Combustion devolatilization

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. 

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. 

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 fluidized bed gasifier, crushed coal is introduced into a fluidized bed of char together with oxygen or air and steam. Coal undergoes drying, devolatilization, gasification and combustion at essentially constant temperature of about 1000 C because of the rapid mixing characteristics of fluidized beds. 

Agarwal, P. K. and la Nauze, R. D. Chem. Eng. Res. Des. 67 (1989) 457. Transfer processes local to the coal particle. A review of drying, devolatilization and mass transfer in fluidized bed combustion. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

Particles of char are produced as a normal intermediate product in the combustion of solid fuels. Following initial particle heating and devolatilization, the remaining solid particle is termed char. Char oxidation requires considerably longer periods (ranging from 30 ms to over 1 s, depending on particle size and temperatur than the other phases of solid fuel combustion. The fraction of char remaining after the combustion zone depends on the combustion conditions as well as the char reactivity. 

Char oxidation dominates the time required for complete burnout of a coal particle. The heterogeneous reactions responsible for char oxidation are much slower than the devolatilization process and gas-phase reaction of the volatiles. Char burnout may require from 30 ms to over 1 s, depending on combustion conditions (oxygen level, temperature), and char particle size and reactivity. Char reactivity depends on parent coal type. The rate-limiting step in char burnout can be chemical reaction or gaseous diffusion. At low temperatures or for very large particles, chemical reaction is the rate-limiting step. At... 

Saastamoinen J.J., Model for Drying, Devolatilization, and Combustion of Small Biomass Particles in Entrained Flow, Iso-thermal Furnaces , project 4450063-4/443-3/30, version 5, Domestic Fuel Laboratory, Jyvaskyla, Finland, (1985)... 

The steady-state permeation model of in situ coal gasification is presented in an expanded formulation which includes the following reactions combustion, water-gas, water-gas shift, Boudouard, methanation and devolatilization. The model predicts that substantial quantities of unconsumed char will be left in the wake of the burn front under certain conditions, and this result is in qualitative agreement with postburn studies of the Hanna UCG tests. The problems encountered in the numerical solution of the system equations are discussed. 

The velocity of the combustion front is determined by the volatile matter content of the raw coal, Wv. If the volatile matter content of the coal is high the frontal velocity will be slowed. With low volatility coals the frontal velocity will be relatively fast for the same excess energy production in the com-bustion/gasification regions of the bed. The velocity vc is set by the requirement that the volatiles content of the coal ahead of the devolatilization zone be essentially that of the raw coal. From an analysis of the energy balance in the region near the front of the devolatilization zone, it can be shown that a particular root of the relation below establishes v (3) ... 

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