Tar Evolution in Heated-Grid Apparatus

Inspection of tar evolution data from heated-grid investigations indicates nearly a 600 °C discrepancy in reported temperatures of tar evolution from HV (high-volatile) bituminous coal samples. For reportedly equivalent heating conditions-1000 °C/s heating to peak temperatures in 1 atm of helium and zero hold time—some investigators report half the tar evolved at temperatures from 550 to 600 K while others report peak temperatures of nearly 1100 K are necessary to achieve the same extent of tar evolution. The temperature dependences of tar yields for long-hold-time conditions in the different investigations are in much closer agreement, indicating that given enough time, the thermocouple and sample temperatures reach equilibrium in the various systems. In all cases, the measured temperature is a thermocouple “placed near” or “with” the coal sample. It is assumed or determined by estimation that the coal particles follow the same temperature versus time trajectory as the thermocouple. Characterization runs performed with nonvolatile and volatile “reference” materials in a typical heated-grid system and under similar heating conditions indicate sample load factors and thermophysical properties are critical in determining the actual sample temperature relative to the thermocouple. Coal devolatilization runs performed with identical total sample masses but changing load factors at different wire screen locations relative to the system thermocouples also indicate the apparent temperatures of tar evolution are a sensitive function of local sample distribution. Thus, it is not surprising that tar evolution times corresponding to measured temperature changes from various investigations are similar, given the similarity in heat-transfer conditions, but that observed temperatures of tar evolution are substantially different in transient heating, zero-hold-time experiments.