Investigation into a gas–solid–solid three-phase fluidized-bed carbonator to capture CO2 from combustion flue gas

A two-dimensional (2D) transient model was developed to simulate the local hydrodynamics of a gas (flue gas)–solid (CaO)–solid (CaCO₃) three-phase fluidized-bed carbonator using the computational fluid dynamic method, where the chemical reaction model was adopted to determine the molar fraction of CO₂ at the exit of carbonator and the partial pressure of CO₂ in the carbonator. This investigation was intended to improve an understanding of the chemical reaction effects of CaO with CO₂ on the CO₂ capture efficiency of combustion flue gases. For this purpose, we had utilized Fluent 6.2 to predict the CO₂ capture efficiency for different operation conditions. The adopted model concerning the reaction rate of CaO with CO₂ is joined into the CFD software. Model simulation results, such as the local time-averaged CO₂ molar fraction and conversion of CaO, were validated by experimental measurements under varied operating conditions, e.g., the fraction of active CaO, chemical reaction temperature, particle size, and cycle number at different locations in a gas–solid–solid three-phase fluidized bed carbonator. Furthermore, the local transient hydrodynamic characteristics, such as gas molar fraction and partial pressure were predicted reasonably by the chemical reaction model adopted for the dynamic behaviors of the gas–solid–solid three-phase fluidized bed carbonator. On the basis of this analysis, capture CO₂ strategies to reduce CO₂ molar fraction in exit of carbonator reactor can be developed in the future. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO₂ capture from combustion flue gases.     

Sulfation Rates of Particles in Calcium Looping Reactors

The sulfation reaction rate of CaO particles in three reactors comprising a post‐combustion calcium looping system is discussed: a combustion chamber generating flue gases, a carbonator reactor to capture CO₂ and SO₂, and an oxy‐fired calciner to regenerate the CO₂ sorbent. Due to its strong impact on the pore size distribution of CaO particles, the number of carbonation/calcination cycles arises as a new important variable to understand sulfation phenomena. Sulfation patterns change as a result of particle cycling, becoming more homogeneous with higher number of cycles. Experimental results from thermogravimetric tests demonstrate that high sulfation rates can be measured under all conditions tested, indicating that the calcium looping systems will be extremely efficient in SO₂ capture.     

Modelling of a fluidized bed carbonator reactor to capture CO2 from a combustion flue gas

In recent years several processes incorporating a carbonation-calcination loop in an interconnected fluidized bed reactor have been proposed as a way to capture CO₂ from flue gases. This paper is a first approximation to the modelling of a fluidized bed carbonator reactor. In this reactor the flue gas comes into contact with an active bed composed of particles with very different activities, depending on their residence time in the bed and in the carbonation-calcination loop. The model combines the residence time distribution functions with existing knowledge about sorbent deactivation rates and sorbent reactivity. The fluid dynamics of the solids (CSTR) and gases (PF) in the carbonator are based on simple assumptions. The carbonation rates are modelled defining a characteristic time for the transition between a fast reaction regime to a regime with a zero reaction rate. On the basis of these assumptions the model is able to predict the CO₂ capture efficiency for the flue gas depending on the operating and design conditions. Operating windows with high capture efficiencies are discussed, as well as those conditions where only modest capture efficiencies are possible.     

Blackened calcium-based composite particles and their apparent kinetics features for solar thermochemical energy storage

The calcium looping (CaL) thermochemical thermal energy storage is one of the best high-temperature heat storage schemes for 3th concentrating solar power (CSP) photothermal power. However, the application of this technology is greatly hindered by the low solar absorption capability and the poor cyclic stability of CaCO₃/CaO-based material. In this article, the solar absorbing capability of CaCO₃ particles is enhanced by doping Mn-Fe oxides, meanwhile, awns of setaria faberis (ASF) and microcrystalline cellulose (MCC) are used as bio-templates to generate pores inside the particles. The pore-making process promotes the cyclic stability and carbonation kinetic features of the composite particles simultaneously. The test results show that the proposed particles possess adequate anti-crushing strength, high cyclic stability, high solar absorption, and high carbonation rate. In addition, the apparent carbonation kinetic features of the composite porous particles are studied with the influencing factors such as CO₂ partial pressure, reaction temperature, and particle morphology taken into consideration. A kinetic equation involving these parameters is developed with the thermogravimetry data of the prepared samples. By employing this kinetic function, the carbonation reaction of the prepared particles inside the carbonator becomes predictable, which is of great significance for the design and regulation of the carbonator achieving highly stable thermal output from the CaL thermochemical heat storage system.     

Experimental investigation of a circulating fluidized‐bed reactor to capture CO2 with CaO

Calcium looping processes for capturing CO₂ from large emissions sources are based on the use of CaO particles as sorbent in circulating fluidized‐bed (CFB) reactors. A continuous flow of CaO from an oxyfired calciner is fed into the carbonator and a certain inventory of active CaO is expected to capture the CO₂ in the flue gas. The circulation rate and the inventory of CaO determine the CO₂ capture efficiency. Other parameters such as the average carrying capacity of the CaO circulating particles, the temperature, and the gas velocity must be taken into account. To investigate the effect of these variables on CO₂ capture efficiency, we used a 6.5 m height CFB carbonator connected to a twin CFB calciner. Many stationary operating states were achieved using different operating conditions. The trends of CO₂ capture efficiency measured are compared with those from a simple reactor model. This information may contribute to the future scaling up of the technology. © 2010 American Institute of Chemical Engineers AIChE J, 57: 000–000, 2011     

Sulfation rates of cycled CaO particles in the carbonator of a Ca‐looping cycle for postcombustion CO2 capture

Calcium looping is an energy‐efficient CO₂ capture technology that uses CaO as a regenerable sorbent. One of the advantages of Ca‐looping compared with other postcombustion technologies is the possibility of operating with flue gases that have a high SO₂ content. However, experimental information on sulfation reaction rates of cycled particles in the conditions typical of a carbonator reactor is scarce. This work aims to define a semiempirical sulfation reaction model at particle level suitable for such reaction conditions. The pore blocking mechanism typically observed during the sulfation reaction of fresh calcined limestones is not observed in the case of highly cycled sorbents (N > 20) and the low values of sulfation conversion characteristic of the sorbent in the Ca‐looping system. The random pore model is able to predict reasonably well, the CaO conversion to CaSO₄ taking into account the evolution of the pore structure during the calcination/carbonation cycles. The intrinsic reaction parameters derived for chemical and diffusion controlled regimes are in agreement with those found in the literature for sulfation in other systems. © 2011 American Institute of Chemical EngineersAIChE J, 2012     

Analysis of iron particle growth in aerosol reactor by a discrete-sectional model

The growth of iron particles by thermal decomposition of Fe(CO)₅ in a tubular reactor was analyzed by using a one dimensional discrete-sectional model with the coalescence by sintering of neighboring particles incorporated in. A thermal decomposition of Fe(CO)₅ vapor to produce iron particles was carried out at reactor temperatures varying from 300 to 1,000°C, and the effect of reactor temperature on particle size was compared with model prediction. The prediction exhibited good agreement with experimental observation that the primary particle size of iron was the largest at an intermediate temperature of 800°C. Model prediction was also compared with Giesen et al.’s [1] experimental data on iron particle production from Fe(CO)₅. Good agreement was shown in primary particle size, but a considerable deviation was observed in primary particle size distribution. The deviation may be due to an inadequate understanding of the sintering mechanism for the particles within an agglomerate and to the assumption of an ideal plug flow in model reactor in contrast to the non-ideal dispersive flow in actual reactor.     

Process performances and characteristics of powders produced using supercritical CO2 as solvent and antisolvent

A carbon dioxide (CO₂) soluble compound (cholesterol) was successfully precipitated either by rapid expansion of SCCO₂ solutions (RESS process, acronym for Rapid Expansion of Supercritical Solution), or from methylene chloride solutions by antisolvent precipitation (SAS-process, acronym for Supercritical Antisolvent process). The same fluid was thus used either as a solvent or as an antisolvent to precipitate cholesterol. Performances of RESS and SAS were compared through the analysis of the particle characteristics and production rates. Differences were related to supersaturation and time scale of nucleation/growth involved in both processes. Polydispersity, large size and elongated shape were characteristics of particles produced by SAS, especially when experiments were performed under conditions of total miscibility of CO₂ and organic solvent. Conditions where vapor-liquid equilibrium exists promoted a confinement of the growth that consequently reduced the final particle size. RESS, by comparison, produced smaller and monodispersed particles. Production of small particles is a key advantage for RESS, but lower production rates and yield might be disadvantages. The combination of the two processes offers the opportunity of tunable sizing of powder, switching from a large production of particles ranging from 10 to 100 μm, to a limited production of fine crystals below 10 μm.     

Preparation of Fe-Si Nanoparticles in Furnace Aerosol Reactor

Crystalline Fe-Si alloy particles ranging from 37 to 150 nm in diameter were produced by thermal decomposition of a mixture of Fe(CO)₅ and SiH₄ in a furnace aerosol reactor. The reactor was made of alumina, 2.4 cm in diameter and 100 cm in length. The operating variables were reactor temperature (800–1400°C), the Fe(CO)₅ concentration (2.5 × 10^?5 to 1.5 × 10^?4 moI/I), the molar ratio of Fe(CO)₅ to SiH₄ (100:0 to 50:50), and the residence time (2.5–10s). The primary particle size increased with reactor temperature increase and decreased when the Si content of the precursor was increased. The sintering of the particles within the agglomerates was an important factor in determining the primary particle size, and the sintering was inhibited by the silicon. The spatial variation of particle morphology was observed by in situ deposition of particles on a TEM grid. At 7 cm from the reactor inlet, nonagglomerated spherical particles encapsulating several smaller iron particles were found. The spherical structure were destroyed downstream to form agglomerates.     

Macrokinetics of Polybutadiene Production on a Titanium Ziegler–Natta Catalyst System Prepared in Turbulent Flows

The research was directed at establishing the dependence of the disperse composition of particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate, and their activity and molecular weight characteristics of polybutadiene formed in their presence on the length of sections and the confuser diameter of the tubular turbulent apparatus of diffuser–confuser design used to prepare a catalytic system in turbulent flows. An approach that combines methods of computational fluid dynamics (ANSYS Workbench 17.1 platform) and formal kinetics, and solves inverse problems of kinetics has been used to solve the stated problem. The following required dependences were established based on numerical experiments on the models developed using this approach: (1) the hydrodynamics of the process of dispersing particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate in a tubular turbulent apparatus with diffuser–confuser design, and (2) macrokinetics of butadiene polymerization on the particles of TiCl₄–Al(i-C₄H₉)₃ catalytically active precipitate dispersed in turbulent flows (batch reactor, the solvent was toluene, and the temperature of the process was 25°С).     

Modeling and analysis of circulation variables of continuous sorbent loop cycling for CO<Subscript>2</Subscript> capture

Carbon capture and storage (CCS) technologies are a cornerstone for reducing CO₂ emissions from energy and energy-intensive industries. Among the various CCS technologies, solid sorbent looping systems are considered to be potentially promising solutions for reducing CO₂ capture energy penalty. We present an evaluation module for a carbonator with sorbent looping cycle to calculate the carbonation efficiency. The module incorporates a simple sorbent activity model, and the solid/gas balances are constructed by assuming simple reactor mixing quality. By conducting simulations, we examine the variation in the carbonation efficiencies as a function of the sorbent looping operation factors and discuss an optimum operating strategy.     

The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles.The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U-U_mf. The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U-U_mf, the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.     

Mechanistic investigations of single-walled carbon nanotube synthesis by ferrocene vapor decomposition in carbon monoxide

The single-walled carbon nanotubes (SWCNTs) were synthesized by the carbon monoxide disproportionation reaction on Fe catalyst particles formed by ferrocene vapor decomposition in a laminar flow aerosol (floating catalyst) reactor. On the basis of in situ sampling of the product collected at different locations in the reactor, kinetics of the SWCNT growth and catalyst particle crystallinity were studied. Catalyst particles captured before SWCNT nucleation as well as inactive particles were determined to have cementite (Fe₃C) phase, while particles with γ- and α-Fe phases were found to be embedded in the SCWNTs. The growth rate in the temperature range from 804 to 915 °C was respectively varied from 0.67 to 2.7 μm/s. The growth rate constant can be described by an Arrhenius dependence with an activation energy of E_a = 1.39 eV, which was attributed to the carbon diffusion in solid iron particles. CNT growth termination was explained by solid-liquid phase transition in the catalyst particles. A high temperature gradient in the reactor was found to not have any effect on the diameter during the SWCNT growth and as a result on the chirality of the growing SWCNTs.     

Controllable preparation of nano-CaCO3 in a microporous tube-in-tube microchannel reactor

In this work, nano-CaCO₃ particles with tunable size have been synthesized via CO₂/Ca(OH)₂ precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO₂ and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (K_La) of CO₂ absorption into Ca(OH)₂ slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)₂ content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO₃ particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles.     

Kinetics of reductive alkylations of phenylenediamines: Influence of substrates isomeric structure

Reductive alkylation of ortho-, meta- and para-phenylenediamines (PDAs) with methyl ethyl ketone (MEK) has been studied in a semi-batch slurry reactor in the presence of a commercial 3% Pt/Al₂O₃ catalyst. It was observed that the PDA isomers differ remarkably from each other in their activity in reductive alkylation and product distribution. The activity was found to decrease in the following order: PPDA>OPDA>MPDA. To understand the substrate structure–activity correlation, the homogeneous equilibrium reactions involved in the alkylation step and the overall catalytic reactions were studied separately. Kinetics of reductive alkylation of PDAs with MEK as a solvent and alkylating agent with 3% Pt/Al₂O₃ catalyst was studied in a semi-batch slurry reactor over a temperature range of 373–453 K and pressure range of 2.07–6.21 MPa. Semi-batch slurry reactor models were developed and kinetic parameters were estimated by fitting the integral batch reactor data at different temperatures to understand the influence of different reaction steps on the activity and selectivity of different products.     

Numerical analysis on particle coating by the pulsed plasma process

The a-Si thin-film growth on particles in the rotating pulsed SiH₄ plasma process was analyzed numerically. The evolutions of chemical concentrations (SiH₄, SiH_x, and polymerized negative ions) in the pulsed plasmas have been shown during the plasma-on and -off. During plasma-on, SiH₄ is consumed by the electron impact dissociative reactions, but, during plasma-off, the disappearance reaction of SiH₄ stops because the electrons disappear in the plasma reactor. During plasma-on, SiH_x and are generated quickly by a fast dissociative reaction of SiH₄, but, during plasma-off, SiH_x disappears rapidly by a reaction with hydrogen and also by the deposition onto the reactor wall and particles, and is consumed quickly by fast neutralization reactions with the negative ions. The negative ions are polymerized by the reactions with SiH₄ during plasma-on, but, disappear by neutralization reactions during plasma-off. The growth rate of the film thickness profile depends on the SiH_x concentration because the particles grow with the SiH_x deposition. As the plasma-on time increases or as the plasma-off time decreases, the thin film thickness on the particles increases more quickly with faster SiH_x deposition onto them. A fraction of the particles falling down in the gas phase (W_FP) increases as the rotation speed of the plasma reactor increases. As W_FP increases, as the particle concentration decreases, or as the particle diameter decreases, the film thickness on the particles increases more quickly because the flux of SiH_x toward the particles increases.The pulsed plasma process can efficiently reduce the growth of polymerized negative ions and particles, both of which are not good for high-quality thin films. We showed that the high-quality thin films on the particles can be prepared successfully by deposition of low mass chemical precursors by pulsed plasma processes.     

Preparation of poly(L-lactic acid) submicron particles in aerosol solvent extraction system using supercritical carbon dioxide

The aerosol solvent extraction system process (ASES), which is one of the supercritical anti solvent processes (SAS), was used to produce poly(L-lactic acid) (PLLA) into the submicron particles. Dichloromethane (DCM, CH₂Cl₂) and carbon dioxide were selected as a solvent and as an antisolvent for PLLA, respectively. The objective of this study was to investigate the effect of the various process parameters such as temperature, pressure, and solution concentration on PLLA particles. With increasing temperature and pressure, particle size was increased. Also, higher PLLA concentration led to larger particle size and broader particle size distribution. A scanning electron microscope (SEM) was used to observe the morphology and size of PLLA particles recrystallized by ASES process. The mean particle size and its distribution of processed particles were measured by using a laser diffraction particle size analyzer (PSA).     

Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor

Results are presented on steam hydration of spent residues obtained from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant unit operating in a CO₂ looping cycle mode. The samples were collected from the unit under various conditions, which included electrical heating of the reactor, as well as firing with coal, and biomass under oxy-fuel combustion conditions. In addition, different operating times, i.e., number of cycles (25 min–455 min/1–25 cycles) were examined, with the carbonator operating at temperatures of 600–700 °C and the calciner at 850–900 °C. The samples collected came from the calciner, carbonator and cyclone. Steam hydration itself was done under atmospheric pressure in saturated steam at 100 °C for periods of 15, 30 and 60 min. The original limestone sample, as well as the spent samples from the pilot plant and the hydrated samples were examined to determine their hydration and carbonation levels, as well as their unreacted CaO content using TGA and XRD analysis. In addition, samples were characterized for pore distribution (nitrogen adsorption/desorption: BET and BJH), skeleton characterization, with density by He pycnometry and particle surface area morphology (SEM/EDX), as well as changes in sample volume during hydration (sample swelling). The results obtained showed successful hydration (typically only ～10% unreacted CaO) even for hydration periods as short as 15 min, and very favorable sample properties. Their pore surface area, pore volume distribution and swelling during hydration are promising with regard to their use in additional CO₂ capture cycles or SO₂ retention. However, their predisposition to fracture is the main disadvantage observed with these samples. This may result in difficulties in terms of their handling in FBC systems, due to intensified attrition and consequent elutriation from the reactor.     

Numerical analysis on premixed combustion of H2–SiCl4–Air system to prepare SiO2 particles

In this study, we used the commercial CFD-code FLUENT to analyze numerically the hydrogen combustion and SiO₂ particle formation in the premixed flame reactor. We considered SiCl₄ as a precursor for SiO₂ particle formation and calculated the profiles of fluid flow, temperature, species concentration and reaction rates for 2D premixed reactor model in FLUENT. Using the data for temperature and velocity extracted from FLUENT, we calculated the trajectories and temperature histories of SiO₂ particles moving inside the premixed flame reactor and those particles starting near the centerline of reactor pass through the maximum temperature histories.