Utilization of rice straw as an alternative fuel source to fossil fuel has been considered. However, properties such as water content, low heating value, less grindability, high bulk density, and high ash content are problems in such a biomass fuel supply chain. This study attempted to produce wood pellets mixed with torrefied rice straw (WPTRS) to solve the problems related to using rice straw as fuel. For that, preferred torrefaction conditions, such as torrefaction temperature and holding time, were determined by indicators of the grindability, energy loss, and heating value. As a result, the preferred grindability was found at torrefaction temperatures of over 220 °C. In terms of energy, we derived two viewpoints: i.e., if minimum energy loss is prioritized, the preferred torrefaction temperature is 220 °C, and for an increase in the heating value of rice straw, 280 °C was preferred. Next, WPTRS was produced using rice straw torrefied at 220 °C and 280 °C, and by changing the mixing ratio of rice straw to wood. Burning tests were conducted to clarify the WPTRS characteristics and the results revealed that the torrefaction at a temperature of 280 °C increased WPTRS's heating value. In addition, the basicity can be used as a good indicator for clinker production when wood and rice straw are used as fuel.

Utilization of rice pellets as biomass fuel has many challenges compared with wood pellets, including low heating value, low bulk density, and high ash content, that cause high logistic costs and clinker formation. To solve these problems, this study focused on upgrading the quality of rice straw pellets to improve logistic efficiency by torrefaction and mixing with wood chips. Torrefaction and mixing with wood chips was effective to improve the energy density, the mechanical durability and the contents of fixed carbon, nitrogen, sulfur and ash, leading to the improvement of both logistic efficiency and combustion behavior. The burning test showed that increases in the fixed carbon content and the mechanical durability contributed to stable increases in the temperature inside the furnace during starting up and reduced the amount of ash in the exhaust gas during stable combustion.