A method for heating raw materials for processing into asphalt

Abstract

Method for heating raw materials for processing into asphalt, comprising the following steps: Providing a stream of dry asphalt granulate (6), Providing a flow of dry minerals (7), Heating the stream of dry asphalt granulate and the stream of dry minerals to a temperature in the range of 100-250 °C, Mixing the heated asphalt granulate and the heated minerals with bitumen and any fillers to obtain asphalt.

Description

[0001] The present invention relates to a method for heating of raw materials for processing into asphalt, as well as to an apparatus suitable for such method.

[0002] Asphalt is a mixture of crushed stone, sand and filler, held together by bitumen, namely a product from the petroleum industries. Filler, sand and bitumen together form the so-called mastic, wherein the mastic surrounds the crushed stone and, together with the choice of stone, provides the properties of the asphalt mixture. After years of intensive use, asphalt is in need of replacement. The old asphalt is milled, after which it is taken to the asphalt plant and made suitable for hot reuse.

[0003] Methods for the reuse of asphalt are well known in the art. U.S. Patent No. 4,096,588 relates to a process for recycling used asphalt-aggregate compositions comprising crushing said used asphalt-aggregate compositions; separating said crushed composition into coarse particles and fine particles; introducing said coarse particles in a first rotatable drum in which hot gases of combustion at a first temperature are provided; introducing said fine particles in a second rotatable drum in which hot gases of combustion at a second temperature are provided, said second temperature being lower than said first temperature; gradually mixing and heating said coarse and fine particles; and combining said heated coarse and fine particles.

[0004] Dutch patent NL2035872 relates to a method for in situ cold recycling of old asphalt, wherein the old asphalt layer is removed from an existing road, after which the resulting milling product is mixed in situ, without the supply of heat, with one or more of foam bitumen, water, very fine material such as dust, cement or similar, and an additive, such as a rejuvenating agent or a softening agent.

[0005] Dutch patent NL1005200 provides for a method for recycling used asphalt wherein the used asphalt is heated by steam to a temperature between 90 °C and 120°C, wherein between 1 and 3 wt.% cement and between 1 and 3 wt.% binder, such as bitumen emulsion, is added to the heated asphalt.

[0006] Dutch patent NL2025239 provides a method for preparing an asphalt composition, comprising a step of providing a first granulate of recovered asphalt from fine aggregates and coarse aggregates, a step of heating the first granulate, wherein the first granulate is heated as it passes through a screw conveyor, a step of mixing the hot first granulate with the second granulate of coarse aggregates and a bitumen-containing binder material to finally obtain an asphalt composition.

[0007] US Patent application US 2012 / 170401 relates to hot mix asphalt (HMA) plants used in road paving and to the use of recycled asphalt (RAP), comprising a pre-heater to preheat said RAP to a temperature approaching, but not more than the boiling point of water inside of said pre-heater; said pre-heater configured for directing such preheated RAP to a mixer. Such a mixer serves to provide direct heating of the preheated RAP to a temperature above the boiling point of water, with heated oil providing heat to the mixer's movable parts. The resulting exhaust gases from the oil heater are exhausted to an open central unit.

[0008] International application WO 2017 / 035670 relates to a method for producing asphalt from a bulk material with residual moisture and bitumen, wherein superheated steam is applied to the bulk material to reduce the residual moisture in the bulk material and then mixing the bulk material with reduced residual moisture with bitumen to asphalt. Bulk solids are a mineral whose residual moisture content has been reduced, i.e. less than 10%. Superheated steam is introduced into a drying chamber wherein the superheated steam is introduced into the drying chamber in countercurrent, opposite to the direction of transport. The application of superheated steam and the associated temperatures will result in substantial emissions of harmful substances.

[0009] From the technologies discussed above, it is clear that the asphalt production is often accompanied by large emissions of harmful components, including polyaromatic hydrocarbon compounds (also referred to as PAHs), benzene, particulate matter, odour, water vapour, CO 2 and NO x . Part of these emissions can be attributed to the heating of recycled asphalt, as old bitumen is heated in the heating process and (depending on the technique used) also partly burned.

[0010] The heating of the minerals usually takes place via direct or indirect combustion of natural gas in a rotating drum, where heat transfer to the minerals takes place via radiation from the flame and convection via hot air. The flue gases from the drum are filtered and discharged to the environment via the chimney. Strict national and European environmental emission requirements apply to such emissions.

[0011] It is known that there are various filter techniques that can capture particulate matter, PAHs, benzene and odour below the standard values. In practice, however, there will always be a certain residual flow that contains, among other things, water vapour, odour, particulate matter, PAH, CO 2 and NO x . The result of this is that there will be a certain degree of nuisance to the environment. There is also a risk that the application emission limits will be exceeded, in particular the emissions of PAH, benzene and NO x , wherein the former is in conflict with the limits set in the Living Environment Activities Decree. The latter is restricted by nature permits. A cloud of steam at the chimney will also be visible to local residents at the production location, which is experienced as undesirable.

[0012] An object of the present invention is to provide a method for heating raw materials for processing into asphalt wherein the emission of harmful substances into the environment is kept to a minimum.

[0013] Another object of the present invention is to provide a method for heating raw materials for processing into asphalt wherein Use is made from sustainable energy sources.

[0014] Another object of the present invention is to provide a method for heating raw materials for processing into asphalt wherein in an efficient manner use is made from the available energy sources resulting in a decrease in energy demand.

[0015] The present invention thus relates to a method for the heating of raw materials for processing into asphalt, wherein the method comprises the following steps: Providing a stream of dry asphalt granulate, Providing a flow of dry minerals, Heating the stream of dry asphalt granulate and the stream of dry minerals to a temperature in the range of 100-250 °C, Mixing the heated asphalt granulate and the heated minerals with bitumen and any fillers to obtain asphalt.

[0016] The present inventors have found that such a method fulfills one or more objects. Under the application of the aforementioned raw materials for obtaining asphalt, the step of heating takes place in such a way that the formation and emission into the atmosphere of harmful substances belonging to the group of NO x , PAHs, CO 2 , odour and particulate matter is kept to a minimum. By using a stream of raw materials that can be regarded as dry, namely streams of raw materials having a high dry matter content, it has been possible to apply heating without any emissions to the atmosphere. By implementing the method in such a way that heating is preceded by an optimal drying process, the emission of harmful substances is reduced to a minimum. In fact, the present method relates to a virtually closed system. It is also possible with the present method to carry out the heating system under the application of renewable energy resources and thus eliminate emissions of CO 2 and NO x . In addition, the closed system improves efficiency, which will reduce energy demand compared to systems wherein natural gas heating and high-moisture output streams are heated, producing significantly large vapour flows. By starting from dry asphalt granulate and dry minerals, the formation of water vapour will hardly occur during the heating step, which is not only beneficial from an emission point of view but also from an energy point of view. The heating in the present method is carried out in such a way that the formation and emission into the atmosphere of harmful substances belonging to the group of NO x , PAHs, CO 2 , odour and particulate matter is kept to a minimum.

[0017] According to an example, the present method for the heating of raw materials for processing into asphalt comprises two sub-steps, in particular a sub-step comprising pre-drying and sub-step comprising heating. The pre-drying step is carried out at a low temperature, for example a temperature in the range of 30-50°C. The aforementioned low temperature means that the moisture in the raw materials does evaporate, but due to the low temperature involved, there is hardly any release of polyaromatic hydrocarbon compounds (PAHs) and other harmful emissions. In the subsequent sub-step of heating, a temperature is applied in the range of 100-250 °C wherein the risk of PAHs are eliminated because there is no formation of steam or other increase in volume in air due to the application of an indirect heating technique without the supply of fresh air. Thus, hardly any flue gases are produced during the sub-step of heating.

[0018] In an example of the present method heating is carried out under application of indirect heating. Such a method of heating ensures that there is no emission of components that are present in the raw material streams to be heated, namely the flow of asphalt granulate and the flow of minerals. The process of heating according to the present method has been adapted in such a way that no gaseous material is released, namely no water vapour by drying and no NO x , PAHs, CO 2 and water vapour by combustion.

[0019] According to an example, pre-drying is carried out under the application of microwaves.

[0020] According to an example, heating is carried out by application of screw jacks which indirectly create heating by means of electrically heated thermal oil.

[0021] By supplying the incoming mass flows completely dry, no water vapour is created during heating. In heating processes according to the art, there is a moisture content of about 2-5 % by weight for the incoming mass flows, which amount of moisture must be evaporated to achieve dry matter. According to the present method it is possible to carry out a 'pre-drying' in a separate process step, wherein such a pre-drying is carried out at a lower temperature than the final heating so that no NO x , PAHs, CO 2 are released during the pre-drying process. By simultaneously no longer allowing heating to take place in the combustion unit by means of the combustion of fossil fuels, such as natural gas, but by using heat pumps and electric heaters, for example, no combustion gases will be released in that step and their emissions into the atmosphere will have decreased. If PAHs are formed when asphalt granulate are heated, such components will be captured via filters. This creates a heating process without releasing gaseous substances. So there is no need for a chimney anymore.

[0022] In an example of the present method the flow of asphalt granulate has a dry matter content of at least 99 % by weight and in particular preferably at least 99.9 % by weight, based on the weight of the asphalt granulate. The application of such high levels of dry matter makes it possible that when heating the stream of dry asphalt granulate to a temperature in the range of 100-250 °C, no energy is lost in the evaporation of the remaining moisture and that the resulting vapour flow is kept to a minimum.

[0023] In an example of the present method the stream of minerals has a dry matter content of at least 99% by weight and in particular preferably at least 99.9 % by weight, based on the weight of the minerals. Here too, the application of such high levels of dry matter has made it possible that when heating the stream of dry minerals to a temperature in the range of 100-250 °C, no energy is lost in the evaporation of the remaining moisture and that the resulting vapour flow is kept to a minimum.

[0024] In an example of the present method the heating of the stream of asphalt aggregate and the heating of the flow of minerals are carried out in separate heating units, for example in rotary drum furnaces. Such furnaces are robust and have proven themselves as efficient heating units in which solid raw materials must be heated.

[0025] In an example of the present method the gas stream coming out of the heating unit, in which the flow of asphalt granulate is heated, is passed through a dust filter. In this way, a possible emission of small particles into the atmosphere is prevented.

[0026] In an example of the present method the gas stream coming out of the dust filter is returned back to the heating unit where the flow of asphalt granulate is heated, in particular that part of the gas stream coming out of the dust filter is discharged into the atmosphere. It should be noted that the method is in fact done according to a closed system, but in practice a certain amount of air will be discharged into the atmosphere through the dust filter, and that amount will be supplemented by a certain amount of fresh air. However, the part that is discharged into the atmosphere as the gas stream coming out of the dust filter is rather small.

[0027] In an example of the present method the gas stream coming out of the dust filter is heated before being returned to the heating unit where the flow of asphalt granulate is heated. The application of such heating prevents excessive fluctuations in temperature in the heating unit.

[0028] In an example of the present method the gas stream to be discharged into the atmosphere is passed through a filter of activated carbon. Such an additional process step serves to capture the harmful components that are still present, in particular odour components.

[0029] In an example of the present method the gas stream coming out of the heating unit, in which the flow of minerals is heated, is passed through a dust filter in order to capture the small components that are still present.

[0030] In an example of the present method the gas stream coming out of the dust filter is returned back to the heating unit where the stream of minerals is heated, in particular that part of the gas stream coming out of the dust filter is discharged into the atmosphere. It should be noted that the method is in fact done according to a closed system, but in practice a certain amount of air will be discharged to the atmosphere through the, and that amount will be supplemented by a certain amount of fresh air. However, the proportion which is discharged into the atmosphere as the gas stream coming out of the filter of activated carbon is rather small.

[0031] In an example of the present method the gas stream coming out of the dust filter is heated before being returned to the heating unit where the stream of minerals is heated. The application of such heating prevents excessive fluctuations in temperature in the heating unit.

[0032] In an example of the present method the gas stream to be discharged into the atmosphere is passed through a filter of activated carbon. Such an additional process step serves to capture any harmful components that may still be present, in particular odour components and PAHs.

[0033] In an example of the present method both the gas stream obtained from the heating of the minerals and the gas stream obtained from the heating of the asphalt granulate are passed through a common dust filter.

[0034] In an example of the present method the gas stream from the common particulate filter is passed through an activated carbon filter in order to capture any harmful substances still present, in particular odour components and PAHs.

[0035] In an example of the present method the heating via indirect heating is carried out under the application of one or more of heat pumps, screw jacks, as discussed above, and electrically controlled heating units. Such heating is desirable from an environmental point of view and prevents the emission of harmful components.

[0036] In an example of the present method in order to obtain the stream of asphalt granulate having a dry matter content as described above, a wet stream of asphalt granulate is dried at a temperature that is lower than the temperature applied when the stream of asphalt granulate is subsequently heated.

[0037] In an example of the present method in order to obtain the stream of minerals having a dry matter content as described above, a wet stream of minerals is dried at a temperature lower than the temperature applied when the stream of minerals is subsequently heated.

[0038] The present invention thus relates to the processing of raw materials for asphalt, in particular the pre-drying of these raw materials so that they are virtually moisture-free before they are subjected to the heating phase, the heating of the dry raw materials by means of convection via heat pumps and possibly electric heaters to a final temperature between 100-250 °C, the recirculation of the hot air and, filtering particulate matter and capturing released PAHs, in the situation of processing recycled asphalt, re-heating via a heat pump and, if necessary, additional electric heating and finally further processing of the hot materials in an asphalt mixing plant to obtain asphalt. It is also possible that ventilation or make-up air, possibly released via the supply and exhaust system of the starting materials, is cleaned using active filter techniques before such air leaves the installation again.

[0039] The present invention will now be explained in more detail on the basis of a figure wherein it is noted that such a figure only serves as an explanation.

[0040] The enclosed figure shows schematically the method for heating raw materials for processing into asphalt according to the present invention.

[0041] The apparatus 1 shown in the figure for heating raw materials for processing into asphalt consists of two sub-steps 30 and 31, wherein sub-step 30 relates to a pre-drying step and sub-step 31 to a heating step. Sub-step 30 includes a step of pre-drying wherein pre-drying can be carried out at a low temperature, e.g. a temperature in the range of 30-50°C, for a period of time such that the target dry matter content is reached.

[0042] The pre-drying according to sub-step 30 includes, for example, drying process 21, where asphalt granulate 3 in a wet state is dried, for example with a moisture percentage of approximately 5 wt.%, with a supply of energy 2 to obtain a stream of dry asphalt granulate 6. For example, stream 6 of asphalt granulate has a dry matter content of at least 99 by weight, at least 99.9 by weight, based on the weight of the asphalt granulate. Pre-drying according to sub-step 30 also includes another drying process 21, where minerals 4 in a wet state are dried with the supply of energy 2 to obtain a stream of dry minerals 7. For example, the stream of minerals 6 has a dry matter content of at least 99 % by weight, preferably at least 99.9 % by weight, based on the weight of the minerals.

[0043] Drying process 21 creates a water vapour stream 5 that is discharged into the atmosphere. It is also possible for water vapour stream 5 to be condensed, wherein the condensed vapour can be reused. It is therefore desirable that dry raw materials are subjected to sub-step 31, i.e. a step of heating, namely a stream of dry asphalt granulate 6 and a stream of dry minerals 7.

[0044] The stream of dry asphalt granulate 6 is returned into a heating unit 22, for example a so-called black drum, and heated there to a temperature in the range of 100-250 °C. Heating unit 22 is supplied with energy 2 and air 8. In heating unit 22, a stream of heated asphalt granulate 9 is created which is returned to mixer 27. In heating unit 22, air flow 14 is supplied to dust filter 23. Dust filter 23 produces air flow 19 which is returned as air flow 16 to heating unit 22. It is therefore clear that there is an almost closed loop of air here, namely between heating unit 22 and dust filter 23. A part of air 17 coming out of dust filter 23 is discharged into the atmosphere via a filter 24 of activated carbon as flow 29.

[0045] The stream of dry minerals 7 is returned into a heating unit 25, e.g. a so-called white drum, and heated there to a temperature in the range of 100-250 °C. Heating unit 25 is supplied with energy 2 and air 8. In heating unit 25, a stream of heated minerals 10 is generated and returned to mixer 27. In heating unit 25, air flow 15 is supplied to dust filter 23. Dust filter 23 produces air flow 19 which is returned as air flow 12 to heating unit 25. It is therefore clear that there is an almost closed loop of air here, namely between heating unit 25 and dust filter 23. A part of air 17 coming out of dust filter 23 is discharged into the atmosphere via a filter 24 of activated carbon as flow 29. Air flow 8 is basically a marginal flow to compensate for the discharge of flow 29 into the atmosphere. Airflow 8 can therefore be regarded as a limited refreshment.

[0046] The figure shows a common dust filter 23, but in certain embodiments it is also possible to use separate dust filters (not shown). It is also possible to heat recirculating air 12 and recirculating air 16 before returning to heating unit 25 and heating unit 22 respectively.

[0047] Bitumen 11 is heated in heating unit 26 under application of energy 26 wherein a heated bitumen stream 12 is obtained which is returned to mixer 27. Bitumen stream 12 and bitumen 11 can be supplemented with one or more additives (not shown). Additives is the collective name for substances that are added to asphalt mixtures to improve the properties of bitumen and asphalt, for example adhesives, which improve the adhesion between bitumen and mineral aggregate, and drip inhibitors, which increase the viscosity of the bitumen. The effect of these substances is mainly to increase the lifespan. In mixer 27, the desired asphalt 13 is prepared from asphalt granulate 9, minerals 10 and bitumen stream 12.

[0048] According to such a process, the production of asphalt does not involve the emission of components present in the streams of materials to be heated, namely the flow of asphalt granulate and the flow of minerals. The process of heating according to the present method has been adapted in such a way that no gaseous material is released, namely no water vapour by drying and no NO x , PAHs, CO 2 and water vapour by combustion.

[0049] From the aforementioned process description, it is clear that heating is carried out at a low temperature in order to minimize emissions. In particular, drying is carried out at a low temperature, which will release moisture but virtually no harmful emissions. The subsequent step of high temperature heating does not increase the volume of the airflow, making it possible for air to be recirculated and no harmful emissions are released.

Claims

1. A method for heating raw materials for processing into asphalt, wherein the method comprises the following steps: - Providing a stream of dry asphalt granulate, - Providing a flow of dry minerals, - Heating the stream of dry asphalt granulate and the stream of dry minerals to a temperature in the range of 100-250 °C, - Mixing the heated asphalt granulate and the heated minerals with bitumen and any fillers to obtain asphalt.

2. A method according to claim 1, wherein heating is carried out under application of indirect heating.

3. A method according to one or more of claims 1-2, wherein the flow of asphalt granulate has a dry matter content of at least 99 % by weight, preferably at least 99.9% by weight, based on the weight of the asphalt granulate.

4. A method according to one or more of claims 1-3, wherein the flow of minerals has a dry matter content of at least 99 % by weight, preferably at least 99.9% by weight, based on the weight of the minerals.

5. A method according to one or more of claims 1-4, wherein the heating of the flow of asphalt granulate and the heating of the flow of minerals are carried out in separate heating units, for example in rotary drum furnaces.

6. A method according to claim 5, wherein the gas stream coming out of the heating unit into which the flow of asphalt granulate is heated is passed through a dust filter.

7. A method according to claim 6, wherein the gas flow coming out of the dust filter is returned to the heating unit in which the flow of asphalt granulate is heated, in particular that part of the gas flow from the dust filter is discharged into the atmosphere, in particular that the gas stream to be discharged into the atmosphere is passed through an activated carbon filter.

8. A method according to claim 7, wherein the gas stream coming out of the dust filter is heated before being returned to the heating unit where the flow of asphalt granulate is heated.

9. A method according to claim 5, wherein the gas stream coming out of the heating unit into which the flow of minerals is heated is passed through a dust filter.

10. A method according to claim 9, wherein the gas flow coming out of the dust filter is returned to the heating unit in which the flow of minerals is heated, in particular that a part of the gas flow from the dust filter is discharged into the atmosphere, in particular that the gas stream to be discharged into the atmosphere is passed through an filter of activated carbon.

11. A method according to claim 10, wherein the gas stream coming out of the dust filter is heated before being returned to the heating unit in which the flow of minerals is heated.

12. A method according to one or more of claims 1-11, wherein the gas stream obtained from the heating of the minerals and the gas stream obtained from the heating of the asphalt granulate is passed through a common dust filter, wherein the gas stream coming from the common dust filter is passed through a filter of activated carbon before its release to the atmosphere.

13. A method according to one or more of claims 1-12, wherein heating via indirect heating is carried out under the application of one or more of heat pumps, screw jacks and electrically controlled heating units.

14. A method according to one or more of claims 1-13, wherein for obtaining the stream of asphalt granulate having a dry matter content as defined in claim 3 a wet stream of asphalt granulate is dried at a temperature lower than the temperature applied when the stream of asphalt granulate is subsequently heated, wherein in particular the step of drying is carried out under the application of microwaves.

15. A method according to one or more of claims 1-14, wherein for obtaining the stream of minerals having a dry matter content as defined in claim 4 a wet stream of minerals is dried at a temperature lower than the temperature applied in the subsequent step of heating of the stream of minerals, wherein in particular the step of drying is carried out under the application of microwaves.

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