Effects of dry/wet torrefaction pretreatments on the combustion reaction characteristics of rice husk

Abstract: Torrefaction has been one of the most significant steps to improve the biomass-based fuel quality. Rice husk has been widely used as typical agricultural biomass, due to a large annual yield in China. It is of great significance to explore the effect of dry/wet torrefaction on biomass pyrolysis and combustion reaction characteristics. In this study, the dry/wet torrefaction pretreatment of rice husk was carried out at various temperatures (200, 250, and 300 ℃; 180, 200, and 220 ℃) in the fixed bed and high-pressure reactor, where the starting temperature was room temperature, and the heating rate was 10 ℃/min. The proximate and ultimate analysis of samples was then conducted by the infrared fast coal quality analyzer (5E-MAG6700) and element analyzer (Vario MACRO), respectively. The Thermogravimetric Analyzer (TGA) was selected for the non-isothermal (room temperature to 800 ℃, 20 ℃/min) combustion of rice husk, rice husk torrefied char, and rice husk hydrochar. Moreover, the kinetics analysis of pyrolysis and combustion of samples were also conducted with the Coats?Redfern method and three common gas-solid reaction mechanism models (first-order model O1, phase boundary controlled reaction model R2 and R3). An evaluation was made on the effects of dry/wet torrefaction on the pyrolysis and combustion characteristics of rice husk. The results show that the dry/wet torrefaction increased the ash, fixed carbon, and carbon content of raw materials, while decreasing the volatile, hydrogen, and oxygen content. van Krevelen diagram displayed that the dry/wet torrefaction pretreatment behaved essentially like the process of dehydration and decarboxylation of raw materials, where the rice husk torrefied char at 300 ℃ (RT-300) was the most significant. Both pretreatments dominated the pyrolysis and combustion reaction parameters of rice husk, leading to the reduction in the pyrolysis reactivity. Dry torrefaction improved the combustion reactivity of rice husk, particularly with the temperature (200-300 ℃) rising. But the wet torrefaction slightly reduced the combustion reactivity of rice husk. The rice husk torrefied char presented a higher temperature of initial weight loss, a lower maximum rate of weight loss, the corresponding temperature and reactivity than the rice husk hydrochar during pyrolysis at 200 ℃. Moreover, the rice husk torrefied char behaved with the lower final temperature of combustion and the maximum rate of weight loss corresponding to temperature, while the higher maximum rate of weight loss and reactivity, compared with the rice husk hydrochar. Interestingly, the linear regression index of model O1 was higher than that of the other two models for the pyrolysis and combustion reactivity of rice husk, rice husk torrefied char and rice husk hydrochar (the regression indexes were all above 0.95). The dry/wet torrefaction increased the pyrolysis activation energy of rice husk, whereas, the wet torrefaction decreased the combustion activation energy. Moreover, the pre-exponential factor of the samples in the two stages increased with an outstanding linear relationship, as the activation energy increased, indicating the kinetic compensation effect between pre-exponential factor and activation energy during combustion. The rice husk torrefied char had a lower pyrolysis activation energy and a higher pre-exponential factor than the rice husk hydrochar at the pretreatment temperature of 200 ℃. In addition, the higher activation energy of combustion and the lower pre-exponential factor were achieved in the rice husk torrefied char, compared with the rice husk hydrochar.