Modification of Proximate Analysis Method for Biochars.

Proximate analysis is widely used for determination of moisture, volatile matter (VM), fixed carbon (FC), and ash content of biochars. The original ASTM D1762-84 method, designed for analysis of wood charcoal, may not be appropriate for biochars, which are produced from diverse biomass feedstocks, under various pyrolysis conditions, and may or may not have been weathered in soil environments. The goal of this work was to modify the proximate analysis method so that it is appropriate for use with diverse biochars, can support analysis of a large sample size, and has low variability. A set of biochars (n= 22) produced from different feedstocks and pyrolysis conditions, of which 11 were freshly made, 6 were aged artificially in the laboratory, and 5 were aged in the field for ≥ 3 years, were analyzed. Here, we compared the original ASTM method with the new method that uses N₂ purge for both moisture and VM determination and considered a range of possible boundary temperatures (350-950°C) for separating VM and FC. Results indicate that weight gain occurred during moisture determination of samples using the standard ASTM method, most likely due to sorption of oxygen, but not with our N₂ purge protocol. The results also suggest that the boundary temperature for VM determination should not exceed 750°C as estimated values of VM peak at this temperature. Furthermore, the percentage of ash free VM or dry ash free FC, and the molar H/C_org ratio was greater for VM determined at 750°C under N₂ purge than by the ASTM method. Our results indicate that use of N₂ purge for determining both moisture and VM is important and that 750°C is the appropriate temperature for determining VM in biochars.