Xingping Kai

The Behavior of Alkali Metals During the Co-combustion of Straw and Coal

Biomass, especially herbaceous fuel, contains high amounts of volatile alkali metals and chlorine, which may cause problems, such as deposition, corrosion, and agglomeration during combustion. The aim of this work was to obtain more information on the influence of temperature on the transformation of alkali metal species. The combustion experiments with rice straw, coal, and additive FeS2 were performed at various temperatures between 600 and 1,100°C in a laboratory scale reactor. Thermodynamic equilibrium calculations were used to predict the behavior of alkali metal species and interpret the experimental results. The results indicate that the amounts of potassium and sodium in flue gas were quite little compared to those retained in the bottom ash. KCl(g) and NaCl(g), which were the major alkali metal species of flue gas, were independent on a temperature below 800°C. KOH(g) also played an important role in the 1,000–1,100°C interval. In the bottom ash, the fraction of potassium in sulfates decreased from 61.8% at 600°C to 0.5% at 1,100°C. Compared to potassium, most of vaporizable sodium was captured by (alumino-) silicates and retained in the bottom ash.

Sub-supercritical liquefaction of rice stalk for the production of bio-oil: effect of solvents.

The effect of solvents (water and ethanol) on liquefaction characteristics of rice stalk (RS) was investigated in an autoclave. The highest conversion and liquid yield in water and ethanol were 84.95 wt%, 72.62 wt% and 78.93wt%, 63.84 wt%, respectively. FTIR and GC-MS of the bio-oils obtained from subcritical water (SubH2O, 300°C) and supercritical ethanol (scEtOH, 300°C) indicated that the behavior of RS liquefaction depended on solvents used. The major components of bio-oil produced in SubH2O were ketones and phenols, while esters and phenols dominated in scEtOH. ICP-OES analysis showed that the concentrations of potassium (K) and sodium (Na) in the bio-oil obtained from scEtOH were 14-15 times higher than that obtained from SubH2O. Ethanol gave rise to an improvement in the bio-oil properties including water content, density, acidity and HHV. It was concluded that the bio-oil from RS can be effectively upgraded in scEtOH.

Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol.

This study investigated the influence of temperature and residence time on liquefaction of rice straw in subcritical CO2-subcritical water (subCO2-subH2O) and in subcritical CO2-supercritical ethanol (subCO2-scEtOH), considering the final reaction temperatures (270-345°C) and residence times (15 and 30min). Residence time was identified as a crucial parameter in the subCO2-subH2O liquefaction, whereas residence time had a marginal influence on subCO2-scEtOH liquefaction. When reaction conditions were 320°C and 15min, solvents have weak impact on the quality of bio-oil, HHV 28.57MJ/kg and 28.62MJ/kg, respectively. There was an obvious difference between the subCO2-subH2O and subCO2-scEtOH liquefaction mechanisms. In subCO2-subH2O, CO2 promoted the carbonyl reaction. In subCO2-scEtOH, supercritical ethanol have the function of donating hydrogen and promoting the reaction of hydroxyl-alkylation.