Recycling at more than 60% thanks to double drum technology and use of economical burner working with diesel and lignite powder
The German contractor JOHANN BUNTE is a growing contractor with increasing activities in road buildings. The road and asphalt division with 11 asphalt plants in Germany has fixed the target to optimize production of hot asphalt and to reduce the consumption of resources and energy. Some kind of green approach for black asphalt. To meet this target, the rates of recycled asphalt pavement (RAP) in the mixture must be dramatically increased.
The production process needs some optimization and the electric consume of all major drives must be minimized by frequency inverters.
The existing batch mix plant from the early 2000’s was retrofitted in 2008 by an ATS/MARINI recycling ring on the existing dryer to increase recycling rates. The existing RAP cold feeding into the mixer was insufficient with only 20 %. By retrofitting the recycling ring, the feed rates could be increased up to 40 % and more.
For big production runs and highest RAP rates it was necessary to install a powerful parallel dryer with burner for lignite powder and gasoil. ATS received the contract to do this retrofit together with installation of inverters for both dryer drums and the bag filter exhauster. The complex control task could not be handled with the existing controls of the competition. So it was necessary to install the latest generation of ATS/MARINI plant controls. By mouse click all processes can be started, changed, and supervised and the batch log registers all parameters, consumes, and events.
The highlights of this project are as follows:
A parallel flow dryer drum for RAP with a nominal output of 180 t/h with variable rotation speed to achieve temperature of the RAP material at dryer discharge between 140 and 160° C.
The exhaust fumes temperature shall not be higher than 10 – 15 K above product temperature. To meet this target, the drum was sized with 2,5 m diameter and a length of 14 m. Drum and drives were designed by ATS and MARINI in close cooperation.
The height of the elevated mixer gave input for the elevation of the drum under consideration of a buffer hopper for hot RAP of 26 t, a separate weighing hopper 3 t to batch the RAP and a highly inclined chute into the mixer. The result was a drum height of 32 m and a very heavy steel structure.
The hoppers for the hot RAP are perfectly insulated and electrically heated. The chute into the mixture is highly inclined with special steel surfaces and is heated and insulated to avoid any adhesion of RAP.
The energy for drying the RAP is generated by double fuel burner for lignite powder / gasoil. The application of these burners in recycling parallel dryers is a new technology and needs special care when designing the combustion area.
Special lifters and forks were designed and installed. So the drum performed in reality much better than expected.
For parallel operation of two dryer drums (virgin aggregates + RAP) the regulation of low-pressure at the burner front wall is quite complicated and needs two servo controlled dampers for both fumes ducts and a constant under pressure in the raw gas duct ahead of the bag filter. Existing exhauster is speed controlled with a new inverter to keep this constant low-pressure in the main duct. The 132 kW exhaustor (operated with inverter) has no further peaks at “switch on” and minimized electric consume during all operational conditions. The reduced noise is an additional benefit.
At high recycling rates, the virgin dryer with only minimum aggregates flow is changing dramatically the heat transfer and temperature conditions. Only with variable drum speed rotation the parameters of drying process can be influenced.
The newly developed ATS/MARINI control module for the burner receives additional input by belt weighing scales indicating material flow into the drum. It calculates any change in material flow for changing of burner capacity in due time for a constant product temperature at outlet. This offers additional savings in energy and improvements of the asphalt quality.
Competitor’s controls, just 3 years old, were completely replaced by latest plant controller of ATS/MARINI to implement the complex controls procedures for ATS multifuel burner system and RAP dryer.
The concept developed with the customer BUNTE, did proof its performance and capacity during the first major productions with a daily production above 2.500 t and a recycling rate far above 60 %.
Assembly, erection and commissioning took place within 2 months without major interruptions of ongoing production of hot mixes.
A success story for the leading asphalt technology, developed in the multinational business area Asphalt Mixing Plants of Fayat Group.