Catalytic Production of Furfural by the Subcritical Hydrothermal Gasification of Flax Straw

Abstract

Developing new sources of energy that can mitigate greenhouse gas (GHG) emissions has generated a strong research interest in the past two decades. Renewable sources of energy have become strong candidates for replacing the conventional resources in order to ameliorate the high level of pollution caused by the use of conventional fossil fuels. Biomass is a type of renewable resource that is considered to be carbon neutral when used in producing fuels and chemicals. Flax straw is an example of biomass that accumulates in Canada in high amounts. It is difficult to dispose of because it does not decompose easily as a result of its tough fibrous nature. However, it can be used through a hydrothermal gasification process to produce gaseous fuels as well as some important liquid products. Hydrothermal gasification process was used in this research because it can deal with wet biomass without the necessity of the drying step. Furfural is an important chemical that has many industrial applications, and as such, was considered to be the major desired product through the hydrothermal gasification of flax straw using a solid acid catalyst. This study focused on the catalytic subcritical hydrothermal gasification of flax straw. The study was performed using a 600 mL autoclave batch reactor using flax straw with a fixed weight (10 g) in all the experimental runs. Three types of solid acid catalysts were explored in this study: γ–alumina, H-ZSM-5, and silica-alumina. Experimental parameters such as temperature (200-325 oC), pressure (0-60 bar), residence time (0-120 min) and weight of solid acid catalysts (0.5-1.5 g) were varied in order to obtain the optimum conditions and to select the best catalyst for producing furfural. The yields of both gas and phenol were also monitored in the study. The yield of gas was quantified using an online gas chromatograph (GC). The gas products included hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2) and trace amounts of methane (CH4) and ethane (C2H6). The yields of furfural and phenol were measured by gas chromatograph/mass spectrometer (GC/MS). The results showed that the production of furfural was affected by all the experimental parameters (temperature, pressure, residence time and weight of the solid acid catalysts). The highest yield of furfural was obtained using γ-alumina with 0.1 g as the optimum weight of catalyst per g of flax straw. The ranking of the three catalysts based on furfural production was: γ–alumina > HZSM- 5 > silica-alumina. This had a direct correlation with the ratio of Lewis to BrÏ•nsted acid sites which decreased similar to the ranking of the performance of the catalysts. A kinetic study of the catalytic subcritical hydrothermal gasification of flax straw using 1 g of γ–alumina was also performed. Kinetic data were obtained using 10 g of flax straw, autogenous pressure, temperatures in the range of 225-325 oC, and residence time in the range of 0-120 min. The data were analysed using an empirical power law rate model. The carbon conversion was calculated using the ultimate analysis, which gave the highest conversion of 66% at 325 °C compared to the conversion of 40% obtained for a previous non-catalytic study. The final kinetic model was: -rA = ð€¯—ð€¯‘ð€®º ð€¯—ð€¯§ = 7.038 * 10-2 e -9463.5/R*T (1 – XA)2. The activation energy achieved in this study was lower than the activation energy of 27,969.6 J/mol obtained by the non-catalytic study thus showing the importance of the catalyst in lowering the energy barrier. The predicted rates from the model showed good agreement with the experimental rates with an average absolute deviation of 8.6%.