CH703226B1 - Procedure for the requalification and energy enhancement of bituminous aggregates. - Google Patents

Abstract

Process and device for the requalification and energy enhancement of CBR waste bituminous aggregates by means of a heat transfer generated by elements (6) which are not in direct contact with said CBR waste bituminous aggregates, where said material is located in absence of oxygen.

Description

[0001] The present invention relates to the field of processes and plants used to transform, by means of a thermal type treatment, waste bituminous inert materials into reusable materials in the same field of applications.

[0002] In the context of the present statement, the following are considered, by way of non-limiting example, as waste bituminous aggregates: the granulated material obtained by milling and screening road surfaces, bituminous residues and other waste from the production of road asphalt, such as for example end-of-production inventories, lots of unused asphalt, lots of road asphalt that are not suitable to the product specifications and therefore not usable and any high material composed of mixtures of mineral aggregates (for example limestone or siliceous) and natural or petrochemical bitumen.

[0003] The present invention relates to a method for the upgrading and energy enhancement of bituminous aggregates as described in the preamble of claim 1, as a means for carrying out said process as described in claim 5.

[0004] The process object of the invention allows to transform the previously mentioned waste material into the following products: Bituminous mixture of inert and bitumen at a suitable temperature to be mixed with additives in order to generate a directly usable product with superior characteristics Bituminous mixture of inert and bitumen at the temperature suitable for its direct use, as a new road surface with intermediate characteristics Clean and decatramated inert that can be used as inert material for the preparation of new bituminous mixtures.

[0005] The use of waste bituminous conglomerates in the asphalt industry has already been implemented for some time. The purpose of reuse is linked to the possibility of transforming a waste material with a cost of disposal into a salable product.

[0006] Currently, the waste conglomerate is recycled by mixing it in well-determined maximum percentages with the virgin bitumen being produced. This recycling therefore takes place forcibly at an asphalt producer, which is sometimes found to be penalizing due to high distances and costs of transporting the waste conglomerate and dependence on the virgin asphalt producer who can impose his own commercial and technical conditions. These limitations mean that in practice a large part of the waste bituminous conglomerate is not recycled but placed in landfills with the inevitable cost and environmental burden that follows.

[0007] In the most common and widespread practice there are two techniques for using the bituminous conglomerate that can be ideally differentiated according to the relative quantity of recyclable waste.

[0008] Both techniques are implemented on the equipment normally used in the production of virgin asphalt, that is, starting from virgin materials such as: quarry aggregates and natural bitumen or from petrochemical refineries.

[0009] In this known type of plant, the virgin aggregate that will be part of the asphalt is heated in a rotating drum with direct flame. The combustion fumes of the burner, placed at the top of the rotating drum, heat the inert material by direct contact. The fumes that have passed through the drum are then extracted and sent to a bag filter which provides for their dedusting.

[0010] The inert material thus heated is then transferred to a suitably thermally insulated storage silo.

[0011] When the asphalt is requested, the hot inert material is mixed together with the bitumen and any other additives and then discharged into the means of transport that will take it to the place of use.

[0012] As part of the recycling of waste bituminous conglomerate, the first possibility offered is to add the waste to the virgin aggregate and heat the two together in the furnace.

[0013] This possibility is limited in the maximum amount of usable waste which usually cannot exceed 30% in the mixture with the virgin aggregate.

[0014] This limitation is a consequence of the fact that the heating is performed with direct flame and the fact that the virgin aggregate must be brought to temperatures of the order of 150 ° C. Higher quantities of bituminous conglomerate waste in the mixture with the aggregate consequently entails large quantities of bitumen which, under the effect of the temperature and the flame of the burner, partially burns or at least generates volatile organic compounds which, not being retained in the filter sleeves, would be released into the atmosphere with consequent air pollution.

[0015] If it is desired to increase the waste bituminous conglomerate fraction in the mixture with the virgin aggregate, the current technology uses two parallel drums. The two drums are of identical design: both are of the direct flame type.

[0016] While one of the drums is used solely for heating the virgin aggregate, the second is used for heating only the waste bituminous conglomerate. While the former can work at normal operating temperature, approx. 150 ° C, the second, for the reasons already set out above, is bound to a maximum temperature of 90 ° C.

[0017] The mixing between inert and bituminous conglomerate of waste heated separately then takes place at the level of mixing during the asphalt production phase.

[0018] The smoke point, the temperature at which a substance generates smoke, for bitumen, in an oxidizing atmosphere, is around 180–200 ° C.

[0019] With this technology it is theoretically possible to produce asphalt starting only from waste conglomerate without requiring virgin aggregate. In practice, this possibility is never used as the bitumen contained in the heated conglomerate has in any case undergone partial oxidation which alters its physical characteristics, due to partial contact with the fumes, and as the low exit temperature of the conglomerate forces the plant to the production of "LEA" (Low Energy Asphalt) (warm) type asphalt, thus requiring the addition of special additives of not insignificant cost.

[0020] The use of direct flame heating also has the disadvantage of being energetically very penalizing, requiring very high specific fuel consumption. The high consumption is above all linked to the fact that the combustion fumes leaving the drum are still very hot and therefore still contain a lot of energy that can only be partially recovered.

[0021] To overcome these problems, a technology for heating the conglomerate has appeared on the market that uses water vapor as a substitute energy vector for direct flame. This technical solution is described in PCT / FI91 / 00221, which expressly mentions the possibility of heating even the waste bituminous conglomerate alone.

[0022] The particularity of this type of technology is linked to the heating energy vector. In this case, a burner physically separated from the container containing the material to be heated is used. The combustion flame is cooled by injection of water which vaporizes, generating a mixture of burnt fumes and steam which is then used for actual heating of the material.

[0023] In summary, current technology has the following disadvantages: the water to be used for vaporization must necessarily be demineralized in order to avoid the formation of limescale in the ducts between the burner and the container for containing the material. Given the necessary water consumption, its demineralization is costly. in transient operating modes or during plant start-up, situations may arise in which part of the vaporized water condenses in the material to be heated, moistening it and thus making it no longer usable. the fumes-steam mixture sent when the material is heated still contains oxygen, due to the fact that the burner fumes always contain an excess of oxygen in order to guarantee total combustion of the fuel. This oxygen can interact with the bitumen contained in the material to be heated, changing its characteristics. the mixture of fumes mixed with steam, after having preheated the material, is released into the atmosphere without any treatment. This flow could, especially in the case of abnormal operating conditions, cause the release of pollutants and dust into the atmosphere. In cold or humid climates, the release into the atmosphere also causes visible plumes of vapor that could make this technology not very usable in inhabited areas. the use of heating with the fumes-steam mixture has, as in the case of direct flame heating, very high specific fuel consumption.

[0024] The object of the present invention is to present a process for the upgrading and energy enhancement of bituminous aggregates which obviates the aforementioned drawbacks, in this case causes a lower emission of vapors, has lower energy consumption and has a better thermal balance. The advantages will be specified in detail in the description of the invention.

[0025] The invention, that is the process for the requalification of bituminous aggregates, is now better clarified with reference to the attached drawing which illustrates in side view an embodiment of the process in question given only by way of example but not limitative as technical variants they can always be made without departing from the scope of protection of the present invention as defined in the claims.

[0026] In said drawing: Fig. 1 shows the scheme of the procedure for the requalification of bituminous aggregates.

[0027] The waste bituminous conglomerate to be treated is loaded into the hopper 1, equipped with a compartmentalization system that allows the environment to be separated from the internal atmosphere of the appliance.

[0028] They can be used as compartmentalization systems, for example, the double flap unloaders, the rotary cells or the pairs of guillotines. From the hopper a handling system 3 such as for example screw conveyor, belt or vibrating chute is responsible for continuously feeding the heating drum 2.

[0029] The heating drum is constantly kept in rotation and heated from the outside until, depending on the process to be implemented, temperatures of the drum skin between about 200 and about 550 ° C.

[0030] The material fed to the drum is deposited inside it and, due to the effect of its temperature in the internal part 4 of the combustion chamber 5, is heated to the desired temperature. The material in the drum occupies only the lower part of the volume: under the effect of rotation, the material is continuously turned over, always putting the new material in contact with the wall, thus increasing the effect of heat exchange and its mixing.

[0031] The final heating temperature of the material is adjusted by acting simultaneously on the fed flow rate and the rotation speed of the drum (control of the residence time) and on the skin temperature of the drum (control of the heat exchange power).

[0032] The drum is inserted in a combustion chamber of large dimensions and lined with refractory material 5. In the combustion chamber there are inserted or more burners 6 with liquid or gaseous fuel such as methane gas, LPG (Liquid Propane Gas), diesel , BTZ (Heavy Fuel Oil). The same burners, if the procedure implemented provides for the production of inert material, are also suitable for burning the gases and vapors obtained from the pyrolysis of the bitumen contained in the starting bituminous mixture. In order to increase the energy yield of the system, the combustion air of the burners can be preheated in a heat exchanger placed on the appliance exhaust fumes 9.

[0033] Pyrolysis is a thermochemical decomposition process obtained by applying heat and in the complete absence of an oxidizing agent, in this case oxygen.

[0034] While heating the material in the presence of oxygen, a combustion occurs which generates heat and produces oxidized gaseous compounds, while carrying out the same heating in conditions of total absence of oxygen, the solid material undergoes the splitting of the original chemical bonds with the formation of molecules simpler in gaseous form.

[0035] The heat exchange between the combustion fumes and the drum occurs mainly due to irradiation phenomena hence the need for a large and refractory chamber in order to increase the thickness of the radiating state and the re-radiation effect.

[0036] The combustion fumes can be used to dry or preheat the conglomerate or other direct uses, or they are passed through an exchanger 9 preferably used for preheating the combustion air of the burners 6 or otherwise for other thermal purposes, such as for example to produce process steam, water for heating or other.

[0037] Depending on the process temperatures, the heating of the conglomerate can lead to the formation of vapors and gases. At low heating temperatures, typically below 150 ° C, there is no formation so that the exhaust 8 is only composed of an exhaust duct into the atmosphere.

[0038] At higher temperatures, gases and vapors are formed, especially if said temperatures are in the pyrolysis range. In this case, the exhaust is coupled to a device for aspirating said vapors and gases, such as fans, blowers, venturi ejectors. The suction device is adjusted so as to keep the discharge hopper 7 and therefore the interior of the drum 4 always in depression at the desired value (a few mbar). Depression is necessary in order to prevent part of the gases and vapors from leaking outside the appliance through the rotating seals with the consequent risk of fire or damage to things or people. The gases and vapors thus aspirated, having a high calorific value, are then conveyed to the burners 6 installed in the combustion chamber 5, integrating or completely replacing the use of primary fuel (natural gas, diesel oil, or other).

[0039] The evaluation of the energy contribution of gases and vapors to the overall consumption of the appliance depends very much on the characteristics of the bituminous conglomerate at the entrance, being in particular determinable according to its bitumen content and its humidity.

[0040] The material which has reached the desired temperature is then extracted from the apparatus from the bottom of the hopper 7 by means of a handling system 10 such as for example screw conveyor, reddler or conveyor belt. Like the entrance, the exhaust is also equipped with a compartmentalization system of the same type.

[0041] If the device is used for the production of aggregates, and in consideration of the high temperatures to which the material must be brought, the exhaust system is also equipped with a cooling system 11 for the inert product. The type of cooler depends on the handling system used. In the exemplary case of the auger, this is jacketed with a duct where the cooling fluid (for example air or water) is made to pass. The recovered heat can be used in a similar way to the heat recovered from the combustion fumes of the appliance.

[0042] If the conglomerate is heated to pyrolysis temperatures, the inert product also contains the carbonaceous part remaining from the pyrolysis. As previously described, said carbonaceous part is limited in size with respect to the fraction of bitumen transformed into gas and steam. In practice, the coal is partly found as a layer that covers the aggregate and partly, under the rolling effect of the material in the drum, is found as dust particles mixed with the aggregate.

[0043] Normally the amount of carbon contained in the aggregate does not create problems but, if necessary, it can be separated and extracted downstream of the device object of the invention by screening.

[0044] The main advantages of the process object of the invention are now specified in relation to the currently known solutions.

[0045] The bituminous conglomerate is heated in a controlled and inert atmosphere. Any bitumen vapors that can form at high heating temperatures remain "pure" and in continuous contact with the material thus maintaining the ideal conditions for maintaining the characteristics of the bitumen.

[0046] The bituminous conglomerate is heated more slowly, thus avoiding the high thermal gradients that occur instead in the case of direct heating.

[0047] In fact, with direct heating there is a tendency to have high temperatures at the surface of the particles while at the core of the particles they are significantly lower. Since the bitumen is deposited on the surface of the particles, it results that, despite having a relatively low average temperature of the particles and below the smoke and flammability point of the bitumen, in reality the surface temperature is significantly higher.

[0048] The flow rate of any vapors produced during heating is considerably lower than the flow rate of vapors, fumes and any steam obtained in the case of direct flame or fluid heating. This makes any fumes treatment system simpler, less expensive and optimized only on the characteristics of the vapors to be treated.

[0049] On the heating fumes side, in the case of using natural gas or diesel burners, no type of treatment is required as these burners are designed to comply with the emission limits (low NOx emission systems, catalytic combustion, etc.) and at the same time guarantee high combustion efficiency.

[0050] The thermal balance of the heating is significantly better than with current technologies. In the case of heating systems with direct flame or with a mixture of fumes and steam, the combustion temperature must be kept below certain limits in order to avoid burning the bitumen. The heat exchange efficiency can be simplistically calculated as: (T = Temperatua)

[0051] For example, in the case of heating with a mixture of fumes-steam and assuming the temperature of the inlet fumes in the material of 350 ° C and the temperature of the fumes outlet of 150 ° C with an ambient temperature of 20 ° C the thermodynamic efficiency of heating becomes: η = (350-150) / (350-20) * 100 = 60.6%.

[0052] In the case of the present invention, since the heat exchange occurs indirectly, it is possible to use combustion fumes at 1200 ° C. The temperature of the fumes at the exhaust of the appliance object of the invention is normally around 250-350 ° C.

[0053] Recalculating the heat exchange efficiency as above, in the worst case scenario: η = (1200-350) / (1200-20) * 100 = 72%, therefore 12% higher than the previous case.

[0054] One possibility offered by the present invention relates to the production of aggregates using the waste bituminous conglomerate as starting material. This is possible using heating temperatures around 350–450 ° C. At these temperatures the bituminous fraction contained in the conglomerate is transformed by pyrolysis into gases - vapors and coal. The most consistent part of the pyrolysis products is given by the gas and vapor fraction which also contains the greatest part of the calorific value of the bitumen.

[0055] With indirect heating this gaseous flow does not come into contact with flames and oxygen, so it can be extracted from the apparatus object of the invention and used as a fuel gas for heating to replace or at least in addition to the primary fuel (gas natural or diesel fuel).

[0056] The exploitation of the energy content of the bitumen therefore makes it possible to significantly reduce the specific energy consumption of the system.

Claims (6)

1. Process for the requalification of bituminous aggregates in a reusable product (P) comprising a scrap bituminous conglomerate (CBR) as starting material for said process, said preceding comprises the loading of said conglomerate (CBR) inside a rotating drum (2) where it is heated, characterized by the fact that the scrap bituminous conglomerate (CBR) is loaded inside the rotating drum (2) in such a way as to be in an oxygen-free environment, inside the rotating drum (2) said conglomerate is brought to a temperature of at least 300 ° C by means of a heat transfer from one or more heating elements (6) which are located outside said rotating drum (2), so that they are not located at direct contact with the scrap bituminous conglomerate (CBR), said temperature of at least 300 ° C being adapted to generate a pyrolysis process of the bitumen contained in the scrap bituminous conglomerate (CBR). 2. The process according to claim 1, wherein the gases and vapors generated inside the rotating drum (2), from said process are recovered by suitable conveying means and used as fuel to generate the heat necessary for heating the conglomerate bituminous waste (CBR). 3. Process according to one of the preceding claims, in which the gases and vapors generated inside the rotating drum (2) by said process are made to pass through a heat exchanger (9). 4. Process according to one of the preceding claims, in which the gases and vapors generated inside the rotating drum (2) by said process are used for a pre-heating or drying of the bituminous waste conglomerate (CBR) before heating inside the rotating drum (2). 5. Device (D) for carrying out the process according to one of the preceding claims, comprising a feeding device which conveys the scrap bituminous conglomerate (CBR) inside a rotating drum (2) characterized in that the rotating drum (2) is housed in a thermally insulated and refractory chamber (5). 6. Device according to claim 5, characterized in that said device is provided with watertight extraction means (7, 10) of the treated conglomerate, suitable for separating the solid product from aeriform products (8).