PERFORMANCE ASSESSMENT OF FIXED-BED UP-DRAFT GASIFICATION BY STEADY STATE MODEL

Coal and biomass co-gasification could be an attractive option to contribute to the reduction of greenhouse gas emissions in distributed combined heat and power (CHP) generation. In this framework, several studies on coal and biomass gasification are currently under development by Sotacarbo in its CO2 Technology Centre of Sulcis (South-West Sardinia, Italy). In particular, an experimental activity is currently under development in the demonstration-scale gasification plant. The unit is mainly composed by a fixed-bed up-draft gasifier (characterized by an inner diameter of 1.3 m) operating at atmospheric pressure. As known, gasification phenomena are very complex, involving drying, devolatilisation, pyrolysis, heterogeneous (solid-gas phase) and homogenous (gas phase) reactions, with a large number of intermediate and final products. Gasification processes are also influenced by reaction kinetics and fluid-dynamical effects, such as temperature and concentration gradients. In order to optimize the experimental campaigns in the demonstration unit (considering the high cost of each test), the performance of the gasifier has been predicted by a steady-state simulation model developed by DIMCM (the Department of Mechanical, Chemical and Materials Engineering of the University of Cagliari). The steady-state fixed-bed gasification (SFBG) model schematizes the gasifier into several different zones: coal preheating and drying, devolatiliza-tion, gasification, combustion and oxidant preheating, assuming char gasification at thermody-namic equilibrium. The model evaluates the mass and energy balance in each zone and the main characteristics of the syngas produced by the gasification process (composition, mass flow, temperature, LHV, etc.), for given fuel composition and fuel, steam and oxidant mass flows. Gasifier mass and energy balances and raw gas composition also influence downstream gas cleaning and treatment systems and they need to be preliminarily estimated for design, analysis and performance evaluation of the overall system (gasification, gas cleaning, desul-phurisation, CO-shift conversion, CO2 and hydrogen separation, etc.). This paper presents a gasifier’s performance comparison between the experimental data col-lected during several tests in the demonstration-scale plant and the main results obtained by the SFBG model.