Effects of torrefaction pretreatment on fuel quality and combustion characteristics of biomass: A review

Biomass is renewable and clean energy source, the utilization of biofuel derived from biomass is an important strategy to reduce CO2 emissions and mitigate the greenhouse effect during power generation process. Biomass poses obstacles to its wide utilization in industry because of the self-characteristics such as low calorific value, high water content, poor grindability and easy corrosion decomposition. In the modern biomass conversion technologies, torrefaction pretreatment technology has attracted much attention. Torrefaction technology can generally be divided into dry torrefaction and wet torrefaction. Meanwhile, the dry torrefaction can be divided into inert torrefaction and oxidative torrefaction. From the perspective of economy and practicality, oxidative torrefaction has more prospects and advantages, such as high efficiency, low energy consumption, short pretreatment duration, et al. Based on the existing researches, a review was conducted on the field of biomass torrefaction pretreatment. Firstly, the detailed effects of torrefaction temperature, atmosphere, pressure and duration on the compositional and elemental variations of biomass were explored. Moreover, the efficiency analysis of inert torrefaction, oxidative torrefaction and flue gas torrefaction for different type biomass were also summarized and reviewed systematically. In this paper, the fuel quality and emission characteristics of torrefied biomass during thermal conversion process were also discussed respectively. Thence, the challenges and prospects of biomass torrefaction technology for the future development were also analyzed. Finally, future directions and technological challenges associated with the torrefaction process are proposed and its application potential is also evaluated.