DEVELOPMENT OF A GAS-SWEEP FACILITY FOR THE DIRECT CAPTURE OF PYROLYSIS TARS IN A VARIABLE HEATING RATE HIGH-PRESSURE WIRE-MESH REACTOR

A variable heating rate high-pressure wire-mesh pyrolysis/hydropyrolysis reactor has been configured to permit the continuous passage of gas through the sample holder in order to remove volatiles away from the reaction zone. The introduction of the gas-sweep facility, hitherto not available at elevated pressures, was made possible by the development and installation of a flow-smoothing section beneath the sample holder. This served to reduce turbulence within the gas stream and even out heat transport from the wire-mesh to the flowing gas. Positive tar recovery was achieved at pressures between 2.5 and 150 bar, at heating rates ranging from 1 to 2500 K s-1 and temperatures up to 850-degrees-C. In line with previous work, hydropyrolysis tar yields obtained at fast heating rates (1000 K s-1) have been observed to diminish with increasing H-2 pressure; the observed decline in high heating rate tar yield with increasing pressure and temperature (at high pressure) was found to be related to the slower release of tar precursors from coal particle surfaces at elevated pressures. However, increases in tar yield with pressure were found at slow heating rates (1 K s-1 to 700-degrees-C with 10 s hold) after an initial decline between 1 and 5 bar. The atmospheric pressure trend showing increasing tar yields with increasing heating rate appears to be reversed at H2 pressures above 10 bar. These observations cover only coals which soften or melt upon heating; the slow hydropyrolysis of two nonmelting Southern Hemisphere (Chilean) coals at 70 bar gave greater tar yields during slow heating rate runs than either 70 bar or atmospheric pressure experiments carried out at high heating rate.