Devolatilization, coal

The devolatilized coal particles are transported to a direct-fired multihearth furnace where they are activated by holding the temperature of the furnace at about 1000°C. Product quaUty is maintained by controlling coal feed rate and bed temperature. As before, dust particles in the furnace off-gas are combusted in an afterburner before discharge of the gas to the atmosphere. Finally, the granular product is screened to provide the desired particle size. A typical yield of activated carbon is about 30—35% by weight based on the raw coal. 

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. 

In the drying zone at the top of the bed, the hot upward-flowing gases remove moisture from the coal. As the coal moves down the bed, its temperature increases, and at about 600-900°F pyrolysis occurs, liberating gases, oils, and tars. As the devolatilized coal (char) further descends, it is gasified by reaction... 

The progress of thermal decomposition is illustrated by the series of spectra for chars in Figure 8. The chars were produced by devolatilizing coal PSOC 212 for 80 sec at the indicated temperature. The results are similar to those observed by Brown (16) and Oelert (17). The rapid disappearance of the aliphatic and hydroxyl peaks is apparent. The aromatic peaks remain to high temperature, and the ether peaks are observed to increase in intensity, possibly from the creation of new ether linkages by... 

After the devolatilization and rapid-rate methane formation stages are completed, char gasification occurs at a relatively slow rate various models to describe the gasification kinetics of this material for various limited ranges of conditions have been proposed. The differential rates of reaction of devolatilized coal chars are a function of temperature, pressure, gas composition, carbon conversion, and prior history. 

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 stmcture of residual char particles after devolatilization depends on the nature of the coal and the pyrolysis conditions such as heating rate, peak temperature, soak time at the peak temperature, gaseous environment, and the pressure of the system (72). The oxidation rate of the chat is primarily influenced by the physical and chemical nature of the chat, the rate of diffusion and the nature of the reactant and product gases, and the temperature and pressure of the operating system. The physical and chemical characteristics that influence the rate of oxidation ate chemical stmctural variations, such as the... 

The main stages of coal combustion have different characteristic times in fluidized beds than in pulverized coal combustion. Approximate times are a few seconds for coal devolatilization, a few minutes for char burnout, several minutes for the calcination of limestone, and a few hours for the reaction of the calcined limestone with SO2. Hence, the carbon content of the bed is very low (up to 1% by weight) and the bed is 90% CaO in various stages of reaction to CaSO. About 10% of the bed s weight is made up of coal ash (91). This distribution of 90/10 limestone/coal ash is not a fixed ratio and is dependent on the ash content of the coal and its sulfur content. 

Devolatilization and combustion occur close to the coal inlet tubes. However, because of rapid mixing in the bed the composition of the soflds in the bed may be assumed to be uniform. 

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... 

The results of an example calculation for a recirculating fluidized bed coal devolatilizer of 0.51 m in diameter handling coal of average size 1200 pm at 870°C and 1550 kPa are presented in Fig. 11. The calculation is based on operating the fluidized bed above the draft tube at 4 times the minimum fluidization velocity. It is also based on the selection of a distributor plate to maintain the downcomer at the minimum fluidization condition. If the two-phase theory applies, this means that the slip velocity between the gas and the particles in the downcomer must equal to the interstitial minimum fluidizing velocity as shown below. 

Yang, W. C., Margaritis, P. J., and Keaims, D. L., Simulation and Modeling of Startup and Shutdown in a Pilot-Scale Recirculating Fluidized Bed Coal Devolatilizer, AIChE Symp. Ser., 74(176) 87 (1978)... 

Large particles heat up more slowly the average particle temperature is lower and therefore the devolatilization rate and yields are lower. Early work with coal particles show that... 

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