Fluidized Bed Biomass Gasifier Research Articles

Tar abatement for clean syngas production during biomass gasification in a dual fluidized bed

Syngas obtained from biomass gasification needs to fulfil strong purity requirements before being used as raw material in power energy generation or chemicals manufacturing. The use of hot catalytic filter candles inside the freeboard of fluidized bed gasifiers allows obtaining clean syngas without dust and low tar content. The tar removal efficiency of four different catalytic filter designs was evaluated with real biomass tar produced in situ in a dual fluidized bed gasifier (DFBG). The tar conversion reached at the outlet of the fluidized bed gasifier was larger for the candles with catalytically active layer design. If a monolith is also incorporated, the tar conversion increases up to 95% which is one of the highest values obtained up to date. In this case, the tar content at the outlet of the catalytic filter was as low as 0.2g/Nm3 (N2 free, d.b.).

Towards an advanced reactor network modeling framework for fluidized bed biomass gasification: Incorporating information from detailed CFD simulations

Fluidized bed biomass gasification (FBBG) is a promising technology to enable the thermochemical conversion of biomass to drop-in fuels. Fluidized bed reactors are utilized for solid to gas conversion processes due to their ability to provide a high degree of gas–solid contact, fast solid–solid mixing and fast gas mixing within the bed-zone due to solids-induced flow. In many reactor models of fluidized bed gasifiers this has lead researchers to assume that the bed zone can be modeled as a continuously-stirred tank reactor (CSTR). In this work the limitations of this model are analyzed with reactive CFD simulations and an improved reactor network model (RNM) framework based on two-phase theory (TPT) is proposed which is capable of capturing the influence of mixing in the bed-zone on the thermochemical conversion. This new RNM framework employs reactive CFD modelling to supply hydrodynamic information to the RNM. It is shown that this improved RNM framework is able to better capture the formation of large polycyclic aromatic hydrocarbon (PAH) compounds implying that their formation is strongly dependent on the availability of rich zones in the emulsion phase.

Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite

Active bed materials are in this work investigated for in situ gas upgrading of biomass-derived gas. Previous research on in situ gas upgrading has focused on assessing gas quality, in terms of the concentrations of tar and permanent gases. Other aspects of fuel conversion, such as char conversion and the impact of oxygen transport on the final gas, are not as well documented in the literature on gasification. In this paper, the overall biomass conversion in a dual fluidized bed biomass gasifier is investigated in the presence of the catalytic material olivine and the alkali-binding material bauxite. The impact of these materials on fuel conversion is described as the combination of four effects, which are induced by the bed material: thermal, catalytic, ash-enhanced catalytic effect, and oxygen transport. Quartz-sand and ilmenite are here used as the reference materials for the thermal and the oxygen transport effects, respectively. Olivine and bauxite show activity toward tar species compared to quartz-...

CFD studies on biomass gasification in a pilot-scale dual fluidized-bed system

A comprehensive CFD (Computational Fluid Dynamics) model using the MP-PIC (Multiphase Particle-In-Cell) method was developed to simulate a pilot-scale (6 tons/day, 1 MW th) dual fluidized-bed biomass gasification system. In this model the particulate phase was described with the blended acceleration model. The momentum, mass, and energy transport equations were integrated with the kinetics of heterogeneous biomass and char reactions and homogeneous gas-phase reactions to predict the particle circulation, producer gas composition, and reactor temperature. The simulation results were compared with experimental data from the pilot-scale gasification system to validate the model at different operating conditions. Parametric studies were conducted to investigate the impact of gasifier temperature, steam to biomass ratio (S/B), and air supply to the combustor on the producer gas composition. The studies showed that increasing gasifier temperature and steam to biomass ratio (S/B) promoted syngas (CO + H2) production and increased hydrogen content in producer gas. The effect of air supply was minor, because for the dual fluidized-bed system air was not directly involved in biomass gasification.

Circulating fluidized bed gasification of biomass for flexible end-use of syngas: a micro and nano scale study for production of bio-methanol

Biomass gasification in circulating fluidized bed gasifiers (CFBGs) is regarded as one of the major options for clean heat and power generation, due to good gas-solid mixing. In this paper, we developed an Aspen Plus model to simulate the methanol production from biomass derived syngas by integrating the bed hydrodynamics and reaction kinetics. Particular emphasis placed on the influence of steam/biomass ratio (S/B), gasification temperature, equivalence ratio (ER), and biomass particle size (BPS) on methanol yield. The simulation results demonstrated that ER and gasification temperature are the most important factors in the process; higher ER and temperature contributed to lower and higher methanol yield, respectively. Model predictions were compared with available data from the literature, which showed fully good agreement.

Experimental investigation of the effect of physical pre-treatment on air-blown fluidized bed biomass gasification

The effect of comminution, drying, and densification on bubbling fluidized bed gasification was investigated by fractionating a forestry residue into a feedstock consisting of different particle sizes, moisture levels, and by densifying to pellets. The gasification performance was evaluated at nominal average bed temperatures of 725°, 800° and 875 °C at a constant fluidizing velocity (0.91 m s−1) with feed input rates between 9 and 24 kg h−1.The gas composition was observed to be influenced by both the particle size and form. Smaller particles led to a gas richer in carbon monoxide and depleted in hydrogen. The gasification of pellets led to a gas with the greatest hydrogen to carbon monoxide ratio. The smallest particles tested resulted in the worst gasification performance, as defined by cold gas efficiency, carbon conversion, and tar production. Despite differences in the gas composition among the larger particles and the pellets, similar carbon conversion and cold gas efficiency was observed.Relative to comparable test conditions with dry feed fractions (having a moisture mass fraction of 7–12%), an average 11% increase in carbon conversion was observed for the wetter feed fractions containing a moisture mass fraction of 24–31%. This increase in carbon conversion offset much of the expected decrease in cold gas efficiency by using a wetter feed material. A slight increase in hydrogen production and negligible change in tar production was observed for the wetter feed fractions relative to the dry feed fraction.

On-line measurement and kinetic studies of sodium release during biomass gasification and pyrolysis

The release of sodium during biomass gasification and pyrolysis in fluidized bed was systematically investigated based on an on-line measurement system. The temperature and atmosphere had strong influences on sodium release behavior. The pyrolysis atmosphere was conducive to the release of sodium than the CO2 gasification atmosphere at 900°C. The overall kinetic parameters of sodium release during two thermal conversion processes were obtained based on the experimental results of instantaneous sodium concentration in flue gas and inverse method. Furthermore, the influence of chlorine on sodium release was studied based on the on-line measurement system. The promoting effect of chlorine on Na release in CO2 gasification was much more obvious than pyrolysis. Thermodynamic equilibrium calculations were conducted to identify the sodium species during biomass gasification and pyrolysis. The formation of Na2CO3 (s, l) mainly accounted for the lower release percentage of sodium in CO2 gasification than pyrolysis.

Tar abatement in a fixed bed catalytic filter candle during biomass gasification in a dual fluidized bed

Catalytic filters are a novel technology for tar conversion in biomass gasification processes. Both particle elimination and tar abatement can be achieved in a single step at high temperatures minimizing energy efficiency penalties. This paper analyses the performance of a fixed bed catalytic filter candle in the reduction of biomass tar generated in situ in a dual fluidized bed gasifier (DFBG). In this study, the temperature of the filter was limited to 800°C. Several variables affecting the performance of the filter were tested. Experiments at different gasification temperatures in the range 750–850°C were performed so that the amount of tar reaching the catalytic filter was varied. The amount of tar at the catalytic filter inlet did not affect the tar conversion achieved which was around 75%. The major tar compound in the gasification gas at the outlet of the catalytic filter was naphthalene. At the highest temperature tested (850°C), the tar content in the clean gas was 0.65g/Nm3. The effect of the face velocity in the filter on the tar conversion reached was evaluated. Tar conversion decreased when the face velocity increased from 40 to 90m/h as the residence time of the gas in the catalytic filter was lower. The ratio H2O to dry biomass was also varied (0.5–0.9) in order to produce tars of different nature. Higher conversions were achieved for higher values of this ratio.

Sequential Modular Simulation of Hydrodynamics and Reaction Kinetics in a Biomass Bubbling Fluidized-Bed Gasifier Using Aspen Plus

A sequential modular simulation (SMS) approach was used to simulate hydrodynamics and detailed kinetics of a fluidized-bed biomass gasifier in Aspen Plus. The kinetics of tar cracking reactions was taken into account in the simulation. The effects of operating conditions including temperature, equivalence ratio (ER), and steam-to-biomass ratio (SBR) on the composition and the lower heating value (LHV) of the effluent gas were studied and compared with experimental data. The model predictions well agreed with the experimental data. The increase of the bed temperature significantly decreased the tar content and increased the hydrogen content of the product gas. At ER = 0.3, the increase of the temperature from 973 K to 1123 K resulted in the increase of H2 molar concentration in the product gas from 7.6% to 11.3% and CO molar concentration from 13.1% to 17.0%. At a temperature of 1073 K, the optimum ER value was 0.3 and the increase in ER from 0.2 to up to 0.3 increased the amount of fuel gases but further increases in ER shifted the system kinetics toward the combustion regime. At 1073 K and ER = 0.3, with an increase in SBR from 0 to 1.0, H2 and CO2 concentrations increased from 9.3% and 13.3% to 10.8% and 14.7%, respectively, and CO concentration decreased from 15.8% to 12.9%. The analysis showed the SMS model with four stages gives the most satisfactory predictions, via comparison with the experimental data.

Three-dimensional full-loop simulation of a dual fluidized-bed biomass gasifier

A three-dimensional CFD model was developed to simulate the full-loop of a dual fluidized-bed biomass gasification system consisting of a gasifier, a combustor, a cyclone separator, and a loop-seal. This full-loop simulation includes the chemical kinetic modeling of biomass drying and pyrolysis, heterogeneous char reactions, and homogeneous gas-phase reactions. In the model, the gas phase is described using Large Eddy Simulation (LES) and the particle phase is described with the Multiphase Particle-In-Cell (MP-PIC) method. The simulation was performed using the GPU-accelerated computing and the simulation results were compared with the gas composition and temperature measurements from a pilot-scale biomass gasification power plant (1MWth, 6tonsbiomass/day). The independence of the accuracy of the model on mesh resolution and computational particle number was determined. The impacts of the particle size distributions (PSD) and drag models on the reactive flows were also investigated.

Sequence-based Process Modeling of Fluidized Bed Biomass Gasification

Despite tremendous effort to model fluidized bed biomass gasifiers, as major sustainable waste-to-energy devices, current equation-oriented approaches suffer from implementation difficulties. In this research, a comprehensive cocurrent sequence-based process model is introduced to simulate bottom-fed bubbling fluidized bed biomass gasifiers (BFBGs). The gasifiers include two operating regions, namely dense bed and freeboard. The dense bed is divided into several sections of logically ordered ideal reactors to describe the behavior of interacting phases, i.e., bubble and emulsion. The bubble phase is well characterized by an ideal plug flow reactor (PFR), and the emulsion phase is simulated as a continuous stirred-tank reactor (CSTR). The freeboard is successfully mimicked with a PFR. Hydrodynamic and kinetic submodels describe physical and chemical phenomena taking place in the gasifiers, respectively. A dynamic two phase model is adopted as the hydrodynamic submodel, and the kinetic submodel is derived f...

Screening method for solvent selection used in tar removal by the absorption process

The aim of this paper is the study of the treatment of flue gas issued from a process of biomass gasification in fluidized bed. The flue gas contains tar which should be selectively removed from the fuel components of interest (e.g. H2, CO and light hydrocarbons) to avoid condensation and deposits in internal combustion engine. The chosen flue gas treatment is the gas–liquid absorption using solvents, which present specific physicochemical properties (e.g. solubility, viscosity, volatility and chemical and thermal stability) in order to optimize the unit on energetic, technico-economic and environmental criteria. The rational choice of the proper solvent is essential for solving the tar issue. The preselection of the solvents is made using a Hansen parameter in order to evaluate the tar solubility and the saturation vapour pressure of the solvent is obtained using Antoine law. Among the nine families of screened solvents (alcohols, amines, ketones, halogenates, ethers, esters, hydrocarbons, sulphured and chlorinates), acids methyl esters arise as solvents of interest. Methyl oleate has then been selected and studied furthermore. Experimental liquid–vapour equilibrium data using bubbling point and absorption cell measurements and theoretical results obtained by the UNIFAC-Dortmund model confirm the high potential of this solvent and the good agreement between experimental and theoretical results.

Experimental study and characterization of a two-compartment cylindrical internally circulating fluidized bed gasifier

This study aims to improve the heating value (HV) of gas produced during the gasification of sawdust using a new type of air-blown gasifier based on a two-compartment cylindrical fluidized bed. The gasification process is based on an internally circulating aerated fluidized bed (ICAFB) and consists of two individual zones; a gasification zone in the annulus and combustion zone in the draft tube. The annulus and draft tube are connected via orifices at the lower section of the draft tube wall to enable the solids to move from the annulus to the draft tube. Char, gasified residue, together with hot bed material move to the draft tube and combust to produce heat. Fluidization ratio (RQ) which is the ratio of air flow rate in the annulus (Qan) to the air flow rate in the draft tube (Qdt) together with equivalence ratio (ER) were used to control the temperature of the gasifier. The effects of ER on the temperature system stabilization, composition, HV of producer gas (PG) and gasifier performance were presented. ER ranges from 0.148 to 0.203 were used in this study. PG compositions of 3.13%, 2.28%, 25.8%, 8.2% and 22.48% for H2, O2, CO, CH4 and CO2 respectively with a HV of about 6.96 MJ m−3 were obtained at an ER of 0.185 and temperature of about 866 °C. Compared with previous air-blown fluidized bed biomass gasification results, the ICAFB gasifier is capable of producing producer gas with higher CO and CH4 fractions.

Prediction and Validation of Major Gas and Tar Species from a Reactor Network Model of Air-Blown Fluidized Bed Biomass Gasification

The thermochemical conversion of biomass via gasification offers a promising approach to producing fungible substitutes for petroleum-derived fuels and chemicals. The kinetic study of the gas-phase reactions of biomass gasification is key to understanding fluidized bed biomass gasification (FBBG). Under typical operating conditions for air-blown FBBG (700–1000 °C), tars exist in the product gas in significant quantities (2–50 g/Nm3). Predicting the formation and evolution of tars in a FBBG reactor model is particularly important as they introduce several operational and cleanup challenges in practice. However, such predictions require implementation of detailed chemical kinetic mechanisms due to the large number of species and competing conversion pathways involved. A detailed gas-phase mechanism has been proposed by the CRECK modeling group at Politecnico di Milano encompassing the secondary pyrolysis, cracking, and oxidation reactions of the devolatilization species of biomass, as well as the oxidation and combustion reactions of the resultant gas-phase hydrocarbon species. In this work, a one-dimensional reactor network model (RNM) of an air-blown fluidized bed gasifier utilizing this detailed chemistry model is developed and validated for the prediction of major gas-phase species and tar compounds. It is found that this RNM is able to accurately predict the syn-gas production and total tar concentration given a modification of water gas shift and/or CO oxidation kinetics to account for catalytic effects of the biomass ash and char. Additionally, validation of the predicted tar composition is attempted against available experimental measurements. Good agreement is achieved for single-ring aromatic and oxygenated tar compounds, while it is found that polycyclic aromatic hydrocarbons are underpredicted by more than an order of magnitude. Finally, the conversion pathways of representative devolatilization products of biomass—levoglucosan, xylofuranose, and p-coumaryl—are analyzed in the context of syn-gas and tar formation routes.

Parametric analysis of a circulating fluidized bed biomass gasifier for hydrogen production

Biomass is considered a potential energy source which can be efficiently converted to useful gaseous products via a gasification process. Circulating fluidized bed (CFB) gasifiers have attracted significant attention due to their high reaction rates and thermal efficiency. This study aims to investigate the CFB biomass gasification process to generate H2-rich synthesis gas. A process simulator is used to analyze the gasifier performance by assuming that the gasification is fast and reach equilibrium. Parametric analysis of the CFB gasifier shows that steam gasification generates the synthesis gas attained the highest H2 content (50–65 vol.%) and the highest product gas quality (higher heating value, HHV = 10–13 MJ/Nm3) at operating temperatures approximately 650–700 °C. High-temperature steam cannot provide enough energy for the gasifier, reducing the gross cold gas efficiency of this process to only 16%. The biomass air-steam gasification process is investigated while avoiding high energy consumption, but less H2 is produced under these conditions.

Segregation of wood particles in a bubbling fluidized bed

Understanding wood segregation in bubbling fluidized beds is of importance for the global analysis, comprehensive modeling, and scale-up of bubbling fluidized bed biomass gasifiers. This study presents measurements of voidage and segregation taken during experiments in fluidized beds containing inert materials mixed with biomass particles. The features of the wood particles affect the bed axial homogeneity; a small particle size or a high density increase homogeneity. When gas fluidization velocity is low, the biomass particles are more segregated from olivine compared to the results achieved with larger velocities. In all experiments, a significant portion of the wood remains within the bed (50–80%), while the rest of the biomass floats at the bed surface. The experimental modeling accurately represents the voidage of the bed, with the exception of the upper part of the bed and the floating portion of the biomass particles. The results of the present study are in agreement with recent publications.

Development of a modelling tool representing biomass gasification step in a dual fluidized bed

The aim of the present study is to develop a 1D modelling tool representing biomass gasification in a dual fluidized bed. This tool should be used with the objective to control and scale‐up the process for different pilot scales and operating conditions. This implies that the modelling tool should be based on a phenomenological description but also that the calculations are not too much time‐consuming. A one‐dimensional steady state model was thus developed for the gasification reactor, combining a description of the biomass thermochemical conversion phenomena and of the hydrodynamic behaviour of the fluidized bed. The reactor is divided into two zones, namely, dense zone and freeboard zone. For the dense zone, the model assumes the existence of two phases –bubble and emulsion– with chemical reactions occurring in both phases (biomass devolatilization, char gasification, water‐gas shift). In the freeboard, a plug flow is assumed and the only reaction considered is water‐gas shift. Mass balance is globally solved across the entire gasifier. The model allows calculating axial concentrations of the gas species in the reactor as well as the global yields. The parameters of the calculations are temperature of the gasifier, biomass composition, biomass and steam feeding rates.

On the flow direction effect in sequential modular simulations: A case study on fluidized bed biomass gasifiers

Feed location is a key parameter in the performance of hydrogen production multi-phase reactors. While the current sequential-based models successfully simulate multi-phase fluidized reactors (FR) with cocurrent feeding, no general countercurrent-feed sequential model is available to predict the behaviour of complex top-feeding systems. In this work, the modular process simulation approach is expanded to simulate the FB-based hydrogen production with opposite feed directions. The model consists of novel logically-ordered plug flow reactors (PFRs) and continuous stirred-tank reactors (CSTRs), accounting for the feed location using a recycle flow, termed as solid circulation factor (XC,i). As a case study, a fluidized bed gasifier for green fuel production is simulated, taking the hydrodynamic and reaction kinetic sub-models into consideration. The dynamic two-phase (DTP) model describes the physical phenomena in the hydrodynamic sub-model, and a comprehensive set of reaction kinetic models derived from literature, including tar conversion, provides the reaction sub-model for a biomass gasifier. By integrating these sub-models, the fluidized bed reactor is divided into two main regions, namely dense bed and freeboard. The dense bed is segmented into several sections, each of which treating the gas flow as a plug flow through the bubbles and as a completely-mixed flow in the emulsion phase. The freeboard section is well simulated employing a plug flow reactor. Satisfactory agreement between the model prediction and the experimental data is achieved when the countercurrent flow and full kinetic model is used, while cocurrent flow and/or equilibrium simulation bring about poor predictions. This work paves the way toward designing a comprehensive fluidized bed sequential module to be integrated with the industrial process simulators.

Conversion of Phenol-Based Tars over Biomass Char under H2 and H2O Atmospheres

This study focused on the effect of a wood char on tar cracking and reforming in the context of fluidized-bed gasification. Phenol was used to create in situ a mixture representative of tars produced by pyrolysis and gasification in a dual fluidized-bed reactor. The wood char was placed in a fixed-bed reactor, and atmospheres composed of H2 and steam were tested both together and separately. Gasification of the wood char under the different atmospheres was characterized. The char was found to be highly catalytic with tars in the presence of steam. The influence of H2 on catalytic activity remained low. A comparison to the results on olivine under the same reaction conditions showed that chars were much more efficient and may have a strong influence on tar reduction in fluidized-bed biomass gasification.

Internal tar/CH4 reforming in a biomass dual fluidised bed gasifier

An internal reformer is developed for in situ catalytic reforming of tar and methane (CH4) in allothermal gasifiers. The study has been performed in the 150 kW dual fluidised bed (DFB) biomass gasifier at Mid Sweden University (MIUN). The MIUN gasifier is built for research on synthetic fuel production. Reduction of tars and CH4 (except for methanation application) in the syngas is a major challenge for commercialization of biomass fluidised-bed gasification technology towards automotive fuel production. The MIUN gasifier has a unique design with an internal reformer, where intensive contact of gas and catalytic solids improves the reforming reactions. This paper presents an initial study on the internal reformer operated with and without Ni-catalytic pellets, by evaluation of the syngas composition and tar/CH4 content. A novel application of Ni-catalyst in DFB gasifiers is proposed and studied in this work. It can be concluded that the reformer with Ni-catalytic pellets clearly gives a higher H2 content together with lower CH4 and tar contents in the syngas than the reformer without Ni-catalytic pellets. The gravimetric tar content decreases down to 5 g/m3 and the CH4 content down below 6 % in the syngas. The tar content can be decreased further to lower levels, with increased gas contact to the specific surface area of the catalyst and increased catalyst surface-to-volume ratio. The new design in the MIUN gasifier increases the gasification efficiency, suppresses the tar generation and upgrades the syngas quality.