New microwave based transesterification techniques for biodiesel production from cultivated microalgae

Abstract

Microalgae biodiesel production has its share of problems such as the high cost in microalgae producti vity, harvesting, dewatering, lipid extraction and transesterification. Thus research in this field focuses on address ing these issues from the cultivation perspective and also the processing aspect. In this dissertat ion both approaches were used to address the problems where marine microalgae (Tetraselmis sp. and Nannochloropsis sp.) were cultivated indoor using light emitting diodes (LED) and processed using direct transesterification (DT). During cultivation, factors influencing microalgae growth rate and lipid content such as the type of LED wavelength and light intensities were investigated in detail. Microalgae were cultivated for 14 days as under blue, red, red-blue LED and white fluorescent light. The intensity of the red, blue and mixed red-blue LED was varied at 100, 150 and 200 umol photons m·2s·l . Findings revealed both species prefer to grow under blue wavelength which showed highest growth rate (reflected by the high cell count and absorbance readings) and lipid content (indicated by the fluorescence intensity). Suitable combination of LED wavelengths and intensity; (red LED: 150, blue: 100 and mixed redblue: 200 umol photons m·2s·l ) produced maximum biomass growth and lipid content. Then, several techniques were used to improve the DT method; i) introducing the use of microwave heating (MWH) to the various extraction methods, ii) varying the type of catalyst under MWH and iii) the use of simultaneous cooling and microwave heating (SCMH). Furthermore, four different methods were used to extract the lipids: Hara and Radin, Folch, Chen and Bligh and Dyer. They were performed under MWH and conventional heating using water bath heating. Lipid yield for Tetraselmis sp. and Nannochloropsis sp. was highest when Hara and Radin (8.19%), and Folch (8.47%) methods were used respectively under MWH. Nannochloropsis sp. wet biomass was transesterified under MWH in the presence of methanol and various alkali and acid catalyst using two different types of DT; one step and two step transesterification. The biodiesel yield obtained from the microwave direct transesterification (MWDT) was compared with that obtained using conventional method (lipid extraction followed by transesterification) and water bath heating DT method. Findings revealed that MWDT efficiency was higher compared to water bath heating DT by at least 14.34% and can achieve a maximum of 43.37% with proper selection of catalysts. The use of combined catalyst (NaOH and H2S04) increased the yield obtained by 2.3 folds (water bath heating DT) and 2.87 folds (MWDT) compared with the one step single alkaline catalyst. Maximum yield was obtained using SCMH when the microwave was set at 50 °C, 800 W, 16 h of reaction with simultaneous cooling at 15°C. When the one step transester ification was performed in SCMH at optimum setting, the biodiesel yield was more than 3.75 folds than conventional method. Gas chromatography analysis depicted that the biodiesel produced from SCMW had shorter carbon chain fatty acid methyl esters « 19 C) and good cetane number and iodine value indicating good ignition and lubricating properties. Thus it was proven that the use of LED, MWH and SCMW can improve microalgae biodiesel yields.