Heteroatom distribution in pyrolysis products as a function of heating rate and pressure

Abstract

Nitrogen and sulphur partitioning between product phases during the pyrolysis of Illinois No. 6 and Tilmanstone (high-rank UK) coals was monitored as a function of heating rate and pressure. An atmospheric-pressure wire-mesh pyrolysis reactor was used at heating rates of 5–5000 K s −1 to 950 °C. High-pressure experiments were performed at 1000 K s −1 to 700 °C in a wire-mesh reactor redesigned to allow tar capture by continuous sweeping of volatile products away from the sample holder into an externally cooled trap. The reactor configurations made it possible to determine yields and recover tars in the relative absence of secondary reactions. Generally, the net transfer into the tar phase of both nitrogen and sulphur was enhanced by increasing heating rate and suppressed by increasing pressure. At 950 °C and 0.1 MPa, depending on coal type and heating rate, between 25 and 50% of the evolved nitrogen was found in the tar. At 700 °C, the split of evolved N between tar and gas as a function of pressure differed sharply between the two coals. Despite comparable organic S contents of the two coals (56% of total S in Illinois No. 6, 51% for Tilmanstone), the fractions of evolved S at 950 °C (0.1 MPa) were different, rising from 60 to 70% for Illinois No. 6 and from 35 to 45% for Tilmanstone with increasing heating rate. With increasing pressure, the distribution of coal-S between product phases differed considerably between the coals: for Illinois No. 6 the volatilized S remained roughly constant over the pressure range, whilst tar-S rapidly shifted (52 to 9%) to the gas phase. For Tilmanstone, volatilized coal-S increased by ~ 10% between 0.1 and 7.0 MPa but the tar-S content also shifted to the gas, suggesting the presence of tar-S within structures thermally more sensitive than those associated with tar-N. Considerably less coal-S in Tilmanstone volatilized. In view of the apparent differences between the combustion chemistries of the different phases formed during devolatilization, the results appear to be of direct relevance to the prediction of NO x and SO x formation during combustion and gasification.