| Abstract | Biofuels derived from microalgae have potential to replace fossil fuel and first generation biofuels, but there are some major indicators that can govern the efficiency and feasibility of biofuels derived from microalgae. Selection of efficient and feasible method to extract the microalgae oil is a major step to minimize the energy investment and GHGs emissions. Performance of microalgae oil in combustion engine and its environmental impact are the two key parameters to decide the feasibility of microalgae oil. This study provides an analysis of the potential, performance and environmental impact of microalgae oil.
A microalgae (Desmodesmus sp.) was treated with Hydrothermal Liquefaction (HTL) process to extract the lipids. Fatty acids were analyzed to examine the quality and potential of microalgae oil. Physical and chemical properties of microalgae oil derived using HTL and other two fuels i.e. fossil diesel and Soybean Methyl Ester (SME100) were used to evaluate the performance and emissions characteristics in agricultural tractor engine. A commercial software “Diesel-RK” was used to simulate the data for selected agricultural tractor engine, Perkins 4.236 that is widely used in Massey Ferguson 255. Environmental impact of microalgae oil from gate to wheel (oil extraction to combustion) was evaluated using SimaPro while taking 1 kWh energy produced by selected engine. From gate to wheel, three processes were considered namely, (i) Oil extraction using hydrothermal treatment, (ii) Transesterification of microalgae oil and (iii) Combustion of microalgae oil in agricultural tractor engine. Three difference cases were covered as (i). Performance of microalgae oil (Base case), (ii). Performance of microalgae oil equivalent to Diesel No. 2 (Case-1) and (iii). Performance of microalgae oil equivalent to SME100 (Case-2).
Study revealed that operating conditions of HTL such as temperature, pressure and physical properties of microalgae feedstock have significant effect on oil yield and content of lipids in oil. Oil obtained has a heating value (LHV) of 34 MJ/kg. Microalgae has a lipid content of 12-20%, which influenced by many factors such as nutrient supply during cultivation, time of harvesting, method of oil extraction etc. Presence of fatty acids in microalgae oil obtained is, C10:0-1.044%, C14:0-0.578%, C16:1-4.226%, C18:0-0.49%, C18:1-8.63%, C18:2-6.48%, C18:3-33.141%, C18:3-2.301% and others 27.574%. Ratio of C:H:O (wt.%) was found to be 74.5:13.01:12.61.
Microalgae oil showed slightly lower power (44.80 kW) and torque (164.9 N-m) compared to diesel No. 2 (54.5 kW, 200 N-m) and SME100 (51.27 kW, 188.34 N-m) at full load and maximum rated speed (2600 rpm). Specific fuel consumption of microalgae oil is higher than other two fuels, which can be explained by lower heating value of microalgae oil. NO2 emission and particulate matter formation is lower when engine was fueled with microalgae oil.
Net Energy Ratio (NER) of base case is higher than case-1 and case-2 that are 15.17, 11.44 and 13.92 respectively. Base case emitted 2.96 kg of CO2-eq while case-1, case-2 emitted 2.4, and 2.81 kg of CO2-eq. Study found that electricity and chemicals used played significant role in energy consumption and GHGs emissions. NER and GHGs emission were reduced by 83% and 50% respectively when biomass based electricity was used instead of coal based.
Keywords: Microalgae; hydrothermal treatment; agricultural tractor engine; performance; emissions; life cycle impact assessment; net energy ratio |