System and method of protecting a NO

1. A method of protecting a NOx reducing catalyst from fouling, where the NOx reducing catalyst is downstream of a circulating fluidized bed (CFB) boiler and upstream of a particulate removal device, the method comprising:

introducing a SO x removing reagent in a calcium to sulfur molar ratio greater than that required for SO 2 removal from a flue gas generated by a CFB boiler, thereby preventing SO 3 formation and fouling of the NOx reducing catalyst.

2. A method according to claim 1 , further comprising adjusting an amount of the SO x removing reagent in response to an amount of SO x detected in the flue gas downstream of the CFB boiler , wherein the amount of SO x in the flue gas is detected by an SO x sensor.

3. A method according to claim 1 , wherein the calcium to sulfur molar ratio is between about 2.0 to about 2.8.

4. A method according to claim 1 , wherein the molar ratio of the SO x removing reagent is between about 2 to about 3 times the molar ratio needed to remove at least 90% of the SO 2 from the flue gas.

5. A method according to claim 1 , wherein the SO x removing reagent is introduced directly to the CFB boiler.

6. A method according to claim 1 , wherein the SO x removing reagent is introduced to fuel combusted in the CFB boiler.

7. A method according to claim 1 , wherein the SO x removing reagent is introduced to the flue gas at a position downstream of the CFB boiler.

8. A method according to claim 1 , wherein the SO x removing reagent is introduced to the CFB boiler and the flue gas at a position upstream of the NO x reducing catalyst.

9. A method according to claim 1 , wherein the SO x removing reagent is limestone.

10. A method of maintaining an amount of SO x removing reagent introduced to a flue gas processing system having a circulating fluidized bed (CFB) boiler adapted to combust one or more types of fuel , the method comprising:

combusting a first fuel in a CFB boiler, thereby producing a first flue gas;

introducing a SO x removing reagent to the flue gas processing system to remove an amount of SO x from the flue gas produced by combustion of the first fuel, the SO x removing reagent introduced in a calcium to sulfur molar ratio greater than that required for SO 2 removal from the flue gas;

removing the first fuel from the CFB boiler; and

combusting a second fuel in the CFB boiler, thereby producing a second flue gas, wherein an amount of SO x present in the second flue gas is removed by the SO x removing reagent, the amount of SO x removing reagent introduced to the CFB boiler combusting the second fuel is maintained at the same amount introduced to the CFB boiler combusting the first fuel.

11. A method according to claim 10 , wherein the SO x removing reagent is limestone.

12. A method according to claim 11 , further comprising independently introducing the first and second flue gas to a NOx removing catalyst downstream of the CFB boiler.

13. A system for protecting a NO x reducing catalyst from fouling, where the NO x reducing catalyst is downstream of a circulating fluidized bed (CFB) boiler and upstream of a particulate removal device, the system comprising:

an injector, sprayer, or feeder to introduce into said system a SO x removing reagent in a calcium to sulfur molar ratio greater than that required for SO 2 removal from a flue gas generated by a CFB boiler, thereby preventing SO 3 formation and fouling of the NO x reducing catalyst.

14. A system according to claim 13 , further comprising a control system for detecting an amount of SO 3 in the flue gas downstream of the CFB boiler and adjusting an amount of SO x removing reagent introduced to the system.

15. A system according to claim 13 , wherein the calcium to sulfur molar ratio is between about 2.0 and about 2.8.

16. A system according to claim 13 , wherein the molar ratio of the SO x removing reagent is between about 2 to about 3 times the molar ratio needed to remove at least 90% of the SO 2 from the flue gas.

17. A system according to claim 13 , wherein the SO x removing reagent is introduced directly to the CFB boiler.

18. A system according to claim 13 , wherein the SO x removing reagent is introduced to fuel combusted in the CFB boiler.

19. A system according to claim 13 , wherein the SO x removing reagent is introduced to the flue gas at a position downstream of the CFB boiler.

20. A system according to claim 13 , wherein the SO x removing reagent is introduced to the CFB boiler and the flue gas at a position upstream of the NO x reducing catalyst.