PURPOSE: To reduce an unburnt content through the acceleration of solid combustion by a method wherein iron or iron and an alkali metal, and a alkali earth metal are mixed in solid fuel of coarse particles with a specified grain size for combustion.

CONSTITUTION: In the distribution of an unburnt content in combustion ash of solid fuel, with a boundary of a particle size of 74-100μm therebetween, a large quantity of unburnt contents are contained in a range of coarse particles with a grain size larger than the boundary. The coarse particle has a ratio surface area per unit weight lower than that of a fine particle. This is a main factor by which since during solid combustion of a boiler, combustion is made in a short time, poor combustibility is caused. Thus, iron or iron and an alkali metal, and an alkali earth metal are mixed in solid fuel of coarse particles with a particle size of at least 74μm for combustion. By adhering a mixture, having ultrafine particles, of iron or iron and an alkali metal and an alkali earth metal to the surface of a coarse particle solid fuel or a porous part of coarse particles, combustion on a solid fuel surface and the porous part is accelerated.