Coal char gasification

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

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

The inhibition effect of CO is widely accepted and reasonably well documented for coal char gasification but not for biomass. The lack of extensive literature for wood char CO2 gasification kinetics including CO has strongly motivated this investigation. 

Roberts, D.G. and Harris, D.J. A kinetic analysis of coal char gasification reactions at high pressures. Energy Fuels, 2006, 20, 2314. 

Molten eutectic salt mixtures have been reported to be active catalysts in graphite and coal char gasification [4, 5]. A major reason probably is the contact between soot and catalyst which is increased by wetting of the soot with the catalyst. 

Kinetics of Bituminous Coal Char Gasification with Gases Containing Steam and Hydrogen... 

Matsuoka, K., Hosokai, S., Kuramoto, K., Suzuki, Y. (2013). Enhancement of coal char gasification using a pyrolyzer-gasifier isolated circulating fluidized bed gasification system. Fuel Process Technology, 109, 43—48. 

Coal char can resemble graphite, depending on the conditions under which it is formed. Coal-char gasification systems generally are bounded by equilibria obtained by considering the carbonaceous solids to be equivalent to graphite. Therefore, it is common to consider char as similar to graphite in a thermodynamic sense. 

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. 

The importance of these concepts can be illustrated by the extent to which the pyrolysis reactions contribute to gas produdion. In a moving-bed gasifier (e.g., producer-gas gasifier), the particle is heated through several distinct thermal zones. At the initial heat-up zone, coal carbonization or devolatilization dominates. In the successively hotter zones, char devolatihzation, char gasification, and fixed carbon... 

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. 

Because coal chars are highly microporous, most of the gasification reactions take place inside the char particles. Therefore, diffusion of gas into, and products out of, porous particles is required. The overall diffusion process can be described by the following steps (1) diffusion of the reactant from the bulk gas to the solid surface (film diffusion) (2) diffusion of the reactant from the particle s surface to its interior (internal diffusion) (3) diffusion of the product from the interior to the particle s surface (internal diffusion) and (4) diffusion of the product from the surface to the bulk gas (film diffusion). 

Exchangeable cations play an important role in the behavior of lignite coals in coal conversion processes. It is, therefore, important that a fundamental understanding be attained, first of the possible interaction of exchangeable cations in lignites with each other, and, second, of the possible effects on the subsequent catalytic activity of the cations for lignite char gasification. 

The pyrolytic gasification of biomass has been interpreted to involve the decomposition of carbohydrates by depolymerization and dehydration followed by steam-carbon and steam-carbon fragment reactions. So the chemistries of coal and biomass gasification are quite similar in terms of the steam-carbon chemistry and are essentially identical after a certain point is reached in the gasification process. Note, however, that biomass is much more reactive than most coals. Biomass contains more volatile matter than coal, and the pyrolytic chars from biomass are more reactive than pyrolytic coal chars. 

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