Thermochemical conversion of microalgae for biofuel production
2013-02
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Thermochemical conversion of microalgae for biofuel production
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2013-02
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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,
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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
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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.
Description
University of Minnesota Ph.D. dissertation. Ph.D. January 2013. Major: Bioproducts/Biosystems Science Engineering and Management. Advisor: Dr. Roger Ruan. 1 computer file (PDF); x, 103 pages, appendices A-B.
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Du, Zhenyi. (2013). Thermochemical conversion of microalgae for biofuel production. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/144467.
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