CO2 Emissions
2.4.1.2.1 Accounting for Biomass-Derived Carbon
Biomass plays a unique role in the dynamics of carbon flow in our biosphere. Biological cycling of
carbon occurs when plants (biomass such as soybean crops) convert atmospheric CO2 to carbon-based
compounds through photosynthesis. This carbon is eventually returned to the atmosphere as organisms
consume the biological carbon compounds and respire. Biomass derived fuels reduce the net atmospheric
carbon in two ways. First, they participate in the relatively rapid biological cycling of carbon to the
atmosphere (via engine tailpipe emissions) and from the atmosphere (via photosynthesis). Second, these
fuels displace the use of fossil fuels. Combustion of fossil fuels releases carbon that took millions of
years to be removed from the atmosphere, while combustion of biomass fuels participates in a process
that allows rapid recycle of CO2 to fuel. The net effect of shifting from fossil fuels to biomass-derived
fuels is, thus, to reduce the amount of CO2 present in the atmosphere.
Because of the differences in the dynamics of fossil carbon flow and biomass carbon flow to and from the
atmosphere, biomass carbon must be accounted for separately from fossil-derived carbon. The LCI
model tracks carbon from the point at which it is taken up as biomass via photosynthesis to its final
combustion as biodiesel used in an urban bus. The biomass-derived carbon that ends up as CO2 leaving
the tailpipe of the bus is subtracted from the total CO2 emitted by the bus because it is ultimately reused in
the production of new soybean oil. In order to ensure that we accurately credit the biodiesel LCI for the
amount of recycled CO2, we provide a material balance on biomass carbon.
The material balance shows all the biomass carbon flows associated with the delivery of 1 bhp-h of
engine work (Figure 7). For illustration purposes, only the case of 100% biodiesel is shown. Lower blend
rates proportionately lower the amount of biomass carbon credited as part of the recycled CO2. Carbon
incorporated in the meal fraction of the soybeans is not included in the carbon balance. Only carbon in
the fatty acids and triglycerides that are used in biodiesel production are tracked. Not all the carbon
incorporated in fatty acids and triglycerides ends up as CO2 after combustion of biodiesel. Some oil loss
occurs in the meal by-product. Glycerol is removed from the triglycerides as a by-product. Fatty acids are
removed as soaps and waste. Finally, carbon released in combustion ends up in the form of CO2, CO,
THC, and TPM. Of the 169.34 grams of carbon absorbed in the soybean agriculture stage, only 148.39
grams (87%) end up in biodiesel. After accounting for carbon that ends up in other combustion products,
148.05 grams of carbon end up as 543.34 grams of tailpipe CO2. This CO2 is subtracted from the diesel
engine emissions as part of the biological recycle of carbon. No credit is taken for the 13% of the carbon
that ends up in various by-products and waste streams.
Comparison of CO2 Emissions for Biodiesel and Petroleum Diesel
Table 7 summarizes CO2 flows from the total life cycles of biodiesel and petroleum diesel and the total
CO2 released at the tailpipe for each fuel. The dominant sources of CO2 for both the petroleum diesel life
cycle and the biodiesel life cycle is the combustion of fuel in the bus. For petroleum diesel, CO2 emitted
from the tailpipe of the bus represents 86.54% of the total CO2 emitted across the entire life cycle of the
fuel. Most remaining CO2 comes from emissions at the oil refinery, which contributes 9.6% of the total
CO2 emissions. For biodiesel, 84.43% of the CO2 emissions occur at the tailpipe. The remaining CO2
comes almost equally from soybean agriculture, soybean crushing, and conversion of soy oil to biodiesel.
At the tailpipe, biodiesel emits 4.7% more CO2 than petroleum diesel, most of which is renewable. The
nonrenewable portion comes from the methanol. Biodiesel generates 573.96 g/bhp-h compared with
548.02 g/bhp-h for petroleum diesel. The higher CO2 levels result from more complete combustion and
the concomitant reductions in other carbon-containing tailpipe emissions. As Figure 8 shows, the overall
life cycle emissions of CO2 from B100 are 78.45% lower than those of petroleum diesel. The reduction isLife Cycle Inventory of Biodiesel and Petroleum Diesel 19 NREL/SR-580-24089
a direct result of carbon recycling in soybean plants. B20, the most commonly used form of biodiesel in
the US, reduces net CO2 emissions by 15.66% per gallon of fuel used.