Modeling of a 120 kW chemical looping combustion reactor system using a Ni-based oxygen carrier

A modeling tool for the investigation of chemical looping combustion (CLC) in a dual circulating fluidized bed (DCFB) reactor system is introduced. CLC is a novel combustion process with inherent CO₂ separation, consisting of two fluidized bed reactors, an air reactor (AR) and a fuel reactor (FR). A solid oxygen carrier (OC) that circulates between the reactors, transports the necessary oxygen for the combustion. In the DCFB concept both AR and FR are designed as circulating fluidized beds (CFBs). Each CFB is modeled using a very simple structure in which the reacting gas is only in contact with a defined fraction of the well mixed solids. The solids distribution along the height axis is defined by a void fraction profile. Different parameters that characterize the gas-solids contact are merged into only one parameter: the fraction of solids exposed to the gas passing in plug flow (φ_s,core). Using this model, the performance of the 120 kW DCFB chemical looping combustor at Vienna University of Technology is investigated. This pilot rig is designed for a Ni-based OC and natural gas as fuel. The influence of the reactor temperatures, solids circulation rate, air/fuel ratio and fuel power are determined. Furthermore, it is shown that with the applied kinetics data, the OC is only fully oxidized in the AR when the AR solids inventory is much larger than the FR solids inventory or when both reactors are very large. To compare different reactor systems, the effect of the solids distribution between AR and FR is studied and both gas and solids conversions are reported.     

Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents

The steam gasification of biomass, in the presence of a calcium oxide (CaO) sorbent for carbon dioxide (CO₂) capture, is a promising pathway for the renewable and sustainable production of hydrogen (H₂). In this work, we demonstrate the potential of using a CaO sorbent to enhance hydrogen output from biomass gasifiers. In addition, we show that CaO materials are the most suitable sorbents reported in the literature for in situ CO₂ capture. A further advantage of the coupled gasification-CO₂ capture process is the production of a concentrated stream of CO₂ as a byproduct. The integration of CO₂ sequestration technology with H₂ production from biomass could potentially result in the net removal of CO₂ from the atmosphere.Maximum experimental H₂ concentrations reported for the steam gasification of biomass, without CO₂ capture, range between 40%-vol and 50%-vol. When CaO is used to remove CO₂ from the product gas, as soon as it is formed, we predict an increase in the H₂ concentrations from 40%-vol to 80%-vol (dry basis), based on thermodynamic modelling and previously published data.We examine the effect of key variables, with a specific focus on obtaining fundamental data relevant to the design and scale-up of novel biomass reactors. These include: (i) reaction temperature, (ii) pressure, (iii) steam-to-biomass ratio, (iv) residence time, and (v) CO₂ sorbent loading. We report on operational challenges related to in situ CO₂ capture using CaO-based sorbents. These include: (i) sorbent durability, (ii) limits to the maximum achievable conversion and (iii) decay in reactivity through multiple capture and release cycles. Strategies for enhancing the multicycle reactivity of CaO are reviewed, including: (i) optimized calcination conditions, and (ii) sorbent hydration procedures for reactivation of spent CaO. However, no CaO-based CO₂ sorbent, with demonstrated high reactivity, maintained through multiple CO₂ capture and release cycles, has been identified in the literature. Thus, we argue that the development of a CO₂ sorbent, which is resistant to physical deterioration and maintains high chemical reactivity through multiple CO₂ capture and release cycles, is the limiting step in the scale-up and commercial operation of the coupled gasification-CO₂ capture process.     

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO₂ ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO₂ into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed for simulating dynamic process of CO₂ ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO₂ absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO₂ gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO₂. The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO₂ concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO₂ into seawater in the CO₂ ocean disposal.     

Numerical simulation of methane fuelled cogenerative SOFCs for the production of synthesis gas and electrical energy

Numerical simulation has been used to show the feasibility of the autothermal cogeneration of synthesis gas and electricity in a solid oxide fuel cell (SOFC) by the electrochemical partial oxidation of CH₄. Owing to the large positive entropy change of the CH₄ partial oxidation reaction and its low heating value, severe cooling effect is being induced in the SOFC due to heat absorbance by the reaction products. For this reason the autothermal operation of the SOFC reactor cannot be secured. As it is shown this can be overcome by combining the dynamic operation of the SOFC under forced periodic reversal of the flow and the bleeding of a small amount of CH₄(<2.5%) in the oxidant stream (cathode). In this respect the catalytic combustion of CH₄, on the perovskite cathodic electrode, provides the necessary energy demand so that in combination with flow reversal operation the SOFC is maintained ignited even at inlet temperature as low as 300 K. It is shown that the overall thermodynamic efficiency of the process can by far exceed unity (η^′>2), thus revealing the unique property of the SOFCs to produce high-quality energy and useful chemicals.     

Characterization of potassium glycinate for carbon dioxide absorption purposes

Aqueous solutions of potassium glycinate were characterized for carbon dioxide absorption purposes. Density and viscosity of these solutions, with concentrations ranging from 0.1 to 3 M, were determined at temperatures from 293 to 313 K. Diffusivity of CO₂ in solution was estimated applying the modified Stokes-Einstein relation. Solubilities of N₂O at the same temperatures and concentrations were measured and the ion specific parameter based on Schumpe's model was determined for the glycinate anion; the solubilities of CO₂ in these solutions were then computed.The reaction kinetics of CO₂ in the aqueous solution of potassium glycinate was determined at 293, 298 and 303 K using a stirred cell reactor. The results were interpreted using the DeCoursey equation for the calculation of the enhancement factor. The rate of absorption as a function of the temperature and solution concentration for the conditions studied was found to be given by the following expression:

     

The effect of specific surface area on the activity of nano-scale ceria catalysts for methanol decomposition with and without steam at SOFC operating temperatures

Nano-particulate high surface area CeO₂ was found to have a useful methanol decomposition activity producing H₂, CO, CO₂, and a small amount of CH₄ without the presence of steam being required under solid oxide fuel cell temperatures, 700-1000 °C. The catalyst provides high resistance toward carbon deposition even when no steam is present in the feed. It was observed that the conversion of methanol was close to 100% at 850 °C, and no carbon deposition was detected from the temperature programmed oxidation measurement.The reactivity toward methanol decomposition for CeO₂ is due to the redox property of this material. During the decomposition process, the gas-solid reactions between the gaseous components, which are homogeneously generated from the methanol decomposition (i.e., CH₄, CO₂, CO, H₂O, and H₂), and the lattice oxygen on ceria surface take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen ( can produce synthesis gas (CO and H₂) and also prevent the formation of carbon species from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). V_O^·· denotes an oxygen vacancy with an effective charge 2⁺. Moreover, the formation of carbon via Boudouard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide with the lattice oxygen .At steady state, the rate of methanol decomposition over high surface area CeO₂ was considerably higher than that over low surface area CeO₂ due to the significantly higher oxygen storage capacity of high surface area CeO₂, which also results in the high resistance toward carbon deposition for this material. In particular, it was observed that the methanol decomposition rate is proportional to the methanol partial pressure but independent of the steam partial pressure at 700-800 °C. The addition of hydrogen to the inlet stream was found to have a significant inhibitory effect on the rate of methanol decomposition.     

Absorption of carbon dioxide into aqueous blends of 2-amino-2-methyl-1-propanol and monoethanolamine

This work presents an investigation of CO₂ absorption into aqueous blends of 2-amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA). The acid gas mass transfer has been modeled using equilibrium-mass transfer-kinetics-based combined model to describe CO₂ absorption into the amine blends according to Higbie's penetration theory. The effect of contact time and relative amine concentration on the rate of absorption and enhancement factor were studied by absorption experiment in a wetted wall column at atmospheric pressure. The model was used to estimate the rate coefficient of the reaction between CO₂ and monoethanolamine at 313 K from experimentally measured absorption rates. A rigorous parametric sensitivity test has been done to identify the key systems’ parameters and quantify their effects on the mass transfer using the mathematical model developed in this work. The model predictions have been found to be in good agreement with the experimental rates of absorption of CO₂ into (AMP+MEA+H₂O).     

Intrinsic kinetics of the oxidative reaction of dichloroacetic acid employing hydrogen peroxide and ultraviolet radiation

In a previous work it has been shown that the combination of H₂O₂ and low wavelength UV radiation is a suitable process for degrading dichloroacetic acid (DCA). The final result provided a validated and complete reaction scheme. That proposal included two possible ways for the hydroxyl radical to react with DCA [Zalazar, C., Labas, M., Brandi, R., Cassano, A., 2007. Dichloroacetic acid degradation employing hydrogen peroxide and UV radiation. Chemosphere 66, 808-815].This work was directed to a single objective: to derive, from the previous reaction sequence, a mathematical model able to represent the kinetics of DCA oxidation and validate its predictive quality with experiments. This representation of the reaction must include all the required variables for an ulterior reactor design and scale-up and, consequently, the kinetic model parameters must be independent of the shape, size and configuration of the laboratory reactor.Working with a complete set of experimental runs that included all the involved variables, the unknown kinetics parameters of the DCA degradation were obtained by comparing predicted concentrations by the model (represented by a set of two ordinary differential equations and two algebraic equations coupled with a mass and a radiation balance inside the reactor) with the experimental values, employing a multi-parameter non-linear regression analysis. Experimental values confirmed the validity of the proposed mechanism. Additionally, an optimal concentration ratio of hydrogen peroxide with respect to DCA was obtained (r=CH₂O₂/C_DCA≈8).The intermediate results of the numerical solution of the complete system of differential and algebraic equations representing the proposed complete reaction mechanism were useful to find simplified, analytical expressions for the reaction rates of DCA and H₂O₂. The obtained rates resulting from these simplifications were compared with those of the complete system showing a very satisfactory concordance. This outcome is, at the same time, a clear indication of the significant influence of the radical in the reaction evolution.     

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.     

Robust control in presence of parametric uncertainties: Observer-based feedback controller design

This paper proposes a complete procedure for the design of a robust controller for a nonlinear process, taking into account the various issues arising in the design and using the main theoretical results from the Literature about this topic. An extended model is set up, linking performance and robustness to the control law: the H_∞ norm of the extended system in closed loop measures the achievement of the objectives. The result is a state feedback control law which guarantees robust performance. The problem of the design of an observer to estimate the state of the system is also addressed, as the complete knowledge of the state is required to calculate the control action; moreover, the implications of the use of the observer in the design of the controller are pointed out. The methodology is illustrated via simulation of a regulation problem in a continuous stirred tank reactor (CSTR). The application of this methodology to more complex systems will be discussed.     

Kinetic analysis of NO-sensitized methane oxidation

The role of NO-sensitized oxidation during the product-gas entrainment of a low-NO_x, multi-jet, natural gas burner is investigated. A detailed kinetic mechanism for the NO-sensitized oxidation of CH₄, consisting of 483 reactions and 69 species, is used for the kinetic analysis. An eigenvalue-eigenvector decomposition is performed on normalized sensitivity coefficients to study the important reactions using principal component analysis (PCA), and the loadings corresponding to the largest eigenvalue are used to identify the reaction pathways of NO-sensitized oxidation. The main reaction pathway is most strongly affected by the temperature profile and equivalence ratio. Also, a reduced kinetic scheme of 110 reactions and 47 species is developed by eliminating reactions with small loadings. The temporal evolution of reactions is investigated using functional PCA, in which the functional loadings reflect the importance of reactions as a function of time. A discretization approach is used to perform the functional PCA.     

The decrease of carbonation efficiency of CaO along calcination-carbonation cycles: Experiments and modelling

Successive calcination-carbonation cycles, using CaO as sorbent, have been performed either in a classical fixed bed reactor or using a thermogravimetric analyser. Significant differences in carbonation efficiencies were obtained, possibly due to different conditions prevailing for CaO sintering during the calcination stage. The effect of the presence of CO₂ on sintering was confirmed.A simple model of the decay of the carbonation capacity along cycles based on the specific surface area of non-sintered micrograins of CaO is able to predict the decrease of the extent of conversion obtained after 40 carbonations along calcination-carbonation cycles. The asymptotic extent of conversion is obtained when all the micrograins present within a grain are sintered. A detailed model of the carbonation shows that the voids present between the micrograins are filled up by carbonate when a critical thickness of the carbonate layer around each micrograin reaches 43 nm. Then, carbonation becomes controlled by diffusion at the scale of the whole grain, with the CO₂ diffusion coefficient decreasing (at ) from 2×10^-12 to as carbonation proceeds from 50% conversion to 76% (first cycle). This scale change for diffusion is responsible for the drastic decrease of the carbonation rate after the voids between micrograins are filled up.     

Further work on cake filtration analysis

Analysis of cake filtration was made by the numerical solution of the appropriate equations of change based on the multiphase flow theory with the assumption that the cake properties are functions of the particle phase compressive stress, p_s. Unlike earlier studies which assume the relationship between p_s and the pressure of the fluid phase, p_l, to be ∇p_s+∇p_l=0, other possibilities were also considered in view of the recent work of Tien et al. (Chem. Eng. Sci. 56 (2001) 5361).In addition to investigating the effect of the p_s-p_l relationship, comparisons of predicted filtration performance with experiments made it possible to substantiate earlier findings that the p_s-p_l relationship is system specific. The results of the analysis were also used to test the parameter sensitivity of predictions, namely, values of the parameters of the constitutive relationships (i.e. ?_s vs. p_s and α vs. p_s, where ?_s and α are the cake solidosity and specific cake resistance). This information, in turn, can be used as a bench mark for improving existing and developing new procedures for determining cake solidosity and permeability.     

Mass transfer and hydrodynamic characteristics of a high aspect ratio self-ingesting reactor for gas-liquid operations

The mass transfer performance of a gas-liquid self-ingesting stirred reactor is reported both for coalescing and non-coalescing systems. The vessel features are a high aspect ratio and a rather narrow multiple-impeller draft tube, through which the gas phase is ingested and led down to the vessel bottom, where it is finely dispersed into the liquid rising in the annular portion of the vessel. Comparison is made between k_La values determined by several variants of the dynamic method, among which pure oxygen absorption in a previously de-gassed liquid phase. Results show that the gas-liquid mass transfer coefficient values obtained with the last approach are remarkably larger than those measured with all other techniques in which nitrogen is initially dissolved in the liquid phase. Possible reasons behind this discrepancy are discussed.The gas-liquid mass transfer performance of the investigated gas inducing contactor is finally compared with literature data on other self-inducing/ingesting devices. Comparison results encourage further development of the investigated apparatus.     

Decomposition of methane in the presence of carbon dioxide over Ni catalysts

Methane decomposition was carried out in the presence of CO₂ over the nickel catalysts. Spherical alumina and glycothermally synthesized zirconia were used as the catalyst supports. In the presence of CO₂, CH₄ was decomposed in the same fashion as pure methane decomposition, and fibrous carbons were formed. However, the formation of hydrogen, carbon monoxide, and water continued even after the apparent carbon formation ceased, and this phenomenon was observed irrespective of the support materials. These results showed a sharp contrast against the results for the pure methane decomposition where the catalyst was completely deactivated when the carbon formation ceased. Further carbon formation was observed when the feed gas containing CO₂ was replaced with pure CH₄. Mechanisms for these phenomena are discussed from the thermodynamical point of view.     

Numerical simulation of the dynamic flow behavior in a bubble column: A study of closures for turbulence and interface forces

Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, C_S, as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737-1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79-95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965-2000] to account for the bubble-induced turbulence in the k-ε model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with C_S=0.08 and 0.10 agree well with the measurements. When C_S is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio (H/D=3 and H/D=6), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22-24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results.     

A multi-scale approach for CFD calculations of gas-liquid flow within large size column equipped with structured packing

This work has been carried out in the framework of post-combustion CO₂ capture process development. Considering the huge amount of gases to be treated and the constraints in terms of pressure drop, it appears that the absorption column will be equipped with high efficiency high capacity packings such as structured packings. The present paper focuses on the CFD modellisation of the two-phase flow within this complex geometry. For limited computational resources reasons, it is presently impossible to run computations at large scales taking into account the gas-liquid interaction and the real geometry of the packing and original approaches must be developed. In the present work, a multi-scale approach is proposed. It first considers liquid-wall and liquid-gas interaction at small scale via two-phase flow calculations using the VOF method. Second, the latter results are used in three-dimensional calculations run at a meso-scale corresponding to a periodic element representative of the real packing geometry. Last, those results are further used at large scale in three-dimensional calculations with a geometry corresponding to a complete column. Results are compared with experimental data and with other CFD simulations in terms of liquid hold-up, pressure drop and unit operation. Some suggestions are made for further development.     

Analysis of the milling rate of pharmaceutical powders using the Distinct Element Method (DEM)

The milling behaviour of microcrystalline cellulose (MCC) and α-lactose monohydrate (αLM) in an oscillatory single ball mill has been analysed by using the Distinct Element Method (DEM). The experimental results suggest that the milling behaviour of αLM is more strongly influenced by the milling frequency as compared to MCC. A similar conclusion is also drawn from the DEM results. The milling behaviour of MCC and αLM is described by a first order rate process, and its rate constant, K_p, is found to correlate very well with the milling power, P_n, determined by the DEM simulation, except for the milling behaviour of αLM at 18 Hz. For the latter, there appears to be an incubation time after which the milling rate increases substantially. The results presented here provide a basis for predicting the milling behaviour of a material systematically based on the fundamental material properties and the machine dynamics without the need for extensive experiment and use of large quantities of materials.     

Theoretical and experimental study on hydrodynamic characteristics of fluidization in air-sand conical beds

This work was aimed at modeling hydrodynamic characteristics of fluidization in conical beds using quartz sand as the inert bed material and air as the fluidizing agent. The minimum fluidization velocity, u_mf, and the minimum velocity of full fluidization, u_mff, were determined by Peng and Fan's models modified for conical fluidized bed. Meanwhile, the pressure drop across a bed, Δp (including Δp_max and Δp_mff corresponding to u_mf and u_mff, respectively), was predicted by using modified Ergun's equations for variable superficial air velocity at an air distributor, u₀. The predicted results were validated by experimental data for some operating conditions. Effects of the sand particle size, cone angle and static bed height on the fluidization pattern and hydrodynamic characteristics are discussed. With the proposed models, the Δp-u₀ diagram were obtained with rather high accuracy for the conical air-sand beds of 30-45^° cone angles and 20-30 cm static bed heights, when using 300- sand particles. For the predicted u_mf and u_mff, the relative computational errors were found to be within 20% for wide ranges of operating variables, whereas Δp_max and Δp_mff could be predicted with lower (10-15%) relative errors. With higher cone angles and/or bed heights, the computational accuracy was found to deteriorate.