Oxygen transfer problems in coal processing bioreactors

Since coal bioprocessing is a heterogeneous reaction a productive bioreactor must contain the highest possible surface area per unit volume. This implies a high concentration of finely ground coal. Heap'' or pile'' type reactors have a high coal concentration, but it cannot be finely ground or it would blow away and the pile would become waterlogged when the solution of nutrients and bacteria was sprayed over it. Waterlogging stops microbial activity by preventing oxygen diffusion into the pile. In the alternative slurry'' type of reactor the coal can be finely ground, but the slurry concentration is limited. Increasing the concentration not only increases the demand for oxygen, it also dramatically reduces the oxygen transfer rate into the slurry. This is due to coalescence of the air bubbles into a few large bubbles which transfer oxygen inefficiently. Thus the productivity of both reactor types is ultimately limited by oxygen transfer. Experimental data and theoretical calculations relevant to this problem are presented. They include oxygen transfer rate versus slurry concentration curves for a 200 liter aerated trough, and liquid holdup versus liquid flow rate curves for coal piles of various particle sizes. The permeability of coal piles to airflow is given asmore » a function of a particle size and liquid holdup. The implications of these data for choosing a reactor type for different bioprocessing operations is discussed. 11 refs., 5 figs.« less