Coal processes, characteristics

Different technology suppliers worldwide have developed gasifiers. The choice of the type of the gasifier is purely a factor of the coal/fuel characteristics. Various technology suppliers for the gasification process are listed in Table 2.2. 

It has been recognized for many years that exposure of coal to air can significantly alter the processing characteristics of the coal. This effect is more pronounced in lower rank coals and in fact some brown coals will undergo spontaneous combustion at or near ambient temperatures. 

Both the initial oxidation and the effect this oxidation has on the processing characteristics of the coal have been an area of interest for many years (1). In the last few years there has been a renewed interest in tTTe oxidation of coal and several new analytical techniques have been applied to this problem (2-6). Several salient features of the process have been revealed including the chemical forms the oxygen takes when bound to the coal. It has been noted that the oxidation is not uniform throughout the particle as evidenced by the formation of a discoloration of the coal near the surface (7). This "oxidation rim" is directly related to the behavior of the coal. 

The petrographical and chemical investigations presented in the following sections were carried out in order to describe the behaviour of the coal types characteristics of the Rhenish brown coal area during refining processes. 

Power and heat production by the process of pressurised air blown fluidised bed gasification of solid fuels is a promising technology with respect to emissions and efficiency. These process characteristics depend strongly on parameters like fuel type, pressure and air stoichiometry. There is a relation between these variables and the actual process configuration, like e.g. staged combustion, advanced PFBC IGCC, or the British Coal Cycle . 

In the 1960s, two direct coal liquefaction processes were under development in the U.S. the Exxon Donor Solvent (EDS) process and the H-Coal process. The distinguishing feature of the EDS process was a separate solvent hydrogenation step to carefully control the hydrogen donor characteristics of the solvent. The most important feature of the H-Coal process was the emulated bed reactor in the process. 

Further experimental data and further model comparisons relate to the rapid pyrolysis of different coals. In the absence of air, this experimental device heats and converts small coal particles (10-200 pm) in gas and distillates. Figure 20 shows a very satisfactory agreement between experimental data relating to a bituminous coal and model results at 1,260 K. It is noteworthy that despite the strong differences between carbon deposit and bituminous coal, the characteristic times for the dehydrogenation processes are practically the same. Further data on this subject, as well as a detailed model for the analysis of the pyrolysis and devolatilization process of coal particles, are available in a recent paper (Migliavacca et al., 2005). 

As in coal formation, with increasing compaction and burial depth bacterial activity declines and temperature increases, so that the low-temperature, biologically mediated processes characteristic of diagenesis are replaced... 

TABLE IV Typical Process Characteristics for Various Coal-Related Combustion Technologies ... 

Sasol produces synthetic fuels and chemicals from coal-derived synthesis gas. Two significant variations of this technology have been commercialized, and new process variations are continually under development. Sasol One used both the fixed-bed (Arge) process, operated at about 240°C, as weU as a circulating fluidized-bed (Synthol) system operating at 340°C. Each ET reactor type has a characteristic product distribution that includes coproducts isolated for use in the chemical industry. Paraffin wax is one of the principal coproducts of the low temperature Arge process. Alcohols, ketones, and lower paraffins are among the valuable coproducts obtained from the Synthol process. 

Operating parameters of this German plant, on the basis of one cubic meter of raw gas, iaclude 0.139 m O2, 0.9 kg briquettes, 1.15 kg steam, 1.10 kg feed water, 0.016 kWh, and 1.30 kg gas Hquor produced. Gasifier output is 1850 m /h and gas yield is 1465 m /t dry, ash-free coal. The coal briquettes have a 19% moisture content, 7.8% ash content (dry basis), and ash melting poiat of 1270°C. Thermal efficiency of the gas production process is about 60%, limited by the quaHty and ash melting characteristics of the coal. Overall efficiency from raw coal to finished products is less than 50%. 

W. Spackman, "What Is Coal ", Short Conrse on Coal Characteristics and Coal Conversion Processes, Peimsylvania State University, University Park, Pa., Oct. 1973, 48 pp. 

Three West Virginia coals were supplied by the West Virgmia Geological Survey (WVGS). The particular coals were chosen on the basis of rank, petrographic composition, and mineral matter content The coals were limited to the bituminous rank since these coals are the most amenable to the NMP solvent extraction process and are mdigenous to the Appalachian region. Some of the coal characteristics are listed in Table 2. 

Adsorption is the process by which liquid or gaseous molecules are concentrated on a solid surface, in this case activated carbon. This is different from absorption, where molecules are taken up by a liquid or gas. Activated carbon can made from many substances containing a high carbon content such as coal, wood and coconut shells. The raw material has a very large influence on the characteristics and performance activated carbon. 

Coke byproduct wastes. Coke, used in the production of iron, is made by heating coal in high-temperature ovens. Throughout the production process many byproducts are created. The refinement of these coke byproducts generates several listed and characteristic wastestreams. However, to promote recycling of these wastes, U.S. EPA provided an exclusion from the definition of solid waste for certain coke byproduct wastes that are recycled into new products. 

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