Effects of secondary reactions on product distribution and nitrogen evolution from rapid coal pyrolysis

We are attempting to understand the roles of primary pyrolysis and subsequent gas-phase secondary chemistry on product and element distributions from coal at typical pulverized-fuel firing conditions. During primary pyrolysis the coal macromolecule undergoes thermally-induced bond scissions, releasing large fragments of its original structure and noncondensible gases into the gas phase. These volatiles will undergo further reactions, termed secondary, if the ambient temperature is high. Product and element distributions are drastically altered by secondary reactions.; In this study we use a novel flow reactor which independently regulates the extent of secondary chemistry and the total volatiles yield. This system relies upon radiant heating of an optically-thin coal suspension entrained into a one-dimensional flow field. By varying the coal loading and residence time, both weight loss and extent of secondary chemistry can be independently controlled. Mass and element closures are achieved in individual runs, so that the effects of various operating parameters can be unambiguously established. The system is supported by rapid quenching to resolve reaction times to several milliseconds, aerodynamic segregation of particulate and gaseous products, and a battery of chemical analyses.; Product and element distributions from primary pyrolysis are reported for four coals ranging in rank from subbituminous to low-volatile bituminous. For all coals, nitrogen distributions show that tar and oils are the main shuttles for nitrogen out of the coal matrix during primary pyrolysis. Furthermore, the evolution of coal nitrogen with tar and oils is proportional to the evolution of coal mass as these products.; Under conditions of secondary pyrolysis we conclude that tar and oils are the precursors to soot. It is also apparent that the expulsion of heteroatoms during tar decomposition is offset by addition of carbon from the gas phase. Our data cannot exclude the addition of tar to soot mass growth during the later stages of secondary pyrolysis. Nitrogen distributions show that nitrogen evolution from char is hindered by char structure development, possibly due to the incorporation of remaining nitrogen functional groups into extensive, condensed-ring structures. Interestingly, the significant fraction of coal nitrogen incorporated into soot is constant and rank dependent.