Modeling of biomass devolatilization in a fluidized bed reactor

A simple model that simulates a single biomass particle devolatilization is described. The model takes into account the main physical and chemical factors influencing the phenomenon at high temperatures (>700 K), where the production of gaseous components far outweighs that of liquids. The predictions of the model are shown to be in good agreement with published data. The model is then applied to the devolatilization of biomass in a fluidized bed, in which attention is focused on heat transfer, particle mixing and elutriation, and gas production. Predictions on the overall devolatilization time for a biomass particle are compared with experimental results obtained in a fluidized bed reactor in which the process was monitored by continuous measurement of the bed pressure. Good correspondence of predicted with calculated values was obtained, supporting the validity of the many approximations made in the derivation of the governing relationships for the pyrolysis process.     

Pyrolysis and gasification of pellets from sugar cane bagasse and wood

Wood pellets have become a popular form of biomass for power generation and residential heating due to easier handling both for transportation and for feeders in the treatment units, improved conversion and storage possibilities. The research on wood pellets as fuel has also been intensified during the past decade. However, other biomass sorts in pellet form, such as sugar cane bagasse, have not yet been extensively studied, especially not physical effects on the pellets during thermal treatment. Bagasse and wood pellets of different origin and sizes, shredded bagasse and wood chips have been studied in a thermogravimetric equipment to compare the effects of sort, origin, size and form of biomass during slow pyrolysis and steam gasification. Physical parameters such as decrease of volume and mass during treatment, as well as pyrolysis and gasification rates are of primary interest in the study. An important observation from the study is that for pellets the char density decreased during pyrolysis to a minimum around 450 °C, but thereafter increased with continued heating. The wood chips behaved differently with a continuous char density decrease during pyrolysis. Another conclusion from the work is that the size of the pellet has larger impact on the shrinkage behaviour throughout the conversion than the raw material, which the pellet is made of.     

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.     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

Parameters of high temperature steam gasification of original and pulverised wood pellets

Experiments on gasification of chars obtained from original and pulverised wood pellets were conducted in atmosphere of water steam and nitrogen under temperatures of 800, 900 and 950 °C. Molar flow rates of carbon containing product gases were measured and approximated using different models with respect to extents of carbon conversion in char of the pellets. Comparison of the random pore, grain and volumetric models revealed the best applicability for approximations of the random pore model. Apparent activation energies obtained as a result of application of the models to the data from experiments with char of original pellets were higher in comparison to those of pulverised pellets, except for a grain model. Approximations under 800 °C showed relatively big deviations from experimental data on the beginning of char gasification. This is attributed to catalytic effects from alkali metals in the pellets.     

Analysis and modelling of wood pyrolysis

In many industrial processes wood is treated as big fragments or pellets. In such conditions kinetics and yields are different with respect to the case of particles with few mg weight. However most of published kinetic models were based on experimental data obtained with very small samples. In this work pyrolysis of wood pellets was investigated by using a special experimental device which allowed to determine kinetics of total weight loss, gas and tar production. Two different heating rates, 0.05 and 1 K/s, were employed to determine kinetic parameters. Dynamic and isothermal pyrolysis tests were carried out on beech and pine wood previously dried in an oven. A simple but realistic kinetic scheme was proposed able to take into account the phenomena that happen in big wood particles. The numerical parameters were determined from the results of experimentation on beech wood. The proposed kinetic model takes into account the presence of two different stages during pyrolysis: a first one involving only unreacted wood and a second one where the products not yet escaped from the solid matrix react further. This kinetic model allowed to fit the experimental data quite well. The model was successfully validated with tests performed at an elevated heating rate (approximately 60–100 K/s) of the external surface of the pellets. In these conditions, the pellets showed a marked gradient of temperature inside, which was suitably considered.     

Breakage behaviour of spherical granulates by compression

This paper describes the deformation and breakage behaviour of granulates in single particle compression test. Three industrial spherical granulates—γ-Al₂O₃, the synthetic zeolite Köstrolith^® and sodium benzoate (C₆H₅COONa) were used as model materials to study the mechanical behaviour from elastic to plastic range. The elastic compression behaviour of granulates is described by means of force-displacement curves, by application of Hertz-Huber contact theory and continuum mechanics. An elastic-plastic contact model was proposed to describe the deformation behaviour of elastic-plastic granules. The effects of granulate size and stressing velocity on the breakage force and contact stiffness during elastic and elastic-plastic displacement are examined. It is shown that the zeolite granulates with elastic-plastic behaviour have viscous properties as well. Breakage mechanisms of granulates during elastic, elastic-plastic and plastic deformation are also explained. The breakage probability is approximated by Weibull distribution function. The behaviour of the granulate during compression under the repeated loading-unloading conditions was investigated.     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (T_bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d_p = 375 and 550 μm) and two fluidizing velocities (u = 5u_mf and u = 10u_mf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s⁻¹ for the wood sizes considered.     

Characterization of coal—liquid fractions. Molar enthalpies of quinoline interaction with coal-derived asphaltenes and heavy oils

The toluene-insoluble (Tl), asphaltene (A), and heavy oil (HO) fractions were isolated from three centrifuged SYNTHOIL liquid product (CLP) samples, prepared under different process conditions at 450 °C, 27.6 MPa hydrogen pressure from the same feed coal, Kentucky hvAb, from Homestead Mine. Run FB 53 was made with CoMo catalyst, 11-min preheater residence time, and 3-min reactor residence time. The much higher viscosity of FB 53–59 compared to FB 53-1 correlates with the larger contents of its toluene-insolubles and asphaltene, larger oxygen and sulphur contents of its asphaltene and toluene-insolubles, larger molecular weight and smaller aromaticity of its asphaltene, and the larger enthalpy of interaction (ΔH^o) of its asphaltene with quinoline in benzene. The average molecular weight and percentage of heteroatoms of the heavy oil from FB 53–59 are also greater than that from FB 53-1, and the value of ΔH^o of the heavy oil with quinoline, follows the same order. Run FB 57 was made with glass pellets, 17-min preheater residence time and 6-min reactor residence-time. Since in FB 53–59 the CoMo catalyst has lost part of its activity, a comparison of FB 53–59 with FB 57 yields information on the effect of residence times on the properties. The toluene-insoluble and asphaltene contents, as well as the viscosity, of FB 53–59 is larger, while the heavy oil content of FB 53–59 is smaller than that of FB 57. This comparison indicates that a larger-residence-time preheater and reactor may, to some extent, favour conversion as well as decrease the viscosity of the product oil. The values of ΔH^o for the interaction of quinoline with the heavy oil and asphaltene fractions obtained from the three CLP samples are in the order: FB 53–59 > FB 57-42 > FB 53-1, and they are attributed to the varying degree of hydrogen-bonding effects involving largely aromatic phenols which serve as hydrogen donors to quinoline.     

Mathematical modelling of slow pyrolysis of segregated solid wastes in a packed-bed pyrolyser

Waste segregation is being explored as one of the potential effective ways for waste management, where wastes are separated for either recycling or energy recovery. In this paper, three segregated wastes, contaminated waste wood, cardboard and waste textile are pyrolysed in a slow-heating packed-bed reactor for the purpose of solid, liquid and gas recovery. The effect of final temperature was investigated and product yields and compositions were measured. Mathematical modelling was employed to simulate the heat, mass transfer and kinetic processes inside the reactor. Both a parallel reaction model and a function group model were used to predict the product yields as well as their compositions. Char yield of 21–34%, tar 34–46% and gas 23–43% were obtained. It is found that packed-bed pyrolysis produces 30–100% more char compared to standard TGA tests and the local heating rate across the packed-bed reactor differs remarkably from the programmed wall-heating rate and varies greatly in both time and space. Mathematical modelling suggests that wood has higher tar cracking ability than cardboard and textile wastes during pyrolysis, and the effects of mineral contents in the fuel need to be explored. CO₂, CO, tar and water are the main released species during the major stage of the pyrolysis processes which occurs between 250 and 450 °C, whereas noticeable quantity of hydrogen and light hydrocarbons is observed only at higher temperature levels and at the final stage.     

Measuring chemical heat production rates of biofuels by isothermal calorimetry for hazardous evaluation modelling

Biofuels are commonly stored in large stacks that may heat up and self‐ignite from microbiological and chemical heat production. This paper shows how isothermal (heat conduction) calorimetry can be used to measure heat production rates of biofuels at relatively low temperatures close to where self‐heating starts to become a problem. Measurements can be made to assess how the reaction rate is a function of such factors as temperature, extent of reaction, oxygen pressure, water content and the presence of catalytic compounds. In the present paper, measurements on pellets made of wood and bark are presented together with an analysis of how the reaction rate of the bark pellets depends on the oxygen pressure. It is also shown that 1% iron or copper ions increased the reaction rate of wood pellets by a factor of three. Copyright © 2006 John Wiley & Sons, Ltd.     

Measurement of heat transfer and change in compressive strength of waste derived solid fuels due to devolatisation

Solid waste derived fuels are being increasingly considered for application in waste-to-energy processes. This thermal treatment methodology is becoming more popular since it deals with two major issues; the disposal of solid wastes and the production of a biomass-derived energy source. This paper is concerned with the analysis of a variety of waste derived fuels including mixed RDF, wood and paper, in comparison with coal. A specially designed chamber was developed to heat the fuels in an inert atmosphere whilst simultaneously measuring the internal temperatures of the pellets. The pellets were also tested for compressive strength, to determine how the devolatisation process affected the integrity of the pellets. Results showed that small RDF pellets devolatise very quickly in a high temperature environment, whereas larger paper pellets have far longer lifespans. Waste derived pellets lose structural integrity far quicker than coal-based fuels, which was identified as being linked to the composition of the materials and the mode by which they are held together.     

Devolatilization characteristics of large particles of tyre rubber under combustion conditions

A systematic experimental study has been performed in order to investigate the effect of particle size and temperature on the devolatilization rate of large tyre rubber particles. Cylindrical tyre particles with diameters between 7.5 and 22 mm were devolatilised in a macro-TGA reactor, at temperatures between 490 and 840 °C in an inert atmosphere. The effect of particle size and surrounding temperature on the rate of devolatilization was observed to be significant, i.e. larger particle diameters and lower temperatures increased the devolatilization time. A detailed mathematical model for the devolatilization process including internal and external heat transfer, three parallel independent devolatilization reactions and reaction enthalpy effects has been developed and solved using an implicit finite difference method. Comparison of the model predictions with experimental data, reveals that the devolatilization process of large tyre rubber particles at temperatures above 490 °C can be considered to be controlled mainly by heat transfer and reaction kinetics. The model analysis further shows that exothermic devolatilization reaction enthalpy effects cannot be neglected in the prediction of the intra particle temperature rise. A sensitivity analysis of the model parameters, demonstrates that the specific heat capacity of the virgin fuel and the activation energies of the devolatilization reactions is the most important model parameters in the prediction of devolatilization times of large tyre rubber particles.     

Devolatilization characteristics of high volatile coal in a wire mesh reactor

A wire mesh reactor was used to investigate the devolatilization process of coal particle during entrained flow gasification. Coal from Indonesia East Kalimantan mine, which has high moisture and high volatile matter, was chosen as a sample. Experiments were carried out at the heating rate of 1,000 °C/s and isothermal condition was kept at peak temperature under atmospheric pressure. The char, tar and gas formation characteristics of the coal as well as the composition of the gas components at peak temperatures were determined. The experimental results showed that devolatilization process terminated when temperature reached above 1,100 °C. Most of tar was formed at about 800 °C, while the rate of tar formation decreased gradually as the temperature increased. CH₄ was observed at temperatures above 600 °C, whereas H₂ was detected above 1,000 °C. The amount of formed gases such as H₂, CO, CH₄ and C _n H _m increased as the temperature increased. From the characteristics of devolatilization with residence time, it was concluded that devolatilization terminated within about 0.7 second when the temperature reached 1,000 °C. As the operating temperature in an entrained flow gasifier is higher than ash melting temperature, it is expected that the devolatilization time of high volatile coal should be less than one second in an entrained flow gasifier.     

Modelling wood combustion under fixed bed conditions

A computer model describing the conversion of wood under packed-bed conditions is presented. The packed bed is considered to be an arrangement of a finite number of particles, typically sized between 5 and 25 mm, with a void space left between them. Each particle is undergoing a thermal conversion process, which is described by a one-dimensional and transient model.Within the single-particle model, heating, drying, pyrolysis, gasification and combustion are considered, whereby each particle exchanges energy due to conduction and radiation with its neighbours. Because of the one-dimensional discretization of the particles, heat transfer and mass transfer is taken into account explicitly. Therefore, no macrokinetic data are needed within the model. For ease of implementation and access, kinetic data and property data are stored in a database. The global conversion of the packed bed is represented by the contributions of single particles, where each particle is coupled to the surrounding gas phase by heat and mass transfer. For gas phase flow through the porous bed, the conservation equations for mass, momentum and energy are solved on a Cartesian mesh by a Finite Volume method.Experiments have been performed to validate the single particle model for the conversion of beech wood during pyrolysis and char combustion. Agreement between experimental and predictions obtained by the model is very satisfactory. However, for wet wood, changes in structure seem to enhance the heat transfer to the solid which is not yet covered in the model.     

Investigation of wood pellets self-heating kinetics and thermal runaway phenomena using lab scale experiments

The phenomena of spontaneous combustion and thermal runaway in wood pellets storage were investigated using lab-scale experiments in the temperature range of 100–200 °C. The critical temperatures were determined for four sizes of reactors. The kinetic parameters of the self-heating were determined using three methods, the Frank-Kamenetskii's method, crossing point method, and numerical curve fitting method. Mean values of activation energy (E) of 78.7 ±0.8 kJ/mol and self-heating rate constant (∆ _rhA) of (4.22 ±2.5) × 10 ⁶ kJ/(kg s) were obtained for four type of wood pellets (made from whitewood) samples from different pellet producers in British Columbia. Finally, a two-dimensional numerical model was developed to predict the temperature development during self-heating and the critical temperature for known sizes of reactors.     

Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within ±20% while the char yield is predicted within ±17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m² K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield.     

TG-FTIR characterization of coal and biomass single fuels and blends under slow heating rate conditions: Partitioning of the fuel-bound nitrogen

The devolatilization behavior of a bituminous coal and different biomass fuels currently applied in the Dutch power sector for co-firing was experimentally investigated. The volatile composition during single fuel pyrolysis as well as during co-pyrolysis was studied using TG-FTIR characterization with the focus on the release patterns and quantitative analysis of the gaseous bound nitrogen species. It was shown that all investigated biomass fuels present more or less similar pyrolysis behavior, with a maximum weight loss between 300 and 380 °C. Woody and agricultural biomass materials show higher devolatilization rates than animal waste. When comparing different fuels, the percentage of fuel-bound nitrogen converted to volatile bound-N species (NH₃, HCN, HNCO) does not correlate with the initial fuel-N content. Biomass pyrolysis resulted in higher volatile-N yields than coal, which potentially indicates that NO_x control during co-firing might be favored. No significant interactions occurred during the pyrolysis of coal/biomass blends at conditions typical of TG analysis (slow heating rate). Evolved gas analysis of volatile species confirmed the absence of mutual interactions during woody biomass co-pyrolysis. However, non-additive behavior of selected gas species was found during slaughter and poultry litter co-pyrolysis. Higher CH₄ yields between 450 and 750 °C and higher ammonia and CO yields between 550 and 900 °C were measured. Such a result is likely to be attributed to catalytic effects of alkali and alkaline earth metals present in high quantity in animal waste ash. The fact that the co-pyrolysis of woody and agricultural biomass is well modeled by simple addition of the individual behavior of its components permits to predict the mixture's behavior based on experimental data available for single fuels. On the other hand, animal waste co-pyrolysis presented in some cases synergistic effects in gas products although additive behavior occurred for the solid phase.     

Generalized model of heat transfer and volatiles evolution inside particles for coal devolatilization

Devolatilization is acknowledged as the first important step in coal conversion techniques. A comprehensive heat transfer and devolatilization model was established, with special consideration of the particle‐scale physics and chemistry, to predict the internal heat transport and pyrolysis behavior of particles. The chemical percolation devolatilization model with corrected kinetic parameters and structure parameters was validated with a lot of experimental data and then adopted to describe the devolatilization behaviors under a broader range of temperatures, heating rates, and coal types. The newly achieved understanding of the integrated effect of heating rate and coal type on coal devolatilization could help to provide a preliminary coal rank selection method for industrial processes. In particular, in‐depth discussion of the influences of heat conduction, volatiles diffusion, and endothermic heat of devolatilization inside particle indicated the dominant roles of these factors when the intensity of heat transfer was strong or the release of volatiles was rapid. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2893–2906, 2014