Jose Manuel Valverde

Ca-based synthetic materials with enhanced CO2 capture efficiency

The Ca-looping process shows a considerable potential for reducing postcombustion CO2 emissions from power plants in the short-term as demonstrated by the recent success of a 1.7 MWt pilot plant. This process involves the carbonation reaction of CaO to capture CO2 and the subsequent calcination of limestone (CaCO3) to regenerate the sorbent. Yet the capture capacity of natural limestones decreases with the increasing number of calcination/carbonation cycles, which is mainly attributed to a decrease of the reactive surface area with the number of cycles as a result of material sintering during calcination. A number of techniques have been developed in the last few years to improve the durability of Ca-based sorbents and minimize their loss in adsorption capacity. The goal is to increase the active surface area and the stability of the pore structure of the sorbent, which would enhance their efficiency for CO2 capture. Material chemistry methods oriented to this objective are generally focused on the use of rigid porous materials as carriers of the Ca-based sorbents, use of additives to improve the sorbent thermal stability, reduction of the sorbent particle size down to the nanometer scale, and use of synthetic precursors to produce novel sorbents with a rich micropore structure. Besides enhancing the thermal stability in newly developed synthetic sorbents, an issue of concern is to promote their mechanical stability. Attrition of natural limestone particles is a main problem affecting the sorbent performance in the Ca-looping process. This paper is devoted to a critical review on the novel Ca-based sorbents developed in the last few years with improved thermal and mechanical stability.

Fluidization of nanopowders: a review

Nanoparticles (NPs) are applied in a wide range of processes, and their use continues to increase. Fluidization is one of the best techniques available to disperse and process NPs. NPs cannot be fluidized individually; they fluidize as very porous agglomerates. The objective of this article is to review the developments in nanopowder fluidization. Often, it is needed to apply an assistance method, such as vibration or microjets, to obtain proper fluidization. These methods can greatly improve the fluidization characteristics, strongly increase the bed expansion, and lead to a better mixing of the bed material. Several approaches have been applied to model the behavior of fluidized nanopowders. The average size of fluidized NP agglomerates can be estimated using a force balance or by a modified Richardson and Zaki equation. Some first attempts have been made to apply computational fluid dynamics. Fluidization can also be used to provide individual NPs with a thin coating of another material and to mix two different species of nanopowder. The application of nanopowder fluidization in practice is still limited, but a wide range of potential applications is foreseen.

Enhancement of Fast CO2 Capture by a Nano-SiO2/CaO Composite at Ca-Looping Conditions

In this paper we show the performance of a new CO(2) sorbent consisting of a dry physical mixture of a Ca-based sorbent and a SiO(2) nanostructured powder. Thermo-gravimetric analysis (TGA) performed at conditions close to the Ca-looping process demonstrate that the rate of CO(2) capture by the mixture is enhanced during the fast carbonation stage of practical interest in applications. Moreover, the residual capture capacity of the mixture is increased. SEM/EDX, physisorption, and XRD analyses indicate that there is a relevant interaction between the nanostructured SiO(2) skeleton and CaO at high temperatures, which serves to improve the efficiency of the transfer of CO(2) to small reactive pores as well as the stability of the sorbent pore structure.

The tensile strength of cohesive powders and its relationship to consolidation, free volume and cohesivity

The tensile strength of a powder is related to the interparticle force and to the free volume, which, in turn, are related to consolidation stress. The relationship between stress and free volume is described by the state diagram that has been measured at zero shear for a set of cohesive powders (xerographic toners) with a range of concentrations of a flow control additive. The toners are 12.7 μm diameter particles of styrene/ butadiene copolymer, and the surface additive is a submicron fumed silica that is used to control the interparticle forces. To overcome problems of sample non-uniformity, powder samples are initially fluidized and then allowed to settle under gravity. The tensile strenghts, σt, of these powders have been measured by means of a powder bed technique in which gas flow through the bed is increased until the bed fractures due to the tensile stress produced by the gas flow. The overpressure required to fracture the bed then provides a measure of σ1. The consolidation stress in the bed, σe, can be altered by varying the weight of the powder per unit area. Tensile strength is found to be linearly related to the consolidation stress in the limited range of stresses we have investigated, and the slope of this relationship is the same for all additive concentrations below 0.1%; above this concentration the slope decreases, consistent with a change from polymer-dominated to silica-dominated contacts between the particles. From the ratio σt/σe, we show that the contacts are fully plastic event at zero load, and that hardness of the contacts increases with increasing additive concentration.

An automated apparatus for measuring the tensile strength and compressibility of fine cohesive powders

This paper describes an apparatus based on a novel use of a powder bed, whereby the relationship between consolidation stress, tensile strength, and free volume of fine powder is measured. The powder to be tested is first initialized to a reproducible state. The initialized powder is next consolidated either beyond its own weight or below its own weight by means of a controlled flow of gas. An ultrasonic device measures the height of the bed, thus providing an average value of the powder free volume. Next the consolidated bed of powder is subjected to a slowly increasing gas flow, so directed as to put the powder under tension. The overpressure causing the powder to break provides a measure of the tensile strength of the powder, which in turn is a function of the consolidation and free volume. The relationship between consolidation stress, tensile strength, and free volume is related to powder flowability.

CO2 multicyclic capture of pretreated/doped CaO in the Ca-looping process. Theory and experiments

We study in this paper the conversion of CaO-based CO2 sorbents when subjected to repeated carbonation-calcination cycles with a focus on thermally pretreated/doped sorbents. Analytical equations are derived to describe the evolution of conversion with the cycle number from a unifying model based on the balance between surface area loss due to sintering in the looping-calcination stage and surface area regeneration as a consequence of solid-state diffusion during the looping-carbonation stage. Multicyclic CaO conversion is governed by the evolution of surface area loss/regeneration that strongly depends on the initial state of the pore skeleton. In the case of thermally pretreated sorbents, the initial pore skeleton is highly sintered and regeneration is relevant, whereas for nonpretreated sorbents the initial pore skeleton is soft and regeneration is negligible. Experimental results are obtained for sorbents subjected to a preheating controlled rate thermal analysis (CRTA) program. By applying this preheating program in a CO2 enriched atmosphere, CaO can be subjected to a rapid carbonation followed by a slow rate controlled decarbonation, which yields a highly sintered skeleton displaying a small conversion in the first cycle and self-reactivation in the next ones. Conversely, carbonation of the sorbent at a slow controlled rate enhances CO2 solid-state diffusion, which gives rise, after a quick decarbonation, to a highly porous skeleton. In this case, CaO conversion in the first cycle is very large but it decays abruptly in subsequent cycles. Data for CaO conversion retrieved from the literature and from further experimental measurements performed in our work are analyzed as influenced by a variety of experimental variables such as preheating temperature program, preheating exposition time, atmosphere composition, presence of additives, and carbonation-calcination conditions. Conversion data are well fitted by the proposed model equations, which are of help for a quantitative interpretation of the effect of experimental conditions on the multicyclic sorbent performance as a function of sintering/regeneration parameters inferred from the fittings and allow foreseeing the critical conditions to promote reactivation. The peculiar behavior of some pretreated sorbents, showing a maximum conversion in a small number of cycles, is explained in light of the model.

The tensile strength and free volume of cohesive powders compressed by gas flow

We report measurements of tensile strength and average free volume for a set of cohesive powders as a function of consolidation stress. Powders with average particle size between 7.8 and 19.2 μm were made from a styrene/butadiene copolymer, and were subsequently surface-treated with different concentrations of a submicron fumed silica. This silica acts as a flow control additive by controlling interparticle forces. The measurement technique consists of initialization of the sample by fluidization and subsequent consolidation by compression under a given gas flow while continuously monitoring the sample volume. By reversing the gas flow, a tensile stress is applied to the sample. For each consolidation state, we determine the tensile strength and the average free volume of the powder. We find that the relation between the free volume and the consolidation stress follows a logarithmic form. The magnitude of the interparticle forces is estimated from bulk measurements. At high consolidation stresses, the average tensile force per contact increases proportionally to the square root of the consolidation force per contact. Physical implications of these results are discussed.

On the breakup of slender liquid bridges: Experiments and a 1-D numerical analysis

Abstract Slender axisymmetric dielectric liquid bridges are made stable by the action of an axial electric field. In this paper, the subsequent dynamics of a slender liquid bridge after turning off the electric field is considered. The evolution in time of the bridge profile is investigated both theoretically and experimentally. A one-dimensional model is used to simulate the dynamic response of the system. Experiments are performed applying an axial electric field to a liquid bridge of 1 mm of diameter, and turning-off the electric field. The evolution of the liquid bridge is recorded using a video camera, and the digitized images are analysed. Good agreement between computations and experiments is found.

Dynamic Analysis of a Light Structure in Outer Space: Short Electrodynamic Tether

The SET (short electrodynamic tether) is an extremely flexible deployable structure. Unlike most other tethers that orbit with their axis of smallest moment of inertia pointing towards the Earths center (natural position), the SET must orbit with its axis of smallest inertia normal to the orbit plane. The Faraday effect allows the SET to modify its orbit in this position. This is due to the interaction of the Earths magnetic field with the tether, which is an electric conductor. In order to maintain the aforementioned operating position, the SET is subjected to a spin velocity around its axis of smallest inertia. If the system were rigid, the generated gyroscopic pairs would guarantee the systems stability.The tether is not perfectly straight after deployment. This fact could make the rotation of the structure unstable. The problem is similar to the instability of unbalanced rotors. The linear study of unbalanced systems predicts the structural instability once a certain critical velocity is exceeded. Instability is due to internal damping forces. The spin velocity of the SET is greater than the critical velocity. Nevertheless, certain works that include the geometric nonlinearities show a stable behavior under such conditions. The object of this paper is to try to verify these results for the SET.The SET consists of a 100-meter tether with a concentrated mass at its end. The system has been modeled using the floating reference frame approach with natural coordinates. The substructuring technique is used to include nonlinearities in the system.

Use of steel slag for CO2 capture under realistic calcium-looping conditions

In this study, CaO derived from steel slag pretreated with diluted acetic acid has been tested as a dry sorbent for CO2 capture under realistic Ca-Looping (CaL) conditions, which necessarily implies calcination under high CO2 partial pressure and fast transitions between carbonation and calcination stages. The multicycle capture performance of the sorbent has been investigated by varying the precalcination time, carbonation/calcination residence times and with the introduction of a recarbonation stage. Results show that the sorbent can be regenerated in very short residence times at 900 °C under high CO2 partial pressure, thus reducing the calciner temperature by about 30–50 °C when compared to limestone. Although the introduction of a recarbonation stage to reactivate the sorbent, as suggested in previous studies for limestone, results in a slightly enhanced capture capacity, the sorbent performance can be significantly improved if the main role of the solid-state diffusion-controlled carbonation is not dismissed. Thus, a notable enhancement of the capture capacity is achieved when the carbonation residence time is prolonged beyond just a few minutes, which suggests a critical effect of solids residence time in the carbonator on the CO2 capture efficiency of the CaL process when integrated into a power plant.