Sequential of acid, alkaline and torrefaction of wood pre-treatments to improve bio oil quality through fast pyrolysis.

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Theses / Dissertations
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Chemical Engineering
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Doctor of Philosophy
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Mohd Safaai, Nor Sharliza

Alternative and renewable energy sources have been explored and investigated over the world to meet the sustained increase in energy demand of and to abate the environmental problems due to heavy reliance on fossil fuels. Woody biomass has abundant resources and is renewable, therefore attracting great interest in the production of energy and fuels. In New Zealand, radiata pine is a dominant species in plantation forests, and residues from forest harvesting and wood processing have a strong potential for energy conversion because it is readily available at relatively low costs. Fast pyrolysis offers a promising way to turn the woody biomass into a liquid product called bio-oil at high yield, together with non-condensable gases (NCGs) and solid char as byproducts. Bio-oil has an extremely complex composition, consisting of hundreds of organic oxygenate compounds as well as water. The qualities of the bio-oil degrade with time during storage. In order to improve the bio-oil quality, bio-oil upgrading has been investigated, however, this normally operates at high temperatures and high pressure thus is a costly operation. Biomass pretreatments have also been reported in which some undesirable components in the biomass are removed before the pyrolysis thus better quality of bio-oil can be produced in the pyrolysis.

This study aims to investigate the effects of various biomass pretreatments on bio-oil yield and composition from the subsequent pyrolysis. The biomass pretreatments include solely torrefaction and chemical pretreatments, as well as their combinations. In the chemical pretreatments, sulfuric acid (0.5%, 1.0%, 1.5% and 2.0%) and sodium hydroxide (4%, 7%, 10% and 13%) were used, while torrefaction was performed at 220 and 250 oC. These pretreatments produced treated biomass samples identified as AC (acid), AL (alkaline), ACAL (acid followed by alkaline), ALAC (alkaline followed by acid), ACTO (acid followed by torrefaction), ALTO (alkaline followed by torrefaction), ACALTO (acid + alkaline + torrefactoion) and ALACTO (alkaline + acid + tporrefaction), representing the pretreatment methods. Based on the analyses, the sample treated with solely acid (AC) showed high volatile matter (80.2%), indicating high volatilization during the subsequent pyrolysis. Biomass treated with alkaline (AL) illustrated a slight reduction of volatile matter (76.8%), which would hinder the volatilization process in the subsequent pyrolysis. ACAL and ALAC samples had 76.6% and 85.7% volatile matters, respectively. With torrefaction pretreatment, the volatile matter was further reduced, ranging from 72.9% to 80.02%.

The reduction of inorganic elements in the biomass was also observed after the chemical pretreatments. Upon torrefaction as the last step, each studied sample portrayed a unique behaviour upon degradation, reflecting its unique elemental composition attained from earlier chemical pretreatment. Of all the samples including the torrefaction, the ACTO-250 sample showed more cracked fragments which could be linked to the effective inorganic removal and small molecular compounds by acid due to low lignin content. All treated samples showed changes in concentration of alkali metals and alkaline earth metals (AAEMs) recorded after analysis using ICP-OES unit. Morphological images from SEM recorded massive destruction after pretreatments that was obviously due to the elimination of lignin and hemicellulose during pretreatments.

Thermogravimetric analysis (TGA) tests of all samples were conducted to investigate the pyrolysis kinetics by determination of the activation energies (Ea) and pre-exponential factors (A) in various kinetic models, including Kissinger, Kissinger-Akahira-Sunose (KAS), Flynn- Wall-Ozawa (FWO), and simplified Distributed Activation Energy Model (DAEM) methods. The results show that the Ea values for all of the treated samples ranged between 170.9 and 270.9 kJ mol−1 for Kissinger model 170.1–262.3 kJ mol−1 for KAS model, 186.2–259.2 kJ mol−1 for FWO model and 169.3–266.3 kJ mol−1 for DAEM model, respectively. The mean Ea values of the pretreated samples increased in comparison with that of the Control sample. In general, the Ea values increased over the degree of conversion from 0.2 to 0.8 attributed to the crystallinity and carbonisation effects upon the pretreatments. The A values determined for all of the treated biomass samples varied over a broad magnitude ranging from 105 to 1026 s−1 for the adopted kinetic models.

Fast pyrolysis of the pretreated biomass samples and the Control sample were conducted in a lab-scale fluidized bed reactor at 400, 450 and 500oC for the production of bio-oil. The bio-oil was analysed for its chemical composition using GC-MS, NMR, FTIR and ICP-MS. Water content, acidity and viscosity were also determined using corresponding techniques. For the chemically-treated samples, the highest bio-oil yield was recorded from AC (39.72 wt%) at pyrolysis temperature of 450°C whereas the highest bio-oil yield from pyrolysis of pretreated samples including torrefaction was 34.05 % for the ACTO sample at the same pyrolysis temperature of 450°C. The bio-oil yields in these two cases were higher than that from pyrolysis of the Control sample (32.79±7.93%) at the same pyrolysis temperature.

This study revealed that both chemical pretreatment and torrefaction of biomass were required to overcome the drawbacks of bio-oil from raw biomass. Using acid as the medium of pretreatment is preferable than using alkaline due to the suppression of inorganic matters in biomass and subsequent bio-oil. Torrefaction lowers the cost of biomass grinding, which is essential in balancing the higher process costs for pretreating biomass. Therefore, the combinatorial approach of pretreatments is required to improve the quality of crude bio-oil, depending on the final usage.

By comparing to the Control bio-oils and other fuels, the results proved that less works are required to further upgrade the oils from ACTO and ALACTO pretreatments. Thus, ACTO and ALACTO bio-oils have the potential to be applied as transportation fuel with further upgrading process. Other applications are bio-adhesive (phenol), bio-based pesticides and biodegradable plastics.

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