Porosity and Surface Area In Wood Chars

Chars formed by wildfires and other natural processes have a wide range of surface areas. It is difficult to determine the thermal regime that produced these natural chars. To better understand the potential range of porosities and surface areas of natural pyrogenic materials in soil, chars were produced from wood and wood components under controlled conditions in the laboratory.

Samples were charred at temperatures ranging from 250 to 500 degrees Celsius for times ranging from 1 hour to 168 hours. Structural changes were examined by changes in porosity as measured by nitrogen gas adsorption. 13C NMR spectrometry, mass loss, and changing elemental compositions were combined to estimate the masses of aromatic and aliphatic carbon remaining in the char.

None of the materials tested developed porosity at or below the charring temperature of 250 degrees Celsius. Porosity began to appear when cellulose, wood, and bark were charred at 300 degrees Celsius for 1 to 8 hours. Porosity began to appear when lignin was charred at 300 degrees Celsius for 24 to 48 hours. For a fixed charring time, porosity generally increased with charring temperature. For a fixed charring temperature, porosity generally increased to a maximum and then decreased with time. However, decomposition, as measured by mass loss, decrease in H/C and O/C ratios, and changes in IR and NMR spectra, progressed continuously. The charring process appeared to initially convert aliphatic carbon into aromatic carbon forming the fused ring matrix in which porosity develops. The porosity did not develop until the conversion of aliphatic carbon was complete and aromatic carbon began to be lost.