Combustion Characteristics of Forest Litter in Larger Laboratory Fires as Determined from Fuel Mass Loss, Fluxes of Evolved Gases and Soot, and Gas Temperatures for CFD Fire Modeling

Over large areas of mixed-oak forest in the eastern US, oaks are being replaced by more mesophytic and less fire tolerant species with different litter decomposition dynamics and fuel bed structure. Historical fire seasonality was bi-modal, however, with fire occurrence peaking in both spring and fall. Combustion tests were done at the Forest Products Laboratory in Madison, Wisconsin, with oak and maple litter collected at the Vinton Furnace State Experimental Forest in southeastern Ohio in the spring and fall. Fuel beds were assembled in wire-mesh baskets connected to a weigh scale and ignited with methanol/paper monitored on a separate scale under a 3MW heat-release rate hood. Obtaining properties for CFD fire modeling is a unique capability of the facility in which measurements of fuel mass rate, chemical heat of combustion, combustion major emissions of gas and soot, chemical heat release rate (HRR), and radiant HRR fraction as a function of time and fuel physical properties are obtained for the larger laboratory fire scale. This data provide both direct inputs and predictive model outputs for use in verifying the dynamic fire models, such as FDS. We have compared oak- and maple-dominated litter collected in spring and fall on the basis of a range of combustion characteristics, including gas and particulate (smoke) emissions, peak heat release rates, total heat release, combustion efficiency, and effective and total heats of combustion. Particular results were instructive concerning the sequential stages of smoldering, flaming, and glowing combustions. For all stages the presence of water vapor as sourced from (1) plume entrained ambient humid air, (2) moisture content of the fuel, and (3) fuel combustion of organic volatiles were elucidated, having relevance to meteorological implications for fire plume behavior predictions. The elemental empirical composition of combusting fuel and its stoichiometric heat of combustion were obtainable from the mass balances of O2, CO2, CO, H2O, and soot as function of time for direct input to the fire model as related to sequential phases of smoldering, flaming, and glowing. The measurement of mass loss and temperature versus time provides an added verifying data for the fire model predictions of fuel pyrolysis and the following combustion process.