Preparation of low-nitrogen chlorella based on proteolysis technology and pyrolysis characteristics of the products

The high protein content in the microalgal biomass leads to the high relative content of nitrogen compounds in the pyrolysis oil, which hinders the efficient application of the microalgal pyrolysis oil. In this paper, a method for preparing low-nitrogen chlorella by using alkaline protease based on proteolysis technology was proposed to reduce the nitrogen content of chlorella, and its process conditions were optimized. The results showed that the maximum nitrogen removal rate and yield could reach 20.32% and 52.36 wt%, respectively. The thermal decomposition characteristics of Chlorella and low-nitrogen chlorella were analyzed by TG-DTG technology, and their main thermal decomposition were calculated by Coats-Redfern method. It was found that compared with Chlorella, the low-nitrogen Chlorella had a larger weight loss rate in main thermal decomposition stage and its average activation energy value reduced by 2.23 kJ/mol. The Py-GC-MS technology was further used to study the distribution of pyrolytic products of Chlorella and low-nitrogen Chlorella at 550 °C. It was found that the relative content of nitrogen compounds in pyrolytic products from low-nitrogen Chlorella was 43.02% lower than that of Chlorella. This is because that the proteolysis pretreatment has a significant effect on reducing the relative contents of some nitrogen compounds in bio-oil, such as pyridines, pyrroles and benzyl nitrile. Based on the above experimental results, a brief mechanism of proteolysis pretreatment on removing nitrogen compounds was proposed, mainly through the proteolytic process to reduce the relative content of various amino acids in Chlorella as precursors for the synthesis of nitrile and N-heterocyclic compounds in pyrolysis. In conclusion, proteolysis technology can effectively reduce the relative content of nitrogen compounds in pyrolysis components and improve the potential of microalgal pyrolysis oil as a high-quality alternative fuel.