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A fuel-air mixer includes an annular shroud, inner and outer counter-rotating swirlers, a hub separating the inner and outer swirlers, a center body extending axially along the annular shroud, a fuel shroud disposed radially outwardly from the outer swirler circumferentially around the annular shrou

A fuel-air mixer includes an annular shroud, inner and outer counter-rotating swirlers, a hub separating the inner and outer swirlers, a center body extending axially along the annular shroud, a fuel shroud disposed radially outwardly from the outer swirler circumferentially around the annular shroud, and a radial swirler disposed downstream of the inner and outer swirlers, the radial swirler being configured to allow an independent radial rotation of a second gas stream entering the annular shroud from a region outside a wall of the annular shroud separately from a stream flowing through the inner and outer swirlers, the radially rotating second gas stream entering the annular shroud at a region adjacent an outer wall of the annular shroud.

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1. A fuel-air mixer, comprising: an annular shroud having a circular cross section defined by a wall, the annular shroud having an axial axis extending along an axial direction, a radial axis extending along a radial direction, and upstream and downstream end portions;a first swirler disposed at the

1. A fuel-air mixer, comprising: an annular shroud having a circular cross section defined by a wall, the annular shroud having an axial axis extending along an axial direction, a radial axis extending along a radial direction, and upstream and downstream end portions;a first swirler disposed at the upstream end portion of the annular shroud;a second swirler disposed at the upstream end portion of the annular shroud outwardly from the first swirler in the radial direction;a hub separating the first and second swirlers, the hub being configured to allow independent rotation of first and a second portions of a first gas stream entering the annular shroud at the upstream end portion thereof, the first and second portions of the first gas stream being rotated by the first and second swirlers, respectively, in which the hub is further configured such that the first and second portions of the first gas stream accumulate at a region adjacent a wall of the annular shroud;a center body extending along the axial axis of the annular shroud;a fuel shroud disposed at the upstream end portion of the annular shroud outwardly from the second swirler in the radial direction and circumferentially around the annular shroud, the fuel shroud being configured to inject a fuel into the annular shroud to be mixed with the first and second portions of the first gas stream and a second gas stream upstream of a combustion chamber; anda third radial swirler is disposed downstream of the first and second swirlers, the third swirler having an axial and radial gap adjacent the wall of the annular shroud and being configured to introduce the second gas stream from a region outside the wall of the annular shroud to the first and second portions of the first gas stream to allow an independent rotation of the first and second gas stream and the fuel from the fuel shroud wherein the second gas stream enters the annular shroud at a region adjacent and within the wall of the annular shroud. 2. The fuel-air mixer according to claim 1, wherein the first and second swirlers are counter-rotating and an axial distance from the third swirler to the first and second swirlers is determined based on a desired fuel concentration value at an exit plane of the annular shroud. 3. The fuel-air mixer according to claim 1, wherein the first and second swirlers are selected from the group consisting of radial swirlers, axial swirlers, and radial/axial swirlers and the third swirler is a radial swirler. 4. The fuel-air mixer according to claim 1, wherein the first swirler comprises vanes and the fuel shroud is in flow communication with the vanes of the first shroud. 5. The fuel-air mixer according to claim 1, wherein the second swirler comprises vanes and the fuel shroud is in flow communication with the vanes of the second shroud. 6. The fuel-air mixer according to claim 1, wherein the first and second swirlers comprise first and second sets of vanes, respectively, and the fuel shroud is in flow communication with the first and second sets of vanes. 7. The fuel-air mixer according to claim 1, wherein the fuel injected into the annular shroud from the fuel shroud is a syngas and the second gas stream is a stream of nitrogen gas to vitiate an amount of oxygen in the first gas stream to control an amount of NOx formed in a combustion region located downstream of the annular shroud. 8. The fuel-air mixer according to claim 1, wherein the second gas stream is a stream of air and the independent rotation of the second gas stream is configured to reduce a local fuel concentration at an exit of the annular shroud in a region adjacent the wall of the annular shroud. 9. The fuel-air mixer according to claim 1, wherein the fuel is selected from the group consisting of natural gas, high hydrogen gas, hydrogen, bio gas, carbon monoxide, syngas, and a combination thereof. 10. The fuel-air mixer according to claim 1, the third swirler further comprising: a first ring having a plurality of vanes disposed on an outer surface thereof and an annular lip extending axially from an inner edge of the first ring; anda second ring disposed axially away from the first ring so as to form a gap therebetween containing the plurality of vanes disposed on the outer surface of the first ring, the second ring including a first surface extending radially inward so as to form a first portion of the gap and a second surface extending axially so as to form a second portion of the gap, the second ring also including a sleeve configured to receive the annular shroud of the fuel-air mixer. 11. A gas turbine combustor comprising the fuel-air mixer of claim 1. 12. The gas turbine combustor of claim 10, wherein the gas turbine combustor comprises a can combustor, or a can-annular combustor, or an annular combustor. 13. A gas turbine, comprising: a compressor;a combustor in flow communication with the compressor configured to burn a premixed mixture of fuel and air, the combustor including a fuel-air mixer disposed upstream of the combustor, the fuel-air mixer including: an annular shroud having a circular cross section defined by a wall, the annular shroud having an axial axis extending along an axial direction, a radial axis extending along a radial direction, and upstream and downstream end portions,a first swirler disposed at the upstream end portion of the annular shroud,a second swirler disposed at the upstream end portion of the annular shroud outwardly from the first swirler in the radial direction,a hub separating the first and second swirlers, the hub being configured to allow independent rotation of first and a second portions of a first gas stream entering the annular shroud at the upstream end portion thereof, the first and second portions of the first gas stream being rotated by the first and second swirlers, respectively, in which the hub is further configured such that the first and second portions of the first gas stream accumulate at a region adjacent a wall of the annular shroud;a center body extending along the axial axis of the annular shroud,a fuel shroud disposed at the upstream end portion of the annular shroud outwardly from the second swirler in the radial direction and circumferentially around the annular shroud, the fuel shroud being configured to inject a fuel into the annular shroud to be mixed with the first and second portions of the first gas stream and a second gas stream upstream of a combustion chamber; anda third radial swirler disposed in the annular shroud downstream of the first and second swirlers, the third swirler having an axial and radial gap adjacent the wall of the annular shroud and being configured to introduce the second gas stream from a region outside the wall of the annular shroud to the first and second portions of the first gas stream to allow an independent rotation of the first and second gas stream and the fuel from the fuel shroud wherein the second gas stream entering the annular shroud at a region adjacent and within the wall of the annular shroud; anda turbine located downstream of the combustor and configured to expand a third gas stream exiting the combustor. 14. A gas-to-liquid system, comprising: an air separation unit configured to separate oxygen from air;a gas processing unit for preparing natural gas;a combustor for reacting oxygen with the natural gas at an elevated temperature and pressure to produce a synthesis gas enriched with carbon monoxide and hydrogen gas;a fuel-air mixer disposed upstream of the combustor to facilitate the premixing of oxygen and the natural gas prior to reaction in the combustor, the fuel-air mixer including: an annular shroud having a circular cross section defined by a wall, the annular shroud having an axial axis extending along an axial direction, a radial axis extending along a radial direction, and upstream and downstream end portions,a first swirler disposed at the upstream end portion of the annular shroud,a second swirler disposed at the upstream end portion of the annular shroud outwardly from the first swirler in the radial direction,a hub separating the first and second swirlers, the hub being configured to allow independent rotation of first and a second portions of a first gas stream entering the annular shroud at the upstream end portion thereof, the first and second portions of the first gas stream being rotated by the first and second swirlers, respectively, in which the hub is further configured such that the first and second portions of the first gas stream accumulate at a region adjacent a wall of the annular shroud;a center body extending along the axial axis of the annular shroud,a fuel shroud disposed at the upstream end portion of the annular shroud outwardly from the second swirler in the radial direction and circumferentially around the annular shroud, the fuel shroud being configured to inject a fuel into the annular shroud to be mixed with the first and second portions of the first gas stream and a second gas stream upstream of a combustion chamber, anda third radial swirler disposed in the annular shroud downstream of the first and second swirlers, the third swirler having an axial and radial gap adjacent the wall of the annular shroud and being configured to introduce a second gas stream from a region outside the wall of the annular shroud to the first and second portions of the first gas stream to allow an independent rotation of the first and second gas stream and the fuel from the fuel shroud wherein the second gas stream entering the annular shroud at a region adjacent and within the wall of the annular shroud; anda turbo-expander in flow communication with the combustor for extracting work from and for quenching the synthesis gas. 15. The gas to liquid system of claim 14, further comprising a Fischer-Tropsch processing unit for receiving the quenched synthesis gas and for catalytically converting the quenched synthesis gas into a hydrocarbon fluid. 16. The gas to liquid system of claim 14, further comprising a cracking unit for fractioning the hydrocarbon fluid into at least one useful product.