Reducing drying energy and costs by process alterations at aggregate stockpiles
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- Thijssen, G.J., Schott, D.L., Demmink, E.W. et al. Energy Efficiency (2011) 4: 223. doi:10.1007/s12053-010-9093-3
In the field of bulk solids, handling knowledge on moisture behaviour in aggregate stockpiles can be useful for process optimisation in terms of energy consumption. In the asphalt industry, an increase in moisture content leads to a significant increase in energy consumption. To determine the characteristics of moisture behaviour, correlations are investigated between theory on soil–water movements and moisture in aggregates. With column drainage experiments with porous bottom, similarities between theory and practice are found. This allows the use of theoretical hydrologic models to determine and predict the moisture behaviour in drained piles. The effect of process alterations within the system of piles on energy consumption was investigated, and a significant reduction of energy consumption was found.
KeywordsEnergy savings Loss prevention Aggregates Asphalt production Stockpiles Moisture behaviour
List of symbols
Weight density of water
Saturated hydraulic conductivity
Volumetric flow rate in the x-direction
Effective water content
Residual water content
Saturated water content
Elevation above arbitrary datum
This paper investigates the effects of process alterations at the aggregate stockpiles on the drying energy and drying costs. Process alterations have a positive effect on the energy for drying and as a result on drying costs, when the moisture content in the aggregates is lower. Knowledge on the moisture content is desirable in all aggregate processing industries like asphalt production, concrete production and coal fired power plants. For instance, 1% moisture content reduction at asphalt production can save up to €200,000 per year for a single plant, while other researches have shown that a coal fired power plant has to fire €87,000 per year extra coal to evaporate the moisture present in the material (Schott et al. 2008; Thijssen 2009).
Earlier research in this field was done by Eckersley for coal stockpiles (Eckersley 1994a, b, c). His research was initiated because of the hazards of flowslides caused by excessive moisture contents at the base of the stockpile. He gained knowledge on the threshold moisture content below which no significant redistribution of water occurred and has indicated that this threshold moisture content changes with height and residence time. However, the results of Eckersley remain at theoretical level and are not translated to practice or concrete recommendations.
This research is inspired on the method of Eckersley, but is extended to practice. Because the aim of the research is to reduce the drying energy and costs, not only knowledge on the moisture behaviour is necessary, but also on the effect of process alterations on moisture behaviour is important. Furthermore, it is based on aggregate drying at asphalt production plant where lowest attainable moisture content is of interest, while Eckersley aimed at the minimal moisture content below which no hazards of flowslides occur.
To determine the effect of drainage, theory on percolation of water in porous media is consulted which belongs to the field of hydrogeology. Hydrogeology is the study of water movements in soils. Soils are a porous media which means that they consist of granular materials with voids in between. These voids can be filled with water, air or both. Large part of the study is the moisture behaviour in the soil during and after a precipitation event.
There are other water movements possible in soils, but for this research all precipitation is assumed to infiltrate and vertically redistributed.
Flow in unsaturated zone
Studies of Eckersley show that the largest part of a coal stockpile behaves like an unsaturated zone. This behaviour has also proven to be true for mineral aggregate stockpiles at, e.g. asphalt production plants (Brooks and Corey 1964).
Here, qx is the volumetric flow rate in the x-direction per unit cross-sectional area of medium [ms−1], z is the elevation above an arbitrary datum [m], p is the water pressure [Nm−2], γw is the weight density of the water [Nm−3] and Kh is the hydraulic conductivity of the medium [ms−1].
Darcy’s law describes the flow at a representative elemental volume of the soil which includes pore spaces and soil particles. Flow occurs in response to the spatial gradients of mechanical potential energy, which has two components: the gradient of gravitational potential energy dz/dx and the gradient of pressure potential energy d(p/γw)/dx per unit weight of flowing water. This means that water always flows to a point where the combination of gravity and pressure is higher than at the current point. The rate of flow is amplified by the hydraulic conductivity Kh which is a measure of the resistance of a material to flow. A high hydraulic conductivity means less resistance to flow and therefore higher flow rates. The hydraulic conductivity is a material property which depends on the pore-size distribution and the water content in a material.
Here, ψ is the pressure head [L] which is defined as p/γw, θ is the volumetric water content of the porous medium which is the ratio of water volume to soil volume and γw is the weight density of the water. The latter is effectively constant for hydrologic problems which do not involve temperature or salinity gradients. The hydraulic conductivity and the pressure head depend on the volumetric water content. Both are crucial determinants of unsaturated flow in soils.
The hydraulic conductivity Kh also depends on the moisture content. Figure 3 shows that initially the hydraulic conductivity drops rapidly and then decreases more gradually with decreasing moisture content.
The Mualem–Van Genuchten model
Both the pressure head–water content and hydraulic conductivity–water content relations need to be known to apply Darcy’s law for flow calculations. The pressure head–water content relation can be determined by field measurements or laboratory experiments, but determination of the hydraulic conductivity–water content relation is more difficult (Dingman 2002). For this reason, often analytical approximations of these relations are used. Various models use analytic approximations ranging from relatively simple equations to extended models. In this paper, the model of Mualem–Van Genuchten will be used because it is best suited for practical applications where the results do not depend on minimal differences in the water flow at small time increments or at the boundaries of full saturation or residual water content (Van Genuchten 1980). Other models, among which the Brooks and Corey model is well known (Brooks and Corey 1964), show the same range of results.
By finding the correct values for the parameters in (3) to (5) the water content–pressure head and hydraulic conductivity–pressure head can be inserted into Darcy’s law to calculate the water flow properties.
The final goal of the method is to determine the influence of residence time on the threshold moisture content. The threshold moisture content is the average moisture content in a stockpile below which no drainage occurs in the associated residence time. This relationship will be depicted as drainage curves in the next section. Since drainage is caused by percolation, the Mualem–Van Genuchten model will be used to determine the percolation effect in the selected materials. The selected materials originate from the asphalt industry and are mineral sand and recycled asphalt. The experiments and simulations deal with a porous bottom or a uniaxial drainage condition.
Column drainage experiments
With these experiments, the drainage behaviour in the aggregates could be observed under more controlled circumstances. Two series of column drainage experiments were conducted. The first series consisted of one column of 159 cm high and was primarily designed for the comparison with literature. The second series consisted of two columns of 268 cm high. These columns were constructed for more resemblance with the situation in practice.
The columns were constructed of PVC cylinders of 12.5 cm outside diameter. At height intervals of 36 cm, holes were made in the column as sampling points for determination of the aggregate moisture content. The column of the first series was equipped with five sampling points and in the second series with eight. The bottoms of the columns were provided with a coarse filter and a glass container for the drained water to simulate the drainage behaviour in practice.
During an experiment, a column is filled with the concerned aggregate. The aggregate is prepared to the desired initial moisture content and remains in the column during a certain drainage period. Aggregate samples are taken from sample points when the drainage period is expired.
The curve parameters α and n are assumed equal to those of typical sand
With bulk density known, the average porosity in the columns is equal to the saturated moisture content
The field capacity can be derived from the smaller column and is converted to the residual moisture content with Hydrus 1D
The saturated hydraulic conductivity Ks is obtained by iteration to the retrieved moisture characteristics with Hydrus 1D
Previous research has indicated that the data from the column drainage experiments complies with the field measurements as well as with the theory on water movement in the unsaturated zone. This means that the data from the experiments can be used as a basis for modelling the moisture behaviour in the aggregates stockpiles (Thijssen 2009).
As described earlier, the model of Mualem–Van Genuchten will be used. The simulation is done with the software program Hydrus 1D which uses a finite element method for calculating the water flow characteristics (Hydrus 1D, version 4.12, 2005–2008). In this program, a vertical column of desired height is specified and constructed with one or more materials of choice. The vertical column is divided in a specified number of nodes. For each node, the spatial flow characteristics are calculated according to Darcy’s law. The vertical column can be accommodated with observation point for which the flow data is logged. This data can be studied after simulation.
With the parameters known, Hydrus 1D is used to extrapolate the moisture behaviour to other heights and residence times, based on a practical situation. With this extrapolation, the drainage curves are constructed to gain insight in the threshold moisture content at different heights.
Column drainage experiments were conducted for the same materials as in the field measurements. With these experiments, the drainage behaviour in the aggregates could be observed under more controlled circumstances. The gained data from the experiments is compared with the results from field experiments and with the literature for reliability of the simulation. With the results of the column drainage experiments, the moisture behaviour is simulated by Hydrus 1D which finally led to the construction of drainage curves. Drainage curves show how the average moisture content in stockpiles at different heights respond to the residence time.
Column drainage experiments
The experimental plan consists of experiments with relatively high or average moisture contents set out to different drainage periods ranging from relatively short to long periods.
Both materials in the first series drain within a measurable time interval to a minimum moisture content. Since the aggregate is not subjected to extreme suctions, this moisture content is the above discussed field capacity θfc
The drainage curve for river sand resembles a typical retention curve for sand
The drainage curve for recycled asphalt does not resemble a typical drainage curve for sand as close as river sand, but has a field capacity which is comparable to sand. As discussed in the results of the field observations, recycled asphalt tends to behave like sand but with an extreme high hydraulic conductivity caused by the smooth bitumen present around the particles
Soil hydraulic properties
Granulated recycled asphalt
The second step is taken by fitting the drainage curves obtained from the column drainage experiments to the moisture data generated by the simulation. This way, the only parameter left for iteration, the saturated hydraulic conductivity Ks, is determined.
Application of results to aggregate stockpiles
In this section, the application of the results to the aggregate stockpiles in order to reduce the drying energy costs is investigated. First, the current moisture behaviour at an aggregate stockpile is analysed followed by the impact of process alterations on this behaviour. As a case study, two stockpiles of mineral sand and recycled asphalt are selected for analysis.
Current moisture behaviour
Granulated recycled asphalt behaves opposite to river sand. This material enters the stockpiles with an initial moisture content of 3.1% which is lower than the threshold moisture content of 4.7%. Due to precipitation, the moisture content increases to 4.9% and drains towards 4.2%. This means that granulated recycled asphalt will benefit from measures which limit or prevent the effect of precipitation.
Altering process parameters: pile capacity and height
Precipitation prevention by roofing
Restrictions on initial moisture content
Increasing capacity and decreasing height are measures which result in lower threshold moisture contents because increasing capacity leads to longer residence times and thus more time for drainage and decreasing height results in a lower drainage curve. In analogy with the previous section, these measures would have a positive effect on materials entering the process with higher initial moisture content than the threshold moisture content. This is applicable to river sand.
Decreasing capacity and increasing height have opposite effects on the threshold moisture content, but have a positive effect on precipitation limiting, because both measures result in an area decrease. For this reason, materials which gain in moisture content due to precipitation would benefit from these measures. This is applicable to granulated recycled asphalt. Furthermore, limiting precipitation by roofing would also benefit the materials with higher threshold moisture contents than initial moisture contents, the most.
Effect on energy costs
Figure 10 also shows that setting a restriction on initial moisture content not only influences the final moisture content directly, but also indirectly. For instance, practice has shown that when a restriction of 3% moisture content is set on the supplied granulated recycled asphalt, the effect of increasing height and reducing capacity becomes amplified as indicated by the grey areas.
By conducting column drainage experiments similarities have been found between hydrologic models and the moisture behaviour in aggregate stockpiles. The moisture in the aggregate stockpiles behaves similar to the unsaturated zone in soils. The drainage curves of river sand are in accordance with part of the retention curve of typical sand and have a field capacity of 4.2%. The recycled asphalt behaves like sand at the field capacity of 4.6% and at high hydraulic conductivities.
The hydrologic model of Mualem–Van Genuchten can be used to determine and predict the behaviour. Usually, the soil parameters to use in the model are determined by extensive laboratory research. This is done by determining the moisture retention curves or creating and counselling soil property databases. However, for most aggregates, no soil property databases exist. For aggregate stockpiles, the data from column drainage experiments can be used to estimate these parameters. This method can be used because the required knowledge of aggregate stockpiles is less than the desired knowledge for standard hydrologic research.
With knowledge on this percolation behaviour, the effect of process alterations can be determined. It becomes clear that stockpiles must be divided into two categories: initial moisture content above the threshold value and initial moisture content below the threshold value.
When the initial moisture content is above the threshold value, the drying energy and costs can be reduced by reducing the stockpile height and increasing the capacity. When the initial moisture content is above the threshold value, a reduction can be realised by increasing the stockpile height and reducing the capacity depending on the practical situation and taking logistics into account.
Knowledge on the drainage curve of a selected material can be used to intelligently operate the stockyard. With knowledge of initial moisture contents and precipitation effects, the minimum required residence time can be determined for different materials. Furthermore, the moisture characteristics have indicated that materials can be significantly wetter at the bottom of the pile, thus for plant operation from reducing energy consumption, it would never be advisable to obtain material from the bottom part of the pile.
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