G.J.Thijssen, October 26, 2009 2009.TEL.7384
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Summary
A significant part of the energy consumption at asphalt production plants results from evaporating the moisture that is present in the aggregates (sand, stones and granulated recycled asphalt). This drying process is generally regarded as an energy loss and a topic of present interest for reducing energy costs. This research investigates which measures should be attempted to reduce the moisture content in the aggregates handling before the dryer that cause a reduction in drying costs. Measures are investigated that do not consume additional energy. The research is initiated by Royal BAM Group which is a construction company in the Netherlands that among other things exploits several asphalt production plants.
This research starts with analysing the aggregate flow by using the Delft Systems Approach to find out which processes can be altered to lower the moisture content. The approach yields that the moisture content in the aggregates originates from the wetting of the material during aggregate production and by precipitation. Mineral sands and granulated recycled asphalt are likely to have the largest contribution to the overall moisture content in the aggregates. The terrain of the asphalt production plant is selected as system in which the measures must be realised, because there Royal BAM has the largest influence. From the equipment present at the terrain, the conveyor belts, storage areas and storage bins are selected for the research, because they are the most accessible for alterations.
A literature research on soil water movement is conducted to investigate which moisture behaviour can be encountered in the aggregates. A small pile experiment showed that the moisture behaviour in the aggregates has analogies with soil water movement in the unsaturated zone and that the final moisture content of an aggregate handling process is influences by the initial moisture content of the incoming aggregates in a process, precipitation, natural evaporation and drainage due to percolation. The effects of precipitation and natural evaporation can be determined by knowledge on process parameters and meteorological data. However, the effect of drainage not only depends on the process parameters, but also on so far unknown material properties of the aggregates with which the moisture behaviour can be approximated by the Mualem-Van Genuchten model.
These unknown material properties are determined by conducting column drainage experiments. First, field measurements were conducted to provide moisture data from practice that consist of two series of moisture sampling at different heights in aggregate stockpiles at an asphalt production plant. The column drainage experiments consist of two series of columns with different height that are filled with aggregate prepared at a desired initial moisture content. After a certain drainage period moisture samples are taken from the
G.J.Thijssen, October 26, 2009 2009.TEL.7384
ii columns at different heights. The results from the field measurements and column drainage experiments indicate that the characteristics of soil water movement are comparable with the moisture behaviour in aggregate stockpiles. By applying the results of the column drainage experiments in the Mualem-Van Genuchten model, the moisture behaviour at the aggregate stockpiles can be determined with software simulation program Hydrus 1D. This program uses the model in combination with a finite element method and calculates the moisture redistribution in time.
The gained knowledge on precipitation, evaporation and drainage due to percolation is combined with the process parameters and data on initial moisture contents in the aggregate supply. Both materials showed a threshold moisture content that varied in time below which no drainage occurred. At average production mineral sand showed a threshold value of 5.1% which is above the field capacity of 4.2%. Recycled asphalt showed a threshold value of 4.6% which is equal to the field capacity which means that although recycled asphalt is capable of retaining higher moisture content, it drains faster resulting in most cases for a lower threshold value than mineral sand. By studying this moisture behaviour it becomes clear that precipitation is responsible for 6% of the drying costs for mineral sand and 28% for recycled asphalt. The effect of natural evaporation is found negligible due to evaporation depth limitations.
The effect of moisture reducing measures is investigated by altering the process parameters height and capacity combined with initial moisture alteration. This investigation showed that alterations of height and capacity lead to a reduction in drying costs depending on whether the initial moisture content is above or below the threshold value. The effects of these alterations are amplified when the moisture content in the aggregate supply is restricted. The full effect of precipitation is prevented by placing roofs over the aggregate stockpiles. However, the current cost price of roofing exceeds the reduction in drying costs. In the current situation roofing becomes profitable below € 38/m2 for recycled asphalt. When roofing is combined with height alterations, capacity changes or initial moisture content restrictions it becomes profitable from € 50/m2.
Furthermore, the research has provided insight in the optimisation of de handling of the aggregates during production like influence of material takeaway and interactive use of the knowledge on drainage behaviour to always achieve the lowest possible moisture contents by passive measures. It is recommended to conduct a case study to the implementation of these measures at an asphalt production plant to quantify the effect of these measures. Moreover, it is recommended to determine the accompanying costs and logistics implications of capacity alterations through a feasibility study to these measures.
