The German contractor Johann Bunte, a renowned asphalt road specialist with three asphalt plants, fixed a target to optimise production of hot mix asphalt, to reduce their carbon emissions and energy consumption. A sure way to meet this target, was to substantially increase the rates of Recycled Asphalt (RAP) in the mixes produced by the company. Bunte’s existing, 2005 built batch plant could only process 20 % RAP thru the mixer. Hence Bunte upgraded their dryer drum, two years ago with a MARINI recycling ring, which helped them increase their RAP processing capability up to 35 %.
With this newly gained confidence, proven success & verifiable numbers, the company then wanted to increase their RAP rates up to 50%. To process up to 50 %, it was necessary to install a powerful dedicated RAP processing dryer drum. Bunte also chose a Burner suitable for lignite dust and gasoil. With the latest generation of MARINI plant controls, installed, Bunte can now, just by a click control the entire process and get real time feedbacks.
The highlights of the project:
To meet Bunte’s requirement of high RAP processing and minimal carbon footprint. Marini specially designed a parallel flow dryer drum for RAP, with variable speeds and low exhaust temperature differential. The drum was rated at 180 Tph, with out let temperature around 140-160°C
The existing plant configuration allowed a RAP buffer hopper of 26 t, a separate weighing hopper 3 t, and a highly inclined chute into the batch plant’s mixer. The hoppers for the hot RAP are perfectly insulated and electrically heated. The chute into the mixture is highly inclined with special steel surfaces, heated and insulated to avoid any blockages.
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 underpressure in the raw gas duct ahead of the bag filter. A 200 Kw, exhaust fan with frequency converter controller, keeps this constant low-pressure in the main duct. This fan now had no further peaks at “switch on” and minimised electric consumption during operation. The reduced noise ís an additional benefit.
Energy Saving Features Added
At high recycling rates, the virgin dryer with only minimum aggregates flow is changing dramatically the heat transfer and temperature conditions. Only with speed controlled rotation the parameters of operation can be influenced. The newly developed MARINI burner control system 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. This offers additional savings in energy and improvements of the asphalt quality.
The concept developed with the German customer Bunte, did prove its performance and capacity during the first major productions with a daily production above 2.500 t and a recycling rate far above 60 %. Erection, commissioning and setting up took place without major interruptions on the ongoing production of hot mixes.