Energy valorization of Sicilian agro-industrial residues by steam gasification

Among the renewables primary energy sources, biomasses are promising candidates that are able to ensure programmable and constant energy production, while providing energy security, especially in rural areas where the access to energy infrastructures is limited. Agro-industrial activities represent important source of residual biomass that could be used as renewable feedstocks for energy purpose, enhancing local economies, while reducing residues/wastes landfilling. Among the different thermochemical biomass conversion technologies, gasification allows to produce a gas mixture (syngas), used for electricity generation, synthesis of liquid fuels and chemicals. Furthermore, it is also considered a preferable route for electricity production from small plants in remote areas, where gasification coupled with internal combustion engine systems are more convenient than combustion coupled with steam engine. In order to realize and encourage the development of local bioenergy economies, further contributions to increase technical data and information about the feasibility of sustainable projects are of fundamental importance. From the above, the intent of this dissertation is to supply fundamental information and tools for the design of steam gasification reactors and systems fed by agro-industrial residues that are typical of the Sicilian economic network and residues from 2nd generation bioethanol production. The first approach used for facing this issue consisted in the study of gas-solid reactions kinetics of steam gasification of different chars obtained from residual biomass. In addition, the kinetic parameters were also determined. The knowledge of steam-char kinetics is of fundamental importance in the design of efficient gasification reactors, since it results to be the controlling reaction when steam is used as gasifying medium. Thermogravimetric analysis showed that the different chars are characterized by different reaction profiles and different reactivity. The influence of ash-forming elements with catalytic (Ca, K) and inhibiting (Si, P) character was evaluated by introducing the Inorganic Composition Ratio (ICR = (Ca+K)/(Si+P)), in order to include in one index both catalytic and inhibiting elements. A good correlation was found between ICR and time of half conversion. It has been noticed that samples with similar ICR show similar reaction profiles and comparable time of conversion. In particular, the samples with ICR < 1 showed marked decelerating reaction profiles, while samples with ICR > 1 exhibited sigmoidal reaction mode. It was further determined that, unlike to other authors, the presence of calcium could not be neglected in order to find an acceptable correlation between kinetic behavior and ash composition, which is the biomass characteristic that has the most influence on the char gasification kinetics. The kinetic study of various feedstocks is not only interesting for reactor design optimization, but it is also a useful tool for planning a proper supply chain for steam co-gasification of local residual biomass. For instance, it was highlighted that in terms of conversion kinetic, the best feedstock integration for co-gasification could be made with orange peels and grape pomace. In addition, both residues from second-generation bioethanol showed a very similar reactivity. The second approach consists in the development of a simulation tool that is able to supply preliminary information for the design of Combined Heat and Power (CHP) systems for energy valorization of agro-industrial residues. The study has been developed using Aspen Plus simulation software, which allows to develop evaluations about process feasibility and optimization of process parameters, in order to maximize the efficiencies, without the necessity to involve commercial-scale facilities. With this approach, it is possible to obtain useful data for the proposed system optimization, while evaluating its sustainability and the potential of the application in a real context. The CHP system that has been studied consisted in the combination of Solid Oxide Fuel Cell (SOFC) with a citrus peel gasificator (using air and steam as oxidants), which used residues from the citrus juice production as fuel. The scope of the feasibility study was to determine the energy balance of the process and verifying the possibility of self-producing the heat required for the drying step, being this the most energy-demanding one. A zero-dimensional simulation model, using Aspen Plus simulation software, was used to analyze the combined system. Mathematical model of the gasification unit was experimentally validated by lab-scale experiments, while the SOFC model was validated with data available in literature. After the stand-alone models validation, the two units’ models were integrated in order to simulate the scale-up of the CHP system, which was able to generate an output of 120 kW DC. Hence, the required feedstock flow-rate and the system energy performances were evaluated. Results showed that 77.2 kg/h of dry biomass (0% H2O) are required in order to produce the syngas necessary to feed the 120 kW DC SOFC unit. If 7,000 h of operational year are considered, the total amount of wet citrus peel with 82% of water content (as it comes out from the extraction process and before the mechanical drying) will be about 429 kg/h and 3,003 t/year, with the possibility to produce 580 MWhe. This information revealed the potential of energy production from a single citrus juice company, which is about 0.193 MWhe/t. Furthermore, it was also possible to find the amount of CO2 savings, that is 189 t/year, considering biomass as a carbon neutral fuel. It can be concluded that by using a simulation model, it was possible to understand the potential of citrus residues energy valorization through the integration of gasification process with a solid oxide fuel cell, the process sustainability, while some of the operative conditions were optimized.