| Torrefaction is a commonly used biomass heat treatment method,which can effectively enhance the physical and chemical properties of biomass.In this paper,the rice straws of southern China were used to study the mechanism of torrefaction at different oxygen content.The effects of temperature and oxygen content on the yield and properties of torrefied products were also studied by using tubular furnace.In the end,the adsorption properties of torrefied products on heavy metal ions in water were studied,which provided a reliable experimental data and basic theory on making high energy density energy materials and high adsorption performance materials by oxygen-containing torrefaction.The experimental results showed that the amount of solid residue decreases with the increase of torrefaction temperature in the range of 230? to 310?,and it showed the same trend when the oxygen content is higher at a same temperature.The kinetic processes of rice straw torrefaction under six oxygen content(0%,3%,7%,11%,16%and 21%,respectively)were carried out.It was found that the first order kinetic model in the reaction series model could describe the process well,and the apparent activation energy decreased with increasing oxygen concentration(from 2.012 kJ/mol to 1.333 kJ/mol).Through torrefaction experiment with tubular furnace,it was found that when the temperature was less than 250?,the time for torrefaction to reach equilibrium increased with the increase of oxygen content in the atmosphere.While when the temperature was higher than 250?,the time increased with the oxygen content increases,and the higher the temperature,the less the time differences among different oxygen contents.At 230?,the heat value of 16%oxygen content was 16.94MJ/kg,and the heat value of 7%oxygen content was 17.21MJ/kg at 250?.When the temperature is higher than 290?,The calorific value of the product decreased with the increase of the oxygen content,and the differences of the calorific value of between products were also increased with oxygen content increase.Torrefaction could effectively reduce the H/C ratio and O/C ratio of rice straw.When the temperature was higher than 270? and the oxygen concentration was more than 7%,the H/C ratio and O/C ratio of the product decreased faster.270?,7%oxygen content may be the inflection point of the torrefaction reaction.When the conditions were greater than the conditions of this condition,there would be a severe oxidation and pyrolysis reaction in the rice straw,making the product quickly carbonized,and ash content increased rapidly.Therefore,the best conditions(the maximum calorific value of the product is 17.21MJ/kg)for the oxygen-containing torrefaction of rice straw is:270 ?,7%oxygen concentration and baking time is 60min.The scanning electron microscopy(SEM)and nitrogen adsorption analysis showed that the increase of temperature and oxygen participation could increase the degree of fineness of the torrefied product and promote the specific surface area and pore development of the product.The specific surface area of the product obtained in the nitrogen atmosphere at 310? and the 11%oxygen content atmosphere was 2.04 and 5.35 m2/g,respectively,and the specific surface area of the product obtained at 230? under a nitrogen atmosphere and 11%oxygen content was not detected to.The results show that the torrefied products can effectively adsorb Cu2+ and Pb2+,and the adsorption capacity is controlled by the torrefaction conditions.The higher temperature and the participation of oxygen are more favorable for the adsorption of heavy metal ions.The terrified biomass produced under the condition of 310? and 11%oxygen concentrate was activated and the adsorption process of the activated products was fitted by Langmuir isothermal adsorption model.It was found that the maximum adsorption capacities of Cu2+ and Pb2+ were 33.36 and 44.22mg/g,respectively,and 0.67 and 1.26 times the commercial activated carbon.Indicating that terrified biomass has the potential to be used as a commercial adsorbent. |
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