A model of devolatilization behavior in lignite pyrolysis with solid heat carriers

Abstract

To elucidate the influence of the process conditions on pyrolytic products, the interactions between transport phenomena and pyrolysis kinetics are quantitatively analyzed at the level of a single coal particle. A comprehensive mathematical model is formulated to predict intraparticle multiphysics and devolatilization behaviors; the model contains two correlative one-dimensional unsteady heat conservation equations and ternary mass conservation equations in conjunction with the simplified dusty-gas model. Moreover, a multistep kinetic model of coal devolatilization is adopted to predict the generating rates of the lumped pyrolytic products. Validation of the model against experimental and literature data showed that can predict the transient temperature profiles of a coal particle and the yields of volatiles. Finally, the effects of the main process conditions on the intra-and extra-temperature history of lignite during pyrolysis with solid heat carriers are analyzed. The interactions between physical transport and the pyrolysis reaction are also examined. K E Y W O R D S devolatilization, heat transfer and mass transfer, lignite pyrolysis, numerical simulation, solid heat carrier 1 | INTRODUCTION Recently, the rapid consumption of high-quality coal resources has promoted innovative technologies for effective and efficient utilization of low-rank coal, that is, lignite, which has become a significant gateway to relieve ever-growing energy demands. To the best of our knowledge, lignite deposits are relatively abundant and have the advantage of low-cost mining; however, extensive application of lignite is stunted by its low degree of coalification, involving high moisture and ash content but relatively low energy density. To overcome these drawbacks, various upgrading technologies, including drying, dewatering, and pyrolysis, have been developed to improve lignite quality in terms of calorific value, heat stability, and moisture absorption. Among these techniques, pyrolysis with heat carriers at moderate temperatures is superior because it makes full use of the characteristics of lignite, such as its enriched volatile content and positive reactive behavior. Compared to the external heating mode, in which heat is input from the inner wall of a pyrolyzer, solid heat carriers (SHCs) as direct heat sources used in an internal heating system can greatly increase the effective heat transfer area, which weakens the bed axial temperature gradients and eliminates local overheating. In this way, the severe problem for tar species depositing or cracking on the inner walls of pyrolyzer is suppressed, although the effective separation between the tar and semi-coke powders remains to be solved. In addition, high value-added components are heavily concentrated in tar, such as aliphatic hydrocarbon, aromatic hydrocarbon, and phenols, since the oxygen-containing functional groups in lignite are well retained during pyrolysis, which leads to improved economic performance after hydrofining. Currently, coal pyrolysis with SHCs in a pi...

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