Time-resolved pyrolysis product distributions of softening coals

Abstract

Time-resolved product distributions of four softening coals pyrolyzed in an electrically heated grid are presented. The data collected over a broad range of temperatures and pressures satisfy a mass balance despite the small sample size (ca. 15 mg). Gaseous product compositions and the weights of tar evolved were determined during heat-up at 1000 K/s and for extended isothermal periods (0-30 s) at various temperatures (450-950 "C). The pressures used ranged from 0.01 to 25 atm of either helium or hydrogen. Quenching with a liquid-nitrogen-cooled helium stream allowed precise time resolution of the pyrolysis reaction over the entire operating range without loss of product.

Rapid coal devolatilization as an equilibrium flash distillation

“…The connection between char and gas formation is motivated by recent transient measurements of gas yields from a highvolatile bituminous coal. In Figure 3 the yields of various light hydrocarbons and the oxides of carbon measured by Bautista et al (1986) have been normalized by their ultimate values, and plotted vs. temperature. These results are for uniform heating of a wire grid at 1,000 K/s to the stated temperatures, immediately followed by rapid quenching (at about 5,000 K/s).…”

“…Yields are enhanced by faster heating at moderate temperatures, but only under vacuum (Niksa et a]., 1985). Similarly, yields are enhanced by reducing particle size under vacuum Heyd, 1982), but not a t atmospheric pressure (Anthony et al, 1975;Suuberg, 1985;Bautista et al, 1986). Ultimate volatiles yields are remarkably constant (at about 50 wt.…”

“…The corresponding tar yields, which appear in Table 1, also show no systematic variation with particle size. In Bautista's (1984) study of size effects, three coals were examined, including a lowvolatile sample that does not soften or swell. As seen in Table 1, neither the total weight loss nor the tar yields from any of these coals varied for particle sizes between 80 and 180 pm.…”

“…Hence, it seems more likely that char forms from intermediates in the condensed phase, not from tar vapor. The char Bautista (1984) formation rate in the condensed phase must be orders of magnitude faster than in the vapor, because the concentration of char precursors is that much greater in the condensed phase. Finally, no quantitative theory has shown why the molecular weight distributions (MWD) of tar shift toward lower molecular weights as the ambient pressure is increased, or why the MWDs are insensitive to temperature (Unger and Suuberg, 1984;Suuberg et al, 1985).…”

An experimental and modeling study of softening coal pyrolysis

“…The need for further progress on intraparticle transport modeling is pressing especially for softening coals since they show the strongest influence of mass transport processes as evidenced by the effects of pressure and, to a lesser extent, particle size on their pyrolysis product yields and compositions (Anthony and Howard, 1976;Suuberg, 1977;Howard, 1981;Bleiket al, 1985;Wagner et al, 1985;Bautistu et al, 1986). The present paper responds to this need by providing a mathematical model of softening coal pyrolysis that includes intraparticle chemical reactions and volatiles transport.…”