The FLAG code consists of the following major modules: Gas Flow (including Gas Turbulence), Gas-Phase Chemistry, Particle Dynamics and Energetics (including Particle Dispersion), Particle Chemistry (including devolatilization and heterogeneous reactions), Particle Collisions, Agglomeration, and Heat Transfer (conduction and radiation). These physical-chemical modules are embodied in computer subprograms. The distinction between a module and a subprogram is that the former is a conceptual division of the significant phenomena, whereas the latter is based on the ease of manipulation within the structure of the computer code. The basic solution procedure is a step-by-step integration of all the governing equations with respect to time, using a semi-implicit, second-order accurate, finite-difference scheme. In this procedure, the field variables, such as pressure, temperature, gas species concentration, velocity, etc. are evaluated at a finite set of spatial locations inside the reactor, as the time goes through a sequence of small, but finite increments. Thus, the evolution of the flow field is analogous to a sequential display of movie frames. To advance the state of the flow field by one time step, a series of calculations must be performed, involving the various modules described above. Figure 2-1 shows the flow chart of the basic computational procedure. Detailed descriptions of each module are given.