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FIGURE G-2 Lipid fraction in the microalgae figures prominently in the amount of coproduct, and, therefore, the total value of the coproduct stream.

NOTES: Fuel costs are estimated using data from (Davis et al., 2011) as a function of the per pound sales price of the coproduct and the lipid fraction. The coproduct is assumed to be the total remaining biomass and the alternative value of the nutrients, and power lost through its sale is accounted for. Coproduct sales price is in dollars per tonne. Curves with a positive slope indicate that the coproduct is providing more of the total value than the fuel component. Negative slopes indicate that fuel is still the most valuable component. Potentially difficult to see, the $200/ton line falls on top of the blue line representing the base case with all nutrients and power production.

be replaced with purchased power and all residual biomass is sold as coproduct. The approximation intends to show trends.

The nutrient requirements can be estimated based on the Redfield molar ratio for algae of 106C:16N:1P (Redfield, 1958). Fertilizer is assumed to be ammonia and diammonium phosphate (DAP). Assuming that all nitrogen (N) and phosphorus (P) remain in the residual biomass, the nitrogen content is approximately 8.6 percent by weight and the phosphorus 1.2 percent, in good agreement with estimates in detailed studies of algal fuel production (Davis et al., 2011). Therefore, the amount of diammonium phosphate (DAP) equivalent in the residual biomass can be approximated as (Eq. G-4):

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The nitrogen present in DAP has to be accounted for, yielding an approximate amount of ammonia equivalent in the residual biomass of (Eq. G-5):



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