The embodiments disclosed herein relate to gas burners and to the firing of such burners.
Burners are known that use fuel to inspirate air through a venturi tube and introduce a premixed air-fuel mixture that then travels into a furnace. The venturi assembly, specifically the throat area of the venturi tube, is designed such that for the desired fuel flow, the amount of air that is inspirated is slightly above the stoichiometric amount of air required for complete combustion. The air required for complete combustion is defined as the air flow that provides the oxygen necessary for combusting the fuel to CO2 and H2O. Typically, there is a deflector, cap or grill assembly downstream of the venturi assembly in order to alter the flow direction of the mixture to control the direction of the flame, and/or to create sufficient velocity exiting the burner to prevent flashback. Flashback is a phenomenon in which the speed of the combustion reaction (burning) is faster than the speed of the effluent from the burner, and the combustion can thus travel backward into the burner itself and result in damage to the burner assembly by the high temperatures of combustion.
U.S. Pat. No. 6,616,442 discloses a burner that is designed to be located in the floor of a furnace for firing vertically up a radiant wall. There is a primary nozzle that inspirates air into a venturi assembly and a grill located downstream of the venturi assembly is designed to increase the velocity of the fuel-air mixture entering the furnace in order to prevent flashback. The venturi assembly is designed such that only a portion of the fuel to be fired in the total burner is used to inspirate all of the required air. Thus, the venturi assembly has an effluent of premixed air-fuel that is air-rich (lean). The balance of the fuel is added in secondary ports located on the edge of the burner.
Burners incorporating lean premix (LPM) technology are known. LPM technology has been used in low NOx burners and uses a venturi assembly to inspirate air. This arrangement is designed to form a lean (air rich) fuel mixture that enters the furnace. Secondary fuel ports that are included in the burner are located outside the venturi assembly and add additional fuel to reach generally slightly above stoichiometric combustion conditions. It is important to note that the location of the fuel injection points for the burner determines the quality of the flame and the NOx production of that flame. If reduced airflow is desired, the fuel to the primary port is reduced. This will inspirate less air. Alternately a damper upstream of the venturi is used to create a pressure drop that would inhibit the flow of air to the venturi. This reduced airflow creates a different air-fuel mixture in the venturi assembly effluent. In the extreme, no fuel is provided at that point and air is drawn through the venturi based only upon the natural draft of the furnace itself. The flame created with an extremely fuel lean mixture (a low amount of fuel premixed with the air) and substantial fuel fired in secondary ports will be unstable.
U.S. Pat. No. 6,607,376 discloses a burner for firing on the wall of a furnace. The burner consists of a venturi assembly in which the air flow is created by the flow of the total fuel through a primary port at the venturi throat. The venturi assembly is designed such that the quantity of air inspirated by the fuel will result in an air-fuel mixture slightly above stoichiometric. The fuel flow at the primary location and the damper assembly are the means for changing air flow. The premixed air-fuel mixture leaving the venturi is then directed along the wall by a cap with orifices to promote radial flow from the wall burner.
U.S. Pat. No. 6,796,790 also discloses a burner for firing on the wall of the furnace. In the described embodiment, primary fuel is used to inspirate air through a venturi assembly. The venturi assembly is designed such that the fuel will provide excess air with respect to the primary fuel. The air rich (fuel lean) effluent from the venturi assembly is then directed through a cap with orifices to direct the flame along the walls of the furnace. In this case, however, additional fuel is injected on the outside of the venturi assembly and cap directly into the furnace. This fuel mixes with the air rich mixture as the mixture exits the cap assembly with the resulting air-fuel mixture in the vicinity of the burner being slightly above stoichiometric.
Stoichiometric combustion is defined as the quantity of air (or oxygen) that will completely combust the fuel to carbon dioxide and water. This corresponds to the maximum flame temperature for the fuel. Typically, combustion is operated at a slight excess of air, typically 10-15%. This provides control over the combustion but minimizes the energy loss created by higher amounts of excess air leaving the furnace at temperatures above ambient. If combustion is operated below stoichiometric conditions (fuel rich) unburned fuel remains in the flue gas representing energy losses as well as pollution. If combustion is operated well above stoichiometric, then there is a significant energy penalty due to the hot excess air leaving the system.
Thermal NOx formation is influenced by flame temperature. The maximum flame temperature is at the point of stoichiometric combustion. This will form maximum thermal NOx. Technology is known such that operation under air rich (above stoichiometric) or fuel rich (sub-stoichiometric) conditions will reduce flame temperatures and hence NOx. Certain low NOx burners are designed for lean conditions from the venturi to lower the primary flame temperature and reduce NOx but then inject (stage) secondary fuel into the primary flame above the burner to give slightly above stoichiometric conditions in total. The net result of staging is a lower combustion temperature since there is also mixing of lower temperature flue gases in the furnace with the combusting gases of the flame.
U.S. Patent Publication No. 2005/0106518 A1 includes a burner layout and firing pattern arrangement in which hearth burners of an ethylene furnace are operated with air in amounts above stoichiometric levels. The excess air is created not by increasing the air flow but by removing fuel from the secondary ports of hearth burners and then injecting that fuel through the wall of the heater just above the hearth burner. This pulls the flame to the wall by creating a low pressure zone behind the principal flame from the hearth burner. The flow of fuel through the primary port still controls the total amount of air inspirated and the air flow for that burner remains the same.
In the design of venturi assemblies for either hearth or wall burners, a very important characteristic is the volumetric heating value of the fuel and the required air to fuel ratio to achieve stoichiometric combustion. Typical gaseous fuel for ethylene plants or refinery heaters is a mixture consisting primarily of methane and hydrogen. This fuel requires approximately 20 pounds of air per pound of fuel to supply the oxygen required for stoichiometric combustion. However in some other combustion cases, other fuels may represent more desirable options. One such fuel is a synthesis gas consisting of a mixture of carbon monoxide (CO) and hydrogen. This mixture has a lower volumetric heat release and requires considerably less air for stoichiometric combustion, on the order of 3 pounds of air per pound of fuel. Volumetric heat release is defined as the heat released from complete combustion per volume of fuel. For example, if a fuel includes CO, the carbon is already partially oxidized (burned) and thus there is less energy released when the CO is burned to CO2 than if that fuel contained only hydrocarbon species.
If a burner with a typical venturi assembly is designed for a given fuel, for example a methane-hydrogen mixture, it is very difficult to operate that burner with a fuel of significantly lower volumetric heat release, for example synthesis gas. For the same mass flow of primary fuel into the venturi throat as a methane-hydrogen fuel, a synthesis gas would inspirate the equivalent amount of air. This would represent considerably more air than required for combustion since the methane-hydrogen mix requires an air to fuel ratio of 20 compared to the synthesis gas required air-fuel of 3 for stoichiometric conditions. Thus, furnaces with burners designed to operate with one gaseous fuel can not be operated efficiently with significantly different fuel requiring different air flows. If a burner is designed for synthesis gas fuel, it can not readily be adapted to combust other fuels in the event the synthesis gas for which it was designed becomes unavailable.