Thermochemical conversion of lignocellulosic biomass into biofuels and petrochemicals

Abstract

The depletion of conventional fossil fuel reserves, including oil, gas, and coal, has intensified concerns over environmental sustainability and energy security. Consequently, there has been a substantial shift towards exploring alternative energy technologies and developing sustainable products and processes. Biomass has gained significant traction as a renewable feedstock of interest in recent decades. Bio-oil derived from biomass holds promise for diverse energy production, chemical synthesis, and potential energy carrier applications. However, crude bio-oil exhibits inherent challenges stemming from its physical and chemical properties that preclude its direct integration into existing fuel infrastructures. Notably, the high acidity, low viscosity, high density, elevated oxygen content, substantial moisture levels, low heating value, complex molecular composition, and instability. These drawbacks can lead to issues such as corrosion, coking during upgrading, and difficulties in storage and transport. Addressing these challenges require advanced refining and treatment techniques to enhance the bio-oil's compatibility and usability within established industrial frameworks. Hence, studies that aimed to improve the properties of the bio-oil organic phase were conducted. Initially, the research embarked on catalytic and non-catalytic fast pyrolysis of Giant Reed using a zeolite as a catalyst. The results indicated that the presence of the HZSM-5 catalyst significantly improved the quality of the pyrolysis oil. Catalytic pyrolysis consistently yielded bio-oil with lower moisture content and higher carbon content than non-catalytic pyrolysis. At 550°C and 10 °C/min, the HHV of the catalytic pyrolysis product was 23.0 MJ/kg compared to 21.3 MJ/kg for non-catalytic pyrolysis. Applying the HZSM-5 catalyst at 650°C and 50 °C/min significantly enhanced the production of aromatic hydrocarbons and phenolic compounds while reducing the presence of undesirable oxygenates in the bio-oil organic phase. However, this observation was a trade-off to lower bio-oil yields and high energy consumption due to a high operating temperature. As a result, 550°C and 10 °C/min was considered as the operating condition for bio-oil organic phase production. Another sub-study focused on investigating the effect of periodic variations on the fuel properties of Giant Reed and assessing its influence on pyrolysis product yield, quality, and distribution. This study was carried out after the incineration of giant reed along the river banks in Ladysmith, RSA, with the aim of minimizing artificial flooding occurrence as a result of stormwater drainage blockages. Four periodic variations, late spring (HS-4), late summer (HS1), late autumn (HS-2), and late winter (HS-3), were considered to investigate the effect of characterization, and bio-oil organic phase (BOP) fuel properties. The considered biomasses herein had average calorific values of 18.86 ± 0.05, 19.73 ± 0.05, 19.23 ± 0.04, and 18.44 ± 0.04 MJ/kg during HS-1, HS-2, HS-3, and HS-4, respectively. The biomass, bio-oil organic phase, biochar, and pyrolysis gas were characterized using thermogravimetric analysis (TGA), gas chromatography-mass spectroscopy (GCMS), Fourier transform infrared spectroscopy (FTIR), micro-GC, and scanning electron microscopy (SEM/EDS). The organic phase of biooil was isolated using a 125 ml separating funnel, allowing natural stratification of the immiscible phases. BOP yield increased from 5 to 11 wt% during HS-4 and HS-3, respectively. The increase in the BOP yield correlated with the lignification effect significantly, as shown by the R2 value of 0.97. Higher heating values (HHV) of the BOP ranged from 19.4 ± 0.03 to 22.6 ± 0.02 MJ/kg in relation to the active growth stage and senescence-dormant phase. Physical and chemical properties (TAN, density, viscosity, water content, and CHNS) and chemical compound groups of organic phase bio-oil were analyzed. The produced BOP was rich in phenolics for all considered periods. The effect of harvest time showed that biomass and bio-oil organic phase fuel properties are improved during the senescence-dormant period. As a result, giant reed biomass should be harvested during autumn to avoid incineration that releases carbon dioxide into the atmosphere and will also reduce the occurrence of artificial flooding. Fast pyrolysis of biomass is crucial for sustainable biofuel production, necessitating thorough characterization of feedstocks to optimize thermal conversion technologies. The third study investigated the isothermal pyrolysis of bamboo and pinewood biomass in a sand-fluidized bed reactor to assess biomass suitability for commercial bio-oil production. The pyrolysis products and biomass species were characterized through proximate and ultimate analyses, GCMS, FTIR, SEM/EDX, and structural analysis, to assess their chemical and physical properties. Results indicated that pine bio-oil possesses superior energy density, with a higher calorific value (20.38 MJ/kg) than bamboo (18.70 MJ/kg). Pine biomass yielded greater organic phase bio-oil (BOP) at 13 wt%, while bamboo produced 9 wt%. Energy yields were also notable, with pine exhibiting an energy yield of 15% for bio-oil organic phase (EBOP), compared to 11% for bamboo. The fibrous nature of bamboo biomass resulted in less reacted biomass at constant reaction time due to flow resistance during pyrolysis. Pine bio-oil organic phase (PBOP) demonstrated a higher heating value (23.90 MJ/kg) than bamboo (B-BOP). The findings suggest that while both biomass types are viable renewable energy sources, pine biomass is more favourable for commercialization due to its superior energy properties and efficiency in pyrolysis. Conventional mild hydrotreatment processes of bio-oil present significant challenges of high degree of polymerization, low oil yield, high coke formation, and poor catalyst recovery. To address these challenges, the fourth study looked into investigating and enhancing the properties of raw bio-oil organic phase samples via a solvent-assisted stabilization approach using methanol (METH), ethanol (ETHA), isopropyl alcohol (IPA), and ethyl ether (ETH). Solvents like methanol (METH) and ethanol (ETH), which are highly polar, yielded higher oil fractions (64% and 62% respectively) compared to less polar solvents like ethyl ether (DME) at 59%. Isopropyl alcohol (IPA), with intermediate polarity, achieved a balanced oil yield of 63%, indicating its ability to dissolve both polar and non-polar components. The moisture reduction in stabilized bio-oils followed the order: IPA > ETH > METH > DME, with IPA showing the highest reduction due to its structural characteristics facilitating dehydration. Viscosity reduction varied with IPA > ETH > DME > METH. Carbon recovery in stabilized bio-oils ranged from 65% to 75% for DME, ETH, and METH and was 71% for IPA. The heating values of stabilized bio-oils ranged from 28 to 29 MJ/kg, with IPA-stabilized bio-oil showing the highest value (29.05 ± 0.06 MJ/kg). METH demonstrated high efficiency (74.8%) in stabilizing bio-oil, attributed to its strong hydrogen-donating capability. ETH followed closely at 69.5%, indicating its comparable performance in bio-oil stabilization. With moderate efficiency (69.3%), IPA presented a balanced alternative considering its molecular structure and hydrogen solubility. In contrast, DME exhibited lower efficiency (63.6%) due to its weaker hydrogenation capability and propensity for undesired side reactions. The study suggested that subcritical conditions up to 200°C are adequate for METH, ETH, and IPA in bio-oil stabilization, comparable to results obtained under supercritical conditions. harvest time on biomass fuel properties, pyrolysis product distribution, non-condensable gas