The homogeneous vapor phase cracking of newly formed wood pyrolysis tar was studied at low molar concentrations as a function of temperature (773–1,073 K), at residence times of 0.9–2.2 s. Tar conversions ranged from about 5 to 88%. The tars were generated by low heating rate (0.2 K/s) pyrolysis of ∼2 cm deep beds of sweet gum hardwood, and then rapidly conveyed to an adjacent reactor for controlled thermal treatment. Quantitative yields and kinetics were obtained for tar cracking and resulting products formation. The major tar conversion product was carbon monoxide, which accounted for over two‐thirds of the tar lost at high severities. Corresponding ethylene and methane yields were each about 10% of the converted tar. Coke formation was negligible and weight‐average tar molecular weight declined with increasing tar conversion. A first‐order distributed activation energy model more closely correlated tar conversion kinetics over a wider range of reaction conditions than did a single‐reaction model.

Char-induced conversion of newly formed wood pyrolysis tars was measured for independent variations in temperature and tar-char space time. Tar vapors were generated by controlled devolatilization of shallow (~2 cm deep) packed beds of 45-250-µ particles of sweet gum hardwood.The tar vapors were extensively diluted in a helium carrier gas and then rapidly conveyed to an adjacent reactor for controlled thermal treatment with (or without), 0.2-12 cm deep beds of fresh char from pyrolysis of the same type of wood. Heterogeneous conversion is defined as the net tar loss from exposure to the wood char (i.e. after subtraction from the total tar lost during thermal treatment the amount of tar destroyed by vapor-phase cracking upstream and downstream of the char bed). Heterogeneous conversion was significant but essentially constant at 14 ± 7 wt % of tar (to 2 standard deviations), for temperatures from 400 to 600 °C and space times from 2.5 to 100 ms. In contrast, vapor-phase tar conversion upstream of the char bed ranged from 0% at 400 °C to 30% (wt % of tar) at 600 °C. The implication is that an otherwise thermally stable fraction of newly formed wood pyrolysis tars is very reactive in the presence of wood char. Literature data and the present results, including measurements of char yields from devolatilization of wood beds of different depths, imply that this fraction contains oxygen, is more aromatic than the whole tar, and may amount to as much as 35 wt % of the total quantity of tar released at the surface of pyrolyzing wood. Lignin is indicated as a major source, but not the only source, of this char-reactive tar fraction. Carbon dioxide and CO are definite products of this char-induced tar conversion, and additional char (coke) is a highly probable product. to this work as part of MIT undergraduate research programs. We also thank Professor H.-M. Chang and his colleagues, Department of Wood and Paper Science, North Carolina State University, who specially prepared samples of sweet gum hardwood, milled wood lignin, and other wood constituents for our biomass research program.Registry No. C02, CO,