| Straw is an important raw material to produce biofuels,which not only can be converted into methane by anaerobic fermentation but also can be converted into densified biomass fuel(DBF) by densification. Due to the inherent characteristics of straw,the primary technical barrier for anaerobic fermentation of straw is its recalcitrance structure,one of the major problems associated with the DBF production from straw is rapid wear of the briquetting unit. Aiming at these problems, we proposed a process of DBF production from straw modified by anaerobic fermentation. In this process,straw is firstly anaerobically digested to produce biogas, and then the solid digestate is pressed into DBF. Biodegradation of anaerobic microorganisms make straw easy to densify,which reduce briquetting pressure and temperature and improve mechanical durability of DBF, thus reducing power consumption of briquetting and prolonging the life of the briquetting unit. Around this purpose, this paper focuses on the mechanism of improving straw briquetting properties by anaerobic fermentation. In addition,the process for DBF production with straw modified by anaerobic fermentation was studied preliminarily.Research on the mechanism of improving straw briquetting properties by anaerobic fermentation. First, in order to study the change rule of the physicochemical properties of straw in anaerobic fermentation,experiments of anaerobic fermentation were performed at different fermentation periods.Second, briquetting experiments were carried out with digestate from anaerobic fermentation, briquetting properties and structure of DBF were analyzed systematically,then combining with the change rule of physicochemical properties of corn straw, the mechanism of improving straw briquetting properties by anaerobic fermentation was proposed. Finally,through by the experiments of mechanism verification were carried out. Lastly is coming to forming a systemic full understanding on the mechanism of improving straw briquetting properties by anaerobic fermentation.Process for DBF production with straw modified by anaerobic fermentation. For the problem of the water constraints on briquetting,on the one hand, anaerobic dry fermentation experiments were performed at different total solid (TS) and TS to inoculum ratio (S:I), the digestate was briquetted alone after dewatering. on the other hand, fermentation straw was briquetted after being mixed with corn straw at different mass ratio.Finally,the reasonable technology process was put forward.Through the above study,the paper draws the following conclusions.Briquetting properties of straw for DBF can be improved by anaerobic fermentation.With the compression pressure of6MPa, retention time of5s and preheating temperature of25℃, the density and strength of DBF produced with corn straw obtained from anaerobic fermentation for20days were respectively increased by10.57%and22.73%compared to the DBF produced with corn straw. When the preheating temperature is raised to120℃, the density and strength were respectively increased by4.35%and48.17%compared to DBF produced with corn straw.In the process of anaerobic fermentation, the degradation of the biodegradable components of the straw by the anaerobic microorganisms and the reproduction of the microorganism, increased the content of the cohesive components in the straw and changed the deformation properties of straw fiber significantly,thus improving straw briquetting properties.Unfermented straw mixed with fermented straw for DBF production can reduce the adverse effects of moisture in briquetting, the appropriate content of modified straw is30%-60%,and the preheating temperature at about100℃should be avoided.Modification by anaerobic dry fermentation can reduce the dewatering requirements of digestate. However, dry fermentation needs to increase inoculum ratio, thus increasing significantly the ash content of raw material for DBF production and improving slagging tendency of DBF. Unfermented straw mixed with the fermented straw for DBF production is a good choice. |
