Hydrothermal Gasification of Flax Straw in Subcritical Water

Abstract

There is substantial interest in the application of biomass as a source for renewable fuels and chemicals. When processed for energy purposes, biomass has the benefit of being either CO2 neutral or a sink. Flax straw, which is rich in lignin content, decays slowly, making it difficult to incorporate it into the soil. Flax straw can be gasified to produce fuel gases or liquid fuels. However, flax straw may contain a higher percentage of water, causing high drying cost if a conventional gas-phase conversion process is employed. This can be avoided if gasification in subcritical or supercritical water is employed. Subcritical water gasification provides a nice alternative to produce chemical intermediates from biomass due to the unique properties of water in that regime. In this study, a 600 mL high pressure autoclave was used to study the effect of reaction temperature, pressure, flax straw concentration, and retention time on the product distribution and conversion of flax straw under subcritical hydrothermal gasification. The objective was to optimize a plausible process of biomass gasification under subcritical water conditions. The gaseous products, analysed by online GC, were CO2, CO, H2, traces of CH4, and C2H6. The liquid products analysed using gas chromatography/mass spectrometry (GC/MS) were furfural, phenol, acetic acid, and formic acid. The results show that the product yields are sensitive to the reaction parameters such as temperature, pressure, and retention time. Furfural, a key compound in the liquid phase, was chosen for its industrial application as a product of interest, and its yield was optimized by using two different kinds of acid catalysts (H-ZSM-5 and HCl). Both catalysts showed good catalytic activity and selectivity towards furfural. A kinetic study for flax straw degradation was also performed at three designated temperatures (225, 275, and 325ºC) using the carbon conversion results obtained from the ultimate analysis. A highest carbon conversion of about 40% was achieved at 325ºC. The experimental results were used to derive an empirical rate model. The model was of the form ; activation energy (Ea) was found to be 27,969.6 J/mol, and the order of the reaction (n) was 2. Estimation of the values of the model parameters was based on the minimization of the sum of the residual squares of the reaction rates using the Gauss-Newton and Levenberg-Marquardt algorithm with non-linear regression (NLREG) software. Good agreement between the experimental and predicted rate was obtained with an absolute average deviation (AAD) of 8.4%.