HAVING REDUCED NO _x EMISSIONS

Technical Field

The present invention relates in general to rotary heating apparatus for the manufacture of asphalt-aggregate material, and in particular to a rotary dryer for heating aggregate in an asphalt plant

Background Art

No single component is more important in the manufacture of hot mix asphalt than the aggregate dryer and its exhaust system. One problem encountered with the use of such apparatus is pollution in the form of NO _x compounds produced by the burner flame. It is known that the formation of NOχ compounds may be inhibited by reducing the amount of nitrogen in the fuel; reducing the flame temperature; reducing the amount of air available for combustion; and reducing the time that combustion gasses spend at elevated temperatures.

It is common in the steam generation industry to lower flame temperature by recirculating flue gas to the burner and thereby reducing NOχ emissions. This reduction in flame temperature is further augmented by staged combustion in which the flame is initially oxygen poor (and therefore cooler) and is charged with additional oxygen a short time later to complete combustion. Multiple stages are preferably utilized to obtain the best results.

Experience has taught, however, that methods useful in the steam industry for reducing the formation of NO compounds are not

applicable to equipment used in the production of asphaltic products, such as aggregate dryers. This is because the two processes utilize different types of flames to provide heat and because aggregate dryers generally are of a shorter dimension unsuitable for implementing staged combustion techniques having multiple stages. Steam generation plants typically utilize lengthy staged combustion and a flame characterized as long and lazy. Lengthy, multiple staged combustion set-ups and long, lazy flames cannot be used in aggregate dryers because aggregate dryers typically provide a smaller combustion area than do steam plants. The recirculation of gases in rotary heating equipment for purposes other than to reduce the level of NO* is known in the art United States Patent No. 4,190,370 discloses a drum mixer having a temperature control system for regulating the temperature of the asphalt-aggregate mix by varying the flow of hot gasses through the drum mixer. The system is also disclosed in connection with an aggregate dryer. The temperature control system withdraws gases exiting the drum before they pass through a baghouse and recirculates them to an input manifold on the drum mixer. This recirculation system reduces the temperature of the burner flame and the energy required to heat the gases within the drum mixer, but does not suggest any effect on NO* emissions.

United States Reissue Pat No. Re. 29,496 discloses another rotary heating device in which combustion gases are recirculated from the outlet of a drum mixer to a burner assembly located at the inlet end of the drum mixer. The recirculation gases are passed through a heating or a cooling heat exchanger before being routed to the burner. This recirculation scheme is said to provide a somewhat isothermal air flow to the burner and to allow more energy efficient operation, but the patent does not discuss any reduction in either flame temperature or flame length. Nor does the patent suggest that the scheme operates to reduce NOχ emissions. Other examples of rotary heating devices incorporating various gas recirculating schemes are disclosed in United States Patent Nos.

3,963,416; 4,143,972; 4,309,113; 4,332,478; 4,600,379; and 4,892,411.

However, none of these recirculation methods are directed to the reduction of NOχ emissions. Therefore, there remains a need for an improved rotary heating

device for use in the production of asphaltic paving materials having reduced NO* emissions.

Summary of the Invention The present invention solves the above-discussed need in the art by providing a rotary heating device in which gases are recirculated from after a baghouse to a burner and to the end of a flame provided by the burner to reduce the level of NO _x emissions created in the combustion process. Generally described, the present invention provides a rotary heating device having reduced NO _x emissions, comprising a rotatable enclosure for heating a material; means for admitting the material into the enclosure for heating the material; means for withdrawing the heated material from the enclosure; a combustion chamber having a first end spaced apart from a second end, the second end positioned adjacent to and in communicating relationship with the enclosure; burner means for providing a flame within the combustion chamber for combustion intermediate the first and second ends to provide combusted gases, the combusted gases passing into the enclosure for heating the material; means for withdrawing the combusted gases from the enclosure; and means for introducing a first portion of the gases withdrawn from the enclosure into the second end of the combustion chamber so as to reduce the temperature of the flame and to shorten the length of the flame.

More particularly, the present invention provides a method for reducing NOχ emissions in a rotary dryer comprising the steps of utilizing burner means to introduce a flame into a first end of a combustion chamber for combustion intermediate a first end and a second end of the combustion chamber; withdrawing combustion gases produced by the flame from the combustion chamber, and recirculating a first portion of the withdrawn gases into the second end of the combustion chamber so as to reduce the temperature and length of the flame.

Another aspect of the present invention provides an apparatus for producing an asphalt-aggregate material, comprising a mixing enclosure for mixing asphalt raw materials and aggregate; means for continuously admitting asphalt raw materials and aggregate into the mixing enclosure to

form the asphalt-aggregate material; means for continuously withdrawing the asphalt-aggregate material from the mixing enclosure; a combustion chamber having a first end disposed outside of the mixing enclosure and a second end disposed inside the mixing enclosure; means for introducing an amount of ambient air into the first end of die combustion chamber, means for introducing a flame into the first end of the combustion chamber for combustion intermediate the ends of the combustion chamber to provide combusted gases, the combusted gases passing through the second end of said combustion chamber into said mixing enclosure; means for withdrawing the combusted gases from the mixing enclosure; means for filtering the gases withdrawn from the mixing enclosure to provide filtered gases having a higher moisture content, a lower temperature, and a lower solids contents than the gases withdrawn from the mixing enclosure; means for introducing a first portion of the filtered gases into the first end of the combustion chamber so as to reduce the temperature and increase the turbulence of the flame; and means for introducing a second portion of the filtered gases into the second end of the combustion chamber so as to reduce the length of the flame and to further reduce the temperature of the flame, whereby the emission of NO* from the combustion chamber is reduced.

Accordingly, it is an object of the present invention to provide an improved rotary heating device.

Another object of the present invention is to provide a rotary heating device which minimizes the amount of NO emissions associated with its operation.

A further object of the present invention is to provide an aggregate dryer which recirculates exhaust gases to control combustion emanating from a burner.

It is yet another object of the present invention to provide a heating apparatus which controls the production of NO _x compounds by controlling the flow of recirculated gases through a burner-heated apparatus.

These and other objects, features and advantages of the present invention will become apparent from a review of the following detailed description of the disclosed embodiment and the appended drawings and

claims.

Brief Description of the Drawings

Fig. 1 is a perspective view of a preferred embodiment of the present invention.

Fig. 2 is a schematic diagram of the device shown in Fig. 1. Fig. 3 is a cross-sectional view of the combustion chamber of Fig. 1.

Detailed Description of the Disclosed Embodiment

Referring to the drawings, in which like numerals indicate like parts, throughout the several views, Fig. 1 shows a counter-flow aggregate dryer 10 adjacent a baghouse 12 and a virgin aggregate bin 14. The aggregate is fed by a conveyor belt 18 from the bin 14 for delivery into the dryer 10 in a manner well known in the art The baghouse 12 filters gases which have passed through the dryer 10, also in a conventional manner.

Referring now to Figs. 1 and 2, the dryer 10 includes an elongate drum 20 rotatably mounted on a support frame 22. Pivotally attached at one end of the support frame 22 are a pair of support legs 24. Attached at the other end of the support frame 22 are a pair of extendable support legs 26. The length of the legs 26 may be adjusted by various methods known in the art, but preferably hydraulically. In their unextended configuration, the legs 26 are generally of a shorter length than the legs 24, which are adjacent to the aggregate feed conveyor 18. In this configuration, the drum 20 is mounted at an angle inclined from horizontal.

As the legs 26 are extended, the angle of inclination of the drum 20 is reduced. However, it is desirable that the drum 20 always be maintained at some inclined angle so that material fed into the drum by the conveyor 18 will feed down die length of the drum 20 due to the affect of gravity as the drum is rotated. The adjustability of the legs 26 therefore provides a means for controlling the rate at which material will feed down the length of the drum 20 at a particular rate of rotation of die drum.

Located at the lower end of the dryer 10 is a flame source, such as a conventional gas burner 28. The burner 28 projects a flame 30 having a temperature of between about 2,200 and 3,000°F into a refractory

combustion chamber 32, shown in more detail in Fig. 3. A discharge manifold 31 is located between the refractory combustion chamber 32 and the drum 20 for discharge of heated aggregate to a hot mix pugmill coater 34 located adjacent the dryer. The hot mix coater 34 is of known construction and operation, as shown in United States Patent No.

4,616,934, incorporated herein by reference.

The pugmill coater 34 is positioned adjacent to and below the combustion chamber 32 with its longitudinal axis sloping with respect to horizontal. The lower end 29 of the pugmill coater is disposed below and adjacent to the discharge manifold 31 so that the dried aggregate from the dryer 10 falls by gravity directly into the pugmill coater 34. Recyclable material may also be introduced into the pugmill coater by a recycle conveyor 27, in a manner well known in the art and recovered fines may also be introduced through a particle return duct 53, described below. Conventional apparatus for heating and conveying liquid asphalt to the pugmill coater is also provided.

Referring now to Fig.3, the refractory combustion chamber 32 is a stepped chamber designed to aid the mixing of recirculated gases and reduce NO _x emissions, as explained below. The combustion chamber 32 is preferably a steel shell 33 lined with a castable refractory material 35 of a type well known in the art To provide a more turbulent flame 30, a first end 37 of the chamber 32 located closest to die burner 28 features a stepped configuration including a reduced diameter throat 36 and a step 37.

Referring further to Fig. 3, the following measurements illustrate the preferred dimensions of die chamber 32. It should be noted however, mat it is the relative dimensions and not the exact sizes which provide the desired flow characteristics.

Approximate QisiaQ££ Measurement ffrl

A 8.0

B 1.0

C 1.0

D 1.5 E 2.0

F 1.0

G 8.0

H 0.5

I 0.5 J 0.5

K 3.5

The reduced throat and stepped construction allows, on its own, for decreased NO _x production with increased efficiency and drying capabilities. The chamber construction provides enhanced mixing of fuel and air which results in a more turbulent flame. The turbulent flame then creates back-swirl or eddy currents which aid in reducing die length of the flame.

To further reduce NO _x emissions, gases may be introduced to the end of die flame to act in a "quenching" manner or to provide an abbreviated version of staged combustion. The steel shell 33 of the chamber 32 is surrounded by an annular duct 40 which is supplied with recirculated gases in a manner described below. A series of quenching holes or nozzles 42 extend through die refractory material 35 to communicate with the interior of the chamber 32 at a second end 41 of the chamber which opens to the drum 20. The nozzles 42 provide a

"quenching ring" for introducing cooler exhaust gases to cool and reduce the length of the flame or may be used as conduits for adding air for staged combustion. As will be explained further, the annular duct 40 is preferably adapted to conduct recirculated gases through the nozzles 42 and direct diem generally toward die center of the chamber at a velocity sufficient to penetrate the flame 30. This promotes turbulence and mixing of the recirculated gases with the end of die flame 30 and thereby reduces the temperature and length of the flame. Experience has taught that a velocity of about 10,000 feet per minute is suitable and may be obtained using a fan or blower generating a pressure of about 16 inches H2O through thirty-six uniformly spaced 2 inch diameter nozzles.

The heated gases from the burner 28 pass from the chamber 32 into the drum 20 to heat and dry die virgin aggregate 14. An exhaust manifold 46 is provided at die upper end of die drum 20 for conducting gases from die drum 20. The exhaust manifold 46 is connected to a

separator duct 48 for conducting gases and suspended paniculate matter (such as small aggregate particles) away from the exhaust manifold. The duct 48 leads to a conventional cyclone separator 50 located above the drum 20 for removal of paniculate matter, such as aggregate fines, from the exhaust gases. The removed paniculate matter is conducted to the pugmill coater 34 by a particle return duct 53 which leads from die bottom of the cyclone separator 50 to die pugmill coater 34. A baghouse duct 54 conducts the separated gases to die baghouse 12 for further paniculate removal. The baghouse 12 is of a design well known in the an and includes an internal filter chamber 56 within which extend a number of fiber filter collectors in the form of filter bags (not shown). Air flow through the baghouse 12 is provided by an exhaust fan 58 having an inlet duct connected to a plenum chamber of the baghouse (not shown). The output of die exhaust fan 58 is connected to an exhaust stack 64 which opens to die atmosphere. A recirculating duct 66 is connected to the exhaust stack 64 for routing an amount of die exhaust gases through the recirculating duct A manual diverter damper 68 is provided on die exhaust stack 64 to route a percentage of the exhaust gases to die recirculating duct 66 according to die damper setting. A modulating control damper 70 is provided on die recirculating duct to vary die flow of gases through the recirculating duct 66 in proportion to die fuel flow to die burner 28. The modulating control damper 70 receives a control signal from a burner controller (not shown) of a type which is well known in the an for controlling the amount of fuel and air introduced to die burner 28. The modulating controller may be calibrated and operated to provide a flow consistent with die values set forth in Tables 1 and 3.

The exhaust gases routed to die recirculating duct 66 may be routed to the burner 28 or to the quenching nozzles 42, or both. A "Y" duct 72 is provided along the recirculating duct 66 to permit the desired routing of die exhaust gases, as explained below.

The recirculating duct 66 is split at the "Y" duct 72 into a primary exhaust gas recirculating ("EGR") feed duct 74 and a quenching EGR feed duct 76. Manual control dampers 78 and 80 are provided on the primary EGR feed duct 74 and the quenching EGR feed duct 76,

respectively. Manipulation of the dampers 78 and 80 allows the desired amount of exhaust gas to be routed through each of die ducts 74 and 76. A primary ambient air duct 82 having a manual control damper 84 and a staging ambient air duct 86 having a manual control damper 88 are provided just downstream of the "Y" duct 72 for introducing ambient air to the primary air feed duct 74 and the quenching air feed duct 76, respectively. The flow rates of gases through each of the ducts 74, 76, 82 and 86 are preferably monitored utilizing conventional pitot tube apparatus (not shown) downstream of die dampers 78, 80, 84 and 88, respectively. Additionally, it will be understood that each of the manual control dampers 68, 78, 80, 84 and 88 may be replaced with electronic control dampers, whose operation may be controlled responsive to signals from the pitot tubes, utilizing conventional microprocessor equipment well known in the an for automatic process control. The contributions of the primary EGR feed duct 74 and the primary ambient air duct 82 are combined at point "R" to form a primary EGR duct 75. Likewise, the contributions of the quenching EGR feed duct 76 and die staging ambient air duct 86 are combined at point "S" to form a quenching EGR duct 79. The primary EGR duct 75 extends to a conventional primary air inlet 77 on the burner 28. For combustion to occur, air and fuel must be supplied to die burner 28 in appropriate amounts. Combustion air is defined as the air or gases required for complete combustion of die available fuel. Excess air is defined as the air or gases supplied in addition to the combustion air. Combustion and excess air may be supplied to die burner 28 utilizing the primary EGR duct 75 and/or a tertiary air duct 89. A primary fan 90, and a tertiary fan 94 are provided along each of die respective ducts 75 and 89 to render available die desired amount of gases from each duct The quenching EGR duct 79 extends via an inlet duct 81 to communicate with die annular duct 40 of the combustion chamber. A quenching fan 92 is provided along die quenching

EGR duct 79 to transmit the desired amount of gases through the quenching EGR duct 79. To obtain die maximum flow rates shown in Example 1 below, a 100 horsepower centrifugal fan was utilized for the primary fan 90; a 40 horsepower centrifugal fan was utilized for the quenching fan 92; and a 150 horsepower axial flow fan was utilized for the

tertiary fan 89.

The dryer 10 operates as follows. A continuous supply of virgin aggregate is introduced into the drum 20 by die conveyor 18. The flame 30 from the burner 28 provides combustion gases to die refractory combustion chamber 32. These gases exit the drum 20 via die exhaust manifold 48 and are routed to the cyclone separator 50 for removal of particulate matter and then to the baghouse 12 for further removal of particulate matter.

It is noted that gases exiting the baghouse 12 are more humid and at a lower temperature man gases within die dryer 10. The present invention uses these cooler, moister gases emerging from die baghouse 12 to accomplish a reduction in the formation of NOχ compounds. The dryer 10 Λereby is a conventional counter-flow aggregate dryer except for the novel features described herein. It is found that NO* emissions may be reduced by maintaining a highly turbulent short flame 30 while reducing die r ... .mum temperature of die flame and the time that the gases spend at a temperature where NO _x is readily created. The dryer 10 operates to produce these conditions by taking the gases from downstream of die exhaust fan 58 and recirculating them to the burner 28 via die primary EGR duct 75 and to die end of the flame 30 via the quenching duct 79, as discussed above. While it will be understood that ambient air or gases recirculated from the exhaust manifold 46 may be used, it is preferred to use air recirculated from after the baghouse 12. Additional benefits of using air recirculated from after the baghouse 12 include die elimination of the back-flow of excessively hot furnace gases dirough die primary fan 90 and the quenching fan 92, and the elimination of dust loading from die fans 90 and 92.

A flow of recirculated gasses dirough the primary EGR duct 75 and die quenching duct 79 may be established by die primary fan 90 and the quenching fan 92, respectively. These moister, cooler recirculated gases are routed to die burner 28 by the primary EGR duct 75 and to the end of die flame 30 via the quenching EGR duct 79 which directs gases to the nozzles 42. Introduction of recirculated gases to the burner 28 and the quenching ring 38 reduces die flame temperature, the flame length, and the free oxygen content. These reductions result in a lower rate of NO _x

production. As stated before, it is preferable to recirculate gases from after the baghouse 12, because the gasses are cleaner and less damaging to die blowers 90 and 92. The trade-off for this benefit of cleaner gases is the disadvantage of a more oxygen rich and cooler recirculation gas stream, because baghouse filtration increases oxygen content. It will be understood that a less oxygen rich exhaust gas stream may be obtained by recirculating the exhaust gas from before the baghouse 12. This, however, has the disadvantage of a more dust laden gas stream.

The amount of exhaust gas recirculated is determined as a mass percentage of the "total gases" supplied by die Primary EGR duct 75, die quenching EGR duct 79, and the tertiary air duct 89. Combustion air is the amount of air or gasses needed for combustion of the available fuel. Excess air is the amount of air or gases supplied in excess of die combustion air.

In the preferred operation of die dryer 10, all of the combustion air and some of die excess air is supplied by die primary EGR duct 75 in combination with die tertiary air duct 89. In this mode, die quenching EGR duct 79 supplies exhaust gases to die nozzles 42 at a velocity sufficient to penetrate the flame 30.

As stated above, the term "total gases" is defined as the sum of all recirculated gases and fresh air supplied by die primary EGR duct 75, die quenching EGR duct 79, and the tertiary air duct 89. In die preferred operating mode, die contributions and compositions of die various gases and air ducts preferably fall within the following ranges set forth in Table 1.

TABLE 1

Approximate % A roximate % b mass b mass in duct

EXAMPLE 1

The below Table 2 sets forth maximum flow rates anticipated to be utilized to perform tests of a dryer embodying the invention. The results of the planned tests are expected to indicate an average reduction in NO emissions, as measured at die exhaust stack 64, from approximately .024 pounds per ton of aggregate to approximately .158 pounds per ton of aggregate.

The above description discloses a mode of operation in which sufficient oxygen is provided to die burner 28 to allow complete combustion. The gases supplied by the quenching nozzles 42 are provided to reduce flame temperature and length. It will be understood, however, that other modes of operation may be practiced to reduce flame temperature and length. For example, the flow rates and die percentage of recirculated gases and fresh air in each duct may be varied to achieve the desired effects. For example, an abbreviated form of staged combustion may be accomplished by supplying insufficient combustion air to the burner. The

remaining air required for combustion of the available fuel may then be supplied by die quenching nozzles 42. When operating in the staged combustion mode, the contributions and compositions of the gases and air ducts preferably fall within the ranges given in Table 3:

TABLE 3

Approximate % Approximate % by mass by mass in duct of t

It will further be noted that by increasing flame turbulence and reducing flame length, as previously stated, the novel design of the combustion chamber 32 is capable of providing reduced NO _x emissions without the introduction of recirculated gas or staged combustion. This result occurs because of the superior mixing of fuel and air obtained by d e geometry of the chamber.

While the foregoing description relates to a counter-flow aggregate dryer, it will also be understood that die foregoing invention may also be utilized to reduce NO _x emissions in connection with parallel flow dryers and drum mixers.

The foregoing description relates to preferred embodiments of the present invention, and modifications or alterations may be made without departing from the spirit and scope of the invention as defined in the following claims.