The combustion or gasification of coal may be thought of as occurring in several steps. First, the coal is heated at high rates, and rapid devolatilization occurs. The volatiles then react in the gas phase. (These processes are discussed in detail in Chapters 8 and 10.) The char remaining after devolatilization consists primarily of carbon, together with the major part of the ash components and some volatile matter. The heterogeneous reactions of the char with oxidizing gases (oxygen, steam, carbon dioxide, etc.) account for the majority of time required for particle burnout. In addition, in the case of gasification, the reaction of char with hydrogen may also become important, especially for high hydrogen partial pressures.

The combustion of char particles including the effects of external mass transfer, surface reaction, pore diffusion, and pore growth is analyzed theoretically. A random pore model is used to relate pore surface area, pore volume and effective diffusivity to the local burnoff. The equations for oxygen concentration and burnoff as functions of time and position are solved analytically for the limiting case of strong diffusional limitations (high temperature or particle size) and numerically for the general case. The results include the variation of particle size and density with burnoff under conditions pertinent to pulverized and fluidized combustion. The analysis is compared with previous analyses from the literature.

Most combustion systems use dense sprays of liquid fuels or pulverized fuel suspension for combustion. Thus the interactive processes between the drops/particles play a crucial role in the ignition and combustion behavior and pollutant formation and destruction. A comprehensive review is presented of the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review originally intended to be in two parts is now divided into three parts. Part I (Prog. Energy Combust. Sci., 18, 1992) is concerned with the interactive processes for arrays, streams and clouds of drops, Part II presents a review of isolated coal/char particles as an introductory material to interactive processes and finally Part III deals with the interactive processes for solid particle arrays, streams and clouds. While drops are totally gasifying, coal is a partially gasifying solid which yields char on gasification. As such there exists separate studies for the pyrolysis of coal and combustion of char particles. The kinetics modeling of pyrolysis, oxidation of volatiles and CO, carbon reactions, heterogeneous and homogeneous ignition and combustion of carbon and coal are reviewed. Because of strong analogy of the group ignition and combustion to porous char ignition and combustion, the literature on porous char combustion is also included. In order to maintain the continuity, new results are presented on the internal ignition of porous char particles using a Frank-Kamenetskii type of analysis. The ignition and combustion results of porous char are later used in Part III of the review which deals with group ignition and combustion of char clouds.

INTRODUCTION An important factor to be considered in developing NO control strategies for pulverized coal combustors is the potential for the reduction of NO by char. Such reaction has been shown to reduce appreciably the emissions of NO from fluidized bed coal combustors (Pereira et al., 1975). A number of experimental studies were carried out of the NO/char reactions using fixed and fluidized bed reactors (Furusawa et al., 1977; Chan, 1977; Sprouse, 1977; Beer et al., 1977) in which the products of the reaction were identified and kinetic parameters of the reaction were determined. The present paper is concerned with the destruction of NO by char at temperatures of interest in pulverized coal flames.

Abstract This review paper presents a comprehensive summary and analysis of research work conducted principally at the Brigham Young University Combustion Laboratory over the past decade. An attempt is made to set forth the philosophy and foundations for the laboratory research, which include commitments to both measurement and modeling of complex combustion processes. Recent work has emphasized pulverized-coal processes. The review paper reports past results, together with some recent, previously unpublished results, and provides an integration and summary of key findings. Information from independent investigators is also included or referenced where comparisons are made or where similar work provided clarification. Among the most significant results include the following: (1) measurement of local properties (e.g. velocity, concentration, temperature, mixing rate) from reacting and nonreacting gaseous and particle-laden flows for a variety of test conditions. These data provide a basis for determining controlling processes and for evaluating comprehensive coal reaction models; (2) development and application of comprehensive pulverized-coal combustion models for premixed and diffusion flames; (3) evaluation of combustion models by extensive comparisons with detailed profile data from this and other laboratories. Research needs and concerns in each area of activity are also identified, and future possible laboratory directions are considered.

Abstract The rates of oxidation of tiny particles (initial diam. ∼115 μm) of a nonporous graphite have been measured in beds of silica sand fluidized by different mixtures of O2 + N2 at 1 atm. Temperatures of 973–1173 K were used. Before burning, the graphite was fully characterized by B.E.T. analysis. Such a fluidized bed has several advantages for measuring the rate of burning of solid carbon: it proved straightforward to ensure that combustion was kinetically controlled, and that such small particles were both isothermal and at the same temperature as the fluidized bed. Also, the oxidation of CO to CO2 is inhibited by the large area of sand, which removes radicals, such as HO2 and OH. It is concluded that in this situation oxidation occurs in Cs + 1/2 O2 → CO (I) whose rate was measured to be half-order in O2 over the temperature range used and for a variety of concentrations of O2. Writing the rate of consumption of carbon in reaction (I) as k1 [O2]1/2 per unit area of carbon, it was found that k1 = 1.1 × 104 exp (−179 kJ mol−1/RT) mol1/2 s−1 m−1/2 correct to 50%. The implications of this are that initially O2 chemisorbs dissociatively on carbon to yield tightly bound oxygen atoms. These are not desorbed; instead, they appear to be in equilibrium with less tightly bound atoms of oxygen, which desorb to give mainly gaseous O2 or, less probably, CO. In such a fluidized bed, CO subsequently burns in the gas phase (usually in bubbles) far away from the original carbon particle. The graphite particles used here were initially not spherical; their shape was found to remain fairly constant during burning, as expected for combustion being kinetically controlled. In combustors other than fluidized beds, the radical HO2, or even OH well above 1300 K, might be alternative reactants instead of O2 in (I).

Abstract The intrinsic reactivities, that is the reaction rates per unit area of pore surface in the absence of any mass transfer restrictions, for combustion of a wide range of carbons have been calculated from published data. The intrinsic reactivities, corrected to a common oxygen pressure of 101 kPa, were ascertained over the temperature range 580 to 2200 K. The reactivities of ‘non-porous’ carbons (diamond, soot, vitreous carbon and pyrolytic graphite) between 770 and 4000 K were also considered. Porous carbons of various origins show intrinsic reactivities that differ by up to 4 orders of magnitude at a given temperature. However, after high-temperature heat treatment, certain carbons of different origin (e.g. sugar and wood charcoals, nuclear and spectroscopic graphites) show closely similar reactivities when reacted with purified oxygen. The reactivities of the ‘non-porous’ carbons show a temperature dependence similar to the average temperature dependence of the porous carbons, but their reactivity expressed per unit external surface area is generally higher than the median intrinsic value of the porous materials.

Abstract It is suggested that the activation energy, E , for the combustion of a char is independent of the coal's properties and only varies with the temperature of the char particle; also, the frequency factor of char oxidation, k o , ch , depends on the coal's properties during its burning in air. We have taken E = 180 kJ/mol as a constant in the oxidation of any coal char burning in air. Based on experiments for different coals, a relationship between k o , ch and the coal's properties, applicable to any coal char burning in air, is derived. The relationship is different from those proposed previously, i.e., with both E and k o , ch varying disorderly with a coal's properties. Now, k o , ch can easily be predicted from the proximate analysis of the coal. Furthermore, it can be demonstrated that k o , ch is also related to the burning regime; a quantitative relationship is presented.

Abstract Combustion rate measurements of Loy Yang brown coal char particles (88 μm median size) over the temperature range 940–1420 K yielded a chemical rate coefficient Rc, expressed as R c = 12.6 exp [ −68.3 (RT p )] kg m 2 s ( kPa O 2 ) 0.4 . Particle size and density decreased with burn off, but in an unusual manner that requires further investigation. The value of the activation energy, the manner of change in size, and density of the particles during combustion, and the observed order in oxygen (0.4), suggest that the burning rate was controlled by the combined effects of pore diffusion and chemical reaction, with a tendency for chemical reaction to dominate.

Abstract Coal is the most abundant fossil fuel and is widely used as an energy source for combustion and gasification. Both experimental methods and computational tools are required for the development of new advanced, innovative clean coal technologies and systems. In particular, 3D computational fluid dynamics (CFD) simulations can provide detailed local and global information on the interaction of fluid dynamics, mixing, and heterogeneous and homogeneous chemical reactions even for complex systems such as combustors, gasifiers, or chemical reactors. The predictive capabilities of CFD simulations depend directly on appropriate models and their mutual interactions. The current state of modeling is reviewed in this paper and the need for further improvements of both individual models and their respective coupling is addressed. In addition, to evaluate and validate the models and their interactions, systems with increasing complexity and well-defined boundary and operating conditions are required that can provide suitable experimental data. A number of reference burners and combustors, developed especially at universities and research institutions, are also presented and recent simulation data for these systems is reviewed.

A two-dimensional, two-phase combustion model of pulverized coal char is presented in this paper. It is of particular importance to reveal the controlling mechanisms, through prediction of the diffusion and reaction rates and to evaluate the effect of initial coal-particle size on the performance of the burner. The partial differential equations of conservation of mass, momentum, energy and chemical species are solved taking into consideration turbulent flow, interphase mass and heat transfer, radiative heat transfer and operational conditions such as coal feed and primary and secondary air. The equations are integrated with the finite volume methods and are solved for the flow field, temperature field of gaseous and solid phases and the concentration of reactants and products. The results are compared with experimental data from the literature and the comparison is satisfactory.

A brief description is given of devolatilization of raw coal and combustion of the residualchar. Major achievements in the study of devolatilization and important areas for further experimentation are noted; however, attention is focussed on the rate processes involved in char combustion. Means of calculating reaction rates are shown which account for temperatures of particles, mass transfer and diffusion of oxygen into the particle's pore structure, and reaction on the pore walls. Theoretical consideration is then given to the changes in the size, density, and pore structure of chars as they burn. Experimental data are shown for observed reactivities of coal chars corrected for external mass transfer effects, and intrinsic reactivities are reported for various coal chars and other carbons corrected for pore diffusion effects. Measured data on mass-transfer rates and changes in particle structure during combustion are given. Recommendations are made for further research on char reactivity and the manner in which pore structure develops during combustion. The need for combustion-oriented kinetics studies of coal develatilization is outlined.