THERMOGRAVIMETRIC ANALYSIS OF BIOCHARS AND BLACK CARBON IN THE ENVIRONMENT

Biochars (charcoal intended for use as a soil amendment) are produced by a wide variety of processes. It has been shown that the formation conditions of these chars have a significant effect on their properties. Often the formation conditions for these production processes are not well known or controlled. Natural chars can also enter the environment as a result of wildfires where the formation conditions are variable and largely unknown. It would be useful to be able to determine the degree of charring in the case of biochars or natural chars of unknown origin in order to estimate the effect of these materials on soil properties.

The thermal behavior of chars was studied by measuring the weight loss of a sample as it undergoes additional pyrolysis under a nitrogen atmosphere in a thermogravimetric analyzer (TGA). The sample was heated sequentially to 8 temperatures, varying by 100-degree increments, between 200 and 900 ^oC, with a hold time of one hour at each temperature. The weight loss at each temperature step was recorded to determine the cumulative weight loss as well as low and high temperature loss profiles.

A series of reference chars were produced in a muffle furnace under inert atmosphere, where key parameters such as the starting feedstock, charring temperature, and charring time were carefully controlled. These chars are used as a reference series against which the unknown chars are compared. The estimate of the degree of charring is based on total weight loss after pyrolysis to 900 ^oC as well as a weight loss profile developed from the weight loss at each temperature step.

The total weight loss from the TGA analysis (loss between 300 ^oC and 900 ^oC) decreases as the char formation temperature increased. The ratio of low temperature weight loss from the TGA analysis (between 300 ^oC and 500 ^oC) to high temperature weight loss from the TGA analysis (between 600 to 900 C) also decreases as the char formation temperature increased.

The thermal behavior of the biochars and natural chars has also been compared with other pyrogenic carbon materials, such as coals, carbon black, activated carbons, and graphite.