OCR Text |
Show 2.1 .20 Xk= Xm(1-e2/3, • 2/3 2/3 E - E (1-E1/3)/X g+ E1/3/Xw m 2/3 w (40) (1-,1/3)/A +(e1/3-e1/3)/A + m g w 1 g "+ 27t^0TT2 4_p_T3L. P* X=G/L, G ' G T=B rl£ "-» BH gas • . ' I . . • I B ' I IGJf solid material water, P . X radiation c »«L /L) ' CONDUCTANCES GSIC , L J-LP'VL sJ g m p Gw2 = LgV*W Fig.5. Idealized pore structure for the calculation of the local heat conductivity It is assumed that the particles are large enough. The local porosities z and z can be calculated, since local dry density p, and moisture content u are known (Fig.2), E= (L /L)3 = K p 1-Pk/Pm and e = (L /L) 3 = 1-pk(1/pm+u/p*w) . p* is the density of water, p*=1000 kg/m3 and p is the density of the material ( w m assumed to remain constant during the pyrolysis), if the porosity were zero, the value p =1500 kg/m3 is used in the calculations. m A is the heat conductivity of the gas inside the pores (which depends on the temperature, heat conductivities for different gases, air, N2, H20, CO, C02except H2 nearly coincide and the values of nitrogen are used in the computer program). In reality, however, the gas is not stagnant, but the contribution of A to A. is small. The values A =0.6 W/mK and X =0.6 W/mK are used in k w m Tthheis c aclocnudlucattiivoint.y model was proposed by prof.H.Ryti for dry porous solids and is extended here to moist particles. Although the pore model is rather rough, it gives fairly good agreement with the measured values for dry and wet virgin wood and peat presented in /8/. This comparison was made by using the constant 3 . values A =0.6 W/mK and p =1500 kg/m ; for a specific fuel these constants could be fitted to give a better correspondence. |