Mass transfer and devolatilization phenomena in very large coal domains at very slow heating rates

The proliferation of nonconventional subsurface hydrocarbon production methods has turned some attention toward production from deep coal seams. There exists little research into coal pyrolysis under conditions relevant to subsurface processing (large coal domains, very slow heating rates, high hydrostatic pressure, volumetric confinement). Basic studies into the phenomena of mass transfer and devolatilization in a high-volatile Utah bituminous coal are described for very large particles (>1 cm) at very slow heating rates (< 10K/min) at atmospheric pressure. Studied systems included large coal blocks heated via immersion heaters and 2 cm-diameter coal cores heated in a tube furnace apparatus. Changes in char porosity during pyrolysis as a function of heating rate are described in large coal blocks. Coal core data show char porosity evolution as a function of temperature and heating rate and demonstrate a distinct threshold for plastic deformation. Volumetric confinement of core swelling was shown to dramatically affect char morphology. Devolatilization data from coal cores are presented, showing little impact of heating rate upon total volatile yield, but a substantial impact upon the yield of tars. A Knudsen flow analysis is also presented to argue that the driving force for mass transfer at very slow heating rates is pressure-driven flow. Several novel pyrolysis phenomena are described, including a pore plugging effect at very slow heating rates. The presented experimental work suggests that many common assumptions for conventional coal pyrolysis would not apply in a subsurface processing environment.