Chemical Composition and Acid Functional Content of Wood Chars

Because of pyrogenic carbon's (charcoal) ability to increase soil fertility and enhance carbon sequestration, interest in using charcoal as an agricultural soil amendment is increasing.

Difficulties in analyzing for pyrogenic carbon in soils and sediments, and the inability to separate pyrogenic materials from other forms of soil organic matter, led us to produce chars from wood and wood components under controlled conditions in the laboratory, varying charring time and temperature. Changes in char composition were examined by Fourier Transform Infrared (FTIR), Nuclear Magnetic Resonance (NMR), mass loss, and elemental composition (carbon, hydrogen, and oxygen). The Boehm titration method was used to measure the content of the acidic functional groups and lactones.

Carbon, hydrogen, and oxygen analysis of the starting and charred materials showed an increase in percentage carbon, and decreases in hydrogen and oxygen percentages, with charring for all tested materials. Analysis of the hydrogen/carbon and oxygen/carbon atomic ratios of the residual char and the material during charring showed that hydrogen and oxygen were lost more rapidly than carbon from the char during the initial periods of charring. This is consistent with either the loss of aliphatic carbon as volatile degradation products or with the conversion of aliphatic carbon into fused aromatic ring compounds.

Charring substantially impacts the acid functional group content of chars. Low temperatures (250 degrees Celsius) and long charring times (greater than 72 hours) produce chars from cellulose and cellulose-containing materials (wood and bark) with the highest acid functional group content (comparable to humic materials as high as 8.8 milliequivalents per gram). Acid functional group and lactone content of cellulose and cellulose-containing materials decreases as charring temperature increases. Total lignin acidity did not change substantially with low temperature charring (250 to 400 degrees Celsius).