Experimental research on the pyrolysis and gasification of randomly packed straw pellets was conducted with an emphasis on the reactive properties of the shrinking porous structure of the pellets. The apparent kinetics of such pyrolysis was approximated by the random pore, grain, and volumetric models. The best approximation results were obtained with the grain and random pore models. The self-organized oscillations of the pellet conversion rate during pyrolysis were observed. Two complementary explanations of the phenomenon are proposed. 1. Introduction Old and modern gasification reactors are fed by various types of fuel, including raw biomass and biomass in pelletized form. Wood chips and chunks of different forms and sizes are also applied, along with a pulverized form of pellets. As the size of a biomass particle decreases, its pyrolysis and gasification occur more quickly. Thus, it may be beneficial for the conditions of industrial scale gasification to feed reactors with a pulverized fuel. However, if one considers a fixed-bed type reactor, the small particles of pulverized fuel may be partially sintered during pyrolysis. The resulting char will have a completely different permeability for gasification agents than that of the pulverized particles in their initially loose state. The release of tars and oils during pyrolysis may also substantially decrease the permeability of the main biomass stock for gasification agents. The bubbling of the particles in fluidized bed-type reactors is among the alternatives that avoid these drawbacks of fixed-bed type reactors. The bubbling, however, has its own disadvantages; for example, it is necessary to prepare particles with a narrow size distribution, and a relatively high flow of a gasification agent is required to maintain bubbling of the particles. Considering these issues and the additional energy and processing costs required for fuel pulverization, exploiters and developers of industrial scale gasifiers and gasifiers of the types described in [1] may prefer the application of raw biomass, wood pellets, chunks, and chips instead of their pulverized form. Despite this choice, the developers and exploiters of reactors should consider the dependence of gasification regimes on the evolution of porous structures that are created in the bed. To achieve the optimal regimes, it should be possible to predict the regimes of both pyrolysis and char gasification in a packed bed as they depend on the temperature, gasification agent, and fuel feeding rates. The evolution of porous structures, from the delivery of
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