Production of gaseous and liquid bio-fuels from the upgrading of lignocellulosic bio-oil in sub- and supercritical water: Effect of operating conditions on the process

Abstract

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).

Bio-oil upgrading in supercritical water using Ni-Co catalysts supported on carbon nanofibres

“…An empirical correlation developed by Channiwala and Parikh [47] was used for estimating the HHV of the treated liquids due to the small amount of sample obtained in each experiment. This experimental correlation was validated experimentally [10]. The original bio-oil and the treated liquid were analysed by GC-MS [10].…”

“…Among these different upgrading technologies, the use of supercritical water (SCW) is emerging as a promising alternative for bio-oil valorisation [11], as it enables an upgraded liquid biofuel to be obtained [10]. However the works dealing with lignocellulosic bio-oil upgrading in supercritical water are extremely scarce and are focused on gas production rather than liquid bio-fuels.…”

“…Further information about the methodology and apparatus used for the characterisation of the bio-oil can be found in a previous communication [10]. An empirical correlation developed by Channiwala and Parikh [47] was used for estimating the HHV of the treated liquids due to the small amount of sample obtained in each experiment.…”

“…Specifically, Penninger and Rep [12] analysed the supercritical water reforming of bio-oil for H 2 production, while Onwudili and Williams [13] studied the supercritical water gasification for the production of CH 4 . Remón et al [10] investigated the use of water in sub-and supercritical state for bio-oil valorisation using a Ni catalyst supported on alumina prepared by coprecipitation (Ni-Co/Al-Mg). The effects of the water regime (sub-and supercritical) and the most important operating conditions on the process were reported.…”

“…However, some of the properties of these liquids (elevated viscosity, high O/C ratio, large H 2 O content and low pH) hider their direct use as liquid biofuels [9]. This scenario makes the upgrading of bio-oil an essential challenge in order to obtain liquid biofuels from biomass and several processes such as hydrotreating, hydrocracking and the use of supercritical water [4,7,8,10] have been developed and tested to achieve this goal.…”