Coal single particle

FIGURE 11.62 Experimental calibration curves for a commercial single-particle counter and two types of calibration aerosols dioctyl phthalate (DOP) and coal dust (adapted from Whitby and Willeke, 1979). 

In order to elucidate the effect of the particle size during the carbonization process, a so-called granular carbonization experiment was carried out (Figure 6). In this experiment single particle-size fractions of coals of different... 

Leaving aside single particle impact tests which fall into section 6.1, the most common type of multiparticle impact tests are drop shatter tests in which a specified quantity of the test material is dropped through a specified height onto a hard surface or into a container. National and international standards exist on drop shatter tests for coke and coal and similar tests are also applied to iron ores and sinters. Drop shatter tests are in general used on coarser solids than those within the scope of this guide the fine fraction is in fact often removed from the bulk material before the test. 

Hock JL and Lichtman D (1982) Studies of surface layers on single particles of in-stack coal fly ash. Environ Sci Technol 16 423-427. 

Since realistic wood feedstocks are heterogeneous, the uniform entity appropriate for fundamental studies to identify improved conditions for tar formation is the single particle. In it, the intraparticle conditions can be measured and related to process conditions that can be manipulated. In addition, the effect of heat and mass transfer rates on reaction products can be determined. The findings can be rationalized to all reactors in which the studied experimental conditions prevail. The investigation of reacting single particles has proven extremely successful in the development of catalytic reactors (2) and coal pyrolysis (3,4,5). 

At high combustion temperatures, up to 70% or more of the reactive coal mass can be consumed through this process. Volatiles combustion in practical systems is complicated by turbulent mixing of fuel and oxidizer, soot formation and radiation, and near-bumer fluid dynamics. Such systems usually exhibit volatiles cloud combustion, rather than single-particle combustioa... 

Worster and Denny (1955) pubUshed data on the terminal velocity of coal and gravel particles, as shown in Table 3-5. The effect of the concentration is clearly marked by a difference in terminal velocity between a single particle and a volumetric concentration of 30%. 

Lee, D.C. and Georgakis, C. (1981) "A single Particle-Size Model for Sulfur Retention in Fluidized Bed Coal Combustors , AIChEJ, 27, 472-81. 

If a coal is exposed to elevated temperatures in the gasifier (<700 °C), it experiences drying and pyrolysis releasing volatile matter. Both result in the breaking of single particles into more pieces, a process called thermal fragmentation. But also the reverse effect (agglomeration) can occur. 

It is the goal of this paper to attempt a systematic organization of material in order that mechanistic as well as phenomenological models xaseful for design and scale-up of coal conversion reactors can he developed. The approach taken here is to examine the current status of coal conversion reactions on a single particle and to select a realistic yet sufficiently simple model capable of describing the phenomena. The deficiencies and limitations of each model are presented carefully. 

In developing a coal conversion reaction model for a single particle system, it is very important to recognize the complexity and heterogeneity of the structure of coal. Coal is a complex. 

BASIC EQUATIONS FOR SINGLE PARTICLE COAL-GAS REACTIONS... 

In the design of a coal conversion reactor, it is necessary to consider not only the reaction kinetics of a single particle but also the hydrodynamics of gas and mixing of solids and the accompanying heat and mass transfer occurring in the reactor. 

However, this mode of flow usually is limited to conveying distances of L < 500 m. The Group D materials that possess wide size distributions (especially a considerable amount of fines) and/or unusual particle shape (e.g., crushed coal, petroleum coke, crushed bath, rice hulls) usually are not suited to low-velocity conveying. In these cases, the options are dilute-phase or single-slug dense-phase (i.e., these materials usually are too coarse for bypass conveying). However, when long distances are involved, dilute-phase may be the only practical option. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

Recent work reported a method for estimating the mineral content of coals based on the electron-mlcroprobe-determlned chemical composition of discrete particles. (1,2) Each particle Is assumed to contain only one mineral component. Possible ambiguities In qualitative Identification of discrete mineral particles can be eliminated by XRD analyses of the bulk material to Identify the minerals present. For most geological materials, such separations are not readily obtainable. Thus, this method Is limited to materials that can be dispersed Into particles composed of single minerals. 

The principal limitations of ESCA include the inability to detect elements present at trace concentrations within the analytical volume, and insufficient lateral resolution to characterize single micrometer-sized particles. The inability to characterize trace species is illustrated in Figure 10 for a sample of coal fly ash particles (11). The fly ash results from the noncombustible mineral components of the coal and consists largely of fused iron oxides and aluminosilicates (42). In addition, most elements are present in at least trace concentrations (22, 42), and many of these elements are highly enriched in the surface region of the particles (evidence for this will be discussed in the next section). However, the ESCA spectrum acquired over several hours of counting time indicates only the presence of detectable surface S and Ca in addition to the fly ash matrix constituents. 

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