Low NOx combustion process and burner therefor

1. A burner assembly comprising:

a fuel injection passage for injecting a jet of fuel into a combustion zone;

a primary oxidizer injection passage for injecting a jet of primary oxidizer into the combustion zone in contact with the jet of fuel to generate a primary flame in the combustion zone;

a first peripheral gas injection passage for injecting a first peripheral gas jet through a first peripheral injection cross section area into the combustion zone around the fuel jet and the primary oxidizer jet, the first peripheral gas injection passage surrounding the fuel injection passage and the primary oxidizer passage and being equipped with a first swirler for conferring to the first peripheral gas jet a first swirl with respect to the primary flame;

a second peripheral gas injection passage for injecting a second peripheral gas jet through a second peripheral injection cross section area into the combustion zone around the first peripheral gas jet, the second peripheral gas injection passage surrounding the first peripheral gas injection passage and being equipped with a second swirler for conferring to the second peripheral gas jet a second swirl with respect to the primary flame; and

a secondary oxidizer injection passage for injecting a jet of secondary oxidizer through a secondary oxidizer injection cross section area into the combustion zone, the secondary oxidizer injection passage being located between the first and second peripheral gas injection passages, wherein the secondary oxidizer injection cross section area is between one tenth and one thirtieth of a sum of the first and second peripheral injection cross section areas;

wherein the first swirler is adapted and configured to confer a swirl to the first peripheral gas jet and the second swirler is adapted and configured to confer a swirl to the second peripheral gas jet different than that of the first peripheral gas jet, the difference between the swirls of the first and second swirlers selected from the group consisting of: a direction of rotation around the primary flame, a rapidity of the swirling, a swirl angle of vanes of the swirlers, and whether the swirl is convergent or divergent.

2. The burner assembly of claim 1 , wherein:

the primary and the secondary oxidizer injection passages are connected to a source of oxygen-rich gas having an oxygen content of between 80% vol and 100% vol; and

the first and second peripheral gas injection passages are connected to a source of air or to a source of a ballast gas containing from 0% vol to 19% vol oxygen and from 0% vol to 50% vol nitrogen.

3. The burner assembly of claim 1 , wherein the primary oxidizer injection passage surrounds the fuel injection passage.

4. A boiler comprising a combustion chamber equipped with at least one the burner assemblies of claim 1 .

5. A method of combusting a fuel with an oxidizer in a combustion zone, comprising the steps of:

injecting a jet of fuel into the combustion zone;

injecting a jet of primary oxidizer into the combustion zone in contact with the jet of fuel to generate a primary fuel-rich flame;

injecting a first peripheral gas jet through a first peripheral injection cross section area into the combustion zone around the fuel jet and the primary oxidizer jet, said first peripheral gas jet having a first swirl with respect to the primary flame;

injecting a second peripheral gas jet through a second peripheral injection cross section area into the combustion zone around the first peripheral gas jet, said second peripheral gas jet having a second swirl with respect to the primary flame, wherein the jet of primary oxidizer injects an amount of oxygen corresponding to between 10% and 50% of an amount of oxygen required for stoichiometric combustion of the jet of fuel; and

injecting a jet of secondary oxidizer through a secondary oxidizer injection cross section area into the combustion zone between the first and second peripheral gas jets, wherein the second oxidizer injection cross section area is between one tenth and one thirtieth of a sum of the first and the second peripheral injection cross section areas;

wherein the first swirler is adapted and configured to confer a swirl to the first peripheral gas jet and the second swirler is adapted and configured to confer a swirl to the second peripheral gas jet different than that of the first peripheral gas jet, the difference between the swirls of the first and second swirlers selected from the group consisting of: a direction of rotation around the primary flame, a rapidity of the swirling, a swirl angle of vanes of the swirlers, and whether the swirl is convergent or divergent.

6. The method of claim 5 , wherein:

each of the primary oxidizer and the second oxidizer is an oxygen-rich gas having on oxygen content of between 80% vol and 100% vol; and

first and the second peripheral gas jets are jets of air or jets of a ballast gas containing from 0% vol to 19% vol oxygen and from 0% vol to 50% vol of nitrogen.

7. The method of claim 5 , wherein the primary oxidizer jet is injected around the fuel jet.

8. The method of claim 5 , wherein the first and second peripheral gas jets are jets of air.

9. The method of claim 5 , wherein the first peripheral gas jet consists essentially of ballast gas and the second peripheral gas jet is an air jet.

10. The method of claim 5 , wherein the first and second peripheral gas jets are jets of ballast gas.

11. A method of generating steam in a boiler, comprising the steps of:

injecting a jet of fuel into the combustion zone;

injecting a jet of primary oxidizer into the combustion zone in contact with the jet of fuel to generate a primary fuel-rich flame;

injecting a first peripheral gas jet through a first peripheral injection cross section area into the combustion zone around the fuel jet and the primary oxidizer jet, said first peripheral gas jet having a first swirl with respect to the primary flame;

injecting a second peripheral gas jet through a second peripheral injection cross section area into the combustion zone around the first peripheral gas jet, said second peripheral gas jet having a second swirl with respect to the primary flame, wherein the jet of primary oxidizer injects an amount of oxygen corresponding to between 10% and 50% of an amount of oxygen required for stoichiometric combustion of the jet of fuel;

injecting a jet of secondary oxidizer through a secondary oxidizer injection cross section area into the combustion zone between the first and second peripheral gas jets, wherein the second oxidizer injection cross section area is between one tenth and one thirtieth of a sum of the first and the second peripheral injection cross section areas; and

using heat from combustion of the jet and oxidizers to generate steam;

wherein the first swirler is adapted and configured to confer a swirl to the first peripheral gas jet and the second swirler is adapted and configured to confer a swirl to the second peripheral gas jet different than that of the first peripheral gas jet, the difference between the swirls of the first and second swirlers selected from the group consisting of: a direction of rotation around the primary flame, a rapidity of the swirling, a swirl angle of vanes of the swirlers, and whether the swirl is convergent or divergent.