The increasing awareness of environmental preservation, coupled with rising global energy demand, has shifted focus towards renewable energy sources [1]. This shift is driven by the volatility of fossil fuel prices and the imminent depletion of these finite resources, emphasizing the urgent need for reliable and sustainable energy alternatives [2]. Unlike conventional fuels, ideal fuel substitutes should not only minimize emissions of harmful gases but also represent qualities such as renewability, non-toxicity, and biodegradability ^[3][4][3,4]. Among many potential alternative energies, biofuels, wind, and solar power stand out the most [5]. In this developing energy scenario, biomass is emerging as an environmentally friendly energy alternative ^[5][6][5,6].

Efficiently converting biomass into useful, low-carbon-footprint chemicals and biofuels stands as an effective strategy to address challenges presented by climate change, the depletion of fossil fuels, and other environmental problems ^[7][8][7,8]

Among thermochemical conversion methods, fast pyrolysis stands out as a particularly promising approach to convert lignocellulosic biomass into liquid bio-crude oils [9]. In this method, biomass is heated to temperatures between 400 and 650 °C without the presence of oxygen, causing the material to decompose into liquid bio-crude oil, solid bio-char, and non-condensable gas [10]. While bio-crude oils are produced in an oxygen-free environment, they tend to retain higher oxygen levels due to the composition of the biomass feedstock, setting them apart from fossil fuels and pure hydrocarbons ^[9][10][9,10]. However, the current commercial viability of pyrolysis bio-crude oil is limited due to its less favorable chemical and physical attributes. These challenges encompass the need for preheating bio-crude oil, the potential for injector corrosion, ignition delays, clogging problems, and overall engine instability. To fully harness its potential, it is crucial to significantly reduce the oxygen content in bio-crude oils and carefully control the boiling point fractions, optimizing their ability to blend with or substitute fossil-based transportation fuels ^{[11][12][13][14][15][16]}[11,12,13,14,15,16].

To date, numerous bio-crude oil upgrading approaches have been explored, as outlined in recent review articles ^{[12][13][14][15]}[12,13,14,15]. The upgrading can be achieved using conventional technologies like esterification, hydrogenation, steam reforming, emulsification, and cracking, as well as emerging methods like non-thermal plasma reactions. Among these, hydrodeoxygenation (HDO) is recognized as a practical approach to oxygen removal from bio-crude oil, leading to increased organic phase yield, improved deoxygenation, and reduced catalyst deactivation ^{[16][17][18][19][20][21]}[16,17,18,19,20,21].

2. Properties of Bio-Crude Oil

After pyrolysis, the common oxygenated compounds found in bio-crude oil include acids, aldehydes, alcohols, ketones, esters, phenolics (20–30 wt%) derived from lignin, sugars, furans, and high molecular weight species like oligomers derived from lignin, which are all present in significant amounts but in low concentrations ^[22][26], are summarized in Table 1. Biomass, by definition, encompasses biologically derived and biodegradable materials, excluding peat and petrified substances. Common feedstocks, such as wood, bagasse, rice straw, switchgrass, and wheat straw, are often used in the production of pyrolysis oil ^[23][27]. Among the various techniques, fast pyrolysis stands out as one of the most widely adopted technologies. In this process, biomass is subjected to moderate temperatures (around 500 °C) and short residence times (typically seconds) in an oxygen-deprived environment ^[24][28]. This results in the rapid depolymerization of biomass, producing pyrolysis vapor, which, upon cooling, condenses into a liquid known as “bio-oil”. This bio-crude oil, a dark brown viscous liquid, comprises a mixture of light hydrocarbons and oxygenated compounds, and remarkably, it contains a high liquid content, constituting up to 75 wt%, while generating gas and char as by-products, albeit in lower yields of about 12 wt% and 13 wt%, respectively ^[25][29]. These oxygenated compounds encompass acids, aldehydes, alcohols, ketones, and esters; although they are typically present in relatively low concentrations. Moreover, a significant portion, accounting for about 20 to 30 wt%, is composed of phenolic compounds derived from lignin, including substances like phenol, catechol, anisole, and guaiacol, and also includes sugars, furans, and high molecular weight species like oligomers originating from lignin ^[9][26][9,30]. Oligomers are primarily found in bio-crude oil due to the depolymerization process, which entails the breaking of C–O and C–C bonds. This process is followed by subsequent dehydration and repolymerization reactions involving the primary biomass polymers ^[27][31]. Most of the typical compounds are summarized in Table 1. However, several factors influence the composition and production of bio-crude oil, including the type of feedstock used, moisture content, particle size, the configuration of the pyrolysis reactor, and various pyrolysis conditions like temperature, time, heat transfer rate, condensation efficiency, and char removal efficiency [9]. For instance, lignocellulosic biomass often contains a wide range of alkali and alkaline earth metal compounds such as potassium (K) and sodium (Na), magnesium (Mg), and calcium (Ca). These elements contribute to various characteristics and can promote the production of acetic acid and water. For example, potassium can catalyze the decomposition of liquid compounds by promoting fragmentation over depolymerization reactions, resulting in higher yields of gas, char, and reaction water ^[28][29][32,33]. However, the characteristics of bio-crude oil, including its high viscosity, acidity, and oxygen and water content, result in adverse consequences when used, such as corrosion of metal components, short shelf-life, and diminished heating value ^[30][31][34,35]. Bio-crude oil exhibits substantial differences from crude oil in several critical aspects. Notably, it contains a water content ranging from 15 to 30 wt%, produced from both dehydration reactions occurring during pyrolysis and the moisture present in the initial biomass feedstock. Traditional distillation techniques are often used to eliminate this water content, but they prove ineffective in reducing the elevated water content of bio-crude oil. During distillation, the application of heat triggers rapid polymerization, resulting in the formation of residual solids and unwanted coke, typically in the range of 30–50 wt% ^[26][30]. The high oxygen content of around 30–55 wt% notably reduces its heating value of around 16–19 MJ/kg, resulting in an energy density that is less than half that of crude oil of around 40 MJ/kg. Bio-crude oil tends to have a low pH between 2–4, primarily due to the presence of acetic acid and formic acid, which can pose challenges for processing, transportation, and storage equipment. Furthermore, the presence of phenols, olefins, and acids can easily lead to the formation of macromolecules through polymerization, increasing the viscosity and density of biocrude oil and diminishing its fluidity of around 40–100 cP and density 1200 kg/m^{3 [9][29][31][32][33]}[9,33,35,36,37]. Some of the characteristics of heavy petroleum fuel oils and bio-crude oils made through fast pyrolysis are listed in Table 2. Therefore, it is essential to improve the standard of bio-crude oils by minimizing or eliminating these negative characteristics prior to its practical application.