Studies on Coal Reactivity: Kinetics of Lignite Pyrolysis in Nitrogen at808°C

Abstract

Lignites pyrolyzed in nitrogen have provided direct evidence for a two-component model of coal constitution. The pyrolysi s was studied in an electrically heated 5.1 ern i.d. laminar-flow furnace using four closely sized cuts (58, 64, 86, and 180 microns). Pyrolysis was under isothermal conditions, at 808°C, for periods up to I sec, following a heating period of 95 msec, at 8000°C/sec rate of heating. Pyrolysis times in excess of 200 msec were obtained by recycling, experiments having shown that this did not affect the kinetics. Variations of weight loss with time were found to be in agreement with equations based on a two component model in which two independent parallel reactions are assumed to occur. Notably, weight loss declined approximately exponentially with time, with a sharp break in the lines obtained for pyrolysis times in excess of 200 msec. Slopes of the lines yielded values of the relevant kinetic constants, with Component "I" decomposition dependent on particle size over the range stud ied, but the slower-reaction of Component "II" being independent of size. The rat io of Component I to Component II is estimated at about 10 to I. In other results, the proportionality between weight loss and ASTM volatile loss of the char, as first reported by Badzioch and Hawksley was reconfirmed, with Q factors for the coals studied of 1.8 and 2.

Characteristics of American coals in relation to their conversion into clean energy fuels. Quarterly technical progress report, October--December 1977

“…These k values are also comparable with those obtained for the pyrolysis 00 of the North Dakota lignite (PSOC-246) 5 • The regression lines in Figure 4 radiate not from the origin (where t = 0), but rather from definite values oo the time axis. This value is approximately 0.095 sec for the 70 x 100, 100 x 140, 140 x 200, and ?.00 x 270 mesh size grades, which is in good agreement with o~r previ~us results 5 • The results for the 230 x 325, 230 x 400, and 60 x 80 mesh s1ze grades do, however, radiate from values other than the 0.095 sec. There is clearly a need to repeat the runs on the 230 x 325 and 230 x 400 mesh size grades.…”

“…The reaction rate constants (k•s) for the pyrolysis of these various size grades of PSOC-412 were obtained using the foregoing information in conjunction with Equation (4) of ~ur previous work, 5 assuming that only component I of the postulated two components comes off in predominance during the first 0.3 sec of the total particle residence time in the hot zone.…”

Characteristics of American coals in relation to their conversion into clean energy fuels. Quarterly technical progress report, July--September 1977. [Coal-fuel oil-water slurries]

“…The char yield (Y) at a given ti111e during pyrolysis or colnbustion can be expressed us a percentuye of the oriuinul dry, ilSh-frec coill ilS follows ( 4) Combining Equations (3) and (4) yields…”

“…2 These results had never been found in the previous experiments involving pyrolysis of 200 x 270 mesh and larger size grades of PSOC-246 in nitrogen at 808°C. 3 It was shown later 4 that extending the probing process to a much earlier stage of pyrolysis indeed yields negative values of weight loss even when a 200 x 270 mesh size grade is used (Figure 12). Figures 11 and 12show that negative weight loss or burn-off is offset by a greater extent of combustion and/or pyrolysis.…”

Improved pyrolysis submodel for the Wen Fixed-Bed Gasifier Program

“…The rate of the devolatilization process has been approximated by a simple first-order decomposition occurring uniformly throughout the particle (Badzcoch and Hawksley, 1970;Rau and Robinson, 1966;Mentser et al, 1970). This model, however, is believed to be inadequate and a series of several first-order processes and variable order models (Shapatina et al, 1960;Pitt, 1962;Wiser et al, 1967;Skylar et al, 1969;Ubhayakar et al, 1976;Nsakala et al, 1977) have been developed to improve the prediction of the devolatilization rate. These models gave better fit to experimental data but still could not I i account for the strong temperature-tinae dependency of the devolatilization rate.…”