The new demands in the paving business, such as the production of sustainable asphalt mixes, more economical mixes and mixes with greater durability, have an effect on the chain of inputs, projects and equipment.
As a result, asphalt plants are evolving significantly with the goal of producing both traditional and more complex mixes that require special inputs such as lime, cellulose fibres and modified asphalt. Likewise, productivity, efficiency and an economical approach are being increasingly prioritised in this space.
Three new technologies in the market are bringing further benefits to the quality of the asphalt mix and/or the optimisation of the production process. These are the new systems developed for control over aggregate drying time, fuel and energy economy through a new combustion system, and technology that helps control the mixing time between the aggregates and asphalt binder. Marcelo Zubaran, Product and Application Engineer at asphalt plant and equipment manufacturer Ciber Equipamentos Rodoviários (Ciber), explains the concepts behind these technologies, and the benefits they bring to asphalt production.
Control over aggregate drying time
To produce a cohesive mix with suitable properties for hot mix asphalt production, the aggregates must be completely dry and heated to obtain adhesiveness with the asphalt binder.
Aggregates from the same region may also have quite distinct characteristics, such as different 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. Subsequently, the drying time is fixed, regardless of the aggregate’s characteristics.
“More porous aggregates should remain in the drying environment longer so that all the moisture can be removed and then heated, according to the task at hand,” explains Mr. Zubaran. “This happens during the creation of the mix in which the aggregates are dried and heated in a kiln until all the moisture is eliminated.”
A new concept for an asphalt plant drying system was developed in order to adjust the aggregate drying time according to the characteristics of those inputs, in regards to adhesiveness and moisture absorption.
Using a frequency inverter, the engine that rotates the drying drum can vary the speed of rotation. “With this technology, aggregates that are more porous spend more time receiving heat from the burner flame, resulting in complete drying,” says Mr. Zubaran. “Less porous aggregates go through the dryer faster, without altering the plant’s production.”
Intelligent combustion system in closed loop
Traditional asphalt plant burners 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 called the damper, placed in the gases’ outlet to the environment that controls the pressure of the plant by manipulating the gas flow. The optimum combustion ratio is 13:1 (13 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.
Mr. Zubaran says the latest combustion technology in plants accurately and automatically controls the optimum air to fuel ratio. Through this new technology, all of 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 in the dryer and provides an automatic variation of the airflow through variations in 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 better fuel economy,” says Mr. Zubaran. “Electric power output is also reduced, as the largest plant engine – in the exhauster – turns according to the demand.”
Control over mixing time between aggregates and bitumen
“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,” explains Mr. Zubaran. “If it’s a large-scale laboratory, the mixing time should also vary according to the properties of the materials.”
He says the latest improvement in this space is a recently developed external pug mill mixer that automatically controls the mixing time according to the requirements of the materials, as determined by the operator before or during production.
“The technology is based on optimising control over the material outlet area between the mixer and the drag elevator,” says Mr. Zubaran. “When this outlet area is reduced, the blend is trapped within the mixer, increasing the material volume and the mixing time, and keeping the plant’s production constant.”
If the outlet area is open, the asphalt mix leaves the mixer more freely, decreasing the volume of material in the mixer and reducing mixing time while still maintaining constant production.
Before the introduction of these technologies, Mr. Zubaran says asphalt plants had to adapt to material 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 for the task.”