Effects of Soybean Varieties on Life-Cycle Greenhouse Gas Emissions of Biodiesel and Renewable Diesel

The transportation sector accounted for about 29% of the total US greenhouse gas (GHG) emissions owing to its high reliance on fossil fuels. Adoption of biofuels is one of the viable options to decarbonize the hard-to-electrify sectors. Biodiesel (BD) and renewable diesel (RD) are two major biomass-derived diesel fuels which are capable for blending in or alternating fusil diesel in long-haul road trucks and maritime ships.

This study presents a life-cycle analysis (LCA) of the greenhouse gas (GHG) emissions of BD and RD produced from soybean varieties with altered oil profiles and meal composition. The varieties evaluated include those with high oleic oil content (78% of oil being oleic oil vs. the current content of 25%), high lipid content (25% vs. the current content of 19%) or high protein content (45% vs. the current content of 35%). The results suggest that high-oleic soybean oil could reduce GHG emissions of RD by 1 g CO2e/MJ by reducing hydrogen consumption in the hydro-treating stage of RD production. With the default allocation method (mass-based allocation between oil and meal), changes in lipid and protein contents have negligible impacts on GHG emissions of BD/RD pathways. Since high-protein soybean meal has a higher nutrient value, we employed the market-based allocation for sensitivity analysis. Assuming a market price of $1.17/dry kg and an average of two substitute feeds with similar protein contents (corn gluten and fish meal), the allocation factor for soybean meal increases from 63 to 81%. Compared with the results for the case with commodity soybean using mass allocation, GHG emissions could be 1.9 and 1.8 g CO2e/MJ lower for BD and RD, respectively. In addition to product-level LCA, we employed a land-based LCA to evaluate the effect of soybean properties on product slates with the same land area and the associated displacement of conventional products. The results suggest that high-lipid soybeans may offer more GHG savings than other varieties as the higher BD/RD yield replaces more fossil diesel.