Natural bed moisture

In the Parr formula for moist, mineral-matter-free calorific value, the moisture basis used is that of the inherent moisture of the coal in the seam (natural bed moisture, capacity moisture) ... 

The subbituminous coals and lignite are then classified into groups as determined by the calorific value of the coals containing their natural bed moisture, i.e., the coals as mined but free from any moisture on the surface of the lumps. The classification includes three groups of bituminous coals with moist calorific value from above 14,000 Btu/lb (32.5 MJ/kg) to above 13,000 Btu/lb (30.2 MJ/kg) ... 

Heating value (calorific value), which is the energy released as heat when coal (or any other substance) undergoes complete combustion with oxygen. Moist calorific value is the calorific value of the coal when the coal contains its natnral bed moisture. The natural bed moisture is often determined as the equilibrinm moistnre under prescribed standard conditions (Chapter 8). In addition, the agglomerating characteristics of coal are used to differentiate between certain adjacent groups. 

Natural Bed Moisture (Equilibrium Moisture, Capacity Moisture)... 

In the ASTM system for the classification of coals by rank and in the International System for the classification of hard coals (Chapter 2), high-volatile coals are classified according to their calorific value on a moist basis. In this instance, the calorific value is quoted for the coal containing its natural bed moisture. 

Indeed, at this point a distinction should be made between the natural bed moisture of coal and the as-received moisture of coal. The natural bed moisture of coal is the amount of water that a particular coal will hold when it is fully saturated at ca. 100% relative humidity (i.e., at the conditions approximating those of an undisturbed coal seam). It is also considered to be an indication of the total pore volume of the coal that is accessible to water. On the other hand, the as-received moisture content of coal relates to the amount of water in the coal at the time the coal is received for analysis. This may be somewhat smaller than the natural bed moisture, especially if the coal has been allowed to dry partially. In contrast, excessive amounts of surface moisture on the coal could lead to values for as-received moistures that are in excess of the natural bed moisture. 

Other relationships also exist, such as variations of natural bed moisture with depth of burial (Figures 8.9 and 8.10) as well as the variations in volatile matter content of vitrinites obtained from different depths (Figure 8.11). This latter observation (i.e., the decrease in volatile matter with the depth of burial of the seam) is a striking contrast to parallel observations for petroleum where an increase in the depth of the reservoir is accompanied by an increase in the proportion of lower-molecular-weight (i.e., more volatile) materials (Figure 8.12). 

FIGURE 8.9 Variation of natural bed moisture with depth for German low-rank coals. (From Murchison, D. and Westoll, T.S., Eds., Coal and Coal bearing Strata, Elsevier, New York, 1968.)... 

The most commonly used system of classification refers to the American standard (ASTM D-388 [13]) using fixed carbon combined with volatile matter on a dmmf basis for higher-rank coals and higher heating value (HHV) on moist mineral matter-free (mmmf) basis for lower-rank coals. The moisture used should be as close as possible to the natural bed moisture content and is obtained by exposing the sample to 30 °C and a relative humidity of 97%. To convert the analysis data to the mentioned reference state, the Parr formulas must be used assuming that a part of the ash sulfur leaves with the volatile matter ... 

Table 3.7 Natural bed and maximum allowable feed coal moisture for dry dense-phase feeding [19].

The existence of an electrical potential causes not only cation and anion movement but also migration of moisture toward the cathode. This movement of water (electroendosmosis) is due to the asymmetrical nature of the polar groups of the water molecule. In arid regions water leaving the anode area may cause the soil surrounding the anodes to become so dry that proper current densities cannot be maintained along the line. To alleviate this, some pipe-line companies have had to transport water into desert areas to re-moisten anode beds. 

Detailed design of fluid spray dryers must, however, be accompanied by experimental work. It has been noted [22] that even quite similar materials may behave very differently in a spray granulator. Experimental work will indicate such information as the natural mechanism of particle growth (coalescence vs layering), attainable product sizes, the preferred temperature and moisture conditions for the bed material, suitable ranges of bed voidage... 

Epstein, Kostrin, and Alpert (1978). The feedstock was pyrolyzed in a 0.5-m ID fluid-bed reactor containing sand and an inert gas generated from compressed air-natural gas combustion with a slight excess of air (about 0.2 to 0.6%). The fluidizing velocities were 0.3 to 1 m/s. The products were low-energy gas (3.89-11.78 MJ/m- (n)), pyrolytic oil (23.3-27.9 MJ/kg), and charcoal. Feed rates were 50-200 kg/h. The moisture contents of the feedstocks were not specified. The balance of the yield for each feedstock is water. 

The testing program was initially designed to evaluate the effect of parameters such as moisture content, bed temperature and the physical nature of the feed. Subsequently, experiments were conducted to examine long term effects, by simulating commercial operation under a variable load/demand situation for extended periods of time (2 periods of 670 hours continuous operation). Also included in the program were stack sampling tests, and tar production measurements. 

Simply stated, the units are sized based on the amount of area required to dry the quantity desired in a finite time period. Unfortunately, each manufacturer uses different designs for their bed screen or porous plate. The way to size the dryer would be based on an effective drying yield defined in units of pounds of solvent per hour-ft. Knowing the amount of solvent or moisture to be removed per hour, one can easily calculate the area of the fluid bed. Getting the manufacturer to divulge the yield may prove difficult since they may not have the data for your particular solvent or may not want to divulge it due to the competitive nature of the business. 

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