Anaerobic digestion and hot water pretreatment of tropically grown C4 energy grasses: Mass, carbon, and energy conversions from field biomass to fuels

Authors: WELLS, JON - University Of Hawaii; CROW, SUSAN - University Of Hawaii; KHANAL, SAMIR - University Of Hawaii; TURN, SCOTT - Hawaii Natural Energy Institute; HASHIMOTO, ANDREW - University Of Hawaii; Kiniry, James; MEKI, NORMAN - Texas A&M Agrilife

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2021
Publication Date: 4/24/2021
Citation: Wells, J., Crow, S., Khanal, S., Turn, S., Hashimoto, A., Kiniry, J.R., Meki, N. 2021. Anaerobic digestion and hot water pretreatment of tropically grown C4 energy grasses: Mass, carbon, and energy conversions from field biomass to fuels. Agronomy. 11. Article 838. https://doi.org/10.3390/agronomy11050838.
DOI: https://doi.org/10.3390/agronomy11050838

Interpretive Summary: As warm season grasses as are used for feedstocks for liquid fuel production, their efficiency of production and their ability to mitigate climate change, remain unresolved in the tropics. We measured final fuels and post-process plant material for warm season grasses produced in two conversion pathways and across 12 species and varieties. Total mass, carbon, and energy in final fuels and post-process biomass were assessed based on field mass and area-based production. We investigated: 1) anaerobic digestion (AD) to methane, and 2) hot water pretreatment and enzyme hydrolysis (HWPEH) to ethanol. We found AD converted lignocellulose to methane more efficiently in terms of carbon and energy compared to ethanol production using HWPEH, though improvements and optimization of each process could change these contrasts. These results will determine if these systems are net carbon sources or sinks to the atmosphere. The highest carbon and energy recovery from lignocellulose were achieved in methane production from a sugarcane hybrid. If agricultural production and conversion of these grasses can be achieved within these carbon and energy limitations then biofuel and bioenergy can be produced as an atmospheric carbon sink while further offsetting fossil fuel use.

Technical Abstract: The efficacy of C4 grasses as feedstocks for liquid fuel production, and their subsequent climate mitigation potential, remain unresolved in the tropics. To identify highly convertible C4 grasses we measured final fuels and post-process biomass produced in two laboratory scale conversion pathways and across 12 species and varieties within the Poaceae (grass) family. Total mass, carbon, and energy in final fuels and post-process biomass were assessed based on field mass and area-based production. Two lignocellulosic processes were investigated: 1) anaerobic digestion (AD) to methane, and 2) hot water pretreatment and enzyme hydrolysis (HWPEH) to ethanol. We found AD converted lignocellulose to methane more efficiently in terms of carbon and energy compared to ethanol production using HWPEH, though improvements and optimization of each process could change these contrasts. Resulting data provide design limitations of agricultural production and biorefinery systems that will determine if these systems are net carbon sources or sinks to the atmosphere. Median carbon recovery in final fuels and post-process biomass from the studied C4 grasses was ~ 5 Mg C ha-1 year-1 for both methane and ethanol, while median energy recovery was ~ 200 MJ ha-1 year-1 for ethanol and ~ 275 MJ ha-1 year-1. The highest carbon and energy recovery from lignocellulose were achieved in methane production from a sugarcane hybrid with ~10 Mg C ha-1 year-1 and ~450 MJ ha-1 year-1. Though several process streams remain unresolved, if agricultural production and conversion of these C4 grasses can be achieved within these carbon and energy limitations then biofuel and bioenergy can be produced as an atmospheric carbon sink while further offsetting fossil fuel use.