Updated energy life-cycle assessment of soybean biodiesel finds fossil energy ratio of 5.54; significant improvement from earlier studies
|Comparing energy requirements for major biodiesel subsystems and total life-cycle energy requirements between the current study and two earlier assessments: Pradhan et al. (2009), and Sheehan et al. (1998). Pradhan et al. (2011) Click to enlarge.|
Researchers from the University of Idaho and the US Department of Agriculture have updated the analysis of the energy life-cycle of soybean biodiesel and found a fossil energy ratio (FER) of 5.54 using 2006 agricultural data. This marks a major improvement over the FER of 3.2 reported in a 1998 NREL study that used 1990 agricultural data and significantly better than the FER of 4.56 later reported using 2002 data.
The FER is the ratio of renewable fuel energy output to the biodiesel share of fossil energy input; only fossil (nonrenewable) energy is included in the input. The improvements are primarily due to improved soybean yields and more energy-efficient soybean crushing and conversion facilities, the team said.
The energy input in soybean agriculture was reduced by 52%; in soybean crushing by 58%; and in transesterification by 33% per unit volume of biodiesel produced. Overall, the energy input reduction was 42% for the same amount of biodiesel produced.
The addition of secondary inputs, such as farm machinery and building materials, did not have a significant effect on the FER—when these inputs were added, the FER decreased to 5.34 (a 3.6% reduction).
They divided their analysis of the life-cycle of biodiesel into four subsystems for the analysis: feedstock production; feed- stock transportation; soybean processing with biodiesel con- version; and product distribution. They then developed an inventory of material and energy that quantifies all fossil energy inputs used in each subsystem.
All direct and indirect sources of energy were included in the inventory, such as the liquid fuel and electricity used to directly power equipment in the system. The energy content of materials that were made from energy resources, such as fertilizers, pesticides, and other petrochemicals, is also included in the inventory. The effect of adding energy used for building biodiesel plants and agricultural machinery was studied separately and not included in the base case to be consistent with.
The soybean crushing and transesterification facilities that have been built in recent times are more energy efficient than older plants. In addition, the continued improvement in soybean yields and reduced overall energy usage on the farm helped increase the energy balance of bio- diesel. The lower chemical uses in recent years can partially be explained by the adoption of GE [genetically engineered] soybeans, which resulted in reduced pesticide use. Five-year average chemical use data showed a general decline in the amount of pesticide use.
...The results from this research suggest a likely improvement of the biodiesel FER over time. All other factors being constant, for every 100 kg/ha (1.5 bu/ac) increase in soybean yield, the FER increases by 0.76%. In addition, the agricultural sector and the biodiesel industry are likely to continue to make energy efficiency gains in order to lower production costs, eventually achieving an even higher FER.—Pradhan et al.
A. Pradhan, D. S. Shrestha, A. McAloon, W. Yee, M. Haas, J. A. Duffield (2011) Energy Life-Cycle Assessment of Soybean Biodiesel Revisited