System and Method for Sequential Injection of Reagent to Reduce NOx from Combustion Sources

• US 20140360168A1
• Filed: 05/30/2014
• Published: 12/11/2014
• Est. Priority Date: 06/07/2013
• Status: Active Grant

• Current Assignee: Ceco Environmental IP Inc. (CECO Environmental Corp.)
• Original Assignee: Peerless Manufacturing Company (CECO Environmental Corp.)
• Inventors: Broderick, Jeffrey Michael, Lindemann, Scott H., Valentine, James M., Lindemann, Eric
• Granted Patent: US 9,869,226 B2
• Time in Patent Office: Days
• US Class Current: 60/274
• CPC Class Codes: F01N 2610/02 the substance being ammonia or urea
• CPC Class Codes: F01N 2900/1404 Exhaust gas temperature
• CPC Class Codes: F01N 2900/1411 Exhaust gas flow rate, e.g. mass flow rate or volumetric flow rate
• CPC Class Codes: F01N 2900/1812 Flow rate
• CPC Class Codes: F01N 3/208 Control of selective catalytic reduction [SCR], e.g. dosing of reducing agent
• CPC Class Codes: Y02T 10/12 Improving ICE efficiencies

Abstract

In a large exhaust duct from a lean burn combustion source, such as a boiler, diesel engine or gas turbine, multiple injectors can be used to inject a reagent, such as an aqueous solution of urea or ammonia, into the exhaust for use in the catalytic reduction of NOx in a process known in the art as selective catalytic reduction (SCR). When operating at low injection rates, such as during low combustor loads, the injectors are operated individually for short periods of time in a sequential manner.

Claims

1. A lean burn combustion source having reduced pollutant emissions, said lean burn combustion source comprising:

an exhaust duct having exhaust gases passing therethrough;

a plurality of injectors positioned so as to inject reagent into the exhaust gases passing through the exhaust duct;

a controller in communication with said plurality of injectors, said controller causing said plurality of injectors to operate in an alternating sequence wherein at least one of the plurality of injectors is caused to inject for a period of time while the remaining ones of the plurality of injectors are caused to be not injecting, and

a catalyst located downstream of said plurality of injectors and spaced apart from said plurality of injectors such that the reagent is allowed to decompose in the exhaust gases and to react across the catalyst to reduce pollutant emissions.

2. The combustion source of claim 1 wherein the reagent comprises urea or ammonia and the reduced pollutant emissions comprise nitrogen oxides (NOx).

3. The combustion source of claim 1 wherein the plurality of injectors comprises a first injector, a second injector and a third injector, and wherein said controller:

(a) causes the first injector to inject while the second and third injectors are caused to be not injecting;

(b) causes the second injector to inject while the first and third injectors are caused to be not injecting; and

(c) causes the third injector to inject while the first and second injectors are caused to be not injecting; and

wherein steps (a)-(c) are sequentially repeated.

4. The combustion source of claim 3 wherein steps (a)-(c) are sequentially repeated for so long as a required on time for the operating injector is less than 20%.

5. The combustion source of claim 1 where the period of time is between about 0.5 seconds and about 2.0 seconds.

6. The combustion source of claim 1 where the plurality of injectors comprises two injectors and wherein said controller causes the injectors to operate individually and sequentially at an injection rate of about 0.07 to about 0.10 gallons per hour of reagent.

7. The combustion source of claim 6 where the injection rate is about 0.5 to about 1.0 gallons per hour of reagent.

8. The combustion source of claim 1 wherein the plurality of injectors comprises an even number of four or more injectors and the injectors are operated and sequenced in groups.

9. The combustion source of claim 8 wherein the plurality of injectors comprises a top left injector, a middle left injector, a bottom left injector, a top right injector, a middle right injector and a bottom right injector and wherein said controller:

(a) causes the top left injector and the bottom right injector to inject while the middle left injector, the bottom left injector, the top right injector and the middle right injector are caused to be not injecting;

(b) causes the top right injector and the bottom left injector to inject while the middle left injector, the top left injector, the bottom right injector and the middle right injector are caused to be not injecting; and

(c) causes the middle left injector and the middle right injector to inject while the top left injector, the bottom left injector, the top right injector and the bottom right injector are caused to be not injecting; and

wherein steps (a)-(c) are repeated.

10. The combustion source of claim 8 wherein the plurality of injectors comprises a top left injector, a middle left injector, a bottom left injector, a top right injector, a middle right injector and a bottom right injector and wherein said controller:

(a) causes the top left injector, the middle right injector and the bottom left injector to inject while the top right injector, the middle left injector and the bottom right injector are caused to be not injecting; and

(b) causes the top right injector, the middle left injector and the bottom right injector to inject while the top left injector, the middle right injector and the bottom left injector are caused to be not injecting; and

wherein steps (a)-(b) are repeated.

11. The combustion source of claim 1 wherein said controller comprises a programmable logic controller (PLC) that triggers operation of the plurality of injectors once a low injection rate limit has been reached and at least one of a low combustor load, fuel flow, gas temperature or emissions rate signal has been received by the PLC controller.

12. The combustion source of claim 1 wherein the lean burn combustion source comprises two lean burn combustion sources exhausting into a common duct and the common duct further comprises a second plurality of injectors that substantially continuously inject reagent into the exhaust gases of the combustion sources during operation thereof.

13. A method of reducing pollutant emissions from a lean burn combustion source equipped with a plurality of injectors that inject reagent into exhaust gases of the combustion source, said method comprising the steps of:

operating the plurality of injectors in an alternating sequence wherein at least one of the plurality of injectors is caused to inject for a period of time while the remaining ones of the plurality of injectors are caused to be not injecting, and

allowing the reagent to decompose in the exhaust gases and to react across a downstream catalyst to reduce pollutant emissions.

14. The method of claim 13 wherein the reagent comprises urea or ammonia and the reduced pollutant emissions comprise nitrogen oxides (NOx).

15. The method of claim 13 wherein the plurality of injectors comprises a first injector, a second injector and a third injector, and wherein said operating step comprises the steps of:

(a) causing the first injector to inject while the second and third injectors are caused to be not injecting;

(b) causing the second injector to inject while the first and third injectors are caused to be not injecting; and

(c) causing the third injector to inject while the first and second injectors are caused to be not injecting; and

wherein steps (a)-(c) are sequentially repeated.

16. The method of claim 15 wherein steps (a)-(c) are sequentially repeated for so long as a required on time for the operating injector is less than 20%.

17. The method of claim 13 where the period of time is between about 0.5 seconds and about 2.0 seconds.

18. The method of claim 13 where the plurality of injectors comprises two injectors and wherein the injectors are operated individually and sequentially at an injection rate of about 0.07 to about 0.10 gallons per hour of reagent.

19. The method of claim 18 where the injection rate is about 0.5 to about 1.0 gallons per hour of reagent.

20. The method of claim 13 wherein the plurality of injectors comprises an even number of four or more injectors and the injectors are operated and sequenced in groups.

21. The method of claim 20 wherein the plurality of injectors comprises a top left injector, a middle left injector, a bottom left injector, a top right injector, a middle right injector and a bottom right injector and wherein said operating step comprises the steps of:

(a) causing the top left injector and the bottom right injector to inject while the middle left injector, the bottom left injector, the top right injector and the middle right injector are caused to be not injecting;

(b) causing the top right injector and the bottom left injector to inject while the middle left injector, the top left injector, the bottom right injector and the middle right injector are caused to be not injecting; and

(c) causing the middle left injector and the middle right injector to inject while the top left injector, the bottom left injector, the top right injector and the bottom right injector are caused to be not injecting; and

wherein steps (a)-(c) are repeated.

22. The method of claim 20 wherein the plurality of injectors comprises a top left injector, a middle left injector, a bottom left injector, a top right injector, a middle right injector and a bottom right injector and wherein said operating step comprises the steps of:

(a) causing the top left injector, the middle right injector and the bottom left injector to inject while the top right injector, the middle left injector and the bottom right injector are caused to be not injecting;

(b) causing the top right injector, the middle left injector and the bottom right injector to inject while the top left injector, the middle right injector and the bottom left injector are caused to be not injecting; and

wherein steps (a)-(b) are repeated.

23. The method of claim 13 further comprising the step of employing computer modeling techniques to locate positions of the injectors within the exhaust gases and to identify a preferred sequence and duration of injection of the plurality of injectors.

24. The method of claim 13 wherein the sequence is programmed into a programmable logic controller (PLC) that triggers operation of the plurality of injectors once a low injection rate limit has been reached and at least one of a low combustor load, fuel flow, gas temperature or emissions rate signal has been received by the PLC controller.

25. The method of claim 13 wherein the lean burn combustion source comprises two lean burn combustion sources exhausting into a common duct and the common duct is further equipped with a second plurality of injectors that substantially continuously inject reagent into the exhaust gases of the combustion sources during operation thereof.