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184   Biofuels for a More Sustainable Future


             glycerin were immediately sold to the market without any further proces-
             sing. Regarding reapplied compost, the amount of compost applied back
             to the field was determined based on nutrient components of its origin
             and requirements from the cultivation.
          – The consequential approach was applied for surplus composts that would
             be used in other fields. Being based on nutrient components of Hibiscus
             leaves and Pongamia and Hibiscus-Vernicia oil cake, this paper assumed
             that surplus composts could be used to avoid the relative amount of min-
             eral fertilizer including urea, phosphorus, and potassium fertilizers.
          – The closed-loop scenario was developed in terms of reapplied composts
             into the field.

          4.3 Net present value (NPV) and discount rate calculation

          The computation of NPV was as follows (Huppes et al., 2004):
                                       n
                                           C t
                                      X
                               NPV ¼            ð US$Þ                 (6.5)
                                               t
                                         ð 1+ rÞ
                                      t¼0
          where n is the period of assessment (year), r is the discount rate, and C t is the
          estimated costs in year t. In Triple I, the time-equivalent value of total one-
          time payment (TP) is considered under the adjustment of NPV, in which n is
          the project lifetime, and C t is the average amount of TP over project lifetime
          period. The discount rate (r) is a key factor in the estimation of NPV mostly
          influenced by the inflation and interest rate (Eq. 6.6)(Davis et al., 2005).
                                             6:5% 2:67%
                      Rate Interest  Rate Inflation
                   r ¼                     ¼              ¼ 3:74%      (6.6)
                          1 + Rate Inflation   1+2:67%
             The average inflation rate of Viet Nam from January 2016 to December
          2016 was 2.67% (calculated based on consumer prices) (Trading Economics,
          2017). The interest rate was 6.5%, according to the State Bank of Viet Nam.


          4.4 Base case assumption
          To date, no practical information on proper fertilizers used in Pongamia cul-
          tivation and Hibiscus-Vernicia intercropping field in Viet Nam has been
          provided. Therefore the base case assumption of annual fertilizer use was
          developed from the previous literature concerning current conditions in
          the North of Viet Nam.
             In mountainous areas of Viet Nam, the propagation of Vernicia was direct
          seed sowing with no care and fertilizer use. Moreover, Vernicia cultivation
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