Carbon stability and morphotype composition of biochars from feedstocks in the Mekong Delta, Vietnam

This study investigates carbon stability of biochars produced from wood, herbaceous, and fruit agricultural waste products from southern Vietnam. The biochars were produced by pyrolysis to 500 degrees C, 700 degrees C, and 900 degrees C at a heating rate of 10 degrees C/min. Organic geochemistry and petrology have well-established measurable parameters that define degree of preservation of organic carbon in the Earth's crust. These parameters are utilized, comparatively, to infer the organic carbon stability in biochars relative to that preserved in the geological carbonaceous rocks. The stability of carbon was assessed by measuring incident-light random reflectance (%Ro) to determine the degree of thermally induced aromatization/ordering of the biochar carbon molecules. Additionally, the organic carbon content of the biochars was quantitatively separated into reactive and non-reactive carbon fractions based on thermodynamic stability of C-H and C-O bonds. The results show increasing carbon stability with increasing pyrolysis temperature and indicate high stability has been achieved at 700 degrees C and 900 degrees C where the biochars have the highest total organic carbon (TOC) contents and lowest Hydrogen Index values. Occurrence of a small quantity of readily degradable, labile free hydrocarbons (soluble organic matter; SOM) is due to condensation of the thermally generated hydrocarbons on the surfaces of the biochars. This fraction is minimized in the biochars produced at 700 degrees C and 900 degrees C due to volatilization of the free hydrocarbons at higher pyrolysis temperatures. The relatively substantial content of labile hydrocarbons associated with particulate organic matter (POM) in the 500 degrees C produced biochars is attributed to the incomplete pyrolyzation of the feedstock at this temperature. This fraction declines to near zero at 700 degrees C and 900 degrees C as complete carbonization is achieved at these higher temperatures. The overall carbon budget within the biochar shows that irrespective of the biochar type, over 97% of the TOC consists of highly refractory, residual carbon that geochemically is considered to possess long-term stability (inert). This is further underpinned by a pronounced increase in biochar mean random reflectance from <2.33% at 500 degrees C to >4.35% when pyrolyzed at 700 degrees C, indicating a highly polyaromatized condensed carbon structure. Rice husk is an exception as it yields a mean reflectance of 4.55% at 500 degrees C suggesting that pyrolysis temperature is not the only factor controlling %Ro and hence carbon stability. Furthermore, the biochar compositions show a relationship between feedstock and pyrolysis temperature. The biochar morphotype composition changes significantly with increasing temperature, but the analyzed biochars are all dominated by fusinoid/solid (primarily fusinoid) morphotypes with the highest initial quantity recorded at 500 degrees C. This quantity decreases with increasing pyrolysis temperature while porous morphotypes (tenuinetwork, crassinetwork, mixed porous, mixed dense) increase. Variations in morphotype composition observed between the biochars is attributed to the structural nature of the feedstocks.

Automated summary

This study examines the stability of biochars produced from different agricultural waste products in southern Vietnam. The carbon stability was assessed based on organic geochemistry and petrology parameters, with higher pyrolysis temperatures resulting in increased stability. The research also found that the biochar composition and morphology were influenced by both the type of feedstock and pyrolysis temperature. Overall, the study highlights the potential of biochars as a stable carbon source.