Anaerobic Digestion and Hot Water Pretreatment of Tropically Grown C4 Energy Grasses: Mass, Carbon, and Energy Conversions from Field Biomass to Fuels

The efficacy of C₄ grasses as feedstocks for liquid fuel production and their climate mitigation potential remain unresolved in the tropics. To identify highly convertible C₄ grasses, we measured final fuels and postprocess biomass produced in two laboratory-scale conversion pathways across 12 species and varieties within the Poaceae (grass) family. Total mass, carbon, and energy in final fuels and postprocess 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 enzymatic hydrolysis (HWP-EH) to ethanol. We found AD converted lignocellulose to methane more efficiently in terms of carbon and energy compared to ethanol production using HWP-EH, although improvements to and the optimization of each process could change these contrasts. The resulting data provide design limitations for agricultural production and biorefinery systems that regulate these systems as net carbon sources or sinks to the atmosphere. Median carbon recovery in final fuels and postprocess biomass from the studied C₄ grasses were ~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 for methane. The highest carbon and energy recovery from lignocellulose was achieved during methane production from a sugarcane hybrid called energycane, with ~10 Mg C ha⁻¹ year⁻¹ and ~450 MJ ha⁻¹ year⁻¹ of carbon and energy recovered, respectively, from fuels and post-process biomass combined. Carbon and energy recovery during ethanol production was also highest for energycane, with ~9 Mg C ha⁻¹ year⁻¹ and ~350 MJ ha⁻¹ year⁻¹ of carbon and energy recovered in fuels and postprocess biomass combined. Although several process streams remain unresolved, agricultural production and conversion of C₄ grasses must operate within these carbon and energy limitations for biofuel and bioenergy production to be an atmospheric carbon sink.