Recent developments and progress on thermal pyrolysis of biomass towards high quality bio-oil: Mechanisms, comparison of conventional and recent technological advances and the potential of using non-lignocellulosic biomass as feedstock

The constant depletion of fossil fuels and their associated negative consequences on the environment, accelerated the shift towards the use of biomass as a waste-based source of energy, through pyrolysis. The thermal breakdown of biomass through pyrolysis results in bio-oil, biogas and biochar as the main products. Bio-oil is a complex and unstable mixture, which needs to undergo through upgrading processes to become high-quality bio-oil with fuel-like characteristics and rich in useful chemicals such as phenolics. The current work discusses the pyrolysis of lignocellulosic and non-lignocellulosic biomass and their effect on quantity and quality of bio-oil, highlighting pyrolysis principles such as types and mechanisms. Additionally, different reactor types that are used in pyrolysis are discussed, where it was concluded that fluidised bed reactor (FBRs) and its subcategories as well as ablative reactors (ABRs) are more suitable to promote high bio-oil yields due to higher heating rates and reduced residence times. Moreover, the effect of operating conditions such as temperature, vapour residence time, biomass particle size and pressure is discussed. The major techniques of physical and chemical upgrading of bio-oil are also covered. Advanced pyrolysis technologies like microwave-assisted pyrolysis (MAP) and solar-powered pyrolysis (SPP) are discussed, highlighting their advantages to produce high-quality bio-oil and minimise the environmental impacts, compared to traditional pyrolysis systems, as well as their challenges regarding the capital cost and scalability. Finally, machine learning (ML) and AI tools, able to predict yield and properties of bio-oil are reported along with their challenges.