Solids backmixing and entrainment in the splash zone of large-scale fluidized bed boilers

This work studies the fluid dynamics of the solids in the splash zone of fluidized bed furnaces, with focus set on solids back-mixing and solids entrainment in order to enhance the understanding and prediction of the solids flow in the bottom region of the furnace. Experimental results show the establishment of a splash zone also for runs in absence of a dense bottom bed. A simple model assuming ballistic trajectories of the ejected solids is shown to satisfactorily estimate the solids back-mixing rate. The flux of non-backmixed solids, which are entrained from the bottom region, is found to be unaffected by the bottom wall configuration (tapered/vertical) for a given gas flow. Finally, an empirical expression is proposed for the solids entrainment from the bottom region which covers wide operational and unit size ranges

Solids flow patterns in large-scale circulating fluidised bed boilers: Experimental evaluation under fluid-dynamically down-scaled conditions

This work aims at gaining novel knowledge of the mechanisms governing the solids flow pattern in the furnace of large-scale Circulating Fluidised Bed (CFB) boilers. A fluid-dynamically down-scaled unit resembling an existing 200-MWth CFB boiler was built and validated against full-scale data. The extensive experimental campaign showed, among others, that the presence or absence of a dense bed governs the entrainment of solids from the bottom region of the furnace, and that the back-flow of solids at the exit region is negligible at low gas velocities although it quickly becomes significant with an increase in gas velocity. Thus, it is shown that the estimation of the external solids flux by the top flux in the furnace is not generally valid

Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed

A semiempirical model for the axial mixing of fuel particles in the dense region of a fluidized bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499) that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solid zone, a zone with sinking emulsion solids, and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upward velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to hot large-scale conditions. When applying values measured in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499), the modeled axial mixing of fuel tracers shows good agreement with the experimental data.\ua0\ua9 2020 American Chemical Society

A comparative exergy-based assessment of direct air capture technologies

The 6th Assessment of the IPCC underlined the need for urgent measures for carbon dioxide removal from the atmosphere, so as to meet the 1.5 \ub0C goal by the end of this century. One option to achieve this is direct air capture (DAC) technologies. This work assesses the thermodynamic performances of different categories of DAC technologies, i.e., adsorption-based, absorption-based, ion exchange, and electrochemical. An exergy analysis is performed on the DAC processes in each category to identify hotspots for efficiency loss within the system. The results show that the consumption of materials is responsible for 5–40% of the exergy consumption of the most-developed DAC processes. Despite their greater use of materials compared to absorption-based processes, adsorption-based processes, together with ion exchange technologies, have the highest exergy efficiencies of the DAC technologies investigated. Moreover, the results highlight the importance of limiting material consumption and electrifying large-scale DAC plants, which cannot run exclusively on waste heat from industrial processes

Experimental investigation of the lateral mixing of large and light particles immersed in a fluidized bed

Fluidized bed reactors for solid fuel conversion are characterized by the presence of a small fraction of fuel particles that are significantly larger (usually 1–2 orders of magnitude larger) and lighter (2–20-fold less dense) compared to the bulk solids. This difference in physical properties strongly influences the mixing of the fuel particles and therefore affects the mass, momentum and heat transfers between the fuel particles and the surrounding bed. This work uses Magnetic Particle Tracking (MPT) to acquire highly resolved trajectories for single tracer particles immersed in a bubbling fluidized bed operated under ambient conditions and with a cross-sectional area of 0.45 m2. This bed size is sufficiently large to abrogate the influence of the reactor walls, allowing data post-processing to study the free movement of the tracer particle, which has not been available to date. This required the enhancement of the MPT system from that in previous works: 12 sensors and a communication protocol in series are here applied, which showed good performance in both spatial accuracy (1 mm) and time resolution (100 Hz). The bed material used in the experiments was glass beads (mean particle size of 106 \ub5m, particle density of 2,486 kg/m3). Two different tracer particles, with diameters of 18 mm but with different densities (572 kg/m3 and 1,015 kg/m3) were used to mimic the sizes and densities of the solid fuel particles. Fluidization velocity was varied within 0.2–0.7 m/s and two fixed bed heights (50 mm and 130 mm) were tested. Based on the trajectories, dispersion coefficients were calculated for quantitative evaluation of the solids mixing. The results reveal that increased bed height yields higher dispersion coefficients with a higher sensitivity for fluidization velocity. The properties of the tracer particles appear, within the tested range, to exert little impact on its lateral dispersion. From the velocity maps generated, a swirling pattern was observed in the vicinity of the walls, while zones of preferential ascendent or descendant movement were observed in the cross-section centre, although clearly defined mixing cells were not exhibited

Biogas upgrading through calcium looping: Experimental validation and study of CO2 capture

The calcium looping technology is one of the most promising technologies for capturing and storing CO2. This technology has been evaluated with a variety of sorbents and conditions in previous works, but the inlet CO2-ladden gas has typically been a flue gas from combustion, which typically has a composition of 10–20% CO2 and 80–90% N2. On the other side, the performance of the calcium looping process for CO2 capture of other gases (i.e., biogas or gases resulting from hydrothermal carbonization) remains largely unstudied. In this work, this knowledge gap is assessed through evaluating the performance of the calcium looping process for biogas (synthesized as 40% CO2, 60% CH4) in terms of carbonation conversion. This experimental study investigates the impact of: (1) using an inlet gas composition representative for biogas instead of combustion flue gas; (2) different biogas compositions; (3) the carbonation temperature; (4) the cooling-down and heating-up of the sorbent material between the reactor and ambient temperatures within cycles; (5) the atmosphere composition during calcination; and (6) the solids particle size. The main result obtained is that the overall CO2-capture performance of calcium looping improves when using biogas as inlet CO2-ladden gas, in comparison with combustion flue gas. One main contribution to this improved performance is shown to be the presence of secondary reactions (i.e., dry reforming, methanation). The impact of the CH4 to CO2 ratio tested is not remarkable, showing that the potentialities of the process in this aspect can be adapted to several biogas producing feedstocks

Investigation on the Performance of Volatile Distributors with Different Configurations under Different Fluidization Regimes

The uniform horizontal distribution of volatiles over the cross section of a fluidized bed with the purpose to obtain good contact between volatiles and bed materials is a key issue to improve the gas conversion in the fuel reactor of chemical looping combustion of solid fuels. The effectiveness of the volatile distributor (VD) concept on the lateral distribution of volatiles in a fluidized bed has been investigated under different operational conditions using a cold-flow model. Furthermore, the performance of the VD has been examined using different configurations of the holes used to distribute the volatiles. The fluidization regimes, i.e., single bubble regime, with only one large bubble formed at a time at the bottom bed, exploding bubble regime, with irregular bubbles containing more particles, and multiple bubble regime, with many small bubbles formed and distributed in the bed, are determined by visual observation of the bottom riser and analysis of the pressure fluctuations, including frequency analysis. The VDs with uneven hole arrangements, which have less distribution holes at the simulated volatile inlet side and a larger open area far from the inlet, provide a more even horizontal distribution of volatiles compared to the VD with equally distributed holes. A larger simulated volatile flow and less open area of the VD increase the pressure drop over the distribution holes and improve the horizontal distribution. In general, the VD gives a more uniform distribution of the volatiles under the exploding bubble regime and better distribution in the single bubble regime compared to the multiple bubble regime. However, the bottom leakage, i.e., the volatile leakage from the bottom of the VD, should be considered, especially in the single bubble regime

Determination of the Apparent Viscosity of Dense Gas-Solids Emulsion by Magnetic Particle Tracking

When designing fluidised bed units a key to ensure efficient conversion is proper control of the mixing of the fuel in both lateral and axial directions in the bed. In order to mechanistically describe the mixing of fuel particles in a fluidised bed, there is a need to determine the apparent viscosity of thegas-solids emulsion, which determines the drag on the fuel particles. In this work the apparent viscosity of a bed of spherical glass beads and air at minimum fluidisation was determined by means of the falling sphere method. Hereto the drag of the bed on a single immersed object was obtained by measuring the velocity of a negatively buoyant tracer with magneticparticle tracking (MPT). MPT allows for highly temporally and spatially resolved trajectories (10-3 s and 10-3 m, respectively) in all 3-dimensions. The bed consisted of glass beads with a narrow size distribution (215 to 250 μm) and tracers with a size from 5 to 20 mm and densities from 4340 to 7500kg/m3 were used. Hence, the literature, which typically covers data for velocities lying within or just above the Stoke flow regime (0.002 < Re < 2.0) could be expanded to Re numbers (53 to 152) well within the transition flow regime. The drag and apparent viscosity was compared to different fluidmodels and agreed well with the Newtonian model, when taking into account possible effects of the bed walls. Comparing the drag coefficient of data of free falling spheres and data of spheres falling with controlled velocities, the latter showed a dependence on the product of tracer diameter andfalling velocity, dput, while the former was constant over dput. This indicates the method with controlled falling velocities to be intrusive and influencing the result of the apparent viscosity of the bed. Using the free falling sphere method this work obtained an apparent viscosity of 0.24 Pa s, which isconsistent with values found in earlier literature for an emulsion of air and sand of similar size and density

Solids back-mixing in the transport zone of circulating fluidized bed boilers

This work investigates the back-mixing of solids in the transport zone of large-scale circulating fluidized bed (CFB) boilers, with the aims of identifying and evaluating the governing mechanisms and providing a mathematical description based on a solid theoretical background rather than on purely empirical correlations. In addition, transient Direct Numerical Simulation (DNS) modeling is used to identify the mechanism that drives migration of the solids from the dilute up-flow in the core region to the down-flow at the furnace walls. Previously published concentration and pressure profiles are collated and analyzed through modeling of the steady-state mass balance of the dispersed solids in the transport zone. The study shows that solids back-mixing at the furnace wall layers is limited (hence governed) by the core-to-wall layer mass transfer transport mechanism rather than by the lateral movement of solids within the core region. The latter is shown by the 3-dimensional (3D) mass balance model, and the transient DNS modeling indicates that this is due to a turbophoresis mechanism. We also show that the use of Pe-numbers to describe the lateral solids dispersion is not straightforward but rather depends on the unit scale, and that Pe-numbers < 26 are needed to yield the solids back-mixing rates measured in large-scale CFB boilers. Finally, we propose a mathematical expression for the core-to-wall layer mass transfer coefficient derived from a Sherwood number (Sh)-correlation fitted to measured values of the characteristic decay constant that result from the solids back-mixing. This expression shows better agreement with the large-scale measurements than do the expressions given in the literature

Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions

Dual fluidized bed systems are indispensable for future energy systems that require solids cycling between different atmospheres. However, controlling the residence time of solids in the reactor, which is crucial for controlling the heat and mass transfer of the fuel, is a significant challenge. This study investigates four experimental techniques to quantify the horizontal flow of solids fluidized under bubbling conditions: integral mass accumulation; differential mass accumulation; thermal tracing; and magnetic solids tracing. Integral mass accumulation entails collecting bed material using a defluidized box within a given time period. Differential mass accumulation measures the material accumulation rate in a section of the bed that is monitored using pressure measurements. Thermal tracing calculates the solids flow rate by solving the heat balance to match the temperature field captured by a thermographic camera. Magnetic solids tracing involves injecting a batch of magnetic tracer solids into the reactor and then measuring the residence time distribution using impedance coils. The experiments were conducted under down-scaled conditions that resemble large-scale operations with a length scaling factor of 0.12. For this study, three operational parameters were varied: the fixed bed height; the volumetric flow rate of the conveying air; and the fluidization velocity in the bed. The horizontal solids circulation rates achieved ranged from 1.7 710−4 to 10 kg/m\ub7s, corresponding to 1.2 710−3 to 70 kg/m\ub7s on a hot up-scaled basis, which is a relevant range to indirect biomass gasification in an industrial setting. The three selected operational parameters led to increases in the horizontal solids flow. While all four methods replicated the trends, quantitative variations in the measured circulation rates occurred due to the inherent characteristics of the methods. High circulation rates resulted in a continuous decrease in the solids inventory, leading to an underestimation of the circulation rate when using the integral mass accumulation method. The accuracy of the differential mass accumulation method relied on transient pressure measurements, which were less-effective at low solids flow rates. Conversely, the accumulation time required for pressure measurements was reduced at high circulation rates, resulting in uncertainties in the analysis. The accuracy of the thermal tracing method decreased drastically with higher solids circulation, resulting in an overestimation of the circulation rate. Moreover, low circulation rates adversely affected the accuracy of the magnetic solids tracing by producing barely discernible tracer concentration gradients