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

Kinetic studies of CO<inf>2</inf> methanation over a Ni/γ-Al<inf>2</inf>O<inf>3</inf> catalyst using a batch reactor

The methanation of CO₂ was investigated over a wide range of partial pressures of products and reactants using a gradientless, spinning-basket reactor operated in batch mode. The rate and selectivity of CO₂ methanation, using a 12 wt% Ni/γ-Al₂O₃ catalyst, were explored at temperatures 453 – 483 K and pressures up to 20 bar. The rate was found to increase with increasing partial pressures of H₂ and CO₂ when the partial pressures of these reactants were low; however, the rate of reaction was found to be insensitive to changes in the partial pressures of H₂ and CO₂ when their partial pressures were high. A convenient method of determining the effect of H₂O on the rate of reaction was also developed using the batch reactor and the inhibitory effect of H₂O on CO₂ methanation was quantified. The kinetic measurements were compared with a mathematical model of the reactor, in which different kinetic expressions were explored. The kinetics of the reaction were found to be consistent with a mechanism in which adsorbed CO₂ dissociated to adsorbed CO and O on the surface of the catalyst with the rate-limiting step being the subsequent dissociation of adsorbed CO.JYL was funded by the Cambridge International Scholarship Scheme. The Cambridge Philosophical Society, the Lundgren Research Award and Corpus Christi College are also gratefully thanked for contributing to the support of his PhD studies.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.ces.2015.10.02

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

11-interval PFG pulse sequence for improved measurement of fast velocities of fluids with high diffusivity in systems with short T2(∗).

Magnetic resonance (MR) was used to measure SF6 gas velocities in beds filled with particles of 1.1 mm and 0.5 mm in diameter. Four pulse sequences were tested: a traditional spin echo pulse sequence, the 9-interval and 13-interval pulse sequence of Cotts et al. (1989) and a newly developed 11-interval pulse sequence. All pulse sequences measured gas velocity accurately in the region above the particles at the highest velocities that could be achieved (up to 0.1 ms(-1)). The spin echo pulse sequence was unable to measure gas velocity accurately in the bed of particles, due to effects of background gradients, diffusivity and acceleration in flow around particles. The 9- and 13-interval pulse sequence measured gas velocity accurately at low flow rates through the particles (expected velocity <0.06 ms(-1)), but could not measure velocity accurately at higher flow rates. The newly developed 11-interval pulse sequence was more accurate than the 9- and 13-interval pulse sequences at higher flow rates, but for velocities in excess of 0.1 ms(-1) the measured velocity was lower than the expected velocity. The increased accuracy arose from the smaller echo time that the new pulse sequence enabled, reducing selective attenuation of signal from faster moving nuclei.CMB acknowledges the Gates Cambridge Trust for funding his research.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jmr.2016.01.02

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

Improving hydrogen yields, and hydrogen: Steam ratio in the chemical looping production of hydrogen using Ca<inf>2</inf>Fe<inf>2</inf>O<inf>5</inf>

A thermodynamic property of Ca2Fe2O5 was exploited to improve the efficiency of the steam-iron process to produce hydrogen. The ability of reduced Ca2Fe2O5 to convert a higher fraction of steam to hydrogen than chemically unmodified Fe was demonstrated in a packed bed. At 1123 K, the use of Ca2Fe2O5 achieved an equilibrium conversion of steam to hydrogen of 75%, in agreement with predicted thermodynamics and substantially higher than that theoretically achievable by iron oxide, viz. 62%. Furthermore, in Ca2Fe2O5, the full oxidation from Fe(0) to Fe(III) can be utilised for hydrogen production – an improvement from the Fe to Fe3O4 transition for unmodified iron. Thermodynamic considerations demonstrated in this study allow for the rational design of oxygen carriers in the future. Modifications of reactors to capitalise on this new material are discussed.Dr Matthew T. Dunstan is acknowledged for help with the XRD analysis. M.S.C.C acknowledges financial support from an EPSRC Doctoral Training Grant. W.L and Y.Y acknowledge funding from the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.cej.2016.03.13

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

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

In situ studies of materials for high temperature CO2 capture and storage.

Carbon capture and storage (CCS) offers a possible solution to curb the CO2 emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO2 capture, such as the CaO-CaCO3 system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation-regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions

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