MXPA97005450A - Method and apparatus for agrega treatment - Google Patents

Abstract

The present invention relates to a method of treating an aggregate comprising the steps of: a) mixing a predetermined proportion of fuel and oxidant in a pre-mix chamber in order to create a premix of fuel, b) admitting the premix fuel within of the combustion region, c) combustion of said premix within the combustion region, d) admitting the premix of combustion into a rotating chamber, e) introducing an aggregate into the rotating chamber in order to evaporate a contaminant of said aggregate; f) flowing the combustion mixture out of the rotation chamber; and g) removing said aggregate from said rotation chamber.

Description

"METHOD AND APPARATUS FOR TREATMENT OF AGGREGATES" BACKGROUND OF THE INVENTION The present invention pertains to the field of drying systems, particularly those of the type used to remove contaminants such as moisture from an aggregate. The present invention has a particular application in rotary dryers such as those used for asphalt aggregates and also for calcinators or the removal of contaminants as used in the recovery of the earth. In the manufacture of asphalt, it is important to remove moisture from the aggregate in order to ensure a product grade that is satisfactory for paving applications. The aggregate components comprising asphalt are typically maintained, away from home on site in the asphalt plants. In this way, these components are exposed to the elements, including humidity and atmospheric precipitation conditions. As such, it is essential that any moisture that can be retained by these aggregates be removed before the processing and application of the asphalt. In order to remove this unwanted moisture, the asphalt plants employ a dryer assembly 10, as shown in Fig. 1. The dryer includes a burner 12 which directs the heat inside the rotating drum 14, like a drum of the known type. in the technology. The aggregate 16 is fed into the drum 14 by an inlet conveyor 18, which receives the aggregate of different sizes from a plurality of funnels 20, 22, 24. The drum 14 is inclined at an acute angle to allow the aggregate 16 to be feed by gravity by means of drum movement 14. As can best be seen in Figs. 2 and 3, the interior of the drum 14 is typically fitted with vanes 26, which extend along the axial length of the drum 14. As the drum 14 rotates, the vanes 26 carry the aggregate 16 from the bottom to the top of the drum 14. where the aggregate 16 is allowed to fall within the thermal flow field, generated by the burner 12 in order to evaporate the moisture. This process is called "cover." The vanes 26 typically have the shape to allow transportation of the aggregate 16 to the top of the drum 14 to produce maximum coverage. The vanes 26 may have a transverse silhouette of "L" or "J". With this covering, aggregate 16 is typically heated to a temperature of about 275-325 ° F. In an exemplary type of the asphalt plant (e.g. "casting plant" is shown in Fig. 1) the aggregate 16 flows towards the bottom of the drum 14 after drying and is placed in the outlet conveyor 28 which carries the aggregate 16 through the steps where it is mixed with Asphalt cement to form hot mixed asphalt, which is then loaded onto trucks to be transported to the paving site. In the dryer assembly 10 it can be a "counter flow" unit (as shown in Fig. 1), where the burner 12 is directed through the drum 14 in the opposite direction from the entry of the aggregate 16. However, the dryer assembly 10 can also be a "parallel flow" unit, in which the aggregate 16 is introduced through the drum 14 in the same direction as the burner 12. In order to maintain an economic and effective production speed, Burner 12 should feed 60-200 MBTU's per hour. Drying assemblies of prior technology have some disadvantages. As shown in Fig. 3, it is common to charge a burner 12 using an enriched fuel / air mixture (30% of the air required for 100% of the fuel supplied, where the air is supplied by a primary blower 38). ). In order to complete the combustion reaction, the secondary air 30 is drawn through an annular opening 32 by the flow of the flame of the burner and an induced suction created by an auxiliary fan (not shown). The secondary air 30 is mixed with the fuel core 34 enriched only at the edges. In order to create a required air flow, the enriched mixture is typically supplied at a pressure of 30 osi (ounces per square inch). This produces a considerable roar of combustion (-110 dBA). Typically, asphalt plants are located within 50 miles of radius of the paving site, therefore, facilities are often located in residential communities where such a level can create local noise nuisance. In these previous systems, only about 10% of the fuel makes combustion at the point where the burner 12 opens inside the drum 14. Because the enriched fuel core 34 mixes with the secondary air 30 only at the edges , the flame becomes very long as the combustion reaction extends over a greater distance. This causes several specific problems. The enriched core of the flame burns at a relatively cool temperature of about 2400-2500 ° F. Also, because the combustion reaction is completed at a great distance from the burner, the thermal distribution within the drum is not uniform. This uneven distribution results in an inefficient fuel consumption per unit of output product because the high temperature differential is not effectively established within the pipe inlet. In addition to the above, the long flame requires that a longer drum 12 be used. As the drum 12 rotates, the aggregate 16 must be prevented from directly covering the flame. The aggregate 16 is by nature, a little cooler than the flame itself. The aggregate 16 cools the flame with the contact, causing the products to "turn off" partially with combustion inside the flame. Carbon monoxide (CO) is created by this "off" because like the products of partial combustion, it "freezes" out of the flame by contact with the cooler aggregate. In addition to CO, other intermediate combustion products are created, mainly Volatile Organic Compounds (VOC's). These VOC 's include, for example, formaldehyde, ethylene oxide and methanol. Carbon monoxide and VOCs are environmental pollutants that are registered and monitored very closely by local, state and federal agencies. In order to reduce these emissions created by the extinguishing of the flame, the vanes 26 must be shorter than the drum 14 by a distance 36, which corresponds to the envelope of the flame. In this way, the drum must be made long enough to accommodate the flame, thus increasing the cost of the unit. In view of this, the above systems typically release 400-2000 ppmvd (parts per million dry volume) of CO. The long flame produced by the previous dryer assemblies also cause other environmental hazards. Because the secondary air 30 is mixed with the core 34 enriched only at the edges, uneven mixing occurs which produces local hot spots within the flame. These hot spots have been identified as sources of nitrogen oxide components (NOx), which are environmental pollutants also controlled by government agencies.

These hot spots can be reduced by maintaining uniform control over the ratio of fuel to air. For this purpose, a secondary blower and control (not shown) are sometimes added to better control the overall supply of air, and therefore the ratio of fuel to air. However, the addition of a secondary blower generates additional installation and operation costs. Even with strict control the above asphalt plants typically emit about 100 ppmvd of NOx. In addition to the noise produced by the roar of combustion, previous systems also produce resonant noise. It sometimes happens that the combustion frequencies coincide with the natural frequency of the drying system, resulting in a low, deep hollow noise produced by the fixed wave that is created in the system. If there is not enough moisture in the system, this oscillation may increase in amplitude, creating a nuisance of noise or resulting in damage to the burner or otherwise producing a potentially undesirable situation. DESCRIPTION OF THE INVENTION In view of the difficulties and disadvantages resulting from the previous systems, it would be advantageous to provide a method and system for the treatment of aggregates that solves the above problems, while providing greater efficiency and versatility.

Therefore, it is an object of the present invention to provide an aggregate drying system and method that reduces the emissions of carbon monoxide and volatile organic components by reducing the incidence of shutdown; It is another object of the present invention to provide a shorter flame, mixed more uniformly, which reduces NOx emissions; It is a further object of the invention to provide an aggregate dryer that reduces the noise produced by the combustion roar; It is another object of the present invention to provide a tuned system and method of aggregate treatment that reduces the resonant noise produced by the dryer system; It is another object of the present invention to provide a system and method of treatment of aggregates that allows a more efficient energy consumption, producing greater fuel savings and increased product yield; It is another object of the present invention to provide a high volumetric heat release from a combustion system and method; It is still another object of the present invention to provide a system and method for safely producing a large volume of the premix.

These and other objects are achieved with the method and apparatus of the present invention, wherein the fuel and the oxidant are mixed in a premix chamber to create a premix of fuel that is then admitted into a combustion region. The premix makes combustion in this region in order to produce a uniform flame. The flame and the products of the combustion are admitted inside a rotating drum that contains an aggregate in order to evaporate a contaminant from the aggregate. After the treatment process, the aggregate is removed from the rotating chamber. In a preferred embodiment, the premix chamber for mixing the fuel and the oxidant consists of a plurality of mixers, each including respective fuel and oxidant inlets, wherein the fuel and oxidizer are mixed within the interior volume of the mixer. , in order to safely generate a large amount of premix. The combustion region of the preferred embodiment consists of a reaction chamber which is measured to thereby recirculate the premix during combustion in order to ensure substantially uniform and complete combustion, thus producing a high volumetric heat release and reducing the partial combustion products such as CO and VOCs, together with the reduction of emissions of NOx. The present invention may also include a nose for supplying additional fuel to the combustion region. The fuel supply can vary between nose and premix to reduce resonant noise. The control for fuel supply can vary the fuel supply between these sources so that the proportion of equivalence in the combustion region remains constant within the limit of about 0.53 to 0.795. As you can see, the invention is capable of these and other different embodiments, and some details thereof are capable of being modified in several aspects, all without departing from the invention. Accordingly, the drawings and description are considered as illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments of the invention will now be described only by way of example, with reference to the accompanying drawings in which like numerals represent the parts with the same numerals and in which: Fig. 1 (in a sectional view) illustrating the operation of the prior aggregate treatment systems: Fig. 2 is a cross-sectional view illustrating an aggregate transport and the coating resulting from drum rotation; Fig. 3 is a sectional view illustrating combustion; and the execution of the burner flow of the previous systems; Fig. 4 is a sectional view illustrating the burner according to the present invention; Fig. 5 is a front view illustrating the arrangement of the mixing tubes according to an embodiment of the present invention; Fig. 6 is a sectional view showing the operation of the burner as used with the rotating drum according to the present invention; FIG. 7 is a sectional view showing the reaction shell of the burner of the present invention, compared to the reaction shell of the above burners; Fig. 8 is a profile showing the heat distribution over the axial length of the rotary drums for the previous burners compared to those of the present invention; and Fig. 9A, 9B and 9C show profiles of the heat distribution along the radius of the drum for the above burners compared to those of the present invention. DETAILED DESCRIPTION OF THE PREFERRED INCORPORATIONS Referring now to the drawings, which are solely for the purpose of illustration of the preferred embodiment of this invention and not for purposes of limiting it, the figures show a heat treatment system for removing a contaminant from an aggregate. , particularly in an asphalt aggregate dryer to remove moisture from an aggregate. However, the invention can also be used for other similar processes, such as the removal of volatiles for the recovery of contaminated soil. Figs. 4-6 illustrate the configuration and operation of an aggregate treatment system in accordance with the present invention. The burner of the present dryer system is similar to those described in related applications 07 / 044,719 registered on April 12, 1993, now assigned, and 08 / 309,198 registered on September 20, 1994, the findings of which are incorporated herein by reference. The burner 40 of the present invention includes a plurality of mixers, in the preferred embodiment, the mixing tubes 42 in which the fuel and the oxidant are mixed together to supply a premix of fuel. The fuel (preferably natural gas) is supplied through a common fuel plenum 44 connecting each mixer tube 42 through the respective fuel inlets 46. The oxidant (preferably air) is supplied through a common oxidant plenum 48 which connects each mixing tube 42 with its respective oxidant inlets 50. The fuel and the oxidant can be supplied at any pressure as proposed by the person skilled in the art. In the preferred embodiment, the fuel is supplied at 5 osi and the oxidant is supplied at 6 osi. In the preferred embodiment, the fuel inlet 46 is a vacuum, but for applications where the fuel is supplied at a higher pressure the fuel inlet 46 can be an inspirer. With any embodiment of the present invention, the air is capable of being supplied at a significantly lower pressure than in the previous systems, thereby reducing the roar of combustion. The cylindrical mixer tubes 42 each include a central obstruction 52 defining an annular flow passage 54 along the substantial length of the interior of the mixing tube 42. The fuel and oxidant are mixed within this flow passage to create a fuel premix. The mixed fuel and oxidant are discharged into a combustion region defined by a reaction chamber 56 in which the premix makes combustion. The flow passages 54 have an effective average diameter equal to the transverse width 58 of the annular passage 54 (see Fig. 5). In this way, the effective width of the flow passage 54 is smaller, thus increasing the effective length-to-width ratio (L / D ratio) of the mixing tube 42. In the preferred embodiment, it has been found that the L / D ratio of of 12 is effective to achieve complete mixing of the fuel and oxidant. The annular silhouette increases the flow velocity and reduces the passage sizes, thereby reducing the risk of an interruption within the flow passage 54, and this allows the production of significant volumes of premix. In a preferred embodiment, the premix is a lean mixture having an equivalence ratio of the limit of about 0.53-0.795. As can be seen from Fig. 5, the plurality of mixing tubes 42 is preferably oriented around the axis of the burner assembly 40 (which is typically collinear with the axis of the rotating drum 14). Each mixing tube 42 securely generates a significant amount of premix. However, when the contributions of each tube are collected (eg eight tubes, as shown in Fig. 5), surprising quantities of premix are produced safely enough to produce a high volumetric safe release of heat (close to 60-200 MBTUs per hour) required for aggregate drying, land reclamation and calcining. After leaving the mixer tubes 42, the premix makes combustion inside the reaction chamber 56. The reaction chamber 56 includes a frusto-conical tapered section 60 in which the diameter of this section is tapered towards the axis of the burner 40. The silhouette of the tapered section 60, in combination with the off-axis location of the tubes 42 mixers, create a flow pattern within the reaction chamber 56 in which the hot products of combustion are recirculated to the combustion site near the opening of the mixing tubes 42. This recirculation promotes a high temperature combustion of the premix very close to the opening of the mixer tubes 42. Recirculation also ensures that the products of partial combustion such as CO and VOCs are partially combusted before leaving burner opening 62. In the burner 40, combustion is completed over a short distance, producing a short, bright flame with a temperature of about 2700 ° F. Because the burner 40 produces a uniformly well-blended combustion product, the incidence of local hot spots is reduced, and thus NOx production is also greatly reduced. It has been found that the present invention produces NOx at levels below 30 ppmvd, typically 10 ppmvd, compared to 100 ppmvd produced by the previous systems. As can be seen in Fig. 6, the burner 40 of the present invention discharges into the rotary drum 14 used for the treatment of aggregates. Because most of the approximately 80% of the combustion can be completed by discharging the burner 40, combustion quenched by the aggregate 16 can be very widely reduced. With the present invention, CO emissions can be reduced to less than 50 ppmvd compared to 400-2000 ppvmd that resulted from previous systems. Accordingly, when the shutdown is reduced, the present invention allows either shorter drums 14, or longer paddles 26, to be used, which extend closer to the end of the drum 1. In any case, the drum 14 can be modified to more efficiently allow the handling of the aggregate 16. Fig. 7 shows the reaction envelope 64 that is achieved with the present invention, compared to the reaction envelope 66 typically produced by the systems previous ones that are directed inside the rotating drum 14. The reaction envelope 66 of the previous systems indicates a long, exhausted flame. The mixture of the oxidant and the fuel is carried out along the edges of the flame so that the combustion combustion occurs in an annular region around the central flame along the distance inside the drum 14. Flame that occurs in the above systems does not extend very far along the radial distance from the axis of the drum 14 to the wall of the drum. Comparatively, the reaction envelope 64 of the present invention indicates a short, compact flame in which the heat distribution extends further along the radial distance from the drum axis to the drum wall. The reaction envelope 64 achieved with the present invention results in a thick layer of triatomic combustion products (carbon dioxide and water vapor), which radiate some of the heat released by combustion, to be directed into the drum 14, to that the present reaction envelope 64 extends very close to the wall of the drum 14. The greater thickness of this radioactive layer allows a greater transfer of radioactive heat to the aggregate 16, according to the well-known thermal relationships. For a burner with an outlet opening of about 3-5 'according to this invention, the radioactive layer has a thickness of about 51"Because the reaction envelope 66 of a comparable prior system does not extend very much. far to the wall of the drum 14, the thickness of the radioactive layer is approximately only 12". In this way, with the present invention, the radial heat transferred to the aggregate 16 (and therefore the drying efficiency) is greater than that produced by the previous systems. The thickness of the radioactive layer resulting from the present invention produces an efficient and more uniform thermal distribution within the volume of the drum than that produced by the previous systems, thereby increasing the effective area of radial heat transfer . As shown in FIG. 8, the radial heat (q) produced by the present invention 68 is more consistent along the axial distance (d) of the drum 14 than that of the pre-vious systems. The difference 76 between these thermal distributions corresponds to the additional amount of heat that is transferred to the aggregate 16 along the length of the drum. In this way, an additional amount of the aggregate, proportional to this difference in heat, can be processed by the present invention. Therefore, the present invention produces a higher amount of dry aggregate product for the drum 14 of a given length. The present invention also offers a greater radial and more uniform efficiency of a given drum 14. As shown in Figs. 9A, 9B and 9C, the prior thermal systems have a thermal distribution 74 that rises concentrically along the axis of the drum. Due to the lower thickness of the radioactive heat layer, the aggregate 16 has only a narrow zone of hot gas through which it can fall, so the heat transfer is less efficient. The present invention has a thermal distribution 72 that is theoretically wider, therefore, increases the area of effective heat transfer during the coating, extending this area closer to the walls of the drum. The difference between these thermal distributions is proportional to a faster rate of aggregate drying during the coating, thus also offering increased aggregate performance. In the previous systems, the thermal radial distribution 74 varies greatly along the axial length of the drum. Fig. 9A shows the theoretical thermal distribution 74 of the previous system at a point near the burner 12, where the heat release is low. At another lower point of the length of the drum 14 (see Fig. 9B), the heat release is greater, but because the greater combustion occurs in an annular region where the mixing is carried out ( along the edges of the flame), the heat release is greater at the nodal points between the shaft and the wall of the drum 14. At a point below the length of the drum 14 (as can be seen in Fig. 9C), the radial thermal distribution begins to flatten as the products are mixed vigorously. However, the thermal profile 72 of the present invention is uniformly distributed along the radius of the drum 14, providing a faster speed of aggregate drying, resulting in increased performance and improved energy efficiency. As is clear from the foregoing description, the apparatus and method of the present invention offers a more efficient energy consumption, allowing a greater yield of the product in less time. Additionally, the present invention also offers energy savings while at the same time reducing CO and NOx emissions. A summary of the results of the present invention compared to that of the prior technology is as follows: Present Present Invention Completed Reaction Systems (at the burner outlet) > 80% = 10% Flame temperature 2700 ° F 2400-2500 ° F (at the burner outlet) Thickness of the combustion products layer 51"12" Air supply pressure 5 osi 30 osi Burst burn < 85 dBA 110 dBA CO < 50 ppmvd 400-2000 NOx 10 ppmvd 100 ppmvd The present invention also includes an adjustment characteristic that reduces the resonant noise produced by the coincidence of the combustion frequencies with the natural frequency of the drying system. As can be seen in Figs. 4-6, the burner 40 includes a nose 80 that supplies additional fuel to the reaction chamber 56. As additional fuel is supplied through the nose 80, the amount of fuel supplied to the mixer tubes 42 is reduced through the fuel inlets 46, to thereby produce a leaner mixture leaving the mixer tubes 42. , in this way the proportion of equivalence is maintained at a constant value within the limits of about 0.53-0.795. By varying the amount of fuel delivered respectively, through the nose 80 and the mixer tubes 42, the combustion frequencies can be altered, and thus provide a "tuner" burner that can be "tuned" to a combustion frequency. sufficiently different from the natural frequency of the drying system in order to reduce undesirable resonant vibrations. Because the present burner 40 can be adjusted to accommodate a variety of different sizes of the drum, the burner 40 must be "tuned" over a frequency limit. The nose fuel supply 80 varies using a nose control 82 (preferably using a regular gas valve). As the fuel is increased to the nose 80, the fuel is decreased to the mixer tubes 42 using a fuel mixer control 84 (also, preferably a regular gas valve), so as to maintain a constant proportion of equivalence. Because the natural frequency of the system is a geometry function and does not change during operation, the burner 40 ordinarily only needs to be manually tuned during installation and thus no need to vary the fuel controls 82, 84. In this way, resonance problems, typically associated with aggregate treatment systems are avoided by the present invention. As described above, the present invention solves many problems associated with prior aggregate sequestration systems and presents an energy efficient aggregate dryer that offers safe operation and lower pollutant emissions, however, it can be appreciated that they can be made changes and arrangements of the parts that have been described here and illustrated in order to explain the nature of the invention by someone skilled in the technology, within the principles and scope of the invention, as expressed in the attached clauses.

Claims (14)

1. NOVELTY OF THE INVENTION Having described the invention, it is considered as a novelty, and therefore, the content of the following clauses is claimed as property. CLAUSES 1. A method of treating an aggregate comprising the steps of: a) mixing a predetermined proportion of fuel and oxidant in a premix chamber in order to create a premix of fuel; b) admit the combustible premix within the combustion region; c) combustion said premix within the combustion region; d) admitting the combustion premix into a rotating chamber; e) introducing an aggregate into the rotation chamber in order to evaporate a contaminant from said aggregate; f) flowing the combustion mixture out of the rotation chamber; and g) removing said aggregate from said rotation chamber.
2. 2. A method of treating an aggregate as in clause 1, wherein the premix chamber comprises a plurality of mixers and wherein the mixing step includes separately introducing the fuel and the oxidant into each plurality of mixers in where mixing occurs within the interior volume of each mixer in order to safely generate a large volume of pre-mix.
3. 3. A method of treating an aggregate as in clause 2, wherein the fuel and the oxidant are admitted into each plurality of mixers at their respective pressures low enough to suppress the combustion roar.
4. 4. The method of treating an aggregate as in clause 1, wherein the combustion region is a reaction chamber and wherein the combustion step includes recirculating the premix within the reaction chamber during combustion in order to produce a substantially complete and uniform combustion before admission to said drum, thereby producing a high volumetric heat release and reducing the products of partial combustion and NOx emissions.
5. 5. The method of treating an aggregate as in clause 1, where the additional fuel is supplied to the combustion region and where the amount of fuel in the premix and the amount of additional fuel may vary in order to reduce the resonant noise.
6. 6. The method of treating an aggregate as in clause 5, wherein the amount of fuel in the premix and the amount of additional fuel vary such that the ratio of equivalence within the reaction chamber remains constant within the limit of about 0.53-0.795.
7. 7. The method of treating an aggregate as claimed in clause 1, where the aggregate is aggregate of asphalt and the contaminant can evaporate as moisture.
8. 8. The method of drying an aggregate as claimed in clause 1, where the aggregate is land that requires recovery and the contaminant is an environmental pollutant.
9. 9. An apparatus for treating an aggregate consisting of: a) a premix chamber for mixing a fuel and an oxidant to create a premix of fuel; b) a combustion region connected to said premix chamber for combustion with said premix in order to produce a flame; and c) a rotating drum, accessible to said combustion region, to receive and treat the aggregate.
10. 10. The apparatus of clause 9, wherein the premix chamber comprises a plurality of mixers, each mixer includes respective fuel and oxidant entries, wherein the fuel and oxidant are mixed within the interior volume of the plurality of mixers. in order to safely generate a large volume of premix.
11. 11. The apparatus of clause 9, wherein the combustion region consists of a reaction chamber with measures to be able to recirculate the premix during combustion in order to ensure a substantially uniform and complete combustion, therefore, producing a release Volumetric heat and reduce the products of partial combustion and NOx emissions.
12. 12. The apparatus of clause 9, which also includes a nose connected to the combustion region to supply additional fuel for combustion.
13. 13. The apparatus of clause 12, wherein the premix chamber includes at least one fuel inlet and wherein the apparatus further includes a fuel supply control to vary the fuel supply between the nose and at least one of the fuel inputs in order to reduce the resonant noise.
14. 14. The apparatus of clause 13, wherein the fuel supply control varies the fuel supply between the nose and at least one of the fuel inlets, respectively so that the proportion of equivalence in the combustion region remains constant within the limits of about 0.53-0.795.