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Technology - New technologies in asphalt plants

By Marcelo Zubaran

As changing economic conditions and technologies combine to create new requirements for construction and road building, so, too, are the requirements for asphalt plants changing.

New demands in the paving business, such as the production of sustainable asphalt mixes, more economical mixes, and mixes with greater durability, influence the chain of inputs, projects, and equipment. Asphalt plants are thus evolving significantly, having the goal of producing both traditional and more complex mixes that require special input (lime, cellulose fibres, and modified asphalt) with maximum productivity, efficiency, and economy.

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Traditional burners applied to asphalt plants use the air from the environment (sucked in by an exhaust fan with constant turning speed) and a mechanical fan (called a blower) to provide the amount of air needed for combustion inside the dryer drum. 
Image credit: Ciber

Three areas that bring benefits to the quality of the asphalt mix and/or optimisation to the production process are: (a) control over aggregates’ drying time; (b) fuel and energy economy through a new combustion system; and (c) control of the mixing time between the aggregates and the asphalt binder.

AGGREGATES’ DRYING TIME

For the production of hot asphalt mixes, the aggregates must be completely dry and heated to obtain adhesiveness with the asphalt binder and to produce a cohesive mix with suitable properties according to the design. It is known that aggregates are natural materials from rock formations whose characteristics derive from particular phenomena of each region. Thus, aggregates of the same origin may present quite distinct characteristics, such as the water absorption capacity and adhesiveness with the asphalt binder.

Traditional plants dry and heat the aggregates in a rotating drum, driven by motors with fixed speed and, therefore, the drying time is fixed, regardless of the aggregates’ characteristics. However, more porous aggregates should remain in the drying environment longer so that all the moisture can be removed and then heated, according to the project. This happens during the creation of the project in which the aggregates are dried and heated in a kiln until all the moisture is eliminated.

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The latest combustion technology in plants accurately and automatically control the optimum air-to-fuel ratio by supplying all combustion air mechanically through two fans: a blower and an axial fan.
Image credit: Ciber

A new concept of an asphalt plant drying system was developed to adjust the aggregates’ drying time according to the characteristics of those inputs, mainly due to adhesiveness and absorbed moisture. To do this, the engines that make the drying drum rotate vary the speed of rotation, thanks to a frequency inverter. With this technology, aggregates that are more porous spend more time receiving heat from the flame of the burner for complete drying, while less porous aggregates go through the dryer faster, without altering the plant’s production.

INTELLIGENT COMBUSTION SYSTEM

Traditional burners applied to asphalt plants use the air from the environment (sucked in by an exhaust fan with constant turning speed) and a mechanical fan (called a blower) to provide the amount of air needed for combustion inside the dryer drum. The control over the airflow for combustion is carried out by a component that is called a damper, placed in the gases’ outlet to the environment, which controls the pressure of the plant by manipulating the flow of the gases. There is an optimum combustion ratio of 13:1 (thirteen volumes of air for one volume of fuel) and the airflow variation is dependent on the position of the damper, since the exhaust flow is constant.

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Aggregates and binder mixing inside the asphalt plant. During the design of an asphalt concrete project in the laboratory, the mixing time between aggregates and binder depends on the characteristics of the materials.
Image credit: Ciber

The latest combustion technology in plants accurately and automatically control the optimum air-to-fuel ratio. In this technology, all the combustion air is mechanically supplied by the plant (without air suction from the environment) through two fans (a blower and an axial fan). Called ‘total air’, it works in a closed loop from an internal pressure input of the dryer and automatic variation of the airflow by the variation of the rotational speed of the exhauster, which transports the gases from the dryer to the filter and later to the atmosphere. This way, only the air used for combustion is heated, resulting in fuel economy. Electric power is also saved, as the largest plant engine, the exhauster, turns according to the demand.

CONTROL OVER MIXING TIME

During the design of an asphalt concrete project in the laboratory, the mixing time between aggregates and binder depends on the characteristics of the materials. If the plant is a large-scale laboratory, the mixing time should also vary according to the properties of the materials. Therefore, an external pug mill mixer has been developed, which automatically controls the mixing time according to the requirements of the materials and as determined by the operator before or during production.

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An external pug mill mixer has been developed, which automatically controls the mixing time according to the requirements of the materials and as determined by the operator before or during production. 
Image credit: Ciber

The technology is based on the control over the material outlet area between the mixer and the drag elevator. When this outlet area is reduced, the blend is trapped within the mixer, increasing the material volume and the mixing time, keeping the production of the plant constant. If the outlet area is open, the asphalt mix leaves the mixer more freely, decreasing the volume of material in the mixer and the mixing time, while still maintaining constant production.

In the past, asphalt plants had to adapt to materials and project characteristics, with variable productivity. With these new technologies, productivity is constant, regardless of inputs and projects, and the quality is reached according to the parameters established in the project.


ABOUT THE AUTHOR

Marcelo Zubaran is a product and application engineer at Ciber Equipamentos Rodoviários, a Brazil-based member of the Wirtgen Group that specialises in developing and manufacturing equipment for building, maintaining, and repairing roads.


 

 

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