| Popis: |
Dried peat is used as a fuel in a number of countries. Drawing on concepts in hydrology and soil physics we present a physically based theory of water movement in unsaturated peat. We apply the theory to the simulation of the drying of a layer of milled peat during a typical production period of four days. The control variables are: (1) the depth to the drained water table, (2) the depth of the milled layer, and (3) the timing and frequency of harrowing. Various ways of characterising peat water content are discussed. Volume wetness is chosen as the natural state variable and related to the heating value of peat as a fuel. As an initial approximation the peat matrix is assumed to be rigid. Extensions are possible for the non-rigid case. We derive the differential equation for liquid and water vapour flow in a column of unsaturated peat of any thickness, subject to boundary conditions (a) at the surface for infiltration of rain or evaporation, and (b) at the bottom of the column, for drainage to a water table or field drains. Two distinct phases of drying are predicted by the theory. In the first phase, evaporation is driven by atmospheric conditions. In the second phase, evaporation is controlled by properties of the peat deposit. The switch is sudden and evaporation declines rapidly, the evaporating surface moving downwards into the peat with the development of a vapour bottleneck. The function of harrowing is to return evaporation to the first phase, in order to capture as much wind and solar energy as possible. |
