Technoeconomic Analysis of Biodiesel and Ethanol Production from Lipid-Producing Sugarcane and Sweet Sorghum

Tolerance of engineered Rhodosporidium toruloides to sorghum hydrolysates during batch and fed-batch lipid production

BACKGROUND

Oleaginous yeasts are a promising candidate for the sustainable conversion of lignocellulosic feedstocks into fuels and chemicals, but their growth on these substrates can be inhibited as a result of upstream pretreatment and enzymatic hydrolysis conditions. Previous studies indicate a high citrate buffer concentration during hydrolysis inhibits downstream cell growth and ethanol fermentation in Saccharomyces cerevisiae. In this study, an engineered Rhodosporidium toruloides strain with enhanced lipid accumulation was grown on sorghum hydrolysate with high and low citrate buffer concentrations.

RESULTS

Both hydrolysis conditions resulted in similar sugar recovery rates and concentrations. No significant differences in cell growth, sugar utilization rates, or lipid production rates were observed between the two citrate buffer conditions during batch fermentation of R. toruloides. Under fed-batch growth on low-citrate hydrolysate a lipid titer of 16.7 g/L was obtained.

CONCLUSIONS

Citrate buffer was not found to inhibit growth or lipid production in this engineered R. toruloides strain, nor did reducing the citrate buffer concentration negatively affect sugar yields in the hydrolysate. As this process is scaled-up, $131 per ton of hydrothermally pretreated biomass can be saved by use of the lower citrate buffer concentration during enzymatic hydrolysis.

Implications of Biorefinery Policy Incentives and Location-Specific Economic Parameters for the Financial Viability of Biofuels

Cellulosic biofuels are part of a portfolio of solutions to address climate change; however, their production remains expensive and federal policy interventions (e.g., Renewable Fuel Standard) have not spurred broad construction of cellulosic biorefineries. A range of state-level interventions have also been enacted, but their implications for the financial viability of biorefineries are not well understood. To address this gap, this study evaluated the efficacy of 20 state-level tax incentives from 14 states and their interactions with other location-specific economic parameters (e.g., state income tax rates, electricity prices). To characterize implications of location-specific policies and parameters on biorefinery cash flows, we developed a new BioSTEAM Location-Specific Evaluation (BLocS) module for the open-source software BioSTEAM. Leveraging BLocS and BioSTEAM, we characterized the minimum ethanol selling price (MESP) for a cellulosic biorefinery (using corn stover as feedstock) and two conventional biorefineries (using corn or sugarcane as feedstock) for comparison. Among state-specific scenarios, nonincentivized MESPs for the corn stover biorefinery ranged from 0.74 $·L^-1 (4.20 $·gallon gasoline equivalent [gge]^-1) [0.69-0.79 $·L^-1; 3.91-4.48 $·gge^-1; Oklahoma] to 1.02 $·L^-1 (5.78 $·gge^-1) [0.95-1.09 $·L^-1; 5.39-6.18 $·gge^-1; New York], while the tax incentive-induced MESP reduction ranged from negligible (Virginia) to 5.78% [5.43-6.20%; Iowa]. Ultimately, this work can inform the design of policy incentives for biorefineries under specific deployment contexts.

Elucidation of Triacylglycerol Overproduction in the C4 Bioenergy Crop Sorghum bicolor by Constraint-Based Analysis

Upregulation of triacylglycerols (TAGs) in vegetative plant tissues such as leaves has the potential to drastically increase the energy density and biomass yield of bioenergy crops. In this context, constraint-based analysis has the promise to improve metabolic engineering strategies. Here we present a core metabolism model for the C4 biomass crop Sorghum bicolor (iTJC1414) along with a minimal model for photosynthetic CO2 assimilation, sucrose and TAG biosynthesis in C3 plants. Extending iTJC1414 to a four-cell diel model we simulate C4 photosynthesis in mature leaves with the principal photo-assimilatory product being replaced by TAG produced at different levels. Independent of specific pathways and per unit carbon assimilated, energy content and biosynthetic demands in reducing equivalents are about 1.3 to 1.4 times higher for TAG than for sucrose. For plant generic pathways, ATP- and NADPH-demands per CO2 assimilated are higher by 1.3- and 1.5-fold, respectively. If the photosynthetic supply in ATP and NADPH in iTJC1414 is adjusted to be balanced for sucrose as the sole photo-assimilatory product, overproduction of TAG is predicted to cause a substantial surplus in photosynthetic ATP. This means that if TAG synthesis was the sole photo-assimilatory process, there could be an energy imbalance that might impede the process. Adjusting iTJC1414 to a photo-assimilatory rate that approximates field conditions, we predict possible daily rates of TAG accumulation, dependent on varying ratios of carbon partitioning between exported assimilates and accumulated oil droplets (TAG, oleosin) and in dependence of activation of futile cycles of TAG synthesis and degradation. We find that, based on the capacity of leaves for photosynthetic synthesis of exported assimilates, mature leaves should be able to reach a 20% level of TAG per dry weight within one month if only 5% of the photosynthetic net assimilation can be allocated into oil droplets. From this we conclude that high TAG levels should be achievable if TAG synthesis is induced only during a final phase of the plant life cycle.

Biodiesel from oil produced in vegetative tissues of biomass - A review

Biodiesel is a green, renewable alternative to petroleum-derived diesel. However, using vegetable oil for biodiesel production significantly challenges the food security. Progress in metabolic engineering, understanding of lipid biosynthesis and storage have enabled engineering of vegetative tissues of plants such as sugarcane, sorghum, and tobacco for lipid production. Such sources could be cultivated on land resources, which are currently not suitable for row crops. Besides achieving significant lipid accumulation, it is imperative to maintain the fatty acid and lipid profile ideal for biodiesel production and engine performance. In this study, genetic modifications used to induce lipid accumulation in transgenic crops and the proposed strategies for efficient recovery of oil from these crops have been presented. This paper highlights that lipids sourced from vegetative biomass in their native form would pose significant challenges in biodiesel production. Therefore, different strategies have been presented for improving feedstock quality to achieve high-quality biodiesel production.

Process optimization, green chemistry balance and technoeconomic analysis of biodiesel production from castor oil using heterogeneous nanocatalyst

In the present work, zinc doped calcium oxide was used as a nanocatalyst for biodiesel production from castor oil. The optimal conditions of biodiesel conversion and green chemistry balance were obtained with response surface methodology. Five green chemistry parameters like carbon efficiency, atom economy, reaction mass efficiency, stoichiometric factor and environmental factor were optimized. The sustainable biodiesel yield 84.9% and green chemistry balance 0.902 was achieved at methanol to oil molar ratio 10.5:1, temperature 57 °C, time 70 min, and catalyst concentration 2.15%. The synthesized biodiesel was characterized by GCMS and FTIR, and physic-chemical properties were determined. Based on experimental study annually 20.3 million kg capacity plant was simulated using SuperPro designer. The sensitivity analysis of oil purchase cost and biodiesel selling price on ROI, payback period, IRR and NPV were investigated. The optimization and technoeconomic analysis provided a sustainable platform for commercial based biodiesel production.

Co-fermentation of magnesium oxide-treated corn stover and corn stover liquor for cellulosic ethanol production and techno-economic analysis

MgO is an effective catalyst to reduce the recalcitrant structure of corn stover and reduce sugar degradation during pretreatment. To evaluate the economic feasibility of MgO pretreatment, techno-economic analysis was performed at a commercial scale of 700,000 MT stover per year based on the collected experimental data. Compared to LHW pretreatment, MgO pretreatment reduced total capital investment due to elimination of solids washing and increased ethanol yield by 78.4 L/MT stover due to higher xylose yield (53.4 vs 10.9%), thus resulted in a lower minimum ethanol selling price (MESP) of $0.72/liter. Although washing of MgO-pretreated solids improved glucose (73.0 vs 69.5%) and xylose (66.0 vs 53.4%) yields, MESP did not decrease but increase by $0.08/liter due to the high capital cost of solid-liquid separation unit. Tween 80 also improved glucose (73.1 vs 69.5%) and xylose (62.6 vs 53.5%) yields. However, its high cost limited its economic feasibility in ethanol production.