The particles that comprise the ash deposits undergo both physical and chemical reactions in the combustor. Physical reactions include melting, vaporization and fragmentation. Chemical reactions include devolatilization and oxidation. All of these processes contribute to the rate of mass growth of a particle. This growth of mass also indicated a volume growth, unless the particle is changing in density as well.
In an effort to develop a comprehensive model of ash deposition, called SAFE, BYU is modeling these processes mathematically. The rate of mass transfer is simply the sum of all the rates of mass loss and mass gain. Each of these has an individual equation that models that mechanism. These equations form the backbone of particle description in SAFE. There are, however, many areas in which the inorganic descriptions lack the detailed physical property descriptions that could be used by the code. Several of the inorganic components undergo additional mass loss by drying or dehydrating (clays), thermal decomposition (pyrite and carbonates), etc. These processes are not yet incorporated in SAFE.