Accumulation of high-value bioproducts in planta can improve the economics of advanced biofuels

Abstract

Coproduction of high-value bioproducts at biorefineries is a key factor in making biofuels more cost-competitive. One strategy for generating coproducts is to directly engineer bioenergy crops to accumulate bioproducts in planta that can be fractionated and recovered at biorefineries. Here, we develop quantitative insights into the relationship between bioproduct market value and target accumulation rates by investigating a set of industrially relevant compounds already extracted from plant sources with a wide range of market prices and applications, including <$10/kg (limonene, latex, and polyhydroxybutyrate [PHB]), $10 to $100/kg (cannabidiol), and >$100/kg (artemisinin). These compounds are used to identify a range of mass fraction thresholds required to achieve net economic benefits for biorefineries and the additional amounts needed to reach a target $2.50/gal biofuel selling price, using cellulosic ethanol production as a test case. Bioproduct market prices and recovery costs determine the accumulation threshold; we find that moderate- to high-value compounds (i.e., cannabidiol and artemisinin) offer net economic benefits at accumulation rates of just 0.01% dry weight (dwt) to 0.02 dwt%. Lower-value compounds, including limonene, latex, and PHB, require at least an order-of-magnitude greater accumulation to overcome additional extraction and recovery costs (0.3 to 1.2 dwt%). We also find that a diversified approach is critical. For example, global artemisinin demand could be met with fewer than 10 biorefineries, while global demand for latex is equivalent to nearly 180 facilities. Our results provide a roadmap for future plant metabolic engineering efforts aimed at increasing the value derived from bioenergy crops.

Keywords: bioenergy crop; biofuel; bioproduct; in planta accumulation; technoeconomic analysis.

Fig. 1.

TCI and AOC of the modeled cellulosic ethanol production facility with and without high-value bioproducts. In the base case scenario, the nonengineered biomass sorghum feedstock is utilized and no value-added bioproduct is produced. Detailed modeled costs can be found in SI Appendix, Table S5.

Fig. 2.

MESP with the selected bioproducts under different extraction efficiencies. Content labeled in the y axis refers to the amount (dry basis) of value-added bioproducts required in biomass sorghum. The sensitivity bars denote the range of content for reaching cost-parity and targeted MESP based on the expected distribution of market prices for each bioproduct (SI Appendix, Table S1).

Fig. 3.

Reported in planta accumulation amount (dry basis) of the selected bioproducts in various crops. Black dashed line is the desired in planta accumulation amount obtained in this study for reaching the target MESP ($2.50/gal). Limonene yields from citrus waste (16) and mandarin peel (63) are based on the total raw materials amount. Yield of limonene from caraway is based on the caraway fruit dry weight (9). Artemisinin yields from A. annua are based on the plant dry weight (6, 28). Artemisinin yield in tobacco is based on the dry weight of leaves (30). Yields of artemisinin from the engineered A. annua are the percentage of the leaves dry weight (7, 29). Latex yields from guayule are based on the stem dry weight (26). Yield of natural latex from rubber tree is the percentage of natural latex content on the whole plant (25). Latex yield from fig tree is based on the whole plant (27). PHB yield from alfalfa (21), sugarcane (22), switchgrass (23), tobacco (24), and Arabidopsis (8) are based on the leaves dry weight. Yields of cannabidiol from hemp (32) are based on the leaves dry weight.

Fig. 4.

Minimum required selling price ranges for bioproducts ($/kg) under different in planta accumulation amount (dry basis) in order to reach the MESP parity ($3.61/gal) and targeted selling price of ethanol ($2.50/gal). The Inset shows the estimated bioproduct selling price of less than $100/kg.

Fig. 5.

Projected global market size for each bioproduct and maximum number of biorefineries needed to meet total global demand, based on per facility coproduction required to bring MESP to the $2.50/gal target. Market projections: Global limonene market (64), artemisinin combination therapy market (65), cannabidiol sales in the United States (61), global styrene butadiene latex market (66), and global PP market (67).

Fig. 6.

A schematic of bioethanol production process with the value-added bioproduct and the integrated one-pot high gravity ionic liquid-based pretreatment process. Biomass sorghum is used as a representative bioenergy crop. In this study, the selected value-added bioproducts are limonene, artemisinin, PHB, latex, and cannabidiol.