Observation of the behavior of coal particles during thermal decomposition

Abstract This paper reports on the observed behavior of coal particles during thermal decomposition. The data presented are the first from a study initiated to address both the physical and chemical behavior of pulverized coal from the time of initial heating through the evolution of soot particulate. In the present study, pulverized (high volatile bituminous) coal was injected through a slit centered in a methaneair flat flame burner, and high resolution holography was employed to record the evolution of volatiles and the structure of soot particulate. Volatiles were observed to evolve in a variety of shapes that range from individual jets to uniformly distributed clouds. Coal particle fragments and/or incipient soot nodules (∼ 3 micron) were found to be present within the volatile gases during the evolution. Further from the burner, stringlike soot particles approaching 1600 microns in length were observed.


INTRODUCTION
particles are known to exist in a given plane of focus, a condition that occurs, for example, when The direct observation of reacting coal particles the particles are stationary or when the number could provide valuable insight into fundamental density is extremely high. In reacting flows, these conditions seldom if ever exist. In contrast to processes associated with pulverized coal combustion and pollutant formation. Such a detailed photographic techniques, holography has the disunderstanding is required for the development of tinct advantage of allowing particles to be imaged in a dynamic field. As a result, holography was physical and theoretical models that could eventselected for the present study. ually lead to the development of advanced pollution control techniques.
The following is a description of the combus-The objective of the present study was to ob-tion and holographic systems employed, together with a presentation of the results and a discussion serve individual coal particle behavior during thermal decomposition. Techniques available for of the basic coal particle combustion behavior observed. These results represent the first in a study such observations include photography and holoof pulverized coal particle behavior during thermal graphy. Photography is convenient only when decomposition.
EXPERIMENT addition, resolution of particles is typically limited to diameters in excess of 50/3xn. By application in Combustion System the present study of special optical and mechanical Coal particles carried by air were injected through techniques (described in more detail in Ref. [4]), a narrow (1.6 ram) slot into a flat, methane-air coal particles in a reacting system were successfully observed down to a size resolution of 3 microns. flame (Fig. 1). The burner consisted of a 13-cm High resolution was achieved by (1) magnifying sintered metal disk, and neither the burner nor the supporting flame was enclosed. The flat flame was with high quality lenses before recording, (2) using operated near methane stoichiometry with a cold near image plane holography to relax the hologram face velocity of approximately 30 cm/sec. The in-requirements further, and (3) precisely aligning the jection velocity of the coal was approximately 5 hologram during reconstruction. A schematic of the holocamera is presented in m/sec. The coal used for this study was a high Fig. 2. The viewing volume recorded on each holovolatile bituminous Utah coal. The samples were pulverized and repeatedly sieved through a 200 gram was 2.54 cm (1 in.) in diameter and roughly mesh screen until microscopic analysis indicated 2.54 cm (1 in.) in depth. that a negligible fraction of fine particulate remained. The size range of the particles injected EXPERIMENTAL RESULTS was approximately 75 to 100 ktm. An auger feeder was used to control the feed rate of the prepared Experimental results are presented for two locacoal at about 70 gin/rain. It is estimated that the tions in the flame. Results are shown in Fig. 3 for coal experienced an initial heating rate of 105 to an area near the burner and in Fig. 4 for an area 106°K/sec. removed from the burner by approximately 5 cm.

Holography System Close to the Burner
Holography has been applied to the study of Close to the burner (Fig. 3), particles are observed several types of combustion with varying degrees to be surrounded by clouds of devolatilization of success (e.g., Refs. [1], [2], [3]). One of the products. As seen, a variety of shapes occur that principal problems is loss of resolution caused by range from jets (e.g., Fig. 3(c)) to uniformly distriimaging through the turbid and variable density buted clouds (e.g., Figs. 3(b), (d), (f)). A few of medium normally associated with combustion. In the forms are dramatic in structure (e.g., Figs. 3(e), (g)), indicating a strong explosionlike or complex soot formation process. It is noteworthy that evidence of devolatilization in Fig. 3(a) is bounded , 111 ! porous MATERIAL within a discernible horizontal band that repre- The clouds are found to contain numerous COAL small particles, many of which are smaller than the ,A~ PER system resolution. However, many clearly defined lent devolatilization process or incipient nodules ~~ of soot. In view of events observed later in the flame (Fig. 4), the latter is highly probable. Figure 3 is an example of a double-pulse image. with a time interval of 150 microseconds). As a re- Figure 4 is an example of holographic interfersuit, two images are captured on the same holo-ometry in which a second exposure is made seconds gram. For particles that can be separately identified (in this case, 30 seconds) after the first exposure. in both images, information on particle dynamics An interference pattern is produced as a result of can be assessed. Because the field is three dimen-some geometrical or density perturbation in the sional, the identification of the same particle in system. A slight change in flame characteristics or both images is difficult and may be impossible in a small movement in either the optical or experimany cases. However, an example of the dynamics mental system is sufficient. of one particle can be observed in Fig. 3(b). The

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The advantage of holographic interferometry is displacement time information suggests a velocity the enhancement to the particle flow visualization of approximately 3 m/sec, which is reasonable and the identification of density variation. In the considering the deceleration that occurs from the dark bands of the interferogram, white, black, and time of injection due to particle drag and spread-gray images can be observed. One explanation for ing.
the particles that appear gray in the dark band (e.g., Fig. 4(c)) is that they are transparent. The Removed from the Burner white images are probably particles present during one of the two exposures that have been rendered At a position removed from the burner (Fig. 4), a white by the additive exposure associated with the variety of particle shapes are observed. Many interference pattern. A remote possibility is that particles are shown to be large in comparison to some of the white images are gaseous pockets. In both the raw coal particles injection into the Fig. 4(g), for example, an image is shown that sugflame and the particle sizes observed close to the gests the presence of a gas pocket surrounding two burner. The unusually long, stringlike structures char particles. (The example is located outside the (Figs. 4(b), (e)) are especially impressive. Note boundary of Fig. 4(a), approximately 5 mm hori. that one photographed particle (Fig. 4(e)), if con-zontally from the top left corner.) The ability to tinuous, exceeds 1600 microns in length. The size resolve pockets of variable density gaseous media and shape of particles in this region suggest that has yet to be established. many of them are soot particles; a conclusion that coincides with visual observations made of luminosity and particulate emission at this location in SUMMARY the flame. It is also of interest to note that all the stringlike structures are generally aligned with the The present study, undertaken to observe the bedirection of the flow. The smaller particles ob-havior of individual reacting coal particles, demonserved are more consistent in size with particles strated that observation of coal particles during observed closer to the burner and are likely char.
thermal decomposition can provide valuable insight  into the fundamental processes associated with • The mechanics of soot formation. pulverized coal combustion. Devolatilization clouds • The mechanics of ash formation. were observed early in the flame, and soot particles were observed late in the flame. Knowledge was These questions are at present the subject of inalso acquired concerning volatile cloud formation quiry in an experiment that has been designed to and structure and soot shape and size. In addition, explore the parametric variation required. double-pulse holography was shown to provide instructive dynamic information, while holographic

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The investigation described in this paper was interferometry demonstrated promise for enhanced conducted at the Energy and Environmental Reflow and particle visualization, search Corporation and supported by the EPA The limited number of events observed in this Fundamental Combustion Research Program, EPA preliminary assessment probably do not encompass Contract Number 68-02-2631. The authors gratethe many that may be amenable to observation, fully acknowledge the helpful comments of W. S. nor do they provide sufficient information to draw Lanier, the EPA Pro/ect Director, and the contribusubstantive conclusions regarding the fundamental tion of Professor R. E. Peck during the developquestions associated with coal particle behavior, ment of the initial concept of the study. Examples of questions amenable to further analyses include: • The time-resolved changes in particle structure