Mathematical modelling of transport processes and degradation reactions in piles of forest fuel material

Abstract

Mathematical models are used in conjunction with laboratory and field experiments for investigating the complex coupling between the transport of oxygen, heat and water and the degradation reactions in piles of chipped forest fuel material. Preliminary investigations indicate that natural convection plays a major role in the supply of oxygen and in the release of heat for the degradation process. Natural convection is governed by temperature differences in the pile and the permeability of the material. Calculations using experimental temperature profiles and permeability distributions give air flow rates (dry basis) of 0.1–0.2 m 3 m −2 h −1 near the surface. The coupled oxygen and heat transport equations have been solved, whereas the transport of water is assumed to be quasi-stationary. The calculations confirm that natural convection plays a major role in the degradation process. The degradation rate in the pile is largely determined by the convective transport of oxygen into the pile. Maximum temperatures of 70–90 °C and average degradation rates of 0.003–0.012 kg (kg DM) −1 month −1 are calculated. The development of the oxygen profile is fast compared with that of the temperature profile. Typically the former is close to steady state after 3–4 d whereas the latter has a time-constant of ~3 weeks. The transport of water is transient in nature, but the change in the moisture content is much slower than the changes in the oxygen concentration and temperature. The degradation rate is strongly influenced (indirectly via the temperature) by the moisture content of the material, which is an important factor in the heating process. The temperature rise is lower in moist material because the heat capacity of the bed is higher. At higher temperatures, evaporation and condensation play an important role in the heating process. Evaporation leads to a lower heating rate because it removes heat efficiently. A large amount of heat is released when cooler parts of the pile are reached. Using the models with reaction rate data from laboratory experiments and permeability data from field measurements, simulations of the oxygen concentration, temperature and degradation in the pile agree reasonably well with observations in the field.