Modelling and analysis of an innovative CHP system based on biomass gasification, hot gas cleaning and conditioning and SOFC by means of Aspen Plus
Author(s)
Marcantonio, Vera
Date Issued
June 14, 2021
Type
Doctoral Thesis
Abstract
In the last decades, global warming, climate change, national energy security and energy dependency issues have led to the need for an alternative to fossil fuels. In this delicate context, biomass gasification has been demonstrated to be a very useful technology to produce power and hydrogen. Nevertheless, in literature, there is a lack of a flexible and fast, but accurate, model of biomass gasification that can be used with all the combinations of oxidizing agents, taking into account both organic and inorganic contaminants and able to give results that are more realistic.
In order to do that, in the present thesis, a model of biomass gasification has been developed using the Chemical Engineering software Aspen Plus. The developed model is based on the Gibbs free energy minimization applying the restricted quasi-equilibrium approach via Data-Fit regression from experimental data. The simulation results obtained, considering different mixes of gasifying agents, were compared and validated against experimental data reported in literature for the most advanced fluidized bed technology. The maximum discrepancy value obtained for hydrogen, with respect to experimental data, is of 8% and all the other values reached by the developed simulations, considering both organic and inorganic compounds, are in good agreement with literature data. The gas yield reached by the developed simulation is in the range of 1.1-1.3 Nm3/kg.
An important aspect in the usage of producer gas is the removal of harmful contaminants from the raw syngas, since the contaminants present in the raw syngas can ruin the equipment of the plant by corrosion, catalyst poisoning, etc.. Thus, the second object of this study is the development of a simulation model for an innovative hot gas cleaning constituted by a combination of catalyst sorbents inside the gasification reactor, catalysts in the freeboard and subsequent sorbent reactors, which is coupled with the gasification model. The gas cleaning chain simulates the raw syngas clean-up for several organic and inorganic contaminants, i.e. toluene, benzene, naphthalene, hydrogen sulphide, hydrogen chloride and ammonia. The tar and inorganic contaminants final values achieved are under 1 g/Nm3 and 1 ppm respectively, that are the allowable level for the main applications (methanol synthesis, gas turbine, solid oxide fuel cell, etc.).
In order to trust the feasibility of the developed model, it was coupled:
- with the model of a Solid oxide fuel cells (SOFC), that have proved to be an excellent energy conversion device. The SOFC modelling was done without external subroutines, unlike most models in the literature, using only the existing ASPEN Plus blocks; making the model simpler and more reliable. The analysis indicate that there must be a trade-off between voltage, electrical efficiency and power with respect to current density and it’s preferable to stay at low steam to biomass ratio. The electrical efficiency achieved under the operating conditions is 57%, and its high value making these systems very attractive;
- with a 100 kWe micro Gas Turbine (mGT) and an Organic Rankin Cycle (ORC) unit, as the bottom cycle. The integrated system operates with higher net electric power than its nominal operation thanks to the bottom ORC unit and steam injection in the combustion chamber. However, results have shown that the overall electric efficiency is penalized slightly by adopting the wet cycle. The syngas-fuelled system, at its maximum electric power, produces the net power of 127.6 kWel with 23.6 % overall electric efficiency using 25 g/s steam injected into the combustion chamber. Moreover, results confirm the reduction of performance of the integrated system especially the overall electric efficiency when the mGT is fuelled with syngas instead of its nominal fuel, natural gas. Nevertheless, the produced thermal power in the condenser of the ORC unit is increased showing the ability of the integrated system to meet higher users’ thermal demands.
Additional information
Dottorato di ricerca in Engineering for energy and environment
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