Fine particles are those less than 10 microns in diameter and are often not detected as single particles by a light microscope. They have a very large relative surface area which tends to accelerate any reaction as well as increasing their ability to adsorb another substance. Flue-gas corrosion is caused by sulphur compounds and water vapor forming sulphuric acid which attacks iron to form ferric and ferrous sulphate. The former is a catalyst in the formation of sulphur trioxide from sulphur dioxide. Sulphur trioxide and ferric sulphate also raise the dew point. Fine particles consist of unburned fuel and ash spheres known as cenospheres or fragments of the latter. They increase the reaction rate as well as the condensation temperature and rate. They can attain a temperature lower than that of the gas stream by radiation and are good condensation nuclei. A qualitative test of the formation temperature, extent, and nature of deposits from cleaned and dirty gas showed that deposits tend to build up in a narrow range and that cleaning the gas greatly decreases the rate of deposit build-up.

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