Abstract
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.
Highlights
Agricultural systems must be sustainably intensified to provide adequate food, fuel, and fiber to support projected increases in global energy and food demand [1,2]
Lignocellulose can vary widely across tropically grown C4 grasses, with total lignin and lignin chemical composition changing across plant parts and within species and varieties based on environmental conditions [11,12]
Acids and further inhibitory sugar breakdown products generated from free sugars and hemicellulose degradation in the liquid fraction are not considered here for fuel generation and will require further work to resolve as waste-to-resource streams [34]
Summary
Agricultural systems must be sustainably intensified to provide adequate food, fuel, and fiber to support projected increases in global energy and food demand [1,2]. Belowground C allocation and total soil organic carbon (SOC) in temperate and (sub)tropical systems can be improved through transitions to perennial bioenergy grasses from conventionally managed crops [4]. Conversion of marginal croplands and areas of low C storage potential to high biomass-producing, deep-rooted, bioenergy grasses under conservation agriculture can improve climate mitigation potential [5,6,7], while the produced bioenergy can directly offset fossil fuels. Lignocellulose can vary widely across tropically grown C4 grasses, with total lignin and lignin chemical composition changing across plant parts and within species and varieties based on environmental conditions [11,12]. As changing biomass composition can affect conversion, life cycle assessments (LCAs) of tropical bioenergy production require further C and energy assessments to evaluate the efficacy of C4 grasses as climate mitigation tools
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