Age 350 300 250 200 150 100 50

Million Years

FIGURE 1-1. Comparison of the geological ages of the world's hard coal and lignite deposits. (From Walker, S., Major Coalfields of the World, IEA Coal Research, London, 2000. With permission.)

of the cellulose of the initial plant material, the conversion of the lignin of the plants into humic substances, and the condensation of these humic substances into larger coal molecules [6]. The kind of decaying vegetation, conditions of decay, depositional environment, and movements of the Earth's crust are important factors in determining the nature, quality, and relative position of the coal seams [1]. Of these, the physical forces exerted upon the deposits play the largest role in the coalification process. Variations in the chemical composition of the original plant material contributed to the variability in coal composition [1,7]. The vegetation of various geologic periods differed biologically and chemically. The conditions under which the vegetation decayed are also important. The depth, temperature, degree of acidity, and natural movement of water in the original swamp are important factors in the formation of the coal [1,8].

The geochemical phase of the coalification process is the application of temperature and pressure over millions of years and is the most important factor of the coalification process. While there is some disagreement as to which has been more important in promoting the chemical and physical changes—high pressures exerted by massive overburdening strata or time-temperature factors—the changes are characterized physically by decreasing

Materials Partial Processes Main Chemical Reactions

Bacterial and fungal life cycles

Air oxidation, followed by decarboxylation and dehydration

Decarboxylation and hydrogen disproportioning

Condensation to small aromatic ring systems

Condensation of small aromatic ring systems to larger ones; dehydrogenation

Complete carbonification

FIGURE 1-2. The coalification process. (From Van Krevelen, D. W., Coal: Typology-Physics-Chemistry-Constitution, Third ed., Elsevier Science, Amsterdam, 1993. With permission).

porosity and increasing gelification and vitrification [9]. Chemically, there is a decrease in moisture and volatile matter (i.e., methane, carbon dioxide) content, as well as an increase in the percentage of carbon, a gradual decrease in the percentage of oxygen, and, ultimately, as the anthracitic stage is approached, a marked decrease in the content of hydrogen [7,9]. For example, carbon content (on a dry, mineral-matter-free basis) increases from approximately 50% in herbaceous plants and wood to 60% in peat, 70% in lignite, 75% in subbituminous coal, 80 to 90% in bituminous coal, and >90% in anthracite [7,10-12]. This change in carbon content is known as carbonifica-tion. The coalification/carbonization process is shown in Figure 1-2, where some of the main chemical reactions that occur during coalification are listed [1].



0 0

Post a comment