Nitrogen Transformations During Pyrolysis Of Microalgae Remnants After Lipid Extraction

Microalgae is a kind of micro-organisms, which can photosynthesize and widely spreads in the land, lake and ocean. Different with the lignocellulosic biomass, microalgae is composed of protein, lipid and polysaccharide. As the cultivated lad that microalgae demands is much less than that of the lignocellulosic biomass. Therefore, microalgae is regard as the third generation biomass energy. Meanwhile, the process of lipid extraction would produce a large amount of microalgae remnants. And the remnants has been regard as inexpensive biomass source. However, the content of nitrogen in microalgae and remnants is much higher than that in the lignocellulosic biomass for high content of protein in algae. The nitrogen in microalgae and remnants would transfer into biofuels in the progress of biofuel production. And then, the nitrogen in biofuel would lead to NOx emission problem. Therefore, the research about nitrogen transformation mechanism during pyrolysis of microalgae or lipid extraction remnants is much significant.In this paper, commercial Nannochloropsis powder was chosen as the experimental sample. Firstly, nitrogen transformations during fast pyrolysis of microalgae were investigated on a fixed bed reactor. The results showed that nitrogen in microalgae was transferred from char to tar and gas at 300℃ to 700℃ and the nitrogen balance during experiment was within 81.4%-91.2%. The nitrogen content in char was decreasing with temperature rising. For functional group, amino in char was decreasing with the rise of temperature, and the peak of cyano side groups was found at 400℃ and 500℃, which proved the existence of nitrile in the char. The yield of tar reached the maximum of 57% at 500℃. With the rise of temperature, that the amidesN was the main nitrogen-containing organic in bio-oil was changed to that heterocyclicN was the main nitrogen contents. And indole was the main content in heterocyclic-N. HCN, NH3 and NO were the main components in pyrolysis gas. With temperature rising, the amount of HCN was increasing significantly. It indicated that the nitrogen in microalgae would convert into HCN at higher temperature.Secondly, after lipid extraction and esterification, saturated and unsaturated fatty acids, such as octadecenoic acid, hexadecanoic acid and tetradecanoic acid, are detected as the main components of lipid. And then, microalgae remnants were pyrolyzed in a fixed bed reactor. The results showed that the nitrogen content in the char of remnants is slightly higher than that of microalgae. And with the rise of temperature, the nitrogen content in char was decreasing. The yield of tar reached the maximum of 58.6% at 400℃. Amides, nitrile and heterocyclic-N were the main components of nitrogencontaining in the tar of remnants. At 300℃ to 400℃, the contents of these three nitrogen-containing were similar. With temperature increasing, the content of amides in tar reduced to less than 1%, the content of nitrile increased slowly, and heterocyclicN was increasing significantly to 21.7%. Meanwhile, pyrroles and pyridines became the main components of heterocyclic-N.Through comparing both experiments, it was found that the lipid had no significant effect on the variations of nitrogenous functional groups between remnants and microalgae. However, the existence of lipid had effect on the trend of yield in progress of pyrolysis and the nitrogen contents in tar. The effect on nitrogen content in tar was that lipid would promote the nitrogen transformation from protein to amides and more nitrogen were transferred to pyridines and pyrroles which belonged to heterocyclic-N without lipid.