Life cycle assessment of the production process of second-generation biodiesel based on waste cooking oil

Waste cooking oil (WCO) is one of the household products of edible oil after cooking at elevated temperatures. A two-stage disposal strategy has been established to firstly convert into fatty acid methyl esters, and then into alkanes. This study aims to explore the feasibility of the two-stage WCO disposal process in China. A series of environmental tests was then carried out using the life cycle assessment software openLCA 2.4.0. The data was also collected from the Tian Gong database v0.2.0. Three stages were finally selected to simulate the entire life cycle of the WCO during second-generation biodiesel production, including the WCO collection and transportation, pre-esterification/esterification, as well as the hydrogenation deoxygenation/hydroisomerization. The results indicate that the three midpoint indicators of the global warming (CO₂ emissions), fine particulate matter formation (PM2.5 emissions), and terrestrial acidification (SO₂ emissions) were 506.78 kg/t (10.93 kg/GJ), 0.04 kg/t (8.63×10^-4 kg/GJ), and 0.14 kg/t (3.02×10^-3 kg/GJ), respectively, for the second-generation biodiesel production using WCO. The lower CO₂ emissions were achieved in a significant environmental advantage, compared with the fossil diesel (about 650 kg/t), coal produced in Chongqing (550 kg/t), jatropha-based biodiesel (3.17 t/t), and loquat seed-based biodiesel (1.129 t/t). The hydrogenation deoxygenation/hydrogenation isomerization accounted for 55.73%, 56.69%, and 58.00% of the total values of the indicators in the entire process, respectively. In the hydrogenation deoxygenation/hydrogenation isomerization, the hydrogenation deoxygenation reaction was carried out at 350℃ and 5 MPa, while the hydroisomerization reaction was performed at 300℃ and 1.5 MPa. The energy consumption was high under the conditions of high temperature and high pressure. Additionally, there was a substantially high hydrogen consumption during the reaction. As a result, the emissions accounted for a relatively high proportion at this stage. After normalization with World (2010) H, the top three categories of the impact assessment were the stratospheric ozone depletion, global warming, and terrestrial acidification, respectively, for the WCO collection and transportation, pre-esterification/esterification, as well as the hydrogenation deoxygenation/hydroisomerization stages. The endpoint indicators for human health (disability-adjusted life year, DALY), ecosystem (loss of species in a year, species. yr), and resource (CNY) were 4.97×10^-4, -1.75×10^-5, and -12.39, respectively, to produce 1 t of the WCO-based second-generation biodiesel. After normalization and weighting with World (2010) H/A, the top three impact categories were the stratospheric ozone depletion, global climate change-human health, and global warming-freshwater ecosystems, with the single scores of 34.11, 25.38, and 25.37, respectively, to produce 1 t of the second-generation biodiesel from WCO. The Monte Carlo method was also used to determine the global warming potential of the WCO-based second-generation biodiesel. The average value was 507.50 kg/t with a standard deviation of 50.74 kg/t, indicating a moderate variability. There was also a wide range from 424.48 kg/t (5th percentile) to 590.67 kg/t (95th percentile). A variety of influencing factors were then attributed to global warming, such as the great variations in greenhouse gas emissions. Therefore, the data can be captured from the pilot production plants at the hydrogenation deoxygenation/hydroisomerization stage, in order to significantly reduce the uncertainty in the future. In addition, much emphasis can also be placed on the emissions of chlorine and bromine gases, greenhouse gas emissions, and the use of hydrogen throughout the process.