Thermochemical conversion of microalgae for biofuel production .

摘要

Concerns about diminishing fossil fuels and increasing greenhouse gas emissions are driving many countries to develop renewable energy sources. In this respect, biomass may provide a carbon-neutral and sustainable solution. Microalgae have received growing interest recently because of their high productivity, high oil content and the ability to grow in a wide range of climates and lands. Pyrolysis is a thermochemical process in which biomass is thermally decomposed to a liquid product known as bio-oil. In this dissertation, pyrolysis and hydrothermal conversion techniques were applied to microalgae for biofuel production and an integrated algae-based biorefinery was proposed which includes algal biomass production, hydrothermal pretreatment, catalytic pyrolysis of microalgae into biofuels, and recycling of the wastewater from conversion as low-cost nutrient source for algae cultivation.;In Chapter 3, Microwavelow-assisted pyrolysis (MAP) of Chlorella sp. was carried out with char as microwave reception enhancer. The results indicated that the maximum bio-oil yield of 28.6% was achieved under the microwave power of 750 W. The bio-oil properties were characterized with elemental, gas chromatography-mass spectrometry (GC-MS), gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric (TG) analysis.;To further elucidate the pyrolysis mechanism of microalgae, the different roles of three major components (carbohydrates, proteins, and lipids) in microalgae were investigated on a pyroprobe. In Chapter 4, cellulose, egg whites, and canola oil were employed as the model compounds of the three components, respectively. Non-catalytic pyrolysis was used to identify and quantify some major products and several pyrolysis pathways of algal biomass were also postulated by analysis and identification of pyrolysis products from the model compounds.;Based on the preliminary catalytic pyrolysis results, a detailed catalyst screening study was carrier out to evaluate the performance of different zeolites for the production of aromatic hydrocarbons in Chapter 5. Three zeolites with different crystal structures (H-Y, H-Beta and H-ZSM5) were used to study the effect of catalyst type on the aromatic yield. All three catalysts significantly increased the aromatic yields from pyrolysis of microalgae and egg whites compared with non-catalytic runs, and H-ZSM5 was most effective with a yield of 18.13%.;Based on the results in Chapter 4 and Chapter 5, nitrogenates are very resistant to catalytic conversion and the aromatic hydrocarbon yield from proteins was the lowest among the three major components of microalgae. However, since nitrogen is an essential element for algal growth, recycling of this nutrient will be important to achieving sustainable algal feedstock production. Therefore, hydrothermal pretreatment (HP) was employed to reduce the nitrogen content in Nannochloropsis oculata feedstock by hydrolyzing proteins without requiring significant energy inputs. The effects of reaction conditions on the yield and composition of pretreated algae were investigated by varying the temperature (150--225 °C) and reaction time (10--60 min). Compared with untreated algae, pretreated samples had higher carbon contents and enhanced heating values under all reaction conditions and 6--42% lower nitrogen contents at 200 °C--225 °C for 30--60 min. The pyrolytic bio-oil from pretreated algae contained less nitrogen-containing compounds than that from untreated samples. (Abstract shortened by UMI.)