Torrefaction pretreatment of straw and gasification reactivity of the solid product

 Torrefaction is a main pretreatment technology for improving the properties of rice straw in order to deal with such problems as dispersed resource, high moisture content, high bulk volume and low heating value. The straw after torrefaction was gasified in an entrained flow gasifier because of high heating value and good grindability. Straw samples were torrefied in a fixed bed reactor at 200℃, 250℃ and 300℃ for 30min under nitrogen atmosphere. The non-isothermal gasification was carried out in a thermogravimetry analyzer (TGA) with heating rate of 20℃/min and final temperature of 1200℃. The results show that the products are mainly solid product and gases, as well as a mount of condensable liquid (water and tar). The ratio of CO₂ in gases is up to 80% remaining a little CO and CH₄. The total weight of solid product decreases with increasing temperature of torrefaction, accordingly that of gas products increases; the liquid products changed little at different temperatures. The energy efficiency of torrefaction is about 40% to 60% and decreases sharply then drops slowly with increasing reaction temperature. The grindability of solid product is improved by torrefaction and easy to be pulverized for utilization in an entrained flow gasifier. The results of TGA indicate the biomass sample pretreated at higher torrefaction temperature has higher gasification reactivity. The mechanism of reaction between torrefied straw and carbon dioxide could be simulated by a two dimension distribution model (2D). The apparent activation energy calculated by Coats-Redfern method is about 73kJ/mol~88kJ/mol.