Experimental investigation on the co-combustion characteristics of rice husk and coal

Rice husk is one of the major biomass sources in the boilers and furnaces in modern agriculture. Particularly, the combustion characteristics can be in the appropriate state. It is very essential to the combustion characteristics of rice husk and coal blend fuels, in order to effectively promote the biomass utilization and mitigate carbon emissions. Fortunately, the thermogravimetric analysis can serve as one of the most significant approaches for the combustion characteristics. In this study, a systematic investigation was made to determine the combustion characteristics of rice husk, coal, and their blended fuel using thermogravimetric analysis. The samples of blended fuel were prepared with the rice husk mass ratio of 10%, 30%, 50%, and 70%. A series of combustion experiments were was carried out in the temperature zone of 25℃ to 1200℃ with three heating rates of 10, 20, and 30℃·min-1. The evaluation indexes were selected as the ignition temperature, burnout temperature, and the integrated combustion characteristic index. Furthermore, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods were used to calculate the combustion kinetic characteristic parameters. The results show that the ignition temperature and burnout temperature of rice husk were lower than those of coal. There were the significantly improved combustion characteristics of rice husk/coal blended fuel, compared with the pure coal. The addition of rice husk with the high volatile matter content was improved the reactivity of the blended fuel at the lower temperatures, and thus improve the ignition performance. The higher content of fixed carbon in the coal was greatly contributed to the more stable combustion process of the blended fuel. There was no influence in of the lower ash content of rice husk on the combustion rate of the blended fuel and fouling. The optimal ignition temperature of rice husk and coal were 250℃ and 360℃, respectively. The ignition temperature of blended fuel was around 250℃, which was independent of the rice husk mass ratio. However, the burnout temperature of the blended fuel decreased from 730℃ to 578℃ with the increase of rice husk blending ratio from 10% to 70%. The burnout degree also gradually increased, with the weight loss increasing from 86.1% to 91.5%. The comprehensive combustion characteristic index also increased from 2.153×10-8 to 1.183×10-7. The blended fuel presented a wider combustion process. At the same time, the combustion characteristics of blended fuel were significantly improved, compared with the pure coal and rice husk. There was little effect of the heating rate on the ignition temperature of blended fuel, which was still around 250℃ at different heating rates. The maximum weight loss rate was also unrelated to the heating rate. However, the burnout temperature of the blended fuel increased from 562.3℃ to 700℃, as the heating rate increased from 10℃·min-1 to 30℃·min-1. The comprehensive combustion index increased from 3.8×10-8 to 1.923×10-7. The experimental curves of blended fuel presented the a larger peak weight loss rate and smaller temperature range, compared with the theoretical TG curves. The combustion weight loss of blended fuel was failed to simply calculate using linear stacking of the components. There was the a synergistic effect between the rice husk and coal during the co-combustion process. Specifically, the synergistic effect parameters gradually increased with the increasing increase of the rice husk mass ratio, indicating the a more outstanding synergistic effect. The combustion activation energy of all samples was calculated by the FWO and KAS, indicating the an excellent correlation coefficient. The activation energy of coal obtained by FWO and KAS were 37.66, and 31 kJ/mol, respectively. The activation energy of rice husk were 20.96, and 16.3 kJ/mol, respectively. The activation energy of blended fuel increased first and then decreased with the increase of combustion temperature, and rice husk mass ratio, respectively. The maximum activation energy was obtained at a 10% blending ratio.