Patent Application
20070070801
This invention relates to a counter-flow asphalt plant used to produce a variety of asphalt compositions. More specifically, this invention relates to a counter-flow asphalt plant having a recycle asphalt pavement (RAP) feed to produce blended virgin aggregate material (VAM)/RAP mixes.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred to by the trade as “hotmix” or “HMA”, are known from the prior art. Particularly relevant to the present invention is the continuous production of asphalt compositions in a drum mixer asphalt plant. Typically, moisture-laden VAM are dried and heated within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the heated VAM flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce an asphaltic composition as the desired end-product. However, often, prior to mixing the virgin aggregate and liquid asphalt, previously crushed RAP is added. The RAP is typically mixed with the heated VAM in the drum mixer at a point prior to adding the liquid asphalt and mineral fines.
The asphalt industry has traditionally faced many environmental challenges. The drum mixer characteristically generates, as by-products, a gaseous hydrocarbon emission (known as blue smoke), various nitrogen oxides (NOx) and sticky dust particles covered with asphalt. Early asphalt plants exposed the liquid asphalt or RAP material to excessive temperatures within the drum mixer or put the materials in close proximity with the burner flame, which caused serious product degradation. Health and safety hazards resulted from the substantial air pollution control problems due to the blue smoke produced when hydrocarbon constituents in the asphalt are driven off and released into the atmosphere.
The earlier environmental problems were further exacerbated by the processing technique standard in the industry which required the asphalt ingredients with the drum mixer to flow in the same direction (i.e., co-current flow) as the hot gases for heating and drying the aggregate. Thus, the asphalt component of recycle material and liquid asphalt itself came in direct contact with the hot gas stream and, in some instances, even the burner flame itself.
Many of the earlier problems experienced by asphalt plants were solved with the development of modern day counter-flow technology as disclosed in U.S. Pat. Nos. 4,787,938 and 6,672,751 to Hawkins, which are incorporated herein by this reference. The asphalt industry began to standardize on the counter-flow processing technique in which the ingredients of the asphaltic composition and the hot gas stream flow through a single, rotating drum mixer in opposite directions. Combustion equipment extends into the drum mixer to generate the hot gas stream at an intermediate point within the drum mixer. Accordingly, the drum mixers have included three zones. From the end of the drum where the VAM feeds, the three zones include a pre-combustion zone to dry and heat material, a combustion zone to generate a hot gas stream for the drying/heating zone, and a post-combustion zone to mix hot aggregate, RAP and liquid asphalt to produce an asphaltic composition for discharge from the lower end of the drum mixer.
Not only did the counter-flow process with its three zones vastly improve heat transfer characteristics, but more importantly, it provided a process in which the liquid asphalt and recycle material were isolated from the burner flame and the hot gas stream generated by the combustion equipment. Counter-flow operation represented improvement with respect to the vexing problem of blue smoke and health and safety hazards associated with blue smoke.
With many of the health and safety issues associated with asphalt production solved by the advent of counter-flow technology, contemporaneous attention has now shifted to operational inefficiencies which are manifest as excessive design and production costs and poor economy of operation from excess energy consumption.
Experience has shown that the environmentally desirable use of a RAP in asphalt production comes with disadvantageous tradeoffs in energy consumption. In some circumstances, for example, all VAM is introduced in one end of the dryer and flows as a falling curtain or veil of material in counter-current heat exchange with hot gases generated at the opposite end of the dryer. The shell temperature is characteristically about 500 degrees F., and the exhaust gas is about 225 degrees F., which is within the normal operating temperature for the baghouse used to filter the exhaust gas of particulate matter. The temperature of the exhaust gas stream is determined by the design of the dryer, but must be kept above dew point to prevent moisture from condensing in the exhaust ductwork and especially in the baghouse itself. A temperature of 225 degrees F. is sufficient, but since varying conditions during operation can cause relatively large temperature swings, most operations are controlled to keep exhaust temperatures in the range of 250 degrees F. to 275 degrees F.
Typically, the addition of RAP material has a significant effect on operating temperatures of the process. Since RAP cannot be exposed to temperatures above a combustion threshold without burning the liquid asphalt and causing hydrocarbon smoke emissions, it is often dried indirectly by superheating the virgin aggregates and then mixing the superheated aggregates with the RAP to achieve a mixed mixture temperature. This results in much higher exhaust gas temperatures and a resulting loss in fuel efficiency. Accordingly, 20 to 40% RAP feeds (that is, operations wherein RAP makes up 20 to 40% of the final asphalt composition) have been close to the upper end of the range heretofore workable in modern counter-flow asphalt plants. Although a 50% RAP feed is achievable, it has been at the cost of high energy and reduced equipment life. Consequently, an upper limit of approximately 40% RAP has been a realistic upper limit for the majority of asphalt plants. The operating conditions necessary are illustrative of the problems. If 50% RAP is introduced midstream in the process, then only 50% virgin aggregates are used. This means that only half the material is present, as compared to the 100% virgin aggregate production, to be heated and only half the veiling of material in the drying section of the drum occurs which yields poor heat transfer characteristics. Under such circumstances, the combustion zone temperature must be elevated significantly to superheat the virgin aggregate. This, in turn, causes shell temperature of the drum to range from 750-800 degrees F. and exhaust gas temperature to increase to about 375 degrees F. Moreover, any time the combustion zone temperature rises to about 2800 degrees F. or greater, then the production of various nitrogen oxides (NOx) as a product of combustion may become a problem.
A need remains in the industry for an improved counter-flow asphalt plant design capable of utilizing high percentage RAP mixes and for operating techniques to address the problems and drawbacks heretofore experienced with modern counter-flow production. The primary objective of this invention is to meet this need.
More specifically, an object of the invention is to provide a counter-flow asphalt plant capable of routinely using high percentage RAP mixes without emitting excessive blue smoke or without excessive energy requirements.
Another object of the invention is to provide a counter-flow asphalt plant capable of processing high RAP mixes with extended equipment life by eliminating the need to superheat virgin aggregates with the associated temperature elevation of the processing equipment.
An alternative object of the invention is to provide a counter-flow asphalt plant capable of processing RAP mixes by utilizing reduced superheating processes, together with the processing techniques which are the subject of this invention.
A further object of the invention is to provide a counter-flow drum mixer with specially designed pre-combustion zone flighting and drum wall orifices to permit virgin material to be pre-mixed with RAP material which has been introduced into a partial outer drum before the beginning of the combustion zone.
Yet another object of the invention is to provide counter-flow drum mixer and method of operation for reducing NOx emissions for processing techniques utilizing RAP with virgin material mixes.
Another object of the invention is to provide counter-flow drum mixer and method of operation for which the exhaust gas temperatures are substantially lower than in many conventional systems (320 degrees F. vs. 375 degrees F. average in a typical 40% recycle plant).
A further object of the invention is to provide a counter-flow asphalt plant of the character described which is both safe and economical in operation. Efficient operation results in improved fuel consumption and in reduced air pollution emissions.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the detailed description of the drawings.
In summary, a counter-flow aggregate dryer for an asphalt plant is equipped with a partial outer drum around a main drum, where the partial outer drum provides for a secondary front loading feeder for early introduction of RAP materials and providing a place for combining RAP with heated virgin material before the beginning of the combustion zone of the main drum. Adjustably sized and located orifices in the wall of the main drum in the pre-combustion zone permit regulation of heated virgin material dropping down into the partial outer drum, which is then premixed with the RAP material and together are carried around the combustion zone and away from direct radiant heat of the combustion zone to the post combustion zone for additional mixing.
In the following description of the drawings, in which like reference numerals are employed to indicate like parts in the various views:
Referring now to the drawings, where like numerals refer to like matter throughout, and referring in greater detail, attention is first directed to a prior art counter-flow asphalt plant as shown in the prior art illustration of
The prior art counter-flow plant includes a substantially horizontal, single drum mixer 10 carried by a ground engaging support frame 12 at a slight angle of declination, typically about 5 degrees. Mounted on the frame 12 are two pairs of large, motor driven rollers 14 which supportingly receive trunnion rings 16 secured to the exterior surface of the drum mixer 10. Thus, rotation of the drive rollers 14 engaging the trunnion rings 16 causes the drum mixer 10 to be rotated about its central longitudinal axis in the direction of the rotational arrow 17.
Located at the inlet or upstream end of the drum mixer 10 is an aggregate feeder 18 to deliver aggregate to the interior of the drum mixer 10 from a storage hopper or stockpile (not shown). The inlet end of the drum mixer 10 is closed by a flanged exhaust port 20 leading to conventional air pollution control equipment (not shown), such as a baghouse, to remove particulates from the gas stream.
Located at the outlet end of the drum mixer 10 is a discharge housing 22 to direct asphaltic composition from the drum mixer 10 to a material conveyor (not shown) for delivery of the final product to a storage bin or transporting vehicle.
A combustion assembly 24 extends through the discharge housing 22 and into the drum mixer 10 to deliver fuel, primary air from a blower 26 and induced secondary air through an open annulus to a burner head 28. Combustion at the burner head 28 generates a hot gas stream which flows through the drying zone of the drum mixer 10. Within the drying zone are fixed various types of flights or paddles 30 for the alternative purposes of lifting, tumbling, mixing, and moving aggregate within the drum mixer 10 to facilitate the drying and heating of the aggregate therein.
Downstream of the burner head 28 is located the recycle feed assembly 34 by which recycle asphalt material may be introduced into the drum mixer 10. A stationary box channel 35 encircles the exterior surface of the drum mixer 10 and includes a feed hopper 36 providing access to the interior of the box channel 35. Bolted to the side walls of the box channel 35 are flexible seals 37 to permit rotation of the drum mixer 10 within the encircling box channel 35. Secured to the outer wall of the drum mixer 10 and projecting into the space defined by the box channel 35 is a plurality of scoops 38 radially spaced around the drum mixer 10. At the bottom of each scoop 38 is a scoop opening 40 through the wall of the drum mixer 10 to provide access to the interior of drum mixer 10. Thus, recycle asphalt material may be delivered by conveyor (not shown) through the feed hopper 36, into the box channel 35 and subsequently introduced into the interior of the drum mixer 10 through the scoop openings 40.
Downstream of the recycle feed assembly 34 is a mixing zone within the drum mixer 10. Mounted on the interior thereof are staggered rows of sawtooth flighting 42 to mix and stir material within the annulus of the drum mixer 10 and combustion assembly 24. A conveyor 44 extends into the drum mixer 10 for feeding binder material or mineral “fines” to the mixing zone. Likewise extending into the drum mixer 10 is an injection tube 46 for spraying liquid asphalt into the mixing zone. At the end of the mixing zone is located the discharge housing 22, as previously discussed, through which the asphaltic product is discharged.
Now referring to
Turning then to the prior art asphalt plant configuration shown in
Located at the inlet or upstream end of the drum 50 is an aggregate feeder 58 to deliver aggregate to the interior of the drum 50 from a storage hopper or stockpile (not shown).
Located at the outlet end of the drum 50 is a discharge housing 62 to direct asphaltic composition from the drum 50 to a material conveyor (not shown) for delivery of the final product to a storage bin or transporting vehicle.
A combustion assembly 64 extends through the discharge housing 62 and into the drum 50 to deliver fuel, primary air from a blower 66 and induced secondary air through an open annulus to a burner head 68. Combustion of the air and fuel within the combustion zone of the drum 50 which generally extends from the burner head 68 to the end of the flame envelope 69 generates a hot gas stream which flows through the drying zone of the drum 50. Within the drying zone, material flights 70 are secured to the interior surface of the drum 50 to lift, tumble, mix, and release aggregate material within the drum 50 to create a substantially continuous veil or curtain of falling material through which the hot gas stream passes in counter-current flow to facilitate the drying and heating of the aggregate.
Early conventional wisdom of asphalt plant design and operation positions the RAP feed downstream of the burner head as illustrated in
Within the combustion zone are mounted a plurality of combustion flights 80 which are spaced apart from the interior surface of the drum shell 50 to provide an annulus region through which material may be carried. It is specifically important to this prior art design that the combustion flights 80 are non-veiling flights to prevent material from falling through the flame envelope 69, as distinguished from the dryer flights 70, which are veiling flights for the intended purpose of creating a continuous curtain of falling material in the heating/drying zone.
Downstream of the burner head 68 is a mixing zone within the drum 50. Mounted on the interior thereof are rows of mixer flighting 82 to mix and stir material within the annulus formed by the drum 50 and combustion assembly 64. An auger 84 extends into the drum 50 for feeding binder material or mineral “fines” to the mixing zone. Likewise extending into the drum 50 is an injection tube 86 for spraying liquid asphalt into the mixing zones. At the end of the mixing zone is located the discharge housing 62 as previously discussed through which the asphaltic product is discharged.
Now referring to
Opening 103 receives virgin aggregate material (VAM) and opening 105 receives recycled asphalt product (RAP).
The main drum 102 is divided into 3 zones, the pre-combustion zone 110, the combustion zone 120, and the post-combustion zone 130. The pre-combustion zone 110 is provided with veiling flights for dispersing the VAM as it passes through the pre-combustion zone 110. Pre-combustion zone 110 further has a plurality of mouse holes 114 which provide for limited amounts of VAM to pass therethrough to help scour the outer drum dispersing flights 118. Pre-combustion zone 110 further has a plurality of adjustable VAM inter-drum holes 116 for regulating the amount of VAM that passes into the partial outer drum 104. The adjustable VAM inter-drum holes 116 may be the results of adding or removing various panels 117 which can be added or removed, depending upon the operational parameters of any particular job.
Combustion zone 120 is where the heat for the pre-combustion zone 110 is generated by the flame 69. Counter-flowing heated air column 122 is shown moving in a direction counter to the direction of flow of the VAM and the RAP.
Post-combustion zone 130 is generally for combining and mixing elements. The pre-mixed RAP and VAM entry hole 132 is where the mixture of heated RAP and heated VAM enters the post-combustion zone 130 and is combined with still more VAM and liquid asphalt via spray 86. The combination is then mixed and output through output 140.
Now referring to
Main inner drum inlet end 1030 receives the VAM and main inner drum spiral intake blades 1022 move the material from the inlet end into the interior of the drum where it can be heated. VAM veiling type drying flights 1024 are disposed on the inside surface of main inner drum 1020 for the purpose of creating a curtain or veil of VAM, as the drum is rotated during operation, so as to improve the efficiency of heating and drying the VAM. As the VAM proceeds downward through the main inner drum 1020, it approaches a flame output by a centrally disposed burner (not shown but disposed along a central axis and upward from the mixing flights). The inside temperature of main inner drum 1020 increases from the main inner drum inlet end 1030 to the burner. Heat shielding plates 1026 are added underneath the VAM veiling type drying flights 1024 in a section of the main inner drum 1020 nearer the burner. The density of VAM veiling type drying flights 1024 is shown as reduced in the area with heat shielding plates 1026; however, this need not be the case. The density of VAM veiling type drying flights 1024 is readily adjustable by having each of the VAM veiling type drying flights 1024 being independently mounted on the main inner drum 1020. As the VAM moves closer to the burner, temperature rises further. Scooping combustion zone insulating flights 1028 are shown in next section. These scooping combustion zone insulating flights 1028 have a relatively small gap along the leading edge; as the main inner drum 1020 is rotated, this gap allows VAM to enter into a shielded or insulated compartment as it proceeds through the main inner drum 1020 at points of increasing internal drum temperatures.
A partial outer drum 1040 is disposed concentrically about main inner drum 1020 with a partial outer drum inlet end 1050 for receiving RAP therein at some intermediate point along main inner drum 1020.
The partial outer drum inlet end 1050 is shown as starting at a point where the main inner drum 1020 has scooping combustion zone insulating flights 1028; however, it could begin at an earlier or later point along the main inner drum 1020 and the flights coinciding with the partial outer drum inlet end 1050 can be any type of flight.
One general purpose of the partial outer drum 1040 is to provide for a preheating of the RAP prior to introduction into the main inner drum 1020 at a point where it is not subject to the high temperatures associated with direct exposure to the flame from the burner.
Now referring to
The VAM will normally proceed down through the main inner drum 1020 into a combustion zone having the scooping combustion zone insulating flights 1028. Main inner drum 1020 may have additional holes therein to permit more VAM to exit the main inner drum 1020 and enter the partial outer drum 1040.
Burner zone flights 1032 are shown disposed above burner zone inner drum adjustable exit openings 1034. A purpose of burner zone inner drum adjustable exit openings 1034 is to permit even more VAM to exit the main inner drum 1020 and thereby avoid traversing the highest temperature areas within the main inner drum 1020. Burner zone inner drum adjustable exit openings 1034 are made adjustable by at least partially covering them with plates (not shown). These partial plates may be stainless steel.
After the burner zone inner drum adjustable exit openings 1034 allow passage of VAM into the partial outer drum 1040, the VAM is deflected by deflectors 1142 and caused to move more in a downward direction through the partial outer drum 1040.
After the workers access opening 1042, the partial outer drum 1040 tapers down to and attaches to the main inner drum 1020. Nearly adjacent to the point where partial outer drum 1040 meets with main inner drum 1020, there are outer drum exit channel deflectors 1146 which help to channel the material in partial outer drum 1040 through the outer drum exit channels 1148.
Once the material from the partial outer drum 1040 is emptied into the main inner drum 1020, it proceeds through the outer drum exit channel chutes 1152. At this time, the VAM and RAP are mixed together with an asphalt liquid in an area behind the burner and out of the flow of hot gases emanating from it and being propelled out the main inner drum inlet end 1030. Mixing flights 1154 are included to facilitate thorough mixing of the RAP, VAM and an asphalt liquid.
The newly formed asphalt is then discharged from the main inner drum 1020 for subsequent use.
It is believed that when these teachings are combined with the known prior art by a person skilled in the art of asphalt drum design and operation, many of the beneficial aspects and the precise approaches to achieve those benefits will become apparent.
It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of the claims.
Since many possible embodiments may be made of the invention without departing from the scope thereof, it is understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.
This application claims the benefit of a provisional application filed on Sep. 23, 2005, and having Ser. No. 60/596,448.
| Number | Date | Country | |
|---|---|---|---|
| 60596448 | Sep 2005 | US |
