Why does the efficiency of a boiler increase when pulverized coal is used in its furnace

For same energy output when energy input is lesser efficiency increase. When maximum heat energy is generated from coal or losses in heat transfer are reduced efficiency increase. Coal in boulder size is burnt combustion may not be complete. There will be more unburnt coal. As per coal chemistry and as per boiler flue gas velocity as designed best coal size for full combustion is designed. Maximum crushed coal is also not the best. Then there will be more coal dust. According to general design in various technologies pulverized coal give best combustion for FBC boilers. For CFB boilers generally crushed coal size is 6 mm.

When you achieve best combustion mean this is main contributor for increase in efficiency. Boiler design, heat transfer, flue gas velocity, heat losses, are other factors for efficiency.