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 degrees C/min. Several unique phenomena are observed, including an absence of plastic deformation at heating rates below 10 degrees 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.