Preface

… in the field of coal science one can hardly distinguish between fundamental investigations, applied research and even process development.

K.H. van Heek [1]

This monograph aims to bridge coal gasification1 technology and computer-based modeling utilizing recent advances in computational fluid dynamics (CFD) including the methodology on numerical heat and mass transfer theories. Latest developments on coal gasification technologies around the world (e.g., China, USA, India, South Africa, Japan, Canada etc.) have demonstrated that coal-derived synthesis gas (syn-gas) utilisation for chemicals and electricity have become an indispensable part in the national energy security policies of that industrially developed countries. Because of the large reserves of coal on Earth, the importance of coal gasification will continue to increase in the future. The basic feedstock used in gasification technologies is crushed and possibly dried raw coal, which is fed into a reactor chamber, the so-called gasifier.

On order to realize sustainable development of new generations of gasifiers with which it is possible to reduce their production and operation costs, it is imperative to use the so-called computer-aided design (CAD) and optimization. In this view, the bottleneck of such virtual design is a mathematical and numerical model describing physical and chemical processes inside a reactor–gasifier. Especially, simple models producing results close to reality are of great interest for the industry. However, it is impossible to develop simple models without understanding the basic fundamental processes characterizing high-temperature conversion in the gasifiers. This book is an effort to explore these fundamentals using the so-called direct or fully resolved numerical simulations of different physical processes related to interphase phenomena during the high-temperature conversion of coal and biomass particles under gasification conditions.

In the design of novel gasifiers operating with solid carbonaceous fuels (particles),the important issue is the adequate prediction of the basic characteristics of such devices. Because of the complexity of the physical and chemical processes inside gasifiers, where high temperatures and pressures prevail, experimental studies are not always capable of characterizing the basic features of all related phenomena. One way to understand, predict, and optimize the complex processes in a gasifier is to use the CFD platform, which is based on the numerical solution of mass, momentum, energy, and chemical species conservation equations.

However, the direct modeling of a gasifier resolving all the different scales, ranging from several meters for the whole reactor down to several micrometers for the coal particles, is impossible because of the lack of computing time to solve the system of equations. Therefore, the CFD modeling of a gasifier requires a multiscale approach in which the physics of the small scales is represented by submodels. In particular, typical submodels in a gasifier simulation calculate the small-scale turbulence, the chemistry–turbulence interaction, and the processes of drying, pyrolysis, and gasification/combustion of the particles. It should be noted that in spite of significant progress in the development of macroscale models for particulate flows and their numerical implementation in many commercial codes (ANSYS-Fluent, ANSYS-CFX) and open-source codes, the submodels which are used in the macroscale simulations, correspond to the models developed for coal combustion modeling in the early 1980s. Therefore, CFD-based models have become well-established tools for the understanding and optimization of fluid-particle flows in gasifiers.

It is rather surprising that, in spite of the subject's importance in the fields of chemical engineering and energy and material conversion, relatively few monographs are available on up-to-date numerical and semiempirical models describing interphase phenomena in high-temperature conversion processes such as gasification. The literature on comprehensive modeling of gasification is concentrated in conference papers or articles in scientific journals only. This book is an attempt to close the gap between the technological progress of gasification, which is well documented in the literature (e.g., see the monograph by Ch. Higman and M. van der Burg Gasification, or by J. Rezaiyan and N.P. Cheremisinoff ‘Gasification Technologies’), and the theoretical understanding and modeling of the interaction between chemically reacting solid particles and the surrounding gas, as applied to coal gasification technology.

This book is designed as a specialized textbook for master's or PhD courses in fields such as thermal sciences and chemical engineering where particulate flows with heterogeneous and homogeneous chemical reactions play a fundamental role. The purpose of this book is to present a description of a gas–particle reaction system taking into account the progress in the development of new models and numerical simulations for single-particle systems in an integrated, unified form. Special attention is paid to understanding and modeling the interaction between individual coal/char particles and a surrounding hot gas (300 K < T < 3000 K) including heterogeneous and homogeneous chemical reactions on the particle interface and near the interface between the solid and gas phases. This book, we hope, will also serve the needs of engineers from industrially oriented R&D engaged in research, development, and design for technologies where chemically reacting particles play a significant role.

This book is divided into 11 major chapters, beginning with an analysis of recent developments in computer-based simulations and mathematical multiscale models for the calculation of high-temperature conversion processes applied to gasification modeling including their validation. Next, Chapter 2 introduces a short review of the state of the art in the gasification of coal, including a brief history and analysis of existing large-scale facilities. Chapter 3 contains a review of the basic approaches used for modeling moving particles, including the treatment of particle–particle collisions and coupling with gas flows. Recommendations and illustrations for the application of these models are given at the end of the chapter. Chapter 4 is devoted to the closure relations for the drag force coefficient and the Nusselt number used for the spherical and nonspherical particles, including the influence of particle porosity on these parameters.

The main new ideas, including the cornerstone of the book, are found in Chapters 5–11. Chapter 5 presents subgrid models and particle-resolved numerical simulations of coal particle heating and drying including validation against experimental data published in the literature. The new models are illustrated by comparing the results with predictions obtained using standard approaches, discussing the advantages and disadvantages of the latter with respect to the new models.

Chapters 6 and 7 describe numerical models based on a fixed-grid method and the results of one-dimensional and two-dimensional numerical simulations devoted to analyzing the unsteady behavior of char particles undergoing gasification and combustion. Two models are illustrated: the so-called surface-based or shrinking-core model, and the so-called shrinking reacted-core model. The shrinking-core model is based on the assumption that the heterogeneous reactions occur at the particle surface. Thus, the carbon consumption is only related to the outer surface of the particle, whose diameter decreases over time. The shrinking reacted-core model takes into account intraparticle diffusion and intrinsic reactivity. In this case, the carbon consumption is related not only to the particle diameter but also to the particle porosity and specific surface. The particle interface tracking is treated using a sharp-interface method coupled with a fixed-grid method.

Chapter 8 describes the pseudo-steady-state (PSS) approach for char particle combustion and gasification. In this chapter, a comprehensive CFD-based model is used for resolving the issues of bulk flow and boundary layer around the particle. The model comprises the Navier–Stokes equations coupled with the energy and species conservation equations. At the surface of the particle, the balance of mass, energy, and species concentration is applied to formulate the boundary conditions on the particle surface, including the effect of Stefan flow and heat loss due to radiation at the surface of the particle. The model is validated against experimental data published in the literature for the laminar and turbulent flow regimes. Finally, the influence of the Reynolds number, the ambient mass fraction, and the ambient temperature on the behavior of char particle is discussed.

Chapter 9 includes descriptions of numerical simulations of the carbon conversion occurring in pores inside the particle. The PSS approach is used to explore the physics of the process. Numerous 2D and few 3D simulations are illustrated and analyzed.

Chapter 10 presents advanced subgrid models for predicting the pyrolysis, gasification, and combustion of a single coal particle moving in a hot environment. Apart from the model formulation and description, this chapter includes numerical examples and validations illustrating new points and showing the robustness of the models.

Finally, Chapter 11 discusses the needs and challenges in modeling the next-generation gasifiers which are under development or in the test phase....