Abstract
This review article summarizes the key published research on the topic of bio-oil upgrading using catalytic and non-catalytic supercritical fluid (SCF) conditions. The precious metal catalysts Pd, Ru and Pt on various supports are frequently chosen for catalytic bio-oil upgrading in SCFs. This is reportedly due to their favourable catalytic activity during the process including hydrotreating, hydrocracking, and esterification, which leads to improvements in liquid yield, heating value, and pH of the upgraded bio-oil. Due to the costs associated with precious metal catalysts, some researchers have opted for non-precious metal catalysts such as acidic HZSM-5 which can promote esterification in supercritical ethanol. On the other hand, SCFs have been effectively used to upgrade crude bio-oil without a catalyst. Supercritical methanol, ethanol, and water are most commonly used and demonstrate catalyst like activities such as facilitating esterification reactions and reducing solid yield by alcoholysis and hydrolysis, respectively.
Highlights
In recent years, the world has been facing an energy crisis due to a combination of several factors including the depletion of fossil fuels, accelerated population growth, increase in global energy demand, and crude oil price fluctuations
Supercritical methanol functioned as a hydrogen donor and promoted the HDO of unsaturated compounds during the upgrading process. In another bio-oil upgrading study by Cheng et al, the results showed a significant reduction in acids in the product oil, the authors predicted the acidic compounds were converted to esters through esterification reactions with alcohol in the supercritical methanol [34]
supercritical fluid (SCF) are recognised for the unique dissolving power which is highly effective during bio-oil upgrading for increasing yield and improving the characteristics of the bio-oil
Summary
The world has been facing an energy crisis due to a combination of several factors including the depletion of fossil fuels, accelerated population growth, increase in global energy demand, and crude oil price fluctuations. The extensive use of fossil fuels has led to climate change and global warming These global issues have motivated research into alternative energy technologies, renewable resources, and more sustainable techniques for energy generation. The properties of crude bio-oil such as low heating value, high moisture, and oxygen content lead to unfavourable effects during application including poor stability and immiscibility with hydrocarbons. This inhibits the direct use of crude bio-oil and its assimilation into existing fuel infrastructures.
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