Reduction of the Volatile Matter Evolution Rate from a Plastic Pellet during Bubbling Fluidized Bed Pyrolysis by Using Porous Bed Material

Rapid volatile matter evolution from high‐volatile fuels such as wastes and biomass is one of problems associated with fluidized bed incinerators and gasifiers. When volatile matter evolves rapidly in the vicinity of the fuel feed point, the mixing of volatile matter with reactant gas is poor, and therefore, unreacted volatile matter is expected to be released from the reactor. In the present work, reduction of the volatile matter evolution rate was attempted by employing porous solids as bed materials instead of nonporous sand. The effect of bed material on the onset of devolatilization was measured by use of a bench‐scale bubbling fluidized bed reactor. Volatile matter capture by the porous solids (capacitance effect) and the heat transfer rate within the bed, both of which affect volatile matter evolution rate, were also measured. Four types of porous solids, both with and without capacitance effect, were employed as the bed material. By employing porous solids without capacitance effect, the contributions of reduced heat transfer rate and capacitance effect to the delay of volatile matter evolution can be evaluated separately. For porous bed materials with a moderate capacitance effect (volatile matter capture of up to 20 %), the delay of the onset of devolatilization, which was measured by detecting the flame combustion of the volatile matter, was explained by the lower heat transfer between the fuel and bed. However, for a porous particle with high capacitance effect (volatile matter capture of 30 %), the capacitance effect also affected the delay of the onset of the flame combustion.     

Reduction of Devolatilization Rate of Fuel During Bubbling Fluidized Bed Combustion Using Porous Bed Material

Plastic waste combustion in bubbling fluidized bed combustors (BFBC) is characterized by the rapid devolatilization of the fuel. Noncombusted hydrocarbons are often formed, which have been reported to promote the formation of dioxins. In this work, porous bed material was employed instead of commonly used non‐porous sand to reduce the devolatilization rate. We measured (1) the heat transfer coefficient between an immersed object (brass sphere) and the bed and (2) the time required for the devolatilization of a plastic pellet after dropping it into the bed at 943 K. For porous particles we found a 30 % lower heat transfer coefficient, delayed onset of devolatilization and prolonged devolatilization time, compared with quartz sand. Therefore, porous particles were found to be effective in suppressing the rapid devolatilization of plastic waste.     

Fluidized‐bed reactor modeling for polyethylene production

Gas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001     

Modelling fluidized bed combustion of high-volatile solid fuels

A model of an atmospheric bubbling fluidized bed combustor operated with high-volatile solid fuel feedings is presented. It aims at the assessment of axial burning profiles along the reactor and of the associated temperature profiles, relevant to combustor performance and operability. The combustor is divided into three sections: the dense bed, the splashing region and the freeboard. Three combustible phases are considered: volatile matter, relatively large non-elutriable char particles and fine char particles of elutriable size. The model takes into account phenomena that assume particular importance with high-volatile solid fuels, namely fuel particle fragmentation and attrition in the bed and volatile matter segregation and postcombustion above the bed. An energy balance on the splashing zone is set up, taking into account volatile matter and elutriated fines postcombustion and radiative and convective heat fluxes to the bed and the freeboard.Results from calculations with a high-volatile biomass fuel indicate that combustion occurs to comparable extents in the bed and in the splashing region of the combustor. Due to volatile matter segregation with respect to the bed, a significant fraction of the heat is released into the splashing region of the combustor and this results in an increase of the temperature in this region. Extensive bed solids recirculation associated to solids ejection/falling back due to bubbles bursting at bed surface promotes thermal feedback from this region to the bed of as much as 80-90% of the heat released by afterburning of volatile matter and elutriated fines. Depending on the operating conditions a significant fraction of the volatile matter may burn in the freeboard or in the cyclone.     

流化床-化学气相沉积技术在先进核燃料制备中的应用进展

流化床-化学气相沉积（FB-CVD）技术是化工流化床技术和材料化学气相沉积制备技术的交叉耦合,兼有流化床处理量大、传热快、温度均匀以及化学气相沉积温度调节范围广、产物丰富多样等优点,其在先进核燃料制备中有着重要的应用,但随着先进核燃料“质”和“量”的不断发展要求,现有的FB-CVD技术有许多方面亟待完善。本文回顾了作者课题组利用流化床-化学气相沉积在高温气冷堆TRISO核燃料颗粒、先进核燃料包覆颗粒、核燃料示踪颗粒、基体SiC纳米颗粒、SiC@Al₂O₃复合纳米颗粒等方面的研究进展,阐述了基本方法、实验过程和典型研究结果,并分析了流化床-化学气相沉积过程中遇到的实际问题。指出了FB-CVD技术未来发展方向,主要涉及反应器规模化放大和连续性生产、孔口沉积消除及温区控制、粉体制备中的纳米颗粒连续收集、新型反应器及工艺设计等方面,具体包括高密度颗粒稳定流化放大准则、床层局部温区控制以及分区流化床结构设计等。     

Model of combustion and dispersion of carbon deposited on porous bed material during bubbling fluidized bed combustion

Porous bed materials capture volatile matter as carbon deposits during fluidized bed combustion of high-volatile fuels such as biomass and wastes. Carbon deposits burn in a dense bed mixed with bed materials; thereby enhancing horizontal dispersion of carbonaceous materials. Commercial scaling-up requires a model that simultaneously assesses carbon deposit combustion and horizontal solid dispersion. This study measured the carbon deposit combustion rate using a fluidized bed. A one-dimensional model of carbon deposit combustion in a fluidized bed is based on the carbon deposit burning rate. A two-dimensional model incorporates reactions and solid dispersion to predict the horizontal concentration profile. Experiments using a bubbling fluidized bed validated that model.     

Multiphase CFD Modeling for a Chemical Looping Combustion Process (Fuel Reactor)

There are growing concerns about increasing emissions of greenhouse gases and a looming global warming crisis. CO₂ is a greenhouse gas that affects the climate of the earth. Fossil fuel consumption is the major source of anthropogenic CO₂ emissions. Chemical looping combustion (CLC) has been suggested as an energy‐efficient method for the capture of carbon dioxide from combustion. A chemical‐looping combustion system consists of a fuel reactor and an air reactor. The air reactor consists of a conventional circulating fluidized bed and the fuel reactor is a bubbling fluidized bed. The basic principle involves avoiding direct contact of air and fuel during the combustion. The oxygen is transferred by the oxygen carrier from the air to the fuel. The water in combustion products can be easily removed by condensation and pure carbon dioxide is obtained without any loss of energy for separation. With the improvement of numerical methods and more advanced hardware technology, the time required to run CFD (computational fluid dynamic) codes is decreasing. Hence, multiphase CFD‐based models for dealing with complex gas‐solid hydrodynamics and chemical reactions are becoming more accessible. To date, there are no reports in the literature concerning mathematical modeling of chemical‐looping combustion using FLUENT. In this work, the reaction kinetics models of the (CaSO₄ + H₂) fuel reactor is developed by means of the commercial code FLUENT. The effects of particle diameter, gas flow rate and bed temperature on chemical looping combustion performance are also studied. The results show that the high bed temperature, low gas flow rate and small particle size could enhance the CLC performance.     

Modeling of volatile organic compounds condensation in fluidized bed

A model of volatile organic compound condensation in dense fluidized bed was used to simulate a new depollution process by separation in fluidized bed. The volatile compounds are condensed and captured by fluidized porous particles before being renewed in an evaporation reactor and recirculated to the condensation riser. This study shows how a large bed height, a small superficial gas velocity, a cold wall temperature provided that the condensation products that do not turn into solid state give a high and stable abatement rate as well as it provides a good use of the internal porosity of the particles. It is also demonstrated that, as already known after thermodynamics constraints, this condensation process is better adapted to high-concentrated effluent and needs a complementary separation system if regulations are severe.     

Investigating Gasification Processes of Solid Fuels by in situ Gaspotentiometric Oxygen Probes

This paper describes a novel analytical method that applies a gaspotentiometric oxygen probe (GOP) for characterizing gasification behavior of solid fuels. On the basis of GOP signals a developed gasification model enables the determination of fuel‐specific properties, like effective reaction rate constant and overall activation energy. For its experimental validation two coal cokes were converted with four different gasification agents in a lab‐scale fluidized‐bed reactor. The results obtained will be discussed and compared with literature data. As this advantageous in situ measurement technique is fast, inexpensive, and easy to handle, it makes the GOP a predestined tool for monitoring and controlling gasification processes.     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Devolatilization and combustion of large coal particles in a fluidized bed

Devolatilization and combustion of large particles of Eastern Canadian coals (Evans and Minto), 5-50 mm dia., were studied in a bench-scale atmospheric fluidized bed reactor at 1023-1173 K with 0.5 mm sand particles as the bed material. The devolatilization time, mass loss history, changes in proximate volatiles content and C/H mass ratio, and temperature history at the centre of the particle during devolatilization were determined. The mass loss during devolatilization is correlated with the proximate volatiles content of the parent coal. The devolatilization time is correlated with the initial particle diameter by a power-law relation with an exponent of 1.54-1.64. The results show insignificant effect of superficial velocity on devolatilization.     

Carbon conversion of solid fuels in the freeboard of a laboratory-scale fluidized bed combustor—application of in situ laser spectroscopy

The carbon conversion of different solid fuels (i.e. beech wood, fir wood, bituminous coal) was investigated in the freeboard of a laboratory-scale fluidized bed combustor by in situ tunable diode laser absorption spectroscopy. A room temperature continuous wave InGaAsSb/AlGaAsSb quantum well ridge diode laser emitting at 2.3-2.35 μm was wavelength tuned at 300 Hz to determine simultaneously CH₄ and CO during devolatilization and char combustion in situ 10 mm above the fuel particles. The lower detection limit was 0.2 vol% (5000 ppm m) for both species. In addition, CO, CO₂ and O₂ were determined ex situ by conventional methods.The experimental results obtained for the bituminous coal were compared to a detailed chemical kinetic model.The in situ measurements proved to be advantageous compared to conventional ex situ concentration measurements. The calculations confirm the determination of the primary products of solid fuel combustion during devolatilization and char combustion. A rather simple model for the devolatilization products was proven to describe well the release rates of CH₄ and CO for the bituminous coal.     

Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed

Single particle devolatilization followed by combustion of the residual coal char particle has been analyzed in a batch-fluidized bed. The kinetic scheme with distributed activation energy is used for coal devolatilization while multiple chemical reactions with volume reaction mechanism are considered for residual char combustion. Both the models couple kinetics with heat transfer. Finite Volume Method (FVM) is employed to solve fully transient partial differential equations coupled with reaction kinetics. The devolatilization model is used to predict the devolatilization time along with residual mass and particle temperature, while the combined devolatilization and char combustion model is used to predict the overall mass loss and temperature profile of coal. The computed results are compared with the experimental results of the present authors for combustion of Indian sub-bituminous coal (15% ash) in a fluidized bed combustor as well as with published experimental results for coal with low ash high volatile matter. The effects of various operating parameters like bed temperature, oxygen mole fraction in bulk phase on devolatilization time and burn-out time of coal particle in bubbling fluidized bed have been examined through simulation.     

Devolatilization of wood and wastes in fluidized bed

Experiments were carried out in a laboratory fluidized bed (FB) to characterize the devolatilization behavior of wood and various wastes at temperatures applicable to FB gasification and combustion, i.e. 750-900 °C. The fuels tested were pellets made of wood, meat and bone meal, and compost (from municipal solid wastes), as well as dried granulates of sewage sludge (DSS). Determination of yields of char, condensate and light gas, as well as the composition of the gas and the time of devolatilization during the pyrolysis of single fuel batches was made. A simple model was developed to analyze the mode of conversion of a single wood pellet and DSS granulate, giving insight on the controlling mechanisms during devolatilization. The devolatilization kinetics of DSS was determined by tests using fine granulates. The model was successfully applied to simulate the conversion of large DSS granulates and wood pellets under the whole range of temperatures analyzed.     

Continuous production of solid polystyrene in back‐mixed and linear‐flow reactors

A mathematical model has been developed to predict the steady state performance of a continuous bulk styrene polymerization process with catalytic initiation for solid polystyrene. The polymerization section contains one boiling CSTR, followed by multiple linear‐flow reactors. The devolatilization section consists of two polymer pre‐heaters and two high‐solids flashes. The polymer moment equations were solved simultaneously with the reactor modeling equations. The non‐linear algebraic equations were solved by a Newton‐Raphson iteration technique to give the steady‐state styrene monomer weight fraction in a CSTR. The coupled, non‐linear ordinary differential equations were numerically integrated using a single‐step, 4th‐order Runge‐Kutta technique, followed by a multi‐step Adams‐Moulton technique. The resulting computer simulation model is capable of evaluating how the production rate and product quality are affected by feed composition, temperature, initiator type, initiator concentration, and residence time. Several case studies were given for commercially important crystal‐clear and impact‐resistant resins. A binary initiation system gives a good balance of monomer conversion, polymer molecular weights, and rubber grafting compared to a single initiation system. The styrene dimer/trimer occur in low concentrations but can be substantially reduced with a low temperature initiator. The ideal mean residence time is approximately one minute or less in a shell‐and‐tube devolatilization pre‐heater. Low flash chamber vacuum is more effective than high polystyrene melt temperature to reduce the volatile content of the final product. The water injected to the low volatile melt shows promising improvement in the second‐stage polystyrene devolatilization.     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Modelling fast pyrolysis of biomass in a fluidized bed reactor

A compartmental one-dimensional model of a fluidized bed pyrolytic converter of biomass is presented. Reference conditions are those of non-catalytic fast pyrolysis of biomass in a shallow fluidized bed with external regeneration of the bed material. The fate of biomass and of the resulting char has been modelled by considering elutriation of biomass and char particles, char attrition as well as bed drain/regeneration. The course of primary and secondary pyrolitic reactions is modelled according to a semi-lumped reaction network using well-established kinetic parameters taken from the literature. A specific focus of the present study is the role of the heterogeneous volatile–char secondary reactions, whose rate has been modelled by borrowing a kinetic expression from the neighbouring area of tar adsorption/decomposition over char. The results of computations highlight the relevance of heterogeneous volatile–char secondary reactions and of the closely associated control of char loading in the bed. The sensitivity of the reactor performance to char elutriation and attrition, to proper management of bed drain/regeneration, and to control of gas phase backmixing is demonstrated. Model results provide useful guidelines for optimal design and control of fluidized bed pyrolyzers and pinpoint future research priorities.     

Comminution characteristics of Korean anthracite in a CFB reactor

Comminution characteristic of Korean anthracite has been determined with operation conditions in a laboratory scale circulating fluidized bed (CFB) combustor. The fragmentation of the anthracite occurs explosively, and generates lots of fine particles at an early stage of devolatilization. The fragmented particles continue to be reduced with generation of the fine particles during an attrition stage in the CFB combustor. With an increase of operation temperature, the coal shows a high degree of fragmentation and generation of fine particles in the CFB reactor. The particle fragmentation occurs actively as its size and Hard Grove Index (HGI) increase. The attrition is also affected with particle size and HGI of the coal. The initial surface crack and the fine clusters on the particle surface are found to be reasons for explosive fragmentation and for generation of fine particles during devolatilization and combustion in the CFB reactor.     

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.