Pyrolysis temperature and feedstock alter the functional groups and carbon sequestration potential of Phragmites australis- and Spartina alterniflora-derived biochars

Abstract

Biochar produced by pyrolysis of biomass under oxygen-limited conditions has recently attracted increasing attention. To investigate the effects of feedstock and pyrolysis temperature on biochar characteristics, Phragmites australis straw and Spartina alterniflora straw were used to produce biochars at different temperatures from 300 to 500°C with an increment of 50°C. The biochars were characterized by their yields, ash contents, elemental compositions (i.e., carbon [C], hydrogen [H], oxygen [O], and nitrogen [N]), functional groups, dissolved organic carbon (DOC) contents, carbon sequestration potential, higher heating values (HHVs), and production costs. The results illustrated that pyrolysis temperature negatively affected biochar yields, H and O contents. In contrast, biochar produced at high temperature showed high ash contents, C contents, HHVs, and stronger aromaticity with low H/C and O/C ratios. In addition, S. alterniflora-derived biochar (SB) contained higher ash content but lower C/H/N/O contents and HHVs than P. australis-derived biochar (PB; p < 0.05). In addition, DOC contents in both SB and PB declined as temperature increased, and SB exhibited higher DOC contents than PB. Fourier transform infrared spectroscopy showed that absorption intensities of –OH, C=O, –CH, and –C–O–C-stretching vibration declined with increasing temperature. The stability of the biochars was enhanced at high temperatures, and the biochar derived from S. alterniflora at 500°C might have a better carbon sequestration potential according to thermogravimetric analysis. Additionally, cost analysis showed that the production cost of biochar with large-scale reactor was lower than that with bench-scale reactor, and SB had a higher cost than PB due to the price of feedstock and drying process for the former. Our results could offer effective information on resource utilization of P. australis and S. alterniflora straw and are valuable for optimizing pyrolysis temperature to tune P. australis and S. alterniflora biochar properties for specific environmental usage.