Lignite, dehydration

The rate of evaporation of water from the carbonaceous residue (to which it is not chemically bonded) during lignite dehydration [83] is only slightly greater than that from alum nuclei [20]. This supports the view that intranuclear water in alums, temporarily retained [43] and believed to promote product recrystalhzation, is not strongly bonded to the residual sulfates and further dehydration rapidly replaces evaporative loss. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Thermoanalytical investigations of sedimented airborne particulates [EINAX and LUDWIG, 1991] confirm experimentally the chemometrically found interpretation that aluminum can serve as an indicator element for lignite combustion. Thermogravimetric analysis of mixed samples of sedimented dusts detect a loss of mass at a temperature of 714°C this can be interpreted as dehydration of aluminosilicates. This loss of mass exhibits a well defined summer minimum and a strong winter maximum. These findings also correspond to the results from factor analysis (see Section 7.2.1.2.4). 

Dimethyl ether is prepared by the reaction of bituminous or lignite coals with steam in the presence of a finely divided nickel catalyst. This reaction produces formaldehyde, which is then reduced to methanol and dimethyl ether. Dimethyl ether may also be prepared by the dehydration of methanol. 

Curves D and E are difficult to distinguish. There may be a short acceleratory stage before onset of the dominant or exclusively deceleratory rate process. Examples of behaviour of fliis type include the dehydrations of Li2S04.H20 [70,92], lignite [93] and Ca(OH)2 [94]. 

The result of a comprehensive literature review by Behrendt consistently confirmed that the hydrothermal treatment of biomass coals by dehydration and decarboxylation yields a product with calorific value of about 30 MJ/kg whose elemental composition is similar to that of lignite [10]. The carbon content and the properties of such cokes strongly depend on the starting materials and the process paramters applied, namely temperature and residence times of 4—14h. The coke yields can reach values of up to 70% (g/g) and even higher values if catalysts such as citric acid are used. Those products have an increased calorific value of 15-20%, relative to the dry weight. About 80-85% of the carbon remains in the coke, about 10-15% remains in the aqueous phase in the form of organic acids and other... 

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