What is left is a simplified approximation of the impact of coproduct sales on the price for sale of an algal biofuel. The power law relationship between the lipid fraction and the price of fuel is used as the starting economic estimate. Coproduct value reduces the price of the fuel, showing the benefit of coproduct sales. Power would have to be purchased to account for the loss of power from the anaerobic digestion, raising the price of the fuel. Fertilizers would have to be purchased to account for the loss of nutrients that are no longer being recycled to the algae cultivation.
Ammonia and DAP prices are nearly equivalent and are assumed to be identical at $450/tonne. Power prices are selected to match published studies at $0.081 per kilowatt hour (Davis et al., 2011).
where δ is 0.146 and is the collection of terms from both the nitrogen and phosphorus requirements (Eq. G-6).
Calculations can then be done to estimate the impact of coproduct sales as deviation from the economics of the reference pathway presented earlier, represented by the blue line in Figure G-2. Discontinuation of the recycle of nutrients from the algal biomass remaining after lipid extraction and elimination of power generation, as would be the case if this was put to other use without returning a revenue stream, requires that additional power and nutrients be purchased, effectively raising the price for sale of the algal biofuel. This is shown as the red line in Figure G-2 and represents an unrealistic, but instructive, case where the value in the residual biomass is not captured. The grey areas between the red and blue lines illustrate that the approximate value of the biomass is $200 per tonne. One commonly suggested outlet for the residual biomass is as animal feedstuff. This would place its value in the $400-500 per tonne range as a bulk feedstuff. At 30 percent lipid fraction, the impact on fuel price is on the order of $0.40 per liter. If algal species begin to express lipids at higher levels, the benefit of the coproduct sales diminishes on a per liter basis because of the reduction in production of the non-lipid biomass.
At points where the coproduct has a value below that of fuel, the shape of the price curve largely follows the curve for no coproduct sales. However, when the coproduct value exceeds the value of fuel, there can be a dramatic drop in fuel price. The interplay of scale and reasonable price of sale, to first approximation, limit the price that the coproducts can reasonably garner. Coproducts certainly can improve the economics of fuel production only modestly because reasonable values for large-volume applications such as animal feedstuffs have established prices that are near the alternative values for the residual biomass