Soy biodiesel energy ratio climbs 42 percent since 1998

By Susanne Retka Schill | November 20, 2009
Soy biodiesel's energy ratio calculated by researchers at the USDA and University of Idaho has improved dramatically in the past decade. Using a life-cycle analysis (LCA) from field to pump, the fossil energy ratio has grown from the 3.2 ratio reported in a 1998 study to nearly 4.6 in a report released this fall.

Biodiesel conversion used the most energy, accounting for about 60 percent of the total energy required in the LCA. Soybean agriculture accounted for 18 percent of the total energy requirements, with diesel fuel for field operations contributing the biggest share. Close behind the agriculture contribution was soybean crushing, which required almost 15 percent of the total energy. The net energy value (biodiesel energy output minus fossil energy input) was about 91,000 Btu per gallon. The Fossil Energy Ratio calculated to 4.56, which is 42 percent higher than the ratio reported in 1998.

A major reason behind the improved number was a 58 percent reduction in the energy requirement for crushing, the study noted. Soybean crushing facilities are far more energy efficient now than a decade ago, plus the U.S. EPA now requires crushers to limit their hexane use. The energy required for transesterification estimated in the new study was about 12 percent lower than the first study. The fossil energy needed for electricity decreased, and methanol usage decreased; however, natural gas and steam usage slightly increased.

The second largest contributor to the improved energy ratio was a 41 percent reduction in the fossil energy inputs for soybean agriculture. The increased adoption of no-till practices by soybean farmers reduced fuel requirements for tillage operations. Widespread use of genetically engineered soybeans has reduced pesticide requirements, and soybean yields have improved due to new seed varieties, improved fertilizer and pesticide applications, and new management practices.

The improvement in agriculture's decreased energy inputs is most likely understated in the new study, which used 2002 soybean production data. The 1998 study suffered from the same limitation using 1990 data. However, the quality of the data is important and, in this case, included actual farming practices employed by soybean producers. USDA periodically conducts a detailed farm survey on crop production practices. The most recent data, collected in a survey conducted in 2006, was not yet available when the newest energy balance study was undertaken. The researchers, however, did look at the impact of improving soybean yields on the baseline data, and found that for each bushel increase in soybean yield, the energy ratio increases by 0.45 percent. They noted that USDA projections call for soybean yields to increase annually by about half a bushel per acre. Holding all other variables constant, the energy ratio of soybean biodiesel is estimated to reach 4.69 in the year 2015, when the average U.S. soybean yield is projected to increase to 45.3 bushels per acre.

While the baseline study was conducted using the same parameters to allow a good comparison with the 1998 study, the researchers reworked the numbers incorporating other energy contributions to assess the impact. The original study, for example, did not incorporate values for agricultural machinery and the energy embodied in building materials. Calculating the ratios with those inputs included lowered the energy ratio from 4.56 to 4.40, which the study noted is still considerably higher than the 3.2 energy ratio reported in 1998. The researchers considered other variables as well. The generic 10 MMgy biodiesel plant modeled in the study included a collocated crushing plant. When the energy value was included for transporting soy oil from an independent crusher to the plant-a common scenario in the industry-the energy ratio dropped to 4.41 from the baseline result of 4.56.

The National Biodiesel Board trumpeted the benefits of biodiesel in announcing the findings. Biodiesel returns more than four times the energy than it takes to make biodiesel, the NBB pointed out, while oil exploration and drilling is becoming more intensive.
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