The initial particle number concentrations in flue gas exhausted from coal-fired boiler were of the order (2.0–3.0) 107 cm3, and it can be regulated by adding diluted air with the same temperature as flue gas into the buffer vessel. The particle size distribution at different amount of air added is shown in Fig. 3b. As can be seen from this figure, the fine particles are diluted by adding air into the flue gas. That is, only the number concentration of each range of particle size decreases, and the size distribution of fine particles is not changed.