Coal conversion factors

Heat recovery efficiency is a consideration of major importance in the conversion of coal to secondary fuels. This parameter is defined as the percent of the heating value of the coal used which is recovered as heating value in the desired secondary fuel. Heat recovery efficiency which can be attained in a coal conversion process depends firstly on the theoretical chemical and thermodynamic requirements of the process, and secondly on the practical realization of the process. The first factor determines the theoretical maximum heat recovery efficiency that can be obtained under ideal circumstances. The second factor determines the extent to which the practical process approaches the theoretical ideal. 

The sulfur distribution from the coal conversion plant is shown in Figure 4. The wt% of sulfur remaining in the ash depends on several factors among which are the relative distribution of organic and inorganic sulfur in the coal and the chemical composition of the ash. High alkaline ashes will capture sulfur as sulphide or sulfate. 

An explosive intended for use in coal mines must be safe to handle and to operate in the presence of material as ignitable as coal. Moreover to produce good lump coal slow-acting explosives should be employed which displace rather than fragment the coal. Conversely, in rock blasting greater explosive power is needed the safety factor is less important. 

Karweil (12) tried to illustrate graphically the correlation between coal rank, rock temperature, and duration of heating on the basis of reaction kinetics. In Figure 18 the ordinate records the temperature, and the abscissa indicates coal rank (in terms of volatile matter and a conversion factor = Z). 

Figure 18. Relations between rank of coal (volatile matter), temperature, and time of coalification (after Karweil (12)) (Z is a conversion factor relating volatile matter to coal rank)...

In this section, unless otherwise stated, coal refers to the internationally traded commodity classified as coal ARA CIF AP 2, while gas refers to the high caloric gas (with a conversion factor 35,17 GJ/m3) from the Dutch Gas Union Trade Supply (GUTS). Moreover, prices for power, fuels and C02 refer to forward markets (i.e. year-ahead prices). 

The important factors in coal conversion are the physical and chemical properties of the coal, heat supply (autothermal or allothermal), reactor type (fixed bed, moving bed, fluidized bed, or entrained bed), gasification agent (air, oxygen, steam, or a combination thereof), and process conditions. Typically, coal conversion is carried out at high temperatures (900—1000 C) be-... 

At first consideration, there may appear to be little, if any, relationship between the physical and chemical behavior of coal but in fact the converse is very true. For example, the pore size of coal (which is truly a physical property) is a major factor in determining the chemical reactivity of coal (Walker, 1981). And chemical effects which result in the swelling and caking of coal(s) have a substantial effect on the means by which coal should be handled either prior to or during a coal conversion operation. 

Lasers have been employed to produce extremely high tanperatures as has microwave heating of coal. Both techniques are reputed to produce considerable quantities of acetylene and the extent of the coal conversion depends on the volatile matter content of the coal, suggesting that the fixed carbon of the coal is not a predominant factor. Flash tubes with short (miCTOsecond or millisecond) irradiation times also produce acetylene as one of the major products and there appears to be little, if any, tar production. Furthermore, the presence of hydrogen appears to reduce the amount of acetylene produced. 

Common Conversion Factors Used in Coal Technology... 

Conversion Factors from a Chemical Equation COAL 1 Given a chemical equation, or a reaction for which the equation is known, and the number of moles of one species in the reaction, calculate the number of moles of any other species. 

A number of environmental implications are involved in the widespread use of coal conversion. These include strip mining, water consumption in arid regions, lower overall energy conversion compared to direct coal combustion, and increased output of atmospheric carbon dioxide. These plus economic factors have prevented coal conversion from being practiced on a very large scale. However, coal conversion does enable relatively facile carbon sequestration (see Section 17.11), which could enable much more sustainable coal utilization. 

In mixed-gas environments, as occur in coal-conversion processes, the factors discussed above remain, but with the added complication of competition between the elements. In addition to oxygen, gases such as SO2 and H2S vie with each other to combine with the various elements in the substrate alloy. 

In general, the design of hydrocarbon absorption systems is straightforward. Since mass U ansfer is not complicated by the occurrence of chemical reactions, conventional absorption coefficient, theoretical plate, and absorption factor concepts can be used for design calculations (see Chapter I). Basic data for such calculations, including the thermodynamic properties of compounds found in coke-oven gas and equilibrium data for several gas-coal liquid systems, are given in the U.S. DOE Coal Conversion Systems Data Book (1982) and other hydrocarbon data compendia. 

Carbon Conversion. Carbon conversion on a once-through basis is a function of the coal composition and is strongly influenced by the oxygen/coal ratio. For some coals, the conversion pattern is also affected by the level of steam in the blast. Another factor is fly slag recycle, which raises the carbon conversion by recycling the unconverted carbon, most of which resides on the fly slag. This results in an overall carbon conversion greater than 99%. 

---