The fundamentals of biocarbon formation at elevated pressure: From 1851 to the 21st century

In 1851, Violette reported studies of wood carbonization in sealed glass tubes which endured considerable pressure at elevated temperatures and evoked unusual pyrolysis chemistry that led to very high charcoal yields. Despite the priority and promise of Violette's work, his experiments were never reproduced and his findings forgotten. In this paper, we describe experiments that closely reproduce those of Violette and confirm many of his prescient observations. We employ small steel tubing bomb reactors rated to pressures above 13.9 MPa (2000 psig) at 400 °C that are heated quickly in a fluidized sand bath. Gas analysis is accomplished by an Agilent micro-GC and the charcoal product is subject to proximate analysis. Carbonization occurs at much lower temperatures than customary at atmospheric pressure. In the case of Avicel cellulose, the experimental fixed-carbon yield of charcoal exceeds the theoretical value derived from thermochemical equilibrium calculations. To the best of our knowledge, this is the first time that a pyrolysis process has produced a cellulosic char whose yield attained or exceeded its thermochemical equilibrium value.