Chemistry: fischer-tropsch processes; or purification or recover – With preliminary reaction to form hydrogen or a carbon oxide – Gaseous oxygen utilized in the preliminary reaction
Reexamination Certificate
1998-09-10
2001-03-13
Richter, Johann (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
With preliminary reaction to form hydrogen or a carbon oxide
Gaseous oxygen utilized in the preliminary reaction
C518S704000, C252S373000, C060S039240, C060S651000, C431S008000, C422S186220
Reexamination Certificate
active
06201029
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to a process for combusting a fuel gas to drive a gas turbine and, more particularly, to a process for combusting a low heating value fuel gas in a combustor by dividing the fuel gas into a plurality of portions and combusting each portion of the fuel gas in a different stage of the combustor.
2. Background Information
Standard commercial combustors for gas turbines employ burner assemblies and combustion chambers typically designed to combust relatively high heating value fuels, such as natural gas which has a heating value in a range from about 50,000 to 60,000 kJ/kg. The stoichiometric demand for air in a conventional commercial burner assembly combusting a high heating value fuel gas is usually at least 10 volumes of combustion air for each volume of fuel gas fed to the burner assembly. In accordance with the swirl-stabilized combustion mechanism utilized by most commercial burner assemblies, the combustion air is fed to the combustion chamber through an annular flow region of the burner assembly which encircles a central burner nozzle. Fixed vanes are positioned in the annular flow region which impart a spin to the combustion air feed creating an air vortex. The burner fuel gas is fed into the interior zone or “eye” of the air vortex via the burner nozzle. The velocities of the feed streams are relatively low at the interface of the fuel-rich edge and the air-rich “eye wall” of the vortex which enables a stable film of laminar diffusion combustion to occur at the interface.
In contrast, the stoichiometric demand for air when combusting a low heating value fuel gas in a burner assembly is substantially less than when combusting a high heating value fuel gas. Low heating value fuel gases contain significant quantities of inert components termed “ballast” gases, such as nitrogen, carbon dioxide, water vapor and the like, which are not combustible and which dilute the remaining combustible components of the fuel gas. Accordingly, the stoichiometric ratio of air to fuel gas in a burner assembly receiving a low heating value fuel gas is generally much less than 10:1 on a volumetric basis, and may be even less than 1:1 for certain fuel gases. As such, a conventional central burner nozzle, which is designed for high heating value fuel gases, is not sufficiently large to accommodate the large volume of low heating value fuel gas required to achieve the necessary heat release for the burner assembly. Conversely, the annular flow region is overly large, permitting an excessive air to fuel ratio in the burner assembly which is outside the flammability envelope of the air/fuel mixture.
Specialized combustors for combusting low heating value fuel gases have been developed at high cost for specific applications, such as for combusting blast furnace gases produced in the manufacture of steel. For example, Asai Brown Boveri Corp. has designed a gas turbine, Model No. 11N-2, which utilizes an oversized burner assembly to accommodate the mass flow of the low heating value fuel gas and utilizes undersized air “swirlers” to ensure a correct ratio of air to fuel in the burner assembly. However, this design is only operable within a narrow range of conditions and cannot be utilized with turbines having internal combustors. Thus, it is apparent that a need exists for an effective process which combusts a low heating value fuel gas and utilizes the resulting gaseous combustion products to drive a gas turbine.
Accordingly, it is an object of the present invention to provide an effective process for combusting a low heating value fuel gas in a combustor associated with a gas turbine and driving the gas turbine with the resulting gaseous combustion products. More particularly, it is an object of the present invention to provide substantially complete combustion of the low heating value fuel gas by dividing the low heating value fuel gas and combusting the divided portions of the low heating value fuel gas in different stages of a combustor. It is another object of the present invention to provide such a combustion process utilizing conventional combustors which are retrofitted at low cost to accommodate the low heating value fuel gas. It is yet another object of the present invention to provide such a combustion process which emits substantially reduced levels of contaminants to the environment. These objects and others are achieved in accordance with the invention described hereafter.
SUMMARY OF THE INVENTION
The present invention is a process for combusting a low heating value fuel gas in a combustor to drive an associated gas turbine. A combustor having utility in the present process comprises a burner nozzle, a combustor casing, and a combustion liner. The burner nozzle and combustion liner are mounted within the combustor casing, with the combustion liner being positioned downstream of the burner nozzle. The combustor casing and combustion liner define an annulus between them, including a mixing zone which is upstream of the burner nozzle and combustion liner. The combustion liner encloses a combustion chamber which includes a flame zone proximal to the burner nozzle and an oxidation zone downstream of the flame zone. A plurality of injectors are provided in the combustor casing upstream of the burner nozzle. A plurality of combustion chamber ports are provided in the combustion liner downstream of the injectors and burner nozzle.
The process comprises dividing a low heating value fuel gas feed into a relatively small minority burner portion and a relatively large majority combustion chamber portion. The combustion chamber portion of the low heating value gas feed is conveyed into the mixing zone through the injectors in the combustor casing. A combustion air is also conveyed through the annulus into the mixing zone and mixed with the low heating value gas feed at a relatively high velocity to form an air/fuel mixture. The ratio of combustible components to air in the resulting air/fuel mixture is substoichiometric and below the normal flammability limit.
A first portion of the air/fuel mixture is conveyed into the flame zone through a burner port which is adjacent to, but fluid isolated from the burner nozzle. A burner fuel gas, which includes the burner portion of the low heating value gas feed, is simultaneously conveyed into the flame zone through the burner nozzle. The burner fuel gas may also include a high heating value fuel gas feed to increase the overall heating value of the burner fuel gas. In any case, the burner fuel gas and first portion of the air/fuel mixture enter the flame zone in relative amounts such that the ratio of combustible components to air in the flame zone approximates a stoichiometric ratio which is within the normal flammability limit and supports a diffusion combustion flame.
The burner fuel gas contacts the first portion of the air/fuel mixture in the flame zone at a relatively low velocity, creating a diffusion combustion flame which combusts the combustible components present in the flame zone and produces flame zone products. The resulting flame zone products are conveyed from the flame zone into the oxidation zone. A second portion of the air/fuel mixture is also conveyed into the oxidation zone from the mixing zone through the combustion chamber ports. The second portion of the air/fuel mixture is combusted in the oxidation zone in the presence of the flame zone products to produce combustion products. The combustion products are conveyed into a gas turbine and drive the gas turbine.
In accordance with a specific embodiment of the present invention, the first portion of the air/fuel mixture is conveyed from the mixing zone to the burner port countercurrent to the direction of flow in the combustion chamber. Similarly, the second portion of the air/fuel mixture is conveyed from the mixing zone to the combustion chamber ports countercurrent to the direction of flow in the combustion chamber. The combustion air is also conveyed into the mixing zone countercurrent to the dir
Ebel Jack E.
Marathon Oil Company
Padmanabhan Sreeni
Richter Johann
LandOfFree
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