Heat transfer and kinetics in the pyrolysis of shrinking biomass particle

The impact of shrinkage on pyrolysis of biomass particles is studied employing a kinetic model coupled with heat transfer model using a practically significant kinetic scheme consisting of physically measurable parameters. The numerical model is used to examine the impact of shrinkage on particle size, pyrolysis time, product yields, specific heat capacity and Biot number considering cylindrical geometry. Finite difference pure implicit scheme utilizing tri-diagonal matrix algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth-order method is used for chemical kinetics model equations. Simulations are carried out for radius ranging from 0.0000125 to , temperature ranging from 303 to and shrinkage factors ranging from 0.0 to 1.0. The results obtained using the model used in the present study are in excellent agreement with many experimental studies, much better than the agreement with the earlier models reported in the literature. Shrinkage affects both the pyrolysis time and the product yield in thermally thick regime. However, it is found that shrinkage has negligible affect on pyrolysis in the thermally thin regime. The impact of shrinkage reflects on pyrolysis in several ways. It includes reduction of the residence time of gases within the particle, cooling of the char layer due to higher mass flux rates of pyrolysis products and thinning the pyrolysis reaction region.     

Modeling the combined impact of moisture and char shrinkage on the pyrolysis of a biomass particle

A detailed computational model of pyrolysis of a moist, shrinking biomass particle is presented. This model is used to examine the effect of varying the moisture content for a single shrinking biomass particle subjected to a constant external temperature. Particle half-thicknesses ranging from 5 μm to 2 cm, temperatures from 800 to 2000 K, moisture contents from 0 to 30% (dry basis), and shrinkage factors from 1.0 to 0.4 are examined. The impact of moisture content and shrinkage was found to be a function of pyrolysis regime. In general, coupling between moisture content and shrinkage was found to result in longer pyrolysis times than if they were considered separately. Additionally, coupling between moisture content and shrinkage increased tar yield and decreased light hydrocarbon yield compared to considering moisture and shrinkage separately.     

Birch wood particle shrinkage during rapid pyrolysis

A study of the shrinkage of cubic (∼5 mm) birch wood particles during pyrolysis is presented. The particles were rapidly injected into a preheated furnace with a constant temperature in the range 350-900°C. The size of the particles in longitudinal, tangential and radial directions was measured until no further mass loss occurs. The volume shrinkage was found to be 45-70% and the shrinkage in the different directions 5-25, 25-40 and 15-40% for longitudinal, tangential and radial directions, respectively. Longitudinal shrinkage commenced after about 60% mass loss and is not strongly dependent on heating rate or on cellulose chain scission. A maximum shrinkage was found for tangential and radial directions at 400 and 500-700°C, respectively, and above these temperatures the shrinkage decreases. The char yield decreases and the char structure becomes more deranged with increasing temperature. Empirical models of shrinkage as a function of conversion are presented.     

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.     

Modeling and simulation of heat and mass transfer during drying of solids with hemispherical shell geometry

A heat and mass transfer model was proposed to describe the moisture and temperature evolution during drying of solid products with hemispherical shell geometry (HSG). The dimensionless form of the model was numerically solved for both several drying conditions and values of a geometrical factor related with the inner radius of the HSG to obtain their moisture and temperature profiles. In addition, average drying kinetics were calculated from the volume integration of local moisture values. A theoretical and numerical approach was used to develop a mass transfer analogy between the proposed HSG and a simpler flat slab-shaped product. These analogies provide simple mathematical expressions for drying process simulation and estimation of diffusion coefficients in solids with the proposed geometry, and may be applicable to other mass and heat transfer operations. Furthermore, the presented procedure may be used to develop similar expressions in other non-traditional or dissection geometries.     

Catalytic pyrolysis of biomass for biofuels production

Fast pyrolysis bio-oils currently produced in demonstration and semi-commercial plants have potential as a fuel for stationary power production using boilers or turbines but they require significant modification to become an acceptable transportation fuel. Catalytic upgrading of pyrolysis vapors using zeolites is a potentially promising method for removing oxygen from organic compounds and converting them to hydrocarbons. This work evaluated a set of commercial and laboratory-synthesized catalysts for their hydrocarbon production performance via the pyrolysis/catalytic cracking route. Three types of biomass feedstocks; cellulose, lignin, and wood were pyrolyzed (batch experiments) in quartz boats in physical contact with the catalysts at temperature ranging from 400 °C to 600 °C and catalyst-to-biomass ratios of 5-10 by weight. Molecular-beam mass spectrometry (MBMS) was used to analyze the product vapor and gas composition. The highest yield of hydrocarbons (approximately 16 wt.%, including 3.5 wt.% of toluene) was achieved using nickel, cobalt, iron, and gallium-substituted ZSM-5. Tests performed using a semi-continuous flow reactor allowed us to observe the change in the composition of the volatiles produced by the pyrolysis/catalytic vapor cracking reactions as a function of the catalyst time-on-stream. The deoxygenation activity decreased with time because of coke deposits formed on the catalyst.     

Dominant design variables in pyrolysis of biomass particles of different geometries in thermally thick regime

In this study, a simultaneous chemical kinetics and heat transfer model is used to predict the effects of the most important physical and thermal properties (thermal conductivity, heat transfer coefficient, emissivity, reactor temperature and heat of reaction number) of the feedstock on the convective-radiant pyrolysis of biomass fuels for different geometries such as slab, cylinder and sphere. The pyrolysis rate is simulated by a kinetic scheme involving two parallel primary reactions and a third secondary reaction between volatile and gaseous products and the char. Finite difference pure implicit scheme utilizing the Tri-Diagonal Matrix Algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth order method is used for solving the chemical kinetics model equations. Simulations are carried out for different geometries considering the equivalent radius ranging from to , and the temperature ranging from to .For conversion in the thermally thick regime (intra-particle heat transfer control), it is found that variations in the properties mainly affect the activity of primary reactions. The highest sensitivity is associated with reactor temperature and emissivity. Applications of these findings in reactor design and operation are discussed. The results obtained using the model used in the present study are in excellent agreement with many reported experimental studies, much better than the agreement with earlier models reported in the literature.     

生物质热解特性研究

以竹材、稻壳、木屑为原料,通过常规热解结合快速热解研究生物质热解特性。结果表明,生物质常规热解的液体得率较低,相比而言竹材最高,稻壳最低,且热解温度是影响竹材和木屑热解的主要因素,其液体得率随温度的升高呈先增后减的变化规律;快速热解方面,利用居里点裂解仪和GC—MS在线分析竹材热解的液相组成,其组成以糠醛和酚类物质为主,它们分别来源于纤维素、半纤维素和木质素的热解。     

Modeling and numerical analysis of counter-current dialyzer at steady state

A mathematical model of a continuous counter-current dialyzer in the case of the transport of one component, which is based on the mass balance and balance of transported component, has been presented. It enables users to calculate the component recovery yield if parameters of the dialyzer (height, dimensions of compartments), parameters of the membrane (thickness, partition coefficients, diffusivity of the component in the membrane, flux of solution through the membrane), parameters of liquids in both the compartments (density, viscosity, diffusivity of the component) and parameters of streams entering the dialyzer (component concentration, volumetric liquid flow rate) are specified.     

Effect of Geometry of Rice Kernels on Drying Modeling Results

Geometry of rice grain is commonly represented by sphere, spheroid, or ellipsoid shapes in the drying models. Models using simpler shapes are easy to solve mathematically; however, deviation from the true grain shape might lead to large errors in predictions of drying characteristics such as moisture content (MC) and moisture gradients (MG). This research was undertaken to determine the impact of such shape considerations on prediction of drying characteristics. Impact of shrinkage of grains caused by drying was also investigated. Three separate mathematical models, each representing rice grain by sphere, spheroid, and ellipsoid shapes, were developed to describe the drying process. These models were solved by the finite element method using Comsol Multiphysics® simulation program. Drying simulations showed important differences in predictions of MC and MG in these three models. The sphere-shaped model predicted a slower drying than the spheroid- and ellipsoid-shaped models, whose MC predictions were similar. In all three models, maximum moisture gradients (MMG) were observed along the shortest axis in the bran region. During drying, MMG increases, reaches a peak, and then decreases. Magnitude and onset of peak of MMG were different in the three models. These differences in drying predictions among the three models make it important to use the appropriate shape to represent the rice grain in mathematical models. Ellipsoid shape, which closely resembles geometry of the rice grain, was found to be the most suitable. Reliable MG predictions from such ellipsoid-shaped models could be correlated to grain fissuring, which thereafter can be employed to optimize the drying process. The impact of shrinkage of rice grains during drying on model predictions is very small. In any drying simulation, maximum error due to neglecting shrinkage would be less than 5% of total moisture loss value.     

Development of a comprehensive free radical photopolymerization model incorporating heat and mass transfer effects in thick films

A comprehensive kinetic model describing photopolymerization is developed which allows variation of temperature, species concentrations, and light intensity through the thickness of a photopolymerized film. Heat and mass transfer effects are included, as is the generation of heat by both reaction and light absorption. In addition to initiation, propagation, and termination mechanisms, both primary radical termination and inhibition are incorporated into the model. The possible presence and diffusion of an inert solvent are also accounted for. Thus, the model is useful for examining complex polymerization kinetics and behavior in industrially and commercially important thick film photopolymerizations, such as the curing of contact lenses, dental restorative materials, photolithographic resists, and optoelectronic coatings. The comprehensive model is used to predict polymerization rate, temperature, and conversion profiles in a variety of systems. The effects of heat generation and the thermal boundary conditions are explored, with the result that heat generation in thick samples leads to greatly increased conversions approaching 100 percent. Increased temperature in these samples also may lead to the appearance of two rate maxima, with the first due to the temperature increase and the second caused by the autoacceleration process. The magnitude of the temperature increase, along with the resultant effects, is more pronounced in insulated systems.     

Examining the Suitability of the Reaction Engineering Approach (REA) to Modeling Local Evaporation/Condensation Rates of Materials with Various Thicknesses

The reaction engineering approach (REA) is examined here to investigate its suitability as the local evaporation rate to be used in multiphase drying. For this purpose, REA is first implemented to model the convective drying of materials with various thicknesses. The relative activation energy, as the fingerprint of REA, generated from one size of a material is used to model the convective drying of the same material with different thicknesses. Because the results indicate that REA parameters can model the drying of materials with various thicknesses, REA can be scaled down to describe the local evaporation rate (at the microscale as affected by local composition and temperature). The relative activation energy is used to describe the global drying rate in modeling the local evaporation rate. REA is combined with a system of equations of conservation of heat and mass transfer in order to yield the spatial reaction engineering approach (S-REA) as a nonequilibrium multiphase drying model. By using S-REA, the spatial profiles of moisture content, concentration of water vapor, temperature, and local evaporation rate can be generated, which can assist in comprehending the transport phenomena.     

Modeling extraction separation of Cu(II) in hollow-fiber modules

The mass transfer problems in the hollow-fiber membrane extractor module with concurrent- and countercurrent-flow were investigated theoretically and experimentally in this study. A two-dimensional mathematical model of the hollow-fiber membrane extractor module was developed theoretically and the shell side flow described by Happel's free surface model was taken into account. The analytical solution is obtained using an eigenfunction expansion in terms of the power series and an orthogonal expansion technique. The theoretical predictions were represented graphically with the mass-transfer Graetz number (Gz), flow pattern and packing density (φ) as parameters and the theoretical results were also compared with those obtained by experimental runs. The highest extraction rate, extraction efficiency and mass transfer efficiency can be achieved by arranging the packing density φ=0.3. The results show that the device performance of the countercurrent-flow device is better than that of the concurrent-flow device. The experiments of the extraction of Cu²⁺ by using D2EHPA with PVDF hollow fibers is also set up to confirm the accuracy of the theoretical predictions. The accuracy of the theoretical predictions for concurrent- and countercurrent-flow are 5.87×10^-2≤E₁≤6.69×10^-2 and 2.46×10^-2≤E₁≤3.48×10^-2, respectively, for Gz_a=40.8.     

Mathematical Modeling of the Fluidized Bed Drying of Cubic Materials

The unsteady‐state simultaneous heat and mass transfer between gas and potato cubes during the drying process in a batch fluidized bed was described by a mathematical model. Mass transfer was considered to occur in three dimensions whereas heat transfer between the gas and dried material was assumed to be lumped. It was found that the model could describe the drying process with acceptable accuracy. The moisture profile inside the material at any cross‐section and at any time can be predicted by the model.     

Modeling Effects of Mass Transfer Rate and Catalyst Concentration on Biodiesel Production in Batch Reactors

This article presents a new approach to investigate the kinetics of sunflower and rapeseed oils methanolysis. Due to its heterogeneous nature, the methanolysis reaction is affected by different physical properties such as mass transfer coefficients and specific surface area of the dispersed phase. Considering these parameters, a model was developed, and was evaluated by comparing the results of the model with the experimental data found in the literature. The mean absolute deviation obtained for sunflower and rapeseed oils is 0.039 mol L^?1, which demonstrates the accuracy of the model. The results show that the mixing speed is more effective in the first few minutes of the process. Furthermore, at mixing speed above 700 rpm, the process is controlled only by the reactions. The rate of biodiesel production increases with increasing catalyst concentration; however, catalyst concentration above 1.5 wt% has little or no significant effect on the rate of biodiesel production. In addition, because of its higher activation energy the rapeseed oil transesterification is more temperature dependent than the sunflower transesterification.     

Mechanistic Modeling of Pollutant Removal,Temperature, and Evaporation in Chemical Air Scrubbers

Chemical air scrubbers reduce the concentration of water‐soluble components such as ammonia from the outgoing ventilation air through absorption in water, followed by chemical conversions and removal of the end products. A mechanistic model for a countercurrent air scrubber was set up. Mass balances for ammonia, hydrogen sulfide, nitrous oxide, and methane were implemented, as well as the water mass balance and heat balances. The model was validated against experimental data from a conventional fattening pig housing facility. The effect of influent characteristics, design parameters, and control handles on the removal efficiency, the temperature profile, and the water evaporation rate were investigated through simulation. The model was able to describe the behavior of a countercurrent chemical air scrubber.     

Modeling the pyrolysis of preceramic polymers: A kinetic study of the polycarbosilane SMP-10

A material model was developed to predict changes in mass, density and thus volume of cured preceramic polymers for CMC matrices as they pyrolyze into ceramics. Because part warpage and delaminations are most likely to occur when matrix strain rates and strain rate gradients are the highest, the ability to accurately predict changes in a matrix material’s volume is essential to determining the processing conditions that will efficiently minimize composite scrap rates. Experimental and model analysis of the SiC forming polycarbosilane, SMP-10, revealed that volume shrinkage is initially driven by mass loss, is quickly dominated by density’s contribution, and has both temperature and time at temperature dependencies, where density is not a simple function of mass yield. While material density is rarely reported in the open literature, the ability to predict changes in density is essential to accurately predicting the volume yield of preceramic polymers used in ceramic matrix composites.     

Dynamic Modeling and Simulation of Steam Cracking Furnaces

The transient modeling of thermal cracking furnaces is developed. This representation is capable of describing and predicting the unsteady‐state behavior of cracking furnaces during start‐up. To accurately predict the heat transfer to the reactor tube, the fireside conditions are coupled with the process side. The mutual interaction of these two sections is found to be very stiff in terms of convergence of the computations. The two‐dimensional transient zone model is developed for the radiative heat exchange calculation. A simplified model for the convection section is also used to predict the crossover temperature at each time increment. The main simulation outputs are the flue gas properties as well as the distributions of heat flux, refractory wall and coil skin temperatures versus time. The dynamic simulation is implemented for a conventional procedure used in the start‐up run of the olefin furnaces.     

A practical approach for modelling and control of biomass pyrolysis pilot plant with heat recovery from combustion of pyrolysis products

A pilot plant of biomass pyrolysis using pyrolysis products as fuel has been tested and shown to improve energy balance of the process and to be environmentally friendly by avoiding rejection of pyrolysis pollutants fumes into the atmosphere. The high number of parameters involved in a pyrolysis process makes it difficult to specify an optimum procedure for charcoal yield and pyrolysis cycle durability. So the knowledge of the essential parameters which govern the kinetics mechanisms of the biomass thermal decomposition and the combustion of pyrolysis gases is very useful to understand the operating cycle of the plant. In the present study a thermochemical model is developed in order to simulate and control the operating cycle of the system. The effect of the inlet molar air flow rate on the temporal evolution of biomass mass loss rate and temperatures in the different active zones of the pilot plant as well as the determination of the critical inlet molar air flow rate for which accidental runaway of combustion reactions occurs are presented. To avoid this accidental phenomenon a Proportional-Integral-Derived (PID) anticipated regulation is used in order to control temperatures evolution in the different zones of the device and avoid the runaway of combustion reactions.