Three Dimensional CFD-based Numerical Study of Chemical Reacting Char Particles in a Fixed Bed Coal Gasifier

摘要

This work is devoted to three-dimensional numerical study of the influence of a hot gas flow and its composition on the carbon consumption evolution within a representative 3D packing of spherical char particles. This field of study is of direct relevance to the understanding of processes inside a fixed bed coal gasifier used for synthesis gas (CO+H_2) production. In particular, the primary interest of this work is the spatial distribution of the temperature and species concentrations in the so-called combustion zone of a fixed bed, where the oxygen reacts with coal particles. In this work the coal-char particles are represented by moisture and ash free nonporous carbon. The model includes six gaseous chemical species (O_2, CO_2, CO, H_2O, H_2, N_2). Three heterogeneous reactions (C+O_2, C+CO_2 and C+H_2O) and two homogeneous semi-global reactions, namely carbon monoxide oxidation and water-gas shift reaction, are employed. The Navier-Stokes equations coupled with the energy and species conservation equations are used to solve the problem in pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration is applied including the effect of the Stefan flow and the heat loss by radiation at the surface of the particle. The range of parameters considered in this study are d = 2·10~(-2) m, T_(in) = 1000 K, Re = 100, where d, T_(in) and Re are the diameter of the particle, the inlet temperature of the gas and the inlet Reynolds number, respectively. Three different inflow gas compositions arc considered: dry-air case corresponding to Yo_2 =.0.233 and Yh_2o = 0.001, gasification case referring to the gas containing less oxygen with water vapor (Yo_2 = 0.11 ,Yh_2o = 0.074), and finally, a humid-gas case characterized by the following gas composition Yo_2 = 0.11 ,Yh_2o = 0.2. The simulations performed for dry-air case showed the effect of flame penetration into the packing of spherical particles at the inlet Reynolds number of 100. In particular, we found out that the penetration length of the flame is about four particle diameters. Parallel, the analysis of results confirmed that the gas radiation plays a key role for the temperature distribution in the gas phase and on the particle surface, since the mass fractions of water vapor and carbon dioxide are high. In particular, comparative simulations showed that the maximum gas temperature is reduced up to 20% comparing to the case when gas radiation is not taken into account. In the operation of most fixed bed gasifiers it is important to limit the temperature below ash melting point. From this point of view, in this work we studied the influence of different ratios of water vapor to oxygen on the flame extinction and temperature distribution between the particles. Further results are discussed.