A model for the devolatilization of a coal particle sufficiently large to be controlled by heat transfer

摘要: This study follows previous experimental work showing that the shrinking-core model applies to pyrolysis (i.e., heating in absence of oxygen) particles (diam.{approx}14 mm) a bituminous coal or lignite fluidized bed at 700-950 {sup o}C. These facts are accord with production volatile matter being endothermic and not thermoneutral, as often assumed. Also, rate which is produced presence oxygen devolatilization) its (pyrolysis) demonstrated here be controlled all by mass transfer, but heat conduction moving reaction front inside particle, provided diameter exceeds {approx}3 mm. The resulting steady-state devolatilization indicates six dimensionless groups required describe of(I)coal->char+volatile for nonfragmenting nonswelling coal. occurs total time c{sub 1}r{sub 0}+c{sub 2}r{sub 0}{sup 2} radius r{sub 0}. Experimental measurements times hot sand N{sub indicate this dependence on 0} holds much better than usually accepted one ar{sub n}. also themore » enthalpy change (I) 9.4{+-}4.0 MJ/(kg matter) thermal conductivity char 1.2{+-}0.5 Wm{sup -1}K{sup -1}. In addition, it was deduced {approx}500 K {eta} (of released burning material, fraction fed back parent particle devolatilizing by, e.g., air) less unity. numerical considerations enable simplified; turns out heated particle's core virgin shrinks mostly constant velocity, characterized only four groups-i.e., two them turn unimportant. important include Biot number char, Bi{sub char}, {omega}=1+{eta}{delta}H{sub comb}/{delta}H-bar{sub v}. Given {eta}=0 {omega}=1 pyrolysis, kinetics case entirely three groups, including char}. conditions shrinking apply discussed found apply, toward end coals low contents. used calculate when starts after an initial period particle. results compared (using gas chromatography volatiles released) various species (e.g., CO, CO{sub 2,} NO{sub x}, C{sub 2}H{sub 4}, 6}, HCN) first appear from pyrolyzing confirm can assumed occur mean temperature K. (author)« less