Process analysis and performance evaluation of updraft coal gasifiers

Coal gasification is becoming commercially even more import due to its potential application in hydrogen, ammonia, methanol and other chemicals and clean fuels production, other than power generation, together with carbon dioxide capture and sequestration. In this framework the technological development is also addressed, with a renewed interest, to simplified processes and plant solutions based, for example, on gasification with air (or air enriched with oxygen) and on moving or fluidised bed gasifiers, of interest for small and medium scale plants. The design, analysis and performance evaluation of the overall system (gasification, gas clean-up, desulphurisation, CO-shift conversion, CO2 and hydrogen separation, etc.) require a preliminary estimation of gasifier mass and energy balances and raw gas composition, which influence the whole downstream gas clean-up and treatment systems. The present study reports a process analysis and performance evaluation of updraft moving bed gasifiers, which have been carried out by a computer simulation model developed using the Aspen Plus 12.1 software, The model schematises the gasifier in several different zones: coal preheating and drying, devolatilization, gasification, combustion and oxidant preheating, under the hypothesis of char gasification at thermodynamic equilibrium. The model allows to appraise the mass and energy balance of the gasifier and the main characteristics of the syngas produced by the gasification process (composition, mass flow, temperature, lower heat value, etc.), being assigned coal composition and coal, steam and oxidant (air eventually enriched with oxigen) mass flows. In this paper the model is applied to predict the performance of two updraft moving bed gasifiers (sized respectively for 35 kg/h and 700 kg/h of low sulphur coal and high sulphur coal (Sulcis). The gasifiers are part of a small pilot gasification and gas treatment plant for hydrogen production under construction at the Sotacarbo Research Centre in Sardinia