THE COALIFICATION PHASE OF COAL SYSTEMS

The coalification phase of coal systems (transformation of peat to coal) results in significant physical reduction in volume and loss of water and organic matter. Modern peat deposits that could result in commercial grade coal generally form in oligotrophic (nutrient poor) environments that are free of detrital sediment influx. Oligotrophic peat-forming environments primarily are acid (pH < 5) and, as a result, microbial degradation is highly restricted. Oligotrophic peat (OP) is well preserved (fibric > humic >>> sapric) and consists of 90% moisture, 9% organic matter, and up to 1% mineral matter (by weight). Well preserved wood also may be an important constituent. Alkaline and alkaline earth elements, Fe, and S, only occur in minor to trace quantities because of the absence of any significant source of these elements. Acid leaching may further reduce available alkaline and alkaline earth elements. The mineral matter in oligotrophic peat, therefore, is primarily composed of relatively insoluble compounds of Si and Al that are precursors to quartz and kaolinite. Significant amounts of Fe and S may be incorporated as pyrite during and after burial.

The transformation of OP to lignite (~ 30% moisture) is primarily the result of burial compaction and expulsion of pore water that results in a volume loss of 70 percent. Experimental data indicate that further compaction and thermal coalification result in additional loss of pore water and loss of organic matter in the form of CO2, CH4, H2O, and N2. Calculations based on the chemical composition of oligotrophic peat and medium volatile bituminous coal (MVBC) indicate that 50% of the original organic matter and > 99% of the original pore water is lost during transformation of OP to MVBC. The coalification processes results in a compaction ratio of 20:1 for OP peat. As a consequence of compaction and coalification, a mineral matter content of 10 % in OP (dry weight basis) will be increased to 18 % in MVBC, assuming that silicate and other mineral matter is neither added nor lost. Experimental results and calculations indicate that most of the bituminous coal mined in the world today has undergone a 20:1 compaction and is derived from peat whose mineral matter content (dry weight basis) was < 10 percent.