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The use of water in hydrothermal and supercritical conditions as a medium to upgrade heavy oil fractions has shown promising results and presents an interesting alternative for heavy oil processing. Water at these conditions improves transport properties increasing the solubility in the medium and reducing the viscosity of the oil, which facilitates the upgrading process. This review focuses on the use of water as a medium to recover and upgrade heavy oils. An analysis of the main reactions occurring, effect of process conditions, and role of water in the reaction mechanism is carried out based on experimental results found in the literature. Studies performed with model compounds that have enabled a proper understanding of the reaction mechanisms, kinetics, and effect of process conditions in the upgrading of heavy oil in near critical or supercritical water are included. An overview of the main challenges of the technology such as corrosion and salt deposition as well as some innovative reactor designs to solve them is provided. Information regarding research carried out in this field has been linked to the growing industrial interest in the technology showing recent developments and registration of patents on reactor designs and processes involving heavy oil upgrading in near and supercritical water.

This work analyses the influence of the temperature (310-450 ºC), pressure (200-260 bar), catalyst/bio-oil mass ratio (0-0.25 g catalyst/g bio-oil), and reaction time (0-60 min) on the reforming in sub-and supercritical water of bio-oil obtained from the fast pyrolysis of pinewood. The upgrading experiments were carried out in a batch microbomb reactor employing a co-precipitated Ni-Co/Al-Mg catalyst. This reforming process turned out to be highly customisable for the valorisation of bio-oil for the production of either gaseous or liquid bio-fuels. Depending on the operating conditions and water regime (sub/supercritical), the yields to upgraded bio-oil (liquid), gas and solid varied as follows: 5-90%, 7-91% and 3-31%, respectively. The gas phase, having a LHV ranging from 2 to 17 MJ/m 3 STP, was made up of a mixture of H 2 (9-31 vol.%), CO 2 (41-84 vol.%), CO (1-22 vol.%) and CH 4 (1-45 vol.%). The greatest H 2 production from bio-oil (76% gas yield with a relative amount of H 2 of 30 vol.%) was achieved under supercritical conditions at a temperature of 339 ºC, 200 bar of pressure and using a catalyst/bio-oil ratio of 0.2 g/g for 60 minutes. The amount of C, H and O (wt.%) in the upgraded bio-oil varied from 48 to 74, 4 to 9 and 13 to 48, respectively. This represents an increase of up to 37% and 171% in the proportions of C and H, respectively, as well as a decrease of up to 69% in the proportion of O. The HHV of the treated bio-oil shifted from 20 to 35 MJ/kg, which corresponds to an increase of up to 89% with respect to the HHV of the original bio-oil. With a temperature of around 344 ºC, a pressure of 233 bar, a catalyst/bio-oil ratio of 0.16 g/g and a reaction time of 9 minutes a compromise was reached between the yield and the quality of the upgraded liquid, enabling the transformation of 62% of the bio-oil into liquid with a HHV (29 MJ/kg) about twice as high as that of the original feedstock (17 MJ/kg).

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This work addresses the co-valorisation in supercritical water of bio-oil obtained from the fast pyrolysis of wood and crude glycerol yielded as a by-product during biodiesel production. The experiments were conducted at 380 ºC and 230 bar for 30 minutes with a Ni-Co/Al-Mg catalyst, analysing the effects on the process of the catalyst loading (0-0.25 g catalyst/g organics) and feed composition (each material alone and all possible binary mixtures). The yields to gas, upgraded bio-oil (liquid) and solid varied as follows: 4-87%, 0-46% and 0-18%, respectively. A synergistic interaction between crude glycerol and bio-oil took place during the upgrading process, which allowed the complete and simultaneous transformation of both materials into gas and liquid bio-fuels with a negligible solid formation. The compositions of the gas and the upgraded liquid can be easy tailored by adjusting the catalyst amount and the composition of the feed. The gas phase was made up of H 2 (7-49 vol.%), CO 2 (31-56 vol.%), CO (0-7 vol.%) and CH 4 (6-57 vol.%) and had a Lower Heating Value (LHV) ranging from 8 to 22 MJ/m 3 STP. The upgraded bio-oil consisted of a mixture of carboxylic acids (0-73%), furans (0-7%), phenols (0-85%), ketones (0-22%) and cyclic compounds (0-53%). The proportions of C, H and O in the liquid shifted between 66-77 wt.%, 7-11 wt.% and 15-25 wt.%, respectively, while its Higher Heating Value (HHV) ranged from 29 to 34 MJ/kg. An optimumfor the simultaneous production of gas and liquid bio-fuels was achieved with a solution having equal amounts of each material and employing a catalyst amount of 0.25 g catalyst/g organics. Under such conditions, 37% of the bio-oil was transformed into an upgraded liquid having a HHV (32 MJ/kg) two times higher than the original material (16 MJ/kg) with a negligible solid formation; the rest of the biooil and all the crude glycerol being converted into a rich CH 4 (55 vol.%) biogas with a high LHV (21 MJ/m 3 STP). This represents a step-change in future energy production and can help to establish the basis for a more efficient and sustainable biomass valorisation.

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This work addresses the preparation, characterisation and screening of different Ni-Co catalysts supported on carbon nanofibres (CNFs) for use in the upgrading of bio-oil in supercritical water. The aim is to improve the physicochemical properties of bio-oil so that it can be used as a fuel. The CNFs were firstly oxidised in HNO 3 and afterwards subjected to a thermal treatment to selectively modify their surface chemistry prior to the incorporation of the metal active phase (Ni-Co). The CNFs and the supported catalysts were thoroughly characterised by several techniques, which allowed a relationship to be established between the catalyst properties and the upgrading results.The use of Ni-Co/CNFs for bio-oil upgrading in supercritical water (SCW) significantly improved the properties of the original feedstock. In addition, the thermal treatment to which the fibres were subjected exerted a significant influence on their catalyticproperties. An increase in the severity of the thermal treatment led to a substantial reduction in the oxygen content of the CNFs, mainly due to the removal of the less 2 stable oxygen surface groups, which allowed their surface polarity to decrease. This decrease resulted in less formation of solid products. However, it also reduced the H/C and increased the O/C ratios of the upgraded liquid. Therefore, a compromise between the yield and the properties of the upgraded bio-oil was achieved with a Ni-Co supported on a CNF with a moderate amount of oxygen surface groups.