Wet torrefaction kinetics and heating value estimation for wet torrefied Japanese cedar and rice straw

Abstract

One effective measure to achieve carbon neutrality is to shift away from coal. Wet torrefied biomass (WTB) can be the promising solid biofuels to replace coal, when high moisture content biomass is used as feedstock. To utilize WTB for industrial use, WTB with predetermined energy properties such as higher heating value (HHV) and solid mass yield (SMY) has to be produced to meet the requirement in coal-fired power plants and industrial boilers. In this study, the analytical approach to estimate SMY and HHV of WTB for given wet torrefaction (WT) process parameters, temperature and residence time is investigated for Japanese cedar and rice straw. The Van Krevelen diagram shows that dehydration is considered as a main reaction of WTB, which is the same result as the previous studies on dry torrefied biomass (DTB). HHVs of WTB as well as DTB are well correlated with its SMY, and the experimental correlations of HHV are proposed as a function of SMY. The difference in HHV of rice straw between WTB and DTB and the difference in HHV between Japanese cedar and rice straw are closely related to the difference in ash content and the ash removal effect in WT process. The WT reaction based on volatile release is modeled as the single reaction with only one reaction rate constant of volatiles evolved. It is found that the solid-state reaction function codes F12 and D1 are the most suitable for Japanese cedar and rice straw, and kinetic parameters in WT process are determined. The estimation method of SMY and HHV is proposed based on the WT kinetic model and the HHV experimental correlation. It is found that the WT temperature and residence time to produce WTB with predetermined SMY and HHV can be provided by the proposed estimation method within an accuracy of about 10%.