Numerical Study of the Pyrolysis of Ellipsoidal Low-Rank Coal Briquettes

Low-rank coal (LRC) upgrading is essential to convert LRC to a more thermal-efficient and environmentally friendly fuel before utilization in coal-based industries. Briquetting and pyrolysis are the dominant route, where briquettes are usually in the shape of an ellipsoid through a two-roll briquetting process. In this work, an integrated numerical model is developed to predict the pyrolysis process of ellipsoidal LRC briquettes in a packed-bed pyrolyzer. A computational fluid dynamics (CFD) model is developed to describe the flow and thermochemical behaviors related to the pyrolysis of ellipsoidal LRC briquettes, including dewatering, pyrolysis, and other homo- and heterogeneous chemical reactions. A discrete element method (DEM) model is used to describe the packing density distribution of ellipsoidal briquettes. The model is validated against the measurements in a pilot-scale test rig in terms of the temperature history and gas species yields. Typical in-furnace phenomena are illustrated, including flow field, temperature field, and product evolution. Then, the effects of key variables, including briquette properties and heating conditions, on the pyrolysis behavior are investigated quantitatively. The effects of key parameters, including briquette moisture, final pyrolysis temperature, and briquette aspect ratio, on pyrolysis are studied, and the optimal values are identified under the given conditions. For example, more H-2 and less CH4 are generated when the moisture content of initial briquettes is increased in the range of 2-15%, but in the range of 15-20%, H-2 is then decreased and CH4 is increased. The maximum packing density is obtained when the aspect ratio of briquettes is 2.0. However, the highest temperature increase rate is observed when the aspect ratio is 1.7. An appropriate final pyrolysis temperature, 973 K in this study, is suggested to balance the pyrolysis rate and energy consumption. This model provides a cost-effective tool for optimizing the design and operation of pyrolyzers for ellipsoidal LRC briquettes.