Devolatilization of coal

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

The primary devolatilization of coal is a very rapid, thermal process and therefore not strongly sensitive to solid residence... 

Solomon, P. R., Report NSF/RA-770422, NTIS, //PB278496/AS, "The Evolution of Pollutants During the Rapid Devolatilization of Coal", 1977. 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

The process is also adaptable to similar uses with other feed materials for example, the devolatilization of coal and lignites. 

The volume of gases generated during devolatilization was determined from the pressure increase in the reactor. Mass spectrometric analyses of the gases were made at many, but not all, of the different test conditions. In this way, the weight of the gases produced by rapid devolatilization of coal was ascertained. 

In addition to the gases produced by rapid devolatilization of coal, heavier products, referred to as tar, also form. This material condenses on the walls of the reactor and is visible as a brown stain on the glass. 

Figure 2. Effect of overall heating rate on devolatilization of coal...

The overall methanation has been described by Moseley and Paterson (10, 11, 12) as consisting of three steps. The first step is the rapid devolatilization of coal which produces, in addition to volatile products, an active carbon species. This active carbon reacts in the second step either with hydrogen to form more methane or with itself in a crosslinking polymerization to form an inactive char. The third step involves the slow reaction of hydrogen with the inactive char. 

Solomon, P. R., Monthly Topical Report, Investigation of the Devolatilization of Coal under Combustion Conditions, Dept, of Energy, Contract ET-78-C-01-3167. (November, 1979)... 

Experimental studies on the devolatilization of coal in a fluidized bed [10] suggest that the devolatilization time, measured as the time between the ignition and extinction of volatile, may be correlated as. 

Most models assume that the fuel-bound nitrogen that is released by the devolatilization of coal is in the form of HCN, or some instantaneous transforms of HCN, which in turn form the base species of NO formation. It is believed that the HCN not only contributes to fuel NOx formation but also to some destruction of NOx and that the net formation might depend on the chemical as well as the thermal state of the mixture. The global chemical reactions involved for coal flames might therefore be expressed as... 

As discussed earlier, pyrolysis of coal, composition and yield of volatiles are strongly dependent on coal particle size, temperature and heating rate. Instantaneous coal devolatilization at the feed point can be e q>ected only when the particle size is small. The time needed for the devolatilization of a 1000 micron coal particle is 0.5 to 1.0 second,which is of the same magnitude as solids mixing time in the bed (21,9 2) This necessitates the consideration for the rate of devolatilization of coal. 

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