CHAPTER 5 Experimental analysis of lignite drying in superheated steam
5.1 Comprehensive analysis of the general drying characteristics of lignite exposed to
The insight into superheating steam drying of a lignite particle was described with reference to particular stages of the process. The temperature profile and mass of the sample as well as the drying indicators introduced in the previous chapter: moisture content and drying rate, were plotted along time to discuss the features of the process.
On the basis of video recording, the features of change in sample surface were evaluated and discussed together with the images of the object captured during the test.
More details related to observation of the examined object are given in section 5.3.
The abovementioned components of drying behavior analysis were supplemented with deliberation on the mechanism of water evaporation. As discussed in chapter 2, moisture in lignite can be roughly divided into free and bound water, depending on the manner in which each type is removed from the porous structure.
As an exemplary case used for the sake of discussion, the instance of 5 mm sample exposed to superheated steam of 170 oC was used. The entire drying can be separated into consecutive stages: {1} preheating period, {2} constant drying rate period (CDRP), {3,4} decreasing drying rate period (DDRP) consisting of two phases and {5} final drying period. The drying behavior in particular parts is shown in Fig. 5.1.
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Figure 5.1 Drying behavior of 5 mm sample at 170 oC, in relation to: A) changes of drying indicators in time, B) changes in object’s appearance, C) stages of water removal,
D) changes of drying indicators in the function of moisture content.
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5.1.1 Preheating period
In the beginning of drying, the exposition of cold sample to the superheated steam atmosphere induces condensation on its surface. The mass of the particle increases and so does the moisture content. The drying rate, consequently, achieves negative value for that reason. The temperature of the surface suddenly increases to reach the saturation temperature of 100oC, followed shortly by the temperature of the center. The end of this period is indicated by moisture content curve achieving its maximum level.
5.1.2 Constant drying rate period (CDRP)
The stage of CDRP is distinguished by the rate of drying stabilized at the invariable value. During the initial phase of this period, marked as {2a} in Fig. 5.1, the removal of water condensed in the preheating period is in progress. At this point the center temperature reaches 100 oC. This subperiod ends when the moisture from the surface is evaporated. It is indicated by stepwise ascend of the surface temperature into value slightly exceeding 100 oC and its subsequent gradual increase. The exact moment is determined by the experimental conditions, such as drying medium temperature and object diameter.
In the latter subperiod of CDRP {2b}, the difference of heat transferred to the sample and heat consumed on evaporation of water in the vicinity of the surface contributes to the increase of temperature in the surface region. Due to slowly inclining temperature gradient between surface and center of the particle, the heat is propagated to the lignite interior. Still, the evaporation occurs in the proximity of the surface, what is induced by capillary forces acting on the free water, which diffuses from the core of the sphere (Fig. 5.1.C). The value of drying rate, which remains constant, is defined by
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the amount of heat exchanged with the superheated steam (Fig. 5.1.D). The decrease in sample diameter becomes observable, however, it is not significant at that stage. The constant drying rate period ultimately ends when the drying rate level decline is observed and the rapid incline in surface temperature occurs.
5.1.3 Decreasing drying rate period 1 (DDRP 1)
As mentioned in the end of previous paragraph, the DDRP, marked as {3} in Fig.
5.1, starts when the drying rate is in decline. Therefore, mass and moisture content of lignite decrease more gradually than in the previous period. The mechanism of drying alters in this stage, with evaporation border shifting towards the sample core (Fig. 5.1.C). The vaporization and resulting water content gradient entail movement of water in the form of multi-phase flow of liquid and gas. Over that period the hygroscopic bound water starts to be removed from the sample, what consumes larger amount of heat than required for the free water. As mentioned in chapter 2, the enthalpy change for the desorption of bound water, removed above the saturation temperature of 100 oC is correlated with the value of moisture content (see Fig. 2.2). The dewatering of lignite during this stage results in formation of gaps and crevices between dry matter, leading to cracking of the sphere’s structure and volumetric shrinkage (Fig. 5.1.B). The first part of DDRP is decided to be finished when the rate of center temperature increase exceeds that of surface temperature, what is indicated by the highest difference between those two.
5.1.4 Decreasing drying rate period 2 (DDRP 2)
The decrease of dewatering rate in the first phase {4a} of DDRP 2 achieves the maximum level regarding the entire process, signalized by the steep inclination of drying rate chart. Evaporation of bound water is still in progress and the cracks already
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formed tend to collapse due to shrinkage of the object (Fig. 5.1.B). The phase marked as {4a} terminates as the rate of temperature change reaches its peak value.
During the second phase {4b}, the rate of temperature alteration declines continually, accompanied by further reduction of the drying rate level. The decreasing drying rate period comes to an end when the temperatures within the sphere equalizes with the temperature of the superheated steam (Fig. 5.1.A).
5.1.5 Final drying period
Though the temperature is no longer subject to observable changes, the evaporation of tightly bonded water continues until the equilibrium moisture content is reached, which value is a function of the drying medium temperature [15]. The drying rate becomes close to none, as the bound water poses great challenge in terms of feasibility of its removal.
This stage takes the longest time to be finished and produces only a minor upgrade of the coal quality, what matches the general trend that subsequent periods of drying consume larger amounts of heat with smaller measurable effect. Thus, drying of coal to such a low moisture content value occurs mostly in the research endeavors [25].
In terms of industrial practice, though, the most crucial stages of drying are the CDRP and DDRP1.