GASIFICATION OF FRUCTOSE IN SUPERCRITICAL WATER FOR PRODUCTION OF HYDROGEN ENRICHED SYNGAS

摘要

The experimental impacts of climate change via increased greenhouse gas emissions and overwhelming usage of fossil fuels are global. Substantial amounts of waste food are obtained globally that, via landfill, contribute majorly to the production of greenhouse gases such as CO_2 and CH_4. Currently, waste food materials are tossed in landfill for composting or anaerobic digestion to produce CH_4. Fructose is a ketonic monosaccharide predominantly found in fruits, berries and vegetables. Supercritical water (temperature ＞ 374℃ and pressure ＞ 22.1 MPa) has found several applications in lignocellulosic biomass gasification for syngas production. With this objective of waste food conversion to biofuels, supercritical water gasification of fructose (as a model sugar compound for waste fruits/vegetables) was performed in this study. Different parameters influencing gasification of fructose were investigated that include temperature, feed concentration and residence time and catalyst concentration. The alkali-based homogenous catalysts, i.e. KOH and NaOH were employed in catalytic gasification of fructose for comparative evaluation of syngas yield and composition. Fructose as a model sugar compound of fruits/vegetables was used as the feedstock in supercritical gasification in a custom-built continuous-flow stainless steel tubular reactor. The gasification apparatus consisted of feed pump, preheater, tubular flow reactor, water-cooled tube, filter, back-pressure regulator, gas-liquid separator, pressure gauges and thermocouples. Supercritical water gasification was performed at 25 MPa to study the impacts of temperature (550-700℃), feed (fructose) concentration (4-10 wt%) and residence time (30-75 s). Homogenous catalysts such as KOH and NaOH at varying concentrations (0.2-0.8 wt%) were used to relatively examine their impacts on syngas yield and composition. The gases were analyzed in an Agilent 7820A gas chromatography with TCD detector including three packed columns and one capillary column. The effects of temperature, feed concentration and residence time were investigated for maximum gas yields for fructose gasification. Total gas yields, carbon gasification efficiency and maximum H_2 yields were recorded at the optimal temperature, feed concentration and residence time of 700℃, 4 wt% and 60 s, respectively. The total gas yield from fructose gasification was higher at 700℃ (1.04 L/g of fructose) compared to that at 550℃ (0.2 g/L of fructose). However, addition of alkali catalysts such as KOH and NaOH enriched the total gas yields to 2.01 L/g and 1.9 L/g, respectively. The H_2 yield at 700℃ (3.4 mol/mol of fructose) was higher than that at 550℃ (0.3 mol/mol of fructose). Lower feed concentration (4 wt% fructose) resulted in greater total gas yields of 1.04 L/g compared to that of 10 wt% fructose (0.8 L/g). Furthermore, 30 s of residence time resulted in lower total gas yields (0.4 L/g) compared to that at 60 s (1.05 L/g). With the increase in catalyst concentration from 0.2 to 0.8 wt%, there was an increase in total gas yields and concentrations of H_2, CO_2 and CH_4. Maximum H_2 yields of 10.7 and 9.9 mol/mol of fructose were obtained with the addition of 0.8 wt% of KOH and NaOH, respectively. The increase in H_2 yield by alkali catalyst addition was due to enhanced water-gas shift reaction. In the non-catalytic gasification of fructose, the lower heating value (LHV) of syngas was higher at 700℃ (2482 KJ/m~3) compared to that at 550℃ (1281 KJ/m~3). However, the LHV of syngas generated at 700℃, 4 wt% fructose with 0.8 wt% KOH was greater (3630 KJ/m~3) than that of 0.8 wt% NaOH (3576 KJ/m~3). Due to high carbohydrate content in waste fruits/vegetables, their gasification in supercritical water could potentially yield H_2-rich syngas and eliminate the cost of feedstock drying and pretreatment. KOH appears to be a promising catalyst in fructose gasification by enhancing the selectivity for H_2. Catalytic gasification has tremendous prospects to generate syngas from waste fruits and vegetables or discarded fruit-derived beverages. This process could supplement the increasing energy demand by producing H_2-rich syngas from waste fruits instead of emitting greenhouse gas CH_4 through their anaerobic digestion.