Patent Application
20110000261
None.
In the production of glass, molten glass is first formed by melting raw materials in a glass melting furnace. This molten glass passes through a forehearth section to achieve temperature uniformity and required properties, for example, viscosity, before reaching the molding section (or forming machines) where the glass is given the desired shape. The forehearth section is usually a long refractory-lined channel. In the forehearth section, multiple burners are installed to supply heat along the length of the channel to maintain the glass at a very specific temperature profile. A typical forehearth contains tens or even hundreds of burners. Due to the large number of burners, it is extremely beneficial to have burners that require very low maintenance.
Due to the high temperature of molten glass in the forehearth section, volatilization of substances present in the flow channel such as glass, boron, sulfur etc. can occur in significant quantities. These volatilized substances can re-condense on any cooler surfaces present within the forehearth. The outer surface of the burner can be one such relatively low temperature surface due to the cooling action of the fuel and oxidant that continuously flows inside the burner body.
Deposition of volatile substances on the burner body can cause significant issues. If the deposition takes place close to the burner nozzle, over time it can start to block the nozzle and interfere with the flame from the burner. Also, deposits on the burner body can build up over time and seal the gap between the burner and the burner block, thus making the burner difficult to remove from the block. To avoid these issues, regular cleaning of burner bodies may be required. Frequent maintenance of these burners can be expensive and impractical due to the large number of burners normally installed in forehearths. Thus, it is highly desirable to have burners that are less prone to the deposition phenomenon and thus require very little maintenance.
Many have proposed solutions to the above problem.
U.S. Pat. No. 5,931,654 discloses the injection of a purge gas coaxially around a nozzle injecting a main gas in order to protect the nozzle from furnace gases entering the burner block passage and attacking the nozzle. It requires that the amount of purge gas injected exceed 50% of the total amount of the main gas and purge gas injected. The purge gas has a velocity of at least 100 ft/s. The nozzle injects only one gas, either fuel or oxidant, and not both simultaneously and hence acts as a lance and not a burner.
U.S. Pat. No. 5,295,816 discloses the injection of low velocity gas around a nozzle injecting high velocity gas such that the low velocity gas forms a protective barrier around the nozzle from combustion zone damage. The amount of protective gas injected is in the range of 10-50% of the total amount of gas injected into the cavity. The nozzle injects a single main gas (not fuel and oxidant combined) and the low velocity gas has a composition substantially similar to the main gas. The high velocity gas is at a velocity of 200-2000 ft/s (60.96-609.60 m/s) while the velocity of the low velocity gas is 5-100 ft/s (1.5-30.5 m/s). The disclosure pertains to a lance and not a burner.
Other burners have been proposed that do not explicitly address the issue of volatile deposits.
Published European Patent Application EP 1 669 669 A1 discloses a single injection hole for oxygen provided concentrically with each fuel injection holes and a plurality of secondary oxygen injection holes forming a ring shape. However it concerns a burner with powder body injection and is dedicated to heavy oil combustion.
Published European Patent Application EP 0 653 591 B1 discloses swirling combustion and also secondary oxidant jet and fuel jet parallels. Some ratios between the velocities of the primary and secondary oxygen flows are defined. However, the invention is directed to liquid fuel combustion.
While U.S. Pat. No. 6,843,185 B1 discloses the use of injection holes for injecting the primary oxygen. It discloses a mixing chamber and is directed to pulverized solid fuels/coal combustion.
While U.S. Pat. No. 6,474,982 B2 discloses injection holes around a central flame, these holes are for both fuel and oxygen. It also uses alternative and annular fuel streams.
While U.S. Pat. No. 5,927,960 discloses a main oxidant outlet, a secondary oxidant supply outlet, and injection holes for oxygen provided concentrically with fuel injection holes, it discloses two oxygen inputs. It also discloses a high secondary oxygen velocity due to accelerating means (convergent or divergent nozzle), a supersonic (subsonic in case of no primary oxidant) velocity of flame gases, and a mixing chamber.
There is disclosed a method for combusting gaseous fuel and an oxidant that includes the following steps. A combustion space is provided that is at least partly defined by a wall having a cavity that communicates with the combustion space. Wall portions adjacent the cavity comprise a burner block. A burner inside the cavity is provided that includes an inner body having an inner bore and coaxial outer body surrounding the inner body. A reactant annulus is defined by outer surfaces of the inner body and inner surfaces of the outer body. The outer body has one or more secondary reactant injection spaces extending therethrough towards the combustion space. A tip of the burner is recessed from the combustion space to define a gap in the cavity therebetween. A first reactant comprising a fuel or an oxidant is provided. A second reactant comprising a fuel or an oxidant is provided, wherein: if the first reactant is a fuel, then the second reactant is an oxidant; and if the first reactant is an oxidant, then second reactant is a fuel. A primary stream of the first reactant is injected from one of the inner bore and the reactant annulus towards the combustion space. A stream of the second reactant is injected from the other of the inner bore and the reactant annulus towards the combustion space. A secondary stream of the first reactant is injected from said one or more secondary reactant injection spaces. The first and second reactants are combusted in the combustion space. The secondary stream of the first reactant exits said one or more secondary reactant injection spaces at a position upstream of where the primary stream of the first reactant and the stream of the second reactant exit the reactant annulus and inner bore.
The disclosed method may include one or more of the following aspects:
molten glass in a flow channel is heated with heat from said combustion step.
the one or more secondary reactant injection spaces comprises a continuous annulus coaxial with and surrounding the reactant annulus.
the one or more secondary reactant injection spaces comprises a plurality of radially spaced holes formed in the outer body.
the plurality of radially spaced holes comprises 8-10 holes.
a sleeve is disposed in form-fitting fashion inside the cavity to decrease a volume between the burner and the burner block.
the secondary stream is injected at a linear velocity greater than that of the primary stream.
the first reactant is the oxidant and the second reactant is the fuel.
the primary stream is injected from the inner bore and the fuel is injected from the reactant annulus.
15-40% by volume of the first reactant is injected as the secondary stream.
the secondary stream comprises at least 3% but no more than 50% of a total amount of the first reactant injected by the burner in terms of mass flow rate.
For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
A primary advantage of the disclosed method is elimination or at least partial decrease in the formation of deposits on the burner from condensed glass vapors. By injecting the secondary reactant around and upstream of the primary reactant and co-reactant, this advantage is realized. This type of injection establishes at least a partial purge of the volume in between the burner and the burner block to shift the recirculation zone away from the burner and towards the combustion chamber. Within the burner itself, one of the reactants is injected from an inner bore while the other reactant is injected from an annulus surrounding the inner bore. While the fuel may be injected in primary and secondary injections and only a single portion of oxidant is injected, preferably, the fuel is injected in a single portion and the oxidant is injected in both primary and secondary injections. Most preferably, the primary oxidant is injected from the annulus surrounding the inner bore, the fuel injected through the inner bore, and the secondary oxidant is injected around and upstream of the injections of the fuel and the primary oxidant. The fuel is preferably natural gas or propane. While the oxidant may be air, pure oxygen, or oxygen-enriched air up to 100% oxygen, preferably it is oxygen having a purity of at least 90% (by volume). Either or both of the injection of the reactants from the inner bore and the annulus surrounding the bore may be swirled. The use of a swirl allows achievement of a same flame length at a different burner power.
The above advantage is better realized when the mass flow rate of the secondary reactant, the linear velocity of the secondary reactant, and the allocation of the reactant between primary and secondary injections is carefully selected.
With respect to the mass flow rate of the secondary reactant, the mass flow rate of the secondary reactant should be in the range of from no less than 3% and no more than 50% of the total flow rate of the reactant (both primary and secondary).
With respect to the linear velocity, it is preferable that the average linear velocity (normalizing for the differences of mass flows) of the primary and secondary reactants be similar to the linear velocity of the primary reactant when the entire reactant requirement is satisfied by primary reactant injection (no secondary reactant is injected). However, it is preferable that the linear velocity of the secondary reactant exceed that of the primary.
With respect to the allocation of reactant between primary and secondary, it is desirable to inject the secondary reactant under the above mass flow rate and linear velocity conditions without requiring too much of the total reactant flow to be injected as the secondary reactant in order to maintain the same properties of the combustion that are achieved with both primary/secondary reactant injection and only primary reactant injection. Preferably, the proportion of the secondary reactant is in the range of from about 5% to about 40% of the total flow rate of the primary and secondary reactant. More preferably, this proportion is in the range of from about 15% to about 40% of the total flow rate of primary and secondary reactant. Even more preferably, this proportion is in the range of from about 25-35% (most preferably about 30%)
The above preferences, goals, and advantage are best realized by the following embodiments.
As best illustrated in
With reference to
In operation, a primary portion or stream of one of the reactants (the fuel or the oxidant), referred to as the first reactant, is injected through either the inner bore 30 or the annular gap 28. The other or second reactant is injected by either of the following ways: through the annular gap 28 when the primary portion of the first reactant is injected through the inner bore 30; or through the inner bore 30 when the primary portion of the first reactant is injected through the annular gap 28. Another or secondary portion or stream of said first reactant is injected from secondary injection space 26 starting from a position adjacent outer tip portions 25′, 25″ and flows along path 26′ across recess R to a position adjacent inner tip portion 25″′ and thence forward to recirculation zone 32. The primary portion of the first reactant is also referred to as the primary reactant, whereas the secondary portion of the first reactant is referred to as the secondary reactant. It should be understood that the primary and secondary oxidants can be supplied with a single source of oxidant and primary and secondary fuels can be supplied with a single source of fuel. Preferably, the fuel is injected through inner bore 30, the primary oxidant is injected through annulus 28 and the secondary oxidant is injected through secondary reactant injections space 26.
Because the injection of the secondary reactant is recessed back from the outer tip portion 25″′ and the tip of the inner body 29, the secondary reactant achieves at least a partial purge of the space in between the burner and the burner block 1. In effect, it shifts the recirculation zone 32 to a position closer to the combustion chamber in comparison to conventional burners without this kind of secondary reactant injection. Because the recirculation zone 32 is shifted away from the burner, formation of deposits upon the burner from condensation of glass vapors are either prevented or at least inhibited.
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In operation, a primary portion or stream of one of the reactants (the fuel or the oxidant), referred to as the first reactant, is injected through either the inner bore 30 or the annular gap 28. The other or second reactant is injected by either of the following ways: through the annular gap 28 when the primary portion of the first reactant is injected through the inner bore 30; or through the inner bore 30 when the primary portion of the first reactant is injected through the annular gap 28. Another or secondary portion or stream of said first reactant is injected from secondary reactant annulus 26″ (secondary injection space 26) starting from a position adjacent outer tip portion 25′ and flows along path 26′ across recess R to a position adjacent inner tip portion 25″′ and thence forward to recirculation zone 32. The primary portion of the first reactant is also referred to as the primary reactant, whereas the secondary portion of the first reactant is referred to as the secondary reactant. It should be understood that the primary and secondary oxidants can be supplied with a single source of oxidant. Since the secondary reactant is injected through a continuous annulus 26″ and not a plurality of holes, the radial thickness (i.e., the difference between the inner and outer diameters) should be minimized, otherwise, too low of a linear velocity for the secondary reactant injection will be realized. Because the injection of the secondary reactant is recessed back from the tip 25″′ of the second inner body portion 25B and the tip of the inner body 29, the secondary reactant achieves at least a partial purge of the space in between the burner and the burner block 1. In effect, it shifts the recirculation zone 32 to a position closer to the combustion chamber in comparison to conventional burners without this kind of secondary reactant injection. Because the recirculation zone 32 is shifted away from the burner, formation of deposits upon the burner from condensation of glass vapors are either prevented or at least inhibited.
Computational fluid dynamic modeling was performed for a burner using 8 secondary reactant injection holes with a diameter of 1 mm. 15% of the total oxidant (O2) was allocated to secondary injection and 85% to primary injection. Based on the model prediction, the linear velocity at the secondary reactant injection holes is 32 m/s while the linear velocity of the primary reactant at the burner tip is 28 m/s. Thus, the secondary reactant linear velocity is larger than that of the primary reactant. One of ordinary skill in the art will recognize that if the number and diameter of holes are kept constant, a relatively greater allocation of the total oxidant to the secondary instead of primary will have the effect of increasing the linear velocity of the secondary.
A series of tests was conducted for the purpose of verifying that injection of a secondary reactant (in this case oxygen) will not significantly change the heat profile and location of hot spots in a furnace comparison with a reference case: the ALGLASS FH burner without any secondary O2 injection (fuel is natural gas). Tests were conducted in a pilot furnace under the following conditions: nominal burner power of 4 kW; O2 ratio of 2.3 (the total oxygen flow rate divided by the total fuel flow rate); and a stable temperature in the combustion space of around 1,300° C. In order to assess differences of temperature profile produced by the various examples, the temperature of the block top surface at three points and the temperature of the bottom of the combustion space at four points were measured with thermocouples. It should be noted that the secondary injection holes were equally spaced (radially) around the inner bore reactant annulus. Each run also included a check for soot formation. The various parameters utilized in the tests are found below in Table I.
As best shown in
Design b was the same as Design a, except that an adjustment of area for the injection of primary oxygen was made so that the average velocity (taking into account differences of mass flow) for the total of the primary and secondary injections could be the same or at least be very close to the original O2 velocity (5.44 m/s) when the burner is operated without secondary oxygen.
Design c was the same as design a, except that the hole diameter was 1.5 mm. The diameter of each hole for Designs a and c was determined so that the cross-sectional areas for the holes represents around:
Design d was the same as Design c, except that an adjustment of area for injection of the primary oxygen was made so that the average velocity (taking into account differences of mass flow) for the total of the primary and secondary injections could be the same or at least be very close to the original O2 velocity (5.44 m/s) when the burner is operated without secondary oxygen.
Several observations may be made.
A check for soot formation did not reveal soot formation for any of the burners.
When the primary oxidant injection area is not adjusted properly in order to obtain the required average velocity for O2 (Designs a and c), the mixing of the fuel and oxygen changes in such way that the heat released inside the block significantly increases. This has the effect of decreasing the heat transferred to the load (as represented by the temperature of the combustion chamber bottom surface). Indeed, obtaining proper mixing conditions is important for achieving suitable combustion conditions and flame specifications. It confirms the relevance of using secondary O2 injection regarding a burner instead of a lance in order to control flame shape and heat transfer. Further, by properly adjusting the linear velocity, one can prevent deposit formation.
Injection of 30% of the oxidant (O2) as the secondary achieves a temperature profile very close to the base reference case for both the block top surface and the chamber bottom surface. Using only 15% instead of 30% slightly changes the temperature profile, but the profile still remains acceptable from a practical standpoint. Nevertheless, it appears to be more suitable to use 30% instead of 15%.
The pressure itself across the secondary holes is not known. However, pressure drops were measured for both O2 and natural gas inlets for each of Designs a-d of Example 2 and for the base reference case during the pilot tests of Example 2. The O2 inlet pressure drop measurements are presented in
Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims.
