Each fuel component has the ability to receive and evaluate a request to ignite or burn; the component's decision to burn will be a function of how much burnable material does it have available, and how ``burnable'' it the material (how moist is it, how densely packed, whether it's already burning, etc.). The fuel component will compare the weight (urgency) of the request to its own burnability, and ``decide'' (via a random number) whether to ignite or not. If the component is dry grass, a very light request could result in a burn; if the component is a moist log, only the most ``urgent'' of requests would result in it igniting.
Once the component is ignited, it ``burns'' at each time step by decreasing its fuel load, and possibly changing its burnability characteristics (dessication, heating, etc.) as a function of the intensity of the burn. Additionally, it will start sending out messages that it is burning, which get translated at higher levels in the simulation as ``requests'' to other fuel components to burn.
In this preliminary model, the fuel components are modeled roughly after the fuel models in the BEHAVE fire simulation (Andrews, 1986), which is one of the more trusted fire simulation packages used by public land managers in the United States. The fuel classes used are 1 hour, 10 hour, and 100 hour fuels. These classes are named according to the estimated time that it takes for them to dry to equilibrium with the atmosphere in idealized conditions; for this discussion it is safe to think of them simply as ``grasses'', ``twigs'', and ``branches'' or ``wood'' respectively. Estimates of loads for each fuel type was taken from simple estimates based on vegetation type from GAP landcover types.
Each component is encapsulated into an object, with its own (possibly unique) burn models and characteristics contained within. While each component is capable of responding to the same requests, the actual way that it responds becomes an internal detail of the object. Thus, an object representing grass can have a burn model that is appropriate for grass, while twigs and branches in the same immediate area can burn according to their own rules.
This model presently is only implementing the BEHAVE fuel types. However, there is no limit on the number of fuel components that a cell may contain, nor are there any constraints on the internal details of the components as long as the ``interface'' (protocols for accepting messages) are consistent. Thus, other kinds of fuels (ground litter, slash from a clearcut, structures, and pickup trucks) can respond to the same requests to ignite, though their internal details of how they burn can be quite variable.