Analysis of solar‐driven gasification of biochar trickling through an interconnected porous structure

The efficient transfer of high‐temperature solar heat to the reaction site is crucial for the yield and selectivity of the solar‐driven gasification of biomass. The performance of a gas‐solid trickle‐bed reactor constructed from a high thermal conductivity porous ceramic packing has been investigated. Beech char particles were used as the model feedstock. A two‐dimensional finite‐volume model coupling chemical reaction with conduction, convection, and radiation of heat within the packing was developed and tested against measured temperatures and gasification rates. The sensitivity of the gasification rate and reactor temperatures to variations of the packing's pore diameter, porosity, thermal conductivity, and particle loading was numerically studied. A numerical comparison with a moving bed projected a more uniform temperature distribution and higher gasification rates due to the increased heat transfer via combined radiation and conduction through the trickle bed. © 2014 American Institute of Chemical Engineers AIChE J, 61: 867–879, 2015     

Three‐dimensional CFD‐PBM coupled model of the temperature fields in fluidized‐bed polymerization reactors

A three‐dimensional (3‐D) computational fluid dynamics model, coupled with population balance (CFD‐PBM), was developed to describe the gas–solid two‐phase flow in fluidized‐bed polymerization reactors. The model considered the Eulerian–Eulerian two‐fluid model, the kinetic theory of granular flow, the population balance, and heat exchange equations. First, the model was validated by comparing simulation results with the classical calculated data. The entire temperature fields in the reactor were also obtained numerically. Furthermore, two case studies, involving constant solid particle size and constant polymerization heat or evolving particle‐size distribution, polymerization kinetics, and polymerization heat, were designed to identify the model. The results showed that the calculated results in the second case were in good agreement with the reality. Finally, the model of the second case was used to investigate the influences of operational conditions on the temperature field. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Splitting CO2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

Thermochemical splitting of CO₂ via a ceria‐based redox cycle was performed in a solar‐driven thermogravimetric analyzer. Overall reaction rates, including heat and mass transport, were determined under concentrated irradiation mimicking realistic operation of solar reactors. Reticulated porous ceramic (RPC) structures and fibers made of undoped and Zr⁴⁺‐doped CeO₂, were endothermally reduced under radiative fluxes of 1280 suns in the temperature range 1200–1950 K and subsequently re‐oxidized with CO₂ at 950–1400 K. Rapid and uniform heating was observed for 8 ppi ceria RPC with mm‐sized porosity due to its low optical thickness and volumetric radiative absorption, while ceria fibers with μm‐sized porosity performed poorly due to its opacity to incident irradiation. The 10 ppi RPC exhibited higher fuel yield because of its higher sample density. Zr⁴⁺‐doped ceria showed increasing reduction extents with dopant concentration but decreasing specific CO yield due to unfavorable oxidation thermodynamics and slower kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1263–1271, 2017     

Flax straw char‐CO2 gasification kinetics and its inhibition studies with CO

Char gasification has been studied in different ways with different feedstock; however, the fundamental studies about the variation of char reactivity with different combination of parameters are still required in order to design the biomass char gasification process in large scale unit. In this work, char from flax straw pyrolysis was used for gasification with different partial pressure of CO₂, different temperature and particle size. Results showed that 375‐µm particle sizes of char has higher reactivity compared to other particle sizes and the inhibition effect was also less at 375‐µm particle size. Kinetic parameters varied for gasification reaction with different particle sizes and the average activation energy was 196 kJ/mol and the order of the reaction was approximately 1. Inhibition studies with the addition of CO in the gasifying agent proved that CO molecules interfere significantly and reduced the reactivity. ANOVA test showed that the temperature plays a vital role in char reactivity. The particle sizes of 375 and 800 µm along with the CO₂ partial pressure of 0.35 bars are the best combination for achieving the maximum reactivity. © 2012 Canadian Society for Chemical Engineering     

A packed‐bed solar reactor for the carbothermal zinc production – dynamic modelling and experimental validation

Integration of concentrated solar energy into the pyrometallurgical Zn production process as clean source of high‐temperature process heat could significantly reduce fossil fuels consumption and its concomitant CO₂ emissions. The solar‐driven carbothermal reduction of ZnO is investigated using a 10‐kW_th solar reactor featuring two cavities, the upper one serving as the solar absorber and the lower one containing a packed‐bed of ZnO and beech charcoal as the biogenic reducing agent. Experimentation in a high‐flux solar simulator is carried out under radiative fluxes of 2300–2890 suns, yielding a peak solar‐to‐chemical energy conversion efficiency of 18.4%. The reactor performance under variable operating conditions is analyzed via a dynamic numerical model coupling heat transfer with chemical kinetics. The model is validated by comparison to the experimental data obtained with the 10‐kW_th packed‐bed solar reactor and further applied to predict the effect of incorporating semi‐continuous feeding of reactants on the process efficiency. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4586–4594, 2016     

Solar gasification of carbonaceous waste feedstocks in a packed‐bed reactor—Dynamic modeling and experimental validation

Thermochemical gasification of carbonaceous waste feedstocks (specifically: scrap tire powder, industrial sludge, and sewage sludge) for high‐quality syngas production is numerically modeled and experimentally validated using concentrated solar process heat. The solar reactor consists of two cavities separated by a radiant emitter, with the upper one serving as the solar radiative absorber and the lower one containing the reacting packed bed. The reactor is modeled by considering combined heat transfer coupled to the reaction kinetics, driven by the applied solar flux at the reactor's aperture. Model validation is accomplished in terms of converted mass, reactor temperatures, efficiency, and solar upgrade based on experiments with an 8‐kW reactor subjected to solar fluxes up to 2560 suns and packed bed temperatures up to 1490 K. The transient operation of a 200‐kW pilot‐scale reactor for gasifying industrial sludge is simulated for a solar day, yielding a maximum solar‐to‐fuel energy conversion efficiency of 89%. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Kinetics of the thermal dissociation of ZnO exposed to concentrated solar irradiation using a solar‐driven thermogravimeter in the 1800–2100 K range

The two‐step H₂O‐splitting thermochemical cycle based on the Zn/ZnO redox reactions is considered for solar H₂ production, comprising the endothermal dissociation of ZnO followed by the exothermal hydrolysis of Zn. A solar‐driven thermogravimeter, in which a packed‐bed of ZnO particles is directly exposed to concentrated solar radiation at a peak solar concentration ratio of 2400 suns while its weight loss is continuously monitored, was applied to measure the thermal dissociation rate in a set‐up closely approximating the heat and mass transfer characteristics of solar reactors. Isothermal thermogravimetric runs were performed in the range 1834–2109 K and fitted to a zero‐order Arrhenius rate law with apparent activation energy 361 ± 53 kJ mol^?1 K^?1 and frequency factor 14.03 × 10⁶ ± 2.73 × 10⁶ kg m^?2 s^?1. Application of L‘vov’s kinetic expression for solid decomposition along with a convective mass transport correlation yielded kinetic parameters in close agreement with those derived from experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Efficient production of uniform nanometer‐sized polymer vesicles in stirred‐tank reactors

Polymer vesicles, so‐called polymersomes, gain more and more attention as potential carriers for medical and biotechnological applications. To put the production of these nanocompartments into action at an industrial scale, an efficient and scalable process has to be established. Moreover, being able to control the resulting particle size distribution (PSD) is vital. In this work, the amphiphilic triblock copolymer poly(2‐methyloxazoline)₁₅–poly(dimethylsiloxane)₆₈–poly(2‐methyloxazoline)₁₅ is formed into polymersomes in miniaturized stirred‐tank reactors. Varying flow conditions have a huge impact on the resulting PSD. Dynamic light scattering measurements show that driving a S‐shaped stirrer at 4000 rpm in unbaffled reactors leads to a monomodal PSD with a low polydispersity index (PDI<0.2). Vesicles with a mean diameter of 200 nm are achieved within less than 1 h in a single production step. The robustness of the established process is shown by producing uniform polymersomes at different temperatures and varying pH and buffer molarities. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43274.     

Three‐dimensional numerical simulation of upflow bubbling fluidized bed in opaque tube under high flux solar heating

Solid particles can be used as a heat transfer medium in concentrated solar power plants to operate at higher temperature and achieve higher heat conversion efficiency than using the current solar heat transfer fluids that only work below 600°C. Among various particle circulation concepts, the dense particle suspension (DPS) flow in tubes, also called upflow bubbling fluidized bed (UBFB), was studied in the frame of the CSP2 FP7 European project. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated and the first values of the tube wall‐to‐DPS heat transfer coefficient were measured. A stable outlet temperature of 750°C was reached with a metallic tube, and a particle reflux in the near tube wall region was evidenced. In this article, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD. Hydrodynamics of SiC Geldart A‐type particles and heat transfer imposed by a thermal flux at the wall are coupled in two‐dimensional unsteady numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter‐particle radiative transfer (Rosseland approximation) are accounted for. Simulations and experiments are compared here and the temperature influence on the DPS flow is analyzed. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3857–3867, 2018     

Particle‐scale modeling of coal devolatilization behaviors for coal pyrolysis in thermal plasma reactors

A generalized heat transfer and devolatilization model coupled with the thermal balance between the heating gas and particles was established to predict the complex coal pyrolysis behaviors in the practical plasma reactors. It was proved that this model could well describe the coal devolatilization behaviors in both the pilot‐scale and lab‐scale plasma reactors as the mechanisms of coal chemistry and particle‐scale physics were incorporated. The achieved understanding on the reactor energy balance demonstrated that the heat recovery of the quenching process was crucial to the thermal efficiency and economic benefit of the overall project. The in‐depth discussion of the influences of coal feed rate and particle size on the reactor performance revealed the dominant roles and presented the optimal values of these two factors. In particular, the simulation results of several coals could help to provide a simple, quick method of coal type selection for industrial plasma processes. © 2014 American Institute of Chemical Engineers AIChE J, 61: 913–921, 2015     

Polymersome formation mechanism and formation rate in stirred‐tank reactors

Uniform polymersomes (polymer vesicles) made of poly(2‐methyloxazoline)₁₅‐b‐poly(dimethylsiloxane)₆₈‐b‐poly(2‐methyloxazoline)₁₅ (PMOXA15–PDMS68–PMOXA15) can be formed in miniaturized‐stirred tank reactors by the aid of a recently published process. In this study, the occurring self‐assembly mechanism was elucidated by using transmission electron microscopy. Subsequent to the initial formation of small spherical micelles and the following fusion to worm‐like micelles, two simultaneously occurring pathways, describing the transformation of further intermediate structures to the desired vesicles, were found. The resulting particle increase was followed by dynamic light scattering. Thus, the vesicle formation rate was judged by the linear increase of the particle diameter over time. While temperature showed no influence, higher initial polymer concentrations and lower final solvent concentrations accelerated the polymersome formation. Besides, the process was crucially dependent on the agitation speed. While spherical micelles did not transform into polymersomes when no stirring or too slow stirring is applied, the self‐assembly process was accelerated by increasing the agitation speed. Uniform polymeric vesicles can be formed under vigorous stirring in stirred‐tank reactors in short process times. In this study, the underlying mechanisms of vesicle formation were elucidated, showing that the polymer forms small micellar structures before undergoing two separate pathways to form the desired vesicular structures. The formation rate of the polymer vesicles was mainly dependent on the agitation speed but also on the polymer and solvent concentrations, highlighting the need for controlled formation conditions. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46077.     

Effect of structural change of pitch on the thermal conductivity of epoxy‐based composites filled with heat‐treated pitch

Petroleum‐based pitches were used as filler materials to study the effects of heat‐treatment‐induced changes in pitch structure on the thermal conductivity of epoxy‐based composites. The heat treatment was performed in two steps: the first involved heating the pitch to 250 °C in order to remove the low‐molecular‐weight compounds from the pitch, and the second involved heating the pitch to either 430 or 450 °C. There was no significant difference in the curing behavior of the diglycidyl ether of bisphenol A (DGEBA)/pitch composites, regardless of the heat‐treatment temperature. However, the thermal conductivity of the DGEBA/pitch composites improved with increasing heat‐treatment temperature, and the epoxy composite prepared with pitch heat‐treated at 430 °C exhibited the maximum thermal conductivity. This can be attributed to structural changes in the pitch, such as the distance between adjacent planes (d‐spacing), crystallite height (L_c) and crystallite width (L_a). Although L_c of the pitch increased with increasing heat‐treatment temperature, the d‐spacings and L_a decreased. These results suggest that the heat treatment of the pitch led to a well‐stacked crystalline structure. However, compared with the pitch heat‐treated at 430 °C, that heat‐treated at 450 °C exhibited lower thermal conductivity in the DGEBA/pitch composite because of the low L_a, resulting in the loss of basal carbon as a consequence of in situ gasification, and pyrolysis of the low‐molecular‐weight compounds in the pitch. © 2013 Society of Chemical Industry     

Potassium‐catalyzed steam gasification of ash‐free lignite coal with CO2 capture

The purpose of this research was to study steam gasification of ash‐free coal integrated with CO₂ capture in the presence of a K₂O catalyst for enhancement of the key water‐gas shift reaction and promotion of hydrogen production. To achieve this goal, gasification experiments on ash‐free coal (AFC) were carried out at varying temperatures (600, 650, 675, 700, and 750 °C) with a sorbent‐to‐carbon (CaO/C) ratio of 2 and a catalyst (K₂O) loading of 0.2 g/g (20 weight percent (wt%)) in a fixed‐bed reactor equipped with a gas chromatography analyzer. The sorbent‐to‐carbon (CaO/C) ratio of 2 is based on dry and ash‐free basis. The CaO/C ratio and K₂O wt% were chosen to maximize hydrogen production based on our previously determined optimal values. The AFC was originally extracted from raw lignite coal using organic solvents, which allowed the sorption‐enhanced gasification to be conducted with minimal ash‐catalyst interactions. The effect of temperature on the yield and the initial reaction rate were investigated. The optimal reaction temperature of 675 °C was determined. Carbon balance and final carbon conversions were calculated based on the residue analysis. Activation energy was also calculated using intrinsic kinetics of the reaction. In this study, using AFC offered the potential advantage of operating the gasification process with catalyst recycle.     

Influence of Particle Size and Single‐Tube Diameter on Thermal Behavior of Fischer‐Tropsch Reactors. Part II. Eggshell Catalysts and Optimal Reactor Performance

The optimal combination of particle and tube size for simulation of a single tube of a wall‐cooled multitubular Fischer‐Tropsch (FT) reactor with cobalt as catalyst was determined. The maximum size of the tubes, realized without temperature runaway, enhances with increasing particle size until an optimal value is reached, thereby improving the production rate of liquid fuels per tube. Reasons for this are that heat transfer to the cooled tube wall for a given tube size is considerably enhanced by increasing the particle size and that the influence of pore diffusion on the effective rate of FT synthesis gets stronger with rising particle size, which reduces the temperature sensitivity of the reactor and decreases the danger of a temperature runaway. The simulations indicate that the use of FT eggshell catalysts is not an option for fixed‐bed reactors. The temperature sensitivity of the reactor is strongly enhanced, which decreases the maximum tube size and with that the productivity per tube. All these effects are valid in general for wall‐cooled fixed‐bed reactors. Respective criteria are presented.     

Numerical Calculations of Spray Roasting Reactors of the Steel Industry with Special Emphasis on Fe2O3‐Particle Formation

This work presents numerical calculations for the lay‐out of spray roasting reactors for the steel industry. In these reactors, a pickling liquor based on water and HCl containing FeCl₂ is regenerated in a combustor leading to the formation of Fe₂O₃ particles. For the lay‐out of these reactors, detailed knowledge of the flow and temperature field, the associated gas phase reactions, and especially, of the formation of the Fe₂O₃ particles is required. An extended particle formation model is presented which is based on earlier work. Finally, results for an industrial spray roasting reactor are given showing the potential of the numerical tools developed for the improvement of the technical lay‐out of such thermal reactors.     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

On the high‐gasification rate of Brazilian manganese ore in chemical‐looping combustion (CLC) for solid fuels

The high rate of char gasification observed when using a Brazilian manganese ore as compared to ilmenite is investigated in a batch fluidized‐bed reactor. Experiments were carried out at 970°C using petroleum coke, coal and wood char as fuel with a 50% H₂O in N₂ as fluidizing gas. A manufactured manganese oxygen carrier was also used, however, which presented a slower char conversion rate than the manganese ore. It is concluded that decrease in H₂ inhibition and oxygen release are unlikely to be the main responsible mechanisms for the ore's unexpected gasification rate. The ore was also mixed in different ratios with ilmenite and it was observed that the presence of even small amounts of ore in the bed resulted in increased gasification rate. Thus, the high‐gasification rate for the manganese ore could be due to a contribution from the impurities in the ore by catalyzing the gasification reaction. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4346–4354, 2013     

Fully resolved simulations of single char particle combustion using a ghost‐cell immersed boundary method

A novel ghost‐cell immersed boundary method for fully resolved simulation of char particle combustion has been developed. The boundary conditions at the solid particle surface, such as velocity, temperature, density, and chemical species concentration, are well enforced through the present method. Two semiglobal heterogeneous reactions and one homogeneous reaction are used to describe the chemical reactions in the domain, and the Stefan flow caused by the heterogeneous reactions is considered. A satisfactory agreement can be found between the present simulation results and experimental data in the literature. The method is then used to investigate the combustion property of a char particle and the interaction between CO₂ gasification and O₂ oxidation. Furthermore, combustion effect on the exchange of mass, momentum and energy between gas‐ and solid‐ phase is explored. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2851–2863, 2018     

Wärmeübergang in kurzen,wandgängig gerührten Apparaten

Scraped surface heat exchanger are often used in industrial settings for the treatment of systems with solid particle formation or higher viscosities. In reactors with a small length‐diameter ratio, the backmixing has thereby a significant influence on the heat transfer. In the experiments presented here, it can be shown that the heat transfer coefficient strongly depends on the flow pattern within the laminar flow regime, and that, depending on the running conditions, an increased rotor speed may have a negative effect on the heat transfer rate.     

Effect of CO<Subscript>2</Subscript> addition on lignite gasification in a CFB reactor: A pilot-scale study

The addition of carbon dioxide to the gasification media during lignite gasification is introduced. The paper presents thermodynamic grounds of CO₂ enhanced gasification using a simplified equilibrium model. Experimental tests conducted using a pilot-scale circulating fluidized bed gasifier are discussed. Detailed analysis of the CO₂/C ratio on process conditions, namely on the process gas composition, lower heating value and H₂/CO ratio, is provided. Process gas composition implies that the gas is suitable for heat and power generation. Alternatively, CO₂ enhanced gasification could be considered as a carbon capture and utilization technology when external, renewable heat supply to the process is used. The results thus obtained are the initial step toward development of the CO₂ enhanced gasification process.