A steady state model for predicting performance of small-scale up-draft coal gasifiers

Small-scale fixed-bed coal and biomass gasifiers represent an attractive option for distributed combined heat and power generation. As known, gasification phenomena are very complex, involving drying, devolatilization, pyrolysis, heterogeneous and homogenous reactions, with a large number of intermediate and final products. Gasification processes are also influenced by reaction kinetics and fluiddynamical effects, such as temperature and concentration gradients. For this reason, simulation models are able to predict gasifiers performance under the assumption of thermodynamic equilibrium only if the gasification process takes place at a known temperature and the reaction time is lower than the reactants residence time. As a consequence, for fixed-bed gasifiers equilibrium models must consider drying and devolatilization taking place at lower temperature in the heat transfer zone, where solid feed is heated by syngas. Therefore, moisture and volatiles are not involved in the gasification reactions since they are released before reaching the reaction zone. Several models based on steady-state and one-dimensional representations have been developed to reproduce gasification processes in fixed-bed reactors. These models have been found adequate to provide information for engineering design and process optimization. In this framework a steady-state simulation model has been developed at the Department of Mechanical Chemical and Materials Engineering (DIMCM) of the University of Cagliari by using the Aspen Plus computer code for predicting performance of small-scale up-draft fixed-bed coal gasifiers. The model can be used to evaluate the mass and energy balance in each zone of the gasifier and the main characteristics of the syngas produced by the gasification process (composition, mass flow, temperature, heating value, etc.). This paper describes the model and presents the main results of a parametric analysis, which shows how the gasification process is influenced by the main operating parameters. Moreover, the results of the model have been compared with the experimental results of an up-draft gasifier fed with an lignite from Alaska. The above-mentioned gasifier is part of a pilot gasification and gas treatment plant built at the Sotacarbo Research Centre in Sardinia, Italy. The comparison shows that the model well represents the performance of the pilot-scale unit