Carbon emissions must be reduced to solve the global warming problem. A biomass alternative to coal, which causes global warming, can be used for carbon emission reduction. The heat generated when biomass is burned in a furnace is used to boil water to generate steam, and the steam generates energy by operating a steam turbine. Hence, high heat must be obtained while greenhouse gas emissions are reduced. In this study, an air-cooled combustion furnace with swirl flow is used. The temperature, carbon monoxide production, and thermal flow characteristics after the combustion reaction were examined using computational fluid dynamics (CFD) in accordance with the angle of the secondary air input inside the combustion furnace. Consequently, it was established that the input angle affected the combustion properties. According to the input angle, there was a temperature differential of up to 137°C (10%) and a carbon monoxide mass fraction difference of up to 0.006(33%) at the furnace's bottom chamber.