Modelling sorption processes of 10-hour dead Pinus pinaster branches

Abstract

Forest fuel moisture content is an important parameter that determines fire risk and fire behaviour. An accurate prediction of moisture content is therefore of great importance in fire management. In the fire risk period, dead forest fuel moisture content changes mainly by water vapour sorption processes so its knowledge enables the development of predictive fire risk models. In the present work, the adsorption and desorption processes and equilibrium moisture content of 10-hour dead Pinus pinaster branches (diameter between 0.6 cm and 2.5 cm) were described in order to develop a moisture content prediction model for this type of fuels. Laboratorial tests were used to determine sorption curves, timelag and equilibrium moisture content for different sets of air temperature (range between 20°C and 40°C) and relative humidity (range between 10% and 90%). The sorption curves and equilibrium moisture were also modelled with forest fuels and agricultural and food products existing models. Field tests were used to evaluate the sorption and equilibrium moisture content models performance. Dead Pinus pinaster branches were collected in central Portugal through the year 2020 and 2021 on the Portuguese fire risk period (15th May to 15th October) between 12:00h and 13:00h LST. Samples with 0.6 cm to 2.5 cm diameter were collected and transported to laboratory to determine moisture content. The laboratorial drying and wetting curves of dead Pinus pinaster branches (0.6 cm to 2.5 cm diameter) show that they are not pure exponential functions, but with different timelag values until equilibrium is reached. Additionally, the results suggest no significant relationship of the timelag periods with air relative humidity but a dependence with air temperature, showing an increase in the sorption rates with temperature. In terms of sorption curves, Modified Henderson and Pabis model provide the best fitting. For this type of fuels, the representation of EMC values as a function of air relative humidity at constant temperature allowed to obtain a typical sigmoid curve. The EMC values obtained were higher for desorption process than for adsorption process, indicated the typical hysteresis effect in these processes. It was found that, besides the models used in forest fires, other EMC models are also suitable to predict fuel moisture content of dead Pinus pinaster branches, as the ones used in agricultural and food analysis.