Dual Fluidized Bed Pilot Plant Research Articles

Development and numerical investigation of a DDPM-KTGF model for modelling flow hydrodynamics and heat transfer phenomena in a bubbling calciner reactor

In this work, a DDPM-CFD model is developed in ANSYS® Fluent for the simulation of the indirectly heated, bubbling calciner of the 300kWth dual fluidized bed pilot plant located at Technische Universität Darmstadt. The calciner is heated by 72 heat pipes that carry the heat from an external combustor. Regarding the heat transfer, both convection and radiation are considered in the model. Regarding the modelling of the drag forces, flow heterogeneity aspects are considered by applying the Energy Minimization Multi-Scale (EMMS) scheme. However, the application of DDPM in such dense, bubbling flows considered here proved to be challenging, demanding several advancements and customizations. To this end, this study proposes mainly three advancements; i) The inter-particle forces are modelled using custom user defined functions incorporating both normal and tangential components. In particular, KTGF-based correlations are applied at dilute regions, while at dense regions the solid pressure is modelled according to Harris and Crighton, and the shear and bulk viscosities are modelled using correlations based on the plastic theory. ii) It is shown that, in order to correctly predict the overall pressure drop, the Lagrangian particle momentum equation should be reformulated according to Model A formulation to be consistent with the solved gas-phase momentum equation. iii) In order to capture the correct heat flux levels, the heat flux on the heat pipe heat exchanger walls is modelled in the Eulerian reference frame scaling the temperature gradient on the wall to take into account the thin thermal boundary layer.Τhe DDPM results are compared against those of an already validated Eulerian TFM model, in terms of calculated flow patterns, volume fractions, pressure profiles and heat fluxes. In addition, both models are assessed for their computational cost. The developed DDPM model predicts practically the same overall pressure drop with the TFM model. However, it overpredicts the bed length by 12% when using the default grid. This reduces to 6% when using a finer grid comprising double computational cells. As for the heat fluxes and the calcination reaction rate, both models predict similar levels and their differences are attributed to the differences in hydrodynamics.

Parametric Study on the Adjustability of the Syngas Composition by Sorption-Enhanced Gasification in a Dual-Fluidized Bed Pilot Plant

Finding a way for mitigating climate change is one of the main challenges of our generation. Sorption-enhanced gasification (SEG) is a process by which syngas as an important intermediate for the synthesis of e.g., dimethyl ether (DME), bio-synthetic natural gas (SNG) and Fischer–Tropsch (FT) products or hydrogen can be produced by using biomass as feedstock. It can, therefore, contribute to a replacement for fossil fuels to reduce greenhouse gas (GHG) emissions. SEG is an indirect gasification process that is operated in a dual-fluidized bed (DFB) reactor. By the use of a CO2-active sorbent as bed material, CO2 that is produced during gasification is directly captured. The resulting enhancement of the water–gas shift reaction enables the production of a syngas with high hydrogen content and adjustable H2/CO/CO2-ratio. Tests were conducted in a 200 kW DFB pilot-scale facility under industrially relevant conditions to analyze the influence of gasification temperature, steam to carbon (S/C) ratio and weight hourly space velocity (WHSV) on the syngas production, using wood pellets as feedstock and limestone as bed material. Results revealed a strong dependency of the syngas composition on the gasification temperature in terms of permanent gases, light hydrocarbons and tars. Also, S/C ratio and WHSV are parameters that can contribute to adjusting the syngas properties in such a way that it is optimized for a specific downstream synthesis process.

CFD modeling of the flow dynamics and gasification in the combustor and gasifier of a dual fluidized bed pilot plant

Numerical simulation of the dense gas-solid reactive flow in a pilot-scale dual fluidized bed for biomass gasification is conducted with large eddy simulation coupled with multiphase particle-in-cell approach. After validating the numerical results with the experimental data, the hydrodynamics of different components operating under different flow regimes are explored. The results show that dense bottom and dilute upper exist in the combustor, while the elutriation of fine particles appears in the freeboard of gasifier. Strong swirling flow together with the fishtailing phenomenon of gas phase appear in the cyclone. Gas density is large in the lower part of combustor, and decreases along the horizontal direction. Large thermal conductivity and specific heat capacity of gas phase exist in the gasifier due to the large mass fraction of low-temperature steam. Biomass inlet results in a large mass fraction of gaseous species in the right part of gasifier. The size distribution of heat carrier negligibly affects the gas thermal property in the lower part of both the combustor and gasifier.

Experimental flow structure analysis in a 1 MWth circulating fluidized bed pilot plant

A water-cooled dual-channel capacitance probe system was developed for in-furnace measurements of solids concentration and solids velocity in a 1MWth dual fluidized bed pilot plant. Measurements were conducted in both reactors of the pilot plant during carbonate looping and combustion test campaigns. Solids velocities were derived by cross correlating the signals of the two channels of the capacitance probe. Thus, horizontal profiles of solids concentration and velocity were simultaneously determined in the reactors. These profiles show a strong core annulus flow structure in both reactors of the pilot plant. The influence of bed material characteristics, operating conditions as well as riser geometries on the particle flow structure is evaluated.

Measurement of solids circulation rate in a high-temperature dual fluidized bed pilot plant

A number of fluidized bed reactor processes operating at high temperature require that solid particles be circulated back and forth between two reactor vessels. Since the circulation rate strongly affects mass and energy balances, and therefore greatly influences hydrodynamics and performance of the system, a reliable technique for its accurate measurement would be helpful in monitoring and modeling the process. However, there are no reported techniques suitable for measuring solid circulation rates at elevated temperatures typical of gasification systems.A novel thermal-tracing technique was developed for measuring the solids circulation rate between two reactors. Particles at room temperatures (cold particles) are injected into a downward-moving packed bed of solids at elevated temperature (hot particles), creating reduced-temperature zones inside the moving bed. The transit time of the cold-particle-clusters between pairs of thermocouples is determined by cross correlation allowing the flux to be estimated. The technique was shown to provide sensitive and reproducible data for a cold model unit with injection of dry ice. The technique was then applied to determine the solids circulation rate between the bubbling bed gasifier and the riser combustor of a pilot scale dual fluidized bed gasification system. A number of conditions are imposed on the data to eliminate unsatisfactory data at high temperatures. Data which satisfy the discrimination criteria are shown to lead to measured solids circulation fluxes up to 133kg/m2-s at temperatures up to 856°C in the gasifier test section. The technique provides high-temperature solids circulation rate information beyond the capability of other techniques.

Attrition of CaO-based pellets in a 0.1 MWth dual fluidized bed pilot plant for post-combustion CO2 capture

This work investigates the attrition of CaO-based pellets in pilot-scale calcium looping cycles for post-combustion CO2 capture. Two groups of pellets with size 250–600μm were prepared from Canadian and Spanish limestones using 10% calcium aluminate cement as a binder. The results showed that all groups of pellets experienced a significant extent of attrition as ~50% of the sorbent fed to the system was collected as fragments with particle size smaller than 250μm. The attrition of pellets was found to be comparable to that of limestone tested under similar conditions. The results also indicated the same tendency of attrition for all groups of pellets, with attrition extent being relatively insensitive to limestone type used for pelletization or injection method of sorbent to the system. The attrition trends suggest that the improvement in the mechanical strength of the pellets was marginal compared to that of limestone.

Co-gasification of biomass and plastics: pyrolysis kinetics studies, experiments on 100 kW dual fluidized bed pilot plant and development of thermodynamic equilibrium model and balances.

Thermo-gravimetric analysis (TGA) of volatilization reaction kinetics for 50 wt.% mixtures of plastics (PE) and biomass (wood pellets) as well as for 100 wt.% plastics was conducted to predict decomposition times at 850°C and 900°C using iso-conversional model method. For mixtures, agreement with residence time of dual fluidized bed (DFB) reactor, treated as continuous stirred-tank reactor (CSTR), was obtained at large conversions. Mono-gasification of plastics and its co-gasification with biomass were performed in DFB pilot plant, using olivine as heterogeneous catalyst and heat transfer agent. It was found that co-gasification led to successful thermochemical conversion of plastics as opposed to mono-gasification. Unknown flow rates were determined applying nonlinear regression to energy and mass balances acknowledging combustion fuel, air, steam, feedstock, but also exiting char, tar, steam and other components in DFB gasification unit. Water-gas shift equilibrium and methanol synthesis requirements were incorporated into gasification model, based on measurements.

Cold model hydrodynamic studies of a 200 kWth dual fluidized bed pilot plant of calcium looping process for CO2 Capture

The calcium looping (CaL) process is a post combustion CO2 capture technology which is currently under development, offering power plants a low cost and energy efficient solution for carbon capture. At IFK, University of Stuttgart, a 200kWth CaL dual fluidized bed (DFB) pilot plant has been built consisting of two circulating fluidized bed (CFB) reactors. This study presents detailed results of tests conducted on a hydrodynamically scaled cold model of the 200kWth CaL DFB facility. The preliminary aim of the cold model studies was to check the workability of the major novelty of this facility which is the implementation of two cone valves to control the solid looping rate between the two CFBs. Furthermore, initial cold model tests, based on the relative CaL process boundary conditions, determined the suitability of the 200kWth CaL DFB system and further tests suggested design improvements for the pilot plant. The novel geometric configurations in a CFB such as a wide bottom CFB reactor and a loop seal with increased weir depth were tested and found to be a useful application for the pilot plant operation. All important process parameters of the cold model DFB system, namely total solid inventory (TSI), riser superficial velocity, air staging ratio, and cone valve opening, were varied in order to fully characterize the DFB operation. The study shows that the proposed solid looping mechanism works satisfactorily and the required operational boundary conditions can be met in the pilot plant with suggested design improvements.

Co-gasification of Plastics and Biomass in a Dual Fluidized-Bed Steam Gasifier: Possible Interactions of Fuels

Co-gasification of biomass and plastics was investigated in a 100 kW dual fluidized-bed pilot plant using four types of plastic material of different origins and soft wood pellets. The proportion of plastics was varied within a broad range to assess the interaction of the materials. The product gas composition was considerably influenced by co-gasification, whereas the changes were nonlinear. More CO and CO2 were measured in the product gas from co-gasification than would be expected from linear interpolation of mono-gasification of the materials. Less CH4 and C2H4 were formed, and the tar content in the product gas was considerably lower than presumed. With the generation of more product gas than expected, co-gasification of wood and plastic materials also had other beneficial effects. Because of the fuel mixtures, more radicals of different types were available that interacted with each other and with the fluidization steam, enhancing the reforming reactions. Wood char had a positive effect on polymer decomposition, steam reforming, and tar reduction. As a result of the more active splash zone during co-gasification of wood and plastics, contact between gas and bed material was enhanced, which is crucial for catalytic tar removal.

Cogasification of Polyethylene and Lignite in a Dual Fluidized Bed Gasifier

This work focuses on the cogasification of lignite and fresh polyethylene (PE) granulate at lignite ratios ranging from 0 to 100% in the dual fluidized bed pilot plant at Vienna University of Technology. The operating parameters for all gasification and cogasification tests were kept constant. The input fuel power was 90 kWth, while the gasification temperature was 850 °C. Olivine was used as the bed material. In addition to standard online measurements of the permanent components of the product gas, the tar content was also measured. Using lignite in the feedstock improved quality in terms of reducing tar load and increasing the possibility of adjusting the main components of the product gas. The beneficial effects of lignite could even be attained at low concentrations of lignite in the fuel mix with PE.

Gasification of lignite in a dual fluidized bed gasifier — Influence of bed material particle size and the amount of steam

The dual fluidized bed steam gasification system, developed by the Institute of Chemical Engineering at Vienna University of Technology, was originally developed for the utilization of woody biomass as feedstock, but during practical operation of the industrial sized plants of this system it turned out that fuel flexibility is the key issue for economic breakthrough. Therefore, gasification tests were carried out with lignite at different operating conditions at the dual fluidized bed pilot plant at Vienna University of Technology. Tests were performed with an input fuel power of 90kWth. Olivine was used as bed material and the applied particle size was 370 and 510μm. The steam-to-carbon ratio was varied between 1.3 and 2.1kgH2O/kgcarbon. In addition to standard online measurements of the permanent gas components of the product gas, impurities like NH3, H2S and tar were also measured. It turned out that a lower amount of steam for fluidization caused a better performance of the gasification reactor in terms of product gas yield, carbon conversion and water conversion. The high catalytic activity of the lignite ash was also a reason for the high product gas quality producing low amounts of condensable hydrocarbons, like tar.

Co-Gasification of Wood and Lignite in a Dual Fluidized Bed Gasifier

Co-gasification tests were carried out with wood pellets and lignite at lignite ratios ranging from 0 to 100% in the dual fluidized bed pilot plant at Vienna University of Technology. The operating parameters for all gasification and co-gasification tests were kept constant; the input fuel power was chosen as 90 kWth while the gasification temperature was held at 850 °C. Olivine was used as the bed material. In addition to standard online measurements of the permanent gas components of the product gas, the tar content was also measured. NH3 and H2S were detected in the product gas for gasification of the pure fuels. It turned out that increasing lignite ratios in the fuel mix with biomass improved the quality in terms of reducing tar load and the possibility to adjust the main components of the product gas. The beneficial effects of lignite can be reached already at low proportions of lignite in the fuel mix with wood which will be presented in this paper.

Operation and results of a 200-kWth dual fluidized bed pilot plant gasifier with adsorption-enhanced reforming

Adsorption-enhanced reforming (AER) is a calcium-based process utilizing the benefits of the reversible carbonation reaction (CaO + CO2 ⇔ CaCO3 + heat) to capture CO2 in situ in a gasifier, thereby shifting product gas distribution toward higher hydrogen yields which can be further combusted in a gas motor for electricity generation or further processed for pure hydrogen or synthetic natural gas production. Following successful EU projects and lab-scale experimentation, a 200-kWth dual fluidized bed pilot plant (consisting of an interconnected gasifier and sorbent regenerator) was designed and constructed to investigate the AER process for purposes of commercialization. The operational pilot plant is hereby described and the designed operating range presented. Detailed characterization of the gasifier and regenerator (e.g., axial temperature and pressure profiles) is shown. The facility operates in a robust manner with constant and controllable temperatures and gas concentrations. Material loss due to attrition is low and the median particle size of the bed only decreases from 450 μm (raw material) to 350 μm (bed) over many days of operation. Selected results of a long-term experiment are presented, where the gasifier temperature is held for 18 h at 650 °C, achieving an average hydrogen concentration of 72 vol% and a carbon dioxide concentration of only 5 vol%. The results from the pilot plant are consistent with previously published dual fluidized bed AER lab-scale results with respect to product gas composition, lower heating value, and tar concentration.

The Effect of Bed Particle Inventories with Different Particle Sizes in a Dual Fluidized Bed Pilot Plant for Biomass Steam Gasification

The paper reviews recent results obtained at the dual fluidized bed pilot plant for steam gasification of biomass at the Vienna University of Technology. The dual fluidized bed reactor system involves the combination of a steam fluidized bubbling bed and an air blown fast fluidized bed. Bed particles of olivine with different mean particle sizes are applied as solid inventory in the fluidized bed system. The applied mean particle sizes are 520 and 260 μm, and therefore differ in coarse and fine solid particle inventories. Increased conversion of higher hydrocarbons is assumed for the fine particle inventory due to higher turbulence (improved gas–solid contact), higher gas residence times in the bubbling bed, and the higher specific particle surface which is exposed to the gas phase. Experimental test runs with conventional wood pellets were conducted by varying the fuel feed, gasification temperature, and the steam-to-fuel ratio. The results are presented according to hydrodynamic considerations, gas composition, tar content, and tar composition, and further specific data (e.g., gas yields, gas residence times, water conversion, and deviation from equilibrium state). The effect of the fine particle inventory is found to significantly influence the tar content of the GC–MS detectable tar fraction; it is reduced by about 50%. Furthermore, it is found that the main gas composition is broadly independent of the particle inventory.

CO2 capture with CaO in a 200 kWth dual fluidized bed pilot plant

Abstract The Calcium Looping (CaL) process using CaO as sorbent for capturing CO 2 from flue gases is a promising postcombustion CO 2 -capture technology that is economically competitive with other capture technologies with an overall increased output in electric power and a low electric efficiency penalty associated with CO 2 capture. The process can be realized in a Dual Fluidized Bed (DFB) system utilizing a Circulating Fluidized Bed (CFB) reactor as carbonator operating at 600–700 °C and a second CFB as regenerator operating at temperatures between 850–950 °C. The separation of CO 2 from the flue gas takes place through the carbonation reaction (CaO+CO 2 →CaCO 3 ). The regeneration of the sorbent is realized via the reverse reaction in the regenerator at higher temperatures. The feasibility of the CaL process has already been shown at IFK, University of Stuttgart on a lab-scale size. In order to investigate the calcium looping process using real combustion flue gas and to evaluate the process on a long-term basis, a 200 kWth pilot plant has been built at IFK. This paper delineates the design of the pilot plant to capture over 90% CO 2 and provides an overview of the main components and the control philosophy. First results from the successful commissioning of the pilot plant are also shown.