Energy Life-Cycle Assessment of Soybean Biodiesel

The first comprehensive life-cycle inventory (LCI) for biodiesel produced in the United States from soybean oil was completed by Sheehan et al. in 1998. The purpose of the study was to conduct a life-cycle assessment (LCA) to quantify and compare the environmental and energy flows associated both with biodiesel and petroleum-based diesel. One of the most often cited results from Sheehan et al. is that the fossil energy ratio of biodiesel is equal to 3.2. In other words, biodiesel yields 3.2 units of energy for every unit of fossil energy consumed over its life- cycle. By contrast, it was found that petroleum diesel’s life cycle yielded only about 0.84 units of energy per unit of fossil energy consumed. The purpose of the following analysis is to update the energy life cycle of the model to determine if any significant changes in the original inventory have occurred since the model was first developed 10 years ago.

The LCI of biodiesel in this analysis includes four subsystems: feedstock production, feedstock transportation, soybean processing with biodiesel conversion, and product distribution. All significant sources of energy are included in the inventory, such as the liquid fuel and electricity used to directly power equipment in the system. The energy requirements to produce materials that are made from energy resources, such as fertilisers, pesticides, and other petrochemicals, are also included in the inventory. The soybean crushing model in this analysis uses the hexane extraction method to extract oil from soybean seed, and transesterification is used to convert soybean oil into biodiesel. Oil extraction and transesterification result in the production of two important coproducts, soybean meal and crude glycerin, respectively. A mass-based allocation method is used to account for the energy associated with the soybean meal and crude glycerin.

The fossil energy ratio (FER), which is used in this study to measure the energy balance of biodiesel, is defined as the ratio of the energy output of the final biofuel product to the fossil energy required to produce the biofuel. The energy requirements of biodiesel include all the fossil energy in the LCI and do not include any renewable energy, such as solar or hydroelectric energy. The analysis first constructed a base case, in which the inventory was kept basically the same as the inventory used in the Sheehan et al. report. Then additional inputs that were excluded by Sheehan et al., such as agricultural machinery and energy embodied in building materials, were added to study their impact on the FER.

The Sheehan et al. study used data from a U.S. Department of Agriculture (USDA) conducted survey on soybean production in 1990, and this study used data from a 2002 USDA survey. Given the long time period between surveys, the newer data would be expected to reflect some changes in soybean production practices over time. One major change that has occurred is the increased adoption of no-till practices by soybean farmers, which reduces fuel requirements. Another change is the widespread adoption of genetically engineered (GE) soybeans, which have had a major effect on pesticide use. Soybean yields have been improving over time because of new seed varieties, improved fertilizer and pesticide applications, and new management practices. Energy savings have also occurred in the soybean crushing industry because facilities that have been built in recent times are far more energy efficient than the older plants.

The first subsystem constructed for the LCI was soybean production, which is the feedstock source for the biodiesel examined in this study. Energy requirements for producing soybeans were estimated for both direct energy, such as diesel fuel, and gasoline, and indirect energy, such as fertilisers and pesticides. Diesel fuel use required the most energy on the farm, followed by fertilisers, and herbicides. Next, the energy required to transport soybeans from the farm to processing plants was estimated based on information from the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model. It requires about 6,393 British Thermal Units (Btu) to transport 1 bushel of soybeans to a processing facility.

The model used in this study was designed to represent a processing facility that combines a soybean processing plant with a biodiesel conversion unit producing 9.8 million gallons of biodiesel, 151,515 tons of soybean meal, 9,000 tons of soybean hulls, and 4,380 tons of crude glycerin. The soybean crusher uses energy in the form of electricity to power motors and provide lighting. Natural gas and process steam are used to provide heat for drying. Hexane is used for oil extraction. The total amount of energy required for removing the soybean oil is about 23,000 Btu per gallon of biodiesel. The soybean oil is converted into biodiesel using a process called transesterification, which is done by reacting the oil with an alcohol and a catalyst in large reactors. This reaction also results in the production of crude glycerin, which is a valuable coproduct. The conversion of the soybean oil into biodiesel and the treatment of the glycerin requires almost 19,000 Btu per gallon of biodiesel. Energy is also required to ship the biodiesel from the processing plant to marketing outlets. Using the GREET model, it was determined that on average it requires about 1,000 Btu to ship a gallon of biodiesel to its final destination.

Combining the energy input estimates from the four subsystems completed the base case life-cycle assessment for biodiesel. After adjusting the energy inputs by energy efficiency factors and allocating energy by coproducts, the total energy required to produce a gallon of biodiesel was 25,696 Btu. Biodiesel conversion used the most energy, accounting for about 60 per cent of the total energy required in the life-cycle inventory. Soybean agriculture accounted for 18 per cent of the total energy requirements, followed by soybean crushing, which required almost 15 per cent of the total energy. The net energy value (i.e., biodiesel energy output, minus fossil energy input) was about 91,000 Btu per gallon. The estimated FER of biodiesel was 4.56, which is about 42 per cent higher than the FER reported by Sheehan et al.

The next step in this analysis was to add secondary energy inputs to the LCI that were not included in Sheehan et al. to determine how they affect the overall results. The secondary inputs added were farm machinery, building materials for a crushing plant, and building materials for a biodiesel conversion plant. When the input energy for both agricultural machinery and building material are added to the inventory, FER declines to 4.40, still considerably higher than the 3.2 FER reported by Sheehan et al. In addition, Sheehan et al. omitted lime from their LCI, whereas this study included lime in the base case LCI. However, lime use only accounted for about 500 Btu per bushel of soybeans, and adding it to the LCI only lowered the FER by 0.22 per cent.

The final step in this analysis was to examine the effect of rising soybean yields on the FER of biodiesel. The analysis found that the FER of soybean biodiesel is expected to reach 4.69 when projected soybean yield reaches 45 bushels per acre in 2015. This is about a 3-per cent increase compared to the 2002 FER estimate. This result suggests that the FER of biodiesel will continue to improve over time. In addition to higher yields, improvements can be expected to occur in other areas of the life cycle as the agricultural sector, along with the biodiesel industry, continues to make energy efficiency gains in order to lower production costs.

Further Reading

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October 2009