The CO2 emission was estimated by using Eqs. (5)–(8) and the
carbon emission factor in Table S7 (details in Section S3 of the supplemental
material). An estimation of CH4 emission was performed
by the same approach (using the CH4 emission factors in Table S7).
The CH4 emission result was converted into CO2-eq by using a factor
of 25 (IPCC, 2007). As mentioned in the previous section, GHG
emission from the chemical production stage was estimated based
on the concept of ‘‘electricity-equivalent’’ energy input. Table S8
(in the supplemental material) provides the key parameters for
estimating GHG emission from NaOH production. The life cycle
inventory of GHG emissions from methanol production was
obtained from 1998 NREL study (Sheehan et al., 1998). The calculation
for GHG emission from methanol manufacturing can be
found in the Section S3 of the supplemental material. The total
GHG emission was 105.44 g CO2-eq/L-biodiesel (398.58 g CO2-eq/
gal) from methanol production. Therefore, the total GHG emissions
for producing one gallon of biodiesel from waste cooking oil was
determined to be 121.33 g CO2-eq/L-biodiesel (458.63 g CO2-eq/
gal).