OCR Text |
Show - 10 - The t e rm dR/dt can be solved from the equations (12) and (1 5 ). The heat of combustion 6 H=32.9 MJ/kgC for the r eaction C+0 2=C0 21 6 H=-14.0 MJ/kgC for the reaction C+C0 2= 2CO and H=8.36 MJ/kgC for the reaction C+,0 2=CO. The effective temperature Tef=TgooI when the combustion product is C02 and there is no flame (model 1). The effective temperature Tef=Tgoo+ f~HCOYooo/CgI when the CO produced is combusted in the boundary layer (model 2) and 6 HCO=10.0 MJ/kgCO. Te f=Tgoo in model 3 with no flame. The Stefan flow has an effect on the heat transfer coefficient, h/ho = !1/St/{exp(±1/St)-1}, where ho denotes the case without the efflux flow. The plus sign corresponds to the model 2, since the heat comes from the flame to the particle surface in the opposite direction to the efflux flow. In other models the minus sign is valid. The heat transfer coefficient ho is calculated by using the correlation (7). CALCULATED EXAMPLES The pyrolysis model and the five char combustion models have been programmed for calculations. The computer program makes it possible to compare different models, the combustion behaviour of different fuels and to obtain the combustion curves of different alternatives on the same screen. It is also possible to see the effect of different parameters, for example the particle size, the gas t~ature, the gas oxygen content on the combustion and make comparisons. The standard conditions from which perturbations are made in the following examples are presented in Table 1. The computer model makes it possible to compare the combustion behaviour of different coals. However, so far the DTG-curve one coal example (Fig. 1) has been programmed. The relative velocity between the particle and the gas is assumed to be zero. |