OCR Text |
Show 2.1.19 SOME CALCULATFD EXAMPLES AND MEASUREMENTS There is a great diversity between the reported values for the specific heats of wood and char /1/,/3/,/8/. The proposed model enables to take into account the dependence of the temperature and the degree of charring on the specific heat, but due to lack of detailed measurements, the values c=(1625p/p0+675)J/kgK is used for the dry partially charred wood. This correlation gives c=2.3kJ/kgK for virgin wood and c=1.0kJ/kgK for char (p/p0=0.2). The reported values for the heats of pyrolysis greatly vary. The expression for the constants H, shows that the specific heat and the heat of pyrolysis have a similar effect. If too small a specific heat is used when calculating (from measurements) the heat of pyrolysis, the pyrolysis reactions would seem to be more endo-thermic, and if the specific heat is too large, the reactions would seem to be more exothermic. It is obvious that in order to evaluate the heat of pyrolysis reactions accurately enough, the dependence of the specific heat on the temperature and on the degree of charring should be well known. The results in this paper have been calculated by using the values 1,= 610 kJ/kg. /10/ for the pyrolysis and 1=2257 kJ/kg for the vaporization of the liquid water. The specific heats c=2.0kJ/kgK for steam, c=1.0kJ/kgK for volatiles and c=4.18kJ/kgK for water are used in the calculations. Due to the anisotrophy of wood the heat conductivity depends on the direction and an average value is used for wood particles. A model for the heat conductivity of partially charred dry wood has been proposed /"] / and some values for dry and wet wood and peat have been tabulated for example in /8/. Due to the lack of detailed knowledge of the heat conductivities of different partially charred biomass fuels and fuels containing moisture, the following model is proposed here.for partially charred and wet fuels. The idealized pore structure is shown in Fig.5, in which the pores are idealized to be cubes and the moisture is assumed to be attached on the pore walls. If the temperature T>373 K, the local moisture content is zero. The electric analogue gives for the local heat conductivity the expression |