Dual Fluidized Bed Research Articles

Hydrodynamic characteristics in a cold model of the dual fluidized bed with mixed particles

Biomass gasification in a dual fluidized bed has been regarded as one of the most promising technologies. The hydrodynamics play a vital role in determining the performance of a dual fluidized bed (DFB) system. In this study, based upon the solid-gas two phase theory, and pressure and mass balances, a hydrodynamic model for the DFB system was developed. The model parameters of mixed particles (rice husk and quartz sand) were fitted using the homogeneous and phase-separated particles model in order to improve the accuracy of predicted results. It was found that the predicted results were in reasonable agreement with the experimental results for the pressure drop, pressure distribution and circulation flow rate of the cold model of the DFB system. Which provides guidance for the operation and design of DFB gasifier equipment.

Dual fluidized bed steam gasification: Change of product gas quality along the reactor height

The impact of the counter-current column of the gasification reactor of a 100 kWth dual fluidized bed steam gasification pilot plant on the product gas quality was investigated. Through the advanced design of the gasification reactor by operating the lower part as bubbling bed and the upper part as counter-current column, the gas-solid interactions between downward flowing hot bed material particles with upwards flowing product gas could be enhanced. This was realized by equipping the counter-current column with constrictions, which increase the residence time and the bed material hold-up. Thus, the conversion efficiency of the fuel including the tar was improved. For the investigations three different experimental campaigns converting softwood pellets using a mixture of olivine and limestone (50/50 wt.-%), a mixture of feldspar and limestone (50/50 wt.-%), and 100 wt.-% quartz as bed materials were conducted. Higher H2 contents and lower contents of higher hydrocarbons could be detected along the height of the counter-current column. Especially heavy tar compounds could be reduced significantly. These two effects are explained by enhanced water gas shift and steam reforming reactions. In case of catalytically inactive quartz, only thermal effects are available and therefore lower effects on tar reduction could be obtained.

Catalytic Gasification of Biomass in Dual-Bed Gasifier for Producing Tar-Free Syngas

Process development for the production of syngas by gasification of biomass, a carbon-neutral source of energy, has been researched very extensively in the recent past. One of the technical challenges in biomass gasification is to produce clean syngas with no tar. Tar removal by employing secondary bed is a solution, but it faces higher cost and reliability issues. Hence, extensive work is directed to produce syngas with no tar formation in the gasifier bed itself, by employing a suitable catalyst or other options. In the present work, a dual fluidized bed process for catalytic gasification of biomass is proposed for the production of high-quality syngas. This process employs an alkali-impregnated alumina or silica-alumina catalyst that eliminates tar formation completely while increasing the H2-to-CO molar ratio to more than 10. The catalyst is able to completely gasify the volatiles and solid char generated from biomass in the temperature range of 500–750 °C, at near-atmospheric pressure, with catalyst-...

Experimental study of Cu-modified manganese ore for O2 production in the CLC + CLOU scheme

In chemical looping combustion (CLC), a necessary oxygen polishing step exerted an additional cost to the technology due to an extra demand of pure O2. However, the additional cost can be lowered if the oxygen carriers can produce gaseous oxygen, i.e. has a chemical looping with oxygen uncoupling (CLOU) property. A CLC + CLOU scheme is proposed in the study. Production of gaseous O2 using a manganese ore and the Cu-modified manganese ore as oxygen carriers were investigated. The reaction kinetics of the two oxygen carriers were studied by thermogravimetric analysis (TGA). Multiple O2 uptake and release cycles were carried out to demonstrate the good stability of the oxygen carriers. Continuous tests with both oxygen carriers were carried out in a dual fluidized bed. Results showed that both oxygen carriers could be fluidized well in the continuous run and a stream of O2 could be obtained stably. The Cu-modified manganese ore showed higher oxidation rate and oxygen transport capacity than the original manganese ore. Both materials have advantage economically and could be promising for O2 production in the CLC + CLOU scheme.

Assessment of correlations between tar and product gas composition in dual fluidized bed steam gasification for online tar prediction

Thermochemical conversion of biomass feedstock via dual fluidized bed steam gasification is a well-proven technology used to produce a medium calorific product gas for various applications in the energy or transportation sector or for chemical syntheses. At unfavorable gasification conditions, undesirable high amounts of tar, which are aromatic hydrocarbons, are present in the product gas. High tar contents are a major problem, and they lead to uneconomic operation due to sharply diminished quality of product gas or unexpected plant shut downs due to fouling of the product gas coolers. Currently, tar content is measured with a discontinuous wet-chemical analysis method, which needs several hours of sample preparation to receive the final tar content. The aim of this study is to establish valid correlations between online measured permanent gas components in the product gas and its tar content. The results show that hydrogen, methane, and ethene concentrations are strongly related to the tar content in the product gas, while the carbon monoxide and carbon dioxide content did not show a clear correlation. Using these correlations with online measured gas components provides the possibility of a direct and prompt response of a plant operator in case of unfavorable gasification conditions. Additionally, an optimization of the plant operation can be conducted and thereby, the operation hours and, consequently, the economic efficiency are improved.

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.

Bark as feedstock for dual fluidized bed gasifiers-Operability, efficiency, and economics

The demand for biofuels and biochemicals is expected to increase in the future, which will in turn increase the demand for biomass feedstock. Large gasification plants fueled with biomass feedstock are likely to be a key enabling technology in a resource-efficient, bio-based economy. Furthermore, the costs for producing biofuels and biochemicals in such plants could potentially be decreased by utilizing inexpensive low-grade residual biomass as feedstock. This study investigates the usage of shredded tree bark as a feedstock for the production of biomethane in the GoBiGas demonstration plant in Gothenburg, Sweden, based on a 32 MWth industrial dual fluidized bed gasification unit. The plant was operated with bark feedstock for 12 000 hours during the period 2014 to 2018. Data from the measurement campaign were processed using a stochastic approach to establish the plant's mass and energy balances, which were then compared with operation of the plant with wood pellets. For this comparison, an extrapolation algorithm was developed to predict plant performance using bark dried to the same moisture content as wood pellets, ie, 8%w.b. Plant operation with bark feedstock was evaluated for operability, efficiency, and feedstock-related cost. The gas quality achieved during the test period was similar to that obtained for operation with wood pellets. Furthermore, no significant ash sintering or agglomeration problems were observed more than 750 hours of operation. The calculated biomass-to-biomethane efficiency is 43% to 47% (lower heating value basis) for operation with wet bark. However, the predicted biomass-to-biomethane efficiency can be increased to 55%–65% for operation with bark feedstock dried to 8% moisture content, with corresponding feedstock costs in the range of 24.2 to 32.7 EUR/MWh; ie, a cost reduction of about 40% compared with wood pellets.

A general method for the determination of the entrainment in fluidized beds

The measurement of the entrainment is complicated in a hot fluidized bed. Therefore, the entrainment or cycle rate of the total bed material inventory is typically estimated via mass and energy balance for e.g. the sorption enhanced reforming process. The sorption enhanced reforming process is an advancement of the conventional dual fluidized bed steam gasification process. It aims for the production of a hydrogen-rich product gas. The process is highly depends on bed material cycle rate, respectively bed material entrainment. Only limited knowledge about simple modelling methods of the entrainment of a fluidized bed are available to validate the cycle rate calculated by mass and energy balances. Therefore, measurements of the bed material entrainment at two different plants were conducted and a model based on easily accessible parameters for the entrainment of fluidized bed risers is presented. It is also verified, that the results from mass and energy balances fit well with the measured data for the entrainment.

Experimental Demonstration and Validation of Hydrogen Production Based on Gasification of Lignocellulosic Feedstock

The worldwide production of hydrogen in 2010 was estimated to be approximately 50 Mt/a, mostly based on fossil fuels. By using lignocellulosic feedstock, an environmentally friendly hydrogen production route can be established. A flow sheet simulation for a biomass based hydrogen production plant was published in a previous work. The plant layout consisted of a dual fluidized bed gasifier including a gas cooler and a dust filter. Subsequently, a water gas shift plant was installed to enhance the hydrogen yield and a biodiesel scrubber was used to remove tars and water from the syngas. CO2 was removed and the gas was compressed to separate hydrogen in a pressure swing adsorption. A steam reformer was used to reform the hydrocarbon-rich tail gas of the pressure swing adsorption and increase the hydrogen yield. Based on this work, a research facility was erected and the results were validated. These results were used to upscale the research plant to a 10 MW fuel feed scale. A validation of the system showed a chemical efficiency of the system of 60% and an overall efficiency of 55%, which indicates the high potential of this technology.

Mobile demonstration unit for fast- and catalytic pyrolysis: The combustion reduction integrated pyrolysis system (CRIPS)

A mobile pyrolysis unit, designed to process biomass at a rate of 83.3 kg h−1 (two metric tons per day, MTPD) has been developed, constructed and its operation demonstrated for on-farm or in-forest production of pyrolysis oil (bio-oil). The trailer-mounted pyrolysis apparatus is based on the combustion reduction integrated pyrolysis system (CRIPS), a patented, dual fluidized bed, biomass pyrolysis design developed by USDA and the University of Pretoria. The mobile design goal was to demonstrate efficacy of on-farm production and coordinate station-to-station operation to simulate a collective system of several unit operations within a distributed/satellite system of pyrolysis biorefinery. Key design features that provide utility for remote operations including in-situ (on-trailer) generation of heat and electric power required for self-sufficiency were tested at this scale. Extensive trials on three feedstocks important to US agriculture, namely woody biomass, switchgrass and horse litter, were conducted. The results show that a processing rate of up to 40 kg h−1 (approximately 1 MTPD) was successful and easily achievable for the said biomass on the trailer; however, beyond this rate operational problems such as pressure imbalances between the dual-bed reactors were encountered, hampering process control. The operation of the CRIPS under catalytic pyrolysis conditions was also successful with catalyst regeneration readily achievable with the design. Bio-oil yield for neat/traditional and catalytic pyrolysis were in the 45% and 5–10% ranges respectively, similar to lab scale, demonstrating production of large volumes of bio-oil on-farm. Bio-oil quality for catalytic pyrolysis was consistent over several hours on stream due to continuous catalyst regeneration that the design affords yielding high levels of BTEX compounds; however, deactivation due to alkali contamination was noticeable at cumulative biomass to catalyst ratios of > 6/1. Compared with laboratory scale results, non-catalytic pyrolysis product quality for the demonstration scale wavered between that of regular pyrolysis oil produced under nitrogen atmosphere and that of the tail-gas reactive pyrolysis (TGRP) as partial recycle of effluent gas was possible with the mounted CRIPS design. With successful demonstration at 1 MTPD capacity the mobile system meets the technology readiness level for pre-commercial design suggested for blueprints (i.e. development of full-scale prototype), however more testing is underway to remove the bottlenecks impeding operation at the full-scale design.

Valorization of Automobile Shredder Residue Using Indirect Gasification

Dual fluidized bed (DFB) gasification offers the possibility to convert solid fuels into a valuable gas, comprised of syngas, and hydrocarbons that can be readily handled in petrochemical units. DFB gasifiers are especially suitable for nonhomogeneous fuels, such as waste fractions. In this work, the possibility to use DFB gasification as a recycling/valorization method of automobile shredder residue is investigated. The gasification tests were carried out in the Chalmers 2–4 MWth gasifier over 4 days. The effects of ash on the gas and tar compositions, as well as on the activity of the bed inventory, were evaluated. The results show that 60% of the total carbon in the fuel can be recovered in the form of a permanent gas, whereby the produced gas contains 12%mol of C2–3 hydrocarbons. The tar levels measured in the produced gas were high, although it was clear that decomposition into monomer-like compounds occurred in the reactor, which resulted in the production of valuable petrochemical compounds, corresponding to 8–9% of the carbon in the feed. Using a higher operating temperature was found to be beneficial in terms of obtaining a higher gas yield, regardless of the level of ash enrichment in the system. The high ash levels in the fuel feed did not negatively affect the technical operation of the fluidized bed. Possible routes of carbon recovery are discussed.

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.

Techno-economic assessment of biomass-based natural gas substitutes against the background of the EU 2018 renewable energy directive

This paper investigates biomethane and BioSNG production processes against the background of the 2018 renewable fuel directive of the European Union (EU). The investigated biomethane processes use manure, clover grass, and grass silage as feedstock, are based on membrane separation gas upgrading processes, and generate 1.0 and 4.8 MW of biomethane. The investigated BioSNG processes use wood chips as feedstock, are based on the dual fluidized bed steam gasification technology and the VESTA SNG process from Amec Foster Wheeler, and generate 6.1, 12.2, and 49.1 MW BioSNG. The techno-economic assessment shows that the biomethane processes have, in general, a lower break-even price for the generated natural gas substitute. However, their scalability is limited and at larger scale (49.1 MW BioSNG capacity), the BioSNG processes become competitive. The 1.0 MW biomethane and all BioSNG plants meet the 2018 renewable fuel directive of the EU. In contrast, the 4.8 MW biomethane process does not meet the directive as the feedstock, which is mainly based on energy crops, causes significant CH4 and CO2 emissions.

Investigation on biomass steam gasification in a dual fluidized bed reactor with the granular kinetic theory

The dual fluidized bed (DFB) reactor is promising to convert biomass into high-quality syngas efficiently. In this work, a three-dimensional model is built based on the granular kinetic theory to predict the biomass steam gasification in dual fluidized bed reactors. The model is firstly validated against a series of experimental results. Then, the effects of some essential operation parameters including the biomass flow rate (Fb), the steam to fuel ratio (Rsf) and the gasification temperature (Tg) on the biomass steam gasification properties in a DFB reactor are comprehensively analyzed with the orthogonal method. In the concerned ranges of the operation parameters, the cold gas efficiency is found to be the most sensitive to Fb and least sensitive to Tg. The optimal cold gas efficiency of the DFB gasifier is 82.9% when Fb, Rsf and Tg are 15 kg/h, 1.5 and 900 °C, respectively, and the H2 mole fraction is 46.62%.

Fuel flexible gasification with an advanced 100 kW dual fluidized bed steam gasification pilot plant

Steam gasification enables the conversion of heterogeneous solid fuels into homogeneous gaseous energy carriers. The utilization of biogenic residues and waste fractions as fuel for this technology offers a sustainable waste management solution to produce heat and power, secondary fuels and valuable chemicals after several cleaning and upgrading steps of the product gas. However, residues and waste fuels show unfavorable properties for gasification and, therefore, cause technical challenges. This paper presents experimental results carried out at an advanced 100 kWth dual fluidized bed steam gasification pilot plant from nine single test runs. In the following the fuels that were gasified will be listed: (i) Five biogenic fuels, mainly residues: softwood, sugar cane bagasse, exhausted olive pomace, bark and rice husks; (ii) two different waste-derived fuels: a municipal solid waste fraction and a shredder light fraction; and (iii) a mixture of municipal solid waste fraction with a 25% blending of lignite based on lower heating value as well as pure lignite. Thereby, various product gas qualities were generated. The presented results offer the basis for a sustainable and promising waste management solution for the tested waste fuels.

Cold model testing of an innovative dual bubbling fluidized bed steam gasifier

Biomass gasification by a dual fluidized bed reactor is a very promising process to produce a hydrogen rich syngas from biomass wastes. In this process, the bed material circulation must be enough to transport heat from the combustor to the steam gasifier, and at the same time siphons/loop-seals must be properly designed to avoid gas leakage between the two reactor chambers (such as N2 from the combustor to the gasifier). A cold model of an innovative pilot scale dual bubbling fluidized bed gasifier (100 kWth as biomass input) has been realized. The Hybrid Lagrangian Particle Tracking (HLPT) technique and tracer gas analysis, both applied to the cold model, have been used to evaluate the flow rate of bed material circulation and the gas leakage between the steam gasification and the combustion chambers. The results have shown that the bed material circulation is 2–3 times the minimum required to assure allothermal gasification, while gas leakage is negligible for every operating condition evaluated experimentally.

Experimental investigation of tar formation and producer gas composition in biomass steam gasification in a 100 kW dual fluidised bed gasifier

Abstract In this study, three types of biomass including corn stover, radiata pine wood and rice husk in the form of pellets were gasified with steam as gasification agent in a 100 kW dual fluidised bed gasifier. Tar formation in initial devolatilization stage and its correlation to the final tar concentration in the producer gas were investigated. In addition, the yields and composition of the producer gas for each type of biomass were also examined. In the gasification experiments, operating temperature was controlled, respectively, at 700 °C and 800 °C. Silica sand was used as the bed material with an inventory of 30 kg. For simulation of the initial devolatilization stage in the steam gasification, N2 was used as fluidization agent. From this study, it is found that there was a positive correlation between tar contents in the devolatilization product gas and those in the final producer gas from gasification. In the devolatilization stage, radiata pine biomass yielded more phenols, while corn stover generated more toluene. Based on the results of this study, tar formation mechanism was proposed which is verified by the observation that more naphthalene was present in the producer gas from gasification of radiata pine while gasification of corn stover produced more biphenyl. The experimental results also show that at gasification temperature of 700 °C, the producer gas yield was the highest for corn stover followed by rice husk and then radiata pine wood. However, for gasification at 800 °C, the trend was reversed with radiata pine having the highest yield followed by risk husk and the corn stover. At both 700 and 800 °C, the radiata pine biomass produced a producer gas with higher contents of H2 and CH4 while the producer gas from rice husk had a higher content of CO and that from corn stover had a higher content of CO2, C2H4 and C2H6. These differences are closely related to the chemical composition of the biomass which was also analysed in this study. Radiata pine had a higher content of lignin (31.96 wt%), rice husk had a higher content of hemicellulose (25.30 wt%) while corn stover was rich in cellulose (69.85 wt%).

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.

The behavior of biomass and char particles in a dual fluidized bed gasification system

Biomass gasification in fluidized beds is a complex process in which particles occur in a wide range of size and density. In this paper, the mixing behavior of the char, biomass and bed material in a gasification reactor of a typical dual fluidized bed (DFB) system was investigated in a cold flow model. Experiments with ternary mixtures were performed in which the size and the density of the used particles were varied. For the experiments, a cold flow model was constructed with a full bed material recirculation loop, similar to DFB systems. Experiments revealed that at low fluidization velocities, the smaller char particles and biomass particles occur more preferentially in the bed material recirculation stream. If the fluidization velocities are increased, this tendency diminishes. Furthermore, the experiments showed that the mass fraction of biomass particles in the recirculation stream is always higher than that of the lighter char particles. It is also shown that the current design of the gasification reactor in DFB systems is not optimal. A way to overcome this issue in existing plants is presented.

Quantifying the exergetic performance of bio-fuel production process including fast pyrolysis and bio-oil hydrodeoxygenation

Fast pyrolysis of biomass in a dual fluidized bed and bio-oil hydrodeoxygenation are simulated by Aspen plus. The process includes the following unit operations: drying, pyrolysis, char combustion, bio-oil vapor condensation, mild hydrodeoxygenation, and depth hydrodeoxygenation. Based on the simulative results, exergetic analysis of the whole process is performed to optimize the overall exergetic efficiency. The overall exergetic efficiency of the process is 57%. The exergetic efficiencies of the fast pyrolysis process and the bio-oil hydrodeoxygenation process are 68% and 78%, respectively. The results indicate that the exergetic efficiency is improved slightly with a decrement in the system pressure drop. Moreover, the exergetic efficiency of pyrolysis is improved from 69% to 76% when the percentage of the pyrolysis char combusted is reduced from 100% to 65%. Compared to other conversion technologies, biomass to liquid fuel conversion via fast pyrolysis and subsequent hydrodeoxygenation has higher exergetic efficiency.