Subsurface
thermal conversion of coal into light gases and oils via pyrolysis
potentially offers a more environmentally benign alternative to conventional
coal combustion. Few studies have examined coal pyrolysis under conditions
relevant to subsurface pyrolysis, such as very large particle sizes,
confining pressures, and very slow heating rates. The presented work
examines structural changes in the porous network of very large particles
of Utah bituminous coal undergoing pyrolysis at atmospheric pressure
at heating rates as slow as 0.1 °C/min. Several unique phenomena
are observed, including an absence of plastic deformation at heating
rates below 10 °C/min, the development of a bimodal macropore
size probability distribution because of confinement effects when
plastic deformation occurs, and a potential trapping mechanism for
residual organic matter in the mesopore system. The pyrolysis behavior
of very large bituminous coal particles at very slow heating rates
is found to deviate substantially from those observed under conventional
conditions.
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