Dual Fluidized Bed Gasification System Research Articles

MFIX–DEM simulation of gas-solid flow dynamics in a dual fluidized bed system

Abstract To overcome the operational difficulties associated with bubbling and circulating fluidized bed gasifiers, a new gasification technology termed dual fluidized bed gasification (DFBG) has evolved. To strengthen the understanding of the gasification and combustion processes in the DFBG system, a thorough analysis of bed hydrodynamics is of paramount importance. Furthermore, the complexity of the hydrodynamics escalates while incorporating diverse fuels like biomass and coal, along with bed materials fluidized in a single reactor due to its nonlinearity and transience. As a result, the study of hydrodynamics in such a system is indispensable. The present study focuses on the discrete element model (DEM) simulation of the bed hydrodynamic behaviour within the gasifier of a DFBG system. The simulation was conducted using Multiphase Flow with Interphase eXchanges (MFiX) software. The influence of superficial velocity on the number of particles in a gasifier was analyzed using the 2-D discrete element model (DEM). In this numerical investigation, transient variation of pressure drop, axial and radial solid volume fraction, and particle velocity was evaluated, and the data were analyzed using the ParaView software. The simulation results of the gasifier indicated an increase in the bed pressure drop with the rise in inlet air velocity. The effective height of solid volume fraction also increased with the surge in superficial velocity. The solid velocity profile and streamlines were also investigated to understand its variation pertaining to the location within the fluidized bed.

Enrichment of fuel properties of biomass using non-oxidative torrefaction for gasification

The abundance and replenishment nature of solid biomass prompt fuel substitution for gasification and thermal power plants. However, many challenges are encountered while utilizing raw biomass, such as seasonality, strong hydrophilicity, low bulk and energy density, excess oxygen content, less compositional homogeneity, and poor grindability. It is, therefore, indispensable to augment the thermo-chemical properties of the solid biomass by performing suitable pretreatment. Among the various pretreatment techniques, non-oxidative torrefaction effectively upgrades solid biomass to coal-like fuel altering its physico-chemical properties. Therefore, in this work, torrefaction of rice husk and sugarcane bagasse have been performed in a fixed bed reactor by varying temperatures from 210–330 °C and residence time from 30–60 min under a non-oxidative environment. The experimental investigation illustrates a decrease in mass and energy yield of the biomass with a rise in temperature and residence time. Conversely, the higher heating value of rice husk and sugarcane bagasse has improved by 119.4% and 128.9%, respectively. The hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratio of the torrefied biomass has reduced to enriched fuel variety as indicated by the van Krevelen plot. The decomposition and structural modifications were assessed using Fourier transform infrared spectroscopy, x-ray diffraction, and morphology analysis. Based on the experimental observations, it has been found that torrefaction of rice husk at 290 °C and 30 min and sugarcane bagasse at 270 °C and 30 min would generate enriched syngas using a dual fluidized bed gasification system. Furthermore, water gas shift reactions will be promoted to enhance the percentage of hydrogen in the gas mixture.

Influence of Gasification Operating Parameters on Performance of the Nong Bua Dual Fluidized Bed Gasification System in Thailand

Gasification system performance generally depends on feed moisture content, activity of bed material, gasifier and combustor temperatures, and scrubber media. The tar concentration and gas composition of product gas are two indicators of the gasification system performance. In this research, the effects of gasifier temperature and the activity of bed material on the tar concentration and gas composition of the product gas produced from a dual fluidized bed (DFB) gasification system power plant were investigated. The DFB gasification system power plant is located in Nong Bua district, Nakhon Sawan province, Thailand. Two periods of gasification operation were examined. These two periods were when the olivine was freshy activated and then after a period of operation. The gasifier temperature had several peaks during the operation, which caused the product gas composition to fluctuate. When the olivine had been used for a period, the percentage of hydrogen was approximately 3% higher than when the olivine had been freshly activated, and a lower heating value was observed, which was probably due to lower heating value of hydrogen. The tar concentration was substantially lower when compared with the freshly activated olivine. When the olivine was used for a period, the average tar concentration was 56±22 mg/Nm3 (this is after 95 h continuous operating time) while the average tar concentration of the freshly activate olivine was 872±125 mg/Nm3 (which was after 34.5 h continuous operating time). It was concluded that the average tar concentration and gas composition were influenced by the activity of the bed material and the gasification temperature.

Analysis of fluidized bed gasification of biomass assisted by solar-heated particles

Although biomass gasification using concentrated solar energy is an attractive technology for the production of storable renewable energy and CO2 reduction, several challenges have stalled its deployment over the last decades: high temperature and/or large reactor volume required for complete fuel (char) conversion, the achievement of a steady syngas generation independent of solar radiation variation, and accomplishing effective heat supply at high temperature in large-scale reactors. An alternative approach overcoming these technical challenges is analyzed in this paper. It is based on conducting the gasification of biomass with steam in a fluidized bed using solid particles that are separately heated in a solid particle receiver using concentrating solar energy. The present study includes the thermodynamic and kinetic analysis of solar steam gasification in a completely allothermal single fluidized bed gasifier as well as in a partially allothermal dual fluidized bed gasification system. A theoretical analysis of the reactor performance and its integration with the solar receiver is examined and the state of the art of the most important aspects for developing the proposed technology is reviewed.

A Detailed One-Dimensional Hydrodynamic and Kinetic Model for Sorption Enhanced Gasification

Increased installation of renewable electricity generators requires different technologies to compensate for the associated fast and high load gradients. In this work, sorption enhanced gasification (SEG) in a dual fluidized bed gasification system is considered as a promising and flexible technology for the tailored syngas production for use in chemical manufacturing or electricity generation. To study different operational strategies, as defined by gasification temperature or fuel input, a simulation model is developed. This model considers the hydrodynamics in a bubbling fluidized bed gasifier and the kinetics of gasification reactions and CO2 capture. The CO2 capture rate is defined by the number of carbonation/calcination cycles and the make-up of fresh limestone. A parametric study of the make-up flow rate (0.2, 6.6, and 15 kg/h) reveals its strong influence on the syngas composition, especially at low gasification temperatures (600–650 °C). Our results show good agreement with the experimental data of a 200 kW pilot plant, as demonstrated by deviations of syngas composition (5–34%), lower heating value (LHV) (5–7%), and M module (23–32%). Studying the fuel feeding rate (22–40 kg/h), an operational range with a good mixing of solids in the fluidized bed is identified. The achieved results are summarized in a reactor performance diagram, which gives the syngas power depending on the gasification temperature and the fuel feeding rate.

Design of experiments and analysis of dual fluidized bed gasifier for syngas production: Cold flow studies

Biomass gasification is a thermo-chemical process widely accepted as a future technology for syngas production. Numerous types of gasification systems have been proposed and studied in the past. Recent developments have shown that Dual Fluidized Bed (DFB) gasifier are commercially more attractive for production of the hydrogen-rich syngas as compared to others. DFB gasification system is very complex in construction and operation. Hence, a detailed understanding of hydrodynamics in such systems is essential for optimum design and scale-up. Hydrodynamics of DFB gasifier mainly depends on the Solid Circulation Rate (SCR). SCR is governed by riser velocity, gasifier velocity, and loop seal velocities. In present work, Central Composite Rotatable Design (CCRD) based Response Surface Method (RSM) was employed to determine the effect of riser velocity, gasifier velocity, recycle chamber velocity, supply chamber velocity, and vertical supply chamber velocity and their interaction on the SCR. Adequacy of regression model developed from RSM was confirmed using ANOVA analysis. The value of coefficient of determination (R2) of the model was 0.9729, which confirms model represents the experimental results satisfactorily. Riser and recycle chamber velocity were found to be most significant parameters, plays an important role in SCR in DFB gasifier.

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.

Effect of ash circulation on the performance of a dual fluidized bed gasification system

During gasification of biomass, ash forming elements are released from the fuel and some of these elements can have a positive impact on the quality of the gas produced. In a dual fluidized bed (DFB) gasifier, a significant amount of these components are found in the fly ash from the gasification and combustion reactors. In order to increase carbon conversion and bed material recovery, these streams are generally circulated back to the combustor for the raw gas fly ash and in some cases to the gasifier for the flue gas fly ash. The impact on the gasification performance has, however, not been investigated. Circulation of flue gas coarse ash was carried out in the Chalmers gasifier, with the aim of assessing the impact on gas quality, in particular in term of tar yields, and how it relates to the flow and properties of ash streams. The coarse ash was first enhanced by an injection of untreated olivine in a fine particle size and was then recirculated, yielding a direct decrease in tar concentration. This effect persisted after the recirculation and the bed activity was seen to increase with time, at a higher rate than a reference aging experiment. Both the internal bed material cycle and the external fly ash loop were found to get enriched in ash components, which was linked to the activity gains observed. These results show the potential of continuous fly ash recirculation as an activity enhancer in industrial dual fluidized bed gasification systems.

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.

Using Ilmenite To Reduce the Tar Yield in a Dual Fluidized Bed Gasification System

Biomass gasification plays an important role in the emerging production of second-generation biofuels. One of the major challenges facing biomass gasification is to find simple and efficient ways to reform tar components. While the tar causes operational problems, it can be reformed to increase the chemical efficiency of the gasification process. With respect to tar reforming, catalytic materials are of special interest. Many of the materials that have been proposed as promising catalysts are metal oxide-based materials. However, metal oxides also have the ability to transport oxygen when subjected to alternating oxidizing and reducing atmospheres, similar to that which occurs in a dual fluidized bed gasification system. In this work, ilmenite was used as the catalytic material in the Chalmers 2–4 MWth dual fluidized bed gasifier to decrease the yield of tar. The ilmenite was mixed with the silica sand, which was used as the bed material, to investigate how the level of ilmenite affected chemical efficiency and tar yield. Furthermore, energy balance calculations were established to elucidate the general aspects of oxygen transport in dual fluidized bed gasification systems. The results presented in this paper reveal that adding low levels of ilmenite reduces the tar yield by ∼50%mass. However, the oxygen transport induced by ilmenite caused a reduction in the chemical efficiency of the gasifier and the heating value of the gas, compared to using 100% silica sand as the bed material. The impact of adding ilmenite was found to be dependent upon the operational conditions of the gasifier; a low fluidization velocity gave the highest reduction of the tar yield, whereas higher fluidization velocities led to increased levels of heavy components. Overall, the use of ilmenite as a catalyst for reduction of the yield of tar appears promising, provided that the level of oxygen transport can be restricted.

Conversion of fuel nitrogen in a dual fluidized bed steam gasifier

During gasification, fuel nitrogen is converted into gaseous species, such as NH3, HCN and others. Several materials are gasified in the dual fluidized bed gasification pilot plant in order to assess the conversion of fuel nitrogen. The fuels tested in this study are different kinds of waste wood, bark and plastic residues. The nitrogen content of these materials ranges from 0.05 to 2.70wt.-%. Detailed measurements of NH3, N2, HCN, NO and nitrogenous tars are carried out during the test runs. It is found that the vast majority of nitrogen is present in the form of NH3. There is a linear relationship with high accuracy between fuel nitrogen and NH3 in the producer gas. The nitrogen balance of the dual fluidized bed gasification system shows the distribution of nitrogen in the two coupled reactors of the gasification system. It is assessed that nitrogen conversion occurs almost exclusively in the gasification reactor. Only minor amounts of nitrogen are found in the char, which is transported to the combustion reactor and is converted to NO there. This result provides important information for the gas cleaning requirements when nitrogen-rich fuels are gasified.

Potential approaches to improve gasification of high water content biomass rich in cellulose in dual fluidized bed

Biomass containing water of 30–65 wt.% and rich in cellulose, such as various grounds of drinking materials and the lees of spirit and vinegar, is not suitable for biological digestion, and the thermal conversion approach has to be applied to its conversion into bioenergy. The authors have recently worked on converting such biomass into middle heating-value gas via dual fluidized bed gasification (DFBG) integrated with various process intensification technologies. This article is devoted to highlighting those technical ways, including the choice of the superior technical deployment for a DFBG system, the impregnation of Ca onto fuel in fuel drying, the integration of gas cleaning with fuel gasification via two-stage DFBG (T-DFBG), and the decoupling of fuel drying/pyrolysis and char gasification via the decoupled DFBG (D-DFBG). The attained results demonstrated that the superior deployment of bed combination for the DFBG should be a bubbling/turbulent fluidized bed gasifier integrated with a pneumatic riser combustor. In terms of improving efficiency of fuel conversion into combustible gas and suppressing tar generation during gasification, the impregnation of Ca onto fuel exhibited distinctively high upgrading effect, while both the T-DFBG and D-DFBG were also demonstrated to be effective to a certain degree.

Buildup of high solids flux conveying flow by coupling a moving bed to the riser bottom

Circulating fluidized bed (CFB) technology was first developed for the fluid catalytic cracking of crude oil in the 1940s, which is mainly characterized by circulating the solids between two adjacent fluidized vessels. Based on the CFB technology, dual fluidized bed gasification (DFBG) was proposed in the 1980s, but it received renewed interest in recent years as a high efficiency solids fuel gasification process for the production of high-quality syngas. In an autothermal DFBG system, heat transfer from the exothermal fluidized bed combustor to the endothermal fluidized bed pyrolyzer/gasifier, especially for pure steam gasification, relies on rapid and large solids circulation between the two vessels. A higher solids circulation rate, Gs, allows small temperature difference between the two reactors, thus making the operation and control of the DFBG system much easier. Transport bed gasification (TBG) is another newly developed thermochemical conversion method based on the CFB technology. The TBG process has the advantages of low tar evolution, modest reaction temperature, and high Correspondence concerning this article should be addressed to G. Xu at gwxu@ home.ipe.ac.cn

Process analysis of a dual fluidized bed biomass gasification system

Process data from a biomass dual fluidized bed gasification system were utilized for process modeling and analysis. The objective of the study was to elucidate the effect of the process operating conditions, i.e., the temperature, the steam, the residence time, and biomass feedstock composition, on the performance of the gasifier. The results indicate that there are two distinct regions, the residence time and the equilibrium-dominated regimes. The breakpoints of the two regions were not contiguous. The temperature-residence time interactions determined the temperatures at which these occurred. For optimum performance, the gasification system temperature was chosen to be at an intermediate point where both of the phenomena prevail. Product gas compositions were found to be dependent, based on experimental and equilibrium studies with a number of feedstocks, on the composition of the feedstock. The hydrogen-to-oxygen ratio in the biomass was determined to be the most important parameter that affected the product distribution.

Pyrolysis of Municipal Solid Waste in Japan

Funabashi City’s Municipal Solid Waste Pyrolysis Plant is the first full-scale plant having a dual fluidized bed gasification system. The plant has the capacity of processing 4.5 × 105 kg/day of mixed municipal solid waste with a very limited emission of air, water and land pollutants. The energy is recovered as high calorific value fuel gas. Since April 1983, the plant has been in continuous operation. The purpose of this paper is to report on the system and the experience obtained during the 5 mo in which it was in operation. Data on the material balance of the pyrolysis, the analysis regarding the gas produced, the flue gas composition and the equipment used are included.