Dual Fluidized Bed Reactor System Research Articles

Computational fluid study of radial and axial segregation characteristics in a dual fluidized bed reactor system

In this work, computational fluid dynamics is adopted to simulate steam-biomass gasification in a dual fluidized bed reactor system. Under the Eulerian-Lagrangian multiphase framework, the large eddy simulation and multiphase particle-in-cell models are selected to track the gas and solid phases, respectively. The compositions of the gaseous products numerically obtained at the gasifier outlet are compared with the experimental data for model validation. Then, the radial and axial segregation of heat carrier with a wide particle size distribution together with the spatial variation of the microscale properties of solid phase are explored. The results show that the apparent size-induced segregation of heat carrier results in the distribution of large particles in the bottom region of both the combustor and gasifier, while the accumulation of fine particles in the freeboard of the gasifier. The mass of heat carrier distributed in the combustor is nearly 10% of that in the gasifier. The particles in the combustor have larger Reynolds number and dispersion coefficient as compared with that in the gasifier. For a specific particle size distribution, reducing the particle size enlarges the temperature, heat transfer coefficient, particle Reynolds number. Different solid residence behaviors appear in different components of the bed. A wider size distribution of heat carrier slightly enlarges the mean residence time in the combustor but obviously diminishes it in the gasifier, and also evidently reduces the solid circulation.

Detailed Oil Compositional Analysis Enables Evaluation of Impact of Temperature and Biomass-to-Catalyst Ratio on ex Situ Catalytic Fast Pyrolysis of Pine Vapors over ZSM-5

The impact of upgrading temperature and biomass-to-catalyst mass ratio on upgrading pine pyrolysis vapors over HZSM-5 was studied in a dual fluidized bed reactor system. Increasing the upgrading te...

Investigations on fluid dynamics of binary particles in a dual fluidized bed reactor system for enhanced calcium looping gasification process

This paper studies the mixing and movement of char in a dual fluidized bed for an enhanced calcium looping gasification process. The gasifier is a compact fluidized bed, consisting of a lower bubbling fluidized bed and an upper riser. The solid recirculation flux can be controlled by three loop seals. Quartz sand was used as a calcium-based sorbent and Chinese rice as the char. No flotsam behavior was observed in the bubbling fluidized bed. Experimental results reveal that the higher fluidization rate in the gasifier and the larger solid recirculation flux promoted char mixing. An increasing system char concentration decreased char mixing but led to higher char concentrations in the recirculation stream. The char holdup in the upper riser was primarily dependent on the fluidization rate in the gasifier. High solid recirculation flux with a low fluidization rate in the gasifier would benefit hot rig operation.

Intensified steam methane reforming coupled with Ca-Ni looping in a dual fluidized bed reactor system: A conceptual design

This study presents an inclusive conceptual design of an intensified steam methane reforming technology coupled with in situ CO2 capture and chemical looping operating in two interconnected fluidized bed reactors. Simulations were run using Aspen Plus® and both the hydrodynamic and kinetic phenomena of the reactors were properly implemented through correlations and kinetic models of literature. High-purity H2 was generated in a bubbling bed reformer, while regeneration occurred at temperatures near 900 °C in a fast fluidization reactor due to the generated CO2 that impeded thermodynamically the calcination in lower temperatures. Simulation results had a satisfactory agreement with experiments in a bench-scale fixed bed apparatus, with both being close to equilibrium. A parametric study including the number of stages that the reformer is divided, the NiO/CaO and CaO/C molar ratios and the type of oxidant (O2/air) of regenerator was conducted. Increasing the NiO/CaO ratio led to lower thermal requirements at the expense of lower H2 yield and no alteration of the CH4 conversion in contrast to thermodynamics. Air utilization in the regenerator avoided the harsh operational conditions but led to the CO2 isolation with lower purity. It was proposed to reduce the energy requirements by incorporating Ni reoxidation with an oxy-fuel calciner.

CO2 gasification of biogenic fuels in a dual fluidized bed reactor system

A 100 kWth dual fluidized bed steam gasification pilot plant has been developed at TU Wien to convert different types of biogenic fuels into a valuable product gas. In this paper, the conversion of different biogenic fuels in combination with the utilization of CO2 as alternative gasification agent was investigated in the mentioned pilot plant. For this purpose, five experimental campaigns were carried out aiming at the investigation of softwood as reference fuel, and rapeseed cake, bark and lignin as alternative fuels. Pure olivine as well as a mixture (90/10 wt%) of olivine and limestone were used as bed materials. The product gas compositions of the different biogenic fuels changed depending on the elemental composition of the biogenic fuels. Thus, a high amount of carbon in the fuel enhanced CO formation, whereas an increased content of oxygen led to higher CO2 contents. Additionally, the presence of alkali metals in the biomass ash favoured the production of CO. The addition of limestone enhanced the H2 and CO contents via the water gas shift reaction as well as steam and dry reforming reactions, but had no significant effect on tar contents. Overall, this paper presents the feasibility of the dual-fluidized bed gasification process of different biogenic fuels with CO2 as gasification agent.

The impact of gasification temperature on the process characteristics of sorption enhanced reforming of biomass

Especially carbon-intensive industries are interested in a decarbonization of their processes. A technology, which can contribute to a significant reduction of the carbon footprint, is the so-called sorption enhanced reforming process. The sorption enhanced reforming process uses a dual fluidized bed reactor system with limestone as a bed material for the thermochemical conversion of biomass into a valuable nitrogen-free product gas. This product gas can be used for further synthesis processes like methanation. The dependency of the product gas composition on the gasification temperature is already a well-known fact. Nevertheless, detailed investigations and models of the effect on elemental balances (especially carbon) of the process are missing in the literature and are presented in this work. Therefore, previously published data from different pilot plants is summarized and is discussed on a mass balance. Based on this information, investigations on the product gas equilibrium composition are presented and conclusions are drawn: it can be shown that the sorption enhanced reforming process can be divided into two sub-processes, namely “carbonation dominated sorption enhanced reforming” and “water-gas shift dominated sorption enhanced reforming.” The sub-process carbonation dominated SER is characterized by a high deviation from the water-gas shift equilibrium and a nearly constant CO content in the product gas over gasification temperature (< 700 °C). The sub-process water-gas shift dominated SER can be identified by a steep increase of the CO content in the product gas over temperature and nearly equilibrium state of the water-gas shift reaction (700–760 °C).

CO2 gasification in a dual fluidized bed reactor system: Impact on the product gas composition

The use of CO2 as gasification agent in the 100 kWth dual fluidized bed gasification pilot plant was investigated at TU Wien. For this purpose, steam was replaced stepwise by CO2 as gasification agent. Softwood was used as fuel and olivine as bed material. Starting from 100 vol.-% steam as gasification agent, substituting it by 32, 45 and finally 68 vol.-% CO2. All loop seals were fluidized further with steam, which resulted in these volume percentages. Additionally, a CO2 gasification test campaign converting softwood with a mixture (90/10 wt.-%) of olivine and limestone was investigated. For this case, the gasification agent was composed of 65 vol.-% CO2 and 35 vol.-% steam. The use of CO2 as gasification agent led to changes of the product gas. Instead of a H2-enriched product gas, which was produced during steam gasification, CO and CO2 occupied the major share of the product gas. Consequently, the H2/CO ratios as well as the lower heating values decreased when substituting steam by CO2. Tar contents were lower for CO2/steam gasification compared to pure steam gasification.

Catalyst deactivation, ash accumulation and bio-oil deoxygenation during ex situ catalytic fast pyrolysis of biomass in a cascade thermal-catalytic reactor system

In this work, we investigated the deactivation of a commercial ZSM-5 based catalyst during ex situ catalytic fast pyrolysis (CFP). The experimental runs were carried out in a novel cascade dual fluidized bed reactor system where thermal pyrolysis of biomass was carried out in the first reactor, while ex situ catalytic conversion of the pyrolysis vapours was carried out in the second reactor. Consecutive reaction-regeneration cycles were realised using the same catalyst batch in order to evaluate catalyst deactivation over time. A comparison between in situ and ex situ CFP revealed that in contrast to what was observed in the case of in situ CFP, no accumulation of biomass-derived metals on the catalyst was observed during ex situ CFP. Both acidity and surface area were less affected compared to in situ CFP and the catalyst maintained higher activity. Product distribution and composition exhibited some variation over time which was attributed to the accumulation of biomass ash in the pyrolysis reactor and not to the poisoning of the catalyst bed. These results clearly demonstrated that ex situ CFP is an effective way to avoid catalyst poisoning during the CFP of biomass and to prolong catalyst lifetime.

The impact of bed material cycle rate on in-situ CO2 removal for sorption enhanced reforming of different fuel types

A dual fluidized bed reactor system produces a nitrogen-free product gas by using steam as gasification agent. Additionally, the usage of limestone as bed material allows for the in-situ removal of carbon dioxide out of the product gas. Hence, a hydrogen-rich product gas with a high reduction potential for the steel industry can be generated. This so-called “sorption enhanced reforming” process has already been proven applicable for wood as fuel, but since the costs for biomass like wood have increased significantly during the last years, cheaper fuels are of interest. The experimental results of three different biogenic fuel types (soft wood, rice husk and bark) and one fossil fuel type (lignite) are discussed in detail. In the past, research mainly focused on temperature dependency of the process since it seemed to be the main factor for carbon dioxide sorption in the gasification reactor and therefore product gas composition. Within this work, it is shown that the bed material cycle rate should not be disregarded and is a key factor within the process. The presented findings allow a detailed understanding of “sorption enhanced reforming”, the influence of bed material cycle rate, and the influence of volatile matter in the fuel.

Production of low-oxygen bio-oil via ex situ catalytic fast pyrolysis and hydrotreating

Catalytic fast pyrolysis (CFP) bio-oils with different organic oxygen contents (4–18wt%) were prepared in a bench-scale dual fluidized bed reactor system by ex situ CFP of southern pine over HZSM-5, and the oils were subsequently hydrotreated over a sulfided CoMo catalyst at 170bar. The goal was to determine the impact of the CFP oil oxygen content on hydrotreating requirements. The CFP oils with higher oxygen contents included a variety of oxygenates (phenols, methoxyphenols, carbonyls, anhydrosugars) whereas oxygenates in the 4wt% oxygen oil were almost exclusively phenols. Phenols were the most recalcitrant oxygenates during hydrotreating as well, and the hydrotreated oils consisted mainly of aromatic and partially saturated ring hydrocarbons. The temperature required to produce oil with <1% oxygen was approximately 350°C for the CFP oil with the lowest oxygen content whereas temperatures around 400°C were required for the other CFP oils. The carbon efficiency during hydrotreating slightly decreased as the CFP oil oxygen content increased but remained above 90% in all cases, and the carbon efficiency for the integrated process was dominated by the efficiency of the CFP process. A preliminary technoeconomic evaluation suggested that with the current zeolite-based CFP catalysts, it is economically beneficial to preserve carbon during CFP, at the expense of higher oxygen contents in the CFP oil.

Comparative life cycle analysis for gasification-based hydrogen production systems

Hydrogen is expected to play a significant role in the future energy systems, given the global increase in energy demand, which is driven by the accelerated growth of the world's population, the on-going industrial development and urbanization as well as the higher standards of living and education. In this paper, the life cycle assessment (LCA) methodology is used to evaluate the environmental impact of two different gasification based hydrogen production technologies: hydrogen production by biomass steam gasification in a dual fluidized bed reactor system (DFB) and hydrogen production by gasification of coal and biomass using entrained flow technology (EF). For both hydrogen production pathways the gasification plant, raw materials production, pre-processing, and transportation are included in the life cycle assessment and also hydrogen delivery to consumers. The DFB cases have lower global warming potential than the EF cases. Also the abiotic depletion fossil potential and the human toxicity potential are lower. The acidification and eutrophication potentials are lower for the EF cases.

Continuous Test of Ilmenite-Based Oxygen Carriers for Chemical Looping Combustion in a Dual Fluidized Bed Reactor System

Raw ilmenite was tested as an oxygen carrier for chemical looping combustion (CLC) in a dual fluidized bed reactor for 100 h. After this 100 h test, 21 wt % of the total used raw ilmenite was impre...

One-dimensional modelling of chemical looping combustion in dual fluidized bed reactor system

Chemical looping combustion (CLC) is an emerging combustion technology with an inherent separation of the greenhouse gas CO2. The technique typically employs a dual fluidized bed reactor system where a metal oxide is used as a solid oxygen carrier that transfers the necessary oxygen from air to the fuel combustion. In this work, a comprehensive simulation tool for the investigation of a CLC system consisting of two interconnected fluidized bed reactors is introduced. One-dimensional, dynamic fluidized bed reactor model is implemented into the Matlab/Simulink environment. The model is based on the conservation of mass and energy, and semi-empirical correlations are used for the calculation of reaction kinetics, hydrodynamics, and heat transfer. The main outputs of the model are the global solids circulation rate, the conversion of the carrier and the gas composition at the reactor exit, the vertical profiles of temperature, reaction rates and gas concentrations, and the distribution of solids in the reactors. For validating and evaluating the capabilities of the model, a reference case based on the operation of a 120kWth CLC test rig was defined and simulated. Good agreement was observed between the experiments and simulations, and the model structure and submodel forms turned out to be appropriate to describe the studied process.

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 from Simulated Syngas via Cyclic Carbonation/Calcination for a Naturally Occurring Limestone: Pilot-Plant Testing

Experiments were performed using a dual fluidized bed reactor system, operated in a batch mode, in order to investigate the effects of steam and simulated syngas on CO{sub 2} capture and sorbent conversion efficiency for a naturally occurring Polish calcitic limestone. In addition, the effect of high partial pressures of CO{sub 2} on the calcination process was examined using either oxygen-enriched air or oxy-fuel combustion in the calciner. As expected, calcination under oxy-fuel conditions resulted in decreased carbonation conversion due primarily to particle sintering and pore pluggage. On average there was a decrease in carbonation conversion of approximately 36.5 and 33.4% for carbonation with steam and steam/simulated syngas, respectively, compared to similar experiments using oxygen-enriched air. However, during the carbonation of the limestone with steam present in the feed gas, it was observed that the high CO{sub 2} capture efficiency period was significantly extended compared to carbonation with only CO{sub 2} present. This resulted in increased CaO conversion from approximately 16.1 to 29.7% for the initial carbonation cycle. A further increase in carbonation conversion from 29.7 to 46.9%, was also observed when simulated syngas conditions (CO, H{sub 2}) were used in the carbonator. Analysis of the outlet gases also confirmedmore » that the calcined limestone catalyzes the water gas shift reaction, which we believe results in enhanced CO{sub 2} concentration levels at the grain surfaces of the sorbent.« less

Modeling of Sorption-Enhanced Steam Reforming in a Dual Fluidized Bubbling Bed Reactor

This paper highlights the use of a dual fluidized bed reactor system for producing hydrogen by sorption-enhanced steam methane reforming. Hydrogen concentrations of >98% are predicted for temperatures of ∼600 °C and a superficial gas velocity of 0.1 m/s, using a simple two-phase bubbling bed model for the reformer. The kinetics of the steam methane reforming and water-gas shift reactions are based on literature values, whereas experimentally derived carbonation kinetics are used for the carbonation of a dolomite. It is shown that the reformer temperature should not be <540 °C or >630 °C for carbon capture efficiencies to exceed 90%. Operating at relatively high solids circulation rates to reduce the need for fresh sorbent is predicted to give higher system efficiencies than for the case where fresh solid is added. This finding is attributed to the additional energy required to decompose both CaCO3 and MgCO3 in fresh dolomite.

The relative coke-inducing tendencies of pyrolysed, gasified and combusted Devonian oil shales

A dual fluidized-bed reactor system was used to examine the isothermal kinetics of carbon deposition when nascent shale oil vapours were passed through beds of pyrolysed, gasified and combusted Devonian shale. Naturally occurring samples of the principal silicate minerals found in Devonian shale (kaolinite, illite and quartz) were also tested. The rate of carbon uptake declined with increasing carbon deposition on the solids. The coke-inducing activities of the processed shales were ranked as: combusted shale gasified shale pyrolysed shale. The effect of coking temperature on carbon deposition was relatively small, which has important implications for solids-recycle oil shale retorts.