Biofuels are an important part of the broader strategy to replace petroleum-based gasoline, diesel, and jet fuels that we use today; however, biofuels have so far not reached cost parity with conventional petroleum fuels.
One strategy to make biofuels more competitive is to make plants do some of the work themselves. Scientists can engineer plants to produce valuable chemical compounds, or bioproducts, as they grow. Then the bioproducts can be extracted from the plant and the remaining plant material can be converted into fuel. When produced in the plant itself, bioproducts can help reduce the cost of the resulting biofuel. But one important part of this strategy has remained unclear — exactly how much of a particular bioproduct would plants need to make in order to make the process economically feasible?
Researchers gathered information on a group of well-studied bioproducts that plants can already effectively produce, ranging from flavors and fragrances to biodegradable plastic. They then designed and simulated what it would take to extract these bioproducts from plant material in the context of an ethanol biorefinery. In this setting, valuable bioproducts would be extracted from the plant, while the remaining plant material would be converted into ethanol.
This helped them answer two important questions: what amount of bioproduct the plant needs to produce in order to make the process of extracting it worthwhile and what amount needs to be made in order to reach the target ethanol selling price of $2.50 per gallon.
Results showed that the amount plants need to make is actually quite feasible. When accumulated at 0.6% of the biomass dry weight, a compound such as limonene — used in flavor and fragrance — would offer net economic benefits to biorefineries. If they can harvest 10 dry metric tons of sorghum biomass from an acre of land, they need to recover only around 130 pounds of limonene from that biomass. The analysis suggests that just five commercial-scale biorefineries could support the entire projected 2025 market demand for limonene.