- Author
- Albini, F. A.
- Title
- Thermochemical Properties of Flame Gases From Fine Wildland Fuels.
- Coporate
- Intermountain Forest and Range Experiment Station, Ogden, UT
- Report
- Research Paper INT-243, March 1980, 46 p.
- Distribution
- AVAILABLE FROM National Technical Information Service (NTIS), Technology Administration, U.S. Department of Commerce, Springfield, VA 22161. Telephone: 1-800-553-6847 or 703-605-6000; Fax: 703-605-6900; Rush Service (Telephone Orders Only) 800-553-6847; Website: http://www.ntis.gov
- Keywords
- wildland fires | fuel fires | heat balance | combustion | fire spread | solid fuels | combustibles
- Identifiers
- process of combustible-gas generation; heat and mass balance model; heat required by a unit mass of fuel; heat available through partial combustion; radiation leakage calculation; structure of the fuel gas-generating zone
- Abstract
- This paper presents a theoretical model for the process by which is generated the gaseous fuel that burns in the free flame at the edqe of a spreading wildland fire in fine fuels. The model is based upon a heat and mass balance for a hypothetical unit mass of solid fuel. As the fuel is heated, it is first desiccated, then pyroloyzed to release a fraction of its weight as combustible (and noncombustible) volatiles, with the reminder converted to char. The heat required to effect this process is supplied by partial combustion of either (1) the volatiles released or (2) equal fractions of volatile fuel and char, leading to two sets of equations. The equations predict the heat of combustion, the stoichiometric air/fuel mass ratio, the mass-averaged temperature, and the mass fraction of unburned fuel in the gas mixture, assuming that the partial combustion occurs at the rich limit (final temperature = 1100 deg c). Also given are equations for the fuel qas mass produced per unit mass lost by the fuel bed within the gas-generating zone. Energy loss from the system is calculated as a fractional loss of radiant energy from the fuel bed. The radiant energy is assumed to be emitted from a tilted plane that represents the surface of contact between burninq and unignited fuel. The fractional energy loss depends upon a single parameter of the fuel bed that is proportional to the fuel weight loading multiplied by the surface/volume ratio of the fuel particles. This functional dependence offers the possibility that a limiting value of fuel moisture content can be related to the geometrical properties of the fuel bed. Several empirical relationships and approximations are introduced to simplify the equations and to reduce data requirements in using the model. It is argued that the speculative and approximate nature of the conceptual model do not warrant greater precision, so the simplification, in themselves, do not weaken the theory. Dependence of the fuel gas properties upon the fraction of fuel converted to char offers the possibility of calculating the synergistic interaction of fuel moisture content and the fire retardant effect of increasinq the char fraction formed. The interplay of intrinsic fuel properties, loading, particle size, and moisture content may permit testing of the theory by compariscn with results from laboratory burns near the extinction limit of moisture content.