Modelling rates of gasification of a char particle in chemical looping combustion

Rates of gasification of lignite char were compared when gasification with CO2 was undertaken in a fluidised bed of either (i) an active Fe-based oxygen carrier used for chemical looping or (ii) inert sand. The kinetics of the gasification were found to be significantly faster in the presence of the oxygen carrier, especially at temperatures above 1123 K. An analytical solution assuming pseudo-binary diffusion of species was developed to account for external and internal mass transfer and for the effect of the looping agent. The model also included the effects of the evolution of the pore structure at different conversions. The results are compared with a full numerical model using the Stefan-Maxwell equations. Excellent agreement was observed between the rates predicted by the two models and those observed experimentally at T ≤ 1123 K. At 1173 K, the pseudo-binary model predicted slightly higher rates than the full numerical solution. It was found that a significant share of the error of the predicted rates with the analytical solution was caused by an underestimation of intraparticle diffusional resistance rather than by assuming a pseudo-binary system external to the particle. Both models suggested that the presence of Fe2O3 led to an increase in the rate of gasification because of the rapid oxidation of CO by the oxygen carrier to CO2. This resulted in the removal of CO and maintained a higher mole fraction of CO2 in the mixture of gas around the particle of char, i.e. within the mass transfer boundary layer surrounding the particle. This effect was most prominent at ~20% conversion when (i) the surface area for reaction was a maximum and (ii) because of the accompanying increase in porosity, intraparticle resistance to gas mass transfer within the particle of char had fallen, compared with that in the initial particle.EPSRCThis is the author accepted manuscript. The final version is now available at http://www.sciencedirect.com/science/article/pii/S1540748914003150

Kinetics of oxygen uncoupling of a copper based oxygen carrier

Here, an oxygen carrier consisting of 60 wt% CuO supported on a mixture of Al_2O_ 3 and CaO (23 wt% and 17 wt% respectively) was synthesised by wet-mixing powdered CuO, Al(OH)_3 and Ca(OH)_2, followed by calcination at 1000⁰C. Its suitability for chemical looping with oxygen uncoupling (CLOU) was investigated. After 25 repeated redox cycles in either a thermogravimetric analyser (TGA) or a laboratory-scale fluidised bed, (with 5 vol% H_2 in N_2 as the fuel, and air as the oxidant) no significant change in either the oxygen uncoupling capacity or the overall oxygen availability of the carrier was found. In the TGA, it was found that the rate of oxygen release from the material was controlled by intrinsic chemical kinetics and external transfer of mass from the surface of the particles to the bulk gas. By modelling the various resistances, values of the rate constant for the decomposition were obtained. The activation energy of the reaction was found to be 59.7 kJ/mol (with a standard error of 5.6 kJ/mol) and the corresponding pre-exponential factor was 632 m^3/mol/s. The local rate of conversion within a particle was assumed to occur either (i) by homogeneous chemical reaction, or (ii) in uniform, non-porous grains, each reacting as a kinetically-controlled shrinking core. Upon cross validation against a batch fluidised bed experiment, the homogeneous reaction mode l was found to be more plausible. By accurately accounting for the various artefacts (e.g. mass transfer resistances) present in both TGA and Fluidised bed experiments, it was possible to extract a consistent set of kinetic parameters which reproduced the rates of oxygen release in both experiments.This work is supported by the Engineering and Physical Sciences Research Council (EPSRC grant EP/I010912/1) and The Cambridge Commonwealth, European & International Trust as well as Selwyn College, University of Cambridge. The authors would also like to thank Mohammad Ismail for the XRD analysis and Zlatko Saracevic for the nitrogen adsorption analysis.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.apenergy.2015.10.00

Limitations on Fluid Grid Sizing for Using Volume-Averaged Fluid Equations in Discrete Element Models of Fluidized Beds

Bubbling and slugging fluidization were simulated in 3D cylindrical fluidized beds using a discrete element model with computational fluid dynamics (DEM-CFD). A CFD grid was used in which the volume of all fluid cells was equal. Ninety simulations were conducted with different fluid grid cell lengths in the vertical (dz) and radial (dr) directions to determine at what fluid grid sizes, as compared to the particle diameter (dp), the volume-averaged fluid equations broke down and the predictions became physically unrealistic. Simulations were compared with experimental results for time-averaged particle velocities as well as frequencies of pressure oscillations and bubble eruptions. The theoretical predictions matched experimental results most accurately when dz = 3-4 dp, with physically unrealistic predictions produced from grids with lower dz. Within the valid range of dz, variations of dr did not have a significant effect on the results.CMB acknowledges the Gates Cambridge Trust for funding his research.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acs.iecr.5b0318

Use of a Chemical-Looping Reaction to Determine the Residence Time Distribution of Solids in a Circulating Fluidized Bed

The residence time distribution (RTD) of solids in various sections of a circulating fluidized bed (CFB) is of great importance for design and operation but is often difficult to determine experimentally. A noninvasive method is described, for which the RTD was derived from temporal measurements of the temperature following the initiation of a chemical-looping reaction. To demonstrate the method, a CuO-based oxygen carrier was used in a small-scale CFB, and measurements were made in the fuel reactor, operated as a bubbling fluidized bed. The measurements were fitted to the tanks-in-series model, modified to account for heat losses from the reactor. There was excellent agreement between the model and the experiment. Limitations and further improvements of the method are discussed, also with respect to larger reactors.This work is supported by the Engineering and Physical Sciences Research Council (EPSRC Grant EP/I010912/1).This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/ente.20160014

High selectivity epoxidation of ethylene in chemical looping setup

We describe the remarkable performance of a new catalyst for the chemical looping (CL-) epoxidation of ethylene, performed at atmospheric pressure and without any promoters added to either the catalyst or the feed gas. To undertake the CL-epoxidation of ethylene, silver was used as the catalyst, supported on either the perovskite SrFeO3 or Ce-modified SrFeO3. Here, the oxygen for the reaction is supplied to the silver catalyst from the active solid support, not from the gas stream. When the support has been reduced and depleted of oxygen, it is regenerated in a separate step with air, which makes the process cyclic and closes a chemical loop. Thus, there is no need to co-feed gaseous oxygen along with the ethylene feed, an important improvement in safety. Two methods were used to synthesise Ce-modified materials, employing either (i) the mechanical mixing of powdered CeO2 and the solid precursors of the perovskite, or (ii) the impregnation of a solution of cerium nitrate into solid particles of SrFeO3. In both cases, the materials were calcined to produce a mixture of CeO2 and SrFeO3. Both CeO2-SrFeO3 materials surpassed the unmodified SrFeO3 for CL-epoxidation. For the CeO2-SrFeO3 prepared by mechanical mixing, the production of ethylene oxide was stable over 15 cycles, giving 60% selectivity at 10% conversion of C2H4. In contrast, the material prepared by impregnation gave up to 85% selectivity but only in the first cycle of reduction, with the performance degrading over subsequent cycles. The reported results are better than the 50% selectivity achieved for the classical epoxidation using pure silver as the catalyst and feeds of gaseous ethylene and oxygen, without reaction promoters

Sensitivity of chemical-looping combustion to particle reaction kinetics

A simple simulation for chemical-looping combustion (CLC) is discussed: two, coupled fluidised reactors with steady circulation of particles of oxygen carrier between them. In particular, the sensitivity of CLC to different particle kinetics is investigated. The results show that the system is relatively insensitive to different kinetics when the mean residence time of particles in each reactor is greater than the time taken for them to react completely.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0009250916302779

Chemical looping epoxidation

Chemical looping epoxidation of ethylene was demonstrated, whereby the sole oxidant was a solid oxygen carrier, 15 wt% Ag supported on SrFeO3. Ethylene reacted with a bed of carrier particles, without any O2(g) in the feed, to produce ethylene oxide (EO) and CO2. Following the reduction by the C2H4 of the SrFeO3, it was regenerated by passing air through the bed. The rate of reoxidation was slow, with full regeneration being achieved only by prolonged oxidation at elevated temperatures. A striking synergy between Ag and SrFeO3 was observed solely when they were in intimate contact, suggesting a basis for a proposed reaction mechanism

Significance of gasification during oxy-fuel combustion of a lignite char in a fluidised bed using a fast UEGO sensor

In oxy-fuel combustion, fuel is combusted in a mixture of O₂ and recycled flue gas, i.e. the N₂ is replaced by CO₂ with the O₂ supplied from an air separation unit. The resulting gas consists largely of steam and CO2, which would be ready for sequestration when dried. In this work, the rate of reaction of particles of lignite char, typically 1200 μm diameter, in a fluidised bed reactor was determined using mixtures of O₂ with either CO₂ (“oxy-fuel”) or N₂. A universal exhaust-gas oxygen (UEGO) sensor enabled rapid measurements of the oxygen partial pressures in the off-gas, representing a novel application of this type of sensor. It was found that the rate of combustion of the particles in oxy-fuel is much more sensitive to temperature than in the equivalent O₂ and N₂ mixture. This is because for bed temperatures >∼1000 K particle combustion in mixtures of N₂ and O₂ is rate controlled by external mass transfer, which does not increase significantly with temperature. In contrast, using oxy-fuel, as the temperature increases, gasification by the high concentrations of CO₂ present becomes increasingly significant. At low temperatures, e.g. ∼1000 K, rates of combustion in oxy-fuel were lower than those in mixtures of O₂ and N₂ containing the same mole fraction of O₂ owing, primarily, to the lower diffusivities of O2 in CO₂ compared to O₂ in N₂ under conditions at which external mass transfer is still a significant factor in controlling the rate of reaction. At higher temperatures, e.g. 1223 K, oxy-fuel combustion rates were significantly higher than those in O₂ and N₂. The point at which oxy-fuel combustion becomes more rapid than in mixtures of O₂ and N₂ depends not only on temperature but also on the ratio of O₂ to CO₂ or N₂, respectively. A numerical model was developed to account for external mass transfer, changes in the temperature of the particle and for the effect of gasification under oxy-fuel conditions. The model confirmed that, at high temperatures, the high concentration of CO₂ at the surface of the burning particle in the oxy-fuel mixture led to an increase in the overall rate of carbon conversion via CO₂ + C → 2CO, whilst the rate of reaction with O₂ was limited by mass transfer. Good agreement was observed between the rates predicted by the numerical model and those observed experimentally.Financial support from the Engineering and Physical Sciences Research Council (Grant reference number: EP/G063265/1) and the Consejo Nacional de Ciencia y Tecnología (CONACYT) is also acknowledged.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.fuel.2014.10.02

The effect of different particle residence time distributions on the chemical looping combustion process

A model for chemical looping combustion has been developed to allow the effect of different residence time distributions of oxygen carrier particles in the air and fuel reactors to be investigated. The model envisages two, coupled fluidised bed reactors with steady circulation of particles between them. The results show that the process is sensitive to the residence time distributions, particularly when the mean residence time of particles in the reactors is similar to the time required for them to react completely. Under certain operating conditions, decreasing the variance of the residence time distribution, leads to a greater mean conversion of the particles by the time they leave the reactors and higher mean rates of reaction in the beds. In this way the required inventory and circulation rate of solids could be reduced, which would lower the capital and operating costs of a CLC process. Since the residence time distribution of solids is important, it should be taken into account when modelling or designing a chemical looping combustion process, e.g. by using a tanks-in-series model. This work indicates that if the number of tanks, N ≤ 5, knowing N to the nearest integer is generally sufficient, unless a high degree of accuracy is needed. As N increases, the sensitivity of the coupled system decreases, so for N > 5, knowing the value to the nearest 5 or 10 tanks is sufficient. This is valid whether N is the same or different in the two reactors. Chemical looping combustion is one example of a reactor-regenerator system, so the results are also relevant for other processes of this type, such as fluidised catalytic cracking

Exploration of the material property space for chemical looping air separation applied to carbon capture and storage

Oxy-fuel combustion is one route to large scale carbon capture and storage. Fuel is combusted in oxygen rather than air, allowing pure CO2 to be captured and sequestered. Currently, the required oxygen is produced via cryogenic air separation, which imposes a significant energy penalty. Chemical looping air separation (CLAS) is an alternative process for the production of oxygen, and relies on the repeated oxidation and reduction of solid oxygen carriers (typically metal oxides). The energy efficiency is governed by the thermodynamic properties of the oxygen carrier material, and how well the CLAS process can be heat-integrated with the process consuming oxygen. In this study, key thermodynamic properties have been identified and assessed using a steady state model of a CLAS-oxy-fuel power plant. It is demonstrated that energy penalties as low as 1.5 percentage points can be obtained for a narrow range of material properties. Based on density functional theory calculations, 14 oxygen carrier systems, which are novel or have received little attention, have been identified that could potentially achieve this minimal energy penalt