Life cycle energy demand from algal biofuel generated from nutrients present in the dairy waste

Abstract

In the USA, approximately 6 million tons of nitrogen and 1 million tons of phosphorus are being produced as a waste stream from the dairy operations. The main aim of this research was to estimate the potential of algal biofuel production, using waste nutrients present in the dairy waste, as well as performing a life cycle assessment to estimate the energy requirement of produced algal biofuel. Four different scenarios for algal biofuel production were simulated using different combinations of the following processes (i) algal-biodiesel-production, (ii) anaerobic-digestion (AD), (iii) pyrolysis and (iv) enzymatic-hydrolysis. Scenario 1 consists of AD and algal biodiesel production. Introduction of pyrolysis in the second scenario decreased biofuel production by ∼6% in the initial cycle and gradually increased to 25% in the later cycles. In the third scenario, biomass liquefaction through enzymatic hydrolysis was introduced to recover nutrients and sugars from the sludge generated from the AD process. Recovered nutrients and sugars were used for additional algal biodiesel production. Remaining sludge after the biomass liquefaction was applied on the agricultural land. As compared to the 1st scenario, further processing of the sludge through liquefaction increased the overall bioenergy production marginally. In the fourth scenario, biomass left after liquefaction was further processed through pyrolysis. In the fourth scenario, a 38% increase in the energy production was observed versus the 1st scenario. Additional energy production (compared to 1st scenario) through pyrolysis (Scenario 2) required additional 1.5 GJ of energy per GJ of energy produced and showed little variability. Additional energy production through the 3rd and the 4th scenario is not energetically favorable as compared to the 1st scenario. With respect to the 3rd scenario, energetically favorable additional energy can only be produced by the 4th scenario. Life-cycle energy-demand of the produced biofuel was varied from 0.35 to 0.68 GJ/GJ of energy produced. This study estimated that using dairy waste at a maximum of 3.14 billion GJ bioenergy could be produced.