Bioenergy production from algae using dairy manure as a nutrient source: Life cycle energy and greenhouse gas emission analysis

This study estimated the potential of algal bioenergy production using nitrogen and phosphorus present in the dairy manure (produced in the US). State wise dairy manure production and energy mixes were used to estimate algal bioenergy production and associated life cycle nonrenewable primary energy demand and greenhouse gas emissions for the four scenarios. These scenarios were constructed using various combination of following processes (i) anaerobic digestion, (ii) algal biodiesel production using effluent from (i), (iii) pyrolysis, and (iv) enzymatic hydrolysis. Bioenergy production, nonrenewable primary energy demand and greenhouse gas emissions of each state were aggregated to estimate the total bioenergy production, nonrenewable primary energy requirement and greenhouse gas emissions for the US. Two different cases were simulated for each scenario, one without taking into account the nutrient values (N, P) of applied sludge generated from the bioenergy production (Case B) while in the other one, nutrient values of sludge were considered (Case A). For incorporation of nutrient values of sludge, system expansion approach was used. It was estimated that by using dairy manure, 0.56 billion GJ/yr bioenergy could be produced. Minimum “nonrenewable primary energy requirement (NRPER)” (GJ/GJ) [Total primary nonrenewable energy requirement/bioenergy produced] and GHG emissions (kg CO2 eq./GJ bioenergy produced) for the four scenarios (1–4) for case B were as follows (1) 0.37, 27 (2) 0.51, −30; (3) 0.55, 47 and (4) 0.70, 15 respectively. In case A, NRPER did not change as compared to case B. GHG emissions increased in case A scenarios as compared to case B scenarios. The increase in GHG emission was mostly due to incorporation of reference scenario (raw dairy manure was applied on the ground) and N2O emission from the sludge amended soil.