Thermochemical conversion of microalgae for biofuel production

Abstract

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, Microwave-assisted pyrolysis (MAP) of Chlorella sp. was carried out with char as microwave reception enhancer. The results indicated that the maximum biooil 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. The algal bio-oil had a density of 0.98 kg/L, a viscosity of 61.2 cSt, and a higher heating value (HHV) of 30.7 MJ/kg. The GC-MS results showed that the bio-oils were mainly composed of aliphatic hydrocarbons, aromatic hydrocarbons, phenols, long chain fatty acids and nitrogenated compounds, among which aliphatic and aromatic hydrocarbons (account for 22.18 % of the total GC-MS spectrum area) are highly desirable compounds as those in crude oil, iii gasoline and diesel. The results indicate that fast growing algae are a promising source of feedstock for advanced renewable fuels production via MAP. 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. Algal bio-oil contains oxygenates and nitrogenates which can lower the heating values and lead to NOx emissions, and thus upgrading processes towards reducing nitrogen and oxygen are necessary. Catalytic pyrolysis was then carried out with HZSM-5 for the production of aromatic hydrocarbons at different temperatures and catalyst to feed ratios. The aromatic yields of all feedstocks were significantly improved when the catalyst to biomass ratio increased from 1:1 to 5:1. Egg whites had the lowest aromatic yield among the model compounds under all reaction conditions, which suggests that proteins can hardly be converted to aromatics with HZSM-5. Lipids, although only accounted for 12.33% of Chlorella, contributed about 40% of aromatic production from algal biomass. 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%. Three H-ZSM5 with silica-to-alumina (Si/Al) ratios of 30, 80 and 280 were used to study the effect of Si/Al ratio on the aromatic yield. The maximum yield was achieved at the Si/Al ratio of 80, which provides moderate acidity to achieve high aromatic production and reduce coke formation simultaneously. Aromatic production increased with the incorporation of copper or gallium to HZSM-5. However, other metals studied either had no significant influence or led to a lower aromatic yield. 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. The bio-oil contained mainly (44.9% GCMS peak area) long-chain fatty acids (C14−C18) which can be more readily converted into hydrocarbon fuels in the presence of simple catalysts. Additionally, the feasibility of using recovered nutrients from HP for cultivation of microalga Chlorella vulgaris was v investigated. Different dilution multiples of 50, 100 and 200 were applied to the recycled process water from HP and algal growth was compared among these media and a standard growth medium BG-11. Algae achieved a biomass concentration of 0.79 g/L on 50× process water after 4 days. Algae removed total nitrogen, total phosphorus and chemical oxygen demand by 45.5-59.9%, 85.8-94.6% and 50.0-60.9%, respectively, on different diluted process waters. The fatty acid methyl ester yields for algae grown on the process water were 11.2% (50×), 11.2% (100×) and 9.7% (200×), which were significantly higher than 4.5% for BG-11 grown algae. In addition, algae cultivated on process water had 18.9% higher carbon and 7.8% lower nitrogen contents than those on BG-11, indicating that they are very suitable as biofuel feedstocks. In summary, HP is a low cost and efficient way to reduce the nitrogen content in microalgae without significant energy inputs. The recovered aqueous nutrients from HP can be recycled for algal cultivation. Pretreated microalgae were very hydrophobic with reduced nitrogen content and retained 73 to 99% lipids of the starting microalgae. These lipids can be easily converted into hydrocarbon fuels in the presence of simple catalysts, such as HZSM5 zeolite.