Coal char

The description given apphes to DR processes that are based on the use of gaseous reductants ia shaft furnaces, batch retorts, and fluidized beds. In the processes that use sohd reductants, eg, coal (qv), the reduction is accomphshed to a minor extent first by volatiles and reduciag gases that are released as the coal is heated and then by CO that is formed by gasification of fixed carbon contained ia the coal char with CO2. Reductioa by sohd carboa and coal volatiles ia kilns is insignificant. 

Summation of Separate Contributions to Gas or Flame Emissivity Flame emissivity g -t-, due to joint emission from gas and soot has already been treated. If massive-particle emissivity ., such as from fly ash, coal char, or carbonaceous cenospheres from heavy fuel oil, are present, it is recommended that the total emissivity be approximated by... 

Coal Char This type of char is the nonagglomerated, nonfusible residue from the thermal treatment of coal. Coal chars are obtained as a residue or a coproduct from low-temperature carbonization processes and from processes being developed to convert coal to hquid and gaseous fuels and to chemicals. Such chars have a substantial heating value. The net amount of char from a conversion process varies widely in some instances, it may represent between about 30... 

Coals mesophase pitch coal chars coal tar pitch carbon mesocarbon microbeads, carbon fibers semi-coke, calcined coke activated carbons premium cokes, carbon fibers, binder and matrix... 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

There can also be substantial particle attrition in cyclones in fluidized-bed systems because particles are accelerated at the inlet of the cyclone and impacted against the cyclone wall. Although there is little information on particle attrition in cyclones in the literature, it has been reported (Sishtla) that increasing system pressure decreases the attrition rate in cyclones operating with coal char. The mechanism by which this occurred was not determined. 

Kojima, T., Yoshitake, H., Kimura, T., Matsukata, M., and Uemiya, S., Contribution of Local Reactions in the Grid Zone to the Performance of a Jetting Fluidized Bed Gasifier of Coal Char, Energy Fuels, 9 379 (1995)... 

Sishtla, C., Findlay, I, Chan, I., and Knowlton, T. M., The Effect of Temperature and Gas Velocity on Fines Generation in Non-Reactive Fluidized Beds of Coal Char, Fluidization VI, (J. R. Grace, L. W. Shemilt, and M. A. Bergougnou, eds.), Banff, Alberta, Canada (1989)... 

Coals coal chars semi-coke, calcined coke activated carbons... 

Chem-Char A process for destroying organic wastes by pyrolysis on devolatilized coal char in a reducing atmosphere, followed by secondary combustion of the product gases. Developed at the University of Missouri-Columbia. 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

Soot can be oxidized by molecular oxygen, through the process described previously for pulverized coal char oxidation, but in combustion systems the dominant oxidation of soot is performed by radical species, especially the hydroxyl radical, OH. This discovery was made by Fenimore and Jones... 

Coal Char Coal char is, generically, the nonagglomerated, non-fusible residue from the thermal treatment of coal however, it is more specifically the solid residue from low- or medium-temperature carbonization processes. Char is used as a fuel or a carbon source. Chars have compositions intermediate between those of coal and coke the volatile matter, sulfur content, and heating values of the chars are lower, and the ash content is higher, than those of the original coal. 

Laurendeau N.M., Heterogeneous Kinetics of Coal Char Gasification and Combustion , Prog. Energy Combust. Sci. 4, 221-270(1978). 

Fig. 35. Oscilloscope tracings of oxygen effects. Top decay of EPR signal of coals, charred sugars and activated carbons heated above 100° with increase in oxygen pressure. Bottom decay of EPR signal in activated carbons with increase in oxygen pressure (181).

Solum, M.S., Pugmire, R.J., Grant, D.M., Fletcher, T.H., and Solomon, P.R., Solid State C NMR Studies of Coal Char Structure Evolution. Western States Section/The Combustion Institute, Pullman, WA (3/89). 

P21 Persistent electron paramagnetic resonance (EPR) signals have been reported in coals, chars, and soots (26-29), but PMj 5 has not been studied by EPR. (Prom Dellinger et ah, 2001)... 

Direct liquefaction processes under development are typically carried out at temperatures from about 450 to 475°C and at high pressures from 10 to 20 MPa and up to 30 MPa. Despite the slow rate at which liquefaction proceeds, the process itself is thermally rather efficient, since it is only slightly exothermic. However, hydrogen must be supplied and its manufacture accounts for an important fraction of the process energy consumption and cost of producing the liquid fuel. The hydrogen itself may be produced, for example, by the gasification of coal, char, and residual oil. 

The refinements that can be made to the model for coal char oxidation are seemingly endless (51). A few of these will be discussed briefly. 

The kinetics of coal char gasification can usually be interpreted in terms of the following set of reactions ... 

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