Remote staged radiant wall furnace burner configurations and methods

The present invention relates to remote staged radiant wall furnace burner configurations, and more particularly, to the placement of secondary gas nozzles remote from the radiant wall burner nozzles resulting in lower NO_X production.

Radiant wall gas burner furnaces are well known and have been used in reforming and cracking operations and the like for many years. Radiant wall burners generally include central fuel gas-air mixture burner tubes surrounded by annular refractory tiles which are adapted for insertion into openings in the furnace wall. The burner nozzles discharge fuel gas-air mixtures in directions generally parallel and adjacent to the internal faces of the refractory tiles. The combustion of the fuel gas-air mixtures causes the faces of the burner tiles to radiate heat, e.g., to process tubes, and undesirable flame impingement on the process tubes is thereby avoided.

Radiant wall burners are typically installed in several rows along a furnace wall. This type of configuration is usually designed to provide uniform heat input to the process from the wall area comprising the radiant wall burner matrix.

More stringent environmental emission standards are continuously being imposed by governmental authorities which limit the quantities of gaseous pollutants such as oxides of nitrogen (NO_X). Such standards have led to the development of staged or secondary fuel burner apparatus and methods wherein all of the air and some of the fuel is burned in a first zone and the remaining fuel is burned in a second downstream zone. In such staged fuel burner apparatus and methods, an excess of air in the first zone functions as a diluent which lowers the temperature of the burning gases and thereby reduces the formation of NO_X. Desirably, furnace flue gas functions as a diluent to lower the temperature of the burning secondary fuel and thereby reduces the formation of NO_X.

Similarly, staged radiant wall burner designs have also been developed wherein the burners radially combust a primary fuel lean mixture of fuel gas and air and stage fuel risers supply the stage tips with secondary fuel. The location of the secondary fuel tips can vary, depending on the manufacturer and type of burner, but they are typically located either in the center of the burner tip or around and adjacent to the perimeter of the tip.

While the staged radiant wall burners and furnace designs have been improved whereby combustion gases containing lower levels of pollutants are produced, additional improvement is necessary. Thus, there is a need for improved methods of burning fuel gas and air using radiant wall burners whereby combustion gases having lower pollutant levels are produced.

<patcit id="pcit0001" dnum="US2002155046A"><text>US 2002/155046</text></patcit> discloses a pyrolysis heater particularly for the cracking of hydrocarbons in the production of olefins, wherein the pyrolysis heater has a burner arrangement in the firebox which directly heats the hearth of the firebox such that it becomes a radiant surface. The base burners may fire horizontally across the hearth or may comprise porous ceramic burners as a part of the hearth. The base burners operate along with vertically firing hearth burners and optional wall burners in the upper portion of the firebox.

A radiant wall furnace burner configuration is provided utilizing rows of multiple radiant wall burners that burn fuel-gas air mixtures inserted in a wall of the furnace with a regular spacing. In accordance with this invention, one or more arrays of secondary fuel gas nozzles are also provided located separate and remote from the radiant wall burners. Secondary fuel gas is introduced into the fuel gas nozzles in an amount that constitutes a substantial portion of the total fuel provided to the combustion zone by the fuel gas-air mixtures and the secondary fuel gas. Preferably the secondary fuel gas nozzles are positioned on the furnace wall adjacent to the rows of radiant wall burners or on the furnace floor, or both, and direct secondary fuel gas to a location on the opposite side of the combustion zone from the radiant wall burners. As a result, NO_X levels in the combustion gases leaving the furnace are substantially reduced.

In a preferred arrangement, the furnace wall is at least substantially vertical and the radiant wall burners are approximately parallel and approximately evenly spaced in rows and columns, and the secondary fuel gas nozzles are positioned in a single row with each nozzle positioned directly below a radiant wall burner in the row above. In another preferred configuration, the radiant wall burners are approximately parallel with the burners approximately evenly spaced in rows and columns, and the secondary fuel gas nozzles are positioned below the radiant wall burners in an upper row and a lower row, wherein each nozzle of the upper row is directly below a burner in the row above and wherein each nozzle of the lower row is midway between the horizontal positions of the nozzles directly above it. In yet another preferred configuration, the radiant wall burners are offset halfway from one another in a staggered positioning, and the secondary fuel gas nozzles are positioned in a single or double row directly below the radiant wall burners with each nozzle positioned to continue the staggered positioning. In still another configuration, a first row of secondary fuel gas nozzles is located below all the radiant wall burners and a second row of secondary gas nozzles is located about midway up the rows of radiant wall burners.

In other preferred arrangements, secondary fuel gas nozzles are also located on the furnace floor, and the furnace can include floor burners (also referred to as hearth burners) with or without secondary fuel gas nozzles on the floor.

Preferably, the secondary fuel gas nozzles have tips with at least one fuel delivery orifice designed to eject fuel gas at an upward angle relative to the longitudinal axis of the nozzle. More preferably, the secondary fuel gas nozzles have multiple fuel delivery orifices.

An embodiment of the present invention provides method of burning fuel gas and air in a radiant wall furnace whereby flue gases of reduced NO_X content are formed comprising the steps of: <ol id="ol0001" compact="compact" ol-style=""><li>(a) providing a fuel lean mixture of fuel gas and air to individual radiant wall burners arranged in rows along a wall of the furnace;</li><li>(b) causing the mixture of fuel gas and air to flow radially outward from each radiant wall burner across the wall of the furnace whereby the mixture contains excess air and is burned at a relatively low temperature and flue gases having low NO_X content are formed therefrom; and</li><li>(c) providing secondary fuel gas from secondary fuel gas nozzles for mixing with flue gases in the furnace and combusting with excess air from the radiant wall burners, lowering the temperature of the burning fuel gas and reducing the formation of NO_X, wherein the secondary fuel gas nozzles are attached to the same wall as the radiant wall burners and wherein said secondary fuel gas nozzles directing secondary fuel gas to a location in the furnace on the opposite side of the combustion zone from the radiant wall burners and wherein the secondary fuel gas nozzles are located separate and remote from said radiant wall burners such that the secondary fuel gas is not encapsulated or surrounded by the mixture of fuel gas and air from said burners thereby allowing secondary fuel gas to mix with flue gases in the furnace prior to mixing with said mixture of fuel gas and air from said burners said mixing occurring in the combustion zone.</li></ol>

Other features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> illustrates the gas flow pattern using conventional staging with secondary fuel gas in the center of each burner.</li><li><figref idref="f0002">FIG. 2</figref> illustrates the gas flow pattern of the present invention with remote staging of fuel gas.</li><li><figref idref="f0003">FIG. 3</figref> is a preferred remote staging burner configuration on the wall of a radiant fuel gas fired furnace.</li><li><figref idref="f0003 f0004 f0005">FIGS. 4A - 4D</figref> illustrate other preferred remote staging configurations on the wall of a radiant fuel gas fired furnace.</li><li><figref idref="f0006 f0007 f0008">FIGS. 5A - 5F</figref> illustrate remote staging configurations that include additional secondary fuel gas discharge nozzles on the furnace floor with and without floor burners.</li><li><figref idref="f0009">FIG. 6</figref> is a side view of a preferred secondary fuel gas discharge nozzle for use in accordance with this invention.</li><li><figref idref="f0009">FIG. 7</figref> is a top view of the secondary fuel gas discharge nozzle of <figref idref="f0009">FIG. 6</figref>.</li><li><figref idref="f0010">FIG. 8</figref> is a graph comparing NO_X emissions from a test furnace with and without the remote staging technique of this invention.</li></ul>

A preferred radiant wall furnace burner configuration of this invention utilizes rows of multiple radiant wall burners that include annular refractory tiles and burn fuel gas lean fuel gas-air mixtures connected to a wall of the furnace in a regular spacing and an array of secondary fuel gas nozzles located separate and remote from the radiant wall burners with means for introducing secondary fuel gas into the secondary fuel gas nozzles and wherein the secondary fuel gas constitutes a substantial portion of the total fuel provided to the combustion zone by the fuel gas-air mixtures and the secondary fuel gas. Preferably, the secondary fuel gas nozzles are positioned on the furnace wall adjacent to the rows of radiant wall burners or on the furnace floor, or both, and direct secondary fuel gas to various locations including a location on the opposite side of the combustion zone from the radiant wall burners. As a result, NO_X levels in the combustion gases leaving the furnace are reduced.

Referring now to the drawings, <figref idref="f0001">FIG. 1</figref> depicts a traditional burner column 11 of staged fuel radiant wall burners 10. The staged fuel radiant wall burners 10 consist of radiant wall burner tips 12 which are provided with a fuel gas lean mixture of primary fuel gas and air. Secondary fuel gas risers 14 supply the secondary fuel gas tips 16 thereof with fuel gas. The location of the secondary fuel gas tips 16 is typically in the centers of the radiant wall burner tips 12 as shown in <figref idref="f0001">FIG. 1</figref>, or around the perimeters of the radiant wall burner tips 12. As shown in <figref idref="f0001">FIG. 1</figref>, the fuel gas-air streams exiting the burner tips 12 form barriers 18 and 20 and encapsulate or surround the secondary fuel gas 22. The fuel gas-air barriers 18 and 20 around the secondary fuel gas 22 prevents sufficient entrainment of flue gas 24 resulting in increased NO_X emissions.

In the remote staged fuel technique of the present invention, the secondary fuel gas from or adjacent each radiant wall burner 10 is eliminated. Instead, the secondary fuel gas is injected into the furnace at a remote location. As shown in <figref idref="f0002">FIG. 2</figref>, by moving the secondary fuel gas to a remote secondary fuel gas nozzle 26 located, for example, below the burner column 11, the secondary fuel gas 22 is able to mix with the furnace flue gases 24 prior to mixing with the fuel gas-air mixture 18 in the combustion zone 28. It has been found that by using one or more remote secondary fuel gas nozzles 26 positioned at remote locations and providing secondary fuel gas patterns, reduced NO_X emissions are achieved as well as improved flame quality compared to state-of-the-art radiant wall burner designs.

Referring to <figref idref="f0003">FIG. 3</figref>, an improved radiant wall furnace burner configuration of this invention is illustrated and generally designated by the numeral 30. Rows 32 of multiple radiant wall burners 10 are inserted in a wall 31 of the furnace. The radiant wall burners 10 discharge fuel gas-air mixtures in radial directions across the face of the furnace wall 31. Radiant heat from the wall, as well as thermal radiation from the hot gases, is transferred, for example, to process tubes or other process equipment designed for heat transfer.

Each radiant wall burner 10 is provided a mixture of primary fuel gas and air wherein the flow rate of air is greater than stoichiometry relative to the primary gas. Preferably the rate of air is in the range of from about 105% to about 120% of the stoichiometric flow rate required to completely combust the primary and secondary fuel gas. Secondary fuel gas is discharged into the furnace by way of secondary fuel gas nozzles 26. The burner configuration of <figref idref="f0003">FIG. 3</figref> shows the secondary fuel gas nozzles 26 arranged in a row 32 with each secondary fuel gas nozzle positioned below a column 34 of radiant wall burners. The secondary fuel gas nozzles are made to discharge fuel gas in a direction generally toward the radiant wall burners as will be explained in detail below.

Additional examples of preferred patterns are illustrated in <figref idref="f0003 f0004 f0005">FIGS. 4A - 4D</figref>. Rows of radiant wall burners 10 can be approximately parallel, the burners 10 can be approximately evenly spaced in columns 34 and the secondary fuel gas nozzles 26 can be positioned in a single row 32 with each nozzle directly below a radiant wall burner 10 in the row above as shown in <figref idref="f0003">FIG. 3</figref>, or offset as shown in <figref idref="f0003">FIG. 4A</figref>. As shown in <figref idref="f0004">FIG. 4B</figref>, in another preferred configuration, the radiant wall burners 10 are in columns approximately parallel, the radiant wall burners 10 are approximately evenly spaced in columns 34 and the secondary fuel gas nozzles 26 positioned below the radiant wall burners 10 are in two rows, an upper row 36 and a lower row 38, wherein each secondary fuel gas nozzle of the upper row 36 is below a burner in the row above and wherein each secondary fuel gas nozzle of the lower row 38 is midway between the horizontal positions of the secondary fuel gas nozzles directly above it in row 36. In yet another preferred configuration shown in <figref idref="f0004">FIG. 4C</figref>, the radiant wall burners 10 are offset halfway from one another, resulting in a diamond shaped pattern with the secondary fuel gas nozzles 26 located below the radiant wall burners and continuing the pattern. In still another preferred configuration, shown in <figref idref="f0005">FIG. 4D</figref>, about half of the radiant wall burners 10 are approximately evenly spaced in rows and columns 40 with a row 42 of secondary fuel gas nozzles 26 positioned directly below. The remaining radiant wall burners 10 are below row 42 of secondary fuel gas nozzles and arranged in columns 44. A second row 46 of secondary fuel gas nozzles 26 is located directly below the burner columns 44.

The furnace walls 31 with the radiant wall burners 10 and secondary fuel gas nozzles 26 connected thereto are described above as if the walls are vertical, but it is to be understood that the walls can be at an angle from vertical or the walls can be horizontal.

Referring now to <figref idref="f0006 f0007 f0008">FIGS. 5A - 5F</figref>, alternate arrangements of secondary fuel gas nozzles 26 in accordance with the present invention are shown with and without floor burners 54 (also referred to as hearth burners). Referring to <figref idref="f0006">FIGS. 5A and 5B</figref>, rows of multiple radiant wall burners 10 are inserted in a wall 31 of a furnace. As previously mentioned, the burners 10 discharge fuel gas-air mixtures in directions across the face of the furnace wall 31. Each radiant wall burner is provided a mixture of primary fuel gas and air wherein the flow rate of air is greater than stoichiometry relative to the primary gas, i.e., in the range of from about 105% to about 120% of the stoichiometric flow rate. Secondary fuel gas is discharged into the furnace by way of secondary fuel gas nozzles 26 disposed below the columns of radiant gas burners 10. In addition, secondary fuel gas nozzles 26 are disposed in the floor of the furnace to provide additional secondary fuel gas that mixes with excess air and furnace flue gases whereby low NO_x levels are produced.

Referring now to <figref idref="f0007">FIGS. 5C and 5D</figref>, a similar arrangement of radiant wall burners 10 and secondary fuel gas nozzles 26 is illustrated. In addition, floor burners 54 are provided adjacent to the wall 31 that mix fuel gas with an excess of air, and the secondary fuel gas nozzles 26 discharge fuel gas toward both the radiant wall burners and the floor burners whereby the secondary fuel gas readily mixes with furnace flue gases and excess air so that low NO_X levels are produced.

Referring now to <figref idref="f0008">FIGS. 5E and 5F</figref>, instead of providing secondary fuel gas nozzles 26 that discharge fuel gas toward both the radiant wall burners and the floor burners, additional secondary fuel gas nozzles can be provided in the floor of the furnace to mix with furnace flue gases and the excess air produced by the floor burners whereby low NO_x levels are produced.

Thus, as will now be understood by those skilled in the art, a variety of combinations of radiant wall burners 10 and separate and remote secondary fuel gas nozzles can be utilized in radiant wall gas burner furnaces in accordance with this invention to reduce NO_X levels in furnace flue gases.

Any radiant wall burner can be used in the present inventive configurations and methods. Radiant wall burner designs and operation are well known to those skilled in the art. Examples of radiant wall burners which can be utilized include, but are not limited to, the wall burners described in <patcit id="pcit0002" dnum="US5180302A"><text>U.S. Pat. No. 5,180,302 issued on Jan. 19, 1993 to Schwartz et al.</text></patcit>, and in <patcit id="pcit0003" dnum="US94900701A" dnum-type="L"><text>U.S. patent application Ser. No. 09/949,007, filed Sept. 7, 2001 by Venizelos et al</text></patcit>. and entitled "High Capacity/Low NOx Radiant Wall Burner," the disclosures of which are both incorporated herein by reference.

Preferably the total fuel gas-air mixture flowing through the radiant wall burners contains less than about 80% of the total fuel supplied to the combustion zone 28.

Secondary fuel nozzles 26 are inserted through the furnace wall or floor extending about 1 to about 12 inches into the furnace interior. Fuel gas is preferably supplied at a pressure in the range of from about 20 to about 50 psig.

The secondary fuel gas nozzles 26, as illustrated in <figref idref="f0009">FIGS. 6 and 7</figref>, have tips 16 with secondary fuel gas delivery openings 48 therein for directing the flow of secondary fuel gas into the furnace space 50. The openings 48 direct secondary fuel towards and away from a wall of the furnace at an angle α in the range of about 60° to about 120° from the longitudinal axis.

In a preferred embodiment, the secondary fuel gas nozzle tips 16 include additional side delivery openings 52 for discharging secondary fuel gas in various directions over angles β in the range of from about 10° to about 180° from both sides of a vertical plane through the longitudinal axis, and more preferably at an angle in the range of about 20° to about 150°. As will be understood by those skilled in the art, the secondary fuel gas nozzle tips can include multiple openings 48 and 52 positioned to discharge fuel gas toward and/or away from the furnace wall depending on the radiant wall and other burner configurations used and other factors.

A method of the present invention for burning fuel gas and air in a radiant wall furnace whereby flue gases of reduced NO_X content are formed comprises the following steps: <ol id="ol0002" compact="compact" ol-style=""><li>(a) providing a fuel lean mixture of fuel gas and air to individual radiant wall burners arranged in rows along a wall of the furnace;</li><li>(b) causing the mixture of fuel gas and air to flow radially outward from each radiant wall burner across the wall of the furnace whereby the mixture contains excess air and is burned at a relatively low temperature and flue gases having low NO_X content are formed therefrom; and</li><li>(c) providing secondary fuel gas from secondary fuel gas nozzles for mixing with flue gases in the furnace and combusting with excess air from the radiant wall burners, lowering the temperature of the burning fuel gas and reducing the formation of NO_X, wherein the secondary fuel gas nozzles are attached to the same wall as the radiant wall burners and wherein said secondary fuel gas nozzles directing secondary fuel gas to a location in the furnace on the opposite side of the combustion zone from the radiant wall burners and wherein the secondary fuel gas nozzles are located separate and remote from said radiant wall burners such that the secondary fuel gas is not encapsulated or surrounded by the mixture of fuel gas and air from said burners thereby allowing secondary fuel gas to mix with flue gases in the furnace prior to mixing with said mixture of fuel gas and air from said burners said mixing occurring in the combustion zone.</li></ol>

In order to further illustrate the furnace burner configuration and method of the present invention, the following example is given.

A comparison was made of the NO_X emissions using radiant wall burners with and without remote staging. The test furnace utilized an array of 12 radiant wall burners arranged in 3 columns of 4 burners each. The burners were spaced 50 inches apart in each column and the columns were spaced 36.5 inches apart. The furnace was operated while supplying secondary gas to the center of the radiant wall burners and the NO_X in the furnace off gas was measured over time. The furnace was then operated after removing secondary gas from the burner centers and conducting the secondary gas to remote nozzles located adjacent to the columns of radiant wall burners.

<figref idref="f0010">FIG. 8</figref> is a plot comparing NO_X emissions from the furnace with and without the remote staging configuration. The data demonstrate that NO_X emissions are reduced by 50% using the remote staging configuration.

Thus, the present invention is well adapted to attain the objects and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the scope of this invention as defined by the appended claims.