Magnetic field induced inversion in the effect of particle size on powder cohesiveness

Experimental measurements are reported on the tensile yield stress of magnetofluidized beds of fine magnetic powders operated in the cross-flow configuration. In the absence of externally applied magnetic field the yield stress of the powder depends on particle size as expected, i.e., it increases as bead size is decreased. This trend is however inverted when an external magnetic field is applied. It is suggested that the average orientation of interparticle contacts relative to the direction of the field as affected by particle size plays a relevant role on the magnetic yield stress of these systems.Ministerio de Ciencia y Tecnología de España-FIS2006-0364

Limestone calcination under calcium-looping conditions for CO2 capture and thermochemical energy storage in the presence of H2O: An in situ XRD analysis

This work reports an in situ XRD analysis of whether the calcination/carbonation behavior of natural limestone (CaCO3) is affected by the addition of H2O to the calciner at a very low concentration under relevant Calcium-Looping (CaL) conditions for CO2 capture in coal fired power plants (CFPP) and Thermochemical Energy Storage (TCES) in Concentrated Solar Power plants (CSP). Previous studies have demonstrated that the presence of steam in the calciner at a high concentration yields a significant increase in the reaction rate. However, a further undesired consequence is the serious deterioration of the CaO mechanical strength, which would lead to particle attrition and mass loss in any CaL process based on the use of circulating fluidized beds. The results presented in this manuscript on the time evolution of the wt% and crystallite size of the phases involved in the calcination/carbonation reactions indicate that the calcination rate is still notably increased by the presence of H2O at very low concentrations whereas the reactivity toward carbonation and crystal structure of the formed CaO are not essentially affected, which suggests that the CaO mechanical strength is not impaired. Thus, the benefit of using steam for calcination in the CaL process could be still retained while at the same time particle attrition would not be promoted.Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-

High viscosity gas fluidization of fine particles: An extended window of quasihomogeneous flow

We explore the role of gas viscosity in the behavior of gas-fluidized beds of fine powders by means of experimental measurements using nitrogen and neon as fluidizing gases, and theoretical considerations. The existence of a nonbubbling fluidlike regime has been recently observed in beds of fine powders fluidized with nitrogen. Our experiments with neon reveal a discontinuous transition from heterogeneous fluidization to a highly expanded homogeneous fluidization state. We point out that increasing gas viscosity enhances the coherence of agglomerate swarms, which promotes a local void-splitting mechanism, thus improving the uniformity of fluidization. Our theoretical analysis predicts that further increase of gas viscosity would produce a full suppression of the bubbling regime, i.e., the uniformly fluidized bed would undergo a direct transition to a turbulent regime as seen in beds of nanoparticles fluidized by nitrogen and in liquid-fluidized beds of moderate-density beads

Types of gas fluidization of cohesive granular materials

Some years ago it was shown that gas-fluidized powders may transit from solidlike to fluidlike fluidization prior to bubbling, shedding light on a long-standing controversy on the nature of “homogeneous” fluidization. In this paper it is shown that some gas-fluidized powders may also transit from the fluidlike regime to elutriation, with full suppression of the bubbling regime. We provide a diagram that can be used to predict these types of fluidization exhibited by cohesive powders based on simple phenomenological equations in which particle aggregation due to attractive forces is a key ingredient

Effect of compaction history on the fluidization behavior of fine cohesive powders

Fine particles agglomerate in the fluidized state due to the strength of interparticle attractive forces as compared to particle weight. Interparticle adhesion can be largely increased by consolidation stresses applied during powder handling. As a consequence, fragments of the consolidated powder may persist when the powder is fluidized, which gives rise to large agglomerates of strongly adhered particles in fluidization. This history-dependent effect can be minimized by coating the particles with surface additives such as silica nanoparticles. In this paper, we investigate the effect of high consolidation stresses _c previously applied to samples of silica-coated fine particles on their fluidization behavior. Our experimental measurements show that, even though homogeneous fluidization is still observed, the average agglomerate size and fractal dimension of the agglomerates increase as _c is increased. Bed expansion in the fluidized state is hindered by previously applied high consolidations, which we attribute to an increase of the largest stable size of mesoscopic fluid pockets. As a consequence, we observe that the initiation of macroscopic bubbling is delayed up to larger values of the fluid velocity

Ca-looping for postcombustion CO2 capture: A comparative analysis on the performances of dolomite and limestone

The low cost and wide availability of natural limestone (CaCO3) is at the basis of the industrial competitiveness of the Ca-looping (CaL) technology for postcombustion CO2 capture as already demonstrated by ~1Mwt scale pilot projects. A major focus of studies oriented towards further improving the efficiency of the CaL technology is how to prevent the gradual loss of capture capacity of limestone derived CaO as the number of carbonation/calcination cycles is increased. Natural dolomite (MgCa(CO3)2) has been proposed as an alternative sorbent precursor to limestone. Yet, carbonation of MgO is not thermodynamically favorable at CaL conditions, which may hinder the capture performance of dolomite. In the work described in this paper we carried out a thermogravimetric analysis on the multicyclic capture performance of natural dolomite under realistic regeneration conditions necessarily implying high calcination temperature, high CO2 concentration and fast transitions between the carbonation and calcination stages. Our study demonstrates that the sorbent derived from dolomite has a greater capture capacity as compared to limestone. SEM analysis shows that MgO grains in the decomposed dolomite are resistant to sintering under severe calcination conditions and segregate from CaO acting as a thermally stable support which mitigates the multicyclic loss of CaO conversion. Moreover, full decomposition of dolomite is achieved at significantly lower calcination temperatures as compared to limestone, which would help improving further the industrial competitiveness of the technology.Junta de Andalucía FQM-5735, TEP-7858, TEP-1900Ministerio de Economía y Competitividad FIS2011-25161, CTQ2011- 2762

Vibration-induced dynamical weakening of pyroclastic flows: Insights from rotating drum experiments

Pyroclastic flows are characterized by their high mobility, which is often attributed to gas fluidization of the usually fine and/or low-density particles. However, the physical mechanism that might drive sustained fluidization of pyroclastic flows over extraordinarily long runout distances is elusive. In this letter it is proposed that a powerful mechanism to weaken the frictional resistance of pyroclastic flows would arise from the prolonged and intense mechanical vibrations that commonly accompany these dense gravitational fluid-particle flows. The behavior of fine powders in a slowly rotating drum subjected to vibrations suggests that fluid-particle relative oscillations in granular beds can effectively promote the pore gas pressure at reduced shear rates. Dynamical weakening, as caused by the enhancement of pore fluid pressure, may be an important mechanism in any geophysical process that involves vibrations of granular beds in a viscous fluid. This is particularly relevant for granular flows involving large amounts of fine and/or light particles such as pyroclastic density currents.Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-RJunta de Andalucía FQM-573

Enhancement of CO2 capture in limestone and dolomite granular beds by high intensity sound waves

The calcium looping (CaL) process, based on the calcination/carbonation of CaCO3 at high temperatures, has emerged in the last years as a potentially low cost technology for CO2 capture. In this work, we show that the application of high intensity sound waves to granular beds of limestone and dolomite in a CaL reactor enhances significantly their multicycle CO2 capture capacity. Sound waves are applied either during the calcination stage of each CaL cycle or in the carbonation stage. The effect of sound is to intensify the transfer of heat, mass and momentum and is more marked when sound is applied during calcination by promoting CaO regeneration. The application of sound would allow reducing the calcination temperature thereby mitigating the decay of capture capacity with the number of cycles and reducing the energy penalty of the technology.Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-

The Ammonia Looping System for Mid-Temperature Thermochemical Energy Storage

Thermochemical reactions have a great potential for energy storage and transport. Their application to solar energy is of utmost interest because the possibility of reaching high energy densities and seasonal storage capacity. In this work, thermochemical energy storage of Concentrated Solar Power (CSP) based on an ammonia looping (AL) system is analysed. The AL process for energy storage is based on the reversible reaction of ammonia to produce hydrogen and nitrogen. Concentrating solar energy is used to carry out the decomposition endothermic reaction at temperatures around 650 ºC, which fits in the range of currently commercial CSP plants with tower technology. The stored energy is released through the reverse exothermic reaction. Our work is focused on energy integration in the system modelled by pinch analysis to optimize the process performance and competitiveness. As result a novel configuration is derived which is able to recover high-temperature heat for electricity production with a thermal-to-electric efficiency up to 27 %. The current study shows a clear interest of the system from an energy integration perspective. Further research should be conducted to access the potential for commercial applications

Magnetic stabilization of fluidized beds: Effect of magnetic field orientation

Fluidized beds of granular materials can be stabilized by interparticle attractive forces which confer the expanded bed an elastic modulus that stabilizes it against flow perturbations. Stabilization in a structure of enduring contacts is seen to occur naturally due to the universal van der Waals forces for ∼50 μm particle size beds albeit in a quite reduced interval of gas velocities over the minimum fluidization velocity vmf. As shown in this work, a magnetic field may induce attractive forces between magnetizable particles thus extending the stable fluidization interval well beyond vmf. The structure of the magnetically stabilized bed is however markedly anisotropic since attractive magnetic forces are maximum along the direction of the externally imposed field which leads to the formation of chain particle aggregates. This paper shows experimental measurements on the magnetic yield stress, gas velocity at the transition to marginal stability and microstructure of magnetostabilized beds as affected by the direction of the magnetic field. Data shows that magnetic stabilization is optimized for co-flow fields as chain aggregates are preferentially orientated parallel to the magnetic field and along the direction of minimum drag. As the magnetic field is tilted, particle chains become tilted according to a balance between the magnetic attractive force between the particles and the vertical drag force, which reduces the magnetic yield stress and therefore shortens the interval of magnetic stabilization.Junta de Andalucía FQM-5735Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-R, FIS2014-54539-