Numerical and experimental study of hydrodynamics in a compartmented fluidized bed oil palm shell biomass gasifier

Numerical and experimental studies of hydrodynamic parameters of fluidized beds formed by either a single component system or a binary mixture in a pilot plant scale model of a Compartmented Fluidized Bed Gasifier (CFBG) have been performed. The numerical study is carried out with an Eulerian-Eulerian description of both gas and particle phases and a standard drag law for multiphase interaction. The numerically simulated results are then compared with the experimental results.The 2D and 3D flow patterns of the combustor and the gasifier are first generated from the numerical study to observe the bubble formation, possible channeling behavior and the binary mixing patterns in the bed.For a single component system, detailed 3D numerical analyses and experimental studies are done to investigate the bed expansion ratio, bubble diameter, bed pressure drop, and fluidization quality in CFBG. Two types of Geldart B inert particles namely river sand and alumina are used in the study.All trends of the aforementioned studies are well-predicted with the numerical values not greater than 15% of the recorded experimental values. Good fluidization is attainable in the combustor side, while the pressure drop behaviour seen for the gasifier with river sand shows that channelling occurs in the bed. The channelling behaviour becomes more severe with alumina bed.The solid circulation rate (SCR) is numerically simulated in this study as well. Solid circulation rate (SCR) increases with the increase in bed height while the main bed aeration does not affect the SCR which is consistent with the experimental data.For a binary mixture system with palm shell and river sand as the second fluidizing material, detailed 3D numerical analysis of the bed expansion ratio is done in parallel with the experimental study. The results of numerical predictions of overall mixing quality and local mixing index are verified by comparing with the experimental results. The actual trends of the studies are modestly captured by the numerical model with under-predicted values of less than 20%. The overall binary mixing quality is enhanced with the smaller palm shell size and larger palm shell weight percent. In addition, increasing the superficial gas velocity increases the local binary mixing index in the experiment.From the studies on bed expansion, bubble formation, steady equilibrium state and overall binary mixing quality, the 2D model provides well over-predicted values compared to the 3D flow model. Also, the local mixing index of the binary system is not captured by the 2D model. The numerical values predicted by 3D model are closer to the actual values.The key findings from the aforementioned studies are used as a guide to develop and operate the pilot plant scale CFBG with 0.5 ton/day of palm shell feed for fuel gas production

CFD modeling of biomass combustion and gasification in fluidized bed reactors

Biomass is an environmentally friendly renewable energy source and carbon-neutral fuel alternative. Direct combustion/gasification of biomass in the dense particle-fluid system is an important pathway to biomass energy utilization. To efficiently utilize biomass for energy conversion, a full understanding of biomass thermal conversion in lab/industrial-scale equipment is essential. This thesis aims to gain a deeper understanding of the physical and chemical mechanisms of biomass combustion/gasification in fluidized bed (FB) furnaces using computational fluid dynamics (CFD) simulations. A three-dimensional reactive CFD model based on the Eulerian-Lagrangian method is developed to investigate the hydrodynamics, heat transfer, and gasification/combustion characteristics of biomass in multiple-scale FB furnaces. The CFD model considered here is based on the multi-phase particle-in-cell (MP-PIC) collision model and the coarse grain method (CGM). CGM is computationally efficient; however, it can cause numerical instability if the clustered parcels pass through small computational cells, resulting in the over-loading of solid particles in the cells. To address this issue, a distribution kernel method (DKM) is proposed. This method is to spread the solid volume and source terms of the parcel to the surrounding domain. The numerical stiffness problem caused by the CGM clustering can be remedied using DKM. Validation of the model is performed using experimental data from various lab-scale reactors. The validated model is employed to investigate further the heat transfer and biomass combustion/gasification process. Biomass pyrolysis produces a large variety of species in the products, which poses great challenges to the modeling of biomass gasification. A conventional single-step pyrolysis model is widely employed in FB simulations due to its low computational cost. However, the prediction of pyrolysis products of this model under varying operating temperatures needs to be improved. To address this issue, an empirical pyrolysis model based on element conservation law is developed. The empirical parameters are based on a number of experiments from the literature. The simulation results agree well with the experimental data under differentoperating conditions. The pyrolysis model improves the sensitivity of gasification product yields to operating temperature. Furthermore, the mixture characteristics of the biomass and sand particles and the effect of the operating conditions on the yields of gasification products are analyzed. The validated CFD model is employed to investigate the fluidization, combustion, and emission processes in industrial-scale FB furnaces. A major challenge in the CFD simulation of industrial-scale FB furnaces is the enormous computational time and memory required to track quadrillions of particles in the systems. The CFD model coupling MP-PIC and CGM greatly reduces the computational cost, and the DKM overcomes the unavoidable particle overloading issue due to the refined mesh in complex geometry. The CFD predictions agree well with onsite temperature experiments in the furnace. The CFD results are used to understand the granular flow and the impact of operating conditions on the physical and chemical processes in biomass FB-fired furnaces

Modelling and simulation of biomass gasification in a circulating fluidized bed reactor

Computational Fluid Dynamics (CFD) has found great acceptance among the engineering community as a tool for research and design of processes that are practically difficult or expensive to study experimentally. One of these processes is the biomass gasification in a Circulating Fluidized Bed (CFB). Biomass gasification is the thermo-chemical conversion of biomass at a high temperature and a controlled oxygen amount into fuel gas, also sometime referred to as syngas. Circulating fluidized bed is a type of reactor in which it is possible to maintain a stable and continuous circulation of solids in a gas-solid system. The main objectives of this thesis are four folds: (i) Develop a three-dimensional predictive model of biomass gasification in a CFB riser using advanced Computational Fluid Dynamic (CFD) (ii) Experimentally validate the developed hydrodynamic model using conventional and advanced measuring techniques (iii) Study the complex hydrodynamics, heat transfer and reaction kinetics through modelling and simulation (iv) Study the CFB gasifier performance through parametric analysis and identify the optimum operating condition to maximize the product gas quality. Two different and complimentary experimental techniques were used to validate the hydrodynamic model, namely pressure measurement and particle tracking. The pressure measurement is a very common and widely used technique in fluidized bed studies, while, particle tracking using PEPT, which was originally developed for medical imaging, is a relatively new technique in the engineering field. It is relatively expensive and only available at few research centres around the world. This study started with a simple poly-dispersed single solid phase then moved to binary solid phases. The single solid phase was used for primary validations and eliminating unnecessary options and steps in building the hydrodynamic model. Then the outcomes from the primary validations were applied to the secondary validations of the binary mixture to avoid time consuming computations. Studies on binary solid mixture hydrodynamics is rarely reported in the literature. In this study the binary solid mixture was modelled and validated using experimental data from the both techniques mentioned above. Good agreement was achieved with the both techniques. According to the general gasification steps the developed model has been separated into three main gasification stages; drying, devolatilization and tar cracking, and partial combustion and gasification. The drying was modelled as a mass transfer from the solid phase to the gas phase. The devolatilization and tar cracking model consist of two steps; the devolatilization of the biomass which is used as a single reaction to generate the biomass gases from the volatile materials and tar cracking. The latter is also modelled as one reaction to generate gases with fixed mass fractions. The first reaction was classified as a heterogeneous reaction while the second reaction was classified as homogenous reaction. The partial combustion and gasification model consisted of carbon combustion reactions and carbon and gas phase reactions. The partial combustion considered was for C, CO, H2 and CH4. The carbon gasification reactions used in this study is the Boudouard reaction with CO2, the reaction with H2O and Methanation (Methane forming reaction) reaction to generate methane. The other gas phase reactions considered in this study are the water gas shift reaction, which is modelled as a reversible reaction and the methane steam reforming reaction. The developed gasification model was validated using different experimental data from the literature and for a wide range of operating conditions. Good agreement was observed, thus confirming the capability of the model in predicting biomass gasification in a CFB to a great accuracy. The developed model has been successfully used to carry out sensitivity and parametric analysis. The sensitivity analysis included: study of the effect of inclusion of various combustion reaction; and the effect of radiation in the gasification reaction. The developed model was also used to carry out parametric analysis by changing the following gasifier operating conditions: fuel/air ratio; biomass flow rates; sand (heat carrier) temperatures; sand flow rates; sand and biomass particle sizes; gasifying agent (pure air or pure steam); pyrolysis models used; steam/biomass ratio. Finally, based on these parametric and sensitivity analysis a final model was recommended for the simulation of biomass gasification in a CFB riser

The effect of particle polydispersion in a gasifier bed dynamics using Eulerian-Eulerian models

The influence of the coal particle size distribution on the performance of a fluidized bed gasifier is investigated by means of a multiphase Eulerian-Eulerian model embodied in the OpenFOAM CFD code. To distinguish the effects of combustion from those of particle segregation we compare the solutions obtained by modeling of solids as, alternatively, monodispersed or polydispersed particles; and as reactive or inert ones. Results reveal that the preferential motion of the smaller particles towards the bed top significantly delays char consumption at the bottom layers, favors volatile oxidation, and results in a more uneven lateral distribution of the off-gas

A hydrodynamic model for biomass gasification in a circulating fluidized bed riser

This study presents a three-dimensional Computational Fluid Dynamic (CFD) model and experimental measurements of the hydrodynamics in the riser section of a Circulating Fluidized Bed (CFB) biomass gasifier consisting of a binary mixture of polydisperse particles. The model is based on multi-fluid (Eulerian-Eulerian) approach with constitutive equations adopted from the Kinetic Theory of Granular Flow (KTGF). The study first presents an assessment of the various options of the constitutive and closure equations for a binary mixture followed by sensitivity analysis of the model to the solution time step, cell size, turbulence and the alternative formulations of the granular energy equation. Accordingly, a robust and reliable hydrodynamic model is recommended and validated using conventional pressure measurements and Positron Emission Particle Tracking (PEPT) technique. Furthermore, the model predictions and experiments revealed evidence of the particle re-circulation within the lower part of the riser, which is an important feature contributing to rapid mass and heat transfer in a CFB gasifier. The present hydrodynamic model can be further developed; by incorporating appropriate reactions and heat transfer equations, in order to fully predict the performance and products of a CFB biomass gasifier

Modeling and Simulation of an Oxygen-Blown Bubbling Fluidized Bed Gasifier using the Computational Particle-Fluid Dynamics (CPFD) Approach

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CFD Modeling of Biomass Gasification Using a Circulating Fluidized Bed Reactor

Biomass, as a renewable energy resource, can be utilized to generate chemicals, heat, and electricity. Compared with biomass combustion, biomass gasification is more eco-friendly because it generates less amount of green gas (CO2) and other polluting gases (NOx and SO2). This research is focused on biomass gasification using a circulating fluidized bed. In the gasifier, fully fluidized biomass particles react with water vapor and air to generate syngas (CO and H2). A comprehensive model, consisting of three modules, hydrodynamics, mass transfer and energy transfer modules, is built to simulate this process using ANSYS Fluent software and C programming language. In the hydrodynamics module, the k-epsilon turbulence equations are coupled with the fluctuating energy equation to simulate gas-particle interaction in the turbulent flows occurring in the riser. In the mass transfer and energy transfer modules, heat transfer and mass transfer in turbulent flows are simulated to solve for the profiles of temperature and species concentration in the gasifier. The impacts of thermal radiation, water gas shift reaction (WGS), equivalence ratio (ER), and char combustion product distribution coefficient are also investigated to gain deeper understanding of biomass gasification process

Multi-scale modelling of fluidized bed biomass gasification using a 1D particle model coupled to CFD

For many fluidized bed applications, the particle movement inside the reactor is accompanied by reactions at the particle scale. The current study presents for the first time in literature a multi-scale modelling approach coupling a one-dimensional volumetric particle model with the dense discrete phase model (DDPM) of ANSYS Fluent via user defined functions. To validate the developed modelling approach, the current study uses experimental data of pressure drop, temperature and gas composition obtained with a lab-scale bubbling fluidized bed biomass gasifier. Therefore, a particle model developed previously for pyrolysis was modified implementing a heat transfer model valid for fluidized bed conditions as well as kinetics for char gasification taken from literature. The kinetic theory of granular flow is used to describe particle¿particle interactions allowing for feasible calculation times at the reactor level whereas an optimized solver is employed to guarantee a fast solution at the particle level. A newly developed initialization routine uses an initial bed of reacting particles at different states of conversion calculated previously with a standalone version of the particle model. This allows to start the simulation at conditions very close to stable operation of the reactor. A coupled multi-scale simulation of over 30 s of process time employing 300.000 inert bed parcels and about 25.000 reacting fuel parcels showed good agreement with experimental data at a feasible calculation time. Furthermore, the developed approach allows for an in-depth analysis of the processes inside the reactor allowing to track individual reacting particles while resolving gradients inside the particle.This project has received funding from European Union's Horizon 2020 Research and Innovation Programme under grant agreement number 731101 (BRISK II). Furthermore, the financial support of the COMET Module project BIO-LOOP (Austrian Research Promotion Agency - FFG - Project Number 872189) funded by the federal government of Austria and the federal province Styria is gratefully acknowledged. The authors want to thank Mario Blehrmühlhuber for conducting cold-flow simulations and evaluating the applicability of the DDPM for the developed model. We further want to thank Markus Braun for his helpful hints when using the DDPM and Simon Schneiderbauer for his advice regarding the coupling strategy.Publicad

Understanding and modeling the formation of syngas contaminants during biomass gasification

The focus of this dissertation was to understand and model how inorganic contaminants (mainly H2S, COS, NH3, and HCN) are formed during biomass gasification to provide information for effective contaminant abatement and producer gas remediation. This dissertation was partitioned into five research studies with specify objectives. In the first study, a simple thermo-gravimetric approach coupled with CHN analyzer and inductively coupled plasma optical emission spectrometry (ICP-OES) was used to track the conversion profile of C, H, N, S, and O during the pyrolysis stage of biomass gasification. The activation energy for the sulfur and nitrogen conversion was drastically lower at 800 °C compared to 600 and 700 °C. Additionally, the elemental concentrations of sulfur and nitrogen were higher for pyrolyzed biomass compared to fresh biomass. In the second study, a non-stoichiometric equilibrium model of biomass gasification was implemented. We demonstrated that the yields of CO, CO2, and H2 during gasification were equilibrium-controlled. However, the yields of CH4 and contaminant species were kinetically-limited. Furthermore, we establish that NH3 + CO ↔ HCN + H2O and H2S + CO2 ↔ COS + H2O reactions were important to nitrogen and sulfur species distribution, respectively. In the third, an inert fluidized bed system was simulated using computational fluid dynamics and discrete element method (CFD-DEM). Also, experimental validation of the developed model was performed on three important hydrodynamic variables of fluidized bed systems (pressure drop, minimum fluidization velocity, and bed height). The CFD-DEM model produced a realistic representation of the particle motion and reasonably predicted the hydrodynamics properties of the experimental system. The fourth and fifth studies were designed to simulate the formation of nitrogen (NH3 and HCN) and sulfur (H2S, COS, SO2) contaminants, respectively, by coupling the developed CFD-DEM model in the third study with appropriate chemical reactions, heat transfer, and particle shrinkage models. We found that the proposed CFD-DEM model gave reasonably prediction for the selected contaminants species. Hence, the proposed model is a valuable tool for gaining insight into the formation and extent of producer gas contaminants

A novel approach for integrating concentrated solar energy with biomass thermochemical conversion processes

Concentrated solar energy provides thermal energy that can be utilised for thermochemical conversion of biomass to produce liquid fuel and gases. This creates an efficient and a carbon-free process. The fast pyrolysis of biomass is an endothermic thermal process that occurs within 400-550oC at fast heating rates of >300 oC/second in the absence of oxygen. This temperature is within the range produced in a parabolic trough arrangement. The process of biomass gasification is the conversion of biomass fuels to non-condensable gases usually for chemical feedstock or as fuel using a fluidising medium. Solar intermittence is a major issue; this can be resolved by proposing a continuous process from concentrated solar energy to fuels or chemical feedstock. Computational fluid dynamics has proven to be a tool for design and optimisation of reactors. The Eulerian-Eulerian multiphase model using ANSYS Fluent has shown to be cost-effective at describing the characteristics of complex processes. The project entails using parabolic trough for fast pyrolysis of biomass; it is integrated with a gasification process with utilities produced entirely from solar energy. The scope of the project are: (i) A Computational fluid dynamic (CFD) model analysis of the novel reactor is to be developed to model biomass pyrolysis (ii) Investigate the potentials of integrating the proposed solar reactor with a conventional circulating fluidised bed (CFB) gasifier to create a highly efficient and sustainable closed loop thermo-solar process (iii) Validate the circulating fluidised bed model with an experimental scale Circulating fluidised bed (CFB) gasifier at Aston University’s European Bioenergy Research Institute. The report studied the use of CFD modelling to investigate fast pyrolysis of switch grass biomass using a solar parabolic trough receiver/reactor equipped with a novel gas-separation system. The separator controls the effect of tar-cracking reactions and achieves high separation efficiency compared to other gas-solid separation methods. The study assumes an average heat flux concentrated along the receiver/reactor. Pyrolysis reaction was represented as a single global first order Arrhenius type reaction with volatiles separated into condensable (bio-oil) and non-condensable products. The drying of moisture of the switch grass was represented as a mass transfer process. The separation efficiency achieved by the conical deflector was about 99%. The proposed reactor at the considered operating conditions can achieve overall energy efficiency of 42%; the product yield consist of 51.5% bio-oil, 43.7% char and 4.8% non-condensable gases. The average reactor temperature, gas residence time, and maximum devolatilisation efficiency were 450 °C, 1.5 s, and 60% respectively. There was good agreement in comparison with experimental findings from literature. A sensitivity analysis was conducted to study the effect of heat flux conditions, heat transfer, sweeping gas temperature, and particle size. The heat flux distribution showed that non-homogeneous provides a greater heating rate and temperature compared to the homogeneous flux. Radiation negligibly affects the final product composition; the radiation heats the biomass mainly rather than cause devolatilisation. The larger the biomass diameter the more bio-oil is produced, when a uniform particle temperature is assumed. An experimental study was conducted for the validation of the hydrodynamic model of a circulating fluidised bed. The experiment measured the pressure profiles and the solid recirculation rate. The experiment result showed that particle size has a negative correlation to the ease of fluidisation. High fluidising gas flowrate has a positive impact on the fluidising regime and pressure in the riser. The following parameters were compared with experimental results: grid size, turbulence model, drag laws, wall treatment, and wall shear properties (specularity coefficient and restitution coefficient). The results proved the optimum hydrodynamic model through comparison of pressure profiles of the model with experimental results. The gasification of char in a circulating fluidised was studied using the optimum hydrodynamic model validated from experiment. The model considered the effect of turbulence on the species evolution and tar reforming with char. Over the range of operating conditions, the results looked into the hydrodynamics and product yield of the gasifier. The product yields obtained for the base case was CO (12%), CO2 (19%), H2 (6%), CH4 (0.7%), and N2 (63%). The results proved that for smaller particles the evolution of species are dominated by kinetics. The catalytic effect of char showed improvement in tar yield and CGE to 15.12g/Nm3 and 67.74%. The product yields showed improvement with the compositions of CO2 and H2 due to reforming reactions. The yields and efficiency were in qualitative agreement with results from literature. The proposed models described will provide details on the procedures for future design of integrated solar biomass thermochemical conversion systems