**7. Conclusions**

Biomass is a sustainable energy source and a promising eco-friendly alternative source of renewable bioenergy. The most abundant biomass, lignocellulose, represents a significant carbon source for bioenergy. Lignocellulosic biomass is a complex mixture of biopolymers. The three major biopolymers are cellulose, hemicellulose, and lignin. Inorganic compounds and organic extractives are among other compounds present in lignocellulosic biomass. The selection of the process to convert biomass to end products depends on several factors, but the desired form of end products and available biomass feedstock are the two key factors usually considered. Thermochemical conversion processes usually offer many advantages over biochemical conversion processes.

Thermochemical conversions convert biomass into liquid, gaseous and solid products. The product distribution depends on the conversion process employed, that is, operating conditions (heating rate, residence time, and temperature). More liquids are produced at moderate temperatures and short residence times. Bio-oil is the desired product in fast pyrolysis and bio-oil yield is maximized at high heating rates, short vapor residence times, and around 500°C. A finely ground dry biomass feed is essential for high heat transfer rates. Due to the higher cellulose and hemicellulose content, woody biomass produces the highest bio-oil yield.

High moisture content is a major barrier in biomass processing for bioenergy. It has a significant impact on the biomass conversion process. High moisture biomass requires a large amount of energy to evaporate moisture to make it suitable for pyrolysis. Hydrothermal processing is useful for biomass feedstocks with high moisture as it does not require drying, thereby reducing energy costs. Hydrothermal processing has been given more attention in recent years and can be classified into HTC, HTL, and HTG based on the reaction temperature, pressure, and residence time. HTC, HTL, and HTG are aimed to maximize the production of solid (hydrochar), liquid (bio-oil/water), and gas (non-condensable) fractions, respectively. More research is required on hydrothermal processing to investigate reaction kinetics and chemistry, heat transfer, energy and heat recovery, combinations with other technologies, such as solar, technical, and economic aspects and the effect of operational parameters.
