Low NOx combustion method and apparatus

Patent metadata

Field	Value
Publication number	US-10281140-B2
Application number	US-201514799091-A
Country	US
Kind code	B2
Filing date	Jul 14, 2015
Priority date	Jul 15, 2014
Publication date	May 7, 2019
Grant date	May 7, 2019

Abstract

A steam generator system employing a fired burner with a flue gas recirculation system with low NOx emission is disclosed. A method to retrofit fired burners for low NOx emission is also disclosed. In the system, the flue gas recirculation system is configured to include a pre-mixer, and the recirculated flue gas (RFG) is routed to the stage of the pre-mixer for mixing with a portion of the fuel stream, forming a secondary RFG fuel mixture. The secondary RFG fuel mixture is routed to the secondary stage of the burner via a plurality of injector ports. The injection of the second RFG fuel mixture results in a reduction of temperature for the NOx emission to be less than 5 ppm at 3% O2, dry basis.

First claim

The invention claimed is:

1. A furnace system comprising:
a structure defining a combustion chamber;
sources of primary fuel, combustion air, and secondary fuel, wherein the primary fuel is natural gas and the secondary fuel is natural gas;
a premix staged combustion burner having at least a port facing into the combustion chamber;
a flue that conveys products of combustion (POC) from the combustion chamber;
a gas pre-mixer for receiving the secondary fuel and the POC, wherein the secondary fuel and the POC are mixed in the gas pre-mixer to form an unignited mixture of the secondary fuel and the POC, and wherein the gas pre-mixer does not receive combustion air;
a plurality of staged fuel injectors configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber through the plurality of staged fuel injectors such that a diffuse combustion zone is created within the combustion chamber downstream of the plurality of staged fuel injectors in which temperatures are too low for thermal formation of oxides of nitrogen (NO x ), wherein the diffuse combustion zone is formed by interaction of a secondary flame envelope downstream of the plurality of staged fuel injectors peripherally surrounding a primary flame envelope downstream of the premix staged combustion burner, and wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber at a controlled volume ratio for the POC to have a NO x concentration of <5 ppm on a dry gas volumetric basis corrected to 3% O 2 ; and
a reactant supply and control system including (a) a primary fuel supply line, (b) a primary fuel control valve for conveying the primary fuel from the primary fuel source to the premix staged combustion burner, (c) a combustion air supply line, (d) a combustion air blower, and (e) a combustion air control valve or a combustion air adjustable speed controller for conveying the combustion air from the combustion air source to the premix staged combustion burner for mixing with the primary fuel, (f) a secondary fuel supply line and (g) a secondary fuel control valve for conveying the secondary fuel from the secondary fuel source to the gas pre-mixer, (h) a POC supply line, (i) a POC blower, and (j) a POC control valve or a POC adjustable speed controller for conveying the POC from the flue to the gas pre-mixer for mixing with the secondary fuel.

2. The furnace system of claim 1 , wherein the port of the premix staged combustion burner is for injection of a mixture of primary fuel and the combustion air, and at least one of the plurality of staged fuel injectors is for injection of the unignited mixture of the secondary fuel and the POC.

3. The furnace system of claim 1 , wherein the combustion air is provided to the premix staged combustion burner at a sufficient rate for the POC to have an oxygen concentration ranging from 0.4 to 3% on a wet basis.

4. The furnace system of claim 3 , wherein the combustion air is provided to the premix staged combustion burner at a sufficient rate for the POC to have an oxygen concentration ranging from 0.75 to 1.5% on a wet basis.

5. The furnace system of claim 1 , wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber at a controlled volume ratio for the POC to have a NO x concentration of <3 ppm on a dry gas volumetric basis corrected to 3% O 2 (<3 ppm at 3% O 2 , dry basis).

6. The furnace system of claim 1 , wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber at a volume ratio of the secondary fuel to the POC of 1:4 to 1:20.

7. The furnace system of claim 6 , wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber at a volume ratio of the secondary fuel to the POC of 1:5 to 1:10.

8. The furnace system of claim 1 , wherein the plurality of staged fuel injectors includes 3 to 8 staged fuel injectors, and wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber through the 3 to 8 staged fuel injectors.

9. The furnace system of claim 1 , wherein the plurality of staged fuel injectors includes 4 to 6 staged fuel injectors, and wherein the plurality of staged fuel injectors is configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber through the 4 to 6 staged fuel injectors.

10. The furnace system of claim 1 , wherein the primary fuel has a higher heating value of 500 to 1200 Btu/scf.

11. The furnace system of claim 10 , wherein the primary fuel has a higher heating value range of 900 to 1180 Btu/scf.

12. The furnace system of claim 1 , wherein the secondary fuel has a higher heating value of 500 to 1200 Btu/scf.

13. The furnace system of claim 1 , wherein the secondary fuel has a higher heating value of 900 to 1180 Btu/scf.

14. The furnace system of claim 1 , wherein the primary fuel and the secondary fuel have different higher heating values.

15. The furnace system of claim 1 , wherein the primary fuel and the secondary fuel are configured to have a volumetric ratio resulting in a primary zone adiabatic flame temperature less than 2600° F. (1427° C.).

16. The furnace system of claim 15 , wherein the primary fuel and the secondary fuel are configured to have a volumetric ratio resulting in a primary zone adiabatic flame temperature less than 2500° F. (1371° C.).

17. The furnace system of claim 1 , wherein flue gas recirculation ranges from 15 to 30% of the POC.

18. A furnace system comprising:
a structure defining a combustion chamber;
sources of primary fuel, combustion air, and secondary fuel, wherein the primary fuel is natural gas and the secondary fuel is natural gas, and wherein the primary fuel and secondary fuel each has a higher heating value of from 500 to 1200 Btu/scf, and wherein the primary fuel and the secondary fuel are configured to have a volumetric ratio resulting in a primary zone adiabatic flame temperature less than 2600° F.;
a premix staged combustion burner having at least a port facing into the combustion chamber;
a flue that conveys products of combustion (POC) from the combustion chamber;
a gas pre-mixer for receiving the secondary fuel and the POC, wherein the secondary fuel and the POC are mixed in the gas pre-mixer to form an unignited mixture of the secondary fuel and the POC, and wherein the gas pre-mixer does not receive combustion air;
a plurality of staged fuel injectors each having a port facing into the combustion chamber, wherein the plurality of staged fuel injectors includes at least 3 staged fuel injectors configured to inject the unignited mixture of the secondary fuel and the POC into the combustion chamber through the at least 3 staged fuel injectors such that a diffuse combustion zone is created downstream of the at least 3 staged fuel injectors in which temperatures are too low for thermal formation of oxides of nitrogen (NO x ), wherein the diffuse combustion zone is formed by interaction of a secondary flame envelope downstream of the plurality of staged fuel injectors peripherally surrounding a primary flame envelope downstream of the premix staged combustion burner, and wherein the plurality of staged fuel injectors is configured to inject the unignited

Assignees

• Chevron Usa Inc

Inventors

• Robertson Thomas F
• Nowakowski John J
• Hannum Mark C
• Storslett Stein J

Classifications

• F23C9/08 Primary

  for reducing temperature in combustion chamber, e.g. for protecting walls of combustion chamber · CPC title

• F23C6/047

  with fuel supply in stages · CPC title

• F23C2202/20

  Premixing fluegas with fuel · CPC title

• F23C5/08

  Disposition of burners · CPC title