Patent Application
20120231400
This application claims priority from United Kingdom Patent Application No. 10 14 969.8, filed Sep. 9, 2010 the entire disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to burners, particularly but not necessarily burners for use in radiant-wall side-fired furnaces of the kind used in endothermic processes such as hydrogen reforming, ammonia reforming, ethylene cracking and ethylene dichloride (EDC) cracking.
2. Description of the Related Art
Considering hydrogen reforming as an example, this is an industrial process for the production of hydrogen by reacting hydrocarbons such as natural gas with steam in the presence of a catalyst. In the primary reaction, methane reacts with water to yield carbon monoxide plus hydrogen:
CH4+H2O→CO+3H2 [Equation 1]
The reaction is highly endothermic, and therefore the process is carried out in a furnace, typically with wall-mounted burners, to maintain a temperature of typically 1050° C. to 1200° C. The burners are commonly fuelled with natural gas, liquefied petroleum gas (LPG) or refinery gas which may contain varying amounts of hydrogen, and sometimes additionally with residual hydrocarbon gas from the reforming process, sometimes blended with or supported by other fuels such as methane. At start-up, natural gas (which is mostly methane) or LPG is usually used as fuel.
NOx formation may be controlled by staging technology in which the supplies of fuel and/or combustion air are adjusted. It follows that the burners are required to handle a range of fuels, and over recent years this range has been extended by a trend towards the use of fuels containing a higher proportion of hydrogen.
The operation of an industrial reformer or cracker gives rise to two notable and related problems, flashback and noise, as will now be discussed.
Hitherto, hydrogen reforming (and other endothermic processes such as ammonia reforming, ethylene cracking and EDC cracking) has used pre-mix burners in which a jet of fuel gas is injected at the outer end of a venturi tube extending through the wall of the furnace and in the venturi tube naturally inspired combustion air is mixed with the fuel to form a combustible mixture ignited at the inner end of the tube, in the furnace. Fuels with relatively high flame speeds, such as those containing relatively high proportions of hydrogen which are now increasingly used, may cause a flashback. At a minimum, this reduces the performance of the plant; and if it results in damage to the burner, the cost of repair or replacement is considerable, especially if the plant has to be shut down. With hundreds of burners in a typical furnace, the risk of flashback in at least one is high.
Hundreds of burners create a great deal of noise, from the combustion itself and from the delivery of combustion air, and the fact that the venturi tubes of the conventional burners provide hundreds of essentially straight passages through the wall of the furnace exacerbates the impact of this noise on the environment of the plant.
Generally, permitted noise levels are limited by environmental controls and/or health and safety regulations. To keep reformer noise levels within such limits it has been necessary to fit a silencer in the form of a sound enclosure on the outer end of each burner. However, in conventional burners this constrains the flow of combustion air, so the risk of flashback is increased.
The constriction in the flow of combustion air can be countered by providing a fan to deliver a forced draught, rather than natural inspiration. However this increases both capital cost and running cost, and also introduces another source of noise outside the furnace.
It is a primary object of the present invention to enable a furnace to operate with a range of fuel gases wider than hitherto, including particularly gases with a higher concentration of hydrogen than hitherto. At the same time the invention facilitates operation without forced draught while meeting permitted noise levels and substantially avoiding a risk of flashback.
Thus according to a first aspect of the invention there is provided a burner for an endothermic process (e.g. hydrogen reforming or ammonia reforming or ethylene cracking or EDC cracking), which burner comprises a fuel duct having an axially extending portion extending axially forwards from a proximal end to discharge fuel at a distal end and an air duct having an axially extending portion extending axially forwards from said proximal end to discharge combustion air at said distal end, wherein after discharge the discharged fuel burns in the presence of the discharged combustion air and draws the combustion air through the air duct by natural inspiration, wherein the fuel duct has at said distal end a laterally extending portion whereby the fuel is discharged laterally and the air duct has at said distal end a laterally extending portion whereby the combustion air is discharged laterally.
Preferably the air duct has an axially extending portion circumjacent an axially extending portion of the fuel duct.
Preferably the air duct comprises at its distal end an annular chamber extending laterally outwards towards a lateral opening for the discharge of the combustion air. The annular chamber may comprise a dished section facing rearwards to turn the combustion air from an axial flow to a lateral flow, and it may be divergent towards its lateral opening. Also, the air duct may adjacent its distal end comprise a section convergent towards the annular chamber.
Preferably the fuel duct comprises at its distal end a plurality of ports extending within the annular chamber towards the lateral opening thereof. These ports may comprise a first set of ports for a main fuel supply and a second set of ports for a stabilising fuel supply, and the ports of the second set may be inclined rearwardly of the ports of the first set. The fuel duct may comprise one fuel passage connected to the first set of ports and a separate second fuel passage connected to the second set of ports and there may also be a third set of ports for fuel staging.
In another aspect, and in contrast with the general use hitherto of pre-mix burners, the invention provides a furnace for an endothermic process, wherein the furnace comprises a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration. The nozzle-mix burners are preferably burners according to the first aspect of the invention with the air and fuel ducts thereof extending through a wall of the furnace. The furnace may be configured and arranged to operate within a specified noise limit, i.e. so that meets permitted noise levels without a substantial risk of flashback.
In a third aspect the invention provides a method of operating a furnace for an endothermic process, wherein the furnace is heated by a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration, and the nozzle-mix burners may be burner according to the first aspect of the invention with the air and fuel ducts thereof extending through a wall of the furnace.
In a fourth aspect the invention provides a method of modifying a furnace equipped with a plurality of wall-mounted burners of the premix type, wherein said method comprises replacing the pre-mix burners with nozzle-mix burners according to the first aspect of the invention.
The invention will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which:
Referring first to
The burner 10 is of the pre-mix type. It comprises a venturi tube 14 extending through the wall 12 from an outer, proximal, end to an inner, distal, end. At its proximal end gaseous fuel G (indicated in the drawings by dark arrows) is injected into the venturi tube 14 by way of a jet 16. Combustion air A (indicated by white arrows) is naturally inspirated into the venturi tube 14 wherein it mixes with the fuel G to form a combustible mixture M (indicated by light arrows). The fuel-air mixture M flows through the venturi tube 14 to burn at its proximal end as indicated schematically at F.
Burners such as the burner 10 are widely used in furnaces for endothermic processes such as hydrogen reforming, and have proved generally satisfactory. However we have identified two problems with them. First, they must work with a range of fuels—typically natural gas or LPG at start-up and subsequently refinery gas of varying composition or some byproduct of the reforming process such as, in the case of hydrogen reforming, tail gas residual to pressure swing adsorption (PSA) purification of the hydrogen—and fuels with a relatively high flame speed increase the risk of flashback. Gases containing hydrogen generally have a higher flame speed than natural gas and so give rise to more flashback incidents when used as a fuel—reducing plant performance, at a minimum, and adding costs in rectification. The second problem with burners such as the burner 10 is that they emit a lot of noise. With a view to containing this, the burner 10 has a silencer 18 fitted around the proximal end of the venturi tube 14. The silencer 18 is effective in containing the noise of operation, but it adds to the cost of each burner and the inevitable constriction of the air flow A further increases the risk of flashback.
At the distal end of the fuel duct 24 is an array of nozzles indicated at 26 each extending radially outwards, i.e. laterally of the axis Y-Y of the fuel duct 24. The nozzles, which are arranged to spray fuel G outwards as indicated at 28, comprise two sets: a first set 26a of twelve main gas nozzles equiangularly disposed around the fuel duct 24, as shown in
Circumjacent the fuel duct 24 and coaxial with it is a primary air duct 30 at the distal end of which is a convergent-divergent section 32. The primary air duct 30 delivers combustion air A to the distal end of the burner 20 so that the spray 28 of fuel G can be ignited (by means not detailed) and burn as indicated at F.
A cap 36 is mounted on the distal end of the fuel duct 24. The cap 36 extends across the distal end of the primary air duct 30 and spaced apart from it to form therewith a generally annular chamber 40, laterally open around its outer periphery, into which the fuel G is sprayed by the nozzles 26. The cap 36 is dished at 42 to help turn the air A into and through the annular chamber 40.
As indicated at 44, the annular chamber 40 diverges outwardly so that it forms, with the convergent-divergent section 32 of the primary air duct 30, a radially extending venturi. This venturi supplements the entrainment effect of the spray 28 and the combustion of the fuel G to draw the air A through the primary air duct 30 by natural inspiration.
Around the periphery of the annular chamber 40 and on each side thereof is a recess 46 bounded by a lip or ring 48 that serves to stabilise combustion when fuels of relatively low flame speed are used, e.g. at furnace start-up.
Circumjacent the primary air duct 30 and coaxial with it is a secondary air duct 50 for delivering secondary air S to the combustion zone. The primary air duct 30 and the secondary air duct 50 each contain air control vanes 34.
Those skilled in the art will appreciate from the foregoing description that a burner 20 according to the present invention operates by nozzle-mixing rather than premixing as in prior art burners like those of
By the above means the present invention secures the benefits of nozzle-mixing in limiting flashback without the need for forced draught air supply. However those skilled in the science will appreciate that the invention is not limited to the specific details set forth above, and in general terms it provides a nozzle-mixed burner with air supplied by natural inspiration.
Other possible modifications will be apparent to those skilled in the science.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 14 969.8 | Sep 2010 | GB | national |
