregnated as a salt into the sorbent composition is selected from the group consisting of metal cations which provide oxidation of sulfur dioxide to sulfur trioxide in an amount sufficient that said layered double hydroxide structure promotes the oxidation of the sulfur dioxide to the sulfur trioxide at sulfur dioxide oxidation conditions.
A process for the preparation of magnesium-containing layered double hydroxide sorbents for removing the SO.sub.x components from a gas stream having compositions before being heated conforming to the formula:
- [Mg.sub.1-x M.sup.III.sub.x-y D.sup.III.sub.y (OH).sub.2 ](A.sup.n-).sub.x/n.pH.sub.2 O
- wherein M.sup.III and D.sup.1II are trivalent metal cations and wherein D.sup.III is a trivalent transition metal cation, A is an interlayer anion of charge n-, x is between 0.8 and 0.12, 0<y<x and wherein p is a number representing moles of water and comprising:
- (a) combining a solution containing stoichiometric amounts of Mg.sup.II, M.sup.III and D.sup.III with a alkali metal hydroxide solution containing a 50% stoichiometric excess of anion A.sup.n- to form a solid precipitate, and
- (b) digesting the mixture at a temperature between 25.degree. to 100.degree. C. for a period of 0.5 to 24 hours, and
- (c) separating the solid product from the solution followed by washing with water to remove contaminated metal salts, and drying.
A process for the preparation of calcium-containing layered double hydroxide sorbents for removing the SO.sub.x components from a gas stream having compositions before being heated conforming to the formula:
- [Ca.sub.1-x M.sup.III.sub.x-y D.sup.III.sub.y (OH).sub.2 ](A.sup.n-).sub.x/n.pH.sub.2 O
- wherein M.sup.III and D.sup.III are trivalent metal cations, A is an interlayer anion of charge n-, x is between 0.8 to 0.12, 0<y<x and wherein p is a number representing moles of water and comprising:
- (a) adding a solution containing stoichiometric amounts of Ca.sup.II, M.sup.III and D.sup.III and a second alkali metal hydroxide solution containing a 50% stoichiometric excess of anion A.sup.n- into water at a temperature between 30.degree.-70.degree. C., more preferably between 50.degree.-60.degree. C., in a manner so as to maintain the pH of the resultant mixture between 12 and 13 and to form a solid precipitate, and
- (b) digesting the resulting solid mixture at a temperature between 30.degree. to 70.degree. C. for a period of 0.5 to 24 hours and
- (c) separating the solid product from the solution followed by washing with water to remove contaminated metal salts, and drying.
A process for the preparation of aluminum-containing layered double hydroxide sorbents for removing the SO.sub.x components from a gas stream having compositions before being heated conforming to the formula:
- [M.sup.II.sub.(1-x-z) E.sup.II.sub.z Al.sub.x (OH).sub.2 ](A.sup.n-).sub.x/n. pH.sub.2 O
- wherein M.sup.II and E.sup.II are divalent metal cations and E.sup.II is a transition metal cation, A.sup.n- is an interlayer anion of charge n-, x is between 0.8 to 0.12, 0<z.ltoreq.(1-x) and wherein p is a number representing moles of water and comprising:
- (a) adding a solution containing stoichiometric amounts of M.sup.II, Al.sup.III and E.sup.II and a alkali metal hydroxide solution containing a 50% stoichiometric excess of anion A.sup.n- into water at a temperature between 20.degree.-70.degree. C. in a manner so as to maintain the pH of the resultant mixture between 11 and 13 and to form a solid precipitate, and
- (b) digesting the mixture at a temperature between 25.degree. to 100.degree. C. for a period of 0.5 to 24 hours, and
- (c) separating the solid product from the solution followed by washing with water to remove contaminated metal salts, and drying.
Dependencies are inferred from the claim text.