Patent Grant
8882899
The invention relates to a method for producing asphalt mixture according to the introductory clause of claim 1 and to a plant for producing asphalt mixture according to the introductory clause of claim 19.
The invention is based in particular upon the recycling of reclaimed or recovered asphalt which arises during the dismantling and reconstruction of asphalt roads and on the basis of legal provisions, including the Recycling Management and Waste Act, is to be correspondingly recycled and returned to use. The reutilization should be carried out for economical and ecological reasons at least as building material, but primarily with reactivation of the binding agent bitumen in newly mixed asphalt. Reclaimed asphalt, for example milled asphalt or scarified asphalt, is thereby reduced to asphalt granulate and mixed with a pre-definable size distribution and corresponding to the classification in a defined quantity together with aggregates and bitumen.
Aggregates are understood in this connection to be fresh solid materials such as gravel, sand and mineral powder, also referred to as filling materials or fillers, which have a defined grain size composition and are used in a pre-definable quantity.
The Deutsche Asphaltverband e.V. describes in the Internet publication “Recycling of asphalt—New system of regulations points the way forward” of May 2008, Annexes 3.1 and 3.2: September 2009, the status of research regarding the recycling of asphalt, in particular in asphalt mixture for asphalt support layers, asphalt support top layers and asphalt foundation layers. Reference is also made to the currently valid legal provisions such as technical rules and standards and data sheets relating to addition quantities of asphalt granulate in new mixed materials.
In principle during the production of asphalt mixture with recycling of reclaimed asphalt in the form of asphalt granulate, a heating and drying of aggregates and asphalt granulate take place in at least one drum device, whereby hot gases serve as a heat source which are supplied in a counter flow or parallel flow to the aggregates and/or asphalt granulate to be heated. Conveyance then takes place, for example with conveyor belts or hot elevators, a classification of the aggregates and mixing with asphalt granulate and with heated bitumen in mixing devices, for example in a paddle vane type mixer, a siloing, in particular in hot silo installations. A hardening of the new binding agent is to be avoided through such pre-mixing.
In the cold method asphalt granulate is heated and dried through contact with fresh aggregates in the mixing device. The aggregates must therefore be correspondingly heated to a higher level, generally over 200° C., in order to achieve the heating and drying of the asphalt granulate and the mixed material temperature of around 160 to 180° C. necessary for incorporation and compaction of the asphalt mixture. In this method the addition quantity of the asphalt granulate is maximum 30%. Besides this low addition of asphalt granulate there is a further drawback in the necessity of pre-mixing the hot aggregates with the cold asphalt granulate and only adding the new binding agent bitumen after removal of the heat excess of the aggregates with simultaneous drying and heating of the asphalt granulate. Besides the thermal overloading of the drying and heating drum and the hot elevator this method leads to irregularities in the operation of the waste gas purification systems. During the heating and drying of asphalt granulate in a mixing plant considerable quantities of vapours are produced discontinuously, for example in a 60-second cycle, whereby these are conveyed into the waste gas system. The waste gas quantity hereby changes considerably in an intermittent manner. The waste gas system must thus be continuously operated with the maximum possible waste gas volume flow, thus inclusive of the maximum possible quantity of vapours. During times in which no vapour arises considerable quantities of false air are drawn into the system. The overall degree of efficiency of the plant is hereby impaired.
In the aforementioned publication of the Deutsche Asphaltverband e.V. further plantspecific schemas and methods for recycling of recovered asphalt are described. In the case of counter flow drums the addition of the asphalt granulate can take place via a central addition or through an adding apparatus on the drum outlet.
The mixture of aggregates and asphalt granulate is then fed via a sieve bypass pocket to the mixing device, for example a mixing tower. Addition quantities of around 40% reclaimed asphalt are intended to be possible with these methods.
An expensive double shell drum is required for a corresponding realisation.
Separate heating of asphalt granulate can be carried out separately from the aggregates in a parallel drum. In order to preserve the binding agent of the asphalt granulate and also to limit the emissions of the binding agent, a temperature of maximum 130° C. is to be observed. Heating to around 110° C. is preferred. Higher temperatures in the drum devices or mixing devices cause an intensive ageing of the bitumen and an impairment of its thermoplastic properties.
In case of continuous mixing installations, wherein the mixing process of the aggregates and the asphalt granulate takes place continuously in a drum or in a subsequently arranged continuous mixer and the asphalt granulate is previously heated together with the aggregates in a drum mixer or, however, is heated separately in a parallel drum, addition quantities of up to approximately 50% asphalt granulate are to be possible.
In principle the addition of asphalt granulate to new aggregates in a drum device, for example a drying drum, is limited in terms of quantity for the abovementioned processrelated reasons. Essential aspects are thereby an overheating of the asphalt granulate which leads to a burden on the environment through the volatile components of the bitumen and/or to a carbonisation of the bitumen contained in the asphalt granulate.
DE 195 30 164 A1 discloses a method and a drying drum for heating and drying asphalt granulate, wherein a separate hot gas generation is carried out in a hot gas generator. A maximum hot gas temperature of 600° C. is thereby to be set. In addition, through a special guidance of the hot gas and the asphalt granulate within the drum, it is endeavoured to ensure gentle heating which is intended to prevent cracking of the bitumen containing asphalt granulate and to minimise the occurrence of harmful substances.
DE 38 31 870 C1 discloses a method for asphalt production using granulated recovered asphalt, wherein hot aggregates dried to around 400° C. and cold asphalt granulate are added in predefined amounts to a mixer and mixed with bitumen and possibly filler material (limestone powder). In order to facilitate a higher proportion of recovered asphalt in the overall mixture a premixing or an addition in two steps into the mixer is carried out. At the end of the first mixing stage the mixture is to have a temperature of 170 to 180° C. and the overall mixing time of a mixer batch is to be around 60 seconds.
DE 10 2004 014 760 B4 discloses an asphalt plant and a method for producing asphalt, wherein aggregates are mixed with bitumen and possibly further additives and old asphalt as asphalt granulate to form a new asphalt mixture which can be incorporated. In order to guarantee a good mixing without damage, vaporisation or ignition of the bitumen a temperature range of from 170 to 190° C. is predefined for the dried and heated aggregates.
EP 0 216 316 A2 discloses a method for recycling of asphalt granulate, wherein aggregates and asphalt granulate are heated and dried in two separate drum driers and then mixed together with supplementary filler and bitumen portions in a mixer to form recycling mixed material. The recycling mixed material is directly supplied for further processing or, however, for intermediate storage for removal as required. Temperatures are not indicated in this document. Reference is only made to the corresponding provisions and to a considerable viscosity increase of the bitumen as a consequence of overheating of the aggregates. The heating of the asphalt granulate in the drying drum with directly connected burner takes place with concurrent flow and the waste gases are fed back to the burner of the drum drier for the aggregates as secondary and tertiary air.
In the methods known from DE 43 20 664 A1 recovered asphalt is heated in a separate drum by flue gas. Gentle heating of the reclaimed asphalt in order to prevent a thermal overheating of the bitumen is to be achieved in that the hot flue gas is conveyed in concurrent flow with the reclaimed asphalt material flow through the drum and in addition a portion of the flue gas leaving the drum at around 170° C. is fed back to the drum on the burner side in order to be able to further reduce the flue gas temperature and hence the temperature difference between the reclaimed asphalt and the flue gas. The flue gas portion not fed back is conveyed through a second drum, in which the aggregates are transported in a counter flow.
DE 20 2008 012 971 U1 discloses a plant for drying and heating granulated material for asphalt production, which comprises a rotating drying drum for drying and heating the aggregates and the asphalt granulate and a hot gas generator for supplying a hot gas flow. Having regard to the compliance with the maximum admissible binding agent temperature and end temperature of the asphalt material at the outlet of the drum dryer and the admissible waste gas temperature at the inlet of the waste gases into a filter installation, to which the waste gases are usually supplied, the degree of energy efficiency is to be increased and the particle load of the filter installation is to be reduced in that a portion of the hot gas flow is fed from the drying drum to the hot gas generator again. In addition the particles and fine parts are to be separated from the waste gas in a settling drum and then added to the material flow comprising recycling asphalt and aggregates. The waste gas flow which is fed back into the hot gas generator is preferably to be enriched with oxygen rich fresh air.
In the method described in DE 10 2006 038 614 A1 the plasticity of the bitumen reduced through thermal ageing is set in the reclaimed asphalt with the aid of a plasticiser. A combined addition of a hardener, preferably in the warm phase of the mixture, is also carried out. It is further described that recovered asphalt is generally heated having regard to the technical environmental provisions relating to air to up to 130 to 140° C. and the aggregates or the new mineral substances have to be heated, with an addition of around 50% reclaimed asphalt, clearly above 200° C. The degree of oxidation (ageing) of the bitumen in the reclaimed asphalt is seen as a problem in association with the degree of heating of the asphalt granulate limited to 140° C. in the recycling of recovered asphalts and the production of asphalt hot mixed materials. Through the addition of a plasticiser/hardener system a reduction of the incorporation temperature of the recovered asphalt from around 170° C. to 140° C. is to be achieved. As a result of the need for plasticisers and hardeners the cost of production of the asphalt mixture is increased considerably.
A disadvantage of the known methods and plants for the production of asphalt mixture, in particular when using asphalt granulate from recovered asphalt, results from the considerable quantities of false air. These are based upon the technology of the plant, lead to a high oxygen content in the hot gas and reduce the degree of heat efficiency of the plants. Generally the burners and/or hot gas generators used in the drying drums are operated with burners with fossil fuels and a waste gas recycling is carried out, wherein up to 50% of the waste gas flow is fed back to a hot gas generator.
A further disadvantage of the known methods and plants is the addition quantity of reclaimed asphalt for asphalting which is limited by technical aspects of process and plants, whereby the asphalt industry cannot adequately meet the constantly increasing requirements for an improved road infrastructure with the recycling of recovered asphalt as completely and with as high a quality as possible.
It is the object of the invention to create a method and an plant which guarantee the production of asphalt mixture in the required quality also with recycling of up to 100% reclaimed asphalt and which extraordinarily improve the efficiency of the asphalt production, in particular by saving raw materials and heat energy.
The object is achieved in terms of the method and devices according to at least one of embodiments of the present invention. Advantageous embodiments are contained in the description of the drawings.
According to the invention in the method for producing an asphalt mixture, wherein recovered asphalt in the form of asphalt granulate and/or new material in the form of aggregates is/are heated and dried together and/or separately in drum devices and then mixed in a mixing device with bitumen and possibly further additives to form an asphalt mixture which can be incorporated, at least the drying and heating of the asphalt granulate and/or the aggregates are carried out in a low oxygen atmosphere. The low oxygen atmosphere is thereby characterised by an oxygen content of 0 to 10%, preferably by an oxygen content of 0 to maximum 5%.
The invention is based on the recognition that through a low oxygen atmosphere at least in the drying and heating of the asphalt granulate and/or the aggregates, advantageously also in the conveyance of the heated and dried asphalt granulate or the heated and dried mixture of asphalt granulate and aggregates, respectively, and in the mixing with bitumen in a mixing device, an oxidation of the bitumen in the asphalt granulate and also in the fresh bitumen is prevented or at least reduced so that the thermoplastic properties of the bitumen are not disadvantageously changed.
It was recognised that solely a temperature increase to up to 250 or 300° C. does not cause any relevant damage to the bitumen, in particular in the reclaimed asphalt or asphalt granulate, respectively. In the production of bitumen in refineries a targeted oxidation process is thus carried out at temperatures in the range of from 250 to 270° C. over two to ten hours. A thermal decay (cracking) of the bitumen only arises at temperatures above 400° C. and leads to a disadvantageous compression of the bitumen structure with a change in the thermoplastic properties of the bitumen. In the presence of oxygen the hydrocarbon compounds are broken up in the aromatic rings of the bitumen. Without oxygen or with a very low oxygen content in the atmosphere, respectively, there is no thermal transformation of the hydrocarbon compounds of the bitumen. This was also ascertained for temperatures above 200° C., for example from 200 to 300° C. Bitumen can be stored in closed containers in long term at temperatures in the range of from 200 to 300° C. without a disadvantageous change in the thermoplastic properties.
Insofar as according to the invention asphalt granulate and/or aggregates is/are heated and dried together and/or separately in one or two drum devices in a low oxygen atmosphere a temperature increase of the asphalt granulate or the mixture of asphalt granulate and aggregates, respectively, to a temperature level in the range of from 180 to 200° C. is facilitated and this temperature level advantageously guarantees the production of asphalt mixture, also with solely recycling of reclaimed asphalt or with 100% asphalt granulate, respectively, with a lower addition of new bitumen and without fresh aggregates.
The low oxygen atmosphere according to the invention during the drying and heating in at least one drum device and preferably also during the conveyance and mixing with new bitumen in a mixing device is achieved using low oxygen gases which have according to the invention an oxygen content of maximum 10% and preferably an oxygen content of maximum 5% so that the oxygen content can preferably amount to 1, 2, 3, 4 or 5% or 6, 7, 8, 9 or 10%.
Low oxygen gases within the scope of the invention are understood to be in particular low oxygen process gases or waste gases of the most varied technical processes.
It is advantageous that asphalt granulate and/or aggregates can be heated and dried using low oxygen gases having a temperature in the range of from 500 to 1000° C. and then be conveyed to a mixing device and that, in order to guarantee a low oxygen atmosphere in the conveyance and mixing, using cold, low oxygen gases having a temperature in the range of from around 20 to around 150° C. or also by means of cooled low oxygen gases having a temperature of from around 150 to 300° C., a mixing of the hot, low oxygen gases with cold, low oxygen gases or respectively a cooling of the hot, low oxygen gases is carried out.
The asphalt granulate can be expelled from the drum devices with a temperature in the range of from approximately 130 to approximately 250° C.
It thereby lies within the scope of the invention for the supply bunkers and/or silos arranged before or after the mixing device, to be supplied with low oxygen gases.
The low oxygen gases can be obtained in particular through combustion of fossil fuels with a combustion air ratio/an air ratio λ=1.0 to 2.0, in particular λ=1.0 to 1.4, whereby the combustion can take place within and/or outside of the asphalt production/asphalt mixing installation.
In case of larger air ratios, for example from around λ>1.4, the combustion air can be mixed with low oxygen gases, for example waste gas, up to 100%.
It is advantageous to cool hot, low oxygen gases, which have been produced through combustion, indirectly, for example in heat exchangers, and/or directly through mixing with cold, low oxygen gases, to a temperature in the range of from 1000 to 400° C., preferably 900 to 600° C.
In principle the cold and hot low oxygen gases can originate from different sources and production plants. It is particularly efficient and also ecologically advantageous to use low oxygen gases which arise in technical processes as by-products or waste products, usefully also outside of the asphalt production. For example the nitrogen arising in metallurgical processes from air decomposition plants or low oxygen gases from gastight boiler plants, oxi-fuel plants and kilns, for example of the glass and metal production, can be used for the production of the inventive low oxygen atmosphere in the production of asphalt mixture. Cold, low oxygen gases can advantageously be brought to a higher temperature indirectly, for example in heat exchangers, or directly through mixing with hot, low oxygen gases. The use of low oxygen gases from processes and production plants outside of asphalt production is not only advantageous for the asphalt production but instead also means improved economy of the technical processes and plants in which the low oxygen gases arise. An improved efficiency in asphalt production is advantageously achieved in that the partial flows of the low oxygen gases from the individual devices can be brought together and fed to a waste gas purification unit and in that recirculation to the plant devices before and/or after waste gas purification is possible.
Cold, low oxygen gases are advantageously supplied for sealing the drum devices and/or conveying devices and/or silo devices and/or mixing devices and the connecting points between these devices and also in the area of the material inlet and outlet devices, in particular the drum devices. Sealing of this nature is to be carried out in particular in regions between the rotating and stationary parts of the drum devices.
It is advantageous that the low oxygen gases are formed and/or used at a positive pressure, for example at around 0.005 to 300 mbar, in particular up to 100 mbar, in a drum device and hot gas generator with burner, whereby a gas suction in the sealing and connecting regions can be carried out and the drawn-off gas can be fed to the burner of the drum device as a primary air portion and/or to the waste gas purification unit and/or to a flue. In this way the emissions of asphalt plants are advantageously reduced.
The low oxygen gases are advantageously supplied at least in part to a waste gas purification unit with drainage and then used as cold, low oxygen gases and used for example to seal the drum devices, conveying devices, mixing devices and/or silo devices.
The inventive installation for production of asphalt mixture, which comprises at least one drum device for heating and drying asphalt granulate made from reclaimed asphalt and/or new material in the form of aggregates and a mixing device for mixing the heated and dried asphalt granulate and/or aggregates with bitumen, is equipped with at least one source for low oxygen gases, in which the low oxygen gases with an oxygen content of maximum 10%, advantageously with an oxygen content of maximum 5%, and/or from which the low oxygen gases can be supplied to the at least one drum device.
Appropriately, at least the drum devices and advantageously also the conveying devices, silo devices and the mixing device are formed to be gastight and provided with seals which prevent false air quantities and a higher oxygen content in the devices. The inventive plant thus differs from the known plant variants which by reason of high false air quantities in the waste gases have an oxygen content of 10% to around 16%.
By way of a drum device a drying and heating drum, to which asphalt granulate and/or the aggregates are fed in counter flow or parallel flow to the hot, low oxygen gases, a counter flow drum for the asphalt granulate and/or the aggregates or a parallel drum for the asphalt granulate can be used, and the mixing devices can be mixing towers, drum mixers or continuous mixers.
In order to supply at least the drum device with low oxygen gases in overpressure, particularly in the range of from 0.005 to 300 mbar, it is useful to provide a suction device at connecting points and to feed the drawn-off low oxygen gases back to a waste gas purification unit, to the burner or into the circuit of the low oxygen gases.
As source for the low oxygen gases, the waste gases of the asphalt mixing installation or waste products and by-products of technical processes and operations outside of the asphalt mixing plant can be used.
A waste gas side connection of the plant for producing asphalt mixture with a coal grinding plant, in which raw coal is reduced in a grinding-drying process and for example ground to form coal dust is particularly advantageous. The low oxygen gases from the asphalt production process and the low oxygen gases from the coal grinding and coal drying can be used at least proportionally and thereby be used both in the installation for asphalt production and in the coal grinding, for example in order to fire the asphalt production installation. The economy of both processes is thus increased.
It is particularly advantageous for the production of hot, low oxygen gases to provide a hot gas generator, in particular with a steel combustion chamber. This generator can comprise a burner for gaseous, liquid and/or solid fuels.
The hot gas generator can have a gas mixer for mixing cold, low oxygen gases, for example from the waste gas purification unit, and the hot, low oxygen gases of the burner.
If the hot gas generator is a LOMA heating system of Loesche, wherein a Loesche perforated jacket (LOMA) heating system is provided, cold, low oxygen gases can be fed to the perforated jacket for mixing with the generated, hot, low oxygen waste gases.
Reference is made to the German patent DE 42 08 951 C2 for a hot gas generator with LOESCHE perforated jacket heating. A production of hot, low oxygen gases with good control facilities is possible with this hot gas generator.
In a particularly preferred embodiment a hot gas generator with a perforated jacket heating system is connected with a counter flow drum as a drum device for drying and heating for asphalt granulate and/or aggregates. The hot, low oxygen gases from the Loesche hot gas generator are transported in counter flow to the asphalt granulate and/or the aggregates in the counter flow drum and an inner recirculation circuit of the volatile hydrocarbon compounds from the bitumen is formed. The concentrations of the volatile hydrocarbon compounds thus increase in the drum to 5 to 15 times in comparison with a parallel drum.
It was found that in the treatment of the recovered asphalt with an increased temperature in comparison with conventional methods, in particular in a counter flow drum, with low oxygen atmosphere, the contact between bitumen and solid substances is improved and a 100% use of asphalt granulate made from recovered asphalt is possible without recognisable, disadvantageous effects upon the properties of the new mixture.
The invention is explained in further detail below by reference to the drawings in which the following are shown in highly schematised manner as plant schemas:
Identical features are provided with identical reference numerals. The gas supplying is indicated with single lines and the transport of the solid materials with double lines.
The installation schema of
Using the conveying device 6 the dried and heated material is fed from the drying and heating drum 4 to a silo device 18, from which the mixture of asphalt granulate 5 and/or aggregates 7 is mixed in a defined proportion with bitumen 9, which is heated with the aid of an oil heater 31. The asphalt mixture 10 can be immediately incorporated or initially supplied to a silo.
A portion of the cold, low oxygen gases 2 is heated in a gas heater 15 using a heat source 37 to a temperature in the range of from 500 to 1000° C. and passed proportionally to the drying and heating drum 4. The flow guidance in the drying and heating drum 4 takes place in a counter flow to the asphalt granulate 5 and/or aggregates 7. A portion of the hot, low oxygen gases 12 from the gas heater 15 thus ensures in the drying and heating drum 4 a low oxygen atmosphere while another portion is mixed with a partial flow of the cold, low oxygen gases 2 and is fed to the conveying device 6, the silo devices 18, 19 and the mixing device 8 for production of a low oxygen atmosphere in these plant devices. The flow guidance within the silo devices 18, 19 and in the mixing device 8 takes place in a parallel flow. A heat source 37 for example an electric heater can be used. A direct or indirect heating of the cold, low oxygen gases 2 into hot, low oxygen gases 12 is also possible. The low oxygen gases from the conveying device 6, the silo devices 18, 19 and the mixing device 8 are collected and fed to a waste gas purification unit 11.
The hot, low oxygen gases 32 from a source 13 outside of the asphalt mixing installation have a temperature >1000° C. and are cooled either directly or, as shown in
A portion of the hot, low oxygen gases 12 from the gas cooler 16 is fed as cooled, low oxygen gases 22 with a temperature in the region of from 150 to 300° C. to the conveying device 6, the silo devices 18, 19 and the mixing device 8 for production of a low oxygen atmosphere with an oxygen content of maximum 10%, in particular 5%. After these devices the partial flows of the low oxygen gases are collected and fed to the waste gas purification unit 11.
The installation according to
In the drying and heating drum 4 of
In the plant of
The plant according to
The plant according to
The fan 40 for the burner 21 of the hot gas generator 20 also draws in, besides the combustion air 39, low oxygen gases 2, 12 from the seals 35 of the drying and heating drum 4 and from the material inlet 33 and material outlet 34 thereof. The waste gases from the waste gas purification unit 11, in particular a waste gas portion from a first stage, serve as a source for the cold, low oxygen gases 2. The remaining waste gases can be fed to a second stage 23 of the waste gas purification unit.
The installation according to
The plant according to
The partial flows of the low oxygen gases from the devices of the plant are collected and fed to a cooler 27 for water separation and subsequently to a waste gas purification unit 11 which serves as a source for the cold, low oxygen gases 2 and thus guarantees an advantageous waste gas recycling.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 050 506 | Oct 2009 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/009292 | 12/28/2009 | WO | 00 | 4/23/2012 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2011/047705 | 4/28/2011 | WO | A |
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| International Preliminary Report on Patentability for International Application No. PCT/EP2009/009292, Apr. 2012. |
| International Search Report; PCT/EP2009/009292; Dec. 28, 2009. |
| Number | Date | Country | |
|---|---|---|---|
| 20120204761 A1 | Aug 2012 | US |
