Feeding spent coffee ground powders with a non-mechanical L-valve: Experimental analysis and TFM simulation
Graphical abstract
Introduction
Soluble coffee and other food industries produce more than 2.5 million tons of Spent Coffee Grounds (SCGs) annually, which is a biomass residue generated from brewing coffee powders [1,2]. Although SCGs have been mainly disposed of in landfills, this practice is not considered a profitable or sustainable option from the resources management point of view [[2], [3], [4], [5], [6]]. The main use of SCGs is as a renewable source to generate thermal energy in the industry itself [7]. Other reported uses include converting SCGs into bio-oil [8,9], hydrogen and ethanol [2], and other value-added products [[10], [11], [12]].
In processing SCG powders for renewable energy generation, gasification or pyrolysis, minimizing the probability of operational hazards in the reactor by maintaining stable powder flow throughout the units that come before and after the reactor is of paramount importance. Thus, SCG powders must be fed continuously under a stable mass flowrate to silos, storage vessels, dryers, and so on; consequently, using appropriate feeding devices is key for effective operations. The powders' particle size distribution, shape and moisture content are not uniform for biomass residues, and they are likely to clog moving parts of mechanical feeders and discharge hoppers' orifices. Therefore, using non-mechanical feeding devices might be appealing for operation with biomass residues as the feeders have simple geometry and no moving parts, hence they suffer less wear and are less expensive compared to mechanical feeders.
L-valves are non-mechanical feeders that rely on hydrodynamic energy to control the solids flowrate. It consists of a vertical standpipe and a horizontal pipe, with an aeration injection located next to the standpipe bottom. The powder is introduced into the feeder through the vertical section, usually by the sole action of the gravitational force, and then it is directed towards the valve's horizontal section by an air flowrate (Q). As summarized in Table 1, different flow features are observed in the feeder depending on the magnitude of Q [[13], [14], [15], [16], [17], [18]]. The unit to be charged with solids is connected at the end of the horizontal section.
Encouraging results and wide operation range have been reported for L-valves feeding conventional powders, such as siderite, gravel, glass, and sand to the atmosphere [[13], [14], [15], [16], [17]]. However, flow behavior can vary significantly with materials properties. Thus, evaluating the performance of L-valves in feeding residue-based biomass powders and classifying the flow regimes shown in Table 1 are very important.
The flow behavior of gas-solid systems can be described by the two-fluid model (TFM) [[19], [20], [21], [22], [23], [24]] with the conservation equations of continuity and momentum. These equations have to be solved simultaneously with turbulence models and with closure equations (drag forces, frictional stress models, among others) related to gas and solid properties, as well as to the operating conditions. The closure equations usually have a certain degree of empiricism and calibration of the parameters is necessary for predicting the gas-solid flow in the feeding device.
Some authors examined feeding sand, dolomite and polyethylene beads to circulating fluidized beds (discharged with back-pressure) by L-valves using both TFMs [25,26] and CFD-DEM [27] approaches. Generally, the solids flowrate in the valve by TFMs has not been compared to experimental data [25,26], and when the predictions by CFD-DEM simulations are compared to experimental results large differences were observed, varying from 0.3 to 2.5 times of the experimental results [27]. Since neither the system presented in this paper (discharge to the atmosphere) nor the properties of the biomass SCGs powders are similar to those available in the literature, a TFM will also be developed and the effects of different model parameters are quantified, in addition to experimental measurements.
This paper is aimed at understanding the feeding of biomass SCG powders by L-valve. The behavior of the SCG powder is examined both experimentally and numerically. First, the transient and stable operations of the L-valve are assessed for a broad range of aeration flowrates. Then, a TFM with the properties of the solid phase described by the Kinetic Theory of Granular Flow (KTGF) is developed to describe the key flow features of the biomass powder in the feeder after the validation against the experimental data. Finally, the effects of different model parameters on the model prediction are quantified. The findings provide useful guidance for studying biomass materials with different materials properties and the operation of non-mechanical L-valve.
Section snippets
Material
The main physical properties of the spent coffee grounds (SCGs) used in the feeding experiments are presented in Table 2. Sample B100 was mechanically sieved to achieve a narrow particle-size distribution for the assays, between 500 and 300 μm. The experimental procedure for sample characterization is described in detail elsewhere [28,29]. The angle of internal friction (φ) was determined by performing shear tests under a normal consolidation of 3 kPa in a powder rheometer FT4 (Freeman
Dynamic variation of solids flowrate and pressure
To assess the dynamic flow behavior during a run, the dynamic variation of the pressures and solids flowrate (Ws) for a typical experiment is discussed below. Fig. 2 shows that different dynamic patterns can be distinguished. At t = 0 s, as soon as the aeration is initiated, the pressures and Ws exhibit transient behavior, which is related to the collapse of the packed-bed in the standpipe and formation of a channel of flowing particles in the horizontal section of the valve. Between 50 and
Conclusions
A promising non-mechanical L-valve for feeding a biomass SCG powder is examined both experimentally and numerically. Key information for the operation and design of non-mechanical L-valves with biomass SCG powder is described. The findings are useful for producing renewable thermal energy and fuels with SCGs and provide useful guidance for studying biomass materials with different materials properties, as summarized below.
L-valve's operation is rather stable with the SCG sample, with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the São Paulo Research Foundation (2016/25946-2 and 2018/11031-8), the Australian Research Council (IH140100035, DE180100266) and CAPES (Finance code 001) for financial support. We thank Dr. Rodrigo Condotta of the Faculty of Industrial Engineering/Brazil for measuring the angle of internal friction for the biomass sample.
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2020, Waste ManagementCitation Excerpt :The turbulence model used was the standard k-ɛ dispersed model with a standard treatment near the wall. The same set of equations has been recently used to simulate the feeding of SCGs to the atmosphere with an L-valve type feeder (Massaro Sousa et al., 2020). In terms of model inputs, the SCG properties considered are as follows: mean diameter (ds) of 0.0004 m, particle density (ρs) of 1,120 kg/m3, packing density (αs,max) of 0.34, and angle of internal friction (ø) of 38.5°.