OCR Text |
Show calculation shows that fuel droplets of 10 ym average size containing 1 ppm of non-volatile elements can produce residual particles of approximately 0.1 urn size. The vaporized compounds of trace elements, especially the organometal1ic compounds, can burn to form oxides or salts of these elements. Most salts under the combustion temperatures of about 1200 C (= 2200 F), are partially dissociated, the extent of which may be determined from their chemical equilibria. In most combustion systems, a steep temperature gradient exists between the combustion gases and the combustor internal walls, especially between the gases and the boiler tubes or cooled turbine blades. The vapors of trace metal compounds, while passing through this temperature gradient region experience sharp temperature changes. The partial pressure of these compounds and their vapor pressure at the prevailing temperature will determine, whether they can condense to form aerosol particles. Thus every condensible compound, depending on its concentration and vapor pressure may tend to condense at a certain distance from the cold surface, which may be called the dew point isotherm surface (Hefner and Lordi, 1981). The multicomponent nature of condensible vapors requires that for every compound a different dew point isotherm surface be defined. The compound with the outermost dew point isotherm, in the absence of residual aerosols and soot particles, will tend to form aerosols by homogeneous nucleation, whereas the compounds with inner dew point isotherms will be formed by heterogeneous nucleation. The time elapsed between the particle formation and its collision with the collector surface and the cooling rate of combustion gases will 1.5.13 |