# Download e-book Thermochemical Processes: Principles and Models

The DOM considers the radiative transfer equation RTE in the direction s i as a field equation, thus the RTE is transformed into a transport equation for radiation intensity in the spatial coordinates:. The standard form DOM suffers from a number of serious drawbacks, such as false scattering and ray effects.

Perhaps the most serious drawback of the method is that it does not ensure conservation of radiative energy. This is a result of the fact that the standard discrete ordinates method uses simple quadrature for angular discretization. Thus, it is a logical step in the evolution of the method to move to a fully finite volume approach, in space as well as in direction. The finite volume method uses an exact integration to evaluate solid angle integrals and the method is fully conservative [ 11 ]. P-1 model is the simplest formulation of the more general P-N radiation model, which is based on the expansion of the radiation intensity I into an orthogonal series of spherical harmonics.

The method of spherical harmonics provides a vehicle to obtain an approximate solution of arbitrary high order i. Using only four terms in the series solution of the respective differential equation, the following relation is obtained for the radiation flux:. The problem is then much simplified since it is only necessary to find a solution for G rather than determining the direction dependent intensity. Then the following expression for q r can be directly substituted into the energy equation to account for heat sources or sinks due to radiation [ 11 ]:. The Rosseland radiation model can be derived from the P-1 radiation model with some approximations.

The radiative heat flux vector in a gray medium is approximated by. The Rosseland radiation model differs from the P-1 model in that the Rosseland model assumes the intensity equal to the black-body intensity at the gas temperature. Substituting this value for G into Equation 27 yields. It is important to keep in mind that the diffusion approximation is not valid near a boundary [ 11 ].

## Design Principles of Perovskites for Thermochemical Oxygen Separation

The main assumption of the Discrete Transfer Radiation Model DTRM is that the radiation leaving the surface element in a certain range of solid angles can be approximated by a single ray. The equation for the change of radiant intensity, dI , along a path, ds , can be written as:. Here, the refractive index is assumed to be unity.

If a is constant along the ray, then I s can be estimated as:. The accuracy of the model is limited mainly by the number of rays traced and the computational grid [ 11 ]. The mixture fraction model is used to present the reaction chemistry in the probability density function PDF method for solving turbulent-chemistry interaction. The equilibrium model is applied which assumes that the chemistry is rapid enough for chemical equilibrium to always exist at the molecular level. Basing on the simplifying assumptions, the instantaneous thermo chemical state of the fluid is related to the mixture fraction f.

An algorithm based on the minimization of Gibbs free energy is used to compute species mole fractions from f. The mixture fraction f is defined in terms of the atomic mass fraction as:. The subscript ox and fuel denote the value at the oxidizer stream inlet and the fuel stream inlet respectively. Under the assumption of equal diffusivities, the species equations can be reduced to a single equation for the mean time-averaged mixture fraction.

The source term S pm is due solely to transfer of mass into the gas phase from reacting sludge particles [ 11 ]. The porous media assumption is generally used in the applications of biomass pyrolysis in fixed bed. The arrangement of biomass particles in the fixed bed forms void spaces. The devolatilization volatiles and gases through the particle voids can be described as flow through a porous media. The particle position may change during the conversion process for the devolatilization, combustion and shrinkage of biomass particles.

In this process to mesh all associated geometry with a complex unstructured or body fitted system is out of both computational power and CFD algorithms levels. At high flow velocities, the modification of this law provides the correction for inertial losses in the porous medium by Darcy-Forchemier equation:.

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The system can be regarded as a two-phase flow [ 3 ]. The flow in biomass fluidized bed gasifier or boilers and furnaces is a typical kind of gas-solid flow with chemical reactions. Thus hydrodynamics of the gas-solid flow can be performed based on the Eulerian—Lagrangian concept. The discrete phase method can be applied to the particle flow when the particle phase can be considered to be sufficiently dilute that the particle-particle interactions and the effects of the particle volume fraction on the gas phase can be assumed neglected. The coupling of the continuous phase and the discrete phase is important and it is solved by tracking the exchange of mass, momentum and energy.

The model computes the particle trajectory using a Lagrangian formulation which includes the inertia, hydrodynamic drag, and the force of gravity. The CFD models used to describe these processes have become an important analysis and design tool to achieve the flow and temperature pattern, the products concentration contour and yields. Table 2 summarizes some of the recent studies.

Fletcher et al. The model is based on the CFX package and describes the phenomena of turbulent fluid flow, heat transfer, species transport, devolatilization, particle combustion, and gas phase chemical reactions. Biomass particulate is modeled via a Lagrangian approach as it enters the gasifier, releases its volatiles and finally undergoes gasification. Figure 2 shows the geometry and surface mesh of the gasifier. The model provides detailed information on the gas composition and temperature at the outlet and allows different operating scenarios to be examined in an efficient manner.

The initial calculations suggest that simulations to examine the effect of gasifier height and the steam flux in the upper inlets can be beneficial in process optimization. The model with further validation against detailed experimental data, will aid with the design process of such gasifiers. The geometry of the gasifier. The lower inlets are used to inject the biomass mixed with air, and the upper inlets are used to inject steam [ 15 ]. Gerun et al. The oxidation zone is crucial for tar cracking.

The simulations fit satisfactorily to the experimental data regarding temperature pattern and tar concentration. Figure 3 shows the temperature profile in the reactor. The heat of reaction is released mainly close to the injector. It induces a very hot zone in this area.

The stream function is shown in Figure 4a , whereas Figure 4b presents the gas pathlines in the reactor. The gas path strongly depends on the initial departure point. The strong recirculation zone is located above the air injection in the centre of the reactor. It plays a major role in air—gas mixing and thus enhances the quality of the gasification.

Temperature profile in the reactor [ 16 ]. Velocity pattern in the reactor [ 16 ]. Table 2 lists the examples of CFD applications in biomass gasification and pyrolysis at present. The submodels used in these examples are summarized in the table. The largest application of CFD models has been to power station boilers and furnaces. Many studies made in relation to coal combustion have been modified to apply to biomass combustion or co-firing. CFD modeling has established itself as a critical tool for the development of new ideas and advanced technologies.

It is capable of predicting qualitative information and quantitative information to within sufficient accuracy to justify engineering design changes on commercial boiler plant. Dixon et al.

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Gas velocity contours a and trajectories for several particle fractions b are shown for the as-constructed design Figure 5 and the modified design Figure 6. The improvements in boiler tube erosion performance can be deduced by visual assessment alone of the predicted flow and trajectory patterns. Flow simulations for the as-constructed design: a Gas velocity; b particle trajectory [ 24 ]. Flow simulations for the modified design: a Gas velocity; b particle trajectory [ 24 ]. He concluded that poor mixing in the furnace is a key issue leading to high emission levels and relatively high amounts of unburnt carbon in the fly ash.

The model was found to correctly predict operational trends same to the boiler experiment. In the future, a significant effort will be put into further improvements and validation of the modeling concept especially with respect to the deposition velocity concept and the tube bank model. Table 3 lists the recent studies of CFD applications in biomass combustion. The co-firing of coal and biomass has been advocated for a number of years as being advantageous on both an environmental and economic basis.

The co-combustion of biomass as a minor component presents an interesting intermediate situation with a high reactivity solid. There are a number of commercially available CFD models, and the suitability of the sub-models available for biomass combustion is a key factor in selecting an appropriate code. Table 4 summarizes the recent CFD applications in biomass co-firing.

Backreedy et al. The effects of the wood particle size and shape on the burnout of the combined wood and coal char were investigated. The effect of varying the devolatilization and char combustion rate constants for the biomass component in the blend was also investigated. Figure 10 shows the biomass particle tracks in the coal-biomass combustion case. Predicted particle traces coloured by particle mass kg for Thoresby coal—biomass combustion cases: 0.

Table 4 lists the recent studies of CFD applications in biomass co-firing. In the case of biomass burner studies there is considerable interest in NO x formation and unburned carbon in ash. The literatures [ 39 — 45 ] described the biomass combustion and NO x formation in detail. Ma et al. The potassium release during biomass combustion is still a subject of current investigation. Figure 11 shows the predicted contours of potassium concentration in the vertical symmetric plane of the furnace.

The particle tracks and temperature distribution are also studied in this work. Good agreement between the predicted and the measured furnace temperature and concentrations of CO 2 and NO x has been achieved. CFD applications in NO x formation of biomass thermochemical conversion. This paper summarized the CFD applications in biomass thermochemical conversion and system design. There is evident that CFD can be used as a powerful tool to predict biomass thermochemical processes as well as to design thermochemical reactors. CFD has played an active part in system design including analysis the distribution of products, flow, temperature, ash deposit and NO x emission.

The CFD model results are satisfactory and have made good agreements with the experimental data in many cases. However, the simulations still have many approximate models as well as some assumptions. To ensure CFD simulations are more than just theoretical exercises, experimental validation is necessary to facilitate the model accuracy.

With the progressing of the computing power and the development of chemical and physical models, the CFD applications in the biomass thermochemical conversion will more widely spread in the future. Europe PMC requires Javascript to function effectively. Recent Activity. The snippet could not be located in the article text.

This may be because the snippet appears in a figure legend, contains special characters or spans different sections of the article. Int J Mol Sci. Published online Jun PMID: Abstract Thermochemical conversion of biomass offers an efficient and economically process to provide gaseous, liquid and solid fuels and prepare chemicals derived from biomass.

Keywords: Biomass, CFD, thermochemical, gasification, pyrolysis, combustion, model. Introduction The use of biomass as a CO 2 -neutral renewable fuel is becoming more important due to the decreasing resources of fossil fuel and their effect on global warming. CFD modeling principles Computational fluid dynamics is a design and analysis tool that uses computers to simulate fluid flow, heat and mass transfer, chemical reactions, solid and fluid interaction and other related phenomena.

CFD sub-models of Biomass Thermochemical Conversion Process Biomass thermochemical conversion refers to the processes of biomass gasification for gaseous fuel or syngas, fast pyrolysis for liquid bio-oil, carbonization for solid carbon or combustion for heat energy. Table 1. Thermochemical conversion variant. Open in a separate window. Two-stage semi-global reaction schemes for: a cellulose; b wood. Additional physical models Although Navier-Stokes equations are viewed as the basis of fluid mechanics describing the conservation laws of mass, momentum, and energy, they have a limited amount of applications in the areas of biomass thermochemical conversion.

P-1 model P-1 model is the simplest formulation of the more general P-N radiation model, which is based on the expansion of the radiation intensity I into an orthogonal series of spherical harmonics. Rosseland model The Rosseland radiation model can be derived from the P-1 radiation model with some approximations. Table 2. CFD applications in biomass gasification and pyrolysis. Model Extra Model Agreement with Exp. Figure 2. Figure 3. Figure 4. Table 3. CFD applications in biomass combustion. Bagasse-fired furnaces [ 31 ] Fluent 3D To gain insight into the effect of moisture on the flame front.

Table 4. CFD applications in biomass co-firing. Model Extra model Agreement with Exp. Co-firing[ 35 ] Fluent 6. Co-combustion boilers[ 37 ] Fluent 6. Biomass utility boiler[ 38 ] Fluent 5. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure Table 5.

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Combustion chamber [ 40 ] Fluent 5. Pilot down-fired combustor [ 41 ] Fluent 5. Fluidized beds [ 42 ] Fluent 6. Wood stove [ 44 ] Spider 2D To model nitric-oxide formation from fuel-bound nitrogen in biomass turbulent non-premixed flames. Conclusions This paper summarized the CFD applications in biomass thermochemical conversion and system design.

References and Notes 1. Blasi CD. Modeling chemical and physical processes of wood and biomass pyrolysis.

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Progress in Energy and Combustion Science. Components, formulations, solutions, evaluation, and application of comprehensive combustion models. Xia B, Sun DW.

Applications of computational fluid dynamics CFD in the food industry: a review. Computers and Electronics in Agriculture. Applications of computational fluid dynamics CFD in the modelling and design of ventilation systems in the agricultural industry: A review. Bioresource Technology. Moghtaderi B. The state-of-the-art in pyrolysis modelling of lignocellulosic solid fuels. Fire and Materials.

Thermal conversion of biomass: Comprehensive reactor and particle modeling. Aiche Journal. Corella J, Sanz A. Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for stationary state. Fuel Processing Technology. Pyrolysis of biomass: improved models for simultaneous kinetics and transport of heat, mass and momentum. Energy Conversion and Management.

Coal devolatilization at high temperatures. Fluent Inc. Modeling biomass devolatilization using the chemical percolation devolatilization model for the main components. Proceedings of the Combustion Institute. Integration of CFD codes and advanced combustion models for quantitative burnout determination. Impact of radiation models in CFD simulations of steam cracking furnaces.

A CFD based combustion model of an entrained flow biomass gasifier. Applied Mathematical Modelling. Numerical investigation of the partial oxidation in a two-stage downdraft gasifier. Validation of kinetic parameter values for prediction of pyrolysis behaviour of corn stalks in a horizontal entrained-flow reactor. Biosystems Engineering. Modelling the pyrolysis of wet wood — I. Three-dimensional formulation and analysis. International Journal of Heat and Mass Transfer. Modelling the pyrolysis of wet wood — II.

Three-dimensional cone calorimeter simulation. Converting moving-grate incineration from combustion to gasification--Numerical simulation of the burning characteristics.

## ISBN 13: 9780080972923

Waste Management. CFD modelling of black liquor gasification: Identification of important model parameters. Sharma AK. Modeling fluid and heat transport in the reactive, porous bed of downdraft biomass gasifier. International Journal of Heat and Fluid Flow. A model of wood flash pyrolysis in fluidized bed reactor.

Renewable Energy. Development of advanced technology for biomass combustion — CFD as an essential tool. Authors: C. Hardcover ISBN: Imprint: Butterworth-Heinemann. Published Date: 18th December Page Count: For regional delivery times, please check When will I receive my book? Sorry, this product is currently out of stock. Flexible - Read on multiple operating systems and devices.

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Institutional Subscription. Free Shipping Free global shipping No minimum order. First ever quantitative approach to the subject of Thermochemical processing Companion volume to Kubachewski et al. Materials scientists, industrial process designers, chemical engineers and students. Powered by. You are connected as. Connect with:.

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